LASER EXCITED POPULATIONS OF ELECTRONS AND PHONONS IN GALLIUM-ARSENIDE: RAMAN SCATTERING PROBE STUDIES

KONG-THON FRANK TSEN, Purdue University

Abstract

This research work has involved the use of Raman scattering as a probe of the solid state plasma and of the phonons in GaAs. A high power YA1G laser capable of operating in Cw and Q-switched modes was employed to investigate both the equilibrium and non-equilibrium distributions of electrons as well as of phonons. The same laser, in the Q-switched mode, was used for changing the electron and phonon distributions and, in situ, for probing those changes by Raman scattering. We have demonstrated that Raman scattering can serve as a good, universal probe for non-equilibrium electron distributions and for excess phonons throughout the Brillouin zone. The work which has been accomplished can be divided into two categories: (1) For the low-laser-intensity, equilibrium case, theoretical analysis of the single particle scattering line shape associated with the charge density, spin density and energy density fluctuation mechanisms have been carried out and were compared with our experimental data. We studied the effects of band structure, carried concentration and collisions on each of these mechanisms. From the combined theoretical and experimental analysis, we found that one had to be very cautious in using single particle scattering as a probe of non-equilibrium electron distribution function. The spin density fluctuation mechanisms was shown to be best suited for this purpose, but not without significant corrections, e.g., for band structure effect and collisions. (2) For the high-laser-intensity, non-equilibrium case, at laser intensity of (TURN) 3 MW/cm('2), we were able to excite (TURN) 5 x 10('15)/cm('3) electrons from deep traps in GaAs at low temperature ((TURN) 25 K). The non-equilibrium phonons generated through non-radiative recombination of these electrons onto the traps and non-equilibrium electron distribution were reflected in the Raman scattering spectrum. The feasibility and general utility of using two-phonon Raman scattering for probing non-equilibrium large wave vector TA (transverse acoustic) phonons was demonstrated. We obtained important new information about the TA phonon bottleneck in GaAs at low temperature recently observed by heat pulse measurements (Ulbrich, Narayanamurti, et al.). Our work provided verification of the existence of the bottleneck and the first specification of the frequencies and populations of the non-equilibrium phonons throughout the Brillouin zone. We discovered that the phonon enhancement was not restricted to the lowest, softest TA branch, but occurred also for the fast TA phonons.

Degree

Ph.D.

Subject Area

Condensation

COinS