US 3322516 A
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May 30, 1967 c. H. DE MINJER 3,322,516
- METHOD OF COATING P-TYPE GERMANIUM WITH ANTIMONY, LEAD OR ALLOYS THEREOF BY ELECTRODEPOSITION AND PRODUCT THEREOF Filed March 5, 1964 INVENTOR.
CLARA HDE MINJER gsu-r United States Patent 3,322,516 METHOD OF COATING p-TYPE GERMANIUM WITH ANTIMONY, L E A D OR ALLOYS THEREOF BY ELEC'I'RODEPOSITION AND PRODUCT THEREOF Clara Henderina de Minjer, Emmasingel, Emdhoven,
Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Mar. 3, 1964, Ser. No. 349,182 Claims priority, applicatiglgr gestherlands, Mar. 5, 1963,
2 5 Claims. (Cl. 29-194) The invention relates to a method of coating p-type germanium with a layer consisting of antimony, lead or alloys thereof by electrodeposition and to the products obtained by this method.
It is an object of the invention to deposit a uniform, well-adherent layer of Sb, Pb or Sb-Pb alloys on a p-type Ge surface in order to make rectifying alloy contacts or alloy-diflusion contacts of uniform depth for use in known semiconductor devices, such as transistors or diodes.
It is another object of the invention to make a uniform Sb layer on a p-type Ge surface in order to make an ntype zone of uniform thickness by diffusing the Sb into the underlying Ge, e.g. for use as a base zone in a known p-n-p transistor.
Various methods are known for coating electrically conductive material by electrodeposition with a layer consisting of antimony, lead or an alloy of these two metals. In addition, for coating germanium with antimony a special alkaline reacting bath is known.
However, it has appeared to be impossible with the known methods to coat p-ty-pe germanium sufliciently uniformly with antimony, lead or an alloy of these two metals, as is necessary for providing very small lowresistance contacts on germanium transistors. The surface of p-type germanium as a rule is coated only incompletely with the metal.
Various chemical and electrochemical pretreatments of the p-type germanium were tested without success, for example the chemical etching by means of a fluoridecontaining bath, the anodic pickling in a potassium hydroxide solution, the anodic connection in the lead bath for a short period of time in which, then, lead is deposited cathodically. Electrolysis at elevated temperature did not give the desired result either.
The invention provides a method by which a complete coating of p-type germanium with the above metals is possible.
According to the invention, one the surface of p-type germanium, before a layer of antimony, lead or an alloy of Pb-Sb is applied in known manner, a thin layer of antimony is deposited, also electrolytically, in a thickness of approximately 0.1-1 at that cathode potential at which the saturation current of antimony is reached but at which no gas development occurs.
For this purpose preferably a bath is used consisting of an aqueous solution which contains per liter 15-19 g. of potassium antimonyl tartrate, 15-52 g. of citric acid and 375-195 g. of sodium citrate, at room temperature and at a cathode potential of approximately --850 to 1000 mv. with respect to a standard hydrogen electrode.
In order to deposit antimony in a compact gray form by electrodeposition, it was known that for this purpose a particular critical value of the current density may not be exceeded, since otherwise black antimony is formed with a more or less porous structure. When carrying the invention into effect it has appeared that exactly at high current densities the p-type germanium is entirely coated with this black antimony.
The current density i as a function of the cathode potential E turned out to have a shape as is shown in the ice diagram of the accompanying drawing which holds for an antimony bath of the composition:
16 g. of potassium antimonyl tartrate,
82g. of sodium citrate, and
25 g. of citric acid per liter of solution in water and a pH of approximately 5.8.
The stated is measured with respect to a standard hydrogen electrode. Proceeding from less negative towards more negative potential, light-gray antimony begins to be deposited at approximately 600 mv. When the cathode potential is further decreased to approximately -850 mv., at which the saturation current density of approximately 0.28 a./dm. is begins to be deposited. If the potential is still further decreased to lower than approximately 950 mv., gas development sets in, in addition to the deposition of black antimony. The point where this gas development sets in, depends upon the pH of the electrolysis solution; at low pH-values, namely at pH 6, the gas development occurred already above 9S0 mv. Therefore, activities are preferably carried out at pH-values above 6 since otherwise the margin in the potential at the beginning of the occurrence of the saturation current and that at which the gas development occurs is small.
It has surprisingly been found that with the so-called black antimony a complete coating of the surface of p-type germanium is possible. Although this deposit has a somewhat porous structure, a layer of antimony, lead or an alloy of antimony and lead on it appeared to adhere excellently to it and to coat it completely. This black antimony is deposited as a flash with a thickness of approximately 0.1-lg. When the layer was provided in a greater thickness, the porous structure appeared to present no difliculties yet as regards the complete coating by the subsequently deposited layer.
When a layer of antimony is to be deposited on p-type germanium, this may be carried out by means of any known electrolyte after the flash of black antimony is deposited. Alternatively, the electrolyte used for providing the flash of black antimony may be chosen for the purpose, in which case the cathode potential must be increased by approximately 200 to 3'50 rnv. after the flash is deposited. For depositing thicker layers of antimony, however, this flash-bath is less suitable.
When lead or an alloy of lead and antimony must be deposited on the p-type germanium provided with a flash, it is to be recommended to deposit, from the same electrolyte as from which the flash has been deposited, a thin layer of gray antimony on the black antimony also by increasing the cathode potential by 200 to 350 mv. before the germanium is connected as the cathode in the electrolyte chosen for the purpose.
If the potential is set up by means of a potentiostatic arrangement, it must be avoided that a great voltage drop occurs at the cathode. In that case special attention should be paid to the connection of the germanium to the current supply wire having a contact resistance which is as low as possible. If this is not the case, a potential diflerence of even 1 volt may occur between the current supply wire and the germanium as a result of which the required value of the cathode potential for the deposition of black antimony would not be attained. This may be done in any known manner, for example by locally depositing a thin layer of indium on the germanium which has been cleaned by scouring and providing on it a copper wire with silver paste. For this purpose, instead of indium also a thin layer of another metal, for example copper, may be deposited on the germanium.
If no potentiostat is used, it is not necessary to provide contacts of very low resistance but the potential may be read and the current may be controlled so that this reached, black antimony potential has the desired value by means of a second contact and a high resistance voltmeter.
In order that the invention may readily be carried into effect, it will now be described more fully, by way of example, with reference to the ensuing specific example.
Strips of germanium having the dimensions 20 x 2 x 0.25 mm. which were of the p-type by doping by means of indium in a quantity of 3.5 atoms per cm. with a resistivity of approximately 1 ohm cm. were sawed out of a single crystal in the [UH-orientation, lapped with Carborundum powder and then pickled for 14 seconds in a mixture of 5 ml. of HNO (d.=1.4), 10 ml. of HF 50%, and 5 ml. of ethanol.
A thin layer of indium was deposited electrolytically on one of the sides of the strips and a copper wire was connected with silver paste.
The side of the strip to which the copper wire was connected, and the copper wire itself, were coated with a layer of nitrocellulose lacquer.
The strips were connected cathodically in an aqueous electrolyte which contained per liter:
16 g. of potassium antimonyl tartrate,
82 g. of sodium citrate, and
25 g. of citric acid and which was adjusted at a pH=6.5.
A platinum strip was used as the anode. The potential was adjusted via a Haber-Luggin capillary and a saturated calomel electrode by means of a potentiostat. The electrolysis was carried out at 20 C.
The potential was connected at -600 mv. (with respect to a standard hydrogen electrode), then rapidly brought to 850 mv., maintained at this value for one minute, then connected back to -600 mv. and maintained at this value for one minute also. This is shown in the E-i diagram of the drawing. The strips were removed from the bath, rinsed, and then a part was coated electrolytically with lead and the remainder with an alloy of lead and antimony.
The lead bath had the following composition per liter of solution in water.
100 g. of lead as lead fiuoborate, 80 g. of fiuoboric acid,
g. of boric acid, and
0.5 g. of gelatine.
The alloying bath contained per liter of solution 3 g. of antimony as antimony fiuoborate or as Sb O and 72 g. of tartaric acid in addition to the above substances.
The anodes consisted of lead. The cathodic current density was 4 a./dm. The bath temperature was maintained at 20 C. The alloy deposit contained approximately 3% of Sb.
All deposits obtained covered the whole surface of the p type germanum strips which was not coated by the nitrocellulose lacquer.
What is claimed is:
1. A method of providing a coating selected from the group consisting of antimony, lead and mutual alloys thereof on p-type germanium comprising first electrolytically depositing on the surface of said p-type germanrum a layer of a thickness of about 0.11,u of black antimony, said electrodeposition being carried out at a cathode potential at which the saturation current of antimony is reached but at which no gas is evolved, and then electrolytically depositing on said layer of black antimony an outer layer of a metal selected from the group consisting of lead, gray antimony and mutual alloys of said metal.
2. A method of claim 1, characterized in that for the deposition of the layer of black antimony a bath is used consisting of an aqueous solution which contains per liter 1519 g. of potassium antimonyl tartrate,
15-52 g. of citric acid, and
37.5- g. of sodium citrate.
3. A method of claim 2, characterized in that the pH of the solution is adjusted at a value above 6.
4. The method of claim 1 wherein subsequent to the deposition of the thin layer of black antimony a thin layer of gray antimony is deposited from the same electrolyte by increasing the cathode potential by 200-350 5. A semiconductor device prepared by the method of claim 1.
References Cited UNITED STATES PATENTS 2,603,693 7/1952 Kircher.
FOREIGN PATENTS 608,181 11/1960 Canada.
JOHN H. MACK, Primary Examiner.
G. KAPLAN, Assistant Examiner.