U.S. patent application number 10/777633 was filed with the patent office on 2004-11-11 for single crystal diamond prepared by cvd.
Invention is credited to Collins, John Lloyd, Dorn, Barbel Susanne Charlotte, Martineau, Philip Maurice, Scarsbrook, Geoffrey Alan, Sussmann, Ricardo Simon, Twitchen, Daniel James, Whitehead, Andrew John.
Application Number | 20040221795 10/777633 |
Document ID | / |
Family ID | 26244497 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040221795 |
Kind Code |
A1 |
Scarsbrook, Geoffrey Alan ;
et al. |
November 11, 2004 |
Single crystal diamond prepared by CVD
Abstract
A single crystal diamond prepared by CVD and having one or more
electronic characteristics; making the diamond suitable for
electronic applications. Also provided is a method of making the
single crystal CVD diamond.
Inventors: |
Scarsbrook, Geoffrey Alan;
(Ascot, GB) ; Martineau, Philip Maurice;
(Littlewick Green, GB) ; Collins, John Lloyd;
(London, GB) ; Sussmann, Ricardo Simon; (Reading,
GB) ; Dorn, Barbel Susanne Charlotte; (Bracknell,
GB) ; Whitehead, Andrew John; (Camberley, GB)
; Twitchen, Daniel James; (Sunningdale, GB) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
26244497 |
Appl. No.: |
10/777633 |
Filed: |
February 13, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10777633 |
Feb 13, 2004 |
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10311215 |
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10311215 |
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PCT/IB01/01037 |
Jun 14, 2001 |
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Current U.S.
Class: |
117/68 |
Current CPC
Class: |
C30B 25/105 20130101;
C30B 29/04 20130101; C30B 25/00 20130101; Y10T 428/263 20150115;
Y10T 428/30 20150115; C30B 25/105 20130101; C30B 29/04
20130101 |
Class at
Publication: |
117/068 |
International
Class: |
C30B 007/00; C30B
021/02; C30B 028/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2000 |
GB |
0014693.6 |
Mar 20, 2001 |
GB |
0106930.1 |
Claims
1. A single diamond prepared by CVD and having at least one of the
following characteristics: (i) in the off state, a resistivity
R.sub.1 greater than 1.times.10.sup.12 .OMEGA.cm at an applied
field of 50 V/.mu.m measured at 300 K; (ii) A high breakdown
voltage in the off state, and high current with long carrier life
time in the on state; (iii) an electron mobility (.mu..sub.e)
measured at 300K greater than 2400 cm.sup.2V.sup.-1s.sup.-1; (iv) a
hole mobility (.mu..sub.h) measured at 300K greater than 2100
cm.sup.2V.sup.-1x.sup.-1; and (v) a high collection distance
greater than 150 .mu.m measured at an applied field of 1 V/.mu.m
and 300 K.
2. A single crystal diamond according to claim 1 which has
resistivity greater than 2.times.10.sup.13 .OMEGA.cm at an applied
field of 50 V/.mu.m measured at 300 K.
3. A single crystal diamond according to claim 1 which has a
resistivity R.sub.1 greater than 5.times.10.sup.14 .OMEGA.cm at an
applied field of 50 V/.mu.m measured at 300 K.
4. A single crystal diamond according to claim 1 which has a
.mu..tau. product measured at 300 K greater than
1.5.times.10.sup.-6 cm.sup.2V.sup.-1 where .mu. is the mobility and
.tau. is the lifetime of the charge carriers.
5. A single crystal diamond according to claim 4 which has a
.mu..tau. product measured at 300 K of greater than
4,0.times.10.sup.-6 cm.sup.2V.sup.-1.
6. A single diamond according to claim 4 which has a .mu..tau.
product measured at 300 K greater than 6,0.times.10.sup.-6
cm.sup.2V.sup.-1.
7. A single crystal diamond according to claim 1 which has an
electron mobility (.mu..sub.e) measured at 300 K greater than 3000
cm.sup.2V.sup.-1s.sup.-1.
8. A single crystal diamond according to claim 7 which has an
electron mobility (.mu..sub.e) measured at 300 K greater than 4000
cm.sup.2V.sup.-1s.sup.-1.
9. A single crystal diamond according to claim 1 which has a hole
mobility measured at 300 K greater than 2500
cm.sup.2V.sup.-1s.sup.-1.
10. A single crystal diamond according to claim 9 which has a hole
mobility measured at 300 K greater than 3000
cm.sup.2V.sup.-1s.sup.-1.
11. A single crystal diamond according to claim 1 which has a
collection distance measured at 300 K greater than 400 .mu.m.
12. A single crystal diamond according to claim 11 which has a
collection distance measured at 300 K greater than 600 .mu.m.
13. A single crystal diamond according to claim 1 which has each of
the characteristics (i), (ii), (iii), (iv) and (v).
14. A method of producing a single crystal diamond according to
claim 1 which includes the steps of providing a diamond substrate
having a surface which is substantially free of crystal defects,
providing a source gas, dissociating the source gas and allowing
homoepitaxial diamond growth on the surface which is substantially
free of crystal defects in an atmosphere which contains less than
300 parts per billion nitrogen.
15. A method according to claim 14 wherein the substrate is a low
birefringence type 1a or 11b natural, 1b or 11a high pressure/high
temperature synthetic diamond.
16. A method according to claim 14 wherein the substrate is a CVD
synthesized single crystal diamond.
17. A method according to claim 14 wherein the surface on which
diamond growth occurs has a density of surface etch features
related to defects below 5.times.10.sup.3/mm.sup.2.
18. A method according to claim 14 wherein the surface on which
diamond growth occurs has a density of surface etch features
related to defects below 10.sup.2/mm.sup.2.
19. A method according to claim 14 wherein the surface on which the
diamond growth occurs is subjected to a plasma etch to minimise
surface damage of the surface prior to diamond growth.
20. A method according to claim 19 wherein the plasma etch is an in
situ etch.
21. A method according to claim 19 wherein the plasma etch is an
oxygen etch using an etching gas containing hydrogen and
oxygen.
22. A method according to claim 21 wherein the oxygen etch
conditions are a pressure of 50 to 450.times.10.sup.2 Pa, an
etching gat containing an oxygen content of 1 to 4%, an argon
content of up to 30% and the balance hydrogen, all percentages
being by volume, a substrate temperature of 600 to 1100.degree. C.,
and an etch duration of 3 to 60 minutes.
23. A method according to claim 19 wherein the plasma etch is a
hydrogen etch.
24. A method according to claim 23 wherein the hydrogen etch
conditions are a pressure of 50 to 450.times.10.sup.2 Pa, an
etching gag containing hydrogen and up to 30% by volume argon, a
substrate temperature of 600 to 1100.degree. C. and an etch
duration of 3 to 60 minutes.
25. A method according to claim 19 wherein the surface on which the
diamond growth occurs is subjected to both an oxygen and a hydrogen
etch to minimise surface damage of the surface prior to diamond
growth.
26. A method according to claim 25 wherein the oxygen etch is
followed by a hydrogen etch.
27. A method according to claim 19 wherein the surface R.sub.A of
the surface on which the diamond growth occurs is less than 10
nanometers prior to that surface being subjected to the plasma
etching.
28. A method according to claim 14 wherein the diamond growth takes
place in an atmosphere which contains less than 100 ppb
nitrogen.
29. A method according to claim 14 wherein the surface on which
diamond growth occurs is substantially a {100}, {110}, {113} or
{111} surface.
30. A method according to claim 14 wherein the dissociation of the
source gas occurs using microwave energy.
31. The use of a single crystal diamond according to claim 1 in an
electronic application.
32. The use of a single crystal diamond according to claim 1 as a
detector element or switching element.
33. The use of a single crystal diamond of claim 1 as a component
in an opto-electric switch.
34. A detector element or switching element comprising a single
crystal diamond according to claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to diamond and more particularly to
diamond produced by chemical vapour deposition (hereinafter
referred to as CVD).
[0002] Methods of depositing materials such as diamond on a
substrate by CVD are now well established and have been described
extensively in the patent and other literature. Where diamond is
being deposited on a substrate, the method generally involves
providing a gas mixture which, on dissociation, can provide
hydrogen or a halogen (e.g. F,Cl) in atomic form and C or
carbon-containing radicals and other reactive species, e.g.
CH.sub.x, CF.sub.x wherein x can be 1 to 4. In addition, oxygen
containing sources may be present, as may sources for nitrogen and
for boron. In many processes inert gases such as helium, neon or
argon are also present. Thus, a typical source gas mixture will
contain hydrocarbons C.sub.xH.sub.y wherein x and y can each be 1
to 10 or halocarbons C.sub.xH.sub.yHal.sub.z (Hal=halogen) wherein
x and z can each be 1 to 10 and y can be 0 to 10 and optionally one
or more of the following: COX, wherein x can be 0,5 to 2, O.sub.2,
H.sub.2, N.sub.2, NH.sub.3, B.sub.2H.sub.6 and an inert gas. Each
gas may be present in its natural isotopic ratio, or the relative
isotopic ratios may be artificially controlled; for example
hydrogen may be present as deuterium or tritium, and carbon may be
present as .sup.12C or .sup.13C. Dissociation of the source gas
mixture is brought about by an energy source such as microwaves, RF
energy, a flame, a hot filament or a jet based technique and the
reactive gas species so produced are allowed to deposit onto a
substrate and form diamond.
[0003] CVD diamond may be produced on a variety of substrates.
Depending on the nature of the substrate and details of the process
chemistry, polycrystalline or single crystal CVD diamond may be
produced. The production of homoepitaxial CVD diamond layers has
been reported in the literature.
[0004] European Patent Publication No. 0 582 397 describes a method
of producing a polycrystalline CVD diamond film having an average
grain size of at least 7 microns and a resistivity, carrier
mobility and carrier lifetime yielding a collection distance of at
least 10 .mu.m at an electric field strength of 10 kV/cm. This is a
diamond film of a quality which makes it suitable for use as a
radiation detector. However, applications for films having
collection distances as low as 7 .mu.m are very limited.
[0005] European Patent Publication No. 0 635 584 describes a method
of producing a CVD polycrystalline diamond film using an arc jet
process with low methane levels (less than 0,07%) and an oxidant.
The diamond material has a narrow Raman peak, a relatively large
lattice constant, and a charge carrier collection distance of
greater than 25 .mu.m. However, the performance of polycrystalline
diamond films in electronic applications is believed to be
adversely affected by the presence of grain boundaries.
[0006] It has not previously been reported that single crystal CVD
diamond can be grown with sufficient control to achieve high
performance detector material. Collection distances measured on
natural single crystal diamond have been reported of about 28 .mu.m
at 10 kV/cm and 60 .mu.m at bias voltages of 26 kV/cm. In high
quality type IIa natural single crystal diamond the collection
distance has been shown to vary nearly linearly with bias voltage
up to 25 kV/cm, unlike polycrystalline material which typically
shows saturation of the collection distance at about 10 kV/cm.
[0007] The collection distance can be adversely affected in a
single crystal diamond by the presence of impurities and lattice
defects which reduce the free carrier mobility and free carrier
recombination lifetime of the carrier.
[0008] Prior art has generally concerned itself with the thermal,
optical and mechanical properties of CVD diamond.
SUMMARY OF THE INVENTION
[0009] According to a first aspect of the invention, there is
provided a single crystal diamond prepared by CVD and having at
least one of the following characteristics:
[0010] (i) in the off state, a resistivity R.sub.1 greater than
1.times.10.sup.12 .OMEGA.cm, and preferably greater than
2.times.10.sup.13 .OMEGA.cm, and more preferably greater than
5.times.10.sup.14 .OMEGA.cm, all measured at an applied field of 50
V/.mu.m and 300 K (or 20.degree. C., which for the purposes of this
invention is considered equivalent)
[0011] (ii) a high breakdown voltage in the off state, and high
current with long carrier life time in the on state and, more
particularly, a .mu..tau. product greater than 1.5.times.10.sup.-6
cm.sup.2N, and preferably greater than 4,0.times.10.sup.-6
cm.sup.2N, and more preferably greater than 6.0.times.10.sup.-6
cm.sup.2N, all measured at an applied field of 50 V/.mu.m and 300
K. .mu. is the mobility and .tau. is the lifetime of the charge
carriers, the product representing the contribution by a charge
carrier to the total charge displacement or current. This
characteristic can also be measured and expressed as a charge
collection distance;
[0012] (iii) an electron mobility (.mu..sub.e) measured at 300K
greater than 2400 cm.sup.2V.sup.-1s.sup.-1, and preferably greater
than -3000 cm.sup.2V.sup.-1s.sup.-1, and more preferably greater
than 4000 cm.sup.2V.sup.-1s.sup.-1. In high quality type IIa
natural diamond electron mobilities, at 300 K, are reported to be
typically 1800 cm.sup.2V.sup.-1s.sup.-1 with exceptional values
reported up to 2200 cm.sup.2V.sup.-1s.sup.-1;
[0013] (iv) a hole mobility (.mu..sub.n) measured at 300 K greater
than 2100 cm.sup.2V.sup.-1s.sup.-1; and preferably greater than
2500 cm.sup.2V.sup.-1s.sup.-1, and more preferably greater than
3000 cm.sup.2V.sup.-1s.sup.-1. In high quality type IIa natural
diamond, hole mobilities at 300 K are reported to be typically 1200
cm.sup.2V.sup.-1s.sup.-1 with exceptional values reported up to
1900 cm.sup.2V.sup.-1s.sup.-1;
[0014] (v) a high charge collection distance greater than 150
.mu.m, and preferably at least 400 .mu.m, and more preferably at
least 600 .mu.m, all collection distances being measured at an
applied field of 1 V/.mu.m and 300 K. In high quality type IIa
natural diamond, charge collection distances are reported to be
substantially less than 100 .mu.m, and more typically about 40
.mu.m at 300 K and an applied field of 1 V/.mu.m.
[0015] In a wide band gap device such as one fabricated from
diamond, the number of free charge carriers present under
equilibrium conditions is extremely small and dominated by the
contribution from lattice defects and impurities; such a device is
said to be in the "off state". The device can be put into the "on
state" by the additional excitation of charge carriers by means
such as optical excitation (primarily using optical energies near
or greater than the band gap) or by charged particle excitation
(e.g. alpha or beta particles). In the on state the free carrier
density exceeds the equilibrium level and when the excitation
source is removed the device will revert to the off state.
[0016] It will be noted from the above that the diamond of the
invention has electronic characteristics which are substantially
greater than those present in natural high quality diamond. This is
surprising and provides the diamond with properties which are
useful, for example, for electronic applications and for
detectors.
[0017] The single crystal CVD diamond of the invention is of high
chemical purity and is substantially free of crystal defects.
[0018] a) Resistivity
[0019] Thus, the single crystal CVD diamond of the invention may,
in one form of the invention, have, in the off state, a high
resistivity at high applied fields and more particularly a
resistivity R.sub.1 greater than 1.times.10.sup.12 .OMEGA.cm, and
preferably greater than 2.times.10.sup.13 .OMEGA.cm and more
preferably greater than 5.times.10.sup.14 .OMEGA.cm, at an applied
field of 50 V/.mu.m measured at 300 K. Such resistivities at such
high applied fields are indicative of the purity of the diamond and
the substantial absence of impurities and defects. Material of
lower purity or crystal perfection can exhibit high resistivity at
lower applied fields, e.g. less than 30 V/.mu.m, but shows
breakdown behaviour with rapidly rising leakage currents at applied
fields greater than 30 V/.mu.m and generally by 45 V/.mu.m. The
resistivity can be determined from a measurement of the leakage
(dark) current by methods known in the art. A sample under test is
prepared as a plate of uniform thickness, cleaned using standard
diamond cleaning techniques in order to accept suitable contacts
(evaporated, sputtered or doped diamond) to which external
connections can be made to the voltage supply, and then partially
or wholly encapsulated to avoid risk of flash-over. It is important
to ensure that the encapsulation does not add significantly to the
leakage current measured.
[0020] Typical sample sizes are 0,01-0,5 mm thick by 3.times.3
mm-50.times.50 mm laterally, but smaller or larger sizes may also
be used.
[0021] b) .mu..tau. Product
[0022] The single crystal CVD diamond of the invention may have a
.mu..tau. product greater than 1.5.times.10.sup.-6 cm.sup.2N,
preferably a .mu..tau. product of greater than 4.0.times.10.sup.-6
cm.sup.2/V and more preferably a .mu..tau. product greater than
6.0.times.10.sup.-6 cm.sup.2N, all measurements at 300 K. The
.mu..tau. product is related to the charge collection distance
using the following equation:
.mu..tau.E=CCD
(cm.sup.2/Vs).times.(s).times.(V/cm)=cm
[0023] where E=applied field
[0024] The single crystal CVD diamond of the invention,
particularly in its preferred form, has a high .mu..tau. product
which translates into a high charge collection distance, much
higher than has been achieved with any other known single crystal
CVD diamond.
[0025] When an electric field is applied to a sample using
electrodes it is possible to separate the electron-hole pairs
generated by photon irradiation of the sample. The holes drift
toward the cathode and the electrons toward the anode. Light with a
short wavelength (ultraviolet or UV light) and a photon energy
above the bandgap of the diamond has a very small penetration depth
into diamond and by using this type of light it is possible to
identify the contribution of one type of carrier only dependent on
which electrode is illuminated.
[0026] The .mu..tau. product referred to in this specification is
measured in the following way:
[0027] (i) A sample of diamond is prepared as a plate in excess of
.apprxeq.100 .mu.m thick.
[0028] (ii) Ti semi-transparent contacts are sputtered onto both
sides of the diamond plate and then patterned using standard
photolithography techniques. This process forms suitable
contacts.
[0029] (iii) A 10 .mu.s pulse of monochromatic Xe light (wavelength
218 nm) is used to excite carriers, with the photocurrent generated
being measured in an external circuit. The pulse length of 10 .mu.s
is far longer than other processes such as the transit time and the
carrier lifetime and the system can be considered to be in
equilibrium at all times during the pulse. The penetration of light
into the diamond at this wavelength is only a few microns.
Relatively low light intensity is used (about 0,1 W/cm.sup.2), so
that N.sub.0 is relatively low and the internal field is then
reasonably approximated by the applied field. The applied field is
kept below the threshold above which mobility becomes field
dependent. The applied field is also kept below the value above
which a significant proportion of the charge carriers reach the far
side of the diamond and the total charge collected shows saturation
(with blocking contacts; non-blocking contacts can show gain at
this point).
[0030] (iv) The .mu..tau. product is derived by relating the
collected charge to the applied voltage using the Hecht
relation.
Q=N.sub.0e.mu..tau.E/D[1-exp{-D/(.mu..tau.E)})]
[0031] In this equation Q is the charge collected at the
non-illuminated contact, N.sub.0 the total number of electron hole
pairs generated by the light pulse, E the applied electric field, D
the sample thickness, and .mu..tau. is the mobility and lifetime
product to be determined.
[0032] (v) As an example, if the illuminated electrode is the anode
(cathode), then the charge carriers are generated within a few
.mu.m of the surface, and the charge displacement of the electrons
(holes) to the nearby electrode is negligible. In contrast, the
charge displacement of the holes (electrons) towards the opposing
contact is significant, and limited by the .mu..tau. product, where
both .mu. and .tau. are specific to the particular charge carriers
moving towards the non-irradiated electrode.
[0033] c) High Collection Distance
[0034] The single crystal CVD diamond of the invention will have a
high collection distance and typically a collection distance of
greater than 150 .mu.m, preferably greater than 400 .mu.m and more
preferably greater than 600 .mu.m, all distances being at 1V/.mu.m
applied field and 300 K.
[0035] Collection distance and its determination are known in the
art. Radiation such as UV, X-rays and gamma rays impinging on
diamond can form electron hole pairs which drift under an applied
voltage between electrodes. Typically, for penetrating radiation
such as beta and gamma rays the electrodes are placed on opposite
surfaces of a diamond layer whose thickness is typically 200-700
.mu.m but can range from less than 100 .mu.m to greater than 1000
.mu.m and the charge carriers (electrons/holes) drift through the
thickness of the layer. For highly absorbed radiation which
penetrates only a few .mu.m into the diamond, such as alpha
radiation or UV radiation with energies near or above that of the
band gap, then inter-digitated electrode arrangements on the same
face of the diamond layer may be used; this face may be planar or
with the electrodes placed in relationship to surface structures
such as grooves.
[0036] However, the electrons and holes have finite mobilities and
lifetimes so they move only a certain distance before recombining.
When an event occurs (e.g. impingement of a beta particle) which
forms charge carriers, then to first order the total signal from
the detector depends on the average distance moved by the charge
carriers. This charge displacement is a product of the carrier
mobility and the applied electric field (which gives the charge
drift velocity) and the recombination lifetime of the carriers
before trapping or recombination stops its drift. This is the
collection distance, which can also be considered as the volume of
charge swept to the electrode. The purer the diamond (or the lower
the level of uncompensated traps) or the lower the level of
crystalline imperfections, then the higher the mobility of the
carriers and/or their lifetimes. The collection distance measured
is generally limited by the thickness of the sample under test; if
the collection distance measurement exceeds about 80% of the sample
thickness, then the measured value is likely to be a lower limit
rather than the actual value.
[0037] The collection distances referred to in this specification
were determined by the following procedure:
[0038] 1) Ohmic spot contacts are placed on either side of the
layer under test. This layer is typically 300-700 .mu.m thick and
5-10 mm square, allowing spot contacts of 2-6 mm diameter.
Formation of ohmic contacts (rather than contacts showing diode
behaviour) is important for a reliable measurement. This can be
achieved in several ways but typically the procedure is as
follows:
[0039] i) the surface of the diamond is oxygen terminated, using
for example, an oxygen plasma ash, minimising the surface
electrical conduction (reducing the `dark current` of the
device);
[0040] ii) a metallisation consisting of first a carbide former
(e.g. Ti, Cr) and then a thicker layer of protective material,
typically Au (to which a wire bond can be made), is deposited onto
the diamond by sputtering, evaporation or similar method. The
contact is then typically annealed between 400-600.degree. C. for
up to about an hour.
[0041] 2) Wire bonds to the contacts are made, and the diamond
connected in a circuit, with a bias voltage of typically 2-10
kV/cm. The `dark current` or leakage current is characterised, and
in a good sample should be less than 5 nA, and more typically less
than 100 pA at 2,5 kV/cm, using 3 mm diameter spot contacts.
[0042] 3) The collection distance measurement is made by exposing
the sample to beta radiation, with a Si trigger detector on the
exit face to a) indicate that an event has occurred, and b) ensure
that the beta particle was not stopped within the diamond film
which would result in a much larger number of charge carriers being
formed. The signal from the diamond is then read by a high gain
charge amplifier, and, based on the known formation rate of charge
carriers of about 36 electron/hole pairs per linear .mu.m traversed
by the beta particle, the collection distance can be calculated
from the charge measured by the equation:
CCD=CCE.times.t
[0043] where t=sample thickness
[0044] CCE=charge collection efficiency=charge collected/total
charge generated.
[0045] CCD charge collection distance.
[0046] 4) For completeness, the collection distance is measured for
a range of values of applied bias voltage, both forward and
reverse, and the characteristic collection distance quoted at bias
voltages of 10 kV/cm only for samples which show a well behaved
linear behaviour for bias voltages up to 10 kV/cm bias. In
addition, the entire measurement procedure is repeated several
times to ensure repeatability of behaviour, as values measured on
poorer samples can degrade with time and treatment history.
[0047] 5) A further issue in measurement of the collection distance
is whether the material is in the pumped or unpumped state.
`Pumping` (also called `priming`) the material comprises of
exposing it to certain types of radiation (beta, gamma etc.) for a
sufficient period, when the collection distance measured can rise,
typically by a factor of 1,6 in polycrystalline CVD diamond
although this can vary. The effect of priming is generally lower in
high purity single crystal; priming by factors of 1,05-1,2 is
common with no measurable priming in some samples. De-pumping can
be achieved by exposing to sufficiently strong white light or light
of selected wavelengths, and the process is believed to be wholly
reversible. The collection distances referred to in this
specification are all in the unpumped state whatever the final
application of the material. In certain applications (e.g. high
energy particle physics experiments), the increase in collection
distance associated with pumping can be used beneficially to
enhance the detectability of individual events, by shielding the
detector from any de-pumping radiation. In other applications, the
instability in device gain associated with pumping is severely
deleterious.
[0048] d) Electron Mobility
[0049] The single crystal CVD diamond of the invention also has a
high electron mobility (.mu..sub.e) and more particularly, an
electron mobility measured at 300 K greater than 2400
cm.sup.2V.sup.-1s.sup.-1, and preferably greater than 3000
cm.sup.2V.sup.-1s.sup.-1, and more preferably greater than 4000
cm.sup.2V.sup.-1s.sup.-1. In high quality type IIa natural diamond
electron mobilities at 300 K are reported typically to be 1800
cm.sup.2V.sup.-1s.sup.-1 with exceptional values reported up to
2200 cm.sup.2V.sup.-1s.sup.-1.
[0050] e) Hole Mobility
[0051] The single crystal CVD diamond of the invention also has a
high hole mobility (.mu..sub.n) and more particularly, a hole
mobility measured at 300 K greater than 2100
cm.sup.2V.sup.-1s.sup.-1, and preferably greater than 2500
cm.sup.2V.sup.-1s.sup.-1, and more preferably greater than 3000
cm.sup.2V.sup.-1s.sup.-1. In high quality type IIa natural diamond
hole mobilities at 300 K are reported typically to be 1200
cm.sup.2V.sup.-1s.sup.-1 with exceptional values reported up to
1900 cm.sup.2V.sup.-1s.sup.-1.
[0052] The characteristics described above are observable in the
majority volume of the diamond. There may be portions of the
volume, generally less than 10 percent by volume, where the
particular characteristic is not observable.
[0053] The single crystal CVD diamond of the invention has
particular application in electronic applications and more
particularly as a detector element or switching element. The high
breakdown voltage in the off state of the diamond makes it
particularly suitable as a component in an opto-electric switch.
The use of the diamond in these applications forms another aspect
of the invention.
[0054] The novel single crystal CVD diamond of the invention may be
made by a method which forms yet another aspect of the invention.
This method includes the steps of providing a diamond substrate
having a surface which is substantially free of crystal defects,
providing a source gas, dissociating the source gas and allowing
homoepitaxial diamond growth on the surface which is substantially
free of crystal defects in an atmosphere which contains less than
300 parts per billion nitrogen.
DESCRIPTION OF EMBODIMENTS
[0055] In addition to the characteristics described above, the
single crystal CVD diamond of the invention may have one or more of
the following characteristics:
[0056] 1. A level of any single impurity of not greater than 5 ppm
and a total impurity content of not greater than 10 ppm. Preferably
the level of any impurity is not greater than 0,5 to 1 ppm and the
total impurity content is not greater than 2 to 5 ppm. Impurity
concentrations can be measured by secondary ion mass spectroscopy
(SIMS), glow discharge mass spectroscopy (GDMS), combustion mass
spectroscopy (CMS), electron paramagnetic resonance (EPR) and IR
absorption, and in addition for single substitutional nitrogen by
optical absorption measurements at 270 nm (calibrated against
standard values obtained from samples destructively analysed by
combustion analysis). In the above, "impurity" excludes hydrogen
and its isotopic forms.
[0057] 2. A cathodoluminescence (CL) emission signal in the 575 nm
band which is low or absent, and an associated photoluminescence
(PL) line, measured at 77 K under 514 nm Ar ion laser excitation
(nominally 300 mW incident beam) which has a peak height
<{fraction (1/1000)} of the diamond Raman peak at 1332
cm.sup.-1. These bands are related to nitrogen/vacancy defects and
their presence indicates the presence of nitrogen in the film. Due
to the possible presence of competing quenching mechanisms, the
normalised intensity of the 575 nm line is not a quantitative
measure of nitrogen nor is its absence a definitive indication of
the absence of nitrogen in the film. CL is the luminescence
resulting from excitation by electron beam at a typical beam energy
of 10 to 40 keV which penetrates about 30 nm to 10 microns into the
sample surface. Photoluminescence is more generally excited through
the sample volume.
[0058] 3. i) A uniform strong free exciton (FE) peak at 235 nm in
the CL spectrum collected at 77 K. The presence of a strong free
exciton peak indicates the substantial absence of defects such as
dislocations and impurities. The link between low defect and
impurity densities and high FE has been previously reported for
individual crystals in polycrystalline CVD diamond synthesis.
[0059] ii) Strong free exciton emission in the room temperature
UV-excited photoluminescence spectrum.
[0060] Free exciton emission can also be excited by above-bandgap
radiation for example by 193 nm radiation from an ArF excimer
laser. The presence of strong free exciton emission in the
photoluminescence spectrum excited in this way indicates the
substantial absence of dislocations and impurities. The strength of
free exciton emission excited by 193 nm ArF excimer laser at room
temperature is such that the quantum yield for free exciton
emission is at least 10.sup.-5.
[0061] 4. In electron paramagnetic resonance (EPR), a single
substitutional nitrogen centre [N-C].degree. at a concentration
<40 ppb and more typically <10 ppb indicating low levels of
nitrogen incorporation.
[0062] 5. In EPR, a spin density <1.times.10.sup.17 cm.sup.-3
and more typically <5.times.10.sup.16 cm.sup.-3 at g=2,0028. In
single crystal diamond this line at g=2.0028 is related to lattice
defect concentrations and is typically large in natural type Ha
diamond, in CVD diamond plastically deformed through indentation,
and in poor quality homoepitaxial diamond.
[0063] 6. Excellent optical properties having a UV/Visible and IR
(infrared) transparency close to the theoretical maximum for
diamond and, more particularly, low or absent single substitutional
nitrogen absorption at 270 nm in the UV, and low or absent C--H
stretch bonds in the spectral range 2500 to 3400 cm.sup.-1
wavenumbers in the IR.
[0064] The CVD diamond of the invention may be attached to a
diamond substrate (whether the substrate is synthetic, natural, or
CVD diamond). Advantages of this approach include providing a
greater overall thickness where the thickness limits the
application or providing support for a CVD diamond whose thickness
has been reduced by processing. In addition, the CVD diamond of
this invention may form one layer in a multilayer device, where
other diamond layers may, for example, be doped to provide
electrical contact or electronic junctions to the CVD diamond, or
merely be present to provide support to the CVD diamond.
[0065] It is important for the production of high quality single
crystal CVD diamond that growth takes place on a diamond surface
which is substantially free of crystal defects. In this context,
defects primarily means dislocations and microcracks, but also
includes twin boundaries, point defects, low angle boundaries and
any other disruption to the crystal lattice. Preferably the
substrate is a low birefringence type Ia or IIb natural, Ib or IIa
high pressure/high temperature synthetic diamond or a CVD
synthesised single crystal diamond.
[0066] The defect density is most easily characterised by optical
evaluation after using a plasma or chemical etch optimised to
reveal the defects (referred to as a revealing plasma etch), using
for example a brief plasma etch of the type described below. Two
types of defects can be revealed:
[0067] 1) Those intrinsic to the substrate material quality. In
selected natural diamond the density of these defects can be as low
as 50/mm.sup.2 with more typical values being 10.sup.2/mm.sup.2,
whilst in others it can be 10.sup.6/mm.sup.2 or greater.
[0068] 2) Those resulting from polishing, including dislocation
structures and microcracks in the form of `chatter tracks` along
polishing lines. The density of these can vary considerably over a
sample, with typical values ranging from about 10.sup.2/mm.sup.2,
up to more than 10.sup.4/mm.sup.2 in poorly polished regions or
samples.
[0069] The preferred low density of defects is thus such that the
density of surface etch features related to defects, as described
above, is below 5.times.10.sup.3/mm.sup.2, and more preferably
below 10.sup.2/mm.sup.2.
[0070] The defect level at and below the substrate surface on which
the CVD growth takes place may thus be minimised by careful
preparation of the substrate. Here preparation includes any process
applied to the material from mine recovery (in the case of natural
diamond) or synthesis (in the case of synthetic material) as each
stage can influence the defect density within the material at the
plane which will ultimately form the substrate surface when
processing to form a substrate is complete. Particular processing
steps may include conventional diamond processes such as mechanical
sawing, lapping and polishing, and less conventional techniques
such as laser processing or ion implantation and lift off
techniques, chemical/mechanical polishing, and both liquid and
plasma chemical processing techniques. In addition, the surface
R.sub.A (arithmetic mean of the absolute deviation of surface
profile from the mean line measured by stylus profilometer,
preferably measured over 0,08 mm length) should be minimised,
typical values prior to any plasma etch being no more than a few
nanometers, i.e. less than 10 nanometers.
[0071] One specific method of minimising the surface damage of the
substrate, is to include an in situ plasma etch on the surface on
which the homoepitaxial diamond growth is to occur. In principle
this etch need not be in situ, nor immediately prior to the growth
process, but the greatest benefit is achieved if it is in situ,
because it avoids any risk of further physical damage or chemical
contamination. An in situ etch is also generally most convenient
when the growth process is also plasma based. The plasma etch can
use similar conditions to the deposition or diamond growing
process, but with the absence of any carbon containing source gas
and generally at a slightly lower temperature to give better
control of the etch rate. For example, it can consist of one or
more of:
[0072] (i) an oxygen etch using predominantly hydrogen with
optionally a small amount of Ar and a required small amount of
O.sub.2. Typical oxygen etch conditions are pressures of
50-450.times.10.sup.2 Pa, an etching gas containing an oxygen
content of 1 to 4 percent, an argon content of 0 to 30 percent and
the balance hydrogen, all percentages being by volume, with a
substrate temperature 600-1100.degree. C. (more typically
800.degree. C.) and a typical duration of 3-60 minutes.
[0073] (ii) a hydrogen etch which is similar to (i) but where the
oxygen is absent.
[0074] (iii) alternative methods for the etch not solely based on
argon, hydrogen and oxygen may be used, for example, those
utilising halogens, other inert gases or nitrogen.
[0075] Typically the etch consists of an oxygen etch followed by a
hydrogen etch and then the process moves directly into synthesis by
the introduction of the carbon source gas. The etch
time/temperature is selected to enable any remaining surface damage
from processing to be removed, and for any surface contaminants to
be removed, but without forming a highly roughened surface and
without etching extensively along extended defects (such as
dislocations) which intersect the surface and thus cause deep pits.
As the etch is aggressive, it is particularly important for this
stage that the chamber design and material selection for its
components be such that no material from the chamber is transferred
by the plasma into the gas phase or to the substrate surface. The
hydrogen etch following the oxygen etch is less specific to crystal
defects rounding off the angularities caused by the oxygen etch
(which aggressively attacks such defects) and provides a smoother,
better surface for subsequent growth.
[0076] The surface or surfaces of the diamond substrate on which
the CVD diamond growth occurs are preferably the {100}, {110},
{113} or {111} surfaces. Due to processing constraints, the actual
sample surface orientation can differ from these ideal orientations
by up to 5.degree., and in some cases up to 10.degree., although
this is less desirable as it adversely affects reproducibility.
[0077] It is also important in the method of the invention that the
impurity content of the environment in which the CVD growth takes
place is properly controlled. More particularly, the diamond growth
must take place in the presence of an atmosphere containing
substantially no nitrogen, i.e. less than 300 parts per billion
(ppb, as a molecular fraction of the total gas volume), and
preferably less than 100 parts per billion. The role of nitrogen in
the synthesis of CVD diamond, particularly polycrystalline CVD
diamond, has been reported in the literature. For example, it has
been noted in these reports that gas phase nitrogen levels of 10
parts per million or greater modify the relative growth rates
between the {100} and the {111} faces with an overall increase in
growth rate, and in some cases crystal quality. Further, it has
been suggested that for certain CVD diamond synthesis processes,
low nitrogen contents of below a few parts per million may be used.
However, none of these reported processes disclose methods of
nitrogen analysis which are sufficiently sensitive to ensure that
the nitrogen content is substantially below 1 part per million, and
in the region of 300 or less parts per billion. Measurement of
nitrogen levels of these low values requires sophisticated
monitoring such as that which can be achieved, for example, by gas
chromatography. An example of such a method is now described:
[0078] (1) Standard gas chromatography (GC) art consists of: A gas
sample stream is extracted from the point of interest using a
narrow bore sample line, optimised for maximum flow velocity and
minimum dead volume, and passed through the GC sample coil before
being passed to waste. The GC sample coil is a section of tube
coiled up with a fixed and known volume (typically 1 cm.sup.3 for
standard atmospheric pressure injection) which can be switched from
its location in the sample line into the carrier gas (high purity
He) line feeding into the gas chromatography columns. This places a
sample of gas of known volume into the gas flow entering the
column; in the art, this procedure is called sample injection.
[0079] The injected sample is carried by the carrier gas through
the first GC column (filled with a molecular sieve optimised for
separation of simple inorganic gases) and is partially separated,
but the high concentration of primary gases (e.g. H.sub.2, Ar)
causes column saturation which makes complete separation of the
nitrogen difficult. The relevant section of the effluent from the
first column is then switched into the feed of a second column,
thereby avoiding the majority of the other gases being passed into
the second column, avoiding column saturation and enabling complete
separation of the target gas (N.sub.2). This procedure is called
"heart-cutting".
[0080] The output flow of the second column is put through a
discharge ionisation detector (DID), which detects the increase in
leakage current through the carrier gas caused by the presence of
the sample. Chemical identity is determined by the gas residence
time which is calibrated from standard gas mixtures. The response
of the DID is linear over more than 5 orders of magnitude, and is
calibrated by use of special calibrated gas mixtures, typically in
the range of 10-100 ppm, made by gravimetric analysis and then
verified by the supplier. Linearity of the DID can be verified by
careful dilution experiments.
[0081] (2) This known art of gas chromatography has been further
modified and developed for this application as follows: The
processes being analysed here are typically operating at
50-500.times.10.sup.2 Pa. Normal GC operation uses the excess
pressure over atmospheric pressure of the source gas to drive the
gas through the sample line. Here, the sample is driven by
attaching a vacuum pump at the waste end of the line and the sample
drawn through at below atmospheric pressure. However, whilst the
gas is flowing the line impedance can cause significant pressure
drop in the line, affecting calibration and sensitivity.
Consequently, between the sample coil and the vacuum pump is placed
a valve which is shut for a short duration before sample injection
in order to enable the pressure at the sample coil to stabilise and
be measured by a pressure gauge. To ensure a sufficient mass of
sample gas is injected, the sample coil volume is enlarged to about
5 cm.sup.3. Dependent on the design of the sample line, this
technique can operate effectively down to pressures of about
70.times.10.sup.2 Pa. Calibration of the GC is dependent on the
mass of sample injected, and the greatest accuracy is obtained by
calibrating the GC using the same sample pressure as that available
from the source under analysis. Very high standards of vacuum and
gas handling practice must be observed to ensure that the
measurements are correct.
[0082] The point of sampling may be upstream of the synthesis
chamber to characterise the incoming gases, within the chamber to
characterise the chamber environment, or downstream of the chamber
to measure a worst case value of the nitrogen concentration within
the chamber.
[0083] The source gas may be any known in the art and will contain
a carbon-containing material which dissociates producing radicals
or other reactive species. The gas mixture will also generally
contain gases suitable to provide hydrogen or a halogen in atomic
form.
[0084] The dissociation of the source gas is preferably carried out
using microwave energy in a reactor examples of which are known in
the art. However, the transfer of any impurities from the reactor
should be minimised. A microwave system may be used to ensure that
the plasma is placed away from all surfaces except the substrate
surface on which diamond growth is to occur and its mount. Examples
of preferred mount materials are: molybdenum, tungsten, silicon and
silicon carbide. Examples of preferred reactor chamber materials
are stainless steel, aluminium, copper, gold, platinum.
[0085] A high plasma power density should be used, resulting from
high microwave power (typically 3-60 kW, for substrate diameters of
50-150 mm) and high gas pressures (50-500.times.10.sup.2 Pa, and
preferably 100-450.times.10.sup.2 Pa).
[0086] Using the above conditions it has been possible to produce
high quality single crystal CVD diamond layers with a value for the
product of mobility and lifetime, .mu..tau., in excess of
1,5.times.10.sup.-6 cm.sup.2N, e.g. 320.times.10.sup.-6 cm.sup.2N
for electrons and 390.times.10.sup.-6 cm.sup.2N for holes.
[0087] An example of the invention will now be described.
EXAMPLE 1
[0088] Substrates suitable for synthesising single crystal CVD
diamond of the invention may be prepared as follows:
[0089] i) Selection of stock material (Ia natural stones and Ib
HPHT stones) was optimised on the basis of microscopic
investigation and birefringence imaging to identify substrates
which were free of strain and imperfections.
[0090] ii) Laser sawing, lapping and polishing processes were used
to minimise subsurface defects using a method of a revealing plasma
etch to determine the defect levels being introduced by the
processing.
[0091] iii) It was possible routinely to produce substrates in
which the density of defects measurable after a revealing etch is
dependent primarily on the material quality and is below
5.times.10.sup.3/mm.sup.2, and generally below 10.sup.2/mm.sup.2.
Substrates prepared by this process are then used for the
subsequent synthesis.
[0092] A high temperature/high pressure synthetic type Ib diamond
was grown in a high pressure press, and then prepared as a
substrate using the method described above to minimise substrate
defects. In finished form the substrate was a plate 5.times.5 mm
square by 500 .mu.m thick, with all faces {100}. The surface
roughness at this stage was less than 1 nm R.sub.A. The substrate
was mounted on a tungsten substrate using a high temperature braze
suitable for diamond. This was introduced into a reactor and an
etch and growth cycle commenced as described above, and more
particularly:
[0093] 1) The reactor was pre-fitted with point of use purifiers,
reducing nitrogen levels in the incoming gas stream to below 80
ppb, as determined by the modified GC method described above.
[0094] 2) An in situ oxygen plasma etch was performed using
30/150/1200 sccm (standard cubic centimetre per second) of
O.sub.2/Ar/H.sub.2 at 333.times.10.sup.2 Pa and a substrate
temperature of 800.degree. C.
[0095] 3) This moved without interruption into a hydrogen etch with
the removal of the O.sub.2 from the gas flow.
[0096] 4) This moved into the growth process by the addition of the
carbon source which in this instance was CH.sub.4 flowing at 30
sccm. The growth temperature at this stage was 980.degree. C.
[0097] 5) The atmosphere in which the growth took place contained
less than 100 ppb nitrogen, as determined by the modified GC method
described above.
[0098] 6) On completion of the growth period, the substrate was
removed from the reactor and the CVD diamond layer removed from the
substrate.
[0099] 7) This layer was then polished flat to a 410 .mu.m thick
layer, cleaned and oxygen ashed to produce a surface terminated by
O and tested for its charge collection distance. This was found to
be 380 .mu.m at an applied field of 1 V/.mu.m (a value invariably
limited by the sample thickness) giving a lower limit for the
product of mobility and lifetime, .mu..tau., of 3,8.times.10.sup.-6
cm.sup.2N.
[0100] 8) The resistivity of the diamond layer, in the off state,
was found to be 6.times.10.sup.14 .OMEGA.cm when measured at
20.degree. C. at an applied field of 50 V/.mu.m.
[0101] 9) The layer, identified as HDS-1, was further characterised
by the data provided below and in the attached FIGS. 1 to 8:
[0102] i) The CL spectra showing low blue band, low 575 nm and high
FE emissions (FIGS. 1 and 2).
[0103] ii) EPR spectra, showing low substitutional nitrogen and a
weak g=2,0028 line (FIGS. 3 to 5).
[0104] iii) Optical spectra showing the near theoretical
transmission (FIG. 6).
[0105] iv) X-ray rocking curves map, showing the angular spread of
the sample to be less than 10 arc sec (FIG. 7).
[0106] v) Raman spectrum showing a line width (FWHM) to be about 2
cm.sup.-1 (FIG. 8).
EXAMPLE 2
[0107] The procedure set out in Example 1 was repeated with the
following variation in conditions:
[0108] Ar 75 sccm, H.sub.2 600 sccm, CH.sub.4 30 sccm, 820.degree.
C., 7,2 kW, less than 200 ppb nitrogen, as measured by the modified
GC method described above.
[0109] The CVD diamond layer produced was processed to a layer 390
.mu.m thick for testing. The .mu..tau. product was found to be
320.times.10.sup.-6 cm.sup.2/V for electrons and
390.times.10.sup.-6 cm.sup.2N for holes (measured at 300 K), giving
a mean value of 355.times.10.sup.-6 cm.sup.2/V.
EXAMPLE 3
[0110] The procedure set out in Example 1 was further repeated with
the following variation in conditions:
[0111] Ar 150 sccm, H.sub.2 1200 sccm, CH.sub.4 30 sccm,
237.times.10.sup.2 Pa and a substrate temperature of 822.degree.
C., less than 100 ppb nitrogen, as measured by the modified GC
method described above.
[0112] The CVD diamond layer produced was processed to a layer 420
.mu.m thick for testing. The collection distance of the layer was
measured to be >400 .mu.m. The resistivity the layer at an
applied field of 50V/.mu.m exceeded 1.times.10.sup.14
.OMEGA.cm.
EXAMPLE 4
[0113] The procedure set out in Example 1 was further repeated with
the following variations in conditions:
[0114] The oxygen plasma etch used 15/75/600 sccm of
O.sub.2/Ar/H.sub.2. This was followed by a hydrogen etch using
75/600 sccm Ar/H.sub.2. Growth was initiated by the addition of the
carbon source which in this instance was CH.sub.4 flowing at 30
sccm. The growth temperature at this stage was 780.degree. C.
[0115] The CVD diamond layer produced had a thickness of 1500
.mu.m, and was processed into a layer 500 .mu.m thick for
testing.
[0116] 1) The charge collection distance was found to be 480 .mu.m
at an applied field of 1 V/.mu.m and 300 K, (a value limited by the
sample thickness) giving a lower limit to the product of mobility
and lifetime, .mu..tau., 4,8.times.10.sup.-6 cm.sup.2/V.
[0117] 2) The .mu..tau. product measured at 300 K using the Hecht
relationship, as described above, was 1,7.times.10.sup.-3
cm.sup.2/V and 7,2.times.10.sup.-4 cm.sup.2/V for electrons and
holes respectively.
[0118] 3) A space charge limited time of flight experiment measured
the electron mobility, .mu..sub.e to be 4400 cm.sup.2/Vs at a
sample temperature of 300 K.
[0119] 4) A space charge limited time of flight experiment measured
.mu..sub.h, the hole mobility, to be 3800 cm.sup.2/Vs at a sample
temperature of 300 K.
[0120] 5) SIMS measurements showed that there is no evidence for
any single defect present in concentrations above 5.times.10.sup.16
cm.sup.-3 (excluding H and its isotopes).
[0121] 6) The measured resistivity was in excess of
5.times.10.sup.14 .OMEGA.cm at an applied voltage of 100 V/.mu.m as
measured at 300 K.
[0122] 7) The PL spectrum showed low blue band and low 575 nm
intensity (<{fraction (1/1000)} of Raman peak). The Raman FWHM
line width was 1.5 cm.sup.-1. The CL spectrum showed a strong FE
peak.
[0123] 8) EPR spectra showed no (<7 ppb) substitutional
nitrogen, and no (<10 ppb) g=2,0028 line.
FURTHER EXAMPLES
[0124] The procedure set out in Example 4 was repeated several
times to produce free standing high quality high purity single
crystal CVD layers with thicknesses ranging from 50-3200 .mu.m.
[0125] Various properties of the diamond were measured (at 300 K)
and the results thereof are set out in the table. The dielectric
breakdown voltage of the samples exceeded 100 V/.mu.m.
1 Plate Resistivity Example Thickness CCD .mu..sub.e.tau..sub.e
.mu..sub.h.tau..sub.h .mu..sub.e .mu..sub.h (.OMEGA. cm) at (Sample
ID) (.mu.m) (.mu.m) (cm.sup.2/V) (cm.sup.2/V) (cm.sup.2/Vs)
(cm.sup.2/Vs) 50 V/.mu.m Ex 1 (HDS-1) 410 >380* 6 .times.
10.sup.14 Ex 2 390 3.2 .times. 10.sup.-4 3.9 .times. 10.sup.-4 Ex 3
420 >400* >1 .times. 10.sup.14 Ex 4 500 >480* 1.7 .times.
10.sup.-3 7.2 .times. 10.sup.-4 4400 3800 >5 .times. 10.sup.14
Ex 5 700 >650* 1.7 .times. 10.sup.-3 6.5 .times. 10.sup.-4 3900
3800 >1 .times. 10.sup.14 Ex 6 1000 3.3 .times. 10.sup.-3 1.4
.times. 10.sup.-3 4000 3800 >5 .times. 10.sup.12 *Minimum value,
limited by sample thickness
BRIEF DESCRIPTION OF FIGURES
[0126] FIG. 1: Free exciton cathodoluminescence spectrum of HDS-1
at 77 K, showing strong emission at 235 nm (transverse optic
mode).
[0127] FIG. 2: Cathodoluminescence spectrum (77 K) of HDS-1,
showing a broad weak band centred at approximately 420 nm, very
weak lines at 533 nm and 575 nm and very intense free exciton
emission (shown in second order at 470 nm).
[0128] FIG. 3: Room temperature EPR spectra of (1) homoepitaxial
CVD diamond containing approximately 0,6 ppm of single
substitutional nitrogen and (2) HDS-1. The spectra were measured
under the same conditions and the samples were approximately the
same size.
[0129] FIG. 4: EPR spectra recorded at 4,2 K of (i) high purity
homoepitaxial CVD diamond grown simultaneously with HDS-1 which was
plastically deformed after growth to demonstrate the influence on
the EPR signal of structural defects created by indentation and
(ii) HDS-1. The spectra were measured under the same
conditions.
[0130] FIG. 5: Room temperature EPR spectra of type IIa natural
diamond and HDS-1. The spectra were measured under the same
conditions and the samples were of the same size.
[0131] FIG. 6: Room temperature ultraviolet absorption spectrum of
HDS-1, showing the intrinsic absorption edge and the absence of the
absorption band centred at 270 nm attributed to single
substitutional nitrogen.
[0132] FIG. 7: Double axis X-ray rocking curve of HDS-1.
[0133] FIG. 8: Raman spectrum of HDS-1 measured at 300 K using the
488 nm line of an argon ion laser.
* * * * *