Control and elimination of cracking in aluminum(0.20) gallium(0.80) nitrogen using aluminum(x) gallium(1-x) nitrogen interlayers and application toward a near ultraviolet vertical cavity surface emitting laser

Thin film heterostructures based on aluminum gallium nitride and gallium nitride are of particular interest for applications such as ultraviolet light emitters and detectors. Due to the elastic mismatch strain between the materials, the AlGaN films tend to crack at thicknesses and aluminum compositions that are useful for device fabrication (e.g., 0.1 μm at 10% AlN mole fraction), and cracking has been shown to be deleterious to device performance. In this work, a method has been developed to control and eliminate cracking in 0.9-μm thick Al_0.20Ga_0.80N films grown on GaN by inserting a thin layer of Al_xGa_1−xN between the Al _0.20Ga_0.80N and GaN films grown by metalorganic chemical vapor deposition. The strain state of the thin interlayers is determined through a combination of in situ and ex situ characterization techniques. It is shown that the interlayer partially relaxes through the nucleation and glide of misfit dislocations to the interlayer/GaN interface, and that although the edge dislocation content in AlGaN overlayers increases by a factor of 5–10, screw dislocations from GaN underlayers are filtered by the interlayer. The number of excess threading dislocations present in AlGaN overlayers can be manipulated by controlling the amount of relaxation of the AlGaN interlayers, which can in turn be controlled by the morphology of the interlayer. It has been found that the morphology of the interlayer is affected by a combination of the strain and the aluminum surface mobility during growth, which can be controlled by adjusting growth parameters such as interlayer composition, growth temperature, interlayer thickness, and V/III ratio. These parameters are explored and guidelines for choosing an interlayer composition, thickness, and morphology are outlined based on the chosen end application for the AlGaN overlayer. Finally, the interlayer technique is applied towards the development of the world's first (at time of announcement) room temperature, quasi-continuous wave optically pumped near ultraviolet vertical cavity surface emitting laser operating at 384 nm. This device employs a crack-free epitaxially grown mirror that consists of 120 pairs of Al _0.20Ga_0.80N/GaN and is over five microns thick.