US3569763A

US3569763A - Multilayer photoconductive device having adjacent layers of different spectral response

Info

Publication number: US3569763A
Application number: US614436A
Authority: US
Inventors: Yuji Kiuchi; Shigeo Tsuji; Hiroo Hori
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1966-02-14
Filing date: 1967-02-07
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09
Also published as: GB1137803A; NL6702246A; DE1614768A1; FR1511816A

Abstract

A photoconductive structure includes first layers of a photoconductive material having one spectral response region and second layers of a different photoconductive material having a different spectral response region laminated one on another either alternately or in a predetermined order at least two times, the thickness of each layer being such that light penetrates therethrough, i.e., less than 2 Mu . Low-resistivity photoconductive materials can be used to provide multilayer structures having a high sensitivity to a predetermined range of wavelengths of light, for example, to the red-light region and a wide spectral region.

Description

United States Patent Inventors Yuji Kiuchi;

Hiroo Hori, Yokohama-shi; Shigeo Tsuji, Fujisawa-shi, Japan Appl. No. 614,436 Filed Feb. 7, 1967 Patented Mar. 9, 1971 Assignee Tokyo Shibaura Electric Co., Ltd.,

Kawasaki-shi, Japan Feb. 14, 1966, Feb. 25, 1966 Japan 41/8334/66, 41/ 11,043, 41/1 1,044 and 41/1 1,045

Priority MULTILAYER PHOTOCONDUCTIVE DEVICE HAVING ADJACENT LAYERS OF DIFFERENT SPECTRAL RESPONSE 7 Claims, 7 Drawing Figs.

U.S. C1 313/65, 313/96, 313/112, 250/213 Int. Cl. ..H0lj 31/26, 1-101j 39/14 Field ofSearch 313/65 (A),

[56] References Cited UNITED STATES PATENTS 2,869,010 1/1959 Gray 2,594,740 4/1952 DeForest et al.

2,890,359 6/1959 l-leijne et a1 2,914,679 1 1/1959 Loebner 2,942,120 6/1960 Kazan Primary Examiner-Roy Lake Assistant Examiner-V. LaFranchi Attorney-Stephen H. Frishauf ABSTRACT: A photoconductive structure includes first layers of a photoconductive material having one spectral response region and second layers of a different photoconductive material having a different spectral response region laminated one on another either alternately or in a predetermined order at least two times, the thickness of each layer being such that light penetrates therethrough, i.e., less than 2 p. Low-resistivity photoconductive materials can be used to provide multilayer structures having a high sensitivity to a predetermined range of wavelengths of light, for example, to

the red-light region and a wide spectral region.

PATENTEDHAR 9m 3.569.763

SHEET 1 [IF 3 PATENTEDMAR 91911 34569763 sum 2 0F 3 PATENTED'HAR slsn 3,569,763

SHEET 3 UP 3 FIG. 5

RELATIVE SENSITIVITY PRIOR ARTB 5bo 4600 WAVELENGTH (mm) 7 5,100 O 80 PRIOR ARTB' so 40 E 20 0 TIME(m sec) MUL'H'HJAYER PHGTOCONDUC DEVIQE HAG ADJACENT LAYERS F DEFFEIRENT SPEC rouse The present invention relates to photoconductive devices such as photoconductive targets for image pickup tubes, photoconductive cells and others and'the method for manufacture manufacturing the same.

rhotoconductive targets for image pickup tubes are usually formed of photoconductive materials having dark resistivity of more than fl-crn. Currently available photoconductive materials which can satisfy the above dark-resistivity requirement, however, are very few. Photoconductive targets made of conventional photoconductive materials such as antimony trisulfide and lead oxide exhibit limited spectral response since a photoconductive material is sensitive to radiations in a particular spectral region. A single photoconductive material, accordingly, can not cover all the desired wide spectral region of wavelengths. By way of example, a target made of lead oxide, though it has a high sensitivity to blue region of visible light, is not sumciently sensitive to red region, so that it can not be used for red-channel pickup tubes for color television broadcast.

To overcome the drawback as described above, it has been proposed to form a photoconductive target from a mixture of two or more photoconductive materials respectively having different spectral response characteristics, for instance, a mixture containing lead oxide which is sensitive to blue region of visible light and lead sulfide which is sensitive to red region. A target formed of such a mixture presents considerably improved spectral sensitivity distribution, but its electrical characteristics as the photoconductive target for the television camera tube are deteriorated because of the incorporated lead sulfide whose dark resistivity is below 10 Q-cm, so that it can not be qualified for the practical use.

As a second alternative method it has also been proposed in US. Pat. No. 2,687,484 to make a double layer structure of the target by laminating layers of different photoconductive materials respectively having different spectral response characteristics, for example, laminating together a red-light sensitive antimony trisulfide layer 0.1 to 0.5 ,u thick and a blue-light sensitive amorphous selenium layer having a thickness of 2 to 5 u. in such laminated structure, however, the first layer exposed to the incidence of light, the antimony trisulfide layer in the above example, should be made thin enough to permit incident light to penetrate therethrough sufficiently and reach the next layer, the amorphous selenium layer in the example, to the result that the first layer can not absorb incident light to a sufficient extent; though the second layer sufficiently absorbs blue light, it is insensitive to the residual red light unabsorbed by the first layer, and incident light can not be fully utilized. Further, in this structure photoconductive materials of low resistivity can not be used, and hence reduced dark current required for photoconductive devices can not be expected.

Accordingly it is an object of this invention to provide a photoconductive device which has sufficient sensitivity over a required spectral region and which permits small dark current therethrough.

it is another object of this invention to provide a method for manufacturing such a device.

SUMMARY OF THE INVENTION According to this invention a novel photoconductive device comprises a plurality of first layers of a photoconductive material having one spectral response region and a plurality of second layers of a photoconductive material having a different spectral response region and each of the first and second layers being less than 2 p. in thickness so that incident light penetrates through and is gradually absorbed by successive layers. it is formed by alternately depositing by evaporation by of a mechanical arrangement first layers of a photoconductive material having a particular spectral response distribution and second layers of a photoconductive material having a difi erent spectral response distribution from separate evaporating sources.

The invention is now described with reference to the accompanying drawings, in which:

FIG. l is a longitudinal section of an image pickup tube having a photoconductive target embodying this invention;

FIG. 2 is an enlarged section partially illustrating the photoconductive target shown in FIG. 1;

FIG. 3 is an elevational section illustrating an apparatus for manufacturing the photoconductive target shown in FIG. 2;

FIGS. 4a and db illustrate a modification of the apparatus for manufacturing the target shown in FIG. 2, with FIG. 4a being elevational section of the apparatus and FIG. 4b being a plan view of a shuttering member included in the apparatus;

FlG. 5 is a diagram comparing relative sensitivity versus wavelength characteristic of the photoconductive device according to the invention with that of a conventional device; and

FIG. 6 is a diagram comparing residual signal of image or lag versus time characteristic of the photoconductive device according to the invention with that of a conventional device.

Referring to the drawings, and particularly to FlG. l, the reference numeral 10 generally designates an image pickup tube comprising an evacuated cylindrical envelope ll which coaxially encloses an electron gun 16 consisting of a cathode 12, a grid electrode 13, a first accelerating electrode 14, a second accelerating electrode 15 and a mesh electrode 17. A faceplate 18 closing the end of the envelope ll remote from the cathode 12 has its inner surface deposited with a trans parent conductive film 19 of, for instance, tin oxide. The conductive film 19 is electrically connected to a signal electrode 20, and has in turn deposited on its side facing the electron gun 16 a multilayer photoconductive structure, which embodies the invention and is generally indicated at 32 and whose manufacture is described later in detail.

As is shown in more detail in FIG. 2, the multilayer photoconductive structure 32 consists of juxtaposed pluralities of first and

second layers

30 and 31, the lamination being, for example, obtained by first vapor-depositing on the conductive film 19 a layer 30 of a photoconductive material particularly sensitive to visible light in short wavelength regions, for instance lead oxide, to have a thickness of 0.8 ;1,, followed by the vapor deposition of a second layer 31 of a different photoconductive material particularly sensitive to visible light in long wavelength regions, for instance lead sulfide, to have a thickness of 0.2 u, and repeating the alternate vapor-deposition of these two photoconductive materials until the thickness of the lamination reaches 10 ,u. The laminated structure 32 may have a thickness ranging from 3 to 20 t and the thickness of individual layers is preferably less than 2 p..

In the operation of the image pickup tube 10, an electron beam 34 from the electron gun 16 is accelerated by accelerating electrodes 14 and 15 and focused and deflected by focusing and deflecting coils not shown to scan the photoconductive layer 32 constituting the target. in the absence of light falling on the target 32, its surface scanned by the electron beam 34 is maintained roughly at the same potential as the cathode. When light reaches the target 32, the target is excited so as to reduce its resistivity depending upon the intensity of the incident light to produce corresponding electrical signals from the electron beam scanning the target.

The method of producing the above-mentioned multilayer photoconductive target is now described with reference to FIG. 3, which illustrates a preferred construction of the apparatus for manufacturing the multilayer photoconductive target.

The apparatus comprises a bell jar 4t) gastight sealedly attached on the peripheral annular margin 42 of a support member all through a rubber packing 43 to form a gastight vessel generally designated at 56. Within the gastight vessel 56 is disposed a coaxial d'mlike rotary substrate support member 46 secured on a vertical shaft 65 sealedly and rotatably extending through the support member 41 to the outside of the vessel 56 and coupled to a rotor of drive means such as motor 44. The disclike rotary substrate support member 46, in this embodiment, is formed with circular apertures 47 of the same diameter and symmetrical with respect to the axis of the shaft 45, the center of the apertures 47 being, for instance, 8 cm. distant from the center of the disclike support member 46. Over each of the apertures 47 is placed a substrate 48 constituting the faceplate of the image pickup tube. Above the substrates 48 is arranged a substrate heating means 49, for instance, consisting of a nichrome wire.

Directly beneath the substrates 48 are disposed a corresponding number of evaporation means, in this embodiment two of such means; one 50 made of a platinum container containing lead oxide and the other 51 made of a quartz container containing lead sulfide and surrounded by a heating coil 52. The platinum container 50 is heated by applying current via conducting leads 54 directly. The platinum container 50 and the heating coil 52 are electrically connected to associated conducting leads 54 which are gastight sealedly brought out of the support member 41 and insulated therefrom by insulating means such as ceramic ring members 53. Each of the evaporation means 55 is made of, for instance, glass to prevent evaporated photoconductive material from being scattered and occupying all the space within the gastight vessel.

In operation, the container 50 is filled with 320 mg; of pulverized lead oxide while the container 51 is filled with 80 mg. of pulverized lead sulfide. Then the substrates 48 are heated to a temperature of from 90 to 200 C., for instance, 150 C. by the heater 49. Thereafter the inside of the gastight vessel 56 is subjected to oxygen atmosphere at a low pressure of from 2 X 10 to 3 X l mmHg, for instance, X mmHg. The most suitable value of said pressure is determined by an experiment. Then the containers 50 and 51 are heated to about 900 C. and to about 700 C., respectively. After lead oxide and lead sulfide have been completely melted by heating, the substrate support member 46 is rotated at a speed of, for instance, l5 r.p.m. With the rotation of the substrate support member 46, the substrates 48 placed over the support member 46 is alternately deposited with lead oxide layers, each layer being 0.8 p. in thickness and lead sulfide layers, each layer being 0.2 u in thickness. For instance a multilayer photoconductive structure having a thickness of 10 u may be obtained if the substrate support member 46 is rotated 10 times before it is stopped. Thus the thickness of one layer and that of the multilayer structure may be determined by specifying the speed and the number of revolutions of. the substrate support member and evaporating conditions.

As in the previous apparatus, the apparatus generally indicated at 61 in FIG. 4a comprises a bell jar 64 secured on a support member 62 through a rubber packing 63 making gastight seal of the apparatus. Within the gastight vessel 61 is disposed a coaxial stationary substrate support means 68 whose top of disclike member is formed with a concentric circular aperture 69 over which is placed a substrate 83 constituting the face plate of the image pickup tube.

Beneath and concentric with the circular aperture 69 is also disposed a disclike shutter member 67 in which is cut a sectorishaped notch as shown in FIG. 4b and which is sealedly and rotatably extending through the support member 62 to the outside of the gastight vessel 61 and coupled to a rotor of a motor 65. As in the previous apparatus a substrate heater 70 is arranged above the substrate 83. Beneath and in the neighborhood of the shutter member 67 there are disposed a'plurality of evaporation means: in this apparatus there are provided two of such means spaced by a predetermined distance and are provided as in the previous apparatus, one of which being platinum container 71 containing lead oxide and the other being quartz container 72 containing lead sulfide and surrounded by heating coil 73. The platinum container 71 is electrically connected to associated conducting leads 8b and 81 which are taken out of the gastight vessel 61 through insulating means to isolate them from the support member 62 such as

ceramic ring members

78 and 79, while the heating coil 73 is electrically connected to associated conducting leads 76 and 77 which are similarly taken out of the gastight vessel 61 through ceramic ring members 74 and 75.

In operation, the platinum container is filled with 320 mg. of pulverized lead oxide and the quartz container 72 with 80 mg. of lead sulfide. Then on the aperture 69 formed in the top of disclike member of the substrate support means 68 is placed a substrate 83, a transparent glass plate which is coated on its side facing the evaporation sources with a transparent conductor. Then the temperature of the substrate 83 is elevated to from to 200 C., for instance, C. by the heater 70. Thereafter the gastight vessel 61 is evacuated and oxygen gas is filled therein to provide low ambient pressure of from 2 X lo mml'ig. to 3 X 101 mmI-Ig, for instance, 5 X 102 mmHg. Then the platinum container is heated to about 900 C. and the quartz container 72 is heated to about 70020 C. After lead oxide and lead sulfide are completely melted by heating, the shutter member 67 is rotated at a speed of, for instance, 15 rpm. by operating the motor 65. When the shutter member 67 is in rotation the substrate 83 is alternately deposited with lead oxide layers, each layer being 0.8 p. thick and lead sulfide layers, each layer being 0.2 ,1. thick during alternate periods during which the notch of the shuttermember 67 proceeds past the evaporating means 71 and 72. As in the preceding embodiment, the multilayer photoconductive lamination structure may be made to have thickness of 10 ,u if the shutter member 67 is rotated 10 times before it is stopped. Also, it may be made to have a desired thickness by suitably specifying the speed and the number of revolutions of the shutter member and evaporating conditions, the size of the notch in the shutter member and other operating conditions.

By fixing speeds of evaporation of individual photoconductive materials and period of evaporation, the thickness of each layer of multilayer structure may be made constant and reproducibility of photoconductive characteristics is very good. Also, since individual photoconductive materials are evaporated from different evaporating means, each layer of the multilayer structure may be made to have a desired thickness by appropriately adjusting the temperature of the evaporating means, which permits controlling photoconductive characteristics of the photoconductive structure, such as spectral response and sensitivity.

As described in the foregoing, according to the invention it is possible to obtain a multilayer photoconductive target by alternately laminating layers of lead oxide 0.8 p. thick and sensitive to short wavelength lights and layers of lead sulfide 0.2 p. thick and sensitive to long wavelengths. Such a multilayer structure is capable of efficient absorption of visible light in a wide range of long wavelength region as represented by characteristic curve A in FIG. 5. As is seen from the FlG., the multilayer structure according to the invention is far sensitive to long wavelength regions without having reduced sensitivity to short wavelength regions as compared to the corresponding characteristic of a conventional lead oxide target as represented by curve B in FIG. 5, which means improved performance and broadened field of use.

Further, as is seen from FIG. 6, the lag characteristic of the lead-oxide-lead-sulfide multilayer photoconductive structure according to the invention is greatly improved as represented by curve A in FIG. 6 in comparison with the similar characteristic of a conventional lead-oxide-lead-sulfide mixture ghotoconductive structure as represented by curve B of FIG.

Heretofore, lead sulfide which has ideal sensitivity to long wavelength regions, could not be applied for the photoelectric converting unit for an image pickup tube as its resistivity is essentially below 10 Q-cm. According to this invention lead sulfide can be used in combination with lead oxide having a high resistivity of over l0 (Lem as a multilayer structure and time constant in the direction of thickness of the resultant multilayer structure can be made to be greater than 1 second.

The foregoing description has been concerned with the manufacture of a leadoxide-lead-sulfide multilayer photoconductive structure. Similar effects would result from multilayer structures of a combination of lead-oxide layers and lead-selenide layers, a combination of lead-oxide layers and lead-telluride layers, and a combination of lead-oxide layers and layers of a mixture of lead telluride and lead sulfide or of lead sulfide and lead selenide, etc. I Y

The photoconductive materials which may be used in accordance with this invention include oxides, sulfides, selenides, tellurides of lead, antimony, arsenic, copper, silver, and cadmium, and suitable mixtures of these compounds.

In the foregoing description the lead-oxide layer and the lead-sulfide layer are respectively formed from intrinsic substances. By doping lead oxide with an impurity element selected from a group consisting of antimony, arsenic and bismuth, to form n-type photoconductivelayer, while doping lead sulfide with an impurity element selected from a group consisting of oxygen, silver, copper and thallium to form P- type photoconductive layer, a multilayer photoconductive structure consisting of alternate NP-N-P....or PN-P- N....layer lamination may be obtained, which structure being effective in considerably reducing dark current.

The foregoing embodiments adapted the use of a photoconductive material having a good sensitivity to light in shorter wavelength region and a photoconductive material having a good sensitivity to light in longer wavelength region. In this point it is also possible to obtain a photoconductive unit which is extremely sensitive to a desired spectral region by laminating together layers of photoconductive material having a relatively narrow spectral response region but good sensitivity and layers of a different photoconductive material whose spectral response region lie near that of the first photoconductive material.

Further, in case of antimony triselenide whose resistivity is too low to be used for television camera tubes, for instance, vidicon, by alternately laminating very thin layers of such low resistive photoconductive material and also very thin insulating layers of, for instance, silicon monoxide, more effective electric field can be established in the laminated structure, than the case of providing only one photoconductive layer having a greater thickness, so that produced free carriers may effectively contribute to conduction. As the insulating material may be used calcium fluoride and magnesium fluoride as well as silicon monoxide. 1

Furthermore, though the foregoing embodiments have been adapted for photoconductive cells and image pickup tubes, similar effects may also be obtained when the multilayer structure is used for photoconductive converter units such as light amplifiers.

Moreover, the characteristics may be remarkably improved by depositing additional photoconductive layer or layers on one side or on both sides of the more than two material multilayer photoconductive structure. By way of example, on the substrate may first be deposited a layer of a mixture containing lead oxide and a trace of an oxide or a sulfide of trivalent metallic element, for instance, antimony trisulfide, on which is then deposited the layer with lead-oxide-lead-sulfide multilayer photoconductive structure, on which is in turn deposited a thin layer of a mixture containing lead oxide and a trace of an oxide on a sulfide of a monovalent metallic element to improve sensitivity, dark characteristic and other various characteristics.

It will be understood that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims. It is intended, therefore, that all the matter contained in the foregoing description and in the drawings are to be interpreted as illustrative only and not as limitative of the invention.

We claim:

l. A multilayer photoconductive device having at least four layers comprising:

first layers of a photoconductive material having a given spectral response characteristic; second layers of another photoconductive material having a different spectral response characteristic from those of said'first layers; and each of said first and second layers having a thickness of less than 2 ;1., said first and second layers which are of different photoconductive material being alternately laminated on one another to form said device having at least four layers. 2. A multilayer photoconductive device according to claim 1 comprising at least five layers.

3. A multilayer photoconductive device according to claim 1 comprising at least seven layers.

4. A multilayer photoconductivedevice according to claim 1 for use as a target for an image pickup tube wherein said target is particularly sensitive to red light and comprises at least two first layers of lead oxide and a corresponding number of second layers of lead sulfide, each of said layers having a thickness of less than 2 ;1., said lead-oxide and lead-sulfide layers being alternately laminated on one another.

5. A The photoconductive target according to claim 4 wherein said first layers have a thickness of about 0.8 p. and

said second layers have a thickness of about 0.2 u.

6. The photoconductive target according to claim 4 wherein said first layers are of one type of conductivity and said second layers are of the opposite type of conductivity.

7. The photoconductive target according to claim 6 wherein said first layers contain at least one impurity element selected from antimony, arsenic and bismuth and said second layers contain at least one impurity element selected from oxygen, silver, copper and thallium.

Claims

2. A multilayer photoconductive device according to claim 1 comprising at least five layers.
3. A multilayer photoconductive device according to claim 1 comprising at least seven layers.
4. A multilayer photoconductive device according to claim 1 for use as a target for an image pickup tube wherein said target is particularly sensitive to red light and comprises at least two first layers of lead oxide and a corresponding number of second layers of lead sulfide, each of said layers having a thickness of less than 2 Mu , said lead-oxide and lead-sulfide layers being alternately laminated on one another.
5. A The photoconductive target according to claim 4 wherein said first layers have a thickness of about 0.8 Mu and said second layers have a thickness of about 0.2 Mu .
6. The photoconductive target according to claim 4 wherein said first layers are of one type of conductivity and said second layers are of the opposite type of conductivity.
7. The photoconductive target according to claim 6 wherein said first layers contain at least one impurity element selected from antimony, arsenic and bismuth and said second layers contain at least one impurity element selected from oxygen, silver, copper and thallium.