DE2350872A1 - ELECTRON TUBE HAVING A A TIEF III B TIEF V SEMICONDUCTOR CATHODE AND METHOD FOR PRODUCING SUCH A CATHODE - Google Patents

Description

7602-73 / Dr.v.B / Ro.

JBJ / RCA 66 550

US Ser. No. 323 746

Filed: January 15, 1973 2350872

RCA Corporation, New York, N ^ Y. (VoSt.A.)

Electron tube with a & γνν ^Β semiconductor

cathode and method of manufacturing such a cathode.

The present invention relates to an electron tube with an evacuated piston and an electron- _{emissive A IT} "B semiconductor cathode arranged therein, which has a surface which is coated with a layer containing cesium which reduces the work function. The invention also relates to a method for producing an Ajj-B -.- semiconductor cathode, which is coated with a layer containing cesium which reduces the work function.

The term "cathode" is intended here to mean, quite generally, an arrangement which. Able to emit electrons into a vacuum. The term "cathode" includes Z ₀ B ₀ photocathodes, dynodes and radiation generating system cathodes for the various

most electron-emitting tubes.

It is known that primary and secondary emission cathodes can be produced from the semiconducting compounds of the elements aluminum, gallium, indium, phosphorus, arsenic and antimony from Groups III and V of the Periodic Table of the Elements, which can be produced by a particularly high electron emission efficiency distinguish. Such cathodes are generally activated by coating them with a layer of cesium in combination with a strongly electronegative element, mostly oxygen or fluorine. Cathodes from A _{II :! .B V} compounds and processes for their activation are described, for example, in US Pat. Nos. 3,387,161, 3,487,213, 3,644,770, 3,669,735 and 3,696,292.

A disadvantage of the known cathodes made from A__B _V compounds is their instability during operation. The binding of the cesium to the surface of the crystalline A ^ .- compound is relatively weak compared to the binding of the cesium to other, more conventional base materials for photocathodes, such as antimony. While photocathodes of conventional base materials such as antimony, at temperatures of 15O ⁰ C and activated about one currently used for the activation of A _{II: v} jB compounds with cesium temperatures below 100 ° C. If an A__ Β ,, - semiconductor photocathode is activated by the methods currently known for such photocathodes and exposed to temperatures above 100 ° C., the layer which reduces the work function does not form on the surface as it would for a practically usable tube or the like is required. On the other hand, cathodes made from A _{111 B.} For a period of time the emission then deteriorates considerably as a result of a loss of cesium from the layer which reduces the work function.

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The present invention is based on the object the shortcomings of the known cathodes from A -.- By connections and to avoid the known methods of making such cathodes.

This object is achieved by the in claims 1 and characterized invention solved.

A tube according to the invention contains a source of cesium buffer in the flask. The buffer source is to buffer a material which has not fully reacted with cesium and provides a small amount of Cesium vapor in the interior of the envelope during a treatment at a temperature above 1OO ⁰ C to the loss of cesium from the cathode at this temperature.

Of the invention for producing a -__ A B ..- containing compound in the embodiment of the method according cathode is exposed to the cathode at a temperature below 100 ⁰ C temperature cesium vapor. Then, the temperature of the cathode is increased to a value above 100 ⁰ C while the cathode cesium vapor suspended until the emission is substantially constant. The concentration of the cesium, the cathode is aucgesetzt is increased during the increase in temperature to a value above about 100 ⁰ C and lowered during the lowering of the temperature from this value.

A cathode made from an A ₁₁₁ B compound which has been activated by the present process has a longer service life than cathodes activated in a known manner. A tube with a cesium buffer source automatically provides easily the appropriate low cesium pressure of the buffer source during the treatment at the temperature about 1OO ⁰ C. The buffer source is cesium in increasing the temperature at an increasing rate, and thereby increases the C.Msiumkonzentration while on the other hand, as the temperature decreases, cesium is absorbed at an increasing rate, thereby reducing the concentration.

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In the following are exemplary embodiments of the invention explained in more detail with the help of the drawing show it?

1 shows a photo enlarging tube according to a preferred one Embodiment of the invention and

Fig. 2 is opposite. 1 shows an enlarged sectional view in a plane 2-2 of FIG. 1.

When in Fig. 1 as an embodiment of the invention The tube 10 shown is a photomultiplier tube or a photo-secondary electron multiplier with a Glass bulb 12 and a foot 14 with a number of metal pins 16, which are used in the usual way as connections for the electrodes of the tube.

As shown in FIG. 2, there is a shield 18 made of metal inside the tube. At a distance from the shield 18 is a metal sheet 20, on one side of which a photocathode 26 in the form of a rectangular disk made of monocrystalline In ₁ Ga As (x = 0.72) is attached, which is about 2 cm long, 0.5 cm wide and 0.5 mm thick. A heating coil 22 is located between the shield 18 and the metal sheet 20, by means of which the metal sheet 20 carrying the photocathode 26 can be electrically heated.

In Fig. 2 counted in the counterclockwise direction is in the angular distance from the photocathode 26 a first grid electrode 28 substantially perpendicular to the direction of the on the photocathode 26 falling light arranged. The grid electrode 28 is followed by a series of dynodes 30 made of a copper-beryllium alloy, and an anode 32. The general direction of the central electron trajectories from the photocathode 26 to the anode 32 is indicated by dashed lines 38. Evaporation boats 40 and 42 are arranged in the vicinity of the foot 14 of the tube 10, which are used in the formation of the photocathode 26 to generate cesium vapor or oxygen and through the passage of current are heatable. In the upper part of the tube 10 is a pearl 43 made of a platinum-antimony alloy with a ceramic shield 45 mounted on an upper part of the wall of the glass bulb

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12 is directed. On the inside of the upper part of the wall of the glass bulb 12, opposite the pearl 43, there is an approximately 10 µm thick layer of antimony which has not completely reacted with cesium. The layer 44 serves as a cesium buffer source during the formation of the cathode described below.

According to a preferred embodiment of the method according to the invention, the photocathode 26 of the tube 10 shown in FIGS. 1 and 10 is activated as follows? After the interior portions of the tube have been mounted 10 in the glass bulb 12, the tube 10 is several hours at a temperature of about 350 ⁰ C with simultaneous evacuation baked by an arranged on the foot 14 pump port to clean the inner parts of the tube. The tube is then allowed to get cold enough for further treatment to be possible. Now, the heating coil 22 is turned on and the cathode 26 _t heated briefly to near its decomposition temperature by itself to the surface of the photocathode 26 to prepare for activation.

Next, add an approximately 10 µm thick layer of antimony evaporated onto a small area of the upper part of the side wall of the glass bulb 12, in which the bead 43 by current passage is heated. The antimony layer reacts with cesium during a later process step to form a Cesium buffer layer 44.

Cesium vapor is now generated inside the tubular flask, if the evaporation boat 40 for the cesium is heated by the passage of current, at the same time the photosensitivity of the photocathode 26 is monitored.The photocathode is exposed to the cesium vapor until the photosensitivity has passed through a maximum, then cesium vapor is generated set. Now is oxygen being produced? in which the evaporation boat 42 is heated for oxygen by the passage of current until the photosensitivity of the photocathode 26 has passed through a maximum again? whereupon the generation of oxygen is stopped. These process _steps in which the photocathode alternates with cesium and oxygen

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is exposed, are repeated at room temperature until maximum photosensitivity is reached. The excess cesium is then expelled from the evaporator boat 40. Following the release of the excess cesium, the tube is heated to about 150 ⁰ C 10 until the photosensitivity of the photo-cathode 26 has become substantially constant, then the tube is allowed to cool to room temperature. The photocathode 26 is now stable. The tube 10 is now melted, the required vacuum being maintained at the pump nozzle.

When cesium vapor is generated in the tube 10 for the first time, part of the cesium reaches the antimony layer on the side of the glass bulb 12, which finally forms the cesium buffer layer 44, and there forms a compound of incompletely reacted antimony with cesium. In addition, cesium is also adsorbed on the surface of the crystal forming the photocathode 26 and there forms a layer which reduces the work function. When the tube 10 is heated from a temperature below 1OO ⁰ C to the elevated temperature of 150 ⁰ C, cesium is discharged at an increasing rate from the cesium buffer layer 44 by the decomposition inside the tube 10, so.daß simultaneously the concentration of the cesium vapor, to which the photocathode 26 is exposed increases. A dynamic state of equilibrium occurs at the surface of the photocathode 26, which prevents a substantial loss of cesium from the layer which reduces the work function. During the subsequent cooling of the tube, the excess cesium in the interior of the tube is again absorbed by the cesium buffer layer 44 by recombination at an increasing rate, and at the same time the concentration of the cesium vapor to which the photocathode 26 is exposed is thereby reduced. Since the _> photocathode 26 can now be activated with cesium at the higher temperature of 150 ^° C. and not just at a temperature below 100 ^° C., the photocathode 26 is much more stable.

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The present invention is applicable to the activation or formation of any A _ ._. N _v compound semiconductor cathode which is untreated with a cesium-containing layer to reduce the work function. In the described preferred exemplary embodiment of the present process, the increased activation ^{temperature was 150 °} C., but a substantial improvement in the stability of the cathode can generally be expected if the cathode is used in the presence of a small amount of cesium vapor, such as that from a buffer source containing a cesium compound is supplied, heated to any temperature above 100 ^{° C.} In some blndungen ^A _II ^B j v ^"Ver" it is expedient to bring oxygen, as in the preferred embodiment, or fluorine to act. Other Aj-By compounds such as gallium phosphide, on the other hand, are generally not exposed to oxygen or fluorine during treatment.

The cesium buffer source can be placed anywhere in the flask where its electron emissivity allows normal operation the tube does not bother. For example, because of the photoemissivity of cesium antimony, it would be impractical to use the buffer source to apply the forming layer at a point where incident light causes the emission of interfering electrons, which can enter the secondary electron multiplier part of the tube. The buffer source should be electrically passive, because neither the ability for primary emission nor that for secondary mission is useful.

The Cäijium-antimony compound of the buffer source of the preferred embodiment of the invention produced apparently in the tube at higher temperatures an optimum cesium equilibrium, since as a result of a weak decomposition enough cesium vapor is generated by it to a überm'lssigen loss of cesium from the A ₁₁ jBy connection to prevent the cathode. At the same time, the cesium is sufficiently bound in the cesium-antimony layer at lower temperatures to prevent the cathode from being adversely affected by excess

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CM ρ i to prevent. Instead of a Ciisium-antimony compound may also be added other cesium compounds which decompose at elevated temperatures above 100 ⁰ C, to use for the buffer source. Examples of such compounds are cesium-bismuth, cesium-gold, or cesium-graphite compounds. The decomposition should, however, begin at such low temperatures that sufficient cesium is available ^{at the elevated temperature used, for example 150 ° C.}

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Claims (1)

Claims 1.j / electron tube with an evacuated piston and an electron-emissive A__ B _v semiconductor cathode arranged in it, which has a surface which is coated with a cesium-containing layer which reduces the work function, characterized in that the piston has an electrically passive cesium buffer source (44 ) made of a material containing cesium. 2.) An electron tube according to claim 1, characterized in that the cesium buffer source is a cesium-containing compound which begins to decompose at a temperature between about 100 ⁰ C and 150 ⁰ C. 3.) electron tube according to claim 1 or 2, characterized ge indicates that the cesium buffer source is an antimony compound that has incompletely reacted with cesium. 4.) Electron tube according to claim 1, 2 or 3 _r, characterized in that the cesium buffer source is applied in the form of a layer on the inner wall of the piston (12). 5.) Method for producing an A _II; _B "-semiconductor compound cathode, in which the cathode is ^{exposed to a first value of cesium at a temperature below 100 °} C. until the photosensitivity has passed through a maximum? characterized in that the cathode (26) is mentioned to a temperature of a second value and is exposed to an increased cesium concentration, that the cathode is kept at the temperature of the second value and exposed to cesium until its photosensitivity has become stable and that then the temperature of the cathode and the concentration of the cesium acting on them can be reduced at the same time. records that the second temperature value is about 15Ο C ) Method according to claim 5, characterized in that;
is. 4098 2 9/0632 7.) The method according to claim 5 or 6, characterized in that the cathode between the treatment exposed to oxygen at the temperature of the first value and heating to the temperature of the second value will. 8.) Method according to claim 5 or 6, characterized that the cathode between the treatment at the temperature of the first value and the heating fluorine is exposed to the temperature of the second value. 409829/0632