US 3806372 A
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April 23, 1974 A. H. SOMMER 3,806,372
METHOD FOR MAKING A NEGATIVE EFFECTIVE-ELECTROH-AFFINITY SILICON ELECTRQN EMITTER Fig. 1
Filed June 2, 1972 I HEAT FOR SEVERAL SECONDS TO NEAR THE MELTING TEMPERATURE SENSITIZE WITH CESIUM AND OXYGEN REPEAT THE ABOVE HEATING AND SENSITIZING STEPS EAT FOR SEVERAL SECONDS TO A TEMPERATURE ABOUT 300 DEGREES LOWER THAN THAT OF THE FIRST HEATING SENSITIZE WITH CESIUM AND OXYGEN REPEAT THE LOWER TEMPERATURE HEATING AND SUBSEQUENT SENSITIZING UNTIL MAXIMUM PHOTOSENSITIVITY IS OBTAINED United States Patent 3,806,372 METHOD FOR MAKING A NEGATIVE EFFECTIVE- ELECTRON AFFINITY SILICON ELECTRON EMI'ITER Alfred Hermann Sommer, Princeton, NJ., assignor to RCA Corporation Filed June 2, 1972, Ser. No. 259,037 Int. Cl. H011 7/34 US. Cl. 1481.5 6 Claims ABSTRACT OF THE DISCLOSURE A method for making a negative effective-electronatfinity silicon electron emitter includes the steps of preparing the silicon by first heating for a short time to near the melting point of the silicon, sensitizing by applying a layer of work-function-reducing material, heating again for a short time to a lower temperature than the first, and again sensitizing by applying a layer of work-function-reducing material.
BACKGROUND OF THE INVENTION The invention relates to silicon negative effective-electron-affinity electron emitters.
A P type silicon crystal which is sensitized by application of work-function-reducing material, such as cesium and oxygen, to its surface can function as an efficient electron emitter. Sensitized silicon cathodes may be used, for instance, as long wavelength-sensitive photocathodes, secondary emitters, and electron gun cold cathodes.
One problem with silicon cathodes is the difliculty of removing contaminants from, and otherwise preparing, the surface region of the crystal before the cesium and oxygen are applied. Such preparation is essential to obtaining a negative etfective-electron-afiinity characteristic in the cathode. If the negative effective-electron-aflinity is not attained, the cathode is not acceptable for commercial use.
It is presently generally accepted in the art of processing silicon cathodes that adequate preparation of the silicon prior to application of the cesium and oxygen must include removal of several monolayers of silicon by sputtering, followed by an annealing at high temperature. The sputtering is by bombardment with non-reactive gas ions. The following references are given to illustrate sputtering and annealing processes and to evidence their general use for silicon cathodes:
Martinelli, R. U Reflection and Transmission Secondary Emission From Silicon, Applied Physics Letters, 17 (8): pp. 313-314, Oct. 15, 1970.
Iona, F., Observations of Clean Surfaces of Si, Ge, and GaAS by Low-Energy Electron Diffraction, IBM Journal, pp. 375-387, September-November 1965.
Lander, J. J. and Morrison, J Structures of Clean Surfaces of Germanium and Silicon I, Journal of Applied Physics, 34 pp. 1403-1410, May 1963.
Farnsworth, H. E. et al., Application of the Ion Bombardment Cleaning Method to Titanium, Germanium, Silicon, and Nickel as Determined by Low-Energy Electron Diifraction, Journal of Applied Physics, 29 (8): pp. 1150-1161, August 1958.
US. Pat. No. 3,591,424, issued to Ward July 6, 1971.
Sputtering is a rather lengthy process which requires relatively complex internal tube structures, such as a special gun. Also, the annealing required after sputtering is likely to degrade the junction characteristic of junction devices, such as PN junction cold cathodes by impurity diffusion. For these reasons, silicon has as yet no significant use commercially for cathodes in electron tubes. Practical considerations require that cathodes for electron tubes be cleaned after they have been mounted in the tube envelope. Sputtering and annealing would require appropriate tube structures therefor to be located inside the tube envelope, which is not presently commercially feasible for electron tubes which might include silicon cathodes.
SUMMARY OF THE INVENTION The novel method of making a silicon cathode comprises heating the silicon in vacuum to near its melting point for a brief time, sensitizing the emitting surface by applying a layer of work function reducing material, heating again for a brief time to a temperature lower than the first heating temperature, and again sensitizing.
By performing the specified heating and sensitizing steps, the necessity for sputtering and annealing is avoided completely. The heating steps are readily performed with the type of equipment already used commercially for electron emissive tubes. Thus, the novel method makes it feasible to use a negative effective-electron-afiinity cathode in existing electron tube types.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart of the preferred embodiment of the novel method.
FIG. 2 is a partially sectional, partially schematic view of a phototube including a silicon cathode processed in accordance with the preferred embodiment of the novel method.
DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a flow chart of a preferred embodiment of the novel method for processing a silicon reflecting photocathode for a phototube 10 shown in FIG. 2. Referring now to FIG. 2, the phototube 10 has a transparent envelope 12, inside of which is mounted a silicon photocathode 14 and an anode ring electrode 16 spaced from the photocathode 12. The cathode 12 is a round P type monocrystalline silicon disc about 2 cm. (centimeters) in diameter and about 200 m. (micrometers) thick, having an emitting surface 18 oriented in the Miller index plane of the crystal. The emitting surface 18 is provided with a work function reducing layer 20 of cesium and oxygen. Electrical leads 21 are clamped to opposite edges of the cathode 12 for ohmic heating of the cathode 12. The envelope is continuously evacuated through exhaust tabulation 22 by a vacuum pump 24. Two appendages 26, 28 on the exhaust tabulation 22 contain compounds which release cesium and oxygen, respectively, when heated. The pump 24 is operated continuously during the entire process to maintain a vacuum of on the order of 10- to 10- torr in the envelope, except during sensitizing, as measured by an ionization gauge.
After a preliminary degassing bake-out of the entire phototube 10, the cathode 12 is first heated to a temperature of about 1300 C. (Celsius) for about 3 seconds by ohmic heating resulting from passing electric current through the leads 21. During the heating, the temperature is monitored by measuring the light radiation output of the silicon with a pyrometer. The cathode 12 is then allowed to cool. After the silicon reaches approximately 100 C., cesium is released from the cesium appendage 26 by heating thereof until the photoemission of the cathode 12 has passed a maximum. Thereafter, at near room temperature, oxygen is released from the oxygen appendage 28 by heating thereof until the photoemission is maximized. If the photosensitivity is less than about 2 microamperes per lumen, the first heating and sensitizing is repeated once. This generally raises the photosensitivity to above that value. The cesium and oxygen together form a portion of the final thickness of the work-function-reducing layer 20.
The cathode 12 is now again heated, this time to a temperature of 1000 C. for about 3 seconds, and allowed to cool to about 100 C. The emitting surface 18 is again sensitized by release of cesium and oxygen from the appendages 26, 28. This last, lower temperature, heating followed by sensitizing, is repeated to the extent necessary to obtain peak photosensitivity from the emitting surface 18.
The novel method of preparing and sensitizing the silicon results in negative eiIective-electron-afiinity without the necessity of additional structures inside the envelope for sputtering and without a lengthy annealing step. Thus, the novel method is readily adapted for processing a silicon cathode in existing commercially-available electron emissive tube types, with only minor modifications in presently used processing. Such a silicon cathode may be used, for example as a photocathode, a secondary emissive dynode, or as a cold cathode in an electron gun.
GENERAL CONSIDERATIONS The first, higher temperature heating should be within about 100 degrees of the melting point of silicon, generally given as 1420 C. However, the temperature may be lower where a longer time at that temperature can be tolerated by the associated cathode structure. For instance, the first heating temperature may be anywhere from about 1100 C. to as near the melting temperature as is practical without resulting in damage to the silicon crystal due to non-uniform heating. For a silicon cathode structure having a PN junction, such as for a forward biased PN junction cold cathode, it is desirable to keep the time to a minimum in order to prevent undesired diffusion of the P type impurities at the thin junction region of the cathode. In such a case, it is desirable to use a high temperature, near the melting temperature, and to keep the time to about seconds or less. Where, however, the silicon has no junction, it may be acceptable to use a temperature on the order of 1100 C. for as long as on the order of one-half minute.
The second heating should be on the order of 300 C. lower than the first heating, or in the range of between about 800 and about 1000 C. Again, the heating should be brief, as for the first heating, and should be minimized for a junction device. However, the time is not as critical for the second heating with regard to the diffusion problem in a junction device since the temperature is lower. The sensitizing may be by application of cesium and oxygen or rubidium and oxygen to obtain negative efiectiveelectron-afiinity. Other work-function-reducing layer materials do not appear to reduce the work function sufliciently to result in negative eifective-electron-afiinity.
If, after the first heating, the photosensitivity of the cathode is below about 2 microamperes per lumen, it is advisable to repeat the first heating, as such a low sensitivity is a sign that the heating was not entirely adequate to prepare the crystal. Also, if the final sensitivity is below about 150 microamperes per lumen, it may be advantageous to repeat the entire heating and sensitizing procedure, since it may be expected that the final sensitivity can be made higher. While these repeating steps are generally necessary, they are not considered at present to be essential.
In the preferred embodiment, the temperature of the silicon was measured with a pyrometer. Other means, such as a thermocouple, may also be used. While there is no assurance that any of the various temperature measurement means will give the absolute temperature of the silicon, the temperature as measured herein is believed to be within about 10 C. of the absolute temperature of the silicon. The optimum temperature varies somewhat with the time which the silicon was held at that temperature. Lower temperatures generally require a longer time.
What is claimed is:
1. A method for making a silicon negative effectiveelectron-aifinity electron emitter, comprising the steps of:
(a) heating a silicon cathode having a surface oriented in the (100) Miller index plane in vacuum to between about 1100 C. and about 14'20 C.; then (b) sensitizing said (100) oriented surface of said silicon cathode by application of a work function-reducing-material thereto; then (c) heating said silicon cathode again in vacuum to a temperature lower than said temperature of said first heating; and
(d) sensitizing said (100) surface by application of a layer of work-function-reducing material.
2. The process defined in claim 1 wherein said first heating is to about 1300 C.
3. The method defined in claim 2 wherein said second heating is to a temperature of about 1000 C.
4. The method defined in claim 1 wherein said workfunction-reducing material is chosen from the group consisting of cesium plus oxygen and rubidium plus oxygen.
5. The method defined in claim 1 comprising repeating steps (c) and (d), in sequence, until electron emission from the silicon is maximized.
6. The method defined in claim 1 comprising repeating, in sequence, steps (a), (b), (c) and (d) at least once.
References Cited UNITED STATES PATENTS 3,669,735 6/1972 Fisher 313-94 X 3,672,992 6/1972 Sohaefer 313 R X 3,632,442 1/ 1972 Turnbull 313-94 X 3,293,084 12/1966 McCaldin 14841.5 3,591,424 7/1971 Ward 1481.5 3,498,834 3/1970 Rome et a1 313--65 R X OTHER REFERENCES Van Laar et al.: Philips Research Reports, vol. 17, No. 2, pp. 101-124 (April 1962).
GEORGE T. OZAKI, Primary Examiner US. Cl. X.R.
148-186, 187, 189'; 252--62.3 E; 313-65 R, 94; 317235 R