Paper EM-ThP16
Characterization of the Buried MgO/Al Interfaces in Multilayer Heterostructures used as Photocathodes with Hard X-ray Photoelectron Spectroscopy

Thursday, November 2, 2017, 6:30 pm, Room Central Hall

Early research and development of photocathode material was based on characterizing compounds with low work function and high quantum efficiency. Recent theoretical and experimental work has shown that the metal-insulator junctions can give rise to changes in the band structure at the interface, which in turn leads to a change in work function and quantum efficiency.

In addition to concerns about work function and quantum efficiency, many modern photoinjector designs also require low beam emittance. Beam emittance is an intrinsic property of the photocathode, therefore it is important to be able control the growth and quantify the factors that lead to such growth. Nemeth [Phys. Rev. Lett. 104, 046801 (2010)] used DFT to model metal- insulator multilayer junction. The model indicate that it is possible to reduce the emittance of the photoemitted beam. Velazquez et al [Appl. Surf. Sci. 360, 762 (2016)] has demonstrated that the work function of lab grown thin film multilayers had trends that match the theory. However the model predicted an exponential decrease of work function, but data suggests a linear decrease.

It has been suggested that the surface roughness of the lab grown thin film multilayers might be the main cause of the deviation from the theory. Thin films are synthesized with a custom build dual laser PLD system, and the surface roughness is systematically altered by growing at different substrate temperature. We utilized a Hard X-ray Photoelectron Spectroscopy (HAXPES) system on the MRCAT undulator beamline at the Advanced Photon Source to characterize the MgO/Ag multilayers to better understand the chemistry at the interface. Understanding the reactivity, can help us better understand how the growth transitions from uniform to rough with increasing layer number.