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.     

Bulk Bridgman growth of cadmium mercury telluride for IR applications

Cadmium mercury telluride (CMT, CD_xHg_1-xTe) is the pre-eminent infrared material, despite the difficulties associated with the production and subsequent processing of this ternary compound. By varying the x value the material can be made to cover all the important infrared (IR) ranges of interest. The first technique developed was the basic vertical Bridgman process with typical crystal dimensions of 13 mm diameter and 150 mm length. We found it necessary to purify both the mercury and the tellurium on-site before use to obtain the required electrical properties. There is marked segregation of the matrix elements in Bridgman growth that is both a disadvantage and an advantage. Its disadvantage is that the yield of material in terms of composition for the two most common regions required (x=0.21 and 0.3 for 8–14 and 3–5 m atmospheric transmission windows, respectively) is low. The advantage is that both regions of interest are produced in the same crystal. A further advantage is that segregation of impurities also occurs and leads to low background donor levels in Bridgman material. This Bridgman material is used exclusively for photoconductive IR detectors that require n-type material. The main disadvantages of the Bridgman technique are that material is non-uniform in composition in the radial direction, as well as in the growth direction, and there are numerous grain and sub-grain boundaries. An improved process was developed at BAE Systems based on the accelerated crucible rotation technique (ACRT). Here, growth ampoules are subjected to periodic acceleration/deceleration in their rotation, rather than constant rotation as in the Bridgman process. The major effect of this is to stir the melt during growth and produce flatter solid/liquid interfaces. This, in turn, improves the radial and axial compositional uniformity of the material, normally by a factor of at least ten-fold. The only drawback is that the material is now p-type as grown and must be annealed in mercury vapor to convert it to n-type. An additional marked 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 acceptable uniformity of composition and good electrical properties for current photoconductive detector programs. © 2001 Kluwer Academic Publishers     

A study of cadmium mercury telluride (CMT) thin film structures: some physical observations and characteristics

Polycrystalline thin film structures of the form Cd_1?x Hg _x Te (0 ≤ x ≤ 0.5) were obtained on ultrasonically cleaned glass substrates in an alkaline medium (pH≈ 10 ± 0.2) at 80^°C. The forerunner solutions were obtained from the equimolar solutions of CdCl₂, HgCl₂ and 0.33 M-refluxed Na₂TeSO₃. The preparation parameters such as temperature, pH, deposition time and speed of mechanical stirring were optimized. The as-prepared samples were tightly adherent to the substrate support, less smooth, diffusely reflecting and were analysed for composition. It appeared that incorporation of Hg²⁺ in the lattice of CdTe replaced divalent Cd²⁺ predominantly. However, a small amount of Te^2? replacement by Hg²⁺ ions has also been observed at higher levels of Hg in CdTe (> 0.1 mol. %). The samples were then characterized optically to determine the absorption coefficient, mode of optical transition and the optical bandgaps. The estimated band-gap decreased, typically from 1.51 to 0.83 eV as Hg content in CdTe was increased from 0 to 0.5. The dc electrical conductivity measurements were made in the temperature range from 300 to 550 K and it was observed that the conductivity increased rapidly up to x = 0.1 and thereafter remained more or less the same. The thermopower measurements showed n-type conduction of these films. Both carrier concentration (n) and mobility (μ) were computed from these studies and found to be dependent on the working temperature and the film composition. The surface morphologies of these as-grown layers were then observed via a scanning electron microscope. CdTe crystals are more or less crystalline spherical and hexagonal prismatic in shape whereas HgCdTe samples appear glassy and show hexagonal prismatic whiskers, some less frequent forms of needles.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Synthesis and characterization of cadmium telluride nanowire

CdTe nanowires with controlled composition were cathodically electrodeposited using track-etched polycarbonate membrane as scaffolds and their material and electrical properties were systematically investigated. As-deposited CdTe nanowires show nanocrystalline cubic phase structures with grain sizes of up to 60 nm. The dark-field images of nanowires reveal that the crystallinity of nanowires was greatly improved from nanocrystalline to a few single crystals within nanowires upon annealing at 200?°C for 6?h in a reducing environment (5%?H(2)+95%?N(2)). For electrical characterization, a single CdTe nanowire was assembled across microfabricated gold electrodes using the drop-casting method. In addition to an increase in grain size, the electrical resistivity of an annealed single nanowire (a few 10(5)?Ω?cm) was one order of magnitude greater than in an as-deposited nanowire, indicating that crystallinity of nanowires improved and defects within nanowires were reduced during annealing. By controlling the dopants levels (e.g.?Te content of nanowires), the resistivity of nanowires was varied from 10(4) to 10(0)?Ω?cm. Current-voltage (I-V) characteristics of nanowires indicated the presence of Schottky barriers at both ends of the Au/CdTe interface. Temperature-dependent I-V measurements show that the electron transport mode was determined by a thermally activated component at T>-50?°C and a temperature-independent component below -50?°C. Under optical illumination, the single CdTe nanowire exhibited enhanced conductance.     

Influence of deposition potential and electrolyte composition on the structural and photoelectrochemical properties of electrodeposited mercury cadmium telluride

Cadmium-rich Cd_xHg_1–xTe (CMT) thin films were grown by cathodic electrodeposition. Two main fabrication parameters were taken into account: deposition potential and mercury content in solution. Different layers of CMT deposited by varying these two parameters were studied by X-ray diffraction, energy dispersive X-ray spectrometry and photoelectrochemical measurements. It was shown that surface structure, composition and quantum efficiency of CMT can be controlled with these two variables. Multigap structures were also been considered.