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Scanning Tunneling Microscopy of III–V Compound Semiconductor (001) Surfaces
Abstract
While the (001) oriented substrate of compound semiconductors are most commonly used in fabrication of wireless and opto-electronic devices by molecular beam epitaxy, metallorganic chemical vapor deposition and related techniques, their surface structures have been puzzling from the beginning of the development of the techniques with which these materials are artificially prepared. This paper reviews the advances in comprehensive understanding of the geometric and electronic structures and chemical properties of the principal reconstructions found on the (001) surface of III-V compound semiconductors including arsenides, such as GaAs, InAs and AlAs, phosphides, such as GaP and InP, antimonides, such as GaSb, AlSb and InSb, and also nitrides (GaN), with the emphasis on the GaAs(001), during the first decade following the invention of scanning tunneling microscopy.
... Semiconductor surfaces, particularly those of silicon (001) and gallium arsenide (001), have been studied for over twenty years ( [1,2], and references therein). Based on this work, a number of principles have been developed to explain the reconstructions observed on these surfaces. ...
... In addition, the As and Ga dimers are arranged within the unit cell to yield the lowest possible electrostatic energy [7]. These rules have been successful in explaining the reconstructions observed on GaAs(001), and on several other compound semiconductor surfaces [1,2]. ...
... The InP(001)-͑2 3 1͒ is unlike any reconstruction observed for GaAs(001). In the latter case, a combination of arsenic or gallium dimers and dimer vacancies provides the conditions necessary to achieve a semiconducting state [1,2]. The difference between these two materials may be related to the relative strengths of the dimer bonds compared to the anion-cation back bonds. ...
An InP(001)- \(2×1\) reconstruction was prepared by metal-organic
vapor-phase epitaxy. Scanning tunneling micrographs and infrared spectra
of adsorbed hydrogen revealed that the \(2×1\) is terminated with
a complete layer of buckled phosphorous dimers, giving rise to
p\(2×2\) and c\(4×2\) domains. A surface band gap of
1.2+/-0.2 eV was measured by scanning tunneling spectroscopy. The
buckling can be explained by electron correlation among the dangling
bonds of pairs of phosphorous dimers. This allows the surface to achieve
a lower energy, semiconducting state. This reconstruction mimics the
Si(100)- \(2×1\), which is terminated with buckled silicon dimers.
... This of course excludes a vast body of work on the gallium-rich reconstructions, kinetics, diffusion, etching, adsorbate induced reactions, growth, etc. that is simply too large to fit into one article. The interested reader should refer to other reviews of this and other compound semiconductor surfaces that have been previously published [6][7][8][9][10][11][12][13]. ...
... The coordinates of the fully relaxed structures for the a; b and b2 models as computed using DFT-LDA by Schmidt and Bechstedt are displayed in Table 1 and Fig. 18 [139]. [62,65,143,13,12]. All three STM images clearly showed the presence of only two dimers and two dimer vacancies per unit cell. ...
... Fig. 17) [83,85,[139][140][141]. In addition, STM studies of surfaces prepared in the a-phase all confirmed that the atomic structure was similar to the b-phase with only two dimers in the outer most surface layer [62,65,143,13,12,144]. The only striking difference that these studies did reveal was that the a phase surfaces were more disordered than surfaces prepared in the b-phase. ...
This article discusses the past 40 years of research covering the equilibrium thermodynamic properties of the arsenic-rich GaAs(0 0 1) surface, which is the starting surface for producing the majority of optoelectronic devices worldwide. A coherent picture of the observed surface structures, theoretical calculations, and experimental results will be presented. The interplay in surface-free-energy-reduction between reconstruction transformation and roughening is now well understood for the GaAs(0 0 1) surface and will be discussed. The recent confirmations of the structural models for the (2×4) and c(4×4) reconstructions as well as the discovery of preroughening aid in this understanding.
... Moreover, these calculations suffer from the DFT-LDA band gap underestimation due to the neglect of electronic self-energy effects. The stoichiometry-dependent surface structures of GaAs(001) have been studied intensively over the last decade [19]. In spite of this, theoretical [20,21] as well as experimental studies [22,23] have only very recently indicated the existence of new structures for the stoichiometric (2 Â 4) reconstructed a phase of GaAs(001) as well as for the Ga-rich (4 Â 2) surface, distinct from the models accepted so far. ...
... We are not aware of any experimental evidence supporting this computational finding. Rather, the formation of (4 Â 6) symmetries is reported for extreme Ga-rich GaAs surfaces [19]. We probed a large number of (4 Â 6) surface structures [34,35]. ...
... One reason for this discrepancy could be that c(8 Â 2) rather than (4 Â 2) reconstructed domains [19,21] are responsible for the measured signal. Unfortunately, our ab initio approach cannot presently be extended to the study of the RAS of c(8 Â 2) reconstructed domains, due to the exceedingly large computational requirements of such a calculation. ...
The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.
... Until recently, the stoichiometry-dependent surface structure of GaAs(0 0 1) seemed to be well understood [1]. New theoretical [2,3] as well as experimental studies [4,5] show, however, that geometries different from the ones accepted so far occur under certain surface preparation conditions. ...
... We are not aware of any experimental evidence supporting this computational finding. Rather the formation of ð4 Â 6Þ symmetries is reported for extreme Ga-rich GaAs surfaces [1]. Xue et al. [1,12] point out that one has to discriminate between a genuine ð4 Â 6Þ reconstruction which is more Ga-rich than the Ga-rich ð4 Â 2Þ reconstruction, and a pseudo-ð4 Â 6Þ phase, which actually consists of a mixture of the ð1 Â 6Þ, the ð4 Â 2Þ and the genuine ð4 Â 6Þ phase. ...
... Rather the formation of ð4 Â 6Þ symmetries is reported for extreme Ga-rich GaAs surfaces [1]. Xue et al. [1,12] point out that one has to discriminate between a genuine ð4 Â 6Þ reconstruction which is more Ga-rich than the Ga-rich ð4 Â 2Þ reconstruction, and a pseudo-ð4 Â 6Þ phase, which actually consists of a mixture of the ð1 Â 6Þ, the ð4 Â 2Þ and the genuine ð4 Â 6Þ phase. To explain the surface structure of the genuine GaAs(0 0 1) ð4 Â 6Þ surface, Xue et al. [1,12] propose a regular array of Ga clusters consisting of 6-8 atoms on top of a ð4 Â 2Þ-reconstructed GaAs surface (see Fig. 3). ...
We re-examine the GaAs(0 0 1) surface by means of first-principles calculations based on a real-space multigrid method. The cð4 Â 4Þ; ð2 Â 4Þ and ð4 Â 2Þ surface reconstructions minimize the surface energy for anion-rich, stoichiometric and cation-rich surfaces, respectively. Structural models proposed in the literature to explain the Ga-rich GaAs(0 0 1) ð4 Â 6Þ surface are dismissed on energetic grounds. The electronic properties of the novel zð4 Â 2Þ structure are discussed in detail. We calculate the reflectance anisotropy of the energetically most favoured surfaces. A strong influence of the surface geometry on the optical anisotropy is found. # 2002 Elsevier Science B.V. All rights reserved.
... Approximately 100 surface structures have been proposed for the GaAs(001) surface and about a dozen surface reconstructions have been experimentally observed [21]. Each of the observed structures has been shown to be dependent upon the vapour and surface composition and surface temperature. ...
... Between 820 and 870 K, the surface structure deviates from the ideal β2(2 × 4) structure with increased surface defect concentration (i.e., atoms are either missing or are displaced from the ideal β2(2 × 4) atomic configuration), while between 750 and 850 K the surface reconstructions appear to be a combination of the c(4 × 4) and the β2(2 × 4) surface structures [31,32]. The remaining (2 × 4) surface reconstructions shown in figure 1 have at one time or another been suggested as low-energy structures and are included for later comparisons of DFT calculated surface free energies [14,21,27]. ...
... Figure 2(a) indicates that over a wide range of the arsenic chemical potential, the lowenergy arsenic-rich β2(2 × 4) surface reconstruction is dominant. As indicated above, the β(2 × 4) had previously incorrectly been thought to be the most stable over this chemical range [21]. More recent DFT calculations indicate that the β(2 × 4) and β2(2 × 4) surface energies are in fact quite close. ...
Current interatomic potentials for compound semiconductors, such as GaAs, fail to
correctly predict the ab initio calculated and experimentally observed surface
reconstructions. These potentials do not address the electron occupancies of dangling bonds
associated with surface atoms and their well established role in the formation of
low-energy surfaces. The electron counting rule helps account for the electron
distribution among covalent and dangling bonds, which, when applied to GaAs surfaces,
requires the arsenic dangling bonds to be fully occupied and the gallium dangling
bonds to be empty. A simple method for linking this electron counting constraint
with interatomic potentials is proposed and used to investigate energetics of the
atomic scale structures of the GaAs(001) surface using molecular statics methods.
... directly illustrates that they are located on the surface of the sample. Therefore, these additional components can be related to Ga-Ga bonds of the Ga-rich (4 × 2)reconstruction [74]. Since the increase of G1 is larger than for G2 it can be assumed that the G1 atoms originate from the topmost layers of the surface. ...
... A surface reconstruction refers to the process where the atoms of a crystal-surface assume a dierent structure than the bulk. Basically, the mechanism of dimerization is responsible for the III-Vcompound semiconductors (001)-surface reconstructions[73].Some of the known reconstructions of GaAs(001) are listed below assorted in a descending order of its Arsenic-concentration[74]. ...
Die vorliegende Arbeit befasst sich mit der Untersuchung der Grenzflächen des Spintronik Mehrschichtsystems MgO/Fe/GaAs(001). Der magnetische Tunnelwiderstand (TMR) und der Riesenmagnetowiderstand (GMR), die in Spintronikbauelementen Anwendung finden, treten an den Grenzflächen auf und werden durch die chemischen und strukturellen Eigenschaften beeinflusst. Die Photoelektronenspektroskopie eignet sich besonders zur Analyse von dünnen Filmen und Grenzstrukturen von Mehrschichtsystemen. Sie ermöglicht eine genaue chemische Untersuchung über hochaufgelöste Spektren der Rumpfniveaus einzelner Elemente. Die spektralen Komponenten in einem Signal beinhalten Informationen über die lokalen Bindungen, z.B. ob der Emitter in einen Oberflächen-Dimer oder in einer tiefer liegende Schicht gebunden ist. Die Signalintensität variiert als Funktion von Polar- und Azimutwinkel. Ursächlich hierfür sind Beugungs- und Streuungseffekte der emittierten Elektronenwelle an benachbarten Atomen. Die Kombination aus hochaufgelösten Spektren und Beugungsbildern erlaubt eine detaillierte Analyse jeder einzelnen Schicht. Das MgO/Fe/GaAs(001)-System wurde in-situ präpariert und sukzessive unter Verwendung von Synchrotronstrahlung der Strahllinie 11 des Speicherings DELTA untersucht. Dabei wurde die Notwendigkeit einer GaAs Oberflächenrekonstruktion für epitaktisches Wachstum des Fe-Films nachgewiesen. Die Spektren der Fe/GaAs(4x2) und Fe/GaAs(001) Systeme weisen eindeutige chemische Bindungen auf. Die Beugungsbilder zeigen ein epitaktisches Wachstum von Fe auf GaAs(4x2) in einer pyramidalen Struktur, die durch das gleichzeitige Insel- und Lagenwachstum verursacht wird. Das Eisen reichert sich auf einer gereinigten GaAs(001) Oberfläche an, allerdings ist die Austausch-Diffusion so stark, dass der Fe-Film komplett amorph ist. Die Ga-reiche (4x2)-Rekonstruktion verhindert eine Diffusion und gewährleistet ein epitaktisches Wachstum der Fe-Schicht. Das MgO wurde auf der wohlgeordneten Fe(001)-Oberfläche aufgebracht und ist dort epitaktisch aufgewachsen. Die Fe-Oberfläche ist aufgrund der MgO-Anlagerung oxidiert, was sich in einer zwei Lagen dünnen FeO-Schicht zeigt. Der MgO-Film liegt in einer Steinsalzstruktur vor, bildet aber einen Gitterfehler in Form von leicht verschobenen Mg-Atomen aus. Diese Verschiebung ist sowohl für den dünnen als auch für einen dickeren Film vorhanden. Dies könnte durch das Substrat induziert sein, da die MgO/Fe Grenzfläche deutlich durch die Fe/GaAs Grenzstruktur beeinflusst wird. Chemische und strukturelle Veränderungen beeinflussen die magnetischen Eigenschaften des Mehrlagensystems. Daher wurden diese durch Magneto-optische Messung mittels T-MOKE untersucht. Die Spektren zeigen eine starke Reaktion des Eisens auf das externe magnetische Feld. Die Analyse der T-MOKE Daten ergab weder magnetischen Eigenschaften des GaAs-Substrats noch der dünne MgO-Schicht. Eine zusätzliche Hysterese Messung belegt die exzellenten ferromagnetischen Eigenschaften der Fe Zwischenschicht trotz der chemischen und strukturellen Veränderungen.
... The GaAs(100) is one of the most studied semiconductor surfaces [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. LEED patterns from the common GaAs(100) reconstructions are shown in Fig. 1.6. ...
... For example, one might not readily conclude whether a pattern is (2 4) or (4 2) when measuring a substrate piece cut from the full wafer, but the surface directions can be unambiguously determined by monitoring the change of the patterns. For the InAs(100), the reconstructions follow a change from c(4 4) to (2 4)/c(2 8) and then c(8 2) when decreasing the amount of As on the surface [28,31,33,[41][42][43][44][45][46]. For the InSb(100), the pattern changes from (4 4) through c(2 6) and (1 3) to c(8 2) when the Sb content on the surface decreases [46][47][48][49][50]. ...
In this chapter, we present the basic concepts of the low-energy
electron diffraction (LEED) and reflection high-energy electron
diffraction (RHEED) experiments. The main goal is to provide an overview
of the exploitation of these instrumental methods for analyzing the
surfaces of technologically important III-V compound semiconductors. In
particular, the interpretation of LEED and RHEED patterns is discussed
for the most representative reconstructions of GaAs(100), GaInAsN(100),
and Bi-stabilized III-V(100) surfaces. Other application examples
concern the use of RHEED for optimizing the growth conditions and growth
rates used in molecular beam epitaxy of III-V device heterostructures.
... Structures of technologically important polar (0 0 1) surfaces of III–V compounds have been under debate for many years. In contrast to (1 1 0) cleavage faces, (0 0 1) surfaces exhibit a large variety of reconstructions depending on surface stoichiometry, which in turn depends on a preparation procedure123 . Surface symmetries of reconstructed surfaces are well established on the basis of diffraction experiments. ...
... In the present study we are using NCAFM to image the (0 0 1) surface of another highly disputed III–V compound, on which surface dimers are present, (2 · 4) InP(0 0 1). Due to lower partial pressure of group V elements, ion sputtering and annealing in UHV conditions lead to creation of surface structures enriched in group III atoms [1,3]. The most common surface symmetry obtained in such conditions is (4 · 2)/c(8 · 2), which has been observed for compounds like GaAs, InSb and InAs [11,12]. ...
A sputter-cleaned indium-rich (2 × 4) InP(0 0 1) surface was investigated by non-contact scanning atomic force microscopy (NCAFM). Atomically-resolved images of the surface exhibit two different patterns. The patterns can be interpreted within the mixed dimer model of (2 × 4) reconstructed InP(0 0 1) surface. It is shown that due to contrast formation mechanism in NCAFM the features resolved are in close correspondence to scanning tunnelling microscopy (STM) data. Due to chemical interaction a P-terminated tip gives the image similar to an empty-state STM image, whereas an In-terminated tip gives the image resembling a filled-state STM one. Moreover, it is shown that due to dipole–dipole interaction, NCAFM can be sensitive to orientation of In-P dimers.
... Nearly a dozen surface reconstructions have been observed experimentally on the (0 0 1) GaAs surface [30,31]. These surfaces often have special surface stoichiometry. ...
... These include the experimentally validated As-terminated b2(2 · 4) [104-106], As-terminated a2(2 · 4) [107], As-rich c(4 · 4) [108,109], and Ga-rich f(4 · 2) [32] surface reconstructions. The surface reconstructions are affected by temperature, vapour composition and deposition rate [31]. ...
Interatomic potentials for modelling the vapour phase growth of semiconductor thin films must be able to describe the breaking and making of covalent bonds in an efficient format so that molec-ular dynamics simulations of thousands or millions of atoms may be performed. We review the der-ivation of such potentials, focusing upon the emerging role of the bond-based analytic bond-order potential (BOP). The BOP is derived through systematic coarse graining from the electronic to the atomistic modelling hierarchies. In a first step, the density functional theory (DFT) electronic structure is simplified by introducing the tight-binding (TB) bond model whose parameters are deter-mined directly from DFT results. In a second step, the electronic structure of the TB model is coarse grained through atom-centered moments and bond-centered interference paths, thereby deriving the analytic form of the interatomic BOP. The resultant r and p bond orders quantify the concept of single, double, triple and conjugate bonds in hydrocarbon systems and lead to a good treatment of radical formation. We show that the analytic BOP is able to predict accurately structural energy differences in quantitative agreement with TB calculations. The current development of these poten-tials for simulating the growth of Si and GaAs thin films is discussed.
... The substrate temperature is kept relatively high to facilitate the film formation and desorption of excess Sb. This strategy of sample growth, which is inherited from the growth of traditional GaAs [23], has been successfully applied in numerous binary vdW compounds [24][25][26]. ...
Exploring new two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a bandgap of 0.93 eV versus the predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. The successful growth of a DLHC material here opens the door for the realization of a large family of novel 2D DLHC traditional semiconductors with unique excitonic, topological, and electronic properties.
... Кроме того, имеется большое разнообразие переходных структур с симметрией (2 × 1), (3 × 1), (2 × 3), (3 × 6), (2 × 6), (4 × 6), (6 × 6) и др. [1,[8][9][10]. ...
The atomic and electronic structures of reconstructions with (2 × 4), (4 × 2), c(4 × 4) and (4 × 3) symmetry on the (001) surface of GaSb and InSb semiconductors were studied by the projector augmented-wave method. It was shown that in the cation-rich limit the β2(2 × 4) reconstruction is stable on the GaSb(001) surface, whereas α2(2 × 4) has the lowest energy in the case of InSb. The c(4 × 4) reconstruction with three Sb dimers is found to be stable in the Sb-rich limit. Near stoichiometric composition the α(4 × 3) and β(4 × 3) structures are stable that is in agreement with experimental data. Electronic structure of the (4 × 3) reconstructions with lowest surface energy is discussed. A weak influence of chemical composition of cations on the surface state structure and their localization at the formation of (4 × 3) structures was revealed. The correlation between the surface energy of some (4 × 2) and (2 × 4) reconstructions and the difference in the atomic radii of anions and cations was established.
... We observed the (√13 × √13) structures all over the surfaces of 1-UC-thick and 2-UC-thick STO islands ( Figure 7(a) and (b)). In analogy with the homoepitaxial growth of GaAs thin films [27], we suggest that the atomic structure of the substrate surface, the TiO 2 nanomesh, is spontaneously transferred to the STO film surface, implying atomic-scale coherent epitaxy at the interface between the STO thin film and substrate (Figure 7(c)). ...
The interfaces of complex oxide heterostructures exhibit intriguing phenomena not observed in their constituent materials. The oxide thin-film growth of such heterostructures has been successfully controlled with unit-cell precision; however, atomic-scale understandings of oxide thin-film surfaces and interfaces have remained insufficient. We examined, with atomic precision, the surface and electronic structures of oxide thin films and their growth processes using low-temperature scanning tunneling microscopy. Our results reveal that oxide thin-film surface structures are complicated in contrast to the general perception and that atomically ordered surfaces can be achieved with careful attention to the surface preparation. Such atomically ordered oxide thin-film surfaces offer great opportunities not only for investigating the microscopic origins of interfacial phenomena but also for exploring new surface phenomena and for studying the electronic states of complex oxides that are inaccessible using bulk samples.
... Techniques that can relate surface geometry and electronic structure to electrical performance are thus highly important, of which scanning tunnelling microscopy (STM) uniquely allows imaging of both surface geometry and electronic structure down to individual atoms and as such already play a prominent role in the strive towards atomic scale semiconductor electronics 2,3,[9][10][11] . In particular, the surfaces of III-V semiconductors are a proven base for STM studies of fundamental atomic scale semiconductor processes 2,[12][13][14][15][16][17][18] . For larger (micrometre sized) surfaces considerable structural rearrangements have been observed by STM under applied bias [19][20][21] . ...
As semiconductor electronics keep shrinking, functionality depends on individual atomic scale surface and interface features that may change as voltages are applied. In this work we demonstrate a novel device platform that allows scanning tunneling microscopy (STM) imaging with atomic scale resolution across a device simultaneously with full electrical operation. The platform presents a significant step forward as it allows STM to be performed everywhere on the device surface and high temperature processing in reactive gases of the complete device. We demonstrate the new method through proof of principle measurements on both InAs and GaAs nanowire devices with variable biases up to 4 V. On InAs nanowires we observe a surprising removal of atomic defects and smoothing of the surface morphology under applied bias, in contrast to the expected increase in defects and electromigration-related failure. As we use only standard fabrication and scanning instrumentation our concept is widely applicable and opens up the possibility of fundamental investigations of device surface reliability as well as new electronic functionality based on restructuring during operation.
... Then, as a result of the diffusion of the excess Ti to the substrate and the film, an interdiffusion region[73]forms near the interface (Fig. 9e), and a different atomic arrangement appears on the film surface. Finally, the (√13 Â √13) structure is transferred to the film surface, which is analogous to the homoepitaxial growth of GaAs[74]. A remarkable point is that both the (2 Â 1) (Fig. 8) and (√13 Â √13) (Fig. 9) RCs allow the epitaxy of the films while maintaining the stacking sequence of the perovskite sublayers (AOeBO 2 eAOe …), concurrent with the conservation of the RC in the grown film surfaces. ...
The growth of perovskite oxide films is known to be strongly influenced by both substrate surface lattice symmetry and stoichiometry. However, this has been postulated mainly based on indirect evidences. Scanning tunneling microscopy (STM) is unambiguously capable of identifying the real-space distribution of the structural and electronic properties of solid surfaces. Therefore, oxide film growth technologies combined with STM are strongly desirable for resolving atomic-scale growth mechanisms of perovskite thin films. Here, we review recent advances in STM studies on initial growth stages of perovskite oxides on SrTiO 3 (001). First, we introduce surface terminations and reconstructions of SrTiO 3 (001), as well as their influence on the initial growth of perovskite films studied by STM on an atomic-scale, followed by a discussion of a feasible model for the surface atomic structures and chem-istries behind such growth behaviors. We then introduce studies on the growth dynamics of perovskite oxides on SrTiO 3 (001) in terms of temperature (T) and thickness-resolved STM: (1) Layer-by-layer identification of T-dependent surface structures of ultrathin SrRuO 3 , suggesting a dramatic change in the surface migration barrier caused by switching of the surface terminations and (2) a prototypical study on the surface diffusion dynamics in perovskite growth realized by the application of the classical diffusion model to the island nucleation stage of SrTiO 3 homoepitaxy.
... AlAs and AlGaAs alloy semiconductors are usually grown on the GaAs (001) substrate as they have matching lattice parameters. AlGaAs/GaAs multilayer structures have been widely used to create various optoelectronic and electronic devices [5][6][7]. In recent years, more and more research turns to study the electrical properties, optical transitions of the AlGaAs/ GaAs by molecular beam epitaxy (MBE) [8,9]. ...
The influence of deposition of aluminum on adatoms diffusion and evaporation during the growth of AlGaAs alloy layer on GaAs (001) surface is investigated. The real space scanning tunneling microscopy (STM) images showed the obvious changes on different AlGaAs surface morphology; reflection high-energy electron diffraction (RHEED) is also used to estimate the deposition. STM images and RHEED patterns showed the pits coverage and roughness of deposition surfaces caused by stronger Al–As bonds and slower surface migration rate of aluminum; Ehrlich–Schwoebel potential is considered as possible explanation of steps forming on the surface morphology. A conjecture for the formation of surfaces morphology and its influence on subsequent growth is proposed.
... Compared with other research ways, scanning tunneling microscopy (STM) is more effective in surface science because it can provide many surface morphology and interaction information of the tip-film or substrate-film at the same time. In addition, it has few damages on the sample surface, [19][20][21][22] which is very suitable for the organic molecules with fragile structure. [23][24][25][26] Thus, the STM technique should be a very powerful tool for exploring the interface behaviors of TPE-An. ...
9,10-bis(4-(1,2,2-triphenylvinyl)styryl)anthracene (TPE-An) material has been paid much attention because of its unique piezofluorochromic characters. Interface properties of TPE-An film on Si (001) substrate were firstly investigated by scanning tunneling microscopy (STM) technique. It shows that both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TPE-An film exhibit the same bending tendency towards the Fermi level with the increase of the applied electric field, which causes the HOMO–LUMO gap shrinking effect. A maximum gap-shrinking can reach 1.21 eV in scanning tunneling spectroscopy (STS) experiments, which results from the shift of HOMO (0.34 eV) and LUMO (0.87 eV) bands. It is also found that the capacitive interface between TPE-An film and substrate determines their physical behaviors. Furthermore, both the polarization effect and sub-surface electric field were proposed to be responsible for different bending behaviors of the HOMO and LUMO at applied field. Our research method may shed new light on comprehending the interaction mechanism between aggregation-induced emission (AIE) materials and substrate.
... AlGaAs/GaAs multilayer structures have been widely used to prepare various opto-electronic and electronic devices, while high quality growth of AlGaAs on GaAs is difficult and often generates rough surface [1]. Interface roughness is harmful to controlled optical transitions in quantum wells (QW), and high-mobility modulation doped heterostructures [2]. ...
... In molecular beam epitaxy (MBE), the (001) surface of III-V semiconductor showed various surface reconstructions under different substrate temperature and group V fluxes. For arsenic compounds (GaAs, InAs, AlAs), arsenic rich (2 × 4) surface reconstructions have been observed under a wide range of temperature and As flux, and most electronic devices were produced on the surface [6,7]. The (4 × 2) reconstruction is found to be metal rich, which has been studied by theoretical calculations and scanning tunneling microscopy (STM) experiment [8]. ...
... The experiments were carried out in a home-built UHV-STM equipped with a field-ion microscope that is used to monitor and fabricate the tip [14]. The base pressure for STM is 1.8 × 10 − 10 Torr. ...
We investigate the atomic behaviour of long-chain 1-dodecene adsorbed on Si(100) using a scanning tunnelling microscope with an exposure of 30 to 2.4 Langmuirs. Unlike previous reports on short-chain molecules, remarkable self-ordered assembly of molecules is not observed at room temperature, which is possibly attributed to the asymmetric molecular structure with long chains of 1-dodecene. After annealing at 500–580 °C, ordered patterns form with a c(4 × 4) structure, accompanied with thermal decomposition of molecules.
... In this model, the three As dimers in the surface unit of the conventional b phase are replaced by three As-Ga ''mixed dimers" or ''heterodimers", bonded again to the underlying complete As layer of the top GaAs bilayer, providing an ordered STM image [5] and retaining reflection high electron energy diffraction (RHEED) patterns. The earlier task of explaining the wide As coverage range of 0.89 ML reported in [6], without break-ing the RHEED diffraction patterns, is thus more acceptable: the b-c(4 Â 4) structure represents the As-rich phase with total As coverage 1.75 ML while the a-c(4 Â 4) represents a stoichiometric phase with total As coverage 1.38 ML. The intermediate surfaces become less ordered [3,7,8]. ...
... Even for ideal, layer-by-layer growth without anion exchange, the different III-V stoichiometries for the AlSb and AlAs reconstructions on the growth surfaces are expected to prevent the formation of ideal interfaces [2,20]. Under these growth conditions, AlAs has a (2 Â 4) reconstruction terminated with 3 4 ML of Al and 1 2 ML of As [21,22], whereas the AlSb has a multilayer (4 Â 3) structure terminated with 1 7 12 ML Sb and 1 12 ML Al in Al-Sb heterodimers [23]. For AlAs-on-AlSb, the excess Sb will create a mixed AlSb-AlAs interfacial layer even on a perfect, planar growth surface. ...
We report a systematic study of how growth temperature affects the quality of AlAs-in-AlSb digital alloy superlattices grown by molecular beam epitaxy for barrier layers in type-II W-structure infrared lasers. Using cross-sectional scanning tunneling microscopy to characterize the atomic-scale structure of the material, we find substantial differences in the superlattice morphology for growth temperatures between 435 and 540°C. At lower growth temperatures, the AlAs forms three-dimensional clusters, with continuous structures threading through multiple periods of the superlattice. With increasing temperature, the morphology of the digitally doped AlAs layers consistently improves, with nearly perfect delta doping observed at the highest temperatures studied. The changes in the superlattice structure can be attributed primarily to the known temperature dependence of the AlSb growth front morphology, with secondary effects associated with anion-exchange at the interfaces and the different surface reconstructions on the two growth surfaces.
... Some different reconstructions are reported on the GaAs(001). [21][22][23][24][25][26][27][28][29] The (4 × 2) reconstruction is one of the structures observed in the typical MBE growth. Among (4 × 2) reconstructions, the most stable structure is the β2(4 × 2) phase, 26) which is similar to the β2(2 × 4) reconstruction on an As-terminated surface. ...
The microscopic migration of an As adatom on a Ga-terminated GaAs(001) surface is investigated by performing first-principle calculations using density functional theory and a slab model of the surface, because the importance of As adatom migration has been reported. The obtained values of barrier energy are compared with those to hopping in the kinetic Monte Carlo simulation. The anisotropic migration of the As adatom is clarified, which agrees with the result of the kinetic Monte Carlo simulation as well as the properties obtained by the scanning tunneling microscopy observation. It is shown that an As adatom is more mobile than a Ga adatom, and the kinetics of both As and Ga atoms on the growing surface are important during molecular beam epitaxial growth.
... Gallium arsenide surfaces are among the most extensively studied surfaces, both for their technological interest and for the fact that GaAs is a material of fundamental importance. 1 While the equilibrium structure of the (110) surface is relaxed but not reconstructed, 2 the (001) surface can exhibit several reconstructions of distinct stoichiometries, whose structure has not been completely elucidated. The As-rich surfaces, prepared under standard molecular beam epitaxy (MBE) conditions, have been extensively studied, and their structures are now well established. ...
While the As-rich 2 × 4 reconstruction of GaAs(001) is well explained by the so-called beta2 structure, the atomic structure of the Ga-rich 4 × 2 phase has been discussed for a long time. In this review, the most important structural models for the GaAs(001) (4 × 2)/c(8 × 2) surface are compared from different theoretical and experimental points of view. The selected reconstructions include the recently proposed zeta model, a new mixed dimer model, and the well-known beta, beta2, Cerdà and Skala models. The different structures are compared on the basis of total energy calculations, simulations of STM experimental images and interpretation of X-ray diffraction data. Only the zeta model satisfies all criteria, and provides therefore a satisfactory explanation of the atomic structure of GaAs(001)-(4 × 2).
... While numerous studies of the GaAs(001) surface abound, including several review articles, 24,25 some recent studies and advances in instrumentation have provided new insight into this important surface. [26][27][28][29] These discoveries, spurred by both in situ scanning tunneling microscopy (STM) and accurate (±2 • C) temperature control of the substrate, center mainly around two areas. ...
A union of the real-space and reciprocal space view of the GaAs(001) surface is presented. An optical transmission temperature measurement system allowed fast and accurate temperature determinations of the GaAs(001) substrate. The atomic features of the GaAs(001)-(2×4) reconstructed surface are resolved with scanning tunneling microscopy and first principles density functional theory. In addition, the 2D lattice-gas Ising model within the grand canonical ensemble can be applied to this surface to understand the thermodynamics. An algorithm for using electron diffraction on the GaAs(001) surface to determine the substrate temperature and tune the nanoscale surface roughness is presented.
... The microscopic structure of the GaP(001) surface is not well understood, in marked contrast to other III-V compounds, in particular GaAs(001) (see Ref. [1] for a recent review). It has been suggested that ion bombardment and annealing of GaP(001) results in a (4 Â 2) reconstructed Ga-rich surface [2,3] in analogy to GaAs. ...
The energetics and atomic structure of the P-rich GaP(001) surface are studied by first-principles total-energy calculations. For (2 × 2) reconstructed surfaces we predict the formation of a P bilayer, which is semiconducting due the formation of inequivalent dimers.
... In surface science, most of the more complex surface structures and reconstructions could be resolved by STM, often together with other surface-science techniques, and are understood reasonably well. The realspace imaging capability proved to be crucial to unravel the structure of the enlarged unit cell of reconstructions [for an example of the richness of reconstructions see Xue, Hashizume, and Sakurai (1997)]. This is even more so for the study of more local phenomena such as surface structures coexisting on short length scales, nucleation and growth phenomena, heterogeneous catalysis, phase transitions, and surface chemistry. ...
Scanning tunneling microscopy appeared as a new method to deal with atoms, molecules, and nanometer-scale structures. It was the first of a growing family of local probes for imaging and measuring, which can serve at the same time as tools. Local probe methods have changed the way we perceive, think about, and treat atomic structures, and have brought about a new appreciation of mechanics. They play a central role for science and technology on the nanometer scale and will allow us to build systems of the same complexity as used by nature, which has built life on nanofunctionality.
... In a subtle experimental study of the dependence of the RAS on the thickness of a GaAs(001) quantum well sandwiched between an As reconstructed surface and an AlAs overlayer, Lastras-Martinez et al [195] demonstrated the existence of both bulk and surface contributions to the RAS in this and other spectral regions. Theoretical work is complicated by the fact that while theory clearly shows that RAS is strongly dependent on surface structure, the GaAs(001) surface exhibits a rich variety of surface reconstructions [199], the occurrence of which depend on the surface preparation and few of which are well characterized structurally. Due to their technological importance the structural parameters of these reconstructions are being refined by first principles total energy calculations [200], and such calculations are a prerequisite for calculations of the optical response. ...
Reflection anisotropy spectroscopy (RAS) is a non-destructive optical probe of surfaces that is capable of operation within a wide range of environments. In this review we trace the development of RAS from its origins in the 1980s as a probe of semiconductor surfaces and semiconductor growth through to the present where it is emerging as a powerful addition to the wide range of existing ultra-high vacuum (UHV) surface science techniques. The principles, instrumentation and theoretical considerations of RAS are discussed. The recent progress in the application of RAS to investigate phenomena at metal surfaces is reviewed, and applications in fields including electrochemistry, molecular assembly, liquid crystal device fabrication and remote stress sensing are discussed. We show that the experimental study of relatively simple surfaces combined with continuing progress in the theoretical description of surface optics promises to unlock the full potential of RAS. This provides a firm foundation for the application of the technique to the challenging fields of ambient, high pressure and liquid environments. It is in these environments that RAS has a clear advantage over UHV-based probes for investigating surface phenomena, and its surface sensitivity, ability to monitor macroscopic areas and rapidity of response make it an ideal complement to scanning probe techniques which can also operate in such environments.
... The surface reconstructions of gallium arsenic and indium phosphide (0 0 1) have been subject to many ultrahigh vacuum studies as they play an important role during the epitaxial growth of thin film devices [1][2][3][4]. While our knowledge of these materials has advanced significantly in the last two decades, it is still unclear whether the structures seen in the vacuum apply directly to those present during processing. ...
The structure of gallium arsenide and indium phosphide (0 0 1) surfaces in the metalorganic vapor-phase epitaxy (MOVPE) environment has been investigated. During growth at V/III ratios in excess of 10, both materials are terminated with group V ad-dimers (As or P), alkyl groups and hydrogen atoms. These species sit on top of a complete layer of the group V atoms. As the V/III ratio decreases, the top layer of arsenic or phosphorous desorbs from the surface. However, the resulting structures are different on GaAs and InP (0 0 1). In the former case, the phase transition occurs with gallium out-diffusion and nucleation of elongated islands. These islands have a b2(2 Â 4) structure that contains only 0.75 monolayer of arsenic dimers. The resulting surface is rough, exposing on average six atomic layers. Conversely, on InP (0 0 1), no indium out-diffusion occurs following desorption of the phosphorous ad-dimers. Instead, the underlying P atoms dimerize, forming a (2 Â 1) structure with a phosphorous coverage of 1.0 monolayer. # 2001 Elsevier Science B.V. All rights reserved.
... The surface temperature, composition of the vapor above the surface, and the deposition rate during film growth all combine to determine the reconstruction structure that forms on a surface. 98 A surface free energy diagram can be used to represent the surface reconstructions that form under equilibrium conditions as the vapor composition is varied. The surface free energy per unit area, ␥, can be calculated by assuming that the arsenic and gallium atoms in the vapor freely exchange between the bulk material ͑the thermodynamic reservoir͒ and an atmospheric vapor ͑atom reservoir͒ 79,99 in a similar manner to that discussed in the previous section. ...
An analytic, bond-order potential BOP is proposed and parametrized for the gallium arsenide system. The potential addresses primary and secondary bonding and the valence-dependent character of heteroat-omic bonding, and it can be combined with an electron counting potential to address the distribution of electrons on the GaAs surface. The potential was derived from a tight-binding description of covalent bonding by retaining the first two levels of an expanded Green's function for the and bond-order terms. Predictions using the potential were compared with independent estimates for the structures and binding energy of small clusters dimers, trimers, and tetramers and for various bulk lattices with coordinations varying from 4 to 12. The structure and energies of simple point defects and melting transitions were also investigated. The relative stabilities of the 001 surface reconstructions of GaAs were well predicted, especially under high-arsenic-overpressure conditions. The structural and binding energy trends of this GaAs BOP generally match experi-mental observations and ab initio calculations.
... A cleaved surface of a bulk crystal is usually Te-terminated with an unreconstructed (1 × 1)-Te structure202122. Similar to the well-known layer-by-layer growth of GaAs with an As4 molecular beam [38], ideal MBE growth of a Sb 2 Te 3 film in units of a QL along the [111] direction should be possible with a Te 2 molecular beam under Te-rich conditions. By systematically varying the Te 2 /Sb flux ratios (θ ) and the substrate temperature (T Si ) we found that the stoichiometric TI could only be achieved under Te-rich conditions (θ = 8 – 20) with the growth temperature satisfying T Sb > T Si T Te [18, 19]. ...
The growth and characterization of single-crystalline thin films of topological insulators (TIs) is an important step towards
their possible applications. Using in situ scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we show that moderately thick
Sb2Te3 films grown layer-by-layer by molecular beam epitaxy (MBE) on Si(111) are atomically smooth, single-crystalline, and intrinsically
insulating. Furthermore, these films were found to exhibit a robust TI electronic structure with their Fermi energy lying
within the energy gap of the bulk that intersects only the Dirac cone of the surface states. Depositing Cs in situ moves the Fermi energy of the Sb2Te3 films without changing the electronic band structure, as predicted by theory. We found that the TI behavior is preserved
in Sb2Te3 films down to five quintuple layers (QLs).
KeywordsTopological insulator-electronic structure-scanning tunneling microscopy-angle-resolved photoemission spectroscopy-molecular beam epitaxy
... By far the most widely studied material is GaAs (100). In connection with the molecular-beam epitaxy (MBE) growth technology, numerous surface reconstructions were prepared and studied extensively by various surface diagnostics in the last decade, namely As-rich (2×4) and c(4×4) or Ga-rich (4×6) and (4×2), next to other less significant surfaces [1]. ...
GaAs (100)-(1X1) surface grown by molecular-beam epitaxy was studied by low energy electron diffraction (LEED). Intensities of diffraction spots were measured in the energy range of (40-300) eV and analysed using dynamical tensor LEED package. Relaxation of surface layers decreased the Pendry''s R-factor to 0.48. Analysis of the LEED intensity-voltage curves for the normal electron incidence shows that the investigated surface structure is more complicated than a simply relaxed ideal surface.
... In this model, the three As dimers in the surface unit of the conventional b phase are replaced by three As-Ga ''mixed dimers" or ''heterodimers", bonded again to the underlying complete As layer of the top GaAs bilayer, providing an ordered STM image [5] and retaining reflection high electron energy diffraction (RHEED) patterns. The earlier task of explaining the wide As coverage range of 0.89 ML reported in [6], without break-ing the RHEED diffraction patterns, is thus more acceptable: the b-c(4 Â 4) structure represents the As-rich phase with total As coverage 1.75 ML while the a-c(4 Â 4) represents a stoichiometric phase with total As coverage 1.38 ML. The intermediate surfaces become less ordered [3,7,8]. ...
We prepared α- and β surface phases of GaAs(0 0 1)-c(4 × 4) reconstruction by molecular beam epitaxy (MBE) using As4 and As2 molecular beams, respectively, and examined them by angle-resolved ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation as an excitation source. The UPS valence band spectra and the XPS 3d core level data show pronounced differences corresponding to the surface composition and the atomic structure of the two phases, as proposed in the literature. In UPS, the β phase is characterized by an intensive surface state 0.5 eV below the top of the valence band at low photon energy, while an analogous peak in the α phase spectra is missing. The surface state is interpreted in terms of dangling bonds on As dimers. The As3d and Ga3d core level photoelectron lines exhibit phase-specific shapes as well as differences in the number, position and intensity of their deconvoluted components. The location of various atoms in the surface and subsurface layers is discussed; our photoemission results support models of the β phase and the α phase with As–As dimers and Ga–As heterodimers, respectively.
... For (b), as a result of the diffusion of the excess Ti to the substrate and the film, an interdiffusion region[25] is formed near the interface, and a different atomic arrangement appears on the film surface. For (c), the ( √ 13 × √ 13) structure is transferred to the film surface, which is analogous to the homoepitaxial growth of GaAs.[26] To determine which growth process inFig. ...
The initial homoepitaxial growth of SrTiO(3) on a (√13 × √13)-R33.7° SrTiO(3)(001) substrate surface, which can be prepared under oxide growth conditions, is atomically resolved by scanning tunneling microscopy. The identical (√13 × √13) atomic structure is clearly visualized on the deposited SrTiO(3) film surface as well as on the substrate. This result indicates the transfer of the topmost Ti-rich (√13 × √13) structure to the film surface and atomic-scale coherent epitaxy at the film/substrate interface. Such atomically ordered SrTiO(3) substrates can be applied to the fabrication of atom-by-atom controlled oxide epitaxial films and heterostructures.
Topological superconductors are characterized by pairing of electrons with nontrivial topology and carry unpaired Majorana fermions at surfaces/boundaries. Using a cryogenic scanning tunneling microscope, we reveal an anisotropic nodeless superconducting gap on epitaxial crystalline beta-Bi2Pd films, which is much larger than that of bulk superconducting beta-Bi2Pd. The newly emerging superconducting gap is found to originate from Dirac-fermion enhanced parity mixing of the surface pair potential, thereby indicates topological superconductivity with spinless odd-parity pairing near the film surface. Majorana zero modes, supported by such a superconducting state, are unequivocally identified by directly probing quasiparticle density of states within the vortex cores under magnetic field. The superconductivity and Majorana zero modes exhibit resistance to intrinsic point and linear defects, characteristic of a time-reversal-invariant topological superconductor.
GaAs(100) c(4×4) surfaces were examined by ab initio calculations, under As2, H2 and N2 gas mixed conditions as a model for GaAs1-xNx vapor-phase epitaxy (VPE) on GaAs(100). Using a simple model consisting of As2 and H2 molecules adsorptions and As/N atom substitutions, it was shown to be possible to examine the crystal growth behavior considering the relative stability of the resulting surfaces against the chemical potential of As2, H2 and N2 gases. Such simple model allows us to draw a picture of the temperature and pressure stability domains for each surfaces that can be linked to specific growth conditions, directly. We found that, using this simple model, it is possible to explain the different N-incorporation regimes observed experimentally at different temperatures, and to predict the transition temperature between these regimes. Additionally, a rational explanation of N-incorporation ratio for each of these regimes is provided. Our model should then lead to a better comprehension and control of the experimental conditions needed to realize a high quality VPE of GaAs1-xNx.
Formation processes of Ga droplets on GaAs(001) have been systematically studied. We present the evidence that the surface atomic structures of the GaAs substrate dominate the surface diffusion of Ga atoms, which plays a key role in determining the size and density of Ga droplets. The Ga droplets are formed on the As-rich (2 × 4) and c(4 × 4)β surface after the modification of the initial surface reconstructions, while droplets are directly formed on the Ga-rich (4 × 6) surface. The density of Ga droplets on the (4 × 6) surface exceeds 1012 cm–2, which is significantly higher than that on the As-rich c(4 × 4)β surfaces.
Structural, morphological, and band offset properties of GaAs/Ge/GaAs heterostructures grown in situ on (100), (110), and (111)A GaAs substrates using two separate molecular beam epitaxy chambers, connected via vacuum transfer chamber, were investigated. Reflection high energy electron diffraction (RHEED) studies in all cases exhibited a streaky reconstructed surface pattern for Ge. Sharp RHEED patterns from the surface of GaAs on epitaxial Ge/(111)A GaAs and Ge/(110)GaAs demonstrated a superior interface quality than on Ge/(100)GaAs. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of Ge about 0.2–0.3 nm. High-resolution triple axis x-ray rocking curves demonstrate a high-quality Ge epitaxial layer as well as GaAs/Ge/GaAs heterostructures by observing Pendellösung oscillations. Valence band offset, ΔEv, have been derived from x-ray photoelectron spectroscopy (XPS) data on GaAs/Ge/GaAs interfaces for three crystallographic orientations. The ΔEv values for epitaxial GaAs layers grown on Ge and Ge layers grown on (100), (110), and (111)A GaAs substrates are 0.23, 0.26, 0.31 eV (upper GaAs/Ge interface) and 0.42, 0.57, 0.61 eV (bottom Ge/GaAs interface), respectively. Using XPS data obtained from these heterostructures, variations in band discontinuities related to the crystallographic orientation have been observed and established a band offset relation of ΔEV(111)Ga>ΔEV(110)>ΔEV(100)As in both upper and lower interfaces.
High-quality epitaxial Ge layers for GaAs/Ge/GaAs heterostructures were grown in situ in an arsenic-free environment on (100) off-oriented GaAs substrates using two separate molecular beam epitaxy (MBE) chambers, connected via vacuum transfer chamber. The structural, morphological, and band offset properties of these heterostructures are investigated. Reflection high energy electron diffraction studies exhibited (2 × 2) Ge surface reconstruction after the growth at 450 °C and also revealed a smooth surface for the growth of GaAs on Ge. High-resolution triple crystal x-ray rocking curve demonstrated high-quality Ge epilayer as well as GaAs/Ge/(001)GaAs heterostructures by observing Pendellösung oscillations and that the Ge epilayer is pseudomorphic. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of ∼0.45 nm and room temperature photoluminescence spectroscopy exhibited direct bandgap emission at 1583 nm. Dynamic secondary ion mass spectrometry depth profiles of Ga, As, and Ge display a low value of Ga, As, and Ge intermixing at the Ge/GaAs interface and a transition between Ge/GaAs of less than 15 nm. The valence band offset at the upper GaAs/Ge-(2 × 2) and bottom Ge/(001)GaAs-(2 × 4) heterointerface of GaAs/Ge/GaAs double heterostructure is about 0.20 eV and 0.40 eV, respectively. Thus, the high-quality heterointerface and band offset for carrier confinement in MBE grown GaAs/Ge/GaAs heterostructures offer a promising candidate for Ge-based p-channel high-hole mobility quantum well field effect transistors.
We show that monolayer-high islands of C60 and C60O can be transferred from Langmuir films on a water or phenol sub-phase to oxide-terminated Si(111) substrates. Faceted islands, in some cases incorporating a foam-like morphology reminscent of that previously observed for Langmuir films at the water–air interface using Brewster angle microscopy, are formed and transferred using small amounts (100–400 μl) of low concentration (of order 10− 5M) solutions of C60 (or C60O) with low target pressures (~ 10 mN/m). However, worm-like monolayer domains are also observed under identical experimental conditions, indicating the key role that inhomogeneous solvent evaporation plays in the formation of two-dimensional fullerene aggregates on the subphase surface. While Langmuir–Blodgett multilayers of C60 and C60O are both granular, there are significant morphological differences observed between the molecular thin films. In particular, C60O multilayers contain a relatively high density of ring (or “doughnut”) features with diameters in the 100–300 nm range which are not observed for C60. We attribute the origin of these features to dipolar or hydrogen bonding-mediated interactions between the C60O molecules at the water surface.
Adsorption of Al atoms on the As-stabilized InAs(001)—(2 × 4) surface induces the formation of the Al-stabilized (2 × 4) reconstruction. The Al-stabilized (2 × 4) surface has mixed In–As dimer at the outermost layer with the Al atoms being incorporated into the subsurface layers. Heating of the Al-stabilized (2 × 4) surface further promotes the diffusion of Al into deeper layers, which results in the formation of the In-rich (4 × 2) structure with the ζa structure.
Low-energy electron diffraction (LEED), scanning-tunneling microscopy (STM), and synchrotron-radiation photoemission results show that Bi induces (2 × 6), (2 × 8), and (2 × 4) reconstructions on the InAs(1 0 0) surface with decreasing Bi coverage. The α2-like structural model, established previously for the clean InAs(1 0 0)(2 × 4) surface, is proposed for Bi/InAs(1 0 0)(2 × 4), and two Bi 5d core-level components of this reconstruction are interpreted within the context of this model. For the Bi/InAs(1 0 0)(2 × 6) surface we propose a tentative model where two topmost atomic layers consist of Bi atoms. Some possible reasons why Bi forms chain-like (2 × 6) and (2 × 8) reconstructions, instead of the prototypical c(4 × 4) stabilized normally by As on III-As(1 0 0), are discussed.
Self-assembly of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on a c(8×2) reconstructed InSb(001) surface as well as on an asymmetrical (1×3) InSb(001) is investigated by means of high resolution scanning tunneling microscopy. The formation of well-ordered one-dimensional molecular lines is observed at low coverage on a former face, whereas on the latter molecules are found as isolated entities. At room temperature spontaneous hopping between energetically similar sites is observed within the lines. At a monolayer coverage flat molecular surface is obtained for both reconstructions, on the c(8×2) the layer is composed of neighboring one-dimensional lines, on the (1×3) molecular structure does not reveal any long range order. Application of the special experimental procedure provides direct comparison of molecule behavior on both reconstructions of the same sample.
We report a combined high-resolution photoemission (XPS) and photoelectron diffraction (XPD) investigation of the three layer system MgO/Fe/GaAs(001). Each layer is investigated with regard to its structure. The two dimers model of the GaAs (4×2) reconstruction was confirmed by XPD patterns. We find the intermediate Fe layer in a crystalline structure. Further, the study clearly shows a well-ordered epitaxial MgO film on Fe. A careful analysis of the interface signals indicates an interdiffusion at the Fe/GaAs interface and partially shifted magnesium layers at the MgO/Fe interface.
This paper reviews the recent experimental findings on the atomic structures on the (001) surface of GaAs. We systematically studied the structure and composition of the GaAs(001) surfaces using reflection high-energy electron diffraction, reflectance difference spectroscopy, scanning tunneling microscopy, and X-ray photoelectron spectroscopy. We found that the As-rich c(4×4)β, c(4×4)α, and (2×4), and Ga-rich (6×6), c(8×2), and (4×6) reconstructions are formed on the GaAs(001) surface critically depending on the preparation conditions. Atomic structures on these reconstructions will be discussed on the basis of the recent findings of experiments and first-principles calculations.
The integration of high carrier mobility materials into future CMOS generations is presently being studied in order to increase drive current capability and to decrease power consumption in future generation CMOS devices. If III–V materials are the candidates of choice for n-type channel devices, antimonide-based semiconductors present high hole mobility and could be used for p-type channel devices. In this work we first demonstrate the heteroepitaxy of fully relaxed GaSb epilayers on InP(001) substrates. In a second part, the properties of the Al2O3/GaSb interface have been studied by in situ deposition of an Al2O3 high-κ gate dielectric. The interface is abrupt without any substantial interfacial layer, and is characterized by high conduction and valence band offsets. Finally, MOS capacitors show well-behaved C–V with relatively low Dit along the bandgap, these results point out an efficient electrical passivation of the Al2O3/GaSb interface.
The molecular beam epitaxy growth of topological insulator Bi2Te3 thin films on Si(111) substrates has been investigated in situ by low-energy electron microscopy. The crystal structure and surface morphology during growth were directly revealed, which enables us to identify the optimal growth conditions for single crystalline Bi2Te3 films. The formation of thin films is preceded by several surface structures, including a wetting layer and a Te/Bi-terminated Si(111)-1×1 reconstruction. Raman scattering spectra and AFM measurements indicate that, under Te-rich conditions, single crystalline films of Bi2Te3 grow along the [111] direction in a layer-by-layer mode. Transport measurements prove the insulating behavior of the films grown in this way.
It has been 30 years since the scanning tunnelling microscope (STM) was invented by G Binnig and H Rohrer. Rapid developments have made STM increasingly powerful as an extremely versatile technique for many disciplines in condensed matter physics, chemistry, biology and other areas. As a state-of-the-art growth method, molecular beam epitaxy (MBE) is a gifted technique for epitaxial growth with atomic-level control. In this paper, by giving several examples, we will show that an STM–MBE combined system is more powerful and unique for studies on low-dimensional and new functional materials.
Single-stranded DNA immobilized on an III-V semiconductor is a potential high-sensitivity biosensor. The chemical and electronic changes occurring upon the binding of DNA to the InAs surface are essential to understanding the DNA-immobilization mechanism. In this work, the chemical properties of DNA-immobilized InAs surfaces were determined through high-resolution X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Prior to DNA functionalization, HF- and NH(4)OH- based aqueous etches were used to remove the native oxide from the InAs surface. The initial chemical state of the surface resulting from these etches were characterized prior to functionalization. F-tagged thiolated single-stranded DNA (ssDNA) was used as the probe species under two different functionalization methods. The presence of DNA immobilized on the surface was confirmed from the F 1s, N 1s, and P 2p peaks in the XPS spectra. The presence of salt had a profound effect on the density of immobilized DNA on the InAs surface. To study the interfacial chemistry, the surface was treated with thiolated ssDNA with and without the mercaptohexanol molecule. An analysis of the As 3d and In 3d spectra indicates that both In-S and As-S are present on the surface after DNA functionalization. The amount of In-S and As-S was determined by the functionalization method as well as the presence of mercaptohexanol during functionalization. The orientation of the adsorbed ssDNA is determined by polarization-dependent NEXAFS utilizing the N K-edge. The immobilized ssDNA molecule has a preferred tilt angle with respect to the substrate normal, but with a random azimuthal distribution.
GaSb (1,0,0) surfaces have been cleaned by chemical passivation and later heat-treatment in ultrahigh vacuum conditions. Four different etching solutions consisting of an oxidant and an acid for oxide dissolution have been studied. With the H2SO4:H2O2:H2O the surfaces become very rough, unevenly covered with oxides, and not suitable for later epitaxial growth. The remaining three etchants studied, HCl:H2O2:NaK(tartrate), HF:HNO3:CH3COOH, and Br:methanol, leave rather smooth surfaces covered with a thin passivating layer, probably consisting of pure Sb, Sb2O3, and Ga2O3 which can be easily removed by heating the samples in ultrahigh vacuum at temperatures between 480° and 510°C.
The structure of AlSb and GaSb (0 0 1) surfaces prepared by molecular beam epitaxy has been studied with in-situ reflection high-energy electron diffraction and scanning tunneling microscopy. Under fixed Sb4 flux, two AlSb reconstructions are observed with increasing temperature (and decreasing surface Sb:Al coverage): c(4 × 4), as observed for InSb, GaAs, AlAs, and InAs, and (1 × 3). In contrast, GaSb reconstructions observed with increasing temperature are: (2 × 5), (1 × 5), c(2 × 6), and (1 × 3). Whereas the (1 × 5), c(2 × 6), and (1 × 3) surfaces are composed primarily of Sb dimer rows on top of an Sb-terminated surface, the (2 × 5) surface is composed of Sb dimer rows on top of two layers of Sb (i.e. the surface is terminated by three Sb layers). We speculate that GaSb is unique in forming the (n × 5) reconstructions due to its excellent lattice match with trigonally bonded elemental Sb.
The zincblende structure of GaAs gives rise to two types of steps on a GaAs(100) surface. In a model using abrupt bulk-termination, these steps would be labelled A or B, depending upon whether they are As or Ga terminated, and in the [011] or [011] directions, respectively. Since these steps are structurally and chemically different, one expects them to have different roles in epitaxy. We have used reflection high-energy electron diffraction (RHEED) to study the morphology of each of these two types of steps on surfaces prepared by molecular beam epitaxy (MBE). The results show large differences in the qualitative and quantitative structural disorder of these two step configurations. During static conditions of no growth, but with an As4 flux provided, the A-steps were straight over long distances. However the terraces between them exhibit large fluctuations in length. Conversely, the B-steps are highly kinked, but are seperated by terraces which show little length variation. For the A-steps, the terrace length fluctuations are greatly reduced by changing the reconstruction from the 2×4 or 1×1 or by initiating growth. During growth, the difference between RHEED intensity oscillations on the two surfaces is striking. For identically prepared and simultaneously mounted wafers that had been oriented 2° from the (100), the A-surface showed intensity oscillations over a range in temperatures. By contrast, the B-surface exhibited only faint oscillations that were rapidly damped. In addition, during growth of GaAs on stepped Si(100) surfaces, growth on surfaces prepared to yield A-steps gave much smoother morphologies than on surfaces prepared to yield B-steps.
We have studied the surface atomic structure and electronic properties of the molecular-beam-epitaxy-grown AlAs(001)-(3×2) film by in situ scanning tunneling microscopy and spectroscopy. A structural model is proposed for AlAs(001)-(3×2), in which the ×2 ordering arises from the formation of unusual double pi-bonded As dimers along the [110] direction and the ×3 ordering results from the missing As-dimer rows in the [1-bar10] direction on the topmost As-terminated layer. Scanning tunneling spectroscopic study directly reveals the energies and spatial locations of the surface states.
Structural and electronic properties of the As-rich GaAs(001)(2×4) reconstructions are investigated by means of converged first-principles total-energy calculations. For an As coverage of Theta=3/4, we find the two-dimer beta2 phase to be energetically preferred over the three-dimer beta phase. As the As chemical potential decreases, the alpha phase of GaAs(001) represents the ground state of the surface. All geometries are characterized by similar structural elements as As dimers with a length of about 2.5 Å, dimer vacancies, and a nearly planar configuration of the threefold-coordinated second-layer Ga atoms leading to a steepening of the dimer block. Consequently, the resulting electronic properties also have similar features. The surface band structures are dominated by filled As-dimer states and empty Ga dangling bonds close to the bulk valence- and conduction-band edge, respectively. The measured Fermi-level pinning cannot be related to intrinsic surface states. The calculated surface states and ionization energies support the beta2 structure as the surface geometry for an As coverage of Theta=3/4.
The Molecular Beam technique has been used to control the surface stoichiometry of GaAs (001). Surface structures as related to surface stoichiometry are studied by LEED. Successive transitions of structures obtained by annealing under ultra-high vacuum are given. LEELS has been studied on surfaces of various arsenic coverage. Surface state associated with gallium dangling bond has been identified. In situ metal deposition on surfaces of different structures gives Schottky contacts with barrier varying from 0. 6 to 0. 8 eV.
A Comment on the Letter by J. Falta et al., Phys. Rev. Lett. 69, 3068 (1992).
The composition and structure of an InAs{100}−(4 × 2) epilayer grown on an InP 100 surface has been studied by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low energy electron diffraction (LEED). Time-of-flight spectra of scattered neutrals plus ions were collected as a function of the crystal azimuthal angle, δ, and the beam incident angle, α, using 4 keV Ne+ ions. The composition of the outermost layer was obtained from grazing incidence scattering. Experimental scattering images were used with the composition from azimuthal specific elemental accessibilities (CASEA) method to define the azimuthal directions with respect to the bulk crystallographic axes. Structural analysis was obtained from the azimuthal anisotropy of the δ-scans, from the features of the incident α-scans, and from the scattering images. These α- and δ-scans and images were simulated by means of classical ion trajectory calculations. The totality of this data is consistent with the InAs{100}−(4 × 2) surface being In-terminated and with the reconstruction being of the missing-row-dimer (MRD) type. The data show that the interatomic spacings are consistent with the coexistence of both one- and two-missing In rows in the surface. The 1st-layer In atoms are found to shift by 0.4 ± 0.1 Å, yielding an In-In dimer spacing of 3.5 ± 0.1 Å. Data for the InP (4 × 2) surface has been included in order to contrast the missing-row-trimerdimer (MRTD) reconstruction model of InP with that of the MRD reconstruction model of InAs.
Reconstructions of the GaN\(0001¯\) surface are studied for the
first time. Using scanning tunneling microscopy and reflection
high-energy electron diffraction, four primary structures are observed:
1×1, 3×3, 6×6, and c\(6×12\). On the basis of
first-principles calculations, the 1×1 structure is shown to
consist of a Ga monolayer bonded to a N-terminated GaN bilayer. From a
combination of experiment and theory, it is argued that the 3×3
structure is an adatom-on-adlayer structure with one additional Ga atom
per 3×3 unit cell.
Scanning tunneling microscopy (STM) images of the arsenic-rich reconstruction of the GaAs(001) surface known as $2\ifmmode\times\else\texttimes\fi{}3$ are presented. It is shown that this surface does not have $2\ifmmode\times\else\texttimes\fi{}3$ symmetry but is composed of domains of $4\ifmmode\times\else\texttimes\fi{}3$ and $c(4\ifmmode\times\else\texttimes\fi{}6)$ unit cells. Analysis of the STM images indicates that occupied and unoccupied surface electronic states are localized at different sites within the $4\ifmmode\times\else\texttimes\fi{}3$ unit cell. A new structural model for the ``$2\ifmmode\times\else\texttimes\fi{}3$'' reconstruction is proposed.
We have studied the surface structures of GaSb, AlGaSb, and alternating GaSb/AlSb layers using 10‐kV reflection electron diffraction. Above 540 °C, the Sb‐stabilized surface (1×3) patterns change to c(8×2), a Ga‐stabilized surface. Because the c(8×2) surface has been observed on all other III–V arsenides, phoshpides, and antimonides, it is now clear that the c(8×2) metal‐stabilized surface is common to all III–V compounds, suggesting that bond pairing occurs on all III–V semiconductor surfaces and is a universal reconstruction mechanism. The smooth, sharp transitions observed in the growth of alternating GaSb and AlSb layers show that atomically smooth interfaces can be formed in this system. In contrast, for the AlAs overgrowth on GaAs, transient structures associated with a Ga surface layer can be observed.
Reflection high energy electron diffraction measurements of the intensity of (elastically) diffracted beams as a function of the incident angle have been carried out for an exactly oriented GaAs(001) surface, using a primary beam energy of 12.5 keV. Different surface reconstructions were prepared in-situ by molecular beam epitaxy and the measurements of the diffracted intensities were made at temperatures and As 2 -fluxes where these structures were stable. Results for the (0,0) beam in the [ 1 10], [110] and [010] azimuths are reported for the 2 × 4, 3 × 1 and c(4 × 4) structures and in addition for fractional and integral order beams for the 2 × 4 surface. In general, very large intensity variations (up to a factor of 10 4 ) are observed. The results are discussed in terms of diffraction processes involving primary Bragg, secondary Bragg and surface resonances, and refraction effects introduced by both bulk and surface “inner potentials” are included. The data cannot be explained using the kinematic approximation, but they are a consequence of multiple scattering events. The reconstructed surface layer appears to have a dominant influence. A complete analysis of the results would require full dynamical calculations based on realistic surface models, but they are not yet available. Finally, the significance of the results to recent observations of oscillations in diffracted intensity during thin film growth by molecular beam epitaxy is considered.
The scanning tunneling microscope (STM) has been used to study the (2×4)-c(2×8) reconstruction on the arsenic-rich surface of GaAs(001). The STM images show that the 4× periodicity is due to a regular array of missing arsenic dimers. The (2×4) units are arranged so as to give small domains of either (2×4) or c(2×8) reconstructions. These images are the first high-resolution STM images obtained from a surface grown by molecular-beam epitaxy. Features are seen which may be important in the mechanism of growth.
This review describes advances in understanding the structural, electronic, and chemical properties of clean low-index semiconductor surfaces during the first decade following the advent of the scanning tunneling microscope (STM). The principles of STM are discussed together with the instrumentation required to perform STM measurements on semiconductor surfaces in ultrahigh vacuum. A comprehensive review of the structures of the clean, low-index surfaces of elemental and compound semiconductors is presented. These structures are discussed using the general physical principles that determine them.
The structure of the GaAs(001)-2 × 4 reconstructed surface is studied using medium-energy ion scattering (MEIS). The MEIS profiles clearly show splitting blocking dips, which originate in the surface reconstruction. We have quantitatively determined the atomic displacement from these blocking dips. The topmost As atoms forming the symmetric dimers are displaced laterally , and the bond length of the As dimer is . The Ga atoms of the second atomic layer, which bond with the As dimer atoms, are also displaced laterally and the displacement is . These surface atoms also move vertically and the surface layers shrink.
Numerical convergence of multi-slice dynamical calculations of reflection high-energy electron diffraction (RHEED) intensities for numbers of slices, atomic layers and beams is reported for a Ag(001) surface. Criterions of the convergence for numbers of slices and atomic layers are delivered by a thermal vibration amplitude of atoms and an electron absorption coefficient, respectively. From comparison of the curves of 11-beam calculation for the zeroth Laue zone and those of 61-beam calculation with higher order Laue zones, it is revealed that low-order RHEED intensities obtained by the 11-beam calculation are not significantly different from those of the 61-beam calculation. Therefore, calculated RHEED intensities for the zeroth Laue zone spots are obtained well by dynamical calculation with a beam set in only the zeroth Laue zone. Effects of the Debye-Waller factor on RHEED intensities of higher order Laue zones are also discussed.
This paper investigates, theoretically, the migration potential and adsorption energies of Ga adatoms on a reconstructed As-rich GaAs (001)-(2 × 4) surface by ab initio calculation. The calculated results show that migration potential depends sensitively on the number of surface Ga adatoms and that the adsorption energy oscillates with the adsorption of every other atom. The dependence on the number of surface Ga atoms is explained by the effects of the surface distortion and the electron counting model. This paper also discusses As incorporation during epitaxial growth and demonstrates the dynamical behavior of Ga adatoms at finite temperature by Monte-Carlo simulation.
Formation energies for a variety of GaAs(100) surface structures have been calculated as a function of the atomic chemical potentials using the first-principles pseudopotential density-functional approach. We find that the surface transforms through four phases as the chemical potential varies across its accessible range. As the Ga chemical potential increases the surface transforms from an As-rich [ital c](4[times]4) through two distinct (2[times]4) structures and finally to a Ga-rich (4[times]2) phase. The predicted structures account for most scanning tunneling microscopy observations for the [ital c](4[times]4), (2[times]4), and (4[times]2) phases.
The infrared spectra of adsorbed hydrogen and deuterium on c(2×8) and (2×6) GaAs(100) contain a series of bands from 2200 to 1200 (1600 to 1000) cm-1 that are due to arsenic hydrides, terminal gallium hydrides, and bridging gallium hydrides (and deuterides). The latter is the first known example of bridge-bonded hydrogen on a semiconductor surface. Polarized spectra reveal that the AsH and GaH bonds orient along the [1¯10] and [110] axis, respectively. These results are consistent with a GaAs surface structure composed of As and Ga dimers with dimer bonds in the [1¯1 0] and [110] directions.
The scanning tunneling microscope (STM) has been used to study the c(8\ifmmode\times\else\texttimes\fi{}2) reconstruction of GaAs(100). High-resolution STM images imply this surface is composed of equal numbers of arsenic and gallium atoms, resolving disagreements in the interpretation of several previous experiments. The c(8\ifmmode\times\else\texttimes\fi{}2) structure arises from an ordered arrangement of (4\ifmmode\times\else\texttimes\fi{}2) subunits, with each subunit containing two As dimers and two Ga dimers.
Scanning tunneling microscopy (STM) has been used to study GaAs(001)-(2×4) surfaces grown by molecular-beam epitaxy. Although reflection high-energy electron diffraction always showed a characteristic (2×4) pattern, STM images indicated significant differences in the composition of the surfaces depending on the nature of the quenching conditions. A previously unreported, locally disordered (1×2) structure was observed under As-deficient conditions. This was found extending from step edges and in missing-dimer holes, where the top layer of As had been removed to expose the second layer. The atoms in this exposed layer are identified as Ga and they form short rows in the [1¯10] direction. The twofold periodicity is due to a vacancy structure, with Ga atoms located at alternate sites along the [110] direction. These results may rationalize some of the recent controversies regarding the composition and structure of the As-terminated (2×4) surface prepared by decapping methods.
We present detailed results of ab initio pseudopotential calculations for equilibrium atomic geometry and chemical bonding on the arsenic terminated GaAs(001)-beta2(2×4) surface. Of particular note is our finding that there are two distinct Ga-As bond lengths between the first and second layers. This feature is due to the presence of both threefold and fourfold coordinated Ga atoms in the second layer. Our results add significantly to the information available from recent first-principles calculations, and from scanning tunneling microscopy, reflection high-energy electron diffraction, and low-energy electron diffraction analyses.
Step structures on InAs (001) vicinal surfaces under two different surface reconstructions were investigated by scanning tunneling microscopy. On an InAs surface misoriented by 1° toward the [110] direction, relatively straight monolayer steps along the [1\=10] direction were observed under an As-stabilized (2×4) reconstruction. On the other hand, step-bunching of about 10 monolayers was detected under an In-stabilized (4×2) reconstruction. On an InAs surface misoriented by 1° toward the [1\=10] direction, ragged monolayer steps roughly running parallel to the [110] direction were seen under the (2×4) reconstruction. However, relatively straight steps along the [110] direction with step-bunching of 2-3 monolayers were observed under the (4×2) reconstruction. These results indicate that thermodynamically favorable step structures are different between the (2×4) and (4×2) reconstructions.
The MBE-grown GaAs(001) surface exhibits various surface phases depending on the surface temperature and As/Ga flux ratio during the growth. Using scanning tunneling microscopy, reflection high energy electron diffraction and theoretical simulations, we have carried out a systematic study of various phases from As-rich c(4×4), 2×4 to Ga-rich 4×2 and 4×6, utilizing migration-enhanced epitaxy. Based on our thorough investigation, wc were able to propose a simple and unified structural model for the evolution of surface phases. The As-rich 2×4 phase consists of two As dimers on the (op layer and another As dimer on the third layer (Chadi's two-dimer model), while there is still a small window where a slightly As-poor phase may exist consisting of two As dimers on the lop layer and second layer relaxation (Northrup-Froyen model). As for the Ga-rich 4×2, we determined that the Beigelsen's two-Ga-dimer model fits best to our experimental and theoretical results, which consists of two Ga dimers on the top layer and an additional Ga dimer on the third layer, a mirror image of Chadi's two-dimer model for the As-rich 2×4 phase. Our direct observations reveal that there arc two distinct 4×6 phases.
The systematic study of the FI-STM images and reflection high-energy electron diffraction of the GaAs(001)2 × 4 surface showed that the GaAs(001)-(2 × 4)-α, β and γ phases all have the same unit cell in the topmost layer consisting of two As dimers and two dimer vacancies.
Reflection high energy electron diffraction measurements of the intensity of (elastically) diffracted beams as a function of the incident angle have been carried out for an exactly oriented GaAs(001) surface, using a primary beam energy of 12.5 keV. Different surface reconstructions were prepared in-situ by molecular beam epitaxy and the measurements of the diffracted intensities were made at temperatures and As2-fluxes where these structures were stable. Results for the (0,0) beam in the [10], [110] and [010] azimuths are reported for the 2 × 4, 3 × 1 and c(4 × 4) structures and in addition for fractional and integral order beams for the 2 × 4 surface. In general, very large intensity variations (up to a factor of 104) are observed. The results are discussed in terms of diffraction processes involving primary Bragg, secondary Bragg and surface resonances, and refraction effects introduced by both bulk and surface “inner potentials” are included. The data cannot be explained using the kinematic approximation, but they are a consequence of multiple scattering events. The reconstructed surface layer appears to have a dominant influence. A complete analysis of the results would require full dynamical calculations based on realistic surface models, but they are not yet available. Finally, the significance of the results to recent observations of oscillations in diffracted intensity during thin film growth by molecular beam epitaxy is considered.
We have studied the interface formation in molecular beam epitaxial (MBE) grown GaAsAlAs (0 0 1) heterostructures using scanning tunneling microscopy in ultrahigh vacuum (UHV-STM) and reflection high-energy electron diffraction (RHEED). High-resolution STM images reveal at the normal interface an indistinct compositional profile due to a strong Ga segregation into the AlAs layer. For the abruptly formed inverted interface the STM images indicate the incorporation of intrinsic point defects at the interface plane. Large-scale STM scans of both interface configurations and of the as-grown AlAs (0 0 1) surface show that the morphological differences are less important as expected. Therefore, we propose that the defect incorporation contributes considerably to the asymmetry in the electronic properties of the two different interface types.
The (2 × 4) reconstruction of the (001) surface of GaAs has been studied using scanning tunnelling microscopy (STM) and spectroscopy. The images, produced at several biases, show coexisting two dimer and three dimer surface unit cell reconstructions. By examining line profiles across the dimers, we find an asymmetry in the two dimer surface unit cell reconstruction which changes with the sample bias. The images of the three dimer surface unit cell reconstruction are also shown to be different at different biases. A spectroscopic spectrum is presented which gives insight into the origin and characteristics of negative and positive bias tunnelling. Our results are compared to theoretical studies of the expected appearance of STM images under different bias conditions and the implications of the findings are discussed.
Atomic resolution scanning tunnelling microscopy (STM) has been used to study the As-terminated reconstructions formed by GaAs(001) surfaces grown in situ by molecular beam epitaxy (MBE). Specific emphasis has been placed on the transition from a (2 × 4) to c(4 × 4) surface with increasing amounts of As. STM images of the initial (2 × 4) surface, corresponding to the β phase, showed an ordered structure with unit cells containing two As dimers. With increasing amounts of As, the intensity of the 24 streak in the RHEED pattern weakened considerably. Although STM images of this (2 × 4) phase again only showed two As dimers per unit cell, the surface was characterized by a considerable degree of disorder and a large number of kinks. The results are consistent with the (2 × 4) β phase having a structure with unit cells based on two As dimers and Gn absent from the missing dimer trenches. The kinks formed on the more As-rich (2 × 4) structure are then caused by the additional As occupying these vacant Ga sites producing an electron rich site. Quenching to lower temperatures in the presence of As leadsto the c(4 × 4) structure. STM images of this surface indicate that the top layer of the structure is based on rectangular units, which when complete, consist of a total of six As atoms. The wide coverage range for which this reconstruction can be maintained is explained by a varying number of missing As atoms from the basic six atom structural unit. A new structural model is proposed for the c(4 × 4) structure based on its formation from the starting (2 × 4) surface and involves a mixed third layer containing both Ga and As.
Scanning tunneling microscopy and current spectroscopy have been used to investigate the p-type arsenic-terminated GaAs(001)-(2 × 4) surface. Images were collected at both positive and negative voltages applied to the sample with respect to the tip. The main features in the images taken at positive sample bias do not differ from those taken at negative bias. This is in contrast to the scanning tunneling microscopy results on the GaAs(110) surface, where, at negative and positive sample bias, the As and Ga atomic orbitals are imaged, respectively, and are, thus, complementary to each other. The different behavior observed on the GaAs(001) surface is attributed to the fact that, on this surface, the Ga atoms are in the second layer. Hence, at positive sample bias tunneling occurs predominantly into the empty As molecular orbital. This conjecture is supported by a calculation of the tunneling current spectrum that contains the contributions of tunneling into the valence band as well as into the conduction band.
RHEED patterns for the InSb(001) asymmetric (1 × 3) structure were taken as a function of the Sb coverage on samples prepared by MBE. A twofold periodicity orthogonal to the asymmetric threefold periodicity has been observed. We propose a simple model based on the (2 × 4) structure to explain the asymmetric (1 × 3) structure, the threefold periodicity being generated by domain formation and the amount of the splitting explained by the degree of Sb coverage. We also show how this model can be used to explain most of the reconstructions observed on the {001} surfaces of III–V compound semiconductors.
We have studied the atomic structures of GaAs (411)A surfaces by using scanning tunneling microscopy, and have found that the surface flatness largely depends on the As coverage. In contrast to the As-rich surface, which has no flat (411)A terraces but has (311)A and (511)A microfacets, the As-deficient surface shows flat (411)A terraces with 0.07-nm-high monomolecular steps. The detailed analysis based on the observed atomic arrangements indicates that the flattening transition can occur because the electron counting rule is broken at all monomolecular steps on the As-deficient surface.
Step structures of InAs (001) vicinal surfaces tilted by 1° towards the [1[bar 1]0] direction were investigated using scanning tunneling microscopy. A transition from an As-stabilized ( 2×4) to an In-stabilized ( 4×2) reconstruction caused a change in the step structure of the vicinal surfaces from ragged to straight along the [110] direction with the incorporation of step bunching. The straight shape of the step structure seems to be caused by desorption of arsenic atoms along the [110] direction.
Thermal stability of the ( 2×4) structures
on InAs and GaAs (001) surfaces is studied
by using scanning tunneling microscopy with atomic resolution.
Even just before the transition to a ( 4×2) structure,
the annealed InAs surface has much lower density of kinks
in the dimer vacancy rows than the annealed GaAs.
This difference leads to a unified model that can explain
both the 1st order phase transition
and the peculiar electric properties of an InAs (001) surface.
The role of steps during the initial stages of GaAs growth on InAs surfaces was investigated by scanning tunneling microscopy (STM). The surface structures of a nominally (001) InAs substrate and a misoriented (001) InAs substrate tilted by 1° towards the [111]B direction were observed by STM after a certain amount of GaAs deposition. In the case of a nominally (001) surface, a growth mode transition from two-dimensional (2D) to 3D island growth occurred when more than 0.75 ML GaAs was deposited. On the other hand, in the case of a vicinal surface, a growth mode transition did not occur when the same amount of GaAs was deposited onto the surface. In this case, GaAs-selective growth attached to the step edges and crack formation extending in the [1\=10] direction were observed in the STM images. These results indicate that the initial growth stages of GaAs heteroepitaxy on an InAs vicinal surface are different from those on a nominally (001) InAs surface due to the existence of steps.
A multi-chamber ultrahigh vacuum scanning tunneling microscopy (UHV-STM) system has been constructed for investigations of processed GaAs surfaces. This system comprises five UHV chambers connected by UHV tunnels. A specially designed STM unit which can be used with a 23-mm-diameter molybdenum block, is installed in an STM-chamber for surface observations. A molecular beam epitaxy (MBE) chamber for epitaxial growth of GaAs layers and a process-chamber for argon ion-bombardment are included in the five chambers. First, the surface of the MBE-grown layer on a GaAs (001) wafer was observed with STM. The sample was then transferred to the process-chamber and bombarded with argon ions. Many defects, which seemed to be caused by the ion-bombardment, were found in an STM image taken with the ion-bombarded sample.
The GaAs(100) (2× 4) surface was studied by molecular beam epitaxy (MBE) and field-ion-scanning-tunneling-microscopy (FI-STM). The results showed that the GaAs(100) (2× 4) surface unit cell consists of two dimers and two missing dimers of As. The c(2× 8) reconstruction was observed to originate by the phase shift induced by the As vacancies. We also observed the domain boundaries on the GaAs(100) (2× 4) surface and they were attributed to two dimensional island growth. A structural model was proposed to explain the observed surface reconstructions.
We have developed a new combined system of a field-ion scanning tunneling microscope (FI-STM) and a molecular beam epitaxy (MBE) growth chamber equipped with six Knudsen-cells and reflection high energy electron diffraction (RHEED). This system allows us to observe compound semiconductor surfaces such as GaAs while keeping their surfaces clean. In this paper, this system was applied to GaAs(100) and Si(110) surfaces. A GaAs sample was heated up to 550°C and a GaAs thin epitaxial film was grown by MBE. 2×, 4× structures were observed by RHEED and the sample was moved to the FI-STM chamber where STM images of 2× 4 reconstructed GaAs(100) surfaces were obtained. Besides the observation of GaAs, FI-STM was also applied to the Si(110) surface. We have studied the change of the Si(110) surface structures at different conditions of heating and cooling.
Two types of cubic and hexagonal GaN films were deposited on (001) GaAs substrates. The film structure proved to be controlled by GaAs pretreatments. By performing a N2H4 (hydrazine) pretreatment of GaAs substrates, the GaN films, which were otherwise hexagonal similarly to ordinary films on sapphire substrates, became cubic. A surface cubic nitride layer was found to be formed on the pretreated GaAs by a RHEED (reflection high-energy electron diffraction) observation, which is thought to be the substantial substrate for the following growth of a cubic GaN film.
The energy-loss spectra of ~ 100-eV electrons were measured for (100) and (1¯1¯1¯) GaAs and (111) Ge surfaces. The portion of the energy-loss spectra attributed to excitations of d electrons is proportional to the density of states in the conduction bands and empty surface states. The GaAs surfaces stabilized into Ga-rich and As-rich conditions permit unambiguous identification of intrinsic surface states. Empty and filled surface states, attributed to dangling Ga and As bonds, are observed near the conduction-band and valence-band edges, respectively.
We use low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to study the In-terminated InAs(001) surface prepared by argon sputtering and annealing. Characterization by LEED shows the formation of a highly ordered surface with a mixture of (4×2) and c(8×2) phases. We systematically vary the sample bias in STM to obtain bias-dependent images over the same surface regions, allowing discrimination between topographic and electronic features. Atomic resolution STM images confirm the existence of both (4×2) and c(8×2) phases and identify an electronic signature at the transition between the two reconstructions. Images of (4×2) regions are consistent with a previously proposed model for this surface in which the unit cell contains one In dimer in the first layer and two In dimers in the third layer. The c(8×2) reconstruction, though similar to the (4×2), is found to arise from a shift in the third- and/or first-layer In dimers by one lattice spacing. Filled-state imaging at the (4×2)-to-c(8×2) boundary shows two bright spots positioned midway between the first-layer In dimer rows. In empty states, these spots are entirely absent, underlining their electronic origin. These electronic features are explained in terms of a localization of charge due either to a structural defect or to the presence of a sulfur doping atom at the transition from (4×2) to c(8×2) reconstructions.
Field-ion scanning tunneling microscopy (STM) and in situ reflection high-energy electron diffraction (RHEED) have been used to study the atomic structures of the As-rich GaAs(001) surfaces grown by molecular-beam epitaxy and migration-enhanced epitaxy. The (2×4) alpha, beta, and gamma phases and the c(4×4) phase have been investigated in a systematic manner, controlling the As surface coverage. The high-resolution STM images show that the (2×4) alpha, beta, and gamma phases all have the same unit structure in the outermost surface layer, which consists of two As dimers and two As dimer vacancies. Various structure models proposed for the (2×4) phases are examined based on the STM observations and dynamical calculation of the RHEED spot intensities. We now propose the following model: The alpha phase is the two-As-dimer model proposed by Farrell and Palmstrom with relaxation incorporated by Northrup and Froyen. The surface coverage of the alpha phase is 0.5 ML of As. The beta phase is the two-As-dimer model proposed by Chadi. The surface coverage of the beta phase is 0.75 ML of As. The so-called gamma phase is characterized by its small domains separated by open areas. The domains consist of the same local structure as the beta phase and the open areas between them have a disordered As double-layer structure similar to that of the c(4×4) phase. The gamma phase is, thus, no more than the mixture of the beta phase and the c(4×4) phase with the surface As coverage varying between 1.75 and 0.75 ML depending on the growth conditions. Our structure model for the As-rich GaAs(001) surface is consistent with most of the observations reported.
We present theoretical studies of the surface dielectric functions of (001) GaAs with As-terminated (2×1) and Ga-terminated (1×2) reconstructions as approximations to the (2×4) and (4×2) reconstructions observed in molecular-beam epitaxy. We assume a surface geometry of symmetric As or Ga dimers. A nearest-neighbor tight-binding model with six orbitals (s,x,y,z, x2-y2, 3z2-r2) per atom is used in the calculation. The interaction parameters and optical matrix elements between atomic orbitals are determined by fitting the band structure and optical matrix elements between Bloch states over the entire Brillouin zone to the corresponding empirical pseudopotential results. The calculated dielectric-function anisotropies for GaAs (2×1) surfaces are analyzed with reference to the electronic structures, and are compared to data.
Structural models characterized by missing rows of surface As dimers are
proposed for the 2×4 and c(2×8) As-stabilized surfaces of
GaAs. Experimental and theoretical support for these models and for a
variable surface stoichiometry are examined. A model with full monolayer
As coverage and either symmetric or asymmetric dimers is found to be
inadequate. Total-energy calculations show that 2×4 or
c(2×8) unit cells are optimal for surface As coverages of
ΘAs=0.5 and ΘAs=0.75. The proposed
model can account in a simple way for the observed one-dimensional
disorder and provides a good description for the amplitudes of
fractional-order structure factors.
Clean GaP(001)-(4x2) and H2S-treated (1X2) surfaces are studied by scanning tunneling microscopy (STM). We have observed a (4X2)/c(8X2) STM image for the cation-stabilized GaP(001) surface. The result suggests that the unit cell of the (4X2) structure consists of two Ga dimers with two missing Ga dimers. For the (1x2)S surface, the previous model that sulfur atoms are adsorbed on the Ga dimer and that a missing row of sulfur is formed along the [1 ($) over bar 10] direction is supported by the STM result. (C) 1995 American Institute of Physics.
The c-4x4 reconstruction of the As(100) face of GaAs is examined using an energy-minimization approach. Other structures with 1x1, c-2x2 and p-2x2 periodicities are also examined in this study. The calculations predict that dimerization of surface As atoms is energetically favorable; however, structures with only one type of dimer per unit cell are found to be unstable. A four atom unit cell with two inequivalent dimers is the minimum size necessary for an adequate description of the surface atomic and electronic properties. The c-4x4 surface is suggested to consist of equal numbers of symmetric and asymmetric As dimers.
Detailed surface phase diagrams of GaAs(001) and AlxGa1−xAs(001) have been determined in a study of MBE growth on vicinal surfaces. Information on the defect structure of the growing surface has been derived from RHEED spot-profile measurements. It is shown that the phase boundaries as well as the defect structure correlate with iso-lines of the surface stoichiometry.
The chemisorption and decomposition of trimethylgallium on the gallium-rich (4 × 6) and (1 × 6) GaAs(100) surfaces have been studied by temperature-programmed desorption, LEED, and Auger electron spectroscopy. Upon adsorption and subsequent heating, a fraction of a monolayer ((1.6 ± 0.7) × 1014 molecules cm−2) of trimethylgallium irreversibly dissociates into monomethylgallium surface fragments. Upon heating, methyl radicals are observed desorbing from the surface with a narrow peak at 710 K. The desorption parameters (activation energy and pre-exponential factor) of the methyl radicals depend on surface coverage, indicating significant adsorbate-adsorbate interactions. The zero coverage limit desorption parameters are: and v = 1.6 × 1014±0.2 s−1. Higher coverages of trimethylgallium desorb molecularly in peaks at 500 and 350 K. After desorption of all hydrocarbon fragments the surface immediately converts into the (1 × 6) reconstruction. The excess gallium released from trimethylgallium decomposition is not incorporated in the GaAs surface and apparently resides in gallium droplets. We present structural models for the gallium-rich GaAs(100) surfaces before and after TMGa chemisorption which are consistent with all of our observations.