Low-temperature franz-keldysh oscillations in uniaxially stressed germanium

In this paper we report some typical results of high-field electroreflectance on single crystals of germanium, stressed uniaxially along the (001) direction. Measurements were made at 10°K near the fundamental direct edge. The electric field was applied by Schottky barrier. Franz-Keldysh oscillations exhibiting up to twenty peaks enabled us to determine the broadening parameter Γ = 1.4 meV and the ratio of the reduced masses μ₀₂/μ₀₁ = 0.97.     

Low-temperature relaxation in spin glasses

We analyze the mechanism governing the long-time, low-temperature relaxation of the thermoremanent magnetization (σ_TRM) in metallic spin glasses. Ideas of quantum tunneling and “screening” of the local magnetic moments are employed to explain the existence of a hierarchy of relaxation times. This results in a time decay which is described by anenhanced power law: $$\sigma _{TRM} = \sigma _0 \exp - A[\ln (\omega t)]^y = \sigma _0 (\omega t)^{ - A[\ln (\omega t)]y - 1} $$ withy≧1. A crossover is predicted below which most parameters become temperature-independent.     

Low-temperature photoluminescence in germanium irradiated by fast neutrons

This study was carried out to investigate low-temperature (T=4.2 K) photoluminescence caused by interdopant recombination transitions in n-germanium irradiated by fast (epicadmium) reactor neutrons and subjected to “complete” annealing (+450°C, 24 h). It is shown that lines of interdopant radiative recombination observed in initial and in irradiated and annealed specimens are caused by both initial impurities and (mainly) dopants (As and Ga) implanted by transmutation as well as by defect sets stable at long-time high-temperature annealing that do not contain fine dopants. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 479–482, July–August, 1997.     

Polarizabilities of shallow donors in silicon

Capacitance measurements on N-type As, P, and Sb-doped Si samples have been made between 4.2 and 1.35°K, from 0.3 to 100 kHz as a function of (N_D ? N_A) [high purity to 2.7 × 10¹⁸/cm³]. From the dielectric constant variation with (N_D ? N_A) donor polarizabilities α_AS, α_P, and α_Sb are found respectively to be $\text{1.0±0.1, 2.4±0.4,}\text{and}\text{3.1±0.3 × 10}{}^5\text{A}\text{?}{}^3$.     

Polarizability of shallow donors in EuSe

Permittivity measurements at a microwave frequency 24 GHz have been made on n-type EuSe at low temperatures 4.2–77 K after various annealing times. Consideration of different models for annealing dependent permittivity leads to the polarizability of Se vacancy defects as the most likely explanation. Donor polarizability $\text{α}{}_{\mathrm{d}}\text{= 3.2 × 10}{}^6\text{A}\text{?}{}^3$is found and further an estimation for electron effective mass $\text{m}{}^1\text{= 0.28m}{}_0$.     

Relaxation of the shallow acceptor center in germanium

Polarized negative muons were used to study relaxation mechanisms of shallow acceptors in germanium. In p-type germanium at low temperatures relaxation of the muon spin was observed, indicating that the muonic atom (gallium-like acceptor center) formed via capture of the negative muon by a host atom is in the paramagnetic state and its magnetic moment is relaxing. The relaxation rate of the muon spin was found to depend on temperature and on concentration of gallium impurity. We conclude that to the relaxation of the magnetic moment of the Ga acceptor in Ge there contribute both scattering of phonons and quadrupole interaction between the acceptors. We estimate, for the first time, the hyperfine interaction constant for the gallium acceptor in germanium as 0.11 MHz.     

Nuclear spin-lattice relaxation in germanium single crystals

The temperature dependence of the spin-lattice relaxation time corresponding to the inelastic scattering of phonons by the⁷³Ge quadrupole moment in Ge single crystals is calculated in the framework of the adiabatic bond charge model. The results obtained agree with the experimental data.     

Far-infrared recombination radiation from impact-ionized donors in germanium

We studied the far-infrared emission spectrum resulting from recombination of an electron with an ionized impurity of As and Sb in germanium under impact ionization at liquid, helium temperatures. The emission peaks at the position corresponding to the transition from the 2p _± excited state to the ground state. This observation indicates that recombination occurs through the capture by the excited states of the donor impurity, which is consistent with the cascade trap model. The intensity of emission radiation is of the order of 10⁻⁷ watts for the excitation power of about one watt, which implies a dominant process of recombination to be accompanied by phonon emission.     

Beryllium-hydrogen and zinc-hydrogen shallow acceptor complexes in germanium

New shallow acceptor complexes with hydrogenically-spaced excited states have been discovered in intentionally-doped crystals of otherwise ultra-pure germanium. The doping consists of >10¹⁴ cm⁻³ of group II impurities, and the crystals were grown under hydrogen atmosphere. The identification proposed in this paper is that of a hydrogen-group II impurity complex, with piezospectroscopic behavior of a 〈111〉-oriented defect.     

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.     

Saturation of ionization edge absorption by donors in germanium

A study of saturation of the absorption and photoconductivity of Sb and P donors in Ge for radiation of 90 m wavelength, i.e., of energy very closely above their ionization edges is presented at T=9.3 K. Under these conditions negligible heating by the excess radiation energy is expected, which provides a convenient opportunity to study the kinetics of photoionization and recombination. From these measurements we have determined the donor capture cross section of electrons at 9.3 K to be _c=(1.2±0.7)×10^–12cm^–2, and the relaxation time from the 2s to the ground state as ₂₁=(5.8±1.0)×10^–10s. The saturation intensity of the absorption coefficient is around three orders of magnitude higher than the saturation intensity of the photoconductivity. We explain the nonlinear photoconductivity by the Debye-Conwell dependence of the mobility on the number of photoionized donors and compensating acceptors.     

DLTS and PTIS of new donors in oxygen-doped germanium

Two new DLTS bands I and II have been observed in n-type germanium containing 10¹⁶ cm^-3 interstitial oxygen. The signature of II is: E_c?E_II = 0.031 eV, K = 4 × 10⁷ cm^?2s^?1; level I is about half as deep. In addision, the excitation spectrum of I is recorded by PTIS. It is composed of three hydrogenic donor series Ia, b, c with activation energies 17.25, 17.6 and 18.1 meV. The donors I and II are thought to correspond with the well known thermal oxygen donors. Energies for the latter were determined at 0.017 and 0.04 eV from Hall effect by Fuller and Doleiden. The possibility of double donors is examined, in analogy with the case of oxygen donors in silicon.     

Low-temperature thermal conductivity of highly enriched and natural germanium

Experimental data on the thermal conductivity K(T) of crystals of natural and highly enriched germanium (99.99%) ⁷⁰Ge with lapped and polished surfaces are analyzed in the temperature range ∼1.5–8 K. In all the samples in the temperature range ∼1.5–4 K the standard boundary mechanism of scattering dominates. As the temperature is raised, an isotopic scattering mechanism is observed in the natural samples. In the highly enriched samples the theoretical values of K(T) turn out to be much smaller than the experimental ones. It is conjectured that a Poiseuille viscous flow regime of the phonon gas emerges in this case. Zh. éksp. Teor. Fiz. 114, 1757–1764 (November 1998)     

Polarizabilities of shallow donors in finite-barrier nanodots

Polarizabilities of shallow donors in finite-barrier GaAs/Ga_1−xAl_xAs of harmonic oscillator nanodots are calculated, within the effective-mass approximation, using the Hasse variational method. The magnetic field dependence of polarizabilities and the diamagnetic susceptibilities are computed.     

Low-temperature magnetization relaxation in magnetic molecular solids

The low temperature relaxation of the magnetization in molecular magnetic solids such as Fe₈ is studied using Monte Carlo simulations. A set of rate equations is then developed to understand the simulations, and the results are compared. The simulations show that the magnetization of an initially saturated sample deviates as a square-root in time at short times, as observed experimentally, and this law is derived from the rate equations analytically.     

Low-temperature nuclear spin relaxation in β-aluminas

²⁷A$\text{l}$ NMR T₁ measurements for Na-, K-, and Na_0.5K_0.5β-alumina in the temperature range ～5–180K are reported. Both Na- and Kβ-alumina exhibit a nearly linear temperature dependence of the spin-lattice relaxation rate T^?1₁ up to ～55K which is attributed to the presence of two-level system (TLS) tunneling modes. A shallow T₁ minimum at ～30K is observed in Na_0.5K_0.5β-alumina. It is suggested that the origin of this minimum is associated with the distribution of TLS parameters, and may be a low-temperature manifestation of the mixed-alkali effect commonly observed in glasses in the higher temperature ionic hopping regime.     

The gallium-site donors germanium and silicon in gallium phosphide

The node in the Bloch part of the electron wave function expected for a Ga-site donor in GaP removes the usual valley-orbit splitting and associated chemical shift. However, the T₂ ground state can still show a small spin-valley splitting into Γ₈ and Γ₇ states, as previously verified for the Sn donor. We find that the optical properties of the Ge and Si donors deviate appreciably from this “normal” behaviour. The Ge donor is anomalously deep, E_D ～ 202 meV, yet binds an exciton by ～63 meV consistent with the Haynes rule for neutral donors in GaP. We find that this exciton possesses the large oscillator strength, f～3.5 × 10^-3, Zeeman and piezo-optical splittings characteristic of a Γ₆, 1s(A₁) ground state, like a P rather than Ga-site donor. However, f and the exciton localization energy are consistent with expectation for E_D ～ 200 meV, as measured from the lowest set of X conduction band minima, if we assume a symmetric A₁-like wave function. A possible explanation for this unexpected result is advanced. The much shallower Si donor, E_D～82 meV, binds an exciton by only ～ 14 meV, also consistent with the Haynes rule. By contrast, we find this Ga-site donor to be normal except that our Zeeman and piezo-optical results indicate an inverted spin-valley splitting, about 25% of that for the still shallower Sn donor. We also discuss the numerous low-lying excited states, some anomalous phonon replicas in the Ge and Si donor bound exciton spectra and the magneto-optical properties of a sharp line near 2.24 eV, attributed to the decay of excitons bound to (S)_p-(Ge)_p donor-acceptor associates.