US2875359A - Photoconductive device - Google Patents

Description

Feb. 24, 1959 A. D. COPE 2,875,359

PHOTOCONDUCTIVE DEVICE Filed Aug. 6, 1956 APPLE-TUN 7, [Upr- United States Patent O PHOTOCONDUCTIV E DEVICE Appleton D. Cope, Hightstown, N. J., assignor to Radio lCorporation of America, a corporation of Delaware Application August 6, 1956, Serial No. 602,187 8 Claims. ('Cl. 313-65) This invention relates to photoconductive devices. In particular, this invention relates to an improved photoconductive element for use in a photoconductive device.

In the past there have been various types of photoconductive devices. A photoconductor is a material which is substantially an insulator when in the dark, but which is highly conductive when exposed to light. This change in conductivity, as a result of exposure to light, is limited to the particular areas of the photoconductor which are exposed to the light. Therefore, photoconductive material is extremely useful as the light sensitive member in photoconductive devices such as phototubes,

pickup tubes and electroluminescent devices.

When selecting a photoconductor for use in photoconductive devices, it is desirable to utilize a photoconductor which has the following properties: a high sensitivity, i. e. a large change in conductivity in response to a given amount of light; a low dark current, i. e. a high resistance when in the dark; and a short photoconductive lag, e. the photoconductor returns to its original high resistance state immediately after the light is removed.

Some of the photoconductive materials that are known at this time have a high sensitivity coupled with a long photoconductive lag. Other known materials have a short photoconductive lag coupled with low sensitivity. Still others have a short photoconductive lag coupled with a high dark current.

It is an object of this invention to provide a new and improved photoconductive material.

It is another object of this invention to provide a novel photoconductive device utilizing a new photoconductive material.

These and other objects are accomplished in accordance with this invention by providing a photoconductive material that includes a combination of antimony tri-sulphide and antimony oxysulphide. The novel photoconductive material may be used as the light sensitive element in a pickup tube, a phototube, an electroluminescent device, or in other known types of tubes wherein photosensitivity is utilized.

The invention will be pointed out in more detail by reference to the following specification when read in conjunction with the accompanying single sheet of drawings wherein:

Fig. 1 is a transverse sectional view of a pickup tube utilizing this invention;

Fig. 2 is an enlarged fragmentary sectional view of the target shown in Fig. 1;

Fig. 3 is an enlarged fragmentary sectional view of an embodiment of a target in accordance with this invention; and

Fig. 4 is a graph of 'the spectral response of a target in accordance with this invention.

Referring now to Fig. l in detail, there is shown a transverse sectionalview of a pickup tube in accordance with this invention. The tube 10 is a version of a tube type that is commercially known as a Vidicon. The particular version shown is that of a tube that has ICC a diameter of approximately half an inch. Thelength of the tube is approximately three inches. It should be understood that this particular version of a Vidicon is shown merely to illustrate an example of applicants invention and that other versons of the Vidicon type pickup tube may be utilized in accordance with this invention. Furthermore, it should be understood that other devices utilizing a photoconductive member as the light sensitive element, for example certain photoconductive cells such as tube type 6694, or electroluminescent devices utilizing an electroluminescent phosphor and a photoconductor are believed to be within the scope of applicants invention.

The tube 10 comprises an evacuated envelope 11 having an electron gun 12 sealed within one end of the envelope and closed by a glass face plate 34 at the other end. The gun 12 includes a heater 14 which is substantially enclosed by a cathode 16. The cathode 16 may have any of the known types of electron emissive material thereon such as barium oxide. Spaced around the cathode 16, and in coaxial alignment therewith, is a control electrode 18 having an aperture in the center of the closed end thereof. Spaced from the control electrode 18 and in alignment therewith is a rst and a second accelerating electrode 20 and 22, each of which has a centrally located aperture to accelerate and focus the electron beam from the gun 12. 'Spaced from the accelerating electrode 22 is a final accelerating electrode 24 which takes the form of an elongated cylinder having one end closed by a mesh screen 26.

Spaced closely adjacent to the mesh screen 26 is a target electrode 28. The target 28, which is shown more clearly in Fig. 2, comprises a photoconductive deposit 30 which is supported on a transparent conductor, or signal plate 32, which in turn is supported upon a transparent support member 34. As shown in the drawings, the transparent support member 34 comprises the end wall of the envelope 11. It should be understood that a separate member could be utilized as the support when desired and be spaced from the end of envelope 11. The transparent support, or face plate 34 is sealed to the envelope 11 by means of a sealing ring 36 which may be of any known metal that readily seals to glass. The opposite en d of envelope 11 comprises a stem 37, including an exhaust tubulation 38. The stem 37 supports a plurality of leadin conductors 40 for energizing the various electrodes within the envelope. The stem 37 is also sealed to the balance of the envelope 11 by means of a sealing ring 42 which may be similar to sealing ring 36.

During operation of the tube shown in Figure l, with potentials such as those shown therein, the electron beam from gun 12 is scanned over the surface of the photoconductive deposit with low velocity type of scanning. When the photoconductive deposit is in the dark, the electron beam drives the scanned surface of the photoconductive deposit to substantially cathode potential. When light from an object to be reproduced is directed onto the photoconductive deposit, the photoconductive deposit becomes conductive in the4 elemental areas which are struck by the light. The conductivity of the elemental areas permits the charge that has been established on the scanned surface of the photoconductive deposit to leakoff to the transparent conductive coating or signal plate 32. The next time the beam strikes the elemental area which has been discharged, the beam will replace the charge and, due to capacity coupling, will produce an output signal in the signal plate 32. The magnitude of the output signal will correspond to the magnitude of the light striking the photoconductive deposit, which is well known. The potentials shown in Fig. 1 are designed for the low velocity type of scanning. As is known to `those skilled in the art high velocity type of scanning may also be utilized.

ln accordance with this invention, the photoconductive deposit 30 comprises a combination of antimony trisulphide and antimony Oxy-sulphide. In the embodiment shown in Figs. l and 2 the antimony tri-sulphide is deposited upon the transparent conductive coating 32 and during the process Vof deposition, the material deposited is gradually changed to antimony Oxy-sulphide. ln other words, the photoconductive deposit 30 comprises a deposit do of antimony tri-sulphide, a deposit 4S which is a mixture of antimony tri-sulphide and antimony oxysulphide, and a deposit 50 of antimony Oxy-sulphide.

The target shown in Figure 2 may be manufactured, as an example, by placing an envelope I l having, a face plate 34 sealed thereto, and a transparent conductive coating 32 therein, in an evacuation chamber such as a bell jar (not shown). While in the bell jar, the face plate and conductive coating may be heated to a temperature within the range of 125 C. to l50 C. The evaporator assembly is pre-heated at a. temperature of approximately 300 for a time of approximately ten minutes. The evaporator assembly may comprise a conductive ribbon containing two pockets, with one pocket containing approximately nine milligrams of antimony tri-sulphide and another pocket containing approximately seven milligrams of antimony Oxy-sulphide. With the evaporator assembly spaced approximately one inch from the face plate, the evapora.or assembly is heated. The temperature of the evaporator is gradually increased from approximately 400 C. tov approximately 600 C. during a period of time of approximately three minutes. At a temperature of approximately 400 C. the antimony trisulphide starts to evaporate and rapid evaporation thereof occurs at a temperature of approximately 450 C. At a temperature of approximately 520 C. the antimony oxysulphide begins to evaporate and rapid evaporation thereof occurs at approximately 580 C.

By the gradual increase of the temperature of the single evaporator ribbon having separate troughs or pockets for the antimony tri-sulphide and antimony oxysulphide, it has been found that a deposit of antimony tri-sulphide is laid down, then a deposit including a mixture of antimony tri-sulphide and antimony oxy- .sulphide, then a deposit of antimony Oxy-sulphide. These layers are designated in Figure 2 by the phantom lines 47 and 49. It has been found that the mixture deposit 48 of antimony tri-sulphide and antimo-ny Oxy-sulphide is approximately 1% microns thick out of total thickness of 2% microns, with deposits 46 and 50 each beingrof approximately the same thickness when utilizing the method preferably described above.

For a one inch diameter envelope, the evaporator assembly is placed approximately two inches from the face plate and is loaded with approximately 20 milligrams of antimony tri-sulphide and 16 milligrams of antimony Oxy-sulphide.

Referring now to Figure 3, there is shown an embodiment of this invention comprising a target 54 having a transparent conductive coating 56 and a deposit of antimony tri-sulphide 58. On the deposit of antimony tri-sulphide is aY deposit of antimony Oxy-sulphide 66. Each of the photoconductive deposits 58 and 60 may be approximately l1/4 microns thick and may be deposited by evaporation.

The embodiment shown in Figure 3 may be manuactured as described above except that separate evaporators are used for the antimony tri-sulphide and antimony Oxy-sulphide. ln the alternative, a single evaporator may be used and heated to a temperature to evaporate all of the antimony tri-sulphide beforeA the temperature is increased to evaporate any of the antimony Oxy-sulphide.

In either of the embodiments of the target shown in Figures 2 and 3, the photosensitive materials are evapo rated ina good vacuum e. g. lO- mm. of mercury. Due to the fact that both of the targets 2S and 54 are substantially inert in the presence of oxygen, after the targets have been deposited, the targets 28 and 54 may be deposited on the face plate 34, which has previously been sealed to the envelope 1l, and before the electron gun parts 12 and the exhaust stem 33 are sealed into the envelope 11.

At times, it has been found that the novel photosensitive material is affected by some transparent co-nductive mate rials used as the signal electrode 32. Therefore, the

transparent conductive material is a material which hasl substantially no eect on the photosensitive material. Materials Within this category and in accordance with this invention are gold, which may be deposited by evaporation to a thickness of several thousand angstrom units; tin chloride which may be deposited by spraying a solution thereof on a heated face plate resulting in a iilm having a resistivity to the order of lil-3 ohms/sq.; or a thin film Vof indium which may be deposited by evaporation and may be of an appropriate thickness.

Due to the extreme smallness of the work area permitted in the particular version of Vidicon shown in Figure l, it is diflicult to exactly measure the amount of material that has been deposited on the transparent conductive coating. Therefore, it is preferable to determine the amount of photosensitive material deposited by using Weighted charges of the components which are then evaporated to completion. Y

Each of the targets 2S and 54 that have been described, have sensitivities of the order of 2000 micro-amps per lumen. Each of the targets have short photoconductive lag that is a great deal shorter than that of either of the materials when used alone. As an example, when using this invention, less than 20% residual signal can be measured 1/0 of a second after removing the light. Each of the targets has a low dark current, as an example dark currents less than .05 micro amps for target potentials of the order of 25 volts have been measured. Both of the targets have a gamma of unity at low light levels and in the range of 0.5 to 0.6 at high light levels.

Referring now to Figure 4, there is shown a spectral typical characteristic 62 of either of the targets in accordance with this invention. It should be noted that the spectral response peaks at approximately 630 millimicrons with the sensitivity at 400 millirnicrons running in the range of 5 to 25% of the peak value. Y

lt should be understood that, even though the inven tion has been described with -particular reference to photoconductive type pickup tube, the invention is also applicable to other photoconductive devices such as phototubes and electroluminescent devices.

What is claimed is:

1. A photoconductive material including antimonyY tri-sulphide and antimony Oxy-sulphide.

2. A photoconductive material including antimony trisulphide, a mixture of antimony tri-sulphide and antimony Oxy-sulphide, and antimony Oxy-sulphide.

3. A photoconductive device including antimony trisulphide and antimony Oxy-sulphide.

4. An electrode structure comprising a conductor, antimony tri-sulphide on said conductor, and antimony Oxy-sulphide supported by said antimony tri-sulphide.

5. An electrode structure as in claim 4 including aY mixture of antimony tri-sulphide and antimony oxysulphide between said antimony tri-sulphide and said antimony Oxy-sulphide.

6. A target for a television pickup tube comprising a transparent conductor, a deposit of antimony tri-sulphide on said conductor, and a deposit of antimony Oxy-sulphide on said antimony tri-sulphide.

7. A target for a; television pickup tube comprising a transparent conductor, a photosensitive deposit on-said conductor, said photosensitive deposit including only a deposit ofv antimony tri-sulphide and a deposit of antimony l Oxy-sulphide.

8. A target for a television camera tube comprising a 5 5 transparent conductor, a photosensitive element on said References Cited in the le of this patent Conductor comprising a deposit of antimony tri-sulphide, UNITED STATES PATENTS a deposit of a mixture of antimony Oxy-sulphide and antimony tri-su1phide, and a deposit of antimony oxy- 2,654,852 Goodrich OC- 6, 1953 sulphide in the order recited with said antimony tn'- 5 2,687,484 Wemr Allg- 24, 1954 sulphide being on said transparent conductor. 2,744,837 Forgue May 8, 1956

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