Theoretical optimization of the hole concentration for GaN photocathode

The theoretical relationship between the hole concentration (n), electron diffusion length (LD), and surface photoelectron escape probability (P) was studied. It is found that with increase of n, LD decreases and P increases. Based on the Spicer's three-step photoemission model and the quantum efficiency (QE) formula of negative electron affinity (NEA) GaN photocathode, the theoretical relationship between n and QE could be obtained. A three-dimension graph was drawn to show the relationship between QE and photon energy (E), which allowed us to observe the QE changing with n intuitively. In the whole wavelength range, QE tended to rise firstly and then fall down. The optimal n shifted to the lower concentration with the increase in wavelength due to the deeper attenuation length of the long-wavelength light. Based on the photo-electrical response of the effective band, the optimal doping concentration of 1.26 × 1017 cm−3 was obtained. The comparison results indicated that the experimental data were consistent with the simulated curves.