We report transport and optical data for ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}}_{0.28}{\mathrm{Ga}}_{0.72}\mathrm{N}$ modulation-doped heterostructures grown by metal-organic chemical-vapor deposition. Variable temperature galvanomagnetic, resistivity, photoluminescence, and photoconductivity measurements have been performed. Evidence for potential fluctuations is provided by the observation of weakly localized transport at low temperatures, together with a negative magnetoresistance due to disorder in the interface region. The deduced localization criteria based on the theoretical modeling from Hall, resistivity and negative magnetoresistance data are in a reasonable agreement with weak-localization conditions. Additional evidence for a built-in electric field caused by the fluctuations near the heterointerface region is given by the observation of photoconductivity dips resonant with free excitons, indicating free-exciton ionization. A theoretical modeling of the transport properties under various limiting scattering conditions is provided, and compared with the experimental data for the transport time and elastic lifetime. The potential fluctuations in the two-dimensional plane from the impurity distribution only are also modeled, and the results are consistent with the experimental indications for strong potential fluctuations. It is concluded that interface roughness, dislocations, and similar structural defects have a strong influence on the transport properties of the two-dimensional electron gas in these structures.

• Received 24 November 1997

DOI:https://doi.org/10.1103/PhysRevB.58.1442