US 3901745 A
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United States Patent [191 Pion [4 1 Aug. 26, 1975 GALLIUM ARSENIDE PHOTOCATHODE  Inventor: Martin Pion, Harlow, England  Assignee: International Standard Electric Corporation, New York, NY.
 Filed: Jan. 30, 1974  Appl. No.; 438,139
 Foreign Application Priority Data Feb. 6, 1973 United Kingdom 5782  US. Cl. 148/171; 148/172; 148/173; 357/30; 313/95; 250/213  Int. C1. H01L 7/38  Field of Search 148/171 173; 357/30; 313/95; 250/213  References Cited UNITED STATES PATENTS 3,478,213 11/1969 Simon et al 357/30 3,575,628 4/1971 Word .1 313/95 3,592,705 7/1974 Kawashima et a1... 148/187 3,631,303 12/1971 Antypas et al 148/171 X 3,823,043 7/1974 Andre et a1 148/171 Primary Examiner-G. Ozaki Attorney, Agent, or Firm John T. OHalloran, Menotti J. Lombardi,lr., Edward Goldberg  ABSTRACT 4 Claims, 1 Drawing Figure GALLIUM ARSENIDE PHOTOCATHODE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to gallium arsenide photocathodes of the transmission type in which light may be directed upon one side of the device in order to produce photon-excited electron emission from the opposite side.
2. Description of the Prior Art The upper limit to the thickness of a GaAs wafer that can be used as a transmission type photocathode is set by the short diffusion length of photon excited electrons in GaAs (typically pm or less). The mechanical properties of GaAs make it difficult if not impossible to produce satisfactory self-supporting wafers of the requisite thickness. One method of overcoming this problem involves growing the wafer on a supporting substrate of GaAlAs which is itself grown on a seed of GaAs. At some stage of manufacture the GaAs seed material is removed by lapping, for if this were not done its optical absorption would prevent the completed structure from being used in the transmission mode. The GaAlAs layer will have to be quite thick, typically 150 to 300 pm, in order to provide a sturdy support for the GaAs layer. This requires the maintenance of comparatively tight manufacturing tolerances during the growth stage in order to produce a homogeneous uniform thickness layer of such depth. The present invention is concerned with the provision of a rim structure which will provide a measure of mechanical support for the photocathode at its periphery.
A prior art semiconductor device with a supporting rim structure is shown in US. Pat. No. 3,478,213, issued Nov. 1 l, 1969. This, however, is used in an electron multiplier of different materials.
SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide an improved semiconductor photocathode structure and method.
According to the invention there is provided a method of making a GaAs transmission type photocathode including the steps of growing an epitaxial layer of GaAlAs upon a GaAs substrate, of growing an epitaxial layer of GaAs upon the layer of GaAlAs, and of removing, without penetrating through either of the epitaxial layers, the central region of the GaAs substrate to form an apertured n'm supporting the periphery of the layers. This permits use of a thinner GaAlAs layer, typically in the range of 40 to 100 pm.
There follows a description of the manufacture of a transmission type photocathode embodying the invention in a preferred form.
BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE schematically shows a crosssection through the photocathode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, a layer 1 of p-type Ga Ah A5, typically 40 to 100 pm thick, is grown by liquid epitaxy upon a GaAs substrate 2 which is at least 150 1.111 thick. (The preferred orientation of the GaAs substrate is the I00 crystal orientation, an altemative preferred orientation is the l l 1 crystal orientation). Next the GaAlAs layer 1 is covered with a thin active outer layer 3 of p type GaAs typically 5 to 10 pm thick. The coated substrate is prepared for etching by attaching it to a support 4 by bedding it face downwards in wax 5. Further wax is painted round the rim 6 of the back of the substrate so that when the support is immersed in an etching solution only the central region 7 of the back of the substrate is exposed to the etch. The width of the masked peripheral region 6 is typically 1 to 2 The central region of the GaAs 7 substrate is next removed chemically by a bubble etching technique which permits uniform removal over a large area. The coated substrate, still secured to its support 4, is placed with its epitaxially grown layers face upwards in a suitable etch contained in a vessel with a porous base through which is pumped nitrogen gas. The exposure of the GaAlAs layer can be seen by inspection of the substrate as it is slightly different in color, and hence it is not necessary to use a GaAs only selective etch. A suitable non-selective etch is a mixture of sulphuric acid and hydrogen peroxide. The lattice constants of GaAs and GaAlAs are closely matched to provide an improved layered structure of higher sensitivity and reduced stress.
Conventional bonding techniques are used for making electrical contact with these devices, such contact being made either directly to the grown layer 3 or to the rim of the supporting substrate 2. The exposed surface of the GaAs layer 3 may be subjected to conven tional caesiating treatment used to improve efficiency by lowering the work function.
It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.
What is claimed is:
l. A method of making a GaAs transmission type photocathode comprising the steps of growing an epitaxial layer of GaAlAs upon a GaAs substrate, growing an epitaxial layer of GaAs upon the layer of GaAlAs, coating the entire outer surface of the GaAs epitaxial layer with a layer of wax extending around the sides of the GaAs epitaxial and GaAlAs epitaxial layers and around the peripheral rim portion on the opposite surface of the GaAs substrate so that only the central region of the substrate is exposed, and removing the exposed central region of the GaAs substrate without penetrating through either of the epitaxial layers to form an apertured rim of GaAs supporting the periphery of the GaAlAs and GaAs layers.
2. The method of claim 1 including the step of attaching the wax-coated outer surface to a support.
3. The method as claimed in claim 2 wherein the central region of the GaAs substrate is removed by etching to expose the GaAlAs layer.
4. The method as claimed in claim 3 and including removing the layer of wax and caesiating at least a portion of the outer surface of the GaAs layer.
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