Advantages and limitations of UV optoelectronics on AlN substrates

AlGaN alloys are the building blocks for deep UV optoelectronics and high-power devices. It has been demonstrated that the highest crystalline quality AlGaN films with high Al content are obtained on AlN single crystal substrates. The resulting mismatch between AlGaN and AlN causes a compressive strain within the AlGaN layers, which varies with composition. Nevertheless, pseudomorphic AlGaN films with Al content higher than 50%, and dislocation densities lower than 10
⁴
cm
^-2
have been achieved, sustaining compressive stresses with thicknesses exceeding 3 μm. Such results demonstrate the advantages of using AlN substrates for this technology, and at such some have been realized on several deep-UV optoelectronics applications. Kinoshita et al. have demonstrated UV LEDs emitting at 265 nm with output powers exceeding 80 mW [1]. UV LEDs grown on these native substrates have higher reliabilities and higher output powers. In addition, optically pumped lasers emitting at wavelengths between 230 nm and 280 nm that display cavity modes and single polarized-state emission with low lasing thresholds have been developed [2, 3]. Nevertheless, there are several limitations related to the performance and further improvement of the active regions as related to their quantum efficiencies and the polarization of their emission is desired. These limitations will be classified in two main categories: (1) identification and control of point defects and (2) growth orientation control.