DISLOCATION ENGINEERING IN SINGLE CRYSTAL SYNTHETIC
DIAMOND MATERIAL
Field of Invention
The present invention relates to a method of manufacturing single crystal diamond
material via a chemical vapour deposition (CVD) technique. Certain embodiments
relate to a method which allows control of the number, distribution, direction and/or
type of dislocations within single crystal CVD diamond material. Certain
embodiments also relate to single crystal diamond materials which can be
manufactured according to the methods described herein. Certain embodiments of the
present invention also relate to the use of these materials in optical, mechanical,
luminescent and/or electronic devices.
Background of the Invention
Dislocations greatly and often detrimentally impact the physical and optoelectronic
properties of crystalline diamond solids. For example, toughness and/or wear
resistance can be affected by dislocation density and direction. Additionally,
dislocations can affect the performance of optical or electronic devices based on
crystalline diamond material.
Diamond is a material that is renowned for its exceptional hardness and mechanical
properties and this has resulted in its use for several applications (e.g. drilling).
Dislocations are known to affect these properties and in particular, in homoepitaxial
CVD synthetic diamond material, dislocations normally propagate in a direction
approximately parallel with the material's growth direction. The resulting parallel
array of dislocations is likely to affect the mechanical properties of the material.
Parallel dislocations of significant density, such as those propagating in a <001>
direction of a synthetic diamond crystal grown homoepitaxially on a (001) substrate,
result in significant strain and therefore birefringence within the synthetic diamond
material, which has been shown to reduce its performance for certain optical
applications such as Raman lasers (see, for example, "An intra-cavity Raman laser
using synthetic single-crystal diamond", Walter Lubeigt et al, Optics Express, Vol.
18, No. 16, 2010). As such, it would be desirable to lower the overall strain or at least
achieve a better distribution of strain within the material to provide better optical
performance. High birefringence is observed when the optical viewing axis is the
same direction as the line direction for parallel dislocations, i.e. parallel to the growth
direction. In optical applications, for simple engineering considerations (e.g.
maximising area) it would be conventional to process the material with major faces
that are perpendicular to the growth direction. This will result in dislocations which
are perpendicular to the major faces of the material and parallel to the viewing axis
resulting in high birefringence.
It is also believed that different dislocation types and directions will affect the
performance of CVD synthetic diamond devices differently. It is postulated that the
ability to select certain line directions of dislocations and not others will allow the
optical and/or electronic properties of a diamond-based device to be influenced and
optimised for the particular application desired.
In light of the above, one problem to be solved is to mitigate the adverse effect of
certain dislocation types and/or directions in single crystal CVD synthetic diamond
material, particularly in relation to optical, mechanical, luminescent, and electrical
applications.
The aforementioned problem has been at least partially solved in the past by
developing methods which reduce the number of dislocations in order to minimize
their detrimental effect. For example, WO2004/027123 and WO2007/066215
disclose methods of forming CVD synthetic diamond material with low dislocation
concentrations so as to provide high quality optical, electronic, and/or detector grade
diamond material. However, it can be relatively difficult, time consuming, and costly
to form CVD synthetic diamond material with a low dislocation density.
Notwithstanding other sources of dislocations, two predominant sources of
dislocations include: (i) threading dislocations from the substrate to the CVD layer;
and (ii) dislocations created at the interface between the substrate and the CVD layer.
With reference to (i), vertically slicing a primary CVD layer to reveal a (001) face and
growing a secondary layer upon this face results in threading dislocations from the
primary to the secondary layer (where the Burger's vector is preserved). Given that
the dislocations in the primary layer are of a <001> direction and edge or mixed 45°
type, there are a number of permutations of threading dislocations within the
secondary CVD layer (see Table 1). However, all of the threading dislocations are in
a <100> direction and either edge or 45° mixed type. Accordingly, while this work
demonstrates some degree of dislocation engineering, it is limited both in terms of
dislocation line direction and type. With reference to (ii), previous studies (see, for
example, M. P. Gaukroger et al., Diamond and Related Materials 17 262-269 (2008))
have shown that substrate preparation has a role in determining the dislocation type in
CVD layers grown upon standard (001) substrates. Dislocations propagating from
surface defects (e.g. a roughly polished substrate) are generally of a 45° mixed type,
the most stable dislocation type in (001) growth.
Table 1: [001] growth upon a (OOl)-grown vertically sliced primary CVD layer,
showing the various dislocation types if the threading dislocations of the secondary
layer are in a [010] line direction.
In light of the above, it should be appreciated that there is a desire to find routes
which minimize the impact of dislocation on specific properties such as electronic and
optical properties, which may or may not be consistent with a total reduction in
dislocation density. For example in some applications (e.g. those requiring
mechanical toughness) a high dislocation density might in fact be preferred, but the
direction and/or type of dislocations may be critical to the functional performance of
the material. There is hence a need to find a route to engineer the type and/or the
direction of the dislocations in homoepitaxially grown single crystal CVD synthetic
diamond.
It is an aim of certain embodiments of the present invention to at least partially solve
the problems outlined above.
Summary of Invention
According to a first aspect of the present invention there is provided a single crystal
CVD synthetic diamond layer comprising a non-parallel dislocation array, wherein
the non-parallel dislocation array comprises a plurality of dislocations forming an
array of inter-crossing dislocations, as viewed in an X-ray topographic cross-sectional
view or under luminescent conditions.
For certain applications, preferably the layer of single crystal CVD synthetic diamond
has a thickness equal to or greater than 1 mp , 10 m i, 50 m i, 100 mp , 500 mp , 1 mm,
2 mm, or 3 mm. Alternatively, or additionally, the layer of single crystal CVD
synthetic diamond may have a density of dislocations in a range 10 cm 2 to 1 x 10
cm 2 , 1 x 102 cm 2 to 1 x 10 cm 2 , or 1 x 104 cm 2 to 1 x 107 cm 2 and/or a
birefringence equal to or less than 5xl0 4 , 5xl0 5 , lxlO 5 , 5xl0 6 , or lxlO 6 . While
embodiments of the invention can be provided by growth on a range of possible non-
{100} oriented single crystal diamond substrates, such as { 1 10}, { 1 13}, and { 111}
oriented substrates, for certain applications the use of a { 110} or { 113} oriented
substrate is preferred. One or more of these features are advantageous to achieve a
relatively thick and/or high quality layer of single crystal CVD synthetic diamond.
For example, growth on a { 111} oriented substrate with a high concentration of
dislocations formed in the layer of single crystal CVD synthetic diamond can result in
poor quality, high strain material which cannot easily be grown to high thicknesses
without fracture.
Preferably, the non-parallel dislocation array extends over a significant volume of the
single crystal CVD synthetic diamond layer, the significant volume forming at least
30%, 40%, 50%, 60%, 70%, 80%, or 90% of a total volume of the single crystal CVD
synthetic diamond layer. The non-parallel dislocation array may comprise a first set
of dislocations propagating in a first direction through the single crystal CVD
synthetic diamond layer and a second set of dislocations propagating in a second
direction through the single crystal CVD synthetic diamond layer, wherein an angle
between the first and second directions lies in the range 40° to 100°, 50° to 100°, or
60° to 90° as viewed in an X-ray topographic cross-sectional view or under
luminescent conditions. As dislocations are known to not propagate in a perfect
straight line, a direction in which a dislocation propagates may be measured in terms
of an average direction over a significant length of the dislocation, where the
significant length is at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a total
length of the dislocation and/or at least 50 mp , 100 mp , 250 mp , 500 mp , 1000 mp ,
1500 mih, or 2000 mih.
According to certain embodiments not all dislocations in the material propagate in the
aforementioned manner. However, in certain embodiments at least 30%, 40%, 50%,
70%, 80% , or 90% of a total number of visible dislocations within a significant
volume of the single crystal CVD synthetic diamond layer form the non-parallel
dislocation array as viewed in an X-ray topographic cross-sectional view or under
luminescent conditions, the significant volume forming at least 30%, 40%, 50%, 60%,
70%, 80% , or 90% of a total volume of the single crystal CVD synthetic diamond
layer.
In certain embodiments, such as { 110} oriented material, the non-parallel dislocation
array may be viewable in an X-ray topographic cross-sectional view but not under
luminescent conditions. In certain alternative embodiments, such as { 113} oriented
material, the non-parallel dislocation array may be viewable under luminescent
conditions but not in an X-ray topographic cross-sectional view. This is because
dislocations in certain line directions emit blue luminescent light while in other line
directions they do not.
In addition to the above, it has been found that material having a non-parallel
dislocation array as described herein has good wear resistance coupled with increased
hardness (e.g. at least 100 GPa, more preferably at least 120 GPa).
According to a further aspect of the present invention there is provided a single crystal
CVD synthetic diamond object comprising a single crystal diamond layer according
to any preceding claim, wherein the single crystal diamond layer forms at least 30%,
40%, 50%, 60%, 70%, 80%, or 90% of a total volume of the single crystal CVD
synthetic diamond object. Such an object can be used in an optical, mechanical,
luminescent, and/or electronic device or application. Alternatively, the single crystal
CVD synthetic diamond object may be cut into a gemstone configuration.
According to yet another aspect of the present invention there is provided a method of
forming a single crystal CVD synthetic diamond layer, the method comprising:
providing a single crystal diamond substrate with a growth face having a
density of defects equal to or less than 5 x 10 defects/mm2 as revealed by a revealing
plasma etch; and
growing a layer of single crystal CVD synthetic diamond as previously
described.
The growth face of the single crystal diamond substrate may have a { 110} or { 113}
crystallographic orientation to form a layer of single crystal CVD synthetic diamond
material having a { 110} or { 113} orientation for the reasons given previously. A
growth rate of the layer of single crystal CVD synthetic diamond may be controlled to
be sufficiently low such that a non-parallel dislocation array is formed. In this regard,
it has been found that at low growth rates on a { 110} oriented substrate, dislocations
form a non-parallel dislocation array whereas if the growth rate is increased then a
parallel network of dislocations is formed. For a { 110} orientation, by growing a
layer of single crystal CVD synthetic diamond on the { 110} growth face at a < 110>
growth rate to <001> growth rate ratio below a certain limit it is possible to form a
non-parallel dislocation array. It is believed that similar comments may also apply to
{ 113} orientations although initial results indicate that a relatively high growth rate
can be utilized with a { 113} oriented substrate while still achieving a non-parallel
dislocation array.
According to certain embodiments, the non-parallel dislocation array comprises a
significant number of dislocations propagating at an acute angle of at least 20° relative
to a growth direction of the layer of single crystal CVD synthetic diamond, said
significant number being at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a total
number of dislocations visible in an X-ray topographic cross-sectional view or under
luminescent conditions. More preferably, the dislocations propagate at an acute angle
in the range 20 to 60°, 20 to 50°, or 30 to 50° relative to the growth direction of the
layer of single crystal CVD synthetic diamond.
Brief Description of the Drawings
For a better understanding of the present invention and to show how the same may be
carried into effect, embodiments of the present invention will now be described by
way of example only with reference to the accompanying drawings.
Figure 1 illustrates a flow diagram showing how different types and orientations of
dislocations can be realized in CVD synthetic diamond material, particularly
highlighting a route by which a non-parallel array of dislocations within a CVD
synthetic diamond material can be achieved;
Figure 2 illustrates the method steps involved in forming a CVD synthetic diamond
material having a non-parallel dislocation array in accordance with an embodiment of
the present invention and possible alternative synthesis routes which result in a
parallel array of dislocations;
Figure 3 illustrates dislocation types which propagate in a direction parallel to the
growth direction in a ( 110)-grown CVD synthetic diamond layer;
Figure 4 illustrates dislocation types which propagate at an acute angle relative to the
growth direction in a ( 110)-grown CVD synthetic diamond layer;
Figure 5 illustrates a single crystal CVD synthetic diamond layer comprising a
parallel array of dislocations;
Figure 6 illustrates a single crystal CVD synthetic diamond layer comprising a nonparallel
array of dislocations;
Figure 7 illustrates a birefringence micrograph for the CVD synthetic diamond
material of Figure 6 which shows comparatively low strain considering the large
dislocation density for this sample; and
Figure 8 illustrates single crystal CVD synthetic diamond layers grown on { 110} and
{ 113} oriented substrates in X-ray topographic cross-sectional view and under
luminescent conditions.
Detailed Description of Certain Embodiments
According to the certain embodiments of the present invention, the present inventors
have developed a technique for manufacturing single crystal CVD synthetic diamond
with a non-parallel dislocation array, particularly thick high quality single crystal
CVD synthetic material. This differs from previous techniques for manufacturing
CVD synthetic diamond in which a parallel array of dislocations forms in the growth
direction of the CVD synthetic diamond film. A parallel array of dislocations has
been found to be the source of several problematic effects. For example, a parallel
array of dislocations of significant density results in strain and birefringence within
the material, which reduces its performance for optical applications such as Raman
lasers. A parallel array of dislocations can affect the toughness and/or wear resistance
of the diamond material. Furthermore, a parallel array of dislocations can also affect
fluorescence and the electronic and optoelectronic properties of CVD synthetic
diamond. For example, for diamond detectors, some workers have speculated that
certain types of dislocations can act both as a carrier trap and also lower the
breakdown voltage.
Previous work as been directed at minimizing the density of dislocations within CVD
synthetic diamond materials. In contrast, the present invention has focused on
providing a non-parallel array of dislocations which propagate in different directions
forming a criss-crossed array of dislocations. The presence of a non-parallel
dislocation array can be beneficial for certain types of optical device as it leads to a
lower overall strain configuration which reduces the birefringence in a CVD synthetic
diamond layer. The presence of a non-parallel dislocation array can also increase
toughness and/or wear resistance of CVD synthetic diamond materials. Further still,
the presence of a non-parallel dislocation array can also improve electronic
performance. For example, certain types of dislocation may preferentially propagate
in favour of other types which act as both a carrier trap and also lower breakdown
voltage.
Certain embodiments of the invention may be applied to CVD synthetic diamond
materials of different chemical types, including but not limited to nitrogen doped,
phosphorus doped, boron doped, and undoped CVD synthetic diamond materials.
Several experimental techniques may be used in order to indicate that a diamond
material is CVD synthetic in origin. Examples include (but are not limited to):
presence of emission features at 467nm and/or 533nm and/or 737 nm in the
photoluminescence spectrum measured using 325nm, 458nm or 514nm continuouswave
laser excitation at 77K, or an absorption feature at 3123cm 1 in the infrared
absorption spectrum. A publication by P.M. Martineau et al. (Gems & Gemology, 40
(1) 2 (2004)) outlines criteria for identifying whether or not diamond material is CVD
synthetic, giving examples of CVD synthetic diamond materials that were grown
and/or annealed under a wide variety of conditions.
The term 'layer' refers to any grown region of CVD synthetic diamond and also refers
to free-standing CVD synthetic diamond material which was originally produced via
deposition of a layer onto a substrate and, optionally, the substrate subsequently
removed. A single crystal CVD synthetic diamond object comprising the previously
described single crystal CVD synthetic diamond layer may be provided, the single
crystal diamond layer forming at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a
total volume of the single crystal CVD synthetic diamond object.
By a non-parallel dislocation array we mean that, by using techniques that are able to
image dislocations in a cross-sectional view (such as X-ray topography, electron
microscopy, or luminescent imaging), within a significant volume of the CVD
synthetic diamond material, one observes the following: (i) dislocations of two or
more line directions (i.e. not all possessing the same line direction) such that one set
of dislocations propagate in a first direction through the single crystal CVD synthetic
diamond layer and a second set of dislocations propagate in a second direction
through the single crystal CVD synthetic diamond layer; (ii) the dislocations from the
first and second set appear to intercross each other; (iii) the angle between the first
and second directions lies in the range 40° to 100°, 50° to 100° or 60° to 90° in a crosssectional
view. The significant volume of the CVD synthetic diamond material is
preferably at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of the total volume of
the CVD synthetic diamond material. Within the significant volume preferably at
least 30%, more preferably at least 50%, more preferably still at least 70%, and most
preferably at least 90% of the dislocations in the CVD synthetic diamond layer
propagate in the manner described above.
It may be useful to characterise the non-parallel dislocations by comparing their line
directions with the CVD synthetic diamond growth direction, which can be
established by investigating the orientation of certain point defects within the
crystallographic lattice. For example, defects such as the nitrogen-vacancy (NV) and
the nitrogen-vacancy-hydrogen (NVH) complex are aligned along < 111> directions,
giving 8 possible configurations (+ve line directions), and the relative populations of
these configurations may show preferential orientation with respect to the growth
direction. Electron paramagnetic resonance measurements varying the angular
alignment of the magnetic field have been used to investigate the orientation of these
defects. For example, the present inventors have observed that in growth upon a ( 110)
surface both of these defects are aligned with the majority (equal to or greater than
50%, 60%, 80%, 95% or even 99%) orientated along the two < 111> orientations outof-
plane with respect to the ( 110) growth surface. This preferential defect alignment is
not observed for the same defects (e.g. NV) in samples grown on substantially {100}
oriented substrates. As the symmetry of the relationship between the < 111> directions
on which these defects lie and the major growth planes for CVD diamond including
the {100}, { 110} and { 111} is unique, then characterisation of the defect population
distribution may be used to uniquely define the growth direction, and in particular
whether growth has taken place on a { 110} plane, and the precise { 110} plane of
growth in the material. The non-parallel dislocations may propagate at an acute angle
in the range of 20 to 60°, or preferably 20 to 50°, or more preferably 30 to 50° relative
to the growth direction (the growth direction being substantially perpendicular to the
major { 110} CVD growth face, which is typically but not always parallel to the
substrate), the significant volume being 30%, 40%, 50%, 60%, 70%, 80%, or 90% of
the total volume of the CVD synthetic diamond material.
Methods already exist to produce CVD diamond that possesses two or more different
separate regions of growth defined by each region possessing parallel dislocations
which propagate in a certain line direction, and dislocations in one region propagating
ostensibly in a different direction to dislocations in another. One might therefore
suggest that dislocations within one region are not parallel to dislocations within
another. An example of two different regions is the case where a secondary CVD
synthetic diamond layer is grown upon a CVD synthetic diamond substrate and the
initial growth direction of the substrate and that of the secondary layer are different
(see, for example, M.P. Gaukroger et al, Diam. Relat. Mater. 17, 262 (2008)). In this
case the substrate and the secondary layer are two different regions. Another example
of different regions is in the growth of CVD synthetic diamond on off-axis substrates,
where individual dislocations vary in their line direction substantially and abruptly, by
crossing different growth pseudosectors, and dislocations within one pseudosector
correspond to those in one region and those in another pseudosector correspond to
those in a different region. The phenomenon of different dislocation line directions in
different regions of material contrasts with aspects of the present invention which are
concerned with providing dislocations which are non-parallel to each other within the
same region of material i.e. dislocations that intercross to form a non-parallel array,
rather than dislocations in different regions of CVD synthetic diamond material that
do not intercross and effectively form two separate parallel arrays in different regions
of material.
X-ray topographs recorded using a Lang camera fitted to an X-ray generator can be
used to identify dislocations in diamond. Section topographs recorded using a Bragg
reflection of the {533} crystallographic plane allow samples to be set up in such a
way that the plane sampled by the X-ray beam is within two degrees of a {001 } plane.
Section topographs recorded using a Bragg {008} reflection allow a {110} plane to be
sampled. X-ray section and projection topography has been extensively applied to
diamond by Lang and others (see, for example, I . Kiflawi et al Phil. Mag., 33 (4)
(1976) 697 and A.R. Lang, J . E. Field. Ed., "The Properties of Diamond", Academic
Press, London (1979) pp. 425-469). Both section and projection X-ray topographic
images may be used to measure the dislocation line directions and majority volume
that the dislocations occupy. The angles between the dislocations themselves and
between the growth direction and the dislocation line directions may be established by
the imaging of two or more section topographs, for example, but not exclusively, by
imaging the {100} and { 110} planes. The majority volume can be established by
either a projection topograph or two or more section topographs.
The contrast that is visible in X-ray topographs is due to the strain imparted on the
crystalline lattice by a dislocation or a bundle of dislocations. Advantageously, by
sampling an area between 10 nm2 and 1 mm2 of a single crystal CVD synthetic
diamond object comprising non-parallel dislocations, it can be establish that it
possesses a density of dislocations/dislocation bundles in the range 10 to lxlO cm 2 .
Within X-ray topographs it is not possible to distinguish between a dislocation or a
bundle of dislocations, but strong contrast in the image would normally imply the
latter. Therefore, the terms 'dislocations' and 'bundles of dislocations' are often used
interchangeably. Projection topographs recorded by translating the sample through the
X-ray beam can be analysed to provide information on the number of dislocations
throughout a sample (see, for example, M. P. Gaukroger et al, Diamond and Related
Materials 17 262-269 (2008)).
In addition to dislocation concentration, the line direction and / or Burgers vector (i.e.
the dislocation type) can also play an important role. It should be noted that that the
reference to the type of dislocation refers to the angle of the Burgers vector relative to
the dislocation line direction. In edge dislocations, the Burgers vector and dislocation
line are at right angles to one another (i.e. 90°). In screw dislocations, they are
parallel (i.e. 0°). In mixed dislocations the Burgers vector is oriented at an acute angle
between these extremes. Dislocation type is established through the analysis of X-ray
topographs recorded for a number of different reflections (see, for example, M. P.
Gaukroger et al, Diamond and Related Materials 17 262-269 (2008)). This type of
analysis is applicable in characterizing single dislocations, but in the case where there
may be bundles the analysis may be complicated in that the bundle may contain
dislocations of more than one type. In this case, a bundle of dislocations of different
type, with no single predominant type, is not characterizable into a particular
dislocation type and is discounted from the analysis.
Different dislocation types possess different degrees of atomic reconstruction, and
therefore impart dangling bonds to a greater or lesser degree which can contribute
towards or affect the optoelectronic properties. For example, the presence/absence of
blue dislocation photoluminescence present in CVD synthetic diamond material is
likely to be determined both by the dislocation line direction and its Burgers vector,
i.e. certain dislocation types exhibit luminescence but not others. This further
emphasises the interest of the inventors in being able to select out and control
dislocation types in CVD synthetic diamond material.
It will be understood that each dislocation does not tend to propagate along a perfect
straight line but rather deviates therefrom due to steps formed during growth of a
CVD synthetic diamond layer leading to the formation of terraces and risers. The
effect of steps on dislocations in CVD synthetic diamond is described by Martineau et
al. in Phys. Status Solidi C6, No. 8, 1953-1957 (2009). Accordingly, it will be
understood that the direction in which a dislocation propagates is described herein in
terms of an average direction over a significant length of the dislocation, where the
significant length is preferably at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of
the total length of the dislocation and/or equal to or greater than 50 mp , 100 mp , 250
mih, 500 mih, 1000 mih, 1500 mih, or 2000 mih in length.
The single crystal CVD synthetic diamond layer (e.g. ( 110) oriented) may comprise a
significant number of non-parallel dislocations oriented within 20°, 10° or 5° of a
<100> line direction, said significant number being at least 30%, 40%, 50%, 60%,
70%, 80% , or 90% of a total visible number of dislocations in, for example, either
section or projection topographs. Optionally, less than 70%, 60%, 50%, 40%, 30%,
2 0% or 10% of dislocations within a significant volume of the single crystal CVD
synthetic diamond layer are oriented within 20°, 10° or 5° of a < 110> line direction
said significant number being at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a
total visible number of dislocations in, for example, either section or projection
topographs. The <100> dislocations may either be a 45° mixed or edge type.
According to certain arrangements, at least 30%, 40%, 50%, 60%, 70%, 80%, or 90%
of a total number of characterizable dislocations in the ( 110) oriented layer are of a
mixed 45° type and/or an edge type. According to certain arrangements, less than
70%, 60%, 50%, 40%, 30%, 20%, 10% or 5% of characterizable dislocations within a
significant volume of the single crystal CVD synthetic diamond layer are of a < 110>
mixed 60° type. Furthermore, according to certain arrangements, less than 70%, 60%,
50% , 40%, 3 0%, 20%, 10% , or 5% of characterizable dislocations within a significant
volume of the single crystal CVD synthetic diamond layer may be of a < 110> screw
or < 110> edge type.
It is important to note that the aforementioned percentages of dislocation types are
relative to the total number of dislocations which are characterizable as having a
particular type in a given method of analysis. For example, as previously indicated, a
bundle of dislocations which contains a number of different dislocation types, with no
single predominant type, will not be characterizable using a topographic method of
analysis and thus will be discarded as non-characterizable. This will be understood by
a person skilled in the art and is discussed in more detail below.
The Burgers vector of a dislocation can be categorised by the acquisition of a number
of different projection X-ray topographs. To produce a projection topograph, it is
required to translate the sample through an X-ray beam in order to expose its
complete volume and to translate the film accordingly to maintain its position relative
to the sample. X-ray projection topographs using different Bragg reflections are used
in order to categorise the Burgers vector of a dislocation-related feature in an X-ray
topograph. This method is summarised by M. P. Gaukroger et al. in Diamond and
Related Materials 18 (2008) 262-269. In diamond, a < 110> Burgers vector is
assumed. In general, the contrast of a dislocation-related feature is dependent on the
angle between its Burgers vector and the atomic layers responsible for its diffraction.
To a good approximation, a dislocation-related feature is invisible in a given X-ray
topograph if its Burgers vector lies parallel to the diffracting plane, has a strong
contrast if its Burgers vector lies perpendicular to the diffracting plane, or has an
intermediate contrast if its Burgers vector lies at an intermediate angle between 0° and
90° relative to the diffracting plane, the contrast being stronger closer to 90° and
weaker when the Burgers vector is closer to 0°. This means that different dislocation
types, having different Burgers vector directions, will have different contrasts in a
particular topographic image. Furthermore, a single dislocation feature having a
particular Burgers vector direction will show a different contrast in different
topographic images taken in different directions relative to its Burgers vector.
By this method it is possible to establish the Burgers vector of a given dislocationrelated
feature and thus characterize its type. A dislocation feature which comprises a
number of dislocations having different Burgers vectors, with no predominant
direction, will tend to have an intermediate contrast in different topographic images
taken in different directions and will not be characterizable.
In practice, other factors must be taken into account. Reflections should also be
chosen in order to achieve suitable perspective in order to image the dislocations
clearly, and to establish accurately their position between different topographs.
Although the { 111 reflection is a good reflection for conventional growth upon (100)
substrates, it may not be the optimal reflection in other cases. For this work, it was
found that it was problematic to use { 111 reflections in order to achieve suitable
perspective such that individual dislocation-related features would be resolved
accurately. The {220} reflection was therefore chosen, as it gave a near "plan" view
of the sample (about 14° off-axis) and this made it easier to recognise the same
dislocation in different views. If four projection topographs employing {220}
reflections are used, defining the growth direction as [ 1 10], four topographs are
recorded using reflections from the (202), (022), (02-2) and (20-2) planes. If the
Burgers vector lies in one of the diffracting {220} planes, then one expects
dislocations to have strong contrast in one topograph, medium contrast in adjacent
topographs, and be invisible in the fourth. If the dislocation-related feature is pure
edge or pure screw-type, we expect medium contrast in all four topographs. One then
needs to take a topograph using a reflection in the ( 110) growth plane in order to
distinguish between the two. Using this method, dislocations of different types can be
characterized. For a bundle of dislocations comprising different dislocation types, the
bundle will tend to have an intermediate contrast in all topographical images and is
non-characterizable. Such a dislocation feature is discarded from the analysis. For a
bundle of dislocations of predominantly one type, the bundle will have a different
contrast in the different topographs according to the predominant type of dislocation
within the bundle and will be characterizable. Such a dislocation feature is counted as
a single dislocation for the purposes of numerical analysis. In order to establish the
presence or absence of a dislocation-related feature in several X-ray topographs, it is
required to match the dislocation-related contrast in different images accurately. This
may be performed manually by visual inspection or may be automated using a
suitable computer algorithm.
Embodiments of the invention are based on the present inventor's understanding that
there are two mechanisms whereby dislocations in CVD synthetic diamond can be
sourced: (i) threading dislocations from a primary (substrate) layer to a secondary
CVD synthetic diamond layer; and (ii) dislocations nucleated at the interface between
substrate surface and the CVD synthetic diamond layer either due to surface defects
or other reasons (e.g. lattice mismatch). What has led to certain embodiments of the
present invention is the acknowledgement that growth upon ( 110) surfaces leads to
vastly greater dislocation engineering scenarios for both (i) and (ii).
With reference to item (i), certain embodiments of the invention are based on research
performed by the inventors in investigating growth upon ( 110) surfaces, by vertically
slicing a primary (001) CVD layer to form a ( 110) growth face and growing upon this
face. The inventors have recognized that there are geometrical arguments which can
result in one type of dislocation in the primary (OOl)-grown layer threading through
and being converted into a second type of dislocation in the secondary (HO)-grown
layer, by changing the line direction but maintaining the Burger's vector, as illustrated
in Table 2 . From this table, we observe that it is possible to create a secondary CVD
layer that only contains specific dislocation types and/or line directions. For example,
it is possible to create a CVD layer that contains 60° mixed < 110> dislocations, if the
primary (OOl)-grown layer contains 45° mixed dislocations. Conversely, it is possible
to create a secondary (HO)-grown layer that contains 45° mixed <100> dislocations,
if the primary (OOl)-grown layer contains edge dislocations. As such, it has been
demonstrated that it is possible to engineer specific dislocation line directions and
types in a secondary CVD layer which are determined by the dislocation type in the
primary layer. This possibility to grow-in some types / line directions of threading
dislocations and not others allows the possibility of separating out and studying
different types of dislocations (edge, screw or mixed). This leads to a far greater
scope in terms of dislocation engineering than standard growth on (001) substrates.
Primary Line Burgers Variety Secondary Line Variety
layer direction vector layer direction
(001) [001] [101] Mixed 45° ( 110) [ 1 10] Mixed 60°
(001) [001] [Oi l ] Mixed 45° ( 110) [ 1 10] Mixed 60°
(001) [001] [ 1 10] Edge ( 110) [ 1 10] Screw
(001) [001] [ 1 10] Edge ( 110) [ 1 10] Edge
(001) [001] [101] Mixed 45° ( 110) [010] Edge
(001) [001] [Oi l ] Mixed 45° ( 110) [010] Mixed 45°
(001) [001] [ 1 10] Edge ( 110) [010] Mixed 45°
(001) [001] [ 1 - 1 0] Edge ( 110) [010] Mixed 45°
Table 2 : ( 110) growth upon a (OOl)-grown vertically sliced primary CVD layer,
showing the various threading dislocation types if the dislocations are in a [ 1 10] and
[010] line direction, respectively.
With relevance to item (ii) mentioned above, the present inventors have observed that
growing on a ( 110) surface gives even more scope for dislocation engineering for
those dislocations that form at the CVD/substrate interface. The present inventors
have observed that when a good ( 110) substrate finish is employed for ( 110) growth,
under specific growth conditions which are discussed later, a non-parallel array of
dislocations possessing a <100> direction can be created, some of which are nucleated
at the interface between the ( 110) substrate and the secondary layer. However, if poor
substrate surface finish is employed, irrespective of the growth conditions, a parallel
array of dislocations possessing a < 110> direction is observed. Poor substrate surface
finish leads to microscale cracks on the substrate surface that act as sources of
dislocations, and the inventors have observed that these dislocations nucleated at
surface defects are of a specific type that grow in a parallel < 110> configuration.
Therefore it is essential to process the substrate surface carefully to avoid the
nucleation of these parallel dislocations.
Certain methods of forming a single crystal CVD synthetic diamond layer comprise:
providing a single crystal diamond substrate with a ( 110) growth face having a
density of defects equal to or less than 5 x 10 defects/mm2 as revealed by a revealing
plasma etch; and growing a layer of single crystal CVD synthetic diamond on the
( 110) growth face at a < 110> growth rate to <001> growth rate ratio below a limit
whereby a non-parallel dislocation array forms in the layer of single crystal CVD
synthetic diamond. The ( 110) growth face may be formed from single crystal CVD
synthetic diamond, a single crystal natural diamond, or single crystal HPHT (high
pressure, high temperature) synthetic diamond. For example, a single crystal CVD
synthetic diamond plate, a single crystal natural diamond, or a single crystal HPHT
synthetic diamond plate may be processed to form the ( 110) growth face having a
density of defects equal to or less than 5 x 10 defects/mm2 as revealed by a revealing
plasma etch. Processing may, for example, involve slicing and polishing and/or
plasma etching.
According to certain embodiments a multi-stage growth process may be used. For
example, certain methods comprise:
providing a single crystal diamond substrate comprising a (001) growth
surface have a density of defects equal to or less than 5 x 103 defects/mm2 as revealed
by a revealing plasma etch;
growing a first layer of single crystal CVD synthetic diamond on the (001)
growth surface;
vertically slicing the first layer of single crystal CVD synthetic diamond to
form the ( 110) growth face;
treating the ( 110) growth face such that it possesses a density of defects less
than 5 x 103 defects/mm2 as revealed by a revealing plasma etch; and
growing a second layer of single crystal CVD synthetic diamond on the ( 110)
growth face at a < 110> growth rate to <001> growth rate ratio below a limit whereby
a non-parallel dislocation array forms in the second layer of single crystal CVD
synthetic diamond.
Figure 1 illustrates a flow diagram showing how the different types and orientations
of dislocations can be realized in a CVD synthetic diamond material, particularly
highlighting a route by which a non-parallel array of dislocations within a CVD
synthetic diamond material can be achieved. In Figure 1, the primary layer refers to a
(001) single crystal CVD synthetic diamond layer grown on a (001) single crystal
diamond substrate. The primary layer is then sliced vertically along a diagonal to
form a ( 110) single crystal diamond substrate on which the secondary layer of ( 110)
single crystal CVD synthetic diamond is grown.
It should be noted that when we talk about a (001) single crystal diamond substrate
we mean a substrate oriented to have a (001) crystallographic plane at the growth
surface. However, the growth surface may not be perfectly aligned with a (001)
oriented plane. Due to processing constraints, the actual growth surface orientation
can differ from this ideal orientation up to 5°, and in some cases up to 10°, although
this is less desirable as it adversely affects reproducibility. Similar comments also
apply for the ( 110) single crystal diamond substrate.
Figure 1 highlights that the dislocation types found in the primary layer are dependent
on the quality of the (001) substrate growth surface on which the primary layer is
grown. If the (001) substrate growth surface has a poor surface finish, mixed 45°
dislocations are formed in the <001> direction (i.e. a parallel array of mixed 45°
dislocations aligned vertically in the growth direction). If the (001) substrate growth
surface has a good surface finish, edge dislocations are the predominant type formed
in the <001> direction (i.e. a parallel array aligned vertically in the growth direction).
Figure 1 also illustrates that the type and orientation of dislocations found in the
secondary layer can be dependent on: (i) the type of dislocations in the primary layer,
(ii) the surface finish of the ( 110) substrate produced from the primary layer, and (iii)
the growth rate used for the secondary layer.
If a poor substrate growth surface finish is initially provided for growth of the primary
layer, resulting in a parallel array of <100> mixed 45° dislocations, and then the
primary layer is sliced vertically along a diagonal to form a ( 110) single crystal
diamond substrate on which the secondary layer is grown, a parallel array of < 110>
oriented dislocations of a mixed 60° type is formed. This is an undesirable route.
In contrast, if a good substrate growth surface finish is initially provided for growth of
the primary layer, resulting in a parallel array of <100> edge dislocations, and then
the primary layer is then sliced vertically along a diagonal to form a ( 110) single
crystal diamond substrate on which the secondary layer is grown, the present
inventors have found that a number of possibilities are available, as shown in Figure
1. If the ( 110) single crystal diamond substrate (upon which the secondary layer is
grown) possesses a poor substrate finish, surface defects will again result in the
creation of parallel < 110> oriented dislocations of a mixed 60° type. If the ( 110)
single crystal diamond substrate is well-prepared, surprisingly there are two
possibilities which are dependent on the growth rate of the secondary layer. If a
relatively high < 110> growth rate to <001> growth rate ratio is used for the secondary
layer, a parallel array of < 110> oriented screw and/or edge type dislocations is
formed. Alternatively, if a relatively low < 110> growth rate to <001> growth rate
ratio is used for the secondary layer, a non-parallel array of <100> oriented mixed 45°
and/or edge type dislocations is formed.
It can be important to avoid surface defects on the ( 110) substrates, as these result in
the nucleation of new dislocations in the secondary layer that adopt a low core-energy
configuration i.e. < 110> mixed 60° (analogous to the <100> mixed 45° dislocations
formed in growth upon poorly-prepared standard (001) substrates). Avoidance of the
nucleation of < 110> mixed 60° dislocations in the secondary layer entails growth
upon well-prepared ( 110) substrates e.g. by scaif polishing and by utilisation of a preetch
prior to growth that avoids the generation of macroscopic pits.
Even when the ( 110) substrate for the secondary layer is perfectly prepared and there
are negligible surface defects, there are still some non-threading dislocations that are
sourced at the interface between the ( 110) substrate and the secondary CVD synthetic
diamond layer. Without wanting to be bounded by theory, having excluded the
aforementioned mechanisms of dislocation nucleation, it is considered that these
dislocations may be due to lattice parameter mismatch between the primary and the
secondary layer. It has been observed that even though these dislocations are sourced
at the interface in a similar manner to those that arise from poor substrate preparation,
these dislocations adopt a non-parallel array of <100> oriented mixed 45° or edge
type configuration, unlike those that arise from poor substrate preparation.
Accordingly, these dislocations are not required to be removed in order to achieve
embodiments of the present invention.
In light of the above, it is evident that to achieve a non-parallel array of dislocations in
single crystal CVD synthetic diamond according to certain embodiments of the
present invention requires: (i) careful preparation of the initial (001) substrate prior to
growth of the primary (001) diamond layer; (ii) careful preparation of the ( 110)
substrate formed from the primary layer; and (iii) careful control of the growth rate of
the secondary ( 110) diamond layer.
Without wanting to be bounded by theory, the previously described results may be
justified as follows.
CVD single crystal diamond growth is usually governed by kinetic rather than
thermodynamic processes. However, the balance between kinetic and
thermodynamically driven process can be changed via a change in growth parameters.
For example, by growing at a low < 110> growth rate to <001> growth rate ratio, the
growth is more likely to be dominated by thermodynamic rather than kinetic factors
and vice versa for high < 110> growth rate to <001> growth rate ratios.
In relation to the above, the present inventors have found a "low" ratio of < 110>
growth rate to <001> growth rate may be one below 1.0 and a "high" ratio may be
greater than 1.0. The < 110> growth rate to <001> growth rate ratio may be controlled
to be equal to or less than 1.0, 0.8, 0.6, 0.4, or 0.2. However, those skilled in the art
will recognize that different conditions, such as a different diamond growth surface
temperature, will impact the detailed kinetics/thermodynamics and might substantially
alter this definition of "low" and "high". The < 110> growth rate to <001> growth
rate ratio is strongly influenced by the < 110> growth rate.
Modifying the < 110>:<001> growth rate ratio in the manner thus far discussed is
achievable for a person skilled in the art. Previously published studies discuss the
modification of growth parameters such as nitrogen doping, boron doping, and
substrate temperature and their relative effect on the growth rate in different
crystallographic directions. Such growth rate ratios are usually characterised in terms
of a, b and g parameters. However, for the purposes of the present invention this is
an overcomplicated scheme and we need refer simply to the growth rate ratio of
< 110>:<100>.
Growing the secondary layer of ( 110) CVD synthetic diamond at a sufficiently low
< 110>:<100> growth rate ratio allows dislocations to adopt a configuration that
minimises their overall energy per unit length. That is, lower core energy per unit
length (more thermodynamically favourable) dislocations are grown-in. We expect
< 110> mixed 60° dislocations to possess the lowest energy per unit length.
Therefore, under low < 110>:<100> growth rate ratios, these < 110> mixed 60°
dislocations will still be grown-in, if there is the possibility to create them either by
the threading of <100> mixed 45° dislocations in the primary layer to the secondary
layer, or by poor surface preparation of the ( 110) substrate produced from the primary
layer. Hence, it is desirable to remove all sources of < 110> mixed 60° dislocations.
As already mentioned, this is done by both correct preparation of the substrate upon
which the primary layer is grown upon in order to minimise <100> mixed 45°
dislocations in the primary layer, and also by good surface preparation of the ( 110)
substrate produced from the primary layer.
In the absence of < 110> mixed 60° dislocations being created, a non-parallel array of
<100> oriented edge and mixed 45° dislocations is grown in. The <100> oriented
edge and mixed 45° dislocations propagate at an acute angle of approximately 45°
from the growth direction and result in a non-parallel dislocation array. This has been
achieved via a combination of correct primary and secondary substrate processing and
secondary layer growth parameters.
At higher < 110>:<100> growth rate ratios, the kinetics of the growth process will
dominate over the thermodynamics. When the growth processes are dominated by
kinetics rather than by thermodynamics, the dislocations will simply revert to the
shortest length configuration which means that they will follow the growth direction,
i.e. the < 110> direction in this case for the secondary layer. Therefore, at high growth
rates, higher core energy dislocations will be preferentially grown-in. These include
< 110> screw and < 110> edge dislocations as illustrated in Figure 1.
More detail regarding how to prepare growth surfaces and how to control growth rates
to achieve the present invention are given later in this specification. It should be
noted that the specific growth rate ratio required to achieve a non-parallel array of
dislocations will depend on the growth chemistry used in the CVD process and will
vary somewhat according to the specific growth chemistries used. However, it will be
understood that a person skilled in the art will be able to optimize the growth rate
ratios by performing a series of trial runs at different growth rate ratios for a particular
process set-up in order to find a growth rate ratio which is near, but does not exceed,
the thermodynamic limit at which dislocations switch from the more
thermodynamically stable <100> orientation to the kinetically driven < 110>
orientation in the secondary growth stage based on a ( 110) substrate which contains
<100> oriented edge type dislocations. Those skilled in the art are aware of factors
that alter growth rate ratios. These include, for example, diamond growth
temperature, carbon fraction in the gas phase, and the presence of certain impurities
such as nitrogen and boron.
Figure 2 illustrates the method steps involved in forming a CVD synthetic diamond
material having a non-parallel dislocation array in accordance with an embodiment of
the present invention and possible alternative synthesis routes which result in a
parallel array of dislocations. Initially, a (001) single crystal diamond substrate 2 is
provided. This may be formed of a natural, HPHT, or CVD synthetic diamond
material. Although each of these different types of diamond material have their own
distinct features and are thus identifiable as distinct, the key feature for this substrate
is that the growth surface is carefully prepared to have a good surface finish.
By good surface finish, we mean a surface having a density of defects equal to or less
than 5 x 10 defects/mm2 as revealed by a revealing plasma etch. The defect density
is most easily characterised by optical evaluation after using a plasma or chemical
etch optimised to reveal the defects (referred to as a revealing plasma etch), using for
example a brief plasma etch of the type described below.
Two types of defects can be revealed:
1) Those intrinsic to the substrate material quality. In selected natural diamond the
density of these defects can be as low as 50/mm2 with more typical values being
102/mm2, whilst in others it can be 106/mm2 or greater.
2) Those resulting from polishing, including dislocation structures and microcracks
forming chatter tracks along polishing lines. The density of these can vary
considerably over a sample, with typical values ranging from about 102/mm2, up to
more than 104/mm2 in poorly polished regions or samples.
The preferred low density of defects is such that the density of surface etch features
related to defects is below 5 x 10 /mm2, and more preferably below 102/mm2. It
should be noted that merely polishing a surface to have low surface roughness does
not necessarily meet these criteria as a revealing plasma etch exposes defects at and
just underneath the surface. Furthermore, a revealing plasma etch can reveal intrinsic
defects such as dislocations in addition to surface defects such as microcracks and
surface features which can be removed by simple polishing.
The defect level at and below the substrate surface on which the CVD growth takes
place may thus be minimised by careful selection and preparation of the substrate.
Included here under "preparation" is any process applied to the material from mine
recovery (in the case of natural diamond) or synthesis (in the case of synthetic
material), as each stage can influence the defect density within the material at the
plane which will ultimately form the substrate surface when preparation as a substrate
is complete. Particular processing steps may include conventional diamond processes
such as mechanical sawing, lapping and polishing (in this application specifically
optimised for low defect levels), and less conventional techniques such as laser
processing, reactive ion etching, ion beam milling or ion implantation and lift-off
techniques, chemical/mechanical polishing, and both liquid chemical processing and
plasma processing techniques. In addition, the surface RQ measured by stylus
profilometer, preferably measured over a 0.08 mm length, should be minimised,
typical values prior to any plasma etch being no more than a few nanometers, i.e. less
than 10 nanometers. RQ is the root mean square deviation of surface profile from flat
(for a Gaussian distribution of surface heights, RQ=1.25Ra. For definitions, see for
example "Tribology: Friction and Wear of Engineering Materials", IM Hutchings,
(1992), Publ. Edward Arnold, ISBN 0-340-56184).
One specific method of minimising the surface damage of the substrate is to include
an in situ plasma etch on the surface on which the homoepitaxial diamond growth is
to occur. In principle this etch need not be in situ, nor immediately prior to the
growth process, but the greatest benefit is achieved if it is in situ, because it avoids
any risk of further physical damage or chemical contamination. An in situ etch is also
generally most convenient when the growth process is also plasma based. The plasma
etch can use similar conditions to the deposition or diamond growing process, but
with the absence of any carbon containing source gas and generally at a slightly lower
temperature to give better control of the etch rate. For example, it can consist of one
or more of the following:
(i) An oxygen etch using predominantly hydrogen with optionally a small amount of
Ar and a required small amount of 02. Typical oxygen etch conditions are pressures of
50-450 x 102 Pa, an etching gas containing an oxygen content of 1 to 4 percent, an
argon content of 0 to 30 percent and the balance hydrogen, all percentages being by
volume, with a substrate temperature 600-1 100°C (more typically 800°C) and a
typical duration of 3-60 minutes.
(ii) A hydrogen etch which is similar to (i) but where the oxygen is absent.
(iii) Alternative methods for the etch not solely based on argon, hydrogen and oxygen
may be used, for example, those utilising halogens, other inert gases or nitrogen.
Typically the etch consists of an oxygen etch followed by a hydrogen etch and then
moving directly into synthesis by the introduction of the carbon source gas. The etch
time/temperature is selected to enable remaining surface damage from processing to
be removed, and for any surface contaminants to be removed, but without forming a
highly roughened surface and without etching extensively along extended defects
such as dislocations which intersect the surface and thus cause deep pits. As the etch
is aggressive, it is particularly important for this stage that the chamber design and
material selection for its components be such that no material is transferred by the
plasma from the chamber into the gas phase or to the substrate surface. The hydrogen
etch following the oxygen etch is less specific to crystal defects rounding off the
angularities caused by the oxygen etch which aggressively attacks such defects and
providing a smoother, better surface for subsequent growth.
Having suitably prepared the growth surface of the (001) single crystal diamond
substrate 2 as illustrated in Figure 2, Step A involves CVD growth of a primary layer
of (001) oriented single crystal CVD synthetic diamond material 4 on the substrate 2 .
This layer will comprise <100> oriented edge type dislocations as previously
discussed in relation to Figure 1.
In Step B, the primary layer of (001) oriented single crystal CVD synthetic diamond
material 4 is vertically sliced along a diagonal (indicated by the dotted lines in Figure
2) to yield a ( 110) single crystal diamond plate 6 as illustrated in Step C. This may be
achieved using a laser. The ( 110) single crystal diamond plate 6 may then be used as a
substrate on which the secondary layer of single crystal CVD synthetic diamond
material 8 is grown. Subsequently, the secondary layer of single crystal CVD
synthetic diamond material can be grown on the growth surface of the ( 110) substrate.
The growth surface of the ( 110) substrate 6 must be treated in a similar manner as
described in relation to the (001) substrate to obtain a good surface finish. By good
surface finish, again we mean a surface having a density of defects equal to or less
than 5 x 10 defects/mm2 and more preferably below 102/mm2 as revealed by a
revealing plasma etch. However, over-etching the substrate will lead to pits being
formed on the substrate surface. Typically these pits consist of (001) and ( 111)
crystallographic planes and if they are of a depth greater than 5 m they will result in
the nucleation of new dislocations in the ( 110) layer which will adopt a low energy
(i.e. < 110> mixed 60°) configuration. This will also manifest itself in pits on the final
growth surface. The conditions whereby over-etching occur will vary strongly
according to reactor geometry but will occur when etching for an excessive duration
(several hours) or at excessive powers and / or temperatures.
The various possibilities for dislocation types and orientations are illustrated in Steps
D, E and F of Figure 2 . According to Step D, if the initial (001) diamond substrate
was not well prepared, resulting in <100> mixed 45° dislocations in the primary layer,
then a parallel array of < 110> oriented mixed 60° dislocations 10 is formed in the
secondary layer. Also according to Step D, if the ( 110) diamond substrate was not
well prepared, then a parallel array of < 110> oriented mixed 60° dislocations 10 may
be nucleated in the secondary layer. According to Step E, if the initial (001) diamond
substrate was well prepared, resulting in <100> edge dislocations in the primary layer,
but a high < 110> growth rate to <001> growth rate ratio is used for forming the
secondary layer 8, then a parallel array of < 110> oriented screw and edge dislocations
12 is formed in the secondary layer. In contrast, according to Step F, if the initial
(001) diamond substrate was well prepared, resulting in <100> edge dislocations in
the primary layer, and a relatively low < 110> growth rate to <001> growth rate ratio
is used for forming the secondary layer 8, then a non-parallel array of <100> oriented
mixed 45° and edge dislocations 14 is formed in the secondary layer.
Growth on a ( 110) surface as described herein offers a route to a greater variety of
dislocation types in the secondary layer than growth on (001) surfaces. The possible
orientations and types of dislocation in the secondary ( 110) layer are summarized
below.
Dislocation type (assuming < 110> Burger's Vector)
Edge
Mixed 45°
Mixed 60° (most favourable in terms of core energy)
Edge
Mixed 45°
Screw (least favourable in terms of core energy)
These different types and orientations of dislocation in the secondary layer are also
illustrated in Figures 3 and 4 . Figure 3 illustrates dislocation types which can
propagate in a direction parallel to the growth direction in a ( 110) CVD synthetic
diamond layer. The growth direction corresponds to the < 110> direction which is the
vertical direction in the Figures. The dislocations propagate from edge or 45° mixed
dislocations in the primary CVD layer (the lower layer in each of the figures). Figure
4 illustrates dislocation types which propagate at an acute angle relative to the growth
direction in a ( 110) CVD synthetic diamond layer. The dislocations propagate in the
<100> direction at an angle of approximately 45° to the vertical growth direction.
Again, the dislocations propagate from edge or 45° mixed dislocations in the primary
CVD layer. As such, Figure 3 illustrates the dislocations which form in accordance
with Steps D and E as previously described in relation to Figure 2 whereas Figure 4
illustrates the dislocations which form in accordance with Step F as previously
described in relation to Figure 2 .
Energetically, the most favourable dislocations (lowest core energy) in the secondary
layer are < 110> mixed 60° dislocations. < 110> mixed 60° dislocations in the
secondary layer are a result of <100> mixed 45° dislocations in the primary layer or
by poor surface preparation of the ( 110) substrates formed from the primary layer.
More generally, mixed dislocation types tend to be of a lower energy than edge or
screw dislocations. <100> mixed 45° dislocations in the primary layer result from
growth on poorly prepared substrates. Growing the primary layer on a well prepared
(001) substrate surface with a good finish will lead to very few <100> mixed 45°
dislocations in the primary layer, and will therefore help to minimise the number of
< 110> mixed 60° dislocations in the secondary layer. Dislocations are also created at
the interface between the secondary layer and the ( 110) substrate produced from the
primary layer. If the surface preparation of the ( 110) substrate is good, the
dislocations that are created at the interface will only be of the <100> edge or mixed
45° type. If the surface preparation of the ( 110) substrate is poor, < 110> mixed 60°
dislocations will additionally be created. Minimising the number of < 110> mixed 60°
dislocations in the secondary layer created at the interface can therefore be achieved
by processing the ( 110) substrate to a high standard. If both of these steps are
performed in order to eliminate < 110> mixed 60° dislocations, < 110> or <100> edge,
<100> mixed 45°, or < 110> screw dislocations will be grown-in and selected through
adjustment of growth process parameters.
In light of the above, it is apparent that embodiments of the present invention allow:
(i) control of the primary layer to remove <100> mixed 45° dislocations so as to
remove < 110> mixed 60° dislocations in the secondary layer; and (ii) control of the
surface preparation of the ( 110) substrate produced from the primary layer in order to
avoid the creation of < 110> mixed 60° dislocations at the interface between the
substrate and secondary layer. The growth rate may then be controlled in the
secondary layer such that the resultant single crystal CVD synthetic diamond material
has a non-parallel dislocation array comprising <100> mixed 45° and/or <100> edge
dislocations.
In addition to controlling the orientation and type of dislocations which can be
provided in single crystal CVD synthetic diamond material, by using substrate surface
treatments and controlling CVD growth parameters, it is also possible to control the
density of the dislocations formed in the material. Generally, a lower concentration of
defects at the growth surface of a substrate leads to a low density of defects in CVD
synthetic diamond material grown on the substrate. Furthermore, careful control of
the CVD chemistry and process parameters such as pressure, substrate temperature,
flow rate of reactants, and plasma temperature can reduce the density of defects
grown into CVD synthetic diamond material. For example, the primary layer of (001)
single crystal CVD synthetic diamond may comprise a density of dislocations in the
range 10 cm 2 to lxlO cm 2 . Furthermore, the secondary layer of ( 110) single crystal
CVD synthetic diamond may comprise a density of dislocations in the range 10 cm 2
to lxlO 8 cm 2 .
Embodiments of the present invention also allow the possibility of separating out and
investigating single types of dislocations (e.g. < 110> 60° mixed dislocations or <100>
45° mixed dislocations), assessing their fundamental properties, and studying which
types of dislocation cause greater or lesser detriment in terms of device performance.
As such, embodiments of the present invention open up the possibility of providing
single crystal CVD synthetic diamond products in which the concentration,
distribution, orientation, and type of dislocations are chosen and controlled to
minimize their impact on material properties or even improve material properties.
These material properties include optical birefringence, electronic (breakdown and m),
luminescence, and mechanical (wear and toughness).
Embodiments of the present invention can provide a single crystal CVD synthetic
diamond product comprising a significant volume having a dislocation density in the
range 10 cm 2 to lxl 0 cm 2 , lxl 02 cm 2 to lxl 0 cm 2 , or lxl 04 cm 2 to lxl 07 cm 2 .
Alternatively or additionally the single crystal CVD synthetic diamond product can
have a birefringence equal to or less than 5xl0 4 , 5xl0 5 , lxlO 5 , 5xl0 6 , or lxlO 6 .
The material may also have single substitutional atomic nitrogen in a concentration
range 0.001 to 20 ppm, preferably 0.01 to 0.2 ppm.
While the previous discussion has largely been directed to { 110} oriented growth, the
present inventors have also found that similar comments apply having regard to CVD
growth on { 1 13} oriented substrates. In fact, initial results indicate that there may be
some advantages to performing the invention on { 113} oriented substrates as growth
rates can be pushed up whilst still retaining a non-parallel dislocation array. It has
been found that thick, high quality single crystal CVD synthetic diamond material
comprising a non-parallel dislocation array can be fabricated having a { 113}
crystallographic orientation. One significant difference between the { 110} orientation
and the { 113} orientation is that for certain { 110} embodiments the non-parallel
dislocation array is visible in an X-ray topographic cross-sectional view but not under
luminescent conditions whereas for certain { 113} embodiments the non-parallel
dislocation array is visible under luminescent conditions but not in an X-ray
topographic cross-sectional view. This is because dislocations in certain line
directions emit blue luminescent light while in other line directions they do not. For
{ 113} embodiments the crystallographic line direction of the dislocations in the nonparallel
dislocation array is such that they emit blue luminescent light whereas for
{ 110} embodiments the crystallographic line direction of the dislocations in the nonparallel
dislocation array is such that they do not emit blue luminescent light.
Some examples of single crystal CVD synthetic diamond layers formed in accordance
with the methods discussed herein are shown in Figures 5 to 8 and described below.
EXAMPLE 1
A synthetic type lb UPHT diamond plate with a pair of approximately parallel major
faces within approximately 5° of a (001) orientation was selected. The plate was
fabricated into a square substrate suitable for homoepitaxial synthesis of single crystal
CVD synthetic diamond material by a process including the following steps:
i) laser cutting of the substrate to produce a plate with all <100> edges; and
ii) lapping and polishing the major surface upon which growth is to occur, the
lapped and polished part having dimensions approximately 6.0 mm 6.0 mm by 400
mhi thick, with all faces {100}.
A defect level at or below the substrate surface was minimised by careful preparation
of the substrate as disclosed in EP 1 292 726 and EP 1 290 251. It is possible to
reveal the defect levels being introduced by this processing by using a revealing
plasma etch. It is possible routinely to produce substrates in which the density of
defects measurable after a revealing etch is dependent primarily on the material
quality and is below 5 x 10 mm 2 , and usually below 102mm 2 . The surface roughness
at this stage was less than 10 nm over a measured area of at least 50 mih c 50 mih.
The substrate was mounted onto a substrate carrier. The substrate and its carrier were
then introduced into a CVD reactor chamber and an etch and growth cycle
commenced by feeding gasses into the chamber as follows:
First, an in situ oxygen plasma etch was performed using 16/20/600 seem (standard
cubic centimetre per second) of 0 2/Ar/H2 at a pressure of 230 Torr, a microwave
frequency of 2.45 GHz, and a substrate temperature of 780 °C, followed by a
hydrogen etch, oxygen being removed from the gas flow at this stage. Then the first
stage growth process was started by the addition of methane at 22 seem. Nitrogen
was added to achieve a level of 800 ppb in the gas phase. Hydrogen was also present
in the process gas. The substrate temperature at this stage was 827 °C. Over the
subsequent 24 hours the methane content was increased to 32 seem. These growth
conditions were selected to give an a parameter value in the range of 2.0±0.2, based
on previous test runs and confirmed retrospectively by crystallographic examination.
On completion of the growth period, the substrate was removed from the reactor and
the CVD synthetic diamond layer removed from the substrate by laser sawing and
mechanical polishing techniques.
Study of the grown CVD synthetic diamond plate revealed that it was free of twins
and cracks on the (001) face, and bounded by < 110> sides and post-synthesis
dimensions of the twin free top (001) face were increased to 8.7 mm 8.7 mm.
This block was then subsequently processed, using the same techniques described
previously (cutting, lapping, polishing and etching) for the production of the lb HPHT
plate, to produce a plate with a major face ( 110) and a well-prepared surface with
dimensions 3.8 x 3.2 mm and 200 mih thick. This was then mounted and grown on
using identical conditions to that described above with the exception that during the
synthesis stage, the substrate temperature was 800°C and nitrogen was not introduced
as a dopant gas. This produced a CVD sample with a ( 110) major face and the CVD
block had typical dimensions of 5.0 x 4.1 mm and 1.6 mm thickness.
In order to study the dislocation structure for this example an X-ray section topograph
using a Bragg {533} reflection was recorded (corresponding to a (100) cross section).
The X-ray topograph is shown in Figure 5 . Figure 5 illustrates a single crystal CVD
synthetic diamond layer grown upon a vertically-sliced ( 110) CVD synthetic substrate
according to an embodiment of the present invention comprising a parallel array of
dislocations. In this cross-section the dislocations form a parallel arrangement that
follows the growth direction, i.e. the [ 1 10] direction. Such a configuration may
correspond to Step D or Step E as illustrated in Figure 2 .
For the sample illustrated in Figure 5, the < 110>:<001> growth rate ratio was 1.1 and
therefore falls within the "high" range where the growth is more likely to be
dominated by kinetic rather than thermodynamic factors. It is clear from the image
that the dislocations in Example 1 form a parallel array. It can be seen that 85% of the
dislocations imaged in the X-ray topograph are between 0° and 2° of the < 110>
growth direction.
EXAMPLE 2
A ( 110) substrate was produced in an identical fashion to that described in Example 1.
The growth conditions for the second growth stage were identical to those for
Example 1 with the exception that the substrate temperature was reduced by 70
degrees to approximately 730°C. This produced a CVD sample with dimensions of
5.7 x 3.5 mm and 1.4 mm thickness. The seemingly minor change to the substrate
temperature reduced the growth rate ratio < 110>:<001> from a high value to a low
value (approximately 0.4).
In order to study the dislocation structure for this example an X-ray section topograph
using a Bragg {533} reflection was recorded (corresponding to a (100) cross section).
The X-ray topograph is shown in Figure 6 . Figure 6 illustrates a single crystal CVD
synthetic diamond layer grown upon a vertically-sliced ( 110) CVD synthetic substrate
according to an embodiment of the present invention comprising a non-parallel array
of dislocations. It can readily be seen that the dislocations form a non-parallel array
and propagate in a direction close to the [100] direction. Such a configuration may
correspond to Step F as illustrated in Figure 2 .
For the sample illustrated in Figure 6, the < 110>:<001> growth rate ratio was 0.4 and
therefore falls within the "low" range where the growth is more likely to be
dominated by thermodynamic rather than kinetic factors. It can be seen from Figure 6
that the dislocations in Example 2 comprise a non-parallel array. The dislocations
form an array of inter-crossing dislocations over the entire volume of the single
crystal CVD synthetic diamond layer. The dislocations propagate in two directions,
with the angle between the first and second direction being between 66° and 72°. 95%
of the dislocations present within the entire volume of the sample are oriented
between 9° and 12° of a <100> line direction. Furthermore, 95% of the dislocations
present within the entire volume of the sample are 33° to 36° from the < 110> growth
direction. Analysis of twelve NV centres showed that they were all grown-in with
preferential orientation in the < 111> direction out-of-plane with respect to the ( 110)
growth surface.
Figure 7 illustrates a birefringence micrograph for the CVD synthetic diamond
material of Figure 6 which shows comparatively low strain considering the significant
dislocation density for this sample.
Initial data also indicates that the CVD synthetic diamond material shown in Figures 6
and 7 has increased material hardness. An earlier review paper by Balmer et al. (J.
Phys.: Condensed Matter 2 1 (2009) 364221) has already disclosed that tools made
using a ( 110) orientation show lower wear and higher chip resistance than those made
with a (001) plane. Initial data for the new material discussed herein indicates that the
new material possesses the advantages of ( 110) diamond in terms of wear resistance,
coupled with increased hardness (e.g. at least 100 GPa, more preferably at least 120
GPa).
EXAMPLE 3
A synthetic type lb HPHT diamond was cut to form a plate with a pair of
approximately parallel major faces within approximately 5° of a ( 113) orientation.
The major surface upon which growth is to occur was further processed by lapping
and polishing.
The substrate was mounted onto a substrate carrier. The substrate and its carrier were
then introduced into a CVD reactor chamber. Etch and growth cycles were performed
as described in Example 1 with the exception that the substrate temperature was
reduced by 70 degrees to approximately 730°C as described in Example 2 .
The resultant single crystal CVD synthetic diamond material was found to comprise a
non-parallel dislocation array visible under luminescent conditions (emitting blue
luminescent light characteristic of dislocations of certain crystallographic line
direction) but not in an X-ray topographic cross-sectional view.
Figure 8 illustrates single crystal CVD synthetic diamond layers grown on { 110} and
{ 113} oriented substrates (as described in Examples 2 and 3) in X-ray topographic
cross-sectional view (top) and under luminescent conditions (bottom). As can be seen
in the Figure, for the { 110} example the non-parallel dislocation array is visible in the
X-ray topographic cross-sectional image but not in the luminescent image whereas for
the { 113} example the non-parallel dislocation array is visible in the luminescent
image but not in an X-ray topographic cross-sectional image.
While this invention has been particularly shown and described with reference to
preferred embodiments, it will be understood to those skilled in the art that various
changes in form and detail may be made without departing from the scope of the
invention as defined by the appendant claims.
Claims
1. A single crystal CVD synthetic diamond layer comprising a non-parallel
dislocation array, wherein the non-parallel dislocation array comprises a plurality of
dislocations forming an array of inter-crossing dislocations, as viewed in an X-ray
topographic cross-sectional view or under luminescent conditions.
2 . A single crystal CVD synthetic diamond layer according to claim 1, wherein
the layer of single crystal CVD synthetic diamond has a thickness equal to or greater
than 1 m i, 10 mpi, 50 mpi, 100 mpi, 500 mpi, 1 mm, 2 mm, or 3 mm.
3 . A single crystal CVD synthetic diamond layer according to claim 1 or 2,
2 8 comprising a density of dislocations in a range 10 cm to 1x10 cm 2, 1x102 cm 2 to
lxlO 8 cm 2 , or lxlO 4 cm 2 to lxlO 7 cm 2 .
4 . A single crystal CVD synthetic diamond layer according to any preceding
claim, comprising a birefringence equal to or less than 5xl0 4 , 5xl0 5 , lxlO 5 , 5xl0 6 ,
or lxlO 6 .
5 . A single crystal CVD synthetic diamond layer according to any preceding
claim, wherein the single crystal CVD synthetic diamond layer is a { 110} or { 113}
oriented layer.
6 . A single crystal CVD synthetic diamond layer according to any preceding
claim, wherein the non-parallel dislocation array extends over a significant volume of
the single crystal CVD synthetic diamond layer, the significant volume forming at
least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a total volume of the single crystal
CVD synthetic diamond layer.
7 . A single crystal CVD synthetic diamond layer according to any preceding
claim, wherein the non-parallel dislocation array comprises a first set of dislocations
propagating in a first direction through the single crystal CVD synthetic diamond
layer and a second set of dislocations propagating in a second direction through the
single crystal CVD synthetic diamond layer, and wherein an angle between the first
and second directions lies in the range 40° to 100°, 50° to 100°, or 60° to 90° as
viewed in an X-ray topographic cross-sectional view or under luminescent conditions.
8 . A single crystal CVD synthetic diamond layer according to any preceding
claim, wherein a direction in which a dislocation propagates is measured in terms of
an average direction over a significant length of the dislocation, where the significant
length is at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a total length of the
dislocation and/or at least 50 mp , 100 mp , 250 mp , 500 mp , 1000 mp , 1500 mp , or
2000 mih.
9 . A single crystal CVD synthetic diamond layer according to any preceding
claim, wherein at least 30%, 40%, 50%, 70%, 80%, or 90% of a total number of
visible dislocations within a significant volume of the single crystal CVD synthetic
diamond layer form the non-parallel dislocation array as viewed in an X-ray
topographic cross-sectional view or under luminescent conditions, the significant
volume forming at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a total volume
of the single crystal CVD synthetic diamond layer.
10. A single crystal CVD synthetic diamond layer according to any preceding
claim, wherein the non-parallel dislocation array as viewable in an X-ray topographic
cross-sectional view but not under luminescent conditions or alternatively the nonparallel
dislocation array as viewable under luminescent conditions but not in an Xray
topographic cross-sectional view.
11. A single crystal CVD synthetic diamond material layer according to any
preceding claim, comprising a hardness of at least 100 GPa or at least 120 GPa.
12. A single crystal CVD synthetic diamond object comprising a single crystal
diamond layer according to any preceding claim, wherein the single crystal diamond
layer forms at least 30%, 40%, 50%, 60%, 70%, 80%, or 90% of a total volume of the
single crystal CVD synthetic diamond object.
13. Use of a single crystal CVD synthetic diamond object according to claim 13 in
an optical, mechanical, luminescent, and/or electronic device or application.
14. A single crystal CVD synthetic diamond object according to claim 13, wherein
the single crystal CVD synthetic diamond object is cut into a gemstone configuration.
15. A method of forming a single crystal CVD synthetic diamond layer, the
method comprising:
providing a single crystal diamond substrate with a growth face having a
density of defects equal to or less than 5 x 10 defects/mm2 as revealed by a revealing
plasma etch; and
growing a layer of single crystal CVD synthetic diamond according to any one
of claims 1 to 12 on the growth face.
16. A method according to claim 15, wherein the growth face of the single crystal
diamond substrate has a { 110} or { 113} crystallographic orientation.
17. A method according to claim 15 or 16, wherein a growth rate of the layer of
single crystal CVD synthetic diamond is controlled to be sufficiently low such that the
non-parallel dislocation array is formed.
18. A method according to any one of claims 15 to 17, wherein the non-parallel
dislocation array comprises a significant number of dislocations propagating at an
acute angle of at least 20° relative to a growth direction of the layer of single crystal
CVD synthetic diamond, said significant number being at least 30%, 40%, 50%, 60%,
70%, 80% , or 90% of a total number of dislocations visible in an X-ray topographic
cross-sectional view or under luminescent conditions.
19. A method according to claim 18, wherein said significant number of
dislocations propagate at an acute angle in the range 20 to 60°, 20 to 50°, or 30 to 50°
relative to the growth direction of the layer of single crystal CVD synthetic diamond.