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 $\text{(∼20\hspace{0.17em}μ}\mathrm{m})$ GaN templates are characterized by low threading dislocation densities ${\text{(∼5×10}}^8\hspace{0.17em}{\mathrm{cm}}^{\text{−2}})$ and by room temperature resistivities of ${\text{∼10}}^8\mathrm{\hspace{0.17em}\Omega }\mathrm{cm}.$ We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing $\text{(x⩽0.05)}$ ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1-x}}\mathrm{N/GaN}$ 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 $n_s{\text{⩽2×10}}^{12}\hspace{0.17em}{\mathrm{cm}}^{\text{−2}}.$ 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 ${\text{∼2×10}}^8\hspace{0.17em}{\mathrm{cm}}^{\text{−2}},$ we have grown an ${\mathrm{Al}}_{\text{0.05}}{\mathrm{Ga}}_{\text{0.95}}\mathrm{N/GaN}$ heterostructure with a record mobility of $\text{75\hspace{0.05em}000\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V s}$ at sheet density of ${\text{1.5×10}}^{12}\hspace{0.17em}{\mathrm{cm}}^{\text{−2}}$ and $\text{T=4.2\hspace{0.17em}}\mathrm{K}.$ The same heterostructure design grown on a semi-insulating HVPE template yielded a peak mobility of $\text{62\hspace{0.05em}000\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V s}$ at a density of $n_s{\text{=1.7×10}}^{12}\hspace{0.17em}{\mathrm{cm}}^{\text{−2}}$ and $\text{T=4.2\hspace{0.17em}}\mathrm{K}.$ The observation of the fractional quantum Hall effect at filling factor $\text{ν=5/3}$ 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 ${\mathrm{Al}}_{\text{0.25}}{\mathrm{Ga}}_{\text{0.75}}\mathrm{N/GaN}$ heterostructures exhibit room temperature density of ${\text{1.0×10}}^{13}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ and mobility of $\text{∼1500\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V s}.$ The dc and rf characteristics of transistors grown by MBE on a HVPE template are presented.