Temperature dependence of the electron effective mass in MnxHg1−xSe

We have investigated the temperature dependence of the electron effective mass in Mn_xHg_1–xSe crystals (0 < x 0.1) in the T=90–300 K temperature range. We have determined that the temperature-dependent changes in the band gap (_g), in the band diagram nonparabolicity, and in the conduction band carrier concentration have a strong effect on the temperature dependence of the carrier effective mass at the Fermi level m ^*=f(T).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 5–9, November, 1987.     

Low-temperature photoluminescence in CuIn<Subscript>5</Subscript>S<Subscript>8</Subscript> single crystals

Photoluminescence (PL) spectra of CuIn₅S₈ single crystals grown by Bridgman method have been studied in the wavelength region of 720–1020 nm and in the temperature range of 10–34 K. A broad PL band centred at 861 nm (1.44 eV) was observed at T = 10 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.5– 60.2 mW cm^?2 range. Radiative transitions from shallow donor level located at 17 meV below the bottom of the conduction band to the acceptor level located at 193 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal has been presented.     

Influence of annealing on the conductivity and photoconductivity of p-type CdTe

On annealing p-type CdTe, considerable change in conductivity takes place. Samples of high resistivity were used for the measurements. Each of a set of samples was annealed at different temperature. After annealing, the temperature dependence of the conductivity and the relaxation curves of the photoconductivity were measured. Analysis of the first set of curves yielded value of energyE _a corresponding to the level occurring in given samples. It was established that this acceptor level is due to V^cd, or V^cd complexes, and is situated at 0.3 eV above the valence band edge. Concentration of these levels is increased by annealing. Furthermore, an energy value ofH=0.79 eV was found, corresponding very probably to the formation energy of a vacancy V^cd.Analysis of the relaxation curves yielded the temperature dependence of _S , _T and the energy distanceE _M of the impurity level that is responsible for photoconductivity. A value of (E _g -E _M )=0.09–0.12 eV was found for all samples studied. This level therefore lies below the conduction band edge and its concentration amounts to 10¹⁴–10¹⁵ cm^–3. The level is probably due to foreign impurities.Two sets of samples were used: both as-grown and Sb-doped. The results for both sets were not much different from each other.Ke Karlovu 3, Praha 2, Czechoslovakia.     

The optical gap of NiO

The nature of the optical absorption gap in NiO at 4.0 eV is investigated. It is found that this gap is due to a band to band transition, where an electron is taken out of the valence band and placed into the conduction band. The optical gap of 6.0 eV found in NiMgO is of a nature, where an electron is taken out of the oxygen 2p band and placed into the first affinity level of the Ni²⁺ ion (3d ⁸L»3d ⁹L^–1). The impurity band created in Ni_1–x Li _x O by the Li ions is found 2.3 eV below the bottom of the conduction band in agreement with model predictions.     

硅中金受主能级在单轴应力下瞬态电容的研究

用恒定温度下瞬态电容法研究了硅中金受主能级在沿〈100〉,〈110〉,〈111〉晶向单轴应力作用下的能级移动。考虑到硅的导带在单轴应力下的分裂,导出了单轴应力下深中心中电子发射率的公式。根据该式和发射率的实验数据以及切变畸变势常数Ξ_u,求出了金受主能级激活能随应力的改变。在实验应力范围内(0—9kbar),激活能与应力成线性关系。当应力平行于〈110〉〈111〉晶向时,激活能随应力改变的斜率分别为α_<110>=-3.2±0.6meV/kbar,α_<111>=-0.3±0.6meV/kbar;当应力平行于〈100〉晶向,若取Ξ_u=9.2eV,α_<100>=-5.8±0.8meV/kbar,若取Ξ_u=11.4eV,α_<100>=-5.3±0.8meV/kbar,α表现出强烈的各向异性。进一步确定了金受主能级相对于零压力下导带底的绝对移动的压力系数。若取Ξ_u=9.2eV,这些系数分别为S_<100>=-1.3±0.8meV/kbar,S_<110>=0.7±0.6meV/kbar,S_<111>=-0.7±0.6meV/kbar。如取Ξ=11.4eV,则S_<100>=-3.5±0.8meV/kbar,S_<110>=0.0±0.6meV/kbar,S_<111>=-1.0±0.6meV/kbar。同一组的三个绝对移动值之间的偏离都超过了实验误差这一事实,说明了金中心具有立方对称性,同时中性金与带负电的金的基态都是非简并的可能性很小。 关键词：     

Electrical and chemical characteristics of nano-meter gold encapsulated in mesoporous and microporous channels and cages of FSM-16 and Y zeolites

The dc and ac conductivities as well as the dielectric constant () were measured for different zeolites encapsulated gold (AuCl₃) samples at different temperatures (300-500 K) and various frequencies (5 kHz-1 MHz). The conductivity was found to change in the order Au/FSM-27>Au/NaY>Au/FSM-47. Sorbed water contained inside zeolites assists greatly the proton mobility (zeolite protons) and the ion mobility (Na⁺ and Au⁺) and hence enhance the electric conduction in the temperature range 300-373 K. Raising the temperature over 373 K induces dehydration effect that assists the metallic gold formation and thus a dramatic loss in conductivity was revealed. The conduction mechanism was expected to be partially ionic and partially electronic. The IR study showed that the exposure of Au zeolites to CO gas produced a characteristic band of Au⁺-CO at 2180 cm⁻¹ that tends to decrease with temperatures and even vanishes at 376 K in favor of Au⁰-CO at 2128 cm⁻¹. Similarly, a phase transition at 338 K, that occurs in the range 300-376 K, was confirmed by DTA to further emphasize the temperature regions of either Au⁺ cations (338 K) or Au⁰ (376 K) formation.     

Thermopower in the region of hopping conduction in TlNiS<Subscript>2</Subscript>

Samples of the composition TlNiS₂ in the hexagonal system with the unit cell parameters a=12.28 Å, c=19.32 Å, and ρ=6.90 g/cm³ are synthesized. The results of the investigation into the electrical and thermoelectrical properties of TlNiS₂ samples in the temperature range 80–300 K indicate that TlNiS₂ is a p-type semiconductor. It is found that, at temperatures ranging from 110 to 240 K, TlNiS₂ samples in a dc electric field possess variable-range-hopping conduction at the states localized in the vicinity of the Fermi level. The density of localized states near the Fermi level is determined to be N_F=9×10²⁰ eV^?1 cm^?3, and the scatter of the states is estimated as J≈2×10^?2 eV. In the temperature range 80–110 K, TlNiS₂ exhibits activationless hopping conduction. At low temperatures (80–240 K), the thermopower of TlNiS₂ is adequately described by the relationship α(T)=A+BT, which is characteristic of the hopping mechanism of charge transfer. In the case when the temperature increases to the temperature of the onset of intrinsic conduction with the activation energy ΔE=1.0 eV, there arise majority intrinsic charge carriers of both signs. This leads to an increase in the electrical conductivity σ and, at the same time, to a drastic decrease in the thermopower α; in this case, the thermopower is virtually independent of the temperature.     

ESR and conductivity studies of doped polyacetylene

Low temperature ESR spectra of cis-rich and trans-rich polyacetylene, lightly doped with pentafluorides (AsF₅, SbF₅) exhibit two clearly distinguishable ESR lines. Detailed studies of these two lines as a function of temperature, doping concentration and microwave power indicate that one of these lines originates from localized spins, the other from conduction electrons. The concentration of the localized spins, generally speaking, decreases with doping. The Pauli-like susceptibility associated with the conduction electrons changes from 1.5×10^–7 to 15×10^–7 emu·cm³/mole for various doping levels and correlates with the conductivity of the same samples. The longitudinal relaxation time of the localized spins in doped trans-rich was found to beT ₁100 s at low temperatures, consistent with values of others in undoped trans rich. The temperature dependence of the homogeneous transverse relaxationT ₂ of the localized spins in doped trans-rich polyacetylene is similar to that of undoped trans polyacetylene. These facts show that the localized spins originate from undoped regions of the doped polyacetylene, perhaps due to inhomogenous doping. Our results are consistent with either formation of metallic islands, or, probably more, with impurity band conduction in strongly disordered semiconductors.     

Radiative transitions to localized Eu states in Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Eu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te solid solutions

Two types of radiative transitions were observed in the low-temperature photoluminescence spectra of Pb_1?x Eu _x Te (0 ≤ x ≤ 0.09) solid solutions: an intense line corresponding to the transitions from the conduction to the valence band and a series (up to 15) of narrow lines corresponding to the transitions from the hybridized conduction band to the split Eu³⁺ levels. The intensity of the discrete lines increases with x, and their energies (and intensities) weakly depend on the temperature. The absorption and emission of the discrete lines are caused by the 4f ⁷(⁸ S _7/2) ? 4f ⁶(⁷ F _J )[L ₆ ^? + 5dt _2g ] transitions.     

EPR of a fullerene-molecule-derived paramagnetic center as a mesoscopic conducting object

Discussion of theg-factor value of fullerene is based on the model of itinerant electrons restricted to the surface of the fullerene molecule C₆₀. The Ag shift, i.e., the difference between the experimentalg-factor and theg-factor of free electron Δg = g ? 2.0023 for C ₆₀ ^?1 is negative as for a very small metallic conducting particle.g-factor value is proportional to the interaction between itinerant electrons in the conduction band, thus the Δg is negative for C ₆₀ ^?1 and C ₆₀ ^?3 having less than half filled conduction band, while Δg is positive for C ₆₀ ⁺ where the conduction band is almost filled.     

On the mechanism of conduction and photoconduction in PbTe films

Conductivity, photoconductivity, thermoelectric, Hall and photothermoelectric measurements have been carried out on vacuum evaporated, air exposed PbTe films of different thickness in the temperature range 300 K-130 K to probe into the mechanism of conduction and photoconduction in these films. The dark electronic transport properties have been explained considering accumulation layers on the free surface and substrate-film interface and also the diffusion of oxygen inside PbTe microcrystals. It appears that both bulk and potential barrier scattering predominate in the high temperature region (300 K-250 K) and only barrier scattering can explain the low temperature carrier mobility at lower temperatures (250 K-130 K). Photoconductivity can be described with trapping of minority carriers within the PbTe microcrystals in the low temperature region and in the high temperature region with diffusion of minority carriers to the surface. The same oxygen trap situated at a depth of 0·06 eV from the conduction band is found to be responsible for both conduction and photoconduction in the low temperature region.Notation N _t Total trap concentration - N _r Total recombination centre concentration - n _t Electron concentration at the trapping centres - n _r Electron concentration at the recombination centres - _n Capture coefficient of electrons by recombination centres - _p Capture coefficient of holes by recombination centres - Capture coefficient of electrons/holes by trapping centres - E _t, E ₁ ^* Trap energies - E _c Conduction band edge - E _v Valence band edge - f Number of electron-hole pairs generated per unit volume per unit time - N _c Effective density of states at the conduction band edge - Potential barrier height - _p Life time of non-equilibrium holes - _s Surface barrier height in dark - _sl Surface barrier height under illumination - n _b Free electron concentration - P _b Free hole concentration - _s Surface mobility of holes - _b Bulk mobility of holes The authors are grateful to Prof. H. N. Bose for helpful discussions.     

Determination of the entropy-factor of the gold donor level in silicon by resistivity and DLTS measurements

The parameters of the gold donor level inp-type silicon are determined from the temperature dependence of the free carrier concentration and from DLTS measurements. The entropy-factorX _T is determined to be 20±2. In addition, the capture cross-section for holes _p =5.5×10^–15 cm² and the reaction enthalpyH _T –H _v =0.35eV for the exchange of holes between the gold donor level and the valence band are obtained in the present investigation.     

Low-Temperature Midinfrared Absorption in GaSe

We report the measurement of the temperature dependence of the absorption spectra of - GaSe over the temperature range 300 K to 5 K. Measurements have been made for both the e-ray (polarized parallel to the crystals c-axis) and the o-ray (polarized perpendicular to the c-axis), over the spectral range 4000 to 10 cm^–1. Nine absorption lines at 417, 440, 499, 546, 891, 945, 1015, 1093, 1270 cm^–1 were recorded at 300 K for the e- ray spectra. Some of these lines were identified using the results of a modified single layer, linear chain model of GaSe. The lines at 417, 440 and 499 cm^–1 were assigned to local impurity absorption originating from N, Mg and O, respectively. The weak lines at 945, 1015 and 1093 cm^–1 were assigned to hole transitions from the acceptor levels to the top of the valence band. Two absorption lines at 891 cm^–1 and 1270 cm^–1 were assigned to hole transitions from the quasi-local acceptor levels to the double degenerate valence sub-bands ₅ or ₆. The origin of lines recorded in the far IR absorption spectra at 20, 37 cm^–1 and 362 cm^–1 were also identified.     

On the effect of non-parabolicity in the quantum-mechanical treatment of cyclotron resonance in n-Ge

The quantum-mechanical expression for _± including non-parabolicity of the conduction band is applied to the case of the transverse-mass resonance of n-Ge in the quantum regime aroundk _B T~ (=5.6×10¹²s^–1). The essential results concern the resonance position in the transmittivity (shift with temperature) and the line shape (asymmetric absorption).     

Variable-range hopping conduction in LaMnO<Subscript>3 + δ</Subscript>

The temperature dependence of the electrical resistivity ρ(T) for ceramic samples of LaMnO_{3 + δ} (δ = 0.100–0.154) are studied in the temperature range T = 15–350 K, in magnetic fields of 0–10 T, and under hydrostatic pressures P of up to 11 kbar. It is shown that, above the ferromagnet-paramagnet transition temperature of LaMnO_{3 + δ}, the dependence ρ(T) of this compound obeys the Shklovskii-Efros variable-range hopping conduction: ρ(T) = ρ₀(T)exp[(T ₀/T)^1/2], where ρ₀(T) = AT ^9/2 (A is a constant). The density of localized states g(?) near the Fermi level is found to have a Coulomb gap Δ and a rigid gap γ(T). The Coulomb gap Δ assumes values of 0.43, 0.46, and 0.48 eV, and the rigid gap satisfies the relationship γ(T) ≈ γ(T _v)(T/T _v)^1/2, where T _v is the temperature of the onset of variable-range hopping conduction and γ(T _v) = 0.13, 0.16, and 0.17 eV for δ = 0.100, 0.125, and 0.154, respectively. The carrier localization lengths a = 1.7, 1.4, and 1.2 Å are determined for the same values of δ. The effect of hydrostatic pressure on the variable-range hopping conduction in LaMnO_{3 + δ} with δ = 0.154 is analyzed, and the dependences Δ(P) and γ_v(P) are obtained.     

Trap spectrum of the “new oxygen donor” in silicon

Electronic properties of thenew oxygen donor generated in phosphorus-doped Czochralski-silicon at 650C are investigated by deep level transient spectroscopy. A continuous distribution of trap states (10¹⁴–10¹⁶ cm^–3 eV^–1) is detected in the upper half of the band gap with increasing values towards the conduction band. The magnitude of the state density observed increases with the oxygen content, the heat duration, and a preanneal at temperatures lower than 650C. The continuous trap spectrum of thenew donor is explained by interface states occuring at the surface of SiO _x precipitates.     

Estimate of the Degree of the Spin Polarization of a Ferromagnet from Data on the Superconductor/Ferromagnet Proximity Effect

The superconductor/ferromagnet proximity effect in the Pb/Co₂Cr_1–xFe _x Al bilayer systems has been studied. Thin films of the Heusler alloy Co₂Cr_1–xFe _x Al have been prepared at different substrate temperatures. It has been established using Andreev spectroscopy of point contacts that the degree of spin polarization of conduction electrons in the Heusler alloy is on the order of 30 and 70% for the films prepared at a substrate temperature of 300 and 600 K, respectively. It has been found that the dependence of the superconducting transition temperature on the thickness of the Pb layer at a fixed thickness of the Heusler layer is determined by the degree of spin polarization of the conduction band in the ferromagnetic layer.     

Thermal quenching of luminescence of BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals activated with Er<Superscript>3+</Superscript> and Tm<Superscript>3+</Superscript> ions

Thermal quenching of interconfigurational 5d-4f luminescence of Er³⁺ and Tm³⁺ ions in BaY₂F₈ crystals is studied in the temperature range of 330–790 K. The quenching temperatures are ~575 and ~550 K for Er³⁺ and Tm³⁺, respectively. It is shown that quenching of 5d-4f luminescence of Tm³⁺ ions is caused by thermally stimulated ionization of 5d electrons to the conduction band.     

Electrical properties of CuTlSe2 in the liquid state

The electrical conductivity and thermoelectric power of CuTlSe₂ have been investigated as a function of temperature up to 230 °C above its melting point. In the liquid state the experimental data are analyzed in terms of a model developed for the density of states and electrical transport in solid amorphous semiconductors (Mott, 1970). Positive thermoelectric power suggests a large predominance of holes in electrical conduction. It appears that the conduction is due to holes in extended states near the band edge. It is found that the energy gap has a large temperature coefficient =5.5×10^–4eV/K.     

A new many-body solution of the Friedel resonance problem

It is numerically shown that the groundstate of the Friedel problem (consisting of a conduction band and a dresonance), occupied with (n+ 1) electrons, can be written as Ψ = (A a ₀ ^* +Bd*) Π _v=1 ⁿ a _v ^* Φ₀, where a ₀ ^* represents a localized conduction electron state, d* is the Friedel resonance state and Π _v=1 ⁿ a _v ⁸ Φ₀ is a Slater determinant of n single electron states a _t ^* , (Φ₀ is the vacuum state). The a _i ^* together with a ₀ ^* are part of a full ortho-normalized basis of the conductions band.