Due to the large electron affinity of InN and In-rich InGaN and InAlN, p-type doping has proven to be a significant challenge. All nominally undoped films have n-type conductivity, and even p-doped films have an n-type surface layer, due to the accumulation of electrons on the surface of the films. Only recently did we demonstrate successful p-type doping of InN with magnesium acceptors, using techniques to account for the effect of the n-type surface layer and allow for a study of the bulk material . Using an electrolyte to make a blocking (Schottky) contact, we applied a voltage to deplete the surface charge and measure the ionized space charge (via a measurement of capacitance) beneath the surface. This showed a net acceptor concentration in the bulk of Mg-doped InN films. This measurement was supported by a study of the electronic properties of InN films subjected to energetic particle irradiation, which showed that Mg-doped films had very different behavior from undoped films. This difference was explained by the presence of p-type material in the bulk of Mg-doped films, beneath the n-type layer on the surface.
In1-xGaxN alloys of x less than or equal to 0.65 are also expected to exhibit a surface electron layer, making verification of p-type doping of the bulk difficult over this composition range. However, using the same electrolyte-based capacitance-voltage measurement as described above, we have been able to confirm successful p-type activity in Mg-doped In1-xGaxN films of x = 0.05, 0.30, 0.33, and 0.81. This is the first demonstration of p-type doping across the whole composition range of In1-xGaxN.  Analysis of the capacitance-voltage curves generated in these measurements reveals an n-type surface accumulation layer masking p-type material beneath in the x = 0.05, 0.30, and 0.33 samples. In the x = 0.81 sample, no surface accumulation layer is observed and the capacitance-voltage measurements show the material to be roughly uniformly p-type.
 R. E. Jones, K. M. Yu, S. X. Li, W. Walukiewicz, J.W. Ager III, E. E. Haller, H. Lu, and W. J. Schaff, Evidence for p-type doping of InN, Phys. Rev. Lett. 96 (2006) 125505.
 D. M. Yamaguchi, J. W. Ager III, R. E. Jones, K. M. Yu, W. Walukiewicz, E. E. Haller, H. Lu, and W. J. Schaff, P-type InGaN alloys, Appl. Phys. Lett., submitted for review.