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

Details

Event SPIE Photonics West

Authors

Suk Choi
Knollenberg, Clifford
Teepe, Mark R.
Northrup, John E.
Noble Johnson
Technical Publications
February 7th 2013
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.

Citation

Choi, S.; Cheng, B.; Yang, Z.; Knollenberg, C.; Teepe, M. R.; Wunderer, T.; Chua, C. L.; Northrup, J. E.; Johnson, N. M. Studies of hole transport in Mg-doped AlGaN layers for deep-ultraviolet light emitters. Gallium Nitride Materials and Devices VIII, Conference 8625 at SPIE Photonics West; 2013 February 2-7; San Francisco, CA USA.

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