United States Patent (19) 11) 4,331,707 Muruska et al. 45 May 25, 1982 (54) PROCESS FOR THN FILM DEPOSTION (56) References Cited OF CADMUMSULFDE U.S. PATENT DOCUMENTS (75) Inventors: H. Paul Muruska, East Windsor; Joseph L. Sansregret, Scotch Plains; 4,095,004 6/1978 Fraas et al.......................... 427/74 Archie R. Young, Montclair, all of Primary Examiner-James R. Hoffman N.J. Attorney, Agent, or Firm-Paul E. Purwin 73 Assignee: Exxon Research & Engineering Co., 57 ABSTRACT Florham Park, N.J. The present invention teaches a process for depositing layers of cadmium sulfide. The process includes depos 21) Appl. No.: 197,082 iting a layer of cadmium oxide by spray pyrolysis of a 22 Filed: Oct. 15, 1980 cadmium salt in an aqueous or organic solvent. The oxide film is then converted into cadmium sulfide by (51) Int. Cl'......................... B05D 1/02; B05D3/02; thermal ion exchange of the O-2 for S-2 by annealing B05D 3/04 the oxide layer in gaseous sulfur at elevated tempera (52) U.S. C. ........................................ 427/74; 427/84; tutes. 427/85; 427/87 58 Field of Search ........................ 427/74, 84, 85, 87 22 Claims, No Drawings 4,331,707 1. 2 sulfide ion whereby a thermally induced ion exchange PROCESS FOR THIN FILM DEPOSITION OF of the sulfur for oxygen produces cadmium sulfide. CADMUMSULFIDE A spray solution comprises a mixture of at least one cadmium salt in solution with either water, an organic The subject invention was made in the course of 5 solvent or a combination thereof. Representative cad research performed for the Solar Energy Research mium salts include cadmium nitrate CdCNO3)2, cad Institute Under Subcontract No. XS-908104-1 from mium acetate CdC2H3O2), and other Cd salts of oxy Prime Contract No.EG-77-C-014042. gen containing anions. Representative organic solvents include methanol, BACKGROUND OF THE INVENTION 10 ethanol, isopropanol and acetone. A solution concentra The present invention relates to thin films of cad tion of cadmium salt ranging from about 0.01 M to mium sulfide and in particular to a chemical spray depo about 0.5 M has been evidenced as operable in the pres sition process employed in conjunction with a thermal ent invention. In a preferred embodiment a solution ion exchange process for producing cadmium sulfide, concentration of about 0.2 M produces strongly adher abbreviated herein as CdS. 15 ent films of cadmium oxide using the spray deposition Cadmium sulfide is a well-known photoconductive parameters set forth hereinafter. semiconductor. Its principal use has been in photodetec In its basic sense, the oxide film is formed by pyrolysis tors and photovoltaic solar cells (CdS/Cu2S cells). In of the cadmium salt; for example: thin film form, these devices display relatively high efficiency at potentially low fabrication costs. Conven 20 tionally, these films of CdS are constructed by one of Cd(NO3)2 - A S Cdo + 2NO+ io, two techniques; at the Institute for Energy Conversion (University of Delaware) for example, vacuum evapo It is known in the art that film quality is often ration techniques are used to deposit 20-40 micron lay strongly dependent on the specific chemistry involved ers of CdS on metal substrates. A second technique, 25 in the film growth process (see for example; Journal of used commercially by Photon Power, Inc. of El Paso, Vacuum Science and Technology, J. Ceranovich et al., Texas, spray deposits CdS from a solution containing 16, 994, (1979)). In the direct spray deposition tech CdCl2 with thiourea as a sulfur source. Specific exam niques known in the art, the impinging flux of molecules ples of this technique may be referenced in U.S. Pat. must undergo a chemical reaction on the deposition Nos. 4,086,101 and 4,095,006. Both of these techniques 30 surface. For example, when CdS is deposited with thio produce cadmium sulfide, CdS, directly. urea as a sulfur source, the sulfur reducing conditions in In contrast, the present invention teaches a process the vicinity of the spray impact must be carefully main for chemical spray deposition of a layer of cadmium tained, typically by control of the spray solvent. In oxide which is converted to cadmium sulfide by thermal contrast, the herein taught formation of an oxide film is ion exchange. The deposition of the intermediary oxide 35 thermodynamically favored and the resultant sulfide is thermodynamically more stable then directly deposit film is both strongly adherent and of controlled compo ing the sulfide, resulting in superior film quality. As sition. The spray solution is atomized by a conventional taught with respect to differing metals and metal oxides, spraying device, modified to permit control and moni in a co-pending application, U.S. Ser. No. 31,421 filed tor of fluid and air flow rates. A suitable substrate, Apr. 29, 1979 and assigned to the assignee of the present 40 heated to a temperature ranging from about 300° C. to invention, a more precise control of the film composi about 500 C., is contacted with the atomized solution tion may be readily obtained by first depositing an inter to form the oxide film. The substrate comprises a ther mediary oxide layer. mally conductive material capable of retaining its physi SUMMARY OF THE INVENTION cal integrity throughout the range of deposition and 45 annealing temperatures. The selection of substrates is The present invention teaches a process for deposit generally dictated by the intended application of the ing films of CdS by first forming a layer of cadmium completed semiconductor device. In photocells, for oxide by spray pyrolysis of a cadmium salt in air and example, optically transparent substrates such as spe secondly converting the oxide to the sulfide by thermal cialty glass, sapphire, quartz or the like may be used ion exchange. Spray deposition in air of solutions of 50 where intended illumination would be through the sub cadmium salts in water and/or methanol produce an strate (so called Backwall Cell). Electrical contact to intermediary cadmium oxide layer which is thermody the cadmium sulfide underlayer may be achieved by namically favorable compared to direct deposition of first coating the transparent substrate with a metallic CdS. Thermal ion exchange comprises annealing the grid or a conductive oxide, such as SnO2 or In2O3, oxide in the presence of a sulfide ion to produce a cad 55 means for which are well-known in the art. Metal sub mium sulfide layer having preferred electrical and crys strates such as molybdenum and tungsten or ceramic tallographic properties. substrates coated with a suitable metal for contacting The CdS layers as taught herein may be used in pho the back layer may be used where substrate transpar toconductive applications such as CdS/Cu2S hetero ency is not contemplated in the finalized device (a so junction solar cells. called Frontwall Cell). DETAILED DESCRIPTION OF THE To promote thermal decomposition of the spray solu INVENTION tion, the substrate temperature is maintained, as a mini mum, at about 300 C. The measurement of this temper In accordance with the teaching of the present inven ature, however, is at the substrate/heater interface. The tion, cadmium sulfide films are fabricated by first depos 65 actual film temperature during the deposition process iting a layer of cadmium oxide by spray pyrolysis of a will be proportionally lower. In a preferred embodi cadmium salt solution followed by an annealing of the ment, however, the substrate temperature is maintained oxide layer at elevated temperature in the presence of a at about 450° C. to assure complete pyrolysis of the 4,331,707 3. 4. spray solution. To accomplish this, there is required embodiment, the oxide film is annealed in H2S, hydro both a relatively high value of thermal conductance gen disulfide gas at about 500 C. for a time period between the heat source and the surface of the substrate depending upon oxide film thickness. As presently un to be coated, and a sufficient thermal mass to sustain the derstood, the conversion reaction rate is predominantly substrate temperature throughout the thermal extrac controlled by the rate of diffusion of the sulfide ion into tion experienced during the spray deposition process. the film (or the diffusion of oxygen ions to the film This has been achieved by either floating the substrate surface). At about 350° C., a conversion rate of cad in a molten metal maintained at the desired temperature, mium oxide to cadmium sulfide in 20% by weight of or, alternatively, using a vacuum chuck to hold the H2S is about 0.3 pl/hr. At about 500 C., the conversion substrate in contact with the heat source during the O rate is about 1.3.pl/hr. The activation energy for the spray process. process is 21,000 cal/mole. X-ray diffraction may be In the spray deposition of the liquid solution, a num used to confirm the extent of conversion to cadmium ber of alternative conventional atomizing devices may sulfide. The CdS film is generally characterizable as be used. For example, ultrasonic, pneumatic, electro being golden yellow in color and the film maintains a static or vibrational atomizing devices may be used to 15 strong adherence to the substrate. The converted film is provide a spray or mist of the liquid solution. It is gener crystalline having relatively large crystallites ranging ally preferred in the practice of the present invention to from about 0.5 micron to about 1 micron. The CdS film use a pneumatic spraying device, in which the gas used is of hexagonal wurtzite structure, evidencing a pre to atomize the liquid solution is compressed air.