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The conduction type in (Zn,Sn,Mn)As2 thin films grown by molecular beam epitaxy (MBE) on InP substrates was found to be controllable from p-type to n-type as a function of Mn content. n-type (Zn,Sn,Mn)As2 thin films were obtained by Mn doping of more than approximately 11 cat.%. It is likely that Mn interstitials (MnI) incorporated by excess Mn doping are located at tetrahedral hollow spaces surrounded by Zn and Sn cation atoms and four As atoms, which are expected to act as donors in (Zn,Sn,Mn)As2, resulting in n-type conduction. The effect of annealing on the structural, electrical and magnetic properties of n-type (Zn,Sn,Mn)As2 thin films was investigated as functions of annealing temperature and time. It was revealed that even if the annealing temperature is considerably higher than the growth temperature of 320 °C, the magnetic properties of the thin films remain stable. This suggests that a MnI complex surrounded by Zn and Sn atoms is thermally stable during high-temperature annealing. The n-type (Zn,Sn,Mn)As2 thin films may be suitable for application as n-type spin-polarized injectors.

The structural, magnetic, and electrical properties of InMnAs thin films were investigated to elucidate their ferromagnetism for potential applications in InP-based spintronics and magneto-optical integrated circuits. The films were prepared on InP substrates by molecular beam epitaxy at growth temperatures of 220–280 °C without further annealing. Except for the 220 °C-grown sample, the films showed ferromagnetism at room temperature. We found that the room-temperature ferromagnetism was predominantly caused by the existence of hexagonal MnAs clusters embedded in a homogeneous InMnAs matrix (InMnAs:MnAs system). High-resolution transmission electron microscope images revealed that the MnAs nanoclusters formed either elongated or round structures in the InMnAs host matrix. The experimental results indicate that the mixed systems of hexagonal MnAs embedded in InMnAs may provide potential ferromagnetic building blocks for InP-based spintronics.

The crystalline structure and quality of ZnSnAs2:Mn films grown by molecular beam epitaxy (MBE) on InP substrates are two of the most important issues in preparing spintronic structures such as magnetic quantum wells and spin-based transistors. We conducted high-resolution X-ray diffraction (XRD) measurements to clarify the crystalline structure of ZnSnAs2:Mn thin films. The XRD measurements reveal with very high accuracy that the samples have a sphalerite structure with high epitaxial quality. These results, within the limits of measurement accuracy support the previous assumption that ZnSnAs2:Mn thin films are coherently clustered ferromagnetic semiconductors without breaking the continuity of the sphalerite structure, leading to ferromagnetism with a high Curie temperature close to that of zincblende MnAs.

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 ferromagnetic transition in (Zn,Sn,Mn)As2 thin films is explained in terms of magnetic percolation in a Mn-As clustering network. We first studied the relationship between the spatial distribution of Mn-As clusters and the Curie temperature (TC). The local atomic structure was reconstructed from datasets of atomic positions in (Zn,Sn,Mn)As2 obtained by atom probe tomography (APT). To probe the local atomic structure and the magnetic properties of Mn-As clusters in ZnSnAs2 thin films, we investigated molecular beam epitaxially grown (Zn,Sn,Mn)As2 samples doped with 2.1 and 3.6 at. % Mn. Representative regions with a low and high Mn concentration were extracted from APT datasets. Mn-As clusters containing 2-36 Mn atoms were identified in regions of high Mn concentration. We also obtained a correlation between TC and Mn-As clustering that was consistent with not only the experimental results but also first-principles calculations using the mean-field approximation.The ferromagnetic transition in (Zn,Sn,Mn)As2 thin films is explained in terms of magnetic percolation in a Mn-As clustering network. We first studied the relationship between the spatial distribution of Mn-As clusters and the Curie temperature (TC). The local atomic structure was reconstructed from datasets of atomic positions in (Zn,Sn,Mn)As2 obtained by atom probe tomography (APT). To probe the local atomic structure and the magnetic properties of Mn-As clusters in ZnSnAs2 thin films, we investigated molecular beam epitaxially grown (Zn,Sn,Mn)As2 samples doped with 2.1 and 3.6 at. % Mn. Representative regions with a low and high Mn concentration were extracted from APT datasets. Mn-As clusters containing 2-36 Mn atoms were identified in regions of high Mn concentration. We also obtained a correlation between TC and Mn-As clustering that was consistent with not only the experimental results but also first-principles calculations using the mean-field approximation.

MnAs thin films were grown by molecular beam epitaxy on InP substrates. The samples were in-situ annealed at different temperatures of 340 °C, 380 °C and 420 °C in an As4 atmosphere. High-resolution X-ray diffraction (XRD) measurements showed that zinc-blende type (zb-) and NiAs type (n-) MnAs co-exist. The XRD peak intensities of zb-MnAs increased with annealing temperatures up to 380 °C. Also, the in-plane XRD results showed that the intensities of zb-MnAs (400) and (220) increased after annealing. On the other hand, the peak intensities of n-MnAs decreased with temperature up to 380 °C. The possible transition between zinc-blende and hexagonal MnAs is estimated to be a temperature of 380 °C. (© 2013 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.

GaSb thin films have been grown on Si(111) substrates by molecular beam epitaxy (MBE), using an Sb template as an initiation layer. The film's crystal properties were investigated by in situ reflection high energy electron diffraction (RHEED), atomic force microscope (AFM), X-ray reciprocal space mapping (RSM) and ϕ - scan X-ray diffraction (XRD). RSMs around Si and GaSb 224 reciprocal lattice points indicated that the GaSb thin film grown on the Sb template is nearly unstrained, in contrast to that grown on an AlSb initiation layer which is under tensile strain. The extra peaks on the ϕ - scan XRD profiles can be assigned to {224} diffraction originating from a sub-domain region that corresponds to the region rotated by 180° with respect to the fundamental domain of epitaxial film. The very weak intensity of the extra peaks suggests that using the Sb template is an effective way to suppress sub-domain generation in the epitaxial film, resulting in a higher quality GaSb thin film on Si(111) substrates. The spotty RHEED patterns observed were analyzed as a superposition of zinc-blende type diffraction spots on the [ 111 ] – [ 2 ¯ 11 ] plane and [ 111 ] – [ 2 1 ¯ 1 ¯ ] plane, indicating that the GaSb quantum dots at the initial growth stage are not randomly orientated polycrystalline structures but a two-domain structure with zinc-blende type crystals.

We have successfully grown MnAs/ZnSnAs2/ZnSnAs2:Mn trilayer heterostructures on InP(001) substrates by molecular beam epitaxy. The magnetization curve showed small double steps due to the difference in coercive force between the ferromagnetic MnAs and ZnSnAs2:Mn layers. The current-in-plane giant magnetoresistance was clearly observed in magnetoresistance (MR) measurements in current-in-plane geometry. The MR ratio was found to be two times higher than that of single MnAs layer. This enhanced MR ratio can be ascribed to the GMR behavior in the trilayers, resulting from the change of the magnetization of the two ferromagnetic layers between parallel and antiparallel configuration. The temperature dependence of MR ratios also differ between the samples of trilayer and MnAs single layer.

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 fabricated (Zn, Sn, Ga)As 2 thin films grown on semi-insulating GaAs(001) substrate by molecular beam epitaxy and characterized their structural, compositional and electrical properties in terms of conduction-type control. The X-ray diffraction peaks of (Zn, Sn, Ga)As 2 films which shift toward lower angle compared with that of (Zn, Sn)As 2 film indicate that Ga incorporating in (Zn, Sn)As 2 decrease their lattice constants. Compositional analysis by electron-probe micro analyzer (EPMA) indicated that the composition ratios of Zn to all the cation sites are smaller than that of Sn. Hall effect measurements of these (Zn, Sn, Ga)As 2 thin films showed n-type conductivity. These results suggest that Ga atoms are substituted for Zn atoms more than Sn atoms and change their conduction type from p-type to n-type.

We investigated the influence of growth parameters on the film properties of multinary ZnSnAs 2 thin films with and without Mn doping grown by molecular beam epitaxy (MBE) on InP substrates. As growth parameters, the substrate temperature and the beam equivalent pressure (BEP) were varied. The most stoichiometric ZnSnAs 2 thin film was obtained when the substrate temperature was 340 °C. We found that the high-quality Mn-doped ZnSnAs 2 (ZnSnAs 2 :Mn) thin films can be realized by decreasing the beam flux of Zn source.

(Zn,Mn,Sn)As2 thin films containing Mn at more than 10% of cation sites were grown on InP substrates by molecular beam epitaxy (MBE). These films showed p -type conductivity and ferromagnetism with a Curie temperature greater than 330 K. They also indicated ferromagnetism, regardless of whether they are semiconducting or metallic. We discovered a linear relationship between the Curie temperature and Mn content in (Zn,Mn,Sn)As2 thin films. The experimental results are expected to provide an important clue to understanding the origin of ferromagnetism in ferromagnetic chalcopyrites, suggesting that the ferromagnetism in these materials derives from the scheme of hole-mediated ferromagnetic interaction. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

For the purpose of investigating their structural and optical properties, GaSb thin films and GaSb/AlSb multiple quantum well (MQW) structures were grown on Si(111) substrates. A GaSb/AlSb MQW structure was also grown on Si(001) substrate as a control sample. Surface morphologies and a XRD measurements of GaSb films grown on Si(111) substrates showed that the GaSb film with a 5 nm thick AlSb initiation layer has good crystal quality. Observation of the RHEED patterns of both MQWs suggests that both GaSb films are under tensile strain at growth temperature. In-plane XRD measurement of MQW on Si(111) showed that the (111) face of the GaSb film is aligned to the Si(111) surface upon rotation by 30 ° . Photoluminescence (PL) spectra consisting of two peaks at 1250∼1400 nm were observed for both MQWs.

We report on the molecular beam epitaxy (MBE) of GaSb films and GaSb/AlGaSb multiple quantum well (MQW) structures grown on Si(1 1 1) and Si(0 1 1) substrates using an AlSb initiation layer. The structural and optical properties of the films on the different oriented Si substrates were characterized by cross-sectional transmission electron microscopy (TEM), high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements. The GaSb films and the MQW structures grown on Si(0 1 1) were found to have rather rough surfaces and of poor crystallinity under the present growth condition with the AlSb initiation layer. In contrast, the films grown on Si(1 1 1) indicated a mirror surface, definite MQW structures, and HRXRD patterns. The PL emissions within the 1.3–1.5 μm for the MQW structures on Si(1 1 1) and Si(0 1 1) were observed at the temperatures up to 300 and 200 K, respectively. We found that the PL peak energy, around 1.5 μm, of the MQW structures grown on Si(1 1 1) is almost temperature independent up to ∼120 K and exhibits a smaller variation with increasing temperature compared to those of the samples grown on Si (0 0 1) substrates. In the case of the MQW on Si (0 1 1), the PL peak energy as a function of temperature showed an intermediate behavior between those grown on Si(1 1 1) and Si(0 0 1) 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.

GaSb/AlGaSb multi-quantum well (MQW) structures with an AlSb initiation layer and a relatively thick GaSb buffer layer grown on Si (0 0 1) substrates were prepared by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated definite MQW structures. The photoluminescence (PL) emission around 1.55 μm wavelength was observed for 10.34 nm GaSb/30 nm Al 0.6 Ga 0.4 Sb MQW structure at room temperature. Dependence of PL emission energy on GaSb well width was well explained by finite square well potential model.

The results of the application of the spectroscopic phase modulated ellipsometry (SPME) to a series of novel ternary compounds from anisotropic to polycrystalline are presented to show the high effectiveness of this optical technique with regard to the dielectric function of the new materials. Besides, SPME in specular reflection mode to study the polarization degree of the light reflected at specular angle is shown to be very informative in determination of the best conditions for optical measurements on rough surfaces such as, for example, surfaces of the polycrystalline ternary thiogallate compounds studied in this work. An incoherent approach relating the measured depolarization with the surface variance and optical constants of a material or thin film is proposed.

The band structure of ternary chain TlInSe 2 is calculated by a pseudo-potential method with allowance for non-locality of ionic pseudo-potentials. In the obtained band structure the symmetry and forbidden character of the direct transitions at band gap are ascertained to be the same as reported earlier from the results of the empirical pseudo-potential calculations. The imaginary part of the components parallel and perpendicular to the c -axis of the dielectric function tensor of TlInSe 2 is calculated at photon energies up to 10 eV. The real part of these components is obtained by extrapolation of the imaginary part to higher energies and subsequent Kramers–Kronig transformation. The obtained dielectric function is compared with the one obtained ellipsometrically in the range 0.85–6.5 eV. The results of comparison are rather favorable.

An approach based on peculiar angles is considered in relation with the problem of restoration of the dielectric function from ellipsometric measurements at and below energy gap on polycrystalline materials with macroscopic roughness. It is shown that at least for macroscopically and mildly rough surfaces, scattering changes only the amplitude diminution while the phase remains unchanged. This offers a possibility to restore the ellipsometric angles unperturbed by scattering provided that the parameters of surface distribution are known. The ellipsometric data on polycrystalline calcium and strontium thiogallates are briefly overviewed in connection with the performed analysis. The results of comparison show that both multiple scattering and overlayer-type effects can be involved in the transient region between diffraction and geometric optics.

The CaGa 2 S 4 and SrGa 2 S 4 with various grain sizes were studied over a wide range of photon energies at and below energy gap by using a spectroscopic phase modulated ellipsometer at room temperature. Complimentary, SEM and XRD techniques were applied. The pseudo-dielectric function of the samples with large grain sizes as compared to the light wavelength was found to be fairly reasonable. The energy gap for direct transitions in CaGa 2 S 4 was estimated at ∼4.10 eV. For samples with small grain sizes (∼100 nm), a splitting between principle angle and angle of minimal elliptic ratio, that could not be assigned to absorption was observed in the transient spectral range between diffraction and geometric optics.

The relationship between Mn concentration and Curie temperature (TC) is studied for Mn-doped ZnSnAs2 ferromagnetic semiconductors, epitaxially grown on InP substrates by molecular beam epitaxy. In the ferromagnetic phase, Mn distributions in a (Zn,Mn,Sn)As2 thin film with 7.2 cation percent (cat. %) Mn are investigated using three-dimensional atom probe tomography. The results indicate an inhomogeneous distribution which spreads to a relatively high Mn concentration of 9.0 at. % (at. %). In the paramagnetic phase, it is found that the paramagnetic to ferromagnetic transition takes place sharply with a TC of 334 K when the Mn doping concentration increases to about 4 cat. % Mn, which corresponds to a magnetic percolation threshold for ferromagnetism in (Zn,Mn,Sn)As2. An effective Curie temperature ⟨TC⟩ is considered to bridge the Curie temperatures obtained experimentally to those calculated theoretically in inhomogeneous magnetic semiconductors. The behavior of magnetism in Mn-doped ZnSnAs2 can be explain...

Thermoluminescence of Eu and Ce co-doped CaGa 2 S 4 has been investigated for the first time. All co-doped samples were found to exhibit a long afterglow of the Eu emission having a peak energy of ∼2.20 eV. Analyses of measured transient thermoluminescence curves showed two activation energies, ∼0.2 eV and ∼0.94 eV, for most co-doped samples. These two activation energies are shown to be explainable by the model which includes one trap level with two activation processes, and hole release from the terminal state of the Eu ion. Experimentally observed thermoluminescence curves are shown to be well reproduced by the theoretically formulated model which takes into account the above-mentioned hole release process.

The resonance-type energy transfer process has been investigated for the first time in Ce and Eu co-doped CaGa2S4 polycrystalline samples with different dopant concentrations in the range from 0.2 at.% to 4 at.%. The energy transfer from Ce to Eu ions increases with increasing Ce and Eu concentrations. The values of the rate determined from measured Ce emission decay time constants with different dopant concentrations showed a reasonable agreement with those obtained from intensity analyses of the Ce emission which takes into account the re-absorption effect by Eu ions. In addition, the observed change of the transfer rate with dopant concentration was discussed in terms of the distance between Ce and Eu ions.

The effects of cationic growth parameters of molecular beam epitaxy on the crystalline and magnetic properties of Mn-doped ZnSnAs2, (Zn,Sn,Mn)As2, thin films were investigated in order to control ferromagnetism at room temperature. The combination of cationic beam equivalent pressures of Zn, Sn, and Mn atoms was varied at an optimal substrate temperature of 320 °C. Clear hysteresis loops were observed for the samples at 300 K, indicating ferromagnetism at room temperature. We found that the change in Zn flux does not significantly affect the composition of (Zn,Sn,Mn)As2 thin films and ferromagnetic properties and that ferromagnetic properties definitely depend on the amounts of Mn and Sn fluxes. X-ray photoelectron spectroscopy analysis revealed that the sum of the amounts of Mn2+ and Mn4+, which may contribute to the ferromagnetism in (Zn,Sn,Mn)As2 thin films, was approximately 60–70% of the total Mn amount with respect to ferromagnetic (Zn,Sn,Mn)As2 samples. This result indicates that the ferromagnetic properties could be controlled in multilayered (Zn,Sn,Mn)As2 by changing the combination of Zn, Sn, and Mn fluxes.

The magnetization of (Zn,Sn,Mn)As2 thin films epitaxially grown on InP(001) substrates exhibited an anomalous temperature dependence, increasing slightly with temperature, compared with estimates based on the mean-field theory (MFT). Assuming that ferromagnetic (FM) and antiferromagnetic (AF) phases coexist at low temperature, these anomalous magnetic properties can be well explained by an AF-FM transition through a paramagnetic phase. The Neel temperature TN of the AF phase is estimated from the threshold of the difference curve between the experimental and theoretical data, assuming the magnetization of the FM phase, which follows the MFT. The estimated TN rapidly increases by ∼50 K in the range of Mn concentrations where the hexagonal MnAs (h-MnAs) phase precipitates. This can be explained by the superparamagnetic behavior of the precipitated h-MnAs nanoclusters, which possess a high blocking temperature.

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)

Atomic-scale Mn distributions in ferromagnetic ZnSnAs2:Mn thin films grown on InP substrates have been studied by applying three-dimensional atom probe (3DAP) microscopy. It is found that Mn atoms in cross-sectional 3DAP maps show the presence of inhomogeneities in Mn distribution, which is characteristic patterns of a spinoidal decomposition phase with slightly high and low concentration regions. The high Mn concentration regions are expected to be coherently clustered MnAs in the zinc-blende structure, resulting in the formation of Mn-As random connecting patterns. The origin of room-temperature ferromagnetism in ZnSnAs2:Mn on InP can be well explained by the formation of atomic-scale magnetic clustering by spinoidal decomposition without breaking the continuity of the zinc-blende structure, which has been suggested by previous theoretical works. The lattice-matching between magnetic epi-layers and substrates should be one of the most important factors to avoid the formation of secondary hexagonal MnAs phase precipitates in preparing ferromagnetic semiconductor thin films.

We prepared boron-doped Si/Si0.8Ge0.2 (Si:B/SiGe) multilayers sputter-deposited on quartz glass substrates, and investigated their structural and thermoelectric properties. These samples were processed by using conventional whole-layer annealing and layer-by-layer annealing methods. Si:B/SiGe multilayer samples annealed layer-by-layer showed good periodicity with well-defined interfaces as compared with conventionally annealed samples (whole-layer annealing). However, Ge diffusion was observed at the interfaces near substrates since the SiGe layer near the substrates suffered a longer integrated annealing time during the layer-by-layer annealing process. It was found that the Si:B/SiGe multilayer annealed layer-by-layer showed thermally stable thermoelectric properties in a wide temperature range from 50 to 800 °C. The present experiment proved that the layer-by-layer-annealing method is an effective way of achieving stable and reliable thermoelectric properties in Si:B/SiGe multilayer micro-thermoelectric devices.

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.

Room temperature spectroellipsometric study of TISe was performed in the photon energy region 1.5-4.6eV. The tensor components of the dielectric function spectrum of TlSe were obtained. Interband density-of-states (DOS) derived from XX (YY) and ZZ components of the obtained dielectric function spectrum was then compared with the calculated intraband DOS, taking into account for selection rules and transition probabilities. It is found that contribution of the interband optical transitions in Γ, T and N points quite plausibly explains main structures in the obtained dielectric function spectra. However, some structures cannot be explained due to optical transitions in these symmetry points. A corresponding assignment is given and discussed.

The first spectroellipsometric investigation of the crystalline and amorphous CaGa2S4 was done in the photon energy region 1.5-5eV and temperature region 290–400K. Dielectric function spectra were obtained. It is shown that two interband critical points (CPs), E0 (4.40eV) and E1 (∼4.75eV), contribute into the direct interband optical transitions near the absorption edge of the polycrystalline CaGa2S4. In amorphous CaGa2S4 the “critical points” are rather smashed and lie at lower energies with as large a shift for E0-CP as 0.58 eV. The energy separation between E0 and E1 approaches ∼0.40eV. The temperature coefficient dE/dT for E0 was found to be -2.7×10-4eV/K. Preliminary study of a monocrystalline CaGa2S4 has confirmed that the E0-CP energy in polycrystalline CaGa2S4 is the gap energy (Eg) for direct transitions in this material, which are strongly polarized. The obtained data are discussed.