NONEQUILIBRIUM EXCITATIONS GENERATED BY NEODYMIUM: YTTRIUM ALUMINUM GARNET LASER RADIATION IN BULK GALLIUM-ARSENIDE (ELZ DEFECT, PHONONS, SHALLOW ACCEPTORS, LEC)

KAM HOI WAN, Purdue University

Abstract

Equilibrium and non-equilibrium excitations in various kinds of GaAs single crystals are probed by 90(DEGREES) Raman scattering. In semi-insulating (SI) material, at room temperature, we are able to separate the three normalized, independent symmetry components of the whole 2-phonon spectrum from 0 to 650 cm('-1) in GaAs single crystals. We identified 25 sharp phonon peaks in such spectra, which includes a few difference frequency combinations less than 100 cm('-1) from the laser line. Our most striking discovery is the generation of non-equilibrium bound holes of concentration (TURN)10('15) cm('-3) on residual, minority, shallow acceptors at low temperatures by a weak, CW Nd:YAlG laser in undoped SI GaAs grown by liquid encapsulated Czochralski technique, but not in n-type or Cr-doped, SI GaAs. We observed sharp-line, well resolved electronic Raman (ER) spectra of these holes on carbon acceptors in some samples and on zinc acceptors in another. These sharp-line spectra are used to explore the various features of the ER spectrum in bulk GaAs and to characterize this technologically important material. We found that the native, mid-gap defects EL2 are responsible for the observation of the long-lived, non-equilibrium bound holes in this material. The combination of facts (1) the compensation between the EL2 defects and the residual, shallow acceptors in this material, and (2) the existence of the metastable state of the EL2 defects, explain the ease of generation of non-equilibrium bound holes by the sub-band gap, Nd:YAlG laser radiation. Under extremely intense, Q-switched YAlG laser illumination the amplitude of these ER spectra decreases due to 2-photon absorption processes, but non-equilibrium ER signals are generated in Cr-doped, SI and n-type materials. For Cr-doped, SI GaAs, we observed a broad peak at 150 cm('-1) which is attributed to the E line of carbon acceptors, broadened by the strong, internal electric fields. For n-type material, we showed that the previously observed non-equilibrium peaks are actually due to the ER scattering of residual, minority, zinc acceptors in n-type samples, instead of non-equilibrium zone boundary TA phonons.

Degree

Ph.D.

Subject Area

Condensation

COinS