AlGaN/GaN devices with metal–semiconductor or insulator–semiconductor interfacial layers: Vacuum level step due to dipole and interface fixed charge

We have systematically investigated effects of metal–semiconductor or insulator–semiconductor interfacial layers (ILs) in AlGaN/GaN devices, where AlOx, TiOx, or NiOx is employed as an IL. From capacitance–voltage characteristics of metal/IL/AlGaN/GaN devices with a metal–semiconductor IL between the gate metal and AlGaN, it is shown that the IL modulates the threshold voltage Vth, attributed to the vacuum level step induced by the dipole of the IL. We find negative vacuum level steps for AlOx and TiOx ILs, and positive for NiOx, from which the IL dipole density is estimated for each IL material. The two-dimensional electron gas carrier concentration in the metal/IL/AlGaN/GaN devices is also modulated by the vacuum level step. On the other hand, from capacitance–voltage characteristics of metal/Al2O3/IL/AlGaN/GaN devices with an insulator–semiconductor IL between Al2O3 and AlGaN, the fixed charge density of the Al2O3/IL/AlGaN interface is evaluated by the Al2O3 thickness dependence of Vth. For AlOx and TiOx ILs, the fixed charge density is higher than that of the Al2O3/AlGaN interface with no IL, while lower for NiOx. The fixed charge density for an IL shows a positive correlation with the IL dipole density, suggesting that the fixed charge is related to the unbalanced IL dipole. Furthermore, using the conductance method, we find a low trap density of the Al2O3/IL/AlGaN interface for AlOx and NiOx ILs, in comparison with that of the Al2O3/AlGaN interface with no IL.