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Schematic diagram of the reaction pathway from disilane to silicon dimer rows. The disilane dissociates upon adsorption, as two SiH 3 groups, which rapidly break up into SiH 2 and adsorbed H. The SiH 2 groups can diffuse and form monohydride ad-dimers, which lose their hydrogen and rotate to form isolated epitaxial dimers. The square structures are formed from two nonrotated dimers, which may then either form the ''2'' structure with a high barrier, or attach an epitaxial dimer, and form a length-3 string.
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We have observed the development of the surfaces during gas-source growth of silicon and germanium in an elevated temperature ultrahigh vacuum scanning tunneling microscopy (STM), with near-atomic resolution under a range of temperature and flux, which are the two dominant parameters, and applied atomistic modeling to the structures seen by STM to...
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... reaction pathway to island formation from disilane Ref. 10 is shown schematically in Fig. 1. The Si 2 H 6 mol- ecules break up on impact into SiH 3 groups, which decay to form SiH 2 H at room temperature. 8 These SiH 2 groups dif- fuse together and form nonepitaxial monohydride ad-dimers, which lose their hydrogen at 150 °C, forming nonrotated ad- dimers. Most of these ad-dimers become mobile at around 200 °C Refs. 19 and 20 ...
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... The upper step is in the top left corner of the STM image; the steps descend in the direction of the image bottom. The surface structure is 2 × N formed by DVLs running perpendicularly to the Ge dimer rows of the 2 × 1 reconstruction [4,50] that corresponds to the initial stage of the WL growth (1-2 ML) at a lower temperature. ...
... The RHEED pattern of this surface presented in Fig.7 (d) [4,50]. As expected, Ge hut clusters are seen to be absent on this surface [32,34]. ...
... additional narrow streaks emerging near the main ones (the former are indicated by the arrows); this corresponds to the 2 × N structure emerging on the Ge (001) surface due to the appearance of DVLs, i.e. shallow trenches formed due to the absence of every N-th dimer in rows in the [110] direction[4,50]. ...
... The upper step is in the top left corner of the STM image; the steps descend in the direction of the image bottom. The surface structure is 2 × N formed by DVLs running perpendicularly to the Ge dimer rows of the 2 × 1 reconstruction [4,50] that corresponds to the initial stage of the WL growth (1-2 ML) at a lower temperature. ...
... The RHEED pattern of this surface presented in Fig.7 (d) [4,50]. As expected, Ge hut clusters are seen to be absent on this surface [32,34]. ...
... additional narrow streaks emerging near the main ones (the former are indicated by the arrows); this corresponds to the 2 × N structure emerging on the Ge (001) surface due to the appearance of DVLs, i.e. shallow trenches formed due to the absence of every N-th dimer in rows in the [110] direction[4,50]. ...
The results of STM and RHEED studies of a thin Ge film grown on the Si/Si(001) epitaxial layers with different surface relief are presented. Process of the partial stress relaxation was accompanied by changes in the surface structure of the Ge wetting layer. Besides the well-known sequence of surface reconstructions (2 × 1 → 2 × N → M × N patches) and hut-clusters faceted with {105} planes, the formation of isolated {105} planes, which faceted the edges of M × N patches, has been observed owing to the deposition of Ge on a rough Si/Si (001) surface. A model of the isolated {105} facet formation has been proposed based on the assumption that the mutual arrangement of the monoatomic steps on the initial Si surface promotes the wetting layer formation with the inhomogeneously distributed thickness that results in the appearance of M × N patches partially surrounded by deeper trenches than those observed in the usual Ge wetting layer grown on the smooth Si(001) surface. Isolated {105} facets are an inherent part of the Ge wetting layer structure and their formation decreases the surface energy of the Ge wetting layer.
... It should be noted that since the letter my Mo et al. pyramidal and "elongated" clusters have alway been considered in the literature as structurally identical ones different only by their base aspect ratio. [57][58][59][60][61][62][63] The only argument for this assumption-the identity of faceting-does not seem to us to be very solid. Stress relaxation via formation of the {105} faceted structures, such as islands or pits, 57 appears to be energetically favorable and the structures are more stable compared to ones with different faceting. ...
... According to this scenario wedge arise by elongation of a pyramid due to the growth of one of its {105} facets. [57][58][59][60][61][62][63] This hypothesis would explain everything unless the necessity to explain why the symmetry of the pyramid is violated. Due to its shape a square-based regular pyramid seems to be stable enough, at least unless some exterior anisotropic driving force affects it removing the degeneracy of its facets. ...
An overview of new results on growth and characterization of Ge/Si(001) heterostructures with dense chains of stacked Ge quantum dots is reported. Ge hut nucleation and growth at low temperatures is discussed on the basis of results obtained by high resolution scanning tunneling microscopy and in-situ reflected high-energy electron diffraction. Atomic-level models of nucleating and growing huts are considered. Recent data of high resolution transmission electron microscopy are presented focusing on long chains of Ge quantum dots synthesized in silicon matrix by means of molecular beam epitaxy at low- Temperature mode. Approaches to formation of three dimensional ordered arrays of Ge cluster and Ge quantum dot crystals are considered in special detail.
... According to this scenario wedge arise by elongation of a pyramid due to the growth of one of its {105} facets. [57][58][59][60][61][62][63] This hypothesis would explain everything unless the necessity to explain why the symmetry of the pyramid is violated. Due to its shape a square-based regular pyramid seems to be stable enough, at least unless some exterior anisotropic driving force affects it removing the degeneracy of its facets. ...
... Remark thatGoldfarb et al. observed also that at 620℃, and hence thinner wetting layer, only hut clusters arose.57 Further, with the increase of Ge coverage, when the capability of pits to release the stress is exhausted huts nucleate in the vicinity of pits.57,58,61 Acknowledging a model by Jesson et al.,63 which explains instability of the hut cluster shapes (read "elongation") by nucleation and growth on the facets, as more common, the authors of Ref. 57 illustrate the elongation process by an example of cluster coalescence.As mentioned above, investigations by Goldfarb with co-authors 62 eventually gave a weighty argument in support of the so-called equilibrium-driven elongation or, in other words, elongation governed by energy minimization. ...
An overview of new results on growth and characterization of Ge/Si(001) heterostructures with dense chains of stacked Ge quantum dots is reported. Ge hut nucleation and growth at low temperatures is discussed on the basis of results obtained by high resolution scanning tunneling microscopy
and in-situ reflected high-energy electron diffraction. Atomic-level models of nucleating and growing huts are considered. Recent data of high resolution transmission electron microscopy are presented focusing on long chains of Ge quantum dots synthesized in silicon matrix by means
of molecular beam epitaxy at low-temperature mode. Approaches to formation of three dimensional ordered arrays of Ge cluster and Ge quantum dot crystals are considered in special detail.
... The latter nuclei consist of two adjacent truncated tetrahedral pyramids, which, upon unification, form a tiny square-based pyramidal critical nucleus, see Fig. 24 [90]. Their subsequent growth was shown to occur from their base where new material arrives, see Fig. 25 [103]. The nuclei of the square and rectangular hut pyramids were also observed and found to have different structures. ...
... The new nucleated material covers the bottom of the hut cluster facet. Source: from[103]. ...
... Evolution of islands for an anisotropy describing(105) and(103) minima beside the (001) orientation. The islands with squared bases are formed first, thereafter develop into a bimodal distribution of islands. ...
Many recent advances in microelectronics would not have been possible without the development of strain induced nanodevices and bandgap engineering, in particular concerning the common SiGe system. In this context, a huge amount of literature has been devoted to the growth and self-organization of strained nanostructures. However, even if an overall picture has been drawn out, the confrontation between theories and experiments is still, under various aspects, not fully satisfactory. The objective of this review is to present a state-of-the-art of theoretical concepts and experimental results on the spontaneous formation and self-organization of SiGe quantum dots on silicon substrates. The goal is to give a comprehensive overview of the main experimental results on the growth and long time evolution of these dots together with their morphological, structural and compositional properties. We also aim at describing the basis of the commonly used thermodynamic and kinetic models and their recent refinements. The review covers the thermodynamic theory for different levels of elastic strain, but focuses also on the growth dynamics of SiGe quantum dots in several experimental circumstances. The strain driven kinetically promoted instability, which is the main form of instability encountered in the epitaxy of SiGe nanostructures at low strain, is described. Recent developments on its continuum description based on a non-linear analysis particularly useful for studying self-organization and coarsening are described together with other theoretical frameworks. The kinetic evolution of the elastic relaxation, island morphology and film composition are also extensively addressed. Theoretical issues concerning the formation of ordered island arrays on a pre-patterned substrate, which is governed both by equilibrium ordering and kinetically-controlled ordering, are also reported in connection with the experimental results for the fabrication technology of ordered arrays of SiGe quantum dots.
... (Goldfarb et al. [21] also observed that at 620 C Ð and hence in a thinner wetting layer Ð only hut clusters arose.) Further, with the increase in Ge coverage of the surface, when the capability of pits to release the stress is exhausted, hut clusters nucleate in the vicinity of pits [21,22,25]. Acknowledging the model by Jesson et al. [27], which explains the instability of the hut cluster shapes (readèlongation') by nucleation and growth on the facets, as more common, the authors of Ref. [21] illustrate the elongation process by an example of cluster coalescence. ...
Ge hut clusters forming quantum dot arrays on the Si(001) surface in the process of low-temperature ultrahigh-vacuum molecular beam epitaxy are morphologically investigated and classified using in situ scanning tunnelling microscopy. It is found that two main Ge hut cluster types---pyramidal and wedge-shaped---have different atomic structures, and it is concluded that shape transitions between the two are impossible. Derivative cluster species --- obelisks (or truncated wedges) and accreted wedges --- are revealed and investigated for the first time and shown to start dominating at high Ge coverages. The uniformity of cluster arrays is shown to be controlled by the scatter in the lengths of wedge-like clusters. At low growth temperatures (360 °C), cluster nucleation during the growth of the array is observed for all values of Ge coverage except for a particular point at which the arrays are more uniform than at higher or lower coverages. At higher temperatures (530 °C), no cluster nucleation is observed after the initial formation of the array.
... (Goldfarb et al. [21] also observed that at 620 C Ð and hence in a thinner wetting layer Ð only hut clusters arose.) Further, with the increase in Ge coverage of the surface, when the capability of pits to release the stress is exhausted, hut clusters nucleate in the vicinity of pits [21, 22, 25] . Acknowledging the model by Jesson et al. [27], which explains the instability of the hut cluster shapes (readèlongation') by nucleation and growth on the facets, as more common, the authors of Ref. [21] illustrate the elongation process by an example of cluster coalescence. ...
Ge hut clusters forming quantum dot arrays on the Si(001) surface in the process of low-temperature ultrahigh-vacuum molecular beam epitaxy are morphologically investigated and classified using in situ scanning tunnelling microscopy. It is found that two main Ge hut cluster types - pyramidal and wedge-shaped - have different atomic structures, and it is concluded that shape transitions between the two are impossible. Derivative cluster species - obelisks (or truncated wedges) and accreted wedges - are revealed and investigated for the first time and shown to start dominating at high Ge coverages. The uniformity of cluster arrays is shown to be controlled by the scatter in the lengths of wedge-like clusters. At low growth temperatures (360 degrees C), cluster nucleation during the growth of the array is observed for all values of Ge coverage except for a particular point at which the arrays are more uniform than at higher or lower coverages. At higher temperatures (530 degrees C), no cluster nucleation is observed after the initial formation of the array.
... Some reports have been published on the surface morphology of Si thin film grown at comparatively low temperature by the use of silicon hydride gas [6,9,10]. It has been reported by Imai et al. that an SEM image revealed a fine structure on the surface of the Si epitaxial layer grown by TM atomic-layer epitaxy (ALE) using Si 3 H 8 gas [6]. ...
... However, in our present experiments, we observed a two-dimensional flat surface grown by the GSMBE mode at the temperature of 5008C, as shown in Fig. 8(a). Observations of the nucleation and growth of Si from Si 2 H 6 on Si(0 0 1) by the use of a real-time elevated-temperature ultrahigh-vacuum scanning tunneling microscope (STM) have been reported [10]. It was reported that increasing the temperature to 4008C leads to a transition to the step-flow growth mode from the island mode. ...
Si thin epitaxial layers were grown by a temperature modulation Si molecular-layer epitaxy (TM Si MLE) method using Si2H6 gas. The surface was observed by the use of a high-resolution field-emission scanning electron microscope (FE-SEM). The surface morphology changed drastically from a three-dimensional island feature to a two-dimensional flat surface with lowering of the modulated temperature from 530°C to 460°C. In conjunction with the result of the in situ observation of the surface hydrogen desorption reaction, the mechanism of the change in the surface morphology was examined.
The results of STM and RHEED studies of a thin Ge film grown on the Si/Si(001) epitaxial layers with different surface relief are presented. Process of the partial stress relaxation was accompanied by changes in the surface structure of the Ge wetting layer. Besides the well-known sequence of surface reconstructions (2 × 1 → 2 × N → M × N patches) and hut-clusters faceted with {105} planes, the formation of isolated {105} planes, which faceted the edges of M × N patches, has been observed owing to the deposition of Ge on a rough Si/Si (001) surface. A model of the isolated {105} facet formation has been proposed based on the assumption that the mutual arrangement of the monoatomic steps on the initial Si surface promotes the wetting layer formation with the inhomogeneously distributed thickness that results in the appearance of M × N patches partially surrounded by deeper trenches than those observed in the usual Ge wetting layer grown on the smooth Si(001) surface. Isolated {105} facets are an inherent part of the Ge wetting layer structure and their formation decreases the surface energy of the Ge wetting layer.
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Structural properties of the clean Si(001) surface obtained as a result of low-temperature (470–650℃) pre-growth annealings of silicon wafers in a molecular-beam epitaxy chamber have been investigated. To decrease the cleaning temperature, a silicon surface was hydrogenated in the process of a preliminary chemical treatment in HF and NH 4 F aqueous solutions. It has been shown that smooth surfaces composed by wide terraces separated by monoatomic steps can be obtained by dehydrogenation at the temperatures 600℃, whereas clean surfaces obtained at the temperatures < 600℃ are rough. It has been found that there exists a dependence of structural properties of clean surfaces on the temperature of hydrogen thermal desorption and the process of the preliminary chemical treatment. The frequency of detachment/attachment of Si dimers from/to the steps and effect of the Ehrlich-Schwoebel barrier on ad-dimer migration across steps have been found to be the most probable factors determining a degree of the resultant surface roughness.
