Impact of exciton diffusion on the optical properties of thin GaN layers

We study the optical properties of thin (0.4-0.7 mu m) GaN layers directly grown on 6H-SiC(0001). These structures are found to always exhibit a broad luminescence band in the blue spectral range, which is identified as exclu sively being due to (D-0,A(0)) transitions in 6H-SiC. It is shown that the SiC substrate is excited indirectly via diffusion of carriers photogenerate d in the GaN layer. The drastic differences found between samples of differ ent electron concentration suggest correspondingly large differences in the diffusivity, which cannot be explained simply by different ambipolar carri er mobilities. Instead, different diffusion mechanisms are required, namely , ambipolar and excitonic diffusion at carrier concentrations above and bel ow the Mott density, respectively. [S0163-1829(98)51344-6].