Molecular beam epitaxy of Zn-polar ZnO/BeMgZnO heterostructure field effect transistors on GaN and c-sapphire: a comparative study

Previously we have conducted researches on the molecular beam epitaxy (MBE) of Zn-polar BeMgZnO/ZnO heterostructure field effect transistor (HFET) structures on GaN templates, which exhibit both high two-dimensional gas (2DEG) density and high electron mobility with relatively low Mg contents via the strain modulation of Be and Mg co-incorporation. In this contribution, we report on the growth of the HFET structures directly on sapphire substrate by employing a buffer consisting of a rock-salt structure MgO layer, a low-temperature (LT) ZnO layer and a high-temperature (HT) ZnO layer. Compared with growth of O-polar on sapphire, in which a thin and wurtzite MgO buffer is deposited at high temperatures above 700 °C, the MgO buffer for Zn-polar growth has to be reduced to 450 °C, in order to obtain smooth interface and surface for the BeMgZnO/ZnO HFETs. The residual electron sheet concentration in Zn-polar ZnO layers is ~2×1012 cm-2 on GaN while semi-insulating Zn-polar ZnO layers on sapphire have been obtained via controlling the buffer growth conditions, which is vital to the realization of HFET device structures.