Spatial distributions of near-band-gap uv and yellow emission on MOCVD grown GaN epifilms

Near-band-gap UV and yellow band emission from metal-organic chemical vapor deposition grown GaN films on sapphires are investigated under laser excitation. The intensities of the UV and the yellow peaks increase at different rates as the entrance slit width of the spectrometer increases. The spatial distribution of the luminescence emission is analyzed through the dependence of photoluminescence intensity on the slit widths of the spectrometer. The yellow emission originates from a spot with a size about 1.5 times larger in diameter than the UV emission. Using an absorption mechanism, a Lorentzian line-shape distribution fit with the data gives estimated effective absorption coefficients of $47{\mathrm{cm}}^{\mathrm{-}1}$ for the UV signal at 364 nm and of $32{\mathrm{cm}}^{\mathrm{-}1}$ for the yellow signal at 546 nm, which agrees perfectly with the ones from an exponential decay fit. Dependence of UV-to-yellow peak ratio on the slit widths of the spectrometer, and consistence with possible origins of yellow luminescence is discussed.