As large native substrates for group III-Nitride (III-N) semiconductors are not yet widely available, III-N films, such as GaN and its alloys, are currently grown by heteroepitaxy on suitable non-III-N substrates. Typically, the films are grown on sapphire (Al2O3), silicon carbide (SiC), or silicon substrates. Silicon substrates are emerging as a particularly attractive substrate candidate for III-N layers due to their low cost, wide availability, large wafer sizes, thermal properties, and ease of integration with silicon-based electronics. However, due to the large lattice mismatch and thermal expansion coefficient mismatch between silicon and III-N materials, there is typically a net tensile stress in III-N epitaxial layers deposited directly on silicon substrates.
This mismatch can result in cracking of the layers and thus thick III-N layers that are crack-free and that exhibit adequate structural quality can be difficult to grow on silicon substrates. For example, the maximum thickness of III-N layers that can be grown without sustaining substantial defects may be limited. If the III-N layers are grown too thick, tensile stress that develops in the layers upon cooling becomes substantial, which can cause cracking of the layers. In many applications in which III-N heteroepitaxial layers are used, it may be necessary that very thick III-N layers of adequate quality be grown on the foreign substrates.