|Abstract:||Two-dimensional (2D) crystals are layered materials with strong in-plane covalent bonding and weak Van der Waals interlayer coupling. Stable 2D materials range from insulators, to semiconductors, to metals and even superconductors. Compared to their bulk counterparts, 2D materials show many inherent unique properties in their atomically thin layers, such as high flexibility, high surface-to-bulk ratio and absence of surface dangling bonds. These distinctive properties make 2D materials promising in many applications, including logic gates, flexible electronics, communication and biomedical sensing. A successful pathway to develop these materials into mature future devices relies on characterization of their electronic properties and incorporating them with other materials.
In this thesis, we studied the intrinsic and extrinsic properties of black phosphorus (BP) by AC conductance and capacitance methods. Black phosphorus capacitors with various types of gate dielectrics were fabricated and measured at varying temperatures. From the temperature dependence of the transition frequency, we determined the bandgap of the ~50 nm black phosphorus is 0.31 eV, while, from electrostatic models, our simulated bandgap is ~0.37 eV and doping is ~3 × 1018 cm-3. From the AC conductance of the BP/Al2O3 and BP/boron nitride (BN) capacitors, we extracted the interface trap density and time constant and found that the interface trap density in Al2O3 is about 10 times that in BP/BN capacitors, indicating the superior quality of the single-crystal BN. We confirmed that the interface trap density increases and the time constant decreases as the trap level approaches the valence band. We also found that BP is naturally p-type doped, and the doping concentration in BP/BN capacitors is much higher than that in BP/Al2O3 capacitors. This may be due to the fact that the Al2O3 deposited by ALD introduces n-type doping in the black phosphorus. These characterization techniques along with the intrinsic and extrinsic properties of the black phosphors obtained in this study will be very important for designing and optimizing electronic devices (such as metal-oxide field effect transistors, tunneling field effect transistor, and resonant tunneling diodes) based on BP.