Electron states at electrolyte/$n$-GaN and electrolyte/$n$-InGaN interfaces

The differential capacitance and differential active conductance of rectifying contacts of $n$-GaN and $n$-In$_x$Ga$_{1-x}$N ($x\approx$ 0.15) with an electrolyte (0.2 M aqueous solutions of NaOH, NaCl, or HCl) have been studied. It was found that electron states with energies corresponding to the upper half of the energy gap of a semiconductor exist at the interface between these semiconductors and a NaOH solution. The density and characteristic recharging time of states noticeably contributing to the differential capacitance and differential active conductance at probe-voltage frequencies of 0.3–1 kHz grow with their binding energy and, for states lying at 0.15–0.3 eV below the conduction-band bottom of $n$-GaN, fall within the ranges 10$^{12}$–2 $\times$ 10$^{13}$ cm$^{-2}$eV$^{-1}$ and 10$^{-4}$–10$^{-2}$ s, respectively. For contacts with NaCl and HCl solutions, there are no states of this kind. It is assumed that the observed states are related to the adsorption of hydroxyl groups.