p- andn-type cubic GaN epilayers on GaAs

Temperature-dependent Hall-effect measurements are performed on cubic GaN layers grown by plasma-assisted molecular-beam epitaxy on (100) GaAs substrates. We find that under N-rich conditions, cubic GaN films are $p$-type with hole concentrations of ≈ ${10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and mobilities of about 350 ${\mathrm{cm}}^2$/V s at room temperature. The acceptors have an activation energy of $E_A=0.445\mathrm{}\pm 0.015$ eV. Ga-rich growth conditions result in $n$-type conductivity with electron concentrations of about ${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and room-temperature mobilities of ${\mu }_n\approx 100$ ${\mathrm{cm}}^2$/Vs. Since for $n$-type samples a strong influence of the underlaying semi-insulating GaAs substrate is observed, a two-layer model is used to evaluate the Hall data, yielding a shallow donor with an activation energy of $E_D=0.16\mathrm{}\pm 0.07$ eV and a deeper donor with $E_{\mathrm{DD}}=0.60\mathrm{}\pm 0.10$ eV.