Growth stresses and cracking in GaN films on (111) Si grown by metalorganic chemical vapor deposition. II. Graded AlGaN buffer layers

Abstract

Growth stress and critical thickness for cracking on cooling in GaN films deposited by metalorganic chemical vapor deposition on (111) Si using graded AlGaN (from AlN to GaN) buffer layers, 0.1–2μm thick, were examined. During the growth of the graded buffer layers, the incremental growth stress was observed to change from tension (∼+1to+1.5GPa) to compression (∼−1to−1.7GPa). An interruption in the composition grading during the compressive segment, followed by continued growth of the AlGaN composition at the interruption, resulted in the eventual transition to a tensile stress in films as thick as 0.75μm. A similar stress evolution, a reduction in the value of the initial compressive stress with thickness, was observed during the growth of 1-μm-thick GaN layers on 2-μm-thick graded AlGaN buffer layers. Such stress evolution, at thickness far beyond coalescence, cannot be explained by initial grain coalescence stresses alone. A combination of compressive stress generation due to the change in AlGaN composition and tensile stress generation due to the lateral increase in grain size with thickness are used to explain these results. In the GaN layer, the thickness at which the compressive-to-tensile stress transition takes place increases and the magnitude of the final stress decreases with an increase in the thickness of the graded AlGaN buffer layer. X-ray diffraction results show that this is due to increasing accommodation of the tensile stress generating microstructural changes within the thickness of the AlGaN buffer layer, thus reducing the level of tensile stress in the subsequent GaN layer. A 1-μm-thick graded AlGaN buffer layer enabled the growth of a 1-μm-thick GaN layer that is crack-free after dicing at room temperature.