Photoacoustic measurement of internal quantum efficiency and observation of the exciton effect in GaSe with photoacoustic phase spectra

We studied the room temperature photoacoustic spectra of GaSe single crystals in the vicinity of the energy gap. Exciton formation was observed in both amplitude and phase spectra. The thermal source that arises in the illuminated sample because of optical absorption without free-carrier generation was incorporated in the heat diffusion equation in order to extend the theoretical approach of photoacoustic signal generation. We calculated the optical absorption coefficient, which shows the exciton formation, and the electron-hole generation quantum efficiency eta(G) using an extended model from the phase and amplitude photoacoustic spectra, respectively.     

Characterization of optical quality of GaSe:Al crystals by exciton absorption peak parameters

The paper reports a first growth of the Al-doped GaSe crystals by modified technology with heat field rotation. These crystals shown from 2 to 3 times lower absorption coefficient at the maximal transparency range to that in crystals grown by conventional Bridgman technology. Possibility of the identification of optimal doping level in grown crystals by analysis of shape of the exciton absorption peak and intensity of the absorption shoulder at the transmission edge was demonstrated. The optimal doping can be attributed to the concentration of Al in the growth charge between 0.02 and 0.05 at.%. The result of the identification is confirmed by frequency conversion experiments: CO₂ laser SHG and optical rectification of fs pulses.     

Photoacoustic thermoelastic effect near Vickers indentations in nanocrystalline nickel

The character of the photoacoustic (PA) signal variations in the vicinity of Vickers indentation zones in nanocrystalline nickel has been studied. PA images revealing the response signal features inside and outside these zones have been obtained. The character of the PA signal variation is compared to that predicted by the existing model relating the PA response to stress-induced changes in the thermoelastic coupling coefficient of the material. It is shown that the existing model adequately describes the observed character of internal stress variations near the indentations sites only in the absence of additional stresses related to the sample preparation technology.     

The effect of neutron irradiation on the exciton absorption in gallium arsenide

We have studied the effect of neutron irradiation on the exciton absorption in n-GaAs crystals. It is shown that the observed decrease in the absorption coefficient, broadening of the exciton peak, and its shift toward higher energies are caused by the electric and strain (compression) fields generated by the radiation-induced defects.     

New polytypes in vapour grown GaSe

The occurrence of new polytypes in GaSe is reported. They have been denoted 9R, 12R and 15R following the Ramsdell notation. These crystals are obtained under certain conditions during sublimation experiments. One of the polytypes (9R) is described more extensively. Apart from X-ray determination techniques these polytypes are detected by measuring the angles of the planes using optical-goniometer techniques. The habit of the different rhombohedral polytypes turned out to be similar to each other.     

Further studies on polytypism in GaSe

Some new rhombohedral polytypes and a hexagonal polytype are reported. They have been denoted 18R, 21R and 6H. These crystals are obtained during sublimation experiments. A model for the 6H-structure is introduced. The phenomenon of syntactic coalescence, viz. the growth of 6H and 18R in one GaSe needle crystal, is shown. A new feature observed in GaSe polytypes, viz. the rotation of the slabs, which build those polytypes, is discussed.     

Properties of Hydrogenated GaSe Crystals

X-ray diffraction evidence is presented for the incorporation of molecular hydrogen into GaSe crystals, which markedly reduces their c-axis resistivity and shifts the excitonic line to higher energies.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 907–909.Original Russian Text Copyright © 2005 by Kaminskii, Kovalyuk, Pyrlya, Gavrilyuk, Netyaga.     

Low frequency dispersion in GaSe thin films

Frequency domain measurements on GaSe thin films have been carried out in the frequency range 10⁻² to 10⁴ Hz and temperature range 300 to 480 K. The measured samples with different contacts have shown low frequency dispersion (LFD) with activation energies 0.66±0.03 eV, 0.72±0.03 eV and 0.65±0.03 eV, respectively. Complex capacitance C(ω) behaviour of GaSe thin film with different contacts suggests possible contribution of diffused contact material.     

Dislocation etching of GaSe single crystals

Dislocation etching of GaSe single crystals was investigated by using a dilute chromic-sulphuric acid mixture, when conical etch pits were revealed on the (0001) surfaces. At the apices of spiral growth hills, bunches of spiral dislocations were revealed, proving that it is not a single screw dislocation of large Burgers vector, but a bunch of co-operating screw dislocations that were responsible for the spiral growth formations of large step-height. In the case of GaSe crystals, grown by vapour transport methods, dislocation densities of 10² to 10⁶ cm⁻² were found. The Bridgman crystals investigated were completely free from non-basal dislocations.     

The identification of non-basal dislocations in GaSe by the etch-pit technique

A detailed study has been made of the etch-pits produced on GaSe crystals, grown by various methods, using etchants consisting of bromine or iodine in methanol. Non-basal dislocations of both screw and edge types, sometimes aligned to form low-angle boundaries, are revealed. Detailed observations of the pit shapes indicate that the dislocations bend and dissociate within the solid. The reorientation of etch-pits within existing pits is discussed in terms of stacking faults. Networks of etch grooves have been observed and are believed to be due to dislocations lying in the basal plane.     

Spectrogoniometric determination of refractive indices of GaSe

A simple interference spectroscopy technique of determining real parts of refractive indices in thin isotropic or anisotropic films is presented. This method is based on the evaluation of the orders of extrema in interference spectra of optical transmittance and/or reflectance measured for various angles of light incidence. It also makes possible to determine thickness of the investigated sample. This method of investigations was used to determine the parameters of gallium selenide (GaSe). The determined spectral characteristics of ordinary and extraordinary refractive indices of GaSe are compared with the data presented in literature by other authors.     

Quantitative Photoacoustic Spectroscopy in the Frequency Domain

In this paper, the development of a new methodology for the quantitative determination of the optical absorption coefficient in simple systems in which the light absorption follows Beer’s law is described. An approximation of the heat diffusion model of the photoacoustic effect for thermally thick samples is explored. It was found that we could combine the amplitude and the phase of the photoacoustic signal to obtain a new analytical expression for the optical absorption coefficient. This expression is directly proportional to the normalized photoacoustic signal amplitude, the sine of the phase difference, and the heat capacity per unit of volume of the sample. The theoretical results were experimentally verified in the visible range (300 nm to 700 nm). The optical absorption coefficient obtained with this methodology was comparable to that obtained by UV–Vis spectroscopy.     

Anisotropic behaviors in polycrystalline Cd-doped GaSe thin films

GaSe thin films were deposited by thermal evaporation technique with Cd doping. X-ray diffraction analysis showed that Cd-doped films have polycrystalline structure with the preferred orientation along (008) direction. Temperature dependent electrical conductivity measurements were carried out in the temperature range of 100–400 K along perpendicular and parallel directions to the growth direction for the films exhibiting p-type conduction determined by hot probe technique. The room temperature conductivity values of the films were found to be as 1.5 × 10⁻⁸ and 4.9 × 10⁻¹² (Ω cm)⁻¹ due to the measurements along both perpendicular and parallel directions, respectively. The difference in the conductivity values is the indication of electrical anisotropy in the samples. Carrier conduction in the films was provided by the thermionic emission in the high temperature region (310–400 K) with almost the same activation energies in both directions. Space charge limited current analysis at different temperatures reveals the existence of two discrete sets of trap levels for both perpendicular and parallel directions. Calculated trap levels and trap concentrations are 99 meV, 3.5 × 10¹² cm⁻³ and 418 meV, 2.2 × 10⁵ cm⁻³ for perpendicular direction, 58 meV, 2.1 × 10¹⁸ cm⁻³ and 486 meV, 1.4 × 10¹² cm⁻³ for parallel direction. The differences in the values of the trap levels and concentrations for both directions confirm the existence of electrical anisotropy in Cd-doped GaSe thin films, because of the structural anisotropy between and inside the crystallites.     

Transport and growth of GaSe from the vapour phase

A comparative study of the sublimation and chemical transport of the compound GaSe is reported. The parameters investigated in the closed system are the iodine concentration, the tube geometry, and under-cooling. The rate of transport in case of sublimation is diffusion-controlled, in the iodine-assisted transport process both diffusion and convection play a role. If the sublimation technique is used, the crystals are not exclusively plate-like, the majority of the crystals exhibit needle-like and ribbon-like habit. With growth in the length direction, the growth axis is parallel to the c-axis, it takes place around screw dislocations and two crystallization mechanisms are assumed to take place simultaneously.     

High field kinetics of photocurrent in GaSe amorphous films

The drift mobility of current carriers μ = (2–7)10⁻⁸cm²/V·s and their lifetime τ = 10–17μs have been determined and a cubic dependence of photogeneration charge on electric field has been obtained in the 5×10⁵ to 10⁶V·cm⁻¹ nange from the results of investigation of photocurrent kinetics in GaSe amorphous films.