Title of Invention

"A METHOD FOR PRODUCING MERCURY CADMIUM TELLURIDE CRYSTALS"

Abstract

A Method of Producing Mercury Cadmium Telluride** A method for producing mercury cadmium telluride comprising the steps of loading into a quartz ampoule or like container Hg, Cd & Te in stoichiometric ratio as per formula Hg 1-x Cdx Te wherein x varies from 0.18 to 0.4 and providing 40-60 mg per cubic centimeter of the empty ampoule space evacuating the quartz ampoule and sealing it using LPG-oxygen flame, heatiang the charge initially to about 660-670°C in 2-3 days and maintaining the temperature for 4-8 hours, again heating the charge up to 750°C and maintaining the temperature for 8-12 hours, heating the charge up to 850°C and maintaining the temperature for 4-6 hours; immediately cooling molten mercury cadmium telluride to the room temperature by forcing compressed air at the bottom of the ampoule; heat treating the cooled mercury cadmium telluride in a constant zone furnace at a temperature below 657°C for 7 days; increasing the temperature 20°C/hour of heated mercury cadmium telluride between 688-725°C for a period of 4-6 weeks to produce mercury cadmium telluride.

Full Text

FIELD OF INVENTION This invention relates to a method for producing crystalline mercury cadmium telluride. PRIOR ART Mercury Cadmium Telluride, popularly known as MCT and expressed by chemical formula Hgi.xCdxTe, is a composition (x) dependent tunable band gap alloy-semiconductor. It is a material of choice for infrared detection in the 3-5 and 8-14 (j.m atmospheric windows for a wide range of strategic, medical, geological and space applications. Compositions with x~0.3 and x-0.2 possess band-gaps suitable for photo-detection in the above two windows respectively. Linear photo-conductor arrays based on bulk MCT (x~0.2) crystals are the most widely used devices for thermal imaging of objects at near ambient temperatures. Some inherent metallurgical problems, associated with Hg/Te-CdTe psuedo-binary system, such as high Hg-vapour pressure and large CdTe segregation in the freezing material, render the standard bulk crystal growth techniques like Bridgman, Czochralski, etc. inapplicable to MCT as such. Solid State Re-crystalisation (SSR) is a specially developed and widely used technique for preparing bulk MCT crystals. The SSR is basically a two-step process. The first step involves melting a stoichiometrically weighed composition in a thick-walled vacuum-sealed quartz ampoule, followed by its fast cooling to obtain a precipitates/pores free cast. The cast so obtained contains acute microscopic compositional non-homogeneity, manifested in the form of a network of CdTe rich dendrites (three dimensional tree like structures) immersed in a matrix with low CdTe content. Typically, the composition of dendrite and inter-dendrite materials for an x~0.2 cast would be x~0.3 and x~0.1 respectively. In the second step, a prolonged (3-4 weeks) heat treatment of the cast at a temperature just below the solidus temperature (melting point) is employed. After which, a compositionally homogeneous ingot containing different single crystalline regions is obtained. The temperature of this heat treatment is desired to be as close as possible to the solidus temperature to make the solid state crystal growth process faster and efficient. However, due to composition variations in the cast, care is required for keeping the temperature below the softening point of the lowest x composition which has the lowest melting point. Partial melting of the ingots is not desirable as it (I) leads to long-range Hg/Te/CdTe segregation and (ii) hampers the solid state crystal growth. The broadly accepted typical pseudo-binary phase diagram of MCT is shown in the Fig 1. The solid curves in this diagram represent the equilibrium solidus and liquidus of MCT. This data is obtained with equilibrium Hg vapour pressure maintained in the system. The dashed curves, on the other hand, represent a condition, where the Hg vapour pressure over the melt was far below the equivalent pressure over the stichiometric composition. However, this SSR method for producing crystalline mercury cadmium telluride, known in the art, suffers from the following disadvantages. Main disadvantage of this process for producing mercury cadmium telluride, know in the art, is that its application is limited for growing x~0.2 compositions as it does not yield large grains in case of higher x compositions. Another disadvantage of this process for producing crystalline mercury cadmium, known in the art, is that the reproducibility of reasonably large sized single crystalline grains is low in general. Yet another disadvantage of this process for producing crystalline mercury cadmium, known in the art, is that the yield of this process is vary low for x ~0.3 and higher compositions. The present invention relates to an improvement in the SSR process vis-a-vis the yield of single crystalline material for all compositions. OBJECTS OF THE INVENTION A primary object of the invention is to propose an improved solid state re-crystallisation (SSR) process for producing crystalline mercury cadmium telluride. Another object of this invention is to provide a method of producing crystalline mercury cadmium telluride, which has a higher yield for the crystal compositions. Yet a further object of this invention is to provide an improved two-stage grain growth method for producing crystalline mercury cadmium telluride. Still another object of the invention is to provide a method of producing crystalline mercury cadmium telluride, which results in higher grain size crystals. Yet another object of the invention is to provide a method of producing crystalline mercury cadmium telluride, which results in highly pure mercury cadmium telluride. Yet further object of the invention is to provide a method of producing crystalline mercury cadmium telluride, which is simpler. still further object of the invention is to provikle a method of producing crystalline mercury cadmium telluride, which is economical. DESCRIPTION OF INVENTION According to this invention there is provided a method for producing mercury cadmium telluride comprising the steps of: (i) loading into a quartz ampoule or like container Hg, Cd & Te in stoichiometric ratio as per formula Hg 1-x Cdx Te wherein x varies from 0.18 to 0,4 and providing 40-60 mg per cubic centimeter of the empty ampoule space evacuating the quartz ampoule and sealing it using LPG-oxygen flame, (ii) heatiang the charge initially to about 660-670°C in 2-3 days and maintaining the temperature for 4-8 hours, again heating the charge up to 750°C and maintaining the temperature for 8-12 hours, heating the chaise up to 850°C and maintaining the temperature for 4-6 hours; (iii) immediately cooling inolten mercury cadmium telluride to the room temperature by forcing compressed air at the bottom of the ampoule; (iv) heat treating the cooled mercury cadmium telluride in a constant zone fuwiace at a temperature below 657°C for 7 days; (v) increasing the temperature 20°C/hour of heated mercury cadmium telluride between 688-725°C for a period of 4-6 weeks to produce mercury cadmium telluride. DESCR1PTION OF FIGURES Fig. 1 describes the typical pseudo-binary diagram of mercury cadmium telluride (MCT) system. Fig. 2 shows cross sectional grain distribution in a typical MCT ingot produced by the present process DESCRIPTION OF THE INVENTION WITH RESPECT TO THE DRAWINGS: According to the process of the present invention, stoichiometricaly weighed composition is melted in a thick walled vacuum sealed quartz ampoule and is followed by quenching to obtain a precipitate and pore free cast. This cast contains acute microscopic compositional non homogeneity manifested in the form of a network of cadmium telluride rich dendrites (three dimensional tree like structure) immersed in a matrix with low cadmium telluride content. Next, the cast is heat treated over a prolonged period of 3-4 weeks just below the solidus temperature. The temperature of this heat treatment process is kept as close to the solidus temperature as possible but at the same time care is taken to keep this temperature below the softening point of the lowest composition. Finally, the temperature of the cast is increased to a temperature between 660- 725 °C and this temperature is maintained for a period of 4-6 weeks. This results into the crystal growth The second step of SSR process is thus split into two stages. In the first stage, homogenisation takes place at temperature 660 °C. After conducting experiments for compositions having the value of x about 0.208 and 0.29 by varying Hg excess pressure, it was found that a temperature of about 658 °C was the most optimum temperature for homogenisation stage for all the compositions. In the second stage, the temperature can be increased up to 725 °C, depending upon the crystal structure, for the excellent grain growth. For example, the crystal growth occurs at about 675 °C for composition having the value of x ~ .208. Similarly, for the composition having the value of x - 0.29, the crystal growth occurs at about 725 °C. According to the present invention, the method of producing crystalline mercury cadmium telluride comprising of following steps (i) Cleaning of quartz ampoule Pre-fabricated thick walled (> 2.5mm wall thickness) quartz ampoule (Internal volume ~15-20 cc with internal diameter ~ 15-20 mm) along with a quartz plug (to be used for its vacuum sealing) is treated with HF based etchant for about 30 minutes followed by thorough cleaning with running deionised water. The ampoule is then dried in a microwave oven. (ii) Loading of charge into the ampoule, evacuation and sealing The dried ampoule is filled with Hg, Cd and Te in stoichiometric ratio as per the formula Hg^Cdx Te. A suitable excess of Hg (typically 40-60 mg per cc of the empty ampoule space after correcting for the estimated charge volume) is also added to provide sufficient Hg over-pressure during formation of MCT ingot. Immediately after loading the MCT charge, the quartz ampoule is then evacuated at 10 ^ mbar or better and sealed using a quartz melting flame. iiii) Synthesis of mercury cadmium telluride The charge is heated gradually in steps in 2-3 days time in a rocking furnace to slowly react and form MCT in molten state. Initially, temperature should be increased from room temperature to about 660-670 °C and the "ernperature is maintained for 4-8 hours. The temperature is further raised to «»bout 750 °C at the rate of 25 °C per hour and the this temperature is maintained -•- mogenisation. (iv) Air quenching of mercury cadmium telluride Next, the molten MCT is quickly cooled down to room temperature by in less than 15 minutes by applying a compressed air jet at the bottom of the ampoule immediately after switching off the furnace. (v) Homogenisation of cast The cast obtained after the step (iv) is then kept in a constant zone furnace at a temperature below the solidus point of the composition ( typically 660°C) for about 7 days for the purpose of compositional homogenization (vi) Grain growth After step (v), the temperature of the cast is increased to a temperature between 668-725 °C depending upon the value of x in the composition. Generally, one large single grain covering most of the ingot volume would grow in a period of 4 weeks. Referring to Fig. 2, the figure shows the cross sectional grain distribution in a typical 6 mm long & 15 mm diameter MCT ingot (x~0.2) grown by the present process. The invention will now be illustrated with the working examples, which are typical examples to illustrate the working of the invention and which are not intended to be taken restrictively to imply any limitation on the scope of the present invention. WORKING EXAMPLE-1 MCT crystal with x = 0.21 and Hg Excess of 50 mg/per cc of empty space was prepared in the lab in the following manner. 42.8990 gm of 79 Triple Distilled grade Hg, 34.2991 gm of 79 grade Te and 6 3448 gm of 69 QZR grade Cd was loaded in a quartz ampoule having 2.5 mm wall thickness and 17cc internal volume. The ampoule was evacuated at ~10"6 mbar with the help of a turbo-molecular pump for two hours. The ampoule was then sealed around a quartz plug using an LPG-Oxygen flame. The sealed ampoule was heated to 660 °C with a ramp of 50°C/hour in a rocking furnace and kopt there for 6 hours to facilitate Hg-Te reaction. The temperature of the furnace w i-s further increased to 740 °C with a ramp of 20°C/hour and held there for 8 hr^'s to complete the reaction of the elements. The temperature was again ra -«;d to 820°C with a ramp of 10 °C/hour and held there for 4 hours to obtain a homogenized melt. The melt was then quenched to room temperature using compressed air in -15 minutes. The ampoule containing the cast was then kept at 657 °C for one week in a constant zone furnace. The temperature of the furnace was then raised to 668 °C and held there for three weeks. The furnace was then switched off and the ampoule was removed after attaining the ambient temperature. The MCT crystal ingot was removed from the ampoule by dissolving the later in cone. HF. The Major portion of the ingot was a single crystal. The Ingot was then carefully sliced with a slow diamond wheel saw and wafers were lapped/polished to obtain mirror shine finish. WORKING EXAMPLE-2 MCT crystal with x = 0.29 and Hg Excess of 40mg/per cc of empty space was prepared in the lab in the following manner. 28.9697 gm of 79 Triple Distilled grade Hg, 25.6394 gm of 79 grade Te and 6.5497 gm of 69 QZR grade Cd was loaded in a quartz ampoule having 2.8 mm wall thickness and 17cc internal volume. The ampoule was evacuated at ~10"6 mbar with the help of a turbo-molecular pump for two hours. The ampoule was then sealed around a quartz plug using an LPG-Oxygen flame. The sealed ampoule was heated to 660 °C with a ramp of 50 °C/hour in a rocking furnace and kept there for 6 hours to facilitate Hg-Te reaction. The temperature of the furnace was further increased to 740 °C with a ramp of 20 °C/hour and held there for 12 hours to complete the reaction of the elements. The temperature was again raised to 820 °C with a ramp of 10 °C/hour and held there for 4 hours. The temperature of the furnace was further raised to 850 °C with a ramp of 5 °C/hour and was held there for two hours to obtain a homogenized melt. The melt was then quenched to room temperature using compressed air in ~15 minutes. The ampoule containing the cast was then kept at 657 °C for one week in a constant zone furnace. The temperature of the furnace was then raised to 678 °C and held there for three weeks. The furnace was then switched off and the ampoule was removed after attaining the ambient temperature. The MCT crystal ingot was removed from the ampoule by dissolving the later in cone. HF. The major portion of the ingot was a single crystal. The Ingot was then carefully sliced with a slow diamond wheel saw and wafers were lapped/polished to obtain mirror shine finish. It is to be understood that the process of the present invention is susceptible to adaptations, changes and modifications by those skilled in the art. Such adaptations, changes and modifications are intended to be within the scope of tho present invention, which is further set forth with the following claims. WE CLAIM: 1. A method for producing mercury cadmium telluride comprising the steps of: (i) loading into a quartz ampoule or like container Hg, Cd & Te in stoichiometric ratio as per formula Hg 1-x Cdx Te wherein x varies from 0.18 to 0.4 and providing 40-60 mg per cubic centimeter of the empty ampoule space evacuating the quartz ampoule and sealing it using LPG-oxygen flame, (ii) heatiang the charge initially to about 660-670°C in 2-3 days and maintaining the temperature for 4-8 hours, again heating the charge up to 750°C and maintaining the temperature for 8-12 hours, heating the charge up to 850°C and maintaining the temperature for 4-6 hours; (iii) immediately cooling molten mercury cadmium telluride to the room temperature by forcing compressed air at the bottom of the ampoule; (iv) heat treating the cooled mercury cadmium telluride in a constant zone furnace at a temperature below 657°C for 7 days; (v) increasing the temperature 20°C/hour of heated mercury cadmium telluride between 688-725°C for a period of 4-6 weeks to produce mercury cadmium telluride. 2.A method as claimed in claim 1, wherein mercury cadmium telluride is heated preferably at 600°C. 3.A method for producing mercury cadmium telluride substantially as described and exemplified herein. .

Documents:

688-DEL-2002-Abstract-(13-01-2009).pdf

688-DEL-2002-Abstract-(18-02-2008).pdf

688-del-2002-abstract.pdf

688-DEL-2002-Claims-(13-01-2009).pdf

688-DEL-2002-Claims-(18-02-2008).pdf

688-del-2002-claims.pdf

688-DEL-2002-Correspondence-Others-(19-02-2008).pdf

688-del-2002-correspondence-others.pdf

688-del-2002-correspondence-po.pdf

688-DEL-2002-Description (Complete)-(13-01-2009).pdf

688-DEL-2002-Description (Complete)-(19-02-2008).pdf

688-del-2002-description (complete).pdf

688-del-2002-drawings.pdf

688-del-2002-form-1.pdf

688-del-2002-form-18.pdf

688-DEL-2002-Form-2-(18-02-2008).pdf

688-del-2002-form-2.pdf

688-DEL-2002-Form-3-(18-02-2008).pdf

688-del-2002-form-3.pdf

688-del-2002-gpa.pdf