We report on holistic and systemic approach of development of Cs-free GaN photocathode structures which utilize polarization band engineering in order to allow for air stable operation and eliminate the need for cesium-based surface treatments. Physics-based simulation of band structure and Monte Carlo simulation of electron transport and emission were used to guide experimental development of photocathode structures. By using an N-polar device, the polarization charge allows for the creation of large surface band bending without the need for δ-doped capping layers. The insertion of a thin AlN interlayer allows for the creation of a quasi-band offset and additional beneficial polarization charge to create a desirable band profile. Samples of both polarities were grown and subjected to chemical surface treatments in order to account for differences in native oxide formation on Ga- and N-polar surfaces. Measured photoemission spectra show quantum efficiencies as high as 23% for a HCl-treated Cs-free N-polar photocathode, which is comparable to cesiated devices.
We report on the impact of growth conditions on surface hillock density of N-polar GaN grown on nominally on-axis (0001) sapphire substrate by metal organic chemical vapor deposition (MOCVD). Large reduction in hillock density was achieved by implementation of an optimized high temperature AlN nucleation layer and use of indium surfactant in GaN overgrowth. A reduction by more than a factor of five in hillock density from 1000 to 170 hillocks/cm −2 was achieved as a result. Crystal quality and surface morphology of the resultant GaN films were characterized by high resolution x-ray diffraction and atomic force microscopy and found to be relatively unaffected by the buffer conditions. It is also shown that the density of smaller surface features is unaffected by AlN buffer conditions.
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