United States Patent (19) 11) 4,096,511 Gurnell et al. (45) Jun. 20, 1978

(54) PHOTOCATHODES 3,699,401 10/1972 Tietjfn ...... 357/16 76) Inventors: Philip Gurnell, Letchworth; Michael 3,765,962 10/1973 Poleshuk et al...... 313/65 AB Charles Rowland, Steeple Morden, FOREIGN PATENT DOCUMENTS both of England 1,239,893 7/1971 United Kingdom. (21) Appl. No.: 309,043 Primary Examiner-Richard E. Schafer 22 Filed: Nov. 28, 1972 Assistant Examiner-Deborah L. Kyle Attorney, Agent, or Firm-Nathan Edelberg; Jeremiah (30) Foreign Application Priority Data G. Murray; Sheldon Kanars Nov. 29, 1971 United Kingdom ...... 5.5355/71 57 ABSTRACT 51 Int. Cl...... H01, 27/14 52 U.S. C...... 357/30; 313/94; A transmission photodetector operable at wavelengths 357/4; 357/16; 357/61 greater than 0.86 micrometers comprising a substrate 58) Field of Search ...... 315/10-12; transparent to the radiation to be detected, at least one 148/177, 178, 179; 313/65 R, 65 AB, 65T, 66, epitaxial intermediate layer comprising (GaAl). 346, 94; 357/4, 16, 30 -InAs and an epitaxial p-type Gai-InAs detector layer. The said one intermediate layer may be p-type. If 56) References Cited desired a second epitaxial intermediate layer comprising U.S. PATENT DOCUMENTS (Gai-Al). InAs may be provided between the sub 3,350,591 10/1967 Asselt ...... 313/65T strate and the said one intermediate layer. In the forego 3,437,890 4/1969 Krohl et al. ... 313/65 AB ing 0

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ZZZZZZZZZZZZZ?!aluminium in a qua detector layer by lattice mismatch and consequently ternary gallium aluminium indium arsenide system dem ruin the performance of the device. 10 onstrating the principal of selection of the concentra Accordingly it is a first object of the present inven tion of the various components for the photocathodic or tion to provide a transmission photocathode structure detector and intermediate layers in a photocathode for photocathode detectors operable at wavelength according to the invention having an indium gallium greater than 0.86 micrometers. arsenide photocathodic layer, A second object of the present invention is to provide 15 FIG. 2 shows a section through a photocathode ac a transmission photocathode structure operable at cording to the invention, wavelengths greater than 0.86 micrometers which may FIG. 3 shows a horizontal system for double liquid be provided with an appropriate window. epitaxy suitable for use with the invention, the lower A further object of the invention is to provide a man view showing the boat and slide in longitudinal section ner of manufacture of such transmission photocathode. 20 and primed ready for use, the upper view is of an empty These objects are achieved in a photocathode com boat in isolation, and prising a substrate transparent to the radiation to be FIG. 4 shows a section through a second photocath detected, at least one epitaxial intermediate layer com ode according to the invention prising (Gai-Al)1-InAs and an epitaxial p-type Gai In FIG. 1 the horizontal axis plots the mole fraction -InAs detector layer wherein 0gallium arsenide formance of the photocathode by raising the conduc of the photocathode layer, whilst the highest curve x = tion level in said one intermediate layer so that a poten 1 is for indium aluminium arsenide. Aluminium to ar tial barrier is provided at the interface with the detector 30 senic and gallium to arsenic atomic spacings are equal, layer preventing the return of photoexcited electrons the lattice spacings of the quaternary alloys formed will from the detector layer into the intermediate layer. It be dependent on the indium content and all vertical has been found that provision of sufficient p-type dop lines on the graph show alloys of equal lattice spacing. ant in said one intermediate layer to produce a potential Therefore in order to achieve lattice compatability be barrier of about volt at the interface effectively stops 35 tween the photocathodic or detector layer and the in all electrons entering the intermediate layer and in termediate layer all that is necessary is the introduction creases the electron yield by 50%, yet is not sufficient to of the same atomic percentage of indium into each. cause a large increase in the radiation absorption. Further examination of the graph shows that this system The p-type dopant may be zinc, cadmium, germa allows the preparation of a wide range of substrate nium, or silicon, although this list is not exclusive. windows with varying cut-offs compatible with one In the present invention by variations of the alumin detector response. For example a photocathode to de ium content of said one intermediate layer, the energy tect radiation at 1.06 micrometers, which is equivalent gap thereof may be varied and it is possible to provide to a band gap of 1.16eV, requires a value of y = 0.28 in photocathodes having different windows. the InGaAs system and the isotaxtic vertical line Where it is desired to construct a transmission photo 45 shows that a range of windows with energy gaps from cathode in accordance with the invention, particularly a 1.16eV are realizable using (Gai-Al), InAs as an transmission photocathode operable at substantially intermediate layer. greater than 0.86 micrometers, it may be desirable to Gallium phosphide is the preferred substrate. It will avoid growing a single thick intermediate layer, but to be seen from the graph that for a photocathode to de provide a second intermediate layer between said sub 50 tect, radiation at 1.06 um having an energy gap of strate and said one intermediate layer wherein said sec 1.16eV the indium gallium arsenide detecting layer ond intermediate layer comprises (Ga1-Al)1-InAs would have a lattice spacing of 5.75A as opposed to the wherein 0szindium arsenide which will accommodate the mismatch so released are indicated as 5. between the gallium phosphide substrate and p-type One method of manufacture of such a transmission gallium indium arsenide detector layer and is at the photocathode is illustrated in FIG. 3. The apparatus same time not detrimental to the performance of the comprises a carbon boat 10 wherein a carbon slider 16 device, indeed the transmission photocathodes prepared operates. The length of the slider 16 is somewhat less 15 have been shown to have very much improved charac than the length of the boat 10. Centrally placed in the teristics over other known types of transmission photo base of boat 10 is a circular recess 12. The slider 16 has cathodes. two cylindrical wells 18 and 20, the same diameter as In embodiments of the invention where it is desired to the boat recess 12. Initially the slider 16 is placed at one construct a photocathode in which the detector layer end of the boat 10 such that neither well overlaps the 20 has a widely different lattice constant from the substrate recess 12. and in which difficulties arise in the growth of a suffi A suitable gallium phosphide seed crystal 14 is chemi ciently thick intermediate layer, it may be preferable to cally cleaned and placed in recess 12. include a second intermediate layer between the first A solution 22 is prepared containing gallium, gallium and the substrate in which the second intermediate layer arsenide, aluminium and indium in the proportions re 25 has a lattice constant between that of the first intermedi quired for the selected values of x and y in the formula ate layer and the substrate. This is simply achieved by a tion (Gai-Al)-InAs. For example in the case where reduction of the indium content of this second interme the detector layer is to be Gao Ino.28As the intermedi diate layer. This construction is illustrated in FIG. 4, a ate layer would be (Gai-Al)07.Ino.28As and values of x second epitaxial intermediate layer 8 comprising the can be selected to give a choice of cut off down to a 30 tertiary compound Gai-AlAs has been interposed wavelength of 0.58pum. between a first intermediate layer 2 comprising p-type A second solution 24 is prepared for the growth of (Gai-Al)1-InAs and a GaP substrate 1. Provision the detector layer. For a detector at 1.06 m using a may be made in the apparatus illustrated in FIG. 3 for detector layer of p-type Gao InoAs the composition the manufacture of this construction by providing an would be 35 additional well in the slider. A three stage process is Indium 5.44g; Indium arsenide 7.536g; then carried out in which the foregoing example of a Gallium 1.811g; Zinc 0.1g. p-type Gai-InAs detector layer would firstly require Additionally 0.1 g zinc is included in solution 22 to the epitaxial growth of a Ga AlAs layer on the GaP provide a p-type intermediate layer. substrate, then the epitaxial growth of p-type (Gal. These solutions are places in the wells 18 and 20 in Al)-InAs, before final epitaxial growth of the p-type slider 16, solution 22 in well 18 nearer the seed crystal Gai-InAs detector layer. 14, solution 24 in well 20 further from seed crystal 14. We claim: The system is assembled and loaded into a single zone 1. A transmission photocathode comprising: furnace. a crystalline substrate transparent to the radiation to Initially the system is flushed for 30 minutes with 45 be detected, pure hydrogen. The furnace is then raised to 1000 C, at least one epitaxial intermediate layer comprising then after 10 minutes taken down to 950 C and left for (Gai-Al)1-InAS, and 20 minutes to stabilise. The system is then programmed an epitaxial detector layer comprising p-type Ga. to cool at the rate of 80 C/hour and the slide 16 moved -InAs wherein 0

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