We investigate the effect of illumination on the velocity of the L& and FT& acoustic-wave propagation along layers of s-Cheese [in the {0001)plane]. Frequency and temperature dependences and also relative changes of various acoustic-wave velocities under hydrostatic pressure in c.-GaSe are used to show that photosensitive absorption and the change of the L~and FT& wave-propagation velocity are associated with the piezoelectric interaction of the acoustic wave with free charge carriers. High anisotropy of anharmonic properties at compressive and shearing strain of the layers relative to each other is a characteristic feature of lamellar crystals and is related to anisotropy of the crystal elasticity. The velocity of piezoactive acoustic waves in P6m 2 crystals has been calculated theoretically. Piezoelectric characteristics and Gruneisen constants for the acoustic branches of the phonon spectrum in c,-GaSe have been determined.
Investigations of exciton luminescence and absorption spectra of InSe (GaSe) lamellar semiconductors doped with different kinds of compounds at T = 1.8 K are carried out. It is found that changes in the luminescence intensity of free and bound excitons are due to screening of Coulomb interaction between electron and hole. Increase of free excitons luminescence intensity in InSe crystals activated by holmium ions is attributed to "healing" of metal and chalcogenide vacancies and also to interlaminar interaction increase due to exchange interaction of local formations, i.e. holmium ions.Es werden bei T = 1,8 K Untersuchungen der Exzitonenlumineszenz-und Absorptionsspektren von InSe (GaSe)-Schichthalbleitern, die mit unterschiedlichen Verbindungen dotiert werden, durchgefiihrt. Es wird gefunden, daB Anderungen in der Lumineszenzintensitat von freien und gebundenen Exzitonen durch die Abschirmung der Coulomb-Wechselwirkung zwischen Elektron und Loch hervorgerufen werden. Der Anstieg der Lumineszenzintensitat freier Exzitonen in durch Holmiumionen aktivierten InSe-Kristallen wird dem ,,Ausheilen" von Metall-und Chalkogen-Leerstellen zugeordnet und auch dem Ansteigen der Interschichtwechselwirkung infolge Austauschwechselwirkung lokaler Formationen, z. B. Holmiumionen.
Complex experimental studies are made of optical phenomena, EPR spectra, Raman scattering, and heat capacity of pure and Nd-doped TIGaS, single crystals in a wide energy (2.1 to 2.7 eV) and temperature (1.8 to 300 K) range. An anomalous temperature dependence of optical spectra, the decay of free excitons, the EPR in the temperature range from 3.6 to 16 K, and the Raman scattering are observed and phase transitions of second order (75 K), and of first (75 K) and second (90 K and above) order in doped and pure TlGaS, crystals, respectively. A model of local formations of Nd3+ion in TIGaS, is proposed.In einem groBen Energie (2,l bis 2,7 eV)-und Temperatur (1,8 bis 300 K)-Bereich wird eine komplexe experimentelle Untersuchung von optischen Phanomenen, EPR-Spektren, Ramanstreuung und WBrmekapazitat von reinen und Nd-dotierten TIGaS,-Einkristallen durchgefiihrt. Es wird eine anomale Temperaturabhangigkeit der optischen Spektren, des Zerfalls der freien Exzitonen, der EPR im Temperaturbereich von 3,6 bis 16 K und der Ramanstreuung beobachtet sowie Phasenubergange zweiter Ordnung (75 K) und erster (75 K) und zweiter (90 K und daruber) Ordnung in dotierten bzw. reinen TlGaS,-Kristallen. Ein Modell der lokalen Bildung von Nd3+-Ionen in TIGaS, wird vorgeschlagen.
TlGaS, crystals possessing some significant and unique properties are of special importance among the crystals with lamellar and chain structure. Among these features are: shift of the maximum of the exciton absorption peak with rising temperature towards high energies [ 11; paramagnetic properties connected with structure defects formed at the expense of bivalent thallium ions similarly as in TlSe crystallized in chain structure [2]; extra-sensitivity of photoluminescence to an external magnetic field 131; intermediate position of TIGaS, structure between lamellar and chain crystals [4] and several other features not inherent in other A"'BV' and A1"B"'CV1 , c ompounds.Due to the discrepancy of available data [5 to 131 additional investigations were necessary for an unambiguous interpretation of the accumulated results.The present note describes an experimental study of exciton absorption and photoluminescence spectra of TlGaS, single crystals grown using the Bridgman-Stockbargcr method (X-ray measurements show two symmetry variants for the monoclinic lattice of TlGaS,, C: or C;,, [14,15]) in wide pressure ((0 to 20) x 10' Pa) and temperature (1.8 to 190 K ) ranges. The values of pressure coefficients of the crystals were determined considering the influence of deformation on the spectra of the extremely narrow photoluminescence lines 2, = 595.72 nm and 1, = 595.56 nm.Measurements were carried out as follows. In a chamber with high hydrostatic pressure, with the use of the laser J'IrH-502, luminescence in TIGaS, crystal was excited, and after passing through a A@C-24 monochromator it was registered by a @3Y-79 receiver with autorecording. The exciting emission i = 475.6 nm was directed perpendicular to the planc of the single crystal wafer and the excited luminescence was registered in the opposite direction.The temperature dependence of the energetic position of the exciton absorption peak E,,,(T) of TlGaS, crystals is given in Fig. 1. One can see that unlike other semiconductors of the A"'BV' and A1"B"'CV' , g r oups here a temperature rise is accompanied by a shift of the exciton line towards the short-wave region, i.e. a temperature rise leads to an increase of the forbidden zone width E,. Note that temperature dependences of the photoluminescence line stipulated by radiative recombination of free excitons and of the line appearing as the result of electron-hole pair formation coincide in the absorption spectrum.Such a behaviour of E , depending on temperature is unaccountable in terms of electron-phonon interaction, as our results concerning the relation between the binding energy of excitons and temperature show that in TlGaS, E,,, changes with the change of I ) 33 Pr. Narimanova, 370143 Baku, Azerbaijan.
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