Stress and Microstructure Evolution in Compositionally Graded Al 1-x Ga x N Buffer Layers for GaN Growth on Si
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Stress and Microstructure Evolution in Compositionally Graded Al1-xGaxN Buffer Layers for GaN Growth on Si Xiaojun Weng,1 Srinivasan Raghavan, Elizabeth C. Dickey, and Joan M. Redwing Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, U.S.A. 1 Penn State Electro-Optics Center, 559A Freeport Road, Freeport, PA 16229, U.S.A. ABSTRACT We have studied the evolution of stress and microstructure of compositionally graded Al1Ga x xN (0 ≤ x ≤1) buffer layers on (111) Si substrates with varying thicknesses. In-situ stress measurements reveal a tensile-to-compressive stress transition that occurs near the half-thickness in each buffer layer. Cross-sectional transmission electron microscopy (TEM) shows a significant reduction in threading dislocation (TD) density in the top half of the buffer layer, suggesting that the compressive stress enhances the threading dislocation annihilation. The composition of the buffer layers varies linearly with thickness, as determined by X-ray energy dispersive spectrometry (XEDS). The composition grading-induced compressive stress offsets the tensile stress introduced by microstructure evolution, thus yielding a tensile-to-compressive stress transition at x ≈ 0.5. INTRODUCTION Growth of GaN on (111) Si substrates is of particular interest due to the high quality, low cost, large wafer size, and good thermal conductivity of Si and the potential integration of highspeed and high-power nitride devices with Si microelectronics. However, due to the large lattice mismatch between GaN and Si, a high density of threading dislocations (TDs) often forms in GaN films during the heteroepitaxial growth. Furthermore, the large tensile coefficient of thermal expansion (CTE) mismatch stress combined with additional tensile growth stress arising from island coalescence and lateral grain growth leads to the development of cracks during the post-growth cooling process in thick (>~250 nm) GaN films grown on Si using thin (~100 nm) AlN buffer layers [1]. Compositionally graded AlGaN buffer layers showed potential to simultaneously reduce the densities of cracks and TDs in GaN films [2-5]. To date, the mechanisms of the reduction of CTE mismatch stress-induced cracks and TDs in GaN/AlGaN/Si heterostructures are not fully understood. Therefore, we have studied the correlations between stress, microstructure, and composition in Al1-xGaxN (0 ≤ x ≤1) buffer layers with various thicknesses, in order to understand the roles of such buffer layers in the reduction of TD and crack densities. EXPERIMENTAL Details of growth and in-situ stress measurements have been reported earlier [1,2]. In brief, both the compositionally graded Al1-xGaxN (0 ≤ x ≤1) buffer layers and the GaN films were grown by metalorganic chemical vapor deposition (MOCVD) in a vertical reactor at 1100°C and 100 Torr on 1×1 cm (111) Si substrates. The total thickness of the Al1-xGaxN buffer layer was varied from 0.1 to 2 µm and the flow rates of trim
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