Overview and Progress Toward High-Efficiency, Air Stable, Cs-Free III-Nitride Photocathode Detectors

We review the recent progress to achieve air stable III-nitride photocathodes for applications as photon detectors. High conductivity
p
-type films are critically important to realize high quantum efficiency (QE) photocathodes with effective negative electron affinity (NEA) and downward surface band bending with narrow surface depletion width. Initial reports of III-nitride photocathodes utilize a Cs-surface activation to achieve effective NEA. To attain air stable, Cs-free photocathodes, novel energy band engineering has been shown using Si delta-doping and an
n
⁺
-GaN cap on the surface of a Ga-polar
p
-GaN layer. Improvement of QE has been achieved utilizing the N-polarity of III-nitrides due to advantageous depletion and polarization charges at the surface. High QE > 25% has been demonstrated by improvements in
p
-type conductivity with improved Mg-dopant incorporation in N-polar hillock structures and by control over unintentional impurity incorporation and distribution. Further, the importance of achieving high
p
-type conductivity films is further shown through reviewing recent simulations of GaN photocathodes with varied band bending and hole concentrations. Through comparison of experimental photoemission with Monte Carlo simulations, band structure parameters such as electron effective mass in the conduction band valleys have been elucidated.