Simulation of Stress Generation during GaN Lateral Epitaxial Overgrowth
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Simulation of Stress Generation during GaN Lateral Epitaxial Overgrowth Zhaohua Feng,1 Edward G. Lovell,1 Roxann L. Engelstad,1 Thomas F. Kuech,2 Susan E. Babcock3 1 Computational Mechanics Center, Mechanical Engineering Department 2 Chemical Engineering Department 3 Materials Science and Engineering Department University of Wisconsin, Madison, WI 53706, U. S. A. ABSTRACT To facilitate an understanding of defect production in gallium nitride during lateral epitaxial overgrowth, computer models have been developed to simulate the complete mechanical stress and strain fields. The virtual process included the deposition of a GaN seed layer on a sapphire substrate followed by a silicon dioxide stencil mask through which, and over which, the GaN product layer evolved and then cooled down to room temperature. Lattice mismatch and thermal strain were continuously assessed. Shear stresses on different crystallographic planes were analyzed to predict dislocation generation. INTRODUCTION Gallium nitride (GaN) is being developed for applications such as blue-light lasers and advanced data storage devices. Lateral epitaxial overgrowth (LEO) continues to be a promising technique for producing GaN films, but during the film growth, high stresses generated by thermal expansion and lattice mismatch between the GaN and sapphire substrates lead to dislocations which adversely affect the film quality and characteristics of subsequent devices. Characterizing the stress distribution at each stage of GaN growth is important for understanding the mechanism of dislocation generation, and consequently, improvements in process procedures. It is very difficult to directly measure the stresses in real time because of the stringent laboratory conditions. Computational simulation of stress evolution is more feasible. The stress distribution in the selectively overgrown GaN hexagonal structure was analyzed and compared with experimental observations by Liu et al. [1]. Finite element (FE) models simulating the stress generation and development during the entire GaN LEO process, (not just one or several special steps), are discussed in this paper. The models were analyzed by transient algorithms, and both lattice and thermal mismatch were considered simultaneously. Shear stresses on different crystallographic planes have been analyzed. Based on the shear stress assessment, the possibility of dislocation generation and development was evaluated. GaN LEO PROCESS AND SIMULATION MODELS The lateral epitaxial overgrowth process was divided into ten principal steps, as shown with the simulation cell in Figure 1. At Step 1 and 2 the seed layer was prepared. At Steps 3, 4 and 5 the SiO2 was deposited on the seed layer and windows were etched to form a stencil mask. Then the GaN was regrown through the windows, vertically and laterally (Steps 6, 7). G3.15.1
½ window Si02 GaN Al2O3 Step 1 Seed layer growth at 1050 °C.
Step 6 GaN regrowth through windows at 1100 °C.
Step 2 Cool-down to 20 °C.
Step 3 SiO2 deposition at 300 °C.
Step 7 GaN vertical and late
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