US3275869A - Low noise electron emitters - Google Patents

Description

Sept. 27, 1966 w, FEIST 3,275,869

LOW NOISE ELECTRON EMITTERS Filed July 12, 1963 2 Sheets-Sheet l lNVE/VTOR WOLFGANG M. FE/ST l B) W ATTORNEY Sept. 27, 1966 w. M. FElST 3,275,869

LOW NOISE ELECTRON EMITTERS Filed July 12, 1963 2 Sheets-Sheet 2 I I 11 I m m E I i VACUUM LEvEL 0 5 cououcnou BAND z I I METAL FORBIDDEN ZONE S BACKING EMPTY ACCEPTOR P'TYPE SURFACE 2 |(STATE$ I1 STATES '5 -L FERMI LEvEL LL! 3 VALENCE BAND THICKNESS OF SEMICONDUCTIVE LAYER MEASURED FROM sAcK|-s- T l I Q I I 11 z m o I {5 i H E 1 1- VACUUM LEvEL 2 m FORBIDDEN ZONE METAL l s N TYPE SURFACE BACKING EMPTY ACCEPTOR g STATES STATES E FERM| LEvEL LU 5 Z/ QQQ QV OCCgLErISACCEPTOR o THICKNESS OF SEMICONDUCTIVE LAYER MEASURED FROM BACKlNG- INVENTO/P WOLFGANG M. FE/ST ATTORNEY United States; Patent 3,275,869 LOW NOISE ELECTRON EMITTERS Wolfgang M. Feist, Burlington, Mass, assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed July 12, 1963, Ser. No. 294,555 8 Claims. (Cl. 31365) The present invention is concerned with an electron emissive device and, more particularly, with an electron emissive device for producing a stream of electrons having a narrow range of energies.

While the invention may be applied to electron tubes in general, it is particularly useful in a travelling wave tube, an electron tube in which an electron beam of very low noise content is especially desirable.

Presently, the large majority of electron tubes utilize thermionic cathodes wherein a stream of electrons is emitted by heating a cathode to high temperatures (e.g. 1500 K.). The kinetic energiesor velocities of electrons produced by thermionic emission vary over a wide range of values which, theoretically has no upper limit, i.e. a Maxwellian distribution. Such a wide spread of electron energies manifests itself as undesirable noise in the output signal of a travelling wave tube or similar device.

It is therefore an object of the present invention to provide a low noise source of electrons which is suitable for use in a variety of electron tubes, including travelling wave tubes.

A low noise electron emitter constructed in accordance with the present invention comprises a cathode having a surface layer of material which responds to electromagnetic radiation by emitting electrons. The material is substantially insensitive to radiation at wave-lengths above a threshold wavelength A The electron emitter further comprises a source of electromagnetic radiation aligned with the cathode surface, the radiation from the source extending over a range of wavelengths beginning at a lower wavelength limit h which is less than, but close to, the threshold wavelength i of the cathode surface material.

In accordance with a further aspect of the invention, means are also provided for cooling the cathode to temperatures substantially below room temperature so as to aid in reducing the energy spread of electrons emitted from the cathode.

The invention will now be further described with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a first embodiment of the invention as applied to a travelling wave tube;

FIG. 2 is a schematic representation of a second em bodiment of the invention as applied to a travelling wave tube;

FIG. 3 is an energy level diagram of a cathode having a semiconductive electron emitting layer with p-type surface states which may be employed in the FIG. 1 or FIG. 2 embodiments of the invention; and

FIG. 4 is an energy level diagram of a cathode having a semiconductive electron emitting layer with n-type surface states which may be employed in the FIG. 1 or FIG. 2 embodiments of the invention.

Referring to FIG. 1, a schematic representation of a travelling wave tube, indicated generally by the reference numeral 10, comprises an evacuated glass envelope 11, a slow wave structure shown as a helix 12, a collector electrode 13 and an electron emissive cathode 14. Additional elements of a conventional travelling wave tube such as the input and output waveguides, the electron accelerating anode and electric and magnetic beam focusing means have been omitted since they are not essential to an understanding of this invention. In accordance with the invention, cathode 14 is sensitive to electromagnetic radiation over a given range of wavelengths, beginning at a threshold wavelength A for emitting electrons. While the invention is not restricted to cathodes sensitive to radiation at wavelengths in the visible spectrum, the invention will be further described with reference to such a cathode since materials sensitive to radiation at such Wavelengths are well known. Cathode 14 may comprise either one of the commonly known photoemissive metals or, as will be described hereinafter, cathode 14 comprises an electron emitting layer of semiconductive material 14a adhered to a metallic backing layer 14b. Preferably, emitting layer 14a comprises a layer of an alkali antimonide such as the intermetallic compound of cesium and antimony 05 8b to which additional p-type acceptor impurities have been added as w ll lbe more fully explained below in connection with FIGS. 3 and 4.

A source of electromagnetic radiation, indicated generally by reference numeral 15, is disposed external to envelope 11 and optically aligned with emitting layer 14a of cathode 14. Source 15 comprises a light source 16, a reflector 17, a low-pass filter 18 and a focusing lens 19. Low-pass filter 18 serves to block radiation at wave-lengths less than a limiting wavelength R where A is close to but less than the threshold wavelength A of cathode 14. From the standpoint of efficiency, the upper wavelength limit of source 16 isalso preferably close to A However, this latter limit is not essential to the invention. Radiation source 15 may, in the alternative, comprise any other type of device which provides radiation having a sharply defined lower wavelength limit. For example, an essentially monochromatic source of radiation such as a gaseous discharge device or a laser may be used to provide the desired radiation or illumination. 7

A means for cooling cathode 14, shown as a refrigerating enclosure 20, is disposed around the outside of glass envelope 11 at the end thereof which contains cathode 14. An inlet conduit 21 and an outlet conduit 22 are attached to refrigerating enclosure 20 to permit entry and discharge of a cooling medium. Refrigerating enclosure 20, or at least that part thereof which lies along the optical path between cathode 14 and radiation source 15, is transparent to radiation at wavelengths between the lower wavelength limit h of source 15 and the threshold wavelength t of cathode 14.

Referring now to FIG. 3 of the drawinggan energy level diagram is shown for a cathode having an electron emitting layer of semiconductive material constructed in accordance with the invention. 'Electron energy is plotted along the axis of ordinates, while the thickness of semiconductive layer 14a measured from metallic backing 14b is plotted along the of abscissae. V

In the diagram of FIG. 3, the metallic backing layer 14!) of cathode 14 is shown to the left of the axis of ordinates, while the emitting layer of semiconductive material 14a is shown to the right of the axis of ordinates. The semiconductive layer is divided into three regions in FIG. 3. The three regions are designated as the region of contact between the metallic backing and the semiconductive layer (I), the bulk region (II) and the surface region (III). Furthermore, three discrete energy level bands are shown, the highest being the conduction band which defines the energy levels normally occupied by some conduction electrons when the material is at temperatures above absolute zero i.e. greater than 0 K.), the lowest being the valence band which defines the energy levels occupied by the more tightly bound valence electrons and the intermediate being the forbidden energy gap or zone which separates the allowable valence and conduction electron energy levels. The Fermi level is indicated by a dashed line and is that energy level at which the probability of occupation by an electron is /2 A plurality of unoccupied acceptor states which may, for example, be produced by diffusion of excess cesium atoms or other p-type impurities within the semiconductive layer 14a, are arranged at an energy level in the forbidden zone close to the upper limit of the valence band. The presence of unoccupied acceptor states close to the valence band serves to shift the Fermi level of the material from a level approximately midway in the forbidden gap to a level close .to the upper. limit of the valence band. A shift of the Fermi level of this nature is indicative of the fact that fewer electrons are available for emission from energy levels above the valence band when such acceptor states are present.

wavelength A of emitting layer 14a aswill be more fully explained below. Light impinging upon emitting layer 14a causes electrons to be emitted from layer 14a towards helix 12 and collector 13. In accordance with the well-known operation of a travelling wave tube, the electrons are focused into a beam by focusing fields (not shown). The beam of electrons passes through the center of helix 12, giving up energy to electromagnetic waves which are coupled to helix 12 by means of a waveguide or similar; device (not shown). The amplified electromagnetic wave is coupled out of helix 12 by means of a second waveguide (not shown) and the electrons are collected by collector 13. 7

The emission of electrons from cathode 14 will now be considered ingreater detail. The maximum energy or velocity of the electrons 'emitted from emitting layer 14a depends upon (1) The wavelength (or frequency) of the incident light, and

(2) The difference between the potential energy of the electron within the layer 14a and the potential en ergy of .an electron at the vacuum level outside the layer 14a .(the latter difference being the photoelectric work function where the emitted electron occupied the hlghest energy level occupied by electrons within the layer 14a).

Specifically, the maximum energy of an emitted electron is given by the expression The threshold wavelength t of emitting layer 14a is 7 related to the work function thereof by theexpression where A is measured in thousands of Angstrom units and is measured in electron volts.

Furthermore, the maximum energy difference between photo-emitted electrons is given by the expression Furthermore, the lower wavelength limit M is close to but less than the threshold.

' where It can therefore be seen that the maximum. energy difference between photo-emitted. electrons may be controlled for a material having a given threshold wavelength by limiting the minimum wavelength of the incident light to a value close to the threshold wavelength.

A In a travelling'wave tube such as is shown ind- IG- 1, it is possible, in'accordance .with the present invention, to restrict the maximum energy. spread of emitted electrons to values not presently attainable from a thermionic cathode itself. For example, in order .to restrict the maximum energy spread of emitted electrons to a value of 0.1 ev. (electron volts),the lower wavelength limit h of source 15 is restricted to a wavelength which where a and )\11 are measured in thousands of Angstrom units.

ures, of the order of 7000 A. (Angstrom units), a case, the above expression is satisfied ifthe lower frequency limit h of source 15 lies .within the. limits Further reduction in the energy spread between emitted electrons may be achieved by further limiting the lower wavelength limit h of source 15.

In order to achieve practical levels of emission from a cathode illuminated by such a narrow band of wave- 7 lengths near the threshold wavelength A th photoelectric yield (i.'e. .unit of emission current per unit of incident illumination) of emitting layer must rise sharply within the range of wavelengthsbetween A and R The desired sharply rising photoelectric yield characteristicis obtained by insuring that a large number ofelectrons' occupy energy levels close to the highest occupied energy level in emitting layer14a. ,Sirice vthemajority of electrons within any material occupy energy. levels in the valence band while only relatively. few electrons are thermally excited to higher energy levels (e.g. in the con duction band or, as with p-type semiconductors, in

defect level states in the forbidden gap), the desired large a number of electrons Within a narrow band of energies is best obtained by preventing electrons from occupying levels above the valence band. Ideally, the photoelectric work function will. then be equal-to. the difference flfitW6l1 the valence .band upper-limit and the vacuum evel.

tained (1). By cooling'cathode 14 by means of refrigerating" enclosure 20 so as to freeze. electronsout of energy; level states above the valence band, and

(2) By introducing p-type acceptor impurities having unoccupied energy levels near. the upper limit of 1 the;

valence band throughout emitting layer 14a.

A cooling medium such :as a liquified gas (e.g. helium) is passed through refrigerating enclosure..20 by means of inlet conduit 21 and outletconduit 22.80 as to cool cathode 14 to a temperature substantially below room temperature (e.g. even approaching absolute zero) and f thereby reduce the number of electrons occupying energy levels above the valence. band energy levels. a

The threshold wavelength A for a cathode having a cesium-antimony surface may. be,'in round fig- In such Furthermore, the photo-emitted electrons will be supplied almost exclusively ,fromvalence bandenergy levels. In accordance with the invention, the. desired sharply rising photoelectric yield over a'narrow. band of 1 wavelengths close to the threshold wavelength t is ob-1.

The unoccupied p-type acceptor states (shown as circles in FIGS. 3 and 4'above the Fermi level) serve to decrease the probability that energy levels above the valence band are occupied by electrons. Suflicient acceptor states may even be added to depress the Fermi level below the upper limit of the valence band.

Referring now to FIG. 2, a second embodiment of the invention in a travelling wave tube is shown. Corresponding parts in FIG. 2 are indicated by the same reference numerals as in FIG. 1.

The FIG. 2 embodiment is generally similar to the FIG. 1 embodiment with the exception that cathode 14 comprises an electron emitting layer 14a and a semitransparent metallic backing layer 14c. In this case, the radiation from source '15 impinges upon semitransparent backing layer 14c and passes through semitransparent backing layer 14c, causing the emission of electrons from emitting layer 14a towards helix 12. The thickness of emitting layer 14a is preferably approximately equal to the maximum escape depth for photoexcited electrons (e.g. about 200-300 Angstrom units for an alkali antim-onide) so as to obtain a high photoelectric yield from cathode 14. In all other respects, the embodiment shown in FIG. 2 corresponds to that shown in FIG. 1.

Referring now to FIG. 4, an energy level diagram similar to the energy level diagram of FIG. 3 is shown. However, FIG. 3 indicates the effect on the energy level diagram of the presence of p-type surface states whereas FIG. 4 indicates the effect thereon of the presence of n-tyoe surface states. Surface states having energy levels within the forbidden gap of a semiconductor result from the presence of impurities on the semiconductor surface. Where, as in FIG. 3, both the bulk region (region H) and the surface states (region III) are p-type, the surface states do not have any noticeable effect on the energy level diagram. However, where, as in FIG. 4, the bulk region (H) is of p-type material and the surface states are n-type, the upper limit of the valence band and the lower limit of the conduction band bend downward. This band bending takes place in the following manner. At equilibrium, the Fermi level must be the same within bulk region (II) as it is within the surface region (111). Therefore, the presence of n-type surface states requires a redistribution of negative charge from the surface states to unoccupied p-type acceptor states within the semiconductive material. As shown in FIG. 4, acceptor states near the surface are filled or occupied by electrons transferred from the n-type surface states. The electric fields set up by this redistribution of charge result in the downward bending of the energy bands (i.e. valence and conduction bands) near the surface of the semiconductive material. The occupied acceptor states in the surface region (III) might shift the threshold wavelengths X towards higher wavelengths. It is therefore desirable to minimize the effect of such n-type surface states. This objective may be accomplished by either absorbing a monolayer of molecules on the surface of emitting layer 14a so as to preclude the formation of n-type surface states or by providing sufiicient p-type acceptor states close to the surface so as to minimize the distance over which the band bending occurs.

While this invention has been described with reference to cathode materials which are sensitive to radiation at wavelengths in the visible spectrum, it will be recognized that the invention contemplates use of radiation sources and emitting devices which operate at other wavelengths as well.

Furthermore, while it has been indicated that, in accor-dance with the invention, an electron beam having a maximum energy spread of 0.1 electron volts may be produced, practical beam currents (eg 20 microamperes) may be produced with a maximum energy spread of, for example, 0.02 electron volt using the type of cathode materials described above.

What is claimed is:

1. A low noise electron emitter comprising a cathode having a layer of material responsive to electromagnetic radiation for emitting electrons, said material being substantially non-responsive to radiation at wavelengths above a threshold wavelength, said electron emitter further comprising means for producing electromagnetic radiation over a range of wavelengths, said means being aligned with said cathode, the shortest wavelength produced by said means being "less than said threshold wavelength of said layer of electron emitting material and said shortest wavelength being limited with respect to said threshold wavelength to limit the maximum energy diiference between electrons emitted from said layer of electron emitting material.

'2. A low noise electron emitter comprising a cathode having a layer of material responsive to electromagnetic radiation for emitting electrons, said material being substantially non-responsive to radiation at wavelengths above a threshold wavelength, said electron emitter further comprising means for producing electromagnetic radiation over a range of wavelengths, said means being aligned with said cathode, the shortest wavelength produced by said means being less than said threshold wavelength of said layer of electron emitting material and said shortest wavelength being limited with respect to said threshold wavelength to limit the maximum energy difference between electrons emitted from said layer of electron emitting material, said electron emitter still further comprising means for cooling said cathode to reduce the difference between said threshold wavelength and the shortest wavelength radiation produced by said means for producing electromagnetic radiation, whereby electrons having a narrow range of energies corresponding to the difference in wavelength between said threshold wavelength and said shortest wavelength are emitted from said cathode.

3. A low noise electron emitter comprising a cathode having a 'layer of material responsive to electromagnetic radiation for emitting electrons, said material being substantially non-responsive to radiation at wavelengths above a threshold wavelength, said electron emitter further comprising a source of electromagnetic radiation aligned with said cathode, means interposed between said cathode and said source for limiting the radiation impinging upon said cathode to a range of wavelengths having a lower limit, the lower wavelength limit being less than said threshold wavelength and selected with respect to said threshold wavelength to limit the maximum energy difference between electrons emitted from said layer of electron-emitting material, said electron emitter still further comprising means for cooling said cathode to decrease the difference between said threshold wavelength and said lower wavelength limit.

4. A low noise electron emitter comprising a cathode, said cathode comprising a *layer of electron-emissive material responsive to electromagnetic radiation over a given range of wavelengths for emitting electrons, said material being substantially non-responsive to radiation above a threshold wavelength, thethreshold, wavelength varying with the temperature of said material, said electron emitter further comprising a source (if-electromagnetic radiation, said source having a short wavelength limit which is within the range of wavelengths to which said material is responsive and which is close to the threshold wavelength for said material at a temperature of absolute zero, said electron emitter still further comprising means for directing radiation from said source of radiation to said cathode and means for cooling said cathode to reduce the difference between said threshold wavelength and said short wavelength limit whereby electrons having a narrow range of energies corresponding to the diiference in wavelength between said threshold and said short wavelength limit are emitted from said cathode.

5. A low noise electron emitter comprising a cathode having a surface of material responsive to electromagnetic radiation for emitting electrons, said material being substantially non-responsive to radiation at wavelengths above a threshold wavelength t said emitter further comprising a source of electromagnetic radiation aligned with said cathode, said source having a lower wavelength limit R which is related to said threshold wavelength a by the expression where R and A are measured in thousands of Angstrom units (A.) whereby the maximum energy difference between electrons emitted from said material is equal to or less than 0.1 electron volt.

6. A lowvnoise electron emitter comprising a cathode, said cathode comprising a layer of semiconductive material responsive to electromagnetic radiation over a given wavelength range for emitting electrons, said material being substantially non-responsive to radiation at wavelengths above a threshold wavelength, said cathode further comprising p-type acceptor states diffused within said semiconductive material, the energy levels of said acceptor states being substantially closer to the upper limit of valence band energy levels than to the lower limit of conduction band energy levels of said semiconductive material, said electron emitter further. comprising a source of electromagnetic radiation, said source having a short wavelength limit less than said threshold wavelength of said semiconductive material'but limited with respect to said threshold wavelength to limit the maximum energy difference between electrons emitted from said layer of semiconductive material, said electron emitter still further comprising means for directing radiation from said source of electromagnetic radiation to said cathode and means for cooling said cathode whereby electrons having a narrow range of energies corresponding to the difference in wavelength between said threshold wavelength and said 4 short wavelength limit are emitted ,from' said layer of semiconductive material.

7. A low noise electron emitter comprising a cathode responsive to electromagnetic radiation over a range of wavelengths beginning. at an upperthreshold Wavelength for emitting electrons, said cathode comprising a metallic backing layer and an electron emitting layer of semiconductive material, one surface of said emitting layer being in contact with one surface of said backing layer,gsaid emitting layer, including diffused p-type acceptor impurities having acceptor energy levels in the vicinity of the maximumvalence band energy level of said emitting layer, said emitting layer being substantially equal in depth to the maximum escape depth for photoexcited electrons from said emitting layer, said electron emitter further comprising a source of electromagnetic radiation, the radiation having a sharply defined short wavelength limit, said short wavelength limit being less thansaidthreshold wavelength of said semiconductive material but limited with respect to'said threshold wavelength to limit the maximum energy difference between electrons emitted from said semiconductive material, said electron emitter still further comprising means for directing radiation from said source of electromagnetic radiation to said cathode and means for cooling said cathode whereby elec.- trons having a narrow range of energies corresponding to the difference in wavelength between said threshold wavelength and said short wavelength limit are emitted from the cathode.

8. A low noise electron emitter comprising a cathode, said cathode comprising a layer of semiconductive ma -r terial responsive to electromagnetic radiation over, a given wavelength range for emitting electrons, said material being substantially non-responsive to radiation at wave lengthsa-bove a threshold wavelength, said semiconductive material comprising a compound of compound alkali metals and antimony with p-type acceptor states diffusedj within said layer of semiconductive material, the energy levels of said acceptor states being substantially closer to the valence band energy level than to'the conduction band energy level'of said semiconductive material, 'said' electron emitter further comprising a source of electro magnetic radiation alignedwith said cathode, the shortest wavelength produced by said source. being :less than the threshold wavelength of said semiconductive material and limited with respect to said threshold wavelength to limit the maximum energy difference between electrons emitted I from said semiconductive material, said electron emitter still further comprising means for cooling said cathode to a temperature in the vicinity of absolute zerowhereby I electronshaving a narrow range of energies correspond ing to the difference in wavelengths between said, threshold wavelength and said shortest wavelength of said source are emitted from said semiconductive material;

References Cited by the Examiner UNITED' STATES PATENTS 3,036,234 3,048,797

JOHN W. HUCKERT,Primary Examiner. J. D. CRAIG, Assistant Examiner.

5/1962 Dacey 3l3105, 8/1962 Linder 3l7--234-

Claims (1)

1. A LOW NOISE ELECTRON EMITTER COMPRISING A CATHODE HAVING A LAYER OF MATERIAL RESPONSIVE TO ELECTROMAGNETIC RADIATION FOR EMITTING ELECTRONS, SAID MATERIAL BEING SUBSTANTIALLY NON-RESPONSIVE TO RADIATION AT WAVELENGTHS ABOVE A THRESHOLD WAVELENGTH, SAID ELECTRON EMITTER FURTHER COMPRISING MEANS FOR PRODUCING ELECTROMAGNETIC RADIATION OVER A RANGE OF WAVELENGTHS, SAID MEANS BEING ALIGNED WITH SAID CATHODE, THE SHORTEST WAVELENGTH PRODUCED BY SAID MEANS BEING LESS THAN SAID THRESHOLD WAVELENGTH OF SAID LAYER OF ELECTRON EMITTING MATERIAL AND SAID SHORTEST WAVELENGTH BEING LIMITED WITH RESPECT TO SAID THRESHOLD WAVELENGTH TO LIMIT THE MAXIMUM ENERGY DIFFERENCE BETWEEN ELECTRONS EMITTED FROM SAID LAYER OF ELECTRON EMITTING MATERIAL.

Images