III-nitride based photocathodes have been the subject of much research in photoemissive devices for ultraviolet (UV) detection in astronomy and defense applications. In order to achieve high quantum efficiency (QE), negative electron affinity (NEA) is necessary to allow for carriers that have relaxed to the conduction band minimum to escape. NEA in III-nitride UV photocathodes is conventionally reached via cesium-based surface treatment of p-type GaN. However, this treatment is highly reactive in air and photocathodes using this technology have been reported to suffer from chemical instability and QE degradation over time.
Recent work has shown the potential to take advantage of the spontaneous and piezoelectric polarization exhibited by III-nitride materials in order to achieve permanent NEA in AlGaN-based photocathode structures without the need for cesiation. The N-polar orientation has potential for improved and expanded device design space due to the reversal of the built-in and stress-induced polarization fields. However, achieving smooth high-quality N-polar material has traditionally been a challenge due to the formation of large hexagonal hillock structures on a typical N-polar surface. Furthermore, achieving high-conductivity p-type material is crucial for high efficiency photocathodes (among other devices), but has been a long-standing challenge in III-nitrides due to the high ionization energy of the Mg dopant and tendancy for self-compensation.
Rapid development and optimization of device design requires accurate modeling of the photoemission process to shorten the feedback loop, but the complexity of the photoemission process makes development of accurate models difficult. Traditionally, it has been assumed that photo-excited hot-carriers are thermalized to the conduction band minimum during transport, which allows for simplified modeling. This assumption breaks down for high-energy excitation or reflection-mode photocathodes, and more accurate treatments are needed. The development in recent years of accurate Monte Carlo modeling of III-nitrides enables simulation of hot-carrier transport. Application of Monte Carlo transport for photoemission modeling has recently been studied in Cs-treated NEA GaAs photocathodes with close agreement to experimental results.
This work outlines development of Cs-free III-nitride photocathodes via use of surface treatments and materials optimization resulting in peak quantum efficiencies of 23\% for N-polar devices. This high QE is comparable to results from cesiated devices. Physics-based device simulations show the promise of N-polar orientation, with the ability to obtain a similar band profile as Ga-polar with 2 orders of magnitude lower p-doping in addition to the potential for substantial narrowing of surface band bending region. Materials development and optimization focused on two aspects: surface morphology and doping efficiency. These optimizations have resulted in a decrease in undesirable surface features by over 3 orders of magnitude via the use of indium surfactant and buffer optimization. For Ga-polar photocathode structures, free hole concentrations in excess of 1 x 1018 cm–3 was achieved in AlGaN absorber of 28% Al composition, via the use of a pulsed deposition technique, representing over a 3 times increase from traditional epitaxy method. Application of the same technique to N-polar films showed reduced doping effectiveness as compared to Ga-polar films due to differences in surface configuration. As part of this study, Monte Carlo simulator based on the open-sourced GNU Archimedes was developed. The Monte Carlo simulation was developed to support III-nitrides and photoexcitation and emission processes in devices based on this material system. Simulated results showed close agreement with experimentally measured values, validating the technique. Results point towards several important factors affecting emission behavior and suggest future research focus. Additionally, photoemission simulation gives evidence towards proper satellite valley band parameters which are subject to much uncertainty.
|Advisor:||Shahedipour-Sandvik, Fatemeh Shadi|
|Commitee:||Bell, Lloyd D., Lifshin, Eric, Thiel, Bradley, Xue, Alex|
|School:||State University of New York at Albany|
|Department:||Nanoscale Science and Engineering-Nanoscale Engineering|
|School Location:||United States -- New York|
|Source:||DAI-B 79/09(E), Dissertation Abstracts International|
|Keywords:||Cs-free ultraviolet III-nitride photocathodes|
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