RAMAN PROBE OF EQUILIBRIUM AND LASER MODULATED POPULATIONS OF ELECTRONS AND PHONONS IN GAAS

DENNIS ALAN ABRAMSOHN, Purdue University

Abstract

This work involved the use of the Raman spectrum as a probe of the free electron solid state plasma and of the phonons in GaAs. A primary goal has been the elucidation of single particle scattering (essentially Compton scattering from a Maxwellian distribution of electrons), manifested as a Gaussian wing around the laser line (the Rayleigh wing). The experiments were divided into two basic categories. The first was a search at high laser intensities for a plasma of photoexcited carriers. Using a high intensity pulsed YAG laser, we photoexcited carriers from deep traps and used the same laser pulse to probe the new carrier distribution. The second was a study at low intensities to more completely understand the mechanisms which produce scattering in the Rayleigh wing. Critical to the success of both objectives was our construction of a computer controlled Raman scattering system. It allowed us to make both gated, high intensity measurements during Q-switched laser excitation and to obtain excellent low intensity spectra. From the high intensity measurements we obtained several interesting results. We observed, for the first time, light scattering from excess carriers photoexcited from deep traps in GaAs and demonstrated the possibility of studying non-equilibrium populations of electrons by pulsed laser techniques. From the strength of the scattering in the Rayleigh wing, we obtained a direct measure of the number of photoexcited carriers. Our results indicated that deep traps are universal in all GaAs samples tested, and are present in a concentration of > 5 x 10('15)/cm('3). From the shape of the wing, we obtained information on the type and distribution function of the excited carriers. We found that the excited carriers are electrons with a Maxwellian velocity distribution at the lattice temperature. We were, however, unable to detect modulation of the phonon populations arising from the thermalization and retrapping of the excited electrons at 300 K. The analysis of the Rayleigh wing lineshape at low intensity led to several new and interesting results. We note that the sole previous study reported a simple Gaussian lineshape at 300 K for all carrier concentrations (10('12) - 10('18)/cm('3)) and for all polarization configurations. We found many departures from this lineshape. At low concentrations (< 10('14)/cm('3)) we observed, for the first time, a highly structured, narrow line, present in equal strength in all samples and overwhelming the scattering from free carriers. This common feature was tentatively identified as representing two phonon difference frequencies, primarily at specific symmetry points in the Brillouin zone. We found that at intermediate concentrations (10('14) - 10('16)/cm('3)) this contribution must be subtracted from the spectrum to obtain an accurate representation of the single particle scattering. This corrected spectrum was then close to the Gaussian, but our detailed studies showed the need for band structure corrections, implicit in the most current theories but never previously demonstrated experimentally. For charge density scattering, we found a regime of single particle-plasmon interaction which again was implicit in the theory but which had not previously been demonstrated experimentally. At concentrations > 5 x 10('15)/cm('3), bulges at the coupled mode frequency greatly broaden the apparent width of the Rayleigh wing. Finally, at the highest concentrations (> 5 x 10('16)/cm('3)), the plasmon exists far away from the single particle region and the single particle lineshape becomes Lorentzian. This regime appears to require extension of the theory to include collisions and departures from a Maxwellian energy distribution for the electrons.

Degree

Ph.D.

Subject Area

Condensation

COinS