Takashi Meguro

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.

Emission Characteristics for , and LaB6 Cathodes

Characteristics for electron beam emitted from , and axial orientation LaB6 cathodes after 100 hours of aging were studied. It is inferred from the emission pattern and cross-over image that electrons are emitted to a great extent from the top surface for the cathode and mainly from the conical surface for and cathodes at a bias voltage that gives maximum brightness. This inference clearly explains the electron beam characteristics such as brightness, emission angle and total beam current.

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.

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.

Superlattice Photocathode with High Brightness and Long NEA-Surface Lifetime

We have suggested that a small momentum spread of photoelectrons and high quantum efficiency can be obtained concurrently by a photocathode using a semiconductor with a superlattice instead of a bulk. We have begun to search for the suitable semiconductor material of a superlattice photocathode for maintaining the surface with a negative electron affinity state for a long time. We measured quantum efficiency degradation of photocathodes using GaAs and AlGaAs semiconductors with various electron affinities. The AlGaAs semiconductor had a quantum efficiency lifetime of 10 times long compared with the GaAs semiconductor. We found that the AlGaAs semiconductor was the suitable material for the superlattice photocathode with the surface maintaining a negative electron affinity state for a long time.

Preparation of Ga-terminated negative electron affinity-GaAs (100) surface by HCl-isopropanol treatment for nanoanalysis by scanning tunneling microscopy

Negative electron affinity (NEA) surfaces can emit electrons by low-energy light illumination that is nearly equal to the bandgap energy of a semiconductor because NEA surfaces lower the vacuum level to below the conduction-band minimum. In particular, NEA-GaAs surfaces show distinct characteristics such as high spin polarization, low emittance, short pulsed operation, and high intensity. NEA surfaces are formed by alternating application of Cs and O2 on a clean GaAs surface. Scanning tunneling microscopy (STM) was used to investigate the surface states of NEA-GaAs (100) surfaces prepared using HCl-isopropanol treatment followed by annealing in an ultrahigh vacuum. The results indicated remarkable improvement in the surface quality of the GaAs (100). The authors have been studying the relationship between electron emission properties and the adsorption structures of Cs on Ga-terminated GaAs surfaces. Here, they report the first observation of NEA-Ga-terminated surfaces with Cs adsorption using STM.Negative electron affinity (NEA) surfaces can emit electrons by low-energy light illumination that is nearly equal to the bandgap energy of a semiconductor because NEA surfaces lower the vacuum level to below the conduction-band minimum. In particular, NEA-GaAs surfaces show distinct characteristics such as high spin polarization, low emittance, short pulsed operation, and high intensity. NEA surfaces are formed by alternating application of Cs and O2 on a clean GaAs surface. Scanning tunneling microscopy (STM) was used to investigate the surface states of NEA-GaAs (100) surfaces prepared using HCl-isopropanol treatment followed by annealing in an ultrahigh vacuum. The results indicated remarkable improvement in the surface quality of the GaAs (100). The authors have been studying the relationship between electron emission properties and the adsorption structures of Cs on Ga-terminated GaAs surfaces. Here, they report the first observation of NEA-Ga-terminated surfaces with Cs adsorption using STM.

Analysis of negative electron affinity InGaN photocathode by temperature-programed desorption method

A III–V semiconductor with a few monolayers of alkali metals (e.g., Cs) forms a negative electron affinity (NEA) surface, for which the vacuum level lies below the conduction band minimum of the base semiconductor. The photocathodes that form an NEA surface (NEA photocathodes) have various advantages, such as low emittance, a large current, high spin polarization, and ultrashort pulsed operation. The NEA-InGaN photocathode, which is sensitive to blue light, has been studied as a material for the next-generation robust photocathode. However, the proper conditions for forming NEA surfaces remain unknown. The authors consider whether the suitable process for NEA surfaces can be understood by investigating the relationship between the electron emission and the adsorption state of alkali metals. In this study, the relationship between the electron emission and the adsorption state of Cs on the p-type InGaN (0001) was analyzed by the temperature-programed desorption (TPD) method using a quadrupole mass spectrometer. From the results of the TPD measurements, it was shown that there were several adsorption states of Cs on InGaN. The quantum efficiency (QE), which indicates the ratio of emitted electrons to incident photons, increased while Cs desorption occurred. The authors divided the formation process of an NEA surface into several sections to investigate the adsorption states of Cs related to the electron emission and to discuss the reasons why the QE increased despite the desorbed Cs. From the results of the NEA activation in each section, it was shown that there were sections where the QE increased by reacting with O2 after Cs supply stopped. There is a possibility that several layers reacting with O2 and those not reacting with O2 are formed by performing NEA activation until the QE saturates. From the results of the TPD measurements in each section, it was suggested that there was a Cs peak at above 700 °C when the TPD method was carried out immediately after confirming the electron emission. Therefore, the adsorption state of Cs that formed a peak at above 700 °C had a close relation to the electron emission. It is considered that the increase of the QE in the TPD was affected by adsorbed Cs compounds that reacted with O2. Although the mechanism is not understood, it is known that the QE was increased by the reaction of Cs adsorbed compounds and O2 in previous studies. It was suspected that layers that reacted with O2 appeared from TPD and then the QE increased by reacting with O2.A III–V semiconductor with a few monolayers of alkali metals (e.g., Cs) forms a negative electron affinity (NEA) surface, for which the vacuum level lies below the conduction band minimum of the base semiconductor. The photocathodes that form an NEA surface (NEA photocathodes) have various advantages, such as low emittance, a large current, high spin polarization, and ultrashort pulsed operation. The NEA-InGaN photocathode, which is sensitive to blue light, has been studied as a material for the next-generation robust photocathode. However, the proper conditions for forming NEA surfaces remain unknown. The authors consider whether the suitable process for NEA surfaces can be understood by investigating the relationship between the electron emission and the adsorption state of alkali metals. In this study, the relationship between the electron emission and the adsorption state of Cs on the p-type InGaN (0001) was analyzed by the temperature-programed desorption (TPD) method using a quadrupole mass spectrom...

Electron Beam Sources using InGaN Semiconductor Photocathodes for Single-shot Imaging Electron Microscope

1. Institute for Advanced Research, Nagoya University, Nagoya, Japan 2. Synchrotron Radiation Research center, Nagoya University, Nagoya, Japan 3. Graduate School of Sciences, The Structural Biology Research Center and Division of Biological Science, Nagoya University, Nagoya, Japan 4. JEOL Ltd., Tokyo, Japan 5. Department of Physics, Faculty of Science Division II, Tokyo University of Science, Tokyo, Japan 6. College of Science and Engineering, Aoyama Gakuin University, Sagamihara-shi, Japan 7. Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya, Japan.