Takashi Saka

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.

GaAs/GaAlAs surface emitting IR LED with Bragg reflector grown by MOCVD

Abstract Surface emitting GaAs/GaAlAs diodes with a Bragg reflector grown by metalorganic chemical vapor deposition (MOCVD) were investigated. They have a double heterostructure and a Bragg reflector consisting of 25 pairs of Ga 1− x Al x As/GaAs multilayers ( x = 1 or 0.45) between the GaAs substrate and the cladding layer. Since the light is reflected by the Bragg reflector, the absorption by the substrate can be avoided and high external quantum efficiency is achieved. The emission spectra are, however, oscillating as a result of the interference of the light reflected by the emitting surface and by the Bragg reflector. The oscillation was removed by coating an anti-reflecting layer on the surface. An output light power of 8 mW operated at 50 mA was obtained, which is four times higher compared with the case of no Bragg reflector.

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).

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.

Bragg reflector of GaAlAs/AlAs layers with wide bandwidth applicable to light emitting diodes

Reflective spectra of the Bragg reflectors applied to GaAs infrared light emitting diodes were simulated. Bragg reflectors are composed of alternating Al0.2Ga0.8As/AlAs layers where the ratio of high‐ and low‐refractive indices is as small as 1.16. The reflectance bandwidth can be broadened by employing chirped structures where the optical thicknesses of the respective layers are changed arithmetically. On the other hand, for chirped Bragg reflectors, resonant reflection drops appeared at some wavelengths. These drops can be corrected by inserting periodic alternating layers. The reflectance band of 80% reflectivity is 2.7 times as wide as that of the conventional reflector of the periodic sequence in the case of 66 layer stack.

Effects of defects and local thickness modulation on spin-polarization in photocathodes based on GaAs/GaAsP strained superlattices

The spin-polarization of electrons from the GaAs/GaAsP superlattice on a GaAs substrate (∼90%) is higher than that from the same superlattice on a GaP substrate (∼60%). Transmission electron microscopy and atomic force microscopy observations revealed that stacking faults were the main defects in the superlattice on the GaAs substrate, while local thickness modulation of the superlattice layers was prominent in the superlattice on the GaP substrate. According to the density of stacking faults and the areal ratio of the thickness modulation, it was concluded that the thickness modulation in the superlattice was the main reason for the spin-polarization reduction in the photocathode on the GaP substrate. Growth of a thin GaAs layer on a GaP substrate prior to superlattice growth eliminated the thickness modulation and the spin-polarization was recovered to 90%.

Spin Relaxation of Electrons in Strained-GaAs-Layer Photocathode of Polarized Electron Source

The luminescence polarization method using a mode-locked Ti:sapphire laser and a streak camera is applied to the measurement of the spin relaxation time and the lifetime of electrons in the strained-GaAs-layer photocathode of a polarized electron source. The spin relaxation time and the electron lifetime are 105 ps and 45 ps at room temperature, respectively. Electron-hole scattering is thought to be the main mechanism of the spin relaxation of our strained-GaAs photocathode.