DE2422250A1 - METHOD AND DEVICE FOR MANUFACTURING SEMI-INSULATING CADMIUM TELLURIDE - Google Patents

Description

DR. HANS ULR: CH WAY

D S MUNICH 2, OTTOSTf = JASSE ld

TELEGRAMS: MAYPATENT MONKS

TELEPHONE CO8113 593682

A-16-F-t / i 284 Munich, May 8, 1974

Dr.M./es

Agence Nationale de Valorisation de la Recherene - AWAR ₉ in

Neuilly-sur-Seine / France

Method and device for the production of semi-insulating

Cadmium telluride

The invention relates to a method and a device for producing semi-insulating cadmium telluride.

If one looks at single crystals of semiconductors such as germanium and silicon which have intrinsic conductivity in a perfectly pure state, than If you want to use detectors »you have to either use the semiconductors in pure Use state and thus as substances with high specific resistance or doped with a suitable dopant »with it Particles of lower energy than a Blektron hole pair lighter than can induce intrinsic conductivity in the pail of the semiconductor.

In applications where one is dealing with a semiconductor with intrinsic conductivity and high resistivity, such as in pest body ionization chambers, it is advantageous to use semiconductors with high atomic weight. For proof of at? - For example, gamma rays are namely detectors made of substances with a high atomic weight more sensitive, since the cross-section for the

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Absorption of gamma rays is proportional to the atomic weight of the substance forming the detector. in order to obtain semiconductors with a high atomic number, one must therefore produce a compound which alloys two elements whose valence electrons in their chemical combination form a semiconductor crystal. Several such combinations are possible, for example Gs.lliuriiarsenid or cadmium telluride. Because of its higher atomic number, cadmium telluride is the better detector. «Tellurium crystallizes like cadmium in the Kabic system. The number of electrons in the outer shell O is two for cadmium and six for tellurium, so a total of eight for cadmium telluride, which corresponds to the stable structure of the higher electron shell of xenon. The pure and perfect cadmium telluride, i.e. without impurities, dopants and with exactly the same number of cadmium and tellurium atoms, is a semiconductor with intrinsic conductivity and high specific resistance. Such semiconductors are referred to as semi-insulators. If tellurium is in excess compared to cadmium, there is an excess of holes and a ¹ * semiconductor is present which is doped by one of its constituents, in this case a semiconductor of the P type with a small excess of cadmium, an N-type semiconductor is present.

If you have a semiconductor with intrinsic conductivity and large to obtain resistivity, one must remove all impurities, such as metals, with one other than cadmium or tellurium Switch off valence, as these metals dop the cadmium telluride and make it conductive.

In the solid-state ionization chambers, where a semiconductor single crystal is inserted as a sandwich between two electrodes, becomes a semiconductor with high specific weight, high specific resistance

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and used free of crystal defects so that the leakage current does not occur is too significant and under the influence of certain rays, for example gamma rays, an electron-hole pair is formed which leads to a change in the current between the electrodes, provided that the semiconductor is sufficiently good. that the electron-hole pair not when it moves through the crystal by recombination disappears. The difficulty lies in using such a substance, i.e. cadmium telluride with intrinsic conductivity a low level of contamination, an accurate stoichiometric composition and a crystal lattice without twinning and get other shifts. If you have made such a substance, these detectors work at room temperature, which is an advantage over the currently used detectors with intrinsic conductivity, i.e. the detectors made with lithium doped germanium, which must be stored and operated at low temperatures in order to diffuse the lithium in the crystal avoid. The detectors with lithium-doped germanium, in front of the Impurities in the germanium are compensated for by lithium in order to obtain a semi-insulator “are also inevitably less good than the detectors made of cadmium telluride, since the capture cross-section for gamma rays is proportional to the atomic number of the as Detector used is semiconductor.

The invention now aims at a method for the production of single-crystal cadmium telluride with intrinsic conductivity, the content of impurities is reduced to a minimum and in which the proportion of displacements, twinning and other crystal defects is reduced. The method of this task serving invention is characterized in that _t the manufacturer

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position takes place in several stages, namely through

a) a synthesis of cadmium telluride from stoechiometric components of high-purity cadmium and high-purity tellurium, with crystallization at a temperature corresponding to the melting point of. stoechiometric mixture and under a certain vapor pressure of the cadmium j, which allows a precise adjustment of the deviation of the compound allowed by the stoechiometric composition;

b) a cleaning of the cadmium telluride block thus obtained by numerous Passes in vertical zone melting;

c) finally a single crystal growth in the solvent tellurium a temperature close to the melting temperature of pure tellurium to finally obtain a cadmium telluride crystal.

In other words, in the method of the present invention, the compound is synthesized at high temperature in a vertical furnace with three temperature zones. The synthesis is followed by gradual cooling the melt, which leads to a separation of the impurities in the end section of the resulting block, under a regulated Vapor pressure of the cadmium (or possibly tellurium), indicating an adjustment of the deviation of the compound from the stoechiometric Composition allows. This step is followed by the cleaning of the block obtained by means of vertical zone melting.

The single crystal is then grown at a low temperature, namely a temperature close to the melting point of pure tellurium, whereby one has a crystal with very small displacements and receives little microprecipitation, which in the case of the high temperature (the melting temperature of the stoechiometric mixture) manufactured crystal is not the case. Only the crystals obtained at low temperature are suitable as detectors for caching

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know of nuclear rays.

The cadmium and tellurium are cleaned by zone melting prior to synthesis after they have been filtered under a hydrogen atmosphere to remove all traces of oxide. The synthesis of the cadmium telluride is carried out at & ώϊλ method according to the invention i: a a would shut "nen quartz tube, the inner layer graphite was provided by cracking methane, Sensol or any other carbon-rich substance in vacuum between 800 and 1200 ⁰ C with a. This tube is closed (sealed) under a secondary vacuum or ultra-vacuum and inserted into the vertical furnace with three heating zones, the central heating zone of which is gradually heated to the melting point of the joint. is while the temperature of the upper part of the furnace is increased and controlled to a value corresponding to a composition of the compound close to the stoechiometric composition.

As can be seen better from FIG. 1 described below, at a temperature of about 900 ° C. the deviations from the stoechiometric composition within which tellurium and cadmium can crystallize together are of the order of magnitude of at most 10 atoms / cm " ³ It is impossible to achieve an exact concentration by weighing, and yet such an excess of one of the constituents is sufficient to reduce the specific resistance to values which preclude the use of such a crystal in a solid-state ionization chamber.

According to the invention, the crystallization is carried out at a high temperature, ie at a temperature T ₂ (FIG. 2) in the vicinity of the melting temperature of the compound cadmium telluride in the synthesis tube and the partial pressure of a component is controlled by regulating the temperature T "

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(Fig. 2) reached in a part of the tube where a crucible is located, which contains at least the component whose partial pressure is to be kept constant.

For example, a pure component, cadmium or tellurium, is added to the crucible in the tube, and the _{temperature of the crucible, designated T 1,} is kept lower than the temperature T _{2 of} the melt during the crystallization.

According to the invention, the crystallization is carried out at a high temperature Relative displacement of the tube against the temperature gradient of the furnace is obtained. The tube can be driven by a suitable drive system be set in continuous or alternating rotation or oscillation. These with the technical expression "normal cooling" designated method leads to a separation of the impurities

*) i.e. displacement perpendicular to the solidification front at the upper end of the block.

According to the invention, the cadmium telluride becomes after this crystallization through several passes through a melting zone in a tube made of silicon dioxide that is internally coated with graphite and degassed at high temperature carried out, which is completely filled with the cadmium telluride, so that no free space remains in it. aside from that a silica plug is placed on the top of the block, and every precaution is taken to especially avoid the transport of contaminants through the original block to prevent the vapor phase.

In the last crystallization of the process according to the invention, a single crystal is grown at a low temperature, namely a near the melting point of pure tellurium, by adding a tellurium-rich liquid zone of cadmium telluride by moving the starting crystal.

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This single crystal growth takes place in a quartz tube closed at the bottom with an inner coating of graphite under ultravacuum, the temperature in the tube being changed from 500 to 900 ° C ^{with high temperature gradients in the order of magnitude of 100 o c / cm.}

According to the invention, instead of under vacuum or ultra-vacuum, it is also possible carry out at least one of the three crystallizations described under an inert gas atmosphere.

According to a modification of the process according to the invention, the crystal obtained by crystallization at low temperature is tempered in a final stage. According to a first modification, this tempering can be carried out in a closed vessel containing the crystal and a crucible filled with cadmium or tellurium. The crystal is brought to a temperature T! J and the crucible to a temperature T ' _g , where T « ₂ <T ¹ ^. The pressure of the element contained in the crucible is determined by the temperature T · of the cold point of the interior of the vessel.

By regulating the thermodynamic parameters T 'and T' ₂ and the duration of the process, it is possible to perfect the stoechiometric composition of the crystal and to obtain crystals with a semi-insulator character. In one embodiment, T ^ is equal to 700 ⁰ C, Τ ¹ "equal to 55O ° C and the duration of treatment three hours.

In a second modified embodiment, a congruent isothermal sublimation is carried out in an isothermal chimney of considerable dimensions at a temperature which is defined by the intersection of the straight line D ₁ , which represents the vapor pressure of Cd of the stoechiometric mixture, and the straight line D ", which represents the vapor pressure of Cd with a congruent isothermal sublimation as-

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that gives, is determined. These straight lines are shown in FIG. 6 and are given in more detail in the publication by M. de Nobel in Philips Research Report 1959, Volume 14, pages 430 to 493. This sublimation takes place, for example, at a temperature of 300 ^° C.

The invention also aims at a device for performing the Procedure, which is characterized by

a) Devices for cleaning cadmium and tellurium by zone melting?

b) Devices for filtering cadmium and tellurium under a hydrogen atmosphere;

c) a Bridgman furnace for performing the synthesis and crystallization at high temperature;

d) a sealed tube containing a crucible in which one can hold the component whose vapor partial pressure is to be kept constant;

e) Devices for cleaning the connection by vertical zone melting, which have:

el) a high frequency induction furnace that only has a narrow zone

the compound heated; e2) mechanical displacement rotors;

f) Devices for growing single crystals by moving a Zone of the tellurium solvent, which have:

f1) a furnace with a heating ring that only heats the tube in which crystallization occurs over a small part of its height,

f2) mechanical devices which hold the pipe vertically and continuously rotate it around a vertical axis.

The invention is illustrated by the following description of a

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Exemplary embodiment. The description refers to the attached Drawings. Show here:

1 shows the liquidus solidus curves of the cadmium-tellurium mixture;

- Fig. 2 schematically that used for high temperature crystallization closed reagent tube;

- Fig. 3 schematically shows the closed pipe and the furnace for the

vertical zone melting;

- Fig. 4 schematically shows a device for crystallization in the solvent Tellurium at low temperature j

- Fig * 5 schematically a tempering device .;

6 is an equilibrium diagram of cadmium telluride.

The method according to the invention is based in particular on that of a logical one Order of the purification and crystallization measures in the following order, which result in a material with a high specific resistance, a so-called "semi-insulator" with better crystal properties and also lead to better electrical properties.

FIG. 1 shows the solidus-liquidus equilibrium curves as a function of the temperature and the relative concentrations of tellurium and cadmium. The temperature is plotted on the ordinate and the concentration of tellurium is plotted on the abscissa in such a way that the concentration is 0 on the left and the concentration is 1 on the right. The liquidus curve is denoted by L and the solidus curve by S. The so-called high-temperature crystallization takes place at a temperature close to point A, which corresponds to the melting temperature of the stoechiometric mixture cadmium-tellurium. Starting from a point B, corresponding to the temperature T ₄ , the concentration of the solid phase crystallizing out is represented by the point C of the solidus curve S to which the concentration at point D of the figure belongs. the

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Single crystal growth takes place at low temperatures & at a point E, which corresponds to a high concentration of tellurium. The concentration of solids obtained by crystallization at this temperature Phase is at point F of the figure. The "belly" of the solidus curve is exaggerated in this figure, which is not to scale. The distance to the stoechiometric composition of the solid mixture never exceeds 10 'atoms / cm.

In FIG. 2, the closed reagent tube 2 is shown on the right, in which the high-temperature crystallization takes place and which is moved downwards at a speed V indicated by the arrow. The temperature of the furnace as a function of the ordinate is shown by curve 4. This shows the temperature changes at various levels along the ordinate x. The furnace is a so-called Bridgman furnace with three temperature zones: The temperature T ₁ at 6 corresponds to a temperature below the melting temperature and controls the partial pressure of one of the constituents in the crucible 10 inside the reagent tube. The temperature T ₂ at 8 is higher than the melting temperature of the stoechiometric mixture. The temperature T ₃ at 12 to which the tube is advanced is below the melting temperature and higher than T ₁ .

In Fig. 3 is the reagent tube and that for vertical zone melting serving furnace shown. The reagent tube 14, which is completely filled with cadmium telluride 16, is plugged by a silicon dioxide 18 is closed, is moved in the direction of arrow V in the figure downwards. The molten zone 20 is opposite the Heating ring 22, the secondary winding of the transformer, its primary winding formed by the turns 24. The temperature is regulated by means of a thermocouple 26.

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In Pig, 4 the reagent tube surrounded by the heating ring, which is used for growing crystals at low temperatures, is shown. The temperature curve as a function of the position of the reagent tube is drawn at 28. The reagent tube filled with cadmium telluride 36 is slowly lowered in the direction of arrow V, which has the effect that the starting material 36 is converted into pure crystallized material 34 after passing through the liquid zone 32. The reagent tube is surrounded by the heating ring 30 and closed by the silicon dioxide plug 38. In one embodiment of the method, the synthesis and crystallization take place at high temperature in a reagent tube which is provided in its upper part with a crucible filled with cadmium. The sealed reagent tube is placed in an oven, the temperature of which is increased very slowly to about 80O ⁰ C in about 24 hours according to a program in order to avoid an excessively vigorous formation reaction, which is strongly exothermic the melting point of the material in the region of 1100 ⁰ C increased. The temperature of the upper zone, above the melt, regulates the vapor pressure of the cadmium and enables the composition of the molten hasse in the middle zone to be adjusted by acting on the equilibrium solid phase - liquid phase - steam. This temperature T _{1 is} generally set at 820 ° C., which corresponds to a cadmium vapor pressure of about 2 atmospheres. The material obtained after crystallization is of type N _; and the impurities are often aluminum atoms. The reagent tube is then left slowly in the oven at a rate of about 3 times per hour. with or without simultaneous rotation to effect the high temperature melting. The "normal" cooling down to a temperature Τ «leads to a migration of the impurities to the upper one

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End of the block. After the synthesis and crystallization at high temperature, the material is after the zone melting process. Heumann cleaned. The massive block of cadmium telluride is completely enclosed in a reagent tube provided with a graphite layer on the inside and degassed at high temperature, so that no free volume remains. A climate zone is generated by high frequency heating and shifted along the block. Ten to twenty passes through the melt zone are made at speeds between 5 and 0.25 cm / hour. carried out, wherein the temperature of the melting zone is generally about 20 ° C above the melting point of cadmiutelluride. Pur this temperature of the melting zone and is obtained for the pressure prevailing in the crystallization cadmium vapor pressure after this last operation, a material of type N. The low temperature (35 ⁰ C) measured electron mobility ^ which record values of 1.46 χ 1Cr cm / V, s achieve, prove the high purity of the material obtained. Trace analyzes with the mass spectrograph show a concentration of residual impurities below 10 ^p atoms / cm. In this state the crystals can be used as alpha particle detectors with surface barriers.

The single crystal is grown on the block obtained by this operation by moving a solvent zone. The thermodynamic conditions of single crystal growth, ie an excess of Telluii favor a shift in the electrical state of the material from type N to type P. This crystal growth takes place in an internally graphitized, closed reagent tube made of silicon dioxide, in the above the ^M output "block of cadmium telluride, obtained by the previous crystallization, a small block of tellurium is placed, which is the solvent

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forms. The tube will melt as before with the removal of zones -.- so closed that no free volume remains, but this precaution is less critical here because of the temperature is lower, the vapor pressures are also lower and the dangers of poisoning are less great.

The reagent tube is placed in an oven consisting of a central ring which is heated by a Thermckoax winding. The dissolution and crystallization interfaces show high temperature gradients on, for example more than 50 ° C / cm. The reagent tube is slowly lowered in the oven and rotated continuously. to improve the thermal symmetry of the system. At a liquid zone temperature of 700 ° C and a growth rate of 7 mm / day you get a very resistant material with a specific electrical resistance over 10 * ohms. cm, which is practical contains no residual impurities. The electronic properties and homogeneity are opposite to that after cleaning by zone melting obtained material greatly improved. When using this single crystal as a detector of gamma particles one can achieve high resolutions obtain. In one application example, the resolution of the energy beam 122 KeV from cobalt 57 was 5.1 KeV (without subtracting the inherent noise of the DetektOT devices that can be used with 3 KeV). This resolution value is several times better than before with a compensated cadmium telluride.

5 shows a tempering device which has a reagent vessel 4 which contains the crystal 42 in the zone 44 and a crucible 48 in which, for example, pure cadmium is placed in the zone 46. The temperature T ^ of the crucible 48 is lower than the temperature T ' _{2 of} the crystal 42. The heating device is not shown.

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posed. Pig. 6 shows the equilibrium diagram of tellurium and cadmium, the cadmium vapor pressure being given as a function of the reciprocal temperature. The hatched area in the interior of curve 50, the so-called three-phase line, corresponds to a solid body. The straight line D ₁ shows the stoechiometric equilibrium of cadmium telluride and the straight line D _{2 shows} the cadmium pressure during the congruent isothermal sublimation of a crystal. During terapuring, one works in an isothermal area at the temperature corresponding to point B.

The cadmium telluride monocrystals are not only used for solid-state ionization chambers, but also for numerous other purposes · Detectors for nuclear rays are mentioned as examples or infrared (of the photon drag type), electro-optical modulators, Windows with permeability in the infrared range, high temperature optocouplers, Laser elements and electrolurainescent cells.

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Claims (12)

-45- claims 1. Process for the production of single crystals of semi-insulating cadmium telluride with a reduced content of impurities and a low proportion of displacements, twinning and other crystal defects _v characterized by the following process steps: a) a first synthesis from stoechiometric fractions of high purity Cadmium and high purity tellurium, followed by crystallization at a temperature corresponding to the melting point of the stoechiometric mixture Temperature; b) a cleaning of the cadmium telluride block obtained by number * rich passages of a vertical zone melting; c) finally a single crystal growth in tellurium as solvent at a temperature close to the melting temperature of pure tellurium to obtain a cadmium telluride single crystal. 2. The method according to claim 1, characterized in that the cadmium and tellurium are first filtered under a hydrogen atmosphere to remove all traces of oxide before the synthesis and then cleaned by zone melting and the synthesis is carried out in a quartz tube, which was previously made by cracking a carbon-rich substance, such as benzene, was provided in a vacuum between 800 ⁰ C and 12OO ° C with a graphite inner layer, and that the filled tube sealed under ultra vacuum and introduced into a vertical oven and made the cooling slowly in a direction from the bottom of the closed tube forth 3. The method according to claim 2, characterized in that the high 409848/1007 temperature crystallization of step (a) in the same graphitized reagent tube as used for the synthesis, carried out and the partial pressure one of the components is controlled by controlling the temperature of part of the reagent tube in front of which a crucible is located, which contains the component whose partial pressure is to be kept constant. 4. The method according to claim 3, characterized in that the temperature T _{1 of} the crucible during the high-temperature crystallization is kept lower than the temperature T "of the melt. 5. The method according to any one of claims 1 to 4, characterized in that that the high temperature crystallization by relative displacement of the reagent tube with respect to the temperature gradient of the furnace carried out by moving, rotating and vibrating (shaking) the reagent tube by a device of known type will" 6. The method according to claim 5 »characterized in that after the high-temperature crystallization of step (a) the cadmium telluride obtained by multiple passes of melting zones in an internally graphitized, high-temperature degassed reagent tube made of silicon dioxide, which completely encloses the cadmium telluride and without any empty volume is closed in its upper part by a silicon dioxide plug, is cleaned by preventing the transfer of impurities from the cadmium telluride starting material through the vapor phase. 7. The method according to claim 6, characterized in that after the zone melt cleaning (stage b) the crystallization at low temperature, namely in the order of 600 to 900 C, durchge- 409848/1007 by creating a tellurium-rich liquid zone of cadmium telluride shifts through the starting crystal. 8. The method according to claim 7, characterized in that the single crystal Cultivation is carried out in an internally graphitized quartz tube under ultravacuum, the quartz tube being introduced into a furnace, the one made of a ring heated by a thermo-coaxial winding and the temperature in the quartz tube with high Teraperaturgradien th of the order of 100 ° c / cm between 500 ° C and 900 ° C is changed. 9. The method according to any one of claims 1 to 7 »characterized in that at least one of the three crystallizations (a., B, c) is carried out under an inert gas atmosphere. 10. The method according to any one of claims 1 to 9, characterized in, that an annealing treatment is carried out at the end. 11. The method according to claim 10, characterized in that the crystal is tempered by heating it in a closed vessel to a temperature T * ^, wherein there is also a container in the vessel which contains tellurium or cadmium and at a temperature T ^f _{2 is} held, which is less than T ^. 12. The method according to claim 10, characterized in that the crystal is annealed in an isothermal chamber at the temperature, which corresponds to the intersection of two curves, one of which the Cacteiumdruck of the stoechiometric mixture as a function on the temperature and the other the cadmium pressure as a function of the temperature in a congruent isothermal sublimation 409848/1007 13 · Device for carrying out the method according to one of the claims 1 to 12, characterized by a) Devices for filtering cadmium and tellurium under hydrogen the atmosphere ; b) devices for purifying cadmium and tellurium by zone melting; c) a Bridgman furnace for carrying out the synthesis and crystallization at high temperature; d) a closed reagent tube which contains a crucible which is suitable for receiving the component, the partial pressure of which is to be kept constant in the steam; e) Devices for cleaning the compound obtained by vertical Zone melting, may have the following components: el) a high frequency induction furnace that only has a narrow zone the connection melts;
e2) mechanical devices for moving and rotating the reagent tube; f) Devices for growing single crystals by moving a Zone of the tellurium solvent, including the following components £ 1) a furnace with a heating ring which heats the reagent tube in which the crystals are grown only over a small part of its height; £ 2.) Mechanical devices, hold the reagent tube vertically and in constant rotation around a vertical axis 409848/1007