Interface charge inhomogeneity at Al2O3/OH-diamond (111) characterized by time-resolved scanning nonlinear dielectric microscopy

Inversion-channel diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) suffer from channel mobility far below the bulk value, yet the microscopic origin of this limitation has not been fully elucidated. Because carriers in the inversion layer are confined close to the MOS interface, spatial inhomogeneity of interface charges is expected to play an important role through Coulomb scattering. However, such effects cannot be fully assessed by conventional macroscopic capacitance–voltage (CV) profiling. Here, we investigated local charge states at the Al2O3/OH-diamond (111) interface used in inversion-channel diamond MOSFETs by local CV profiling based on time-resolved scanning nonlinear dielectric microscopy. The measured local CV profiles exhibit substantial spatial fluctuations with clustered non-uniformity. The corresponding interface charge fluctuation, estimated from the local CV profiles, was smaller than that previously obtained for an as-oxidized SiO2/SiC interface but larger than that for a nitrided SiO2/SiC interface. Numerical device simulations incorporating the experimental data indicate that these spatially fluctuating interface charges can induce Coulomb scattering more strongly than expected from the macroscopic interface characterization. This leads to a significant reduction in the peak field-effect mobility.