Bias voltage dependent field-emission energy distribution analysis of wide band-gap field emitters

We have studied the origin of field emission from wide band-gap semiconductors by a combination of voltage dependent field-emission energy distribution and I-V measurements. For this purpose, tip-shaped molybdenum emitters were coated with 100-1000 nm thick layers of nominally undoped diamond and cubic boron nitride (c-BN) powders. Electron energy spectra revealed that significant band bending occurred due to field penetration into wide band-gap materials. Voltage drops on the order of several volts were measured across the coatings, for applied voltages on the order of 1 kV, and a cathode-gate distance of 500 mu m. These voltage drops showed a linear dependence with the applied bias voltage for well-annealed diamond coatings and a strongly nonlinear behavior for unannealed diamond and c-BN coatings. In general, annealing of diamond coated Mo tips led to improved emission current stability and lower ''turn-on'' voltages due to the removal of oxide and the formation of conductive carbide layers between the metal and semiconductor. From the extrapolation of the linear behavior to the flat-band condition, we concluded that the emission from diamond, as well as c-BN, originated from the conduction-band minimum. Nonlinear behavior was attributed to positive space-charge accumulation at the field-emitting surface, which arose due to electron depletion by field emission from wide band-gap material. (C) 1997 American Institute of Physics.