A thin film P-type gallium nitride photocathode: Prospect for a high performance electron emitter.

摘要

The study of the electronic structure of gallium nitride (GaN) surfaces is undertaken in order to evaluate a wide band gap photocathode as a high performance electron source. In considering detailed studies targeting the starting surface of GaN (0001) and the nature of the activation layers using Cs only and Cs/O, an efficient and robust emitter is proposed.; Achieving clean surfaces is a major and challenging requirement for the study of any semiconductor surface. The use of synchrotron radiation (SR) to probe the electronic structure of the GaN (0001) surface that has undergone wet chemical cleaning sequences followed by heating is described. The refractive properties of GaN allow a simple and non-destructive surface preparation to be successful in removing C and O contaminants, involving chemical cleaning followed by thermal desorption.; The electron affinity for the clean surface measured is 3.3 +/− 0.2 eV using SRPES. The maximum reduction achieved in the electron affinity is approximately 3.0 +/− 0.2 eV by depositing ¾ ML of cesium at room temperature. In addition, the threshold for photoemission emission in spectral yield curves is at the band gap energy of GaN (3.4eV), demonstrating the NEA activation of GaN with Cs alone.; The chemistry of the traditional co-deposited cesium and oxygen (Cs/0) adlayer commonly used with small band gap III-V's is also investigated. These are the first studies reporting a molecular form of oxygen incorporated in the thin NEA activation layers and the charge state is found to determine the net dipole strength leading to the maximum yield obtainable with the Cs/O activation.; Last, a comparative study is performed between Cs/0 activated GaN (0001) and GaAs (100) investigating the decay in the quantum yield using low photon density. The quantum efficiency (QE) from the GaN (0001) photocathode remains constant within a few percent over a 10 hour period at ≥20% QE and decays by less than a factor of 2 over the subsequent 7 hours. In comparison to GaAs, the delayed and slowly decreasing monotonic yield for GaN gives preliminary evidence that wide band gap materials are more robust emitters.