AlN/GaN and (Al,Ga)N/AlN/GaN two-dimensional electron gas structures grown by plasma-assisted molecular-beam epitaxy

We report on an extensive study of the two-dimensional electron gas (2DEG) structures containing AlN layers. It is shown that the presence of large polarization fields in the AlN barrier layer in AlN/GaN heterostructures results in high values of the 2DEG sheet density of up to ${\text{3.6×10}}^{13}\hspace{0.5em}{\mathrm{cm}}^{\text{−2}}.$ Room-temperature sheet resistance of $\text{180\hspace{0.5em}Ω/□}$ is demonstrated in the AlN/GaN structure with a 35 Å AlN barrier. As a result of reduced alloy disorder scattering, low-temperature electron mobility is significantly enhanced in AlN/GaN heterostructures in comparison to AlGaN/GaN structures with similar values of the 2DEG sheet density. The growth of GaN cap layers on top of AlN/GaN structures with relatively thick (∼35 Å) AlN barriers is found to lead to a significant decrease in the 2DEG sheet density. However, inserting a thin (∼10 Å) AlN layer between ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ and GaN in the ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ $\text{(x∼0.2–0.45)}$ 2DEG structures does not affect the 2DEG sheet density and results in an increase of the low-temperature electron mobility in comparison to standard AlGaN/GaN structures. At room temperature, a combination of the high 2DEG sheet density of ${\text{2.15×10}}^{13}$ ${\mathrm{cm}}^{\text{−2}}$ and high electron mobility of 1500 ${\mathrm{cm}}^2\mathrm{/V s}$ in ${\mathrm{Al}}_{\text{0.37}}{\mathrm{Ga}}_{\text{0.63}}\mathrm{N/AlN/GaN}$ yielded a low sheet resistance value of $\text{194\hspace{0.5em}Ω/□.}$