Radiative and Nonradiative Relaxation of Excitons in GaN
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**Groupe d'Etude des Semiconducteurs, Case Courrier 074, Universite Montpellier II
ABSTRACT
We report on investigations of the excitonic quantum efficiency in GaN epilayers as function of buffer layer thickness, buffer layer material, sample thickness and residual oxygen content. These values are compared to that of GaN bulk material. The quantum efficiency of the free excitons rises with increasing buffer layer thickness, increasing sample thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. Additionally, the homoepitaxial growth of GaN shows higher quantum efficiencies than the growth with an AIN buffer layer. In general, the observed quantum efficiencies in GaN epilayers are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range. Only, GaN bulk material shows quantum efficiencies of 25 % for the free A-exciton XA and of 50 % for the donor-bound exciton complex D",X). INTRODUCTION
Nonradiative recombination processes are detrimental to the laser action in III-V-nitride devices [1]. Since there is still scarce information on these nonradiative processes we performed
low-temperature time-resolved photoluminescence and calorimetric experiments near the band gap on a series of GaN epilayers grown on (0001) sapphire by MOCVD. The excitonic luminescences dominating in this spectral range exhibit decay times between 10 and 300 ps. Using the simultaneous and highly sensitive detection of the calorimetric absorption, transmission, and reflection at 50mK we determine the quantum efficiency of the excitonic decay processes involved [2]. From these data a detail picture of radiative and nonradiative relaxation and recombination processes in the spectral range near the band gap energy is obtained. EXPERIMENTAL SETUP
We investigated GaN epilayers grown by metal organic chemical vapor deposition (MOCVD) on sapphire with an AIN or a GaN buffer layer. The V/II molar ratio used during growth differs from 8000 to 10000. The thickness of the buffer layers varied from 250 to 500 A and the thickness of the epilayers from 1 to 2 ptm. In addition, samples with different amounts of residual oxygen content (10"6 - 1018 cm-3 ) were investigated [3].
The measurement of the radiative relaxation of the epilayers was done using time-integrated and time-resolved photoluminescence (PL). The time-resolved measurements were performed with a Nd:YAG pumped dye laser with a time resolution of 15 ps. To obtain information about the nonradiative relaxation we used the calorimetric absorption spectroscopy (CAS) at mK temperatures. The excitation light source is a xenon lamp XBO. With this technique it is possible 637 Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
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