I. N. Yassievich

Room‐temperature photoluminescence of erbium‐doped hydrogenated amorphous silicon

A comparison of the photoluminescence of Er‐doped hydrogenated amorphous silicon and crystalline silicon a‐Si:H(Er) and c‐Si(Er), is presented. It is shown that a‐Si:H(Er) exhibits efficient room‐temperature photoluminescence at 1.537 μm which is as strong as the emission from optimized c‐Si(Er) at 2 K. Most remarkably, there is practically no temperature quenching of the emission intensity in the range 2–300 K. The experiments suggest that the lifetime connected with the Er‐induced emission is considerably shorter in a‐Si:H(Er) than in c‐Si(Er) which may be responsible for the different dependences of the photoluminescence intensity on the temperature, chopping frequency, and excitation power.

Spin-dependent resonant tunneling in symmetrical double-barrier structures

A theory of resonant spin-dependent tunneling has been developed for symmetrical double-barrier structures grown of noncentrosymmetrical semiconductors. The dependence of the tunneling transparency on the spin orientation and the wave vector of electrons leads to (i) spin polarization of the transmitted carriers in an in-plane electric field and (ii) generation of an in-plane electric current under tunneling of spin-polarized carriers. These effects originated from spin-orbit coupling-induced splitting of the resonant level have been considered for double-barrier tunneling structures.

Deep impurity-center ionization by far-infrared radiation

An analysis is made of the ionization of deep impurity centers by high-intensity far-infrared and submillimeter-wavelength radiation, with photon energies tens of times lower than the impurity ionization energy. Within a broad range of intensities and wavelengths, terahertz electric fields of the exciting radiation act as a dc field. Under these conditions, deep-center ionization can be described as multiphonon-assisted tunneling, in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field. The field dependence of the ionization probability permits one to determine the defect tunneling times and the character of the defect adiabatic potentials. The ionization probability deviates from the field dependence e(E) ∝ exp(E2/Ec2) (where E is the wave field, and Ec is a characteristic field) corresponding to multiphonon-assisted tunneling ionization in relatively low fields, where the defects are ionized through the Poole-Frenkel effect, and in very strong fields, where the ionization is produced by direct tunneling without thermal activation. The effects resulting from the high radiation frequency are considered and it is shown that, at low temperatures, they become dominant.

Room-temperature electroluminescence of erbium-doped amorphous hydrogenated silicon

We have observed strong room-temperature electroluminescence at 1.54 μm induced by erbium ions in amorphous hydrogenated silicon (a-Si:H). The device consisted of an Al/a-Si:H(Er)/n-c-Si/Al structure. A mechanism for electronic excitation of the erbium ions in the amorphous matrix is proposed that is based on defect-related Auger excitation.

Light emission from silicon nanocrystals

The main experimental results of studies of the photoluminescence of silicon nanocrystals and theoretical methods developed for the description of optical processes occurring in them are reviewed. Special attention is focused on silicon nanocrystals in the SiO2 matrix that were the object of most of the studies. Two fundamental theoretical methods described in detail are the multiband effective-mass method and the tight-binding method which have found wide application in simulating various processes occurring in nanostructures. A phenomenological model for excitons self-trapped on the surface of oxidized silicon nanocrystals, which has been recently developed on the basis of experimental results obtained by femtosecond spectroscopy, is reported.

Mechanism of terahertz lasing in SiGe/Si quantum wells

Intense terahertz (THz) stimulated emission from boron-doped SiGe/Si quantum well structures with internal strain has been observed recently. We present a theoretical calculation which shows the formation of resonant states, and explains the origin of the observed temperature dependence of the dc conductivity under low bias voltage. Thus, the mechanism of THz lasing is population inversion of the resonant state with respect to the localized impurity states. This is the same mechanism of lasing as in uniaxially stressed p-Ge THz lasers.

Resonant acceptor states and terahertz stimulated emission of uniaxially strained germanium

The stimulated emission spectrum of uniaxially strained p-Ge is presented. The energy spectrum of the states of a shallow acceptor in Ge under uniaxial compression is calculated. The threshold pressure at which the acceptor state split off from the ground state becomes resonant is found. The pressure dependence of the width of this resonant level is calculated. The stimulated emission lines are identified. In particular, it is shown that the principal emission peak corresponds to the transition of holes from the resonant 1s (1sr) state to the local p±1 state. The probabilities of optical transitions are calculated. A mechanism of population inversion due to the intense resonant scattering of hot holes with an energy corresponding to the position of the 1sr level is proposed.

Optical transitions and energy relaxation of hot carriers in Si nanocrystals

Dynamics of hot carriers confined in Si nanocrystals is studied theoretically using atomistic tight binding approach. Radiative, Auger-like, and phonon-assisted processes are considered. The Auger-like energy exchange between electrons and holes is found to be the fastest process in the system. However, the energy relaxation of hot electron-hole pair is governed by the single optical phonon emission. For a considerable number of states in small nanocrystals, single-phonon processes are ruled out by energy conservation law.

Tunnelling ionization of deep centres in high-frequency electric fields

Experimental and theoretical work on the ionization of deep impurity centres in the alternating terahertz field of high-intensity far-infrared laser radiation, with photon energies tens of times lower than the impurity ionization energy, is reviewed. It is shown that impurity ionization is due to phonon-assisted tunnelling which proceeds at high electri cfi eld strengths into direct tunnelling without involving phonons. In the quasi-static regime of low frequencies the tunnelling probability is independent of frequency. Carrier emission is accomplished by defect tunnelling in configuration space and electron tunnelling through the potential well formed by the attractive force of the impurity and the externally applied electric field. The dependence of the ionization probability on the electric field strength permits one to determine defect tunnelling times, the structure o ft he adiabatic potentials of the defect, and the Huang–Rhys parameters of electron–phonon interaction. Raising the frequency leads to an enhancement of the tunnelling ionization and the tunnelling probability becomes frequency dependent. The transition from the frequency-independent quasi-static limit to frequency-dependent tunnelling is determined by the tunnelling time which is ,i nt he case of phononassisted tunnelling, controlled by the temperature. This transition to the highfrequency limit represents the boundary between semiclassical physics, where the radiation field has a classical amplitude, and full quantum mechanics where th er adiation field is quantized and impurity ionization is caused by multiphoton processes. In both the quasi-static and the high-frequency limits, the application of an external magnetic field perpendicular to the electric field reduces the ionization probability when the cyclotron frequency becomes larger than the reciprocal tunnelling time and also shifts the boundary between the quasi-static and the frequency-dependent limits to higher frequencies. At low intensities, ionization of charged impurities may also occur through the Poole–Frenkel effect by thermal excitation over the potential well formed by the Coulomb potential and the applied electric field. Poole–Frenkel ionization precedes the range of phonon-assisted tunnelling on the electric field scale and

Energy transfer between silicon nanocrystals

It is shown that the energy migration between silicon nanocrystals embedded into a silicon dioxide host is caused by the “nonresonant” dipole-dipole interaction. This process is efficient only for a part of small nanocrystals among the whole ensemble of nanocrystals. The nonresonant dipole-dipole energy transfer has such a feature as the emission of two optical phonons at each step of the process. The time of the excitation transfer has been experimentally determined for nanocrystals 1.5 nm in size existing in the ensemble of nanocrystals with a density of 1018 cm−3 and the size distribution with a standard deviation of 20%. A value of 30 μs obtained for this time is in good agreement with the performed theoretical estimation.