US2967254A - Composite photoconductive layer - Google Patents

Description

Jan. 3A, 1961I s. v. FoRcauE 2,967,254

COMPOSITE PHoTocoNDUcTIvE LAYER Filed Feb. 18, 1955 /f in lll/l as" a 500D 1//070- nited States Patent i COMPOSITE PHOTOCONDUCTIVE LAYER Stanley Vincent Forgue, Cranbury, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Feb. 18, 1955, Ser. No. 489,203

11 Claims. (Cl. 313-65) 'the visible portion of the color spectrum coupled with ioptimum time lag characteristics and good sensitivity.

It is therefore a principal object of this invention to :provide a new and improved photoconductive target having high sensitivity in the visible portion of the spectrum -coupled with improved lag characteristics.

It is another object of this invention to provide an improved photoconductive target having suicient re- .sponse in the red portion of the visible color spectrum coupled with optimum sensitivity.

It is a further object of this invention to provide a new and improved television pickup tube utilizing a target having usable lag characteristics coupled with high sensitivity.

These and other objects are accomplished in accordance with this invention by providing a pickup tube having, within an evacuated envelope, a target including a porous layer of photoconductive material and a solid layer of photoconductive material. The target may also include still another layer of photoconductive material.

The novel features which are believed to be characteristic of this invention are set forth in the appended claims. The invention itself will best be understood by reference to the following specification when read in connection with the accompanying single sheet of drawings, in which:

Figure 1 is a sectional view of a pickup tube utilizing a photoconductive target in accordance with this invention;

Figure 2 is an enlarged fragmentary sectional view of the photoconductive target schematically shown in Figure 1;

Figures 3 4 are enlarged fragmentary sectional views of embodiments of this invention; and

Figure 5 is a light characteristic of a target in accordance with this invention.

Although this invention relates to photoconductive targets for use in various types of electron discharge devices, it will be explained with reference to television pickup, or camera, tubes in which it is particularly useful.

Referring now to the drawings in detail, and particularly to Figure 1, pickup tube comprises a vacuum tight envelope 11 having an electron gun 12 mounted in one end thereof. The several electrodes of the electron gun 12 include the usual cathode 36, control electrode 37, and one or more accelerating anodes 38 which are connected through lead-in pins 39 to appropriate sources of potential (not shown) for forming an electron beam 25. The specic design of the gun 12 is conventional and forms no part of the present invention and further discussion thereof is not deemed necessary.

The electron beam 25 from the gun 12 is directed upon a target y13 mounted in the other end portion of ,i 2,967,254 Patented Jan. 3, 1961 ICC tube 10. Target 13 is schematically shown in Figure 1 and will be described in detail in connection with Figure 2. Means are provided for focusing the electron beam and scanning the beam over target 13 to form a raster. These means may include a focus coil 14, deflection yoke 15, and an alignment coil 16. A mesh screen electrode 17, which is permeable to the electron beam 25, is positioned adjacent to target 13 and during operation, together with focus coil 14, functions to insure that the electron beam 25 is normal to the surface of target 13 during its approach to the target 13. On the interior surface of envelope 11 is an accelerating electrode 19 which is in the form of a conductive coating on the envelope 11.

The above recited elements of the tube 10 are conventional and form no part of this invention, except insofar as these elements are combined with the new and improved target electrode of this invention.

Referring now to Figures 1 and 2 for a description of target 13. Target 13 is conventionally supported adjacent to a transparent window 20 and is electrically energized by means of a lead-in 26 extending through the Window 20, or through other portions of the envelope 11.V The lead-in 26 is connected to an appropriate potential source (not shown). As shown more clearly in Figure 2, target 13 comprises a support plate 21 which is preferably of transparent glass.- The support plate 21 may comprise the transparent window 20, shown in Figure l, or the support plate 21 may be a separate transparent support member which is supported by the lead-in 26 adjacent to the window 20. A transparent conductive coating, or signal plate 22 is disposed on the surface of support plate 21 toward the electron gun 12, and may be of a material such as tin chloride or tin oxide.

In accordance with this invention, the target 13 includes a porous layerof photoconductive material 23 and a solid layer of photoconductive material 24. In the embodiment of the invention shown in Figure 2 the porous layer of photoconductive material 23 is deposited on the surface of the transparent conductive coating 22, while the solid layer of photoconductive material is deposited on the exposed surface of the porous layer of photoconductive material 23. Examples of materials for both the solid and porous layers of photoconductive material 23 and 24 are antimony tri-sulphide,

cadmium selenide, germanium sulphide, and antimony` thiosulphide. Both the solid layer and the porous layer of photoconductive material may be deposited by evaporation.

A specific method of manufacturing the target 13 is to apply the transparent conductive coating 22 ontoy the supporting plate 21 by spraying the tin oxide and condensing it on the support plate. After this is done the support plate 21 is placed in an atmosphere of argon at a pressure within the approximate range of 10.-1 to several millimeters of mercury at which time a layer of antimony tri-sulphide is deposited by evaporation from a heated source of the material. Because of gaseous atmosphere the antimony tri-sulphide is deposited in a porous form. Next, the support plate is placed in a good vacuum of approximately 5 10-6, or higher, millimeters of mercury, and a second layer of antimony tri-sulphide is deposited by evaporation.

This second layer is of a solid form due to the high vacuum used. The antimony tri-sulphide may be evaporated by heating a crucble containing the material to a temperature within the range of 500 to 600 C. Both of the layers may be evaporated from the same crucble (not shown). Approximately milligrams of antimony tri-sulphide in'thercrucible has been found to be sufficient to deposit both of the photoconductive layers'.

and form the composite photoconductive layer for a target of l square inch area.

A porous layer of cadmium selenide, may be deposited by `evaporating the material in an atmosphere of argon at apress'ure from-2X 1,0-1 to several mm. of mercury, while a porous ,layer of germanium sulphide may be deposited by evaporating the material in an atomsphere of argon at a. pressure of 5 1Ol mm. of mercury or lower, and a porouslayer of antimony thiosulphide may be deposited by `evaporating the material in an atmosphere of argon at a `pressure of approximately 5 l01 mm or lower. The solid layers of cadmium selenide, germanium sulphide, and antimony thiosulphide may be deposited by evaporation in a good vacuum of the orderof lil-6mm. of mercury.

VIt should be understood that other atmospheres may be utilized while forming the porous layer of photoconductive material 23 examples of which are neon, oxygen,

I' lau.

,Physically a porous antimony tri-sulphide layer, i.e. one evaporated-in a poor vacuum, may be said to have a dull or smokey appearance which contrasts with the smooth and shiny surface obtained when the same material is evaporated in a higher vacuum. This porous layer may also be described as spongy, or soft, to distinguish it from the solid layer which has a hard surface.

The thicknesses of the layers of photoconductive materil are not criticalfor purposes of this invention. It has been found that a solid layer of antimony tri-sulphide within the approximate range of .0l to .4 Vmil operates satisfactorily Vin the pickup tube target previously described. The porous layer of antimony tri-sulphide may also be within the vsame approximate range and operate satisfactorily in the type of target under consideration. The comparative thicknesses between the two layers of photoconductive material are also not critical. However, to eliminate capacity lag when the target described is used in a television pickup tube, it is necessary that the thickness of the composite photoconductive layer be greater than a minimum value. By capacity lag is meant the lag caused by a charge being built up across the target because of the capacity thereof. Since the capacity is increased as the target is made thinner, a minimum total thickness of 0.1 mil is preferred in order for the target to reproduce a transient change in light, i.e. the ability of the target to erase a signal in a given period of time without showing a shadow of trail or light. This thickness given is based on a targetarea of about a square inch. Greater thicknesses of layers would be necessary with larger area targets.

A target in accordance with the invention has a better response in the red portion of the spectrum than a target comprising a porous layer alone. This feature is shown schematically in Figure which compares the characteristics of a solid, porous, and composite layer of antimony tri-sulphide targets. `Figure 5 also shows the comparative sensitivities of porous, solid, and composite layers of antimony tri-sulphide in microamperes per microwatt of radiant energy. As is well known in the art, the total sensitivity of each of the layers over the visible spectrum is a function of the area under the respective curve of Fig. 5, the response of the human eye and the characteristie of a standard tungsten light source, and is measured in microamperes per lumen. The porous target has a sensitivity of the order of 50 to 75 micro amperes per lumen. A solid target alone has a sensitivity of 300 to 400 micro amperes per lumen, however, the lag characteristics thereof are undesirable. The composite target has a sensitivity of approximately 200 micro amperes per lumen in combination with good lag characteristics.

Furthermore, composite layer targets in accordance with this invention, have good photoconductive lag characteristics, i.e. they are capable of reproducing pictures of fast moving objects in approximately l frame time. The photoconductive lag characteristic` of thenovel composite .4 lantimony tri-sulphide target such as is shown in Figure 2 is better than that of a solid antimony tri-sulphide target alone and is as good as, or better than, the lag characteristics of a porous antimony tri-sulphide target alone. Still further, the exposed surface of the composite target is mechanically rugged, while the composite target thickness is suitable for the avoidance of capacity lag.

Referring now Vto Figure 3V there isV shown anenlarged fragmentary sectional view of an embodiment of this invention.` This embodimentof a target 2S differs from that previously described-in that a solid layer of photoconductive material 27 is deposited on the signal plate "22. On the solid layer of photoconductive material 27 is deposited a porous layer of photoconductive material 29. The means for depositing the layers 27 and 29 as well as the operation and advantages of the composite layer may be similar to those previously described,

Referring now to Figure 4 there is shown an enlarged fragmentary sectional View of another embodiment of.

this invention. In this embodiment, a target 30 is provided for a pickup tube as shown in Figure l, whichicornprises a support member 21 having a signal plate 22 on one surface thereof. On the signal plate 22 is deposited a porous layer of photoconductive material 32. On the porous layer of photoconductive material `32 is a solid layer of photoconductive material 33. On the exposed surface ,of the .solid layer of photoconductive material 33 is deposited a third layer of photoconductive material 35. The third layer of photoconductive material 35 may be a porous layer of photoconductive material, or in the alternative may be material different from those described above such as selenium, or 4other photoconductive material.

Itshould be understood that it is lwithin the contemplation of this invention that the order of photoconductive layers in the embodiment shown in Figure 4 maybe changed. The means for depositing the porous layer 32 yandsolid layer 33, as well as the operation andadvantages of the triple layer target may be similar to those pointed out above. The third layer of photoconductive material 35 may be deposited by evaporation.

In each of the embodiments shown in Figures ,3 Vand 4f the photoconductive materials may b e those described in connection with Figure 2, i.e. antimony tri-sulphide, cadmium selenide, germanium sulphide, or antimony thiosulphide. It should be understood that the porous layery of photoconductive material may be constructed of a different material than the solid layer in each of the em` bodiments showneg. in Figure 2 the porous photoconductive layer 23 may be antimony tri-sulphide while the solid photoconductive layer 24 may be lead selenide .or cadmium selenide.

What is claimed is:

`l. A photosensitive device comprising, a porous photoconductive material, a solid photoconductive material extending over said porous photoconductive material, and a conductive electrode extending over one of said photoconductive materials. g

2. A photosensitive device comprising, a porous layer of photoconductive material, a solid layer of photoconductive material extending substantially over said porous layer of photoconductive material, a transparent signal electrode on one of said layers of photoconductive material, and a member supporting said layers.

3. A photosensitive device comprising, a porous layer of antimony tri-sulphide, a solid layer of antimony trisulphide deposited on said porous layer, a signal electrode on one of said layers of antimony tri-sulphide, and a member supporting said layers.

4. A target for a photoconductive camera tube-comprising, a signal electrode, a porous layer of photocon-v ductive material disposed on one surface of said signal electrode, and a solidlayer of photoconductive-materi-al disposed on said porous layer.

5. A .target for ,a i photoconductiye lcarriera tube com.

prising, a signal electrode, a solid layer of photoconductive material disposed on one surface of said signal electrode, and a porous layer of photoconductive material disposed on said solid layer.

6. A target for a television pickup tube comprising, a porous layer of photoconductive material, a solid layer of photoconductive material extending over said porous layer of photoconductive material, and a third layer of photoconductive material extending over one of said layers of photoconductive material, and a signal plate extending over the other of said layers of photoconductive material.

7. A target for a pickup tube comprising a porous layer of antimony tri-sulphide, a solid layer of antimony tri-sulphide extending substantially over said porous layer of antimony tri-sulphide, and a signal electrode extending substantially over one of said layers of antimony trisulphide.

8. A target for a photoconductive camera tube comprising a signal electrode, a porous layer of antimony trisulphide disposed substantially over one surface of said signal electrode, and a solid layer of antimony tri-sulphide disposed substantially over said porous layer of antimony tri-sulphide.

9. A target for a camera tube comprising a transpareut signal electrode, a solid layer of antimony tri-sulphide disposed over one surface of said signal electrode, a porous layer of antimony tri-sulphide disposed over said solid layer of antimony tri-sulphide.

10. A photosensitive device comprising, a porous photoconductive material, a solid photoconductive material in Contact with said porous photoconductive material, and a conductive electrode in contact with one of said photooonductive materials, said photoconductive materials being selected from the group consisting of antimony tri-sulphide, cadmium selenide, germanium sulphide and antimony thiosulphide.

1l. A photo sensitive pick-up tube having an envelope the end Wall of which comprises a light transparent window, a light translucent conductive coating on the inner surface of said window, a light translucent porous layer of a photo-conductive material on said conductive coating and a solid layer of a photo-conductive material in Contact with said porous layer and which exhibits the photo conductive effect due to light reaching it through 20 said porous layer.

Weimer Aug. 24, 1954 Forgue May 8, 1956

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