Characterization of heterointerfaces in GaAs/MnAs/MnZn-ferrite structures

We have grown GaAs epitaxial films on MnZn-ferrite substrates using MnAs buffer layers and investigated their heterointerfaces with glazing incidence-angle X-ray reflectivity and X-ray photoelectron spectroscopy. It has been found that the heterointerfaces for this structure are quite abrupt and the roughness at the GaAs/MnAs and MnAs/MnZn-ferrite interfaces are 1.1 and 0.2 nm, respectively. We also found that the diffusion of atoms through the GaAs/MnAs interface into the GaAs film is negligible. These results indicate that the MnAs buffer layer for the GaAs/ferrite structure is chemically stable and promising for the application to the future magnetic electronics.     

Controllable cubic and hexagonal GaN growth on GaAs(0 0 1) substrates by molecular beam epitaxy

Two kinds of GaN samples were grown on GaAs(0 0 1) substrates. One is grown on nitridized GaAs surface, the other is grown on nitridized AlAs buffer GaAs substrate. X-ray diffraction and photoluminescence measurements find that the GaN sample directly grown on GaAs substrate is pure cubic phase and those grown on AlAs buffer is pure hexagonal phase. The present study shows that the phase of GaN samples grown on GaAs substrates can be controlled using different buffer layers.     

Effect of rapid thermal annealing on Zn1‐xCdxO layers grown by radio‐frequency magnetron co‐sputtering

Zn_1‐xCd_xO layers were deposited on the sapphire substrate using the radio‐frequency magnetron co‐sputtering system. The grown Zn_1‐xCd_xO layers were carried out in the post‐annealing treatment for 1 min at the 800 °C oxygen‐ambient by the rapid thermal annealing (RTA) method. X‐ray diffraction (XRD) experiment shows that the Zn_1‐xCd_xO layers are changed from the single phase of the hexagonal structure at 0≤x ≤0.08 to the double phase of hexagonal‐and‐cubic structure at x =0.13. Thus, the maximum Cd‐composition ratio with the hexagonal structure was found out to be x =0.08. Also, the crystallinity of Zn_1‐xCd_xO layers at x =0.13 was remarkably improved by the RTA annealing treatment. This crystal quality improvement was thought to be associated with the relaxation of the compressive strain remaining in the Zn_1‐xCd_xO layers. Therefore, the results of XRD and transmittance lead that the crystal quality of the Zn_1‐xCd_xO layers forming the hexagonal ZnO phase is better than that forming the cubic CdO phase. Consequently, the reliable formation and the crystallinity of the Zn_1‐xCd_xO layers were achieved by using the RTA method of short‐time thermal‐annealing at the high temperature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of surface reconstruction on CdTe/GaAs(001) interface structure

CdTe/GaAs(001) heterostructures were fabricated by molecular beam epitaxy on chemically etched and thermally deoxidized GaAs(001) substrates, as well as GaAs(001) (3×1) buffer layers grown in situ by molecular beam epitaxy. Different growth protocols were also explored, leading to Te-induced (6×1) or (2×1) surface reconstructions during the early growth stage. High-resolution cross-sectional transmission electron microscopy was used to examine the final interface structure resulting from the different substrate preparations, and surface reconstructions. The (2×1) surface reconstruction led to pure (001) growth, while the (6×1) reconstruction led to an interface which included small (111)-oriented inclusions. In addition, deposition on etched and deoxidized GaAs(001) wafers led to preferential CdTe growth within etch pits and resulted in a macroscopically rough interface region.     

Zinc-blende MnAs thin films directly grown on InP (001) substrates as possible source of spin-polarized current

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 Å 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/cm³ 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.     

Phonon behavior and interfacial stress in the strained (InAs)m/(GaAs)n ultrathin superlattices

Phonon behavior in the strained (InAs)_m/(GaAs)_n ultrathin superlattices grown by molecular beam epitaxy has been investigated by means of Raman scattering spectroscopy. The phonon frequency in the GaAs layers shifts toward lower energy with increasing InAs layer thickness under fixed thickness of GaAs layers. The frequency in the InAs layers does not change significantly, as deduced from the behavior of the InAs-like mode in In_xGa_1−xAs alloys. These observed results are phenomenologically discussed on the basis of the combined effect of phonon confinement in the respective layer and stress at the alternating interfaces. Furthermore, a large softening of the phonon confined in the GaAs layers decreases the frequency gap, resulting in traveling of the optical phonons confined in both layers. The strain at the interfaces is estimated by an empirical method, i.e., by comparing the frequency in the superlattice and in the alloy of equivalent composition. In the In_xGa_1−xAs alloys, the composition dependence of the mode frequency is considered as being due to the local strain. The group III elements are considered to be in the local strain state from an extended X-ray absorption fine structure (EXAFS) analysis.     

Metalorganic vapour phase epitaxy growth of InGaAs/GaAs strained layer quantum wells beyond the pseudomorphic limit

Low pressure metalorganic vapour phase epitaxy growth and characterization of strained InGaAs/GaAs quantum well structures beyond the pseudomorphic limit are reported. Photoluminescence and X-ray diffraction measurements of these structures show considerable gallium/indium interdiffusion at the interfaces and partial strain relaxation in the layers.     

Strain analysis of InGaN/GaN multi quantum well LED structures

Five period InGaN/GaN multi quantum well (MQW) light emitting diode (LED) structures were grown by a metalorganic chemical vapor deposition (MOCVD) system on c‐plane sapphire. The structural characteristics as a strain‐stress analysis of hexagonal epilayers MQWs were determined by using nondestructive high resolution x‐ray diffraction (HRXRD) in detail. The strain/stress analysis in AlN, GaN, and InGaN thin films with a variation of the In molar fraction in the InGaN well layers was conducted based on the precise measurement of the lattice parameters. The a‐ and c‐lattice parameters of the structures were calculated from the peak positions obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in‐plane and out‐of‐plane strains. The biaxial and hydrostatic components of the strain were extracted from the obtained a‐ and c‐direction strains values.     

Growth temperature dependence in molecular beam epitaxy of gallium arsenide

Epitaxial layers of GaAs were grown on GaAs(100) at substrate temperatures ranging from 400° to 600°C by molecular beam epitaxy. Surface structures of the substrate and the epitaxial layers were investigated by means of low-energy electron diffraction. Two new structures of c(4 × 4) and c(8 × 8) were observed from layers grown at the low temperature of 400°C. The electrical and optical properties of layers doped with Si were investigated by measurement of Hall effect and photoluminescence as a function of growth temperature. It is found that a semi-insulating layer is grown below a critical temperature, and the layer is useful as a buffer layer for GaAs FET's. Variation of carrier concentration was observed near the interface between layers grown at different temperatures under a constant Sn beam flux. The effect is attributed to defect-induced segregation of Sn.     

Neutron diffraction investigation of the atomic and magnetic structures of MnAs at high pressures

The atomic and magnetic structures of the MnAs compound are investigated using neutron diffraction at high pressures up to 38 kbar in the temperature range 15–300 K. It is found that, as the temperature decreases under high pressure, the MnAs compound undergoes a spin-reorientation transition from the ferromagnetic orthorhombic phase to a new phase whose magnetic moment has both the ferromagnetic and antiferromagnetic components. The structural and magnetic parameters of the high-pressure phases of the MnAs compound are determined.     

水热法合成Mnx Zn1-xO微晶体

本文采用水热法合成了MnxZn1-xO晶体,水热反应条件为3mol·L-1KOH作为矿化剂,填充度为35;,温度为430℃,在Zn(OH)2中添加一定量的MnO2为前驱物,反应时间为24h.通过X射线能谱仪测量了晶体中的Mn含量,随着前驱物中MnO2含量的增加,晶体中Mn的原子百分比随着增加,Mn最大原子百分比含量超过了2;,晶体的形貌具有纯ZnO晶体的六角柱形特征.显露柱面m{1010}、锥面p{1011}、负极面O面{0001}和正极面{0001}.晶体直径为50～200μm,高度为20～100μm.     

Pressure assisted evolution of defects in silicon

The effect of enhanced hydrostatic pressure following heat treatment on the evolution of point defects in neutron‐irradiated Czochralski‐grown silicon is investigated using infrared spectroscopy. The behavior of oxygen‐related defects, particularly of the VO and the VO₂ centers, is mainly studied using samples subjected to heat treatment under hydrostatic pressure. It is observed that (1) pressure accelerates the annealing process of the VO defects and enhances the growth of the VO₂ complexes and (2) the VO₂ concentration is larger than expected from the corresponding decay of the VO defects. The faster decay of the VO defects is attributed to a pressure‐induced decrease of their migration energy. The larger VO₂ concentration is also discussed. One possible explanation is that pressure stimulates an additional mechanism for the formation of the VO₂ defects, which involves the reaction of oxygen dimers with vacancies. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman scattering study on Ga1–x Mnx As prepared by Mn ions implantation,deposition and post‐annealing

Raman scattering measurements have been performed in Ga_1–x Mn_x As crystals prepared by Mn ions implantation, deposition, and post‐annealing. The Raman spectrum measured from the implanted surface of the sample shows some weak phonon modes in addition to GaAs‐like phonon modes, where the GaAs‐like LO and TO phonons are found to be shifted by approximately 4 and 2 cm^‐1, respectively, in the lower frequency direction compared to those observed from the unimplanted surface of the sample. The weak vibrational modes observed are assigned to hausmannite Mn₃O₄ like. The coupled LO‐phonon plasmon mode (CLOPM), and defects and As related vibrational modes caused by Mn ions implantation, deposition, and post‐annealing are also observed. The compositional dependence of GaAs‐like LO phonon frequency is developed for strained and unstrained conditions and then using the observed LO_GaAs peak, the Mn composition is evaluated to be 0.034. Furthermore, by analyzing the intensity of CLOPM and unscreened LO_GaAs phonon mode, the hole density is evaluated to be 1.84×10¹⁸ cm^‐3. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of annealing and composition on crystal structure,surface morphology and optical absorption of chemically deposited Cd1‐xSnxS films

Cd_1‐xSn_xS thin films were successfully deposited on suitably cleaned glass substrate by chemical bath deposition method at 74 °C. Hydrated Stannous Chloride (SnCl₂.2H₂O) in aqueous solution was added to the CdS growing solution in different proportions. The experimental results indicate, a successful doping for lower concentration of Sn, saturation for intermediate doping levels, and a degradation of the doping process for higher concentration of Sn. Indirect (X‐ray diffraction) and Direct (Scanning electron microscopy) measurements were performed to characterize the growth and the nature of crystallinity of the different Cd_1‐xSn_xS films. The effect of annealing on the crystal structure and morphology of the deposited films has also been discussed. The X‐ray diffraction spectra show that the thin films are polycrystalline and have both cubic and hexagonal structure. The Interplanar spacing, lattice constant, grain size, strain, and dislocation density were calculated for as‐deposited and annealed films. The grain size was found to decrease from 5 nm to 0.89 nm with doping concentration of Sn. The grain size further decreased due to annealing at 400 °C. SEM studies show layered growth and long needle like structures along with some voids. After annealing the densification and smaller size of the particles was also observed. The optical absorption spectra show shifting of absorption peaks towards lower wavelength side (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure of SiC‐Quantum Wells Studied by TEM and CBED

Multi‐quantum well structures of 3C/4H‐SiC polytypes grown either on stepped or on on‐axis hexagonal SiC by molecular beam epitaxy have been investigated by conventional and high resolution TEM. The 3C‐SiC layers were nearly free of defects and the interface between different polytypes was abrupt. For the 3C‐SiC layers the strain state and the lattice parameters have been investigated to a high accuracy by convergent beam electron diffraction (CBED).     

Formation and characterization of GaAs/As superlattice grown by molecular beam epitaxy at low substrate temperature

High-resolution X-ray diffractometer and transmission electron microscope (TEM) are used to characterize the redistribution of As precipitates in Si δ-doped GaAs grown by molecular beam epitaxy at low substrate temperature (230°C). The superlattice satellite peaks are observed for samples annealed at 700-800°C for 10 min, which is attributed to the formation of a GaAs/As superlattice during the annealing period. The degree of As precipitates confined on the δ-doped planes is revealed on the intensity of satellite peaks in the X-ray rocking curves, as confirmed by the TEM observations. The lattice expansion and contraction of the annealed low-temperature epitaxial layers can be easily observed from the asymmetry of the satellite peaks.     

MOVPE growth of GaInP/GaAs hetero-bipolar-transistors using CBr4 as carbon dopant source

Carbon doping of GaAs with carbon tetrabromide (CBr₄) in low pressure MOVPE has been investigated and applied to the fabrication of GaInP/GaAs HBTs. Especially the hydrogen incorporation and the associated acceptor passivation has been studied. The hydrogen found in single GaAs:C layers is predominantly incorporated during cooling the sample under AsH₃ after growth, n-Type capping layers can block this H indiffusion and GaAs:C base layers in HBTs show much lower H concentrations than GaAs:C single layers without a cap. A further reduction of acceptor passivation is possible by optimization of the growth procedure. First HBTs processed from layers with a base that was doped using CBr₄ show promising DC and HF performance (β = 45, for 2 × 20 μm² devices).     

Element incorporation in vapour grown III–V compounds

Some recent developments and ideas in the field of element incorporation in vapour grown III–V compounds are highlighted in this paper. The study is based mainly on GaAs and GaP. Specific areas of consideration are as follows. The role of equilibrium thermodynamics in predicting element incorporation. Here, theoretical developments are discussed and the difficulties in testing models in vapour growth in contrast to melt growth are noted. The residual impurity problem is considered, particularly the effects of vapour pressure parameters on the properties of epitaxial layers. The role of surfaces in relation to growth rate and the orientation-dependence of element incorporation are discussed. In connection with the field of defect chemistry, the effect of III–V ratio on element incorporation is debated. Also in the context of defects, the role of ionised As vacancies in GaAs is considered as a controlling parameter in element incorporation. This development allows one to account for the well-known insensitivity of doping to the intrinsic electron concentration.     

Chemical Deposited Pd on GaAs

The chemical deposition of palladium on n and p‐GaAs from an aqueous solution of PdCl₂, in strong acid medium can be an alternative technique for contacts on GaAs. This paper presents the variations of palladium films thickness (measured by an interferometric method) as a function of solution concentration, reaction time and substrate conductivity. There are presented data concerning the adhesion of palladium films (measured by a mechanical method) related to the film thickness. From a comparative study concerning the adhesion of evaporated films such as: Au/n‐GaAs, Ag/n‐GaAs, AuGe/n‐GaAs and Pd/n‐GaAs, it results the conclusion that the adhesion of palladium films are of the same order of magnitude as the metallic films evaporated without an annealing procedure.     

Surface morphology of carbon-doped GaAs grown by MOVPE

The development of the surface structures of carbon-doped epitaxial GaAs layers grown by metalorganic vapor phase epitaxy was investigated by atomic force microscopy (AFM). Carbon-doped GaAs layers were grown using trimethyl gallium and a mixture of AsH₃/TMAs. The AFM micrographs were quantitatively analyzed through the determination of the height-height and height-difference correlation functions, which yields both the short and long range surface structures. The incorporation of carbon leads to the progressive roughening of the GaAs surface as well as an increase in surface correlation length. The high concentration of surface-adsorbed methyl radicals are suggested to lead to the diminution of growth rate and change in surface structure.