R.K. Bagai

Precipitation in CdTe crystals studied through Mie scattering

Below gap optical losses in as-grown n-type CdTe crystals were analyzed in terms of free carrier absorption and Mie extinction due to Te precipitates. Experimental absorption spectra measured between 2 to 20 μm exhibited the well-known free carrier absorption behavior αFCA∼λx with x=3 due to scattering by polar optical phonons. In shorter wavelength regions below 6 μm, however, additional contributions to the light loss due to absorption and scattering by precipitates were also observed. Assuming a log-normal size distribution, the precipitate extinction spectra were calculated according to Mie theory within the electric and magnetic dipole and electric quadrupole approximation. A comparison with the experimental spectra identifies the precipitates and enables estimation of their sizes and total number density. In this investigation, both undoped and In-doped CdTe crystals grown from stoichiometric melts by vertical asymmetric Bridgman method were used. It was found that In doping, in general, suppresses Te precipitation. At high doping level (melt containing∼1019 In atoms cm−3), the formation of In2Te2 is also indicated. It is demonstrated that the Mie extinction analysis offers an, expedient method to rapidly analyze the precipitates in CdTe and in similar other wide gap materials in a nondestructive manner.

Asymmetrical bridgman growth technique for larger and better quality mercury cadmium telluride crystals

Abstract An innovative and simple technique of modified vertical Bridgman growth is reported for making the solid-liquid interface planar. Instead of the usual radial symmetrical temperature conditions, a linear temperature gradient is created along the diameter of the cooling ingot during the growth. Single crystals of Hg 1- x Cd x Te with fairly uniform composition across the cross-section could be grown. This method has advantages over the other contemporary methods being adopted by various workers for Hg 1- x Cd x Te crystals.

Preferential etchant for revealing crystallographic defects on (111) Te surface of CdTe crystals

Abstract A new preferential etchant has been developed to delineate etch pits on the (111) Te surface of CdTe crystals. The etch pits formed are well defined conforming to the crystallographic orientation of the surface and reveal dislocations and other crystallographic defects.

Shallow donor neutralization in CdTe:In by atomic hydrogen

Passivation effects in In doped n-CdTe upon exposure to rf hydrogen plasma are studied by electrical and photoluminescence measurements and were found to be maximum at about 150°C. Depth profiling by capacitance-voltage measurements show passivation of approximately an order of magnitude at 150°C and 50% at 170°C. No visual surface damage is seen. Reverse bias annealing experiments show that atomic hydrogen drifts in a charged state in n-CdTe, with reactivation kinetics of the donors different from the other well studied semiconductors. Manifestation of the donor passivation in photoluminescence is seen by the reduction of the donor bound luminescence.

Nature of the crystallographic defects on the (111) Te surface of CdTe delineated by preferential etching

Abstract The crystallographic defects on the (111) Te surface of CdTe, revealed by a new preferential etchant called “B” solution, have been studied. Distinctly, two types of etch pits are observed: one type is shallow triangular and smaller in size and the other type is deep pyramidal and bigger in size. These have been designated as due to α and β dislocations, respectively, on the basis of a simplified atomistic model. Clustering of dislocation etch pits around Te precipitates and stacking-fault-like defects have also been observed.

Dislocation etch pits on various crystal planes of silicon

Abstract Dislocation etch pits on various crystallographic planes of silicon are reported and the nature of their shapes in the plane of observation has been discussed in confirmation with the crystal structure. The crystallographic planes studied are divided into two categories: (i) low energy planes, (ii) high energy planes. It has been observed that the solution containing HF, CrO3 and H2O is versatile and by proper adjustant of CrO3 the dislocation etch pits on both low energy and high energy planes can be delineated. Our observations lead to the conclusion that the minimum angle for getting the trace of {111} planes delineated on the plane of etching is around 35° and this seems to be the controlling factor for the geometry of the etch pits.

Homogenization of zinc distribution in vertical Bridgman grown Cd0.96Zn0.04Te crystals

One of the most pressing issues in the growth of high quality single crystal Cd0.96Zn0.04Te material, is to achieve homogenization of the high axial variation of Zn concentration, caused by the larger than unity segregation coefficient of Zn in CdTe. This is achieved in our crystals (i) by thermal annealing of the CdZnTe crystal, which redistributes the as grown Zn distribution by solid state diffusion of Zn (this solid state diffusion of Zn occurs at three stages (a) during the growth when the solidified crystal is near to the melting point temperature, (b) during the post growth annealing of the crystal at a high temperature and (c) during the cooldown to room temperature) and (ii) by the reduction of Zn segregation during the growth stage by enhanced convective mixing of the melt, through a proper choice of ampoule and furnace dimensions. By adopting suitable growth parameters and sufficient post growth annealing it has been possible to grow Cd0.96Zn0.04Te crystals, which have nearly 75% of their fraction within 1% Zn concentration variation.

Semiconducting behaviour of Zinc doped PbI2 single crystals

Abstract PbI 2 has been purified using zone refining technique by giving 20 passes. Single crystals of pure and doped PbI 2 were grown by the zone melting and Bridgman-Stockbarger technique. X-ray diffraction (oscillation photographs) studies revealed the absence and presence of arcing i the pure and doped crystals respectively. The reflections corresponding to 4H, 12R and other higher polytypes were observed. DC conductivity (σ) studies were made on these crystals at room temperature. Typical values for σ were found to be ≁10 -8 and ≁2×10 -6 ohm -1 cm -1 for the undoped and doped crystals respectively.

Nature of preferentially etched sites on (100) surface of silicon crystals

Abstract Formation of mounds and etch-pits on the (100) surface of silicon crystal by preferential etching is reported and explained. A fast and reliable preferential etchant for delineating dislocations by etch pit formation on the (100) surface of silicon single crystals is developed.

In-situ delineation of defects in ISOVPE-grown Hg1−xCdxTe layers

Abstract Well-defined, triangular features conforming to the crystallographic orientation have been observed on the (111)Cd surface of the as-grown epilayer of Hg 1− x Cd x Te grown by isothermal vapour phase epitaxial method. It is found to be the result of preferential condensation of Te at defect sites during controlled terminal cooling of the growth tube.