US3423237A

US3423237A - Photoconductive device

Info

Publication number: US3423237A
Application number: US479992A
Authority: US
Inventors: Ann Hutchinson
Original assignee: General Electrodynamics Corp
Current assignee: General Electrodynamics Corp
Priority date: 1965-08-16
Filing date: 1965-08-16
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21

Description

Jan. 21, 1969 A. HUTCHINSON 3,423,237

PHOTOCONDUCTIVE DEVICE Filed Aug. 16. 1965 A NN H u TCH/NSON INVENTOR.

ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE A process for preparing a target for a vidicon camera tube wherein a mixture of selenium and sulfur is deposited on a conductive coating on the face plate and is heat treated and then a layer of arsenic disulfide is deposited and heat treated so that the sensitivity and the stability of the photoconductive coating are substantially increased.

This application is a continuation-in-part of my copen'ding application Ser. No. 458,671, filed May 25, 1965 and now abandoned.

This invention relates to photoconductive devices, and more particularly it relates to photoconductive image pickup tubes of the type commonly known as the vidicon.

Television camera tubes employing photoconductive targets and known as vidicons are now well known in the art, having been described in an article in the May 1950 issue of Electronics magazine, and in a number of patents including, for example, Patent No. 2,745,- 032 to Forgue et al. As described in the prior art, and as is well known, a vidicon camera tube consists of an electron gun and a target assembly contained in a glass envelope, usually about six inches long and one inch in diameter. The electron gun may be of the conventional type used in other types of television pickup tubes. The target assembly comprises a film of light-transparent, electrically conductive material on the face plate of the envelope, and a coating of photoconductive material deposited upon the electrically conductive film. The target and the gun are so arranged within the envelope that the electron beam from the gun scans the photoconductive surface of the target.

In commercial television camera tubes the photoconductive target accumulates, or stores a charge throughout the period of time, say of a second, between interrogations by the scanning electron beam. By selecting some of the more insulating type photoconductors for use in such a target, charges accumulated by exposure of the photoconductor to an image can be usefully stored for periods of time considerably longer than of a second before the beam reads out the charges. The length of storage time available before the image deteriorates varies, within limits, with the resistance characteristics of the selected photoconductor.

Some applications of vidicon tubes require photoconductors which are capable of storing images for several minutes. For example, vidicon tubes used in cameras carried on interplanetary and other space probes are required to be of the slow scan type because weight limitations prohibit the use of equipment necessary for rapid scanning of the vidicon target. The photoconductive targets of such vidicon tubes are also required to be as sensitive as possible in order to produce usable pictures of dimly lighted subject matter, and furthermore it is highly desirable that these vidicon tubes be resistant to deterioration from exposure to heat.

Photoconductive materials used for vidicon targets are 3,423,237 Patented Jan. 21, 1969 electric insulators in the dark, but become electrically conductive when exposed to light. The conductivity is proportional to the amount of light striking the material, and is limited to the immediate area under the influence of the light. A number of different photoconductive materials are usable for various electronic devices, and one of the most widely used photoconductors is elemental selenium, in which, in fact, the phenomenon of photoconductivity was first observed, in 1873. Selenium has two common allotropic forms, both of which are widely used as photoconductors. The crystalline gray form, sometimes referred to as the metallic form, is a fair conductor even in the dark, and is used principally in photocells and rectifiers, and the like. The amorphous red form is useful as a photoconductor for photoconductive television image pickup tubes and then film detectors because it is characterized by a resistivity sufficiently high (approximately 10 ohm-cm.) to permit charge storage operation. It is the latter allotropic form to which the invention described herein relates.

A major difiiculty in the use of amorphous red selenium in photoconductive television image pickup tubes is that the material is unstable and will slowly convert to the more conductive crystalline form even at ordinary room temperature. At temperatures above about 45 C. the time required for such conversion is only a few hours. Thus where amorphous selenium has previously been used in television camera pickup tubes, it has been necessary to strictly limit the operating temperature of the tube.

It is an object of this invention to provide a selenium base photoconductor which does not have the thermal instability of red amorphous selenium photoconductors previously used.

It is another object of this invention to provide a photoconductive coating, such as is used on a vidicon camera tube face plate, which is formed principally of amorphous selenium, but which has a high photosensitivity and is far more stable at relatively higher temperatures than is ordinary red amorphous selenium.

Still another object of the invention is to provide a :method for forming a photoconductive surface from selenium which provides a photoconductive surface having high resistivity in the dark and good sensitivity combined with a long life at ordinary operating temperatures.

Yet another object of the invention is to provide a vidicon camera tube which can be scanned at relatively long intervals and will still give good current to light response, and which has a long, useful life.

These and other objects of the invention are accomplished by forming a photoconductive surface from red amorphous selenium which has been combined with a small proportion of sulfur in such a manner that the selenium and sulfur form a complex, or an inter-molecular compound, whereby the sulfur modifies the reaction of the selenium to temperature and makes the amorphous selenium more stable, and by then diffusing arsenic disulfide into the selenium-sulfur compound.

For a better understanding of the invention, reference is now made to the following description and to the single figure of the accompanying drawing, which shows a longitudinal and partly sectional view of one form of vidicon camera tube embodying the present invention.

The drawing shows a vidicon type camera tube, indicated generally by the reference numeral 10, which comprises an evacuated envelope 12 having an electron gun 16 in one end thereof. The electron gun 16 may be any of the known types of electron guns, and produces an electron beam directed toward the target electrode 18 in the other end of the envelope 12. The electron beam is focussed and scanned over the exposed surface of the target electrode by any conventional means (not shown).

The target electrode 18 is attached to a metal ring 19, made of a metal such as Kovar, which is sealed by means well known in the art to the edge of the target electrode and to the end of the envelope 12. The target electrode comprises a transparent face plate or substrate 20, preferably made of fused quartz or some similar material which has a low coeflicient of expansion. The face plate has applied thereto a layer 22 of a conductive material, which comprises a signal plate which in turn is covered by a layer 24 of a photoconductive material.

The transparent conductive layer or signal plate 22 may be made of tin oxide or a thin evaporated metal film such as gold, and may be deposited by any known technique such as spraying or evaporation. The signal plate should be highly electrically conductive and should be transparent to the particular radiant energy for which the device is designed to respond.

The photoconductive coating 24 is made up of a particular form of red amorphous selenium which is stabilized by the combination therewith of a comparatively small amount of sulfur, and by the addition of arsenic disulfide (AS 3 Such a coating is prepared by first combining sulfur and selenium in such a way that an amorphous atomically intimate and homogeneous mixture of selenium and sulfur is obtained. Such a mixture may be obtained, for example, by co-melting the two elements in a protective atmosphere, or by co-precipitation, or by co-evaporation of the two elements. It has been found that co-melting of the two elements in an evacuated tube at a temperature of around 400 C. is a convenient and reliable method of obtaining the desired homogeneous mixture.-

After the mixture of selenium and sulfur is prepared it is fabricated into a photoconductive thin film lying on the signal plate of the target electrode. Such a thin film may be prepared by means well known in the art, such as, for example, by evaporating either in a high or low vacuum, or by settling, or by electrophoretic plating, or by other well known methods for applying such photoconductive layers. Such methods of depositing such coatings are well known in the art and do not form a part of this invention. However, good results have been achieved in forming the coating of this invention by evaporating the selenium-sulfur mixture onto the conductive layer at a temperature of about 300 C. in a vacuum of approximately 0.1 mm. absolute pressure.

The photoconductor as thus formed comprises a red coating, amorphous by X-ray diffraction, which has good photoconductive sensitivity and has better heat stability than ordinaly red amorphous selenium. However, the characteristics of the coating are substantially improved by following the coating process with a heat treatment which comprises holding the photoconductor at a temperature in the range of about 40 C. to about 100 C. for a time suflicient to convert the initial amorphous form to a second form which is also amorphous by X-ray diffraction, but which is characterized by a darker red color and a lower transparency than the original form, and which also has excellent resistance to conversion into the gray crystalline form under exposure to moderate temperatures, under 100 C. The precise time required for the conversion from the first amorphous form to the second amorphous form will vary according to the particular temperature selected. It has been found that if the heat treatment is carried out at about 70 C., heat treatment for one hour is sufficient to obtain substantial conversion to the second amorphous form.

The product thus prepared appears to be a mass of very small spherical particles of the second amorphous form with a small amount of the first amorphous form filling the interstices. The fact that this amorphous form is different from the usual red amorphous form is made evident by the performance of the material. Vidicons which previously used red amorphous selenium for the photoconductor were limited to operations at temperatures used 45 C., and even at this temperature had a life of only a few hours before the red amorphous material would begin to convert to the gray crystalline form, which has such a low dark resistivity that it cannot be used in a vidicon. However, a vidicon which is provided with a photoconductor prepared in the manner hereinbefore described has a much longer life even though operated at temperatures as high as C. or greater. Apparently the addition of the sulfur to the selenium stabilizes the amorphous form of the selenium and prevents its ready conversion to the gray form upon heating.

It is not known precisely what proportion of sulfur is present in the coating applied to the signal plate of the target. However, it is thought likely that the proportions of the materials in the photoconductive layer are very near the proportions which are in the mixture which is evaporated. It has been found that the advantageous results of this invention are obtained when the mixtures evaporated in forming the photoconductive layer comprise sulfur amounting to from about one-half percent to about twenty percent of the weight of the selenium present, although a preferred amount of sulfur is between about five percent to about ten percent of the weight of selenium.

According to the present invention, the photoconductive coating prepared in the foregoing manner is then modified by the addition thereto of arsenic disulfide (AS252). The arsenic disulfide may be conveniently added by evaporating it onto the photoconductive coating in a thin layer and then diffusing this layer into the seleniumsulfur complex by heating.

It has been determined that the addition of the arsenic disulfide to the selenium-sulfur photoconductor tends to arrest the conversion of the first amorphous form of selenium to the second amorphous form, thereby producing additional thermal stability. In addition, however, such addition substantially increases the photosensitivity of the photoconductive layer. The increase in sensitivity appears to be a result of the final heat treatment. For maximum sensitivity it has been found that heat treatment at a temperature of about C. for about hours is suitable. Such heat treatment can be performed at temperatures as high as about 100 C. for shorter periods of time, but only with danger of possible conversion of some of the amorphous material to crystalline material. Also, some increase in sensitivity can be obtained by heat treatment at temperatures as low as about 40 C. but only with extremely long periods of heating. Within these limits, the increase in sensitivity appears to be more or less directly related to the time and temperature of heat treatment.

This final heat treatment is normally carried out in a vacuum, since, as is well known in the art, vidicon tubes are sealed and pumped out after deposition of the photoconductive coating. The heat treatment is preferably performed after such sealing and evacuating.

It has also been determined that desirable modifications in the characteristics of the photoconductive coating of this invention can be achieved by variations in the conditions under which the first heat treatment is carried out. If the heat treatment is performed in air or other gaseous atmosphere at or near atmospheric pressure, a coating of good sensitivity with comparatively long image storage is obtained and its blue response is relatively higher, but if the coating is baked in a vacuum, substantially greater sensitivity results, and the image storage period is much shorter and the red response is relatively higher. The vacuum may conveniently be in the range of from about 25 microns to about 10 mm. Hg absolute with good results. The period of this first heat tretment may vary from one to several, perhaps ten, hours, depending upon temperature and degree of conversion desired. It will be apparent that conditions of this thermal treatment may be modified within the limits herein set forth as necessary to change the coating characteristics for specific applications.

A vidicon tube incorporating the photoconductive coating prepared in the manner hereinbefore described has been found to possess a substantially higher photosensitivity than has previously been obtained with a selenium photoconductor, together with good image storage for slow scan operation and far greater thermal stability than has heretofore been obtained.

Although preferred embodiments of the invention have been shown and described herein, the invention is not limited to such embodiments, but only set forth by the following claims.

I claim:

1. A process for forming a photoconductive target comprising simultaneously evaporating selenium and sulfur and depositing it on a substrate,

heating said deposit until it is converted to a less transparent form, and

depositing a thin layer of arsenic disulfide on the first deposit.

2. A process as defined by claim 1, wherein said heating is carried out in a vacuum.

3. A process for preparing a photoconductive target comprising preparing an amorphous, atomically intimate, substantially homogeneous mixture of selenium and sulfur,

fabricating said mixture into a thin coating on a sub strate,

heating said coating to transform it to a less transparent amorphous form having high resistance to conversion to a crystalline form,

depositing a thin layer of arsenic disulfide on said coating, and

heat treating at a temperature and for a period suflicient to substantially increase the sensitivity of the coating without any substantial conversion to a crystalline form. 4. A process as defined by claim 1, wherein said heating is carried out in a gaseous atmosphere.

5. In a process for forming a photoconductive target comprising forming a thin film of amorphous seleniumsulfur on a substrate, the improvement which comprises depositing a thin layer of arsenic disulfide on the selenium-sulfur film.

6. A process as defined by claim 5, and including heat treating the target so formed.

7. A process as defined by claim 6, wherein the heat treatment is at a temperature of from about 40 to about 100 C. and is continued until the arsenic disulfide is diffused into the selenium-sulfur film.

8. In a process for forming a photoconductive target comprising preparing an amorphous, atomically intimate, substantially homogeneous mixture of selenium and sulfur, fabricating said mixture into a thin coating on a substrate, and heating the coating to transform it to a less 9. A process as defined by claim 8 wherein the said heat treatment is carried out at a temperature of from about 40 C. to about 100 C. and causes diffusion of the arsenic disulfide into the selenium-sulfur coating.

10. In a process for forming a photoconductive target comp-rising co-melting amorphous selenium and from about one-half percent to about twenty percent by weight of sulfur to form an amorphous, atomically intimate and homogeneous mixture, fabricating said mixture in a thin coating on a substrate, and heating the coating at a temperature of from about 40 C. to about 100 C. for a period sufficient to convert it to a less transparent amorphous form, the improvement which comprises depositing a thin layer of arsenic disulfide on said coating, and

heat treating at a temperature and for a period sufficient to substantially increase the sensitivity of the coating without any substantial conversion to a crystalline form.

11. A process as defined by claim 10 wherein the said heat treatment is carried out at a temperature of from about 40 C. to about 100 C. long enough to cause difiusion of the arsenic disulfide into the selenium-sulfur coating and to substantially increase the sensitivity of the coating without any substantial conversion to a crystalline form.

12. In a photoconductive target comprising a substrate, a conductive coating, and a thin film of amorphous selenium-sulfur intermolecular compound, the improvement which comprises a thin layer of arsenic disulfide diffused into the selenium-sulfur film.

13. In a photoconductive target comprising a transparent substrate, a thin film of an amorphous, atomically intimate and homogeneous mixture of selenium and from about one-half percent to about twenty percent by weight sulfur, the improvement which comprises a thin layer of arsenic disulfide diffused into the selenium-sulfur film.

References Cited UNITED STATES PATENTS 2,662,832 12/1953 Middleton et al. 117-62 XR 2,994,621 8/1961 Hugle et al. 117201 3,249,783 5/1966 Santilli et al 313- WILLIAM L. JARVIS, Primary Examiner.

US. Cl. X.R.