Magnetoplasma resonance of exciton drop system in germanium

Two new absorptions which are believed at least partially connected with magnetoplasma resonance within high density exciton drops are observed below 2.2.K in 35GHz cyclotron resonance at germanium under the configuration that E ∥ 〈100〉 and H ∥ 〈001〉. One of them appears very close to the light hole cyclotron resonance signal and is qualitatively interpreted in terms of a simplified magnetoplasma theory, while the other shoots up near zero magnetic field and remains yet unexplained.     

Features of hole scattering in electroplastically deformed germanium crystals

The effect of specific features of scattering center formation during electroplastic deformation on the electric properties of germanium is investigated.     

Resonant states of shallow acceptors in uniaxially deformed germanium

The states of shallow acceptors in uniaxially deformed germanium are studied theoretically. A non-variational numerical computational method is developed for determining the energy and wave functions of localized states of holes in the acceptor field as well as the states of the continuous spectrum (including resonant impurity states). The dependence of the energy of the lower resonant state on strain is studied. It is found that this state is formed from the excited 4Γ ₈ ⁺ state with a binding energy of 1.3 meV (in the absence of deformation) and not from the ground state. The results presented in this work may be useful in the study of the conditions for the generation of far IR radiation in deformed p-Ge, which involves optical transitions between resonant and localized acceptor states.     

The fundamental absorption band of amorphous germanium

The index of attenuationk() of evaporated films of amorphous Ge was measured in the spectral range of 1.95 to 5.3 eV by measuring the transmissivity of samples with thicknesses varying from 560 to 1510 Å. It is shown that in comparison with crystalline Ge the absorption is significantly shifted towards lower energies and the peak at 4·5 eV is absent. Using these data and the data onk() determined in other spectral ranges up to 25 eV by various authors, the index of refractionn() was calculated by Kramers-Kronig analysis. The absorption of amorphous Ge is compared with that of crystalline Ge and the differences are discussed.The authors thank Mr. Z. Balej for the preparation of the thin films used in our measurements, Dr. J. Nadrchal for help with the calculations, Dr. T. M. Donovan for the information on his measurements of photoemission before its publication and Prof. R. Grigprovici for helpful discussions.     

The influence of exciton motion on spin-resonance absorption

We use a stochastic model for the exciton motion which comprises both the coherent and the incoherent motion. The incoherent part is taken care of by a stochastic process which allows the local excitation energy and the transition matrix element to fluctuate by means of a Markovian process. The interaction between the spins and their surroundings is described by the usual spin-Hamiltonian which is, however, simplified to a spin 1/2 particle (instead of the triplet state). In the present paper we solve exactly the two limiting cases of completely coherent and incoherent motion (for two molecules). In the incoherent case the influence of the exchange interaction integral is taken into account by perturbation theory. We find expressions which are immediately comparable with ESR-experimental data. This comparison and additional information derived from optical absorption measurements allow us to determine all free parameters of our model uniquely. In particular, the fluctuations of the exchange interaction integral (with strength γ₁) play an important role. From these parameters we may furthermore calculate the correlation time of the proton spin resonance in agreement with experimental data. The results show clearly that at room temperature in anthracene crystals the exciton undergoes a hopping process.     

Nonequilibrium carrier and exciton kinetics in silicon and germanium crystals

Asymptotic small-parameter expansion is applied to the solutions for singularly perturbed systems in order to examine the kinetics of laser excitation for nonequilibrium carriers and excitons in silicon and germanium crystals, which incorporates nonequilibrium carrier recombination, linear recombination, and exciton formation and ionization. Asymptotic evaluations can be used that apply throughout the range in pulse length in order to provide indirect and rough estimates of the probabilities for various recombination and ionization processes.Baku State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 24–26, December, 1993.     

The exciton absorption spectrum of KPbI3 thin films

The absorption spectrum of KPbI₃ thin films is studied in the spectral range 2–6 eV within the temperature interval 90–470 K. Localization of excitons in the sublattice of the compound containing lead ions is established. Excitons are shown to be of two-dimensional nature.     

Two-photon absorption in the vicinity of the exciton absorption in CdS

The two-photon absorption spectrum of CdS in the wavelength region of the free and bound exciton lines has been measured using a pulsed dye laser and a differential transmittance technique. An intense absorption band is observed which is possibly due to the interference of the contributions of different intermediate states to the interband two-photon absorption. No absorption is observed which can be attributed to the formation of free excitonic molecules.     

Surface photovoltage in exciton absorption range in CdS

The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120–160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.     

Interpretation of far-infrared absorption spectra in germanium

The high resolution far-infrared absorption spectra of free excitons in germanium are analyzed for the first time using a model which accurately includes both the anisotropy of the valence and condition bands and the degeneracy of the valence band. The observed structure is in good agreement with the calculated transitions between the anisotropy split ground state and the various p-like excited states.     

Effects of spatial dispersion on exciton absorption

The exciton absorption is studied in the spatially dispersive case. The energy propagation properties of the medium are used to define an appropriate absorption coefficient. The specific example of PbI₂ is considered in order to display the role of spatial dispersion.     

Indirect exciton transitions in the two-photon absorption in semiconductors

The theory of indirect two-photon exciton transitions is developed from third-order time-dependent perturbation theory. The problem is treated by introducing three different band models; a four-band, a two-band and an intermediate (three-band) model. Selection rules and numerical estimates show that the three-band model is the most favourable for this type of process. An application for the case of GaP is performed.     

Free exciton optical absorption in direct gap semiconductor alloys

Disorder effects on the optical absorption spectra of free excitons in direct semiconductor alloys are discussed as function of their Bohr radius. The variation of disorder-induced line broadening with alloy composition is described, with a particular emphasis on the violation of the Nordheim's rule for such spatially extended quasiparticles.     

Indirect exciton absorption and Raman scattering in GaAs-AlGaAs superlattices

Longitudinal optic (LO) phonon assisted indirect exciton creation (X_LO), hot carrier relaxation ((e-h)_LO) and Raman scattering phenomena are reported in the optical spectra of GaAs-AlGaAs superlattices. Structures of the same dimensions both with and without double heterostructure confining barriers are studied. For the structures without confining barriers, continuum transitions are suppressed in photoluminescence excitation (PLE) spectra, and as a result the X_LO, (e-h)_LO and Raman peaks are observed. The X_LO absorption peaks are identified from the observation of a clear threshold in PLE at ℏω_LO (36.4 meV) above the heavy hole exciton peak. The intensity of X_LO is a maximum at 6 meV above the threshold, probably due to dissociation into free carriers at the exciton binding energy (6meV) above ℏω_LO. The influence of non-radiative processes on incoherent (PLE) and coherent (Raman) processes is compared.     

Fine structures of indirect exciton absorption in phosphorus-doped silicon

A new indirect exciton absorption structure has been observed in phosphorus-doped silicon crystals by using a wavelength modulation method. The structure is interpreted in terms of two-electron transitions involving a free exciton and the valley-orbit states of a phosphorus donor, in which the donor is left in the 1s (A₁) singlet state.     

Nonparabolicity and exciton effects in two-photon absorption in zincblende semiconductors

The two-photon absorption coefficient is calculated from perturbation theory for direct-gap zincblende semiconductors using exact nonparabolic energies and matrix elements. The enhancement due to exciton effects is included. The results are compared to experiment and to other theories including the tunneling or Keldysh theory.