High mobility AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy

We report on an extensive study of the growth and transport properties of the two-dimensional electron gas (2DEG) confined at the interface of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on thick, semi-insulating GaN templates prepared by hydride vapor phase epitaxy (HVPE). Thick (~20m) GaN templates are characterized by low threading dislocation densities (~5x108cm-2) and by room temperature resistivities of ~108cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x0.05) AlxGa1-xN/GaN heterostructures is investigated. The use of low Al containing heterostructures facilitates the study of the 2DEG transport properties in the previously unexplored regime of carrier density ns2x1012cm-2. We detail the impact of MBE growth conditions on low temperature mobility. Using an undoped HVPE template that was residually n-type at room temperature and characterized an unusually low dislocation density of ~2x108cm-2, we have grown an Al0.05Ga0.95N/GaN heterostructure with a record mobility of 75,000cm2/Vs at sheet density of 1.5x1012cm-2 and T=4.2K. The same heterostructure design grown on a semi-insulating HVPE template yielded a peak mobility of 62,000cm2/Vs at a density ns=1.7x1012cm-2 and T=4.2K. The first observation of the fractional quantum Hall effect at filling factor =5/3 in the AlGaN/GaN system is reported. It is also demonstrated that thick semi-insulating GaN templates grown by HVPE are a viable substrate for the growth of high electron mobility transistors. Typical Al0.25Ga0.75N/GaN heterostructures exhibit room temperature density of 1.2x1013cm-3 and mobility of ~1500cm2/Vs. The DC and RF characteristics of the first transistors grown by MBE on a HVPE template are presented.