US 3809584 A
Description (OCR text may contain errors)
May 7, 1974 SHN-16H1 AKAl ETAL 3,809,584
METHOD FOR OONTINUOUSLY GROWING EPITAXIAL LAYERS OF SEMICONDUCTORS FROM LIQUID PHASE 5 Sheets-Sheet 1 Filed Jan. 18, 1973 wk O O Or May 7, 1974 3,809,584
METHOD FOR CONTINUOUSLY GROWING EPITAXIAL LAYERS OF SEMICONDUCTORS FROM LIQUID PHASE Filed Jan. 18, 1975 3 Sheets-Sheet 2 May 7, 1974 SH|N|CH|AKA| ET AL 3,809,584
METHOD FOR CONTINUOUSLY GROWING EPITAXIAL LAYERS OF SEMICONDUCTORS FROM LIQUID PHASE Filed Jan. 18, 1973 3 Sheets-Sheet E,
72 77 Eupen/m95 United States Patent Oce 3,809,584 Patented May 7,
Im. c1. H611 7/38 U.S. Cl. 148-172 8 Claims ABSTRACT oF THE DISCLOSURE Single-crystal epitaxial layers of semiconductors are grown on suitable substrates from the liquid phase.
The substrates are moved consecutively on a plurality of lvessels in a reaction furnace from a high temperature regionto a low temperature region maintained within the reaction furnace, which is provided with a temperature gradient in the longitudinal direction, all the while the substrates are kept in contact-with the liquid solution containing a source material of the epitaxial layers, whereby the epitaxial layers are grown successively and on a continuous basis.
BACKGROUND OF THE INVENTION The present invention relates to a method for successively growing liquid phase epitaxial layers which enables the epitaxial growth of semiconductors on a commercial basis.
As to techniques of growing epitaxial layers on singlecrystal substrates from the liquid phase, there has been widely employed the tilting technique (Nelson method), dipping technique, sliding technique, etc. These techniques comprise the so-called slowly-cooling liquid phase epitaxial growth method, wherein the single-crystal substrates are contacted with the liquid solution containing a source material and then the temperature in the reaction furnace is lowered at a suitable rate. In those cases where the liquid phase epitaxial layers are to be grown on many substrates on a commercial scale according to the conventional slowly-cooling liquid phase epitaxial growth method, the temperature of the reaction furnace after completion of each growth operation has been reduced to a point that is lower than the initial temperature by a predetermined number of degrees and, accordingly, the temperature in the reaction furnace must be elevated again to the initial temperature point before performing the next operation. Consequently, the epitaxial growth is inevitably not accomplished on a truly continuous basis and is inefficient, since a time is required for elevating temperature between a growth operation and the next growth operation. For a detailed description, see H. Nelson, Epitaxial Growth From the Liquid State and Its Application to the Fabrication of Tunnel and Laser Diodes, RCA Review 24, p. 603, 1963, or see U.Sl. Pat. No. 3,565,702. These difficulties are inevitable in the liquid phase epitaxial growth method. 'Ihe liquid phase epitaxial growth method has not been suitable enough for the production on a commercial basis.
SUMMARY OF 'II-IE INVENTION The present invention relates to an improvement in or relating to said slowly-cooling liquid phase epitaxial growth method. By the method of this invention, epitaxial layers of the following various semiconductors can be grown successively on suitable substrates: Group III- V compound semiconductors such as GaAs, GaP, InP,
InSb, InAs4 and GaN; Group III-V mixed semiconductors such as Ga1 xAlXAs, InAsLXSb,' In'xGalQxP, In`Ga1 ,Sb, InxAl1 XP, GaxAl1 XP and In1 xAlXAs (0 x` 1 in the foregoing semiconductors); Group II-VI compound semiconductors such as CdTe, CdS, ZnSe, ZnTe, ZnOy and BeTe; mixed crystals of Group II-VI semiconductors such as ZnSe1 xTex, Cd1 ZnXS and Hg1 XCd,Te (0 x 1 in the foregoing semiconductors) and other compound semiconductors such as (ZnS)1 X(GaP)X, (ZnSe)1 (GaAs)x (0 x 1 in thev foregoing semiconductors) and ZnSip2.
According to the method comprising this invention, a suitable substrate consisting of a single crystal of a semiconductor is moved through a reaction furnace having a temperature gradient from a high temperature region to a low temperature region within the reaction furnace, all
the while the substrate is kept in contact with the liquid.
solution in which a source material of the 'single-crystal epitaxial `layer of the semiconductor to be grown is -dissolved, thereby growing the epitaxial layers successively.
Thus, the principal object of this invention is`to provide a method of growing liquid phase epitaxial` layers of semiconductors successively, as well as continuously, on many suitable substrates from the liquid phase.
A feature of thisinvention resides in a method'of grow` ing epitaxial layers of semiconductors on suitable singlecrystal substrates from the liquid phase, wherein the single-crystal substrates are moved through a reaction furnace from a high temperature region to a low temperature region within the reaction furnace, which is provided to have a temperature gradient in the longitudinal direction thereof, all the while the substrates are kept in contact with the liquid solution containing a source material of the semiconductor, thereby growing epitaxial layers of said semiconductors on said single-crystal substrates.
Another feature of this invention resides in a method of growing epitaxial layers of semiconductors on suitable single-crystal substrates from the liquid phase wherein substrate-supporting vessels are employed with each containing at the bottom of the vessel the single-crystal substrate and a liquid solution containing a source material of the semiconductor is supplied onto the substrate in the substrate-'supporting vessels from a main container of liquid solution having at least five times as much Volume as the vessel. The vessels are moved through a reaction furnace from a high temperature region to a low temperature region within the reaction furnace, which is provided to have a temperature gradient in the longitudinal direction of the furnace, all the while the single-crystal substrates are kept in contact with the liquid solution, thereby growing epitaxial layers of the semiconductors on the single-crystal substrates.
Still another feature of this invention resides in a method of forming epitaxial growth layers of semiconductors on suitable single-crystal substrates from the liquid phase wherein substrate-supporting vessels are employed with each containing at the bottom of the vessel the single-crystal substrate and these vessels are vertically moved in a vertically elongated reaction furnace having a vertical temperature gradient, from a high temperature region to a low temperature region within the reaction furnace, all the while the single-crystal substrates are kept in contact with a liquid solution containing a source material of the semiconductor so that the single-crystal substrates in the vessel are positioned on the low temperature side of the reaction furnace and the solution is positioned on the high temperature side of the furnace.
BRIEF DESCRIPTION OF I'HE DRAWINGS The foregoing and other objects, features and advantages of this invention will be apparent from the following more particular description of preferred embodiments 3 of the invention as illustrated in the accompanying drawings.
In the drawings:
FIG. 1 illustrates in cross section a horizontal reaction furnace or apparatus for continuously growing the epitaxial layers on suitable substrates from the liquid phase together with a graphic illustration of the longitudinal temperature profile of the reaction furnace.
FIG. 2 is a perspective view of a portion of the apparatus shown in FIG. 1.
FIG. 3 illustrates in cross section a vertical reaction furnace r apparatus for continuously growing the epitaxial layers on suitable substrates from the liquid phase together with a graphic illustration of the vertical temperature profile of the reaction furnace.
DESCRIPTION OF PREFERRED EMBODIMENTS THROUGH EXAMPLES The following are examples of employing the method and apparatus comprising this invention for growing the single-crystal epitaxial layers of semiconductors on suitable substrates by the continuous liquid phase epitaxial growth method.
Example 1 FIG. 1 is a cross sectional View of a liquid phase epitaxial growth reaction furnace apparatus for carrying out the continuous liquid phase epitaxial growth used in this example together with an illustration of the temperature profile in the reaction furnace. FIG. 2 is a perspective view of a portion of the inside of the reaction furnace shown in FIG. 1.
In FIG. l, a main container 2 of a liquid solution 1 containing a source material is supported from within the reaction tube S and is kept at a temperature T1 by an electric heater 9. In the main container 2, the liquid solution 1 in which a source material consisting of constituent elements or the semiconductor has been dissolved is contained. A frame 4 for supporting the liquid solution is positioned upon a slider or carrier 3 to provide a plurality of substrate-supporting vessels 20. At the bottom of each vessel 20 or on the slider surface, a single-crystal substrate 5 is positioned. The substrate-supporting vessels 20 containing the single-crystal substrate 5 are continuously moved in the direction of an arrow 19 into the reaction furnace heated by an electric furnace and thereafter introduced into the lower portion or area of the main container 2.
An opening 11 is provided in the bottom of the main container 2 through which the liquid solution is supplied into each vessel 2t) passing therebeneath. The main container 2 is provided with at least tive times as much volu-me as a single vessel in order to feed the liquid solution successively into a plurality of vessels 20 on a continuous basis. On the right side of the container 2, a temperature gradient of 0.1-15 C./cm. is provided horizontally through the furnace by an electric heater 12 as depicted in the temperature profile curve at 21. The temperature gradient through the furnace is controlled depending upon the desired conditions of the epitaxial growth and moving rate of the vessels 20 through the furnace. It is desirable to select the value of the product GV C./min.) of the temperature gradient G C./cm.) and the moving rate V (cm/min.) in the range of 0.1-10 C./min. Temperature gradient in the vertical direction through each vessel 20 is not necessarily required but preferably the temperature of the singlecrystal substrate 5 is lower than that of the solution. While each vessel 20 containing the single-crystal substrate 5 and the deposited liquid solution is moved through a remainder of the furnace, an epitaxial layer is grown on the single-crystal substrate 5. The right side or area of the reaction tube 8 is kept at a predetermined temperature T2 by an electric heater 13. Each vessel 20 is then separated into the frame 4 and the slider 3, whereby the liquid solution is removed from the surface ofA the epitaxial layer grown on the single-crystal substrate. 5. By providing the proper temperature T2, the epitaxial layers are subjected to suitable heat treatment for annealing purposes. FIG. 2 illustrates an embodiment for separation of the frame 4 from the slider 3. As the vessel 20 moves along, the frame 4 for supporting the liquidsolution is caused to slide by a xed arm member 14 in therdirection of an arrow 22 which is substantially perpendicular to the direction of vessel movement depicted by Varrow 19, so that the liquid solution is separated by member 14 from the surface of the epitaxial layer grown on'the single-crystal substrate 5.
In employing the method of this invention, a solid or integral substrate-supporting vessel 20 may also be used. In this case, the liquid solution is .removed after the entire vessel has been taken out from the reaction tube.
In FIG. l, hydrogen gas or an inert gas such as nitrogen gas is introduced through a gas inlet 6 of the tube 8, and the gas is exhausted through a gas outlet 15. Hydrogen gas or an inert gas such as nitrogen is introduced at outlets 7 and 16 and exhausted, respectively, through outlet 17 and outlet 18 of the reaction tube 8. Through the .employment of this construction, the absence of an air atmosphere within the reaction tube 8 is maintained.
A semicontinuous liquid phase growth method is also contemplated within the scope of this invention wherein the temperature region of T1 is longer than that shown in FIG. 1 because the quartz reaction tube 8 is longer than that shown in FIG. 1. Also, the main container 2 would be positioned at the left end of the tube to supply the liquid solution into each vessel 20 which are fixed so that the reaction furnace itself is moved-in a direction toward the container 2, that is, toward the left of FIG. l.
In this example, GaP epitaxial layers were grown successively on GaP single-crystal substrates from the liquid phase. In FIG. 1, the liquid solution 1 comprises about 30 g. of Ga and 1.0 g. of GaP. Temperature T1 of the solution was kept at 1050 C.`In the formation of nepitaxial growth layers, Te or S was doped as impurity. As the single-crystal substrates 6, wafers obtained from GaP single crystals grown bythe liquid encapsulated Czochralski process were used. Surface area of the'wafer was about 2 cm?.
The single-crystal substrate 5v was coveredk 'withmthe liquid solution to a depth of about 2 mrn. as supported within the frame 4 which is about 2 mm. in'height'for supporting the liquid solution. The substrate Sand the liquid solution were moved in this state in the 'reaction tube 8 having a temperature gradient of 2 C./cm. at a rate of 3 cm./min. in the direction of arrow 19. At the growth completion portion of the furnace,i the temperature is kept at T 2=950 C. Finally,`frame 4 is separated from the moving slider 3 by means of the xed arm member 14. Thus, there is obtained epitaxial growth layer with a thickness of about 30 um.' Generally, the height of the frame may be selected inthe range of 0.5"- 5 mm. The depth of th'e liquid solution over the substrate 5 may be 0.5-5 mm., depending upon the height of the frame 4. If the depth is too small, the epitaxialgrwth is diflicult. Too large of a'depth is undesirable from 'an economicalviewpoint. It is desirable to provide a solid cover (not shown in the figure) on the'liquid solution jso that the depth of the liquid solution is uniform.
In growing the liquid phase epitaxial layers of the III-V compound semiconductors and mixed crystal semiconductors, volatile components of the semiconductorsor volatile doping impurities may be evaporated out of the solutions. In such a case, the loss from the solution by evaporation may be covered up by putting a lid on the top of container 2 and also each vessel 20 to prevent the evaporation or may be compensated by introducing the volatile compound into the reaction tube 8 together with the carrier gas through the inlet 6. Moreover, it is possible to counter-dope an impurity of the different type by in;
troducing the impurity in vapor form, thereby growing the epitaxial layers containing a p-n junction.
Reference is made to FIG. 3 which illustrates in cross section a vertical reaction furnace for continuously growing the epitaxial layers used in this particular example together with the temperature profile vertically through the reaction furnace. This process in which the vertical reaction furnace is used is superior to that in Example 1 wherein the horizontal furnace is used for` the following reasons:
(1) In the process in which the horizontal furnace is used, the temperature gradient is also produced on the surface of the semiconductor substrateand convection of the solution is produced since the semiconductor substrate is placed horizontally in the substrate-supporting vessel and is in contact with the solution in the reaction furnace having a horizontal temperature gradient. Therefore, the temperature across the deposited solution in the vessel is notuniform. On the other hand, according to the process in whichthe vertical reaction furnace is used, the direction of the temperature gradient is perpendicular to the surface of the semiconductor substrate and the temperature on the surface of the semiconductor substrate is uniform and, therefore, more uniform epitaxial growth layer :can be obtained.
(2) In the liquid phase epitaxial growth, it has been known that for .obtaining uniform epitaxial layers, conditions are selected so that temperature of the semiconductor substrate is lower than that of the solution, as, for
example, the substrate is cooled with a special cooling device. For this purpose, it is not desirable that temperatures of the semiconductor substrate and the saturated lsolution be the same. The conditions can be satisfied by l(3) In effecting the successive epitaxial growth, the
semiconductor substrate wafers are naturally arranged in the horizontal direction if the horizontal furnace is used. However, temperature gradient in each semiconductor substrate mustbe minimized and, consequently a sharp temperature gradient 'should not be produced. Therefore, for providing a necessary temperature gradient between the lowtemperature regionl andV the high` temperature region, a very long furnace must be used. On the other hand, in the vertical reaction furnace, temperature on the surface of each semiconductor substrate is uniform and a sharp temperature gradient in the direction perpendicular to the wafer surface can be obtained and, consequently, the furnace of a small size and length may be used. In the case of the vertical furnace, it is desirable to select the temperature gradient G C./cm.) in the range of l,-50 C./cm.
As shown in PIG. 3, a substrate-supporting vessel 25 made of carbon, quartz or BN is provided for containing a semiconductor substrate 23 and a solution 24 saturated Withthesource material. An elevator conveyor is provided to have many shelves for lowering the individual vessels 25 to the lower portion of the vertical furnace 43. Heaters 27 through 41 and 52 are provided in the furnace 43. The T-shaped quartz tube 42 is housed within the internal area of the furnace 43. The overall length of the furnace 43 is about 1 m. The temperature gradient is produced so that high temperature (T1 C.) region is :positioned in the upper portion of the furnace 43 and low temperature (T2 C.) region is positioned in the lower portion thereof, with the middle portion adapted to have nearly a straight temperature gradient (see temperature profile 51 on the left of FIG. 3). The temperature gradient, T1 and T2 can be changed selectively depending upon variety ofthe compound semiconductor to be epitaxilly grown, and thickness of theepitaxial layer to be grown. Each vessel 25 having a frame for supporting the liquid solution 24 and containing the GaP semiconductor substrate 23 is introduced in the direction of an arrow 44 beneath the lower or bottom inlet 47 of a main container 46 containing a Ga solution saturated with GaP source material. The Ga solution 24 saturated with GaP is fed into the frame of the vessel 25 and then introduced into the conveyor 26. In this particular example, the GaP semiconductor substrate 23 had a thickness of 450 am. and the surface area on which the epitaxial layer is to be grown was about 2 om?. The liquid solution 45 comprised about 30 g. of Ga and 1.8 g. of GaP. The solution 45 was kept at a temperature T1 of 1050 C. In the formation of n-epitaxial growth layer, Te or Ga2S2 was added as impurity. Temperature gradient in the furnace 43 was about 2 C./ cm. The low temperature region in the lower portion of the furnace was kept at about 950 C Each vessel 25 was lowered at a rate of 3 cm./min. from the high temperature region to the low temperature region in the furnace 43, which took about 20 minutes. The vessel 25 lowered to the low temperature region was then moved in the direction of an arrow 48 by means of a push bar 49. The frame for supporting the solution of the vessel 25 was removed by a jig bar 50 having the same structure as the arm member 14 in FIG. 2. Thickness of the resulting epitaxial growth layer was about 30 nm.
Although temperature gradient in this example was nearly straight, the temperature profile may be produced so that the temperature gradient in a high temperature region changes slowly and the temperature gradient in a low temperature region changes sharply. Further, the middle part of the temperature prole may be kept at a temperature l0-50 C. higher than the high temperature region and then the temperature may be made gradually lower thereafter in order to dissolve 1-5 um. thickness of the surface of the semiconductor substrate 23 in the solution 24 and bring about epitaxial growth.
While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.
1. The method of continuously growing epitaxial layers of semiconductors on single-crystal substrates from the liquid phase comprising the steps of moving a plurality of consecutively arranged vessels through a reaction furnace provided with a high temperature region and a low temperature region, each of said vessels including a single-crystal substrate, depositing the liquid solution containing a source material of the semiconductor within each of said vessels on the surface of the substrate as they proceed through the furnace within the high temperature region, and thereafter continuously moving each of said vessels from the high temperature regionv through a decreasing temperature gradient region and thence into the low temperature region of the furnace while said substrates are maintained in contact with the liquid solution thereby growing epitaxial layers of said semiconductors consecutively on said substrates.
2. The method of claim 1 characterized in that said semiconductor is a Group III-V compound semiconductor.
3. The method of claim 2 characterized in that said Group III-V compound semiconductor is GaP.
4. The method of claim 1 characterized in that said liquid solution in contact with said substrate has a height ranging from 0.5 to 5 mm.
5. The method of claim 1 characterized by the step of separatingl the liquid solution from each, of the epitaxial layers grown on said substrates as said vessels move through said low temperature region of said furnace.
6. The method of claim 5 characterized by providing van inert atmosphere withinfthe interior of the furnace.
7. The method of claim 1 characterized bygco-ordinat- Iing the temperature gradient in the furnace with the rate of movement of said 'vessels therethrough thereby controlling the nature and rate of epitaxial growth.
8. The method of continuously growing epitaxial layers of semiconductors on `single-crystal Isubstrates from the liquid phase vcomprising the steps of moving a plurality of consecutively arranged vessels 10 through a vertical reaction furnace provided with an upper high temperature region and a lower low yternperature region, each of said Vessels including` a single-crystal substrate, I
` depositing the liquid solutioncontaining a source ma- 15 terial of the'semiconductor within each of saidvessels on the surface of the substrate as they proceed through the high temperature region, and
continuously but uniformly moving each of said vessels consecutively from the high temperature region 20 downwardly through a decreasing temperature gradient'region of the furnace and thereafterliinto said low temperature region while said substrates are maintained in contact with 'the' liquid-solt'ion y'thereby growing epitaxial layers 0f Said 'slen'iicor'b'y `A `ductors on said substrates.
i References Cited UNITED STATES PATENTS 3,e15,944
10/11971 Sheng et al 148,-18-9 13,163 1,836 1/1972 Jarvela et al 14S- 171 X .3,665,888 5/ 1972 Bergh et al. 148-,1711.X-
3,690,965 9/ 1972 Bergh et al. f1,48,- 1-7,2
Solomon 148-.-7-1-71 GEORGE. T. OZAKI, Primary Examiner U.S. Cl. X.R.
117-201; HSL-415; 148-17*1, 173