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We have grown Ga doped ZnSnAs2 thin films on InP substrates using all-solid source MBE technique. Using the optimum substrate temperature of 340 oC and Zn:Sn:As4 beam equivalent pressure ratio (BEPR) of 24:1:52, samples were prepared with different Ga concentration achieved by changing the Ga flux. Compositional analysis was performed using electron microprobe microanalysis (EMPA). We found that Ga concentration on the cation sites is 20.9% and 8.7%. XRD patterns of Ga doped samples showed only assignable to ZnSnAs2:Ga and InP substrate. The lattice constant decreases linearly from 0.5862 to 0.5829 nm with decreasing Ga concentration. The observed concentration dependence indicates that the lattice constant of the thin films can be controlled by Ga doping concentration. Hall coefficient and resistivity of Ga 8.7% doped sample can be well explained by the impurity band model. We were able to resolve the experimentally obtained carrier concentration pexp and mobility μexp into valence band carrier concentration pv with mobility μv and acceptor band carrier concentration pa with mobility μa. The computed apparent values papp and μapp are in good agreement with pexp and μexp, respectively. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied the epitaxial growth of InAlAs on ZnSnAs2 thin films to establish magnetic heterostructures involving ferromagnetic Mn-doped ZnSnAs2 (ZnSnAs2:Mn) thin films. These heterostructures were successfully grown at temperatures around 300 °C to maintain room-temperature ferromagnetism in ZnSnAs2:Mn. Reflection high-energy electron diffraction, X-ray diffraction measurements and cross-sectional transmission electron microscopy revealed that the InAlAs layers were pseudomorphically lattice-matched with ZnSnAs2, even at the low temperature of 300 °C. We attempted to prepare magnetic quantum well structures from the InAlAs/ZnSnAs2:Mn magnetic multilayer structure. We found that InAlAs layers heteroepitaxially grown on ZnSnAs2 and ferromagnetic ZnSnAs2:Mn films are suitable for preparing InP-based magnetic semiconductor quantum structures. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The detailed crystalline structure of molecular beam epitaxially grown MnAs thin films on InP(001) substrate has been investigated using high resolution X-ray diffraction techniques. Reciprocal space mapping of the MnAs/InP(001) samples indicates that the MnAs has a cubic zinc-blende (zb) structure with the epitaxial relationship zb-MnAs[110]|InP[110]. The lattice constant of zb-MnAs is ∼6.06 A. The MnAs lattice is relaxed and is mosaic-like likely due to large lattice mismatch between the film and InP substrate. The isotropic nature of the magnetic properties supported our conjecture that the MnAs epitaxial film under study has indeed a cubic structure.

We have observed anomalous temperature dependence of magnetization of ZnSnAs 2 :Mn epitaxial thin films grown on InP(001) substrates compared with the values estimated by the mean-field theory using S = 5/2, as a function of temperature: The ZnSnAs 2 :Mn epitaxial thin films exhibits slightly increasing magnetization with rising temperature. From the first-principles calculation, when Mn substitutes for neighboring Zn sites in ZnSnAs 2 , MnZ n -MnZ n pairs result, which prefer antiferromagnetic (AFM) ordering. On the other hand, MnZ n -MnS n and MnS n -MnS n pairs can introduce both holes and spins, leading to ferromagnetic (FM) ordering. Thus, assuming that the magnetic property of ferromagnetic phase follows the mean-field theory, it can be explained by coexistence of FM and AFM phases.

We investigated the effect of film thickness on structural and magnetic properties of ZnSnAs 2 :Mn thin films. Samples with 80, 170 and 250 nm values of thickness were epitaxially grown on semi-insulating InP(001) substrates, followed by ZnSnAs 2 buffer layers using molecular beam epitaxy (MBE). The lattice constants and crystal structures were determined by X-ray diffraction (XRD). The reciprocal lattice mapping revealed that the 80-nm-thick sample could be completely lattice-matched with InP (001) substrates. Ferromagnetic hysteresis was clearly observed at 300 K for the 170 nm and 250 nm samples.

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.

We report the magnetic and electrical properties in Mn-doped and undoped (Zn,Sn)As2 epilayers, that were annealed at slightly higher temperatures than the growth temperature. (Zn,Sn)As2 :Mn were epitaxially grown on InP (001) substrates at 300°C, and showed room-temperature ferromagnetism. The hole concentration, saturation magnetization and Curie temperature were measured and evaluated as a function of annealing temperature. The Curie temperature had a tendency to slightly increase at annealing temperatures up to 340°C, and completely disappeared at 400°C. The ferromagnetism could be attributed to hole-mediated ferromagnetism resulting from Mn ion substitutions at both the II-group Zn and IV-group Sn sites, especially from the large solubility of Mn2+ substitution at Zn sites. The disappearance of ferromagnetism may be explained by several types of mechanisms : migration of mobile interstitial Mn atoms, diffusion of substitutional Mn ions to the surface, substitution of interstitial Mn atoms on Zn vacancies, and formation of MnAs clusters. It is noteworthy that the growth of magnetic semiconductor thin films from substrate lattice matching is essential for avoiding magnetic secondary phases such as MnAs clusters.

GaSb thin films using an Sb template layer on Si(111) substrates were grown by MBE to investigate the effect of the Sb template on the crystalline quality of the GaSb films. Structural properties were compared with those of the GaSb films grown on Si(111) substrates using an AlSb initiation layer and a GaSb film directly grown on Si(111) substrate. For the GaSb thin films prepared using an Sb template layer, streaky RHEED patterns were observed throughout growth of GaSb layer and a typical surface RMS roughness value of 7.4 nm was obtained. The results of θ-2θ scan XRD investigations indicated that GaSb film on Sb template is nearly unstrained. The φ-scan XRD measurements with respect to {422} reflection peaks of GaSb films and Si(111) substrates did not show any sound evidence of alignment upon rotation by 30° and rather suggest two domain growth of the GaSb films. The smaller percentage of the domain which is rotated by 60° suggests that the GaSb film grown using an Sb template is more single crystal than the one grown using an AlSb initiation layer. These structural comparisons indicated that the Sb template layer is favorable in obtaining good GaSb epitaxial films. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the growth of GaSb layers and GaSb/-AlGaSb multiple quantum well (MQW) structures using an AlSb initiation layer on Si(111) substrates. The entire growth was monitored using reflection high-energy electron diffraction (RHEED). The GaSb epitaxial films and MQWstructures were characterized by X-ray diffraction (HRXRD), atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. Our results suggest that high-quality (111)-oriented GaSb films with a mirror surface can be obtained using the present growth condition. The PL peak energy of the MQWs on Si(111) substrates is almost temperature independent up to ∼120 K and exhibits relatively smaller variation with temperature compared to that of the MQWs grown on Si(001) substrates. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mn-doped ZnSnAs2 (ZnSnAs2 :Mn) thin films with 5.0 and 6.5% Mn composition were epitaxially grown by molecular beam epitaxy on InP (001) substrates. These films had a Curie temperature of 334 K, corresponding to room-temperature ferromagnetism. The local structures around Mn atoms in ZnSnAs2 :Mn were studied by analysis of the X-ray absorption fine-structure spectra. It was found that the Mn atoms substitute into Zn or Sn cation sites, and the Mn‒As bond length is 2.50 A, which is slightly smaller than the value of 2.53 A in a sphalerite (zinc-blende) ZnSnAs2 bulk crystal. The Mn‒As bond length in ZnSnAs2 :Mn is consistent with the value obtained from GaMnAs, and has a smaller value than that obtained from the zinc-blend MnAs thin films grown on a InP substrate.

The photoluminescence (PL) properties of heavily Mn-doped ZnSnAs2 layers epitaxially grown on nearly lattice-matched semi-insulating InP substrates are studied. PL spectra are obtained for samples with Mn concentrations of 5, 12 and 24 mol% relative to the combined concentrations of Zn and Sn. A broad emission band centered at ~ 1 eV is detected for Mn-doped layers at room temperature. The emission is a intense broad asymmetric line at low temperatures. The line is reconstructed by superposition of two bands with peak energies of ~ 0.99 and 1.07 eV, similar to those reported for InP. These bands are superimposed onto a 1.14 eV band with well-resolved phonon structure for the layer doped with 12 % Mn. Recombination mechanism involving the split-off band of the ZnSnAs2 is suggested. Temperature dependence of integrated intensities of the PL bands indicates to thermally activated emission with activation energies somewhat different from those found for InP. Mn substitution at cationic sites increases the concentration of holes which may act as recombination centers. Recombination to the holes bound to Mn ions with the ground state located below the top of the valence band has been proposed as a possible PL mechanism.