US 3729348 A
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P 1973 R. H. SAUL METHOD FOR THE SOLUTION GROWTH OF MORE PERFECT SEMICONDUCTOR CRYSTALS Filed Sept. 29, 1970 COOLING CYCLES .IOII I o w W 8 O O O TIME (ARBITRARY UNITS) INVENTOR R. H. SA UL United States Patent O 3,729,348 METHOD FOR THE SOLUTION GROWTH OF MORE PERFECT SEMICONDUCTOR CRYSTALS Robert H. Saul, Scotch Plains, NJ., assiguor to Bell Telephone Laboratories, Incorporated, Murray Hill, NJ. Filed Sept. 29, 1970, Ser. No. 76,550 Int. Cl. H011 7/38, 7/00; B01j 17/20 US. Cl. 148172 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION (1) Field of the invention The invention relates to the growth of semiconductor crystals from solution.
(2) Prior art One method which has been used to grow crystals of semiconducting materials especially compound semiconductors is the deposition of these materials from solution onto seed substrates. Seeds of many of the subject materials can be most conveniently obtained by the Czochralski process. However, most of these Czochralski materials are characterized by high dislocation densities. These high dislocation densities are detrimental to the performance of the finally fabricated devices. Attempts to produce material of higher crystalline perfection by epitaxial deposition from the vapor phase has been largely unsuccessful, such epitaxial material exhibiting dislocation densities equal to or greater than the dislocation densities of the seed crystal. However, epitaxial deposition from the liquid phase has been somewhat more successful in decreasing the dislocation density. Some of the improvement has been attributed to the solution of a surface layer of the seed crystal before the deposition starts. It is felt that such surface layers contain impurities and crystal damage introduced during seed preparation. However, it is clear that further improvements in crystal perfection are required if improved device performance is to be realized.
SUMMARY OF THE INVENTION Futher improvement in the crystalline perfection of semiconductor crystal grown from solution has been obtained by halting the growth of the crystal after some crystal material has been deposited. By halting the growth for a period of time, the halt possibly including the remelting of some of the already deposited material, decreases in dislocation density of nearly an order of magnitude have been observed. Material deposited after the halt is observed to contain nearly an order of magnitude lower disclocation density than the material deposited before the halt. Such an increase of crystal perfection allows the fabrication of improved devices from the more readily available Czochralski grown seeds since the major defects of the Czochralski materials is their high dislocation content.
BRIEF DESCRIPTION OF THE DRAWING The figure is a graph which shows an exemplary temperature vs. time schedule experimentally used to illus- ICC trate the disclosed invention as applied to the liquid phase epitaxial growth of gallium phosphide.
EXEMPLARY GROWTH METHODS The crystal growth from solution of semiconducting materials has proven most useful in the production of crystals of compound semiconductors. Much of the work in Group III-V semiconductors has been done using such processes. For example, the best gallium phosphide electroluminescent diodes to date have been produced by the epitaxial deposition of gallium phosphide together with desired dopant impurities from gallium solution. In this process the entire seed and solution are maintained at a uniform temperature and cooled together, the gallium phosphide precipitating from solution onto the seed as the cooling proceeds.
A modification of this process is known as the Traveling Solvent Method. This method starts with a seed crystal at the bottom, above which is a zone of solvent material typically one to several millimeters thick. Above the solvent and in contact with it is a body of polycrystalline or powdered material of the composition to be grown. A temperature gradient is maintained across the solvent zone, the polycrystalline material being maintained at a higher temperature than the single crystal material below the solvent. The temperature and the temperature gradient is adjusted so that material is dissolved at the upper end of the solvent zone. diffuses through the solvent and is deposited as a single crystal at the lower end of the solvent zone. As the single crystal grows the temperature gradient is moved upward and more of the polycrystalline or powder dissolves.
Crystal growth with halts In accordance with the invention disclosed here, crystal growth by methods such as the exemplary methods described above are caused to proceed with a brief halt in the growth schedule while maintaining the crystal tn contact with the solvent and adding nothing to the solvent. In uniform temperature methods, cooling is halted and in temperature gradient methods, the motion of the temperature gradient with respect to the single crystal and polycrystalline material is halted. The period of the halt can be as little as one minute and still yield discernible improvement; halts of the order of five minutes are usually to be preferred in order to assure appreciable improvement; while interruptions greater than five minutes may in some situations be desirable. However, it would usually be uneconomic to interrupt crystal growth for greater than two hours. The halts can take two general forms-crystal growth can be halted and the growth face of the material held stationary during the halt period or the growth process can be reversed and some of the already deposited material dissolved. Further improvements in crystal perfection have been observed by both repeated growth and halt cycles.
EXAMPLES The figure shows three exemplary cooling cycles used to exhibit the invention as applied to liquid phase epitaxial growth of gallium phosphide from gallium solution. In the figure segment 11 between points A and B represent the warm up of the system to the temperature at which growth will be initiated. The segments 12, 13, 14 between points B and E represent the control experiment which defines cooling cycle I. This cooling cycle includes no halts. Cooling cycle II includes segments 12, 13, 16, and 17 between points B, D, F, and G. This is a cooling cycle in which the halt includes a partial solution of the already deposited material. Cooling cycle III includes segments 12, 15 and 17 between points B, C, P, and G. In this cooling cycle the temperature is held stationary during the halt.
Upon examination of epitaxial layers produced by the three cooling cycles, it was observed that the dislocation density was essentially uniform throughout the epitaxial layer deposited by the control experiment, COOling cycle I. Several experiments performed according to cooling cycle II, which includes a partial solution, showed an average decrease in dislocation density of a factor of 4.5. Cooling cycle III yielded a 36 improvement. In each case the improvement occurred at the growth plane corresponding to the halt. The experiment indicated in Table I which included three separate halts each of ten minute duration showed improvement from one increment layer to the next with a total overall improvement of a factor of 6.2.
TABLE I.VARIATION OF DISLOOATION DENSITY IN AN INOREMENTALLY-GROWN LAYER Average dislocation density (10 cm.-*)
Temperature Incremental layer range, C.
The above experiments were, of course, exemplary and do not represent optimum conditions or maximum avail: able improvements.
What is claimed is:
1. A method for growing of crystalline compound semiconductor matter of a conductivity type by epitaxial depo sition of the matter on a surface of a crystalline solid of the same conductivity type which surface is in contact With a liquid body comprising a solvent and the compound semiconductor matter in solution therein, characterized in that, after the deposition has proceeded and an epitaxial layer has been produced, the deposition is halted for a time period while maintaining the surface in contact with the liquid body and maintaining the composition of the liquid body essentially constant, after which time period the deposition is resumed thereby producing an improvement in the crystalline perfection of the material deposited subsequent to the time period.
gallium phosphide. w
3. A method of claim Zjin which the time period is between 5 minutes and 2 hours.
4. A method of claim. 1 in which the crystalline solid coalisists essentially of a Group III-Vsemiconductor mate- I'l 1 5. A method of claim 4 in which the crystalline solid consists essentially of-gallium phosphide, the solvent consists essentially of gallium and the liquid body comprises 2. A method of claim in which the time period at 6. A crystalline body of a semiconducting material produced by the method of claim 1. r
References Cited 7 3 UNITED STATES PATENTS OTHER REFERENCES I Saul, R. H.: Defect Structure of GaP Liquid Phase Epitaxial Deposition, I. Electro. Chem. Soc, vol. 115, No. 11, November 1968, pp. 1184-1190.
Woodall et al.: Liquid Phase Epitaxial Growth of Ga A1 As, ibid., vol. 116, No. 6, June 1969, pp. 899- 903.
L. DEWAYNE RUTLEDGE, Primary Examiner W. G. SABA, Assistant Examiner V Us. 01. X.R.