We report the comprehensive spectroscopic results of AlN thin films deposited on the A-, R- and C-surfaces of sapphire substrates by radio frequency magnetron sputtering. The optical and structural properties of the epitaxial-grown AlN films were characterized using various techniques of high-resolution x-ray diffraction spectroscopy, x-ray photoelectron spectroscopy, Raman scattering spectroscopy, spectroscopic ellipsometry and associated analytical tools. Our large number of measurement results clearly show that sapphire substrates of different polarities have effects on the surface roughness, dislocation density, grain size, microstrain, and surface oxygen binding capacity of the film grown on its surface. The results obtained from Ellipsometry measurements show that the thickness, band gap and roughness of AlN films grown on C-plane sapphire are the smallest among the three samples. After careful analyses of the variable temperature Raman spectra, as the temperature rises from 80 K to 800 K, the AlN film has always exhibited tensile stress. In the same temperature range, the tensile stress of the AlN film grown on the C-plane sapphire has the greatest effect with temperature. The lifetime of E2 (high) phonons gradually decays with the increase of temperature.
Surface plasmon (SP) enhancement of photoluminescence (PL) from a green-emitting InGaN/GaN quantum well (QW) using nanoparticles (NPs) made of different metals and their combinations was investigated. The NPs were formed by annealing the metal films in N2 followed by rapid cooling. Four-fold enhancement in PL intensity was achieved using random metal NPs made of Cu on Mg (Cu-Mg) double metal film that was more than two folds of the enhancement observed by AgNPs. Reversing the order of metal film deposition (Mg on Cu) resulted in much lower PL intensity due to significantly different NPs size distribution as the given annealing conditions did not cause homogeneous alloying of the two metals. The results pave the way for the application of NPs of relatively low-cost unconventional metals and their combinations in the SP enhancement of LEDs.
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