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Ferromagnetic semiconductor EuO and GdxEu1-xO (0.01

Vapor phase epitaxy (VPE) is a promising method to produce GaN substrates due to its high growth rate. In this paper, we first describe ZnO buffer layer deposition by pulsed laser deposition (PLD) on sapphire substrates for subsequent GaN VPE growth. Previously, ZnO buffer layers for GaN growth were RF-sputtered films with only poly or highly oriented crystal structure. PLD-grown ZnO buffer layers are single crystalline with streaky RHEED patterns and exhibit a sharp band-edge peak in photoluminescence. We examined the effect of ZnO buffer layer thickness on film quality of VPE-grown GaN layers on c-plane sapphire substrates. The VPE-grown GaN surfaces on ZnO buffer layers exhibit terrace-like flat surfaces, whereas three-dimensional growth with sharp rock-like structure occurs without the buffer layer. X-ray rocking curve (XRC) measurements showed that inserting the ZnO buffer reduced the GaN peak width by more than a factor of two, suggesting better crystalline quality. From the XRC measurement, buffer layers upto 50 nm thickness improve the GaN growth, while the optical properties measured by photoluminescence (PL) remain unchanged. With a 200 nm thick ZnO buffer layer, cracks occur in the subsequent GaN layer, resulting in a broader XRC peak width. In addition, the GaN film on a thick ZnO buffer shows strong peaks from donor–acceptor pair recombination and deep acceptor level from 2.6–3.2 eV in the PL spectra which are associated with Zn-doping of GaN. This implies that a thick ZnO buffer results in Zn diffusion from the buffer layer into the VPE-grown GaN film.

Zinc-blende type nanoscale MnAs dots were grown on a sulfur-passivated GaAs (0 0 1) surface by molecular beam epitaxy. We report on the dot density dependence of the X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) in the Mn 2p core-level region. The XAS spectrum of low-density MnAs dots showed “localized” behavior as in the case of (Ga,Mn)As, while the spectrum of high-density MnAs dots changed to metallic behavior. The magnetic-field dependence of the XMCD revealed ferromagnetic behavior in the case of high-density MnAs dots.

The polarization characteristics of a pn-junction Si photodiode and a Au-GaAsP Schottky photodiode which were revealed when the detectors were used obliquely to the incident radiation were investigated theoretically and experimentally in the spectral region from the vacuum ultraviolet to the visible. The experimental results in both cases were well explained by an optical model in the ultraviolet to the visible spectral range. It was found that the effect of beam divergence on the detector response could be negligible in the visible and resulted in a smaller response below about 90 nm for both the Si and the Au-GaAsP photodiode. The silicon photodiode showed complex spectral variation of polarization properties between 100 nm and 400 nm.

We have investigated the electronic and magnetic properties of a MnAs:GaAs granular film with MnAs clusters embedded in the GaAs matrix fabricated by high-temperature annealing of Ga1−xMnxAs using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The XAS line shapes in the Mn 2p core level changed from a localized structure to an itinerant NiAs-type one. Magnetic-field dependence of the XMCD revealed no hysteresis curves at the fixed photon energy where the large XMCD signals were observed, suggesting the superparamagnetic behavior at 100 K.

A type of GaAs-based ferromagnetic semiconductor, (Ga, Cr)As, was successfully prepared by low-temperature molecular-beam epitaxy. The (Ga, Cr)As thin film shows a flat surface up to the Cr content x=0.18. However, when Cr content x exceeded 0.20, the roughness of the film became larger. The ferromagnetic ordering of the semiconductor sample with the Cr content of x=0.11 was observed. The electronic structures of (Ga, Cr)As were characterized by x-ray absorption spectroscopy (XAS). The XAS spectra show different line shapes in different Cr content.

Zinc-blende CrAs/GaAs multilayers were grown by molecular beam epitaxy. It was certified that each CrAs layer maintains an epitaxial relationship with the zinc-blende GaAs structure judging from the reflection high-energy electron diffraction observation. The film contains thicker zinc- blende CrAs layers in total than the CrAs thin film directly grown on the GaAs substrate which has the critical thickness of 3 nm. It was clarified that the optimum thicknesses of CrAs and GaAs to keep a good epitaxial relationship are 2 ML and 2 ML, respectively. The electronic structure of the multilayer is thought to be close to that of the (Ga, Cr)As thin film which has 50% of Cr content judging from x-ray absorption spectroscopy measurements.

A GaAs MCM power amplifier has been developed for 1.9-GHz digital cordless telephones. Power-added efficiency of 40.2% and P/sub 1dB/ of 22.2 dBm have been obtained at drain supply voltage of 3.6 V. Adoption of the MCM structure and on-chip ferroelectric capacitors has successfully reduced the GaAs total chip area to be 1.1 mm/sup 2/. >

GaN films with good crystalline quality are grown on sapphire by atmospheric pressure vapor phase epitaxy using gallium tri-chloride (GaCl3) and ammonia (NH3). Epitaxial growth is carried out over temperature and V/III-ratio ranges of 800–1000°C and 100–1000, respectively. Typical growth rate obtained is in the range of 5–20 μm/hr. The films grown below 925°C typically show three dimensional (island) growth, while above that temperature, continuous films are obtained. Films grown at 975°C with a V/III ratio > 300 exhibit a smooth surface. XRD analysis shows that the films are single crystal with hexagonal polytype. Strong band-edge photoluminescence is observed with a FWHM of 60 meV at room temperature and 25 meV at 77K. The results indicate that this simple growth technique is effective for growing high quality bulk GaN, which can be used as a substrate for subsequent epitaxy. In order to further improve the surface morphology, a preliminary experiment on GaN growth on a thin GaN buffer layer prepared by gas source MBE is also presented.

We have developed a new real refractive index guided GaAlAs laser which satisfies both low operating current and low noise characteristics by self‐sustained pulsation. The structure has a planar active layer and a GaAlAs optical confinement layer which give large saturable absorber, and small internal loss. As a result, as operating current as low as 19.7 mA at an output power of 3 mW is obtained under self‐sustained pulsation, leading to a relative intensity noise level less than −135 dB/Hz.

A new monolithically integrated low-noise (RIN ≤ –135 dB/Hz) and high-power (≥ 150 mW) GaAlAs laser array has been developed. The laser array consists of two real refractive index guided lasers with different stripe widths.

High temperature operation at up to 90/spl deg/C has been achieved in 65O nm-band AlGaInP visible laser diodes with a real refractive index guided self-aligned structure using an AlInP current blocking layer. The operating current is as low as 45 mA under CW operation at 60/spl deg/C and the characteristic temperature is 120 K from 20 to 60/spl deg/C.

With the synchrotron radiation, a transverse magneto-optical effect in bismuth substituted gadolinium iron garnets was measured by a reflection method. Differences of resonance energy shifts between the lights polarized parallel and perpendicularly to the direction of magnetization of the material were determined on reflectivity peaks in the near ultraviolet and a peak at the photon energy of about 18 eV. This reflectivity peak at about 18 eV was assigned to the 3d-4p transitions of iron ions in the octahedral and tetrahedral sites. A mechanism of the transverse magneto-optical effect in the 3d-4p transitions is discussed.