GB2213403A

GB2213403A - Method and apparatus for growing CdxHg1-xTe

Info

Publication number: GB2213403A
Application number: GB8529946A
Authority: GB
Inventors: Anthony Worswick Vere; Dennis John Williams; Peter John Bradford
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1984-12-06
Filing date: 1985-12-05
Publication date: 1989-08-16
Also published as: GB2213403B; GB8529946D0; GB8430896D0

Abstract

The material CdxHg1-xTe is grown in a thin walled sealed ampoule contained in a pressure vessel. Problems of uniformity of the material is reduced by a specific combination of energetic mixing of a melt of Cd, Hg, and Te and a rapid controlled freezing of a disc of CdxHg1-xTe following by annealing. The mixing may be a rapid rotation, e.g. 900-930 r.p.m., of the ampoule with rapid reversal of direction every few seconds. Controlled freezing is obtained by cooling a base plate, supporting the ampoule, to 300 DEG C or lower with liquid or gaseous coolant. Annealing is obtained by raising the ampoule to 650 DEG C - 690 DEG C at the hottest end with a temperature gradient of 5 DEG to 20 DEG C per cm along the ampoule length. These temperatures and gradient values are held for 24 hours or more before the ampoule is cooled to 450 DEG C or below.

Description

This invention concerns a method and apparatus for growing the ternary alloy Cd HgTe.

x i-x The material CdxHg1-xTe is now widely used as a detector material for infra red imaging systems in the 10-14 um wavelength regions. It was first described for such use in 1957 and details were published in G.B. 859,588.

Although a very good detector material difficulties in growing good quality materials have hindered its use. Vany years after its introduction Inany problems still exist which prevent growth of uniform and reproducible quality material. Most of these problems arise from the nature of the alloy CdxHg1-xTe. Both CdTe and HgTe are relatively stable and easy to grow in the bulk but Cd Mg Te tends to segregate into Te and Cd rich regions and x l-x lose Hg. The value of x is found to vary across a slice of bulk grown Cd Hg Te and from slice to slice and this affects x detector device sensitivity; ideally x should be constant.

The to currently preferred bulk growth processes for CdxHg1-xTe are the Eridgman process and the solid state recrystallisation process variously not;n as Cast Quench Anneal or Cast Recrystalline Anneal (C.R.A.). In the Briden process a sealed ampoule of material is heated and the material is solidified progressively from one end by lowering the ampoule through a temperature gradient. The C.R.A. process involves melting the elements and a relatively rapid cooling followed by an anneal to recrystallise and homogenise the material with the desired properties.

An improved C.R.A. process is described in Journal of Crystal Growth 59 (1982) 121-129 by A. W. Vere et al. This uses a thin walled (0.2 cn) ampoule contained in a pressure vessel during the growth process. Use of a thin walled ampoule allows rapid quenching of the melt but needs a pressurised growth chamber to balance the high mercury vapour pressure inside the ampoule and prevent anpoule fracture. Gas jets are directed onto the ampoule to enhance the cooling rate.

Using the above pressure grown material samples of 1.3 GM diameter CdxHg1-xTe were produced. These were an improvement over previous samples but showed axial solidification for only about 1 cn with unwanted radial solidification further along the samples.

This diameter represents the approximate maximum achievable with the previously patented techniques. At larger diameters the rate of neat extraction which can be achieved in thick.alled quartz, or with sample gas-jet cooling, is inadequate to give the required unifo=ity.

The above probiems are reduced according to this invention by a specific combination of energetic mixing of a melt of Cd, Eg, and Te and a rapid controlled freezing of a disc cf CdxHg1-xTe followed by annealing with the ampoule held at a set temnerature gradient and a gradual cooling.

According to this invention a method of growing the material Cd Mg Te (0 < x < 1) 1) comprises the steps of I -x ti) containing a desired proportion of Cd, Mg, and Te in a sealed ampoule, (ii) containing the ampoule in a pressure chamber within the confines of a furnace and with the base of the ampoule resting in intimate thermal contact with a base plate, (iii) maintaining a gas pressure inside the chamber at a value near the pressure inside the ampoule as the temperature of the ampoule is changed, (iv) operating the furnace to gradually raise the tenrerature o of the ampoule and contents to above 800 C to provide a melt, (v) energetically mixing the melt constituent whilst the r.elt temperature remains reasonably uniform, (vi) cooling the base plate to 30 C or lower, (vii) withdrawing the ampoule from the furnace whilst maintaining the furnace temperature constant and cooling the base plate until the whole melt has solidified to an ingot, (viii) raising the temperature of the ampoule to a value of o o 650 C - 690 C at the hottest end with a temperature gradient of o about 50 to 2C C per cm along the ampoule the highest temperature being at the last to solidify end of the ingot, (ix) holding this temperature value and gradient for at least 0 24 hours, slowly cooling the ingot to 45C C or lower.

Step (iv) is preferably achieved by rapid rotation of the ampoule about a vertical axis with rapid reversal of direction of rotation every few seconds. For example rotating at 900-930 r.p.m. with reversal of rotation every 3 sec. Each reversal is preferably achieved in about 1 sec or less.

The pressurising gas may be nitrogen or helium.

Step (vi) is preferably achieved by directing cooled gas onto the base plate. For example the gas may be cooled to less than -40 C before being directed onto the base plate. Alternatively the gas may be replaced by a liquid such as cooled water or a water/ethylene glycol mixture. In this case the base plate may be cooled down to 1000C or lower.

Step (v) may be discontinued before step (vi) or continued until the melt has solidified.

Step (vii) may continue until the ingot reaches room temperature.

The ampoule may then be placed within a different furnace capable of attaining the desired temperature variation along its length. A typical withdrawal rate is 1 cm/min. * Preferably the temperature along the length of the ampoule is reasonably constant during steps (iv) and (v).

Step (ix) may reduce the ingot temperature to room temperature at a slow rate e.g. 10 to 1.50C per hour. Alternatively the temperature may be reduced slowly to the range 4000 to 4500 C, and then cooled rapidly e.g. several degrees per minute, to room temperature.

Preferably the elements in the sealed ampoule are cast into a sample ingot by a synthesis furnace as a separate step prior to the recrystallising. Such synthesis may be achieved by heating the sealed ampoule and contents in a furnace inside a pressure vessel whilst rocking the ampoule, furnace, and pressure vessel to stir the melt compontents. After a suitable time of mixing e.g. half hour, the sample is solidified over a period of e.g. 5 minutes by reducing the furnace temperature rapidly.

According to this invention apparatus for growing the material CdXHgl xTe (0 < x < 1) comprises: a pressure vessel, a furnace mounted inside the pressure vessel, an ampoule holder having a base plate for supporting an ampoule containing elements of Cd, Hg, and Te, a pull rod for rotating the holder inside the furnace and for moving the holder and ampoule into and out of the furnace, means for moving the pull rod axially and rotationally, sensors for detecting the temperature of the ampoule, sensors for measuring the pressure inside the vessel, means for varying the pressure inside the vessel, and means for supplying coolant gas or liquid onto the ampoule holder base plate.

An additional pressure vessel and synthesis furnace may be used to provide a sample of cast Cd Sg1 xTe for use in the above apparatus.

Such synthesis apparatus is mounted for a rocking motion to stir the elements of Cd, Hg, and Te whilst molten.

One form of the invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a diagrammatic sectional view of apparatus for casting samples of CdxHgl xTe; Figure 2 is a diagrammatic sectional view of a rocking synthesis furnace; Figure 3 is a part view of a modification to Figure 1.

As shown in Figure 1 a stainless steel pressure vessel 1 contains a furnace 2 and ampoule 3 with sample 4. Pressure is monitored by a transducer 5 and adjusted by an outlet pipe 6 with valve 7, and an inlet pipe 8 with valve 9 supplied from a nitrogen filled gas bottle 10.

The furnace 2 is an electrically heated furnace incorporating a cylindrically shaped heat pipe 11. This heat pipe 11 maintains a uniform vertical temperature distribution by the known process of evaporating and condensing a working fluid in different parts of the heat pipe. One example is an isothermal furnace liner supplied by Dynotherm Corp., U.S.A.

Inside the furnace 2 is a holder 12 mounted for rotary and linear motion on a pull rod 17. This pull rod 13 passes through a bearing and seals 14 outside the vessel 1 and is driven by a rotary motor 15 and linear motion motor 16. The holder 12 supports the ampoule 3 on a base plate 17 and is provided with a cooling chamber 18 under the base plate 17. Cooling gas to this chamber 18 is supplied from the gas bottle 10 through a cooler 20, flexible pipe 21, to an annular chamber 22 mounted on the pull rod in bearings 23. From this annular chamber 22 the cooling gas passes upwards through the hollow pull rod 13 to the cooling chamber 18. It is important that the base of the ampoule 3 makes good thermal contact with the base plate 17.

Thermo couples 24, 25 measure ampoule and furnace temperature respectively.

Figure 2 shows a rocking synthesis furnace for initial preparation of Cd < Rgl so from its elements. A stainless steel pressure vessel 30 is mounted in bearings 31 for a rocking motion by a motor 32. Inside the vessel is an electrically powered furnace 33 into which the ampoule is placed. The furnace 33 incorporates a heatpipe in order to maintain an isothermal temperature profile.

A thermocouple 34 monitors ampoule temperature. Gas pressure, measured by transducer 35, is maintained at required values through an inlet pipe 36 and outlet pipe 37 and controlled by valves 38, 39: A pressurised bottle supplies nitrogen.

Figure 3 shows a modification of a part of Figure 1. As before a holder 12 is mounted for rotary and linear motion on a pull rod 13.

Coolant liquid is supplied via a pipe 21 into an annular chamber 22 mounted in bearings and seals 23. Apertures in the hollow rod 13 channel coolant into a cooling chamber 18 under the holders 12 base plate 17. Coolant is removed by a return pipe 41 concentric with the rod 13. The chamber 22 is extended by an outlet chamber 42 mounted around the return pipe 41 in bearings and seals 43, 44. A return hose 45 removed coolant from the chamber 42. The coolant may be water, or water with ethylene glycol (ethane diol), or other suitable liquid.

Sanples of CdxHg1-xTe may be prepared as follows: Elements of high purity Cd, g, and Te are weighed to give the desired value of x and then sealed under a vacuum to exclude air in the silican ampoule 3. A narrow jet of a flare may be used to seal the ampoule.

The sealed ampoule 3 is then placed in the rocking furnace 33, Figure 2, and the vessel 30 sealed. The furnace 37 and vessel 30 o are set into a rocking movement of + 15 from the rest position at a frequency of 3 cycles per minute. At the same tine the 0 furnace 33 is operated to bring the ampoule up to 810 C over a 3 o hour period and then maintained at 810 C for half an hour. The result is a homogenised melt of Cd, Hg, and Te. This melt is solidified over a period of about 5 minutes by rapidly reducing the furnace 7 power. Throughout the melting, mixing, and solidification periods the pressure inside the vessel is balanced with that existing inside the ampoule 3.Pressure inside the ampoule 3 varies with temperature and can be calculated from Steininger's equation InPHg (at.) = 1C.2G6 - 7149/r Mg (K) where ln is natural logarithm.

P is pressure of Hg in atmospheres Hg o T ) is temperature inKelvin.

(J. Steininger, Journal of Electronic Materials, 5, 299 (1976).) The result of this is an ingot sample of CdxHg1-xTe but with inferior crystalline structure.

To produce a high quality, directionally solidified uniform sample the apparatus of Figure 1 is used as follows: The sealed ampoule 3 is placed inside the furnace 2 in the pressure vessel 1 with the flat base of the poule 3 in intimate thermal contact with the base plate 17. The furnace 2 is operated to raise the sample o temperature to 810 C over a period of about 5 hours. During this time, and later until the sample has again been cooled, the pressure inside the vessel is adjusted to equal the pressure inside the ampoule 3. As already noted ampoule internal pressure is related to its temperature. Thus by monitoring the ampoule temperature by the thermocouple 24 the pressure can be adjusted as required.Preferably this monitoring and pressure balancing is controlled by a microprocessor (not shown).

The pull rod 13 is operated to rotate the holder 12 and ampoule 3 at 90C-930 r.p.m. with a reversal of direction every 3 seconds ar.d with about 1 sec of decelaration and acceleration time. This rapid rotation and rapid reversal of rotation centrifuges the melt 4 up the ampoule walls 3 interspersed with a large and strong vertical movement during the reversal periods. As a result the Cd, Hg, Te elements are thoroughly mixed throughout the sample.

o After half an hour rotation, with reversals, at 810 C the ampoule o 3 is stopped. Gas cooled on to less than -40 C is injected into the chamber 18 below the ampoule. Typical fiow rate is about 50 to 100 standard litres per minute. This is maintained for about 2 minutes after which time the base plate 17 has been cooled o to about 300 C. Meanwhile the furnace 2 temperature is o maintained uniform at 810 C.

When the base plate 17 has been cooled do to about 300 C the pull rod 15 is operated by motor 16 to lower the ampoule 3 at about 1 cr/minute whilst maintaining the cooling gas flow and the furnace temperature contact, and balancing the pressures inside and outside the ar;oule. The result is a solidification of the sample 4 from the botton with a steep temperature gradient towards the top of the sample.

After about 12 minutes the solidification procedure is complete; the furnace 2 power supply and the cooling gas supply is shut off.

As cast the sample 4 is fine grained with axial dendritic growth of Cd Mg Te with interdentritic arterial of a Te rich x 1-x composition. Typically the sample is of 28mm diameter and 25mm height. Another sample had 20mm diameter and 20rm height.

Recrystallising or high temperature annealing, homogenises the material by solid state diffusion of the components throughout the sample and is achieved as follows. The sealed ampoule 3 is placed in a furnace 33 inside a pressure vessel 0, e.g. as in Figure 2 but without rocking movements. The ampoule 3 temperature is o o raised to 650 to 690 C at its upper end with a temperature o gradient of about 5 to 20 C per vertical om. This contrasts with the casting temperatures where a flat temperature profile is required. The rise in temperature is achieved over about 2 hours.

At this high tellperature the sample is ieft for 2 to 10 days and then progressively cooled. The cooling rate is about 1 to 1Y2 o o C/hour don to about 400 to 45G C. The sample may then 0 continue to be slowly cooled to room temperature, 20 C.

Alternatively it may be cooled rapidly e.g. a few deprees/min. down o to 2C C. The different final cooling rates varies the carrier concentration of the sample.

After completion of annealing the sample can be removed from the ampoule and sliced into wafers for subsequent processing into detectors.

Claims

Claims:

1. A method of growing the material Cd S g1 xTe (O < x < 1) comprises the steps of (i) containing a desired proportion of Cd, Hg, and Te in a sealed ampoule, (ii) containing the ampoule in a pressure chamber within the confines of a furnace and with the base of the ampoule resting in intimate thermal contact with a base plate, (iii) maintaining a gas pressure inside the chamber at a value near the pressure inside the ampoule as the temperature of the ampoule is changed, (iv) operating the furnace to gradually raise the temperature of the ampoule and contents to above 8000C to provide a melt, (v) energetically mixing the melt constituent whilst the melt temperature remains reasonably uniform, (vi) cooling the base plate to 3000C or lower, (vii) withdrawing the ampoule from the furnace whilst maintaining the furnace temperature constant and cooling the base plate until the whole melt has solidified to an ingot, (viii) raising the temperature of the ampoule to a value of 650"C - 6900C at the hottest end with a temperature gradient of about 50 to 200C per cm along the ampoule the highest temperature being at the last to solidify end of the ingot, (ix) holding this temperature value and gradient for at least 24 hours, slowly cooling the ingot to 450oC or lower.

3. The method of claim 1 wherein step (iv) includes rapid rotation of the ampoule about a vertical axis with rapid reversal of direction of rotation every few seconds.

4. The method of claims 1 or 2 wherein step (vi) includes directing a gas coolant onto the base plate.

5. The method of claims 1 or 2 wherein step (vi) includes directing a liquid coolant onto the base plate.

6. The method of any one of claims 1 to 4 wherein step (v) is discontinued before step (vi).

7.- The method of any one of claims 1 to 4 wherein step (v) is continued until the melt has solidified.

8. The method of any one of claims 1 to 6 wherein step (vii) is continued until the ingot reaches room temperature.

9. The method of any one of claims 1 to 7 wherein step (ix) reduces the ingot temperature to room temperature at a rate of 10 to 1.500 per hour.

10. The method of any one of claims 1 to 7 wherein step (ix) reduces the ingot temperature to the range 4000 to 4500C at a rate of 1" to 1.50C per hour followed by a rapid cooling of several degrees per minute to room temperature.

11. The method of any one of claims 1 to 9 wherein the contents of the sealed ampoule are cast into a sample ingot prior to step (i) of claim 1.

12. Apparatus for carrying out the method of claim 1 to grow CdxHgl~xTe . (O < x < 1 ) comprising a pressure vessel, a furnace mounted inside the pressure vessel, an ampoule holder having a base plate for supporting an ampoule containing elements of Cd, Hg, and Te, a pull rod for rotating the holder inside the furnace and for moving the holder and ampoule into and out of the furnace, means for moving the pull rod axially and rotationally, sensors for detecting the temperature of the ampoule, sensors for measuring the pressure inside the vessel, means for varying the pressure inside the vessel, and means for supplying coolant gas or liquid onto the ampoule holder base plate.

13. The apparatus of claim 11 constructed, arranged, and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of growing the material CdxHgl-xTe (O C x < 1) comprises the steps of: (i) containing a desired proportion of Cd, Hg, and Te in a sealed ampoule, (ii) containing the ampoule in the confines of a furnace within a pressure chamber and with the base of the ampoule resting in intimate thermal contact with a base plate, (iii) maintaining a gas pressure inside the chamber at a value near the pressure inside the ampoule as the temperature of the ampoule is changed, (iv) operating the furnace to gradually raise the temperature of the ampoule and contents to above 800"C to provide a melt, (v) energetically mixing the melt constituent whilst the melt temperature remains reasonably uniform, (vi) rapidly cooling the base plate to 3000C or lower, (vii) withdrawing the ampoule from the furnace whilst maintaining the furnace temperature constant and maintaining cooling of the base plate and thermal contact between the base plate and the ampoule until the whole melt has solidified to an ingot, (viii) high temperature annealing, in the confines of a furnace within a pressure chamber, by raising the temperature of the ampoule to a value of 6500C-690"C at the hottest end with a temperature gradient of about 50 to 2O0C per cm along the ampoule the highest temperature being at the last to solidify end of the ingot.

(ix) holding this temperature value and gradient for at least 24 hours, and slowly cooling the ingot to 4500C or lower.

2. The method of Claim 1 wherein step (v) includes rapid rotation of the ampoule about a vertical axis with rapid reversal of direction of rotation every few seconds.

3. The method of Claims 1 or 2 wherein step (vi) includes directing a gas coolant on to the base plate.

4. The method of Claims 1 or 2 wherein step (vi) includes directing a liquid coolant on to the base plate.

5. The method of any one of Claims 1 to 4 wherein step (v) is discontinued before step (vi).

6. The method of any one of Claims 1 to 4 wherein step (v) is continued until the melt has solidified.

7. The method of any one of Claims 1 to 6 wherein step (vii) is continued until the ingot reaches room temperature.

8. The method of Claim 1 wherein step (vii) is followed by removal of te ampoule from the confines of the pressure chamber of steps (ii) to (vii) and placed within the confines of a separate furnace within a pressure chamber.

9. The method of any one of Claims 1 to 8 wherein step (ix) reduces the ingot temperature to room temperature at a rate of 1" to 1.5 per hour.

10. The method of any one of Claims 1 to 8 wherein step (ix) reduces the ingot temperature to the range 4000C to 450"C at a rate of 1" to 1.50C per hour followed by a rapid cooling of several degrees per minute to room temperature.

11. The method of any one of Claims 1 to 10 wherein the contents of the sealed ampoule are cast into a sample ingot prior to step (i) of Claim 1.

12. Apparatus for carrying out the method of Claim 1 to grow Cd,Hgl,yTe (Oxl) comprising: a pressure vessel, a furnace mounted inside the pressure vessel, an ampoule holder having a base plate for supporting in intimate thermal contact an ampoule containing elements of Cd, Hg, and Te, a pull rod for rotating the holder inside the furnace and for moving the holder and ampoule into and out of the furnace, means of moving the pull rod axially and rotationally, sensors for detecting the temperature of the ampoule, sensors for measuring the pressure inside the vessel, means for varying the pressure inside the vessel, and means for supplying coolant gas or liquid on to the ampoule holder base plate.

13. The apparatus of Claim 12 constructed, arranged, and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.