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BACKGROUND OF THE INVENTION
This invention relates to semiconductor photoelectron emission devices.
It is possible to obtain photoelectron emission by cleaning the surface of
a semiconductor and activating with cesium or cesium and oxygen. However,
since the pull-out or emission probability of the electrons cannot be made
sufficiently high with semiconductors whose forbidden band widths are less
than about 1 eV, it has been proposed to form heterojunctions of small
forbidden band width semiconductors are large forbidden band width
semiconductors, and to excite the electrons from the former and emit them
from the surface of the latter into the environment, such as vacuum.
However, there may arise a high rate of loss by recombination of the
excited electrons in the course of reaching the junction interface and the
outer surface, so that this was difficult to carry out in actual practice.
Lattice matching between the different semiconductors has also been
previously considered in order that the loss at the junction could be
alleviated. For example, the lattice constants of germanium and zinc
selenide are in good matching. However, since the two semiconductors will
not make a solid solution in a wide range of concentrations, grain
boundaries appear at the junction and form a large obstacle to injections
of minority carriers. Further, since the zinc selenide is a direct
transition type semiconductor, injected electrons are lost by
recombination in the course of passing through this region. Consequently
the region has to be made extremely thin, but this is quite difficult to
obtain technically.
SUMMARY OF THE INVENTION
The present invention removes the foregoing and other defects and
disadvantages of the prior art and encompasses a device capable of
emitting photoelectrons with high efficiency.
The photoelectron device comprises a heterojunction formed with mixed
crystals of two or more semiconductors including a first region of a
direct transition type semiconductor of small forbidden band width and a
second region of an indirect transition type semiconductor with a
comparatively wider forbidden band width. Means excite the photoelectrons
in the former and emit them from the surface of the latter into the
exterior, such as a vacuum.
It is required that there be a homogeneous solid solution. In order to
inject the excited electrons from the direct transition type semiconductor
of the first region to the indirect transition type of the second region,
an electric field may be applied between them. Also, the emission
efficiency as markedly raised by activating the surface of the second
region with cesium or cesium and oxygen. It is further desirable that the
semiconductors comprising the heterojunction have the same type of crystal
structure and that their crystal orientations be identical or
substantially similar and that the differences in their lattice constants
be as small as possible.
Since the present invention can, in this manner, form a heterojunction by
using semiconductors that will mutually go into solid solution in any
desired proportions and by matching their lattice constants, defects at
the junction may be substantially reduced to be very few and the electron
injection loss be reduced to be very small. Also, since the photoelectrons
are excited in the direct transition type first region, the transition
probability is high, and it is possible to raise the number of
photoelectrons generated per unit of incident light. In addition, by
making at least the greater portion of the region in which the electrons
are injected of an indirect transition type semiconductor, it is possible
to have only an extremely small amount of electrons lost by recombination
during their passage to the emission surface. That is, since the electron
transport factor is extremely high and the forbidden band of the region
having the emission surface is widened, it is possible to attain high
electron emission efficiency by activation treatment.
A feature of the invention is the use in a photoelectron emission device of
mixed crystals of two or more semiconductors to form a heterojunction. In
such a device another feature is the crystals defining a first region of
direct transition type semiconductor and a second region of an indirect
transition type semiconductor having a forbidden band wider than that of
the first region.
Advantageously, the mixed crystals being mutually soluble in solid solution
enable substantial matching of crystal structures and lattice constants.
Electron injection loss is substantially reduced. The use of direct
transition type first region and indirect transition type second region
with wider forbidden band gap enables high probability of electron
transition and increase of photoelectrons generated per unit of incident
light, and furthermore, only a small amount of electrons are lost by
recombination.
A further feature of the invention is the use of mixed crystals selected
from the group consisting of GaSb, AlSb, InSb, InAs, AlAs; and impurities
of Zn, Cd, Te, Si, Ge, and Sn; and any combination thereof; and use of
atoms of Groups III and V to control the lattice parameters.
Another feature of the invention is the use of an intermediate layer of
intrinsic semiconductor or n-type semiconductor between the first region
and the second region.
Advantageously, the second region can have a thickness equal to or less
than the diffusion length of the electrons.
A further feature of the invention is the physical arrangement of the
different materials alone or in combination with other types of materials,
such as an insulating or high resistance material.
The foregoing and other features, objects and advantages of the invention
will become clearer with the reading of the below drawing and detailed
description, both of which are to be construed to be illustrative of the
invention and not in any limiting sense.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 depicts an illustrative embodiment of the invention with two
regions;
FIG. 2 depicts another illustrative embodiment of the invention with three
regions;
FIG. 3 depicts a still further illustrative embodiment of the invention
with three regions, similar to FIG. 2, except for the use of a different
material in the intermediate layer;
FIG. 4 depicts the relation between the forbidden band gap and composition
of a specific example of the invention;
FIG. 5 depicts a vessel in which surface activation is carried out;
FIGS. 6A, 6B, 6C, and 6D depict an illustrative embodiment of one
arrangement of layers to form the invention device;
FIGS. 7A, 7B, 7C and 7D depict another illustrative embodiment of another
arrangement of layers to form the invention device; and
FIGS. 8A, 8B, 8C and 8D depict a further embodiment of a further
arrangement of the layers of the invention device.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 depicts an arrangement of an illustrative embodiment of the
invention, wherein heterojunction 12 is formed in crystal 20 by first
region 1 comprising p-type conductivity direct transition type
semiconductor whose effective forbidden band gap is comparatively narrow
and by second region 2 comprising a p-type conductivity indirect
transition type semiconductor whose forbidden band gap is wider than that
of the first region. This crystal 20 may be enclosed in high vacuum vessel
7. After surface 4 of the second region 2 is cleaned it is given zero or
negative electron affinity by activating with cesium or cesium and oxygen.
Anode 5 may be installed in the vessel 7 facing this surface 4. In the
first and second regions, ohmic contacts or electrodes 51 and 52 may be
furnished, which apply a suitable bias voltage between the regions by
means of power source 61, together with application of suitable positive
voltage to anode 5 by means of power source 63. When light rays 8 or 9 of
photon energies greater than the forbidden band gap of this portion are
made to be incident on first region 1, the photoelectrons are excited
toward the conduction band.
These electrons are passed through heterojunction 12 by action of the
electric field generated by power source 61 and are injected into the
conduction band of second region 2 and emitted from surface 4 into vacuum
and collected by anode 5. Consequently, there is a flow of photoelectron
current ip. In this case, the electric field has the effect of raising the
response speed and increasing the transport factor of electrons excited by
the light rays 9. In order to obtain such a drift electric field, a slope
may also be given to the impurity concentration, or it is also possible to
give the slope to the composition of the mixed crystal. Also, in second
region 2, there must be strong prevention of loss of the injected
electrons by recombination. Because of this the present invention is one
that uses indirect transition type semiconductors. It is further desirable
that the thickness of second region 2 be equivalent to or less than the
diffusion length of the electrons. It is also advantageous to form a drift
electric field by such means as providing a slope in the impurity
concentration or a slope in the composition of the crystal.
The device of FIG. 1 consumes electric power with diode current i.sub.d
flowing to heterojunction 12. FIG. 2 depicts an embodiment wherein this
power is decreased and the injection rate is increased. The embodiment
comprises an intermediate transition type second region 22 whose forbidden
band is over 1 eV and which has p-type impurities doped thereto and a
first region 1 as described above. Between these two regions is interposed
region 21 of an intrinsic semiconductor whose forbidden band is wider than
that of the second region. The region 21 may be a semiconductor having a
low impurity concentration close thereto. Consequently, a barrier is
formed to the holes injected from second region 22 to region 21. Because
of this, the injection of the holes is blocked and the diode current
i.sub.d is decreased, and the injection rate of the electrons is
increased. IN addition, ohmic contact electrode 53 is furnished in region
21 and bias power 61' and 62 are interposed between electrodes 51 and 52.
However, it is also possible to eliminate this electrode 53. Also, when
region 21 is made of an indirect transition type semiconductor, the
recombination loss during passage of the injected electrons through this
region is alleviated. Consequently, while it is possible to increase its
thickness and its manufacture is made more easily, the acceleration of the
electrons is made still more effective when a slope is imparted to the
mixed crystal composition or a slope is made in the impurities
concentration, in at least one of the several regions.
FIG. 3 depicts an embodiment similar to FIG. 2 except region 21 is an
n-type semiconductor region 21'. In this case, there is extended a
depletion layer by applying a reverse bias with power source 61' on the
heterojunction between the first region 1 and region 21'. Consequently,
the photoelectrons that are particularly excited in the depletion layer of
first region 1 are accelerated by the high electric field of the depletion
layer and injected into region 21' with good efficiency, and are further
transported to region 22 by electric field generated by power source 62.
Various semiconductor materials may be used in the present invention.
Mixtures of gallium antimonide (GaSb); and aluminum antimonide (AlSb) are
preferred. Both of these crystal structures are zinc blend structures,
being considerably alike with the former having a lattice constant of
6.0954 Angstroms and the latter of 6.1355 Angstroms. They will also go
into solid solution in any desired proportion of composition, and the
former is a direct transition type and the latter is an indirect
transition type.
FIG. 4 is a chart showing the relation between the forbidden band width at
300.degree. K and composition x of a crystal mixture of GaSb and AlSb,
namely, Al(x)Ga(1-x)Sb, wherein x is a positive number less than 1, and
wherein the indirect transition forbidden band width Egi(x) in the former
is about 1 eV, while that of the latter is 1.62 eV. Also, the direct
transition forbidden band width Egd(x) is 0.7 eV for GaSb and 2.218 eV for
AlSb, and the transition type of the mixed crystal is determined by the
smaller value of the curves Egi(x) and Egd(x). That is, when the
composition at the intersection of the curves at c is taken as x.sub.c,
composition x is a direct transition type in the range smaller than this,
and is an indirect transition type in the range larger than this, so that
this relation will give the transition type and the forbidden band width.
Consequently, the first region 1 is taken as having composition x lower
than x.sub.c, and the second region 2 as having higher than x.sub.c, and
the forbidden band gap of the former is selected at between 0.7 to 1.25 eV
and that of the latter at 1.25 to 1.62 eV.
There is also a minute difference in the lattice constants of GaSb and AlSb
as before stated. However, it is possible to get much better matching of
these crystals by substituting portions of the lattice points or sites by
using other atoms of Groups III or V for those occupied by atoms of Groups
III or V, or by using substitute other atoms. For example, when a portion
of the Ga lattice sites in GaSb is substituted by indium (In) having a
larger covalent ion radius, the lattice constant is increased to be close
to that of AlSb. Also, when a portion of the Sb is substituted with
bismuth (Bi), the same effect is obtained. When a portion of the Sb
lattice sites in AlSb are substituted with such as arsenic (As) and
phosphorus (P) which have small covalent ion radii, the lattice constant
decreases and approaches that of GaSb. It is further possible to
substitute with impurities such as Zn, Cd, Te, Si, Ge and Sn that
determine the conductivity type of the semiconductor and thus control the
matching of the lattice constants while simultaneously controlling the
conductivity type and the resistivity.
It is further possible to vary the effective forbidden band width by the
substitutions described above. Since the width of the forbidden band of
the first region which the photoelectrons excited determines the response
threshold of the long wave length, it is very important that this is made
small. Consequently, when the first region is GaSb, the threshold of the
long wavelength will be about 1.8 microns, but the response wavelength
will be extended by making this zone a mixed crystal of GaSb and InSb or
InAs. On the other hand, when there is a need to increase the forbidden
band width of the second region, part of the Sb can be substituted with As
or P.
Although the foregoing has been an explanation for the case when the device
is mainly constructed of GaSb, AlSb and their mixed crystals, it is also
possible to use, for example, GaAs, AlAs and their mixed crystals, GaAs
being a direct transition type semiconductor whose effective forbidden
band width is 1.43 eV and whose lattice constant is 5.642 Angstroms and
AlAs being an indirect transition type semiconductor whose effective
forbidden band width is 2.13 eV and whose lattice constant is 5.661
Angstroms. These can be made into solid solutions in any proportions and
so comprise types of materials suitable for obtaining the device of this
invention.
In a preferred embodiment of this invention, as shown, for example, in FIG.
1, the inventive device was manufactured with GaSb as first region 1 and
AlSb as the second region 2. First, a p-type GaSb monocrystal was given a
mirror finish by mechanical means, and the damaged layer was removed by
etching. This crystal was washed and dried, then inserted in a vapor phase
growing apparatus, and Al(x)Ga(1-x)Sb was grown to form second region 2.
In this case it was difficult that the GaSb first region was made thin.
However, a transmission type photoemission device is obtained as follows.
First, Al(x)Ga(1-x)Sb, wherein x is greater than x.sub.c, secondly GaSb
are successively grown on AlSb monocrystal substrate, for example, using a
slide method of liquid phase epitaxial growth. And then the AlSb substrate
is easily removed from the other portions because of its high etching
speed. In this device GaSb is the first region and Al(x)Ga(1-x)Sb is the
second region. It is also possible to obtain a reflection type
photoemission device by growing AlSb or Al(x)Ga(1-x)Sb, wherein x is
greater than x.sub.c, on a GaSb substrate.
Crystal 20 obtained in the aforestated manner is formed in the desired
shape and electrodes 51 and 52 may be attached by such means as metal
deposition. The device may be inserted into a vacuum vessel 7, as shown in
FIG. 5.
This vessel 7 is furnished with a branch tube with cesium generating source
10 contained therein, and silver tube 13 which is connected with a gas
exhaust tube via cover seal 14. When this vessel 7 is connected to an oil
free very high vacuum exhaust system and evacuated to a pressure degree of
at least 10.sup..sup.-7 Torr, vessel 7 may be heated to 350.degree. C to
degas. When a pressure degree of about 10.sup..sup.-8 Torr is reached, the
heating is stopped. Then cesium source 10 is heated. The cesium is
liberated inside the branch tube, and is cooled by dry ice or liquid
nitrogen to condense it inside the branch tube.
The electron emission surface of crystal 20 is purified by heating for a
number of minutes at about 500.degree. C in a very high vacuum or by argon
ion bombardment. After this cleaning treatment is performed, the electron
emission surface is irradiated with white light, and either a number of
tens of volts is applied between electrode 52 and cathode 5 while
observing the photoelectric current; or else voltage is applied between
electrodes 51 and 52 without any irradiation of light rays while observing
the cold electron emission.
In this state the cooling of the branch tube is stopped, the cesium is
gradually fed into vessel 7, and when the maximum photoelectric current or
cold electron flow has been achieved, the cesium feeding is stopped, and
oxygen in air is introduced into vessel 7 by heating tube 13. This
introduction of oxygen is done with care so that the partial pressure does
not exceed 10.sup..sup.-7 Torr. Thus, the current after temporary rising,
will decrease. When the current has declined to about one tenth, it is
increased by again introducing cesium. When these operations are repeated
and a maximum current is observed, the surface 4 is achieved. The branch
tube is sealed off, vessel 7 is sealed off from the vacuum system and the
device is completed. It is possible to use a cesium ion gun in introducing
the cesium and, in this case, it is also possible to perform
quantification of the inlet amount.
Since it is particularly important that there be little dark current in a
photoelectron emission device, it is necessary to prevent thermal
excitation of the electrons. In order to do this, it is necessary that the
Fermi level in the semiconductor of the first region be as close as
possible to the valence band. Consequently, it is useful that the
concentration of the p-type impurities be as high as possible considering
the diffusion length, although if it is over 10.sup.17 atom/cm.sup.3, it
will be sufficient. However, it is possible, such as in the embodiment of
FIG. 3, to have a low concentration of impurities in the portion adjoining
the depletion layer of the first region. That is, the impurity
concentration is selected so that a suitable thickness of a depletion
layer extends toward the first region. For example, the layer can be
intrinsic one.
It is necessary to consider the structure of the device further in order to
lower the dark current and raise the photoelectric sensitivity. FIG. 6A-D,
is an embodiment to effect such results. As shown in FIG. 6A, an n-type
GaSb layer 31 of about 10 .mu.m in thickness is epitaxially grown on
p-type GaSb substrate having impurity concentration of about 10.sup.17 to
10.sup.19 atom/cm.sup.3. Substrate 1 forms the first region, its thickness
is for example about 100 .mu.m, and the surface has a suitable orientation
such as (111), (100), or (110), and the impurity concentration of n-type
layer 31 is about 10.sup.16 to 10.sup.17 atom/cm.sup.3.
Next, as shown in FIG. 6B, a suitable mask 36, such as a synthetic resin
film or photoresist is formed on growth layer 31 and a portion of growth
layer 31 is removed by etching. Mask 36 is removed, and as shown in FIG.
6C, Al(1-x)Ga(x)Sb layer 21 with a wide forbidden band gap, and a low
impurity concentration to form a barrier against the holes, and p-type
Al(1-x)Ga(x)Sb layer 22 with a narrower forbidden band gap are grown. In
these formulae, the x is a positive number less than one. Layer 21 should
suitably be about 500 Angstroms to 10 .mu.m, of an order that the holes
will not tunnel through from region 22 to region 21. It is also necessary
that the thickness of layer 22 be less than the diffusion length of the
injected electrons.
Finally, as shown in FIG. 6D, ohmic contact electrodes 51 and 52 are
attached and surface layer 4 is formed by cesium or cesium and oxygen in a
very high vacuum vessel. That is, region 31 is n-type and region 1 is
p-type, so that a depletion layer is made at their boundary, and this has
the action of an insulating film and restricts the range of the electron
emission. Consequently, the emission of electrons thermally excited in the
unnecessary part of the region 1 can be prevented. Furthermore, the region
contributes to decrease bias current. Also, the photoelectrons which have
been attained or reached the ohmic contacts 52 are lost by recombination
and are not emitted. But, since in the device of FIG. 6A-D, the contact of
electrode 52 is separated from the electron injection zone of the second
region by more than the electron diffusion length, the loss of this can be
disregarded.
FIG. 7A-D is an example of a transmission type device, wherein as shown in
FIG. 7A, successive epitaxial growth layers are made on p-type GaSb base
35 of high concentration p-type Al(x)Ga(1-x)Sb, wherein x is a positive
number smaller than 1, layer 34 and n-type GaSb layer 33, low impurity
concentration and wide forbidden band gap Al(x)Ga(1-x)Sb layer 21 and
narrower forbidden band gap and high impurity concentration p-type
Al(x)Ga(1-x)Sb layer 22. After that, base 35 is lapped off, for example,
mechanically, up to the position shown in the Figure by the broken line
71. Still another portion is pared by such as sand blasting as shown by
the broken line 72 in FIG. 7B, forming a hole reaching to layer 34.
FIG. 7C shows a state where a prescribed portion of layer 34 has been
selectively removed by utilizing the difference in etching speeds between
GaSb and the Al(x)Ga(1-x)Sb layer, and then the first region is formed in
the portion shown by slanted lines in FIG. 7D close to GaSb layer 33 by
diffusing a p-type impurity such as zinc using a mask such as silicon
oxide (SiO.sub.2) or aluminum oxide (Al.sub.2 O.sub.3). Then, electrodes
51 and 52 are provided, and evacuating and activation treatment are
performed to complete the device.
This device can respond to both light rays 8 and 9, and is made so that the
impurity concentration of region 1 is highest on the reverse surface side.
Consequently, there is formed a drift electric field such that the
photoelectrons formed by region 1 are accelerated in the direction of
emission surface 4. There are particularly many electrons that are excited
at the reverse surface side of region 1, and recombination in this part is
a problem. Since these excited electrons move directly toward the junction
interface because of the drift electric field, there is little loss. Such
a drift electric field can also be formed by providing a slope in the
effective forbidden band. Also, since region 33 is an n-type, a depletion
layer is made between it and region 1 and there is the same action as in
region 31 of the embodiment of FIG. 6A-D, but there is an even greater
effect given by applying a reverse bias as required between regions 33 and
1. Since the forbidden band gap of region 34 is wider than that of region
1, the electrons excited in region 1 can be prevented from diffusing into
region 34. The GaSb layer of region 35 is useful in lowering the ohmic
contact resistance. Regions 33 in FIG. 3 and 31 in FIG. 6 form high
resistance, and these parts can be insulation layers of such materials as
SiO.sub.2 or Al.sub.2 O.sub.3.
FIG. 8A-D depict the construction of a transmission type device using
transparent support base 32, where there may be used such materials as
sapphire, corundum, quartz, transparent alumina, and wide forbidden band
gap semiconductor crystals such as ZnSe, SnS, Sec, ZnTe, GaP and AlP. The
compound ZnTe has the same crystal structure as GaSb, and their lattice
constants are close to each other. Then GaSb-ZnTe system is a preferred
compound to use.
As shown in FIG. 8A, there are grown on base 32, which may be of ZnTe
suitable thicknesses of p-type GaSb layer 1 and high resistance and wide
forbidden band Al(x)Ga(1-x)Sb layer 21. Then the portion of region 21 in
FIG. 8A is etched as in FIG. 8B using a mask, and then as shown in FIG.
8C, there is furnished insulation film or layer 30 of SiO.sub.2 or
Al.sub.2 O.sub.3. After this, region 22 of a p-type Al(x)Ga(1-x)Sb layer
with a narrower forbidden band gap than region 21 and a high impurity
concentration is grown, electrodes 51 and 52 are provided as shown in FIG.
8D, and then the active surface 4 is formed. In this case, since the
forbidden band gap of ZnTe is 2.26 eV, base 32 acts as a window for the
lower energy photon than this gap.
In transmission type photoelectron emission devices, since the incident
light rays at the surface of the first region have maximum strength,
recombination is a problem at this part. Since the recombination velocity
at the surface of the semiconductor is greater than other parts, an
effective means of preventing it is to apply a suitable surface treatment.
It is also effective to widen the forbidden band gap of the first region
near the illuminated surface to decrease the recombination at the surface,
and it is also possible to furnish an antireflecting film to enhance the
sensitivity.
The foregoing description is for purposes of illustrating the principles of
the invention. Numerous variations and modifications thereof would be
apparent to the worker skilled in the art. All such variations and
modifications are to be considered to be within the spirit and scope of
the invention.
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