Free and bound excitons in silver bromide under hydrostatic pressure

Abstract

Employing resonant Raman and luminescence spectroscopy, various exciton states and phonon modes are studied at low temperature in AgBr under hydrostatic pressure up to 0.7 GPa. The deformation potential for the indirect free exciton gap and mode Grüneisen parameters for various phonons are determined. Excitons bound to neutral donors and isoelectronic iodine are found to essentially derive from L-point valence band states.     

Vaccum-ultraviolet low-tenperature spectroscopic study of free excitons in alkali iialide crystals under hydrostatic pressure

Abstract

Effect of pressure up to 14 kbar on free exciton reflection bands of alkali bromide crystals in VUV spectral region at 77 K was studied for the first time, In CsBr a strong resonance coupling of three exciton levels (an exciton analog of pressure-scanned Fermi resonance) was observed and computer-calculated. In NaBr, KBr and RbBr the increase of the spin-orbit splitting of the valence band was found, Besides, the exciton binding energy was estimated to decrease with pressure in NaI, KI and RbI.     

Photoluminescence of GaSb under hydrostatic pressure

Photoluminescence measurements on GaSb samples were carried out at low temperatures (2 – 5 K) and high pressures (0 – 9 kbar). The energy shift of the direct gap was determined: $\text{d}\text{E}{}^{\mathrm{\Gamma }}\text{/dP= 14.5± 0.3 meV/kbar}$. At pressures above 8 kbar the spectra showed additional structure from the indirect L-conduction band minima. The energy shift of the L-conduction bands were determined: $\text{d}\text{E}\text{L/d}\text{P}\text{= 5.5± 1. meV/kbar.}$ From these data the critical pressure for the inversion of the two conduction bands was calculated: $\text{p}{}^{\mathrm{h}}{}_{\mathrm{c}}\text{= 10.5 ± 1. kbar}$.     

Visualizing interfacial structure at non-common-atom heterojunctions with cross-sectional scanning tunneling microscopy

We describe how cross-sectional scanning tunneling microscopy (STM) may be used to image the interfacial bonding across the nearly lattice-matched, non-common-atom GaSb/InAs heterojunction with atomic-scale precision. The method, which takes advantage of the length difference between interfacial and bulk bonds, appears equally applicable to AlSb/InAs and suggests how one might recover the complete structure of either heterojunction from atomic-resolution STM data.     

Electroreflectance in GeSi alloys under hydrostatic pressure

From the electroreflectance spectra measured under hydrostatic pressure to 7 kbar we have determined the pressure coefficients for germanium (dE₁/dP = 7.8 ± 0.4⁻⁶eV/bar, dP = 1.4 ± 0.8 10⁻⁶eV/bar), for Si (dE′_o/dP = 1 ± 1 10⁻⁶eV/bar, dE₁dP = 6.2 ± 0.4 10⁻⁶eV/bar) and for GeSi alloys in the entire composition region. For the composition 80–100% of Si which is widely discussed in the literature, we could distinguish two maxima with substantially different pressure coefficients. The absolute experimental values of dE/dP agree rather well with theoretical values which, together with composition shift of electroreflectance peaks, enable us to connect the peak E₁ predominantly with λ and L and E′_o with Г and Δ transitions in the entire composition region.     

Raman scattering in GaTe under hydrostatic pressure

Raman-active lattice vibrational modes of GaTe have been investigated at 300 K in the range 15–300 cm^?1 in equilibrium conditions under different hydrostatic pressures up to 11.25 kbar. The spectra of the Bridgman grown crystals were excited with the 1.06 μm line of the continuously operated YAG:Nd³⁺ laser. The mode-Grüneisen parameters $\text{Γ}{}_{\mathrm{j}}\text{= (}\text{1}\text{β}\text{)(}\text{1}\text{ν}{}_{\mathrm{j}}\text{) =}\text{dν}{}_{\mathrm{j}}\text{d}{}_{\mathrm{p}}$ were determined for all fourteen Raman bands observed. It is shown that there is no low frequency rigid-layer modes or Davydov pairs in Raman spectra of GaTe crystals.     

Deep traps in GaAs under hydrostatic pressure

The influence of hydrostatic pressure on the energy levels of eight different deep traps in GaAs is determined by means of a transient capacitance technique. Hydrostatic pressure coefficients are shown to be decisive parameters in order to distinguish isoenergetic traps of different origin. For the EL6-, the M3- and the E3-traps (E_c?0.31eV), strongly different pressure coefficients are found. The HK1- and HK2- hole traps exhibit only small pressure coefficients relative to the valence band edge.     

DLTS investigation of GaP under hydrostatic pressure

Abstract

Deep Level Transient Spectroscopy (DLTS) was applied to nitrogen related deep electron trap 0.4 eV in green emitting diodes of GaP under hydrostatic pressure. The pressure coefficient of the level energy is determinated as equal -31 meV/kbar with respect to the valence band edge.     

Effect of hydrostatic pressure on characteristics of GaAs-Ge1−x(GaAs)x heterojunctions

The conditions of obtention of nGaAs-pGe_1–x(GaAs)_x alloy heterojunctions and the behavior of the electrical and photoelectric characteristics of these structures under hydrostatic compression were investigated. It is shown that the mechanism of current transport in this structure is of the tunneling-recombination type. The pressure coefficients of the forbidden gap width in the solid solution, the effective electron mass in gallium arsenide, and the height of the potential barrier were experimentally determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 90–93, February, 1979.     

Spatial and orientational ordering of charge-transfer excitons at intense pumping. Absorption and fluorescence spectra

The non-linear properties of charge-transfer exciton dynamics at intense pumping are investigated. The shift of an absorption band of an arbitrary cell, induced by the electric field of the available excitons, results in spatial and orientational ordering of the excitons. The side bands appear in the absorption and fluorescence spectra.     

Thermoelectric properties of tin clathrates under hydrostatic pressure

We report the temperature dependence of electrical resistance (R) and thermopower (S) of clathrate Cs₈Sn₄₄ under high pressure up to 1.2 GPa. We observe a reversible gap widening, prominent relaxation effect of R, irreversible increase of |S| under high pressure. We also find that the power factor S²σ (σ: electrical conductivity) reaches a maximum at pressure of 0.3 GPa. Comparison of the experimental results with band structure calculations suggests that the intrinsic vacancy in the clathrate structure of Cs₈Sn₄₄ plays an important role in transport properties under high pressure. Measurements on Cs₈Zn₄Sn₄₂, a clathrate which has defects other than vacancies, are compared with Cs₈Sn₄₄. The results indicate that replacing Sn by Zn has similar effect as the intrinsic vacancy on S.     

Optical absorption edge of GaS under hydrostatic pressure

The pressure variation of the optical edge of GaS has been measured. The direct exciton has been studied up to 6 kbar at 77 K and the indirect edge up to 40 kbar at 300 K. The exciton is shown to have a coefficient of ?2 ± 0.5 × 10^?6eV/bar and the indirect edge of ?ll ± 1.5× 10^?6eV/bar. A discussion of the values of the pressure coefficients for direct and indirect transitions in gallium chalcogenides is given.     

Elasticity of BaFCl single crystal under hydrostatic pressure

The sound velocities for longitudinal and transverse waves have been measured in single crystalline BaFCl at room temperature using ultrasonic pulse echo and Brillouin scattering techniques. The complete set of elastic constants is deduced and lead to the bulk moduli values of BaFCl at ambiant conditions (, , ) which are compared with those obtained by a shell model. Moreover, using the ultrasonic technique under pressure, the pressure derivatives of the second order elastic constants at 298 K have been determined up to 0.3 GPa. All moduli increase linearly with pressure in this pressure range, allowing to determine directly and separately the first derivative of the bulk modulus B'₀ = 5.8. These data are used to calculate a Murnaghan equation of state. A detailed comparison is given between our results with those recently obtained by X-ray diffraction on powder or calculated using the local density approximation method. Finally, the anisotropy of BaFCl under pressure is discussed. Received: 19 March 1998 / Revised: 15 May 1998 / Accepted: 19 May 1998     

The fractional quantum hall effect under hydrostatic pressure

We report on experiments performed on a high quality, high carrier concentration heterostructure at a range of hydrostatic pressures. At a pressure of 1 bar a clear _xx minimum and Hall plateau were observed associated with a fractional state while no such structure was observed for the fractional state. At increasing pressure however structure in the resistivity components _xx and _xy for the state became increasingly pronounced. As the disorder in the sample and the measured activation energy for the state remained relatively unchanged with increasing pressure we speculate that the enhancement of the state with increasing pressure is due to an increase in the energy gap for this state.     

Photoluminescence on CdSe and CdTe under hydrostatic pressure

Photoluminescence measurements on CdSe and CdTe samples have been made under hydrostatic pressure up to the phase transition. Transition pressure values are reported. Nonlinear pressure dependence of the direct gap is observed for both materials. Pressure coefficients are in good agreement with values calculated from the dielectric theory of the chemical Bond.     

CuCl nanocrystals in alkali-halide matrices under hydrostatic pressure

CuCl nanocrystals in crystalline alkali-halide matrices have been investigated under hydrostatic pressures up to 18 GPa. The pressures of structural phase transitions in CuCl have been determined both for different nanocrystal sizes and for different matrices (NaCl, LiCl, KCl). For CuCl nanocrystals in NaCl an increase of the transition pressure with decreasing nanocrystal size is observed, which is explained by the increasing importance of surface pressure for small nanocrystals. We found higher transition pressures for the LiCl matrix than for the NaCl matrix. The reason for this is that the pressure which acts on the nanocrystal differs from the external pressure. A simple elastic model describes the effective pressure transmitted from the matrix to the nanocrystal. With CuCl nanocrystals embedded in KCl we have studied the behavior of nanocrystals during a phase transition of the matrix. Additionally we have determined the pressure coefficients of the exciton energies of the CuCl nanocrystals, which depend on the elastic properties of the matrix. Received 4 March 1999     

Tunable InSb detectors under uniaxial and hydrostatic pressure

We report on n-InSb photoconductive detectors, which are either mounted on different substrates (metals and synthetic materials) or embedded in a special glue. If these detectors are cooled to 4.2 K, the different coefficients of expansion provide either a uniaxial or a hydrostatic pressure. This pressure causes a shift of the resonances (for the mounted detectors up to 5.5%, for the embedded detector 5.8% at 2.3 T) and a freeze out of electrons, which results in a decrease of the cyclotron resonance (CR) signal in comparison to the impurity cyclotron resonance (ICR) signal for the embedded detector.     

Resonant Raman scattering in polyacetylene under hydrostatic pressure

The results of resonant Raman scattering experiments on trans-poly-acetylene under hydrostatic pressure are reported. The measurements were performed in a diamond anvil cell. The spectra could be measured up to 44 kbar. The pressure dependence of the 1295 cm^-1 line was measured in a sapphire cell up to 17 kbar. The results show that the changes in the phonon frequencies are very small. By comparing the pressure dependence of the Raman bands with their dependence on the photon energy of the exciting laser line it is possible to determine the pressure variation of the electronic energy gap. The results are consistent with previous measurements of the absorption spectrum under hydrostatic pressure which were carried out up to 13.5 kbar. The gap is found to decrease rapidly with pressure but the decrease tends to saturate at high pressures. The results are consistent with a model in which chain-chain interaction plays a dominant role.