U.S. patent application number 12/523968 was filed with the patent office on 2010-02-18 for diamond electronic devices and methods for their manufacture.
Invention is credited to Richard Stuart Balmer, Ian Friel, Geoffrey Alan Scarsbrook.
Application Number | 20100038653 12/523968 |
Document ID | / |
Family ID | 39644161 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100038653 |
Kind Code |
A1 |
Scarsbrook; Geoffrey Alan ;
et al. |
February 18, 2010 |
DIAMOND ELECTRONIC DEVICES AND METHODS FOR THEIR MANUFACTURE
Abstract
The present invention relates to a diamond electronic device
comprising a functional interface between two solid materials,
wherein the interface is formed by a planar first surface of a
first layer of single crystal diamond and a second layer formed on
the first surface of the first diamond layer, the second layer
being solid, non-metallic and selected from diamond, a polar
material and a dielectric material, and wherein the planar first
surface of the first layer of single crystal diamond has an Rq of
less than 10 nm and has at least one of the following
characteristics: (a) the first surface is an etched surface; (b) a
density of dislocations in the first diamond layer breaking the
first surface is less than 400 cm.sup.-2 measured over an area
greater than 0.014 cm.sup.2; (c) a density of dislocations in the
second layer breaking a notional or real surface lying within the
second layer parallel to the interface and within 50 .mu.m of the
interface is less than 400 cm.sup.-2 measured over an area greater
than 0.014 cm.sup.2; and (d) the first surface has an R.sub.q less
than 1 nm.
Inventors: |
Scarsbrook; Geoffrey Alan;
(Berkshire, GB) ; Friel; Ian; (Berkshire, GB)
; Balmer; Richard Stuart; (Berkshire, GB) |
Correspondence
Address: |
BRYAN CAVE LLP
211 NORTH BROADWAY, SUITE 3600
ST. LOUIS
MO
63102-2750
US
|
Family ID: |
39644161 |
Appl. No.: |
12/523968 |
Filed: |
January 22, 2008 |
PCT Filed: |
January 22, 2008 |
PCT NO: |
PCT/IB08/50219 |
371 Date: |
August 28, 2009 |
Current U.S.
Class: |
257/77 ;
257/E21.049; 257/E29.068; 438/105 |
Current CPC
Class: |
G01N 2201/0636 20130101;
H01J 37/321 20130101; G01N 21/87 20130101; H01L 21/02104 20130101;
H01L 29/1602 20130101; H01L 29/167 20130101; G01N 27/308 20130101;
H01L 29/36 20130101; C23C 16/27 20130101; C23C 16/278 20130101;
C30B 25/20 20130101; Y10T 428/24355 20150115; H01J 2237/3341
20130101; C30B 25/105 20130101; C30B 29/04 20130101; H01L 29/045
20130101; H01L 21/041 20130101; H01L 21/02579 20130101; H01L
21/02527 20130101; C01B 32/28 20170801; H01L 21/0262 20130101; C23C
16/274 20130101; H01J 2237/08 20130101; H01L 21/02376 20130101;
H01L 21/02634 20130101; G01N 21/95 20130101 |
Class at
Publication: |
257/77 ; 438/105;
257/E21.049; 257/E29.068 |
International
Class: |
H01L 29/12 20060101
H01L029/12; H01L 21/04 20060101 H01L021/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 22, 2007 |
GB |
0 701 186.9 |
Mar 22, 2007 |
GB |
0 705 523.9 |
Mar 22, 2007 |
GB |
0 705 524.7 |
May 21, 2007 |
GB |
0 709 716.5 |
Jul 11, 2007 |
GB |
0 713 464.6 |
Claims
1. A diamond electronic device comprising a functional interface
between two solid materials, wherein the interface is formed by a
planar first surface of a first layer of single crystal diamond and
a second layer formed on the first surface of the first diamond
layer, the second layer being solid, non-metallic and selected from
diamond, a polar material and a dielectric material, and wherein
the planar first surface of the first layer of single crystal
diamond has a surface roughness R.sub.q of less than 10 nm and has
at least one of the following characteristics: (a) the first
surface is an etched surface; (b) a density of dislocations in the
first diamond layer breaking the first surface is less than 400
cm.sup.-2 measured over an area greater than 0.014 cm.sup.2; (c) a
density of dislocations in the second layer breaking a notional or
real surface lying within the second layer parallel to the
interface and within 50 .mu.m of the interface is less than 400
cm.sup.-2 measured over an area greater than 0.014 cm.sup.2; and
(d) the first surface has an R.sub.q less than 1 nm.
2. A diamond electronic device according to claim 1, wherein the
planar first surface of the first layer of single crystal diamond
is a mechanically processed surface.
3. A diamond electronic device comprising a functional interface
between two solid materials, wherein the interface is formed by a
planar first surface of a first layer of single crystal diamond
wherein the planar first surface has been mechanically processed,
and a second layer formed on the first surface of the first diamond
layer, the second layer being solid, non-metallic and selected from
diamond, a polar material and a dielectric material, and wherein
the planar first surface of the first layer of single crystal
diamond has a surface roughness R.sub.q of less than 10 nm and
wherein the planar surface of the first layer of single crystal
diamond is substantially free of residual damage due to mechanical
processing.
4. An electronic device according to claim 2, wherein the number
density of defects revealed by a revealing etch in the functional
planar surface is less than 100 per mm.sup.2
5. A diamond electronic device according to claim 1, wherein the
first surface of the first diamond layer is an etched surface.
6. A diamond electronic device according to claim 5 where the
etched first surface is an isotropically etched surface.
7. An electronic device according to claim 6, wherein the first
surface has been isotropically etched using a gas mixture
comprising a halogen and an inert gas.
8. An electronic device according to claim 6, wherein the etch
removed at least 0.2 .mu.m from the first surface of the first
diamond layer.
9. An electronic device according to claim 8, wherein the etch
removed at least 0.2.times. the largest grit particle size used in
the last stage of lapidary processing.
10-16. (canceled)
17. A diamond electronic device according to claim 1 wherein the
first surface of the diamond layer is a surface of a diamond layer
grown on a single crystal diamond layer.
18. A diamond electronic device according to claim 17 wherein the
grown diamond layer has a thickness of less than 20 microns.
19. A diamond electronic device according to claim 1 wherein the
first surface of the first diamond layer is substantially free from
damage introduced by post-growth mechanical processing of an
as-grown surface to a depth of at least 1 nm.
20-24. (canceled)
25. A diamond electronic device according to claim 1 wherein
diamond on one side of the interface is intrinsic diamond and the
diamond on the other side of the interface is boron-doped
diamond.
26-31. (canceled)
32. A method for producing a diamond electronic device comprising:
(i) providing a diamond layer having a thickness of greater than
about 20 .mu.m; (ii) preparing a first surface of the diamond layer
by mechanical means to a have a surface roughness R.sub.q of less
than about 10 nm; (iii) etching the first surface of the diamond
layer to form a planar first surface having a surface roughness
R.sub.q of less than about 10 nm; and (iv) forming a second layer
on the planar first surface of the diamond layer to form a
functional interface between the diamond layer and the second
layer, wherein the second layer is solid, non-metallic and selected
from diamond, a polar material and a dielectric material.
33. A method for producing a diamond electronic device comprising:
(i) providing a diamond layer having a thickness of greater than
about 20 .mu.m; (ii) preparing a first surface of the diamond layer
by mechanical means to a have a surface roughness R.sub.q of less
than about 10 nm; (iii) growing a thin layer of diamond, preferably
having a thickness of less than about 20 .mu.m, on the first
surface of the diamond layer to from a planar first surface having
a surface roughness R.sub.q of less than about 10 nm; and (iv)
forming a second layer on the planar first surface of the diamond
layer to form a functional interface between the diamond layer and
the second layer, wherein the second layer is solid, non-metallic
and selected from diamond, a polar material and a dielectric
material.
34. The method according to claim 32, wherein the etch is an
isotropic etch.
35. The method according to claim 34, wherein in step (iii), at
least about 10 nm is removed.
36. The method according to claim 32, wherein the diamond layer is
a boron-doped single crystal diamond.
37. (canceled)
38. The method according to claim 33, wherein the thin layer of
diamond on the first surface has a thickness of less than 20
.mu.m.
39. The method according to claim 34, wherein a gas mixture
comprising a halogen and an inert gas is used in the isotropic
etch.
Description
[0001] The present invention relates to electronic devices
fabricated in diamond, and to methods of manufacture of these
electronic devices in order to obtain high performance.
BACKGROUND OF THE INVENTION
[0002] The present generation of high frequency (HF) and microwave
signals is mostly based on Si and GaAs devices. Due to physical
limitations, these devices cannot achieve power levels higher than
a few hundred watts (depending on the frequency to be amplified) in
simple solid-state device configurations. Wide band gap materials
(diamond, SiC, GaN, etc), in principle, allow for higher power
amplification per unit gate length at microwave frequencies. This
is because a larger bias voltage, and hence a larger voltage
amplitude on the microwave signal, can be supported across the
transistor channel region over which the current is modulated. In
effect, the higher breakdown electric field of a wide band gap
semiconductor is exploited. In microwave transistors, the ability
to support high voltage is particularly desirable since, generally,
power has to be transferred to a relatively high impedance (for
example 50.OMEGA.) load.
[0003] The use of diamond in manufacturing transistors of various
types has been described in, for example, JP-A-60246627, EP 0 343
963 B1 and WO 2006/117621 A1.
[0004] WO 2006/117621 A1 discloses a metal semiconductor
field-effect transistor (MESFET). The MESFET is manufactured by
providing a single crystal diamond material substrate having a
growth surface on which further layers of diamond material can be
deposited, depositing a plurality of further diamond layers on the
substrate growth surface, and attaching appropriate contacts to the
respective diamond layers, thereby defining a transistor structure.
The further diamond layers deposited on the substrate include a
boron doped interface layer (a "delta-doped" layer). Such a design
presents several synthesis challenges. The main challenge is the
requirement to produce nanometer-thin boron layers which transition
very abruptly to an intrinsic layer (e.g. a change in B
concentration from about 10.sup.15 B atoms per cm.sup.3 to about
10.sup.20 B atoms per cm.sup.3 in a few nm). Growing such boron
layers (delta layers) is dependent upon a number of crucial steps
including substrate surface preparation for flatness and smoothness
and diamond growth conditions. In this type of device, the holes
(acting as charge carriers) are essentially localised in a thin
intrinsic diamond layer in the immediate vicinity of the boron
acceptors in the delta layer.
[0005] An alternative design, described in co-pending application
number GB0701186.9 provides a structure in which the charge
carriers and ionised acceptors/donors are spatially separated
leading to particular advantages in terms of device manufacture and
performance. This is achieved by putting a polar layer in contact
with the diamond surface in order to substantially confine the
carriers in the diamond within a thin diamond surface layer
adjacent to the polar layer.
[0006] Work has also taken place on diamond surface devices. These
are not generally perceived as being practical devices in the long
term, because they are in general intrinsically unstable, but they
do offer a route to characterising the behaviour of diamond. A
surface device utilises the fact that under certain circumstances a
hydrogen terminated diamond surface has free carriers in a surface
layer formed by band bending which can then be used in the
fabrication of a device. The instability arises in these devices
because further species need to be adsorbed to the hydrogen
terminated surface in order to induce the band bending, and these
species, and the hydrogen termination itself, can be lost, for
example if the device is heated.
[0007] Preparation of diamond surfaces has historically focused on
providing flat surfaces. Flat surfaces in diamond can generally
only be prepared in the first instance by mechanical processing.
Subsequently any further treatment tends to roughen or pit the
surface because of anisotropic behaviour. WO 01/06633 reported that
in homoepitaxial CVD diamond synthesis there is benefit in
mechanically preparing a substrate surface which is flat and where
the process is optimised to minimise sub-surface damage.
Subsequently these surfaces are etched using an anisotropic etch
such as a hydrogen etch or an oxygen etch prior to synthesis
(preferably in-situ and immediately preceding growth), and this
etch, being anisotropic reveals the sub-surface damage in the form
of pits, so that synthesis takes place on a surface of reduced
surface damage, but which is no longer completely flat, being
roughened or pitted by the etch. This relatively damage free but
etch roughened surface is then suitable for growth according to
that disclosure.
[0008] WO 2006/117621 reveals that in fabrication of some
electronic devices mechanical processes can be used to obtain
parallel faces to the electronic material, and that this processing
can be optimised to achieve both flatness or smoothness and the
minimisation of subsurface damage, although the latter is not
eliminated.
[0009] Electronic devices are manufactured in a number of
materials. Typically fabrication of electronic devices comprises
the preparation of a substrate and the synthesis of one or more
`epi` or epitaxial layers on this substrate. The epitaxial layers
can differ from the substrate in a number of ways: [0010] Higher
purity and/or lower dislocation content, since these can be
difficult to control in bulk grown substrate material [0011] Dopant
concentrations, for example the substrate can be insulating to
provide isolation, and the epilayers doped to provide the active
device regions. [0012] In the case of heteroepitaxial layers, the
basic material in the epilayer can be different.
[0013] The situation in diamond is different: [0014] The highest
purity material can be grown in thick layers, although the final
surface of such thick layers is not flat. [0015] Any interface, or
new start of growth, in the diamond can be a source of generation
of new dislocations, so the number of interfaces is in general
minimised. [0016] True single crystal diamond cannot be grown
heteroepitaxially, so a diamond single crystal substrate is always
used. Heteroepitaxial material can sometimes be described as single
crystal from, for example, visual inspection of the growth surface,
but still retains regions of crystal misoriented with respect to
one another and separated by low angle boundaries.
[0017] One area of similarity between diamond and more conventional
electronic materials is that diamond can be doped, typically using
boron. Doped layers are generally formed by CVD growth, generally
in a separate growth stage to the intrinsic layer.
SUMMARY OF THE INVENTION
[0018] The present invention provides a diamond electronic device
comprising a functional interface between two solid materials,
wherein the interface is formed by a planar first surface of a
first layer of single crystal diamond and a second layer formed on
the first surface of the first diamond layer, the second layer
being solid, non-metallic and selected from diamond, a polar
material and a dielectric material, and wherein the planar first
surface of the layer of single crystal diamond has an R.sub.q of
less than 10 nm and has at least one, preferably at least two,
preferably at least three, preferably all four of the following
characteristics: [0019] (a) the first surface is an etched surface,
preferably an isotropically etched surface; [0020] (b) a density of
dislocations in the first diamond layer breaking the surface is
less than 400 cm.sup.-2 measured over an area greater than 0.014
cm.sup.2; [0021] (c) a density of dislocations in the second layer
breaking a notional or real surface lying within the second layer
parallel to the interface and within 50 .mu.m of the interface is
less than 400 cm.sup.-2 measured over an area greater than 0.014
cm.sup.2; and [0022] (d) the first surface has an R.sub.q less than
1 nm.
[0023] Features (a)-(d) refer to the preparation of the diamond
surface, and it is generally preferred that the diamond surface has
at least one, more preferably 2 (two), more preferably 3 (three),
more preferably all 4 (four) of the characteristics (a)-(d).
[0024] In addition to having at least one of the characteristics
(a) to (d), preferably the functional interface of the diamond
electronic device of the present invention has regions with a layer
of charge carriers adjacent thereto such that the charge carriers
form the active device current where, in use, the charge carriers
either move substantially parallel to the interface or the charge
carriers move substantially perpendicular to and through the
interface.
[0025] An interface prepared according to the above method will be
termed a `damage free planar interface`.
[0026] Preferably the interface formed, by a planar first surface
of a first layer of single crystal diamond and a second layer
formed on the first surface of the first diamond layer, is an
internal interface.
[0027] In a further aspect, the present invention provides a
diamond electronic device comprising a functional interface between
two solid materials, wherein the interface is formed by a planar
first surface of a first layer of single crystal diamond wherein
the planar first surface has been mechanically processed and a
second layer formed on the first surface of the first diamond
layer, the second layer being solid, non-metallic and selected from
diamond, a polar material and a dielectric material, and wherein
the planar first surface of the first layer of single crystal
diamond has an R.sub.q of less than 10 nm and wherein the planar
surface of the first layer of single crystal diamond is
substantially free of residual damage due to mechanical
processing.
[0028] Preferably the number density of defects revealed by a
revealing etch in the functional planar surface is less than about
100 mm.sup.2, preferably less than about 50 per mm.sup.2,
preferably less than about 20 per mm.sup.2, preferably less than
about 10 per mm.sup.2, preferably less than about 5 per
mm.sup.2.
[0029] Preferably the planar surface of the first layer of single
crystal diamond material is prepared from a processed surface,
preferably a mechanically processed surface, preferably a
mechanically prepared surface.
[0030] As used herein, the term "mechanically processed" means that
the surface has been subjected to a step involving conventional
polishing and lapping techniques. As used herein, the term
"mechanically prepared" refers to a surface that has been
mechanically processed such that it is suitable for a specific
intended purpose. This might include processing by a route
optimised to minimise the amount of sub-surface damage as opposed
to an arbitrary combination of lapping and polishing steps.
[0031] In a further aspect, the present invention provides a method
for producing a diamond electronic device comprising providing a
diamond layer having a thickness of greater than about 20 .mu.m;
preparing a first surface of the diamond layer by mechanical means
to a have a surface roughness R.sub.q of less than about 10 nm;
etching the first surface of the diamond layer to form a planar
first surface having a surface roughness R.sub.q of less than about
10 nm; and forming a second layer on the planar first surface of
the diamond layer to form a functional interface between the
diamond layer and the second layer, wherein the second layer is
solid, non-metallic and selected from diamond, a polar material and
a dielectric material.
[0032] In a further aspect, the present invention provides a method
for producing a diamond electronic device comprising providing a
diamond layer having a thickness of greater than about 20 .mu.m;
preparing a first surface of the diamond layer by mechanical means
to a have a surface roughness R.sub.q of less than about 10 nm;
growing a thin layer of diamond, preferably having a thickness of
less than about 20 .mu.m, on the first surface of the diamond layer
to from a planar first surface having a surface roughness R.sub.q
of less than about 10 nm; and forming a second layer on the planar
first surface of the diamond layer to form a functional interface
between the diamond layer and the second layer, wherein the second
layer is solid, non-metallic and selected from diamond, a polar
material and a dielectric material.
[0033] Preferably the diamond layer is single crystal diamond.
[0034] In the context of this invention, a planar interface is an
interface which is not necessarily flat over large dimensions, e.g.
over dimensions larger than about 1 .mu.m, more preferably larger
than about 10 .mu.m, more preferably larger than 100 about .mu.m,
more preferably larger than about 1 mm, but on this scale may show
a degree of curvature. However the interface is planar because it
is free of sharp features which may degrade the performance of the
device by causing scattering of the charge carriers. In particular,
the first surface of the first layer, and preferably the interface
formed on it, preferably has root-mean-square roughness R.sub.q of
less than about 10 nm, preferably an R.sub.q of less than about 5
nm, preferably an R.sub.q of less than about 3 nm, preferably an
R.sub.q of less than about 2 nm, preferably an R.sub.q of less than
about 1 nm, preferably an R.sub.q of less than about 0.5 nm
preferably an R.sub.q of less than about 0.3 nm, preferably an
R.sub.q of less than about 0.2 nm, preferably an R.sub.q of less
than about 0.1 nm. Furthermore, the surface of the second layer
facing the first layer preferably has an R.sub.q of less than about
10 nm, preferably an R.sub.q of less than about 5 nm, preferably an
R.sub.q of less than about 3 nm, preferably an R.sub.q of less than
about 2 nm, preferably an R.sub.q of less than about 1 nm,
preferably an R.sub.q of less than about 0.5 nm, preferably an
R.sub.q of less than about 0.3 nm, preferably an R.sub.q of less
than about 0.2 nm, preferably an R.sub.q of less than about 0.1
nm.
[0035] A functional interface is one which forms part of the
operational design of the device, such that in the absence of the
interface the design of the device would be different and/or its
operation would be significantly changed. More specifically, the
charge carriers which are, in use, the active current of the
device, move in proximity to the functional interface, either
substantially parallel thereto or substantially perpendicular and
therethrough.
[0036] The electronic device of this invention can comprise natural
single crystal diamond, synthetic single crystal diamond made by
high pressure-high temperature (HPHT) techniques, and synthetic
single crystal diamond made by CVD techniques (`single crystal CVD
diamond`). Alternatively it may comprise a combination of these,
for example a first layer comprising boron doped HPHT diamond
providing a first surface, and single crystal CVD diamond providing
a second layer.
[0037] Preferably the first layer of the electronic device of this
invention comprises single crystal CVD diamond. Preferably where
the second layer is diamond this comprises single crystal CVD
diamond.
[0038] Preferably, either the first layer and/or the second layer,
between which there is the interface, is high purity single crystal
diamond, preferably high purity single crystal CVD diamond.
[0039] The high purity single crystal diamond preferably has a
total impurity content, excluding hydrogen and its isotopes of
about 5.times.10.sup.18 atoms per cm.sup.3 or less, preferably
about 1.times.10.sup.18 atoms per cm.sup.3 or less, preferably
about 5.times.10.sup.17 atoms per cm.sup.3 or less.
[0040] Alternatively or in addition, the high purity single crystal
diamond has a nitrogen content of about 5.times.10.sup.17 atoms per
cm.sup.3 or less, preferably about 1.times.10.sup.17 atoms per
cm.sup.3 or less, preferably about 5.times.10.sup.16 atoms per
cm.sup.3 or less, preferably about 1.times.10.sup.16 atoms per
cm.sup.3 or less.
[0041] Alternatively or in addition, the high purity single crystal
diamond has a boron content of about 1.times.10.sup.17 atoms per
cm.sup.3 or less, preferably about 1.times.10.sup.16 atoms per
cm.sup.3 or less, preferably about 5.times.10.sup.15 atoms per
cm.sup.3 or less, preferably about 1.times.10.sup.15 atoms per
cm.sup.3 or less.
[0042] The total impurity, nitrogen and boron concentrations can be
measured by techniques including secondary ion mass spectroscopy
(SIMS). SIMS can be used to provide bulk impurity concentrations
and to provide `depth profiles` of the concentration of an
impurity. The use of SIMS is well known in the art, for example the
measurement of boron concentrations by SIMS is disclosed in WO
03/052174.
[0043] The interface may be formed by etching or regrowth.
Preferably the interface is formed by etching, preferably by
isotropic etching. Where the interface is formed by isotropic
etching preferably it is prepared by ICP etching using a gas
mixture containing a halogen and an inert gas. Preferably the
halogen is chlorine and the inert gas is argon.
[0044] Advantageously, by use of the technique of isotropic
etching, the surface(s) which form the interface are etched at
approximately the same rate irrespective of crystal orientation.
This is particularly advantageous as it means that the interface
may be formed from single crystal or polycrystalline diamond. This
also means that the surface(s) can be etched without preferentially
removing the damaged regions, as would otherwise be the case were
an anisotropic etch to be used. Thus, the isotropic etch removes
damage from the surface without significantly roughening the
surface.
Etching
[0045] An etched surface means the removal of a minimum thickness
of material from the surface.
[0046] In one embodiment, an etched surface means the removal of a
minimum thickness of material from an as mechanically processed
surface, preferably a mechanically prepared surface, based on grit
size of last mechanical process, to provide a surface which is free
or substantially free of mechanical processing damage, and is also
free or substantially free of damage etch features.
[0047] As indicated above, an isotropically etched surface means
that the surface roughness of the surface is not substantially
increased by the etch. Surface roughness measurements R.sub.q.sup.B
and R.sub.q.sup.A are taken on the same area of the diamond. By
"same area" is meant an equivalent area as close as reasonably
practical, using multiple measurements and statistical analysis
where necessary to verify the general validity of the measurements,
as is known in the art. In particular the isotropically etched
surface of the invention has a roughness R.sub.q.sup.A (After the
etch) and the original surface a roughness R.sub.q.sup.B (Before
the etch), such that R.sub.q.sup.A/R.sub.q.sup.B is preferably less
than about 1.5, more preferably less than about 1.4, more
preferably less than about 1.2, more preferably less than about
1.1, and in addition, the isotropic etch preferably provides at
least one, preferably at least two of the following features:
[0048] an etched surface which is smooth and preferably smoother
than the initially prepared surface, and in particular where the
R.sub.q of the etched surface (R.sub.q.sup.A) is preferably less
than about 10 nm, preferably less than about 5 nm, preferably less
than about 2 nm, preferably less than about 1 nm, preferably less
than about 0.5 nm, preferably less than about 0.3 nm. [0049]
Removal of a thickness of material exceeding at least about 0.2
.mu.m, more preferably at least about 0.5 .mu.m, more preferably at
least about 1.0 .mu.m, more preferably at least about 2 .mu.m, more
preferably at least about 5 .mu.m, more preferably at least about
10 .mu.m.
[0050] Removal, by etching, of a minimum thickness of material from
the as mechanically processed surface based on grit size of last
mechanical process, to provide a surface which is free or
substantially free of mechanical processing damage, requires the
removal of sufficient depth to significantly reduce the surface
damage and thus needs removal by etching of the same order of
thickness as the surface damage layer. Typically surface damage
layers have thicknesses in the range of about 0.2 .mu.m to about 20
.mu.m (or thicker with very aggressive lapidary techniques). Thus
preferably the etch removes a thickness of material from the
surface, where the thickness of material removed is at least about
0.2 .mu.m, more preferably at least about 0.5 .mu.m, more
preferably at least about 1.0 .mu.m, more preferably at least about
2 .mu.m, more preferably at least about 5 .mu.m, more preferably at
least about 10 .mu.m. The surface damage layer typically has a
thickness that is about the same as the size of the largest diamond
grit particle used for the last stage of lapidary processing; for
example a surface scaife polished with 1-2 .mu.m sized diamond grit
will typically have a surface damage layer about 2 .mu.m thick.
Therefore, to minimise the amount of damage from lapidary
processing that remains after etching by the method of the
invention, the amount of material removed by the method of the
invention should preferably be at least about 0.2 times the size of
the largest grit particles, more preferably at least about 0.5
times the size of the largest grit particles, more preferably at
least about 0.8 times the size of the largest grit particles, more
preferably at least about 1.0 times the size of the largest grit
particles, more preferably at least about 1.5 times the size of the
largest grit particles, more preferably at least about 2 times the
size of the largest grit particles. After the etch, the surface of
the single crystal diamond preferably has a surface roughness after
the etch, R.sub.q, of less than about 10 nm, more preferably less
than about 5 nm, more preferably less than about 2 nm, more
preferably less than about 1 nm, more preferably less than about
0.5 nm, more preferably less than about 0.3 nm.
[0051] Where the interface is formed by etching it can extend
across the whole of a surface of the first diamond layer, or across
a proportion of the surface such as structural features etched into
the surface, using known techniques such as photolithography, this
portion of the surface then forming the first surface.
[0052] Where the interface is formed by etching, the interface is
preferably a functional interface in the design of the electronic
device, and is preferably one of the following interfaces deemed to
be an internal surface or interface of the final device: [0053] a
diamond to diamond interface, such as intrinsic diamond to boron
doped diamond, or vice versa, or between two diamond layers of
different doping concentration, where a dopant concentration
changes across the interface by at least a factor of about 2,
preferably by at least a factor of about 5, preferably by at least
a factor of about 10, preferably by at least a factor of about 20,
[0054] a diamond to diamond interface where the level of at least
one impurity changes at the interface, such that: [0055] the
impurity concentration in at least one layer is greater than
10.sup.15 atoms/cm.sup.3, preferably greater than about
3.times.10.sup.15 atoms/cm.sup.3, preferably greater than about
10.sup.16 atoms/cm.sup.3, preferably greater than about 10.sup.17
atoms/cm.sup.3, preferably greater than about 10.sup.18
atoms/cm.sup.3, or [0056] where the change in impurity
concentration at the interface is by at least a factor of about 5,
preferably by at least a factor of about 10, preferably by at least
a factor of about 30, preferably by at least a factor of about 100,
and preferably where the impurity is other than hydrogen; [0057] a
diamond to non-diamond polar material interface; [0058] a diamond
to a non-diamond dielectric material interface.
[0059] Where the interface is formed by etching, more preferably
the interface is functional interface in the design of the
electronic device, and is preferably one of the following
interfaces deemed to be an internal surface or interface of the
final device: [0060] a diamond to diamond interface, such as
intrinsic diamond to boron doped diamond, or vice versa, or between
two diamond layers of different doping concentration, where a
dopant concentration changes across the interface by at least a
factor of about 2, preferably by at least a factor of about 5,
preferably by at least a factor of about 10, preferably by at least
a factor of about 20; [0061] a diamond to diamond interface where
the level of at least one impurity changes at the interface, such
that: [0062] the impurity concentration in at least one layer is
greater than 10.sup.15 atoms/cm.sup.3, preferably greater than
about 3.times.10.sup.15 atoms/cm.sup.3, preferably greater than
about 10.sup.16 atoms/cm.sup.3, preferably greater than about
10.sup.17 atoms/cm.sup.3, preferably greater than about 10.sup.18
atoms/cm.sup.3, or [0063] where the change in impurity
concentration at the interface is by at least a factor of about 5,
preferably by at least a factor of about 10, preferably by at least
a factor of about 30, preferably by at least a factor of about 100,
and preferably where the impurity is other than hydrogen; [0064] a
diamond to non-diamond polar material interface.
[0065] Where the interface is formed by etching, more preferably
the interface is functional interface in the design of the
electronic device, and is preferably one of the following
interfaces deemed to be an internal surface or interface of the
final device: [0066] a diamond to diamond interface, such as
intrinsic diamond to boron doped diamond, or vice versa, or between
two diamond layers of different doping concentration, where a
dopant concentration changes across the interface by at least a
factor of about 2, preferably by at least a factor of about 5,
preferably by at least a factor of about 10, preferably by at least
a factor of about 20; [0067] a diamond to diamond interface where
the level of at least one impurity changes at the interface, such
that: [0068] the impurity concentration in at least one layer is
greater than 10.sup.15 atoms/cm.sup.3, preferably greater than
about 3.times.10.sup.15 atoms/cm.sup.3, preferably greater than
about 10.sup.16 atoms/cm.sup.3, preferably greater than about
10.sup.17 atoms/cm.sup.3, preferably greater than about 10.sup.18
atoms/cm.sup.3, or [0069] where the change in impurity
concentration at the interface is by at least a factor of about 5,
preferably by at least a factor of about 10, preferably by at least
a factor of about 30, preferably by at least a factor of about 100,
and preferably where the impurity is other than hydrogen.
[0070] Furthermore the etched diamond surface with low R.sub.q
preferably is substantially free of processing damage such that the
number of defects revealed by the revealing etch test is less than
about 100 per mm.sup.2.
[0071] In the context of this invention the term `impurity` refers
to atoms other than Sp.sup.3-bonded carbon (that is carbon bonded
as diamond) or hydrogen (and their isotopes) that are either
intentionally or unintentionally present in the diamond of the
invention. A dopant is such an impurity added to modify the
electronic properties of the diamond, and the material containing
the dopant described as `doped diamond`. An example of an impurity
which is intentionally present in the invention is boron, which is
added so as to provide a source of carriers and is thus a dopant.
An example of an impurity which may be unintentionally present in
the invention is nitrogen, which may have been incorporated as a
result of being present in the source gases used for synthesis or
as a residual gas in the CVD synthesis system.
[0072] Impurity concentrations can be measured by techniques
including secondary ion mass spectroscopy (SIMS). SIMS can be used
to provide bulk impurity concentrations and to provide `depth
profiles` of the concentration of an impurity. The use of SIMS is
well known in the art, for example the measurement of boron
concentrations by SIMS is disclosed in WO 03/052174.
Regrowth
[0073] Formation of the interface by regrowth is advantageous
because it has the effect of distancing any damaged layer(s) from
the surface(s) which forms the functional interface(s) of the
device.
[0074] Where the interface is formed by growth it can be restricted
to a portion of a surface of the first diamond layer by using
masking techniques, this portion corresponding to the first
surface, or, more preferably, it can extend across the whole of a
surface of the first diamond layer, this whole surface forming the
first surface.
[0075] As growth is a much slower process than etching, e.g.
.about.1 .mu.m/hr as compared to .about.0.1 .mu.m/min, there is
greater scope for the control of the thickness of the layer.
[0076] In some circumstances, the technique of regrowth may be more
attractive than an etching technique, specifically where it is
possible to reduce the effect of mechanical damage sufficiently by
regrowth alone. An example of such a situation might be the
deposition of a buffer layer on to a substrate where the charge
carriers do not move in the buffer layer.
[0077] An interface formed by regrowth means growing a new thin
diamond layer, where the surface of this thin layer is then used as
the first surface in its as grown state.
[0078] The interface between the mechanically processed surface and
the regrowth layer preferably does not itself serve an inherent
part of the device design (or as a functional interface) other than
to provide a layer of material to displace or separate an interface
which is designed to act as an interface in the electronic device
design (a functional interface) away from an interface where there
is mechanical processing damage.
[0079] Such a thin diamond layer is preferably grown by CVD
synthesis, and is thin to limit the formation of macroscopic growth
steps. The thickness of this layer, grown onto a previously
mechanically prepared surface, is less than about 20 .mu.m,
preferably less than about 10 .mu.m, preferably less than about 3
.mu.m, preferably less than about 1 .mu.m, preferably less than
about 100 nm, preferably less than about 50 nm, preferably less
than about 20 nm, preferably less than about 10 nm.
[0080] Such a thin layer may be prepared using a number of
techniques including monolayer growth techniques and use of
off-axis surfaces to control the propagation of surface steps, and
thus retain a very flat and smooth surface.
[0081] Where the surface upon which the thin layer is grown has
Miller indices close to those of a {001} surface, this being the
surface upon which homoepitaxial CVD diamond growth is most easily
accomplished, the normal to the surface is preferably between
0.degree. and about 5.degree., preferably between about 0.5.degree.
and about 1.degree., of the normal to a {001} or a {111} surface.
Where the surface is close to a {001} surface, the normal to the
surface is preferably within about 10.degree. of the great circle
passing through the pole of the {001} surface and the pole of an
adjacent {101} surface.
[0082] Such a thin layer may comprise high purity intrinsic
diamond, more preferably comprising high purity intrinsic diamond
with material properties conforming to the disclosures in WO
01/96633.
[0083] Alternatively such a thin layer may comprise conductive
doped diamond, for example B doped diamond.
[0084] The surface of this thin as-grown layer forms the first
surface and preferably has an R.sub.q of less than about 10 nm,
preferably an R.sub.q of less than about 5 nm, preferably an
R.sub.q of less than about 3 nm, preferably an R.sub.q of less than
about 2 nm, preferably an R.sub.q of less than about 1 nm,
preferably an R.sub.q of less than about 0.5 nm preferably an
R.sub.q of less than about 0.3 nm, preferably an R.sub.q of less
than about 0.2 nm, preferably an R.sub.q of less than about 0.1 nm.
Thus, this surface has very low surface roughness and in addition
is free of processing damage.
[0085] The prepared surface onto which this layer may be grown
could be any form of diamond, but is preferably CVD synthetic
diamond, preferably boron doped CVD diamond.
[0086] Furthermore, where the interface is formed by regrowth,
preferably the interface is one of the following interfaces deemed
to be an internal surface or interface of the final device: [0087]
A conductive doped diamond to conductive doped diamond interface,
such as a boron doped diamond to boron doped diamond, where both
layers contain a dopant at a concentration preferably greater than
about 10.sup.17 atoms/cm.sup.3, preferably greater than about
10.sup.18 atoms/cm.sup.3, preferably greater than about 10.sup.19
atoms/cm.sup.3, preferably greater than about 10.sup.20
atoms/cm.sup.3, and preferably where any difference in boron doping
between the layers is not relevant to device performance and the
damaged layer is essentially encapsulated in a region of conducting
diamond away from any active device interfaces. Preferably the
dopant is boron.
[0088] A diamond to diamond interface, such as intrinsic diamond to
intrinsic diamond, wherein the properties of the diamond either
side of the layer are sufficiently similar for the interface not to
be designed to act as an interface in the electronic device design.
Preferably the intrinsic diamond comprises high purity intrinsic
diamond with material properties conforming to the disclosures in
WO 01/96633.
[0089] More preferably, where the interface is formed by regrowth,
the interface is a conductive doped diamond to conductive doped
diamond interface, where both layers contain a dopant at a
concentration preferably greater than about 10.sup.17
atoms/cm.sup.3, preferably greater than about 10.sup.18
atoms/cm.sup.3, preferably greater than about 10.sup.19
atoms/cm.sup.3, preferably greater than about 10.sup.20
atoms/cm.sup.3, and preferably where any difference in boron doping
between the layers is not relevant to device performance and the
damaged layer is essentially encapsulated in a region of conducting
diamond away from any active device interfaces. Preferably the
dopant is boron.
Combined
[0090] The techniques of etching and regrowth may be combined, such
that a surface is first etched and then a thin layer regrown to
form the first surface of the first layer and subsequently the
interface. This approach is generally advantageous only if the etch
has not been completed to sufficient depth to remove all mechanical
processing damage. However, by use of a combination of the two
techniques, it is envisaged that it is possible to produce an
interface which has minimal surface damage. This is because the
damage has first been removed by etching and then any residual
damage is distanced from the functional interface by the growth of
the thin diamond layer.
PREFERRED EMBODIMENTS OF THE INVENTION
[0091] It is desirable that the first layer has a low dislocation
density in the region of the first surface. In particular, it is
desirable that the density of dislocations breaking the first
surface of the first layer is less than about 400 cm.sup.-2,
preferably less than about 300 cm.sup.-2, preferably less than
about 200 cm.sup.-2, preferably less than about 100 cm.sup.-2,
measured over an area of greater than about 0.014 cm.sup.2,
preferably greater than about 0.1 cm.sup.2, preferably greater than
about 0.25 cm.sup.2, preferably greater than about 0.5 cm.sup.2,
preferably greater than about 1 cm.sup.2, and preferably greater
than about 2 cm.sup.2.
[0092] Methods of preparing and characterising diamond and diamond
surfaces with low dislocation density are reported in the prior art
of WO 01/96633, WO 01/96634, WO 2004/027123, and co-pending
application PCT/IB2006/003531. The preferred methods of
characterising the dislocation density are the use of a `revealing
plasma etch` and the use of X-ray topography.
[0093] It is further desirable that the first surface of the first
layer forming one side of the interface is substantially free from
damage introduced by post-growth mechanical processing of the
as-grown surface to a depth of at least about 1 nm, preferably at
least about 2 nm, preferably at least about 5 nm, preferably at
least about 10 nm, preferably at least about 20 nm, preferably at
least about 50 nm, preferably at least about 100 nm, preferably at
least about 200 nm, preferably at least about 500 nm. The presence
of such damage, which includes microfractures and
mechanically-generated point and extended defects, can have a
detrimental effect on the performance of a device through carrier
scattering and trapping, perturbation of the local electric field
and degradation of the breakdown electric field.
[0094] In the case of diamond and in particular single crystal CVD
diamond, such defects can be introduced into the material by
mechanical processing of the as-grown surface, such as by using
conventional lapping and polishing techniques. These issues are
particularly relevant to diamond in view of its hard and brittle
nature, and its chemical inertness which limits the number of
chemical and physical etching processes available. The requirements
for processing an electronic surface to obtain low roughness, and
those for processing an electronic surface to obtain low surface
damage, are quite distinct. The preparation of an electronic
interface showing both these features is a further aspect of this
invention.
[0095] Generally, thick layers of single crystal CVD diamond in the
as-grown state are not suitable for use as the first layer and
their surfaces are not suitable for use as the first surface
because of the presence of non-planar features that can develop
during thick growth. The presence of non-planar features, even if
they are epitaxial to the underlying surface, results in surfaces
being present that do not have the same crystallographic
characteristics. For example, hillocks with surfaces formed by
{111} planes may be present on the {001} surface. This is
undesirable as in subsequent growth it can result in the presence
of regions of different growth sector and produce regions which
have different properties. Further, boundaries between regions of
different growth sectors can be the source of dislocations which
are detrimental to the electronic properties of the device.
Therefore, it is desirable to ensure that the surface is flat i.e.
has an Rq as defined above and is free from surface features.
[0096] Conversely, the diamond layer on which the electronic
surface is to be prepared needs to sufficient rigid and robust for
processing and handling, and consequently the fabrication of an
electronic device usually starts from a thick diamond layer. There
are a number of methods provided in this invention of producing a
suitable diamond surface from the as-grown surface of a thick
diamond layer, which processing steps are included in the method.
In the context of this invention, a single crystal CVD layer is
considered to be thick when its thickness exceeds about 20
.mu.m.
[0097] Firstly, a first surface may be prepared on the thick
diamond layer using mechanical lapping and polishing processes,
which have been optimised for minimum surface damage by using
feedback from, for example, a revealing etch. Such a technique is
described in for example WO 01/96633. Whilst such a surface may
have a low damage level, it is unlikely to be sufficiently free of
damage to obtain more than adequate performance from the
device.
[0098] The first surface may then be prepared from a processed
surface, preferably from a mechanically processed surface,
preferably a mechanically prepared surface itself optimised for
minimum surface damage by using the method above, by using a
further processing stage comprising chemical etch or other forms of
etching, such as ion beam milling, plasma etching or laser
ablation, and more preferably plasma etching. Preferably the
etching stage removes at least about 10 nm, preferably at least
about 100 nm, more preferably at least about 1 .mu.m, more
preferably at least about 2 .mu.m, more preferably at least about 5
.mu.m, more preferably at least about 10 .mu.m. Preferably the
etching stage removes less than about 100 .mu.m, preferably less
than about 50 .mu.m, preferably less than about 20 .mu.m. This
further processed surface preferably has an R.sub.q of less than
about 10 nm, preferably an R.sub.q of less than about 5 nm,
preferably an R.sub.q of less than about 3 nm, preferably an
R.sub.q of less than about 2 nm, preferably an R.sub.q of less than
about 1 nm, preferably an R.sub.q of less than about 0.5 nm
preferably an R.sub.q of less than about 0.3 nm, preferably an
R.sub.q of less than about 0.2 nm, preferably an R.sub.q of less
than about 0.1 nm.
[0099] Alternatively, the first surface may be prepared from a
processed surface, preferably from a mechanically processed
surface, preferably a mechanically prepared surface itself
optimised for minimum surface damage by using the method above, or
from an etched surface such as those described above, by growing a
further thin layer of diamond on the surface, preferably using a
CVD process. Prior to deposition of the further thin layer of
diamond (termed regrowth), the processed surface has an R.sub.q of
less than about 10 nm, preferably an R.sub.q of less than about 5
nm, preferably an R.sub.q of less than about 3 nm, preferably an
R.sub.q of less than about 2 nm, preferably an R.sub.q of less than
about 1 nm, preferably an R.sub.q of less than about 0.5 nm
preferably an R.sub.q of less than about 0.3 nm, preferably an
R.sub.q of less than about 0.2 nm, preferably an R.sub.q of less
than about 0.1 nm. After deposition of the further thin layer of
diamond (termed regrowth), the new as grown regrowth surface has an
R.sub.q of less than about 10 nm, preferably an R.sub.q of less
than about 5 nm, preferably an R.sub.q of less than about 3 nm,
preferably an R.sub.q of less than about 2 nm, preferably an
R.sub.q of less than about 1 nm, preferably an R.sub.q of less than
about 0.5 nm preferably an R.sub.q of less than about 0.3 nm,
preferably an R.sub.q of less than about 0.2 nm, preferably an
R.sub.q of less than about 0.1 nm.
[0100] Where the first surface is prepared by plasma etching,
preferably the etching is achieved by ICP etching, preferably using
a gas mixture containing a halogen and an inert gas, preferably
where the inert gas is argon, and preferably where the halogen is
chlorine.
[0101] The electronic device may be a 2-terminal device, such as a
diode.
[0102] The electronic device may have at least 3 terminals, such as
a 3-terminal transistor.
[0103] The electronic device is preferably a transistor, preferably
a field effect transistor.
[0104] In one embodiment of the present invention, the electronic
device comprises a functional interface between two solid
materials, wherein the interface is formed by a planar first
surface of a first layer of single crystal diamond, wherein the
planar first surface has preferably been mechanically processed and
subsequently isotropically etched and a second layer formed on the
first surface of the first diamond layer, the second layer being
solid, non-metallic and selected from diamond, a polar material and
a dielectric material, and wherein the planar first surface of the
first layer of single crystal diamond has an R.sub.q of less than
about 10 nm, preferably an R.sub.q of less than about 5 nm,
preferably an R.sub.q of less than about 3 nm, preferably an
R.sub.q of less than about 2 nm, preferably an R.sub.q of less than
about 1 nm, preferably an R.sub.q of less than about 0.5 nm
preferably an R.sub.q of less than about 0.3 nm, preferably an
R.sub.q of less than about 0.2 nm, preferably an R.sub.q of less
than about 0.1 nm.
[0105] In another embodiment of the present invention, the diamond
electronic device comprises a functional interface between two
solid materials, wherein the interface is formed by a planar first
surface of a first layer of single crystal diamond and a second
layer formed on the first surface of the first diamond layer, the
second layer being solid, non-metallic and selected from diamond, a
polar material and a dielectric material, and wherein the planar
first surface of the first layer of single crystal diamond has an
R.sub.q of less than about 1 nm and wherein the first surface of
the diamond layer is a surface of a diamond layer, preferably
having a thickness of less than about 20 .mu.m, preferably less
than about 10 .mu.m, preferably less than about 3 .mu.m, preferably
less than about 1 .mu.m, preferably less than about 100 nm,
preferably less than about 50 nm, preferably less than about 20 nm,
preferably less than about 10 nm, grown on a single crystal diamond
layer.
[0106] In another embodiment of the present invention, the
electronic device comprises a functional interface between two
solid materials, wherein the interface is formed by a planar first
surface of a first layer of single crystal diamond, wherein the
planar first surface has preferably been mechanically processed and
subsequently isotropically etched and a second layer formed on the
first surface of the first diamond layer, the second layer being
solid, non-metallic and selected from diamond, a polar material and
a dielectric material, and wherein the planar first surface of the
first layer of single crystal diamond has an R.sub.q of less than
about 10 nm, preferably an R.sub.q of less than about 5 nm,
preferably an R.sub.q of less than about 3 nm, preferably an
R.sub.q of less than about 2 nm, preferably an R.sub.q of less than
about 1 nm, preferably an R.sub.q of less than about 0.5 nm
preferably an R.sub.q of less than about 0.3 nm, preferably an
R.sub.q of less than about 0.2 nm, preferably an R.sub.q of less
than about 0.1 nm and wherein the first surface of the diamond
layer is a surface of a diamond layer, preferably having a
thickness of less than about 20 .mu.m, preferably less than about
10 .mu.m, preferably less than about 3 .mu.m, preferably less than
about 1 .mu.m, preferably less than about 100 nm, preferably less
than about 50 nm, preferably less than about 20 nm, preferably less
than about 10 nm grown on a single crystal diamond layer.
Defining Measurement Techniques
[0107] For the purposes of this invention the roughness of a
surface is described by its R.sub.q value. R.sub.q is also known as
the `root mean square` (or RMS) roughness. R.sub.q is defined as
the square root of the mean squared deviations from the centre-line
or plane of the surface profile:
R.sub.q= ((y.sub.1.sup.2+y.sub.2.sup.2+ . . .
+y.sub.n.sup.2)/n)
where y.sub.1.sup.2 etc are the squared deviations from the
centre-line or plane of the surface profile and n is the number of
measurements.
[0108] A surface may also be quantified by its R.sub.a value (also
referred as `average roughness` or `centre line average`):
R.sub.a=(|y.sub.1|+y.sub.2|+ . . . |y.sub.n|)/n
where |y.sub.1| etc are the moduli of the deviations from the
centre-line or plane of the surface profile and n is the number of
measurements.
[0109] For a surface with a Gaussian distribution of deviations
from the centre-line or plane of the surface profile, the value of
R.sub.q=1.25.times.R.sub.a.
[0110] R.sub.a and R.sub.q may be measured along lines (a
one-dimensional measurement) or over areas (a two-dimensional
measurement). An area measurement is essentially a series of
parallel line measurements.
[0111] For the purposes of this invention the R.sub.q value is
normally measured over a 1 .mu.m by 1 .mu.m area or 2 .mu.m by 2
.mu.m area using a scanning probe instrument such as an atomic
force microscope (AFM). In certain circumstances, it is considered
more appropriate to measure the R.sub.q using a stylus profilometer
over a 0.08 mm scan length (or over whatever length is standard
within the art for the roughness of the surface).
[0112] The extent of sub-surface damage can be revealed and
quantified using a deliberately anisotropic thermal revealing etch.
The revealing etch preferentially oxidises regions of damaged
diamond and therefore allows such regions to be identified and
thereafter quantified. Regions containing sub-surface damage from
mechanical processing are typically darkened or even blackened by
the revealing etch.
[0113] The revealing etch consists of: [0114] (i) examining the
surface at a magnification of 50 times using reflected light with a
typical metallurgical microscope to ensure that there are no
surface features present, [0115] (ii) exposing the surface to an
air-butane flame thereby raising the diamond surface to a
temperature of typically about 800.degree. C. to about 1000.degree.
C. for a period of about 10 seconds, [0116] (iii) examining the
surface at a magnification of 50 times using reflected light with a
typical metallurgical microscope and counting the damage features
revealed by the revealing etch, in the manner described below, to
determine their number density, [0117] (iv) repeating steps (ii)
and (iii) and comparing the measured density of defects with that
of the previous cycle until the following condition is met: if the
number density of defects counted is less than or equal to 150%,
preferably less than or equal to 120%, of the number density
determined in the previous cycle, then all the defects are deemed
to be revealed and the measurement recorded is the average of the
measurements of the last two cycles, if not the cycle is repeated
again.
[0118] The number density of defects is measured by the following
method: [0119] (i) the defects to be counted are those defects
visible at a magnification of 50 times with a typical metallurgical
microscope which fall totally or partially within a rectangular
area 1 mm.times.0.2 mm projected onto the surface being
characterised, [0120] (ii) the area is selected at random over the
surface or portion of the surface to be characterised and randomly
oriented, [0121] (iii) the defects are counted in a minimum of 5
such areas, [0122] (iv) the number density of defects is calculated
by dividing the total number of defects counted by the total area
examined to give a number density in defect per mm.sup.2.
[0123] To measure the number density of defects in areas less than
1 mm.sup.2 the above method is adapted by completing the defect
count over the whole area as a single measurement.
[0124] For the surface to be considered to be substantially free of
residual damage due to mechanical processing the number density of
defects revealed in a surface of single crystal CVD diamond
prepared by the method of the invention is less about 100 per
mm.sup.2, preferably less than about 50 per mm.sup.2, preferably
less than about 20 per mm.sup.2, preferably less than about 10 per
mm.sup.2, preferably less than about 5 per mm.sup.2.
[0125] Methods of preparing and characterising diamond and diamond
surfaces with low dislocation density are reported in the prior art
of WO 01/96633, WO 01/96634, WO 2004/027123, and co-pending
application PCT/IB2006/003531. The preferred methods of
characterising the dislocation density are the use of a `revealing
plasma etch` and the use X-ray topography.
[0126] As used herein, the term "about x" is intended to include
the value x itself.
EXAMPLE
[0127] By way of example, in the case of a diode, preferably the
damage free interface functional interface is formed between doped
conducting diamond and intrinsic diamond, and is formed by one of
the following methods: [0128] By regrowth, wherein the boron doped
layer is formed, and a planar surface formed on the doped diamond
by lapidary or mechanical processing. A further thin B doped layer
is then grown onto this layer, preferably using growth conditions
selected to minimise roughening and preferably keeping the layer
sufficiently thin and in the thickness range 10 nm-20 .mu.m, more
preferably in the thickness range 100 nm-10 .mu.m, more preferably
in the thickness range 1 .mu.m-10 .mu.m to minimise roughening, and
thus encapsulating the surface with mechanical damage between two
regions of doped conducting diamond. Then a high purity intrinsic
diamond layer is grown onto the regrown layer surface, this layer
preferably comprising high purity intrinsic diamond with material
properties conforming to the disclosures in WO01/96633 thus forming
a damage free interface between the thin doped conducting diamond
layer and a further layer of intrinsic diamond which is displaced
from the damage layer encapsulated within the boron doped layer.
[0129] By etching, wherein the conducting doped diamond layer is
formed, and a planar surface formed on the doped diamond by
lapidary or mechanical processing. This surface is then etched,
preferably using a plasma etch, more preferably an Argon/Chlorine
plasma etch. Optionally, a further thin B doped layer may be
regrown onto this layer, preferably using growth conditions
selected to minimise roughening and preferably keeping the layer
sufficiently thin and in the thickness range 10 nm-20 .mu.m, more
preferably in the thickness range 100 nm-310 .mu.m, more preferably
in the thickness range 1 .mu.m-10 .mu.m to minimise roughening.
Preferably this optional layer is not used. Then a high purity
intrinsic diamond layer is grown onto the etched surface, or
optional regrown layer surface, this layer preferably comprising
high purity intrinsic diamond with material properties conforming
to the disclosures in WO 01/96633, thus forming a damage free
interface between the conducting diamond layer and a further layer
of intrinsic diamond.
[0130] Alternatively the diode structure above may be formed
between a heavily boron doped layer providing a highly conductive
layer, and a lightly boron doped layer providing the reverse
voltage hold-off.
[0131] In the case of a transistor, preferably the damage free
interface is formed by etching or regrowth so that the damage free
surface is prepared in the intrinsic diamond. Preferably the damage
free surface is parallel to the primary current flow in the device,
with this current flow taking place primarily in the intrinsic
diamond layer adjacent to the damage free surface, and that current
flow is in close proximity to the interface, typically less than 1
.mu.m and more typically less than about 100 nm. Thus, there is a
2-dimensional charge carrier gas, where the meaning of the term
"2-dimensional charge carrier gas" is as is normally understood in
the art, present in the intrinsic diamond adjacent to the damage
free surface.
[0132] It will of course be understood that the present invention
has been described above purely by way of example, and that
modifications of detail can be made within the scope of the
invention as defined by the claims.
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