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.     

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.     

Solution growth of cadmium telluride

Cadmium telluride crystal has been grown by solution method from Te-rich (Cd_0·3Te_0·7) melt. Ingots having 9 mm diameter and length up to 30 mm were grown by cooling the melt slowly (1°C/h) under a vertical temperature gradient of about 30°C/cm. As-grown ingots were characterized for optical transmission and resistivity. The middle portion of the ingots exhibited better optical transmission properties. Resistivity (p-type) was found increasing, towards the last-to-grow end, from 10³ to 10⁶ Θ-cm. Surface barrier type of detectors, made from low resistivity (≅ 10⁴Θ-cm) materials, were found suitable for detection ofX- and low energy gamma radiations. In case of high resistivity (≅10⁶Θ-cm) detectors, the performance was seen to be affected by polarization. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

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.     

Bulk growth of gallium antimonide crystals by Bridgman method

Gallium antimonide crystals were grown by the vertical Bridgman technique. Effects of ampoule diameter and dopant impurities (Te, P and In) on growth were studied. Crystal stoichiometry and homogeneity were verified with electron-probe microanalysis. Impurity distribution was investigated by secondary ion mass spectrometry (SIMS) and electron probe micro analysis. Variations of etch pit density (EPD) along the length and the diameter were studied by image analysis method. Resistivity, mobility and carrier concentrations were measured along the length of the crystal.     

Ion-beam-induced modification of the electrical properties of vacancy-doped mercury cadmium telluride

The effect of ion-beam milling (IBM) on the electrical properties of vacancy-doped mercury cadmium telluride (MCT) p-Hg_1−x Cd _x Te (x ∼ 0.22) has been studied. The samples were prepared by thermal annealing of molecular beam epitaxy (MBE)-grown heterostructures and the films and single crystals grown by liquid-phase epitaxy (LPE). The etching of samples by IBM resulted in the formation of donor centers. In MBE-grown heterostructures (but not in LPE-grown samples), the concentration of these centers reached ∼10¹⁷ cm⁻³. It is established that the appearance of a high concentration of donor centers in the heterostructures is caused by the IBM-induced activation of neutral defects formed during epitaxial growth. The probable nature of defects is discussed.     

Photon absorption coefficient and the Franz-Keldysh shift of cutoff wavelength for mercury cadmium telluride detectors

Based on the calculation of photon absorption coefficient alpha and width W of the depletion region of mercury cadmium telluride detectors, we derived the equation for alpha, W and the effective Franz-Keldysh shift Dlambda(ce). An interpretation of the substantial difference between Dlambda(ce) and the maximum Franz-Keldysh shift Dlambda(cm) of the cutoff wavelength is presented. We also propose a method to increase the Dlambda(ce) of mercury cadmium telluride detectors.     

HgInTe晶体的生长及其性能研究

利用垂直Bridgman法生长了HgInTe单晶体,并采用X射线衍射分析、FT-IR光谱分析对晶体的形态结构及红外透过性能进行了检测,结果表明所生长的晶体是高质量的单相完整单晶体,其在400～4000cm-1范围内的红外透过性能较好,达到50%～55%,晶体对红外光的吸收主要为晶格吸收和自由载流子吸收引起的.     

Defect formation in epitaxial layers of cadmium mercury telluride solid solutions highly doped with indium

We have studied the effect of strong (up to 5×10²¹ cm^?3) indium doping on the behavior of components in cadmium mercury telluride solid solutions. At an indium concentration in a modified subsurface layer on the order of 10²¹ cm^?3, these layers are depleted of mercury and cadmium; simultaneously, cadmium is segregated in the region immediately below the indium-doped surface layer. The strong doping with indium leads to the formation of extended defects, which is manifested by characteristic patterns in electron micrographs observed after chemical etching. The observed redistribution of the solid solution components upon doping is explained by peculiarities of the defect formation in cadmium mercury tellurides.     

Direct analysis of solid cadmium mercury telluride by flameless atomic absorption using interactive computer processing

This paper describes a solid sampling technique for the quantitative measurement of impurities in cadmium mercury telluride (CMT) in the ppb region. It has been used to calibrate the spark source mass spectrograph (A.E.I. MS7) analysis of this material. Typical detection limits obtained are: 0.0002 ppmw Ag, 0.002 ppmw Cu, 0.001 ppmw Fe, 0.0002 ppmw Mn, 0.002 ppmw Cr, 0.004 ppmw Al, 0.0001 ppmw Na, 0.000 06 ppmw Mg, 0.01 ppmw Si. The solid sample is directly analysed by flameless atomic absorption and the method is shown to give results precise to about 10%. Aqueous standards are used throughout and area measurements are found to be considerably better than peak-height calibration curves for this material. The data collection, calculation and display use a shared computer system developed at these laboratories.     

Correcting nonlinear response of mercury cadmium telluride detectors in open path Fourier transform infrared spectrometry

The effect of a nonlinear response of mercury cadmium telluride (MCT) detectors to photon flux is to cause a large offset and a slow variation in the zero-line of single-beam Fourier transform infrared (FT-IR) spectra, which dramatically reduce the accuracy to which strongly absorbing bands or lines can be measured. We describe a noniterative numerical technique by which the baseline offset can be corrected by adjusting the values of the maximum point in the interferogram (the "centerburst") and the points on either side. The technique relies on the presence of three spectral regions at which the signal is known to be zero. Two of these are found in all spectra, namely, the region below the detector cutoff and the high-wavenumber region just below the Nyquist wavenumber where the interferogram has been electronically filtered. In open path FT-IR measurements there are several regions where atmospheric water vapor and CO2 are totally opaque. We have selected the region around 3750 cm(-1). This algorithm is even shown to work well when the interferogram is clipped, i.e., the value at the centerburst exceeds the dynamic range of the analog-to-digital converter.     

Bulk crystal growth of Mg2Si by the vertical Bridgman method

Mg₂Si were grown by the vertical Bridgman (VB) method in crucibles made of chemical vapor deposition (CVD) pyrolytic graphite (PG) in order to minimize the reaction and sticking of molten Mg-Si during growth. Congruent crystallization was derived from a stoichiometric melt of Mg₂Si, and incongruent crystallization was derived from nonstoichiometric melts having Mg/Si ratios of 85:15, 70:30 and 60:40. Grown samples were characterized by X-ray diffraction and electron-probe microanalysis, and their power factors were calculated from the Seebeck coefficients and electrical conductivities measured from room temperature to 773 K. The grown crystals were single-crystal-like and had high Seebeck coefficients at the temperatures from 500 to 773 K. A sample derived from a stoichiometric melt had a Seebeck coefficient of −470 μV/K and the highest power factor, 7.8×10⁻⁶ W/cm K² at 373 K, was calculated for the sample derived from a melt with an Mg/Si ratio of 70:30.     

Electrical properties of mercury telluride films

Vacuum deposited HgTe films formed at 120°C were studied for resistivity, activation energy, Hall constant, mobility μ and thermoelectric power α between 78 and 420 K. These films are semiconducting and p-type. Graphs of μ against temperature T showed peaks around 310 K suggesting the predominance of ionized impurity scattering below 310 K and piezoelectric scattering above 310 K. A reasonable agreement between the experimental and calculated values of α was also observed.     

Electro-optic coefficient spectrum of cadmium telluride

A spectrum for the electro-optic coefficient of cadmium telluride measured from 3 to 14 μm is reported. The spectrum shows that the quantity n(3)r(41) has a nearly constant value of 1.09 × 10(-10) m/V over this spectral band, with a slight (5%) dip at the weak absorption band centered at 6 μm. Measurements were performed with an infrared Mueller matrix spectropolarimeter. Transmission spectra of the Mueller matrix were acquired at a set of applied voltages. Retardance spectra were calculated from Mueller matrix spectra, and then the electro-optic coefficient was calculated at each wavelength by a least-squares fit to the resulting retardance as a function of voltage.     

Proper point defects in cadmium telluride with excess of cadmium

Calculation of concentrations of free charge carriers and point defects in monocrystals of cadmium telluride is carried out at the boundary of the region of existence of the compound. The type of dominating proper point defects determining the electrical properties of the material at an excess of cadmium is identified. The maximum annealing temperatures and partial vapor pressures of two-temperature annealing at which a material with electron conductivity is obtained are determined.