Method of making a transmission photocathode device

Abstract

A transmission photocathode device of the negative-electron-affinity type is disclosed. The device comprises epitaxially grown P type semiconductor layers and an alkali metal or alkali metal-oxygen work-function-reducing activation layer. Also disclosed is a novel method for making a negative-electron-affinity transmission photocathode device. The method enables a photocathode device to be made by the serial epitaxial growth of p type layers of a II-VI or III-V semiconductor on a semiconductor substrate. The method provides for a virtually perfect lattice match between the semiconductor layers thereby increasing the efficiency of the photocathode by eliminating lattice defects which would otherwise exist at the interface between the transmitting material and the absorbing material.

Description

BACKGROUND OF THE INVENTION

The present invention relates to transmission photocathode devices and to a method of making the same and more particularly relates to transmission photocathodes which make use of the negative-electron-affinity principle and to a method for making such devices.

Photocathodes comprising P type semiconductor layers activated with an alkali metal or an alkali metaloxygen combination having a low work function are known in the photocathode art. One such photocathode is described by Van Laar and Scheer in U.S. Pat. No. 3,387,161. These photocathodes consist of a material that emits electrons when exposed to radiant energy. Van Laar and Scheer describe an opaque photocathode of the type which emits electrons from the same side as that struck by light which is incident on the photoemissive material.

A second type of photocathode, the semitransparent or transmission photocathode, has a photoemissive layer or absorber on a transparent medium. Electrons are emitted from the side of the photocathode opposite the side upon which the radiation is incident on the transparent medium.

It is possible to epitaxially grow a P type layer of semiconductor material on a layer of transparent material and then to activate the P type layer with an alkali metal or alkali metal-oxygen activation layer and thereby make a negative-electron-affinity device. This constitutes the prior art method of making such devices. The problem which is created by this method of epitaxially growing one crystal on another is that of lattice defects. Lattice defects occur either when the lattice parameters of the different semiconductors are different or when defects have been introduced into the semiconductor during crystal growth.

The problem that is created by lattice mismatch is that conduction band electrons which have been generated in the photocathode material are able to recombine in centers located at the defects in the crystal lattice. This recombination of electrons prevents them from diffusing to the emitting surface of the photocathode where they can be liberated from the photocathode. This has the effect of degrading the efficiency of the photocathode.

SUMMARY OF THE INVENTION

A transmission photocathode is presented which comprises an absorption layer of P type semiconductor material having a first side and a second side; a transmission layer of P type semiconductor material adjacent the first side of the absorption layer, the transmission layer comprising a semiconductor material having lattic parameters which are within 0.5% of the lattice parameters of the absorption layer, and the transmission layer having a higher bandgap energy than the absorption layer, the bandgap energy of the transmission layer being at least 1.1 electron volts; and a coating of a work-function-reducing activation layer on the second side of the absorption semiconductor layer.

Also presented is a method for making a transmission photocathode device comprising the steps of epitaxially growing a P type layer of a semiconductor material on a substrate of a like semiconductor material; growing a second semiconductor layer upon the P type layer, the second semiconductor layer being of a material with a higher bandgap energy than the P type layer; removing the substrate to expose the P type layer; and coating the exposed P type layer with a work-function-reducing activation layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of one embodiment of a transmission photocathode device made by the method of the present invention.

FIG. 2 is a sectional view of a semiconductor substrate with two epitaxially grown semiconductor layers.

FIG. 3 is a sectional view of the transmission photocathode device of FIG. 1 prior to the application of the activation layer.

FIG. 4 is a sectional view of a multiple-bin refractory furnace boat which is suitable for carrying out the method of the present invention.

DETAILED DESCRIPTION

Referring generally to FIG. 1, the preferred embodiment of a transmission photocathode 10 made by the method of the present invention is shown. The transmission photocathode 10 comprises a transmission layer 12 on one side of an absorption layer 14. Coated onto the other side of the absorption layer 14 is a work-function-reducing activation layer 16. The transmission photocathode 10 of the preferred embodiment is fabricated with III-V semiconductor material. However, II-VI compounds may also be used, either alone or in conjunction with III-V compounds, to construct a transmission photocathode 10 according to the method of the present invention. In the preferred embodiment shown in FIG. 1, the absorption layer 14 comprises the binary III-V compound gallium arsenide, GaAs, doped to provide a P type conductivity and preferably having an acceptor concentration of at least 5 .times. 10.sup.17 atoms per cm.sup.3. The transmission layer 12 comprises the ternary III-V compound aluminum-gallium arsenide, (AlGa)As. The activation layer 16 comprises an alkali metal such as cesium or an alkali metal-oxygen combination such as cesium-oxygen which is used in the preferred embodiment.

In the growth of semiconductor crystal layers which do not contain lattice defects, it is very important to start out with a low defect substrate. In order to minimize the lattice defects in the epitaxial layers, it is also important to have a close match of lattice parameters between the substrate and the layer being grown. Once lattice defects have been introduced into the crystal structure, they will propagate throughout the growth of crystalline layers. Therefore, in the preferred embodiment of the method of the present invention, a low defect substrate of gallium arsenide, GaAs, 18 will be used as the basis for fabricating the transmission photocathode 10. A low defect substrate is one which has less than 10.sup.3 dislocations per cm.sup.2.

Referring generally to FIG. 2, one starts with the aforementioned GaAs substrate 18 which may be melt grown. On the GaAs substrate 18 a P type GaAs layer 14 is grown. In the preferred embodiment the method of liquid phase epitaxy is used to fabricate the photocathode 10. The acceptor impurity of the preferred embodiment is germanium, a Group IV element, which replaces atoms of arsenic for the most part in the crystal structure of GaAs to yield a P type material. The P type GaAs layer 14 is typically 2 .mu.m thick. Following the growth of the P type layer 14, an aluminum-gallium arsenide (AlGa)As layer 12 is grown on the P type GaAs layer 14. The growth of the (AlGa)As layer is also accomplished through the method of liquid phase epitaxy in the preferred embodiment of the method of the present invention. The (AlGa)As layer 12 may have an aluminum concentration of about 30 to 50%. In the preferred embodiment the aluminum concentration is about 30%.

(AlGa)As is a ternary III-V compound having a higher bandgap energy than the binary III-V compound GaAs. The increased bandgap of (AlGa)As makes it transparent to light of a lower wavelength than will pass through GaAs. (AlGa)As has a lattice structure having parameters which are extremely close to the lattice parameters of GaAs. Therefore, relatively few lattice defects will occur at the interface between the GaAs layer 14 and the (AlGa)As layer 12. This means that the growth of the (AlGa)As layer 12 will not be initiated with many lattice defects and therefore there will be few dislocations to propagate through the growth of the (AlGa)As layer 12. The thickness of the (AlGa)As layer 12 is not important but will generally be made on the order of about 125 .mu.m in order to provide support for the photocathode 10. The (AlGa)As layer may also be made P type in order to prevent the formation of a junction in the device, as a junction would impede the free flow of electrons.

Referring generally to FIG. 4, a multiple-bin refractory furnace boat 22 such as that described by Nelson in U.S. Pat. No. 3,565,702 is shown. The boat 22 is provided with three wells 24, 26, 27 and a movable slide 28 which is suitably made of a refractory material such as graphite. The slide 28 has an upper surface which is coplanar with the plane of the bottom of each of the bins 24, 26, 27. A slot 34 is provided in the upper surface near one end of the slide 28. The slot 54 is large enough to accommodate the GaAs substrate 18 which is positioned in the slot 34 so that the substrate has the surface upon which layers are to be grown uppermost. Suitably, the exposed upper surface of the substrate 18 is cleaned and polished before the substrate 18 is positioned in the slot 34 of the slide 28. A first charge is introduced into the bin 24 and a second charge is introduced into the bin 26. The first charge may consist of 5 g. of Ga, 550 mg. of GaAs, and 100 mg. of Ge, and a second charge may consist of 5 g. of Ga, 250 mg. of GaAs, 200 mg. of Ge, and 6 mg. of Al. The charges may be granulated solids at room temperatures. The loaded furnace boat 22 is then positioned in a furnace. A flow of high purity hydrogen is maintained through the furnace and over the furnace boat 22 while the temperature of the furnace and its contents is increased from about 20.degree.C. to about 920.degree.C. in about 20 minutes.

The power is then turned off and the furnace boat with its contents is allowed to cool at a rate of about 3.degree. to 5.degree.C. per minute. At the temperatures thus attained, the first charge becomes the first melt or solution 36, which in this example consists principally of GaAs dissolved in molten gallium with a germanium conductivity modifier capable of acting as an acceptor and inducing P type conductivity in GaAs. The second charge becomes the second melt or solution 38 which consists principally of (AlGa)As dissolved in molten gallium as the solvent.

When the temperature of the furnace boat 22 and its contents have reached about 900.degree.C. the slide 28 is pulled in the direction shown by the arrow so that the substrate 18 becomes the floor of the first bin 24. The substrate 18 is allowed to remain in this position until the temperature reaches 880.degree.C. During this time, some of the GaAs dissolved in the first melt 36 precipitates and deposits on the substrate 18 as a first epitaxial layer 14 as shown in FIG. 2. The epitaxial layer is of P type conductivity because some germanium is incorporated in the crystal lattice of the epitaxial layer 14.

The slide 28 is now moved in the direction shown by the arrow so that the substrate 18 becomes the floor of the second bin 26. The substrate 18 is now permitted to cool to a temperature of about 850.degree.C. while in contact with the second melt 38. During this time, a second epitaxial layer 12 is deposited on the first epitaxial layer 14. Some of the aluminum present in the second melt 38 is also incorporated in the second epitaxial layer replacing some of the Ga atoms in the layer so that the second epitaxial layer is also a mixed compound semiconductive material having the general formula Al.sub.x Ga.sub.1.sup.-x As, where x is less than 1 and is 0.3 in the preferred embodiment.

When the temperature of the furnace boat 22 reaches 850.degree.C. the slide 28 is again moved in the direction shown by the arrow so that the substrate 18 becomes a floor of empty bin 27. The substrate 18 with its successive epitaxial layers 14, 12, is then cooled in the empty bin 27 to room temperature in a non-oxidizing ambient. The use of an empty bin 27 for the cooling step is convenient in order to prevent the additional growth of unwanted Al.sub.x Ga.sub.1.sup.-x As of a possibly undesirable composition.

While the preferred embodiment of the method of making a photocathode has been described with reference to the methods of liquid phase epitaxy, as will be obvious to one skilled in the art, vapor phase epitaxy methods can also be used. If vapor phase epitaxy is used, though, zinc should be substituted for germanium as the acceptor impurity as germanium acts as a donor when vapor phase epitaxy on GaAs is used. For vapor phase epitaxy the substrate 18 is placed in a chamber into which is passed a gas containing the element or elements of the particular semiconductor material. The chamber is heated to a temperature at which the gas reacts to form the semiconductor material which deposits on the surface of the substrate. The Group III-V compound semiconductor materials and alloys thereof can be deposited in the manner described in the article of J. J. Tietjen and J. A. Amick entitled "The Preparation and Properties of Vapor-Deposited Epitaxial GaAs.sub.1.sup.-x P.sub.x Using Arsine and Phosphine," JOURNAL ELECTROCHEMICAL SOCIETY, Vol. 113, p. 724, 1966. The Group II-VI compound semiconductor materials can be deposited in the manner described in the article by W. M. Yim et al., entitled "Vapor Growth of (II-VI) -- (III-IV) Quarternary Alloys and Their Properties," RCA REVIEW, Vol. 31, No. 4, p. 662, December 1970.

Following the epitaxial crystal growth processes, the substrate 18 is etched away in order to expose the surface 20 of the absorption layer 14 as shown in FIG. 3.

Any commonly-known etching solution, such as Caro's acid, may be used for this etching process. Portions 19 of the substrate 18 may be protected during the etching process by a wax coating. The wax coating is removed following the etching process in order to provide for structural members 19 which may be used to strengthen and as a means for handling the photocathode 10.

Following the etching process, the newly-exposed surface 20 of the absorption layer 14 has a work-function-reducing activation layer 16 coated onto it. This activation layer 16 is comprised of a low work-function alkali metal or alkali metal-oxygen layer. In particular, a coating of cesium-oxygen may be used for the activation layer 16. Cesium may be generated using either a vapor source consisting of a mixture of cesium chromate and silicon contained in a nickel tube, or an ion source consisting basically of sintered alumina impregnated with cesium carbonate. The coverage of the absorption layer 14 may be effected by the alternate exposure of the surface of the absorption layer 14 to cesium and oxygen at room temperature as per the method described in the article of A. A. Turnbull and G. B. Evans entitled "Photoemission From GaAs-Cs-O," BRIT. J. APPL. PHYS., Ser. 2, Vol. 1, p. 155, 1968.

The advantage of this method of fabrication is that a low surface recombination interface will exist between the GaAs layer 14 and the (AlGa)As layer 12. Furthermore, because the GaAs layer 14 is grown on a GaAs substrate 18 there is a perfect lattice match. If the (AlGa)As layer 12 was grown first this would not have been the case, as pure crystalline (AlGa)As is generally not available. The dislocation density and hence the diffusion length in the P type region 14 will be unaffected by the growth of the (AlGa)As layer 12. While the present embodiment has been described with GaAs, a binary III-V compound, and (AlGa)As, a ternary III-V compound, other binary and ternary III-V compounds such as gallium antimonide, GaSb, and aluminum-gallium antimonide, (AlGa)Sb, or gallium phosphide, GaP, and aluminum-gallium phosphide (AlGa)P, may be used with slight differences between lattice parameter matching and transmission frequency of the photocathodes formed with these other materials. It is also possible to use the method of the present invention to form transmission photocathodes using combinations of binary III-V compounds such as aluminum phosphide, AlP, and gallium phosphide, GaP, wherein the higher bandgap AlP would act as the transmission material and the lower bandgap GaP would be the absorption material. Similarly II-VI semiconductors may be used as will be obvious to those skilled in the art.

Claims

I claim:

1. The method for making a transmission photocathode device comprising the steps of:

a. epitaxially growing a first layer of a P type semiconductor material on a substrate of the same semiconductor material as that of the first layer; b. epitaxially growing a second semiconductor layer upon said P type first layer, said second semiconductor layer being of a P type semiconductor material having a higher bandgap energy than the semiconductor material of said P type first layer;

c. removing said substrate to expose said P type first layer; and

d. coating said exposed P type first layer with a workfunction-reducing activation layer.

2. The method of claim 1 wherein said steps of epitaxially growing the first and second semiconductor layers are accomplished by the method of liquid phase epitaxy.

3. The method of claim 2 wherein said step of removing said substrate to expose said P type first layer is accomplished by etching in an acid.

4. The method of claim 1 wherein the steps of epitaxially growing the first and second semiconductor layers are accomplished by the method of vapor phase epitaxy.