Low Dislocation Density AlGaN Epilayers for UV Laser Diodes and Devices for Power Electronics

The AlGaN alloy system has been the material system of choice for development of solid-state UV emitters and has promising properties for high-voltage power electronics.  The performance and reliability of both optical emitters and power devices are critically dependent on the density of threading dislocations.  While the binary substrates, GaN and AlN, are becoming commercially available, only a limited range of AlGaN compositions can be grown pseudomorphicly to device-relevant thicknesses of a micron or more.  This his restricts the range of heterostructures available for designing advanced devices, and creates a need for a ternary-like pseudo-substrate with a tunable lattice constant. 

 We present a method for fabricating crack-free Al_xGa_1-xN templates over the entire alloy range with threading dislocation densities (TDD) in the low 10⁸ cm^-2 ranges.  We employ Al_xGa_1-xN overgrowth of previously grown Al_xGa_1-xN/AlN/sapphire templates that have been patterned with parallel trenches on a 2 μm pitch consisting of submicron-wide-mesas.¹  Following overgrowth, the remaining threading dislocations are uniformly distributed over 2-inch-wafer-size areas enabling large area devices such as LEDs and PIN diodes to consist of only low TDD material.  As a result, laser diodes do not need to be aligned to the underlying trench pattern.  Wafer bow is reduced to less than 15 μm and epilayer cracking is essentially eliminated by using 1.3 mm-thick sapphire substrates (three times thicker than typical 2-inch wafers).  The utility of the described overgrowth process for various alloy compositions is demonstrated by growing 5 to 10 μm thick Al_0.3Ga_0.7N, Al_0.7Ga_0.3N and AlN epilayers on sapphire substrates.  While AlN substrates continue to improve, they are still relatively immature, expensive, and small in area compared to GaN substrates.  Using the described overgrowth process and a modest growth temperature of 1100 °C, AlN epilayers grown on sapphire have achieved FWHM linewidths < 300 arc seconds for the (10-11) x-ray diffraction reflection and an RMS roughness of 1 Å over a 10 x 10 μm²region.  Such x-ray linewidth and surface roughness values are comparable to AlN grown in specialized MOCVD reactors capable of growth in excess of 1300 °C. 

These low TDD AlGaN layers have been used for various devices.  For example, the AlN overgrowth templates are used to demonstrate 2DEG formation in MODFETs formed by Al_0.85Ga_0.15N / Al_0.7Ga_0.3N heterostructures.   Additionally, UV laser diodes were grown on Al_0.3Ga_0.7N overgrowth templates that achieved room-temperature, pulsed-current operation with emission at 352 nm and peak output powers > 2.5 mW per facet.  Finally, we will also discuss the potential of using >10 μm thick Al_0.3Ga_0.7N overgrowth templates for PIN diodes. 

[1.]  Allerman, et. al. J. Crystal Growth 38876 (2014).

 This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.