Electron mobility in modulation-doped AlGaN–GaN heterostructures

We report on the measurements of the electron mobility in modulation-doped ${\mathrm{Al}}_{\mathrm{0.2}}{\mathrm{Ga}}_{\mathrm{0.8}}\mathrm{N–GaN}$ heterostructures grown on sapphire, conducting 6H–SiC, and insulating 4H–SiC substrates as a function of the sheet electron density, $n_s,$ at the heterointerface in a wide temperature range. The mobility increases with an increase in $n_s$ up to approximately ${\text{1×10}}^{13} {\mathrm{cm}}^{\mathrm{-2}}$ and decreases with a further increase in $n_s.$ This is explained by the electron spillover at high values of $n_s$ from the two-dimensional states at the AlGaN/GaN heterointerface into the delocalized states in the doped GaN channel. The maximum electron Hall mobility in excess of $\text{2000\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V s}$ at room temperature and $\text{11\hspace{0.05em}000\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V s}$ at 4.2 K was measured in the heterostructures grown on 6H–SiC at the values of $n_s$ close to ${10}^{13} {\mathrm{cm}}^{\mathrm{-2}}$ and ${\text{7×10}}^{12} {\mathrm{cm}}^{\mathrm{-2}},$ respectively.