Weak localization in compositionally graded Al_xGa_1-xN

We report on the observation of weak localization in the compositionally graded Al_xGa_1-xN. The polar discontinuity at the epitaxial interface of dissimilar nitrides could introduce a two-dimensional electron gas. Compositionally graded structures, however, could introduce similar sheet carrier density distributed across the film. The various heterostructures were grown on AlN single crystal substrate using metal-organic chemical vapor deposition. The concentration of aluminum was graded, from x=0.56 to x=1, uniformly across the film thickness. The Hall resolved carrier density (1.35×10¹⁸ cm^-3) closely matches the theory predicted values, hinting an extremely low density of compensating defects. Room temperature capacitance measurements reveal a three-dimensional distribution of charge carriers. The sheet resistance shows a metallic behavior (dR_s/dT>0) with negligible Hall carrier freeze-out, cooling down to helium liquid temperature. Signatures of the weak localization were observed in the longitudinal magnetoresistance below 30 K. The magnetoresistance shows a well-known parabolic behavior at higher temperatures. The low-temperature longitudinal magnetoresistance shows good agreement a two-dimensional weak localization model, suggesting a dimensional crossover with temperature.