激光超高温度梯度快速定向凝固研究

研究了激光重熔工艺参数对三种不同成分的Cu Mn合金重熔区微观组织生长方向的影响。结果表明 ,熔池中微观组织的生长方向强烈地受激光工艺参数 (激光输出功率和扫描速度 )和合金成分的影响。通过选择合适的工艺参数 ,实现了与Bridgman法类似的超高温度梯度快速定向凝固 ,其温度梯度可高达 10 6 K m ,速度可高达 2 4mm s。利用激光表面熔凝技术实现超高温度梯度快速定向凝固的关键在于 :1)在激光熔池内获得与激光扫描速度方向一致的温度梯度 ;2 )根据合金凝固特性选择适当的激光工艺参数以获得胞晶组织。     

Resistivity variation of semi-insulating Cd1−xZnx Te in relationship to alloy composition

We investigated the resistivity variation of semi-insulating Cd_1−xZn_xTe used as room temperature nuclear radiation detectors, in relationship to the alloy composition. The resistivity and the zinc composition were determined using leakage current measurements and triple axis x-ray diffraction lattice parameter measurements, respectively. While the zinc content of the nominally x_Zn∼0.1 ingot varied monotonically according to the normal freezing behavior with an effective segregation coefficient of k_eff=1.15, the resistivity was found to vary non-systematically throughout the ingot. Furthermore, the “expected” relationship of higher zinc content with higher resistivity was not always observed. For example, wafer regions of x_Zn∼0.12 and x_Zn∼0.08 exhibited resistivity values of ∼10¹⁰ and ∼10¹¹ Ω·cm, respectively. In general, the experimental resistivity values can be explained by calculated values which take into account a compensating deep level defect and various electron and hole mobility values. The relative influence of the parameters that govern the resistivity (n,p, μ_e, and μ_h) are quantitatively investigated.     

Vapor phase growth of Hg1−xCdx Te epitaxial layers

Epitaxial layers of Hg_1−xCd_xTe have been grown on CdTe substrates by a new vapor-phase method which yields compositions covering the entire alloy range (0≤x≤1). Independent control of the four principal parameters that determine the composition — the substrate temperature (400-550‡G) and the partial pressures of Hg, Cd and Te — is achieved in an open-tube system that utilizes separate elemental sources. The choice of growth parameters is discussed in terms of a model that treats the deposition of the alloys as the simultaneous deposition of HgTe and CdTe. In the direction perpendicular to the layer surface there is an initial region of uniform alloy composition, followed by a graded region extending to the substrate. Absorption edge measurements showed the composition parallel to the surface to be quite uniform. All the layers are n-type, with carrier concentrations of 10¹⁶-10¹⁷/cm³ to a depth of several micrometers, below which there is a region of greatly reduced concentration extending into the substrate.     

Vertical bridgman techniques to homogenize zinc composition of CdZnTe substrates

In order to improve the Zn homogeneity along the axial direction of CdZnTe boule, we have employed a modified Bridgman technique using a (Cd, Zn) alloy source in communication with the melt, whose temperature has been gradually changed from 800 to 840°C during growth. Electron probe microanalysis (EPMA) measurements of Zn composition in the boule shows an excellent homogeneity of Zn along the axis of the CdZnTe boule compared with results in a boule grown by using a fixed source temperature. We have performed a numerical simulation to obtain the approximate temperatures of additional heating and cooling needed to improve the radial Zn homogeneity. CdZnTe boule has been grown by seeded vertical Bridgman furnace with two zones of heater and cooler. Ultraviolet/visible spectroscopic measurements of Zn composition over the length of the boule indicate that the radial distribution of Zn composition is very homogeneous in the body region of the boule, where the radial variation of Zn composition is ±0.0005.     

Evaluation of Mn Uniformity in CdMnTe Crystal Grown by the Vertical Bridgman Method

Cd_1−x Mn _x Te is a typical diluted magnetic semiconductor, as well as substrate for the epitaxial growth of Hg_1−x Cd _x Te. In this paper, the homogeneity of a Cd_1−x Mn _x Te (x = 0.2) single-crystal ingot grown by the vertical Bridgman method was studied. The crystal structure and quality of the as-grown ingot were evaluated. Near-infrared (NIR) transmission spectroscopy was adopted to develop a simple optical determination of the Mn concentration in the as-grown ingot. A correlation equation between cut-off wavelength λ _co from NIR transmission spectra and Mn concentration by inductively coupled plasma atomic emission spectrometry (ICP-AES) was established. Using this equation, we investigated the Mn concentration distribution in both the axial and radial directions of the ingot. It was found that the segregation coefficient of Mn in the axial direction of the ingot was 0.95, which is close to unity. The Mn concentration variation in the wafers from the middle part of the ingot was 0.001 mole fraction. All these results proved that homogeneous Cd_0.8Mn_0.2Te crystals can be grown from the vertical Bridgman method.     

Temperature dependence of the optical properties of Hg1−xCdxTe

The alloy composition of Hg_1−xCd_xTe should be controlled during growth, so that the desired band gap and the lattice-matched layer may be obtained. In-situ spectroscopic ellipsometry, now commercially available, enables one to acquire spectral data during growth. If one knows the optical dielectric function as a function of alloy composition and temperature, the technique can be fully used to monitor and control temperature, the thickness, and the alloy composition. For this purpose, we first obtained temperature dependent spectral data of Hg_1−xCd_xTe by spectroscopic ellipsometry (SE). The spectral data of Hg_1−xCd_xTe with x = 1,0.235, and 0.344 were obtained from room temperature to 800Kin the photon energy range from 1.3 to 6 eV. The spectral data revealed distinctive critical point structures at E₀, E₀+Δ₀, E₁, E₁+Δ₁, E₂(X), and E₂(Σ). Critical point energies decreased and linewidths increased monotonically as temperature increased. The model for the optical dielectric function enabled (i) the critical point parameters to be determined accurately, and (ii) the spectral data to be expressed as a function of temperature within and outside the experimental range.     

Basic principles of postgrowth annealing of CdTe:Cl ingot to obtain semi-insulating crystals

The process of annealing of a CdTe:Cl ingot during its cooling after growth was studied. The annealing was performed in two stages: a high-temperature stage, with an approximate equality of chlorine and cadmium vacancy concentrations established at the thermodynamic equilibrium between the crystal and vapors of volatile components, and a low-temperature stage, with charged defects interacting to form neutral associations. The chlorine concentrations necessary to obtain semi-insulating crystals were determined for various ingot cooling rates in the high temperature stage. The dependence of the chlorine concentration [Cl _Te ⁺ ] in the ingot on the temperature of annealing in the high-temperature stage was found. The carrier lifetimes and drift mobilities were obtained in relation to the temperature and cadmium vapor pressure in the postgrowth annealing of the ingot.     

Microcharacterization of composition modulations in epitaxial ZnSe1-xTex

Molecular beam epitaxial growth of the ZnSe_1-xTe_x (x=0.44-0.47) alloy on vicinal (001) GaAs substrates tilted four, six, and nine degree-[111]A or B results in partial phase separation of the alloy with a vertical modulation between different compositions. Transmission electron microscopy images of samples grown on four degree-tilted substrates showed superlattice-like structures, with periods in the range 13.4-28.9Â. Lattice images reveal diffuse interfaces between light and dark bands. Period variations were detected in isolated regions of some samples. We present evidence that the modulation develops at the growth surface, and remains stable in the bulk at temperatures up to 450°C. Satellite spot pairs with approximate indices (h k 1 + δ) were present near the zinc-blende spots in electron diffraction patterns and x-ray diffraction data, as expected from material with a sinusoidal composition profile. The orientation of the spots reveals that the modulation vector is parallel to the growth direction, rather than to [001]. The [111]A- and B-tilted samples showed significant modulation, while the five degree-[110] and on-axis material showed no detectable modulation. The modulation wavelength did not strongly depend on growth temperature in the range examined (285–335°C). Samples showing composition modulation did not exhibit significantly altered low-temperature luminescence spectra from material with no modulation.     

GaAs1−xSbx growth by OMVPE

The system GaAs_1−xSb_x has a solid phase miscibility gap ranging from x = 0.2 to x = 0.8 at the typical growth temperature of 600 C; however, metastable alloys covering this entire composition range have been grown by organometallic vapor phase epitaxy (OMVPE) using trimethylgallium, antimony and -arsenic as the source materials. The solid composition is studied for various values of growth temperature, the ratio of Sb to total group V elements in the vapor phase and the ratio of group III to group V elements in the vapor phase. The fraction of trimethylarsenic (TMAs) pyrolyzed in the vapor phase is found to be < 1 and to vary with temperature. Taking into account the incomplete pyrolysis of TMAs, solid composition is found to be controlled by thermodynamics. The effects of temperature and vapor composition are all accurately predicted using a simple thermodynamic model assuming equilibrium to be established at the solid-vapor interface. The properties of these metastable GaAs_1−x Sb_x alloys were explored using interference contrast microscopy, room temperature and 4 K photoluminescence, Hall effect and conductivity measurements. The alloy GaAs_0.5Sb_0.5 grown lattice matched to the InP substrate has excellent surface morphology, is p-type, and the photoluminescence spectrum consists of a single, intense, fairly broad (23.5 meV half width) peak at a wavelength of 1.54 Μm.     

Monitoring the composition of the Cd<Subscript>1 − <Emphasis Type="Italic">z</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">z</Emphasis></Subscript>Te heteroepitaxial layers by spectroscopic ellipsometry

A hardware-software complex based on a spectroscopic ellipsometer integrated into a molecular beam epitaxy installation and destined to monitor the composition of the Cd_{1 − z} Zn _z Te alloy at small values of z is described. Methodical features of determination of the composition of growing layers by the spectra of ellipsometric parameters are considered. The procedure of determination of the composition by the absorption edge that allows measuring this parameter accurate to 1.2% is developed. Problems are considered the solutions of which will allow one to increase the resolution by the composition. In particular, maintaining a stable temperature during growth is required for this purpose.     

Influence of alloy composition,substrate temperature,and doping concentration on electrical properties of Si-doped n-Alx Ga1−x As grown by molecular beam epitaxy

Capacitance and Hall effect measurements in the temperature range 10-300 K were performed to evaluate the deep and shallow level characteristics of Si-doped n-Al_xGa_-xAs layers with 0 × 0.4 grown by molecular beam epitaxy. For alloy compositions × 0.3 the overall trap concentration was found to be less than 10⁻² of the carrier concentration. In this composition range the transport properties of the ternary alloy are comparable to those of n-GaAs:Si except for lower electron mobibities due to alloy scattering. With higher Al content one dominant electron trap determines the overall electrical properties of the material, and in n-Al_0.35Ga_0.65As:Si the deep trap concentration is already of the order of the free-carrier concentration or even higher. For the composition × = 0.35 ± 0.02 the influence of growth temperature and of Si dopant flux intensity on the deep trap concentration, on shallow and deep level activation energy, and on carrier freeze-out behaviour was studied and analyzed in detail. Our admittance measurements clearly revealed that the previously assumed deepening of the shallow level in n-Al_x Ga_1-x As of alloy composition close to the direct-indirect cross-over point does actuallynot exist. In this composition range an increase of the Si dopant flux leads to a reduction of the thermal activation energy for electron emission from shallow levels due to a lowering of the emission barrier by the electric field of the impurities. The increasing doping flux also enhances the concentration of the dominant electron trap strongly, thus indicating a participation of the dopant atoms in the formation of deep donor-type (D,X) centers. These results are in excellent agreement with the model first proposed by Lang et al. for interpretation of deep electron traps in n-Al_x Ga_1-x grown by liquid phase epitaxy.     

Growth and characterization of lattice-matched epitaxial films of GaxIn1−xAs/InP by liquid-phase epitaxy

We determined the conditions for successful lattice-matched growth by liquid-phase epitaxy near T = 620‡ C of Ga_XIn_1−XAs on [111B] InP substrates. We have used the results of the growth of both lattice-matched and intentionally lattice-mismatched epitaxial layers, (0.4 ≤ X ≤ 0.7) to calculate a phase diagram which gives the correct liquidus temperature, (T_L ± 1‡ C), and the correct solid composition, (± 5 % of the nominal composition), for the entire range of growth solutions considered for this important ternary semi-conductor system. The parameters appropriate to this calculation are significantly different from those used to describe the growth of Ga_XIn_1−XAs on GaAs. The results of this calculation play an important part in the better understanding of the quaternary alloy Ga_XIn_1−XAs_yP_1−y. Our measurements show that the ternary alloy lattice-matched to InP is Ga_0.47In_0.53As, semiconductor with a direct band gap about 0.75 eV at room temperature. We have grown p-n junction homostructures and double-heterostructures on InP substrates. These wafers have been used to make detectors in the 1.0 – 1.7/um range of the optical spectrum.     

Structure of Bi<Subscript>2</Subscript>Se<Subscript>0.3</Subscript>Te<Subscript>2.7</Subscript> alloy plates obtained by crystallization in a flat cavity by the Bridgman method

The property anisotropy in Bi₂Se_0.3Te_2.7 alloy is analyzed by constructing index surfaces for the thermoelectric figure of merit and thermal expansion coefficient. Texture is an important factor forming the property anisotropy and technological applicability of an ingot for fabricating modules. The property anisotropy is analyzed based on studying the texture in ingots produced by the modified Bridgman method (thermoelectric plate growth in a flat cavity). Analysis of the texture shows that not only the crystallization rate, but also the crystallization cavity design is an important factor for the proposed crystallization method, affecting the formation of the thermoelectric-material structure. As the plate thickness is decreased by changing the heat removal conditions in a thin gap, a more perfect structure can be obtained.     

Microstructure of a lead-free composite solder produced by an in-situ process

A composite solder, which consisted of solder matrix and intermetallic reinforcements, was manufactured by an innovative method. The cast ingot of the Sn-6.9Cu-2.9Ag alloy has primary Cu₆Sn₅ intermetallics in dendritic form. After rolling, the intermetallic dendrites are crushed into fine particles and distributed uniformly in the solder matrix. The crushed intermetallic particles do not melt, and they retain their shape during spheroidizing and reflow because of their high melting temperature. Coarsening and gravitational segregation of the particles was observed during reflow and analyzed theoretically.     

Oxygen incorporation in AllnP, and its effect on P-type doping with magnesium

Oxygen incorporation in Al_yIn_1−yP (y ∼ 0.5) grown by metalorganic chemical vapor deposition (MOCVD) has been studied as a function of PH₃ flow, growth temperature, and alloy composition. Both O₂ and diethylaluminum ethoxide (DEAlO) were employed as sources of oxygen. The incorporation of oxygen was found to be a superlinear function of O₂ or DEAlO flow. When multiple sources of oxygen are present, a surface interaction leads to enhanced oxygen incorporation. Oxygen incorporation cannot be adequately described by a simple dependence on growth temperature or V/III ratio due to strong interactions between these variables. In Mg-doped AlInP co-doped with oxygen, the incorporation of both Mg and O is strongly affected by an interaction between the two species, and roughly 10% of the oxygen atoms act as compensating donors.     

Characterization of p-type multicrystalline silicon prepared by cold crucible continuous melting and directional solidification

A solar-grade boron doped silicon ingot with the cross section of 62 mm× 62 mm was cast by cold crucible continuous melting and directional solidification (CCDS). The characterization of this p-type multicrystalline silicon (mc-Si) was measured and evaluated. The results indicate that the ingot mainly consists of uniform columnar grains preferentially aligned parallel to the ingot growth direction. The average density of dislocations (N_dis) in the center area varies from 4 × 10⁴ cm⁻² to 4 × 10⁵ cm⁻², and it is much lower than that in the peripheral area. Comparing with the raw material, the oxygen concentration in the cast ingot is much lower while the carbon holds the same level. The electrical resistivity distributes uniformly and its average value is same as that of the raw material. The minority carrier lifetime (MCLT) is higher than that of the raw material and no region with obvious low MCLT is observed. CCDS has shown to be a potential process to produce mc-Si for solar cells with no crucible contamination and consumption, high production efficiency and uniform quality.     

Influence of cation-vacancy imperfection on the electrical and photoelectric properties of the Cu1 − x Zn x InS2 alloy

The growth technology of single crystals of Cu_{1 ? x} Zn _x InS₂ alloys (x = 0–12) of n-type conductivity is developed. The formation mechanism of the alloy is investigated by X-ray structural analysis. It is shown that the single crystals have chalcopyrite structure, and the unit-cell parameters depend on the alloy composition. The temperature dependence of the electrical conductivity in the temperature range T = 27–300 K and the spectral distribution of the photoconductivity at T ≈ 30 K are investigated. Induced photoconductivity is found for CuInS₂-ZnIn₂S₄ with a content of ～8 and ～12 mol % ZnIn₂S₄ and thermally stimulated currents are investigated.     

Low-temperature photoluminescence and X-ray diffractometry study of InxGa1−x As quantum wells

The structures grown by molecular-beam epitaxy with In_xGa_1?x As quantum wells (QWs) in GaAs were studied by X-ray diffractometry and low-temperature photoluminescence techniques. The inhomogeneity of the QW composition along the growth direction was established. Energy positions of the exciton recombination lines in the QWs with step-graded In distribution were calculated, and good agreement with the experimental data was obtained.     

Modulation spectroscopy as a tool for electronic material characterization

Modulation spectroscopy is an optical characterization tool that can be of great utility to the materials scientist. We present here numerous examples where a simple photo-reflectance and electroreflectance setup is used in our laboratory to determine such important material parameters as alloy composition and carrier concentration in a very short time. For determining alloy composition in semiconductors, contactless room temperature photoreflectance is nearly as sensitive as low temperature photoluminescence. Examples will be given on how to determine: the effects of surface preparation and implant damage; alloy composition and carrier homogeneity for large area wafers to better than 1%; the segregation coefficient of isoelectronic impurities in bulk semiconductors; the sub-band energies in quantum well structures; and the presence and homogeneity of built-in electric fields in MODFET structures. Particular emphasis will be placed on band edge and exciton effects on the photoreflectance and on the criteria used to distinguish between them. Materials studied included Si doped GaAs, Al_xGa_1-xAs for variousx grown by OMVPE and MBE, bulk InP doped with iso-electronic As and Sb, and MODFET structures.     

Effects of VLS and VS mechanisms during shell growth in GaAs-AlGaAs core-shell nanowires investigated by transmission electron microscopy

We investigated by means of transmission electron microscopy (TEM) the final growth stage of GaAs/AlGaAs core-shell nanowires (NWs) self-assembled by Au-catalyst assisted metalorganic vapor phase epitaxy (MOVPE). TEM observations and energy dispersive x-ray spectroscopy revealed the presence of an AlGaAs tapered region of varying chemical composition nearby the NW extreme end (i.e. between the core-shell NW trunk and the Au nanoparticle catalyst). Our findings evidence that this region exhibits an unintentional Al_yGa_1−yAs/Al_xGa_1−xAs core-shell structure, a result of the combined axial (vapor-liquid-solid, VLS) self-assembly and conventional (vapor-solid, VS) overgrowth of the material. While the VS-grown Al_xGa_1−xAs alloy retains the Al composition (x=0.3) of the AlGaAs shell along the NW trunk, the central Al_yGa_1−yAs section is made of an Al-rich (y≈0.8–0.9) alloy segment formed during AlGaAs shell overgrowth, followed by a graded-alloy segment formed upon deposition of the terminating GaAs cap layer, the latter segment due to the effect of the Al reservoir left in the Au catalyst nanoparticle (NP).