US 3102828 A
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t. 1953 J. COURVOISIER 3,102,828
METHOD .OF MANUFACTURING SEMICONDUCTOR BODIES Filed June 1, 1960 Temperature "C I INVEYNTOR JEAN c. F. COURVOISIER.
BY MK AGEN United States Patent G 3 102,828 METHOD F MKNUFACTURING SEMI- CGNDUCTOR BODlES l Jean Conrvoisier, Geneva, Switzerland, assignor to This invention relates to the manufacture of a semiconductor body consisting of two elements, for example silicon and germanium, which body contains at least one zone in which the ratio between the proportions of these elements shows a continuous variation.
It has already been suggested to manufacture such a body by alloying a small amount of germanium to a single crystal of silicon. On cooling, a zone will grow to the silicon; in the material which is the first to solidify and which adjoins the initial crystal, the proportion of silicon will predominate over the proportion of germanium, while the subsequently solidifying parts contain progressively less silicon and more germanium.
it has also been suggested to manufacture a semiconductor body by depositing a semiconductor material, such as germanium or silicon, on a suitable base from vapour or by producing such material by the decomposition of a compound and precipitating it on a base layer. In these processes, the semi-conductors themselves could be used as bases.
This method has a limitation in that it is diflicult to produce single crystal bodies in this manner.
It is an object of the invention to obviate these disadvantages.
According to the invention, at least one element is deposited on the surface of a base which contains the other element and has a melting point higher than that corresponding to the composition of the deposited material, while the temperature of the base is maintained at a value such that the surface is liquid.
Preferably, the base layer is silicon and the deposited material is germanium; however, the deposited material may alternatively be a mixture of germanium and silicon. The invention can also be employed for manufacturing bodies consisting at least partly of two elements forming a semiconductor compound.
It should be noted that the elements may contain impurities, which may be added deliberately.
In order that the invention may readily be carried into effect, an embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:
FIGURE 1 shows schematically an apparatus for the deposition of semiconductor material.
FIGURE 2 is a constitutional diagram for mixtures of silicon and germanium, and
FIGURE 3 is a sectional view of a semiconductor body made by the method in accordance with the invention.
The apparatus shown in FIGURE, 1 comprises an annular cathode 1 of tungsten wire connected to a filament current supply. A molybdenum screen 2 reflects parts of the electrons emitted from the cathode and protects a base 3, on which the material is deposited, against thermal radiation from the cathode. An electrode 4, likewise 3,102,828 Patented Sept. 3, 1963 made of molybdenum, which is maintained at a positive potential of a few hundreds of volts, promotes the flow of electrons towards two anodes 5 and 6 made from silicon and germanium, respectively. The anodes are arranged on graphite supports 7 and 8. The potentials of the anodes 5 and 6 can be adjusted independently by means of a double potentiometer 9, having two sliding contacts 10 and 11. As an alternative, however, these contacts may be coupled to one another so that, when one is at maximum potential, the other is at minimum potential, and conversely. The supports 7 and 8 may be cooled in a manner not shown in the drawing, so that only the upper parts of the semiconductor bodies 5 and 6 are melted by the electron bombardment.
The base 3, which [comprises a single silicon crystal, can be heated by means of a filament 12.
The entire apparatus is arranged in an envelope adapted to be evacuated, which is not shown.
FIGURE 2 is a so-called phase diagram for mixtures of silicon and germanium, in which the ordinate represents the temperature in C. and the abscissae the silicon proportion of the mixture in atomic percent. Both the solidus 2t and the liquidus 21 have a continuous variation from the melting point of silicon at 1420 C. to the melting point of germanium at 960 C. Owing to the fact that these elements can form a continuous series of mixed crystals but do not form an azeotropic system, there can be formed on the surface of the silicon base a layer in which the concentration of the silicon decreases gradually.
The process is initiated by heating the silicon base 3 to about 1400 C. Then a voltage of 5000 volts is applied to the silicon anode 5 and an appreciably lower voltage to the germanium anode 6. The vapour produced will consist substantially of silicon. Gradually the first voltage is reducedand the second voltage is increased, until the vapour produced consists only of germanium. In the meantime, the temperature of the base 3 is gradually reduced to the melting point of germanium, after which the entire heating system is switched out of circuit. Throughout the entire process, the temperature of the base is kept at a value such, depending upon the composition of the deposited material, that the surface layer is liquid.
It is not necessary to use a source supplying silicon vapour. If germanium only is deposited from vapour, the base can be heated to a temperature such that a liquid mixture of silicon and germanium is produced on the surface, and this mixture gradually contains less silicon in proportion as more germanium is deposited and the temperature is reduced.
A semiconductor body made in this manner is shown diagrammatically and greatly enlarged in FIGURE 3. It comprises a silicon layer 30, a layer 31 consisting of a mixture of silicon and germanium and a germanium layer 32. In the layer 31, the proportion of silicon gradually decreases towards the germanium layer 32. Obviously, the process can be stopped at an intermediate stage, for example, before the layer of pure germanium has been formed.
It will be appreciated that the invention can be employed in an analogous manner for manufacturing a body consisting at least in part of a semiconductor compound of two elements which should have a constitutional diagram similar to that of silicon and germanium, that is to say, a non-azeotropic diagram.
What is claimed is:
1. A method of making a semiconductive body containing an alloy portion of silicon and germanium comprising providing a single crystal base member of silicon,
depositing on the surface of said base member a vapor containing both silicon and germanium in a ratio in which the silicon predominates while heating the base member to maintain it at a temperature just above the melting point of the surface composition whereby the surface only is molten, continuing to deposit onvthe surface a vapor with a gradually decreasing proportion of the silicon and a gradually increasing proportion of the germanium while reducing the heating of the base member to maintain a temperature just above the gradually-decreasing melting point of the surface composition which is increasing in germanium, causing a gradual crystallization of the surface melt to form a generally single crystal growing on the base, which grown single crystal has a gradually decreasing proportion of the silicon and a gradually increasing proportion of the germanium.
2. A method as set forth in claim 1 wherein the deposition is continued and the silicon content reduced until only germanium is being deposited on the base.
References Cited in the file of this patent UNITED STATES PATENTS 2,739,088 Pfann Mar. 20', 1956 2,780,569 Hewlett Feb. 5, 1957 2,855,334 Lehovec Oct. 7, 1958 FOREIGN PATENTS 737,527 Great Britain Sept. 28, 1955 742,237, Great Britain Dec. 21, 1955 805,493 Great Britain Dec. 10, 1958 3 4 Great Britain Aug. 19, 1959