•Ta2O5 has been deposited by a sputter-deposition technique at room temperature.•Electronic band configuration of Ta2O5/hydrogenated-diamond has been investigated.•Valence band offset has been found to be 1.5 ± 0.2 eV.•Large band offset makes it probably suitable for application of power devices.Ta2O5 films have been deposited on hydrogen-terminated diamond (H-diamond) by a radio-frequency sputter-deposition technique at room temperature. Electronic band structure of Ta2O5/H-diamond heterojunction has been investigated by X-ray photoelectron spectroscopy. Based on the binding energies of core-levels and valence band maximum values, valence band offset has been found to be 1.5 ± 0.2 eV for the Ta2O5/H-diamond heterointerface. It shows a type-II band configuration with conduction band offset of 2.4 ± 0.2 eV. The large ΔEV value makes the Ta2O5/H-diamond heterojunction probably suitable for the application of high power and high frequency field effect transistors.

Photoconductivity gain (PG) and persistent photoconductivity (PPC) properties observed in diamond photodetector are theoretically explained by solving rate equations which describe capture and emission processes of photo-generated hole and electron through a boron acceptor and a hole trap in a diamond epilayer and a nitrogen donor in a diamond substrate. Formation of one-sided pn-junction between the epilayer and substrate and slow hole capture rate of acceptor and hole trap levels provide accumulation of photo-generated hole in the epilayer, which produces the PG larger than the ideal responsivity. The PPC current after turning off the deep ultraviolet light is interpreted as due to the slow hole capture rate of the acceptor and trap levels.

Thermally stable, deep-ultraviolet (DUV) photodetectors are developed by fabricating a semi-transparent tungsten carbide (WC) or hafnium nitride (HfN) Schottky contact and an annealed Ti/WC Ohmic contact on a boron-doped homoepitaxial p-type diamond layer. Thermal annealing at 500 °C improves the rectifying current–voltage characteristics of the photodiode, resulting in a dramatic enhancement (by a factor of 103) of DUV responsivity at 220 nm. The discrimination ratio between DUV and visible light is measured to be as large as 106 at a reverse bias voltage as small as 2 V, and it remains almost constant after annealing at 500 °C for 5 h. The short-circuit photocurrent of the HfN Schottky photodiode gradually decreases as the annealing temperature increases, which is well explained by the dependence of the depletion layer width beneath the contact interface on the annealing temperature. In contrast, the short-circuit photocurrent of the WC photodiode is rapidly reduced as the temperature increases. The B doping is found to affect the time response property and reducing the B concentration significantly reduces the response time. Metal carbide and nitride contacts for diamond are thus useful for developing a thermally stable diamond UV photodetector.

Generation and recombination processes of electrons through phosphorus (P) donor are analyzed by admittance spectroscopy for a Ni/Au Schottky contact on an n-diamond epilayer. The dependence of capacitance and conductance frequency on temperature is interpreted by Shockley–Read–Hall statistics. The thermal ionization energy and capture cross-section of P donor are evaluated to be 0.54 ± 0.02 eV and (4.5 ± 2.0) × 10− 17 cm2, respectively. Broadening of the conductance–frequency curve is observed, which is believed to be evidence of a long Debye tail of electron distribution at the depletion layer edge.