We present measurements of enhanced quantum efficiency (QE) in thin film photocathodes due to optical interference in the cathode-substrate multilayer. Modulations in the quantum efficiency of Cs$_{3}$Sb films grown on multilayer 3C-SiC substrates are observed over a range of visible wavelengths, and are shown to increase the QE by more than a factor of two at certain wavelengths. We derive a model to describe the modulations in QE based on a three step photoemission process incorporating cases of both constant density of states and density functional theory (DFT) derived density of states, which is shown to be in good agreement with the measurements. Predictions from the model show that the QE can be enhanced by more than a factor of four by optimizing the cathode and substrate layer thicknesses. We also find that by optimizing layer thicknesses of the cathode-substrate system in the calculation, optical interference can enhance the QE beyond optically dense films. Advantages of this interference effect for electron accelerator sources are discussed.

We present measurements of quantum efficiency (QE) modulations in CsSb and Cs3Sb photocathodes that arise from optical interference of reflections from the underlying substrate that has multiple semi-transparent layers. The photocathode films are grown on a cubic silicon carbide layer (3C-SiC) which itself is grown epitaxially on Si(100) during fabrication. We find that the QE modulates by up to a factor of two over a laser wavelength range of 30 nm, and that a modulation peak can be tuned to coincide with a desired laser wavelength by changing the thicknesses of both the photocathode and the silicon carbide layer in the substrate. A model for the QE modulations is derived and fitted to QE measurements of CsSb and Cs3Sb films, which have different indices of refraction, in addition to QE measurements of Cs3Sb films grown on 3C-SiC substrates with two different silicon carbide layer thicknesses. Good agreement is found between the model and measurements, confirming the optical interference effect can be exploited to enhance quantum efficiency at desired visible wavelengths.

Alkali-based semiconductor photocathodes are widely used as electron sources and photon detectors. The prop-erties of alkali-based semiconductor materials such as crystallinity and surface roughness fundamentally de-termine the performance merits like quantum efficiency and thermal emittance. In BNL, pulsed laser deposition (PLD) was utilized to assist the growth of alkali-based photocathode materials, providing precise control of material growth and improving film quality. In the pre-sented work, films prepared with thermal and PLD sources are compared. The film quality of K2CsSb, Cs3Sb and Cs2Te grown with PLD assisted technique are reported.

Photocathodes play an integral role in the development of electron accelerators and photon detectors. The emitted beam brightness can be limited by the surface and bulk disorder of the polycrystalline photocathode material. Epitaxial growth of photocathodes has the potential to overcome this problem and achieve high brightness electron beam. This work demonstrates the epitaxial growth of K2CsSb photocathode on varied single crystal substrates. In our study, streaky pattern aligned with latticed matched substrates from reflection high energy electron diffraction (RHEED) were observed from the K2CsSb thin film. Further, azimuthal angular dependence of the crystalline structure of the K2CsSb thin film was also observed for RHEED, which confirms the growth of the epitaxial layer with flat surface and high crystallinity. We obtained quantum efficiency (QE) of about 4.5 % at wavelength 530 nm light from the ~ 20 nm film with a roughness of ~ 0.8 nm. The stoichiometry and crystallinity of the K2CsSb thin films are confirmed by X-ray diffraction (XRD) and X-ray fluorescence (XRF). High QE over 9 % at 530 nm has been achieved for epitaxial K2CsSb photocathode thin films.