DE2422251C2 - Process for the production of a doped cadmium telluride single crystal - Google Patents

Description

2. The method of claim 1 wherein the step (A) carried out in a closed vessel and the solid body obtained from its, the blowholes containing the upper part is freed, stripped and rinsed.

3. The method according to claim 1 and 2, wherein one chooses such an excess of tellurium for step (c) that the crystal dissolves at 900 ⁰ C with alloy formation

4. The method according to claim 3, wherein the dopant is added to the batch in a vessel with graphitin nenschicht and closes the vessel under ultra vacuum as close as possible to the batch

5. The method according to claim 3 or 4, wherein the vessel is moved in the temperature gradient of the furnace while rotating and vibrating

6. The method according to claim 1 or 2, wherein in step (c) the melt is displaced as a zone through the crystal while keeping the concentration of dopant in the zone constant in that one the solubility limit of the dopant means in the Tellurium melt exceeds, or that one uses a crystal with the same dopant has been endowed

7. The method according to claim 6, wherein one works in a quartz vessel with a graphite inner layer under ultravacuum between 500 and 900 ⁰ C with a gradient of the order of magnitude of 180 ° / cm.

8. The method according to any one of claims 1 to 7, wherein at least one of steps (a) to (c) in is carried out in an inert gas atmosphere.

The invention relates to a method for producing a doped cadmium telluride single crystal.

Single crystals of pure or doped semiconductors are required for use as detectors. if the semiconductor must have a high specific resistance, such as in solid-state ionization chambers, it is advantageous to use semiconductors with a high atomic weight. These are for example for Detection of gamma rays more sensitive, as the effective absorption cross-section for gamma rays is proportional to the atomic weight of the compound making up the detector. For the manufacture of semiconductors with a high atomic weight one has to combine two elements with a high atomic weight, the valence electrons of which lead to the formation of a semiconductor crystal in their chemical combination. Are particularly suitable instead cadmium and tellurium. Both crystallize in the cubic system. The number of electrons in the outer Shell 0 is two for cadmium and six for tellurium, so a total of eight for cadmium telluride, which corresponds to the stable outer electron shell of xenon Pure and perfect csdmium telluride with exactly the same number of cadmium and tellurium atoms, Contains no impurities or dopants is a high resistivity semiconductor, sometimes referred to as a "semi-insulator" Wenn if tellurium is present in excess to cadmium, there is an excess of holes and thus a semiconductor "doped" with one of its components, in in this case a p-type semiconductor. When the cadmium is in In contrast, if there is a slight excess, an n-type semiconductor is present.

One can also dop a cadmium telluride crystal with an electron donor or acceptor in order to To obtain a p-type or n-type semiconductor, or to dose the dopant so that its concentration corresponds to the excess of one of the constituents compensates In the case of a cadmium telluride semiconductor, which is initially of the η type due to excess cadmium, For example, you can add indium with a valence of 1 and thus intercept the mobile electrons and manufacture a high resistivity semiconductor. In the solid-state ionization chambers, where a semiconductor single crystal is in a sandwich is arranged between two electrodes, a semiconductor with a high specific weight is used and high resistivity with no crystal defects, so that the leakage current is not too great and is not For example, under the influence of certain gamma rays, electron-hole pairs form, resulting in a Changing the current between the electrodes leads, provided that the semiconductor is sufficiently good is that the electron-hole pair is not destroyed by recombination during its migration through the crystal will. The difficulty is to find such a semiconductor with low impurities, one precisely complied with stoichiometric composition or a precisely defined doping and with a To produce crystal lattices without twinning and interference. Once you've made them, detectors like this work at room temperature, what over the currently used, lithium-doped germanium detectors is an advantage, since this Must be operated at low temperature and also in so far inevitably less advantageous than the Cadmium telluride detectors are proportional, as the capture cross section for gamma rays, for example is the atomic weight of the semiconductor used for detection.

A. Libicky in H-Vi Semiconducting describes compounds for the production of cadmium telluride crystals 1967 International Conference (1967), page 389-401 a process in which initially cadmium telluride from high-purity cadmium and high-purity tellurium by melting at up to 1170 ° C in a closed, with Pyrolytically deposited graphite-lined quartz ampoule is synthesized, whereupon in a second stage the obtained bars of cadmium telluride melted in a closed graphite crucible under vacuum and at high temperature (up to I14O ° C) is recrystallized by zone melting. The ingots of pure cadmium produced in this way

telluric! however, were polycrystalline.

The use of zone melting for zone cleaning and zone equalization of cadmium telluride, the is in a graphite boat in a closed quartz vessel, which is in a separate area contains metallic cadmium or tellurium, in a horizontal two-zone furnace by D. De Nobel investigated (briefly presented in Wilke, Methods of Crystal Growth, Verlag Harri Deutsch, Frankfurt 1963, page 359). Due to high Cd or Te pressures, the Sublimation of CdTe can be avoided. The growth of single crystals is not described.

With reference to the aforementioned work by D. De Nobel, N. R. KyIe describes 5n J. Electrochem. Soc. (Solid State Science) Vol. 113. No. 11 (Nov. 1971), pp. 1790-1797 "Growth of Semi-Insulating Cadmium Telluride" a modified Bridgman method for growing CdTe. In turn, the composition of the melt and thus the solid phase is regulated the pressure of one of the components of the CdTe set over the melt. The synthesis and pre-purification of the CdTe by sublimation takes place in a vertical held, internally coated with graphite closed Quartz tube under high vacuum or hydrogen. The single crystal was grown here in a vertical Two-zone oven, on the other hand, as with De Nobel, in a vessel with two compartments with a controlled temperature or Te printing carried out For the optimal growth of large CdTe single crystals, a number of conditions were such as temperature gradient, speed, vapor pressure of the components, quenching speed indicated, which obviously all must be strictly adhered to when doped CdTe single crystals should be obtained with a high specific resistance, which was not always successful. It also leads to the precipitation of Cd or Te crystals in the CdTe on cooling (retro-solubility), which leads to Proposal leads to further investigate the single crystal growth of stoichiometric CdTe from the vapor phase.

In view of the difficulties of zone melting, the US-PS 34 94 730 proposes a simple process for the production of CdTe-Poly or single crystals, wherein CdTe from equimolar amounts of Cd and Te in the presence of cadmium chloride at over 49O ⁰ C, z. B. 650 ⁰ C. is obtained in a liquid phase, from which an excess of CdTe can initially be crystallized as a single crystal on cooling. If the temperature falls below the eutectic temperature, a mixture of polycrystalline cadmium telluride and cadmium chloride crystallizes out. After cooling, the cadmium chloride is dissolved out of the mixture with water. However, only very small single crystals were produced using this process.

The invention now aims at a method for producing a doped cadmium teliuride single crystal, whose doping is precisely regulated in order to either use a semiconductor of a certain type with a certain Establish specific resistance or the excess of one of the constituents by introduction of a suitable dopant to give a high resistivity crystal obtain. In particular, a doped cadmium telluride single crystal is to be produced as a radiation detector can be used at room temperature. The method according to the invention should be easier to carry out and deliver better products than the known processes.

It has been found that the production of a cadmium teliuride single crystal with the required properties, namely with a very low proportion of defects and few micro-precipitates, only succeeds if the last crystallization, which leads to the single crystal, is at a low temperature well below the melting temperature of the stoichiometric mixture cadmium-tellurium is carried out.
The object set is achieved by the method according to the invention for producing a doped cadmium teliuride single crystal which has the features specified in claim 1. Preferred embodiments are specified in the subclaims. The operations of purification and crystallization of the cadmium telluride are carried out in a logical sequence in the process according to the invention, so that a material with a high specific resistance is obtained, the properties of which are both crystallographically and electrically better than those of the cadmium telluride monocrystals which have hitherto been practically produced.

In the method according to the invention, three crystallization operations are carried out one after the other First in stage a) a coarse synthesis from the pure elements cadmium and tellurium, which become one still imperfect cadmium telluride bar leads In stage b) this bar is at a temperature ir the Near the melting point of cadmium telluride remelted to form an ingot of purer polycrystals from cadmium telluride, which, however, still have numerous crystallization errors and impurities. This purer cadmium telluride bar is then made from a melt of doped tellurium in stage c) recrystallized at a low temperature close to the melting point of pure tellurium, giving the desired Cadmium telluride single crystal without impurities and with a very low degree of imperfections, Twinning and micro-precipitates obtained, which is suitable as a detector for nuclear purposes is.

The basic materials cadmium and tellurium used in stage a) of the process according to the invention are previously cleaned separately by filtering under hydrogen to remove any trace of oxide, and then zone melting. The synthesis of cadmium telluride from approximately stoichiometric proportions of pure cadmium and pure tellurium takes place in stage a) of the method according to the invention, preferably in a test tube made of quartz without OH ions and having an inner coating made of graphite. This coating is obtained by cracking of methane, benzene or other carbon-rich substance in vacuo 1000-1200 ⁰ C. After filling, the tube is sealed in an ultra vacuum and introduced into a tilting furnace, so that the synthesis in the horizontal position and the subsequent cooling the melt can take place in a vertical position, the cooling being carried out slowly in one direction from the bottom of the test tube. The bar obtained after cooling is freed from its upper part containing the cavity, then stripped and rinsed, for example with bromine alcohol.

In a modified embodiment of the invention, a batch is used in step a) whose

Composition a small excess of tellurium compared to the stoichiometric composition having. The setting of this deviation determines the choice and necessary concentration of the doping

medium.

In stage b) of the process according to the invention a recrystallization of the solid obtained in step a) at a high temperature close to Melting point of cadmium telluride made in a vessel sealed under ultravacuum. In one embodiment of the method, this is a vessel a test tube with a graphite inner layer similar to the one used in the first stage is sealed as close as possible to the solid material after adding a slight excess of additional tellurium containing dopants, if that is necessary for crystal growth in Stage c) is necessary according to the conditions given below

As explained in the following FIG. 1, the fluctuations in the stoichiometric composition, within which the tellurium and cadmium can crystallize together, are of the order of 10 ¹⁷ atoms / cm ³ at about 900 ^° C. However, such an excess of one of the constituents is sufficient for the To allow the specific resistance to drop sufficiently that the use of such a crystal in a solid-state ionization chamber is out of the question. These differences in concentration with respect to the stoichiometric concentration, however, correspond to the order of magnitude of the concentration of dopants that can be built into the crystal lattice in order to compensate for the excess of one of the constituents and to come back to a higher specific resistance of at least 10 ⁵ ohm / cm.

One generally works with an excess of tellurium, especially since the slope of the liquidus curve of FIG. 1, which indicates the concentration of the liquid as a function of the temperature, is steeper on the tellurium side than on the cadmium side and consequently the concentrations by adjusting the Temperature can be better controlled, and because, on the other hand, knowledge of the excess of the concentration of tellurium determines the nature of the η-type dopant to be introduced

In a modified embodiment, the crystallization is carried out at high temperature, & h. a temperature close to the melting temperature of the cadmium telluride in a sealed test tube made of quartz performed with graphite coating and the partial pressure of a component is regulated by adjusting the temperature of a part of the test tube where a Is located in a crucible that contains at least one of the substances whose partial pressure is to be maintained.

In a modified embodiment of the invention, a pure component, cadmium or tellurium, is poured into the crucible contained in the test tube, and during the crystallization at high temperature, the temperature of the crucible, which is denoted by 7}, is lower than the temperature T \ der Melt held.

In a second modification of the invention, in addition to one of the two components cadmium or tellurium, a volatile dopant is added to the crucible. In order to set the partial pressure of one of the constituents of the gas above the melt, the atomic absorption of the light is measured and, in accordance with the measurement result, the temperature Ti is set by a known type of control device so that the partial pressure in the gas remains at the desired value

According to a further modification of the method according to the invention, the substance placed in the crucible is dissolved in a slightly volatile amphoteric solvent, the introduction of which into the crystal does not change its electrical properties. In this case, the temperature T _{2 of} the crucible during the crystallization is kept equal to that of the melt, which simplifies the furnace construction and process management. The partial pressure of the component / is determined by ίο the following formula:

D iy ν ν

η « r, · Λί · Τι
wherein

P ₁ Vapor pressure of the pure component / at the temperature Tu

Xi is the mole fraction of the component /, Yi is the thermodynamic activity coefficient of the component / at the temperature Ti and the concentration Xi.

For example, binary alloys of cadmium with tin, indium, gallium, zinc or bi are used nary alloys of tellurium with tin, indium, gallium and zinc, or cadmium halides.

In all modifications of the invention, the crystallization at high temperature by relative displacement of the test tube and the temperature gradient The test tube can be operated continuously by means of an appropriate drive system or rotated continuously or vibrated. This method of so-called "normal" cooling in the direction perpendicular to the solidification tion front leads to a deposition of the impurities at the top of the bar.

In the last crystallization of the process described above, the crystal is dissolved and then as Single crystal at low temperature, d. H. a temperature crystallized near the melting point of pure tellurium

In the first modification of the production of single crystals at low temperature, the process with "exhaustion of a solution", the ingot is filled with a

very large excess of tellurium introduced into a test tube made of quartz which is optionally coated with graphite, the relative concentrations of cadmium and tellurium being chosen so that the mixture is in the range of about 900 ^° C., which is close to melting point of pure tellurium is completely liquid The test tube is placed under ultravacuum or under the partial pressure of a neutral or reducing Tightly sealed gas. The dopant is added to the batch and crystallization is started in egg in a vertical oven as above for crystallization Described at high temperature, carried out after the test tube as close as possible to the material was locked. The difference is that the temperature of crystallization in the presence of the solution by means of tellurium is much lower than that which would correspond to cadmium telluride.

In a modification of the doping method, the dopant is either pure or dissolved in a crucible suspended in a test tube amphoteric solvent is introduced and the partial pressure of the dopant is controlled as in the case of crystallization at high temperature In a second modification of the single crystal manufacturing

7 8

treatment at low temperature becomes a zone of the doped drawings. Herein shows

liquid tellurium through the original crystal shows the liquidus-solidus curves of the mixture

pushed so that the solubility limit of the doping Cadmiuni-Tellurium;

is not exceeded by means of the tellurium solvent. Fig. 2 schematically the test tube for the crystalline

One chooses a crystal, which in the crystallization at 5 sation at high temperature;

high temperature with the same dopant Fig.3 schcmasst the Heiz.ring and the reagent was doped to the concentration of the latter in the glass, for crystallization from the tellurium melt at deeper Solvent zone is used in accordance with its continued temperature;

step to keep constant and to achieve that if Fig. 4 shows schematically the device for crystallization

the solubility limit of the dopant in the solution at low temperature using a

solvent zone is exceeded, like the test tubes for cadmi- inclined, which are continuously

Umchlorid can be the case in tellurium, the concentration turns.

As stated above, the method according to the invention consists of the crystallization at high temperature grain method, at high temperature in the vicinity of the menden crystal, which in this case is not doped, is the melting temperature of the stoichiometric mixture , must pre-dop. According to the invention, the tellurium-cadmium polycrystals of doped cadmium are produced in an ultravacuum. Subsequently, a Einkriführt, wherein the temperature in the test tube between the stable production which crystals supplies which is / changed a 500 and 900 ⁰ C and the high temperature gradients in the 20 very low level of impurities and little micro order of magnitude of 180 ° C cm. have precipitations, which is not the case with high

According to the invention, crystals produced in vacuo or in temperature can be used. The specific Wi-ultra vacuum to work at least one of the three resistances of the crystals is carried out by doping the program crystallizations in an inert gas atmosphere with halogens, for example in the form of cadsphere. 25 mium chloride, fluoride, iodide, bromide adjusted, whereby

The inventive method is carried out the dopant is introduced by the tellurium melt.

in a system that has the following devices: is performed. You can also use indium, tin, etc.

In Fig. 1 are the equilibrium curves Solidus-Li

- Devices for cleaning the cadmium and quidus as a function of the temperature (ordinate) Tellurium by zone melting; 30 and the relative concentrations of tellurium and cad-

- Devices for filtering the cadmium and mium shown, with the concentration of the tellurium Tellurium in a hydrogen atmosphere; increases in the abscissa from 0 on the left to 1 on the right. The Liqui-

- Vessels in which the mixture of cadmium and dus curve is denoted by L and the solidus curve by S. Tellurium is subjected to a first crystallization. The so-called crystallization at high temperature is; 35 door takes place at a temperature close to point A,

- a tilting furnace to carry out the synthesis in which the melting temperature of the (stoichiometric horizontal position and the cooling in the vertical mixture) corresponds to cadmium telluride. starts from a point B , which is the temperature Ta

- Furnaces for carrying out the crystallizations at corresponds to the concentration of the crystallizing high and low temperature; 40 solid at point C of the solidus curve S, for which

- Test tubes which contain crucibles in which the concentration D is obtained from the figure The Einkridie components can be given, the vapor partial stable production takes place at low temperature and should be kept constant at a pressure; Point E, which is the result of a high concentration of tellurium

- Devices for measuring the atomic absorption speaks the concentration of the crystallization at of the light in the gas phase above the melt; 45 solid product obtained at this temperature lies in

- Devices for controlling the temperature Ti the figure at point F.

according to the concentration of the gas phase The "belly" of the solidus curve is not measured in this

on the desired component; mortal figure exaggerated. The distance to the stoichio-

- Devices for carrying out the single crystal metric composition of the solid mixture production by shifting a zone of the solvent 50 never exceeds about 10 ¹⁷ atoms / cm ³ , which in turn have: In Fig. 2, the test tube 2 is shown on the left, in which an oven with a heating ring, which heats the crystallization at a higher temperature durühgciuhri the test tube in which the crystallization takes place and which follows at a speed V in the direction over a small part of its height enabling arrow is moved downwards. The temperature of the light; 55 furnace as a function of the ordinate is shown in the curve mechanical devices to the test tube 4, namely to keep the changes in Tempes perpendicular and around a vertical axis ratur as the ordinate as a function of location x. The furnace to rotate continuously. has three temperature zones, with the temperature im

Area 6 of a temperature above the melting temperature - For carrying out the inventive method of driving the stoichiometric mixture of cadmium telluride, an arrangement of furnace test and the temperature T ₂ in area 8 of a lower limit glass, which can be tilted around the, are used Touch corresponds to temperature that is used to maintain the partial liquid bar and gas bubbles between the pressure of one of the components, which is located in the crucible 10 in the liquid, the test tube wall and the bar test tube, the temperature T ₃ in the Bezur side to drive out, so that through this device 65 is rich 12 to which the test tube is fed, a more uniform crystallization is achieved. ter melting temperature. The liquid 9 crystallizes

The invention is illustrated by the following description of the solid 11.. Spelling. This refers to the attached. In Fig.3 the test tube is shown on the left, where the

IS

Crystallization takes place at low temperature. This test tube is moved downwards at speed V and is surrounded by a heating ring 13 which generates temperature gradients, the values of which are given in curve 14 as changes in temperature as a function of location χ . When the test tube is lowered, ie when it passes through the heating ring, the crystal passes through a zone of liquid doped tellurium 16, and the crystal dissolved at 18 crystallizes again at 19, the first crystallization starting from a nucleus 20

In the case of one shown in FIG. 4, the test tube is displaced in the direction of arrow V and continuously rotated in the direction of arrow 22. The raw material 23 passes through the zone of liquid doped tellurium 24 in the area of the heating ring 26 and crystallizes again at 28 to form the finished single crystal.

example 1

For the production of cadmium telluride rods of tellurium and cadmium of 1500 g and 500 mm each are used Length first cleaned by zone melting To do this, they are placed in test tubes in a hydrogen atmosphere Brought out of quartz and placed in a furnace. For the purpose of cleaning, the zone melting takes place in twelve passes at 25 mm / hour. The purified metals are then used for synthesis with a A small excess of tellurium is placed in a horizontally held test tube. The temperature is increased at only 12 ° C. per hour, so that the very exothermic The formation reaction does not proceed suddenly. The temperature is then kept at 1200 ° C. for two hours then brings the test tube into an upright position and cools within 12 hours, whereby the Cooling begins at the bottom of the test tube and the oven used is, for example, an oven with heating rings

During the second stage, i.e. After crystallization at a high temperature, a small excess of tellurium is added to the test tube containing the alloy previously obtained and the temperature of the test tube is increased to 1150 ^° C. in order to melt the ingot. The test tube containing the molten material is then shifted downwards, where a temperature of 1090 ^° C. prevails, in order to bring about the crystallization. The lowering speed is 5 mm / hour.

In the third stage, i.e. the crystallization at deeper Temperature, the bar is placed in a quartz test tube into which 60 g of tellurium with a content of 700 ppm of cadtnium chloride have been previously filled. That Test tube is slowly moved at a speed of 03 mm / hour. moved down, taking it with one revolution per minute is rotated around itself. The temperature of the solid-liquid interfaces lies at this temperature and lowering rate at 660 ° C. This method gives an ingot of cadmium telluride doped with about 3 ppm of cadmium chloride

Example 2

The first bar is produced as in Example 1. The it Crystallization of the alloy at high temperature takes place in a test tube, which is in its upper part is provided with a crucible, the pure tellurium and about Contains 300 ppm cadmium chloride. The lower part of the oven is heated to 1150 ° C. and the test tube is then lowered. At 1000 ^° C., the cadmium telluride doped with 3 ppm cadmium chloride crystallizes. The temperature of the top of the test tube where the crucible is located must remain at about 900X. Crystallization at low temperature takes place as in Example 1, but replacing the 60 g of tellurium doped with 700 ppm cadmium chloride with 60 g of tellurium doped with only 200 ppm cadmium chloride. The obtained single crystal is similar to that obtained in Example 1.

Apart from the solid-state ionization chambers, single crystals of cadmium telluride find numerous other applications, of which we can mention, for example: detectors for nuclear rays or infrared, electro-optical modulators, infrared-permeable windows, active components such as diodes or photodiodes, high-temperature optocouplers, bipolar transistors, field-effect transistors, junction transistors and those with insulated gate electrodes, thyristors, diacs, triacs and lasers and electroluminescent cells.

For this purpose 3 sheets of drawings

25th

30th

35

45

50

55

Claims (1)

Patent claims: 1. A method for producing a doped cadmium telluride single crystal, wherein one a) pure cadmium and pure tellurium in proportions close to the stoichiometric composition of cadmium telluride melts and becomes one Solidifies solid, b) the solid melts and lets it crystallize again at a temperature close to the melting point of cadmium telluride and finally c) the crystal in a melt of doped is Tellurium dissolves and crystallizes out at a temperature close to the melting point of pure tellurium