quasiharmonic equations of state for dynamically-stabilized soft-mode materials

We introduce a method for treating soft modes within the analytical framework of the quasiharmonic equation of state. The corresponding double-well energy-displacement relation is fitted to a functional form that is harmonic in both the low- and high-energy limits. Using density-functional calculations and statistical physics, we apply the quasiharmonic methodology to solid periclase. We predict the existence of a B1--B2 phase transition at high pressures and temperatures

Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and \alpha-Al_2O_3 (corundum). A continuum theory at arbitrary electron-phonon coupling is used for calculation of the energies of thermal dissociation, photoionization (optically induced release of an electron (hole) from the ground self-consistent state), as well as optical absorption to the non-relaxed excited states. Unlike the case of free strong-coupling polarons, where the ratio \kappa of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Froehlich coupling constant \alpha and the screened Coulomb interaction strength \beta. Reasonable variation of these two parameters has demonstrated that the magnitude of \kappa remains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation energy for the ground self-consistent state and the energy of the optically induced charge transfer process (hops of a hole between O^{2-} ions) have been calculated using the quantum-chemical method INDO. Results obtained within the two approaches for hole O$^-$ polarons bound by the cation vacancies (V^-) in MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to experimental data and to each other. We discuss a surprising closeness of the results obtained on the basis of independent models and their agreement with experiment.Comment: 13 pages, 2 figures, 2 tables, E-mail addresses: [email protected], [email protected]

Carbon-doped MOCVD InP is semi-insulating up to 700°C

InP:C layers grown by metal organic vapour phase epitaxy at 500°C and doped with CCl(ind 4) remain semi-insulating following anneals up to 700°C. The compensation of grown-in CP acceptors ([CP]about 5×1018cm-3) is attributed to the presence of C-C deep donor defects, revealed by Raman scattering. [CP] and the concentration of grown-in CP-H pairs both decrease with increasing temperatures. CIn, VInH4 or PIn shallow donors are not detected. Reductions in the internal random electric fields upon annealing lead to narrowing of the CP localised vibrational mode. Samples annealed at 800°C become lightly n-type (about 10(exp 16)cm-3)

The calibration of the strength of the localized vibrational modes of silicon impurities in epitaxial GaAs revealed by infrared absorption and raman scattering.

N-type silicon-doped epitaxial layers of gallium arsenide grown by molecular-beam epitaxy (MBE) or metal-organo chemical vapor deposition (MOCVD) have been investigated by measurements of the Hall effect and the strengths of the localized vibrational modes (LVM) of silicon impurities using both Fourier transform absorption spectroscopy and Raman scattering at an excitation energy of 3 eV close to the E sub 1 band gap. Lines from Si(Ga) donors, Si(As) acceptors, Si(Ga)-Si(As) pairs, and Si-X, a complex of silicon with a native defect, were detected and correlated for the two techniques. The maximum carrier concentration (n) found for samples grown under standard conditions was 5.5 x 10 high 18 cm high minus 3. At higher doping levels Si-X becomes dominant and acts as an acceptor, so reducing (n). An integrated absorption of 1 cm high minus 2 in the Si(Ga) LVM line corresponds to 5.0 plus minus 4 x 10 high 16 atoms cm high minus 3: a similar calibration applies to the Si(As) line, but fo r Si-X, an absorption of 1 cm high minus 2 corresponds to only 2.7 plus minus 1.0 x 10 high 16 defects cm high minus 3. Possible structures for Si-X are discussed but a definitive model cannot yet be proposed. MBE samples grown at 400 degree C had values of (n) close to 10 high 19 cm high minus 3, and a negligible concentration of Si-X. On annealing, (n) decreased and Si-X defects were produced together with site switching of Si(Ga) to Si(As). These results are important to the understanding of the mechanism of silicon diffusion at low temperatures. The infrared absorption and Raman measurements are complementary. Absorption measurements made at a resolution of 0.1 cm high minus 1 require layers greater than or equal to 1 mym in thickness doped to a level of 3 x 10 high 17 cm high minus 3 but require the prior elimination of free-carrier absorption. Raman measurements can be made on as-grown layers only 10 nm in thickness doped to a level of 2 x 10 high 18 cm high minus 3, but with a

A local vibrational mode investigation of p-type Si-doped GaAs

Infrared absorption (IR) and Raman scattering measurements have been made of the localized vibration modes (LVM) due to defects incorporating silicon impurities in p-type Si-doped GaAs grown by liquid phase epitaxy (LPE) on (001) planes and by molecular beam epitaxy (MBE) on (111)A and (311)A planes. Analysis of a closely compensated LPE sample indicated that an existing calibration factor for the Si subAs LVM (399 cm high-1) relating the integrated absorption coefficient (IA) to the concentration (Si subAs) should be increased by 40 %, so that IA = 1 cm high-2 corresponds to (Si subAs) = 7 x 10 high16 cm high-3. The Si subAs LVM appeared as a Fano dip in the hole absorption continuum at about 395 cm high-1 in the highly doped p-type material, some 4 cm high-1 lower in frequency than its normal position in compensated GaAs. Electron irradiation of samples led to the progressive removal of the Fano dip and a shift with the emergence of the expected Si subAs LVM absorption line at 399 cm high-1. In MBE samples the irradiation also generated Si subGa donors, but the site switching was not detected in LPE material. By contrast, Raman spectra of as-grown p.type samples exhibited a symmetrical peak at 395 cm high-1, which also shifted towards 399 cm high-1 as the free carriers were removed. MBE (111)A GaAs:Si compensated by Sn subGa donors revealed the Si subAs LVM at its normal position. After hydrogenation of MBE and LPE samples, only stretch modes due to H-Si subAs were observed. Passivated MBE GaAs (111)A codoped with Si and Be showed stretch modes due to both shallow acceptors. It was thereby concluded that only one type of acceptor (Si subAs) was present in p-type Si-doped GaAs, contrary to previous proposals. There was no evidence for the presence of Si subAs pairs or larger clusters