US3519480A - Process for treating photoconductive cadmium sulfide layers - Google Patents

Description

United States Patent 3,519,480 PROCESS FOR TREATENG PHOTOCONDUCTIVE CADMIUM SULFIDE LAYERS Franz Trautweiler, Pittsford, and Otis G. Peterson, Rochester, 'N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Jan. 13, 1967, Ser. No. 608,957 Int. Cl. B44d 1/48; G03g 5/02 US. Cl. 117201 4 Claims ABSTRACT OF THE DISCLOSURE maximum by subsequently treating the overexposed cadmium sulfide layer to vapors of cadmium metal under similar conditions of elevated temperature and reduced pressure.

This invention relates to photography and in particular to processes for increasing the photoconductivity of vacuum-deposited cadmium sulfide layers or films.

Deposition of photosensitive compounds upon a support material by vacuum or reduced pressure evaporation techniques is old in the art. Cadmium sulfide is a well known photoconductor, and it has been deposited on suitable support compositions to provide light-sensitive photoconductive elements. The light and dark conductivity of evaporated cadmium sulfide film layers can be determined, and it is the ratio between the values for light and dark conductivity (hereinafter termed L/D) which provides a commonly employed index of photoconductive utility. The L/D ratios for evaporated cadmium sulfide layers are extremely low, however, and such layers, without more, are generally not susceptible of suitable use where photoconductors can be advantageously employed.

The resultant of vacuum deposition, wherein the usual substrate temperatures are not amenable to a favorable growth rate of both the deposited layer and the crystals contained therein, is a cadmium rich, non-stoichiometric layer, wherein an excess of electrons not involved in bonding operates to increase dark conductivity. Additionally, the many small crystals formed on evaporation produce intercrystalline barriers which lower electron mobility and thus lower the potential light conductivity. These two factors combine to decrease the L/D ratio and hence photoconductive utility.

Accordingly, it is well known thta suitable post-evaporative treatment, generally annealing at elevated temperatures (which often range to 600 C. and higher) and in the presence of copper or silver doped cadmium sulfide powder, causes the substitutional diffusion of silver or cuprous copper into the vacuum-deposited cadmium sulfide layer, which diffusion will continue until the layer and the powder are in equilibrium.

The silver or copper functions as an electron acceptor with reference to cadmium, since an additional electron is required to form a bond with sulfur. As such, the number of unshared or non-bond-involved electrons decreases, thus descreasing dark conductivity and raising the L/D ratio. Moreover, recrystallization of the vacuum-deposited sulfide layer can occur at suitably elevated temperatures, wherein selected crystalline growth is experienced at the expense of surrounding crystals. Such crystalline growth 3,519,480 Patented July 7, 1970 lowers the number of total crystals in the layer, which, in turn, reduces the number of intercrystalline barriers and increases the L/D ratio by raising the maximum light con ductivity.

Diffusion techniques, however, involve the use of a dopant, such as silver doped cadmium sulfide powder, which must be prepared separately. Additionally, the silver or copper is diffused into the evaporated camium sulfide layer at a severely limited rate, since the ratio of silver or copper to cadmium sulfide powder is extremely low. Treating temperature can be raised to increase the diffusion rate, but a more elevated treating temperaturue narrows the range of suitably employed support materials to such compositions (quartz, for example) as will Withstand the increased heat without softening.

Increased treating temperature also tends toward a greater incidence of pinholes, small points where the vacuum-deposited cadmium sulfide layer has burned through, which provide conducting paths and promote shorting through the layer when a charge is placed across it. Then too, when diffusion is carried out under vacuum and at a temperature of about 550 C. for about 2 /2 hours, a cadmium sulfide layer of five microns or less completely evaporates off of the support. Extending the treating period without raising temperature increases the amount of copper or silver which is diffused intothe vacuum-deposited cadmium sulfide layer, but this too is accompanied by increased pinholing.

Alternatively, a thin layer of copper metal or silver metal can be vacuum deposited upon the surface of such cadmium sulfide layers as are described above, after which the contiguous, vacuum-deposited layers of cadmium sulfide and electron acceptor are annealed in an inert gas such as argon. This diffusion technique permits lower treatment temperatures and shorter treating time. When the copper or silver metal is annealed in directly, however, a co-activator must be annealed in concurrently to provide charge neutrality; such a coactivator in this case must be a lattice defect which adversely affects the photoconductivity. Additionally, such a procedure diffuses the electron acceptor into the cadmium sulfide layer at a non-uniform rate, causing the formation of conductive areas Where higher concentrations of the sulfide of said electron acceptor are present. In such fashion, dark conductivity is increased, which decreases the L/D ratio. Shortening the treatment period or lowering the treating temperature prevents the formation of exess sulfide, but then the L/D- ratio of the vacuum-deposited cadmium sulfide layer is not sufficiently increased.

Accordingly, an object of the instant invention is to provide a novel, post-deposition reaction for increasing the photoconductivity of vacuum-deposited cadmium sulfide layers, wherein an electron acceptor can be introduced into the cadmium sulfide layer without use of a doped carrier.

Another object of this invention is to provide a new post-deposition reaction for increasing the photoconductivity of vacuum-deposited cadmium sulfide layers wherein an electron acceptor can be introduced into the cadmium sulfide layer at a rapid rate.

Another object of the present invention is to provide a novel post-deposition reaction for increasing the photoconductivity of vacuum-deposited cadmium sulfide layers wherein an electron acceptor can be introduced into said cadmium sulfide layer at lower treatment temperatures.

An additional object of the present invention is to provide a novel post-deposition reaction for increasing the stoichiometry of vacuum-deposited cadmium sulfide layers.

Still another object of this invention is to provide a new post-deposition reaction for increasing the photoconductivity of vacuum-deposited cadmium sulfide layers wherein an electron acceptor can be introduced into said layer without any significant tendency to promote pinholing.

Yet another object of this invention is to provide a novel, post-deposition reaction for increasing the L/D conductivity ratio of thin, vacuum-deposited cadmium sulfide layers wherein the effect of the reaction is readily reversible.

These and other objects of the invention described herein are accomplished by reacting thin, vacuum-deposited cadmium sulfide photoconductive layers with the vapor of a cuprous halide or a silver halide, such reaction being carried out at an elevated temperature and reduced pressure. At the optimum duration of the reaction, a maximum L/D ratio is obtained, and continued reaction results in a lessening in L/D ratio from the previously obtained maximum. This falling oil? of L/D ratio is subject, however, to reversible regulation; a subsequent contacting of the cadmium sulfide layer with vapors of cadmium metal, under conditions of elevated temperature and reduced pressure similar to those noted for the prior reaction with a cuprous halide or a silver halide vapor, can raise the L/D ratio from the low level produced by the over-reaction of such cadmium sulfide layer with vapor of a cuprous halide or a silver halide.

The halide salts suitable for practicing the invention described herein are the cuprous and silver halides, such as cuprous chloride, silver chloride, cuprous bromide, silver iodide, silver fluoride and the like, silver chloride being preferred.

Thin, vacuum-evaporated cadmium sulfide layers which are advantageously reacted, as described herein, to increase their L/D ratio, and hence, their photoconductive utility, can be prepared by any of the well-known techniques for binderless deposition of photosensitive compositions such as silver halide or cadmium sulfide upon a support material by vacuum-evaporation means. Such means are described, for example, in US. 1,970,496 and in a co-pending US. application, Ser. No. 428,204. More generally, such deposited layers vary in thickness from about one micron to about five microns, with one and one-half to three microns being preferable. At thicknesses of substantially less than one micron, pinholes are easily formed during the post-deposition reaction, and with thicknesses substantially greater than five microns, light absorption in the cadmium sulfide layer is obtained only in about the first five microns of thickness, with the resistance of the unexposed portion preventing the increase of light conductivity to an acceptable degree.

A wide variety of suitable support materials of the type commonly utilized in electrophotography can be used for the cadmium sulfide layer. Advantageously employed nonconducting supports include, for example, quartz, hard and soft glass, ceramics and ceramic coated metals. Conducting support materials which can be used include metals such as aluminum, titanium, palladium and others. Glass coated with tin oxide, gold, or other conducting materials is also advantageously employed as a conducting support. Such supports are chosen to withstand the reaction temperature of the invention.

Post-deposition reaction is typically carried out by plac ing a suitable support, upon which has been deposited, by vacuum evaporation means, a thin layer of cadmium sulfide, into a sealable enclosure, which combination of enclosure and cadmium sulfide coated support is then placed into a furnace or other suitable heating means. Also, a silver halide or a cuprous halide is placed inside the enclosure, along with the cadmium sulfide coating. After sealing the enclosure, it is heated to a temperature which can vary from about 350 C. to about 500 C.,- and evacuated to a pressure which is not greater than the vapor pressure of the halide salt at the selected reaction temperature. Reaction time of the cadmium sulfide and 4 the halide compound is variable and depends largely upon reaction temperature.

The reaction mechanism is not known positively, but one explanation is that, upon heating, the silver or copper of the halide displaces cadmium from its sulfide, producing silver sulfide in the vacuum-deposited cadmium sulfide layer since silver or copper is substantially interposed in place of cadmium. Cadmium halide is also formed, but this apparently passes oif as a vapor and does not affect the cadmium sulfide layer. Additionally, the passing off of cadmium halide prevents the detrimental introduction of a halogen, an electron donor, into the cadmium sulfide layer. Silver and copper, as electron acceptors with reference to cadmium, involve additional electrons in bond formation; this effects a decrease in free electrons, producing a decrease in dark conductivity, and hence, a rise in the layers L/D ratio. The elevated temperatures can also produce a recrystallization of the evaporated cadmium sulfide layer, wherein selective crystalline growth at the expense of neighboring crystals lessens inter-crystalline barriers and tends to increase the L/D ratio by raising light conductivity. These two reaction efiects result in an increase in the cadmium sulfide layers L/D ratio. The introduction of a large amount of silver or copper metal into the cadmium sulfide layer, however, results in the production of excess silver sulfide or cuprous sulfide, whose high inherent conductivity dominates over the advantageous lessening of free electrons and operates to raise dark conductivity and lower the L/D ratio.

Heating is continued, then, for a duration sufiicient to raise the L/D ratio of the evaporated cadmium sulfide layer to its maximum, but not for a sufficient time so that the L/D ratio (since excess cuprous sulfide or silver sulfiide has been formed), declines from a previously obtained maximum. During the period of heating, the yellow color of the cadmium sulfide layer shifts, moving toward that of the darker colored silver sulfide or cuprous sulfide, although the predominant color remains that of cadmium sulfide. When heating is ceased, the enclosure and its contents are cooled.

If the cuprous halide or silver halide reaction is continued to a point at which something less than a previously obtained maximum L/D conductivity ratio is obtained (the subsequent fall-off being presumably due to the presence of excess silver sulfide or cuprous sulfide in the vacuum-deposited cadmium sulfide layer), the decrease can be efiicaciously reversed, and the L/D ratio raised by contacting the deposited cadmium sulfide layer, which bears excess silver sulfide or cuprous sulfide, with cadmium metal vapor. Such an operation is carried out by placing the over-reacted vacuum-deposited cadmium sulfide layer in an enclosure along with cadmium metal. The enclosure is sealed, heated to a temperature which can vary from about 200 C. to about 500 C. and evacuated to a pressure which is not greater than the vapor pressure of cadmium metal at the chosen temperature. After heating for a period of time sufiicient to advantageously raise the L/D ratio, the reaction is stopped by reducing the temperature. Cadmium metal evaporates rapidly under conditions of temperature and pressure as described herein; if evaporation is complete prior to the lowering of the reaction temperature, then for the remainder of that period of heating, the vacuum-deposited cadmium sulfide layer (which has been overreacted with vapor of a cuprous halide or a silver halide) is exposed to reduced pressure and elevated temperature.

The heating in the presence of cadmium vapor appears to convert part of the silver sulfide or cuprous sulfide to silver metal or copper metal and cadmium sulfide, with such silver metal or copper metal then tending to evaporate out of the cadmium sulfide layer. Whatever silver or copper metal remains in the cadmium sulfide layer, does so as discrete specks which do not promote shorting through the cadmium sulfide layer when a charge is placed across it, or otherwise deleteriously affect its photoconductive properties.

Measurements from which the L/D conductivity ratios referred to herein are calculated are taken first in resistivity units by Van der Pauw technique as described in Phillips Research Reports, 13, 19 (1958), and then converted into the reciprocal conductivity values.

The practice of the invention described herein is further illustrated by the following examples, which represent preferred embodiments of the present invention and should not be considered as limitations thereof.

EXAMPLE I A piece of soft glass, upon which a 2 micron layer of cadmium sulfide (with an L/D ratio of 1.5) had been deposited by vacuum evaporation means (175 C., 10- torr) was placed inside a glass tube wherein it faced crystals of solid silver chloride at a distance of 2 cm. The tube was sealed, heated to 400 C. and evacuated to a pressure of 10' torr, after which the sample and silver chloride were held at that temperature for 2 hours and 50 minutes, during which time the yellow cadmium sulfide layer darkened. Upon cooling, the L/D ratio was No pinholing was observed. Similar results can be obtained with cuprous chloride.

EXAMPLE II According to the procedure of Example I, a similar sample was reacted with silver chloride for 2 hours and 50 minutes at 400 C. and a pressure of 10' torr. After cooling, its L/D ratio was 10 EXAMPLE III According to the procedure of Example I, a similar sample was reacted with silver chloride for 3 hours and 10 minutes, at 400 C. and a pressure of 10 torr. After cooling, its L/D ratio was 10 EXAMPLE IV A piece of soft glass, upon which a 2 micron layer of cadmium sulfide (with an L/D ratio of 1.5) had been deposited by vacuum evaporation (175 C., 10* torr) was placed inside a glass tube wherein it faced crystals of solid silver chloride at a distance of 2 cm. The tube was sealed, heated to 425 C. and evacuated to a pressure of 10- torr, after which the sample and silver chloride were held at that temperature for one hour and 37 minutes, during which time the yellow cadmium sulfide layer darkened. Upon cooling, the L/D ratio was 7x10 The sample was then placed in another glass tube together with some cadmium metal, after which the tube was heated to 400 C. and evacuated to 10- torr, where it was held for 50 minutes and cooled to room temperature. The cooled sample then had a more yellow color. No pinholing was observed. The L/D ratio after the treatment with vapor of cadmium metal was 1.3 10 Similar results can be obtained with cuprous chloride.

EXAMPLE v 0 ments thereof, modifications and variations can be effected within the spirit and scope of the invention as described above and as defined in the following claims.

We claim:

1. A process for improving the photoconductivity of thin, vacuum-deposited cadmium sulfide layers which comprises reacting said cadmium sulfide at a temperature of from about 350 C. to about 500 C. and a reduced pressure with vapor of a halide selected from the group consisting of cuprous halide and silver halide, said reduced pressure being not greater than the vapor pressure of said halide at the reaction temperature.

2. A process as described in claim 1 wherein the halide is silver chloride.

3. A process as described in claim 1 wherein the thickness of said deposited cadmium sulfide is about one micron to about five microns.

4. A process for reversibly increasing the photoconductivity of vacuum-deposited cadmium sulfide layers which have been over-reacted with vapors of a halide selected from the group consisting of cuprous halide and silver halide which comprises reacting said over-reacted vacuumdeposited cadmium sulfide with vapor of cadmium at a temperature of from about 200 C. to about 500 C., and a presure not greater than the vapor pressure of cadmium metal at the reaction temperature.

References Cited UNITED STATES PATENTS 2,765,385 10/1956 Thomsen 252-501 X 2,995,474 8/1961 Pearlman 961.5 X 3,202,609 8/1965 Reisman et al. 252501 3,238,150 3/1966 Behringer et al. 252-501 3,355,321 11/1967 Vecht 117201 X 3,361,591 1/1968 Dill et al 117-201 3,377,200 4/1968 Chamberlin et al. 117201 3,379,527 4/1968 Corrsin et al. 96-1.5' 3,391,021 7/1968 Esbitt et al. 117201 GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant Examiner U.S. Cl. X.R.