CA1055300A - Electrophotographic film member with r.f. sputtered inorganic photoconductor - Google Patents

Abstract

ABSTRACT

An improved electrophotographic film of the type which comprises a thin film coating of an inorganic photoconductive, electronically anisotropic material such as cadmium sulfide, r.f. sputtered onto a thin, stable substrate member, a thin film ohmic layer inter-posed between the substrate member and the photoconductive coating and an ultra thin film layer of r.f. sputter-coated photoconductive material between the substrate and the ohmic layer and serving as a bond enhancing medium between the same.

Description

~L~S~3~
In earlier applications th~Le was proviclecl an electro-photographic film member which i5 not only capable oE being u-tilize(l Eor -the same purposes as conventlonal xerographic and elec-troEax memb~rs but also is capable of being used in the same manner as silver halide ernulsion photographic films.
The electrophotographic film comprises a thin film coating of an inorganic, photoconductive, electronically anisotropic material such as r.f. sputtered cadmium sulfide bonded to a thin film layer of ohmic ma-terial, such as indium oxide which in turn is honded to a thin, stable substrate memher, preferably of flexible, plastic shee-ting.
- The thickness of the photoconductive coating is about 3000 Angstroms, of the ohmic layer, about 500 angstroms ; 15 and of the substrate member, about à fraction of a milli-meter. The resulting electrohotographic member has a hard, abrasion resistant surface, is highly transparent and is flexible notwithstanding the fact that the coating is microcrystalline. It has high photoelectric gain with speed and sensitivity to enable its use in high speed photography. As such it can accept a charge at a rapid rate and will retain the same selectively after exposure to enable practical toning with an almost infinite gradation of pigment values.
It is desirous nevertheless to improve upon this already superior electrophotographic member.

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Accorclingly, the invention herein provides an electrophotographic member o~ the type inc]ucling substrate means, a thin ~ilm coatincJ of a wholly inorganic, r.f.
sputtered, photoconductive material on said substra-te S means, sald coating being very dense, microcrystalline, substantially transparent, having a dark resistlvity of at least 10 ohm-centimeters and a ratio between dark and light resistivity oE at least 10 , having the capability of accepting a rapid charge and retaining same to enable toning and being electrically anisotropic,and a thin film layer of ohmic material sandwiched between the coating and substrate means for facilitating charging of said coating before exposure; bond enhancing means comprising an ultrathin film layer of a transparent wholly inorganic material between the ohmic layer and the substrate means.
The preferred embodiments of this invention will now be described, by way of example, with reference-to the drawings accompanying this specification in which the figure is a diagrammatic cross-section of the electro-photographic film according to the invention.
' Before reference is made to the drawings for a de-tailed description of the invention herein, certain expressions that are used hereinafter and in the art to define observed phenomina should be reviewed.
The expression "thin film" is used herein, both in the specification and in the claims. As a general rule t~he scientific literature in some way attempts to def:ine 10553C~) ~hin fi1m in terms oE ~he properties oE the substance beiny discussed, calling attelltion to those properties in contrast to the properties of the same substance in bulk. This la-tter is called "bu1k properties" herein. Speaking in relatively simple terms, some ma-terials act diEferently when constituted as a "skin" than they clo as a "body".
Reference may be had, for example, to a publication entitled "Thin Films" by Leaver & Chapman, Wy]ceham Publications (London)Ltd.) 1971 for a general discussion of the differences between thin film and bu1k properties of the same type of material. In that publication, the thickness of a "thin film" is given as "usually less than one micron". This general definition is required in view of the breadth of the subject covered in that publication.
When one considers the purposes and requirements of ; -the structures in which a certain category of material is to be used, the boundary or boundaries between the thin film and the bulk properties must take these purposes and requirements into consideration. Properties which are of no importance or interest to the solution of a problem do not enter into the matter and hence should not establish the physical criteria. For example, if a radical change in the sound reflecting property of a certain material occurs when the material is made about 2 microns thick and less 2S because of skin effect, then if that ma-terial is going to be used in an environment which uses the sound reflecting property, it is e~hibiting a thin film effect. On the .. .

553~t) other h~nd, :L:E that identi~al mate.rial changes i.ts .resistivi.ty radically only when iks thickness i5 decreclsed to .5 micxons or less then, for the condi-t.ions of use in which its resist:ivity is o.E importance, the material is still a bulk material at thicknesses grea-ter than about .5 micron.
The use of the materials involved here.in relate to several properties which are beneficial and advantayeous for the invention, and the meaning of the expression "thin film" as used herein will be related only to these properties, irrespective of the properties of any other materials for any other purposes which may have been referred to as thin films in thicknesses other than those which will be defined The words "thin film" ~hen used in the specification and in the claims wi.ll be taken to mean a thickness at which the desired properties of the material in question cease acting as bulk properties and commence acting as a skin or thin film. The thickness in all known examples which have actually been made is substantially less than a micron 20 . (lO,000 Angstroms).and ver~ few of the coatings or layers tested exceed 5,000 Angstroms. Accordingly, a "thin film"
will be considered one that is substantially less than a micron thick.
The expression "photoelectric gain" as used herein has a meaning requiring explanation. The speed and efficiency of an electrophotographic member is d~rectly rblated to the "hole-electron pairs" produced when 1~)553()V
subjec~ed to ]ight. In prior art photoconductiv~ coati.nys usecl in xero~raphy or electrofax, it requires many photons (extreme]y br:ight light) to produce a si.ny].e hole~elec-tron pair. The number is usu~lly upwards o:E a thousand~ It ~ollows that if an el.ec-trop}lo-tographic ~ilm can produce a hole-election pair upon the incidence of a single photon or even two photons its "photoelectric gain" is very substantially great. Aceordingly, in order to provide an expression for the gain of the eleetrophotographic members of the type with whieh we are concerned "high photoelectric gain" will be intended to mean a condition in which, at most, several photons are re~uired to produce a single hole election pair. The term "high photoelectrie gain"
implies also the ability of the member to which the term is applied to permit the recom~ination of the pairs result-ing in discharge.
The expression "electrophotographic film" or "photo- ::
graphic film" as used herein is intended to mean a eomplete article with several layers or lamina for use in some photographic process. Reference to the substrate or subtrate member or substrate means will not include the use of the word "film" although the substrate whieh is eontemplated by the invention eould be eonsidered a film in the ordinary meaning of the word. As will be seen, it is preferred that the substrate be a thin flexible transparent member of plastie sheeting, eommonly known a~s plastic film.

~.~)$5300 The improved electrophotographic Eilm or the herein invention comprises a -thin E:ilm coating oE a wholly inorganic, crystalLine, r.E. sputtexed photoconductive material overly:ing thin Eilm layer oE an ohmic or conductive material which in turn is bondecl to an ul-txathin coating of the same ~ype of r.f. sputtered photoconductive material as above mentioned applied to the substrate means. The preferred form of substrate means is a thin, flexible, insulative, plastic sheeting of high stability~ such as polyethylene terpthalate (sold under the well known Trademark "Mylar").
The photoconductive coating or layer 12 is the most important element of the improved electrophotographic film as well as the basic ~ilm since it represents the func-tional and physical characteristics which make the same advantageous over the prior art.
The material from which the photoconductive layer or coating is made and which will be described in detail below is one of several known photoconduc-tive compounds.
These compounds have been used in the past, but so far as known, have not been successfully incorporated into an electrophotographic member having the properties of the member of the t~pe concerned herein. For example, the preferred compound which will be discussed in considerable detail below is cadmium sulfide. The compound had been incorporated in thick photoconductive coatings comminuted a~d embedded in organic matrices and has even been 16~5S3~
reactively sputtere~d in Eorming wholly inoryanic coatinys, but without achievement oE the advantageous results characteristic of the herein descîibed and the earlier electrophotographic mem~er oE which this is an improvement.
Like the earlier electrophotographic members of the type concerned, the best results here have been with cadmium sulfide (CdS). Other photoconductive materials are suitable such as zinc indium sulfide (ZnIn2S~), arsenic trisulfide (As2S3), zinc selenide (znSe), zinc sul~:ide (ZnS), zinc telluride (ZnTe), cadmium selenide (CdSe), cadmium telluride (CdTe), yallium arsenide (GaAs), antimony trisulfide (Sb2S3) and perhaps others.
The ~ollowing are characteristics of the earlier as well as the subject improved electrophotographic member.
The photoconductive covering is wholly inorganic microcrystalline and several thousand Angstroms thick. The only known useful cadmium sulfide coatings have been mixtures with organic binders and matrices, of great thickness and no substantial transparency or flexibility.
The photoconductive coating herein is deliberately made O O
crystalline and thin - being 3500 A to 5000 A in thickness -and is thus extremely flexible and transparent. The conduction of electrons and holes through the coating is enhanced by the manner in which the coating is produced.
25 ~ The crystals are believed to be vertically oriented, that is normal with respect to the surface upon which same are deposited, this resulting from a sputteri~lg process in S53~t~

wh:ich a second dark space is establlshed between the plasma and anode ln addition to the cathodic dark space conventionally achieved usln~ r.f. sput~ering techniques.
It has been found that the "edge effect" characteristic of prior art xerography, -~or example, is eliminated to a substantial degree in toning the surEace of the photo-conductive layer 12 of the invention. This "edge effect"
consists of the cen-ter of a reproduction of an image having a solid pigmented area being light and the edges being dark.
The larger the area the more obvious the results of the "edge efEect" so that large solid areas that are required to be black throughout come out while in the center. Photographs are impossible to reproduce with even a fraction of their oxiginal quality without the use of relatively course screens overlying the original. Negative originals, that is, documents which are illustrated as fine white lines on a black background are incapable of being reproduced with the modern xerographic and electrofax methods because of this "edge effect".
The earlier type as well as the subject electrophoto-graphic film as described is capable of faith-Eully reproducing documents and photographs without the use of any intervening screens and without toner biasing. This includes negatives which come out clear and sharp without "edge effect". Toner biasing practically eliminates any v,estiges of "edge effect" making possible the very highest quality of photographic reproduction. As a matter of fact, .

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the qua]ity oE which tlle photoconductive coatiny 1~ is capable is greater than that available from most ordinary photographs today because the latter have grain oE
macroscopic size while the only limi-ting factors for the texture of reproduced images on the coating 12 is the size of the toner particles and the size of the crystals comprising the coating. Both of these are typically of the order of a small fraction of a micron, that is microscoplc.
It is believed that this benefit is achieved because each crystal is arranged perpendicular to the substrate and forms an individual ~ield when affected by electrons in its vicinity. The toner particles are thus attracted by the myriads of fields and not to the areas where the gradients between presence and absence of charge are the greatest. This latter condition is the reason for "edge effect" commonly experienced.
As an example of the flexibility which is achieved, when the photoconductive and ohmic layers are deposited upon a sheet of flexible polyester .005 inch in thickness, the resulting electrophotographic film can be wrapped around a cylinder 0.25 inch in diameter without cracking or crazing even though the photoconductive layer is crystalline. The ability to be wrapped around cylinders a fraction of an inch in diameter is representative of the ability of transporting the electrophotographic film through handling `
and display machinery without problem.
Another characteristic which is related to the ~act ~)553(~0 :
that the layer 12 is inorganic, thin and crystalline in character is lts extreme density and hardness. rrhe surEace is mentioned above as being hard as glass. Abrasion resistance is important in handling the film since it obvîates scratches, scores and the like ~hich can cause loss of detail and data, especially in fine subject matter. In the manufacture o~ ~he electrophotographic film no difficulties are met ~here it is necessary frictionally to move the same by engagement of the surface by friction rollers and the like.
The abrasion resistance of the photoconductive coàting 12 is believed related to ~ e density of the compound oc-casioned by the manner in which it is deposited. This ~-radically improves the electrical properties as well, over ;
known coatings. ~
The material is electrically anisotropic due to its ;
thinness and semiconductor properties, among other reasons.
This means that the material will, at least for a suhstantial period of time retain a nonuni-form charge `~
pattern applied thereto or produced therein as re~uired in ~ , its utilization electrophotographically and as a photo-conductor. It also means that the finest resolution pattern can be accurately and faithfully produced in the latent image.
The coating 12 has a high photoelectric gain (as defined hereinabove) characteristic. Thus, instead o the large number of photons being needed to create a hole-electron pair in the photoconductors of the prior 11~ ' l~S5300 art, only one or two photons are needed to drive the charge carriers to the trapping or recombination centers, thus producing a coating of much greater electrophoto-graphic efficiency. This mechanism is what is intended S to define "gain" as referred to herein. The "gain" for doped film of the invention is many times greater than that of undoped film.
The high gain charac-teristic is of importance because it increases the sensitivity of the electrophotographic --film of type concerned to a point where it is commensurate with the sensitivities of most high speed photographic films, but not necessarily with the same characteristic loss of detail due to large grain. There is no grain in the material of the invention, the crystalline structure being microscopic.
The gain increase of the photoelectric material of the type concerned is believed to be the result of the release of free electrons from energy levels in the for-bidden band oE the photoconductor and is exponentially ;
related to the thinness of the photoconductor. In other words, the thinner the layer, the greater the release of ~ ~;
electrons and the more sensitive the electrophotographic film.
Since the absorption of photons of light i5 needed ;~.~
-to discharge the photoconductive coating, it is clear that there must be a certain degree of absorptivity o~
visible light or other electromagnetic radiation by . ^ : , . . . -- .- :
- .. . . . -l~S5300 the photoconductive coat:ing. ~n the other hand, the gain is higher for thinner coa-tln-;s.
It becomes clear that the thickness of the layer 12 should be such that there is suEficient material to give the desired light absorptivity and abrasion resistance qualities and yet thin enough to give the desired gain.
What one can do is to deposit a thickness of the layer which gives the ma~imum of gain with the minimum of pxactical thickness. This is easily ascertained ex-perimentally for any given material by measuring the light absorption and gauging the abrasion resistance and strength by su.itable means, continuing to deposit the material until a practical compromise is made between these qualities and the desired photoelectric gain. -The requirements of light absorption must be met, in any event. ~ ' The photoconductive coating 12 has a high dark resistivity which promotes charge acceptance and charge retention~ The cadmium sulfide coating which is the preferred photoconductive coating is inherently n-type and ' in its purest form, as deposited according to the method described has a dark resistivity of 10 2 to 1014 ohm-cen~imeters. Its light resistivity is about 10 ohm-centimeters. Its energy gap is about 2.45 eV. These measures of resistivity are static and are made by known methods of bonding electrodes to the surface or surfaces of the photoconductive coating, applying d.c. voltage, ~553(~
measuriny current and computiny the values from the geometry.
The dark resistivity measurements are made in darkness.
It is pointed out, however, that this is done without a charge on the photoconductive layer. Since the photo-conductive layer of the type concerned is very thin, when ~ ~
the charge is applied to the surface it enters such surface ~ -and drives free carriers toward the ohmic layer. Its effect is felt to a great extent through the photoconductive layer.
Absent such carriers, during the period after charying, discharge is inhibited, and the dark resistivity should be increased. Dynamic measurement of dark resistivity can be efected by considering the dark decay characteristic i;~;; ;
to be a conventional RC discharge of a condenser, and comparlng such characteristics with the computed and graphed characteristics for different resistivities. Using such techniques it has been determined that the dark resistivity of a cadmium sulfide layer of the invention that has been charged is substantially increased at least several times ~ ~
at the beginning of the characteristic to as much as lO00 ~ -time ~hereafter. Obviously the dynamio ratio of dark to ~light resistivity also increases.
The reference hereinafter, both in the specification and claims to resistivities will be considered static.
As stated, the dark resistivity 15 10 to 10 ohm-centimeters and higher. So far as known, the resistivities of relatively thicker art photoconductive members of types other than the electxophotographic members concerned, are ,, . . - . . . .
. .. - .. . . . . .. . . .. . . . ..

~055300 `~`
the same or little diferent whether considered statically and dynamically.
The high dark resistivity of the coating 12 represents an e~cellent insulating material; the h:Lgh ratio of dark to light resistivity i5 of the order of 105 represents a radical change in the resistance. This coating is one which had a thickness of about 3500 A and an optical trans-missivity of between 70% and 85%. The conductivity increase when illuminated is related to the sensitivity of the ;;
coating.
Zinc indium sulfide, one of the other useful photo-conductive compounds has a dark resistivity of about the same order as that o~ the cadmium sulfide with a light resistivity somewhat higher so that the ratio is not as great. The energy gap of zinc indium sulfide is about 2.3. Its performance as a photoconductive coating is not as good as that of cadmium sulfide, at least in the electro-photographic films that were tested using the zinc indium sulide as the photoconductive layer.
Although not required, cadmium sulfide can be doped `~
~ .
with known dopants, such as, for example, minute quantities of copper, iodine and the like, to provide additional carriers of electrons. This should rsnder the coatiny even more n-type than the pure cadmium sulfide and give a greater gain.
It must bs understood that the proportions of the elements which make up the photoconductive layer must be ~S530~

stoichiome~rically correct, this being achieved by con-trol of the conditions of deposit. The dopant proportions, if dopant is used, must also be controlled, but since the entire layer is inorganic, conventional control methods make this feasible and relatively easy.
The photoconductive coating of the type describea which is made from cadmium sulfide is practically panchromatic.
Tlle photoconductive coating ofthis invention as well as the earlier electrophotographic member of the type described is easily deposited in the special manner which gives it its unusual properties. This guarantees uniform deposit and high speed controlled production.
The photoconductive coating L2 in all cases is deposited by r.f. sputtering in a vacuum chamber~ All o~
the materials which go to rnake up the coating, whether dopants are included or not, are introduced into the vacuum chamber. The materials are introduced either by way of the consumable target or by gases or sublimed compounds introduced into the atmosphere of the vessel after the process has been started. Stoichiometrically correct proportions are easily controlled by techniques which are known to result in a substantially perfect and uniform product.
The sputtering of the photoconductive layer 12 is a critical part of the invention in that, so far as known, the vast improvement over the prior art is achieved by establishing a second dark space. This can be done by ~0553~0 connecting the radio frequency circuit of the sputtering apparatus in a bias arranyement. In certain causes, the second dark space can be self-induced.
The characteristics ~hich have been described above are not exclusive, but are believed to be the most important.
Many other advantages accrue concurrent]y, either as a result of the characteristics which have been men-tioned or in ad~
dition thereto.
The ohmic layer 14 is a conductive layer that is ~-deposited on the substrate member 16 before the deposition of the photoconductive layer 12. Its primary purpose is ta facilitate the charging of the surface of the photoconductive layer. It also may serve to assist in bonding the photo-conductive layer to the substrate member. Under circum-lS stances that a p-type coating or layer 12 is used, the ohmic layer 14 may assist in discharge. In the use of the coating 12 to produce an electrophotographic film, the layer 14 is transparent.
This ohmic layer is very much thinner than the photo-conductive layer 12, preferably being of the order of 500 Angstroms. This thickness will not interfere with the transparency or flexibility of the final electrophotographic film product. It forms the interface between the photo-conductive layer 12 and the substrate member 16. It functions as one element of the capacitive circuit during çharging of the surface of the photoconductor.
A purity grade of semiconductor indium oxide either lOS53~)0 alone or co~bined with a small percentage ~about 10%) of tin oxide is a suitable material for use as the ohmic layer 14. It is easily bonded to aluminum edges or conducting strips. It is aLso easily and preferably applied by sputtering techniques in the same apparatus as used to apply the photoconductive layer. This latter is the method used to make the practical embodiments of the inventionO
Vacuum or vapor deposition may be used but will probably not provide as dense and smooth a layer nor one which is so well-bonded to the substrate.
The substrate member 16 is the carrier or mechanical support for the photoconductive layer 12, the ohmic layer 14 and as will be seen the bond enhancing layer 18. The mechanical properties are flexibility, strength, trans-parency, ability to adhere to the deposited layers and of great importance - stability. The stability refers to dimensional stability, stability in retaining thickness, stability in resisting any changes which may occur due to being subjected to the temperatures and elec~rical phenomena which occur within the pressure vessel during the depositing processes~ Resistance to abrasion is a good property to include in choosing the substrate material.
Polyetheylene terphthalate sheeting oE .005" thickness has been mentioned above as one example of substrate that has been satisfactory. This material is an organic polymer.

105S30~
of excellellt characteristics is such material made by the E. I. duPon-t cle Nemours company and sold under the Trademark "M~lar." Internal stresses thereof are preferably required to be removed prior to use, the process of doing so being referred to as normalization. This can be done by subjecting the film to a temperature of about 190 celsius for a period of about 30 minutes. Such steps are known.
The su~strate material should not have any occluded gases, and these can be removed by outgassing the same in suitable chambers. Likewise, the sheeting should be perfectly clean.
The above descriptions comprise the details concern- `
ing the principal elements of the electrophotographic film 10 o the electrophotographic film of the type lS concerned.
In particular, a major aspect of the improvement herein relates to the provision of an electrophotographic film member having a bonding layer 18 of ultrathln dimension, ; namely, of the order of 5~ to 300 Angstroms thick which is deposited directly upon the substrate between the ohmic layer 14 and the substrate 12. The adhesive affinity of the substrate or the overlying ohmlc and photoconductive . . .
.
layers 1~ and 12 respectively, is improved. The so-called bonding layer 18 is formed of cadmium sulfide r~f.
sputtered directly upon the substrate under the conditions as employea in the deposition of the photoconductive layer 12. It should be noted that the thickness of said bonding layer are of the order not readily measurable, even by -19- :

. , ~ .. ., ~ . . . . , :

30al ~e~nl ~les : :
interferometric ~ ~ but are estimated b~ comparison with the measurable thickness of the photoconductive coating deposited. The ohmic layer 14 of the order of 300 Angstroms preferably ic r.f. sputtered on the bonding layer 18, and the photoconductive layer 12 o-f cadmium sulfide r.f. sputtered upon the ohmic layer 14. The bonding layer ~ ;
18 of cadmium ~sulfide is believed to become effectively a part of the substrate but its thickness is such as to have no dlscernible effect on total transparency of the film member.
As shown in the Figure, in use, co~ can be made at 19 with the ohmic layer by reason of the photoconductive layer being noncoextensive with said ohmic layer, leavlng a portion exposed. The reference numeral 20 signifies a high voltage source and the reference numeral 21 represents a corona generator, the circuit being symbolic of a charging circuit for sub~ecting the photoconductive thin film layer 12 to a surface charge. ~ battery is no-t intended by s~mbol 21.
The cathode or target of such apparatus is formed of the material from which the layer is to be made, or several of the elements to be used. Other elements can be added by introduction into the chamber. In one example carried out for testing purposes the cathode was semiconductor grade' indium oxide. This was for the deposit of the ohmic layer 14. ~he cathode is spaced from the anode in accordance ~ith the physical characteristics of the particular chamber, ,-~0553~0 ~ ~ ~
considering the geometry, the voltages to be used, etc.
The chamber in the example was pumped do~l to near the lO torr pressure range. This, of course, is a substantial 1 ~ vacuum. The~ ~ltrapure argon, that is, containing less S ~ ~ than lO ppm ~ and N2 was admitted to the sputtering chamber through a servo-leak valve until a pressure of about 20 millitorr is achieved.
At a suitable point, the radio frequency field is established and the ionization of the argon produced electrons which bombard the target or cathode, knocking ''".. ~, "'.
the particles of indium oxide out of the target thereby producing the plasma vapor between the cathode and anode and carrying the particles toward the anode there to be deposited upon the previously deposited bond enhancing ~-layer on the substrate member.
This sputtering is carried out at a rate which is determined by the conditions within the chamber, typically about 15 to 40 Angstroms per second for a commercial ;~
version using approximately 1 to 2 square feet of target area. Thickness is monitored by optical means known in the art until a thickness o~ about 500 Angstroms is reached.
,. . , ~
The substrate member is now removed from the chamber ., ~ .
and passed into or placed within another chamber in production. I the process is a laboratory process or in very small production, the same chamber may be used, but the cathode or target must be changed. Likewise stringent steps must be taken to remove all possible residual ~:

:

1(~55300 material to avoid contamination Careful shielding oE the `~
target or targets and the plasma can minimiæe contamination in the chamber.
In any event, the substrate mernber 16 with its first coating of the ohmic layer 14 and a previous underlying bonding coating 18, in the case of the example being described being indium oxide alone or com~ined with tin oxide,is again mounted on an anode carrier or led over a rotating anode or the like.
For a photoconductive layer of cadmium sulfide,the cathode or target will be made out of cadmium sulfide or even cadmium alone. The pressure is first dropped to 10 6 torr before being adjusted to 20 millitorr with later admitted argon gas and hydrogen sulfide. The hydrogen sulfide provides the correct amount of sulfur to the vapox plasma so that a stolchiometrically correct proportion o cadmium and sulfur is deposited on top of the ohmic layer.
Actually, the hydrogen sulfide serves as a background gas . .
to counterbalance the vapor pressure of sulfur. Tbis prevents decomposition of the cadmium sul~ide and thus controls stoichiometry. It will be appreciated that in Z ~
both depositing procedureZs the rear surface of the substrate ~; member 16 is blocked or masked to prevent any deposit thereon in normal processes. A first dark space induced by a .~ ....
, 25 shield around the target inhibits side and back deposits.

In the case that a cadmium sulfide cathode is used, the àmount of hydrogen sulfide admitted is about 500 to 15,000 ': " . ~' ... . . ~ - - . . .

l~5S30~

ppm in argon. In other cases when a caclmium cathode is used, these proportlons may be increased. The final pressure o~
deposit was between 7 and 15 millitorr.
A small arnount of copper deposit in the form of sub-limated copper chloride may be admitted into the sputterlng chamber, this being effected by keeping -the copper salt in an evacuated vessel which communic~tes with the sputtering chamber through a control valve. Copper is the dopant in this case, increasing the trapping levels in the inherently n-type cadmium sulfide. Hydrogen iodide may alternatively be used to provide iodine dopant to give additional trapping levels in the cadmium sul~ide deposi-t.
Other methods of doping are ion implantation, diffusion migration and the 1ike.
The application of the radio frequency high voltage creates the necessary plasma to effect deposit of the cadmium ;
sulfide onto the ohmic layer to form o~ the photoconductive layer 12. The rate of deposit in tests conducted was about 6 to 15 Angstroms per second. Greater rates as mentioned above can be achieved in commercial e~uipemnt. The copper or hydrogen iodide if used is admitted in small controlled quantities sufficient to dope the cadmium sulfide on the ohmi~ layer in an amount of 5 x 10 percent by weight.
Most practical examples were totally pure. The sputtering is continued until the thickness o~ the coating 12 reaches 3000 to 3500 Angstroms.
As previously mentioned, one of the most important ... . , . ~- .

~155300 aspects of the invention revolves around the special method of sputtering which is used. ~lile used ~or the deposit of the bond enhanc.ing layer 18, the ohmic layer 14 and the photoconductive coating 12, the most important application o~ -this method is in sputtering the photoconductive material to :Eorm both the bond enhancing layer/and the photoconductive coating 12.
In the method o~ sputtering which is conventional, the cathode or target is connected to the "hi~h" side o:E the .
output of the radio requency generatorJ normally through a matching network, and the anode or substrate sùpport is connected to ground. The radio frequency energy ionizes the argon gas which is introduced into the chamber and there is a plasma ~ormed between the target and anode, there being ~ ::
.
a first dark space of relatively short dimension just at the ~
surface of the target. Atoms of the.target are literally . ~ .
knocked out o~ the target by the ions OL the argon gas and are driven across the intervening space through the plasma and impinge against any article that overlies the anode.
This would be a substrate member and the particles themselves or after reacting with other reagent elements which may have been introduced into the chamber are deposited onto the substrate.
It has been discovered that by biasing the radio requency circuit in the manner to be described the atoms of depos.ited material are deposited in a very dense manner and that the unusual electrical properties descr.ibed result ' ~-~055300 ~ -there:Erom. Thls biasing arrangement produces a second dark space immediately above the anode.
It has also been discovered that the second dark space can be achieved sometimes by adjusting the geometry o the target, shields, anode, etc. within the chamber. When this second dark space appears, the desirable qualities of the deposit are achieved without changing the circuit configura-tion which would indicate, o course, that the presence of the second dark space is the desidera-tum rather than the circuitry.

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Claims (7)

The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows:

1. An electrophotographic member of the type including substrate means, a thin film coating of a wholly inorganic, r.f. sputtered, photoconductive material on said substrate means, said coating being very dense, microcrystalline, substantially transparent, having a dark resistivity of at least 1012 ohm-centimeters and a ratio between dark and light resistivity of at least 104 having the capability of accepting a rapid charge and retaining same to enable toning and being electrically anisotropic and a thin film layer of ohmic material sandwiched between the coating and substrate means for facilitating charging of said coating before exposure; bond enhancing means comprising an ultrathin film layer of a transparent wholly inorganic photoconductive non-metallic material between the ohmic layer and the substrate means.

2. The electrophotographic film member defined in claim 1 in which said ultrathin film layer is a photo-conductor and has a thickness that is a small fraction of the thickness of the ohmic layer.

3. The electrophotographic film member as defined in claims 1 or 2 in which said ultrathin film layer is r.f. sputtered directly on the substrate.

4. The electrophotographic film member as defined in claims 1 or 2 in which said ultrathin film layer is formed primarily of r.f. sputtered cadmium sulfide.

5. The electrophotographic film member as defined in claims 1 or 2 in which said ohmic layer is primarily indium oxide.

6. The electrophotographic film member according as defined in claims 1 or 2 in which said ohmic layer is primarily indium oxide including tin oxide in a concen-tration of the order of ten per cent by weight.

7. The electrophotographic film member according as defined in claims 1 or 2 in which said bond enhancing layer and said photoconductive coating are formed of the same photoconductive material.