Orientation of cracks in AlGaN epilayers with sapphires with sapphire substrates

摘要

Epitaxial layers when deposited on a substrate whose lattice constant is larger experience a tensile stress. As thicker layers are deposited the stored elastic energy increases until, as has been shown by Matthews and Klokholme [1] and Murray et al. [2], a critical thickness t_c is reaced; cracks are then formed. The energy required to form the two surfaces of the crack is harvested from the strained material within a distance t from the crack. Equating the stored elastic energy to the energy of the new surfaces allows the critical thickness to be predicted as.