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BACKGROUND OF THE INVENTION
A substantial effort has been expended to extend the detectable wavelength
of the III-V semiconductor photoemitters beyond the wavelength of about
.lambda. = 1.24.mu., i.e., below 1.0 eV photon energy.
In general, there are basic limitations on the wavelength response of
photoemitters, for example, the work function .epsilon.. In so-called
"negative affinity" photoemitters, it has been shown that the
"interfacial" barrier between the semiconductor and the activator is also
an important limitation. In order to extend the detectable wavelength,
such limitations must be overcome.
The most effective semiconductor photoemitters are P-type structures where
the Fermi level more or less coincides with the valence band of the
electrons in the bulk semiconductor. In the so-called multialkali
photocathodes, the energy difference between the Fermi level at the
semiconductor surface and the vacuum level, known as the work function
.epsilon., is greater than the energy difference between the valence band
and the conduction band of the electrons in the semiconductor, and the
energy h.nu. of photons impinging on the semiconductor must be great
enough to promote the electrons from the valence band to the higher energy
conduction band and from the conduction band over the vacuum level at the
surface of the semiconductor, i.e., h.nu. > .epsilon..
The negative affinity III-V form of semiconductor photoemitter lowers the
effective .epsilon. by the well known band-bending at the electron
emitting surface of the semiconductor. The bending lowers the edges of the
valence band and the conduction band at the semiconductor surface relative
to the band edges in the bulk semiconductor, and effectively lowers the
vacuum level relative to the conduction level, decreasing the height of
the barrier to the electrons seeking to escape over the vacuum level.
Since the difference between the conduction band and the vacuum level is
known as electron affinity, semiconductor devices employing band-bending
wherein the vacuum level has effectively been made lower than the
conduction band are referred to as negative-affinity devices.
In these negative affinity devices, it is also necessary that the photon
energy h.nu. be greater than the band gap, i.e., the energy E.sub.G
between the valence band and the conduction band, or
h.nu..gtoreq. E.sub.G
in order to obtain absorption of the photon and creation of the electron
hole pairs. If the conduction level, while higher than the vacuum level so
that no problem exists in having the electrons escape from the conduction
level to the vacuum, is so high as to create a large bandgap, absorption
of the photons and promotion of the electrons from the valence band to the
conduction band does not take place and few electrons are emitted.
By proper selection of the III-V material, the bandgap E.sub.G can be
lowered such that a profusion of electrons are promoted to the conduction
band, but, as a result of the E.sub.G lowering, the conduction band falls
below the vacuum level and the work function .epsilon. may prevent the
electrons from escaping from the conduction band into the vacuum.
As a consequence of those conditions, a significant problem with electron
emission from photoemitters such as III-V semiconductors is the work
function .epsilon.. A considerable amount of work has been directed to
lowering the work function of semiconductors; it has been shown that
suitable surface activation with Cs.sub.2 O can substantially reduce the
work function, to as low as 0.6 eV, i.e., corresponding approximately to
photons of 2.mu. wavelength.
It would seem then that by selecting a III-V semiconductor with band
bending and with a low E.sub.G, and by activating the surface with
Cs.sub.2 O to reduce the work function, a very good long wavelength
photoemitter should result. However, although such a device provides
negative-affinity as desired, the junction between the III-V semiconductor
and the other semiconductor material Cs.sub.2 O forms a large
heterojunction barrier, and this interfacial barrier is higher than the
vacuum level and the conduction band. This interfacial barrier height
E.sub.B is typically about 1.15 eV and it prevents the desired long
wavelength responsive photoemission. For a discussion of the interfacial
barrier on III-V compounds see "Behavior of Cesium Oxide as a Low Work
Function Coating" by J. Uebbing and L. James, Journal of Applied Physics,
Vol. 41, No. 11, October 1970, pages 4505 to 4516, inclusive. Although
some disagreement exists relative to the exact nature of this interfacial
barrier as seen by reference to the articles "Long-Wavelength
Photoemission From Ga.sub.1-x In.sub.x As Alloys" by D. G. Fisher et al,
Applied Physics Letters, Vol. 18, No. 9, May 1, 1971, pages 371-373,
"Interfacial Barrier Effects in III-V Photoemitters" by R. Bell et al,
Applied Physics Letters, Vol. 19, No. 12, Dec. 15, 1971, pages 513-515,
and "Photo-electron Surface Escape Probability of (Ga, In) As : Cs-O in
the 0.9 to .about. 1.6.mu. m Range," Journal of Applied Physics, Vol. 43,
No. 9, September 1972, pages 3815-3823, this barrier is clearly seen to
prevent the escape of electrons from the conduction band to the vacuum,
and to prevent attainment of efficient long wavelength infrared
photoemission from III-V compounds.
The devices previously proposed for increased photoemission at long
wavelengths involve multiple layer growths, uniform large-area operation
of heterojunctions, biased layers, etc. leading to problems in surface and
bulk nonuniformities in the grown layers, particularly where more than one
heteroepitaxial layer is to be grown. In the case of approaches relying on
tunneling through thin insulator layers, non-uniformity, already a serious
problem with other than the simplest unbiased III-V negative affinity
system, is a dominant and disabling phenomenon.
SUMMARY OF THE PRESENT INVENTION
The present invention provides a long wavelength photoemitter, for example,
a negative affinity III-V type, having a work function reducing activation
layer and produced by straightforward growth, fabrication, and activation
procedures, the photocathode employing novel techniques for overcoming the
energy barrier between the semiconductor conduction band edge and the
vacuum, resulting in high, uniform sensitivity over large areas as desired
for infrared photocathodes.
In the photocathodes of the present invention, the electrons in the
semiconductor are first promoted by photo-excitation from the valence band
into the conduction band, and the electrons are thereafter promoted by
thermal energy means into higher levels in the conduction band from where
they may pass over the interfacial barrier and into the vacuum. In a
preferred embodiment of the invention, a simple biased Schottky barrier is
employed between the semiconductor and the activation layer, and the
photoexcited electrons are promoted into higher levels of the conduction
band by application of a moderate electric field, this effect being the
intervalley electron transfer effect responsible inter alia for the
microwave Gunn effect.
In another embodiment of the invention, the semiconductor compound is
chosen such that the upper conduction band or valley of the semiconductor
has a relatively high density of electron energy states, as for example,
the zone-edge L or X band edges, compared to the gamma minimum, such that
an appreciable number of the photoexcited electron population, when
thermalized at 300.degree. K (room temperature), will be excited to the
upper state, which may be as high as 0.1 to 0.2 eV above the gamma
minimum. Electron emission into vacuum directly from the upper conduction
band state over the surface energy barrier (work function or interfacial
barrier) takes place with a significantly higher transmission efficiency
(escape probability) than possible from the gamma minimum.
In other embodiments to be described below, photo-excitation may occur in
one region of a composite semiconductor body and promotion to a higher
conduction band in another region. The different materials of these
regions may be designed to optimize their separate functions, the common
requirements being that it should be possible to grow one on the other by
some convenient method, for example, epitaxial growth, and that it should
be possible to transfer electrons from the conduction band of the photon
absorbing layer to the conduction band of the electron promotion layer.
These requirements can be satisfied in the systems discussed below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a model for a photoemitter cathode which incorporates one
embodiment of the present invention.
FIG. 2 is a plot showing low energy and high energy valleys in the
conduction band of a photoemitter cathode of FIG. 1.
FIG. 3 is an illustration of the band edge energy in a particular III-V
compound (GaAlAsSb) utilized in one embodiment of this invention.
FIG. 4 is a model of a second form of photoemission cathode incorporating a
second embodiment of the present invention and utilizing a Schottky
barrier therein.
FIG. 5 is a plot of the band edge energies of a quaternary form of III-V
cathode.
FIG. 6 is a plot of the satellite valley population versus electric field
in a semiconductor of a type similar to that used in FIG. 4.
FIG. 7 is a band edge plot of a ternary system utilized in the cathode of
FIG. 4.
FIG. 8 is a band edge plot of an InP/InGaAsP/InP cathode.
FIG. 9 is a band edge plot of a Ge/GaAs cathode.
FIG. 10 is a band edge plot of a Si/GaP cathode.
FIG. 11 is a band edge plot of a low dark current GaAs/InGaAs/InGaP
cathode.
FIG. 12 is a series of band edge plots showing a technique for handling
discontinuities in the conduction band edges at the heterojunction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1 there is shown a model of the well known III-V
semiconductor negative affinity photocathode with the interfacial barrier
11 between the III-V surface and the activation layer Cs.sub.2 O. By
choosing a III-V compound with a relatively low bandgap E.sub.G, for
example, 0.8 eV, a profuse number of electrons may be excited by the
incoming photon energy h.nu. from the valence band (VB) to the conduction
band (CB). By band bending, the vacuum level (VL) has been reduced below
the conduction band edge (CB) in the bulk semiconductor for negative
affinity, and the work function .epsilon. between the Fermi level (FL) and
the vacuum level (VL) has been reduced by the activation layer Cs.sub.2 O.
Therefore, a low bandgap, low work function, negative affinity device is
provided; however, efficient photoemission is prevented by the height
E.sub.B of the interfacial barrier formed between the two semiconductor
surfaces. The Uebbing and James article cited above shows this interfacial
barrier height E.sub.B to be about 1.15 eV.
A very strong laser line exists at about .lambda. = 1.06.mu., or 1.17 eV,
and therefore a barrier E.sub.B of such height causes the photoemissive
response of the cathode represented by this model to fall off appreciably
at this laser line. Reducing the E.sub.B height even slightly enables the
photocathode to operate well on this laser line.
It is the purpose of the present invention to make use of thermal agitation
of the photoexcited electrons in the conduction band to overcome the
energy barrier, for example, the interfacial barrier, so that the
electrons escape into the vacuum. In one method, the thermal energies of
the photo-excited electrons are used to overcome the energy barrier. With
electrons in a single system and at different energies, for example,
electrons in free space, the Boltzmann distribution establishes the
probability of the electrons being at a particular kinetic energy E of
p (E).about. exp (-E/kT)
and, therefore, the higher the kinetic energy E, the less probability of
having an electron at that higher energy. But, with a finite temperature T
of 300.degree. K, a finite probability does exist.
In the semiconductor body of the cathode, the electrons are not found in a
single system, and the electrons in the conduction band are found in
several different systems with different masses. The energy E versus
momentum is plotted in FIG. 2 for electrons in two of the possible systems
in a III-V compound, with the lowest conduction band in the III-V material
having a very high curvature 12, i.e., a low mass, and therefore a low
density of quantum-mechanical energy states. The other region 13 of the
conduction band has a low curvature, i.e., high mass and therefore a high
density of energy states. As a result, the probability relationship given
above includes a density-of-states function such that
p (E) .about. [g (E)] exp (-E/kT)
Therefore, with one region 13 of the conduction band having a high density
of states while another region 12 of the conduction band has a low density
of states, the ratio of the upper band g(E) to the lower band g(E) can be
relatively large. Electrons promoted into the lower energy, light mass
conduction band region 12 by photoexcitation, will, because of the higher
probability factor, be transported into the high state density band 13
because of the large number of energy states that will accept them. This
will occur by thermal excitation provided T is finite, as for example room
temperature, and provided the energy difference between the lower and
higher energy conduction band regions is small enough, e.g. 3kT.
Certain III-V alloys will give the correct conduction band structure, i.e.,
those in which the zone-edge L and X band edges have very high energy
state densities relative to the gamma minimum, and an appreciable fraction
of the photoexcited population, even when thermalized at room temperature,
will be found in the upper states. These upper states are as high as 0.1
to 0.2 eV above the gamma minimum, and this energy difference is
significant when only a slight energy increase is needed to overcome the
energy barrier as in the laser line illustration mentioned above. Thus
electron emission into vacuum directly from the upper states over the
heterojunction barrier takes place with a significantly higher
transmission efficiency ("escape probability") than possible from the
gamma minimum. Assuming parabolic band edges, at energies E1 and E2, with
multiplicities g1 and g2, effective masses m1 and m2, and an absolute
temperature T, the ratio of upper to lower populations is given by
N2/n1 = (g2/g1) (m2/m1).sup.3/2 exp(E1-E2)/kT
For g2/g1 = 7, m2/m1 = 10, T = 300.degree.K, and E2-E1 = 0.1 eV,
N2/n1 = 4.6, a satisfactorily high value.
Some loss in diffusion length can be expected because of the higher mass of
the upper valley electron population, and the lower mobility. This effect
is small, being proportional to the square root of the effective mass,
whereas the population varies as the 3/2 power of the effective mass.
Moreover, the lifetime against recombination in the upper (indirect)
levels may be significantly higher than for the gamma electrons,
offsetting any decrease in mobility.
The requisite band structure is realized by the quaternary III-V alloy
system GaAlAsSb as illustrated in FIG. 3. It is seen that the band edge
energies E.sub.X and E.sub.L are 1.35 eV and the gamma minimum is 1.1 eV
for the illustrative compound
Ga.sub.0.85 Al.sub.0.15 As.sub.0.45 Sb.sub.0.55
A more preferred embodiment of the invention is shown by the model of FIG.
4 wherein a biased Schottky barrier is formed between the III-V
semiconductor and the Cs.sub.2 O activation layer by a thin (100 A) silver
layer on the semiconductor surface under the activation layer. Such a thin
silver layer with Cs activation has been employed on cold cathode
emitters; for example, see Stolte and Archer, "pn - Schottky Hybride
Cold-Cathode Diode," Applied Physics Letters, Vol. 19, No. 11, Dec. 1,
1971.
As in the unbiased photocathode, the initial action is the absorption of
the photon across a direct bandgap E.sub.G of, for example, 0.8eV (giving
a sharp onset of optical absorption and a high .alpha. under operating
conditions, conducive to efficient operation). As discussed below, it may
be possible to operate with bandgaps as low as 0.46 eV, corresponding to a
long wavelength cut-off of 2.7 microns. The additional energy provided to
the photoexcited electrons in order for them to be emitted into vacuum
relies on hot-carrier effects and intervalley electron transfer, phenomena
which are the basis of the microwave Gunn effect, and are now relatively
well understood. The Gunn effect and other useful microwave effects ensue
when a moderately strong electric field F.sub.E -- a few kV/cm-- is
applied to a semiconductor with a light-mass .GAMMA. conduction band
minimum and heavy mass minima at a higher energy. Electrons in the .GAMMA.
minimum gain energy from the electric field until they are energetic
enough to transfer into the upper valleys. Here the mobilities are low
enough and the scattering mechanisms strong enough that for the fields
applied, the electron population gains no further significant energy, and
ionization across the forbidden gap is prevented. The avalanching range of
fields is moved to much higher values (.about.10.sup.5 V/cm) by the
presence of these higher "satellite" valleys of the conduction band.
Because of the relatively high densities of states in the upper conduction
band minima, a significant fraction of the photoelectrons are transferred
into these upper levels; this high fractional transfer is important for
the quantum efficiency and noise properties of the resulting photocathode.
In utilization of this effect, the equivalent electron effective
temperatures obtained are much higher than the lattice temperature of the
order of several thousand .degree.K. As discussed above, electrons present
in the conduction band can be promoted much higher up in the energy levels
of the conduction band to more easily pass over the energy barrier at the
surface. Alternatively, the gamma minimum for the semiconductor can be
lower and still obtain promotion up to the X or L minimum to pass over the
barrier, while still obtaining absorption at long wavelengths due to
absorption into the gamma minimum.
In Gunn-effect devices, very large currents flow under the action of the
applied electric field, whereas in the present structure the only current
flowing is due to photoexcited electrons, a substantial fraction of which
are emitted into vacuum. Very high photoelectron quantum efficiencies are
therefore available.
The important consequences of a proper choice of band structure for the
cathode are: (a) efficient absorption of the incoming photons near the
surface (direct bandgap), (b) excitation of the photoelectrons to
well-defined upper levels of the conduction band on applying moderate
electric fields, and (c) the action of these upper levels in preventing
further runaway and ionization (avalanching) of the semiconductor, before
a significant fraction of the photoelectron population has been excited to
the higher energy levels.
In order to apply such electric fields, the material under stress must
contain few free carriers. This is achieved by compensating the material
using "deep" trapping centers, or more easily by sweeping out the
carriers, as in the depletion region of a back-biased pn-junction or
Schottky-barrier contact. FIG. 4 illustrates the use of the depletion
layer of a Schottky barrier, formed by the thin Ag overlayer on the p-type
semiconductor. In this configuration, the field increases towards the
surface, giving the most rapid available band-bending rate, favorable to
emission, just at the surface. While this field can even rise into the
higher "avalanching" range just at the surface, without serious detriment
to the noise performance, the very high surface band-bending fields of the
conventional negative affinity photocathode (.about.10.sup.6 V/cm) are not
used, since this gives rise to tunneling of holes from the Schottky
barrier into the semiconductor and very large currents which cannot be
supported by the thin Ag layer.
Avalanching in high-field devices occurs under conditions in which
electrons in the conduction band can reach energies sufficient to create
new electron-hole pairs across the bandgap. The relevant kinetic energy is
clearly at least equal to the bandgap energy at the gamma point E.sub.G,
and on a simple theory accounting for momentum conservation is at least as
high as 1.5 E.sub.G. To avoid avalanching at low fields, before transfer
to satellite valleys is effective, the conduction band gamma electron
energies are limited to less than 1.5 E.sub.G by placing the satellite
valleys at or below this energy.
These considerations, coupled with available surface barrier heights,
determine the long wavelength limit of the present device in the following
manner. Assuming location of the Ag Fermi level at one-third the bandgap
from the valence band (Mead's one-third rule), and an Ag/Cs.sub.2 O
barrier height of 1.0 eV, this surface barrier will lie at E.sub.BC = 1.0
- 2E.sub.G /3 above the conduction band edge at the surface. For emission
from satellite valleys over this barrier, the height of the satellite
valleys E.sub.S above the conduction band edge must be at least equal to
E.sub.BC. However, there is already a limit on E.sub.S due to avalanche
prevention: E.sub.S < 1.5 E.sub.G. Therefore,
1.5 E.sub.G > E.sub.S > E.sub.BC = 1.0 - 2E.sub.G /3 (eV)
from which is derived a minimum value of the bandgap
E.sub.G > 0.45 eV.
The corresponding long wavelength cut-off is 2.7 microns. Since this value
is based on a number of assumptions, it is not easily met in actual cases,
and the indicated limit is difficult to approach closely in practice.
A more practical approach is to operate at a bandgap E.sub.G of 0.7 eV,
corresponding to a cut-off of 1.77 microns. From the one-third rule, the
surface Fermi level is pinned at about 0.47 eV below the conduction band
edge. The top of the Ag/Cs.sub.2 O barrier is then 1.0 eV above this, or
0.53 eV above the conduction band edge of the III-V at the surface. The
band structure is therefore such as to generate L or X minima to about
0.53 eV above the bottom of the .GAMMA. minimum, or at a level 0.7 + 0.53
= 1.23 eV on the diagram of FIG. 4.
FIG. 5 illustrates the In.sub.x Ga.sub.1-x As.sub.Y Sb.sub.1-y quaternary
system suitable for use in the FIG. 4 Schottky barrier embodiment with the
ternary system GaAs/InAs represented on the left and the ternary GaSb/InSb
on the right. The binary end-point band structure critical points are
taken from Herman et al, Vol. 8, Methods in Computational Physics,
Academic Press, 1968, pages 193-250. For illustrative purposes, the higher
points are joined by straight lines to represent linear variation across
the ternary composition, although it is well known that the actual curves
are slightly parabolic.
A bandgap at the .GAMMA.-point of 0.7 eV mentioned above is indicated by
the horizontal dashed line, and the assumed barrier height of 1.25 eV by
the horizontal chain-dashed line. The composition A is Ga.sub..75
In.sub..25 As, the composition B is Ga.sub..75 In.sub..25 Sb, and the
composition C is a .64/.36 combination of these, or Ga.sub..75 In.sub..25
Sb.sub..64 As.sub..36. The resulting lattice constant is close to that of
GaSb which is therefore a convenient substrate for epitaxial growth. The
.GAMMA. minimum lies at 0.7 eV, and the L.sub.1 minima at 1.25 eV. This
composition gives field-assisted, hot electron photoemission, when
activated with Cs and oxygen, out to wavelengths of 1.77 microns. Although
the discussion has been in terms of front-surface photoemission, GaSb is
transparent to 0.7 eV radiation at reduced temperature, and the
possibility of transmission operation of this photocathode is clearly
apparent.
FIG. 6 shows the calculated ratio of electrons in the upper valleys of GaAs
as a function of electric field from Butcher and Fawcett, Proc. Phys. Soc.
86, 1965, pages 1205 to 1219. It is clear that over 90% of the population
can be transferred at reasonable fields. Similar transfer occurs in all
semiconducting compounds with similar band structure to GaAs; Gunn
oscillations have been observed in many of these, e.g. CdTe, InGaSb,
GaAsP, InP, etc. InP is particularly relevant since the conduction band
gamma to satellite valley spacing for InP is about 0.6 eV, i.e., greater
than that of the quaternary system discussed here.
The ternary GaAsSb alloy series of FIG. 7 provide another suitable band
structure. The gamma-point bandgaps near the mid-composition range are
suitable for strong absorption of 1.06-micron radiation, and the L.sub.1
and X.sub.1 satellite valleys are at a convenient height for obtaining
emission over the surface barriers. Referring to the model of FIG. 4, for
a barrier of height E.sub.B, the lowest satellite valley would be at a
height E.sub.X,L = E.sub.B + E.sub.G /3. Using E.sub.B = 1.0 eV as for
Cs.sub.2 O on Ag, the intersection of the dashed lines on FIG. 7
(E.sub.X,L - 1.0 eV and E.sub.G /3) gives the composition of the lowest
bandgap GaAsSb alloy that functions in the prescribed fashion-- about 1.0
eV.
We discuss next embodiments in which the functions of photon absorption and
electron promotion are performed in separately-designated regions of a
heterogeneous but continuous semiconductor body. FIG. 8 shows
schematically such an arrangement. The active absorbing layer is a p-type
InGaAsP quaternary layer grown lattice-matched on an InP substrate.
Lattice-matched growth of this system yields a high performance
1.06-micron photocathode with excellent uniformity over large areas. The
InGaAsP quaternary system with the InP lattice constant can generate
bandgaps spanning the region from 1.35 eV (InP) to about 0.7 eV
(In.sub.0.63 Ga.sub.0.37 As). The latter would be suitable for 1.75-micron
operation. The doping of the active layer need not be as high as for an
unbiased photocathode. Zinc doping of the order of 10.sup.16 -10.sup.17
/cm.sup.3 would be adequate, and would generate a correspondingly long
diffusion length for this layer.
A few-micron thick (say 3 microns) lightly-doped emitter layer of InP is
then grown on the quaternary. This layer is automatically lattice matched.
It can be n or p-type, preferably the latter, and should be doped to less
than 10.sup.15 /cm.sup.3. The device is completed by a 100-A thick
metallic layer (e.g. Ag) deposited by vacuum evaporation, and activated by
Cs and oxygen. This cathode would be illuminated by transmission through
the InP substrate, transparent to all radiation beyond 0.9 micron.
In operation, a positive bias on the metallic contact depletes the
lightly-doped InP emitter and establishes a field in it greater than
10.sup.4 V/cm. For such fields, which are low compared with breakdown
fields (10.sup.5 - 10.sup.6 V/cm), conduction electrons in InP are
promoted into upper satellite valleys. These lie about 0.65 eV above the
lowest central conduction band valley. FIG. 8 shows that at the surface
these valleys are 1.2 eV above the Fermi level of the metal contact. The
Ag-Cs.sub.2 O barrier is of the order of 1 eV as determined by J. J.
Uebbing and L. W. James in Journal of Applied Physics Vol. 41, pages
4505-4516, October 1970. Electrons from the upper valleys are therefore
easily emitted into the vacuum. Promotion to the upper valleys in InP is
an efficient process for fields greater than 2 .times. 10.sup.4 V/cm.
An important practical advantage of InP is the characteristically high
barrier height obtained on forming a metallic Schottky barrier to p-type
material (or in other words the hole barrier height when biased in the
direction shown in FIG. 8). This height is typically of the order of 0.75
eV. Such a high barrier prevents the flow of current from the Schottky
barrier contact into the biased layer when the bias voltage is applied,
preventing current drain on the biasing source and preventing also
destruction of the thin Schottky barrier metallization and overlying
(Cs,O) work-function-lowering layer referred to above as the activation
layer
FIG. 9 shows Ge/GaAs system analogous to the InGaAsP/InP system just
described. Germanium possesses an indirect gap of 0.67 eV and a direct gap
of about 0.8 eV at room temperature. The indirect gap rises to 0.7 eV at
200.degree.K. The onset of direct absorption means that a high resolution
photocathode can be fabricated for 1.5-microns wavelength, and the lower
indirect gap implies that some imaging should be possible out to 1.75
microns. The lattice matched GaAs layer is doped to function as the
electron promotion and hole barrier layer.
The inter-doping difficulties of growing GaAs on Ge can be minimized by use
of a low-temperature transient liquid phase epitaxial technique. A cool Ge
substrate is suddenly introduced into a Ga melt saturated with As. The
rapid cooling of the melt by the substrate forces rapid growth of a GaAs
layer on the Ge without appreciable inter-contamination. Some grading,
however, would be beneficial in order to remove any heterojunction
band-discontinuities.
The thermal bandgap of Ge is the limiting element determining dark current
in this cathode due to a thermally-excited diffusion current of magnitude
given by a well-known expression
J.sub.d = 4(2.pi.kT/h.sup.2).sup.3 (m.sub.e m.sub.h).sup.3/2 (qD/Lp.sub.o)
exp(-E.sub.g /kT).
Where the symbols have their usually accepted meanings, For Ge at T =
300.degree.K and a hole density p.sub.o = 10.sup.18 /cm.sup.3, we have
J.sub.d = 10.sup.-.sup.6 A/cm.sup.2, which is of course much too high, and
indicates that all photocathodes relying on materials with bandgaps in the
region of 0.67 eV or less must be cooled.
For a somewhat lower temperature of 200.degree.K and allowing for an
increase of E.sub.g to 0.7 eV at this temperature, we have J.sub.d = 2 +
10.sup.-.sup.14 A/cm.sup.2, a more reasonable value for dark current. At
200.degree.K, the thermionic emission from the Cs.sub.2 O and its
interface states is negligible by comparison.
The bias current at this temperature can be computed from the thermionic
emission over the hole barrier of the Schottky contact on GaAs, which is
about 0.5 eV. We have
i.sub.b = 4 .times. 10.sup.-.sup.7 A/cm.sup.2 at T = 200.degree.K
and
i.sub.b = 10 mA/cm.sup.2 at T = 300.degree.K.
FIG. 10 shows a similar situation with GaP grown on p-type Si. This growth
can be carried out by vapor-phase methods. The resulting device will give
some photoemission at 1.06 microns, which owing to the indirect Si bandgap
will have a rather low efficiency-resolution product, but is of
theoretical interest in that the GaP emitter can emit directly from the
lowest conduction-band minimum over a Ag/Cs.sub.2 O barrier.
One of the problems with present low bandgap negative affinity
photocathodes is a dark current one or more orders of magnitude too high
for some applications, at room temperature. This is a consequence of a low
work function, which must of course be slightly lower than the photon
energy in a passive cathode. Using the biased Schottky barrier cathode, a
significantly higher work function surface may be used on the biased
emitter, thereby reducing the dark current to the level of the
thermally-excited diffusion current from the active layer, which will be
adequately low at room temperature (about 10.sup.-.sup.15 A/cm.sup.2 for a
10.sup.18 p-doped active layer).
FIG. 11 shows GaAs/InGaAs/InGaP field assisted photoemitter. The active
layer is In.sub.0.15 Ga.sub.0.85 As, some microns thick (e.g. 5 microns),
grown by graded vapor phase epitaxy process on a GaAs substrate. Light
will be incident through this substrate. Such growth produces material of
adequate quality when suitably graded. The lattice constant of the active
layer is about 5.7 A.
A depletion layer about 3 thick of lightly-doped lattice-matching InGaP
(approximately In.sub.0.6 Ga.sub.0.4 P) is grown on the active layer. At
the lattice constant of 5.7 A, the .GAMMA. conduction bandgap of InGaP is
about 1.8 eV, and the X and L valleys lie at about 2.15 eV. Assuming a
hole barrier of 0.75 eV for a surface metallic contact, the upper valley
electrons have an energy in the metallic layer of 1.4 eV. They can
therefore surmount a surface barrier of this magnitude, e.g. as provided
by a layer of Cs over the Schottky barrier. The contribution of the
metallic contact to thermionic emission is less than 10.sup.-.sup.16
A/cm.sup.2 at 300.degree.K. The contribution of the InGaP depletion layer,
having a minimum bandgap of 1.75 eV, will be negligible (<10.sup.-.sup.23
A/cm.sup.2) if the light doping is p-type.
Since this cathode is freed of the bandgap constraints imposed on a passive
(unbiased) cathode by the surface interfacial barrier with Cs.sub.2 O, the
basic absorption and transport processes are very efficient, and the
ultimate cathode efficiency is determined by the transmission of the
Schottky barrier, which can be high.
Yet another method for increasing the thermal energy of the electrons in
the conduction level to promote them over the energy barrier employs the
known optical pumping technique for pumping the electrons from the lower
energy levels in the conduction band to higher levels. A source of pumping
radiation illuminates the photocathode of the type shown in FIG. 1. of
high intensity and of lower energy than the bandgap E.sub.G so that this
pumping light does not promote electrons from the valence band to the
conductance band. Because of the low cross-section for this process and
the high intensity of light needed, this method of optical excitation is
much less desirable than the intervalley transfer by electric field,
described above.
The above discussion has ignored discontinuities in the conduction band
edges at a heterojunction, which must be removed for proper operation of
the two-layer embodiment. This removal can be effected by use of
compositional grading combined with appropriate doping. Considering the
discontinuity shown in FIG. 12 (a), improvement can be made (FIG. 12b) by
n.sup.+ doping of the early growth of the whole bandgap material.
10.sup.19 -level doping accomplishes band-bending in a distance of the
order of 100 A. If now the bandgap is graded (via a lattic compositional
change) over the region indicated by the vertical lines in FIG. 12b, a
smooth conduction band edge is obtained as shown in FIG. 12c. The natural
occurrence of smooth changes of composition, rather than the fictitious
step change of FIG. 12a, is a feature of liquid phase epitaxial growth,
due to melt-back and regrowth on contacting a substrate with a
nonequilibrium melt. In the case considered here, the rest of the emitter
layer would be grown lightly p-type.
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