Surfactant Effects of Indium on the Growth of AlN/GaN Distributed Bragg Reflectors via Metal Organic Vapor Phase Epitaxy

Abstract

Distributed Bragg Reflectors (DBRs) remain critical to the fabrication of various nitride based optoelectronic devices. In particular, DBRs are often employed for cavity formation in Resonant Cavity Light Emitting Diodes (RCLEDs) to enhance and obtain a more directional emission and also in Vertical Cavity Surface Emitting Lasers (VCSELs). As a result, epitaxially grown reflectors are attractive for direct integration in the device, reduced processing requirements, and the formation of narrow cavities. In the III-Nitride material system, Aluminum Nitride (AlN) and Gallium Nitride (GaN) offer a large contrast in refractive index and are therefore well suited for fabricating DBRs with high reflectivity and wide bandwidths using relatively few periods. However, material cracking arising from to the 2.4% lattice mismatch and difference in thermal expansion coefficient decreases reflectivity and is detrimental to the efficiency of subsequent device fabrication. Several techniques, such as superlattice insertion layers or the growth of Al_xIn_1-xN layers, have been employed to reduce strain and cracking in such structures. In this work, results of the use of indium as a surfactant in the Metal Organic Vapor Phase Epitaxy (MOVPE) of AlN/GaN DBRs will be discussed. Specifically, this study targets AlN/GaN DBRs with peak reflectivity at ranging from 465 nm to 540 nm. Indium has been used as a surfactant during growth by introducing trimethylindium into the system. It has been shown that crack formation is dependent on the flow of the indium precursor despite minimal indium incorporation into the lattice. Image processing techniques were used to quantify the crack length per square millimeter and it was observed that indium has a significant effect on the crack formation and can be used to reduce the total crack length in these structures by a factor of two.