Tomohiro Nishitani

Highly polarized electrons from GaAs-GaAsP and InGaAs-AlGaAs strained-layer superlattice photocathodes

GaAs–GaAsP and InGaAs–AlGaAs strained-layer superlattice photocathodes are presented as emission sources for highly polarized electron beams. The GaAs–GaAsP cathode achieved a maximum polarization of 92(±6)% with a quantum efficiency of 0.5%, while the InGaAs–AlGaAs cathode provides a higher quantum efficiency (0.7%) but a lower polarization [77(±5)%]. Criteria for achieving high polarization using superlattice photocathodes are discussed based on experimental spin-resolved quantum efficiency spectra.

High-Brightness Spin-Polarized Electron Source Using Semiconductor Photocathodes

We developed AlGaAs photocathodes with low electron affinity for long negative electron affinity (NEA) lifetime. AlGaAs photocathodes achieved 10 times longer NEA lifetime than conventional NEA-GaAs photocathodes. We estimated the appropriate superlattice structure for small conduction mini band width, high density of state in the conduction band and high splitting energy between the heavy- and light-hole bands by theoretical energy band calculation. We conclude that the AlGaAs–GaAs superlattice semiconductor is a suitable NEA-GaAs photocathode that not only has acceptable NEA lifetime but also fulfills the requirements of small energy spread of photoelectrons, high quantum yield and highly spin-polarized electrons.

Surface photovoltage effect and its time dependence in GaAs–GaAsP superlattice studied with combination of synchrotron and laser radiation

The surface photovoltage (SPV) effect and its temporal profiles in a GaAs–GaAsP superlattice (SL) were measured by core-level photoelectron spectroscopy with the combination of synchrotron radiation and laser. It was found that the SPV effect in the SL is remarkably suppressed as compared with that in a bulk GaAs. The difference in the temporal profile of the SPV between SL and bulk samples was observed in microsecond range. The suppression of the SPV effect in the negative electron affinity surfaces of the SL was also observed. It is concluded that the SL with a high-doping surface layer is suitable for the spin-polarized electron source without the SPV effect.

SURFACE-PHOTOVOLTAGE EFFECT IN A GaAs–GaAsP SUPERLATTICE STUDIED WITH COMBINATION OF SYNCHROTRON RADIATION AND THE LASER

Core-level photoelectron spectroscopy with the combination of synchrotron radiation (SR) and a laser was used for exploring the surface-photovoltage (SPV) effect and its temporal profiles in a GaAs–GaAsP superlattice (SL). It was observed that the SPV value in the SL is suppressed as compared with a bulk GaAs. However, no significant difference was found in the temporal profile between the bulk and the SL. It is suggested that the suppression of the SPV in the SL is dominantly due to the small value of band bending under thermal equilibrium.

High Spin Polarization of Conduction Band Electrons in GaAs-GaAsP Strained Layer Superlattice Fabricated as a Spin-Polarized Electron Source

We measured a spin-dependent luminescence from a GaAs–GaAsP strained layer superlattice and GaAs substrate to evaluate the spin polarization of conduction band electrons excited by circularly polarized light. The GaAs–GaAsP strained layer superlattice with a mixture of group-V elements, As and P, was considered as a suitable spin-polarized electron source because the discrepancy of the valence band was reported to be larger than that of the conduction band. The observed maximum circular polarizations of the luminescence from the GaAs–GaAsP strained layer superlattice and GaAs substrate were 68% and 15%, respectively. The dependence of the circular polarization of the luminescence on the excitation photon energy was well explained by the calculated band structure. The initial spin polarizations of conduction band electrons excited in the GaAs–GaAsP strained layer superlattice and GaAs substrate were estimated to be 95% and 46%, respectively, from the luminescence polarization, lifetime and spin relaxation time. The high initial spin polarization of conduction band electrons proved the high performance of a photocathode with the GaAs–GaAsP strained layer superlattice as the spin-polarized electron source.

Atomic hydrogen cleaning of GaAs photocathode with a load-lock system

We are constructing a new polarized electron source for Japan Linear Collider. It is designed to operate the gun at 200 kV. The “load-lock” mechanism is employed to avoid the dark current due to the Cs accumulation on the electrodes and to exchange the activated NEA photocathode quickly. We have developed superlattice photocathode which has advantages of high spin polarization, high quantum efficiency and high resistance against surface charge limit phenomenon. However, it seems difficult to clean the surface of such a thin layer photocathode by the normal etching procedure without destruction of its delicate structure if it has no As caplayer. Atomic hydrogen is expected to clean the surface of superlattice effectively. We have introduced these techniques to the new source design.

In situ Observation of Formation Process of Negative Electron Affinity Surface of GaAs by Surface Photo-Absorption

We have used surface photo-absorption (SPA) to investigate the formation of negative electron affinity (NEA) surfaces on p-GaAs during the Yo-Yo method, under an alternating supply of Cs and O2. The SPA spectra showed that the surface during the first Cs step was different from those in the following Cs and O2 steps. This suggests that the surface structure did not change after the initial surface was formed, indicating that there could be two Cs adsorption sites on the GaAs surface, which is different from previously proposed models.

Development of spin polarized electron photocathodes: GaAs-GaAsP superlattice and GaAs-AlGaAs superlattice with DBR

We have tested two kinds of spin-polarized electron photocathodes, the GaAs-GaAsP strained layer superlattice and the GaAs-AlGaAs superlattice with distributed Bragg reflector. The experimental results of these photocathodes are briefly reported.

Photocathode electron beam sources using GaN and InGaN with NEA surface

A photocathode electron source using p-type GaN and p-type InGaN semiconductors with a negative electron affinity (NEA) surface has been studied for its ability to maintain an extended NEA state. The key technology of NEA photocathodes is the formation of electric dipoles by atoms on the surface, which makes it possible for photo excited electrons in the conduction band minimum to escape into the vacuum. This means that in order to keep the electron energy spread as small as possible, the excitation photon energy should be tuned to the band gap energy. However, the NEA surface is damaged by the adsorption of residual gas and the back-bombardment of ionized residual gas by photoelectrons. The p-type GaN and InGaN semiconductors were measured a lifetime of quantum yield of excitation energy corresponding to the band gap energy in comparison to the p-type GaAs as the conventional NEA photocathode. Lifetime of NEA-photocathodes using the GaN and InGaN were 21 times and 7.7 times longer respectively than that using the GaAs.

Photoemission lifetime of a negative electron affinity gallium nitride photocathode

A photocathode electron source using p-type GaN semiconductor with a negative electron affinity (NEA) surface has been studied for its ability to maintain an extended NEA state. The key technology of NEA photocathodes is the formation of electric dipoles by cesium and gallium atoms on the surface, which makes it possible for photoexcited electrons in the conduction band minimum to escape into the vacuum. This means that in order to keep the electron energy spread as small as possible, the excitation photon energy should be tuned to the band gap energy. However, the NEA surface is damaged by the adsorption of residual gas and the back-bombardment of ionized residual gas by photoelectrons. The p-type GaN semiconductor was measured time evolution in quantum yield during NEA surface activation, and a lifetime of quantum yield of excitation energy corresponding to the band gap energy in comparison to the p-type GaAs as the conventional NEA photocathode. In NEA surface activation process, the quantum yield of the GaN was more than 3 orders of magnitude higher than that of the GaAs by only cesium deposition. The exposure amount of cesium in the NEA surface activation of the GaAs was 1.5 times as that of the GaN, even though the quantum yield of the GaAs was the same value as the GaN. Lifetime of NEA-photocathodes using the GaN was 21 times longer than that using the GaAs. The decrease of quantum yield of the GaAs was well correlated in the form of the exponential decrease function with a decrease time of 4.4 h, while the decrease of quantum yield of the GaN was well correlated in the form of the exponential decrease function with two decrease times of 47 and 174 h.