Bulk growth of near-IR cadmium mercury telluride (CMT)

Cadmium mercury telluride (CMT, Cd_xHg_1–xTe) is still the pre-eminent infrared material, despite the difficulties associated with its production and subsequent processing. By varying the x value, the system can be made to cover all the important infrared (IR) ranges of interest. The two most common regions required are x0.21 and 0.3 for 8–14 and 3–5 m atmospheric transmission windows, that is, long wave, LW, and mid wave, MW, respectively. Recently we have extended the growth process to produce both very long wavelength and near-IR material for various applications. This paper focuses on the work undertaken to produce near-IR material, where higher starting x values are used. Growth takes place in simple 2-zone furnaces with the pure elements contained in thick-walled high-purity silica ampoules. The thick ampoule walls are needed to contain the high (up to 70 atm) mercury vapor pressures within the ampoules. An improved ampoule seal-off procedure was developed to enable us to grow at the higher temperatures (hence higher pressures) needed for these higher x start crystals. The accelerated crucible rotation technique (ACRT) modification to the basic Bridgman process is used to grow the crystals. Here, ampoules are subjected to periodic acceleration/deceleration in their rotation, rather than constant rotation as in the Bridgman process, which stirs the melt during growth and produces flatter solid/liquid interfaces. This, in turn, improves the radial and axial compositional uniformity of the material. An additional advantage of ACRT is that the improved radial compositional uniformity enables larger diameter material to be considered. We are currently growing 20 mm diameter, 200 mm long crystals of 0.5 kg weight with good uniformity of composition. The assessment of the near-IR material has included wavelength mapping of both radially cut slices and axially cut planks. The latter gives useful information on the shape and change in the solid/liquid interface as growth proceeds. Quenching experiments reveal actual solid/liquid interfaces that confirm the findings of the wavelength mapping. Images taken with an IR camera reveal features in slices, for example, cracks, inclusions of second phase and swirl patterns, the origin of the latter is unknown.     

Bulk growth of near-IR cadmium mercury telluride (CMT)

Cadmium mercury telluride (CMT, Cd_xHg_1?xTe) is still the pre-eminent infrared material, despite the difficulties associated with its production and subsequent processing. By varying the x value, the system can be made to cover all the important infrared (IR) ranges of interest. The two most common regions required are x～0.21 and 0.3 for 8–14 and 3–5 μm atmospheric transmission windows, that is, long wave, LW, and mid wave, MW, respectively. Recently we have extended the growth process to produce both very long wavelength and near-IR material for various applications. This paper focuses on the work undertaken to produce near-IR material, where higher starting x values are used. Growth takes place in simple 2-zone furnaces with the pure elements contained in thick-walled high-purity silica ampoules. The thick ampoule walls are needed to contain the high (up to ～70 atm) mercury vapor pressures within the ampoules. An improved ampoule seal-off procedure was developed to enable us to grow at the higher temperatures (hence higher pressures) needed for these higher x start crystals. The accelerated crucible rotation technique (ACRT) modification to the basic Bridgman process is used to grow the crystals. Here, ampoules are subjected to periodic acceleration/deceleration in their rotation, rather than constant rotation as in the Bridgman process, which stirs the melt during growth and produces flatter solid/liquid interfaces. This, in turn, improves the radial and axial compositional uniformity of the material. An additional advantage of ACRT is that the improved radial compositional uniformity enables larger diameter material to be considered. We are currently growing 20 mm diameter, 200 mm long crystals of ～0.5 kg weight with good uniformity of composition. The assessment of the near-IR material has included wavelength mapping of both radially cut slices and axially cut planks. The latter gives useful information on the shape and change in the solid/liquid interface as growth proceeds. Quenching experiments reveal actual solid/liquid interfaces that confirm the findings of the wavelength mapping. Images taken with an IR camera reveal features in slices, for example, cracks, inclusions of second phase and swirl patterns, the origin of the latter is unknown.     

Composition Distribution in the MnxCd1-xIn2Te4 Ingot Grown by ACRT-B Method

MnxCd1-xIn2Te4 (x=0.1) ingot was successfully grown by the modified Bridgman technique, which applied the accelerated crucible rotation technique (ACRT) in Bridgman process, or briefly ACRT-B. The growth interface profile shape and the composition distribution in the MnxCd1-xIn2Te4 (x=0.1) ingot were analyzed. Even though the stoichiometric composition was synthesized in the original ingot, the composition has been redistributed during the ACRT-B growth process. Mn and Cd contents decrease while In increases along the longitudinal axis. The partition ratios of solutes Mn, Cd and In at the growth interface are evaluated by a mathematical method based on the experimental data, which are found to be 1.286, 1.926 and 0.729 in α phase growth process, and 1.120, 1.055 and 0.985 in β phase growth process, respectively. In the radial direction,Mn and Cd contents increase while In decreases with the distance from the centerline of the ingot.     

垂直布里奇曼法CdZnTe晶体生长过程的数值分析

模拟计算了半导体材料CdZnTe布里奇曼法单晶体生长过程，分析了熔体的过热温度、坩埚侧面强化换热以及坩埚加速旋转(ACRT)等因素对结晶界面的形态和晶体组分偏析的影响。结果表明：当熔体的过热温度减小时，熔体中自然对流的强度显著降低，固液界面的凹陷深度有所增加，晶体的轴向等浓度区显著加长，而晶体组分的径向偏析明显增大，坩埚的侧面强化换热增加了自然对流强度，也增大了固液界面的凹陷，但是对溶质成分的偏析影响较小，坩埚加速旋转引起的强迫对流强度远大于自然对流，显著增大了固液界面的凹陷，使熔体中的溶质分布成为均一的浓度场，显著减小了晶体组分的径向偏析，增加了晶体组分的轴向偏析。     

Cd0.9Mn0.1Te晶体生长及其缺陷分析

采用Bridgman法和ACRT-B法生长了两根Cd0.9Mn0.1Te晶锭(简称CMT-B和CMT-A).采用光学金相显微镜和扫描电镜研究了这两种方法生长的晶体中出现的各种缺陷,并分析了其形成机理.采用JEOL-733电子探针测定了两根晶锭中Mn的分布.对比CMT-B和CMT-A两根晶锭,发现ACRT所产生的对流可提高Cd0.9Mn0.1Te晶体的结晶质量.     

Cd0.96Zn0.04Te单晶生长及晶体成分偏析

采用常规Ｂｒｉｄｇｍａｎ法和ＡＣＲＴ－Ｂ法进行Ｃｄ０．９６Ｚｎ０．０４Ｔｅ晶体生长实验。结果表明：ＡＣＲＴ产生的强迫对流在很大程度上消除了侧壁形核，有利于获得大的晶体；ＡＣＲＴ的加入提高了有效分凝系数ｋｅｆｆ，使其向平衡分凝系数ｋｅｑ趋近，导致轴向的偏析增大。     

Doping effects on indentation plasticity and fracture in germanium

Vickers micro-indentation tests have been performed in the temperature range 20 to 420° C on the {0 0 1} surfaces of germanium crystals of three different dopings: intrinsic, heavily doped p-type and heavily doped n-type. Indentation sizes, dislocation rosette sizes and median/radial crack lengths were measured. Rosette sizes were found to depend strongly on doping, being respectively larger and smaller than in intrinsic material for n-type and p-type specimens, over the temperature range 20 to 420° C. This result correlates well with dislocation velocity measurements in germanium. Indentation size (hardness) was found to vary with doping above ~ 300° C, hardness increasing from n-type through intrinsic to p-type material. Crack lengths, as a function of temperature, showed a sharp transition (to much shorter crack lengths) at a well-defined temperature; this ductile/brittle transition temperature was found to depend on doping, being lowest for n-type (~ 290° C) and highest for p-type (~ 400° C). This is the first observation of a relation between a fracture parameter and bulk electronic doping.     

An X-ray topographic assessment of cadmium mercury telluride

This paper describes an X-ray topographic (Berg-Barrett) assessment of cadmium mercury telluride grown by the Bridgman and cast-recrystallize-anneal (CRA) methods. Reflection topographs reveal that the Bridgman material studied consists of large numbers of small grains (0.05 to 0.6 mm) with misorientations from 1 to 9 minutes per grain. In contrast, the CRA material studied had only a few grain boundaries and features consistent with a strained lattice, possibly caused by compositional variations.     

High temperature solution growth of yttrium aluminium garnet crystals using accelerated crucible rotation

The value of ACRT stirring has been confirmed in the case of Y₃Al₅O₁₂ crystal growth, its main advantage being to suppress nucleation in the bulk solution and increase the yield of crystals. Restriction of nucleation to 1 to 5 crystals has been achieved, particularly by close control of temperature stability and by the provision of an anchor to prevent crystals from becoming detached due to the stirring action.     

Microrotation analysis applied to material cracking and toughness

Micro cracking in a solid material is a phenomenon of solid plane bifurcation with incompatible microrotation. Therefore analysis of a microrotation field in a strained material is important in fracture mechanics. Based on the strain-rotation (S-R) decomposition theorem newly developed in non-linear mechanics of finite deformation, a new geometric criterion of cracking: critical value of gradient of local mean rotation at a point i.e. || is proposed. At the same time, the concept of microrotation toughness is established. The advantage and practical value of the new criterion are elucidated through experimental exemplification in the displacement field within 0.2 mm domain around a crack tip. Microrotation toughness values of marble, structural steel and bronze are estimated also.     

Preparation of Semi-insulating Vanadium-Doped CdTe Crystals

The effect of Bridgman growth conditions on the transport and optical properties of CdTeV crystals with a vanadium concentration of 5 × 10²⁵, 10²⁵, and 5 × 10²⁴ m^–3 is studied.     

Carrier polarity reversal with sodium addition in Bridgman-grown CuInSe2

Ingots containing monocrystals of CuInSe₂, grown in this laboratory from stoichiometric melts by a one-ampoule Bridgman method, have always been found to be p-type but become n-type if 0.3 at.% or more of elemental sodium is added to the pre-growth melt. However, if excess selenium is present in the melt, corresponding to the formula CuInSe_2 + x, with x = 0.2 or 0.4, the ingots remain p-type with sodium additions up to 3 at.% at least. Even so, with smaller Se excesses of x = 0.005, 0.02 and 0.05, conversion from p- to n-type has been observed in the material with sufficient sodium additions. From the results, a relationship between x and the amount of sodium required for type conversion is beginning to emerge.     

Vickers indentation anisometry on thin cylindrical materials

The required anisometric correction to be made when using Vickers indentations on filaments with radial dimensions of the order of those of the microindenter, is evaluated. For an isotropic plastic material the anisometry of indentation. l=l{-l ₁ increases with decreasing cross-section of the filament. If the calculated correction, /^c, is in agreement with experiment the interest lies in the fact that the material preserves the indentation shape after removal of the indenter. Values of //^c reveal the presence of a mechanical anisotropy on metallic filaments due to processing. Furthermore, in the case of thin highly oriented polymeric fibres the inherent microindentation anisotropy is masked by the anisometry effect. Consequently, the proposed correction is very important and particularly valuable when anisotropic cylindrical material surfaces are being studied.     

Experimental setup for rapid crystallization using favoured chemical potential and hydrodynamic conditions

The rapid crystallization of KH₂PO₄ (KDP) from solution is demonstrated at a rate up to ≈ 7.5 mm/day along [100] and 22 mm/day along [001] in a crystallizer of 5 l capacity, using accelerated crucible rotation technique (ACRT) and simulated platform geometry for controlling the hydrodynamic conditions. On an experimental basis we have grown the crystals up to 40 · 43 · 66 mm3 size in about 3 days. Comparative analysis of the main structural and optical properties of crystals grown by conventional and rapid crystallization technique, is discussed.     

Microstructural investigation and indentation response of pressureless-sintered α- and β-SiC

The microstructure and indentation response of pressureless-sintered - and -SiC were studied using a high-resolution electron microscope and analytical electron microscopy. The materials were manufactured with boron and carbon as sintering aids. It was found that the overall porosity of the materials was very low but a large number of carbon inclusions were present. X-ray diffraction revealed the fabricated -SiC material was of the same 3C polytype as the initial starting powder; however, electron microscope observations indicated that the material contained a high density of faulting of the -forms. High-resolution imaging of grain boundaries in these materials indicated that the boundaries were very clean, and when they contained an amorphous intergranular film it was at most 0.5 to 1 nm thick. The presence of boron was not detected. Deformation due to identation took several forms. Firstly, radial cracks extending from the corners of the indent suffered little hindrance from the matrix microstructure, such that transgranular fracture was the dominant mode. Secondly, the deformation zone beneath the indentations showed copious lattice microcracks with some preferred orientation during crack formation and propagation.     

Investigation of silicon wafering by wire EDM

The new technology of silicon wafering by wire electrodischarge machining (EDM) was investigated to determine its mechanism of current-conducting and material removal. Target materials were n-type single-crystal silicon ingots with the resistivity of 7–15 cm. It was found that the surface potential barrier of the semiconductors had a dominating effect on EDM cutting speed. Technological experiments were performed to determine the correlation between cutting speed and machining parameters. The machined surfaces were examined by scanning electron microscopy and X-ray energy dispersive spectrometer to test the surface finish and surface impurity. The results obtained show that the technique is effective for silicon wafering.     

Some recent advances in bulk growth of mercury cadmium telluride crystals

The inherent metallurgical problems associated with the HgTe/CdTe pseudobinary alloy system render the standard crystal growth processes inapplicable to the preparation of mercury cadmium telluride crystals for infrared detector applications. A variety of rather nonconventional techniques have been developed to overcome these problems. Two such techniques, viz. asymmetrical Bridgman and horizontal casting for solid-state recrystallization, developed at Solid State Physics Laboratory for the bulk growth of mercury cadmium telluride crystals are reviewed in this communication. Due to the poor thermal conductivity of mercury cadmium telluride melts and solids, and the use of thick-walled quartz ampuoles, it is extremely difficult to obtain a flat solid-liquid interface during Bridgman growth of this material. The technique of asymmetrical Bridgman has been successful in overcoming this problem to a great extent. Solid-state recrystallization has been widely accepted as one of the most successful techniques for obtaining large quantities of acceptable-quality mercury cadmium telluride crystals for infrared detector applications. This is a two-step process—the melt is first quenched to obtain a good cast, which is then subjected to a grain-growth annealing. The horizontal casting procedure developed for solid state recrystallization growth has been successful in improving the overall quality and yield of bulk mercury cadmium telluride crystals.     

Growth of lead molybdate crystals by vertical Bridgman method

The growth of PbMoO₄ crystals by the modified Bridgman method has been reported in this paper. The feed material with strict stoichiometric composition is desirable for the Bridgman growth of the crystals. The continuous composition change of the melts during growth can be avoided because the volatilization of melts is limited by sealed platinum crucibles. By means of the optimum growth parameters such as the growth rate of < 1.2 mm/h and the temperature gradient of 20 ∼ 40°C/cm across the solid-liquid interface under the furnace temperature of 1140 ∼ 1200°C, large size crystals with high optical uniformity were grown successfully. The distribution of Pb and Mo concentration along the growth axis was measured by X-ray fluorescence analysis. The single crystallinity of the grown sample was evaluated by the double-crystal X-ray rocking curve. The transmission spectra were measured in the range of 300–800 nm at room temperature.     

The change of the electric conductivity type in crystals of Bi2−xInxTe3 solid solutions

Room-temperature values of the Hall constantR _H (Bc), Seebeck coefficient (Tc), and the temperature dependence of the electric conductivity _c in the 120–360 K temperature interval have been investigated on samples of Bi_2–x In _x Te₃ crystals prepared by the Bridgman technique. It has been established on the basis of the changes of these transport coefficients with increasing content of indium that for values close tox=0.1 the conductivity type changes from p-type to n-type. Suppression of the concentration of holes due to the incorporation of In atoms into the crystal lattice of p-Bi₂Te₃ is accounted for by a model of point defects in the Bi₂–xIn _x Te₃ crystal lattice. It is assumed that the arising uncharged substitutional In _Bi ^x defects polarize the Bi₂Te₃ lattice and thus lower the concentration of anti-site defects Bi_Te, whose charges are compensated by holes. The dominant defects in the crystal lattice of n-Bi_2–x In _x Te₃ are the tellurium vacancies V _Te ^. .     

Finite element analysis of the effect of fibre shape on stresses in an elastic fibre surrounded by a plastic matrix

The finite element (FE) method was used to calculate the axial and radial stress distributions as a function of axial distance, z, from the centre, and radius, r, in an elastic fibre surrounded by a plastic matrix. Plastic deformation of the matrix was considered to exert a uniform interfacial stress, , along half the length of the fibre. Axisymmetric models were created for uniform cylindrical, ellipsoidal, paraboloidal and conical fibres characterised by an axial ratio, q, and half length, L. Young's modulus for the material of the fibre and L were arbitrarily assigned values of unity, since they act as scaling factors; q also acts as a scaling factor but was assigned a value of 10 to create models with a fibrous appearance. For the cylindrical fibre, the axial stress increased linearly from the end towards the centre; the radial stress was more evenly distributed. At the other extreme, the conical fibre showed a uniform distribution of axial and radial stress. Results for ellipsoidal and paraboloidal fibres were intermediate between these two extremes. In general, the effect of taper is to lower peak stress at the fibre centre and make the stress distribution throughout the fibre more even. These results are in good agreement with recent analytical theories for the axial distribution of surface radial stress and axial stress along the fibre axis. However, FE models have the advantage of predicting full three-dimensional stress distributions.