A New Growth Method for CdTe: A Breakthrough toward Large Areas

Journal of Electronic Materials, Jun 2005 by Pelliciari, B, Dierre, F, Brellier, D, Verger, L, Et al

CdTe and CdZnTe are well-investigated II-VI semiconductors, mainly used as substrates for the HgCdTe-IR detection and as detectors for x-ray and γ-ray detection. For both applications, the demand is toward larger and larger dimensions to make larger infrared (IR) and x-ray and γ-ray arrays. This paper presents a new method to grow large dimension CdTe (or CdZnTe), mainly devoted to x-ray and γ-ray detection. This method is based on solvent evaporation from Te-rich solution made of cadmium and tellurium (optionally zinc); it operates in an open tube, and growth proceeds in a crucible maintained at a constant temperature. At the end of growth, a disc of CdTe (or CdZnTe) is obtained, the thickness of which is in the range 1-10 mm. The 65-mm-diameter discs appear as polycrystals with large grains. The electrical properties strongly depend on the presence of voluntarily introduced dopants to obtain high-resistivity material. Two different impurities are commonly used to obtain resistivity in the 10^sup 10^ Ωcm range: aluminum and chlorine. Characterization of both doped materials and results of detectors under x-ray and γ-ray illumination will be given; spectrometric grade performance has been obtained and will be presented. The originally 65-mm-diameter crucible can be scaled up to 300 mm in diameter; this will be discussed in the paper.

Key words: CdTe, CdZnTe, growth method

INTRODUCTION

The detection of high-energy radiations such as x-rays and γ-rays has been an extensive topic these last years, particularly in the direction to devices operating at room temperature.

Among a number of detection materials used in these systems, CdTe (and CdZnTe), whose properties are well suited for this application, have come to the forefront. A lot of effort has been directed to understanding and to improving CdTe (and CdZnTe) material, either in terms of a better crystalline quality by a better control of the growth methods (traveling heater method (THM), standard Bridgman (BM), high-pressure Bridgman (HPB)), or in terms of a better control of the electrical properties (zinc content, doping).

Nevertheless, some points remain difficult to solve in this situation.

* First, ingots grown by BM and HPB are plagued with zinc nonuniformity along the ingot axis (due to its segregation coefficient) and also with impurities segregation;1 some authors2 report that the best part for producing detectors is in the middle region of each ingot; this means that production yield is not high.

* Second, all of the growth techniques using quartz sealed tubes (THM, BM) are reasonably limited in dimension to diameter less than or equal to about 100 mm; and it seems difficult to imagine a HPB equipped with a crucible well larger than 100 mm in diameter, even if eV-Products (Saxonburg, PA) recently published that they are working on a 140mm-diameter crucible;3 this means that large dimensions, which are a demand for the future, are not easily attainable by these growth methods.

A NEW GROWTH METHOD

These two remarks led us to determine a new growth approach that would be scalable to large dimensions and that would give an ingot not thicker than what is needed for the device applications. To fit the first point, it is mandatory to work in an open tube; the aim of the second point is to grow 1-mm to 10-mm-thick plates, and, in this study, we focus on 5-mm-thick plates.

Generally, supersaturation, which allows growth to proceed from a liquid solution, is created by temperature ramp cooling. Another possibility is to change the composition of the liquid at a constant temperature: this seems well adapted to an open tube system, particularly, in the case of a Te-rich cadmium telluride liquid solution, where tellurium evaporation changes the liquid composition toward that of the liquidus. So, growth can proceed at a constant temperature, by tellurium solvent evaporation. Let us call this new method process by solvent evaporation (PBSE).

EXPERIMENTAL PROCEDURE

Growth Apparatus

A crucible is enclosed in a semiclosed quartz tube reactor filled with slowly flowing high-purity argon maintained at a pressure chosen in the range of 1-0.5 atm. The quartz tube reactor is surrounded by a furnace that allows the temperature to reach 1100°C.

The opened end of the reactor is the access port for loading and unloading. A cold point is maintained close to the access port, where evaporated atoms from the crucible are condensed (Fig. 1).

Growth Thermal Cycle

A tellurium-rich precompounded solution made of high-purity (6N) cadmium and tellurium is loaded into the crucible. The crucible is loaded into the reactor, which is closed by a high vacuum designed tap.

The reactor is evacuated; when the pressure reaches 10^sup -4^ atm, argon is allowed to flow and pressure in the reactor is regulated at the desired value.

The furnace is then put on. It reaches the growth temperature (T^sub g^), which is some 10° above the equilibrium temperature (T^sub eq^) of the solution, to ensure complete homogenization of the solution. When the solvent is completely evaporated, growth is over and the system is allowed to cool to room temperature.