Yoshio Jinbo

Electrotransport Properties of p-ZnSnAs2 Thin Films Grown by Molecular Beam Epitaxy on Semi-insulating (001) InP Substrates

ZnSnAs2 thin films were prepared by molecular beam epitaxy (MBE) on semi-insulating (001) InP substrates using the same growth conditions as previously reported. High-resolution X-ray diffractometry (HRXRD) and Raman spectroscopy studies suggest the presence of both the chalcopyrite and sphalerite phases. The transport properties were measured from 5 K up to room temperature. We observed a pronounced peak in the Hall coefficient temperature dependence curve at ~130 K, similar to those observed only from chalcopyrite-phase bulk ZnSnAs2 in earlier studies. A hole concentration of p = 5.98 ×1018 cm-3, hole mobility of µ= 23.61 cm2/(Vs) and resistivity of ρ= 4.43 ×10-2 Ωcm were obtained at room temperature.

Optical Characterization of Heavily Sn-Doped GaAs1-xSbx Epilayers Grown by Molecular Beam Epitaxy on (001) GaAs Substrates

We have optically investigated ternary GaAs1-xSbx (x<0.58) epilayers and Sn-doped GaAs1-xSbx (x=0.10–0.14) epilayers grown by molecular beam epitaxy on GaAs (001) substrates. Sn-doped GaAsSb layers were grown as a function of Sn Knudsen-cell temperature, and then characterized by low-temperature photoluminescence (PL) measurements and Hall effect measurements. The Sn-doped GaAsSb films grown at a K-cell temperature of 670°C changed from exhibiting p-type conduction to exhibiting n-type conduction, and showed a maximum PL intensity and a maximum electron mobility of 1900 cm2/V s. The PL intensities obtained for Sn-doped GaAsSb films showed a relatively good correlation with the variations in Hall mobility.

Characterization of GaSb/AlGaSb Multi-Quantum-Well Structures Grown on Si(001) Substrates

We prepared GaSb/AlGaSb multi-quantum-well (MQW) structures on n-type silicon (001) substrates by molecular beam epitaxy (MBE). To minimize dislocations in MQW layers and decrease the total thickness of the epitaxial layer, we employed not only an AlSb initiation layer but also a superlattice buffer layer (SL-BL) in a moderately thick GaSb buffer layer. For comparison we also fabricated other MQW structures on a considerably thick GaSb buffer layer without SL-BL. The obtained atomic force microscopy (AFM), transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated that the definite MQW structures for both the samples and the quality of MQW layers prepared using SL-BL were generally better than those of the reference sample. The PL emission of these samples at about 1.30–1.55 µm was observed at room and low temperatures. The dependence of PL emission energy on GaSb well width was well explained by the finite square well potential model.

Magnetic anisotropy of (Ga,Mn)As films grown on Si (001) substrates

We investigated the anisotropic magnetotransport and magnetic properties of (Ga,Mn)As films on GaAs buffer layers grown on Si (001) substrates. The in-plane magnetoresistance (MR) showed similar dependence on the applied magnetic field at 10K for crystallographically equivalent [110] and [1¯10] directions. On the other hand, the in-plane MRs for [110] and [100] directions in an as-grown sample were observed to have slightly different magnetic field dependence, which disappeared after low-temperature annealing. The behavior observed in this experiment was different from that observed from (Ga,Mn)As∕GaAs systems. This difference was probably related to the competition between uniaxial magnetic anisotropy and cubic magnetic anisotropy induced by the antiphase domains observed for our samples grown on Si (001) substrates.

Anomalous Hall Effect and Magnetoresistance in Mn-Doped ZnSnAs2 Epitaxial Film on InP Substrates

We present for the first time the temperature dependence of resistivity, anomalous Hall effect, and extraordinary magnetoresistance (MR) in 6.5% Mn-doped ZnSnAs2 epitaxial film prepared by molecular beam epitaxy (MBE) on InP(001) substrates. The magnetic field dependence of magnetization (M–H curve) show clear hysteresis loops at 300 K for magnetic fields applied both perpendicular and parallel to the sample surface. The Curie temperature was evaluated to be 350 K. Near-zero-field hysteresis loops in the anomalous Hall resistance were also observed at various temperatures corresponding to the hysteretic out-of-plane magnetization of the sample. Negative and positive values of MR were observed in the low-field region. The behavior of the MR can be properly described by the Khosla–Fischer semi-empirical model for spin scattering of carriers in an impurity band. These characteristics strongly indicate a carrier-spin interaction in Mn-doped ZnSnAs2.

Annealing Effects on Impurity Band Conduction of ZnSnAs2 Epitaxial Films

Un-doped II-IV-V2 ZnSnAs2 thin films have been grown epitaxially on semi-insulating InP(001) substrates by molecular beam epitaxy using a substrate temperature of Ts=300°C. In-situ reflection high-energy electron diffraction observations during the growth revealed streaky patterns indicating an atomically flat surface. After verification of the resulting stoichiometry by electron probe microanalysis and the crystalline structure by high-resolution X-ray diffraction studies, three samples were cleaved from the as-grown sample and subsequently annealed at temperatures of 300, 320, and 340°C for two hours with face-to-face proximity capping by GaAs wafers to simulate an arsenic atmosphere. The temperature dependence of the Hall coefficient of the as-grown sample and the samples annealed at 300 and 320°C showed equal carrier concentration at the exhaustion and freeze-out ranges, suggesting the validity of the impurity band model for these samples. The measured temperature dependence of the Hall coefficient and resistivity, analyzed within the framework of the impurity band model proposed by Isomura, revealed decreasing total carrier concentration p and impurity band carrier mobility μa, increasing acceptor activation energy Ea and maximum valence band mobility μv with increasing annealing temperature.

Room-Temperature Ferromagnetism in (Zn,Mn,Sn)As2 Thin Films Applicable to InP-Based Spintronic Devices

We investigated the growth and magnetic properties of ternary ZnSnAs2 thin films doped with a various degrees of Mn content. It was confirmed that Mn-doped ZnSnAs2 thin films are pseudomorphically grown on nearly lattice-matched InP(001) substrates. Magnetization measurements on Mn-doped ZnSnAs2 thin films revealed a ferromagnetic transition temperature of around 330 K, and clearly showed hysteresis loops even at room temperature. No evidence of magnetic secondary-phase MnAs formation in the host ZnSnAs2 thin films was observed within the limit of our measurement system. We also prepared a trilayer structure consisting of Mn-doped ZnSnAs2 layers and an undoped ZnSnAs2 intermediary layer as a preliminary structure for a tunneling magnetic junction. This structure was confirmed to demonstrate ferromagnetism at room temperature. The present results suggest that diluted ferromagnetic (Zn,Mn,Sn)As2 thin films are one of the most promising building blocks for InP-based spintronic devices.

Dependence of MBE-grown ZnSnAs2:Mn epitaxial film properties on Mn doping level

We have prepared Mn-doped ZnSnAs2 thin films with varying Mn doping content (2.1%, 2.7%, and 5.0%) using molecular beam epitaxy, by changing the Mn-to-Sn beam equivalent pressure ratio during growth. All the samples were grown on InP(001) substrates using the optimum substrate temperature Ts=300°C previously reported. As a reference, an un-doped ZnSnAs2 epitaxial film was also prepared. In-situ reflection high-energy electron diffraction observations revealed the transformation from streaky to spotty suggesting increasing surface roughening with increasing Mn-content. From high-resolution X-ray diffraction studies, the lattice constant was found to increase with increasing Mn-doping level. The 5% Mn-doped ZnSnAs2 epitaxial film exhibited a Curie temperature of ~314 K, as revealed from measurement of the zero-field cooled temperature dependence of the remanent magnetization using a magnetic property measurement system superconducting quantum interference device magnetometer. A hysteretic M-H curve was also obtained even at 300 K. From the M-H curve measured at 5K, the magnetic moment was computed to be 1.1μB per Mn atom.

Evidence of Pseudomorphic Growth of ZnSnAs2 Epitaxial Layers on Nearly Lattice Matched InP Substrates

ZnSnAs2 epitaxial films of various thicknesses have been grown on InP(001) substrates by molecular beam epitaxy using different growth times. No manifestations of epitaxial layer lattice relaxation such as broadening of the full width at half maximum of high-resolution x-ray diffraction rocking curves, reduction of the ratio of the diffraction peak intensities of the epitaxial layer and the substrate, and extinction of the Pendellos?ng fringes were observed with increasing sample thickness. This implies that the epitaxial films remain pseudomorphic with the InP substrate to a thickness of at least 285 nm. Micro-Raman spectroscopy measurements revealed that all the samples exhibited A1 vibrational modes that were similar to those observed in chalcopyrite semiconductors

Experimental evidence for site preference of Mn substitution in ferromagnetic (Zn,Mn,Sn)As2 films

We investigated the site preference for Mn atoms in ferromagnetic (Zn,Mn,Sn)As2 thin films grown by molecular beam epitaxy. All the (Zn,Mn,Sn)As2 samples used exhibited ferromagnetism with Curie temperatures above room temperature. The chemical compositions of the (Zn,Mn,Sn)As2 thin films were measured by electron probe microanalysis. Under the present growth conditions, increasing the Mn concentration in (Zn,Mn,Sn)As2 reduced the Sn content almost three times more than the Zn content. This result reveals for the first time that Mn substitution in (Zn,Mn,Sn)As2 thin films prefers Sn sites over Zn sites. To explain the carrier-mediated ferromagnetism of (Zn,Mn,Sn)As2 with p- or n-type conductivities, we discuss the possibility that Mn3+ substitution occurs at both Zn and Sn sites in addition to Mn2+ and Mn4+ substitutions.