Hiromichi Horinaka

Large enhancement of spin polarization observed by photoelectrons from a strained GaAs layer

Abstract We have observed a large enhancement of the spin polarization of photoelectrons emitted from a 0.08 μm thick strained GaAs(001) layer grown on a GaPxAs1−x substrate by the MOCVD method with x=0.17. For this fraction of phosphorus, the lattice-mismatch was estimated to be ∼0.6% and the energy splitting between heavy-hole and light-hole bands at the valence band maximum to be ∼40 meV. The maximum polarization of ∼86% was observed with a quantum efficiency of ∼2 x 10−4, under the conditions that the cathode was at room temperature and the excitation photon wavelength was λ≈860 nm.

Photoluminescence Properties of CuGaSe2 Grown by Iodine Vapour Transport

The photoluminescence spectra of CuGaSe2 are measured on melt-solidified crystals, iodine vapour transported as-grown crystals and annealed crystals in various conditions. From the change in the spectra among them and the influence of annealing on luminescence, it is concluded that the iodine impurity acts as a deep donor which has been unintentionally incorporated into crystals during growth, and that the selenium vacancy acts as a shallow donor. The latter defect can be filled back by annealing in selenium vapour at 600°C. All photoluminescence bands observed in the iodine vapour transported crystals are explained in trems of these two donors (0.38 eV and 80 meV) and one acceptor (40 meV), which is probably due to the copper vacancy.

Excitonic Structure of CuGaS2xSe2(1-x) and CuAlS2xSe2(1-x)

Phase-shift-difference (PSD) spectra upon reflection are measured at 110 K and 20 K on CuGaS2xSe2(1-x) and CuAlS2xSe2(1-x) alloy systems. The spectra reveal so-called A, B and C excitonic structures which are associated with the splitting of the uppermost valence band. In addition, the first excited state of the A-exciton is observed at 20 K in CuGaS2xSe2(1-x) system, and the binding energy of the exciton is estimated by using the hydrogen model. Analysis by Hopfields quasi-cubic model for the spin-orbit and crystal field splittings is found to be inapplicable to these compounds. The crystal field splitting is always larger than expected in these compounds. This effect as well as anomalously small spin-orbit splitting are explained by p-d hybridization of the valence band.

Surface charge limit in NEA superlattice photocathodes of polarized electron source

The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.

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.

Super-High Brightness and High-Spin-Polarization Photocathode

Using a newly developed transmission-type photocathode, an electron beam of super-high brightness [(1.3±0.5)×107 Acm-2sr-1] was achieved. Moreover, the spin-polarization was as high as 90%. We fabricated a transmission-type photocathode based on a GaAs–GaAsP strained superlattice on a GaP substrate in order to enhance the brightness and polarization greatly. In this system, a laser beam is introduced through the transparent GaP substrate. The beam is focused on the superlattice active layer with a short focal length lens. Excited electrons are generated in a small area and extracted from the surface. The shrinkage of the electron generation area improved the brightness. In addition, a GaAs layer was inserted between the GaP substrate and the GaAsP buffer layer to control the strain relaxation process in the GaAsP buffer layer. This design for strain control was key in achieving high polarization (90%) in the transmission-type photocathode.

High brightness and high polarization electron source using transmission photocathode with GaAs-GaAsP superlattice layers

In order to produce a high brightness and high spin polarization electron beam, a pointlike emission mechanism is required for the photocathode of a GaAs polarized electron source. For this purpose, the laser spot size on the photocathode must be minimized, which is realized by changing the direction of the injection laser light from the front side to the back side of the photocathode. Based on this concept, a 20kV gun was constructed with a transmission photocathode including an active layer of a GaAs–GaAsP superlattice layer. This system produces a laser spot diameter as small as 1.3μm for 760–810nm laser wavelength. The brightness of the polarized electron beam was ∼2.0×107Acm−2sr−1, which corresponds to a reduced brightness of ∼1.0×107Am−2sr−1V−1. The peak polarization of 77% was achieved up to now. A charge density lifetime of 1.8×108Ccm−2 was observed for an extracted current of 3μA.

Real Time Magnetic Imaging by Spin-Polarized Low Energy Electron Microscopy with Highly Spin-Polarized and High Brightness Electron Gun

We developed a spin-polarized low energy electron microscopy (SPLEEM) with a highly polarized and high brightness spin electron gun in the present study. Magnetic structures of Co/W(110) were observed with an acquisition time of 0.02 s with a field of view of 6 µm. We carried out a dynamic observation of magnetic structures with the SPLEEM during the growth of Co on W(110).

A criterion for Applying Chalcopyrite Semiconductors to Optical Line Elimination Filters

The optical line elimination filter consisting of a birefringent, optically active crystal and two polarizers has a narrow band at the wavelength of the accidental optical isotropy of crystal. Chalcopyrite semiconductors applicable to this filter are predicted from the contribution of anisotropy of transition intensity to birefringence. CuAlS2, AgAlS2, AgAlSe2, AgAlTe2 and AgInS2 are thought to satisfy the condition of the filter, i.e. the accidental optical isotropy. Experimental results show that CuAlS2 had the optical isotropic wavelength at 378 nm. The dependence of center wavelength of AgInxGa1-xSe2 filter on x is estimated from the band parameters. The center wavelength of AgInxGa1-xSe2 filter (x=0.5) is shown experimentally to be adjusted to the emission line of Nd:YAG laser.

Strain dependence of spin polarization of photoelectrons from a thin GaAs layer

Abstract We studied the strain dependence of the spin polarization of photoelectrons from a thin GaAs epilayer which was strained by a lattice mismatched GaP x As 1− x buffer substrate. The polarizations were measured for seven samples with different residual strains between 0.34% and 0.80%. Our result shows the clear dependence of the polarization on the residual strain in this strain region. The maximum polarization attained by each sample increases gradually from 67% up to 87% with an increase of the residual strain which corresponds to that of the band splitting from 22 to 52 meV. A brief discussion about this behavior is given.