V. Yu. Chalkov

Synchrotron investigations of the specific features in the electron energy spectrum of silicon nanostructures

The Si L2, 3 x-ray absorption near-edge structure (XANES) spectra of porous silicon nanomaterials and nanostructures with epitaxial silicon layers doped by erbium or containing germanium quantum dots are measured using synchrotron radiation for the first time. A model of photoluminescence in porous silicon is proposed on the basis of the results obtained. According to this model, the photoluminescence is caused by interband transitions between the energy levels of the crystalline phase and oxide phases covering silicon nanocrystals. The stresses generated in surface silicon nanolayers by Ge quantum dots or clusters with incorporated Er atoms are responsible for the fine structure of the spectra in the energy range of the conduction band edge and can stimulate luminescence in these nanostructures.

Confocal Raman microscopy of self-assembled GeSi/Si(001) Islands

The method of confocal Raman microscopy is used for the first time to study the spatial distribution of elemental composition and elastic strains in self-assembled GexSi1 − x/Si(001) islands grown by the method of sublimation molecular-beam epitaxy in the GeH4 ambient. The lines related to vibrational modes Si-Si, Ge-Ge, and Si-Ge are identified in the Raman spectra measured in the regions with dimensions <100 nm on the surface of the samples. The spatial distribution of the Ge atomic fraction x in the GexSi1 − x alloy and of the elastic strain ɛ (averaged in depth over the island layer) have been calculated from the maps of the Raman shifts of the corresponding lines over the sample surface. The dependences of x and ɛ on the islands’ growth temperature and on the nominal thickness of the deposited Ge layer have been studied.

Photodiodes based on self-assembled GeSi/Si(001) nanoisland arrays grown by the combined sublimation molecular-beam epitaxy of silicon and vapor-phase epitaxy of germanium

We investigate the photosensitivity spectra of photodiodes based on Si p-i-n structures with single-layered and multilayer self-assembled GeSi/Si(001) nanoisland arrays in the i region, which are grown using a technique combining Si molecular-beam epitaxy and Ge vapor-phase epitaxy, in dependence on the temperature, diode bias, and GeSi nanoisland parameters. We show that the temperature and field dependences of the diode photosensitivity in the spectral range of the interband optical absorption in GeSi nanoislands are determined by the ratio between the rate of emission of photoexcited holes from the nanoislands and the rate of the recombination of excess carriers in them. We demonstrate the possibility of determination of the hole recombination lifetime in GeSi nanoislands from the temperature and field dependences of the photosensitivity.

Investigation of the local density of states in self-assembled GeSi/Si(001) nanoislands by combined scanning tunneling and atomic-force microscopy

The local density of states in self-assembled GeSi/Si(001) nanoislands is investigated for the first time by combined tunneling and atomic-force scanning microscopy. Current images and tunneling spectra of individual GeSi/Si(001) islands are obtained. These measurements yield the spatial and energy distributions of the local density of states in GeSi islands, respectively. The tunneling spectroscopy data demonstrate that uncapped Ge0.3Si0.7/Si(001) islands behave as type-I heterostructures.

Optically active centers in Si/Si1 − x Ge x :Er heterostructures containing Er3+ ions

The basic types of optically active erbium centers that make the major contribution to the photo-luminescence signal from the Si/Si1 − xGex:Er heterostructures with the Ge content from 10 to 30% are analyzed in detail. It is shown that the origin of the optically active centers containing Er3+ ions correlates with the molar composition of the Si1 − xGex:Er layer and the content of oxygen impurity in the layer. The major contribution to the photoluminescence signal from the Si/Si1 − xGex:Er heterostructures with the Ge content below 25% is made by the well-known centers containing Er3+ ions and oxygen. An increase in the Ge content in the Si1 − xGex:Er layer (x ≥ 25%) yields the formation of a new type of centers, specifically, the Gecontaining optically active erbium centers unobserved in the Si-based structures previously.

Photoluminescence of GeSi/Si nanoclusters formed by sublimation molecular-beam epitaxy in GeH4 medium

The morphology and photoluminescence spectra of GeSi/Si(001) heterostructures with nanoclusters formed by sublimation molecular-beam epitaxy in GeH4 medium are investigated as functions of growth conditions. It is established that the clusters nucleate by the Stranski-Krastanow mechanism; however, the coalescence processes substantially affect their morphology during further growth. Doubling of photoluminescence lines in nanoclusters associated with the radiative recombination inside the clusters and the blue shift of lines with increasing growth time associated with the Si diffusion from substrate into the clusters are observed. The conditions of forming uniform nanocluster arrays emitting at room temperature are determined.

Methods for the doping of silicon layers in growth by sublimation MBE

Epitaxial layers doped with various impurities were grown by sublimation MBE on Si (100) substrates. Doping with phosphorus was controlled at electron densities ranging from 2×1013 to 1019 cm−3. A high dopant concentration of ∼1020 cm−3 was obtained from the evaporation of partly molten Si sources. It shown that the type and concentration of an impurity in the sublimation MBE process can be controlled by the fabrication of multilayer p+−n+ structures.

Simultaneous doping of silicon layers with erbium and oxygen in the course of molecular-beam epitaxy

Epitaxial silicon layers codoped with erbium and oxygen were grown by molecular-beam epitaxy using a silicon sublimation source. For growing the erbium-doped silicon layers, two types of impurity sources were used: (i) erbium-doped silicon plates were used as a source of fluxes of Er and Si atoms; and (ii) metallic erbium plates were used as an impurity-vapor source in combination with the silicon sublimation source. If gaseous oxygen was used for in situ codoping with erbium and oxygen, then concentrations ranging from 1018 to 1020 cm−3 were attained. When oxygen is in the growth chamber, the erbium-entrapment efficiency of a layer increases substantially, with the surface segregation of erbium also being suppressed by oxidation.

Photodetectors on the basis of Ge/Si(001) heterostructures grown by the hot-wire CVD technique

The fabrication of photodetectors for the wavelength range of communications λ = 1.3–1.55 µm on the basis of Ge/Si(001) heterostructures with thick (~0.5 µm) Ge layers grown by the hot-wire technique at reduced growth temperatures (350°C) is reported. The single-crystal Ga layers are distinguished by a low density of threading dislocations (~105 cm–2). The photodetectors exhibit a rather high quantum efficiency (~0.05 at λ = 1.5 µm and 300 K) at moderate reverse saturation current densities (~10–2 A cm–2). Thus, it is shown that the hot-wire technique offers promise for the formation of integrated photodetectors for the wavelength range of communications, especially in the case of limitations on the conditions of heat treatments.

Epitaxially Grown Monoisotopic Si, Ge, and Si1–xGex Alloy Layers: Production and Some Properties

The technology of the growth of Si, Ge, and Si1–xGex layers by molecular-beam epitaxy with the use of a sublimation source of monoisotopic 30Si or 28Si and/or gas sources of monogermane 74GeH4 is demonstrated. All of the epitaxial layers are of high crystal quality. The secondary-ion mass spectroscopy data and Raman data suggest the high isotopic purity and structural perfection of the 30Si, 28Si, 74Ge, and 30Si1–x74Gex layers. The 30Si layers doped with Er exhibit an efficient photoluminescence signal.