RU2341594C2 - Mehgod of growing cadmium telluride monocrystal - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of manufacturing cadmium telluride monocrystal lies in loading polycrystal half-product into crucible, hermetization with further crucible vacuuming, melting of half-product, cooling of obtained ingot, its standing at certain temperature and further cooling to room temperature; polycrystal half-product is loaded into crucible together with pure cadmium sample, whose weight is determined by Clapeyron-Mendeleev equation, crucible is exhausted to pressure 10^-6-10^-7 mm of mercury, half-product is melted, ensuring temperature gradient on height 1-5°C/cm, half-product melt is stood at melting temperature during 2-4 hours, then half-product is cooled at rate 0.5-1,0°C/hour to full crystallization; obtained crystal is cooled at rate 40-60°C/hour to temperature 920-960°C, crystal is stood at said temperature during 8-12 hours, then it is cooled again at rate 40-60°C/hour to temperature 820-860°C and stood at during 8-12 hours, then crystal is cooled to temperature 700-720°C and stood during 8-12 hours, after which crystal is cooled at rate 10-20°C/hour to room temperature and removed from crucible as end-product.

EFFECT: high perfection of microstructure and high optical characteristics.

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Description

The invention relates to the field of crystal production, and more particularly to a method for growing cadmium telluride single crystals — CdTe.

Currently, crystals of this material, which has semiconductor properties, are beginning to be widely used in optical and electronic technologies. Due to the optimal combination of electrical and optical properties, the use of cadmium telluride for the manufacture of elements of infrared optics is very promising. This material is used for the manufacture of solar panels, lasers, photo resistances, radiation counters. Cadmium telluride is one of the few semiconductors in which the Ghana effect is noticeably manifested. The essence of this effect is that the very introduction of a small plate of the corresponding semiconductor into a sufficiently strong electric field leads to the generation of high-frequency radio emission. The Ghana effect has already found application in radar technology. Thus, the potential industry demand for high-quality cadmium telluride crystals is very high.

An obstacle to the widespread use of the material is the presence of precipitates in the crystal, which leads to a decrease in the quality of detectors and modulators. In these devices, precipitates are scattering centers and large-scale traps of charge carriers, impairing charge collection and reducing the efficiency of radiation detection. In electro-optical modulators, tellurium precipitates reduce the transmission of infrared radiation and increase the absorption coefficient, making them unsuitable for use.

In this regard, an active search is underway for new methods for producing pure cadmium telluride crystals containing no precipitates.

A known method of growing crystals of cadmium telluride, which includes heating a polycrystalline charge and elemental cadmium located in different sections of a sealed container. The charge is melted, the melt is held, and then cooled. During cooling, the melt section is maintained in a gradientless temperature field, and the temperature in the cadmium section is maintained in the range 755-765 ° C. Cooling is carried out at a rate of less than or equal to 3 ° C / h. (RF patent 1431391, IPC С30В 29/48, publ. 1993.03.15).

The disadvantage of this method is the need for heating separately load and elemental cadmium, which are located in different containers. To implement this method, the presence of two furnaces is required, which complicates the design of the installation and causes high energy consumption for the production of crystals. In addition, the method does not allow to obtain precipitate single crystals of optical quality.

There is also known a method for producing crystals of CdTe and CdZnTe with a low content of tellurium precipitates by the Obreimov-Shubnikov method (also referred to in the foreign literature as the “VGF” method, ie, crystallization by cooling with a vertical temperature gradient), which is described in US patent No. 6,299.680, NKI 117/2, publ. Oct 9, 2001.

In accordance with this method, the process of obtaining crystals is as follows: after the initial mass is melted and the formation of the crystal begins, cooling in the temperature range from 1050 to 700 degrees Celsius is carried out with simultaneous pressure control over the crystal surface in order to maintain the stoichiometric ratio of the crystal. After reaching a certain temperature, determined by calculation, the crystal is thermostated for a period of time, which is determined on the basis of a logarithmic dependence, in which the diameter and length of the grown crystal are variables. After completion of thermostating, the crystal is again cooled, and the temperature of the crystal and the reservoir with Cd are logarithmically changed. To implement this method, it is necessary in the crystallization unit to have a separate reservoir communicating with the crucible, which is filled with Cd. To change the pressure in this tank, an additional heater is used. The reservoir communicates with the crucible in which the crystal is grown.

The disadvantage of this method is the presence of high energy costs associated with the need to heat both the crucible and the tank with Cd. In addition, it is necessary to maintain a rather complex logarithmic dependence between the time of the processes and the maintained temperature regime. The method is implemented only if there is a separate tank heater control system. The system for regulating the relationship between the temperatures of the crystal and the walls of the reservoir with Cd, changes in the pressure of the gas phase, which is based on observing the conditions of inequalities containing variables that vary according to the logarithmic law, cannot provide accurate stoichiometry. The result of this is the presence of precipitates in the resulting crystal. The process of obtaining the crystal itself is very lengthy. So in the example given in the patent description, it is indicated that the thermostating time at a temperature of 950 ° C was 20 hours, and the pressure of gaseous Cd was maintained at 0.15 atm.

The present invention is the creation of a simple, low-energy, providing a high degree of reproducibility of the results of the method of production of precipitate-free CdTe.

The technical result of the invention is to obtain semi-insulating single crystals of CdTe and CdZnTe with high perfection of the microstructure and high optical characteristics. The crystals obtained by the considered method for several years retain a high resistivity (approximately 10 ⁹ Ohm × cm), which is important for their successful application in detectors and modulators.

The solution to this problem is realized by the method of manufacturing a cadmium telluride single crystal, which consists in loading a polycrystalline billet into a crucible, sealing and then evacuating the crucible, melting the billet, cooling the obtained ingot, holding it at a certain temperature and subsequent cooling to room temperature.

In this case, a polycrystalline charge, which is individual fragments of 0.5-1.0 cm in size, is poured into the crucible simultaneously with elemental cadmium. The mass of cadmium m _Cd is determined by the Klaiperon-Mendeleev equation:

m _Cd = V _t M _Cd P _Cd / T _a R

Where:

V _t is the free volume of the ampoule;

M _Cd is the atomic mass of cadmium;

P _Cd is the partial pressure of cadmium;

T _a - the average pressure in the ampoule;

R is the gas constant.

The crucible is pumped to a pressure of 10 ^-6 -10 ^-7 mm Hg, the preform is melted, providing a temperature gradient in height of 1-5 ° C / cm, the preform is melt at the melting temperature for 2-4 hours, the workpiece is cooled at a speed 0.5-1.0 ° C / h until the workpiece melt crystallizes completely, the resulting crystal is cooled at a speed of 40-60 ° C / h to the temperature of the second monophase region, the crystal is kept at the temperature of the second monophase region for 8-12 hours, again cool it at a speed of 40-60 ° C / h to the temperature of the third monophasic region, the crystal is kept at the temperature of the third monophasic region for 8-12 hours, it is cooled to the temperature of the fourth monophasic region, the crystal is kept at the temperature of the fourth monophasic region for 8-12 hours, after which the crystal is cooled at a rate of 10-20 ° C / h to room temperature and take it out as a finished product from the crucible. The temperature of the crystal in the second monophase region is 920-960 ° C, the temperature in the third monophase region is 820-860 ° C and the temperature of the crystal in the fourth monophase region is in the range of 700-720 ° C.

The meaning of the term "monophasic region" can be explained by looking at the state diagram near the CdTe compound (Fig. 1), published in [1]. Figure 1 shows the region of existence of solid cadmium telluride, consisting of four monophasic regions, indicated in figure 1 by Roman numerals I-IV. The single-phase regions above are limited by the retrograde solubility lines of the components (Cd and Te), respectively, at the crystallization temperature (phase I) and at the temperatures of two phase transitions (phases II and III). The third phase transition is located at the intersection of the lower solidus lines of phase III at a temperature of about 750 ° C, below which phase IV is indicated by dashed lines. The presence of these three solid-phase transitions was experimentally confirmed by the dilatometric method in [2]. The essence of the processes implemented in the invention is illustrated in figure 2, which illustrates a graph of the temperature of the billet of the crystal depending on time.

The following is a description of the processes of the method based on theoretical and experimental studies.

During the cooling of the crystal after its crystallization, the presence of a phase transition causes a number of negative phenomena: precipitation of precipitates due to a decrease in the solubility of the excess component; a constant change in the concentration of the excess component, which stimulates the occurrence of microinhomogeneity; the appearance of mechanical stresses during the simultaneous existence of two phases having different specific volumes, which causes the polygonization of the dislocation and the appearance of a cellular microstructure; instability of the electrical properties of the crystal due to partial hardening of the high-temperature phase.

To reduce the influence of these negative phenomena, it is necessary to maximize the cooling rate of the crystal in the phase transition temperature range. In this case, a binary transition of the first kind proceeds as a quasi-unary transition in the middle of the region of discontinuity in the dissolution of two phases. The invention is illustrated in the drawings. Figure 1 - a fragment of a state diagram near the connection CdTe is given according to [1]. Figure 2 - mode of cooling the crystal.

From the diagram shown in Fig. 1, it follows that in a binary compound, for example, CdTe, during the first-order transition in the case of slow cooling, the phase compositions change along the solidus lines bounding the two-phase regions (regions (I + II), (II + III ), Fig. 1). When the crystal is rapidly cooled in the temperature range of the phase transition, the composition of the low-temperature phase does not have time to change and the phase transition proceeds as in a single-component system, i.e. without changing the composition of the crystal or, as a quasi-unary transition [3]. Such a process proceeds through intermediate states of very high disorder, accompanied, for example, by transitions of Cd and Te atoms to internodes, which can be partially frozen. This implies the need to bring the crystal into equilibrium by holding it for a sufficiently long time at a temperature that corresponds to its monophasic state and is below the phase transition temperature. To implement the above provisions, the process of forming the electrophysical and structural parameters of the crystal must be carried out in a stepwise manner, alternating the processes of rapid cooling with periods of exposure of the crystal for a certain period of time, as shown in figure 2.

An example implementation of the method.

A polycrystalline charge of cadmium telluride weighing 725 grams and elemental cadmium weighing 0.542 grams, which was determined by calculation based on the above Mendeleev-Klaiperon equation, were loaded into a glassy carbon crucible with a diameter of 90 mm. The crucible was installed in a quartz container and fixed inside it. The cavity of the container was pumped out to a pressure of 10 ^-6 mm Hg. and soldered. The sealed container was placed in a furnace and heated to a temperature of 1100 ° C, at which a cadmium telluride melt was formed. The melt was kept at the indicated temperature for 3 hours. After this time, the furnace was cooled at a rate of 0.5 ° C / h. Next, the grown crystal was subjected to stepwise cooling with exposure at temperatures of monophase regions: 950 ° C, 840 ° C, and 720 ° C. The exposure time in this particular example in each of the areas was 10 hours. After the last temperature exposure, the furnace was cooled at a rate of 10 ° C / h to room temperature. The container was removed and the crystal was removed from the crucible. The influence of the modes of processes on the crystal parameters is shown in the table.

Experiment Number The exposure time of the crystal at temperatures of 950, 850 and 710 ° C (h) The resistivity of the crystal, measured after manufacture (Ohm / cm) The resistivity of the crystal, measured one year after manufacture (Ohm / cm) one 8 1 · 10 ⁸ -2 · 10 ⁸ 1 · 10 ⁸ -5 · 10 ⁸ 2 10 1 · 10 ⁸ -1.5 · 10 ⁸ 1 · 10 ⁸ -3 · 10 ⁸ 3 12 1 · 10 ⁸ -5 · 10 ⁸ 1 · 10 ⁸ -5 · 10 ⁸ four 7 1 · 10 ⁸ -5 · 10 ⁷ 3 · 10 ⁵ -5 · 10 ⁶

As can be seen from the table, a crystal subjected to post-crystallization thermostating (holding for a certain time at a given temperature) for 8-12 hours, retains its resistivity for one year without change, which is especially important for crystals intended for optoelectronic devices.

The crystals, thermostatically controlled for 7 hours, reduce the resistivity more than an order of magnitude a year after manufacture. At the same time, the duration of temperature control for more than 12 hours does not lead to an improvement in the quality of crystals, but lengthens the technological process of manufacturing crystals. Thus, the optimal time range for temperature control is 8-12 hours.

The study of natural chips of the entire single crystal by cathodoluminescence showed an average dislocation density of 3 · 10 ⁴ cm ^-2 , the absence of small-angle boundaries and microtwins. Examination of samples on an IR microscope showed the absence of tellurium precipitates larger than 3 μm, and on a transmission electron microscope, the absence of precipitates in nanometer sizes. The parameters obtained as a result of the study indicate the high perfection of the microstructure, which, combined with the large size of the single crystal, allows its use in the manufacture of substrates for IR detectors. The absence of tellurium precipitates in the crystal makes it possible to create on its basis electro-optical modulators and other elements of nonlinear optics.

Literature

1. Yu.M. Ivanov. // J. Crystal Growth, 1996, V.161, p. 12

2. Yu.M. Ivanov, A.N. Polyakov, V. M. Kanevski et. al. // Phys. Stat. Sol. C, 2003, N3, p. 899.

3. Albers V., in the book. Physics and Chemistry of Compounds A ² B ⁶ , per. from English under the editorship of Medvedeva S.A., M, Nauka, 1970, p. 173.

Claims (1)

A method of manufacturing a cadmium telluride single crystal, which consists in loading a polycrystalline preform into a crucible, sealing and then evacuating the crucible, melting the preform, cooling the resulting ingot, holding it at a certain temperature and then cooling to room temperature, characterized in that the polycrystalline preform is loaded into the crucible together with a portion of pure cadmium, the mass of which is determined by the Klaiperon-Mendeleev equation, the crucible is pumped to a pressure of 10 ^-6 -10 ^-7 mm Hg, distribution melt the workpiece, providing a temperature gradient of 1-5 ° C / cm in height, maintain the melt of the workpiece at a melting point for 2-4 hours, cool the workpiece at a speed of 0.5-1.0 ° C / h until complete crystallization, the resulting crystal cooled at a speed of 40-60 ° C / h to a temperature of 920-960 ° C, kept the crystal at this temperature for 8-12 hours, cooled again at a speed of 40-60 ° C / h to a temperature of 820-860 ° C and stand for 8-12 hours, then cool the crystal to a temperature of 700-720 ° C and stand for 8-12 hours, after which the crystal is cooled rate of 10-20 ° C / h to room temperature and removed it from the crucible as the final product.

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