Comparative study of BeMgZnO/ZnO heterostructures on c-sapphire and GaN by molecular beam epitaxy

Abstract

Growth of ZnO directly on c-plane sapphire, with a large in-plane lattice mismatch of 18%, is relatively well understood for O-polar variety. However, the two-dimensional (2D) growth of Zn-polar ZnO on c-sapphire, needed for 2D electron gas formation, with low background electron density is in its infancy. While Zn-polar ZnO can be grown on GaN with the resulting small lattice mismatch (1.8%), the parallel conduction through GaN (bulk and/or surface) in fabricated BeMgZnO/ZnO heterostructure field effect transistors (HFETs) limits its utility for the time being. In this contribution, the authors report on the growth of high-quality Zn-polar BeMgZnO/ZnO HFET structures directly on sapphire substrates, in an effort to avoid the aforementioned parallel conduction, by employing an MgO and low-temperature ZnO buffer stack. The residual 2D equivalent concentration in the HFET structure on GaN is ∼2 × 1012 cm−2, which is detrimental, while highly resistive (>100 Ω.cm) Zn-polar ZnO layers with smooth surfaces on c-sapphire have been obtained via controlling the buffer stack growth conditions, which is vital to the realization of HFET device structures. Compared with the highest room temperature electron mobility of ∼250 cm2/V s in BeMgZnO/ZnO HFETs on GaN templates, a slightly lower electron mobility of ∼220 cm2/V s was achieved on c-sapphire due to a somewhat lower overall crystalline quality.