We have carried out the growth and systematic studies of the optoelectronic and structural properties of AlN epilayers. The results revealed that the threading dislocation (TD) density, in particular the edge TD density, decreases considerably with increasing the epilayer thickness. The screw dislocation density was estimated to be about 5x10^6 cm^-2 in the 4 ?m thick AlN epilaye and is more than one order of magnitude lower than that in GaN of the same thickness. The improved materials were utilized to fabricate deep UV Schottky barrier photodetectors. The fabricated AlN/n-SiC hybrid Schottky barrier detectors exhibited a peak responsivity at 200 nm with very high reverse breakdown voltages and high responsivity and DUV to UV/visible rejection ratio. Furthermore, we have developed MOCVD growth processes for depositing a-plane AlN epilayers and photonics structures on r-plane sapphire substrates, which will partially address the low efficiency issue of deep UV emitters based on AlGaN. We have also achieved p-type conduction in Al0.7Ga0.3N alloys at elevated temperatures. Although Mg doped AlGaN alloys with high Al contents are generally highly resistive at room temperature, our work has provided a more coherent picture for the conductivity control of AlGaN and AlN.
