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 (∼20μm) GaN templates are characterized by low threading dislocation densities (∼5×10 ⁸cm ^-2) and by room temperature resistivities of ∼10 ⁸cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x≤0.05) Al _xGa _1-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 n _s≤2×10 ¹²cm ^-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 ∼2×10 ⁸cm ^-2, we have grown an Al _0.05Ga _0.95N/GaN heterostructure with a record mobility of 75000cm ²/Vs at sheet density of 1.5×10 ¹²cm ^-2 and T=4.2K. The same heterostructure design grown on a semi-insulating HVPE template yielded a peak mobility of 62000cm ²/Vs at a density of n _s=1.7×10 ¹²cm ^-2 and T=4.2K. The observation of the fractional quantum Hall effect at filling factor ν=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 Al _0.25Ga _0.75N/GaN heterostructures exhibit room temperature density of 1.0×10 ¹³cm ^-3 and mobility of ∼1500cm ²/Vs. The dc and rf characteristics of transistors grown by MBE on a HVPE template are presented.