US3667007A - Semiconductor electron emitter - Google Patents

Abstract

An electron emitter comprising a body of a semiconductor material which is adapted to generate light therein when properly biased but which is a poor absorber of the generated light. On a surface of the body is a thin region of a semiconductor material which is a good absorber of the generated light and which has an index of refraction which substantially matches the index of refraction of the material of the body. The thin semiconductor material region is adapted to absorb the light from the body and convert the light into free electrons. On the surface of the semiconductor material layer is a thin film of an electropositive work function reducing material which is adapted to emit the electrons formed in the semiconductor material layer.

Description

United States Patent Kresselet al.

[ 51 May 30,1972

[54] SEMICONDUCTOR ELECTRON EMITTER [72] Inventors: Henry Kressel, Elizabeth; Jacques Isaac Marinace, I.B.M. Technical Disclosure Bull., Vol. 6, No. 2,

July 1963, page 82.

Fischler, IBM Technical Disclosure'BulL, Vol. I 1, No. 3, Aug. 1968, page 284. I Marinace, I.B.M. Technical Disclosure Bull., Junctions with Varying Band Gap," Vol. I 1, No. 4, Sept. 68

Nicol], R.C.A. Technical Notes, April, 1968.

Primary ExaminerJ0hn W. Huckert Assistant Examiner-Martin H. Edlow Att0rney-Glenn H. Bruestle [57] ABSTRACT An electron emitter comprising a body of a semiconductor material which is adapted to generate light therein when properly biased but which is a poor absorber of the generated light. On a surface of the body is a thin region of a semiconductor material which is a good absorber of the generated light and which has an index of refraction which substantially matches the index of refraction of the material of the body. The thin semiconductor material region is adapted to absorb the light from the body and convert the light into free electrons. On the surface of the semiconductor material layer is a thin film of an electropositive work function reducing material which is adapted to emit the electrons formed in the semiconductor material layer. 1

5 Claims, 2 Drawing Figures Patented May 30, 1972 I 3,667,007

INVENTIYIRS Henry Kressel and Jacques I Pan kove.

QMJMM ATTORNEY SEMICONDUCTOR ELECTRON EMITIER BACKGROUND OF INVENTION The present invention relates to semiconductor electron emitters, and more particularly to a semiconductor electron emitter which includes an electroluminescent semiconductor element. 1

One type of electron emitter or cathode which has been used in electron discharge devices is known as a cold cathode in that it does not use heat to generate the electrons. One type of cold cathode uses a semiconductor element as the source of the electrons and a layer of an electropositive work function reducing material, such as an alkali or alkaline earth metal, on the surface of the semiconductor element as the means for emitting the electrons. To obtain a high degree of electron emission from such a cold cathode it is necessary that the semiconductor element produce an abundance of electrons and that the electrons produced be delivered to the emitting surface, usually necessitating that the emitting element be very close to the region where free electrons are available.

Another type of cold cathode uses two parts, one an emitter of light, and the other a photoemitter that absorbs at least part of the emitted light. See for example J .M. Lavine et a], Cold Cathode Electron Emitter, Solid-State Electronics, Vol. 6, pp. 674-676, 1963, and TS. Moss and J.B. Coombes, Ari Opto Electronic Cold Cathode Ray Tubes," Solid-State Electronics, Vol. ll, pp. 661-666, 1968. These two parts have been of widely different materials, and although they were optically coupled together, the two parts did not match well with respect to the good absorption of the emitted light and good transfer of the light from the emitter to the absorber. As a result, the overall quantum efiiciencies achieved were as small as 10' to I SUMMARY OF INVENTION An electron emitting element including a body of semiconductor material having adjacent P type and N type regions with a PN junction therebetween. The semiconductor material of the body being capable of generating light when the PN junction is properly biased and the semiconductor material of at least one of said regions being a poor absorber of the generated light. A region of a semiconductor material which is a good absorber of light is provided on a surface of the region of the body which is a poor absorber of light. A layer of an electropositive work-function-reducing material is on the surface of the good absorbing semiconductor material region.

BACKGROUND OF INVENTION FIG. 1 is a sectional view of one form of the electron emitting element of the present invention.

FIG. 2 is a sectional view of another form of the electron emitting element.

DETAiLED DESCRIPTION Referring to FIG. 1, a preferred form of the electron emitting element of the present invention is generally designated as Ill Theelectron emitting device 10 comprises a monolithic body 12 of a semiconductor material having opposed flat surfaces. The body 12 has aP type region 14 contiguous to anN type region 16 so as to provide aPN junction 18 therebetween. For reasons which will be explained, theP type region 14 can be relatively thick, at least 5 microns in thickness. The body 12 is of a semiconductor material which is capable of generating light in he vicinity of thePN junction 18 and preferably over a broad area of theP type region 14 when the PN junction is biased so as to inject charge carriers of one type into the P type region which combine with charge carriers of the opposite type in the P type region to generate the light. The preferred semiconductor materials for the body 12 are the Ill-V compound semiconductors and alloys thereof, such as the nitrides, phosphides, arsenides and antimonides of boron, aluminum, gallium and indium. Preferably, the body 12 is of the compound semiconductor Al,Ga, As, where x is less than 0.34, which has a high energy band gap and is capable of efficiently producing light at room temperature. In addition, the semiconductor material of at least theP type region 14 should be a poor absorber of the generated light.

One form of the body 12 is a body of Al ,Ga As, where x is less than 0.34, with the body containing silicon as a conductivity modifier to form theP type region 14 and theN type region 16. The single conductivity modifier can be used to form the two regions of opposite conductivity type since silicon is amphoteric in the III-V compound semiconductor materials. The silicon is incorporated in different portions of the crystal lattice of the lIl-V compound semiconductor material depending on the temperature at which it is incorporated. Thus, the silicon will act as a donor if incorporated at one temperature and will act as an acceptor if incorporated at a lower temperature. For details, see for example I-I. Kressel et al, Luminescence in Silicon-Doped GaAs Grown by Liquid-Phase Epitaxial," Journal Applied Physics, Vol. 39, No. 4, pp. 2006-201 1, March 1968. The silicon compensated P type andN type regions 14 and 16 can be formed in the body 12 by liquid-phase epitaxy such as described in the article of H. Kressel et a], Properties of Efficient Silicon-Compensated Al,Ga, As Electroluminescent Diodes," Journal Of Applied Physics, Vol. 40, No. 5, pp. 2248-2253, April 1969. Also, by using silicon as a conductivity modifier to form the? type andN type regions 14 and 16, theP type region 14 will be a poor absorber of the light emitted in the body 12.

Another suitable form of the body 12 is a wafer of an N type lII-V compound alloy semiconductor material having a high energy band gap, such as Al Ga As containing tellurium as a conductivity modifier, as theN type region 16. On a surface of the wafer is a layer of a P type Ill-V compound alloy semiconductor material, such as Al,Ga, ,As containing germanium or silicon as a conductivity modifier, as theP type region 14. The semiconductor material of theP type region 14 should have a band gap energy which is lower at the surface of theN type region 16 than the band gap energy of theN type region 16 and which increases to the surface of theP type region 14. By using Al,Ga ,As as the semiconductor material for theP type region 14, the band gap energy can be varied by varying the amount of aluminum in the material. The band gap energy increases with increasing amounts of aluminum. Also, the PN junction between the P type region and the N type region must be a heterojunction.

' This body can be formed by epitaxially growing P type Al,- Ga, ,As on the surface substrate of gallium arsenide by liquidphase epitaxy. The first portion of the Al,Ga, ,As deposited on the substrate will have a high concentration of aluminum and the concentration of aluminum will gradually decrease as the thickness of the epitaxial layer increases. When the P type epitaxial layer is of a thickness of at least 5 microns, N type Al,Ga, As having a high concentration of aluminum is epitaxially grown on the P type layer by liquid-phase epitaxy. After the N type epitaxial layer of the desired thickness is grown, the gallium arsenide substrate is removed, such as by etching and polishing. This provides a P type region on an N type region with a heterojunction between the two regions and the P type region has the desired graded band gap energy.

This body can also be formed by epitaxially growing the P type region on a substrate of the N type semiconductor material using vapor phase epitaxy. Vapor phase epitaxy comprises forming a mixture of gases containing the elements of the semiconductor material to be deposited and pyrolytically reacting the gaseous mixture in the presence of the substrate to deposit a mixture of the elements on the substrate. For an example of vapor phase epitaxy see 1.]. Tietjen' et al, The Preparation and Properties of Vapor-Deposited Epitaxial GaAs PBx Using Arsine and Phosphine, Journal Electrochemical Society, Vol. 1 13, pg. 724, 1966. By gradually increasing the amount of the aluminum containing gas in the mixture as the P type layer is deposited on the substrate, the desired graded band gap energy can be achieved.

Aregion 20 of a P type semiconductor material is provided on the surface of theP type region 14. Theregion 20 is of a semiconductor material which is a good absorber of the light generated by the body 12 and which has an index of refraction which at least substantially matches the index of refraction of the semiconductor material of theregion 14 of the body 12. Suitable semiconductive materials for theregion 20 are the Ill- V compound'semiconductor materials or alloys thereof. Theregion 20 should be of a thickness of l to 5 microns and should provide a heterojunction withP type region 14. Theregion 20 can be provided on the body 12 by epitaxially depositing a layer on the surface of theregion 14. This can be achieved either by liquid-phase epitaxy such as described in the article by H. .Nelson, Epitaxial Growth From the Liquid State and its Application to the Fabrication of Tunnel and Laser Diodes,"RCA Review 24, pg. 603, 1963, or by vapor phase epitaxy such as previously described.

Athin layer 22 of an electropositive work function reducing material is provided on the surface of the P typesemiconductive material region 20. Theelectropositive layer 22 comprises an alkali or alkaline earth metal and oxygen, and is monomolecular or has a thickness not exceeding a few atomic diameter of the electropositive material. The alkali or alkaline earth metal of theelectropositive layer 22 may be cesium, potassium or barium, with cesium being the preferred metal. Theelectropositive layer 22 may be applied by evaporation in a vacuum..Athin film 24 of alight reflecting material, such as silicon monoxide covered with gold, may be coated on the peripheral edge surface of the body 12 completely around the body so as to prevent any of the light generated in the body from being emitted from the periphery of the body and to reflect more of the light toward theelectropositive layer 22. The reflectingfilm 24 may be applied to the body by evaporation in a vacuum.-

Terminal wires 26 and 28 are electrically connected to theP type region 14 andN type region 16 of the body 12. As shown, theterminal wire 28 is connected to theN type region 16 through acontact layer 30 on the surface of the N type region. Thecontact layer 30 is a film of a metal, such as tin, which will make good ohmic contact to the semiconductor material of theN type region 16. The tin film may be coated with a film of nickel and a film of gold to provide for greater ease of securing theterminal wire 28 to thecontact layer 30. Theterminal wire 26 is connected to theP type region 14 through theP type region 20. As shown, theterminal wire 26 is fused directly to theP type region 20. However, theterminal wire 26 can be secured to the P type region through a small contact layer on the surface of theP type region 20. Such a contact layer could be a film of a metal which would make a good ohmic contact to the semiconductor material of theP type region 20, such as nickel, which can be coated with a film of gold.

In the use of electron emitting device theterminals 26 and 28 are connected to a source of voltage with the terminal 28 being connected to the negative side of the voltage source and the terminal 26 to the positive side. The voltage biases thePN junction 18 so as to inject charge carriers of one type from theN type region 16 into theP type region 14 where they combine with charge carriers of the opposite type and generate light in theP type region 14. By having a relatively thickP type region 14 current crowding at the contact between the terminal 26 and theP type region 20 is prevented and thecurrent spreads across the entire PN junction18 to achieve light generation across substantially the entirebulk of theP type region 14. Thus, substantial generation of light is achieved even though the contact betweenten'ninal 26 and theP type region 20 is small.

Y Since the semiconductor material of theP type region 14 is a poor absorber of light and the semiconductor material of theP type region 20 is a good absorber of light and has an index of refraction which at least substantially matches the index of refraction of the region. 14, thelight generated in the body 12 is absorbed in theP type region 20. The reflectinglayer 24 prevents any light from being emitted from the edge of the body 12. In theP type region 20 the light absorbed is converted into free electrons. Since theP type region 20 is thin, a high concentration of electrons is provided at the surface of theelectropositive layer 22. Also, the thickness of theP type region 20 is such that the electrons generated therein have sufficient energy to pass into theelectropositive layer 22 and be emitted therefrom in the manner described inthe article of RF. Simon et al, Electron Emission from a Cold Cathode GaAs P-N Junction," Applied Physics Letters, Apr. l, 1969, Vol. 14, No. 7. pgs. 214-216. The heterojunction between theP type region 20 and theP type region 14 prevents the electrons generated in theP type region 20 from passing back into theP type region 14. Thus, theelectron generating element 10 delivers to the emitting layer 22 a large number of electrons so as to provide a high degree of electron emission.

An electron emitting device of the construction shown in H6. 1 was made with a body 12 of Al,l xAs containing silicon as a single conductivity modifier to form the P type region l4 andN type region 16 in the manner previously described. TheP type region 14 was of a thickness of about 10 microns. TheP type region 20 was a layer of gallium arsenide containing zinc as a conductivity modifier at a doping level of about l0, cm TheP type region 20 was about 1 micron in thickness. Theelectropositive layer 22 was of cesium and oxygen and was of an area of approximately 5 X l0 c 11 Contacts were made to theP type region 22 and theN type region 16 by pressure contact only. When a current was passed through emitting electron emitting device light at a wave length of 8,700 A, value, was generated in the body 12. Electrons were emitted from the deviceinto a vacuum and it was found that the efficiency of emission was 10*, i.e. it took 1,000 electrons crossing the junction to get one electron into the vacuum. This efficiency is an improvement by a factor of about 1,000 over the reported efiiciencies for PN junction semiconductor cold cathode electron emitting devices previously developed.

Referring to FIG. 2, another formof the electron emitting element is generally designated as 32. Theelectron emitting element 32 comprises amonolithic body 34 of a semiconductor material having anN type region 36 contiguous to aP type region 38 so as to provide aPN junction 40 therebetween. As in theelectron emitting device 10 of FIG. 1, thebody 34 is of a semiconductor material which is capable of generating light in the vicinity of thePN junction 40 when the junction 'is properly biased, such as the Ill-V compound semiconductors and alloys thereof. In thebody 34, theN type region 36 should be relatively thick, at least 5 microns in thickness, and at least theN type region 36 should be of a semiconductor material which is a poor absorber of light.

A fonn of thebody 34 is a wafer of a P type Ill-V compound alloy semiconductor material, such as GaAs, P where x is greater than 0 and equal to or less than 1, containing a P type conductivity modifier, such as zinc, beryllium or cadmium, as aP type region 38 having on surface thereof a layer of N type lll-V compound alloy semiconductor material, such as GaAs ,P, containing tellurium or selenium as a conductivity modifier as anN type region 36. The semiconductor material of theN type region 36 should have a band gap energy which, at the surface of theP type region 38 is larger than or equal to the band gap energy of theP. type region 38. By using GaAs P, as the semiconductor material for theN type region 36, the band gap energy can be varied by varying the amount of phosphorous in the material with the band gap energy increasing with increasing amounts of phosphorous. This body can be formed by epitaxially growing P type GaAs, ,P on the surface of a substrate of gallium arsenide by vapor phase epitaxy in the manner described in the previously referred to article of JJ. Tietjen et al. Then N type GaAs, ,P, is epitaxially grown on the P type region. By gradually increasing the amount of phosphorous containing in the mixture as the N type layer is deposited, the desired graded band gap energy can be achieved. After the N type epitaxial layer of the desired thickness is grown, the gallium arsenide substrate is removed, such as by etching and polishing.

Aregion 42 of a P type semiconductor material is provided on the surface of theN type region 36. TheP type region 42 is the same as theP type region 20 of the electron emitter device of FIG. 1. Thus, the P type region should be of a semiconductor material which is a good absorber of light and which has an index of refraction which substantially matches the index of refraction of the semiconductor material of thebody 34. Also, theP type region 42 should be of a thickness of between 1 and 5 microns. Athin layer 44 of an electropositive work function reducing material is provided on the surface of theP type region 42. Theelectropositive layer 44 is of the same composition as and is formed in the same manner as theelectropositive layer 22 of theelectron emitter device 10 of FIG. 1.

A pair ofterminal wires 46 and 48 are electrically connected to theN type region 36 andP type region 38 of thebody 34. As shown, theterminal wire 46 is connected to theN type region 36 through asmall contact layer 50 on the surface of the N type region. Thecontact layer 50 is a film of a metal which will make good ohmic contact to the semiconductor material of theN type region 36, such as tin which may be coated with nickel and gold. A small portion of theP type region 42 is removed, such as by etching, to permit thecontact layer 50 to be applied to the surface of theN type region 36. Theterminal wire 48 is connected to theP type region 38 by acontact layer 52. Thecontact layer 52 is a film of a metal which will make good ohmic contact to the semiconductor material of the P type region, such as nickel which may be coated with gold. Athin film 54 of a light reflecting material, such as silicon monoxide coated with gold, may be coated on the peripheral edge surface of thebody 34 so as to prevent any of the light generated in the body from being emitted from the periphery of the body.

In the use of theelectron emitting device 32 theterminals 46 and 48 are connected to a source of voltage with the terminal 46 being connected to the negative side of the voltage source and the terminal 48 being connected to the positive side. Theelectron emitting device 32 operates in substantially the same manner as described with regard to theelectron emitting device 10 of FIG. 1 except that the light is generated in theP type region 38 and passes through theN type region 36 to theP type region 42. The light is absorbed in theP type region 42 and converted to free electrons which are emitted from theelectron emitting device 32 by theelectropositive layer 44.

We claim:

1. An electron emitting element comprising:

a. a body of semiconductor material having adjacent P type and N type regions with a PN junction therebetween, the semiconductor material of said body being capable of generating light when the PN junction is properly biased and the semiconductor material of at least the P type region being AI Ga AS a region of P type gallium arsenide on a surface of the P type region of the body and forming a heterojunction therebetween, said P type gallium arsenide region being a better absorber of light than the semiconductor material of the P type region of the body,

a layer of an electropositive work function reducing material on the surface of the P type gallium arsenide, region, and a separate terminal connected to each of the P type gallium arsenide region and the N type region of the body.

2. An electron emitting element in accordance with claim 1 in which the P type region of the body is at least 5 microns in thickness.

3. An electron emitting element in accordance with claim 1 in which the P type region of the body contains silicon as the conductivity modifier to lower the light absorption properties of the P type region of the body.

4. An electron emitting element in accordance with claim 1 in which the concentration of the aluminum in the P type region of the body varies from a minimum at the PN junction to a maximum at the heterojunction so that the band gap energy of the P type region of the body increases from the PN junction to the heterojunction.

5. An electron emitting element in accordance with claim 1 in which the P type gallium arsenide region is between 1 and 5 microns in thickness.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N 3667007 DatedMav 30. 1972 InVentm-(S) Henry Kressel and Jacques Isaac Pankove It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 lines 3 Z6 and 55 change "A-l Ga As" to Al Ga As- Column 2, line 71, change "GaAs PBx" to GaAs P-.

Column 4 line 19 change "Al l XAs' to Al Ga As Column 4, line 25, change "l0 cmf to lO cm Column 4,line 30, change "emitting" (first occurrence) t0 -the-.

Signed and sealed this 5th day of December 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attestlng Officer Commissioner of Patents FORM (0459) uscoM M-oc 60376-5 69 a U.$. GOVERNMENT PRINTING OFFICE 2 I95!) O366"334

Claims (5)

1. An electron emitting element comprising: a. a body of semiconductor material having adjacent P type and N type regions with a PN junction therebetween, the semiconductor material of said body being capable of generating light when the PN junction is properly biased and the semiconductor material of at least the P type region being AlxGa1 xAs b. a region of P type gallium arsenide on a surface of the P type region of the body and forming a heterojunction therebetween, said P type gallium arsenide region being a better absorber of light than the semiconductor material of the P type region of the body, c. a layer of an electropositive work function reducing material on the surface of the P type gallium arsenide, region, and d. a separate terminal connected to each of the P type gallium arsenide region and the N type region of the body.

2. An electron emitting element in accordance with claim 1 in which the P type region of the body is at least 5 microns in thickness.

3. An electron emitting element in accordance with claim 1 in which the P type region of the body contains silicon as the conductivity modifier to lower the light absorption properties of the P type region of the body.

4. An electron emitting element in accordance with claim 1 in which the concentration of the aluminum in the P type region of the body varies from a minimum at the PN junction to a maximum at the heterojunction so that the band gap energy Of the P type region of the body increases from the PN junction to the heterojunction.

5. An electron emitting element in accordance with claim 1 in which the P type gallium arsenide region is between 1 and 5 microns in thickness.

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