R.C.C. Leite

Non equilibrium phonon distribution and electron-phonon coupling in semiconductors

Abstract A comparison is made for different semiconductors, of the non equilibrium phonon distributions induced by photon absorption. It is found that energetic carriers create preferentially LO-phonons for GaAs where Frohlich type interaction dominates. For Si and the intermediate case of InSb the energetic carriers still relax through optical phonons. For InSb LO- and TO-phonons participate in the same proportion. Highest phonon temperatures are obtained in the layer semiconductor GaSe.

Resonant Raman scattering of TO(A1), TO(E1) and E2 optical phonons in GaN

Abstract Resonant Raman Scattering by optical phonons in GaN was observed under conditions that permitted comparison with the ‘bare exciton’ theory. Excellent agreement was obtained. Results covered an energy range from ≈ 800 to 300 meV below the gap energy, which shows that even at such energies exciton effects are dominant for Raman scattering.

Single particle scattering from hot electrons in GaAs

Abstract Two photon-absorption in GaAs from a pulsed Nd : Yag laser induced non-equilibrium distributions of electrons and optical phonons in the bulk of the material. The Nd : Yag laser radiation itself is used as a probe to search for population changes of optical phonons and carriers by means of Raman scattering. This allowed comparison of electron and phonon temperatures at different excitation levels. Electrons reached a temperature of 560° K for a lattice temperature of 300° K.

Very high non-thermal equilibrium population of optical phonons in GaAs

Abstract Extremely high equivalent temperatures of optical phonons were obtained in GaAs without detectable increase in the lattice temperature. The improvement upon previous experiment was due to the use of a pulsed Nd:Yag laser which was doubled by the GaAs sample itself and absorbed in its bulk. The non-equilibrium LO-phonon population attained before crystal breakdown was ∼ 27 times larger than the thermal equilibrium population at the room temperature (290K). The equivalent temperature observed for LO-phonons was ∼3700K.

Dependence of hot carriers temperature on lattice temperature in CdS

Abstract Non-equilibrium carrier distributions were obtained in CdS at various temperatures from 77 to 400K. A study is made of the influence of the lattice temperature on the carrier temperature. It is found that the higher the lattice temperature the lower is the difference between carrier and lattice temperatures, though carriers are always thermalized among themselves. The results can be accounted for by carrier relaxation through optical polar phonon emission.

Band gap reduction in CdS due to high density of photo-injected carriers

Abstract At high photoexcitation levels the intrinsic radiative emission band in CdS, displays strong distortions which increase with excitation intensity. The two main features are a shift towards lower energies and a broadening of the lower energy side of the emission band. These effects are attributed to the high density of injected carriers

Hot electron and hot phonon contributions to radiative emission spectra in CdS at high excitation intensities

Abstract The dependence of excitation frequency of photoluminescence spectral shape of CdS at high excitation intensities allowed discrimination between hot phonon and hot electron contributions to the broadening of the emission band on one side and the broadening due to many body effects on the other. It was found that the shift towards lower energies of the emission peak is mainly due to induced high carrier density and that the broadening is related to the presence of high densities of optical phonons and hot carriers.

Carrier assisted radiative recombination of free excitons in GaSe

Abstract Photoluminescence spectrum of GaSe at very high excitation intensities displays two emission bands. At lower excitation levels emission is dominated by free exciton recombination. As the excitation increases the spectrum becomes dominated by recombination of an exciton-carrier complex. Arguments are based on line shape analysis, and spectral behaviour with temperature and excitation intensity.

Wave-vector dependence of electron-optical phonon coupling in GaAs

Induced non equilibrium distribution of optical phonons allows direct measurement of electron-phonon coupling as a function of phonon wave-vector K. Results indicate that near K=0(K<20, 000 cm−1 coupling between TO phonons and electrons is independent of K whereas LO phonons show a K-2 dependence. Results also suggest that electron relaxation in the conduction band by multiple phonon production is quite significant.

Photoluminescence of Mn doped epitaxial GaAs

Abstract We have measured the near band edge photoluminescence of Mn doped liquid phase epitaxially grown GaAs. The photoluminescence spectra at 2°K shows, at low excitation intensities, a structure of up to eight sharp peaks (widths .2 to 1.0 meV) between 1.517 and 1.512 eV, besides the lower energy bands near 1.41 eV due to the deep Mn acceptor level and the usual donor-acceptor bands around 1.47 eV. Attempts to relate the sharp lines to the Mn electronic states, introduced by doping, were unsuccessful. It is our belief that the presence of this particular impurity in our samples allows for whatever states are responsible for the sharp line structure, to reveal themselves in the emission spectrum. A most unespected result is that near band edge sharp line luminescence is observed for impurity concentration as high as 10 18 cm -3 .