3.3.3 Hole Transport

As in GaN, p-type conductivity in InN has proven to be difficult to achieve. Even though the valence band edge lies 1.6 eV below the Fermi level stabilization energy [236], the low position of the conduction band edge makes efficient p-type doping very difficult. Another issue is the pinning of the surface Fermi level above the conduction band edge, due to native donor defects, which leads to a n-type accumulation layer at the surface [237,238]. Any study of p-type bulk material has to isolate the effects of this accumulation layer. This was achieved by Jones et al., who provided the first indirect evidence of a net concentration of acceptors, but who were however unable to verify the presence of free holes [239]. Later works were not yet able to demonstrate net p-type conductivity [240], but an activation energy for the Mg acceptor of about 61 meV was extracted by photoluminescence measurements [241]. Using the same value for the activation energy Wang et al. [242] calculated a hole mobility in the range of 17$-$36 cm$^2$/Vs for a hole concentration of about (1.4$-$3.0)$\times$10$^{18}$ cm$^{-3}$. However, they used a suggested effective hole mass value (0.42m$_0$) [243], which was not experimentally confirmed. The same value was adopted by Fujiwara et al. [244], who reported mobilities of 25$-$70 cm$^2$/Vs. Recent works agree [245], that free holes can be detected only for moderate Mg contents. Most of the evidences of electrical conductance related to free electrons are yet to be confirmed.