Ohmic Contacts to P-Type Mercury Cadmium Telluride

The present invention is concerned with mercury cadmium telluride semiconductor devices. In particular, the present invention is directed to the preparation of reliable ohmic contacts to p-type mercury cadmium telluride. For the purposes of this specification, the common chemical equations for mercury cadmium telluride, (Hg,Cd)Te or Hg.sub.1.sub.-x Cd.sub.x Te, will be used.

Mercury cadmium telluride is an intrinsic photodetector material which consists of a mixture of cadmium telluride, a wide gap semiconductor (E.sub.g =1.6 eV), with mercury telluride, which is a semi-metal having a negative energy gap of about -0.3 eV. The energy gap of the alloy varies approximately linearly with x, the mole fraction of cadmium telluride in the alloy. By properly selecting x, it is possible to obtain (Hg,Cd)Te detector material having a peak response over a wide range of infrared wavelengths. High performance (Hg,Cd)Te detectors have been achieved for wavelengths from about 1 to 30 microns.

In the prior art, techniques have been developed for making ohmic contact to p-type (Hg,Cd)Te. For example, one technique involves the deposition of gold on a p-type (Hg,Cd)Te surface. The gold and the (Hg,Cd)Te are then heated to cause diffusion of the gold into the (Hg,Cd)Te. In this manner, a higher conductivity p-type region (a "p+ region") is formed near the gold contact. Electrical leads are then attached to the gold with a bonding material such as an indium solder.

The prior art techniques, however, have not been completely satisfactory in making reliable ohmic contacts to p-type (Hg,Cd)Te. In particular, the prior art contacting techniques have proved to be unsatisfactory at very low temperatures (for example, less than about 50.degree. K). At these low temperatures, the prior art contacts tend to exhibit non-ohmic behavior and excess noise.

Because of the vapor pressures of the constituents of (Hg,Cd)Te, the material cannot be heated above about 700.degree. K for any extended period without changing the electrical properties. Any successful contacting technique for (Hg,Cd)Te, therefore, must be capable of being performed at less than about 700.degree. K. This limits the bonding materials which may be used to materials having a low melting point.

The present invention is a reliable ohmic contact for p-type (Hg,Cd)Te. The ohmic contact of the present invention includes a Column IB metal which is in contact with the p-type (Hg,Cd)Te. A buffer material is in contact with the Column IB metal. The buffer material, in turn, is contacted with a bonding material. The buffer material prevents alloying of the bonding material with the Column IB metal.

FIGS. 1 and 2 show mercury cadmium telluride photodiodes utilizing the ohmic contact of the present invention.

FIGS. 3a - 3i show the fabrication steps of a mercury cadmium telluride photoconductive detector having the ohmic contacts of the present invention.

FIG. 1 shows a mercury cadmium telluride photodiode. The photodiode is formed from the body 10 of (Hg,Cd)Te having n-type region 10a, p-type region 10b, and p+ region 10c. Attached to n-type region 10a is a contact 12, which is preferably of a donor material such as indium. Lead wire 14 is attached to contact 12.

The ohmic contact of the present invention is made to p-type region 10b. The ohmic contact includes p+ region 10c, layer 16 of a Column IB metal, buffer layer 18, and bond 20. Electrical lead 22 is attached by bond 20.

Bond 20 is a low melting point material. Indium, aluminum, and alloys thereof are the preferred materials for bond 20. However, any low melting point metal or alloy is suitable.

Suitable low melting point materials generally act as donor materials in (Hg,Cd)Te. It has been discovered that donor materials forming bond 20 are generally capable of alloying with the Column IB metal (i.e. gold, silver, or copper) of layer 16 at relatively low temperatures (for example, less than about 700.degree. K). If buffer layer 18 is not provided, the donor material of bond 20 alloys with the Column IB metal during fabrication of the (Hg,Cd)Te device. The resulting alloy contains ions which are donors in (Hg,Cd)Te. A donor type alloy can cause undesirable surface states or potential barriers such as junctions which result in non-ohmic or noisy contacts to p-type (Hg,Cd)Te.

Buffer layer 18 is interposed between layer 16 and bond 20 to prevent this undesirable alloying. The buffer material is any material which is capable of preventing alloying between layer 16 and bond 20. The buffer material is generally effective if it does not alloy with the Column IB metal or react with the bonding material at temperatures less than about 700.degree. K.

In a preferred embodiment of the present invention, the buffer material is a transition metal (e.g. iron, nickel, chromium, or cobalt). These metals are relatively easy to deposit and adhere well to Column IB metals. Nickel has been found to be particularly advantageous as the buffer material.

P+ region 10c, which is proximate layer 16, is typically formed by diffusion of the Column IB metal into the p-type (Hg,Cd)Te. The p+ region 10c assures good ohmic contact between the (Hg,Cd)Te and layer 16.

FIG. 2 shows another mercury cadmium telluride photodiode utilizing the ohmic contact of the present invention. The device of FIG. 2 is similar to the device of FIG. 1, and similar numerals and letters have been used to designate similar elements.

The photodiode of FIG. 2 differs from that of FIG. 1 in that the photodiode is attached to a conductive substrate 24 by bond 20. In preferred embodiments of the present invention, conductive substrate 24 is a copper substrate, bond 20 is an indium solder, buffer layer 18 is nickel, and layer 16 is gold.

FIGS. 3a - 3i show the fabrication steps used to form a p-type (Hg,Cd)Te photoconductive detector having the ohmic contacts of the present invention. For the purposes of simplifying the discussion, a preferred embodiment in which the Column IB metal is gold, the buffer material is nickel, and the donor or contact material is indium will be described. It will be understood, however, that the process described is also applicable to other materials.

In FIG. 3a, gold layers 30a and 30b have been deposited on a surface of p-type mercury cadmium telluride body 32. Layers 30a and 30b are deposited by the standard evaporation techniques.

The gold layers 30a and 30b and the p-type body 32 are then heated in a controlled atmosphere to diffuse gold into p-type body 32. The resulting higher conductivity (p+) regions 32a and 32b are shown in FIG. 3b. In the preferred embodiments, the diffusion is at a temperature of between about 500.degree. K and about 600.degree. K for about 10 minutes in a reducing atmosphere such as hydrogen.

In FIG. 3c, the remaining portions of p-type body 32 are covered by mask 34. Mask 34 is any one of many mask materials which are well-known in the semiconductor fields.

The initial contacts 30a and 30b formed by vapor deposition have thickness of about 2000A. It has been found to be advantageous to increase this thickness by electroplating additional gold. FIG. 3d shows the device after the thickness of layers 30a and 30b have been increased by electroplating. In one embodiment, an additional 2000A of gold is deposited on contacts 30a and 30b using a commercial acid gold process, Engelhard E-56.

In FIG. 3e, buffer layers 36a and 36b are deposited on layers 30a and 30b, respectively. In one preferred embodiment, layers 36a and 36b are formed by electroless nickel plating. The nickel plating solution is as follows:

Prior to plating, a small amount of ammonia is added to the solution and the solution is heated to about 330.degree. K. The thickness of 36a and 36b is preferably about 2000A.

Mask 34 is removed from p-type body 32. The resulting structure is shown in FIG. 3f.

Mask 38 is then deposited over the entire contact area, as shown in FIG. 3g. The remainder of (Hg,Cd)Te body 32 is left exposed.

Body 32 is then cleaned and etched, preferably by etching body 32 for about 30 to 60 seconds in a 5% bromine-alcohol solution. The resulting structure, after mask 38 is removed, is shown in FIG. 3h.

In FIG. 3i, lead wires 40a and 40b are attached by bonds 42a and 42b, respectively. Bonds 42a and 42b are preferably an indium containing solder.

Experiments have shown that nickel buffer layers 36a and 36b are effective in preventing alloying between the gold of layers 30a and 30b and the indium of bonds 42a and 42b. The resulting contacts have been tested at temperatures from 300.degree. K to as low as 4.2.degree. K and have exhibited ohmic behavior at all temperatures.

In conclusion, the present invention is a highly advantageous ohmic contact for p-type (Hg,Cd)Te. The contact of the present invention is particularly advantageous when the (Hg,Cd)Te device will operate at temperatures of less than about 50.degree. K. Even for higher temperature operation (T> 50.degree. K), this contacting technique results in a higher yield of good ohmic contacts than the prior art techniques.

Although the present invention has been disclosed with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.