Dielectric-induced interface states in black phosphorus and tungsten diselenide capacitors

The interfaces between two-dimensional (2D) materials and gate dielectrics play an important role in the performance and reliability of 2D electronic devices. In this work, we systematically studied the capacitance and interface states of a narrow bandgap material (black phosphorus, BP) and an intermediate bandgap material (tungsten diselenide, WSe₂). We found that their capacitance-voltage (CV) characteristics are drastically different. The BP capacitor CVs demonstrate ambipolar and low-frequency properties, while WSe₂ capacitor CVs shows unipolar (p-type) and high-frequency behavior. The narrow bandgap of BP (∼0.3 eV) enables large amounts of minority carriers, low generation-recombination resistance, and short minority carrier lifetime, giving low-frequency behavior of the CVs, while the wide bandgap of WSe₂ (∼1.21 eV) leads to the high-frequency behavior of the CVs. The nearly intrinsic (low) doping of the BP flake results in ambipolar CVs which are symmetric about the midgap. The naturally p-type doping in WSe₂ gives unipolar CVs similar to p-type silicon. In both materials, the interface state density is as high as 10¹³ cm^-2eV^-1. Although 2D materials are free of dangling bonds, their intimate contact with high-k dielectrics like Al₂O₃ could generate a larger number of interface states and degrades the device performance. Hexagonal boron nitride (hBN) effectively reduces the interface state density as dielectrics. The interface state for BP/hBN capacitor shows much lower density than counterpart with Al₂O₃ gate dielectric. We also found that the interface state density increases exponentially with the gate voltage when the surface Fermi level is swept from the midgap toward the band edge.