Metalorganic chemical vapor deposition of gallium nitride on sacrificial substrates
Fenwick, William Edward
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GaN-based light emitting diodes (LEDs) face several challenges if the technology is to make a significant impact on the solid state lighting market. The two most pressing of these challenges are cost and efficiency. The development of alternative substrate technologies shows promise toward addressing both of these challenges, as both GaN-based device technology and the associated metalorganic chemical vapor deposition (MOCVD) technology are already relatively mature. Zinc oxide (ZnO) and silicon (Si) are among the most promising alternative substrates for GaN epitaxy. This work focuses on the development of MOCVD growth processes to yield high quality GaN-based materials and devices on ZnO and Si. ZnO, because of its similar lattice constant and thermal expansion coefficient, is a promising substrate for growth of low defect-density GaN. The major hurdles for GaN growth on ZnO are the instability of ZnO in a hydrogen atmosphere and out-diffusion of zinc and oxygen from the substrate. A process was developed for the MOCVD growth of wurtzite GaN and InxGa1-xN on ZnO, and the structural and optical properties of these films were studied. High zinc and oxygen concentrations remained an issue, however, and the diffusion of zinc and oxygen into the subsequent GaN layer was studied more closely. Silicon is the most promising material for the development of an inexpensive, large-area substrate technology. The challenge in GaN growth on Si is the tensile strain induced by the lattice and thermal mismatch between GaN and Si. A thin atomic layer deposition (ALD)-grown Al2O3 interlayer was employed to relieve strain while also simplifying the growth process. While some strain was still observed, the oxide interlayer leads to an improvement in thin film quality and a reduction in both crack density and screw dislocation density in the GaN films. A comparison of GaN-based LEDs grown on sapphire and Al2O3/Si shows similar performance characteristics for both devices. IQE of the devices on silicon is ~32%, compared to ~37% on sapphire. These results show great promise toward an inexpensive, large-area, silicon-based substrate technology for MOCVD growth of GaN-based optoelectronic devices.