Progress in Cesium-Free III-Nitride Photocathodes Based on Control of Polarization Charge

III-nitride photocathodes have many applications for ultraviolet (UV) detection; the wide bandgaps available in the AlGaN family provide intrinsic solar blindness, and the wavelength cutoff may be tuned by control of composition. Negative electron affinity (NEA) is desirable for these structures in order to maximize quantum efficiency (QE). Normally surface cesiation is used to create low or negative electron affinity; however, the resulting reactive surface must be protected from air during fabrication and use, necessitating a sealed-tube configuration. Cesium-free photocathodes would offer lower cost, improved robustness, and greater chemical stability, in addition to the major advantage of higher QE. We report on the use of polarization engineering in order to achieve permanent low electron affinity without the use of Cs. We will discuss progress in design, fabrication, and characterization of polarization-engineered III-nitride photocathodes. An important component of these designs is the use of N-polar GaN and AlGaN. The nitride polarity affects the interface and surface polarization charge, and the ability to achieve NEA depends critically on this charge. We will present results demonstrating high (>10%) QE for non-cesiated N-polar GaN photocathodes.