A model of hopping and band DC photoconduction in doped crystals

Expressions for the screening length and the ambipolar diffusion length are derived, for the first time, for the case where hopping conduction and band conduction coexist in semiconductors with hydrogen-like impurities. A method is proposed for calculating the diffusion coefficient of electrons (holes) hopping between impurity atoms from data on the Hall effect, in the case where the hopping and band conductivities are equal. An interpretation is given of available experimental data on hopping photoconduction between acceptors (Ga) and donors (As) in p-Ge at T=4.2 K doped by a transmutation method. It is shown that the relative magnitude of the mobilities of electrons hopping between donors and holes hopping between acceptors can be found from the hopping photoconductivity measured as a function of the intensity of band-to-band optical carrier excitation.     

Long-lived excited impurity states in diamond-like semiconductors

The lifetime of charge carriers in the lowest excited states of some impurities of groups III and V in diamond, silicon, and germanium can be several (four to six) orders of magnitude longer that the lifetime of free carriers. Accumulation of carriers in these long-lived states may give rise to several new effects, such as hopping photoconductivity via long-lived excited states of impurities in dc and microwave electric fields, slow relaxation of induced absorption, and infrared absorption at energies lower than the impurity ionization energy. Zh. éksp. Teor. Fiz. 112, 221–236 (July 1997)     

Impurity photoconductivity in epitaxial layers of gallium arsenide

Impurity photoconductivity spectra in the range 0.5–1.5 eV are studied in epitaxial layers of n-GaAs grown on substrates of semiconductive GaAs withρ > 10⁶ Ω·cm in the system Ga-AsCl₃ -H₂. The effect of uncontrolled acceptor impurities on the impurity photoconductivity spectrum is evaluated.     

Investigation of acceptor centers in semiconductors with the diamond crystal structure by the μ − SR method

The residual polarization of negative muons in crystal silicon samples with phosphorus (P: 1.6×10¹³ cm⁻³) and antimony (Sb: 2×10¹⁸ cm⁻³) impurities is investigated. The measurements are made in a 1000 G magnetic field oriented in a direction transverse to the muon spin in the temperature range 4–300 K. The relaxation rate and shift of the precession frequency in the silicon sample with the phosphorus impurity are measured more accurately than previously. It is found that in antimony-doped silicon the acceptor center _μ A1 at temperatures below 30 K can be in both ionized and neutral states. The experimental data are interpreted on the basis of spin-lattice relaxation of the magnetic moment of an acceptor center, formation of acceptor-donor pairs, and recombination of charge carriers at the acceptor. Preliminary measurements showed a nonzero residual polarization of negative muons in germanium. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 61–66 (10 July 1998)     

Temporal dynamics of impurity photoconductivity in n-GaAs and n-InP

The dynamics of impurity photoconductivity in n-GaAs and n-InP under photoexcitation with a short light pulse has been calculated. It has been shown that the photoconductivity dynamics in a nanosecond time range is determined by cooling of electrons, while the role of cascade capture of electrons by impurity is insignificant in this range. A nonmonotonic time dependence of photoconductivity caused by the competition between different relaxation mechanisms of the electron pulse has been predicted.     

Features of laser-induced luminescence and photoconductivity of layered Cu<Subscript>3</Subscript>In<Subscript>5</Subscript>S<Subscript>9</Subscript> crystals

Luminescence and photoconductivity of layered Cu₃In₅S₉ crystals at high levels of optical excitation are studied experimentally. A pulsed nanosecond Nd:YAG laser with built-in second and third harmonic generators to generate 1064-, 532-, and 355-nm radiation is used as a light source. It is found that the photoluminescence spectra exhibit two emission bands due to zone–acceptor level and impurity donor–impurity acceptor transitions. It is shown that the photoconductivity in Cu₃In₅S₉ is monopolar. The waveform of the photoconductivity consists of fast and slow components associated with two channels of recombination.     

Impurity-induced stabilization of Luttinger liquid in quasi-one-dimensional conductors

It is shown theoretically that the Luttinger liquid can exist in quasi-one-dimensional conductors in the presence of impurities in a form of a collection of bounded Luttinger liquids. The conclusion is based on the observation by Kane and Fisher that a local impurity potential in Luttinger liquid acts, at low energies, as an infinite barrier. This leads to a discrete spectrum of collective charge and spin density fluctuations, so that interchain hopping can be considered as a small parameter at temperatures below the minimum excitation energy of the collective modes. The results are compared with recent experimental observation of a Luttinger-liquid-like behavior in thin NbSe₃ and TaS₃ wires.     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

Photoconductivity of lead telluride-based doped alloys in the submillimeter wavelength range

Persistent photoconductivity in a Pb_0.75Sn_0.25Te(In) alloy initiated by monochromatic submillimeter-range radiation at wavelengths of 176 and 241 µm was observed at helium temperatures. This photoconductivity is shown to be associated with optical excitation of metastable impurity states.     

Polarization kinetics of a photosensitive relaxor ferroelectric

Polarization switching in alternating quasi-static electric fields of frequency 10^?4 Hz and polarization relaxation in dc fields were studied in a photosensitive La-and Ce-doped barium-strontium niobate relaxor ferroelectric. Experimental data obtained in the thermal activation stage of the relaxation were used to reconstruct the relaxation time distribution spectrum. The characteristics of the polarization kinetics of an illuminated and a dark crystal are compared. It is shown that, in the crystal illuminated by light, the photoconductivity compensates for random electric and depolarization fields, thereby giving rise to a growth in amplitude of the dielectric hysteresis loops in the polarization versus field relation and to longer polarization relaxation times or increased heights of the potential barriers separating stable states from metastable states.     

Impurity bands in Ga1-xAlxAs/GaAs quantum wells

We calculate the Density-of-States (DOS) for electrons bound to impurities in a thin sheet inside a Ga_1-xAl_xAs/GaAs quantum well. Impurities are considered at the center, midway to the interface and at the interface of the GaAs layer. It is shown that for reasonable impurity concentrations an impurity band appears separated from the lowest subband. The bandwidth is comparable with that obtained due to diagonal disorder assuming a uniform distribution inside the whole well.     

Infrared absorption by pairs of group III and V impurities in silicon

Infrared absorption bands, shifted to regions of lower energies relative to narrow lines of transitions of impurities to excited states, are investigated in silicon doped with group III and V impurities in concentrations above 10¹⁶ cm^?3. It is found that the band structure is peculiar to each of the investigated impurities but independent of their concentrations, and the absorption coefficients in the bands increase approximately quadratically with concentration. This leads one to infer that the bands are caused by the excitation of charge carriers bound on impurity pairs localized within several Bohr radii.     

Mechanisms of absolute negative photoconductivity in solids

The microscopic mechanisms governing the absolute negative photoconductivity in solids (j_ph = σ_ph, σ_ph < 0) discovered recently in concentrated ruby crystals are described. It is shown that in hopping models of photoconductivity the contribution to the current, connected with the field-induced asymmetry of processes of electron photoexcitation and recombination, dominates. It is predicted that the sign of σ_ph(ω) changes as a transition over an absorption resonance is performed.     

Correlation between the anisotropy of hopconduction magnetoresistance and reciprocal band masses in Ge

Near 70 kG the d.c. magnetoresistance of phonon-assisted hopping of charge carriers between shallow impurities is found to have an anistropy depending on the reciprocal effective mass calculated from the L₁ conduction band minima in the case of n-type Ge and from the heavy hole component of the Γ′₂₅ valence band maximum in case of p-type Ge. These correlations are discussed and it is suggested that they are connected with the symmetry of the impurity wave functions and that the magnetic field tends to remove the light hole component of an acceptor wave function.     

Magnetic impurity interactions in a quantum well on the base of IV-VI semiconductors

We study the indirect exchange interaction of magnetic impurities via 2D excitations in a quantum well on the base of IV-VI narrow-gap semiconductors. The energy spectrum of 2D excitations takes into account the nonparabolicity of dispersion as well as the strong spin-orbit interaction. The calculations are performed for the case when the main mechanism is an exchange by virtual electron-hole pairs. It means that we assume the Fermi level to lie inside the energy gap, and the temperature to be small, T << ϵ₀. It is shown that at large distances. R >> ν/ϵ₀ (2ϵ₀ is the excitation energy for nearest size-quantized subbands, ϵ the interband interaction parameter), the antiferromagnetic interaction of pairs dominates, so that the impurity spins tend to be directed along the heterojunction plane perpendicular to the vector connecting the impurities. The interaction contains both the Heisenberg and pseudodipole terms.     

Nearly magnetic antiparallel pairs in dilute alloys

When a dilute alloy exhibits exchange enhancement of the isolated impurity susceptibility, it is shown that pairs of impurities may exhibit a low lying anti-parallel fluctuation mode such that the pair susceptibility in a uniform field is less enhanced than that of the isolated impurity, while the specific heat may be strongly enhanced. N.M.R. properties of such a pair should exhibit a small shift in frequency and a fast nuclear spin lattice relaxation time, leading to possibly large deviations from the Korringa relation.     

A lattice model of nearest-neighbor hopping conduction and its application to neutron-doped Ge: Ga

A model of hopping conduction between nearest neighbors is developed in which the majority and compensating dopant atoms are assumed to form a unified simple cubic lattice in a crystalline matrix. The hopping of carriers occurs when thermally activated “equalization” of majority impurity levels takes place, while the compensating impurities block the corresponding sites. The range of relatively high temperatures is considered in which the interactions giving rise to a Coulomb gap can be neglected and the density of states of the majority impurity band is Gaussian. The concentration dependences of the activation energy for hopping conductivity ? ₃ (nonmonotonic and having a maximum) and the preexponential factor σ₃ are found. The results are compared with experimental data obtained by different authors for neutron-doped Ge: Ga.     

Magnetism without magnetic ions in non-magnetic perovskites SrTiO3, SrZrO3 and SrSnO3

Using the full-potential linearized augmented plane-wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential, we studied spin polarization induced by replacement of oxygen atoms by non-magnetic 2p impurities (B, C and N) in non-magnetic cubic SrMO₃ perovskites, where M=Ti, Zr and Sn. The results show that the magnetization may appear because of the spin–split impurity bands inside the energy gap of the insulating SrMO₃ matrix. Large magnetic moments are found for the impurity centers. Smaller magnetic moments are induced on the oxygen atoms around impurities. It is shown that SrTiO₃:C and SrSnO₃:C should be magnetic semiconductors while other compounds in this series (SrTiO₃:B, SrTiO₃:N and SrZrO₃:C) are expected to exhibit magnetic half-metallic or pseudo-half-metallic properties.     

Effect of Cd impurity on the electrical properties of a-Se80Te20

Temperature dependence of dark and photoconductivity is studied in amorphous films of Se₈₀Te_20−xCd_x in the temperature range (300 K to 380 K) with a view to see the effect of Cd impurity on a-Se₈₀Te₂₀ binary alloy. It is observed that, at low concentration of Cd (x = 0.5), the dark and photoconductivity increase at all temperatures. However, at higher concentration of Cd (x = 10), an appreciable reduction in these parameters occurs in the same temperature range. The photosensitivity (σ_ph/σ_d) remains unchanged at x = 0.5 but decreases quite significantly at x = 10. The results are explained in terms of impurity doping at x = 0.5 and alloying effect at x = 10.     

Mössbauer investigation of dilute Ag : Er alloys: Spectra of isolated atoms and clusters

Mössbauer spectra of dilute $\text{Ag}$ : Er alloys indicate that only a fraction of the impurity ions are in isolated cubic symmetry sites of the host lattice. From the spectrum of the isolated ¹⁶⁶Er impurities the hyperfine coupling constant of + 3.80 ± 0.05 mm/sec characteristic of the Γ₇ ground state has been deduced. Clusters of rare earth atoms give a quite different contribution to the hyperfine spectrum which is easily identified. The influence of such clusters on the relaxation times and the EPR residual linewidth of isolated rare earth impurities is pointed out.