This technology relates to semiconductor devices and growth techniques in the field of III-N semiconductors. For example, the technology provides a semiconductor device with a substrate comprising a groove. A buffer layer is formed on a surface of the groove. The buffer layer has at least one material chosen from AIN, GaN or AlxGa1-xN, where x is between zero and one. An epitaxially grown semiconductor material is disposed over the buffer layer, and at least a portion of the epitaxially grown semiconductor material having a cubic crystalline phase structure.