US3321659A - Radiation sensitive electron emissive device - Google Patents

Description

May 23, 1967 T. P. BRODY 3,321,659

RADIATION SENSITIVE ELECTRON EMISSIVE DEVICE Filed Dec. 12, 1965 4 (D O! x m Z Fl 9.4.

DISTANCE WITNESSES INVENTOR Mex? 9744M Thomas I? Brody United States Patent 3,321,65 RADIATION SENSITIVE ELECTRON EMISSIVE DEVICE Thomas P. Brody, Penn Hilts Township, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 12, 1963, Ser. No. 330,139 8 Claims. (Cl. 31394) This invention relates to a solid-state electron device and more particularly to a radiation-sensitive electron emitter.

The present state of the radiation imaging art is particularly limited in the infrared range. Present photomissive surfaces with a vacuum work-function of less than 1 electron volt are not available. The obvious result of this limitation is that photoemissive devices cannot operate at wavelengths'longer than 1.2 microns. Present thermal-imaging devices, whose operation is dependent on phenomena other than that photoemission, are severe- 1y limited by factors such as speed and resolution. It would therefore be desirable to provide surfaces that would enable photoelectrons of infrared energies to the extracted into a vacuum. This would enable one to utilize conventional electron multiplication techniques as employed in present-day imaging devices and thus highsensitivity would be obtainable with the well-known high speed and high resolution capabilities of photoemissive surfaces.

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

It is another object to provide a solid-state imaging device which permits vacuum extraction of low energy photoelectrons.

It is another object to provide a solid-state infrared sensitive electron emissive cathode.

Briefly, the present invention accomplishes the above objectives by providing a metallic photosurface having a thin film of insulating material provided thereon and a thin electrically conducting film provided on the exposed surface of the insulating material. The thin electrical conducting film is treated to have a low vacuum work function.

These and other objects will become more apparent when considered in view of the following specification and drawings, in which:

FIGURE 1 is a schematic drawing embodying the techniques of the present invention;

FIGURE 2 is an energy level diagram versus distance of the radiation sensitive electron source of FIGURE 1;

FIGURE 3 is a schematic drawing illustrating a modification of the radiation sensitive electron source shown in FIGURE 1; and

FIGURE 4 is an energy level diagram versus distance for the electron source shown in FIGURE 3.

Referring to FIGURE 1, a photoelectric device is shown including an evacuated envelope 10. ;The. en-

velope includes an input window 12 and an output window 14. A cathode structure 16 is provided at the input end of the envelope 10 and an output electrode 18 is provided at the opposite end of the envelope adjacent the output window 14. The output electrode 18 is comprised of an electron sensitive phosphor layer 20 such as zinc sulfide deposited on the inner surface of the output window 14. An electrically conductive layer 22 is deposited on the exposed surface of the phosphor layer 26'. A lead-in 23 is connected to the conductive layer 22 to provide an external terminal. The output window 14 is transmissive to the light generated in the phosphor layer 20 so that an observer may view the output image.

The cathode structure 16 consists of a metal photosurface layer 30 which may be deposited directly on the input window 12. The photosurface layer 30 is of a suitable material such as aluminum, magnesium, bismuth or beryllium. These materials exhibit the property of absorbing input radiation, giving up energy and increasing the kinetic energy of an electron by an amount hu: u is the frequency of the input radiation, and h is Plancks constant. These increased energy electrons may be referred to as photoelectrons. The layer 30 is of a suitable thickness of a few hundred angstroms and may be deposited on the input window 12 by suitable process such as evaporation in a vacuum of about 10- torr. In the case of an infrared device, the input window 12 should be transmissive to infrared and may be of a suitable material such as sodium chloride or barium chloride.

Deposited on the exposed surface of the layer 30 facing the output electrode 18 is a layer 32 of a suitable insulating material such as aluminum oxide and of a thickness of about angstroms. The insulating layer 32 may be formed on an aluminum'layer 30 by admitting oxygen to the system at a pressure of a few microns. The thickness of the insulating layer 32 is selected so that the layer 32 will have a tunneling characteristic when a suflicient potential is applied across the layer 32. In the tunneling characteristic, the current voltage characteristic of the device should have a steep rise in current for a relatively small increase in voltage. Such a characteristic need not entirely result from tunneling alone, but may also result from electron injection over the barrier at the barrier at the photoemissive layer 30 and insulating layer 32 interface. Another suitable insulator for the layer 32 is bismuth oxide when using a bismuth for layer 30.

An electrically conductive layer 34 is evaporated onto the exposed surface of the insulator 32 facing the output electrode 18 to provide a low Work function layer andlateral electrical conductivity. A voltage source 24 is connected across the layers 30 and 34 with polarity as shown. The electrode 34 may be comprised of an alkali metal, such as potassium, or an earth alkali metal such as barium, deposited onto the insulating layer 32. The layer 34 may also be comprised of a metal layer that is oxidized at its surface to provide a low work function, such as barium oxidized to form barium oxide. The layer 34 may also be formed by alloying a metal layer with another material, for example, gold alloyed with barium. Another method of providing the low work function layer 34 would be to deposit a monolayer of a low work function material on a metal surface such as a monolayer of caesium de- Thus, by utilizing a thin layer 34 of a material having a Q11 low work function surface to vacuum, electrons may readily be emitted from the cathode structure 16 into the vacuum portion of the envelope and then be accelerated on into the output electrode 18.

Suitable focusing and acceleration means may be provided, as well known in the art, in the form of electrostatic or electromagnetic means (not shown) to provide focusing of the electron image emitted from the cathode 16 onto the output electrode 18.

In the specific device shown, an electron multiplier structure 40 is provided between the cathode 16 and the output electrode 18. The structure 40 may consist of one or more multipliers and may be of the type described in US. Patent 2,905,844 entitled, Electron Discharge Device, by E. I. Sternglass and assigned to the same assignee as this invention. A source of direct potential 42 is connected between the cathode 16 and the multiplier electrode 40 with the positive terminal connected to the electrode 40 and the negative terminal connected to the cathode 16. The potential of the battery 42 may be about 300 volts. A second voltage source 44 is connected between the electrode 40 and the output electrode 18. The voltage of this source 44 may be 10,000 volts.

In order to further explain the operation of the device shown in FIGURE 1, reference is made to the energy diagram of FIGURE 2. The input radiation which may be in the form of infrared photons, is focused on the front metal film 30. The photons on impingement on the layer excite electrons within the layer 30 to an energy ltu above the Fermi level of the material in layer 30. If these excited electrons or photoelectrons are sufiiciently energetic to overcome the barrier at the interface between the metallic layer 30 and the insulating layer 32, they will pass through or over the barrier 35 provided at the interface between the layers 30 and 32. A bias voltage is applied by the battery 24. The negative terminal is connected to the layer 30 and the positive terminal to layer 34. The voltage source may be of a potential of about 2 volts. The photoelectrons will pass through the insulator layer 32 and the layer 34 and be emitted into the vacuum portion of the envelope. The electrons emitted into the vacuum are then accelerated by the voltage source 42 and impinge on the multiplier electrode 40. The impingement of the photoelectrons on the electrode results in an amplification of the input and provides an amplified electron image output. The output of the multiplier electrode 40 is then accelerated to the output electrode 18. The electrons will pass through the layer 22 and bombard the phosphor 20 causing emission of light from layer 20 which in turn may be viewed through the output window 14.

It is of course obvious that several modifications may be utilized to derive the signal sensed by the cathode structure 16. For example, the electron image generated by the cathode 16 may be directed onto a storage element such as found in an image orthicon type of pickup tube and an electron beam utilized to scan the opposite surface of the storage electrode to derive an electrical signal representative of the electron image directed onto the storage electrode.

The transport of the electrons into the insulating layer 32 may take place by tunneling through or by being excited'over the potential barrier 35 between the layer 30 and the insulating layer 32. The contact between the insulating layer 32 and the layer 30 should be nearly ohmic or an electron injecting contact. This type of contact will provide only a small barrier to the electrons. The material in layer 30 should be matched with the material in layer 32 to provide that the Fermi level is above or within a few tenths of an electron volt of the conduction band edge of the material in layer 32 when placed in intimate contact. Infrared photoelectrons, by virtue of their excess energy (hu) would then have a considerably enhanced probability of tunneling into the conduction band of the 4 insulating layer 32 and hence move into and through the film 34.

Instead of utilizing a tunneling barrier such as illustrated and explained with regard to FIGURES l and 2, a thicker insulating layer may be used in the cathode assembly such as shown in FIGURE 3. In this device, a cathode 43 consists of a layer 45 of an electrically conductive material such as indium of a thickness of a few hundred angstroms. An insulating layer 41 of a material such as cadmium sulfide and of a thickness of angstroms to 1 micron is provided on the layer 45. The conductive layer 34 is provided on the layer 41. The photoelectrons from the layer 45 are transported into the conduction band of the insulator layer 41 by space charge limited processes. The energy diagram of this embodiment is shown in FIGURE 4. This structure requires the presence of a small potential barrier 50 between the metal layer 45 and the insulating layer 41.

In the operation of this device, photoelectrons are injected into the insulating layer 41 by an ohmic injecting contact in the form of the layer 45 and are transported across the insulating layer 41 by the field applied by the battery 24. As is the case in the tunneling device described with regard to FIGURE 1, the film 34 is made sufficiently thin that a large fraction of the electrons are able to penetrate through it into the vacuum. By varying the potential of battery 24, the height of the potential barrier 50 may be varied.

The role of the low conductivity region 32 and 41 11'! FIGURES 1 and 3 respectively is to enable a sufficiently large bias voltage to be developed across the region so that electrons arriving at the barrier between the insulating layer 32 or 41 and the layer 34 have sufiicient energy to allow them to escape through the layer 34 into the vacuum. The presence of the two bias terminals from the battery 24 to the layers 30 or 45 and 34 also affords the possibility of modulating the photo-output of the cathodes 16 or 43. This feature may have advantages from the point of view of amplification and signal enhancement. On the other hand, the provision of the uniform bias over an extended area will almost certainly require the provision of a conducting grid structure over the exit film of such dimensions that the transverse voltage drop in the film regions between the grid wires is not significant.

As with all photodetectors of the quantum type, the thermal energy kT of the electrons must be less than ha of the incident radiation. This means that the cathode will probably have to be cooled for wavelengths greater than about 5 microns.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and numerous changes in the details of fabrication, materials used and the combination arrangement of elements may be resorted to without departing from the scope and spirit of the present invention.

I claim as my invention:

1. An electron device comprising a photosensitive solid state electron emissive source and-an output electrode for collecting electrons emitted from said electron emissive source, said electron emissive source comprising a first layer of electrically conductive material exhibiting the property of absorbing radiations directed on a first surface thereof and permitting substantially all the photoelectrons generated within the layer in response to the radiation to reach a second surface of said layer; a second layer of insulating material in intimate contact with said first layer and providing a potential barrier to transmission of electrons of an energy less than the energy of said photoelectrons and a third layer of electrically conductive material in intimate contact with said second layer.

2. An electron device comprising a photosensitive solid state electron emissive source and an output electrode for collecting electrons emitted from said electron emissive source, said electron emissive source comprising a first layer of electrically conductive material capable of absorbing substantially all of the input radiation directed thereon and provide transmission of substantially all excited photoelectrons through said layer, a second layer of insulating material in intimate contact with said first layer and providing a tunneling barrier, the height of said tunneling barrier being such as to suppress transmission of all electrons from said first layer with the exception of said excited photoelectrons by said radiation, and a third layer of electrically conducting material in intimate contact with said second layer of low work function material and thin enough to permit transmission of electrons through said third layer into a vacuum.

3. An electron discharge device comprising an evacuated envelope and having therein a radiation sensitive cathode, said radiation sensitive cathode comprising a first layer of electrically conductive material exhibiting the property of absorption of input radiation directed on a first surface and generation of photoelectrons within said first layer in response to said radiation and in which said photoelectrons are able to reach a second surface of said first layer, a second layer of insulating material on said second surface of said first layer in intimate contact with said first layer and providing a potential barrier to the transmission of the electrons having less energy than said photoelectrons, a third layer in intimate contact with said second layer, said third layer of electrical conductive material to permit the photoelectrons energetic enough to overcome said potential barrier to pass through said third layer into the vacuum of said envelope, an electron multiplier positioned in front of the emission surface of said radiation sensitive cathode for collecting the electrons emitted from said cathode and emitting a larger number of electrons and an output electrode positioned on the opposite side of said electron multiplier with respect to said cathode for collecting the output of said electron multiplier to derive an amplified input signal.

4. An image converter comprising an evacuated envelope and having therein a large area radiation photocathode surface for receiving and emitting an electron image corresponding to the radiation input directed on said photocathode, a transmission type electron multiplying structure positioned in front of said photocathode for receiving the electron image from said photocathode and multiplying the electron image, an output electrode of a material exhibiting the property of emitting light in response to electron bombardment and means for directing the electron image from said electron multiplier onto said output electrode to produce an output light image corresponding to said radiation input image, said radiation sensitive photocathode comprising a first layer of electrical conductive material exhibited the property of absorption of said input radiation and creation of photoelectron within said first layer in response to said radiation, a second layer of insulating material in contact with said first layer, a third layer of electrically conductive material in contact with said second layer, said second layer providing a potential barrier with regard to electrons within said first layer having an energy less than the energy of said photoelectron and means for impressing a potential across said first and third layers for transporting said photoelectrons from said first layer through said second and third into the vacuum between said photocathode and said electron multiplier electrode.

5. An electron device comprising a photosensitive solid state electron emissive source and an output electrode for collecting electrons emitted from said electron emissive source, said electron emissive source comprising a first layer of electrically conductive material exhibiting the property of absorbing radiations directed on a first surface thereof and permitting substantially all the photoelectrons generated Within the layer in response to the radiation to reach a second surface of said first layer, a second layer of insulating material in intimate contact with said first layer and providing a potential barrier to transmission of electrons of an energy less than the energy of said photoelectron, a third layer of electrically conductive material in intimate contact with said second layer, and a voltage source connected across said first and third layers to vary said potential barrier.

'6. An electron device comprising a photosensitive solid state electron emissive source and an output electrode for collecting electrons emitted from said electron emissive source, said electron emissive source comprising a first layer of electrically conductive material capable of absorbing substantially all of the input radiation directed thereon and provide transmission of all excited photoelectrons through said layer to the opposite surface thereof, a second layer of insulating material in intimate contact with said first layer and providing a tunneling barrier, the height of said tunneling barrier being such as to suppress transmission of all electrons from said first layer with the exception of said excited photoelectrons by said radiation, a third layer of electrically conducting material in intimate contact with said second layer of low work function material to provide transmission of electrons through said third layer into a vacuum, and a voltage source connected across said first and third layers to modulate the number of transmitted photoelectrons.

7. An electron discharge device comprising an evacuated envelope and having therein a radiation sensitive cathode, said radiation sensitive cathode comprising a first layer of electrically conductive material exhibiting the property of absorption of said input radiation and generation of photoelectrons Within said first layer in response to said radiation, a second layer of insulating material on the surface of said first layer in intimate contact with said first layer and providing a potential barrier to the transmission of electrons from said first layer having less energy than said photoelectrons, a third layer in intimate contact with said second layer, said third layer of electrical conductive material to permit photoelectrons energetic enough to overcome said potential barrier to pass through said third layer into the vacuum of said envelope, a voltage source connected to said first and said third layer to accelerate said photoelectrons, an electron multiplier positioned in front of the emission surface of said radiation sensitive cathode for collecting the electrons emitted from said photocathode to multiply the number of electrons and an output electrode positioned on the opposite side of said electron multiplier and said radiation cathode for collecting the multiplied output of said electron multiplier to derive an amplified input signal.

8. An image convertor comprising an evacuated envelope and having therein a large area radiation photocathode surface for receiving and emitting an electron image corresponding to the radiation input directed on said photocathode, a transmission type electron multiplying structure positioned in front of said photocathode for receiving the electron image from said photocathode and multiplying the electron image, an output electrode of a material exhibiting the property of emitting light in response to electron bombardment, means for directing the electron image from said electron multiplier onto said output electrode for producing an output light image corresponding to said radiation input image, said radiation sensitive photocathode comprising a first layer of electrical conductive material exhibited the property of absorption of said input radiation and creation of photoelectron within said first layer in response to said radiation, a second layer of insulating material in contact with said first layer, a third layer of electrically conductive material in contact with said second layer, said second layer providing a potential barrier with regard to electrons within said first layer having an energy less than the energy of said photoelectron and means for impressing a potential across said first and third layers for transporting said photoelectrons from said first layer through said second and third in a vacuum between said photocathode and said electron multiplier electrode.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS De Forest 31395 Teves 250-213 Kossel 31394 Garbung 250-833 Mead 317234 Giaever 317-234 Kanter 313-94 X OTHER REFERENCES C. A. Mead: Operation of Tunnel-Emission Devie's, Journal of Applied Physics, vol. 32, N0. 4, April 1961, pp. 646-652.

RALPH G. NILSON, Primary Examiner.

JAMES W. LAWRENCE, WALTER STOLWEIN,

Claims (1)

1. AN ELECTRON DEVICE COMPRISING A PHOTOSENSITIVE SOLID STATE ELECTRON EMISSIVE SOURCE AND AN OUTPUT ELECTRODE FOR COLLECTING ELECTRONS EMITTED FROM SAID ELECTRON EMISSIVE SOURCE, SAID ELECTRON EMISSIVE SOURCE COMPRISING A FIRST LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL EXHIBITING THE PROPERTY OF ABSORBING RADIATIONS DIRECTED ON A FIRST SURFACE THEREOF AND PERMITTING SUBSTANTIALLY ALL THE PHOTOELECTRONS GENERATED WITHIN THE LAYER IN RESPONSE TO THE RADIATION TO REACH A SECOND SURFACE OF SAID LAYER; A SECOND LAYER OF INSULATING MATERIAL IN INTIMATE CONTACT WITH SAID FIRST LAYER AND PROVIDING A POTENTIAL BARRIER TO TRANSMISSION OF ELECTRONS OF AN ENERGY LESS THAN THE ENERGY OF SAID PHOTOELECTRONS AND A THIRD LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL IN INTIMATE CONTACT WITH SAID SECOND LAYER.

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