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The structural and optical properties of ultrathin Ge insertions in an Si matrix were studied. Transmission electron microscopy
revealed the spontaneous formation of arrays of disk-shaped quantum dots (QDs) with a small lateral size (3–10 nm) at a nominal
Ge insertion thicknesses, from submonolayer to nearly critical, for the transition to 3D growt...
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We compare results obtained in several tens of samples grown by
molecular-beam epitaxy under different growth conditions with a
substantial amount of data found in the literature. By plotting the
photoluminescence (PL) peak energy (Ep) of the quantum dot
(QD) bands as a function of the nominal thickness of deposited InAs (L)
three regions are clear...
Cu thin films sandwiched between Nb-doped TiO2 (NTO) thin
films were grown on glass substrates using tilted-dual target DC
magnetron sputtering deposition. The thicknesses of the top and bottom
NTOs were nominally 30 nm, and the thicknesses of the Cu films (t)
varied between 1.5 nm and 50 nm. We measured the ellipsometric angles
(\Psi,Δ) of the NTO...
We present a detailed comparison of the optical properties of a quantum well (QW) and a two-dimensional (2D) quantum dot superlattice (QDSL) made of In 0:4 Ga 0:6 As on GaAs(311)B substrates under the same growth conditions. The lowest electron to hole transition energy in strained In 0:4 Ga 0:6 As/GaAs(311)B QWs is calculated. While good agreement...
The authors present a modulated reflectivity study of the wetting layer (WL) states in molecular beam epitaxy grown InAs/GaAs quantum dot (QD) structures designed to emit light in the 1.3-1.5 mu m range. A high sensitivity of the technique has allowed the observation of all optical transitions in the QD system, including low oscillator strength tra...
Nanoparticles of Zn1−xCuxO system with nominal compositions x = 0.0, 0.01, 0.02 and 0.03 were prepared by co-precipitation method at room temperature. Structural, morphological, optical and chemical species of grown crystals were investigated by X-ray diffraction (XRD) technique, Scanning Electron Microscopy (SEM), UV-visible and FTIR spectroscopy,...
Citations
... However, we point out that radiative recombination will typically be indirect in real space (type II), suitable for instance for photodetector operation. Type I structures suitable for stimulated emission are possible at even smaller sizes, as was demonstrated from submonolayer Ge QDs [23]. In the present work we describe an experimentally observed growth mode change from Stranski–Krastanow (SK) in the absence of C to Volmer–Weber (VW) when depositing Ge on a C-enriched Si substrate. ...
We follow the growth of islands with different shapes by monitoring the strain relaxation
by reflection high energy electron diffraction (RHEED). Comparing a bimodal ensemble of
pyramids and domes with a monomodal distribution of C-induced domes, we observe
different relaxation pathways and a growth mode change from Stranski–Krastanow to
Volmer–Weber. We also study the changes induced by the capping process with Si.
Small strains in thin cap layers are revealed by spectroscopic ellipsometry. Raman
spectroscopy is employed to probe the built-in strain and silicon intermixing in
different types of islands, evidencing that smaller islands are enriched in Si and
effectively recompressed, whereas bigger relaxed dots remain substantially unaffected.
... Assuming comparable electron effective masses in Si and Ge and a height of the potential spike of 0.1 eV one may conclude that the exciton ground state has the lowest binding energy to effective Ge layer thickness of 0.6-0.7 nm. Indeed, photoluminescence studies [18,19] performed in a wide range of excitation densities demonstrated that ultrathin Ge insertions do not demonstrate a characteristic high energy shift of the luminescence with excitation density, which is characteristic for type-II structures. This is quite the opposite to the situation with thicker Ge insertions [19], or Stranski-Krstanow SiGe QDs [20] and in agreement with a relatively small potential spike in the conduction band, which can be estimated as ~0.1 eV for strained Ge inclusions in Si [21]. ...
... At the same time the localisation energy of electrons remain fairly small and the thermal excitation of electrons into the electron miniband in Si occurs at fairly low (20-30 K) temperatures. This process is accompanied by reduction of the energy separation between the Ge-related photoluminescence (PL) and the corresponding Si-related PL lines and quenching of the Ge-related emission [18]. The effect of a strong decrease of the Ge-related PL is mostly linked to the trapping of thermally escaped electrons by nonradiative surface or Si substrate states. ...
... Doping of the active region with Sb creates significant equilibrium concentration of electrons preventing electron depletion. This strongly reduces temperature dependence of the PL emission from ultrathin Ge insertions in Si and allows it's observation up to room temperature [18]. ...
Si-Ge system offers a significant extension to traditional Si-based microelectronics. However, applications of the system
would be further greatly expanded if it can be used for high-speed optical transmitters and interconnects. A straightforward
idea to achieve this goal is to use the device designs, which are already successfully applied for direct- gap semiconductor
materials, particularly, to III–V materials. These are diode lasers based on double heterostructures [1] and on heterostructures with reduced dimensionality [3], so called, quantum wells (QWs), quantum wires (QWWs) and quantum dots (QDs). An idea to achieve lasing in indirect gap
materials by using double hetrostructure concept was first mentioned by Kroemer in 1963 [2]. In his paper H. Kroemer proposed to use the double heterostructures (DHS) for carrier confinement in the active region
of the diode laser and wrote that “laser action should be obtainable in many of the indirect gap semiconductors and improved
in the direct gap ones, if it is possible to supply them with a pair of heterojunction injectors”. Attempts to achieve lasing
SiGe-Si DHSs and QWs did not result is significant success, however, as also in the case of other types of indirect-gap materials,
for example, AlGaAs DHSs with high Al content (x>0.5), or in type-II GaAs-AlAs quantum QWs. A different approach to achieve
lasing in semiconductors was first proposed by Basov, Vul and Popov in 1959 [4], who considered unipolar carrier injection. Population inversion between ionised impurities and free carriers was thought
as a gain mechanism through impurity ionisation upon application of pulsed electric field. Boundaries of the sample providing
the reflection of light were proposed for a laser feedback mechanism. For Si-based optoelectronics such an opportunity is
particularly important, because optical transitions in the latter case are linked only to one band and the problem of indirect
crystal band structure in silicon is lifted. In 1971 an extension of the unipolar laser approach was proposed by Kazarinov
and Suris [5]. The authors proposed to use population inversion between different electron subbands in a specially designed QW superlattice.
The laser based on such approach (cascade laser) was realised in 1985 by Faist et al. [6]. The success of the cascade laser is linked, however, to direct-gap III–V materials and not to Si-based systems, in spite
of the fact that the hystory of intraband lasing in indirect gap materials (e.g. in p-doped Ge) is quite long [7].
The study of the structural characterizations, physical properties and fabricated methods on the Si-based nanometer materials has attracted much attention because of their potential applications in the optoelectronic-integrated technology. Self-assembled growth methods are of increasing interest as a main formed technology of high quality nanostructures such as nanoquantum dots, nanoclusters and nanoscale films. In particular, self-assembled formations of the nanometer materials with controlled crystallite size and density distribution are very important for optoelectronic device applications. Two routes for the fabrication of these materials were proposed. First, controlling the orderliness of preferential nucleated sites on solid-state surface can obtain the materials. Second, controlling the orderliness of nucleated process during self-assembled growth will form them. New progress of these fabricated methods was reviewed, and the tendency of development in the near future was predicated.
Quantum dot (QD) arrays have now been attracting tremendous attention due to the potential applications in various high performance devices (e.g., QD lasers, 3rd generation solar cells, single photon emitters, QD memories, etc.), the fundamental investigation of quantum computing and quantum communication, and in the exploration or observation of novel physical phenomena. Uniform and regular QD arrays with precisely controlled positions and sizes may serve as a template for the next generation of nanoelectronic and optoelectronic devices. Currently, the major challenging issues in commercialized application of QD arrays include fabrication of large-area, defect-free, highly uniform and ordering QDs, accurate positioning for individual QD nucleation site, and reproducibility in size and spatial distribution, which all crucially determines optoelectronic performance and consistency for these QDs-based functional devices and the investigation of fundamental physical properties for QDs. Over the past decade, enormous attempts have been made to improve the ordering, positioning, uniformity, and defect reduction for obtaining perfect QD arrays over a large area with long range ordering. This article provides a review of some major attempts and progresses recently made for enhancing the ordering, positioning and uniformity for QD arrays, with an emphasis on the problems which has been well addressed to reach the current state of the arts. Furthermore, the prospects, challenges and trends for producing high quality QD arrays with high ordering, uniformity, positioning and defect reduction, are addressed. Finally, some potential or promising solutions for achieving perfect QD arrays are discussed.
Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molecular-beam epitaxy at low
temperatures (
Porous inorganic materials such as zeolites and zeolitelike crystalline molecular sieves are of great interest because of
their range of commercial applications such as catalysis, adsorption/separation, and ion exchange. The term zeolite refers to the specific class of aluminosilicate molecular sieves, although the term is frequently used more loosely to describe
compounds other than aluminosilicates that have frameworks similar to known zeolites.
The growth and properties of semiconductor quantum dots have been studied extensively in the last decade. These novel nanostructures
offer interesting prospects for the development of new electronic or optoelectronic devices. In particular, if the size, shape,
and positioning of those structures can be controlled, they become very attractive for applications such telecommunication
wavelength-integrated photodetectors or tunable or single-photon light sources.
The initial stages of Ge growth on Si(111) vicinal surfaces tilted in the [
[`1][`1] 2\overline 1 \overline 1 2
] and [
11[`2]11\overline 2
] directions were studied in situ in the temperature range 350–500°C using scanning tunneling microscopy. It was shown that,
at low Ge deposition rates of 10−2 to 10−3 BL/min, ordered Ge nanowires can form on surfaces tilted in the [
[`1][`1] 2\overline 1 \overline 1 2
] direction under conditions of step-layered growth. The height of a nanosized Ge wire is one or three interplanar distances
and is determined by the initial height of a silicon step. It was established that, during epitaxial growth, steps with a
[
11[`2]11\overline 2
] front are replaced by steps with a [
[`1][`1] 2\overline 1 \overline 1 2
] front. As a result, the step edge is serrated and the formation of smooth nanosized Ge wires uniform in width is hampered
on the serrated Si(111) surfaces tilted in the [
11[`2]11\overline 2
] direction.
The electronic states of silicon with a periodic array of spherical germanium clusters are studied within the pseudopotential
approach. The effects of quantum confinement in the energies and wave functions of the localized cluster states are analyzed.
It is demonstrated that clusters up to 2.4 nm in size produce one localized s state whose energy monotonically shifts deep into the silicon band gap as the cluster size increases. The wave function of
the cluster level corresponds to the single-valley approximation of the effective-mass method. In the approximation of an
abruptly discontinuous potential at the heterointerface, the quantities calculated using the effective-mass method for clusters
containing more than 200 Ge atoms are close to those obtained by the pseudopotential method. For smaller clusters, it is necessary
to take into account the smooth potential at the interface.
