Ultra-Low Emittance Iii-V Semiconductor Photocathodes

Abstract

Performance of large (km scale) electron accelerators (used for high energy physics experiments and as x-ray sources) as well as small (m scale) ultra-fast electron diffraction setups is limited by the source of electrons. Low energy (< 1 eV) electrons obtained using visible light from III-V semiconductors activated to negative electron affinity (NEA) are essential for many of these applications. Much of the physics behind the photoemission of electrons from such semiconductors is not well understood. A good understanding of this photoemission will enable design of novel materials that will have enhanced photoemission properties to improve the performance of the fore-mentioned applications. This thesis presents our theoretical, computational and experimental advances to achieve greater understanding of the photoemission process and their application to develop novel III-V semiconductor based structures that enhance photoemission properties. First, using Monte Carlo based electron transport in conjunction with the three step photoemission model, we develop a photoemission simulation that explains the experimentally observed photoemission properties of NEA III-V photoemitters. Based on this simulation, novel layered III-V semiconductors have been designed and grown using molecular beam epitaxy (MBE) to enhance photoemission characteristics. Second, we identify and discuss the various possible causes of the discrepancy between the theoretically predicted and experimentally observed energy distributions. Effects of surface roughness and work-function non-uniformities at various length scales have been explored in detail. Ab-initio calculations using density functional theory are used to obtain properties of the photoemitting surface of GaAs based photocathodes and explore possible reasons behind surface non-uniformities. Last, to improve photoemission diagnostics, a 2-D electron energy analyzer, which is capable of measuring the longitudinal (along the surface normal) and transverse (perpendicular to the surface normal) energy distributions simultaneously, has been designed and built. This energy analyzer uses the motion of low energy electrons in a strong magnetic field along with the principle of adiabatic invariance to measure the energy distribution of electrons with a resolution better than 6 meV rms.