Trap‐assisted photon‐enhanced thermionic emission from polycrystalline diamond films

Abstract

In this work, the photoelectron emission (PEE) in the temperature range of 298–873 K was investigated from undoped hydrogenated polycrystalline diamond (Poly‐Di) film. Poly‐Di film was deposited by hot filament chemical vapor deposition (HF CVD) method on silicon substrate. The sample was illuminated with monochromatic wavelengths of 360, 380, 400, and 440 nm and electron emission intensities were recorded as a function of temperature. The obtained results were compared with thermionic electron emission (TEE) from the same sample measured in the dark. In the temperature range 298–550 K, the photoemission intensity decreases with temperature for all wavelengths, most likely due to reduction of electron diffusion length with temperature. In the ∼550–873 K temperature range a very pronounced effect of photon‐enhanced thermionic electron emission (PETE) was detected. It was observed that the PETE process is a function of the incoming light wavelength and more prominent in the lower wavelength range. Data analysis shows that such photocurrent enhancement with temperature is well described by the Richardson–Dushman equation with low‐activation energy of 0.14 eV. Based on the obtained results the effect of PETE may be rationalized as a combined three‐step process: (i) photon excitation of electrons to the conduction band; (ii) electron–phonon scattering and electron capture by trapping centers; and (iii) thermal excitation from near conduction band trap levels into vacuum. Our results suggest that diamond can be considered as an active cathode with thermally stable negative electron affinity (NEA) and shallow trap levels as a platform for thermal electron emission concurrent with optical illumination.