Studies of hole transport in Mg-doped AlGaN layers for deep-ultraviolet light emitters

Abstract

AlGaN epilayers with Al composition higher than 50% are required for the realization of high-performance nitride-based light-emitting devices operating in the deep ultraviolet (DUV) region ( < 300 nm), with their wide bandgap, UV transparency, and high optical confinement effect. In DUV light emitters, the growth of p-type AlGaN layers have been a major challenge because of the dramatic increase of Mg-acceptor thermal activation energy in AlGaN with increasing Al composition. Mg-doped AlGaN superlattice structures that utilize polarization fields to reduce Mg acceptor energy level have been reported as an alternative method to improve p-type doping efficiency of AlGaN layers. However, the existence of a large energy barrier in these structures severely degrades hole mobility and layer conductivity in the vertical direction, which makes these structures less useful in actual device structures. We have explored a number of designs for the p-layer for DUV light emitters and will report the results. The studies were conducted with p-layers of AlGaN grown on bulk AlN substrates. Van der Pauw devices were fabricated for variable-temperature Hall-effect measurements. The AlGaN p-layers show very small effective dopant activation energies in the range of 20 meV. This is to be compared to that for p-type GaN (146 meV) or AlGaN (323 meV). The results indicate that the acceptors in our AlGaN structure are activated via an athermal process. AlGaN p-n junction devices were grown on bulk AlN to confirm high vertical conductivity of the p-layer structure. Current-voltage and four-wire measurements on the p-n junction devices revealed that our p-layer provides higher conductivity, smaller voltage drop, and higher current drive compared to standard homogeneous AlGaN p-layer with similar average Al composition. A UV test diode employing our p-layer displayed DC-mode current injection level of 550 mA, which is equivalent to 11 kA/cm2.