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 [2].  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.

     In_1-xGa_xN 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 In_1-xGa_xN 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 In_1-xGa_xN. [3]  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.

[2] 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.

[3] 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.