U.S. patent application number 12/523963 was filed with the patent office on 2010-04-01 for diamond electronic devices including a surface and methods for their manufacture.
Invention is credited to Richard Stuart Balmer, Ian Friel, Geoffrey Alan Scarsbrook.
Application Number | 20100078652 12/523963 |
Document ID | / |
Family ID | 39644161 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100078652 |
Kind Code |
A1 |
Scarsbrook; Geoffrey Alan ;
et al. |
April 1, 2010 |
DIAMOND ELECTRONIC DEVICES INCLUDING A SURFACE AND METHODS FOR
THEIR MANUFACTURE
Abstract
The present invention relates to a diamond electronic device
comprising a functional surface formed by a planar surface of a
single crystal diamond, the planar surface of the single crystal
diamond having an Rq of less than 10 nm and at least one of the
following characteristics: (a) the surface has not been
mechanically processed since formation by synthesis; (b) the
surface is an etched surface; (c) a density of dislocations in the
diamond breaking the surface is less than 400 cm''2 measured over
an area greater than 0.014 cm2; (d) the surface has an Rq less than
1 nm; (e) the surface has regions with a layer of charge carriers
immediately below it, such that the regions of the surface are
normally termed conductive, such as a hydrogen terminated {100}
diamond surface region; (f) the surface has regions with no layer
of charge carriers immediately below it, such that these regions of
the surface are normally termed insulating, such as an oxygen
terminated {100} diamond surface; and (g) the surface has one or
more regions of metallization providing electrical contact to the
diamond surface beneath these regions.
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.: |
12/523963 |
Filed: |
January 22, 2008 |
PCT Filed: |
January 22, 2008 |
PCT NO: |
PCT/IB08/50218 |
371 Date: |
August 28, 2009 |
Current U.S.
Class: |
257/77 ;
257/E29.082 |
Current CPC
Class: |
H01L 21/02104 20130101;
G01N 21/95 20130101; H01J 2237/3341 20130101; C23C 16/27 20130101;
H01L 21/02579 20130101; H01L 29/167 20130101; Y10T 428/24355
20150115; C01B 32/28 20170801; G01N 27/308 20130101; C23C 16/274
20130101; C30B 25/20 20130101; C23C 16/278 20130101; H01L 21/02376
20130101; H01J 37/321 20130101; H01L 21/041 20130101; G01N 21/87
20130101; C30B 29/04 20130101; H01J 2237/08 20130101; H01L 21/02634
20130101; H01L 29/1602 20130101; G01N 2201/0636 20130101; H01L
21/02527 20130101; C30B 25/105 20130101; H01L 21/0262 20130101;
H01L 29/045 20130101; H01L 29/36 20130101 |
Class at
Publication: |
257/77 ;
257/E29.082 |
International
Class: |
H01L 29/16 20060101
H01L029/16 |
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 surface
formed by a planar surface of a single crystal diamond, the planar
surface of the single crystal diamond having a surface roughness Rq
of less than 10 nm and at least one of the following
characteristics: (a) the surface has not been mechanically
processed since formation by synthesis; (b) the surface is an
etched surface; (c) a density of dislocations in any diamond
material breaking the planar surface is less than 400 cm.sup.-2
measured over an area greater than 0.014 cm.sup.2; (d) the surface
has an Rq less than 1 nm; (e) the surface has regions with a layer
of charge carriers immediately below it, such that the regions of
the surface are conductive; (f) the surface has regions with no
layer of charge carriers immediately below it, such that these
regions of the surface are insulating; and (g) the surface has one
or more regions of metallization providing electrical contact to
the diamond surface beneath these regions.
2. A diamond electronic device according to claim 1, wherein the
planar surface of the single crystal diamond has at least one of
the following characteristics: (b) the surface is an etched
surface; (c) a density of dislocations in any diamond material
breaking the planar surface is less than 400 cm.sup.-2 measured
over an area greater than 0.014 cm.sup.2; (d) the surface has an Rq
less than 1 nm; (e) the surface has regions with a layer of charge
carriers immediately below it, such that the regions of the surface
are conductive; (f) the surface has regions with no layer of charge
carriers immediately below it, such that these regions of the
surface are insulating; and (g) the surface has one or more regions
of metallization providing electrical contact to the diamond
surface beneath these regions.
3. (canceled)
4. A diamond electronic device according to claim 3 wherein the
planar surface of the single crystal diamond has least two of the
characteristics (a)-(d), and at least one of the characteristics
(e)-(g).
5-8. (canceled)
9. A diamond electronic device comprising a functional surface
formed by a planar surface of a single crystal diamond, the planar
surface of the single crystal diamond having been mechanically
processed and having an Rq 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.
10. A diamond electronic device according to claim 9, wherein the
number density of defects revealed by a revealing etch in the
functional planar surface is less than 100 per mm.sup.2.
11. A diamond electronic device according to claim 1 further
comprising an intrinsic diamond layer wherein total impurity
concentration (excluding hydrogen and its isotopes) is less than 1
ppm.
12. A diamond device according to claim 2 further comprising an
intrinsic diamond layer wherein nitrogen impurity concentration
(excluding hydrogen and its isotopes) is less than 0.1 ppm.
13. (canceled)
14. A diamond electronic device according to claim 1 where the
planar surface is an etched surface.
15. A diamond electronic device according to claim 14, wherein the
etched surface is an isotropically etched surface.
16-20. (canceled)
21. A diamond electronic device according to claim 1 wherein the
surface of the diamond layer is a surface of a diamond layer grown
on a single crystal diamond layer, the grown diamond layer having a
thickness of less than 20 microns.
22. A diamond electronic device according to claim 21 wherein the
grown diamond layer has a thickness of less than 20 microns.
23. A diamond electronic device according to claim 1 wherein the
surface of the 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.
24-31. (canceled)
32. A diamond electronic device according to claim 1 wherein the
planar surface is an external diamond surface.
33-40. (canceled)
41. A method of producing a diamond electronic device comprising:
(i) providing a diamond layer having a thickness of greater than 20
.mu.m; (ii) preparing a first surface of the diamond layer by
mechanical means to a have a surface roughness Rq of less than 10
nm; and (iii) etching the first surface of the diamond layer to
form a functional surface having a surface roughness Rq of less
than 10 nm.
42. A method of producing a diamond electronic device comprising:
(i) providing a diamond layer having a thickness of greater than 20
.mu.m; (ii) preparing a first surface of the diamond layer by
mechanical means to a have a surface roughness Rq of less than 10
nm; and (iii) growing a thin layer of diamond on the first surface
to from a functional surface having a surface roughness Rq of less
than 10 nm, wherein the thin layer of diamond on the first surface
has a thickness of less than 20 .mu.m.
43. The method according to claim 41, wherein the etch is an
isotropic etch.
44. The method according to claim 43, wherein in step (iii), at
least about 10 nm is removed.
45. The method according to claim 41, wherein the diamond layer is
a boron-doped single crystal diamond.
46. (canceled)
47. (canceled)
48. A diamond electronic device according to claim 1, wherein the
surface has regions with a layer of charge carriers immediately
below it, such that these regions of the surface comprise
conductive hydrogen terminated {100} diamond surface regions.
49. A diamond electronic device according to claim 1, wherein the
surface has regions with no layer of charge carriers immediately
below it, such that these regions of the surface comprise
insulating oxygen terminated {100} diamond surface regions.
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 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.
[0018] Electronic devices made according to the prior art, however,
have limited performance. This invention recognises these
limitations exist and that a need exists to provide a solution to
overcome these limitations and thus for devices with substantially
enhanced performance.
SUMMARY OF THE INVENTION
[0019] The present invention provides a diamond electronic device
comprising a functional surface formed by a planar surface of a
single crystal diamond, the planar surface of the single crystal
diamond having an R.sub.q (where R.sub.q is the root-mean-square or
`RMS` roughness of the surface) of less than about 10 nm and at
least one, preferably at least two, preferably at least three,
preferably four, preferably five, preferably six, preferably all
seven of the following characteristics:
(a) the surface has not been mechanically processed since formation
by synthesis; (b) the surface is an etched surface, preferably an
isotropically etched surface; (c) a density of dislocations in the
diamond breaking the surface is less than about 400 cm.sup.-2
measured over an area greater than about 0.014 cm.sup.2; (d) the
surface has an R.sub.q less than about 1 nm; (e) the surface has
regions with a layer of charge carriers immediately below it, such
that the regions of the surface are normally termed conductive,
such as a hydrogen terminated {100} diamond surface region, the
region preferably extending over the whole of the surface; (f) the
surface has regions with no layer of charge carriers immediately
below it, such that these regions of the surface are normally
termed insulating, such as an oxygen terminated {100} diamond
surface the region preferably extending over the whole of the
surface; and g) the surface has one or more regions of
metallization providing electrical contact to the diamond surface
beneath these regions.
[0020] A surface prepared according to the above method will be
termed a `damage free planar surface`.
[0021] Of the characteristics (a)-(g) described above, (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, more preferably 3, more preferably all 4 of the
characteristics (a)-(d). Of the characteristics (a)-(g) described
above, (e)-(g) refer more to the use of the diamond surface in the
device, and it is generally preferred that the diamond surface has
at least one, more preferably 2, more preferably all 3 of the
characteristics (e)-(g).
[0022] Preferably the surface has at least one characteristic from
(a)-(d) and at least one characteristic from (e)-(g). Preferably
the surface has at least two characteristics from (a)-(d) and at
least one characteristic from (e)-(g). Preferably the surface has
at least three characteristics from (a)-(d) and at least one
characteristic from (e)-(g). Preferably the surface has at least
one characteristic from (a)-(d) and at least two characteristics
from (e) (g).
[0023] In particular, preferably the surface has characteristic
(d), that is, an Rq less than 1 nm. Preferably the surface also has
at least one of characteristics (a) and (b), that is the surface is
either an etched surface, preferably an isotropically etched
surface, or else the surface has not been mechanically processed
since formation. More preferably the surface may have both
characteristics (a) and (b). Finally the surface preferably also
has characteristic (d), that is a controlled low level of
dislocations penetrating the surface, and in particular a density
of dislocations in the diamond breaking the surface is less than
about 400 cm.sup.-2 measured over an area greater than about 0.014
cm.sup.2. Such a surface, because of the low level of damage at the
surface from processing, and the low level of defects due to the
intersection of extended defects such as dislocations, has
surprisingly much better electronic properties and is thus
particularly suitable for diamond electronic device applications.
In use therefore, the surface is typically further characterised by
one or more of characteristics (e)-(g).
[0024] Preferably the functional surface is prepared from a
processed surface, preferably a mechanically processed surface,
preferably a mechanically prepared surface.
[0025] 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.
[0026] In a further aspect, the present invention provides a
diamond electronic device comprising a functional surface formed by
a planar surface of a single crystal diamond, the planar surface of
the single crystal diamond having been mechanically processed and
having an R.sub.q of less than about 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.
[0027] Preferably the number density of defects revealed by a
revealing etch in the functional planar surface is less than 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.
[0028] 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; and
etching the first surface of the diamond layer to form a functional
surface having a surface roughness R.sub.q of less than about 10
nm.
[0029] 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; and
growing a thin layer of diamond, preferably having a thickness of
less than about 20 .mu.m, on the first surface to from a functional
surface having a surface roughness R.sub.q of less than about 10
nm.
[0030] In the context of this invention, a planar surface is a
surface 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 about 100 .mu.m,
more preferably larger than about 1 mm, but on this scale may show
a degree of curvature. However the surface 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 surface 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.
[0031] A functional surface is one which forms part of the
operational design of the device, such that in the absence of the
surface 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 to
and therethrough.
[0032] Where the surface has not been mechanically processed since
formation by synthesis (characteristic (a)), this is typically
formed by regrowth of a thin layer onto a surface which has been
mechanically processed in order to achieve the required flatness,
optionally followed by a plasma etch, preferably an isotropic
plasma etch, to reduce or remove the damage associated with the
mechanical processing. This technique is advantageous because the
presence of such damage at or adjacent to the functional interface
will degrade the electronic performance of the device. Therefore
the elimination of such damage will enable improved device
performance.
[0033] 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.
[0034] 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.
[0035] The single crystal CVD diamond of the first layer is
preferably high purity single crystal CVD diamond. In this regard,
the high purity single crystal diamond preferably has a total
impurity content, excluding hydrogen and its isotopes of about
5.times.10.sup.18 per cm.sup.3 or less, preferably about
1.times.10.sup.18 per cm.sup.3 or less, preferably about
5.times.10.sup.17 per cm.sup.3 or less.
[0036] Alternatively or in addition, the high purity single crystal
diamond has a nitrogen content of about 5.times.10.sup.17 per
cm.sup.3 or less, preferably about 1.times.10.sup.17 per cm.sup.3
or less, preferably about 5.times.10.sup.16 per cm.sup.3 or less,
preferably about 1.times.10.sup.16 per cm.sup.3 or less.
[0037] Alternatively or in addition, the high purity single crystal
diamond has a boron content of about 1.times.10.sup.17 per cm.sup.3
or less, preferably about 1.times.10.sup.16 per cm.sup.3 or less,
preferably about 5.times.10.sup.15 per cm.sup.3 or less, preferably
about 1.times.10.sup.15 per cm.sup.3 or less.
[0038] 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.
[0039] Thus the diamond surface of this invention may, in the final
device structure, be an internal surface of one diamond layer onto
which a second diamond layer has been formed. Alternatively the
surface of this invention may, in the final device structure, be an
exposed external surface, or a surface partly or wholly covered or
encapsulated by non-diamond material such as a dielectric, a polar
material, a metal, for example a metal contact, or an adsorbed or
chemically attached fluid layer such as an adsorbed gas layer or a
bonded organic layer. Preferably the surface is an internal surface
of one diamond layer onto which a second diamond layer has been
formed. Preferably the surface of this invention is, in the final
device structure, an exposed external surface with an adsorbed or
chemically attached fluid layer such as an adsorbed gas layer, or a
surface partly or wholly covered or encapsulated a metal, for
example a metal contact. Those skilled in the art will recognise
that even where a second, such as a metal or polar layer, may be
present on the diamond surface of the invention, there may be an
intermediate layer comprising an absorbed gas layer or similar at
the interface.
[0040] The surface may be formed by etching or regrowth. Preferably
the surface is formed by etching, preferably by isotropic etching.
The formation of the surface by isotropic etching is particularly
advantageous as the surface can be etched without preferentially
removing the damaged regions which means that the etch removes
damage without significantly roughening the surface.
[0041] The surface may be formed by etching followed by
regrowth.
[0042] The prior art, for example WO2006/117621, teaches that a
mechanically processed surface is sufficient to enable the
fabrication of functioning diamond electronic devices i.e.
providing a functional surface and that flatness (i.e. low R.sub.q
or R.sub.a) is a suitable parameter for the characterisation of the
surface
[0043] The present invention recognises that in preparing a flat
surface by mechanical means, subsurface damage introduced can
result in substantial reduction in electronic properties of the
diamond. In particular, this damage can reduce the breakdown field,
it can cause charge trapping and/or it can reduce mobility,
particularly where the carriers move adjacent and parallel to the
surface/interface. Whilst the final device may function even with
the mechanical damage present, the performance is severely
compromised and would be much better in the absence of the
damage.
[0044] As such, the solution to the above identified shortcomings
in the art is to first prepare a flat substrate surface by
mechanical means on a diamond layer thick enough for mechanical
stability, optionally mounted onto a non diamond layer for further
stability, and then use one of two simple or novel methods to
provide the necessary surface in the electronic devices which
retain the flatness of the original mechanical surface and in
addition, remove or displace to an unimportant location the
associated subsurface damage. These methods are an etch, preferably
an isotropic etch, or a regrowth step.
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 the as mechanically processed,
preferably 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] 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 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 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 1 nm, more preferably less than about 0.5 nm, more
preferably less than about 0.3 nm.
[0051] Where the surface is formed by etching it can extend across
the whole of a surface of the 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 surface, per se.
[0052] Where the surface is formed by etching, the surface is
preferably a functional surface in the design of the electronic
device. It will be appreciated that interfaces in an electronic
device and in particular one of the following interfaces is deemed
to include a surface according to the present invention: [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 about
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.
[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 about 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 about 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 surface is formed by growth it can be restricted
to a portion of a surface of the diamond layer by using masking
techniques, this portion corresponding to a surface, or, more
preferably, it can extend across the whole of a surface of the
diamond layer, this whole surface forming the surface according to
the invention.
[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. 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.
[0076] An interface or surface formed by regrowth means growing a
new thin diamond layer, where the surface of this thin layer is
then used as the surface in its as grown state.
[0077] 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 the surface
or interface which is designed to act as a surface or interface in
the electronic device design (a functional interface) away from a
surface or interface where there is mechanical processing
damage.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] Alternatively, such a thin layer may comprise conductive
doped diamond, for example, B doped diamond.
[0082] The surface of this thin as-grown layer forms the surface or
first surface of an interface 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.
[0083] 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.
[0084] Furthermore, where the interface including a surface
according to the present invention 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: [0085]
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. [0086] 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.
[0087] More preferably, where the surface or 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
[0088] 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
[0089] It is desirable that the layer of diamond has a low
dislocation density in the region of the surface. In particular, it
is desirable that the density of dislocations breaking the surface
of the 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.
[0090] 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.
[0091] Alternatively, the diamond layer can be formed of material
known to be totally free of dislocations and stacking faults. The
ability to produce this material in HPHT diamond is disclosed in
WO2006/061707, and in CVD diamond in co-pending application
PCT/IB2006/003531. This material would be particularly suitable for
the preparation of surfaces which have intersecting them a low or
zero density of dislocations.
[0092] It is further desirable that the surface according to the
present invention of the layer forming one side of an 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. The extent to
which this occurs is surprising, for example whilst the theoretical
breakdown field of diamond is at least about 10 MV/cm, values
obtained in practice can be as low as about 100 KV/cm, and are
often as low as 1-2 MV/cm, and which has been demonstrated here to
be largely limited by subsurface damage causing local field
enhancement and breakdown.
[0093] Other effects can contribute to low breakdown voltages, in
particular high levels of impurity in the diamond, and high levels
of dislocations in the bulk of the diamond providing a conductive
path. In order to fully realise the benefit of the present
invention it is important to minimise the contribution of these
effects. As such, intrinsic diamond layers required to support high
fields, preferably have, preferably in addition to one or more of
the characteristics (a)-(d) in the combinations described earlier,
total impurity concentrations (excluding hydrogen and its isotopes)
of less than about 1 ppm, preferably less than about 0.3 ppm,
preferably less than about 0.1 ppm, preferably less than about 0.03
ppm, preferably less than about 0.01 ppm. In particular, intrinsic
diamond layers required to support high fields preferably have,
preferably in addition to one or more of the characteristics
(a)-(d) in the Combinations described earlier, nitrogen impurity
concentrations less than about 0.1 ppm, preferably less than about
0.03 ppm, preferably less than about 0.01 ppm, preferably less than
about 0.003 ppm, preferably less than about 0.001 ppm. The most
relevant dislocation density is that aligned along the direction of
the field. Ordinarily, in devices such as diodes, this is
perpendicular to the major faces of the device and thus these
dislocations intersect the major face and are already limited by
the need to limit the dislocations intersecting the surface which
is normally the major face. In some devices, such as FETs (field
effect transistors), the field can be applied in part along the
device, and thus the dislocation density in principle should be
maintained below a threshold for a conceptual measurement plane
normal to the local field, which is most easily characterised by
limiting the maximum dislocation density for any conceptual
measurement plane in the material. As such, preferably for any
conceptual measurement plane in the material, preferably the
dislocation density in the material, preferably in addition to one
or more of the characteristics (a)-(d) in the combinations
described earlier, 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.
[0094] In the case of diamond and in particular single crystal CVD
diamond, subsurface 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
surface and/or interface including such a surface showing both
these features is a further aspect of this invention.
[0095] Generally, the surface of a thick layer of single crystal
CVD diamond in the as-grown state is not suitable for use because
of the presence of non-planar features that can develop during
thick growth. Conversely, the diamond layer on which the electronic
surface is to be prepared needs to be sufficiently 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 20 .mu.m.
[0096] 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.
[0097] The surface may then be prepared from a processed surface,
by using a 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.
[0098] 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 v10 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] 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.
[0100] The electronic device may be a 2-terminal device, such as a
diode or a detector.
[0101] The electronic device may have at least 3 terminals, such as
a 3-terminal transistor.
[0102] The electronic device may be a transistor, for example a
field effect transistor.
[0103] A diamond detector is a good example of where external
surfaces may be beneficially prepared according to the present
invention. The simplest bulk diamond detector comprises a layer of
intrinsic diamond with two metal contacts on opposing major faces
and a bias applied across them. Alternatively, the electrodes may
be on the same surface and structured in an interdigitated form. In
this application, generally fields are well below breakdown and the
advantage to the performance of the detectors in reducing
subsurface damage in the diamond below the contacts is in reducing
trapped charge, giving better stability in the device in use,
particularly where the device is undergoing radiation damage. More
complex detectors, for example comprising two devices back to back
with a common boron doped diamond layer electrode, may also have
internal surfaces according to the present invention. In a detector
device, the charge carriers which are the active current of the
device move substantially perpendicular to the functional
surface.
[0104] A diamond Schottky diode is a good example of where both
internal and external surfaces may be beneficially prepared
according to this invention. The simplest bulk diamond diode
comprises a thin layer of intrinsic diamond on a thick (to provide
mechanical support as well as electrical conductivity) with a
Schottky contact to the free intrinsic diamond surface and an ohmic
contact to the boron doped diamond. Both surfaces of the intrinsic
diamond layer benefit from being free of subsurface damage, as does
the surface of the boron doped layer in contact with the intrinsic
layer. A particularly beneficial route to producing this structure
is to mechanically process a flat surface on the boron doped layer,
regrow a thin layer of boron doped material on top, and then grow
the thin intrinsic layer directly onto this as grown surface, and
to not mechanically process the final surface but simply add the
necessary contacts etc.
[0105] A diamond surface FET is a good example of a three terminal
device where an external surface may be beneficially prepared
according to this invention. Here one region of the external
surface may be for contacts, for example metal contacts, and the
active device region may have, for example, hydrogen termination in
order to provide the carriers in the device region. Since these
carriers move parallel and close to the diamond surface the device
benefits particularly from surfaces prepared by the method of this
invention. In an FET device, the charge carriers which are the
active current of the device move substantially parallel and
adjacent to the functional surface. The devices of the present
invention are particularly advantageous in this application. This
is because, in devices where the charge carriers move parallel to
the functional surface, the charge carriers are permanently exposed
to damage at the functional surface (as opposed to the situation
where the charge carriers travel perpendicular to and across the
surface and are thus only exposed to any damage for a limited
period of time). Thus, minimising the damage at the functional
surface as is the case in the present invention dramatically
improves the performance of such devices.
[0106] A diamond bulk FET is a good example of a three terminal
device where an internal surface may be beneficially prepared
according to this invention. Since in this type of device the
carriers move parallel and close to the diamond surface the device
benefits particularly from surfaces prepared by the method of this
invention.
[0107] In a device such as a polarisation enhanced diamond FET
disclosed in co-pending application GB0701186.9, again the carriers
move parallel and close to the diamond surface. The device benefits
particularly from surfaces prepared by the method of the present
invention.
[0108] Preferably the electronic device comprising the surfaces
according to the present invention are selected from detectors,
diodes, and surface and bulk transistors.
[0109] A particular application of the present invention includes
external diamond surfaces which have metallization attached. The
surface termination of the diamond surface appears to be retained
in some form even after metallization, at least to the extent that
the properties of the contact between metallization and diamond can
be modified by the surface termination prior to metallization.
After providing a diamond surface the surface modification may be
modified prior to metallization according to the needs of the
contacts desired. In most applications, portions of the
unmetallized surface, i.e. those between metallization/contact pads
need to be either non-conducting (e.g. detectors, diodes), achieved
on a {100} surface using oxygen termination, or conducting (e.g.
surface FETS in the active region), achieved on a {100} diamond
surface using H termination. Where the device is a surface FET,
optionally the surface of the diamond acting as the region of
charge mobility, e.g. the gate region of the transistor, may be
further terminated with organic species to provide an FET whose
operation is modified by the fluids (gas or liquid) to which the
surface is exposed. Alternatively, where the device is a stabilised
surface FET, rather than having, for example, a hydrogen terminated
{100} surface exposed to air, it may use more stable forms of
termination, such as a chemically sealed or terminated surface. A
number of suggestions for this are known in the art, for example
the use of Buckminsterfullerene (C.sub.60) to provide a stable
organic surface sealant providing beneath it an electrically
conducting region in the diamond.
[0110] In one embodiment the diamond electronic device comprises a
functional surface formed by a planar surface of a single crystal
diamond, wherein the planar first surface has preferably been
mechanically processed and subsequently isotropically etched, the
planar surface of the single crystal diamond after etching having
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.
[0111] In another embodiment of the present invention, the diamond
electronic device comprises a functional surface formed by a planar
surface of a single crystal diamond, wherein the 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, the
planar surface of the single crystal diamond having 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.
[0112] In one embodiment the diamond electronic device comprises a
functional surface formed by a planar surface of a single crystal
diamond, wherein the planar first surface has preferably been
mechanically processed and subsequently isotropically etched, the
planar surface of the single crystal diamond after etching having
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 the surface of the diamond
layer is a surface of a diamond layer, preferably having a
thickness of less than 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
[0113] 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.
[0114] 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.
[0115] 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.
[0116] 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.
[0117] 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).
[0118] 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.
[0119] The revealing etch consists of: [0120] (i) examining the
surface at a magnification of 50 times using reflected light with
typical metallurgical microscope to ensure that there are no
surface features present, [0121] (ii) exposing the surface to an
air-butane flame thereby raising the diamond surface to a
temperature of typically 800.degree. C. to 1000.degree. C. for a
period of about 10 seconds, [0122] (iii) examining the surface at a
magnification of 50 times using reflected light with typical
metallurgical microscope and counting the damage features revealed
by the revealing etch, in the manner described below, to determine
their number density, [0123] (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.
[0124] The number density of defects is measured by the following
method: [0125] (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, [0126] (ii) the area is selected at random over the
surface or portion of the surface to be characterised and randomly
oriented, [0127] (iii) the defects are counted in a minimum of 5
such areas, [0128] (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.
[0129] 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.
[0130] 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.
[0131] As used herein, the term "about x" is intended to include
the value.times.itself.
[0132] 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.
EXAMPLES
[0133] The following examples are for illustrative purpose only and
should not be considered to limit the scope of the present
invention.
Example 1
Detectors 1 (External Surface)
[0134] A high purity substrate is carefully prepared using
mechanical means to form a flat surface. An etch, preferably an
isotropic etch, is optionally used to the remove subsurface damage
layer resulting from mechanical processing and to provide a better
surface for either metallization (for example to act as contacts)
or further growth. A thin high purity layer is grown on top (e.g.
<20 .mu.m) of the etched surface, the final surface being a
surface free of damage caused by mechanical processing. Optionally
etching, preferably isotropic etching, is also used on this
surface, preferably to add structure to the surface (e.g. contact
recesses). However, the as-grown surface is most preferably used.
Surface detector(s) using contacts, for example metal contacts, to
the external surface, is fabricated.
Example 2
Detectors 2 (Internal and External Surfaces)
[0135] Electrically conductive diamond (e.g. boron doped) substrate
is carefully prepared using mechanical means to form a flat
surface. Further thin layer(s) of B doped material (e.g. <20
.mu.m, preferably <5 .mu.m) are grown to produce an as-grown
surface, or alternatively the B doped layer is etched using
preferably an isotropic etch to remove the mechanically damaged
layer. A thin high purity layer is grown on top (e.g. <20 .mu.m)
of the etched surface, the final surface being a surface free of
damage caused by mechanical processing. Etching, preferably
isotropic etching is optionally used on this surface, preferably to
add structure to the surface, but most preferably the as-grown
surface is used. Using contacts, for example metal contacts, to the
external surface, and using the B doped layer as the second
contact, bulk detector(s) are fabricated.
Example 3
Diodes (See Detectors 2 Above)
[0136] An electrically conductive diamond (e.g. boron doped)
substrate is carefully prepared using mechanical means to form a
flat surface. A further thin layer of B doped material (e.g. <20
.mu.m, preferably <5 .mu.m) is grown thereon to produce an as
grown surface, or alternatively a B doped layer is etched using
preferably an isotropic etch to remove the mechanically damaged
layer, or alternatively the B doped layer is etched and then
re-grown. A thin high purity layer is grown on top (e.g. <20
.mu.m), the final surface being a surface free of damage caused by
mechanical processing. Optionally etching, preferably isotropic
etching is added to this surface, preferably to add structure to
the surface, but it is preferable to use the as grown surface. Bulk
diodes(s) are fabricated using contacts, for example metal
contacts, to the external surface, and using the B doped layer as
the second contact. In view of edge effects, the diode structures,
and in particular near the edge of the conducting contact regions,
may include edge termination technology to minimise field focusing
effects exceeding the local breakdown voltage.
Example 4
Surface Transistors
[0137] A high purity substrate is carefully prepared using
mechanical means to form a flat surface. Optionally an etch,
preferably an isotropic etch, is used to remove the subsurface
damage layer resulting from mechanical processing and to provide a
better surface for further growth. A thin high purity layer is
grown on top (e.g. <20 .mu.m) of the etched surface, the final
surface being an as-grown surface and thus a surface free of damage
caused by mechanical processing. Etching, preferably isotropic
etching is optionally used on this surface, preferably to add
structure to the surface (e.g. contact recesses), but it is
preferable to use the surface as-grown. Steps are taken to ensure
the correct surface termination of the diamond to ensure surface
conductivity. On a diamond ((100)) surface, this is normally
hydrogen termination or oxygen termination, normally resulting in
an insulating surface. On other crystallographic orientations the
preferred termination may differ. Using contacts, for example metal
contacts to the external surface, surface transistor(s) are
fabricated, such as surface FETs. Optionally the surface of the
diamond acting as the region of charge mobility, e.g. the gate
region of the transistor, may be further terminated with organic
species to provide an FET whose operation is modified by the fluids
(gas or liquid) to which the surface is exposed.
[0138] Note that in each instance of using an etch to remove the
damage layer, this can itself comprise of two steps: first
implantation to cause a uniformly damaged layer to at least some of
the depth of the subsurface damage layer, preferably the whole of
that depth, and which will etch isotropically even if the etch is
not isotropic in defective crystalline diamond, and secondly
continuing the etch sufficiently into the diamond to remove all of
the ion damaged region such that the material being etched is
crystalline diamond (potentially defective but largely free of the
subsurface damage from the mechanical processing) and thus an
isotropic etch is preferable.
[0139] Further by way of example, in the case of a diode,
preferably the damage free surface functional interface is formed
between doped conducting diamond and intrinsic diamond, and is
formed by one of the following methods: [0140] 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 WO 01/96633 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. [0141] 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
[0142] 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.
[0143] 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.
[0144] In the case of a transistor, preferably the damage free
surface/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.
[0145] 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.
* * * * *