Hot Electrons and Holes in Highly Photoexcited GaN Epilayers

Abstract

In GaN epilayers an increase of the carrier temperature was deduced from the luminescence spectra under intense quasi-steady-state photogeneration. To account for the measured dependence of the electron temperature on the excitation power density, all relevant mechanisms of the nonequilibrium-phonon production were considered. Finally, theoretical modeling yielded strong implications that, contrary to electrons, hot photogenerated holes maintain no Maxwellian distribution function above the threshold for the optical phonon emission.Application of GaN-based materials in strongly nonequilibrium conditions is related to the high-density and high-temperature electron±hole plasma regime, which recently received much attention. In particular, nonequilibrium-carrier distribution functions (DFs), which are of crucial importance for relaxation and recombination rates, optical gain, band gap renormalization, and screening of built-in piezoelectric fields, are being actively discussed in connection with electron and phonon spectra of III-nitrides [1 to 4]. One of the most effective ways to get an insight into the energy exchange processes in highly excited semiconductors is optical spectroscopy, which allows one to directly monitor the electron DF. In the present work we report on the dependence of the carrier effective temperature on the off-resonant photoexcitation intensity in GaN epilayers. We present strong implications that the DF of the photogenerated holes may not attain a conventional Maxwellian shape even at high excitation.Semi-insulating GaN epilayers were grown on sapphire and 6H-SiC substrates by low-pressure MOCVD. The samples were photoexcited by the fourth harmonic of a Q-switched YAG : Nd 3+ laser radiation (incident photon energy hn inc 4X66 eV, pulse duration 10 ns). Luminescence spectra were recorded by means of a double monochromator, photomultiplier and box-car integrator. The measurements were performed at T 80 K. Fig. 1 depicts high-energy tails of some luminescence spectra in a GaN/SiC sample. The spectra indicate that the electron DF is a smooth exponential function of energy.

Ultrafast Dynamics of Lasing Semiconductor Nanowires

“…(ii) The hole relaxation rates in the three materials are higher due to their higher effective masses (see Table 1 in Supporting Information). ,, Figure b depicts the conduction band (CB) of CdS (black), GaN (cyan), and ZnO (red) with the respective energies for the longitudinal (LO) and transverse (TO) optical phonons. Electrons release their excess energy mainly by successive emission of LO phonons and relax toward the CB minimum, until the electrons do not possess enough energy and the LO phonon emission is forbidden by energy conservation .…”