Photoconductive target of an image pickup tube comprising graded selenium-tellurium layer

Abstract

A photoconductive target of an image pickup tube comprising a light-transmitting substrate, an N-type conductive layer deposited on the substrate and a P-type conductive layer making a rectifying contact with the N-type conductive layer, in which the P-type conductive layer includes at least selenium and tellurium, the composition of the P-type layer changes along the direction of the thickness of the layer, the average content of selenium in the P-type conductive layer is not less than 50 atomic percent, the content of tellurium at both surfaces of the P-type conductive layer is not more than 10 atomic percent, and the maximum tellurium content of 10 to 40 atomic percent is located on a plane in the P-type conductive layer nearer to the N-type conductive layer than the middle plane of the P-type conductive layer.

Description

The present invention relates generally to the construction of a photoconductive layer used for the target of an image pickup tube of the vidicon type, or more particularly to a photoconductive layer with a rectifying contact which has an increased sensitivity and an improved spectral sensitivity to red light.

Sb.sub.2 S.sub.3, PbO and Si are widely used as materials of photoconductive layers for the target of the vidicon type image pickup tube. Among these materials, Sb.sub.2 S.sub.3 constitutes a photoconductive layer of injecting contact type, while PbO and Si are used for the photoconductive layers of rectifying contact or junction type. The advantages of the photoconductive layer of rectifying contact or junction type over that of injection type are a higher response speed, small dark current and higher sensitivity. However, the photoconductive materials capable of forming a rectifying contact successfully used as a target of the image pickup tube are limited, and it is difficult to obtain a photoconductive material with properties suitable in all respects.

The peak of spectral sensitivity, for example, is located in the proximity of infrared range for Si and on the side of visible short wavelength for PbO, with the result that if they are used for a color image pickup tube, Si and PbO have insufficient sensitivity to blue and red respectively. The inventors have found that amorphous selenium is also capable of forming a rectifying contact suitable for the target of the image pickup tube, but this material also has the disadvantage of insufficient sensitivity to red light.

A method to improve the sensitivity to red light is disclosed in U.S. Pat. No. 3,350,595. According to this method, a conductive thin film is deposited on an insulating substrate which is in turn covered with a photoconductive layer comprising mainly a mixture of tellurium and selenium. The surface portion of the photoconductive layer adjacent to the conductive thin film contains selenium of 70 to 80 % by weight while the opposite surface portion thereof includes selenium of 90 to 100 % by weight, the selenium content gently changing between both the surface portions. However, the fact that the tellurium content in the photoconductive layer near the interface thereof with the conductive thin film is high is accompanied by the disadvantage of increased dark current. In order to overcome this disadvantage, U.S. Pat. No. 3,350,595 discloses a method in which a blocking layer of such metal as cesium with small work function is interposed between the conductive film and the photoconductive layer. This method, however, is disadvantageous in that the manufacturing processes are complicated.

The object of the present invention is to obviate the above-mentioned disadvantages and to provide a photoconductive target of an image pickup tube which is high in sensitivity to red light, small in dark current and is easily manufactured.

In order to achieve the above-mentioned object, the photoconductive target of the image pickup tube according to the invention comprises a light-transmitting substrate, a first N-type conductive layer deposited on said substrate and a P-type conductive layer making a rectifying contact with said first N-type conductive layer, said P-type conductive layer including at least selenium and tellurium, the composition of said P-type conductive layer being different along the direction of the thickness thereof, the average content of selenium in said P-type conductive layer being not less than 50 atomic percent, the content of tellurium at both surfaces of said P-type conductive layer being not more than 10 atomic percent, the maximum tellurium content of 10 to 40 atomic percent being located on a plane in said P-type conductive layer nearer to said first N-type conductive layer than the middle plane of said P-type conductive layer. The N-type conductive layer is a light-transmitting conductive film preferably including as its main component an oxide of tin, indium or titanium. Further, to prevent the crystallization of the P-type conductive layer, an N-type conductive layer formed of one substance selected from the group consisting of CdS, CdSe, ZnS, ZnSe and a mixture thereof may be interposed between the P-type conductive layer and the light-transmitting conductive film or a translucent metal making up the surface portion of the light-transmitting substrate nearer to the side of the N-type conductive film. The P-type conductive layer may include As, Sb, P, Bi, Ge and/or Si in addition to selenium and tellurium.

Amorphous selenium is generally of P conduction type and forms a rectifying contact with a variety of N-type materials including films of single crystals or crystallites of such IV group semiconductors as Ge and Si, III - V group semiconductors such as GaAs and GaP and II - VI group semiconductors. Among them, the most suitable material of N-type conduction to form a photoconductive layer for the target of an image pickup tube in combination with selenium are an oxide of tin used for a transparent conductive film, an oxide of indium, an oxide of titanium, and II - VI group semiconductors such as ZnS, ZnSe, CdS and CdSe. Although the low intrinsic resistance of conductive films of the above-mentioned oxides makes it possible to use them also as an electrode for taking out a signal from the image pickup tube, the II - VI group semiconductors cannot be used as such an electrode at the same time without additional provision of the transparent electrode of any of the above-mentioned oxides or a translucent metal laid thereon.

The better understanding of the present invention will be gained from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a sectional view showing the fundamental construction of the target of the image pickup tube according to the present invention; and

FIGS. 2 to 5 are graphs illustrating the distribution of component elements along the direction of the thickness of the P-type photoconductive layer.

An embodiment of the invention is shown in FIG. 1. The target of an image pickup tube according to the invention generally comprises a glass substrate 1, a transparent electrode 2 extended on the glass substrate 1, an N-type conductive layer 3 and a P-type conductive layer 4. Reference numeral 5 shows incident light, and numeral 6 a scanning electron beam. A rectifying contact is formed between the N-type conductive layer 3 and the P-type conductive layer 4.

In the case where the transparent electrode 2 is formed of an oxide of N-type conductivity, the transparent electrode 2 and the N-type conductive electrode 3 are actually integrated into a single layer. In the event that amorphous selenium and the above-mentioned oxide or a II - VI group semiconductor are used as the materials of the P-type conductive layer and the N-type conductive layer respectively, sensitization is necessary to improve the sensitivity to red light since the above-mentioned materials other than CdSe do not have sensitivity to red light.

A well-known method to improve sensitivity of II - VI group to red light consists in doping into it Cl, Br, I, In, Ga or other elements acting as a donor together with Cu, Ag or the like element forming an acceptor. In the case where a P-type conductive layer including selenium is involved as in the present invention, the sensitivity to red light is successfully improved in combination with the above-mentioned method concerning the sensitization of the N-type conductive layer of II - VI group semiconductor.

It is also well known that the sensitivity to red light can be improved by adding Te to amorphous selenium. The electrical resistance of Se to which Te has been added is sharply reduced, thereby degrading the characteristics of the target of the image pickup tube. If, for example, the concentration of Te in the neighborhood of the interface between N-type conductive layer 3 and P-type conductive layer 4 is increased, the reverse breakdown voltage of the rectifying contact is decreased thereby to increase the dark current in the image pickup tube.

On the other hand, if the Te concentration of the P-type conductive layer 4 as a whole is increased, the resistance of the P-type layer is decreased or the carrier mobility in the layer is reduced, with the result that the dark current is increased or the time response characteristics are degraded.

The results of research by the inventors show that in improving the sensitivity of the Se conductive layer to red light by the use of Te, it is recommended that Te concentration in the Se conductive layer be made progressively higher toward the side of the N-type conductive layer rather than uniformly distributing it, and the Te concentration at and in the vicinity of the interface between N-type conductive layer and P-type conductive layer is maintained at a low level, thus making it possible to prevent dark current from being increased without adversely affecting the characteristics of the target of the image pickup tube. For this purpose, it is necessary to limit the Te concentration at and in the vicinity of the interface to a level ranging from 0 to 10 atomic percent. Also, if there is Te included in a considerable portion of the P-type conductive layer toward the opposite surface thereof, the intensity of electric field in such a portion is decreased for the degradation of the response characteristic thereof, and therefore it is necessary to maintain the Te concentration in that portion in the range from 0 to 10 atomic percent.

The Te concentration of as little as 10 atomic percent in the P-type conductive layer cannot achieve sufficient improvement in sensitivity to red light. In view of this, the most effective method to improve the sensitivity to red light is to provide a portion between the surfaces of the P-type conductive layer with the main component of Se where the Te concentration is maximum. If the excitation of carriers is to be effected satisfactorily at the point of high Te concentration, it is desirable that such a portion be located at a position the nearest possible to the plane of incidence of a light signal into the P-type conductive layer, that is, the interface thereof with the N-type conductive layer. In other words, the portion of maximum Te concentration should be located on a plane in the P-type conductive layer nearer to the N-type conductive layer than the middle plane of the P-type conductive layer.

Since an excessively high Te concentration in that portion results in an increased dark current, the maximum value of Te concentration should preferably be between 10 to 40 atomic percent. As will be explained later with reference to an embodiment, Te is not necessarily required to be smoothly distributed in the P-type conductive layer, but it may consist of a laminated structure of a multiplicity of films each about 10 A thick comprising films of high Te concentration and films of low Te concentration laid one on another. For this reason, it should be noted that the above-mentioned range of Te concentration from 10 to 40 atomic percent represents an average value in the region several hundred A thick including the portion of maximum Te concentration.

The progressive change in Te concentration in the transverse direction of the P-type conductive layer may be abrupt in the microscopic order of, say, several tens of A. Somewhat macroscopically, however, the curve of the change should preferably be gentle in the order of several hundreds of A. If macroscopically there is a portion where the Te concentration is discontinued, the burning effect of the image pickup tube may be promoted.

The disadvantage of the photoconductive layer with Se as a main component resides in that the layer is easily crystallized by heat, with the result that the picture produced is accompanied by defects in the form of white dots. By a well-known method to prevent the defects, an element such as As, Sb, P, Bi, Ge or Si is added to the material for the layer thereby to increase the viscosity thereof and delay the speed of crystallization. This principle also applies to the present invention, wherein the life of the target may be lengthened by adding such an element to the P-type conductive layer thereby to reduce the speed of crystallization. The addition of excessive amount of the element adversely affects the response characteristic of the target, the desirable amount of the element to be added being less than 20 atomic percent.

When the element for prevention of the crystallization coexists with tellurium for improving the sensitivity, the maintaining of superior dark current and response characteristics require that selenium accounts for at least 50 atomic percent.

The fact that the emission of secondary electrons from the photoconductive layer containing much Se used as a target is comparatively great disturbs the landing of the scanning beam and often causes abnormal phenomena including the distortion of an image and the reversal of a polarity of a video signal at a high target voltage.

An effective method to prevent the above-mentioned phenomena is to deposit by vacuum or gaseous evaporation on the P-type conductive layer a film of Sb.sub.2 S.sub.3, As.sub.2 Se.sub.3 or As.sub.2 S.sub.2 approximately 1000 A thick.

Embodiments of the invention will be explained below.

Embodiment 1

Se, Ge and Te contained in different evaporation boats of tantalum are deposited simultaneously by evaporation in the vacuum of 3 .times. 10.sup.-.sup.6 Torr on a transparent N-type conductive layer with tin oxide as a main component which is formed on a glass substrate. In this way, a P-type photoconductive layer as thick as 3 .mu.m is produced.

The compositional profile of the P-type photoconductive layer is adjusted by controlling the current in each boat so as to be in consistence with the graph as shown in FIG. 2. Further, an Sb.sub.2 S.sub.3 film approximately 1000 A thick is deposited by evaporation on the surface of the P-type photoconductive layer in the low-pressure argon of 5 .times. 10.sup.-.sup.2 Torr thereby to improve the landing characteristic of the scanning beam for the target of an image pickup tube.

Embodiment 2

A transparent N-type layer consisting mainly of indium oxide is deposited on a glass substrate. A CdSe film 2000 A thick is further deposited in the vacuum of 2 .times. 10.sup.-.sup.6 Torr at the substrate temperature of 200.degree.C, on the surface of a transparent N-type conductive layer by evaporation. On the other hand, a first photoconductive material of Se containing Te of 40 atomic percent and a second photoconductive material of Se containing As of 10 atomic percent are prepared in a quartz ampule. These two types of photoconductive materials are crushed and put into different evaporation boats of tantalum and then they are simultaneously deposited on the CdSe layer in the vacuum of 3 .times. 10.sup.-.sup.6 Torr. In the process, the speed of evaporation of the first and second photoconductive materials is continuously changed to form a film 4 .mu.m thick with the composition distribution of Se, Te and As as shown in the graph of FIG. 3. On the surface of the resulting P-type conductive layer is deposited by evaporation a film of As.sub.2 Se.sub.3 approximately 500 A thick in the vacuum of 3 .times. 10.sup.-.sup.6 Torr. This As.sub.2 Se.sub.3 film is further covered by a similar method with an As.sub.2 Se.sub.3 film about 500 A thick in the low-pressure argon of 5 .times. 10.sup.-.sup.2 Torr. This double layer of As.sub.2 Se.sub.3 is formed for the purpose of improving the landing characteristic of the scanning electron beam.

Embodiment 3

A translucent Al film is formed on a glsss substrate in the vacuum of 1 .times. 10.sup.-.sup.6 Torr at the substrate temperature of 150.degree.C, and then on this translucent Al film is deposited a CdS film 3000 A thick in the vacuum of 5 .times. 10.sup.-.sup.6 Torr at the substrate temperature of 150.degree.C.

Subsequently, thin films of Se, As.sub.2 Se.sub.3 and Te are deposited one after another on the CdS film in a rotary vacuum evaporator of 5 .times. 10.sup.-.sup.6 Torr. As a result, a film 5 .mu.m thick comprising 3000 to 6000 layers of Se, As.sub.2 Se.sub.3 and Te each having the average thickness of 10 A or less is obtained.

During the process of forming the multiple layer, the current in the Te boat or the opening of a slit interposed between the Te evaporation boat and the substrate is controlled continuously thereby to produce a macroscopic profile of transverse distribution of composition as shown in FIG. 4. On this multiple layer is further deposited an As.sub.2 S.sub.3 film about 500 A thick in the vacuum of 2 .times. 10.sup.-.sup.6 Torr thereby to improve the landing characteristic of the scanning electron beam.

Embodiment 4

Films of CdSe, CdTe and Se are deposited one after another in the vacuum of 5 .times. 10.sup.-.sup.6 Torr on a transparent N-type conductive electrode with tin oxide as a main component which is in turn deposited on a glass substrate. In this way, a multiple layer 2 .mu.m thick comprising 2000 to 4000 films of CdSe, CdTe and Se each having the average thickness of 10 A or less is obtained. In the process of forming the multiple layer, the macroscopic composition of the elements in the transverse direction of the layer is controlled by similar means to those employed in the preceding embodiment thereby to obtain the profile of composition as shown in FIG. 5.

An excessive amount of Cd tends to be included in the CdSe film by the ordinary method of production thereof, often resulting in the CdSe film having an N-type of conduction. This problem is overcome by adding Se as in the embodiment under consideration thereby to obtain an intrinsic or nearly P-type layer. On this layer is further deposited an Sb.sub.2 S.sub.3 film approximately 1000 A thick in the low-pressure argon of 5 .times. 10.sup.-.sup.2 Torr for improving the landing characteristic of the scanning beam.

It will be understood from the above explanation of the embodiments that according to the invention the sensitivity especially to red light is improved without adverse effect on the rectifying contact with the N-type photoconductive layer by distributing Te in a P-type photoconductive layer containing Se or 50 or more atomic percent in such a manner that the maximum Te content is located on a plane in the P-type conductive layer nearer to the N-type conductive layer than the middle plane of the P-type conductive layer. The spectral sensitivity of the P-type conductive layer is thus changed greatly, making it possible to produce a photoconductive layer suitable for a specific purpose. Although the above explanation of the embodiments involves a light-receiving film of the target for the image pickup tube, it is possible to use the above-mentioned photoconductive layer for a solid-state light-receiving element, solid-state image pickup element or the like by employing an appropriate metal electrode in place of an electron beam.

Claims

What we claim is:

1. A photoconductive target of an image pickup tube comprising a light-transmitting substrate, a first N-type conductive layer deposited on said substrate and a P-type conductive layer making a rectifying contact at a first surface thereof with said first N-type conductive layer and having a second outer surface to be scanned by electrons, said P-type conductive layer including at least selenium and tellurium, the composition of said P-type conductive layer being different along the direction of the thickness thereof, the average content of selenium in said P-type conductive layer being not less than 50 atomic percent, the content of tellurium at said first and second surfaces of said P-type conductive layer being not more than 10 atomic percent, the maximum tellurium content of 10 to 40 atomic percent being located on a plane in said P-type conductive layer between said first N-type conductive layer and the middle plane of said P-type conductive layer.

2. A photoconductive target of an image pickup tube according to claim 1, wherein said first N-type conductive layer is a transparent conductive film including one substance selected from the group consisting of an oxide of tin, indium, and titanium as a main component.

3. A photoconductive target of an image pickup tube according to claim 1, wherein a second N-type conductive layer formed of one substance selected from the group consisting of CdS, CdSe, ZnS, ZnSe and the mixture thereof is interposed between said P-type conductive layer and said first N-type conductive layer or a translucent metal electrode constituting the surface portion of said lighttransmitting substrate on the side of said first N-type conductive layer.

4. A photoconductive target of an image pickup tube according to claim 1, wherein said P-type conductive layer contains one substance selected from the group consisting of As, Sb, P, Bi, Ge, Si and the mixture thereof in addition to selenium and tellurium.

5. A photoconductive target of an image pickup tube according to claim 4, wherein the concentration of said one substance in said P-type conductive layer is substantially uniform therethrough.

6. A photoconductive target of an image pickup tube according to claim 1, wherein the minimum concentration of said selenium in said P-type conductive layer is located at the location of said maximum tellurium content in said P-type conductive layer.

7. A photoconductive target of an image pickup tube according to claim 1, wherein said P-type conductive layer contains Cd in addition to selenium and tellurium.

8. A photoconductive target of an image pickup tube according to claim 7, wherein the maximum concentration of said Cd in said P-type conductive layer is located at the location of said maximum tellurium content in said P-type conductive layer.

9. A photoconductive target of an image pickup tube according to claim 1, wherein the absolute rate of increase of the tellurium content in said P-type conductive layer as the location of said maximum tellurium content is approached is equal to the absolute rate of decrease of the tellurium content as the location of said maximum tellurium content is passed.

10. A photoconductive target of an image pickup tube according to claim 1, wherein the absolute rate of increase of the tellurium content in said P-type conductive layer as the location of said maximum tellurium content is approached is greater than the absolute rate of decrease of the tellurium content as the location of said maximum tellurium content is passed.