U.S. patent application number 15/921225 was filed with the patent office on 2018-07-19 for phosphorus doped diamond electrode with tunable low work function for emitter and collector applications.
The applicant listed for this patent is Arizona Board of Regents on Behalf of Arizona State University. Invention is credited to Franz A. M. Koeck, Robert J. Nemanich.
Application Number | 20180204702 15/921225 |
Document ID | / |
Family ID | 60243605 |
Filed Date | 2018-07-19 |
United States Patent
Application |
20180204702 |
Kind Code |
A1 |
Koeck; Franz A. M. ; et
al. |
July 19, 2018 |
PHOSPHORUS DOPED DIAMOND ELECTRODE WITH TUNABLE LOW WORK FUNCTION
FOR EMITTER AND COLLECTOR APPLICATIONS
Abstract
An apparatus includes an emitter electrode including a
phosphorus doped diamond layer with low work function. The
apparatus further includes a collector electrode and a vacuum gap
disposed between the emitter and the collector. The collector has a
work function of 0.84 eV or less.
Inventors: |
Koeck; Franz A. M.; (Tempe,
AZ) ; Nemanich; Robert J.; (Scottsdale, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Arizona Board of Regents on Behalf of Arizona State
University |
SCOTTSDALE |
AZ |
US |
|
|
Family ID: |
60243605 |
Appl. No.: |
15/921225 |
Filed: |
March 14, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15588362 |
May 5, 2017 |
9922791 |
|
|
15921225 |
|
|
|
|
62332338 |
May 5, 2016 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 19/06 20130101;
C30B 25/16 20130101; H01J 45/00 20130101; C30B 25/20 20130101; H01J
9/14 20130101; C30B 25/186 20130101; H01J 9/04 20130101; H01J 19/30
20130101; C30B 29/04 20130101 |
International
Class: |
H01J 19/30 20060101
H01J019/30; C30B 29/04 20060101 C30B029/04; C30B 25/16 20060101
C30B025/16; C30B 25/18 20060101 C30B025/18; C30B 25/20 20060101
C30B025/20; H01J 9/04 20060101 H01J009/04; H01J 9/14 20060101
H01J009/14; H01J 19/06 20060101 H01J019/06; H01J 45/00 20060101
H01J045/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This disclosure was made with government support under
N00014-10-1-0540 awarded by the Office of Naval Research. The
government has certain rights in the disclosure.
Claims
1. A method for forming an electrode, comprising: loading a
substrate into a plasma enhanced chemical vapor deposition (PECVD)
reactor and preparing a surface of the substrate for doped diamond
growth; exposing the substrate to a pure hydrogen plasma for a
preset duration; and depositing a phosphorus doped diamond layer on
the substrate to form the electrode, wherein the electrode
comprises a collector that has a work function of 0.84 eV or
less.
2. The method of claim 1, wherein the wet-chemical cleaning
procedure comprises: boiling the substrate in H2SO4/H2O2/H2O at
220.degree. C. for at least fifteen minutes; treating the substrate
for at least five minutes using a solution comprising hydrofluoric
acid; and boiling the substrate in NH4OH/H2O2/H2O at 75.degree. C.
for at least fifteen minutes.
3. The method of claim 2, wherein the wet-chemical cleaning
procedure further comprises: rinsing the substrate with deionized
water after each step.
4. The method of claim 1, wherein exposing the substrate to the
pure hydrogen plasma for the preset duration comprises: exposing
the substrate to the pure hydrogen plasma at a preset temperature
range for at least fifteen minutes.
5. The method of claim 4, wherein the preset temperature range
comprises a low temperature not greater than 800.degree. C. and a
high temperature of at least 900.degree. C.
6. The method of claim 1, wherein depositing the phosphorus doped
diamond layer on the substrate comprises: depositing the phosphorus
doped diamond layer using a methane flow rate of 2 sccm and a 200
ppm trimethylphosphine/hydrogen (the phosphorus source) flow rate
between 10 sccm and 30 sccm with hydrogen as carrier gas.
7. The method of claim 6, wherein the methane flow rate is 0.5% of
a total gas flow rate of 400 sccm.
8. The method of claim 1, wherein depositing the phosphorus doped
diamond layer on the substrate comprises: depositing the phosphorus
doped layer at around 900.degree. C.-1200.degree. C. using a
microwave power of at least 2500 W and a preset chamber pressure
range.
9. The method of claim 17, wherein the preset chamber pressure
range comprises a range of 75 to 85 Torr.
10. The method of claim 1, further comprising: controlling
impurities during layer deposition by using a water-cooled sample
stage in the PECVD reactor.
11. The method of claim 10, further comprising: timing a growth
period of the doped diamond growth for less than or equal to seven
minutes; terminating methane and TMP/H2 flow after the growth
period; and inducing a negative electron affinity to the surface of
the substrate using hydrogen plasma exposure.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 15/588,362, filed on May 5, 2017, which claims
the benefit of U.S. Provisional Application Ser. No. 62/332,338,
filed May 5, 2016, and entitled, "Phosphorus doped diamond
electrode with tunable low work function for emitter and collector
applications," both of which are incorporated herein by reference
in their entireties.
BACKGROUND
[0003] Low work function electrodes are highly sought after as they
can significantly advance and enable technologies that rely on
electron transfer in devices such as electron sources for
communications and in particular direct energy converters that
transform heat into electricity without mechanically moving
components. Electron sources utilizing thermionic electron emitters
are widely deployed in high power/high frequency communications
(travelling wave tubes, TWT's), radar, free electron lasers,
directed energy weapons, X-ray sources and space propulsion.
Conventional electron sources based on metallic cathodes operate at
temperatures exceeding 1000.degree. C. Lowering the operating
temperature would lead to a less involved design, a reduced power
demand, and a lighter and smaller payload for operation in mobile
terrestrial and satellite applications.
[0004] Thermionic energy converters operate through the generation
of an electron emission current from a thermionic electron emitter
or cathode which is held at a temperature optimized for its
emission barrier or work function, typically in excess of
1000.degree. C. for refractory metal based emitters. A second lower
work function electrode is coupled to the thermionic emitter,
through a small vacuum gap, which establishes a configuration that
can generate electrical power. The efficiency can then directly be
related to the work function of the counter-electrode, the
collector, where an ideal value of 0.5 eV was reported. This
ultra-low work function would enable predicted efficiencies greater
than 50%. To achieve a similar efficiency with solid-state
thermo-electric conversion would require a material with
ZT.about.10. However, the current best materials exhibit
ZT.about.2. It is notable that traditional thermal power plants can
be characterized as operating with ZT.about.3. Additionally,
establishing a means to control the electrode work function would
enable devices to operate with optimum performance at the desired
temperature.
SUMMARY
[0005] According to a first aspect, an apparatus is provided. The
apparatus includes a collector including a phosphorus doped diamond
layer. The apparatus further includes a thermionic emitter and a
vacuum gap disposed between the emitter and the collector. The
collector has a work function of 0.84 eV or less.
[0006] According to a second aspect, a method is provided for
making an apparatus. The method may include one or more of the
following acts: preparing a substrate using a wet-chemical cleaning
procedure; loading the substrate into a plasma enhanced chemical
vapor deposition (PECVD) reactor and prepare a surface of the
substrate for doped diamond growth; exposing the substrate to a
pure hydrogen plasma for a preset duration; and depositing a
phosphorus doped diamond layer on the substrate.
[0007] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 illustrates simulation results of the electron
emission current using the Richardson-Dushman expression for
various work functions .phi. according to an aspect of the
disclosure.
[0009] FIG. 2 illustrates a band diagram according to an aspect of
the disclosure.
[0010] FIG. 3 illustrates an example vacuum thermionic energy
converter device according to an aspect of the disclosure.
[0011] FIG. 4 illustrates conversion efficiency for various power
generators in comparison with the thermo-electric ZT figure of
merit and the corresponding efficiency of a vacuum-thermionic
converter with collector work function .phi. of 1 eV and 0.5
eV.
[0012] FIG. 5 illustrates thermionic emission vs temperature for
samples with an epitaxial phosphorus doped diamond layer on
nitrogen doped single crystal diamond (100) substrates according to
an aspect of the disclosure and the work function (phi) extracted
from a fit to the Richardson formalism.
[0013] FIG. 6 illustrates a schematic of the doped diamond
electrode device according to an aspect of the disclosure.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0014] Before the present disclosure is described in further
detail, it is to be understood that the disclosure is not limited
to the particular embodiments described. It is also to be
understood that the terminology used herein is for the purpose of
describing particular embodiments only, and is not intended to be
limiting. The scope of the present disclosure will be limited only
by the claims.
[0015] As used herein, the singular forms "a", "an", and "the"
include plural embodiments unless the context clearly dictates
otherwise.
[0016] Specific structures, devices, and methods relating to
thermionic and solar energy conversion have been disclosed. It
should be apparent to those skilled in the art that many additional
modifications beside those already described are possible without
departing from the inventive concepts. In interpreting this
disclosure, all terms should be interpreted in the broadest
possible manner consistent with the context. Variations of the term
"comprising" should be interpreted as referring to elements,
components, or steps in a non-exclusive manner, so the referenced
elements, components, or steps may be combined with other elements,
components, or steps that are not expressly referenced. Embodiments
referenced as "comprising" certain elements are also contemplated
as "consisting essentially of" and "consisting of" those elements.
If a series of numerical ranges are recited, this disclosure
contemplates combinations of the lower and upper bounds of those
ranges that are not explicitly recited. For example, if a range
between 1 and 10 or between 2 and 9 is recited, this disclosure
also contemplates a range between 1 and 9 or between 2 and 10.
[0017] Diamond based electrodes are a preferred material for
electron sources in general and energy conversion in particular as
the material exhibits excellent high temperature stability,
mechanical properties exceeding that of the hardest metals and
exceptional resistance to radiation. Additionally, with its high
thermal conductivity and electron mobility and the ability to
accept shallow donors in its lattice, a diamond based electrode may
overcome limits seen with conventional materials.
[0018] Low and ultra-low work function doped diamond electrodes
were prepared by deposition of a thin phosphorus doped diamond
layer on single crystal nitrogen doped, high-pressure,
high-temperature (HPHT) substrates through epitaxial layer growth
using plasma enhanced chemical vapor deposition (PECVD). The
nitrogen doped single crystal substrate allows electrical
conduction that improves with temperature suitable for electron
sources operating at elevated temperatures. Prior to diamond
deposition the surface is cleaned by a wet-chemical procedure
including H2SO4, H2O2, NH4OH and HF followed by exposure to a pure
hydrogen plasma at a higher temperature to prepare a clean surface.
Utilizing hydrogen (H2) as carrier gas, methane (CH4) as carbon
source and a phosphorus source, here a 200 ppm
trimethylphosphine/hydrogen (TMP/H2) gas mixture, a thin phosphorus
doped layer is deposited by establishing a short growth period
typically timed for 7 minutes. Optionally, a finishing step in a
pure hydrogen plasma establishes a hydrogen passivated surface that
induces a negative electron affinity (NEA) characteristic and
allows further lowering of the work function or emission
barrier.
[0019] The devices characterized with respect to the thermionic
emission law of Richardson-Dushman indicated an ultra-low work
function of 0.67 eV. This is the one of the lowest work functions
reported and the lowest work function material operating at
temperatures exceeding 800.degree. C. Adjustment of growth
conditions enabled tuning of the work function value where a
readily achieved alternate deposition regime achieved a work
function of 0.84 eV.
[0020] FIG. 1 illustrates simulation of the electron emission
current using the Richardson-Dushman expression for various work
functions cp. The results are presented for three different values
of the Richardson constant, the theoretical value for metals
(A.sub.R=120 A/cm.sup.2K.sup.2), the theoretical value for diamond
(A.sub.R=68 A/cm.sup.2K.sup.2) and the diamond value reduced by a
factor of 100 that reflects the range of anticipated experimental
values. The upper region 110 presents an emission current density
>20 A/cm.sup.2 that would compete with or surpass the present
emitter technology. Moderate work function metals of 1.3-1.4 eV
with the theoretical Richardson constant of 120 A/cm.sup.2K.sup.2
may provide higher current densities at higher temperatures. Here,
line 120 illustrates the change of emission current density values
as the emitter temperature increases when the A.sub.R is the
theoretical value (68 A/cm.sup.2K.sup.2) for diamond and the work
function is 0.67 eV. Line 122 illustrates the change of emission
current density values as the emitter temperature increases when
the work function is 0.67 eV and the A.sub.R is one hundredth the
theoretical value (0.68 A/cm.sup.2K.sup.2) for diamond as an
example when A.sub.R deviates to lower values
[0021] In FIG. 1, line 124 illustrates the change of emission
current density values as the emitter temperature increases when
the A.sub.R is the theoretical value for metals and the work
function is 1.3 eV. Line 126 illustrates the change of emission
current density values as the emitter temperature increases when
the A.sub.R is the theoretical value for metals and the work
function is 1.4 eV. Line 128 illustrates the change of emission
current density values as the emitter temperature increases when
the A.sub.R is the theoretical value for diamond and the work
function is 1.6 eV. Line 130 illustrates the change of emission
current density values as the emitter temperature increases when
the A.sub.R is the theoretical value for diamond and the work
function is 1.8 eV. Line 132 illustrates the change of emission
current density values as the emitter temperature increases when
the A.sub.R is the theoretical value for diamond and the work
function is 2.0 eV.
[0022] FIG. 2 illustrates a band diagram 200 in a flat band
configuration for diamond 210 indicating the donor states for
nitrogen, and phosphorus and the acceptor states for boron which
reside between the valence, EV, and conduction band, EC. The vacuum
level 220 for a hydrogen terminated and oxygen terminated surface
is marked, which corresponds to a negative electron affinity (NEA)
and a positive electron affinity (PEA), respectively.
[0023] FIG. 3 illustrates a vacuum thermionic energy converter
device 310 comprised of a thermionic electron emitter 312, a vacuum
gap 314, and a collector 316 (left image). The corresponding band
schematic 320 indicates emitter and collector work function,
.phi..sub.E and .phi..sub.C, respectively, and the self-generated
voltage, V, due to the work function difference. The thermionic
electron emitter 312 may include a N-doped single crystal diamond.
The collector 316 may include a phosphorus doped diamond layer. The
phosphorus doped diamond layer may be a P-doped homoepitaxial
diamond layer.
[0024] FIG. 4 illustrates conversion efficiencies for various power
generators in comparison with the thermo-electric ZT figure of
merit and the corresponding efficiency of a vacuum-thermionic
converter with collector work function .phi. of 1 eV (line 410) and
0.5 eV (line 420). The feasibility of higher ZT values is
indicated. The ZT value for different power conversion processes
are indicated as is the operation of thermal power plants (yellow
region 430).
[0025] FIG. 5 illustrates thermionic emission vs. temperature for
samples with an epitaxial phosphorus doped diamond layer on
nitrogen doped single crystal diamond (100) substrates. The
phosphorus doped diamond films were grown with 10 sccm TMP/H2
(curve 510) and 30 sccm TMP/H2 (curve 520). An ultra-low work
function of 0.67 eV was deduced from the Richardson relation for
the film grown with 10 sccm TMP/H2. An increase of the work
function to 0.84 eV was observed for the film grown with a 30 sccm
TMP/H2 flow rate. (The solid lines present a fit to the
Richardson-Dushman expression.)
[0026] FIG. 6 illustrates a schematic of the low work function
doped diamond electrode device 600 according to an aspect of the
disclosure. In FIG. 6, the electrode device 600 may include the
conducting (doped) single crystal diamond substrate 610 with (100)
or similar orientation (nitrogen doped single crystal,
high-pressure, high-temperature (HPHT) or CVD diamond) with a
prepared surface (the controlled interface 615); the thin (5 nm-100
nm) phosphorus doped diamond layer 620 with varying and controlled
doping concentration and with diamond surface 630 generated by
optional surface treatment to induce negative electron affinity
properties (hydrogen passivation). Electrical contacts 640 are
prepared using metallic(gold) or conducting, doped nano-structured
diamond conductors
[0027] Electron Sources--Introduction:
[0028] Thermionic electron sources are an integral part in today's
advanced terrestrial and space based telecommunications through
travelling wavetubes (TWT's), space exploration through propulsion
and energy generators, national security through radar and directed
energy weapons, science, health care and industry through electron
microscopy, free electron lasers, X-ray sources, electron beam
lithography and microwave generators. While each application group
requires specifics for operation a common criterion is the power
requirement to establish necessary output levels. As the power
requirements are directly related to the device (cathode) operating
temperature, establishing efficient emission at a lower temperature
would translate into a refined system design and reduced power
consumption. This becomes more critical for mobile or space
applications where part of the payload needs to address power
generation. To account for the increased power requirements in
satellites which range from 1 kW to 25 kW, silicon based solar
panels are being replaced by triple-junction InGaP/InGaAs/Ge solar
cells with a conversion efficiency of about 30%. As satellites
present relay stations for terrestrial communications via
travelling wave tubes, the impact of improved electron sources may
be significant. Likewise, it is apparent that more efficient power
sources are necessary for space communications and exploration. In
fact, electrical power for the Mars Curiosity rover is supplied by
a multi-mission radioisotope thermo-electric generator (MMRTG)
utilizing a 2000 W thermal power heat source that is converted by
thermo-electrics to about 120 W electrical power in a bulky
configuration measuring 25 in. in diameter (fin tip to fin tip), 26
in. in length and weighing about 94 lbs.
[0029] Increasing demand for high power/high frequency
communications (THz) can be achieved by novel cathodes capable of
delivering high current densities. For current TWTs the cathodes
are typically heated to 1100.degree. C. and electrons are extracted
via thermionic emission and the application of a high voltage. The
cathode voltage can range from thousands to hundreds of thousands
of volts and establish cathode currents greater than 10 A which
requires a cathode material capable of the high emission current
densities.
[0030] Physics of Thermionic Electron Emission:
[0031] Vacuum thermionic electron emission, first formulated by
Richardson-Dushman, relates the emission current density J(T) to
the emission barrier or work function .phi. and the Richardson or
emission constant A_R through the expression
J(T)=A_R*T 2e (-.phi./(k_B*T)) (1)
where
A_R=(4.pi.m_0*k_B 2*e 2)/h, (2)
[0032] with Boltzmann's constant, k_B, electron effective mass, m0,
electronic charge, e, and Planck's constant h. For diamond a value
for A_R of 68 A/cm2K2 was reported. A simulation of (1) is shown in
FIG. 1) for various work function values. The plot shows the
expected current density that may be obtained from a low work
function of 0.6 eV emitter where A_R ranges from 68-0.68 A/cm2K2.
With .phi.=0.6 eV and A_R=0.68 A/cm2K2 the cathode is able to
provide a current density of .about.20 A/cm2 at around 500.degree.
C. surpassing any present emitter technology. Some of the lowest
work functions have been reported for Ba--Ca--Al oxides. One report
of 12-CaO 7-Al2O3 (C12A7) indicates a work function of 0.6 eV.
However, thermionic applications are apparently limited to
temperatures less than 500.degree. C. More importantly, for single
crystal C12A7, a shift in the work function to 2.1 eV was observed
at a higher temperature regime up to 950.degree. C. which limits
use for most energy conversion and communication applications.
[0033] Diamond as Electron Emitter:
[0034] Considering diamond for electron emitter applications
requires evaluation of its band structure that is schematically
drawn in FIG. 2. Here, the doping levels are indicated for a flat
band configuration, for donor levels of nitrogen at 1.7 eV below
the conduction band minimum (CBM) and phosphorus at 0.6 eV below
the CBM. A unique band configuration arises when the diamond
surface is terminated by hydrogen. In this case the vacuum level is
shifted below the CBM eliminating a surface barrier for electron
emission. Thermal promotion of electrons from shallow states into
the CBM would then allow their unhindered release into vacuum. For
clean and oxygen terminated surfaces the vacuum level is positioned
above the CBM establishing a positive electron affinity (PEA)
surface. Oxygenated and clean diamond surfaces with various
orientations were reported with PEA values ranging from 0.6 eV to
1.45 eV. In conjunction with shallow donors, PEA doped diamond
surfaces can provide work functions less than 2 eV suitable for
emitter applications.
[0035] Ultra-low work function diamond as collector electrode for
direct energy conversion: The direct conversion of heat into
electricity by means of vacuum thermionic energy conversion
presents an approach for electrical power generation where high
conversion efficiencies are feasible as hot and cold side of the
generator is separated by a vacuum gap (see FIG. 3). A vacuum
thermionic converter is comprised of a hot side, a thermionic
electron emitter and a vacuum gap providing a thermal barrier to
the counter-electrode, the collector. This is contrasted by
solid-state thermo-electric conversion techniques that rely on
materials with low thermal conductivity to establish a sufficiently
large thermal gradient between the hot and cold side. The
conversion efficiency is typically described by the ZT value where
the best current materials exhibit a ZT of about 2 which is reduced
to about 1.5 for operating temperatures greater than about 700K. As
a comparison, the efficiency of today's thermal power plants would
correspond to a ZT value of about 3 and is shown in a comprehensive
plot for various heat engines (see FIG. 4). For vacuum thermionic
energy conversion a similar figure of merit can be related to the
collector work function, and its relation to thermo-electric ZT
values is displayed in the efficiency plot in FIG. 4.
[0036] Engineering of Low Work Function Doped Diamond Cathodes:
[0037] One of the focus of this disclosure is on the preparation of
an ultra-low work function collector that can also act as a
thermionic emitter or cathode. As electron emitters have to support
the electron current across the device an electrically conducting
diamond substrate, here, a nitrogen doped, single crystal,
high-pressure, high-temperature substrate with (100) surface
orientation was selected. These substrates were characterized at
elevated temperatures where a Richardson constant A_R of 62 A/cm2K2
was observed, a value approaching the theoretical value for diamond
of 68 A/cm2K2. This established the capability of doped diamond to
enable high values for the emission or Richardson constant crucial
for efficient thermionic emitters. Preparation of the low work
function collector and emitter device commenced with a wet-chemical
cleaning procedure that consisted of:
[0038] Boil in H2SO4/H2O2/H2O, 3:1:1 at 220.degree. C. for 15
min
[0039] HF treatment for 5 min
[0040] Boil in NH4OH/H2O2/H2O, 1:1:5 at 75.degree. C. for 15
min
[0041] rinse with DI water after each step
[0042] The thus prepared substrate was then loaded into the PECVD
reactor and exposed to a pure hydrogen plasma at 800.degree.
C.-900.degree. C. for 15 min. This process prepares the surface for
the doped diamond growth. A thin phosphorus doped diamond layer was
then deposited using a methane flow rate of typically 2 sccm (0.5%
of the total gas flow) and a 200 ppm trimethylphosphine/hydrogen
(the phosphorus source) flow rate between 10 sccm and 30 sccm with
hydrogen as carrier gas with a total gas flow rate of 400 sccm. The
phosphorus doped layer is deposited at around 900.degree.
C.-1000.degree. C. using a microwave power of 2500 W and a chamber
pressure of 75-85 Torr. With these growth conditions and by
utilization of plasma focusing geometry of the sample holder,
heating of the substrate is achieved by the plasma discharge.
Furthermore, a water-cooled sample stage and PECVD reactor reduce
incorporation of impurities during deposition. The growth period is
timed for about 7 minutes after which methane and TMP/H2 flow is
terminated and the sample cooled under hydrogen plasma exposure
which induces a negative electron affinity surface. Results from
two devices are presented here, where the TMP/H2 flow rate was
established at 10 sccm and 30 sccm. The average thicknesses of the
films are estimated to be between 5 nm and 100 nm, and the layer
thickness can be further optimized to achieve a low resistivity and
work function.
[0043] Thermionic Emitter Characterization:
[0044] To prepare the collector and thermionic cathode, electrical
contacts are prepared using a thin gold layer on the top surface as
well as the backside of the substrate (see FIG. 6). In an emission
characterization chamber, a bias voltage is applied to the device
to overcome space charge effects, and the emission current is
recorded as a function of temperature as shown in FIG. 5. For the
sample prepared with 10 sccm of TMP/H2 an ultra-low work function
of 0.67 eV was extracted from a fit to the Richardson expression.
As the TMP/H2 flow rate is increased to 30 sccm, the work function
was observed to increase to 0.84 eV. The devices were operated at
temperatures up to 950.degree. C. indicating high temperature
stability of the device.
[0045] A novel device structure based on doped diamond has been
developed where a donor state establishes a surface with a low work
function that can be further reduced through suitable surface
terminations. This device can provide a work function as low as
0.67 eV where the operating temperature exceeds 800.degree. C. The
cathodes are readily prepared on (100) oriented single crystal
diamond substrates a preferred crystallographic orientation for
device manufacturing. Tuning the value of the work function was
achieved by adjusting the growth parameters. With a modified growth
regime a cathode with a work function of 0.84 eV was engineered.
These low work function thermionic electron emitters may advance
high power/high frequency telecommunications, as well as
applications where electron sources may be a key component.
Secondly, in direct energy conversion application of the low work
function electrode as collector conversion efficiencies approaching
50% may be realized.
[0046] Thin layers of phosphorus doped diamond with controlled
doping concentration on (100) single crystal nitrogen doped diamond
that allow preparation of ultra-low work function collectors and
thermionic emitters with tunable work function.
[0047] The phosphorus doped diamond collector/emitter with a work
function of 0.67 eV presents the only known material that can
maintain this low emission barrier across a wide temperature regime
which has been measured up to 950.degree. C. Additionally, we have
established a means to control the work function by adjusting the
phosphorus doping, where a variation of the work function to 0.84
eV was demonstrated. The low work function is achieved without
adsorbates or coatings.
[0048] Diamond based electrodes are a preferred material for
electron sources in general and energy conversion in particular as
the material exhibits excellent high temperature stability,
mechanical properties exceeding that of the hardest metals and
exceptional resistance to radiation. Additionally, with its high
thermal conductivity and electron mobility and the ability to
accept shallow donors in its lattice, a diamond based electrode may
overcome limits obtainable by conventional materials.
[0049] Low and ultra-low work function doped diamond electrodes
were prepared by deposition of a thin phosphorus doped diamond
layer on single crystal nitrogen doped, high-pressure,
high-temperature (HPHT) substrates through epitaxial layer growth
using plasma enhanced chemical vapor deposition (PECVD). The
nitrogen doped single crystal substrate allows electrical
conduction that improves with temperature suitable for electron
sources operating at elevated temperatures. Prior to diamond
deposition the surface is cleaned by a wet-chemical procedure
including H2SO4, H2O2, NH4OH and HF followed by exposure to a pure
hydrogen plasma at a higher temperature to prepare a clean surface.
Utilizing hydrogen (H2) as carrier gas, methane (CH4) as carbon
source and a phosphorus source, here a 200 ppm
trimethylphosphine/hydrogen (TMP/H2) gas mixture, a thin phosphorus
doped layer is deposited by establishing a short growth period
typically timed for no more than 7 minutes. Optionally, a finishing
step in a pure hydrogen plasma establishes a hydrogen passivated
surface that induces a negative electron affinity (NEA)
characteristic and allows further lowering of the work function or
emission barrier.
[0050] The disclosed devices may be characterized with respect to
the thermionic emission law of Richardson-Dushman and indicated an
ultra-low work function of 0.67 eV. This is one of the lowest work
functions reported and the lowest work function material operating
at temperatures exceeding 800.degree. C. Adjustment of growth
conditions may be enabled by tuning of the work function value
where a readily achieved alternate deposition regime achieved a
work function of 0.84 eV.
[0051] The disclosure also provides a low work function electrode
as a thermionic electrode emitter that utilizes shallow diamond
donors such as phosphorus. The disclosed devices may include
thermionic cathodes and may be employed in a wide variety of
applications. The low work function electrode may include a
negative electron affinity surface for ultra-low work function
electrodes, a positive or zero electron affinity for a low work
function where the ultra-low work function of the negative electron
affinity surface is increased by the value of the electron affinity
(typically 1 eV). The low work function electrode has a reduced
upward band bending and a thin 5-100 nm thin phosphorus doped
diamond layer. The low work function electrode can enable efficient
energy conversion devices.
[0052] Although the invention has been described in considerable
detail with reference to certain embodiments, one skilled in the
art will appreciate that the present invention may be practiced by
other than the described embodiments, which have been presented for
purposes of illustration and not of limitation. Therefore, the
scope of the appended claims should not be limited to the above
description contained herein.
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