A PROCESS FOR GROWING A MULTI-COMPONENT CRYSTAL

Abstract: A process for growing a multi-component type crystal from the melt of a starting material consisting of more than three elements of the multi-component crystal in a sealed reaction container. The sealed reaction container is divided into a reaction zone and a vapour pressure control zone which are communicated to each other through a through-­ opening or openings. The starting material for the multi-­ component crystal is charged in the reaction zone while a vapour pressure control material consisting of more than two components of the multi-component is charged in the vapour pressure control zone. And then, the temperature in the reaction control zone is adjusted and maintained at a predetermined value to grow the crystal from the melt under a controlled vapour pressure. This process is applicable to produce a wide variety of compound semiconductors of II-VI, III-V or the like, or oxide crystals.

Description: The present invention is related to a process for growing a crystal comprising three elements (hereinafter, referred as a multi-component crystal). Single crystals of compound semiconductor, oxide or the like are used in a wide variety of solid-state devices in the field of microelectric, optoelectronics, etc. Recently, new materials consisting of a plurality of elements have been developed in order to improve functions of the devices and to give them additional functions. For example, mixed crystals or solid solutions consisting of three or more elements are used, since physical properties (for example, the lattice constant, the band gap, the light reflectance, the thermal expansion coefficient, mobility of electrons and hold) and chemical properties (surface energy, the resistance to chemicals, etc.) of two-component compound semiconductors are fixed unconditionally according to elements of which the two-component compound semiconductor is made, and hence these properties cannot be improved or controlled artificially. The merits of such multi-component mixed crystals are that there is a wider range of selection in their constituent elements, their proportions or the like and hence it is possible to obtain a variety of compound semiconductors having desired properties. This fact is the same in the case of a single crystal of oxide which consists of more than three constituent elements. The multi-component crystal can be prepared by a variety of methods. In a method in which a multi-component crystal is grown from the melt of a material composed of plural elements in a sealed reaction tube, Vertical Bridgman method, Traveling Heater method or the like can be utilized. JP-A-58-91096 discloses a method of growing crystals in a sealed container subject to a temperature gradient that travels down the length of the reaction chamber. Figure 5 illustrates an example of the process for growing a crystal of a multi-component compound semiconductor by the conventional Vertical Bridgman method, in which Figure 5(a) shows a stage of mixed crystal growth and Figure 5(b) shows the temperature distribution of a furnace employed for this process. Now, we will explain an outline of this process by describing a case in which a mixed crystal having a composition: A 1-x B x C (wherein A,B,C denote consistent elements, and 0 < x < 1) is produced. After a mixed crystal (a seed crystal) having the composition of A 1-x B x C is placed at the bottom of a reaction tube 51 made of quartz etc., predetermined amounts of the material 52 for a mixed crystal, for example, A 1-x C 1-x and B x C x are charged onto the seed crystal. Then, the pressure in the reaction tube 51 is reduced to create a vacuum condition. The resulting reaction tube 51 under vacuum is placed at the determined position in a furnace (not shown) having the temperature distribution presented in Figure 5 (b) so as to melt the materials 52 of mixed crystal. The seed crystal starts to grow with being in contact with the melt 52 of the materials. Thereafter, the reaction tube 51 is moved slowly downward in the direction of an arrow shown in Figure 5 (a), so that the liquid-solid interface moves upwardly in reaction tube 51. In this manner, a mixed crystal is prepared by growing the resulting crystal part 53. Figure 6 illustrates an example of the process for growing a mixed crystal for compound semiconductors by the conventional Traveling Heater Method, in which Figure 6 (a) shows briefly a stage of mixed crystal growth and Figure 6 (b) shows the temperature distribution of a furnace employed for this process. In this example, the starting materials, for instance, A 1-x C 1-x (64) and B x C x (65) are placed in the manner to confront each other along the vertical axis of reaction tube 61 and the temperature distribution is controlled so as to form partially the molten zone 62. Other operation of this process for producing a mixed crystal 63 is substantially same as the Vertical Bridgman Method. In these prior arts, however, it is difficult to produce a uniform crystal having no lattice defects such as vacancies and also it is impossible to control a stoichiometrical composition, because a part of consistent elements in the starting material having a higher vapour pressure evaporates when the starting material is melted. In fact, the starting material of the mixed crystal and also the resulting mixed crystal itself are mostly such compounds having higher vapour pressures under a molten condition. For example, when a mixed crystal of Cd 1-x Zn x Te is produced according to the prior arts, its components Cd and Zn having higher vapour pressures are apt to evaporate and hence the resulting crystal contains vacancies of Cd and Zn and becomes mostly a P-type conduction type. Thus, the prior arts are thoroughly imperfect in such aspect as of control of composition in a crystal and stoichiometrical control. Therefore, in the conventional technique, it is difficult to produce multi-component crystals having stable or constant properties with a higher yield due to fluctuation of composition and deviation of stoichiometry. FR-A-2255949 describes an apparatus for growing a multi-component crystal having a sealed reaction container with a reaction zone and a vapour pressure control zone communicating with the reaction zone through a through opening. In use, a starting material for the multi-component crystal is charged in the reaction zone, and a vapour pressure control material charged in the vapour pressure control zone. The temperature in the vapour pressure control zone is adjusted and maintained at a predetermined value to grow the crystal from a melt under controlled conditions. Thus, this document corresponds to the pre-characterising part of claim 1. In FR-A-2255949, a two component crystal is grown and the element of one of those components is also used as the vapour pressure control zone. However, according to the present invention, the multi-component crystal is a mixed crystal of II-VI group compound semiconductor of Cd-Zn-Te, and the vapour pressure control material is an alloy of Cd-Zn. In practice, when the stoichiometrical multi-component crystal is grown from the melt of starting material in the sealed container for reaction, it is necessary to control the vapour pressure in the sealed reaction container at the value appropriate for obtaining the stoichiometric compound. According to the present invention mentioned above, it is possible to control the vapour pressure in the whole reaction zone by providing the sealed reaction container with the vapour pressure control zone containing therein the vapour pressure control material consisting of an alloy of Cd-Zn and by maintaining said vapour pressure control zone at a predetermined temperature to evaporate said vapour pressure control material so as to make such a vapour pressure as is appropriate for obtaining the stoichiometric compound. Thus, the present invention allows to grow the multi-component crystal having little change in composition and in deviation from the stoichiometry and reduced number of lattice defects. Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 (a) illustrates a schematical drawing of an apparatus used when the process of the present invention is applied to the three-Temperature Horizontal Bridgman Method, Figure 1 (b) shows an example of the temperature distribution in the apparatus of Figure 1 (a), Figure 2 (a) illustrate a schematical drawing of an apparatus used when the process of the present invention is applied to the three-Temperature Vertical Bridgman Method, Figure 2 (b) shows an example of the temperature distribution in the apparatus of Figure 2 (a), Figure 3 is a drawing showing a relation between the temperature in the vapour control zone and the specific resistance of crystals produced according to the present invention and a comparative example. Figure 4 illustrates the concentration of Zn in the mixed crystal produced according to Example 2 to 4 with respect to the solidification ratio. Figure 5 and Figure 6 are schematical drawings which show conventional method for growing crystals. Figure 1 (a) shows an apparatus employed when the process of the present invention is applied to the three-Temperature Horizontal Bridgman Method. Figure 1 (b) shows an example of the temperature distribution controlled by a furnace in the apparatus of Figure 1 (a), wherein T₁ represents a temperature which is higher than the melting point of a mixed crystal, T₂ represents a temperature which is lower than the melting point but higher than T₃ and T₃ represents a temperature which is necessary to heat vapour pressure control material 6 required to hold the vapour pressure appropriate for obtaining the stoichiometric compound in the reaction system. The apparatus shown in Figure 1 (a) consists mainly of reaction tube 1 made of a material such as quartz, divided by separating member 2 having a small opening into reaction zone 3 and vapour pressure control zone 4, and being under vacuum or an inert gas atmosphere. In reaction zone 3, there is placed a boat made of material such as quartz, graphite or BN to be loaded with the starting material for a multi-component crystal. On the other hand, the vapour pressure control zone 4 is provided with a thermocouple 7 for measuring the temperature of said vapour pressure control material 6, so that the temperature in the furnace 8 is adjusted according to the measured temperature. The furnace 8 is mounted around the reaction tube 1 along the horizontal axis of reaction tube 1 and is composed of a plurality of parts, each of which can be controlled independently. A process according to the present invention will be explained by reference to an example for growing mixed crystal of ternary II-VI group compound semiconductor having a composition of Cd 1-x Zn x Te in the apparatus shown in Figure 1 (a). At first, boat 5 shown in Figure 1 (a) is loaded with a mixture of Cd 1-x Te 1-x and Zn x Te x as a starting materials for a mixed crystal. Among the elements forming said mixed crystal of Cd 1-x Zn x Te, Cd and Zn have higher vapour pressures, and hence it is necessary to control the vapour pressures of these two elements simultaneously in order to obtain a stoichiometrical mixed crystal of Cd 1-x Zn x Te. Therefore, an alloy having a composition Cd 1-y Zn y (0 < y < 1) as the vapour pressure control material 6 is charged in the vapour pressure control zone 4 so as to create a vapour pressure appropriate for obtaining the stoichiometric compound when the temperature in vapour pressure control zone 4 is maintained T₃. The value of y in the alloy can be determined so as to create above mentioned vapour pressure according to a desired value of x. Then, the reaction tube 1 is depressurized so as to create a vacuum. Under this condition, the vapour pressure control zone 4 is heated externally by means of the furnace 8 to the temperature T₃ to evaporate Cd and Zn and maintain the vapour pressure in the reaction tube 1 which is appropriate for obtaining the stoichiometric compound. On the other hand, the reaction zone 3 is heated by means of the furnace 8 to the temperature T₁ to melt said materials of mixed crystal charged in boat 5. The resulting melt 9 comes in contact with a mixed crystal (seed crystal) placed in one end of boat 5 [the right end in Figure 1 (a)] to start crystal grow. Thereafter, the reaction tube 1 is moved slowly in the direction shown by an arrow in Figure 1 (a), so that the liquid-solid interface is also moved to grow a crystal 10 [the state shown in Figure 1 (a)] After the crystallization is completed, the reaction tube 1 is opened to take out the grown mixed crystal of Cd 1-x Zn x Te. Although the reaction tube (1) is moved with respect to the stational furnace 8 in above mentioned embodiment, this is not an indispensable feature of the present invention. Thus, the liquid-solid interface between the melt 9 in the boat 5 and the growing crystal 10 can be moved by controlling the distribution or profile of temperature in the reaction tube 1. In this case where the distribution or profile of temperature in the furnace 8 is controlled, the boat 5 is kept stationary so that it is possible to prevent occurrence of lattice defects due to vibration to improve ordering in crystal. Figure 2 (a) illustrates another embodiment of the present invention showing an apparatus which is applied to the three-Temperature Vertical Bridgman Method. Figure 2 (b) illustrates a temperature distribution in the apparatus of Figure 2 (a), wherein T₁, T₂ and T₃ means the same temperatures as Figure 1 (b). In this embodiment, a reaction tube 21 is installed in a furnace 28 placed vertically. The reaction tube 21 has an upper reaction zone 23 and a lower vapour pressure control zone 24 which are separated by a partition wall 22 having a small opening. In the vapour pressure control zone 24, vapour pressure control material 26 is charged and is contacted to a thermocouple 27 through a wall of the tube to detect temperature of the material 26. In the upper reaction zone 23, a crucible 25 is hung from the top of the reaction tube 21 and is charged with a seed crystal and starting material. When a multi-component crystal is produced in this apparatus, operation is substantially same as Figure 1. Namely, after the vapour pressure control material 26 and the starting material are charged in the reaction tube 21, the tube 21 is reduced in pressure to seal them under vacuum. The tube 21 is installed at the fixed position in a furnace 28 to evaporate the vapour pressure control material 26 to create a predetermined vapour pressure in the tube 21. The resulting melt 29 is contacted with the seed crystal and then the tube 21 is moved along an arrow shown in Figure 2 (a) to grow a crystal. After the crystal growth completes, the resulting crystal is taken out of the reaction tube 21. Furthermore, capillary can be used in place of the partition wall having the through-opening. Furthermore, the furnace can be moved relative to a stationary reaction tube. Now, we will show several Examples according to the process of the present invention. Example 1 There was prepared a mixed crystal of Cd 0.95 Zn 0.05 Te by using the apparatus shown in Figure 1 (a). At first, the boat 5 in the reaction zone 3 as shown in Figure 1 (a) was charged with starting material elements, Cd, Zn and Te in a predetermined amount. At the same time, a vapour pressure control material 6 of an alloy composed of Cd 0.99 Zn 0.01 was charged in the vapour pressure control zone 4. Then, the reaction tube 1 was evacuated to a pressure of lower than 266,6 × 10⁻⁶ Pa (2 × 10⁻⁶ Torr). After that, the temperatures, T₁, T₂ and T₃ of the furnace 8 were adjusted to 1110 °C, 1050 °C and 800 to 830 °C respectively so as to melt the starting material elements in the boat 5 and hold the vapour pressure in the reaction zone 1 at a determined pressure. Then, the reaction tube 1 is moved in the direction of arrow shown in Figure 1 (a) at a rate of 3 mm/h to grow a crystal. Figure 3 shows the relation between the specific resistance or resistivity of the resulting mixed crystal and the temperature in the vapour pressure control zone 4. The boat 5 was made of quartz coated with a carbon film formed by thermal cracking of high purity benzene. In the starting material elements, Cd was etched with a 10 % nitric acid-ethanol solution, while Zn and Te were etched with a 10 % hydrochloric acid before use. It is apparent from Figure 3 that, according to the present invention, it is possible to control the conductivity of p-type and n-type by controlling the temperature in the vapour pressure control zone and to obtain a crystal of CdZnTe having a higher specific resistance in the order of 10⁸Ω-cm as a non-doped crystal and being extremely excellent in quality. As a comparative example, a mixed crystal was prepared under similar conditions to the above example, except that only Cd is charged in the vapour pressure control zone 4 to control the vapour pressure in the reaction system only with Cd. There is also shown in Figure 3, the relation between the specific resistance of the resulting mixed crystal and the temperature in the vapour pressure control zone 4. In this comparative example, when Cd was used in a small amount, Zn was condensed in the vapour pressure control zone. As a result, Te exceeded the stoichiometry in the boat 5 and hence in the vapour pressure control zone 4 Te condensed to form an alloy with said Cd. As the result, the vapour pressure in the reaction system fell greatly so that the crystal growth could not be performed normally. Moreover, as shown in Figure 3 the specific resistance or resistivity of the resultant crystal was also low. Example 2 A mixed crystal of Cd 0.96 Zn 0.04 Te was produced in the same apparatus shown in Figure 1 (a). The boat 5 of quartz coated with carbon was charged with predetermined amounts of Cd, Zn and Te in the reaction tube 3. In the vapour pressure control zone 4, an alloy of Cd 0.96 Zn 0.04 was placed as the vapour pressure control material. Then, the interior of the reaction tube 1 was displaced with high purity Argon gas and reduced in pressure below 1066,4 × 10⁻⁷ Pa (8 × 10⁻⁷ Torr) and then sealed. Temperature gradient in the furnace 8 was adjusted to T₁ = 1140 °C, T₂ = 1080 °C and T₃ = 820 °C respectively to melt the starting material in the boat 5 with maintaining the vapour pressure in the reaction tube 1 at a predetermined value. Then, the temperatures T₁ and T₂ were brought down at a rate of 0.2 °C/hour by controlling portions of the furnace 8 corresponding to the T₁ and T₂, while both of T₃ and the vapour pressure in the reaction tube were kept constant, to change the distribution of temperature in the tube in order to grow a mixed crystal from the melt of the starting material. Figure 4 shows the concentration of Zn in the resulting mixed crystal with respect to the solidification rate thereof. Example 3 and 4 The same procedure as in Example 2 was repeated except that alloys of Cd 0.98 Zn 0.02 and of Cd 0.99 Zn 0.01 were used as the vapour pressure control material. The above mentioned relations between the concentration of Zn and the solidification rate in Examples 3 and 4 are also shown in Figure 4. It is apparent from Figure 4 that the mixed crystals obtained by Example 2 to 4 have lower segregation coefficients K determined by the following equation: C = KCo(1-g) k-1 (wherein, C is the concentration of Zn in a crystal, Co is the initial concentration of Zn, and g is the solidification ratio). Particularly, in a mixed crystal obtained from an alloy containing higher content of Zn, the segregation coefficient became much lower and the distribution in concentration of Zn became much more constant, resulting in an improvement in the productivity or yield of acceptable crystal which falls within a predetermined allowable range (for example, in Example 2 to 4, the concentration range of Zn in the crystal falls within 4.0 ± 0.5 mol %). The resulting mixed crystal showed higher specific resistance of more than 10⁵Ω-cm under all vapour pressures and hence can become a crystal of high specific resistance or resistivity without any dopant. Furthermore, the crystal obtained by the present invention showed an improved etch-pit density of 5000 to 40,000 pits/cm² which is one order higher with respect to conventional crystals. In this crystal, no lattice defects such as twin, polycrystal etc. were observed and no cellular structure was observed in the distribution of etch-pits. Thus, this method for growing a crystal by controlling the distribution of temperature in the furnace can improve greatly properties of crystal in comparison with a method in which either of the furnace or the reaction tube is moved. In conclusion, the process of the invention makes it possible to control the composition of the resulting crystal stoichiometrically to produce a uniform multi-component crystals of high quality and to improve the yield due to lowered segregation coefficient.

Claims: A process for growing a multi-component crystal in a sealed reaction container (1), the container having a reaction zone (3) charged with a starting material for the multi-component crystal, and a vapour pressure control zone (4) communicating with the reaction zone through a through opening or openings (2) a vapour pressure control material being charged in the vapour pressure control zone (4), and the temperature in the vapour pressure control zone (4) being adjusted and maintained at a predetermined value to grow the crystal (10) from a melt (9) under controlled conditions; characterised in that: the multi-component crystal is a mixed crystal of II-VI group compound semiconductor of Cd-Zn-Te, and the vapour pressure control material is an alloy of Cd-Zn. A process for growing a multi-component type crystal as according to claim 1, wherein the crystal is grown by the three-temperature horizontal Bridgman method.

Inventors: MIYAZAKI, KENJI C/O OSAKA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1987
Main classification: C30B11
IPC codes: C30B11, C30B29