Study of Near-Threshold Gain Mechanisms in MOCVD-Grown GaN Epilayers and InGaN/GaN Heterostructures
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performed in an edge emission geometry. In order to avoid distortion of the spontaneous emission spectra due to re-absorption processes, the laser beam was focused on the sample surface and spontaneous emission was collected from a direction near normal to the sample surface. 8 RESULTS AND DISCUSSIONS GaN epilayers To determine if the SE threshold density occurs above or below the Mott density (the critical density beyond which no excitons can exist), we studied the temperature behavior of the SE threshold in GaN epilayers grown on sapphire and SiC substrates over a wide temperature range (20 to 700 K), as shown in Fig. 1. For temperatures above 200 K, the SE thresholds
roughly followed an exponential dependence: IIh= Io exp(T/To), with To =_-170 K (Ref. 2). This exponential behavior of the SE threshold is qualitatively similar to that observed in other material structures. However, as the temperature is reduced to below 200 K, a significant (faster than exponential) reduction in the SE threshold was observed in GaN epilayers. This is associated with a change in the gain mechanism indicating a drastic increase in the SE efficiency at low temperatures. Recently, Fischer et al. convincingly demonstrated the presence of excitonic resonances in GaN epilayers well above RT through optical absorption measurements.'° Excitons in GaN epilayers cannot be easily ionized due to the relatively large exciton binding energy. However, at near-SE-threshold (near-Ih) pump densities the picture is not straightforward due to screening of the Coulomb-interaction by the photo generated free carriers. In general, the existence of excitons depends on the strength of the Coulomb-interaction which in turn depends on the density and distribution of carriers among bound and unbound states. It has been predicted theoretically that in a material system with a relatively large exciton binding energy, one could expect inelastic ex-ex scattering to have the lowest SE threshold at low temperatures." Schmidt et al. performed pump-probe experiments and confirmed the presence of excitons at pump 10
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for GaN thin films grown on SiC (open triangles) and sapphire (filled circles),
and SE peaks as a function of temperature for GaN thin films grown on SiC (open triangles) and sapphire (filled circles).
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densities above the SE threshold at 10 K in reflection spectra.t 2 These observations support the idea that excitons persist above the densities required to observe SE in GaN at cryogenic temperatures. The effects of excitons on the SE process can be better understoo
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