Search

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.

The electronic and magnetic properties of pure as well as Mn-doped chalcopyrite semiconductor ZnSnAs2 were studied by the DFT+U ab initio method within the spin generalized gradient approximation. The band structure and DOS calculations resulted in a direct gap scenario for semiconducting ZnSnAs2. The energy gap of 0.34 eV occurs at the center of the Brillouin zone and linearly increases with increasing Mn concentration x at a rate of 2.05 eV for x < 0.03. The total energy calculations for a number of supercells showed that a ferromagnetic rather than antiferromagnetic ordering is favorable in Mn-doped ZnSnAs2 when Mn replaced Sn. The energy deficient substitution of Zn by Mn led to the antiferromagnetic ordering. The magnetic moment per Mn atom, together with contribution of each atom into the total moment of ZnSnAs2: Mn was determined. The contributions from Mn and As are dominant but have opposite signs. Besides, the As atoms chemically bounded to Mn dopant were found to be most contributive. At last it was shown that the vacancies such as V(Zn), V(Sn), and V(As) affect the magnetic properties and may even strengthen the magnetization of Mn-doped ZnSnAs2. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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)

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 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.

We have directly grown zinc-blende (zb)-type MnAs thin films on InP (001) substrates without the aid of any buffer layer using molecular beam epitaxy (MBE). From the High-resolution X-ray diffraction (XRD) data, assuming face-centered cubic (fcc) MnAs structure, the average lattice constants values were calculated to be 6.068 and 6.060 A for growth temperatures of 250 and 300 °C, respectively. High-resolution transmission electron microscopy (TEM) investigations and selected-area electron-diffraction (SAD) verified the successful growth of zb-type cubic MnAs coexisting with the NiAs-type hexagonal MnAs. The saturation magnetization was estimated to be 300 emu/cm 3 determined from the magnetic field dependence of the magnetization curves. From the temperature dependence of magnetization, the Curie temperature was found to be approximately 308 K. Success in the growth of zb-type MnAs thin films could be reasonably explained by the existence of a monolayer of InAs at the interface between the MnAs and InP substrates.

(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)

Ternary ZnSnAs 2 thin films heavily doped with nominal 10 and 20% Mn content on InP (001) substrates are grown using low-temperature molecular beam epitaxy and their magnetic and transport properties are investigated for the first time. It is found that the Mn-doped ZnSnAs 2 thin films are pseudomorphically grown on nearly lattice-matched InP (001) substrates, and a trace amount of secondary phase MnAs formation is observed by high-resolution X-ray diffraction (HR-XRD) measurements. Magnetization measurements on Mn-doped ZnSnAs 2 thin films reveal that the Curie temperature is around 334 K. Nominal magnetic moments per Mn atom measured from the saturation magnetization of hysteresis loops at 5 K have been estimated as 5.28 and 4.17 μ B for 10% and 20% Mn-doped ZnSnAs 2 thin films, respectively. We have found from Hall effect measurements that the 10% and 20% Mn-doped ZnSnAs 2 films exhibit n-type conduction, in contrast to p-type conduction in ZnSnAs 2 doped with less than 10% Mn. This is likely related to the presence of a certain amount of Mn interstitials or Mn 3+ substitution on Zn site in the samples.

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.

Since the ferromagnetism in ternary chalcopyrite semiconductor CdGeP2:Mn was first reported by Medvedkin et al., several ferromagnetic ternary compounds such as II-IV-P2 and II-IV-As2 doped with Mn have been exploited from the viewpoint of experimental and theoretical aspects. Almost of these compounds have been synthesized in forms of bulk II-IV-V2 chalcopyrite crystals. In this article, we review recent experimental results of diluted ferromagnetic ZnSnAs2:Mn thin films among various ternary ferromagnetic compounds exhibiting room temperature ferromagnetism. We expect that ZnSnAs2:Mn epitaxial thin films is a promising candidate as a ferromagnetic building block in InP-based semiconductor spintronics, although the origin of ferromagnetism is not yet clearly understood. The state of development of ZnSnAs2:Mn thin films is still at a very early stage, and basic information on the ZnSnAs2:Mn is required to elucidate the origin of the ferromagnetism. [DOI: 10.1380/ejssnt.2011.95]

ZnSnAs 2 epitaxial film has been grown on epi-ready semi-insulating InP(001) substrates by low-temperature molecular beam epitaxy (LT-MBE) technique. The MBE-grown sample was then cleaved into pieces, three of which were subjected to lowtemperature annealing at different temperatures of 300 ∘ C, 320 ∘ C and 340 ∘ C which are equal or slightly higher than growth temperature using face-to-face proximity capping by GaAs wafers to simulate arsenic atmosphere. HR-XRD measurements showed that increasing annealing temperature decreases the lattice constant towards the bulk value. This suggests that indeed the relatively higher lattice constant of ZnSnAs2 epitaxial films is partly, if not wholly, due to defects consequence of low temperature growth. For the as-grown control sample, resistivity of 4.31×10 −2 Ω cm, mobility of 17.7 cm 2 /V −s and hole concentration of 8.18×10 18 cm −3 were obtained at room temperature. After annealing at 340 ∘ C, the resistivity was increased to 21.0×10 −2 Ω cm, the mobility increased to 60.9 cm 2 /V −s, and the hole concentration was decreased to 4.88×10 17 cm −3 .

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.

ZnSnAs2 epitaxial films were grown by molecular beam epitaxy (MBE) on nearly lattice matched InP substrates. Four samples namely samples A, B, C, and D were prepared at different growth times of 15, 30, 50, and 83 mins, respectively, using the optimum growth conditions earlier reported to obtain samples of different values of thickness for the purpose of structural characterization using High-Resolution X-ray Diffraction (HR-XRD). HR-XRD investigations revealed unrelaxed lattice constant a ⊥ values along the growth direction of 5.8991 A, 5.8991 A, 5.8886 A, and 5.8928 Afor samples A, B, C, and D, respectively. Reciprocal Space Mapping on one of the samples indicates pseudomorphic growth with respect to the InP substrate. The absence of full-width at half maximum (FWHM) broadening of the of the HR-XRD rocking curves with increasing thickness of the samples suggests that all the epitaxial films in this work are pseudomorphic with the InP substrate. Assuming a Poisson ratio ν of 1/3, the corrected values of the lattice constant, i.e. values of the free-standing lattice constant afs, were calculated to be 5.8840 A, 5.8840 A, 5.8788 A, and 5.8809 Afor samples A, B, C, and D. These results suggest that the elongation due to the pseudomorphic growth is substantial in the computation of the true lattice constant of ZnSnAs2 epitaxial films.

The previously reported transport properties data of the undoped ZnSnAs2 grown on semi-insulating InP substrates by molecular beam epitaxy (MBE) have been reviewed and analysed. We have found out that the temperature dependence of Hall coefficient and resistivity can be well explained by the impurity band model proposed by Isomura and Tomioka. By using the said 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. To support the validity of the model, we also have confirmed the presence at low temperatures of negative magnetoresistance expected if impurity band conduction is predominant. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have prepared Ge 0.92 Mn 0.08 thin films by molecular beam epitaxy (MBE) on semi-insulating (0 0 1) GaAs substrates and investigated their transport and magnetic properties. We have found out that the resulting properties of the thin films are highly dependent on the substrate temperature T s . The sample grown at T s =130 °C exhibited a pronounced negative magnetoresistance and anomalous Hall effect even up to room temperature most probably due to the presence of substitutional Mn in the Ge host matrix and the formation of the Ge 5 Mn 3 phase. Because these phenomena were not detected for samples grown above 130 °C, we believe that these are due to the T s dependence of Mn distribution in Ge 0.92 Mn 0.08 thin films and the degree of the interdiffusion of Ge, Ga and As atoms at the Ge 0.92 Mn 0.08 /GaAs interface.

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.

ZnSnAs 2 thin film doped with ∼4% Mn was grown on (0 0 1) InP substrates by all elemental solid source molecular beam epitaxy using the previously determined optimum substrate temperature of 300 °C and Zn:Sn:As 4 beam equivalent pressure ratio of 24:1:52. In this work, we have employed slower growth rate by halving the beam equivalent pressures of Zn, Sn and As 4 we have used in our earlier reports, while maintaining the same beam equivalent pressure ratio. From the high-resolution X-ray diffraction (HRXRD) data, the computed lattice constant a is 5.867 A, which is in very good agreement with the reported lattice constant value of the bulk ZnSnAs 2 chalcopyrite, suggesting that lowering the growth rate leads to a transition to a more chalcopyrite structure. The average composition ratio of Zn:Sn:As:Mn according to electron probe micro-analysis (EPMA) studies is 1:0.826:2.42:0.076. Hysteretic M – H curves were obtained at 5, 100, 300 and 320 K using a superconducting quantum interference device (SQUID) magnetometer. The Curie temperature was estimated to be ∼330 K from the zero-field cooled temperature dependence of magnetization. The possible origin of this above-room-temperature ferromagnetism is discussed.

(Ga,Mn)As/Zn-doped GaAs superlattices (SLs) have been prepared by molecular beam epitaxy (MBE), and the effect of low-temperature thermal annealing on their structural, electrical, and magnetic properties has been investigated. The SL structures studied in this work consists of 5-periods of 20-nm (Ga,Mn)As/5-nm GaAs. In order to introduce extra holes in (Ga,Mn)As layers, the GaAs spacer layers were doped with Zn acceptor atoms. Low-temperature thermal annealing was also performed to increase the hole concentration by reducing lattice defects like Mn interstitials and As antisite defects in (Ga,Mn)As layers. As-grown SL sample showed a ferromagnetic transition temperature T C of ∼50 K, and the presence of noticeable abrupt change in magnetization around ∼35 K in the M – T curve which is possibly due to the rotation of in-plane uniaxial anisotropy with changing temperature. Low-temperature annealing led to an increase in T C up to 65 K accompanied with a decrease in the resistivity.

To compare the annealing effects on GaMnAs-doped with Zn (GaMnAs:Zn) and undoped GaMnAs (u-GaMnAs) epilayers, we grew GaMnAs thin films at 200 °C by molecular beam epitaxy (MBE) on GaAs substrates, and they were annealed at temperatures ranging from 220 °C to 380 °C for 100 min in air. These epilayers were characterized by high-resolution X-ray diffraction (XRD), electrical, and magnetic measurements. A maximum resistivity at temperatures T m close to the Curie temperatures T c was observed from the measurement of the temperature-dependent resistivity ρ ( T ) for both the GaMnAs:Zn and the u-GaMnAs samples. We found, however, that the maximum temperature T m observed for GaMnAs:Zn epilayers increased with increasing annealing temperature, which was different from the result with the u-GaMnAs epilayers. The formation of GaAs:Zn and MnAs or Mn–Zn–As complexes with increasing annealing temperature is most likely responsible for the differences in appearance.

We investigated the formation of ~1–3 nm ultrathin SiON films by the plasma nitridation of oxidized Si substrates having different orientations, namely, (100), (110), and (111). Oxidation was performed by dry, wet, and plasma oxidation processes with rapid thermal oxidation (RTO), in situ steam generation (ISSG), and a slot plane antenna (SPA), respectively. Nitridation was performed by a plasma nitridation process with SPA. The thickness of base SiO2 films prepared by a plasma oxide process was independent of substrate orientation unlike in the dry and wet oxidation processes. Furthermore, no significant discrepancy in nitrogen areal density was observed among the SiON films grown by the plasma nitridation of differently oriented oxidized Si substrates. Our results suggest that SiON films with arbitrary nitrogen densities and thicknesses can be prepared by oxidation and plasma nitridation irrespective of the Si substrate orientation.

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.

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.

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.

We report the molecular beam epitaxy (MBE) growth of lightly Mn-doped Zn–Sn–As-based compounds on (0 0 1) InP substrates. Our XRD measurements showed peaks assignable to sphalerite-type ZnSnAs 2 :Mn, a supposition further supported by our EPMA and TEM analyses. The optimum substrate temperature T S resulting into the most stoichiometric ZnSnAs 2 epitaxial films was first determined by growing undoped ZnSnAs 2 at different T S of 265, 280, 300, and 320 °C. The sample grown at T S =300 °C showed the best stoichiometry according to our EPMA composition studies. We then prepared lightly Mn-doped ZnSnAs 2 at this optimum substrate temperature of 300 °C. Room temperature SQUID measurements revealed paramagnetic response with weak ferromagnetism as evident from the slightly S-shape M – H curve superimposed on a linearly increasing magnetization with increasing field.

GaMnAs epitaxial films with high Zn incorporation level were prepared by molecular beam epitaxy. Our results indicate an increasing hole concentration p accompanied by decreasing Curie temperature T c and transition towards metallic behavior of the epitaxial film properties with increasing level of Zn incorporation.

We investigated the growth of GaSb layers on Si (001) substrates by molecular beam epitaxy (MBE). Epilayers were grown as a function of Sb4/Ga beam equivalent pressure ratio (BEPR). They were then characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), photoluminescence (PL), micro Raman scattering analysis. We confirmed that the optimum condition to grow relative high quality GaSb layer in this study was Sb4/Ga=20. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Heavily Sn-doped GaAs1-xSbx epitaxial films were grown on SI-GaAs (001) substrates by solid source molecular beam epitaxy. A 5 nm-thick AlSb buffer layer was employed to relax the lattice mismatch between the epilayer and the substrate. X-ray diffraction (XRD), Hall effect measurements and photoluminescence measurements were performed to characterize the epitaxial films. The heavily Sn-doped GaAs1-xSbx / AlSb films with x ≤ 0.24 indicated n-type conduction while the epitaxial films with x ≥ 0.43 indicated p-type conduction. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence spectra of stoichiometric rare-earth molybdate and tungstate compounds MNd(MoO4)2, MNd(WO4)2 (M = Li, Na, K) have been investigated. All the compounds exhibit emissions around 0.89 μm, 1.06 μm and 1.34 μm due to the transitions from 4F3/2 to 4I9/2, 4I11/2 and 4I13/2, respectively, in Nd3+. The Nd3+ emissions from the stoichiometric Nd molybdate and tungstate, which are charge-compensated by alkali atoms of Li and K are the first observations to the best of our knowledge. Amoung these compounds, the efficiency of the Nd emission, defined as ratio of emission intensity measured with integration semi-sphere to the real absorption, is the highest and the line width of the 1.06 μm emission is the narrowest for KNd(WO4)2. These results suggest a sufficient potentiaslity of KNd(WO4)2 as a stoichiometric laser material. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In order to fabricate a (Ga,Mn)As trilayer structure with tunneling magnetoresistance (TMR) effect on Si, GaAs buffer layers grown on Si (0 0 1) substrates were investigated as a function of growth sequence by employing relatively low-temperature (LT) processes less than 800 °C. The present results showed that both the thick film growth and the several intermittent annealing of initial LT-GaAs buffer layers were effective in improving the crystalline quality of GaAs films on Si substrates without high-temperature cleaning. The GaAs buffer layer was prepared on Si substrate using the optimized growth condition, and then a (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunneling junction structure was fabricated. TMR ratio was measured to be approximately 13% at a temperature of 5 K.

We investigated the growth of GaAs 1− x Sb x ( x =1.0, 0.82, 0.69, 0.44, 0.0) layers on Si (0 0 1) substrates using AlSb as a buffer layer. Epilayers were grown as a function of As beam equivalent pressure (BEP) under a constant Sb BEP, and they were then characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), and micro-Raman scattering analysis. We confirmed that GaAs 1− x Sb x layers have been successfully grown on Si substrates by introducing AlSb layers.

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.

We investigated the properties and annealing effects of substrate-free (Ga,Mn)As films prepared by etching Si substrates from (Ga,Mn)As/Si structures, and compared the results with those from (Ga,Mn)As/Si heterostructures. The substrate-free (Ga,Mn)As films with 6% Mn content were annealed at 250 °C as a function of time. From Hall-effect measurements, the Curie temperature of substrate-free (Ga,Mn)As films was estimated to be 87 K for an as-grown film, enhanced up to 152 K after low-temperature annealing for 60 min. We found that the (Ga,Mn)As films grown on Si substrates show a relatively high Curie temperature.

A boron doped Si1−xGex thin film has been successfully prepared on a quartz (SiO2) glass substrate at room temperature (RT) by irradiating an intense, pulsed, ion beam on a Si–Ge–B pellet. As a result, in situ boron doped Si1−xGex thin film was found to be crystallized. In addition, by van der Pauw measurement, the resistivity, carrier density, and mobility of the thin film were observed as ρ=2.3×10−3Ωcm, n=7.2×1020cm−3, and μ=3.8cm2∕Vs, respectively. Thus, the boron doped Si1−xGex thin film was clarified to be not only crystallized but also activated without heat treatment.

We investigated ternary MnAs1-xSbx thin films on GaAs (001) substrates by low-temperature molecular beam epitaxy under varying growth conditions, studying their crystal structures and magnetic properties. It was found that MnAs1-xSbx film with x=0.2 seems to be lattice-matched to GaAs (001) substrates and is suitable for the growth of magnetic multilayers. We explored the possibility of growing magnetic MnSb/MnAs/MnAs1-xSbx multilayers on GaAs (001), and successfully prepared this kind of magnetic multilayer. We also report herein preliminary results regarding the magnetotransport properties of MnAs1-xSbx films in relation to the Sb content.

We describe the growth of diluted magnetic semiconductor (DMS) (Ga1−x, Mnx)As epitaxial layers on n-type Si(1 0 0) substrates using low-temperature-molecular beam epitaxy (LT-MBE). The Mn content of the (Ga1−x, Mnx)As layers was relatively high (6.2%). The ferromagnetic transition temperature TC was estimated to be 80 K for the as-grown film, and it strongly depended on the annealing temperature. The p-(Ga, Mn)As/n-Si heterostructures showing a ferromagnetic nature indicated a change in the sign of the Hall coefficient. We found that the transition temperature from n-type to p-type conduction considerably correlates with the TC.

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...

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.