N. A. Bekin

Self-organization of germanium nanoislands obtained in silicon by molecular-beam epitaxy

Nanometer germanium islands in epitaxial layers of silicon are obtained by molecular-beam epitaxy. The dimensions and shapes of the islands are determined in an atomic-force microscope. The photoluminescence spectra are found to contain lines that can be interpreted as quasidirect optical transitions in the islands. It is concluded on the basis of optical and microprobe measurements and theoretical calculations of the energies of electronic states that silicon is dissolved in the germanium islands. Values of the germanium and silicon contents in the solid solution are presented.

Electron and hole spectra and selection rules for optical transitions in Ge1−xSix/Ge heterostructures

The electron and hole spectra in strained Ge1−xSix/Ge heterostructures grown on a (111) plane have been investigated. It is shown that the structure of the conduction band in these structures can be determined by investigating the polarization of their photoluminescence. The selection rules for indirect optical transitions have been found.

Terahertz Emission from Phosphor Centers in SiGe and SiGe/Si Semiconductors

Terahertz-range photoluminescence from silicon-germanium crystals and superlattices doped by phosphor has been studied under optical excitation by radiation from a mid-infrared CO2 laser at low temperature. SiGe crystals with a Ge content between 0.9 and 6.5 %, doped by phosphor with a concentration optimal for silicon laser operation, do not exhibit terahertz gain. On the contrary, terahertz-range gain of ~ 2.3 - 3.2 cm-1 has been observed for donor-related optical transitions in Si/SiGe strained superlattices at pump intensities above 100 kW/cm2.

Terahertz Luminescence of GaAs-Based Heterostructures with Quantum Wells under the Optical Excitation of Donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ∼3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO2 laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm−1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers ND ≈ 1011 cm−2 can be attained under the excitation-flux density 1023 photons/(cm2 s).

The conduction band and selection rules for interband optical transitions in strained and heterostructures

The conduction-band alignments of and heterostructures grown on (111) and (001) crystallographic planes, respectively, are analysed. We have obtained the selection rules for interband dipole optical transitions in the heterostructures, and discussed the possibilities for specifying the types of the lowest conduction-band minima. We show that this can be done by, for example, exploring the polarization of different phonon-assisted band-edge optical transitions. The conduction-band minima may be at different L or points of the Brillouin zone, depending on the structure parameters. Although bulk Ge, Si, and their alloy are indirect-gap semiconductors, the heterostructures and can have a direct band gap. We found the parameter regions where type-I and type-II band alignment were realized, and those where the band gap was direct in quantum well (QW) structures. It is shown that in direct-gap QW structures grown on (001) planes, direct interband optical transitions between the nearest electron and hole subbands are allowed, but the same transitions are forbidden for direct-gap structures grown on (111) planes.

A cascade laser on shallow-donor transitions in δ-Doped GaAs/AlGaAs superlattices

The possibility of amplification of terahertz-range electromagnetic waves due to optical transitions between a two-dimensional continuum and shallow-level donor states in GaAs/AlGaAs superlattices with selectively δ-doped quantum wells is analyzed theoretically. The population inversion required for the gain is attained under the conditions of vertical transport in the superlattice due to the hybridization of states in neighboring quantum wells coupled via electron tunneling through the barrier. It is shown that, at a doping level of 5 × 1010 cm−2 per period, the gain can be as high as 50 cm−1 at wavelengths 100–120 μm. The current density under operating conditions is ∼50 A/cm2.

Determination of the density of states in quantum wells and quantum dot arrays by the capacitance-voltage method

The possibility of determining the electronic density of states in quantum wells and quantum dot arrays in heterostructures from the capacitance-voltage curve is investigated. In heterostructures fluctuations of the composition and geometrical dimensions play an important role. It is shown that to reconstruct the exact density of states from the measured capacitance-voltage curve is impossible, because this problem is ill-posed from the mathematical point of view. An approximate method is proposed for solving the problem, involving the determination of a “reduced” density of states. It is shown that the reduced density of states is close to the true density if the characteristic energy scale governing the variation of the latter is much greater than the thermal energy kT. The proposed method is used to find the density of states in the conduction band of a quantum well in an In0.22Ga0.78As/GaAs heterostructure.

Infrared radiation from hot holes during spatial transport in selectively doped InGaAs/GaAs heterostructures with quantum wells

The infrared radiation from hot holes in InxGa1−xAs/GaAs heterostructures with strained quantum wells during lateral transport is investigated experimentally. It is found that the infrared radiation intensities are nonmonotonic functions of the electric field. This behavior is due to the escape of hot holes from quantum wells in the GaAs barrier layers. A new mechanism for producing a population inversion in these structures is proposed.

Exciton luminescence in Ge–Ge1−xSix multiple‐quantum‐well structures

Photoluminescence spectra of quasi‐2D excitons in Ge–Ge1−xSix multiple‐quantum‐well structures (MQWS) on Ge (111) substrates are investigated. The properties of the energy spectra for electrons and holes in strained nanometer Ge layers are discussed on the basis of experimental measurements and theoretical estimations. We found experimentally the Ge band gap in Ge–Ge1−xSix MQWS and its dependence on a layer thickness, and also the binding energy of quasi‐2D excitons in strained nanometer Ge layers. It was shown that the no‐phonon transitions intensity increases with a decrease in a quantum‐well width by enhanced scattering on the short‐range component of the potential perturbation near the interfaces and in the barriers.

Far-infrared emission and possibility of population inversion of hot holes in MQW InGaAs/GaAs heterostructures under real space transfer

Abstract Hot holes in strained MQW In x Ga 1− x As/GaAs heterostructures excited at lateral charge transport are probed by far-infrared emission and band gap photoluminescence. Highly nonequilibrium phenomena discovered are shown to result from the real space transfer. New mechanism for the population inversion between continuum and shallow bound states in quantum wells in high lateral electric fields is put forward.