US3268764A

US3268764A - Radiation sensitive device

Info

Publication number: US3268764A
Application number: US250268A
Authority: US
Inventors: Robert A Simms
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1963-01-09
Filing date: 1963-01-09
Publication date: 1966-08-23
Anticipated expiration: 1983-08-23
Also published as: DE1201865B; GB1043430A

Description

g- 1966 R. A. SlMMS 3,268,764

RADIATION SENSITIVE DEVICE Filed Jan. 9, 1965 2 Sheets-Sheet 1 VOLTA J: OUTPUT WITNESSES INVENTOR fi ug K. Robert A. Simms ATTGRNEY Aug. 23, 1966 R. A. SIMMS RADIATION SENSITIVE DEVICE Filed Jan. 9, 1965 2 Sheets-Sheet 2 LIGHT TRANSFER CHARACTERISTICS Fig.3.

DI. mm L O P. m mm 0 0 SP 30. Vu TM L v m 0A A u w 1 VI m0 V I v 20 VOLTS I =.OOl4LLAMP V =3OVOLTS I =.2UAMF? v 2o VOLTS DOS TARGET ILLUM|NATlON- FOOT CANDLES (Z870KELVIN TUNGSTEN SOURCE) STORAGE CHARACTERISTIC 2 FOOT CANDLE ILLUMINATION ESi =I5 VOLTS 0 m o 0 M F E G A O R 2 O T S O m mm IIIIIII IIVII d -0 6 N w 0 M '4 R G E m l [O 2 I\I\\|.\L O 0 O O O O O 0 m 0 9 TIME-SECONDS ice 325,754 I Patented August 23, 1966 3,268,764 RADIATION SENSHTHVE DEVICE Robert A. Sirnms, llorseheads, N.Y., 'assignor to Westinghouse Electric Corporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed Jan. 9, I963, Ser. No. 250,268 9 Claims. (Cl. 315-16) This invention is directed to an electron discharge device and more particularly to an improved radiation sensitive target electrode.

One particular application of this invention is in a photoconductivity type of pick-up tube. The most common type of photoconductivity pick up tube is a vidicon. The vidicon is comprised of an evacuated envelope in which there is provided a transparent ttace-plate portion. An electrically conductive coating of a radiation transmissive material is positioned on the inner surfiace of the face-plate. The conductive layer may be referred to as the back plate or signal plate of the tube. A layer of photoconductivity material is deposited upon this back plate and may be referred to as the target. An electron gun is provided at the opposite end of the envelope with respect to the face plate for providing an electron beam. Suitalble scanning means is provided for scanning the electron beam over the target member. The beam of electrons may be of high energy type that is, of an energy between the first and second crossover potential of the target surface or it may be of the more common type of low energy type in which operation is below the first crossover potential of the photoconductive material.

In the latter type of operation, the electrons are substantially slowed down as they approach the target and are deposited upon the exposed photoconductive surface to drive the surface to substantially the potential of the cathode of the electron gun. The conductive signal plate of the target is normally held at a potential of several volts to 100 volts) positive with respect to the cathode potential. The exposed surface of the target is normally maintained at the cathode potential. In this manner, an electric field is provided across the photoconductive layer. When a radiation image is directed onto the photoconductive material, the film of photoconductivity material is excited. The excitation of the photoconductivity material by the photons causes a generation of electrons and holes. As a result of the field impressed across the photoconductive layer, a current flow will take place across the film in the illuminated areas and will cause those areas on the exposed surface of the photoconductive layer to tend to charge towards the potential of the conductive back plate. Those areas of the photoconductive layer which are not illuminated will remain at substantially cathode potential. The electron beam upon scanning over the exposed surface of the photoconductive layer will return the illuminated target areas to cathode potential. Since the signal plate is capacitively coupled with the scanning surface of the target, the instantaneous charging of the target by the beam to cathode potential will be evidenced by a voltage change in an output circuit provided to the conductive back plate. This voltage change is the output signal of the tube. In those areas that were not illuminated there will be no change in potential and there will not be an output signal derived from those unillurninated areas. The term radiation means not only electromagnetic radiation such as light but also particle bombardment such as electrons.

The vidicon type of pick-up tube provides a tube which is simple, reliable and rugged. This type of tube, then, is very desirable for use in such critical environments as space vehicles. The vidicon has had several drawbacks in the past. One is lack of light sensitivity. An-

other is sutficient dark resistivity to give adequate storage in the operational encvironmtent. The improvement in darkresistivity of the [device will provide a better sig nal to noise ratio. Another disadvantage of the vidicontype tube is the inherent photoconductive time lag, which causes objectionable trailing in moving objects in a transmitted picture.

It is accordingly an object of this invention to provide an improved radiation sensitive device.

It is a further object to provide an improved radiation sensitive target having low dark current and the minimum time lag.

It is another object to provide an improved electron bombardment induced conductivity type of target.

It is another object to provide an improved light sensitive target for use in a pick-up tube.

It is another object to provide an improved radiation sensitive target of high sensitivity.

In accordance with my invention, I provide an improved radiation sensitive member comprised of an electrical conductive layer having a thin layer of insulating material provided on one surface thereof and a radiation sensitive coating provided on the insulating layer.

Further objects and advantages of the invention will become apparent :as the following description proceeds. The features of novelty which characterize the invention will be pointed out in particularity in claims annexed to and forming a part of the description.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a view in section of a pick-up tube embodying the teachings of this invention;

FIG. 2 is an enlarged sectional view of the target schematically shown in FIG. 1;

FIG. 3 is a graphical representation ofthe output signal of the target versus illumination according to this invention in comparison with the prior art; and

FIG. 4 is a graphical representation illustrating the advantages of this invention over the prior art.

Referring now to FIGS. 1 and 2, a pick-up tube is illustrated including an evacuated envelope 12 containing an electron gun assembly 20 and a target assembly 30. The electron gun 20 consists of at least a cathode 22, a control grid 24 and one or more accelerating

anodes

26 and 28 connected through suitable lead-in to appropriate sources of potential for generating and forming an electron beam. The specific design of the gun 20 is conventional and any suitable type of electron gun for generating a pencil type electron beam is suitable for this application. The envelope 12 includes a face-plate portion 14 of a material such as glass transmissive to radiations from the scene. A light transmissive electrically conductive coating or film 32 is provided on the inner surface of the face plate 14. An insulating coating 34 is provided on the conductive layer 32 and a photoconductive layer 36 is provided on the insulating coating 34. The conductive film or coating 32 is the signal electrode or back plate of the target 30. An electrical lead-in 38 is provided to the exterior of the envelope 12. The lead-in 38 is connected through a resistor 37 to a voltage source 39. The signal output from the tube is derived across the resistor 37. The target 30 will be described in more detail as to structure and fabrication in connection with FIG. 2. Means are provided for focusing the electron beam gen erated by the electron gun 20 and scanning the beam over the target 30 to form a raster in a conventional wellknown means. This may include a focus coil 40, deflection yoke 41, and alignment coil 42. An electrical conductive screen electrode 33 is positioned adjacent the target 30 and during operation together with the focus coil 40, functions to insure that the electron beam from 25 the gun 20 is directed onto the target 30 normal to the surface thereof.

The electron discharge device described above is substantially conventional and any suitable type of structure may be utilized with the exception of the target assembly which is the new and improved target electrode of this invention.

Referring now to FIG. 2 for a more detailed description of the target 30. The target 30 is supported on the light transmissive face plate 14 of a material such as glass. The target 30 consists of an electrically conductive coating 32 of a thickness of about 500 angstroms and transmissive to the input radiation. The resistance of the material in the layer 32 should be less than 200 ohms per square. The electrical conductive coating 32 may be of a suitable material such as tin oxide and is formed by spraying a solution of tin salt over the heated support face plate 14. It is also possible to evaporate an electrical conductive coating such as gold according to well-known and established techniques onto the face plate 14 to provide the conductive coating.

After the electrically conductive coating has been provided upon the face plate, the retina structure is placed in a vacuum of approximately 10* millimeters of mercury. A tantalum boat containing a precisely measured weight of one milligram of a suitable insulator such as silicon monoxide is positioned at a distance of not less than inches from the retina structure. The spacing of the boat with respect to the retina assures the uniformity of the evaporated insulating layer. The tantalum boat is then heated to a temperature of 1350 C. for a period of 5 minutes that the silicon monoxide is evaporated onto the conductive layer 32. If the retina is weighed before and after this evaporation, the thickness of the insulator coating 34 can be calculated from the knowledge of its density and geometry. The resulting insulating layer 34 has a thickness of 50 to 100 angstroms. Other suitable insulating materials such as magnesium oxide may be used for this layer. The resistivity and thickness of the layer 34 must be such that a high field of something greater than 10 volts per centimeter can be contained across the insulator upon application of normal vidicon operating target voltages. This has been found to call for thickness of about 50 to 100 angstroms and a material whose bandgap is somewhat greater than 4 electron volts.

The radiation sensitive layer 36 is a semiconductor whose dark resistivity is equal to or greater than 10 ohms centimeters and whose bandgap is compatible with the spectral range ultimately desired in the tube, less than 3.5 electron volts. It is a desirable feature that the dielectric constant of the semiconductor be as low as feasible. Typical semiconductors which have proved to be feasible in this application are As se As Se S, AsSe, and Sb2S3.

One suitable semiconductor i-s As Se and this semiconductor is evaporated onto the insulator coating 34 using essentially the same techniques as used in evaporating the insulating layer 34. The retina is placed in an atmosphere of approximately 10 millimeter of mercury and a given quantity of arsenic selenium is provided in a tantalum boat and positioned at a distance of about 10 inches from the retina. A typical weight of semiconductor material placed in the boat is 500 to 600 milligrams. In the case of As Se 580 milligrams of stoichiometric As Se compound is used. The compound is prepared by placing 1.498 grams of As and 2.369 grams of Se in a quartz bulb. The bulb is sealed off in 10- mm. Hg vacuum and heated to 650 C. The bulb is shaken to insure mixing of the melt and air quenched to room temperature. The tantalum boat is then heated to a temperature of 700 C. for a period of 3 minutes until a suitable thickness of the semiconductive material is deposited on the insulating layer 34. The control of the thickness of the semiconductor layer 36 is usually accomp'lished by well-established methods of monitoring the transmission of light through the retina during the evaporation until the desired characteristic is obtained. It has been found that in this specific application that a thickness of about 1000 angstroms is necessary and in view of the type of deposition, the layer is of the hard or metallic type of deposit. The deposit is at its normal bulk density. It is, of course, within the teachings of this invention to utilize suitable evaporating atmospheres such as an inert gas to provide what is known in the art as a porous-type of deposit. This deposit is at less than its normal bulk density. In the case of the porous deposit, the thickness of the layer would be of about 1200 angstroms.

A target fabricated in accordance with this invention has better response, better sensitivity, minimum lag, and low dark current.

Some of these features are illustrated in FIG. 3. The curves illustrate the signal response of diflferent tube types and different target voltages. Curves 1 and 2 illustrate the signal output from a WL7325 tube with target voltages of 20 and 30 volts respectively. The target voltage is the voltage applied to the back plate 32 normally positive with the scan surface at substantially the potential of the cathode, normally ground. The WL7325 type target illustrates the prior device and consists of a porous layer of Sb S on a conductive back plate of tin oxide.

Curves

3 and 4 illustrate the signal output from a WX5035 tube with target voltages of 20 to 30 volts respectively. The WX5035 target consists of a conductive back plate of tin oxide, a layer of SiO on the tin oxide and a porous layer of Sb S on. the SiO. The dark current is reduced from 0.2 microampere to 0.08 microampere with a target voltage of 30 volts by utilizing the SiO layer.

Curves

5 and 6 illustrate the signal output from a WX4914 with target voltages of 20 and 30 volts respectively. The WX4914 consists of a back plate of tin oxide, a layer of SiO on the tin oxide and a vacuum deposit of As Se on the SiO.

Curves

5 and 6 also illustrate the advantages and anticipated results of the invention over the prior art.

It has also been found that by the provision of the insulating layer between the photoconductive layer and the conductive back plate in a device such as described in US. Patent No. 3,046,431 by J. Nicholson and assigned to the same assignee that it is possible to obtain a vidicon of high sensitivity without the storage phenomena as described therein. It was found that the use of the insulator substantially eliminated the storage effect and the integration eifect and provided a high sensitivity type of target.

This is illustrated in FIG. 4. The WX4914 does not provide integration of the input and provides destructive read out in less than $4 of a second as shown. The signal rises to a maximum in less than & of a second, see curve 7, and decays to 25 percent of the signal in about $6 of a second with the scan beam turned on, see curve 8. Information may be stored with scan beam off for several seconds, see curve 9, and then read out in about of a second by scan beam.

A WL7383, which consists of a target of a layer of As Se on tin oxide, illustrates by

curves

10, 11 and 12, the action of the device described in U.S. Patent No. 3,046,431. Curve 10 illustrates the integration properties of the device. Curve 11 illustrates the mtulticopy read out with the scan beam. Curve 12 illustrates the decay with the scan beam turned off.

Another application of invention is as a target in an electron bombardment induced conductivity type of device such as described in US. Patent No. 2,900,555 and assigned to the same assignee.

The theory of the operation of this target is not entirely clear but the following explanation is given in hopes of clarifying the matter. In the case of the ideal target, in which a conductive coating is utilized and a semiconductive photoconductor deposited thereon, an ohmic contact is required to the semiconductor which by definition supplies the free carriers required by the photoconductive process. The retina functions as follows. A photon is absorbed forming a free electron hole pair. Under the applied field, the photo-excited electrons move toward the positive or anode potential; similarly the photo-excited holes would move toward the negative electrode or cathode. After the initial electron leaves the photoconductor at the anode, the residual hole left in the material provides for the entrance of another electron into the material via the cathode. This charge transfer constitutes the video signal. It is however believed that in the prior art structures, assuming that the electron beam provides an ohmic injection contact, that the problem really exists in the contact between the conductive back plate and the photoconductor. It is believed that the real difference between what should be the ideal situation is due to the presence of recombination centers in the photoconductor and a mismatch in the work function of the semiconductor and the conductive coating. The presence of recombination centers means that free carriers (in this case the discussion will center around electrons) suffer a reduction in life-time depending upon the amount of time spent in the recombination centers and the direct result of this reduced life-time is a decrease in sensitivity as demonstrated by the following mathematical relationship. G is equal to where G is the gain of the photoconductor, U is the mobility of the carrier, V is applied voltage, L is the inter-electrode spacing, and R is the free electron lifetime. A further reduction in gain results from the presence of the barrier due to the mismatch between work functions of the semi-conductor and the conductor. This reduction is caused by the inability of electrons to cross the barrier which could conceivably be as high as a few electron volts. Those electrons which cannot, because of their energy, surmount this barrier are reflected back into the semiconductor where they are susceptible to recombination. As the objective control of recombination centers as well as the possibility of matching Work functions, is beyond the state-of-the-art, it follows that one must effectively reduce the barrier by indirect means in order to achieve the full intrinsic sensitivity or gain of the photoconductor. By insertion of the insulator between the photoconductor and the conductor, and with the normal operating range of target voltage applied, and under the conditions of photon input, the majority of the voltage gradient appears across the insulator. [by photon absorption are directed toward this high-field gradient and as they come under its influence, they achieve a gain in energy according to the established physical laws and hence, have enough energy to surmount the barrier. This effectively reduces the probability of recombination resulting in an increase in gain or sensitivity.

While there have been shown and described what are presently considered to be preferred embodiments of the invention, modifications thereof will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. A radiation sensitive device comprising a layer of radiation sensitive material, said radiation sensitive film exhibiting the property of change in electrical conductivity in response to radiation, an electrically conphotoconductive Electrons freed ductive layer and a film of insulating material sand wiched between said conductive layer and said radiation sensitive layer, an electron beam for scanning the exposed surface of said radiation sensitive material so as to substantially maintain the surface thereof at an equilibrium potential, means for establishing a potential on said electrically conductive layer different from said equilibrium patential to establish a field across said radiation sensitive material and said film of insulating material, said film of insulating material being non-responsive to radiations to which the radiation sensitive device is adapted for use.

2. A radiation sensitive device comprising an electrically conductive layer, a film of insulating material deposited on one surface of said electrically conductive layer of a thickness of about angstroms and a layer of material exhibiting the property of photoconductivity deposited in intimate contact with the exposed surface of said insulating film, said insulating film comprised of a material exhibiting the property of being non-responsive to radiations to which said target is adapted for use and means for impressing a field across said photo conductive material and said insulating material to provide means of conduction of charge carriers within said photoconductive layer in response to radiation directed thereon.

3. A light sensitive pick-up tube comprising an evacuated envelope and including therein means to develop a scanning electron beam, a light sensitive target assembly within said envelope to be scanned by said electron beam and cooperating with said beam to produce output signal currents corresponding to light excitation on elemental areas of said target assembly, said target assembly comprising a light transmissive conductive film provided with a lead-in conductor to apply a biasing potential thereto, a layer of light transmissive insulating material on said conductive film of a thickness of substantially 50 to 100 angstroms, said insulating film of a material exhibiting the property of maintaining a high resistance in the presence of light and a coating of photoconductive material responsive to light on said insulating material and covering the surface of said insulating layer thereof and spaced from said conductive film by substantially the thickness of said insulating layer.

4. An image tube comprising an evacuated envelope containing means to develop a scanning electron beam, an electron sensitive target assembly within said envelope to be scanned by said electron beam and cooperating with said beam to produce an output signal current corresponding to the electron excitation on elemental areas of said target assembly, said target assembly comprising a conductive film provided with a lead-in conductor to apply a biasing potential thereto, a layer of insulating material on said conductive film of a thickness greater than 50 angstroms, a coating of electron bombardment induced conductivity material on said insulating layer covering the surface of said insulating layer and spaced from said conductive film by the distance of at least 50 angstroms.

5. A light sensitive pick-up tube comprising an evacuated envelope containing means to develop a scanning electron beam, a light sensitive target assembly within said envelope to be scanned by said electron beam and cooperating with said beam to produce output signal currents corresponding to light excitation on elemental areas of said target assembly, said target assembly comprising an electrically conductive film provided with a lead-in conductor to apply a biasing potential thereto, a layer of light transmissive insulating material provided on said conductive film, said insulating film of a material exhibiting the property of maintaining a high resistance in the presence of light and a coating of photoconductive material on said insulating material covering the surface of said insulating material and spaced from said conductive material by at least 50 angstroms.

6. A light pick-up tube comprising an evacuated envelope containing means to develop a scanning electron beam, a light sensitive target assembly within said envelope to be scanned by said beam and cooperating with said electron beam to produce output signal currents corresponding to light excitation on elemental areas of said target assembly, said target assembly comprising an electrically conductive film provided with lead-in conductor to apply a biasing potential thereto, a layer of light transmissive insulating material having a bandgap greater than four electron volts provided on said conductive film, said insulating film of a material exhibiting the property of maintaining a high resistance in the presence of light and a coating of photoconductive material on said insulating material layer covering the surface of the insulating material layer and spaced from said conductive film by at least 50 angstroms.

7. A light pick-up tube comprising an evacuated envelope containing means to develop a scanning electron beam, a light sensitive storage target assembly within said envelope to be scanned by said beam and cooperating with said beam to produce an output signal current corresponding to the light excitation on elemental areas of said target assembly, said target assembly comprising a conductive film provided with a lead-in conductor to apply a biasing potential thereto, a layer of light transmissive insulating material of a porous deposit of a thickness greater than 50 angstroms, said insulating film of a material exhibiting the property of maintaining a high resistance in the presence of light and a coating of photoconductive material on said insulating material covering the surface and spaced from said conductive layer by at least the thickness of said insulating layer.

8. A light sensitive pick-up tube comprising an evacuated envelope containing means to develop a scanning electron beam, a light sensitive target assembly Within said envelope to be scanned by said beam and cooperating with said beam to produce output signal currents corresponding to light excitation on elemental areas of said target assembly, said target assembly comprising a conductive film provided with a lead-in conductor to apply a biasing potential thereto, a layer of light transmissive insulating material on said conductive film, said insulating film of a material exhibiting the property of maintaining 8 a high resistance in the presence of light and a coating of a photoconductive material including arsenic and selenium covering the surface of said insulator and spaced from said conductive film by at least the thickness of said insulating layer.

9. A light sensitive pick-up tube comprising an evacuated envelope containing means to develop a scanning electron beam, a light sensitive target assembly Within said envelope to be scanned by said beam and cooperating With said beam to produce output signal currents corresponding to light excitation on elemental areas of said target assembly, said target assembly comprising an electrically conductive film provided With a lead-in conductor to apply a biasing potential thereto, a layer of light transmissive insulating material having a bandgap greater than four electron volts, said insulating film of a material exhibiting the property of maintaining a high resistance in the presence of light and a coating of a photoconductive material having a bandgap less than 3.5 electron volts and consisting essentially of arsenic and selenium.

References Cited by the Examiner UNITED STATES PATENTS 2,277,013 3/1942 Carlson 250-211 X 2,277,101 3/ 1942 Heimann 250211 2,881,340 4/1959 Rose 250-211 X 2,944,155 7/1960 Mayer 313 X 2,997,614 8/1961 Morton et al 313-65 3,046,431 7/ 1962 Nicholson 313--65 3,048,502 8/1962 Nicholson 313-65 X FOREIGN PATENTS 146,368 3/ 1949 Australia.

OTHER REFERENCES Knoll and Kazan Storage Tubes and Their Basic Principles, Wiley and Sons, New York, 1952, pp. 28-29.

DAVID G. REDINBAUGH, Primary Examiner. WALTER STOLWEIN, Examiner.

Claims

1. A RADIATION SENSITIVE DEVICE COMPRISING A LAYER OF RADIATION SENSITIVE MATERIAL, SAID RADIATION SENSITIVE FILM EXHIBITING THE PROPERTY OF CHANGE IN ELECTRICAL CONDUCTIVITY IN RESPONSE TO RADIATION, AN ELECTRICALLY CONDUCTIVE LAYER AND A FILM OF INSULATING MATERIAL SANDWICHED BETWEEN SAID CONDUCTIVE LAYER AND SAID RADIATION SENSITIVE LAYER, AN ELECTRON BEAM FOR SCANNING THE EXPOSED SURFACE OF SAID RADIATION SENSITIVE MATERIAL SO AS TO SUBSTANTIALLY MAINTAIN THE SURFACE THEREOF AT AN EQUILIBRIUM POTENTIAL, MEANS FOR ESTABLISHING A POTENTIAL ON SAID ELECTRICALLY CONDUCTIVE LAYER DIFFERENT FROM SAID EQUILIBUM PATENTIAL TO ESTABLISH A FIELD ACROSS SAID RADIATION SENSITIVE MATERIAL AND SAID FILM OF INSULATING MATERIAL, SAID FILM OF INSULATING MATERIAL BEING NON-REPSONSIVE TO RADIATIONS TO WHICH THE RADIATION SENSITIVE DEVICE IS ADAPTED FOR USE.