Stable ohmic contacts to thin films of p-type tellurium-containing II-VI semiconductors

Abstract: A stable ohmic contact for thin films of p-type tellurium-­ containing II-VI semiconductors and photovoltaic devices incorporating such contacts. An ohmic contact according to the invention includes a contact-forming layer deposited on a p-­ type thin film of a tellurium-containing II-VI semiconductor. Preferably, the contact-forming layer is copper having a thickness of about 2 nanometers. An isolation layer is deposited on the contact-forming to isolate subsequently deposited layers from the thin film. The isolation layer may be carbon or a thin layer of nickel. A connection layer for attaching an external electrical conductor is deposited on the isolation layer. The connection layer may be aluminum, chromium or a layer of copper, provided a copper layer is covered with one of silver, aluminum or a thin layer of nickel, preferably covered with aluminum. The stable, ohmic contact may be used as a back contact in a photovoltaic device incorporating a thin film of a tellurium-containing II-VI semiconductor as one of the active semiconductor layers in the device.

Description: This invention concerns stable ohmic contacts for, and a process for making stable ohmic contacts to, thin films of p-type tellurium-containing II-VI semiconductors and electronic devices, particularly photovoltaic devices, incorporating the novel stable contacts. Photovoltaic devices incorporating layers of tellurium-containing II-VI semiconductors offer substantial promise in terms of conversion efficiency, cost and ease of manufacture. As used here, the term tellurium-containing II-VI semiconductors refers to semiconductors containing tellurium and one or more elements selected from Group IIB of the Periodic Table of the Elements, as published by the American Chemical Society, including mercury cadmium telluride, cadmium telluride, cadmium zinc telluride, mercury zinc telluride, zinc telluride and mercury telluride. Of these semiconductors, cadmium telluride, mercury cadmium telluride, cadmium zinc telluride and zinc telluride offer particular promise. Of the II-VI tellurium-containing semiconductors, cadmium telluride and mercury cadmium telluride have probably been the subject of most research. These materials can be prepared by a variety of techniques. For example, monocrystalline cadmium telluride can be prepared by the Bridgeman technique. Thick films of cadmium telluride can be prepared by sintering a screen printed dried paste of cadmium and tellurium particles. These thick films, e.g. 20 micrometers thick, are sintered at rather high temperatures (about 600°C to 700°C) to react the particles of cadmium and tellurium to form cadmium telluride. Thin films of cadmium telluride can be conveniently electrodeposited as disclosed in US-A-4 400 244 and US-A-4 425 194. Thin films of cadmium-rich mercury cadmium telluride can be electrodeposited following the disclosure of US-A-4 548 681. The electrodeposited cadmium telluride-containing thin films generally exhibit n-type conductivity, but may be converted to p-type conductivity by heat treating in accordance with the disclosure of US-A-4 388 483. An ohmic contact to electrodeposited thin films of cadmium telluride-containing semiconductors may be prepared according to the disclosure of US-A-4 456 630. EP-A-0 186 350 discloses a photovoltaic device based on II-VI semiconductor in which contact is made to p-type cadmium telluride by a thin copper layer (2 nm) on which a relatively thick layer of nickel (140 nm) is deposited. There is no suggestion that there is any problem with the use of this relatively thick layer of nickel. DE-A-3 328 899 discloses a photovoltaic device with a layered contact. There is no disclosure of making contact to a tellurium-containing II-VI semiconductor. The layer in contact with the semiconductor is gold, palladium, or graphite. While ohmic contacts to crystals and films of p-type tellurium-containing II-VI semiconductors have been successfully made by various processes, including the method disclosed in US-A-4 456 630, we have observed that some ohmic contacts to thin films of p-type tellurium-containing II-VI semiconductors do not remain stable over time. For example, gold contacts on electrodeposited cadmium telluride and mercury cadmium telluride that were prepared according to US-A-4 456 630 have been observed to react with or diffuse into the tellurium-containing semiconductor. In general, gold reacts with and/or diffuses into tellurium-containing II-VI semiconductors so that the contact is lost. It is now known in the art that copper makes a good ohmic contact to the surface of a thin film of a p-type cadmium telluride-containing semiconductor that has been prepared according to the sequential acidic and alkaline treatment steps disclosed in US-A-4 456 630. However it is also known that copper readily diffuses into tellurium-containing II-VI semiconductors. If sufficient quantities of copper are available, copper will diffuse through the entire thickness of a thin film of a tellurium-containing II-VI semiconductor, making the thin film useless in a semiconductor or photovoltaic device. However, copper is useful for forming a contact to tellurium-containing thin films because of its excellent adhesion and electrical properties. One solution to the copper diffusion problem is limitation of the amount of copper available on the semiconductor surface as a diffusion source. The amount of copper available may simply be controlled by limiting the thickness of the copper layer. For example a copper layer about 1 to 5 nanometers thick deposited on a film containing cadmium and tellurium and about 1.5 micrometers in thickness contains too few atoms to damage the film by diffusing into it. However such a thin copper contact-forming layer has an inherent resistivity which is intolerably high and is almost impossible to contact directly with a wire or lead without severely damaging the layer and underlying thin film. In order to increase the thickness of the ohmic contact so that an external electrical connection can be made to it, a different material must be deposited on the thin contact-forming layer. Copper-nickel contacts to cadmium telluride films are known but are unsatisfactory. Nickel has a relatively low resistivity and therefore must be deposited in a relatively thick layer to achieve an acceptable resistance. For example, a nickel film 200 nm or more in thickness is required to produce an acceptably low resistance layer. However, we have found that thick films of nickel are mechanically unstable. Evaporated nickel films 100 nm or more in thickness on copper tend to retain internal stresses. Unprotected thick nickel films on copper have been observed to oxidize over time and to flake off under the influence of the internal film stresses. In order to prepare photovoltaic devices containing thin films of p-type tellurium-containing II-VI semiconductor that are useful over reasonable life times, ohmic contacts having long term stability must be easily and cheaply prepared. It is a principal object of this invention to prepare ohmic contacts to thin film of p-type tellurium-containing II-VI semiconductors which are stable over a long period of time. It is another object of this invention to make an ohmic contact to a thin film of p-type tellurium-containing II-VI semiconductor requiring only simple low cost processing steps and materials. It is a further object of this invention to achieve a photovoltaic device including a thin film of a p-type tellurium-containing II-VI semiconductor and incorporating a stable ohmic contact. According to one aspect of the present invention there is provided a method of making an ohmic contact to a thin film of a p-type tellurium - containing II-VI semiconductor which comprises: depositing a copper contact-forming layer (13) on a thin film (9) of a p-type tellurium-containing II-VI semiconductor in a thickness sufficient to form an adherent, electrically conductive contact to the thin film, but insufficient to act as a source of dopant atoms throughout said film, depositing an electrically conducting connection layer (17) over said metallic contact-forming layer and depositing an electrically conducting barrier layer (15) on said copper contact-forming layer (13) before depositing the electrically conductive connection layer, said barrier layer being carbon or a layer of nickel less than 100 nanometers in thickness. According to another aspect of the present invention there is provided a photovoltaic device comprising: a light transmitting substrate (3); an electrically conductive, transparent layer (5) disposed on said substrate as a first electrode; a layer (7) of a first semiconductor disposed on said first electrode; a p-type thin film (9) of a tellurium-containing II-VI semiconductor disposed on said first semiconductor to form a photo responsive junction with it; a second electrode contacting said thin film, said second electrode comprising a copper contact-forming layer (13) disposed on said film in a thickness sufficient to form an adherent, electrically conductive contact to said thin film, but insufficient to act as a source of dopant atoms throughout said film, an electrically conductive layer (17) being disposed over said contact-forming layer; and an electrically conducting barrier layer (15) is disposed between the copper contact forming layer (13) and the electrically conductive layer (17), the barrier layer being carbon or a layer of nickel less than 100 nanometers in thickness. In one embodiment of the invention, the surface of a cadmium telluride-containing thin film is prepared using a known technique. Thereafter, a contact-forming layer of copper, which forms a good mechanical and electrical contact, is deposited on the prepared surface. For example a layer of copper 2 nm thick adheres well to p-type cadmium telluride-containing semiconductors, but includes too few atoms to act as a doping source that could short circuit the thin film. A barrier layer is deposited on the contact forming layer. The barrier layer avoids undesirable diffusion of metal atoms from subsequently deposited layers into the semiconductor and/or formation of a blocking (Schottky) contact between those layers and the semiconductor. Carbon or a thin layer of nickel (less than 100 nanometers) is used as the barrier layer. A relatively thick connection layer is deposited on the barrier layer to facilitate connection of an external electrical conductor to the ohmic contact. The connection layer can be a single layer of aluminium or silver or multiple metallic layers, such as a layer of copper covered with a thin layer of nickel or aluminum. All of the layers of materials in the novel contact may be deposited by established vacuum deposition techniques. The novel contact requires no heat treatment that would damage the underlying semiconductors. In a photovoltaic device, the novel ohmic contact may form a non-transparent or "back" contact to the device. The device may conveniently include a transparent substrate, such as glass coated with a transparent, electrically conducting layer, such as tin oxide or indium tin oxide, to form a first electrode or contact to the device. A layer of a first semiconductor material, such as cadmium sulphide, is deposited on the first electrode by any convenient method such as electrodeposition. A thin film of a p-type tellurium-containing II-VI semiconductor, such as cadmium telluride or mercury cadmium telluride, is disposed on the first semiconductor layer to form a photoresponsive junction. The ohmic contact described in the previous paragraph is then formed on the tellurium-containing II-VI semiconductor film. The ohmic contact described in the previous paragraph is then formed on the tellurium-containing II-VI semiconductor film. The ohmic contact may be formed by the sequential vacuum evaporation of sources of the constituents of each layer and condensation of the vapors. In the accompanying drawings:- Figure 1 is a schematic sectional diagram of a photovoltaic device according to the invention. Figure 2 is a graph of the normalized short circuit current density over time of photovoltaic devices according to and not according to the invention, measured during and after illumination. Figure 3 is a graph of the normalized efficiency over time of photovoltaic devices according to and not according to the invention, measured during and after illumination. In the invention, stable ohmic contacts to thin films of p-type tellurium-containing II-VI semiconductor materials are made. The term "thin film" as used here means a film having a thickness not exceeding about 3 micrometers. Tellurium-containing II-VI semiconductors include cadmium telluride, mercury cadmium telluride, cadmium zinc telluride, and mercury telluride. These semiconductors may exhibit p-type conductivity either as deposited or after a heat treatment changing their conductivity type and/or may be intentionally doped by incorporation of electronically active impurities. While the invention is not limited to use with electrodeposited thin films of II-VI semiconductors containing cadmium and tellurium, it is particularly useful with those polycrystalline films electrodeposited as disclosed in US-A-4 400 244, US-A-4 425 194, US-A-4 548 681, US-A-4 388 483, and US-A-4 456 630 mentioned above. A method of making ohmic contacts to p-type II-VI semiconductors is mentioned in US-A-4 456 630 (the ′630 patent). Reference is made to this patent for information on the treatment of the semi-conductor surface prior to deposition of the contact. The US patent discloses etching an electrodeposited thin film of cadmium telluride briefly (e.g 2 seconds) in a solution containing one volume of 98% weight sulfuric acid and one volume of saturated potassium dichromate (sold under the trademark Dichrol). The acid etched surface was immersed in a hydrazine solution (an alkaline solution) for about one minute. We have found the acid-alkaline surface preparation steps described in US-A-4 456 630 are useful. We prefer to modify the steps slightly by reducing the acidity of the Dichrol solution so that the etching time is longer and need not be so precisely controlled. We have found that a solution of 19 volumes of 50% weight sulfuric acid mixed with one volume of dichromate produces good results in a 5 second etch. We have found that satisfactory nickel-copper contacts can be produced by depositing a relatively thin and therefore stress-free layer on a copper contact forming layer. For example, a 50 nm thick layer of nickel on a 2 nm thick copper contact-forming layer provides a good mechanical and electrical contact. The thin nickel layer is covered with another metal layer, such as aluminum, for protection of the nickel from oxidation and to provide sufficient metal thickness for bonding of an external contact without unacceptable damage to the nickel and copper layers. External contact via wire bonding or the like to an aluminum layer can readily be made by known technology. As is known, aluminum cannot be used to contact p-type tellurium-containing II-VI semiconductors directly because it forms a blocking (Scholtky) contact. Thus, a nickel layer is an appropriate barrier layer for isolating the outermost connection layer, e.g., aluminum, from the semiconductor. We have found that another useful barrier layer can be formed from carbon. The surface of a cadmium telluride-containing semiconductor is first sequentially treated with the acid and alkaline solutions as already described. Then a thin contact-forming layer, such as 2 nm of copper, is deposited. A layer of carbon is deposited on the contact-forming layer, preferably by vacuum evaporation and condensation. The carbon layer should be thick enough to be opaque, at least 50 nm and, preferably, 100 nm thick. Carbon is a very effective barrier layer and does not permit diffusion of copper through it. Therefore, a second copper layer may be deposited on the carbon for making a wire bond or other external contact without concern that copper from the second layer will diffuse through to the tellurium-containing II-VI semiconductor. Preferably, the second copper, connection layer is coated with another metal, such as aluminum or silver, to protect the second copper layer from oxidation. As an alternative to a second copper layer, a layer of aluminum or a layer of copper covered with a thin layer of nickel may be deposited on the carbon layer as a connection layer to complete the ohmic contact. In the latter case, aluminum may be deposited on the nickel as a protective coating, if desired. Deposition of nickel directly on the carbon is not desirable, but other metals that make good electrical and mechanical contact to carbon, such as chromium, may be employed in direct contact with the carbon layer as a connection layer. A photovoltaic cell employing embodiments of the invention is shown in Fig. 1 in a schematic, sectional view. Fig. 1 is not drawn to any scale. Cell 1 includes a light transmissive substrate 3, typically glass. A layer 5 of a transparent, electrically conductive material is disposed on substrate 3 as a first electrode. Appropriate materials for layer 5 are transparent, electrically conductive oxides, such as tin oxide or tin oxide doped with indium. A wire 6 forming a first electrical terminal of the cell makes electrical contact to layer 5 possibly via a thin metallic film (not shown). A thin film of a first semiconductor layer 7, preferably n-type cadmium sulfide, is disposed on electrode 5. Preferably, a cadmium sulfide layer is electrodeposited, although it may be deposited by other known techniques such as vacuum evaporation and condensation. A thin film of a p-type tellurium-containing II-VI semiconductor layer 9 is disposed on layer 7 to form a photoresponsive heterojunction. Thin film 9 may be electrodeposited cadmium telluride or mercury cadmium telluride that has been converted to p-type conductivity by heat treatment. An ohmic contact 11 is formed on thin film 9 having the structure described above. After the surface of the tellurium-containing thin film 9 is sequentially treated in acid and alkaline solutions as already described, a contact-forming layer 13 is deposited on it, such as about 2 nm of copper. Thereafter, an barrier layer 15, such as carbon or nickel is deposited on the contact-forming layer to isolate it from subsequently deposited layers. Finally, a connection layer 17 of one or more metals is applied to isolation layer 15 so that the second electrical terminal to cell 1 may be completed. Unlike known contacts to screen printed thick film cadmium telluride-containing semiconductors, no heat treatment that might damage thin film 9 is required to make the multiple layer ohmic contact operational. A wire 19 is bonded to the connection layer 17 of ohmic contact 11 by any known means such as welding, soldering or adhering with a mass of an electrically conductive adhesive, to form the second terminal of cell 1. Alternatively, for testing purposes, an electrically conducting probe may touch the connection layer to establish electrical communication with an external device. Embodiments of the novel ohmic contact have shown substantial improvements in stability over known contacts. Figs. 2 and 3 are measured responses over time of cadmium sulfide-cadmium telluride photovoltaic cells of the type depicted in Fig. 1. The transparent electrode of the cells was indium tin oxide contacted by a one micrometer thick layer of aluminum. All cells had an area of about one square centimeter. On the conventional cells, the rear contact or second electrode was a layer of copper 5 nm thick coated with a layer of nickel 100 nm thick. The cells according to an embodiment of the invention included a rear contact having a copper contact-forming layer 5 nm thick covered with 50 nm of carbon. The carbon layer was coated with 3 nm of copper and 70 nm of nickel. The layers in the rear contacts were deposited by sequentially heating sources of each of the layers in a vacuum chamber with an electron beam. The normalized, measured values of short circuit current density and efficiency of the conventional cells and of cells according to the invention, over time, are plotted in Figs. 2 and 3, respectively. The measured values for the cells according to the invention are indicated by + symbols. All measurements were taken at intervals during otherwise continuous light exposure in a solar simulator for a period of about 460 hours. The continuous exposure in the solar simulator represented an accelerated life test since the cells would ordinarily be exposed to the light-dark diurnal cycle. Temporary electrical connections to the cells for the measurements were established with probes. After the period of continuous light exposure, all cells were removed from the solar simulator and further measurements were made while the cells were in darkness, i.e., ambient light. As seen in Figs. 2 and 3, short circuit current density and efficiency improved for all cells in darkness after the light exposure. More importantly, Figs. 2 and 3 show that no degradation in short circuit current density or efficiency appeared in cells according to the invention until after 150 hours of light exposure. Cells with known copper-nickel contacts showed earlier and larger degradation and never performed at higher normalized short circuit current density or efficiency than the novel cells. In fact, only at the final measurement, in the dark, did any cell perform nearly as well as the novel cells in terms of normalized short circuit current density and efficiency (see Fig. 2). Based on our measurement experience with cadmium telluride cells having gold and other contacts, these results indicate that the novel contact when used at the rear (the non-illuminated contact) of the cell is far more stable and performs much better than previously known contacts. The elimination of gold and the use of more common metals in the novel ohmic contact also means that cost is reduced. Moreover, the process of making the novel contact is simple and only requires conventional equipment.

Claims: A method of making an ohmic contact to a thin film of a p-type tellurium - containing II-VI semiconductor which comprises: depositing a copper contact-forming layer (13) on a thin film (9) of a p-type tellurium-containing II-VI semiconductor in a thickness sufficient to form an adherent, electrically conductive contact to the thin film, but insufficient to act as a source of dopant atoms throughout said film, depositing an electrically conducting connection layer (17) over said metallic contact-forming layer, and depositing an electrically conducting barrier layer (15) on said copper contact-forming layer (13) before depositing the electrically conductive connection layer, said barrier layer being carbon or a layer of nickel less than 100 nanometers in thickness. The method of Claim 1 wherein the thickness of the contact-forming layer is between 1 and 5 nanometers. The method of either of Claims 1 or 2 wherein a layer of carbon having a thickness of at least 50 nanometers is deposited as said barrier layer. The method of either of Claims 1 or 2 wherein the a layer of nickel less than 100 nanometers in thickness is deposited as the barrier layer. The method of any one of the preceding claims wherein the barrier layer is carbon and a layer of one of aluminium, copper and chromium is deposited as said connection layer. A photovoltaic device comprising: a light transmitting substrate (3); an electrically conductive, transparent layer (5) disposed on said substrate as a first electrode; a layer (7) of a first semiconductor disposed on said first electrode; a p-type thin film (9) of a tellurium-containing II-VI semiconductor disposed on said first semiconductor to form a photo responsive junction with it; a second electrode contacting said thin film, said second electrode comprising a copper contact-forming layer (13) disposed on said film in a thickness sufficient to form an adherent, electrically conductive contact to said thin film, but insufficient to act as a source of dopant atoms throughout said film, an electrically conductive layer (17) being disposed over said contact-forming layer; and an electrically conducting barrier layer (15) is disposed between the copper contact forming layer (13) and the electrically conductive layer (17), the barrier layer being carbon or a layer of nickel less than 100 nanometers in thickness. The device of Claim 6 wherein the thin film (9) is selected from the group consisting of cadmium telluride, mercury cadmium telluride, zinc telluride, mercury zinc telluride, and mercury telluride and wherein said first semiconductor (7) comprises n-type cadmium sulphide. The device of either of Claims 6 or 7 wherein said contact forming layer (13) comprises a layer of copper having a thickness of between 1 and 5 nanometers. The device of any one of Claims 6 to 8 wherein the barrier layer (15) is carbon and has a thickness of at least 50 nanometers. The device of Claim 9 wherein the electrically conductive layer (17) comprises a layer of one of aluminium, copper, and chromium disposed on said barrier layer of carbon. The device of any one of Claims 6 to 8 wherein said barrier layer (15) comprises a layer of nickel less than 100 nanometers in thickness.

Inventors: BITER WILLIAM JOSEPH, SZABO LOUIS FRANK, BITER, WILLIAM JOSEPH, SZABO, LOUIS FRANK
Applicants: STANDARD OIL CO OHIO, THE STANDARD OIL COMPANY
Application year: 1987
Main classification: H01L21
IPC codes: G06F9, H01L31, H01L21