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Low-temperature photoluminescence spectra of nominally undoped high quality AlN layers on SiC and Al2O3 substrates are reported. Under high excitation conditions, we observe several bands that increase superlinearly with the excitation density. Based on temperature and excitation level dependences recorded on different samples, we identify a band 36 meV below the free A-exciton transition as due to exciton-exciton scattering (P2 band) and a second band down-shifted from the A-exciton transition by 27 meV as due to biexciton recombination. The combined data yield an exciton binding energy of 48 meV.

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The effects on surface morphology, crystal quality, and growth rate of undoped AlN layers grown on c-plane sapphire substrates due to the changes in growth parameters, such as V-III ratio, N 2 2H 2 ratio, and growth temperature in LP-MOVPE are studied here. The optimized growth process resulted in an almost flat surface morphology with a significantly reduced number of hexagonal pits ð3 Â 10 7 cm À2 Þ and good crystalline quality having a rms value of roughness of 0.4 nm measured by atomic force microscopy and a high resolution X-ray diffraction (HRXRD) FWHM value for (0 0 0 2) reflection of 200 arcsecs. The threading dislocation density of the AlN layer is estimated approx. 10 9 cm À2 from cross-sectional transmission electron microscopy (TEM) measurements. Additionally, these optimized samples show a strong donor-bound exciton luminescence signal with a FWHM of 20 meV. Furthermore, small period AlN/ GaN superlattice structures with excellent uniformity were grown, as proven by our HRXRD and TEM studies.