Synthesis of Ultra-High-Purity CdTe Ingots by the Traveling Heater Method

Journal of Electronic Materials, Jun 2005 by Audet, N, Cossette, M

For telluride compounds, the presence of electrically active contaminants in the grown crystals is one of the critical factors that can severely limit the ensuing device performance. In order to minimize contamination from both precursor material handling and environmental sources during the compounding operation, a fully integrated traveling heater method (THM) CdTe synthesis process was developed. Based on glow discharge mass spectroscopy (GDMS) analysis, the average total impurity content, excluding carbon, nitrogen, and oxygen, is in the order of 100 PPB atomic, with zinc being the major contaminant and accounting for more than 40 pct of this value.

Key words: CdTe, traveling heater method (THM), ultra high purity

INTRODUCTION

Telluride compounds, such as CdTe and CdZnTe (CZT), are materials of choice for the fabrication of infrared and high-energy radiation detectors. For both of these applications, the presence of electrically active contaminants in the grown crystals is one of the critical factors that can severely limit the ensuing device performance.

While tellurium, cadmium, and zinc precursor elements can be readily obtained at 7N purity levels, the CdTe or CZT crystals produced from these usually suffer from significant degradation in terms of residual impurity concentration.

Given the challenges associated with the crystal growth of these II-VI materials, a synthesis operation is often seen as an obligatory step to be performed prior to the growth process.1 The additional manipulations and high-temperature environments typically associated with this synthesis process can play an important role in increasing the contaminant levels of the final product.

This article presents a fully integrated traveling heater method (THM) approach for the synthesis of ultra-high-purity polycrystalline CdTe ingots of large dimensions. This novel process addresses the issues of contamination from both precursor material handling and environmental sources during the precrystal growth compounding operation.

RATIONALE AND EXPERIMENTS

In the THM method, a tellurium-rich molten zone is vertically swept across the length of the charge consisting of the precursor elements.2 The movement of the molten zone through the charge leads to progressive dissolution of the charge at the top liquid to solid interface, while simultaneously leading to synthesis and growth of the compound at the bottom interface. The THM technique was identified as the method of choice for the preparation of CdTe polycrystalline ingots because it offers a number of advantages over the near-stoichiometric melt synthesis techniques. Some of these benefits include the following.

* A low-temperature environment: Whereas CdTe Bridgman synthesis is performed at nearly 1,100�C, THM can be done well under 900�C as the synthesized compound never reaches its melting point. This has a significant impact in reducing contamination from the crucible walls, typically made from high-purity quartz.3 Indeed, the diffusion coefficients of a number of elements in quartz, including sodium, lithium, calcium, and potassium, increase rapidly with temperature, and become significant at temperatures reaching 1,000�C or more.

* A low-pressure environment: The compounding of tellurium and cadmium to form CdTe is a highly exothermic process. The rapid increase in temperature that results from this process can lead to high vapor pressure for the unreacted cadmium, which in turn can lead to crucible failures. In THM, the low vapor pressure associated with the tellurium-rich environment practically eliminates any risk of such failures. The low pressure environment also allows the use of nonsealed and re-usable crucibles. These features have a strong impact in reducing the synthesis costs of ultra-high-purity CdTe and CZT.

* A capability to synthesize large ingots: Whereas the CdTe synthesis with conventional Bridgman is typically limited to 1.5 kg or so, mainly because of crucible failure concerns, CdTe ingots produced by THM can easily exceed 3.5 kg. This offers yet another cost advantage, as well as benefits in terms of reduced material contamination resulting from postsynthesis handling given the decreased surface to volume ratio provided by larger ingots.

For this development work, commercial grade tellurium and cadmium were first transformed into 7N precursor materials using 5N Plus Inc.'s purification processes, which include successive vacuum distillations, followed by multipass zone refining operations.

These precursor ultra-high-purity elements were then formed into specific shapes via directional solidification performed in semiconductor-grade quartz boats. The shapes of the tellurium and cadmium precursors were defined such as to allow for their direct insertion inside dedicated THM quartz crucibles. This removes any need for standard material manipulations such as cutting, weighing, or etching, each of which has the potential to lead to material contamination. Typical contaminants resulting from these operations include sodium, aluminum, magnesium, silicon, and sulfur, which are common impurities present in clean room environment. These impurities may originate from a number of sources, including the processes performed in the clean room, the clean room personnel, the cleaning agents, and the ventilation system and general air quality. Additionally, contamination with iron and chromium can be expected if stainless steel tools come in violent contact with the charge material, as would be the case when breaking or crushing tellurium ingots in order to obtain smaller pieces better suited to the crucible charging operation. The contamination levels associated with these operations can easily exceed 100 PPB atomic for the individual impurity elements.