US 3780427 A
A zinc sulfide body is treated to form an ohmic contact by applying a Group II metal or alloy thereof to a surface region of the body in the presence of a source of donor precursor such as a Group IIIa metal or a halogen and heating the region to a temperature above the melting temperature of the metal or alloy.
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Description (OCR text may contain errors)
United States Patent [191 Jenkins et al.
[4 1 Dec. 25, 1973 OHMIC CONTACT TO ZINC SULFIDE DEVICES  Inventors: Robert Jenkins, Cupertino; Carver A. Mead, Pasadena; James McCaldin, So. Pasadena, all of Calif.
 Assignee: Monsanto Company, St. Louis, Mo.
 Filed: Oct. 28, 1970 21 Appl. No.: 84,910
Related U.S. Application Data  Division of Ser. No. 824,898, April 25, 1969, Pat.
 U.S. Cl. 29/590, 148/15  Int. Cl B01j 17/00  Field of Search 29/589, 590;
 References Cited UNITED STATES PATENTS 3,515,954 6/1970 Maruyama et a1 317/234 3,518,511 6/1970 Koelmans 317/237 3,549,434 12/1970 Aven 148/186 Primary Examiner-Charles W. Lanham Assistant Examiner-W. C. Tupman Atr0rneyLindenberg, Freilich & Wasserman  ABSTRACT 12 Claims, No Drawings OIIMIC CONTACT TO ZINC SULFIDE DEVICES CROSS-REFERENCE TO RELATED APPLICATION The present application is a division of Ser. No. 824,898, filed April 25, i969, now U. S. Pat. No. 3,614,551.
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electroding zinc sultide, and more particularly, the present invention relates to processing zinc sulfide devices to form electron-injecting contact regions exhibiting ohmic characteristics at room temperature.
2. Description of the Prior Art Currently available gaseous light emitting devices operate at relatively high voltages and therefore are incompatible with conventional integrated circuits. This increases the cost of construction and operation of electronic instruments utilizing these devices A substantial effort is in progress to develop solid state, electroluminescent devices that emit light at wave lengths at which the eye is most efficient and which are compatible with standard transistor or integrated circuit voltages.
Light emission in solid state electroluminescent devices occurs by radiative recombination of injected electrons and holes which combine at recombination centers in a manner favoring the emission of a photon. The maximum available energy of the photon is limited by the band-gap of the material utilized to fabricate the device. The currently available low-power devices that have reasonable levels of light emission at room temperature have been fabricated from materials having a narrow band-gap, of the order of about 2.5 electron volts or less, and emit radiation in the red region at wavelengths longer than 6,500 Angstroms. The eye is 30 times less efficient in the red region of the spectrum than in the green.
Devices capable of emitting light at a variety of wavelengths would permit communication of an enormous quantity of information by the color variation in a multicolor display. 7
Zinc sulfide (ZnS) is known to be a very efficient phosphor and has a wide band-gap of 3.6 electron volts. It would appear that light at the desired shorter wavelengths would be emitted by the radiation resulting from the recombination of electrons and holes injected into a body of zinc sulfide.
Although it is possible to prepare crystals of ZnS with relatively high n-type conductivity, one of the major problems in the developement of zinc sulfide electroluminescent devices has been the difficulty in forming ohmic contact regions without simultaneously introducing large concentrations of defects which interfere with desired injection. Another major problem in electroding zinc sulfide stems mainly from its very low electron affinity and the very large energy barrier that exists between the zinc sulfide surface and metal contact interface.
The barrier energy behavior of a covalent semiconductor metal interface such as silicon, or germanium differs considerably compared with the more ionic wide band-gap semiconductors such as zinc sulfide. With the covalent semiconductors, the barrier energy does not depend very strongly on the metal which is in contact with the semiconductor surface and is largely a property of the semiconductor surface. In contrast the barrier energy between a more ionic semiconductor and a metal is a function of both the electronegativity of the metal and of the semiconductor.
With a few semiconductors, an ohmic contact can be made by decreasing the barrier energy of the metalsemiconductor junction such that the thermal current which flows in the reverse direction is large enough for the particular device application. However with zinc sulfide, metals with an electronegativity small enough to reduce the barrier energy sufficiently for device purposes do not exist. Metals that can be effectively electroded to zinc sulfide exhibit a barrier energy of about i to 2 electron volts from the conduction band edge.
Thermal current however, is not the only current which can flow in a metal-semiconductor system. It is known that as the net ionized impurity concentration in the semiconductor depletion region beneath the metal contact is increased, the width of the depletion layer is decreased. At very high carrier concentrations, the depletion layer becomes sufficiently thin that quantum mechanical tunneling can take place. This tunneling results from the fact that the electron probability distribution in the forbidden region decreases exponentially with distance and hence an electron can penetrate a barrier if it is sufficiently thin.
A tunneling contact requires a net ionized impurity density in the region of the semiconductor body under the metal contact preferably above about l0" carrier cm It is very difficult to introduce such a high density of atoms into a wide band-gap material such as zinc sulfide without concomitantly introducing compensating defects which negate the effect of the desired impurities. If a donor precursor such as indium is placed on a clean, cleaved surface of n-type conducting zinc sulfide crystal and the surface is heated until the indium melts and is then cooled, the indium wets and otherwise reacts with the surface. However, the current voltage characteristics of the contact indicates that the indium has not been introduced into this surface and the electrode presents essentially the same barrier as before the processing. The contact will rectify and cannot be used to supply electrons to the n-type crystal, the polarity necessary for an ohmic contact.
The most satisfactory prior art technique for forming contacts has been reported by Aven and Mead in Volume 7, No. l of Applied Physics Letters. This technique relies on the combination of very powerful chemical gettering agents and a chemically etched zinc sulfide surface. Contacts with the best overall performance are obtained according to this technique by etching the zinc sulfide crystal in pyrophosphoric acid at 250C and immediately scribing the indium contacts onto the phosphate phase with a liquid indium-mercury amalgam and firing at 350C in a hydrogen atmosphere. It is believed that the zinc atoms are extracted and held in the phosphate phase and the much larger amount of indium passes through this phase and enters the lattice in numbers sufficient to form a net ionized donor density of at least lO cm However, even under these corrosive conditions, the final contact is not always ohmic at room temperature. Furthermore, the technique will not work on a cleaved or mechanically prepared surface and the known photo resists are not capable of protecting the edges and back of the body of zinc sulfide during the treatment with pyrophosphoric acid.
OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the invention to provide an ohmic contact on a body of conducting n-type zinc sulfide.
A further object of the invention is to provide a technique for electroding zinc sulfide under a wider variety of conditions which are compatible with available photo resist processing.
Yet another object of the invention is the provision of an electron-injecting contact on zinc sulfide surfaces that can be provided on unprepared, or mechanicallyprepared surfaces that can be effected in inert, reducing or oxidizing atmospheres; or vacuum.
These and other objects and many attendant advantages of the invention will become apparent as the description proceeds.
Zinc sulfide is treated to form an ohmic electrode according to the invention by applying to a surface region of a body of n-type zinc sulfide in the presence of a source of a donor precursor, a Group II metal or alloy containing Group II metal, and heating said region to above the melting temperature of the metal or alloy. The donor precursor is preferably a Group Illa metal such as aluminum, gallium or indium or halogen such as Cl, Br, I, and must be present in the surface region in a density of at least l' cm' before treatment or may be substitutionally introduced into the surface region during the treatment by being present on the surface alloyed with the Group II metal.
Best results have been achieved by referring to the temperature-composition phase diagram for the particular Group II and Group Illa elements considered and selecting an alloy on the Group II rich side of the eutectic composition. The final contacts have been found to exhibit a resistivity at room temperature of less than 25 ohm-cm and in preferred embodiments of less than 1 ohm-cm? Lower resistivity contacts have been formed with cadmium as compared to zinc.
The final device is in the form of a body of n-type zinc sulfide provided with an ohmic electrode. The electrode comprises a Group II metal or Group llla metal alloy thereof in a firm and stable metalurgical contact with the thin surface region of the body which has a net donor density of greater than lo cm It is to be understood that zinc sulfide devices according to the invention are intended to include electroded bodies containing a mixture of zinc sulfide and other wide band gap materials such as cadmium sulfide.
In one procedure, according to the invention the Group II metal or Group Illa alloy thereof is brought into intimate contact with the surface region of a zinc sulfide body. This may be accomplished by evaporating the metal on the surface of the region or by pressing a preform of the metal on the surface. The condition of the surface is not critical and it may be sawed, abraded, cleaved or chemically etched. The treatment is facilitated by initially wetting the surface with a liquid metal such as mercury-indium amalgam or gallium.
The region in intimate contact with the metal is then heated to above the melting temperature of the metal. suitably for a short period which can be as short as a few seconds. The temperature is sufficiently high to enable zinc atoms to become disrupted from the lattice of the crystal. The temperature typically ranges from about 350 to 450C.
The processing can be carried out in an inert atmoshpere such as argon, in vacuum or even in an oxidizing atmosphere such as sulphur vapor. The processing chemicals utilized in the pretreatment of the surface are compatible with available photo resists which may be present to protect the non-treated surfaces of the crystal body.
Though the manner in which the contact is formed and operates has not been definitely determined, it is believed that the Group II metal and especially cadmium is introduced into the surface region. Cadium sulfide tends to become metal-rich when heated while zinc sulfide tends to become metal poor when heated. During the brief period of heating the surface region, cadium atoms enter the crystal and occupy the metal vacancies which are present compensating the donor atoms. Thus, a thin layer of very high net impurity density is produced just under the contact which allows the aformentioned electron. tunneling to take place. Thus an effective ohmic contact results.
The following examples are offered only by way of illustration, it being understood that many substitutions, alterations and modifications can readily be made without departing from the scope of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE I A slice of low-resistivity n-type zinc sulfide crystal was mechanically cleaved from a zinc sulfide material doped with =10 aluminum donor atoms per cm. The net donor density of this crystal was about 10 donor atoms per cubic centimeter. A surface region of the slice was chemically etched in HCl at 50C for 5 minutes.
The etched surface was then scrubbed with an indium-mercury amalgam to wet the surface. A preformed slug of a slightly cadmium-rich indium-cadmium alloy was pressed onto the surface and the slice was heated on a platinum strip heater for one minute at 350450C. The heating was conducted in an argon atmosphere. The slice was cooled to room temperature. The contact resistance of the electrode was measured and was found to exhibit a resistance of about 1 ohm-cm The slug was in firm metallurgical contact with the surface.
The procedure was successfully repeated on a cleaved zinc sulfide surface and on an abraded surface. When the procedure was repeated with a percent cadmium 10 percent indium alloy slug, the resistance of the contact was only slightly higher.
EXAMPLE II The procedure of Example I was repeated by substituting a cadmium-gallium alloy having a weight ratio of l3:l:Cd:Ga, somewhat on the Cd rich side of the Cd-Ga eutectic preform and an electrode having a contact resistance of about 20 ohm-cm resulted.
EXAMPLE Ill The procedure of Example I was repeated utilizing a slightly zinc-rich slug of a zinc-indium alloy, and an electrode having a contact resistance of about ohm-cm was formed.
EXAMPLE IV A slice of n-type zinc sulfide was mechanically cleaved from a zinc sulfide material doped with aluminum to a level of about atoms cm? The net donor density was about 10 cm indicating that a large precentage of the aluminum atoms were not in a donor state but were complexed with Zn vacancies.
A preformed slug of cadmium was pressed onto the surface of the slice wetted with a Cd-Hg amalgorn and the slice was heated on a platinum strip heater for about 5 seconds at 350 to 450C in an argon atmosphere. The slice was cooled to room temperature and the contact resistance of the electrode was measured and was found to exhibit a resistance of about 10 ohmcm It is evident that a substantial percentage of the aluminum atoms in the thin surface region under the cadmium slug have been converted to donor atoms to form a net ionized donor density in the thin region of at least l0 cm It is to be realized that only preferred embodiments of the invention have been disclosed and that numerous substitutions, alterations and modifications are permissible without departing from the scope of the invention as defined in the following claims.
What is claimed is:
1. A method of electroding zinc sulfide comprising the steps of:
applying to a surface region of a body of n-type zinc sulfide both a Group II metal and a donor precursor; and heating said surface region to a temperature above the melting temperature of said metal to form a contact ohmic at room temperature.
2. A method according to claim 1 in which said body is heated to a temperature of at least 350C.
3. A method according to claim 2 in which said metal is brought into intimate contact with the surface of said region.
4. A method according to claim 1 in which said Group II metal is cadmium.
5. A method according to claim 1 in which said donor precursor is a Group Illa metal.
6. A method according to claim 5 in which said Group Illa metal is indium, aluminum or gallium.
7. A method according to claim 1 in which said donor precursor is a halogen selected from the group consisting of chlorine, bromine or iodine.
8. A method according to claim 5 in which said donor precursor is present in the surface region of said zinc sulfide body.
9. A method according to claim 8 in which said donor precursor is present in said region in a density of at least l0 cm' 10. A method according to claim 9 in which the net donor precursor density in said region after the heating step is at least l0 cm' 11. A method according to claim 5 in which said Group Illa metal source is alloyed with said Group II metal prior to its being applied to said body.
12. A method as recited in claim 1 wherein the surface region of said body of n-type zinc sulfide is first wetted with an amalgam including mercury.