[en] Thermal oxidation of MBE-grown silicon nanowires with a gold droplet on their tips was investigated. Two kinds of oxidation behavior were observed: (i) enhanced axial oxidation, if there was a direct contact between the gold droplet and the nanowire, and (ii) conventional oxidation for nanowires when there was no direct contact between the gold and silicon. For a dry atmosphere, such enhanced oxidation takes place at temperatures down to 500 °C. Under a wet atmosphere, remarkable oxidation was observed even for temperatures down to 250 °C

[en] Deep level transient spectroscopy (DLTS) measurements were performed to study electron emission from quantum states in a 20-layer Ge quantum-dot superlattice (QDSL) in a Ge/Si p-n heterostructure. It was established that the changes in the DLTS spectra depend heavily on the magnitude of the applied reverse bias U_r. Three regions of the reverse bias U_r were identified, corresponding to the manifestation of the three modes of the Wannier-Stark effect: Wannier-Stark ladder mode, Wannier-Stark localization, and nonresonant Zener tunneling mode. Furthermore, it was found that the appearance of DLTS peaks for all three modes is associated with electron emission from deep-level defects via Wannier-Stark localized states arising as a result of the splitting of the electron miniband of the Ge/Si QDSL

[en] Deep level transient spectroscopy (DLTS) measurements were performed to study electron emission from quantum states in a 20-layer Ge quantum-dot superlattice (QDSL) in a Ge/Si p-n heterostructure. It was established that the changes in the DLTS spectra depend heavily on the magnitude of the applied reverse bias U_r. Three regions of the reverse bias U_r were identified, corresponding to the manifestation of the three modes of the Wannier-Stark effect: Wannier-Stark ladder mode, Wannier-Stark localization, and nonresonant Zener tunneling mode. Furthermore, it was found that the appearance of DLTS peaks for all three modes is associated with electron emission from deep-level defects via Wannier-Stark localized states arising as a result of the splitting of the electron miniband of the Ge/Si QDSL.

[en] An original approach to selective doping of Si by antimony (Sb) in molecular beam epitaxy (MBE) is proposed and verified experimentally. This approach is based on controllable utilization of the effect of Sb segregation. In particular, the sharp dependence of Sb segregation on growth temperature in the range of 300-550 deg. C is exploited. The growth temperature variations between the kinetically limited and maximum segregation regimes are suggested to be utilized in order to obtain selectively doped structures with abrupt doping profiles. It is demonstrated that the proposed technique allows formation of selectively doped Si:Sb layers, including delta (δ-)doped layers in which Sb concentrations can be varied from 5 x 10¹⁵ to 10²⁰ cm^-3. The obtained doped structures are shown to have a high crystalline quality and the short-term growth interruptions, which are needed to change the substrate temperature, do not lead to any significant accumulation of background impurities in grown samples. Realization of the proposed approach requires neither too low (<300 deg. C), nor too high (>600 deg. C) growth temperatures or any special equipment for the MBE machines.

[en] The electroluminescence (EL) of multilayered p-i-n structures with the self-assembled Ge(Si)/Si(001) islands are investigated. It is found that the structures with islands grown at 600^oC have the highest intensity of the electroluminescence signal at room temperature in the wavelength range of 1.3-1.55 μm. The annealing of structures with the Ge(Si) islands leads to an increase in the EL-signal intensity at low temperatures and hampers the temperature stability of this signal, which is related to the additional Si diffusion into islands during annealing. The found considerable increase in the electroluminescence-signal intensity with the thickness of the separating Si layer is associated with a decrease in the elastic stresses in the structure with an increase in this layer's thickness. The highest EL quantum efficiency in the wavelength range of 1.3-1.55 μm obtained in investigated structures amounted to 0.01% at room temperature.

[en] Scanning electron microscopy, spectroscopic ellipsometry, and current-voltage and current-temperature measurements were employed to characterize nanowhisker structures grown by molecular-beam epitaxy on Si(111) substrates. Small clusters of gold deposited on the Si surface were used as the seeds for nanowhisker growth. The diameter of grown nanowhiskers and their length ranged from 70 to 200 nm and from 580 to 890 nm, respectively. The whiskers were found to inherit the (111) orientation of the Si substrate. By means of spectroscopic ellipsometry in the range 1.5-4.77 eV, lateral optical inhomogeneity of the nanowhisker layer was revealed, with optical properties of the layer substantially differing from those of single-crystal Si. Electrical measurements point to the presence of a Schottky barrier with height 0.70 eV in the structure and to the presence of electrically active centers non-uniformly distributed over the nanowhisker length

[en] The effect of structure parameters on the electroluminescence and photoconductivity of multilayer structures with self-assembled Ge(Si)/Si(0 0 1) islands has been studied. The highest intensity of the room-temperature electroluminescence in the wavelength range of 1.3–1.55 µm has been observed for the islands grown at 600 °C. The same diode structures with Ge(Si)/Si(0 0 1) islands have demonstrated room-temperature photoconductivity signals in the wavelength range of 1.3–1.55 µm. The observed overlap of the electroluminescence and photoconductivity spectra obtained for the same structures with Ge(Si) islands makes these structures a promising material for the fabrication of a Si-based optocoupler. Less degradation after neutron irradiation has been observed for the electroluminescence and photoconductivity signals from multilayer structures with Ge(Si) self-assembled islands in comparison with bulk silicon structures. This result is associated with more effective confinement of charge carriers in the multilayer structures with Ge(Si) islands

[en] Methods of optical spectroscopy and electron microscopy have been used to study tunnel-injection nanostructures the active region of which consisted of an upper In_0.15Ga_0.85 As quantum-well layer and a lower layer of In_0.6Ga_0.4As quantum dots as a light emitter; both layers were separated by a GaAs barrier layer. Deviations from the semiclassical Wentzel-Kramers-Brillouin model are observed in the dependence of the tunneling time on barrier's thickness. Reduction of the transfer time to several picoseconds at a barrier thickness smaller than 6 nm is accounted for by formation of InGaAs nanobridges between tops of quantum dots and the quantum-well layer; the nanobridges include those with their own hole state. The effect of an electric field induced by tunneling on the carriers' transfer time in a tunnel-injection nanostructure is taken into account.

[en] Photoluminescence of multilayer structures with InAs quantum dots grown in the p-n junction in GaAs by molecular-beam epitaxy is studied. Formation of vertical columns of quantum dots is verified by the data of transmission electron microscopy. It is shown that a natural increase in the size of quantum dots from layer to layer brings about their vertical coalescence at the upper part of a column. An unbalance of electronic levels caused by the enlargement of quantum dots was compensated by an external electric field, so that the resonance of ground electronic states in the column was attained. The onset of resonances was checked by the methods of steady-state and time-resolved photoluminescence. It is shown that, in the case of a resonance, the photoluminescence intensity and the radiative lifetime of excitons increase (up to 0.6-2 ns), while the time of tunneling of charge carriers becomes shorter (shorter than 150 ps). Outside the resonances, tunneling of electrons is appreciably enhanced owing to the involvement of longitudinal optical phonons. If only these phonons are involved, the time of nonresonance tunneling between quantum dots becomes shorter than the time of relaxation of charge carriers from the barrier (100 and 140 ps, respectively)

[en] The energy band diagram of the multilayered Ge_0.8Si_0.2/Ge_0.1Si_0.9 heterostructures with vertically correlated quantum dots is analyzed theoretically. With regard to fluctuations of the thickness layer in the columns of quantum dots and to the exciton-phonon coupling, it is shown that the electron states constitute a miniband. The hole wave functions remain localized in the quantum dots. The spectrum of optical transitions calculated for a 20-layered structure at room temperature is in good agreement with the experimental photoluminescence spectrum that involves an intense band at about 1.6 μm. From theoretical considerations and experimental measurements, specific evidence for the miniband in the superlattice is deduced; it is found that the overlap integrals of the wave functions of electrons and holes and the integrated intensity of the photoluminescence band of the Ge quantum dots are described by quadratic functions of the number of the structure periods