Record high electron mobilities in high-purity GaN by eliminating C-induced mobility collapse

Impact of carbon impurities on the electrical properties of lightly doped n-type GaN [electron concentration ∼(1–2)×1015 cm−3 at room temperature] was investigated using temperature-dependent Hall effect measurements. GaN crystals with a threading dislocation density of (1–3)×106 cm−2 were grown by our originally developed quartz-free hydride vapor phase epitaxy method, which enabled the background Si, O, and C concentrations to be suppressed to below the mid-1014 cm−3 range. We prepared three samples with different C concentrations ([C]) by intentional C doping. The C incorporation induced severe mobility collapse at temperatures greater than 60 K, where the measured mobility decreased and deviated from the theoretical value as [C] increased. The mobility collapse was eliminated for the purest GaN crystal with [C] ∼1.4×1014 cm−3, exhibiting a record high room-temperature mobility of 1480 cm2/(V·s), as well as a record high maximum mobility of 14 300 cm2/(V·s) at 62 K. The latter was almost double the previous record. We found that the overall mobility behavior can be well reproduced by adding an empirical [C]-dependent mobility component expressed as μUNK=K/TnUNK with 1≤nUNK≤2 and K∝C−1 to the conventional mobility theory (phonon and impurity scattering). Although the mechanism of the component remains uncertain, our findings provide insight into the unsolved issue of mobility collapse.