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We have grown Fe δ-doped (In,Fe)Sb thin films by low-temperature molecular beam epitaxy for magnetic-field sensors with ultrahigh Hall sensitivity. Magnetic circular dichroism spectroscopy measurements indicate intrinsic ferromagnetism in the Fe δ-doped (In,Fe)Sb layers. Although the Fe δ-doped (In,Fe)Sb thin films have lower Curie temperature than the Fe uniformly-doped (ln,Fe)Sb with the same average Fe concentration, their anomalous Hall effect is much stronger. The Hall sensitivity of the Fe δ-doped (In1−〈x〉,Fe〈x〉)Sb sample with an average Fe concentration 〈x〉 = 17% reaches 1522 Ω/T at room temperature, which is much higher than those of Fe uniformly-doped (In,Fe)Sb, (Ga,Fe)Sb, and commercial InSb Hall sensors.

In order to integrate spintronic devices into well-established Si device processes, thermal stability as well as improvement of device characteristics by thermal processes is important. We investigated the influence of the post-growth annealing on a Schottky barrier metal-oxide-semiconductor field-effect-transistor (MOSFET) with epitaxial ferromagnetic MnAs/Si(0 0 1) source and drain, by evaluating the magnetic properties of MnAs and the transport characteristics of the MOSFET structure. When the post-growth annealing was performed at 500 °C in a N 2 atmosphere, the transport characteristics were improved preserving the ferromagnetic behavior of MnAs. As a result, the MnAs/Si(0 0 1) junction was found to have sufficiently high thermal stability for the device process up to 500 °C.

We fabricated In 0.52 Al 0.48 As:MnAs granular thin films by annealing [(In 0.52 Al 0.48 ) 1− x Mn x ]As alloy thin films at 600 °C. The [(In 0.52 Al 0.48 ) 1− x Mn x ]As thin films were grown on InP(0 0 1) substrates with Mn concentrations x of 0.09 and 0.12 by low-temperature molecular beam epitaxy. We investigated the structural and magneto-optical properties of the In 0.52 Al 0.48 As:MnAs granular thin films by X-ray diffraction and magnetic circular dichroism, and compared with those of the as-grown [(In 0.52 Al 0.48 ) 1− x Mn x ]As thin films. The In 0.52 Al 0.48 As:MnAs granular thin films showed a large magneto-optical effect at room temperature. This large magneto-optical effect of the granular materials lattice-matched InP will be useful for InP-based magneto-optical devices operating at room temperature.

We have grown ferromagnetic MnAs thin films on InP(001) substrates by low-temperature molecular beam epitaxy (MBE) and investigated their structural and magnetic properties. We found that the MnAs c-axis (MnAs[0001]) lies along the InP[110] axis, and the easy magnetization axis lies in-plane, along the MnAs [1120] axis, which is parallel to the InP[110] axis. The Curie temperature of the MnAs films on InP(01) was estimated to be 321 K. We have also grown the ferromagnetic MnAs on In 0.65 Ga 0.35 As 0.75 P 0.25 thin films, and their structural and magnetic properties were very similar to those of the MnAs thin films directly grown on InP(001). The feasibility of growing monocrystalline ferromagnetic layers on InP(001) will open up the possibility of the monolithic integration of magnetic devices with InP-based optoelectronics.

This article reviews the recent advances of epitaxial ferromagnetic thin films and heterostructures as well as devices towards semiconductor-based spin-electronics or often called spintronics. We review the developments of III–V based ferromagnetic thin films and heterostructures grown by molecular beam epitaxy, with focus on the Mn-delta-doped GaAs/p-AlGaAs heterostructures with high ferromagnetic transition temperature, and control of the spin-dependent properties. As a new direction in spintronics research, we present our recent study on a new silicon-based spin device, spin metal-oxide-semiconductor field-effect transistor (spin MOSFET), and novel reconfigurable or reprogrammable logic gates using spin MOSFETs, which are compatible with the current CMOS technology and promising for reconfigurable computing.

We have investigated the growth, structural characterization and magnetic properties of epitaxial MnAs/NiAs/MnAs heterostructures grown on exact GaAs(1 1 1)B substrates by molecular beam epitaxy. Reflection high energy electron diffraction observation during the growth and cross-sectional transmission electron microscopy analysis revealed that the epitaxial orientation of [0 0 0 1]MnAs//[0 0 0 1]NiAs// [ 1 ¯ 1 ¯ 1 ¯ ] GaAs is maintained throughout the growth with atomically abrupt heterointerfaces. The magnetization loops of the heterostructures show double-step features when the thickness of the NiAs interlayer was more than 8 nm. The magnetic properties, such as the temperature dependence of magnetization, were sensitive to the growth conditions, which mainly results from the crystalline quality of the top MnAs layer.

We have successfully grown single-crystal MnAs/AlAs/MnAs magnetic tunnel junctions (MTJs) having extremely smooth interface morphology on exact (1 1 1)B GaAs substrates, where a several-monolayer-thick ultrathin GaAs buffer layer was used to avoid the unwanted formation of pyramidal facets that would otherwise be spontaneously evolved on a thick GaAs buffer layer surface. The surface morphology of the MnAs film grown on an ultrathin GaAs buffer layer is strongly dependent not only on the growth temperature of the MnAs film but also on the thickness of the ultrathin GaAs buffer layer, and became completely flat by optimizing them. When MnAs/AlAs/MnAs MTJs with flat interfaces were epitaxially grown on an ultrathin GaAs buffer layer, they exhibited clear double-step features in magnetization loops and tunneling magnetoresistance. On the other hand, magnetic properties of the MTJs grown on a thick GaAs buffer layer were severely deteriorated by interface-roughness-induced interlayer coupling.

We have grown Mn δ-doped GaAs layers by molecular beam epitaxy with various Mn sheet concentrations (δMn defined in the monolayer (ML) unit. Structural analyses using cross-sectional transmission electron microscopy and secondary ion mass spectroscopy revealed that when δMnis below I ML, Mn dopants are abruptly confined within 2-3 ML without lattice dislocations and clusters, and also without diffusion and surface segregation. Characterizations on the thermal stability of the doping profiles showed that the profiles retained abruptness even at elevated growth temperature T, = 400°C without diffusion and significant segregation. Strong surface segregation of Mn atoms, lattice dislocations, and MnAs clusters appeared when θMn exceeds 1ML. The transport properties of Mn δ-doped GaAs layers without and with selectively p-type doping are also presented.

We have grown a new alloy semiconductor (Ga 1− x , Er x )As by low-temperature molecular beam epitaxy (LT-MBE) on GaAs (0 0 1) substrates, and have studied its structural, optical, electrical, and magneto-optical properties. By LT-MBE at 200–400°C, homogeneous ternary alloy (Ga 1− x , Er x )As thin films with (Erbium) Er concentration from 2×10 16 up to 7.7×10 20 cm −3 ( x =3.5%) can be grown without major defects or clustering. (Ga, Er)As is highly resistive, suggesting that Er is 3+ substituting the Ga sublattice. Photoluminescence (PL) at 1.54 μm was observed for the (Ga, Er)As samples grown at 400°C. From magneto-optical spectra, it was found that there is little hybridization of 4f and sp orbitals in (Ga, Er)As, in contrast with the strong sp – d hybridization in (GaMn)As.

We have successfully grown ferromagnetic-metal (MnAs)/III–V semiconductor (GaAs, AlAs, InAs)/ferromagnetic-metal (MnAs) trilayer heterostructures on GaAs(1 1 1)B substrates by molecular beam epitaxy. In magnetoresistance (MR) measurements in current-in-plane (CIP) geometry, the spin valve effect was clearly observed, which was caused by the change of the magnetization of the two ferromagnetic MnAs layers between parallel and antiparallel directions. All the trilayer structures showed peculiar temperature dependence of the MR, which is very different from the conventional all-metallic magnetic multilayers. The MR ratio was found to be larger with decreasing the thickness and decreasing the energy band gap of the semiconductor spacer layer.

It is shown that magneto-optical effect can be controllably enhanced in a molecular beam epitaxy (MBE) grown semiconductor-based multilayer structure consisting of a magnetic layer and GaAs/AlAs distributed Bragg reflectors (DBR), that is a one-dimensional semiconductor-based magneto-photonic crystal. Here, the magnetic layer is GaAs containing MnAs nanoscale clusters (GaAs : MnAs), which can be formed by annealing (GaMn)As alloys at 500–750°C. Structural and magneto-optical properties of GaAs : MnAs are presented and its dielectric functions are derived. It is shown that DBR/GaAs : MnAs/DBR structures were successfully grown by MBE and their magneto-optical effect is enhanced due to the localization of light at the Bragg wavelength. Comparison with theoretical calculations of the magneto-optical properties and device potentials are also discussed.

We have investigated the initial growth of MnSb and MnAs films and multilayers on GaAs(1 0 0) and (1 1 1)B substrates by molecular beam epitaxy (MBE) and successfully grown epitaxial MnSb/MnAs multilayers on GaAs(1 1 1)B. We propose a possibility of the novel heat-induced giant magnetoresistance (GMR) effect in the MnSb/MnAs multilayer system.

Ferromagnetic MnAs thin films were grown on clean, S- and Se-passivated GaAs(1 0 0) substrates by molecular beam epitaxy (MBE). Growth modes of MnAs films on GaAs substrates depend on the surface treatment. On the clean and the Se-passivated surfaces MnAs films grow by a layer-by-layer mode, whereas on the S-passivated surface MnAs films grow by an island growth mode, and as a result polycrystalline films are obtained. Photoelectron spectroscopy study revealed that the Se overlayer on the Se-passivated GaAs substrates prevented Ga interdiffusion. We also found that the saturation magnetization of MnAs films on the Se-passivated surface was slightly larger than that on the clean and the S-passivated surfaces. This difference can be explained by the role of Se overlayer in preventing interdiffusion between MnAs and GaAs.

Recent studies on two types of ferromagnet/semiconductor hybrid structures grown by molecular-beam epitaxy (MBE) are described; (1) ferromagnet [MnAs]/semiconductor [GaAs] layered heterostructures, and (2) III–V (GaAs)-based magnetic semiconductor alloy [(GaMn)As] thin films. Their MBE growth, structures, and magnetic properties are presented.

We have studied tunneling spectroscopy of two types of ultrathin magnetic semiconductor heterostructures; Au/Al 2 O 3 /(GaMn)As, and (GaMn)As/AlAs/(GaMn)As. The latter is all MBE grown heterostructures with abrupt and smooth interfaces and atomically controlled barrier thickness. It was found that (GaMn)As has a small energy gap, which is similar to the soft Coulomb gap, near the Fermi energy, like heavily doped semiconductors. Temperature dependence of the tunneling conductance, which is proportional to the transverse density-of-state of the electrode, follows the power law T 0.5 . Furthermore, large tunneling magnetoresistance (TMR) was observed in a (GaMn)As/AlAs/(GaMn)As tunnel junction.

We have grown novel III-V diluted magnetic semiconductors, (Ga 1-x Mn x )As, on GaAs substrates by low-temperature molecular beam epitaxy using strong nonequilibrium growth conditions. When the Mn concentration x is relatively low (≤ 0.08), homogeneous alloy semiconductors, GaMnAs, are grown with zincblende structure and slightly larger lattice constants than that of GaAs, whereas inhomogeneous structures with zincblende GaMnAs (or GaAs) plus hexagonal MnAs are formed when x is relatively high. Magnetization measurements indicate that the homogeneous GaMnAs films have ferromagnetic ordering at low temperature.

We have studied epitaxial orientations and template effects in molecular beam epitaxy (MBE) of ferromagnetic MnAs thin films on (001) GaAs substrates. The MBE-grown MnAs thin films take two different epitaxial orientations, [1100] (type-A) and [1101] (type-B), depending on the surface reconstruction and one monolayer template formation on (001) GaAs surfaces just prior to the growth of MnAs. The easy axis of the magnetization in type-A and type-B MnAs films is along the [110] and the [110] of GaAs, respectively, 90° different with respect to the GaAs substrates. Furthermore, when a very thin epitaxial ErAs template layer was formed on c(4 × 4) GaAs and then MnAs was grown, the growth direction of the MnAs was found to be [1102], and no strong magnetic anisotropy was observed within the film plane. These results indicate the importance of the first few monolayers and starting surface stoichiometry to control the epitaxial orientation and magnetic properties of the epitaxial ferromagnetic MnAs thin films.

Molecular beam epitaxial (MBE) growth of GaAs, AlAs, and AlGaAs on V-grooved GaAs substrates are studied. Observing the morphology of the heterostructures grown on the various part of V-grooves with scanning electron microscopy (SEM), it is shown that GaAs, AlAs and AlGaAs have different elemental growth parameters such as surface diffusion and atom incorporation, leading to the difference in growth behavior. Under appropriate growth conditions, in particular, the growth of AlAs results in very smooth surface on the (111)A sidewalls and drastic sharpening at the bottom of the V-grooves, whereas the growth of AlGaAs and GaAs do not show such phenomena. MBE growth mechanisms, both on the V-grooved substrates and on the (111)A flat surface, are discussed based on the phenomena observed in our experiments. Furthermore, we investigate the growth temperature dependence of the morphology and the surface diffusion of atoms during the growth of GaAs/AlAs heterostructures on the V-grooves, and demonstrate the successful fabrication of GaAs multiple quantum-wire structures at the bottom of the V-grooves by high-resolution SEM observations and cathodoluminescence measurements at 16K.

We describe the coherent epitaxy of MnNiAl fenomagnetic thin films on AlAs/GaAs heterostructures using molecular beam epitaxy. The epitaxy helps to stabilize the τ phase, which is known to be a metastable ferromagnetic phase with a tetragonal lattice. The easy axis of the ferromagnetic thin film is oriented perpendicular to the surface as a consequence of the coherent epitaxy. The results of structural and magnetic analysis are given for thin MnxAl1-x films with 0.40 < x < 0.60. We have further assessed the change in structural and magnetic properties of the thin films when Ni is added as a substitution for Mn in Mn0.60-yNiyAl0.40, with 0 < y < 0.18. We find that coherent epitaxy is obtained over these wide compositional ranges and that the change in magnetic properties using various Ni additions adds a degree of freedom in the design of ferromagnetic thin layers on AlAs/GaAs for future device structures.

We have measured the critical temperature where the growth mode transition between step-flow and 2D nucleation takes place by RHEED measurements during the molecular beam epitaxial growth of GaAs and AlAs on (001) and (111)B GaAs vicinal surfaces. Combining the experimental results with our theory and taking into account the surface diffusion and supersaturation ratio of adatoms on the terraces, the intrinsic surface diffusion length λs of Al and Ga atoms is estimated on both (001) and (111)B GaAs vicinal surfaces with various misorientation directions.

We have studied the electron mobility in novel MBE grown GaAs QWs in which one monolayer AlAs potential barriers are inserted one or three times in the well. The electron mobility parallel to the QW layers at 4.2K was found to decrease drastically (from 1.7×105 to 4.8×104 cm2/V·s or lower) due to the inserted AlAs layers. By comparing measured mobilities with theory, it was shown that a small fluctuation of thickness of inserted barrier (one monolayer deviation in one twentieth of the total area) can account for the decrease of mobility. This result suggests the need to control precisely the layer thickness of short period superlattices (SP-SLs) if one needs to achieve high mobility.

Atomic-scale models of heterointerface structures of GaAs-AlxGa1−xAs quantum wells (QWs) are presented. For QWs with x > 0.5, top (AlGaAs-on-GaAs) interfaces formed by continuous MBE have the roughness of one atomic layer height whose atomic step interval is 200 A, resulting in a well-known photoluminescence (PL) broadening from QWs. This roughness can be smoothed by growth interruption for tens of second prior to the formation of top interfaces, leading to a drastic sharpening of the PL spectra by a factor of 4–5. In contrast, bottom (GaAs-on-AlGaAs) interfaces have atomic steps of 40 A in width irrespective of growth interruption; this step interval is so small as compared with excition size that they act as a “pseudo-smooth” interface. For QWs with x < 0.3, however, the bottom interfaces become similar to the top interface, and can be smoothed by growth interruption.

We report Raman measurements of the LO phonon modes confined in GaAs/AlAs quantum wells grown by refined MBE with and without growth interruption and by migration enhanced epitaxy (MEE). The energies of the confined LO phonon modes lie close to the values expected from the bulk GaAs phonon dispersion. We show that the higher order confined phonon modes are more sensitive to roughness than the lower order modes.