Interface studies during vertical Bridgman CdTe crystal growth

doi:10.1016/0022-0248(84)90290-2

Journal of Crystal Growth

Volume 70, Issues 1–2, December 1984, Pages 379-385

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Abstract

The purpose of this study was to reveal the shape of the melt/solid interface, and its influence on boule quality, during the vertical Bridgman melt growth of CdTe crystals. ¹¹¹In, with a half-life of 2.8 days, was used as a radiotracing dopant, and sharp growth rate perturbations were used to incorporate it in bands corresponding to the melt/solid interface during otherwise normal crystal growth. Autoradiography was used to reveal their actual shapes, which were found to be almost flat. Slightly convex interface shapes were generated by thermal modifications, but no significant improvements in grain structure (large grain, polycrystalline) were noted. Growth rate histograms showed that furnace end effects can be a dominant factor in Bridgman systems, and can cause significant differences between imposed and actual growth rates.

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This chapter discusses suitable base elements, namely, cadmium, tellurium, and zinc, required to synthesize and grow high-quality CdTe and CdZnTe crystals. It implies pure to the extent required to avoid that the final device properties be dominated by extrinsic impurities coming from these base elements. Breakthroughs in analytical techniques are perhaps the most important factor that have been responsible for the improvements in the base element purity over the last 3 decades. It also focuses on the routine use of glow discharge mass spectrometry (GDMS) has enabled the direct measurement of purity down to ppba levels for a wide range of impurities in a rapid and reproducible way. This has enabled significant progress to be made in the purification process itself and highlighted the impact of environmental contamination resulting. Commercial vendors and their customers alike are now well aware of such matters and have widely adopted the use of glove boxes, clean rooms, special packaging practices, and materials in an effort to maintain purity at the required levels. The purpose of this chapter is to review the purification methods used for the base elements, namely, cadmium, tellurium, and zinc, and also examine the approach consisting in the purification of the compounds namely CdTe and/or CdZnTe.
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This chapter discusses suitable base elements, namely, cadmium, tellurium, and zinc, required to synthesize and grow high-quality CdTe and CdZnTe crystals. It implies pure to the extent required to avoid that the final device properties be dominated by extrinsic impurities coming from these base elements. Breakthroughs in analytical techniques are perhaps the most important factor that have been responsible for the improvements in the base element purity over the last 3 decades. It also focuses on the routine use of glow discharge mass spectrometry (GDMS) has enabled the direct measurement of purity down to ppba levels for a wide range of impurities in a rapid and reproducible way. This has enabled significant progress to be made in the purification process itself and highlighted the impact of environmental contamination resulting. Commercial vendors and their customers alike are now well aware of such matters and have widely adopted the use of glove boxes, clean rooms, special packaging practices, and materials in an effort to maintain purity at the required levels. The purpose of this chapter is to review the purification methods used for the base elements, namely, cadmium, tellurium, and zinc, and also examine the approach consisting in the purification of the compounds namely CdTe and/or CdZnTe.
Control of growth interface shape using vibroconvective stirring applied to vertical Bridgman growth
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The ability of vibroconvective stirring to control the growth interface shape during vertical Bridgman growth was demonstrated using the coupled vibrational stirring (CVS) technique. CVS involves the application of low-frequency vibrations to the outside of the growth ampoule and produces strong flows emanating from the fluid surface. Previous studies showed that the growth rate and interface shape fluctuations occurred under growth with a constant vibrational frequency. In order to eliminate these fluctuations, this paper examines methods to lower the vibrational frequency as growth progressed to maintain a constant fluid flow rate in the vicinity of the growth interface. These flow control methods were tested on two growth systems: NaNO₃ and lead magnesium niobate–lead titanate (PMNT). The previously observed fluctuations were greatly reduced when flows were appropriately controlled. Using this control method, CVS flows were then applied to reduce the concavity of the growth interface in the NaNO₃ system at fast growth rates (>10 mm/h).
In situ studies of Cd<inf>1-x</inf>Zn<inf>x</inf>Te nucleation and crystal growth
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The nucleation and growth of Cd_1−xZn_xTe crystals in a multi-zone vertical Bridgman growth furnace have been observed and measured using in situ eddy current sensor techniques. A two-coil eddy current sensor measured coil impedance changes for multifrequency which were then interpreted using an electromagnetic finite element analysis. The sensor was used to characterize the initial melting of a charge and the subsequent nucleation of solid during solidification. Fully remelted in situ compounded charges were exposed to significant melt superheating and were found to undergo large melt undercoolings (of up to 20°C), spontaneous crystal nucleation and rapid solidification (velocities approaching 60 mm/h which was more than 10 times the furnace translation rate). Post-growth metallography revealed that about 20 mm of polycrystalline solid was formed in this way before recalescence arrested the solidification interface. In partially remelted charges neither undercooling nor unstable growth were observed. These results indicate that eddy current sensors can be used to monitor critical aspects of the vertical Bridgman crystal growth of semiconducting materials and may simplify the implementation of seeded crystal growth concepts in this, and other, semiconductor crystal growth processes.
The effect of temperature gradient and ampoule velocity on the composition and other properties of Bridgman-grown indium antimonide
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A systematic study of the role of the melt–solid interface position in determining the crystal quality of Bridgman-grown indium antimonide ingots is made. The crystals are grown under various conditions of imposed gradients and velocities. The gradient-to-velocity ratio Λ in the crystal is seen to move the interface position above or below the melting-point isotherm and this in turn affects crystal quality and grain structure. Post-growth investigations reveal that the growth takes place under off-stoichiometric conditions, for large as well as for small values of Λ, undesirable for growth of high-quality homogeneous ingots of InSb. However, crystals grown under `near-equilibrium' conditions are stoichiometric, show low defect density, improved grain structure and comparatively narrow rocking-curve width, with very high mobility.
Eddy current sensing of vertical Bridgman growth of Cd<inf>0.96</inf>Zn<inf>0.04</inf>Te
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An encircling two coil eddy current sensor has been integrated into the ceramic liner of a commercial six zone vertical Bridgman furnace and used to monitor the growth of 72 mm diameter Cd_0.96Zn_0.04Te crystals. The sensor was maintained at a fixed location with respect to the ampoule and monitored melt cooling/composition, detected the position of the liquid-solid interface, measured the interface's curvature and determined the electrical resistivity of the solid during post-solidification cooling. The study confirmed earlier predictions that a two coil, multifrequency sensor approach can independently recover the liquid-solid interface location and provide insight about its curvature using data collected in the 50 kHz to 5 MHz frequency range. It suggests that the directional solidification of Cd_0.96Zn_0.04Te most probably initiates from a Cd-depleted melt, occurs in a colder than anticipated region of the furnace and progresses with a convex interface shape.

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Interface studies during vertical Bridgman CdTe crystal growth

Abstract

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth