Comparison study on light trapping of GaN UV photocathode

Abstract

To obtain optimal light absorption of GaN photocathode, the absorption spectra of GaN photocathode have been studied under different emission layer thicknesses, buffer layer thicknesses and buffer layer materials in detail based on finite difference time domain (FDTD) method. The results show that, whether photocathode is reflection-mode or transmission-mode, its absorption spectra has same characteristics and trends. The appropriate thickness of GaN emission layer is in range of 100–150 nm, and at this time, maximum absorption efficiency can be achieved. For GaN photocathode with ultra-thin emission layer, AlN buffer layer becomes an important factor affecting light trapping. Compared with thickness of buffer layer, different Al composition in buffer layer cause a more significant change in absorption spectrum. In addition, absorption spectrum of AlxGa1−xN with variable Al composition as emission layer has also been studied. It is found that although emission layer with variable Al compositions can generate a built-in electric field to help electrons escape, it would reduce trapping of incident light, narrow absorption sideband, and shift cutoff wavelength below 300 nm, which is suitable for “sun blind” detection applications. Our research can provide theoretical guidance for designing GaN photocathodes with optimal light absorption properties.