0000000000134470
AUTHOR
Clemens Ulrich
Investigation of conduction-band structure, electron-scattering mechanisms, and phase transitions in indium selenide by means of transport measurements under pressure
In this work we report on Hall effect, resistivity and thermopower measurements in n-type indium selenide at room temperature under either hydrostatic and quasi-hydrostatic pressure. Up to 40 kbar (= 4 GPa), the decrease of carrier concentration as the pressure increases is explained through the existence of a subsidiary minimum in the conduction band. This minimum shifts towards lower energies under pressure, with a pressure coefficient of about -105 meV/GPa, and its related impurity level traps electrons as it reaches the band gap and approaches the Fermi level. The pressure value at which the electron trapping starts is shown to depend on the electron concentration at ambient pressure an…
Band-to-Band and Band-to-Acceptor Photoluminescence Studies in InSe under Pressure
We report on photoluminescence (PL) measurements under pressure on p-type N-doped InSe at 10 K and on n-type Si-doped InSe at room temperature. Low-temperature PL of N-doped InSe is dominated by a band-to-acceptor peak. From the pressure dependence of the ionization energy of the N related shallow acceptor, the pressure change of the hole effective mass is estimated through the Gerlach-Pollmann model for hydrogenic levels in uniaxial crystals and discussed in the framework of a k p model. Room temperature PL in Si-doped InSe is dominated by a band-to-band peak exhibiting a pressure shift in agreement with previous works. This PL peak has been measured up to 7 GPa and a steep reversible decr…
Electric-field-induced Raman scattering in GaAs: Franz-Keldysh oscillations
We have studied the influence of strong electric fields on the Raman scattering intensity from LO phonons in GaAs (100) at room temperature using laser excitation energies above the fundamental ${\mathit{E}}_{0}$ gap. Striking oscillations are found in the scattering intensity for configurations where either the deformation potential or Fr\"ohlich electron-phonon interaction contribute. The oscillations in the deformation-potential-mediated scattering intensity can be related to Franz-Keldysh oscillations derived from the ${\mathit{E}}_{0}$ gap, whereas a more complicated mechanism has to be invoked for processes where Fr\"ohlich interaction is responsible.
Effect of Pressure on Direct Optical Transitions of ?-InSe
We have investigated the effect of hydrostatic pressure on direct optical transitions of the layered semiconductor γ-InSe by photoreflectance (PR) spectroscopy (T = 300 K). In addition, electroreflectance (ER) measurements were performed at ambient pressure. Six structures are resolved in the ER spectra in the energy range from 1.1 to 3.6 eV. The pressure dependence of four of these structures was determined by PR spectroscopy for pressures up to 8 GPa. In order to assign the features observed above the fundamental gap we have carried out band structure calculations for InSe at ambient pressure using a full-potential linear augmented plane wave method. Based on calculated band gap deformati…
Raman Scattering in CuCl under Pressure
Raman spectra of CuCl were measured under hydrostatic pressures up to 14 GPa at low temperatures (T = 5 K). The anomaly in the Raman lineshape of zincblende CuCl at frequencies near the transverse-optic (TO) mode, which consists of a broad structure with several maxima, disappears at a pressure near 3 GPa. This effect is well reproduced by a model calculation of the anharmonic coupling of the TO mode to acoustic two-phonon states (Fermi resonance). Alternative interpretations of the TO Raman anomaly in terms of local vibrational modes of Cu atoms in off-center positions are not supported by the present results. Raman spectra indicate the existence of the phase CuCl-IIa in a narrow pressure …
Lattice phonon modes of the high-pressure phase CuCl-IV
Raman spectra of isotopically pure CuCl samples ${(}^{63}\mathrm{CuCl}$ and ${}^{65}\mathrm{CuCl})$ were measured under hydrostatic pressure up to 14 GPa at low temperature $(5 \mathrm{K}).$ Up to six Raman modes were resolved for the binary BC8-analog phase CuCl-IV in the range from 4.1 to 14 GPa. The phonon dispersion relations of this phase were calculated within a rigid ion model. Based on this calculation an assignment for the observed Raman-active modes is proposed. The calculated pressure dependences of mode frequencies reproduce the experimental results. Raman spectra support the existence of the phase CuCl-IIa in a narrow pressure range from 3.3 to 4.1 GPa.
Vibrational Properties of InSe under Pressure: Experiment and Theory
The pressure dependence of the phonon modes in the layered semiconductor γ-InSe has been investigated experimentally and theoretically for pressures up to 11 GPa. The mode Gruneisen parameters of all Raman-active zone-center phonons have been determined by Raman scattering under pressure. In addition, features corresponding to second and third-order scattering processes are apparent in the Raman spectra under resonance conditions, from which information about zone-edge modes can be obtained. For the assignment of the observed Raman features to vibrational modes we have calculated the phonon dispersion curves using a rigid-ion model including couplings to first-nearest neighbors and long-ran…
Effect of pressure on structural properties and energy band gaps of γ-InSe
We have investigated theoretically the effect of hydrostatic pressure on interatomic bond lengths and energy band gaps of γ-InSe. Total energy calculations were performed using the linear augmented plane wave (LAPW) method, taking into account scalar relativistic corrections as well as spin-orbit coupling. Internal structural parameters were optimized for different pressures by adopting as input the unit cell parameters known from experiment. Our theoretical results for the nearest-neighbor In-Se bond length are in excellent agreement with a recent experimental determination from high-pressure EXAFS measurements. The covalent In-In bond is found to be more compressible than the partially io…
Effects of Conduction Band Structure and Dimensionality of the Electron Gas on Transport Properties of InSe under Pressure
We report Hall effect and resistivity measurements in InSe under pressure. The electron concentration strongly decreases under pressure in samples exhibiting 3D transport behaviour. This is explained by the existence of an excited minimum in the conduction band moving to lower energies under pressure. The related impurity level traps electrons as it reaches the band gap and approaches the Fermi level. In samples exhibiting 2D behaviour the electron concentration remains constant. This behaviour, together with the pressure dependence of the Hall mobility, is consistent with a previous model which considers high mobility 3D electrons and low mobility 2D electrons to contribute to charge trans…