Impurity conduction and magnetoresistance in lightly dopedn-type GaAs

Measurements of the resistivity and magnetoresistance of epitaxially grownn-type GaAs (N _D ~ 1–2 × 10¹⁵ cm^–3) at 4.2 K and below are reported. The hopping resistivity ₃ depends onN _D according to ₃= ₀ exp (1.88/N _D ^1/3 a) in agreement with predictions of percolation theory, wherea is the Bohr radius of the impurity ground state. The experimentally obtained preexponential factor ₀ is very close to a recent theoretical prediction, which was derived from the computation of the topology of an infinite cluster. In magnetic fields below 1.3T the resistivity is found to be proportional to exp ,t having a value of 0.06 in disagreement with a recent theoretical calculation oft=0.04. In the high magnetic field region above 3T the data are described by exp [q ( ² a _B N _D)^–1/2], where is the characteristic magnetic length anda _B is the magnetic field dependent Bohr radius. From the experiments a value ofq=0.68 is deduced, while the theory predictsq=0.98.     

Impurity conduction and magnetoresistance in lightly dopedn-type GaAs

Measurements of the resistivity and magnetoresistance of epitaxially grownn-type GaAs (N _D ~ 1–2 × 10¹⁵ cm^–3) at 4.2 K and below are reported. The hopping resistivity ( \fractal⁴ N_D )\left( {\frac{{ta}}{{\lambda ^4 N_D }}} \right) ,t having a value of 0.06 in disagreement with a recent theoretical calculation oft=0.04. In the high magnetic field region above 3T the data are described by exp [q ( ² a _B N _D)^–1/2], where is the characteristic magnetic length anda _B is the magnetic field dependent Bohr radius. From the experiments a value ofq=0.68 is deduced, while the theory predictsq=0.98.     

Impurity dipole interactions in alkali halides at low temperature

Calculations of dipole correlations and internal-energy densities are reported for dilute dipole systems in the NaCl and CsCl lattices. The tensorial dipole interaction potential is used in thermal averaging in a two-dipole approximation. Actual lattice geometries are taken into account in the three-dimensional spatial averages. The dipoles are assumed to have equilibrium orientations along [100] in the NaCl lattice, and along either [100] or [111] in the CsCl lattice. It is found thatlocal antiparallel ordering is favored in both lattices at low temperatures. Comparison with recent, low-temperature dielectric data indicates that relaxation mechanisms are as important as local antiparallel ordering in limiting theT ^?1 increase of (??1)/(?+2) with decreasing temperature for OH^? doped KCl. A simplified, nonequilibrium mechanism is studied whereby parallel-correlated dipoles with interaction energies greater than2 kT in absolute value become nonalignable and do not contribute to?. This mechanism together with antiparallel ordering are found to describe the experimental data satisfactorily; but additional relaxation mechanisms are indicated. The contribution of the dipole interactions to the specific heat is also calculated, and is in qualitative agreement with measured specific heat data for OH^? doped KCl.     

On the negative magnetoresistance puzzle at variable range hopping at low temperatures

In this work, we report the results of magnetoresistance studies of variable range hopping down to 30 mK in isotopically engineered Ge with low compensation, and in n-CdTe crystals. Experimentally we find a decrease and disappearance of the negative magnetoresistance with decreasing temperature down to 200 mK, in weak magnetic fields. Such behaviour is in disagreement with the quantum interference theory of Nguen, Spivak and Shklovskii.     

Impurity band conduction in a high temperature ferromagnetic semiconductor

The band structure of a prototypical dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [sigma1(omega)] with carrier density are only consistent with EF lying in an IB. Furthermore, the large effective mass (m*) of the carriers inferred from our analysis of sigma1(omega) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.     

The implantation behaviour of indium and silver in iron at low temperature

¹¹¹In and ¹⁰³Ag have been implanted at very low temperatures (30 mK) in iron. Using the nuclear orientation technique it can be shown that the nuclei are ending up almost exclusively in substitutional lattice sites.     

Free and bound exciton decay in indium phosphide

Excitonic recombination and photoconductivity near the energy gap of vapour grown InP epitaxial layers are investigated. Besides the free exciton several bound exciton complexes are observed and studied as a function of temperature. A new value of the band gap is derived (E_g = 1.424 ± 0.001 meV).     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.     

Electron mobility in compensated semiconductors at low temperatures

Electron mobility in compensated semiconductors for ionised impurity scattering is calculated. A simple screened dipole potential is used. The calculations differ from those of Stratton due to the fact that in this paper the mobility is exactly calculated.     

Impurity absorption in gallium phosphide doped with zinc and oxygen

The extrinsic optical properties were investigated in homogeneous GaP crystals at room temperature. The transmission of extrinsic light is influenced by a simultaneous illumination of an additional light of different frequency. This light changes the occupancy of an impurity level. A simple theory is given, which relates this change in transmission of the extrinsic light to the optical cross sections of the impurity level. We had to use steady light because of the large relaxation time, but still obtained a resolution of 2 × 10⁻⁵ in the relative transmission change. The spectral dependence of the optical cross sections for an impurity level 0·9 eV below the conduction band were calculated. Another level approximately 1·85 eV from a band edge was also seen in these measurements, but it had complex optical properties. Both levels were seen in a direct measurement of the absorption coefficient.     

Free and bound electron transitions to acceptors in indium phosphide

In epitaxial InP a new emission band at 1.3776 eV is identified as the free electron-acceptor transition. It is investigated in connection with the corresponding donor-acceptor pair band at 1.374 eV. This permits an individual determination of the acceptor and donor binding energies (48 ± 1 meV and 7.3 ± 0.6 meV respectively). A second new emission band at 1.396 eV is reported and discussed.     

Phonons and related crystal properties in indium phosphide under pressure

Based on the pseudopotential approach, the lattice vibration and polaron properties in InP under pressure up to 8 GPa have been investigated. At zero pressure, our calculated values for all features of interest are in reasonable agreement with data available in literature. Upon compression, the optical phonon modes at Γ point in the Brillouin zone are shifted towards high energies and their frequencies display a blueshift with rising pressure. The splitting of the longitudinal and transverse optical phonons narrows under pressure, indicating thus a decrease of the ionicity of InP crystal with pressure. It is observed that all physical quantities being studied in this contribution behave monotonically upon compression. To the best of the author's knowledge, the pressure dependence of the polaron properties for InP in the present study is reported for the first time.     

Impurity dependence of the low temperature annealing in n-type germanium

Lightly doped n-type Ge, irradiated at liquid He temperatures with 1 MeV electrons, exhibits a large thermal recovery stage at 50–70°K. We have found that the rate at which this stage anneals depends on the type of group V impurity used to dope the sample. We Propose that impurity complexes are involved in this annealing stage. We have also observed the same impurity dependence when this annealing stage is destroyed by radiation annealing at liquid He temperatures. This suggests that one of the defects produced during irradiation is free to migrate at very low temperatures.     

Anomalously large negative magnetoresistance of 1T-TaS2 at ultra low temperatures

The magnetoresistance of 1T-TaS₂ in the commensurate CDW phase was measured from 8 K down to 50 mK. Very large negative magnetoresistance was observed below 0.5 K for both transverse and longitudinal configurations of the magnetic field with respect to the current. This result is explained by Fukuyama and Yosida's theory based on the variable-range hopping in the Anderson localized states.     

Spin relaxation of conduction electrons in highly-doped n-type germanium at low temperature

We present a theory of the spin relaxation time of the conduction electrons in highly-doped n-type germanium at liquid helium temperature. The theory is compared with some of our measurements and the experimental data available in the literature on As-doped germanium. The observed linewidth at T = 10 K is accounted for in the whole metallic concentration range (N_D > 3 × 10¹⁷cm^?3).In the lower concentration range (3 × 10¹⁷ < N_D < 10¹⁸cm^?3), the relevant mechanism is the random jumping of the g factor upon intervalley scattering. The agreement with experiment is good without any adjustable parameter.In the higher concentration range (N_D > 10¹⁸cm^?3), the dominant process for the linewidth is the spin-flip scattering by ionized donors (Elliott process); the usual theory is shown to be insufficient and the greater effectiveness of scattering by the localized part of the donor potentials is pointed out. The calculated linewidth is related to the intervalley scattering time T_iv. The agreement with experiment is good and predictions are given for the linewidth in the case of other shallow donors in the same concentration range.     

Specific heat at low temperature due to negative U centres in disordered solids

The negativeU-Anderson model is considered and energy spectrum is obtained using the Gorkov’s decoupling scheme for one-electron Green’s function. The correlation of localized electron pair (bipolaron) is explicitly taken into account in this scheme. The electronic specific heat of disordered solids with negativeU-centres and having a distribution of negativeU is then calculated. At low temperature the specific heat shows linear temperature dependence, and this linearT-term is a combined effect of distributedU and of the existence of localized electron pairs.     

Electronic conduction in quasicrystals at low temperatures

At low temperatures, a perfect quasicrystal is in the “critical” state of metal-insulator transition. A power-law temperature dependence of conductivity, which was experimentally observed at T<5 K in the icosahedral phase of Al-Pd-Re, was obtained using the critical wave functions. Mott’s hopping law was also observed in the Al-Pd-Re samples and explained by the delocalization of electronic states in the momentum space.     

Effects of passage of 200 MeV Ag9+ ions in indium phosphide at different depths

Indium phosphide sample was irradiated with 200?MeV Ag⁹⁺ ions for the fluence of 2?×?10¹³?ions?cm^?2. The sample was chemically etched down up to 240?nm depth to investigate the distribution of defects at different regions. Raman scattering and glancing incidence X-ray diffraction spectra were recorded at different depths. The stress estimated from Raman shift was found to increase with depth up to 160?nm and thereafter it decreased and at a depth of 224?nm sample did not show any stress. Phonon coherence length estimated from the Phonon Confinement Model was found to vary between 43 and 18?nm with respect to depth. Glancing incidence X-ray diffraction results revealed the decrease in crystallite size from 16.12 to 1.00?nm in different depth regions.