Development of UV-photocathodes using GaN film on Si substrate

摘要

We developed GaN photocathodes for detecting ultraviolet radiation by using Mg-doped GaN. Crack-free, 200 nm thick GaN:Mg layers were grown by metal organic chemical vapor phase epitaxy (MOVPE) on a GaN template having a structure of undoped GaN/(AlN/GaN) multilayers on Si (111) substrate. The Mg concentration was varied in the range from 7 × 10~(18) to 7 × 10~(19) cm~(-3). The grown film was mounted in a phototube to operate in reflection mode; i.e. the light was incident from the photoemission side. The photoemission surface was activated by sequential adsorption of cesium and oxygen to reduce electron affinity, ensuring efficient electron emission. Photoemission spectrum was measured in the range of 200-600 nm. We found that the quantum efficiency of photoemission was affected by the crystallinity of GaN:Mg, depending on the concentration of Mg dopant and the growth pressure of GaN:Mg top photoemissive layer. The lower Mg concentration and higher growth pressure resulted in higher quantum efficiency. The obtained maximum quantum efficiency was 45% at 200 nm (6.2 eV) and 25% at 350 nm (3.54 eV). The elimination ratio between visible and UV light was 4 decades and the slope of cutoff was 10 nm per decade.