U.S. patent number 3,914,136 [Application Number 05/309,756] was granted by the patent office on 1975-10-21 for method of making a transmission photocathode device.
This patent grant is currently assigned to RCA Corporation. Invention is credited to Henry Kressel.
United States Patent |
3,914,136 |
Kressel |
October 21, 1975 |
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.
Inventors: |
Kressel; Henry (Elizabeth,
NJ) |
Assignee: |
RCA Corporation (New York,
NY)
|
Family
ID: |
23199555 |
Appl.
No.: |
05/309,756 |
Filed: |
November 27, 1972 |
Current U.S.
Class: |
438/20;
148/DIG.65; 148/DIG.107; 252/62.3GA; 313/105R; 427/78; 148/DIG.67;
148/DIG.72; 148/DIG.120; 257/466; 313/542; 427/77; 438/500 |
Current CPC
Class: |
H01J
1/34 (20130101); H01J 9/12 (20130101); H01J
2201/3423 (20130101); Y10S 148/065 (20130101); Y10S
148/107 (20130101); Y10S 148/067 (20130101); Y10S
148/12 (20130101); Y10S 148/072 (20130101) |
Current International
Class: |
H01J
1/02 (20060101); H01J 1/34 (20060101); H01J
9/12 (20060101); H01L 007/36 (); H01L 007/38 () |
Field of
Search: |
;313/95,105,94
;148/171-173,175 ;427/77-87 ;357/30 ;252/62.3GA |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Ozaki; G.
Attorney, Agent or Firm: Bruestle; G. H. Cohen; D. S.
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.
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.
* * * * *