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Growth and Structural Properties of Low Defect, Sub-Grain Free CdTe Substrates Grown by the Horizontal Bridgman Technique
Reprint from the Journal of Electronic Materials
Khan, A. A.,
W.P. Allred, B. Dean, S. Hooper, J.E. Hawkey, and C.J. Johnson.
Abstract
Large area, low defect CdTe substrates are essential for high quality epitaxy of HgCdTe in infrared detector applications. Vertical
Bridgman (VB) CdTe normally exhibits higher than desired dislocation density and sub-grain structure. A seeded Horizontal Bridgman (HB) technique has been used to grow CdTc single crystals which exhibit superior crystalline qualities when
compared to standard VB substrates. The HB grown CdTe crystals were not intentionally doped and had resistivities in the 107 ohm-cm range. The etch pit density (EPD) near the seed and the tail end sections is 5 x 104 cm-2, while wafers from
the middle section of the ingot have EPDs in the 104 cm-2 range. Furthermore, HB EPD patterns indicate the absence of sub-grain boundaries. X-ray rocking curves are very sharp and exhibit FWHMs as low as 9 arc-sec. By comparison, the best samples from standard VB CdTe ingots exhibit x-ray rocking curves with FWHMs in the > 30 arc-sec range. The IR transmission of HB material is as high as 57% in the 2.5 to 20 m m region. Results of electrical and optical characterization are presented.
Cadmium telluride is a member of the II-VI family of compound semiconductors. Because of its large mean atomic number (50), reasonably high bandgap (1.44 eV), adequate carrier mobilities and existence of both n and p type conductivities, CdTe has been investigated for a number of applications such as gamma ray detectors, solar cells, laser windows, electro-optic modulators and as a substrate for the epitaxial growth of mercury cadmium telluride for use in the fabrication of infrared detectors and arrays. These applications notwithstanding, the crystal growth technology of CdTe lags behind other important compound semiconductors such as GaAs. This is due in part to its unique thermal and chemical properties. Some crystal growth difficulties relate to CdTe's low thermal conductivity and low bond-energies. The former makes it difficult to control growth interface shape, while the latter provides an abundance of crystal defects in response to thermo-mechanical stresses encountered during growth and/or post-growth cooling.
We have found that it is possible to reduce the magnitude of thermo-mechanical stress on bulk crystals in Horizontal Bridgman (HB) growth by a combination of real-time temperature profile control and careful furnace design. Such
control has allowed growth of high quality single crystal GaAs and CdTe ingots. The growth and characterization of undoped semi-insulating GaAs technique is described elsewhere.
The purpose of this paper is to discuss the salient
features of the electrical and optical characteristics of HB grown CdTe, especially as they contrast with Vertical Bridgman (VB) material.
It has been shown that relatively large, high quality single crystal CdTe can be grown by the
Horizontal Bridgman technique. These crystals exhibit dislocation densities in the low 104 cm -2 range, excellent crystalline quality with x-ray rocking curves approaching the theoretical FWHM limit of 9 arc-sec, and absence of subgrain structures. Semi-insulating and IR transmission characteristics are also good with resistivities in the 107 ohm-cm range and IR transmission .50% from 2.5 m m-20m m.
PHOTOLUMINESCENCE AND
ABSORPTION STUDIES OF DEFECTS IN ZnxCd1-xTe CRYSTALS
Cheryl Barnett Davis*, David D. Allred*, A Reyes-Mena*>, J. Gonzalez-Hermandez ~, Ovidio Gonzalez^, Bret C. Hess+, and Worth P. Allred#
*Department of Physics
and Astronomy, Brigham Young University, Provo, Utah 84602;
*> On leave from Departamento de Fisica, Centro de Investigacion y de Estudios Avanzados del IPN, 07000, Mexico, D.F.;
~Dentro de Investigacion y de Estudios
Avanzados, Unidad Saltillo, Instituto Politecnico Nacional, Carr. Saltillo-Monterrey Km. 13, Apartado Postal 663, 25000 Coahuila, Mexico;
+Department of Physics, California State University, Fresno, California, 93740;
#
Galtech Semiconductor Materials Corporation, Mt. Pleasant, Utah 84647
Abstract
We have studied at cryogenic temperatures photoluminescence features which lie more than 0.15 eV below the ban edge in ZnxCd1-xTe (0 £ x £ 0.09) crystals. The
same features, a defect band which lies at about 0.13 to 0.20 eV below the band gap energy and a peak of 1.1 eV, that are observed in pure CdTe samples are observed in these alloy materials. We have annealed samples and we have observed
that the 1.1 eV feature, which has been attributed to tellurium vacancies, increases drastically in fast-cooled samples. Increased concentrations of tellurium vacancies can be understood in terms of the phase diagram of CdTe which
tolerates higher concentrations of excess Cd in CdTe quenched from higher temperatures. We also observe an absorption transition near 1.1 eV by photothermal deflection spectroscopy (PTDS). This is the first reported use of PTDS in studying
CdTe and ZnxCd1-xTe crystals. The PTDS phase shifts show that the deep defect is a bulk rather than a surface effect. The well-defined absorption peak suggests that the states contributing to
the 1.1eV transition are both localized. Our results also suggest that the defect band which lies 0.13 eV below the band gap (1.48 eV in CdTe) may be related to tellurium vacancies. However, the fact that the ratio between this defect band
and the 1.1 eV feature is highly variable suggests that the relationship is not simple. The origin of the defect band and its phonon replicas nevertheless remains controversial.
INTRODUCTION AND BACKGROUND
Zinc cadmium telluride (ZnxCd1-xTe) is a direct band gap semiconductor with possible applications in near-infrared optical electronic
devices such as lasers, photovoltaic and photoconductive infrared detectors, high-efficiency solar-cell structures, and electroluminescent devices.
ZnxCd1-xTe is also a new
substrate material for the epitaxial growth of mercury cadmium telluride (Hg1-yCdyTe). High crystalline quality is needed in its use as a substrate. Defects in the substrate often
cause defects in the epitaxial layers. ZnxCd1-xTe is a good substrate material because its lattice parameters can be matched to those Hg1-yCdyTe; thus, defects due to "misfit-dislocation" are minimized. ZnxCd1-xTe also could be useful for the
formation of superlattices in combination with other II-VI compounds. For these applications, understanding the annealing behavior and identifying and eliminating impurities are essential. This material has a narrow band gap that is
tailorable from that of cadmium telluride, CdTe (1.44 eV at room temperature), to that of zinc telluride, ZnTe (2.26 eV at room temperature). It has the same characteristics as other direct band gap semiconductors with the added advantage
that the band gap ranges from the infrared into the visible. Its study as an electronic material is in its infancy.
A large effort has been expended to studying Hg1-yCdyTe and other similar materials, but ZnCdTe has not been investigated to as large an extent. This deficiency is mainly due to the lack of experimental work in the infrared region and the
lack of high-quality samples. Basic research on the typical photoluminescence spectrum of CdTe has been done. The spectra for ZnxCd1-xTe are similar except that the
near-gap emission lines are all shifted to higher energies (shorter wavelengths) and the bound exciton lines are broader that in CdTe.
Other members of our group have previously published studies of ZnxCd1-xTe in the near-gap region (energies greater than 1.4 eV). Now we have extended this study down to about 0.7 eV. We have studied features in ZnxCd1-x Te at approximately 1.4 eV and 1.1 eV which are similar to features studied well in CdTe. In the literature the 1.4 eV feature has been termed in the defect band. The peak at 1.1 eV (which we also term the deep level feature) has long been known and is also probably due to impurities or defects in the sample. The exact nature of these defects remains a matter of controversy, though it is suggested that this feature is connected with tellurium vacancies.
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