Dislocation cell structures in CdZnTe substrates and its behavior of threading into HgCdTe LPE epilayers were studied.
A kind of dislocation cell structure of which dislocations linearly pile up to three <1 1 0> orientations on the (1 1 1) B
face of CdZnTe crystal was found. The formation of this cell structure can be demonstrated by the enrichment of the
dislocations through slip during the growth or cooling process. By comparing the dislocation densities and distributions
of HgCdTe LPE epilayers with it of CdZnTe substrates at the constant region using an optical microscopy system, the
behavior of cell structures of CdZnTe substrates threading into HgCdTe LPE epilayers were also discussed. The results
show that the stored dislocation densities at the dislocation walls can significantly affect the behavior of dislocation cell
structures threading into HgCdTe LPE epilayers. Dislocation cell structures of which dislocation walls have higher
stored dislocation densities can appear in constant region of HgCdTe epilayers. But the dislocations of HgCdTe epilayers
growth on CdZnTe substrate with lower stored dislocation densities at the dislocation walls are almost uniformly
ZnTe crystal has been grown at a temperature as low as 1060°C using Te solvent method. X-ray diffraction showed that
the ZnTe crystals were grown from <110> oriented. The transmission was over 60% from 2 μm to 22 μm by using
Fourier Transform Infrared Spectrometer. The etch pit density in the sliced wafer was about 2×10<sup>4</sup> cm<sup>-2</sup> detected by Scan
Electronic Microscopic. The transmission spectrums were measured from 0.2 to 3 THz by using Terahertz Time Domain
Spectroscopy. And the refractive index and extinction coefficient were obtained through analysis of the time domain
The dislocation densities in HgCdTe films grown on CdZnTe by Liquid Phase Epitaxy (LPE) are calculated based on their effects on the x-ray rocking curves. The dislocation densities derived from three kinds of methods, i.e. FWHM of X-ray double axis diffraction, Williamson-Hall plot and Pseudo-Voigt function, are approximately the same. It is found that the thickness of HgCdTe epilayers about 10 um is large enough so that effect of crystallize size on the rocking curves width can be ignored. Because the intrinsic FWHM of HgCdTe and the instrumental function of high resolution X-ray diffraction are neglected in Williamson-Hall plot and Pseudo-Voigt function, the dislocation densities obtained by these methods are a little larger than those derived from the first kind of method. Among three kinds of methods, Pseudo-Voigt function method is the easiest one to fit the rocking curves and calculate the dislocation densities.
The Cd-annealing effects on Cd<sub>1-x</sub>Zn<sub>x</sub>Te wafers were studied by means of IR transmission and micro-Raman spectrum. The experiments and theoretical analysis demonstrated that the free carrier absorption related to the Cd vacancies resulted in the IR extinction as observed in the transmission spectra. The Raman spectra showed that Raman scattering is a more sensitive method to detect the fine Te precipitates in the Cd<sub>1-x</sub>Zn<sub>x</sub>Te substrates. The Raman scattering peaks related to the Te precipitates could be found in both the as grown and the annealed samples. The relative intensity of the Te scattering peaks became weaker after Cd-annealing. This result also indicated that it was quite difficult to eliminate the fine Te precipitates entirely through annealing process.
The surface morphologies of both MBE and LPE HgCdTe films were observed by optical microscope and atom force microscope. Both HgCdTe films were grown on CdZnTe substrates. It was found that the surface morphology of LPE film is very sensitive to the orientation of CdZnTe substrate, while MBE film surface morphology is mainly dependent on the surface morphology of the substrate. The characteristic of the surface morphologies and the non- flatness of HgCdTe LPE and MBE films in different scales are studied.
The recent progress in MBE growth of HgCdTe at the Research Center for Advanced Materials and Devices, and the National Laboratory for Infrared Physics is reported. It is found that the excellent compositional uniformity and reproducibility of HgCdTe can be archived by MBE technique. The results of surface morphology, dislocation density, electrical properties and focal plane array detectors are described in the paper.
Based on empirical rules for the intrinsic absorption coefficient and refractive index of Hg<SUB>1-x</SUB>Cd<SUB>x</SUB>Te in Hougen's model, a novel calculation method determining the composition profile of epitaxy layer from room- temperature infrared transmittance spectroscopy is presented. The composition depth profile of Hg<SUB>1-x</SUB>Cd<SUB>x</SUB>Te film samples grown by liquid-phase epitaxy and after annealing is determined using this method. The expression of the composition interdiffusion coefficient for Hg<SUB>1-x</SUB>Cd<SUB>x</SUB>Te was deduced.
The recent progress in MBE growth of HgCdTe at the Epitaxy Research Center for Advanced Materials, and the National Laboratory for Infrared Physics is described. It was found that the surface morphology is sensitive to the growth temperature and the flux ratio. The compositional reproducibility studied in a limited number of samples showed that a STDDEV for x-values of 0.0017 deviated from an average value of 0.229 was obtained. The epilayers showed excellent compositional uniformity across 2-inch wafers, the relative deviations for x-value and thickness were found to be 0.18 percent and 2.19 percent respectively. Ellipsometer was used for real-time monitoring the compositional variations during growth. The post growth annealing process was found to be effective in reducing the dislocation density, a reduction in dislocation density by approximately 50 percent could be obtained even by approximately 250 degrees C low temperature annealing. Electrical properties of epilayers are described, and a p-type in situ vacuum annealing process was demonstrated. MBE grown p-HgCdTe epilayers were successfully incorporated into 32 X 32 focal plane arrays detectors.