Mixed photoconductivity analysis of Cr-doped semi-insulating GaAs based on the study of current instability phenomenom

An observation of a type of photocurrent instabilities which appears systematically in the spectral region of 0.87 eV photoconductivity peak of Cr-doped semi-insulating GaAs is interpreted in terms of a mixed conductivity. The separate contributions of electrons and of holes to the photoconductivity spectrum at 143 K are shown.     

Defect and photoluminescence profiles from thermally converted Cr-doped GaAs

Specimens of Cr-doped semi-insulating (SI) GaAs have been annealed in quartz ampoules under vacuum at elevated temperatures. Some samples, depending on temperature and time of anneal, were partially or wholly converted to p-type. In these cases CV measurements have been combined with serial sectioning to produce carrier concentration profiles. The As overpressure dependencies indicate acceptors to be associated with Ga vacancies. The diffusion coefficient of the Ga vacancies was estimated to be about 3.35 × 10^-14cm²sec^-1 at 950°C. Low temperature photoluminescence on the converted samples show a reasonably good correspondence between carrier distribution profile and the intensity of copper luminescence peak on photoluminescence spectra taken at various depth in the crystals.     

Mixed anionic conduction in PbFCl

We have measured transference numbers of fluoride ions and chloride ions in undoped and doped samples of PbFCl. The results indicate pure anionic conduction. The electrical conductivity of doped single crystals support the assumption that the thermal defects in PbFCl are of the Schottky-type. The defect chemistry of PbFCl involved is described.     

Spin relaxation of conduction electrons in GaAs

We have used optical spin orientation techniques to measure T₁ of conduction electrons in GaAs (NinA ≈ 10¹⁷ cm^-3) for 4.7 K ? T ? 200 K. From Hall effect measurements we estimated the electron momentum relaxation time τ_p. For 50 K ? T ? 200 K, the product T₁τ_p agrees with our earlier order of magnitude estimate of the D'yakonov-Perel' mechanism, in which band structure induced precession is strongly narrowed by momentum relaxation. The Elliott mechanism is one to two orders of magnitude weaker.     

Mixed conduction in sodium vanadium bronze

The ionic and electronic conduction in α'-Na_xV₂O₅ prepared by the solid state reaction has been investigated. The electronic conductivity is nearly equal to the total conductivity and depends on the V⁴⁺ ion concentration. The Na⁺ ion conductivity was measured by a dc technique. The building up and decaying curves obtained at the transient polarization phenomena were analyzed by using a certain approximation for the steady state in order to save experimental time. The typical ionic conductivities were relatively low, namely 3×10^?6 and 2×10^?1 S cm?1 at 300°C for the samples of x=1.0 and 0.8, respectively which seemed to be dependent on the number of vacant Na⁺ ion sites.     

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.     

Nonparabolicity and warping in the conduction band of GaAs

The dispersion of the conduction band in GaAs is calculated using k·p models which in different ways take into account the coupling to the p-bonding and p-antibonding states. Nonparabolicity, warping and spin-splitting are accurately described up to energies about 50 meV above the conduction band minimum by the 8×8 Kane model. For higher energies a 14×14 matrix is required.     

Mapping of conduction bands in GaAs by angle-resolved photoemission

Using angle-resolved photoemission spectra in the photon energy range 12 ? hv ? 20 eV, we have mapped selected conduction bands for the (110) direction in GaAs using a simple direct-transition analysis. At higher energies the spectra suggest a one-dimensional density of states interpretation. The two mechanisms are discussed.     

Photoluminescence of thermally treated n+ Si-doped and semi-insulating Cr-doped GaAs substrates

Photoluminescence measurements are used to investigate the nature of the surface layers formed on n⁺ Si-doped and semi-insulating Cr-doped GaAs substrates after heat-treatment at 780–830°C in H₂ or He flow. At 5.5 K the heat-treated n⁺ substrates exhibit a band near 1.44 eV while the semi-insulating substrates are characterized by a phonon assisted transition with the zero-phonon band at 1.41 eV. Both these bands are identified with donor-acceptor pair recombination. The ionization energy of both the donor and acceptor for the 1.44 eV band is estimated to be ～ 35–40 meV and it is suggested that the acceptor is Si_As. The identities of the donor in the 1.44 eV band as well as that of the centers responsible for the emission at 1.41 eV are unknown.     

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.     

Optimization of pulse width of double passively Q-switched lasers with GaAs and Cr-doped saturable absorbers

Compared with a single passively Q-switched laser, a double passively Q-switched laser with a GaAs saturable absorber and a Cr⁴⁺-doped saturable absorber can produce more symmetric and shorter pulses with high pulse peak power. New normalized coupled rate equations for a double passively Q-switched laser with both a GaAs saturable absorber and a Cr⁴⁺-doped saturable absorber are solved numerically, where the single-photon absorption (SPA) and two-photon absorption (TPA) processes of GaAs are combined. The Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density are considered. The optimization of a double passively Q-switched laser to obtain the shortest pulse width is performed, and a group of general curves are generated for the first time. The curves clearly show the dependence of the optimal normalized parameters on the parameters of the gain medium, the GaAs saturable absorber, the Cr⁴⁺-doped saturable absorber and the spatial distributions of the intracavity photon density. Sample calculations for a diode-pumped Nd³⁺:YVO₄ laser with both a GaAs saturable absorber and a Cr⁴⁺:YAG saturable absorber are presented to demonstrate the use of the curves and the related formulas.     

Mixed ionic conduction in solid KHF2

From EMF measurements it can be concluded that there is no significant electronic conductivity in KHF₂. In the relatively good conducting high temperature β-phase the following transport numbers are found: t_K≈0,25 and t_F≈0,75 while t_H≈0 as was concluded from changes in chemical composition on electrolysis in Hittorf-Tubandt type experiments. From current-time characteristics it follows that in the low-conducting low temperature α-phase, the proton is the only mobile charge carrier in this phase, i.e. t_H≈1. It also proved that the d.c. conductivity of single crystals measured with hydrogen permeable electrodes coincides with the zero-frequency conductivity found by extrapolation of a.c. data.     

Stress-induced splitting of the conduction bands of GaAs and GaSb

The splitting induced by a [110] strain in the conduction bands of GaAs and GaSb can be obtained from the measured spin relaxation of photoexcited carriers. These experimental results are shown to be in satisfactory agreement with predictions based on $\text{k}$.$\text{p}$ perturbation theory, which use recently calculated values of the coupling between the Г₁ and the two nearest Г₁₅·bands induced by the strain.     

Mixed polaronic-ionic conduction in lithium borate glasses and glass-ceramics containing copper oxide

The effect of electric field strength on conduction in lithium borate glasses doped with CuO with different concentration was studied and the value of the jump distance of charge carrier was calculated. The conductivity measurements indicate that the conduction is due to non-adiabatic hopping of polarons and the activation energies are found to be temperature and concentration dependent. Lithium borate glasses are subjected to carefully-programmed thermal treatments which cause the nucleation and growth of crystalline phases. X-ray diffraction analysis confirmed the amorphous nature for the investigated glass sample and the formation of crystalline phase for annealed samples at 650 °C. The main separated crystalline phase is Li₂B₈O₁₃. The scanning electron micrographs of some selected glasses showed a significant change in the morphology of the films investigated due to heat treatment of the glass samples. It was found that the dc-conductivity decreases with an increase of the HT temperature. The decrease of dc conductivity, with an increase of the HT temperature, can be related to the decrease in the number of free ions in the glass matrix. There is deviation from linearity at high temperature regions in the logσ-1/T plots for all investigated doped samples at a certain temperature at which the transition from polaronic to ionic conduction occurs. The hopping of small polarons is dominant at low temperatures, whereas the hopping of Li⁺ ions dominates at high temperatures. PACS 71.55.Jv; 72.60.+g; 72.80.Ng     

Diffusion of 51Cr in Cr-doped MgO

An arc fusion technique was used to grow single crystals of MgO and Cr-doped MgO. Diffusion coefficients for ⁵¹Cr in Cr-doped single crystals were measured at three temperatures 1383, 1444 and 1495°C using a high specific activity isotope ⁵¹Cr. An approximately linear relationship between the concentration of Cr-ions in MgO and diffusion coefficients of ⁵¹Cr was obtained.It is shown that the activation energy of 19.6 $\text{kcal}\text{mole}$ obtained for the doped crystals is the difference between the energy for motion and the energy for association of the Cr-vacancy complexes. Using a previously determined value of 39.9 kcal/mol for the energy of motion, the energy of association for the Cr-vacancy complex is calculated to 20.3 ± 3 kcal/mol or 0.88 ± 0.13 eV.     

Electrical properties of Cr-doped CoO

This paper reports the results of the electrical conductivity measurements for polycrystalline specimens of undoped and Cr-doped CoO in the ranges of p(O₂) (10⁻⁵ – 10⁵ Pa) and temperature (1223 – 1373 K). The experimental data are considered in terms of the effect of Cr on semiconducting properties of CoO. It is shown that Cr results in a decrease of the reciprocal of the p(O₂) exponent of electrical conductivity, however, the obtained experimental values are substantially lower than those predicted by defect chemistry. The activation energy of the electrical conductivity remains independent of p(O₂) and Cr content (at the level of about 0.5 eV) except strongly reduced CoO, at p(O₂)=2.10⁻⁴ Pa, of which the activation energy is substantially higher. Thermopowervs p(O₂) exhibits maximum at p(O₂)=10 Pa (except of thermopower data for Cr-doped CoO at the highest temperature). The experimental data are considered in terms of the effect of both p(O₂) and Cr on semiconducting properties.     

The screening influence of the conduction electrons on the shallow donor levels in n-type GaAs

Using the approximative formula found by the author for the eigenvalues of Schrödinger equation for an electron in the screened coulombic field and the condition of the charge neutrality in partially compensated semiconductor, the ground-state and the first excited-state energy levels of shallow donors in n-type GaAs are computed for various donor concentrations and compensation ratios. The energy levels are found to be temperature dependent, which enables to explain the discrepancy between the ionization energies experimentally determined by optical methods at 4·2 K and those found by fitting the experimental temperature variation of the carrier concentration. The different values of the energies for the transition from the ground-state to the first excited-state at 4·2 K and at 15 K experimentally determined by other authors also confirm the temperature dependence of shallow donor levels which can be explained by the screening effect of conduction electrons.