Fumiaki Ichihashi

High-Performance Spin-Polarized Photocathodes Using a GaAs/GaAsP Strain-Compensated Superlattice

Optimized transmission-type photocathodes with a GaAs/GaAsP strain-compensated superlattice were developed. The strain-compensated superlattice structures were of high crystal quality, and electron beams from the photocathodes had a maximum spin polarization of 92% and a quantum efficiency of 0.4% without an antireflection coating. The strain-compensated superlattice structure effectively prevented strain relaxation, and the high spin polarization was maintained up to a superlattice layer thickness of 300 nm. Increasing the superlattice layer thickness effectively improved the quantum efficiency while keeping the super high-brightness.

Electron spectroscopy of conduction electrons excited by visible light utilizing NEA surface

In this paper we propose an angle-resolved photoemission spectroscopy to observe the conduction electrons emitted from a surface of negative electron affinity state. In actual, we have measured conduction electrons in a GaAs bulk crystal and obtained the electron dispersion around Γ point. In addition, we could also observe hot electrons excited by the light which energy was much larger than band gap energy. These results suggest that the method we proposed is one of the most powerful tools for the evaluation method of the conduction band.

Temperature dependence of carrier relaxation time in gallium phosphide evaluated by photoemission measurements

For understanding of carrier behavior in semiconductors, it is important to measure the carrier relaxation time. In the present study, the relaxation times of inter-valley transition from the Γ valley to the X valley in GaP were evaluated by near-band-gap photoemission spectroscopy of electrons emitted from a surface with a negative electron affinity state. In the energy distribution curves, two peaks, which originate from the electron population accumulated in the Γ valley and the X valley, were observed. From the temperature dependence of the energy of these two peaks, we could successfully evaluate the temperature dependence of the energies of the Γ valley and the X valley. Furthermore, the relaxation times of the inter-valley transition from the Γ valley to the X valley were estimated from the ratio of the electron concentration of the Γ valley and the X valley. The values of the relaxation time are good agreement with the previous studies. These results indicate that the near-band-gap photoemission s...

Direct measurement of conduction miniband structure in superlattice by visible-light photoemission spectroscopy

We have proposed a novel angle-resolved photoemission spectroscopy to measure the conduction electrons emitted from a surface with negative electron affinity state. We call this method visible-light photoemission spectroscopy (VPS). In the present study, the conduction miniband structure formed in a superlattice was evaluated by the VPS method. As a result, we succeeded to obtain the dispersion of the miniband. In addition, the effective mass of the miniband was quantitatively determined from the observed dispersion. Therefore, we believe that VPS is a quite powerful tool for the evaluation of intermediate-band solar cells.

Ultrahigh-resolution direct observation of mini-bands formed in InGaAs/AlGaAs superlattice

Mini-bands formed in superlattice structures are often used for an intermediate-band solar cell. In order to design an optimized structure, it is important to measure the mini-band dispersions which influence transport properties and solar-energy conversion efficiency. In this study, we directly observed the dispersions of mini-bands formed in InGaAs/AlGaAs quantum-well superlattices by angle-resolved photoemission spectroscopy with synchrotron radiation light. The short-period energy dispersions due to the mini-bands around the X-valley were clearly obtained. In addition, the effective mass coefficient of each mini-band could be individually evaluated.

Development of angle-resolved spectroscopy system of electrons emitted from a surface with negative electron affinity state

We developed an angle-resolved photoemission spectroscopy system for the analysis of conduction-band electrons. By forming a negative electron affinity surface on a semiconductor surface, electrons in conduction bands are emitted into a vacuum and measured by using an analyzer. This method enables us to determine the energy and momentum of the conduction electrons. Furthermore, it can be used to determine unoccupied conduction band structures. The main challenges of this method are that the energies of the emitted electrons are extremely low and the trajectories of the electrons change due to various influences. We overcame these problems by placing the shielding mesh close to the sample and parallel to the sample surface. The entire chambers, including the shielding mesh, were grounded, and a negative bias voltage was applied only to the sample. This configuration realizes the acceleration of electrons while preserving the momentum component parallel to the sample surface. Another problem is the establishment of a method for converting a detected angle into the corresponding wavevector. We focused on the emission angle of electrons emitted from a sample and their minimum energy and then established an analytical method for converting detected angles into corresponding wavevectors on the basis of the minimum energy.

Measurement of energy distribution of conduction electrons in superlattice by visible-light photoemission spectroscopy

Visible-light photoemission spectroscopy (VPS) is a novel angle-resolved photoemission spectroscopy developed by us for measuring conduction electrons emitted from a surface with negative electron affinity state. In this study, InGaAs/GaAsP quantum well superlattice was analyzed by the VPS method with changing the excitation photon energy. By comparing with comparison of the energy distribution curves obtained by different excitation energy, the first and second conduction mini-band structures were directly clarified.

Fourfold Increase in Quantum Efficiency in Highly Spin-Polarized Transmission-Type Photocathode

We developed a new transmission-type GaAs/GaAsP strained superlattice photocathode with an AlGaAs transparent inter-layer and Si3N4 anti-reflection coating. The electrons emitted from this photocathode demonstrate a high spin-polarization of 90% with quantum efficiency as high as 0.4% that is a fourfold increase at room temperature compared with our previous photocathode.