NL2032889B1

NL2032889B1 - Heteroepitaxial growth of single crystalline diamond on a substrate

Info

Publication number: NL2032889B1
Application number: NL2032889A
Authority: NL
Inventors: Ishihara Ryoichi
Original assignee: Univ Delft Tech
Priority date: 2022-08-29
Filing date: 2022-08-29
Publication date: 2024-03-12
Also published as: WO2024049294A1

Abstract

A method of forming single crystalline diamond on a substrate is described wherein the method comprises: providing an epitaxial base layer over a substrate, wherein the epitaxial base layer has a crystalline structure that closely matches the crystalline structure of diamond, preferably the epitaxial base layer being an lridium layer; forming at least one inorganic insulating layer over the epitaxial base layer; forming nanoholes in the at least one insulating layer, the base of each nanohole exposing at part of the epitaxial base layer; and, hetero-epitaxially growing single crystalline diamond grains over the nanoholes using a chemical vapour deposition technique.

Description

NL34342 -Viltd

Heteroepitaxial growth of single crystalline diamond on a substrate

Technical field

The embodiments relate to heteroepitaxial growth of diamond, and, in particular, though not exclusively, to a method for hetero-epitaxially growing single crystalline diamond, such as single crystalline diamond grains or a single crystalline thin film layer.

Background

Diamond single crystal substrates are typically made by high pressure and high temperature diamond (HPHT) growth methods, that mimic the thermodynamic conditions in nature wherein diamond is formed. The maximum size, of approx. 2 x 2 mm of single crystalline diamond substrate with low impurity level that can be realized by such growth methods is however very small. Because of such small substrate sizes, advance nanofabrication equipment which use wafer sizes of two inch or larger cannot be used. Additionally, the price of such single crystal substrates is very high.

It is possible to grow polycrystalline diamond on a large surface using

CVD, however the crystalline size, or the grain size, is very small and hence there are many defects between the grains. Consequently, the properties of such layers are not suitable for high performance applications.

M. Schreck et al in their article Heteroepitaxial growth of diamond using

Ir/SrO3 buffer layer on 4-inch silicon substrate, Scientific Reports, describe heteroepitaxial growth of diamond on a four-inch Ir/SrTiO3 substrate. Similarly, Aida et al, in their article Fabrication of freestanding heteroepitaxial diamond structure via micropatterns and microneedies Applied Physics Express 9, 035504 (2016) http://doi.org/10.7567/APEX.9.035504 describe the fabrication of heteroepitaxial diamond substrates via a process of etched diamond micropatterns. However, these films include many defects and impurities and have a high strain. Additionally the growth time (one day or more) of these films is too long.

Hence, from the above, it follows that there is a need in the art for improved methods for epitaxial growth of diamond thin-film layers. In particular, there is a need in the art for methods for epitaxial growth of diamond thin-film layers that can be used for realizing wafer-scale epitaxial thin-film layers.

Summary

It is an objective of the embodiments in this disclosure to reduce or eliminate at least one of the drawbacks known in the prior art. In an aspect, the embodiments may relate to a method of forming single crystalline diamond on a substrate wherein the method comprises: forming at least one inorganic insulating layer over a top surface of the substrate, wherein the top surface of the substrate approximately matches the crystalline structure of diamond; forming nanoholes, preferably a two-dimensional array of nanoholes, in the at least one insulating layer, the base of each nanohole exposing at part of the top surface of the substrate; and, hetero-epitaxially growing single crystalline diamond grains over the nanoholes using a chemical vapour deposition technique, preferably a microwave plasma enhanced chemical vapour deposition (MWCVD) technique. .The embodiments in this application enabling growth of single crystalline diamond thin film layers on a large surface that have reduced defects.

Nano-patterns are formed on a large host substrate and a high quality crystal will be grown out of these patterns and connected on the surface to from a continuous epitaxial diamond layer.

In an embodiment, the substrate may comprise an epitaxial base layer wherein the top surface of the epitaxial base layer forms the top surface of the substrate. In an embodiment, the epitaxial base layer may comprise an Iridium layer.

In an embodiment, the method may further comprise: hetero-epitaxially growing a single crystalline diamond layer, preferably a single crystalline diamond thin film layer 1 to 500 micron thick single crystalline diamond thin film layer, over the single crystalline diamond grains using a chemical vapour deposition technique.

Hence, the nanoholes may be used as sites for forming an array of single crystal diamond grains that may form a template for growing a single crystalline diamond layer. When using such template single crystalline diamond grains may be grown such that they connect, coalescent and from a continuous epitaxial diamond layer that covers the surface of the insulating layer.

In an embodiment, a microwave plasma enhanced chemical vapour deposition (MWCVD) technique is used for the formation of the single crystalline diamond grains and/or the single crystalline diamond layer.

In an embodiment, before the single crystalline diamond layer is grown over the crystalline diamond grains, the inorganic insulating layer is removed, preferably using a selective etching technique. Hence, in this embodiment, the inorganic insulating layer is removed to form diamond nanopillars on the substrate, wherein the nanopillars on the substrate are used as a template to form a high quality single crystalline diamond layer.

In embodiment, the epitaxial base layer may be in direct contact with the substrate, preferably a MgO substrate.

In an embodiment, at least one buffer layer is provided between the epitaxial base layer, preferably the substrate being a Si or a SiC substrate. The buffer layer may be selected to have a thickness selected between 10 nm and 10 micron, preferably 20 nm and 5 micron, more preferably 40 nm and 2 micron.

In an embodiment, the at least one buffer layer may include a an SrTiOs layer and/or YSZ layer.

In an embodiment, the inorganic insulating layer comprise an oxide layer, such as SiO2 or an Al203 layer. In another embodiment, the inorganic insulating layer may comprise a nitride layer, such as a Si3N4 layer. In yet another embodiment the inorganic insulating layer may comprise an oxynitride layer.

In an embodiment, the thickness of the inorganic insulating layer may be selected between 10 nm and 10 micron, preferably 20 nm and 5 micron, more preferably 40 micron and 2 micron.

In an embodiment, the nanoholes may form a grid or array of nanoholes over the substrate, wherein the pitch between two neighboring nanoholes may be selected between 20 nm and 1000 nm.

In an embodiment, the cross-sectional shape of the nano-holes may be a rectangular, preferably a square

In an embodiment, the sides of the rectangular nano-holes may be aligned with crystalline orientations of the substrate and/or the epitaxial base layer.

In an embodiment, the horizontal cross-sectional sides of the nanoholes have a width selected between 10 nm and 500 nm and/or a depth between 10 nm and 10 micron, preferably 20 nm and 5 micron, more preferably 50 nm and 1 micron.

In an embodiment, the nanoholes may form a two-dimensional grid, wherein the sides of the nanoholes, e.g. square nanohole or triangular nanoholes, are aligned with the crystallographic orientation of the substrate surface.

In an embodiment, the substrate temperature during the growing may be selected between 600 and 900 °C. In an embodiment, the microwave power may be selected between 100W and 2000W. In an embodiment, the total pressure during growth may be selected between 1 and 50 Torr. In an embodiment, the molar ratio of

CH4 and H2 used during the MWCVD may be selected between 0.1% and 10%

Applications of the method and the products produces by these methods will be in the semiconductor field including but not limited to high-power electronics used in electric cars, heat spreader for high-power LED used in lighting or for high frequency devices using in 5G base stations.

In an embodiment, during growing the single crystalline diamond grains on the basis of a microwave plasma enhanced chemical vapour deposition, applying a DC bias for enhancement of nucleation of diamond in the nanoholes, preferably the voltage applied to the substrate being selected in the range between -50V and - 500V.

The invention will be further illustrated with reference to the attached drawings, which schematically will show embodiments according to the invention. It will be understood that the invention is not in any way restricted to these specific embodiments.

Brief description of the drawings

Fig. 1A and 1B depict a nanostructure template for growing a single crystalline diamond grain according to an embodiment;

Fig. 2 depicts a schematic for realizing an array of single crystalline diamond grains and for growing an epitaxial diamond thin film layer according to an embodiment;

Fig. 3 depicts a top view of a structure for forming an array of single 5 crystalline diamond grains according to an embodiment;

Fig. 4 depicts a nanostructure comprising a single crystalline diamond grain according to an embodiment;

Fig. 5A and 5B depict top views of a structure for forming an array of single crystalline diamond grains according to an embodiment;

Fig. 6A-6D depicts a method for growing an epitaxial diamond layer according to an embodiment;

Fig. 7A and 7B depicts a top view of a structure for forming an array of single crystalline diamond grains according to another embodiment of the invention

Description of the embodiments

The embodiments in this application aim to solve or at least substantially reduce problems related to the realisation of single crystalline diamond thin films. The embodiments aim to solve these problems using nano-patterns, in particular nanoholes, that are formed in an insulating layer formed over substrate which has a crystalline structure that substantially matches a crystalline structure of single crystal diamond. In some embodiment, the nano-patterns may be formed in an insulating layer formed over an epitaxial base layer which has a crystalline structure that substantially matches a crystalline structure of single crystal diamond.

This epitaxial base layer may be formed over a substrate, e.g. a Silicon substrate.

These nanoholes may be used as sites for forming an array of single crystal diamond grains that may form a template for growing a single crystalline diamond layer. In particular, when using such nanoholes template in an embodiment, single crystalline diamond grains may be grown such that they connect, coalescent and from a continuous epitaxial diamond layer that covers the surface of the insulating layer.

Fig. 1A and 1B depict cross-sectional schematic of exemplary embodiments of a nano-hole structure for growing a single crystalline diamond grain according to an embodiment. Such structure may be referred to a nano-hole template. As shown in Fig. 1A, the structure may include a substrate 100 having a top surface 101, wherein the crystalline structure of the substrate approximately matches a crystalline structure of a single crystal diamond. An insulating layer 108 is provided over the substrate and a nanohole may be formed in the insulating layer such that the base of the hole exposes part 109 of the top surface of the substrate.

The thickness of the insulating layer may be selected between 10 nm and 10 micron so that holes may have a depth between 10 nm and 10 micron. The horizontal cross- sectional shape of the nanohole may have different geometrical shapes depending on the crystalline orientation of the substrate. For example, in an embodiment, the substrate may have a (100) orientation. In another embodiment, the substrate may have a (111) orientation.

As will be described hereunder in greater detail, the substrate comprising the nanoholes may be used as a template for forming a thin film epitaxial diamond layer. In particular, a heteroepitaxial growth technique such as a plasma-enhanced CVD technique may be used to form crystalline diamond in the nanohole. During the vertical growth, defects will be trapped and grains 110 will be filtered out so that a small single crystalline diamond 112 may be formed over the nano hole. This way, the nanohole may form a nucleation site for the formation of a single crystalline diamond grain.

In some embodiments, the substrate may have a (multi) layered structure. An example of a nanohole structure comprising a multi-layered substrate is depicted in Fig. 1B, the structure may include base substrate 102, e.g. a silicon substrate, on an which an epitaxial base layer 104 is formed that closely matches the crystalline structure of diamond, materials for such layter may include at least one of beta-SiC, MgO, c-BN, Al203, Ni, Ir, Cu, TiC, Co, NizSis, Ni3Ge. In an embodiment the epitaxial base layer may be an iridium Ir layer. In some embodiments, to form the epitaxial base layer on the silicon base substrate an buffer layer 104 may be used wherein the buffer layer may have a thickness between 10 and 100 nm. In an embodiment, the buffer layer may include a SrTiO; layer and/or YSZ layer 106. Further, an insulating layer 108, e.g. an Al20s and/or

SiO2 layer, may be formed over the epitaxial base wherein nanoholes may be formed in the insulating layer such that the base of the nanohole exposes the epitaxial base layer. Crystalline diamond may be formed in the nanohole in a similar way as described with reference to Fig. 1A, wherein during the vertical growth, defects will be trapped and grains 110 will be filtered out so that a small single crystalline diamond 112 may be formed over the nano hole..

Fig. 2A-2H depicts a method of epitaxial growth of diamond according to an embodiment. In particular, the method may start with a substate 200 (Fig. 2A) which a crystalline structure that approximately matches the crystalline structure of diamond. As already described with reference in some embodiment, the substrate may be a single substrate. In other embodiments, the substrate may be a multi- layered substrate including a base substrate 202 e.g. such as a silicon base substrate, e.g. Si (100) or Si (111) substrate, a silicon carbide 8-SiC (100) or B-SiC (111) substrate or a manganese oxide MgO (100) or MgO (111) substrate. In an embodiment, the substrate may be a wafer. An epitaxial base layer 206, which has a crystalline structure that approximately matches the crystalline structure of diamond may be formed over the base substrate. In some embodiments, the epitaxial base layer may be deposited directly onto the base substrate. For example, in case of a

MgO substrate, an Ir layer may be formed directly onto the substrate to form the epitaxial base layer. Here, sputtering technique at high substrate temperatures 800- 900 °C.

In further embodiments, before formation of the base layer, a buffer layer 204 may be deposited on the substrate, wherein the buffer layer may form a buffer between the substrate and the epitaxial base layer 206. In an embodiment, the buffer layer may include SrTiO3 layer. In a further embodiment, the buffer layer may include YSZ layer. Such buffer layer may be used in case the crystalline mismatch between the substrate and crystalline Ir layer is too large, e.g. in case of a Si substrate. The buffer layer may be epitaxially grown on the substrate using a suitable deposition method such as MBE or pulsed-laser deposition.

Then in a further step (Fig. 2B) an insulating layer 208, e.g., SiO2 or

Al203, may be deposited on the substrate surface by PECVD, LPCVD or thermal oxidation. The thickness can be in the range of 10 nm — 1000 nm with a typical value of 500 nm. A mask layer 209 may be formed over the insulating layer. Then holes 210 may be formed in the mask layer (Fig. 2C). In an embodiment, the holes may have dimensions of 10 nm — 500 nm by 10 nm — 500 nm. Then, holes 212 may be dry etched in the insulating layer (Fig. 2D). The holes may be formed using an anisotropic etch, such as a reactive ion etching (RIE) technique or an another suitable anisotropic etch such as the known Bosch process which ix a higheaspaat ratio

B Dased protection fim deposition Dy quick gas swiiching ovalas, wieren the SE, plasma ovals atchay silicon, and the GF plasma ovals creates a protection layer,

The holes may be etched through the insulating layer so that the base (bottom) of the holes expose the epitaxial base layer. The pitch between the holes can be in the range between 20 nm and 1000 nm. In an embodiment, the cross- sectional shape of the holes may be rectangular, preferably a square. In a further embodiment, the edges of the rectangular holes may be aligned with the crystal direction of the substrate and/or the epitaxial base layer.

Then, diamond may be grown hetero-epitaxially in the holes using a chemical vapour deposition technique. In an embodiment, a microwave plasma enhanced chemical vapour deposition (MWCVD) technique may be use. In an embodiment, the diamond may be grown at temperatures selected in a range between 600 and 900 °C, preferably around 750°C. Further growth parameters may include CH4 and H2 at a pressure of 10 and 50 mBar and an applied microwave power selected between 100 and 2000W. During growth, single crystalline diamond grains 214 will nucleate at the surface of the base layer (Fig. 2E) which is exposed at the base of the holes. This way, the holes 216 will be filled with diamond grains (Fig. 2F) wherein the holes act as grain filters so that when the diamond exits the holes, a single crystalline grain is formed over the holes. Typically, these single crystalline diamond grains will have a pyramid shape (Fig. 2G) due to preferred growth direction of the crystalline diamond material. During deposition, the total pressure can be in the range of 1-50 Torr. Further, the molar ratio of CH4 and H2 may be selected in the range of 0.1% to 10%.

In an embodiment, the deposition system may have a substrate DC bias for enhancement of nucleation in the initial phase of the growth. In an embodiment, the voltage applied to the substrate may be selected in the range between -50V and -500V.

Then, the epitaxial growth of diamond may be continued so that the pyramid shaped diamond grains grow towards each other until the top surface of the patterned insulating layer is completely covered with a diamond layer. Diamond growth may be continued to form a thin-film single crystalline diamond layer over the substrate. The diamond crystals which exit from those holes will coalesce and become a large single crystalline diamond. If the holes are made over entire wafer, wafer-sized single-crystalline diamond can be formed. In an embodiment, the thin- film single crystalline layer may be selected between 10 and 100 micron. In another the thin-film single crystalline layer may be selected between 10 and 10 micron. In yet another embodiment, the thin-film single crystalline layer may be selected between 10 and 10 micron.

The shape and dimensions of the holes and the arrangement of holes may be varied and optimized for effective removing of defects and coalescence of small crystals. An exemplary embodiment is illustrated in Fig. 3, which shows a top view of a silicon substrate 300 with (100) orientation including a layer structure provided over the substrate, wherein the layer structure includes a plurality of nanoholes 302 arranged in a regular grid as described with reference to Fig. 2.

The horizonal (i.e. in plane) cross sectional shape of nanoholes may be square with sides parallel to the crystal directions of <110>. For example, first sides 304, ; of nanohole 302 may be parallel to the [110] direction and second sides 3086» of the nanohole 302 may be arranged parallel to the [110] direction of the substrate.

For this particular shape and orientation an expected shape of a small diamond grain grown out from the holes can be determined.

As for the patterns of array of holes shown in Fig.3, for a (100) crystalline oriented substrate, in an embodiment, the (centre of) square holes may be arranged on the cross points, e.g. cross points 3124.4, of a diagonal tiling, e.g. a diagonal square tiling pattern, formed by lines 3084.4,3104.4., wherein the sides of the square holes are arranged parallel to the <110> orientation of the substrate as shown in Fig. 3.

Fig. 4 depicts a cross-sectional view of a nanohole 402 which is used as a seed structure to grow a small single crystalline diamond structure 404. As shown in the figure, the slowest growth speed of the crystalline structure is expected on (111) crystal surfaces 4064 ; and the facet will be formed by the corresponding surfaces. The edges 408, ; of the crystal grain with the top surface 410 of the substrate is a straight line parallel to <110> orientations (as is shown in more detail in

Fig. 5). The shape and orientation of the hole will thus effectively filter out crystals grains 403 that have other crystallographic orientations.

Fig. 5A-5C illustrates the epitaxial growth of the diamond structures using nanostructured template according to an embodiment. In particular, Fig. 5A and 5B show horizontal and vertical cross sections respectively of the nanohole template including single crystal diamond structures grown from the nanoholes. Due to the alignment of orientation of the holes with the crystalline orientation of the substrate, effective coalescence of adjacent small crystals 504 grown in holes 502 will occur. The small diamond crystals will have an expected pyramidal shape with edges 506 (denoted by the dashed lines) that are parallel to the <110> crystalline orientation of the substrate. The dashed lines in Fig. 5A show the orientation of the edges of the pyramidal single crystal diamonds that are formed over nanoholes with respect to the crystalline orientation of the substrate at a first time instance during growth. Fig. 5C shows the same array of single crystal diamonds at a later second time instance wherein the diamonds have grown in their preferential crystalline direction so that the edge of the diamonds border each along the <110> orientation.

Fig. 6A-6D depict a method of epitaxial growth of diamond according to another embodiment. In this particular example, single crystalline grain diamonds may be formed over nanoholes in a similar way as described with reference to Fig. 2A-2G. As shown in Fig. 6A such structure may include a substrate 602, an epitaxial base layer 606 and, optionally, one or more buffer layers 604 and an insulating layer comprising nanohole wherein single crystal diamond grains are grown out of and over each nanohole. Then, before the single crystalline grain diamonds coalescence into a diamond layer the insulating layer is removed, thereby forming an array of freestanding diamond nanopillars 620 with a single crystalline diamond grain on top (Fig. 6B). This substrate with freestanding diamond nanopillars may be used as a template 621 to grow a diamond layer 622 (Fig. 6C). In an embodiment, the diamond layer may be a bulk diamond layer of 500 microns or thicker (Fig. 6C). Thereafter, the template and the bulk diamond layer may be separated at the height 624 of the nanopillars (Fig. 6D).

Fig. 7A and 7B show another embodiment of a nanostructured template according to an embodiment. In particular, the figure shows of a top view of a silicon substrate 702 that has a surface with an (111) orientation. A layer over the substrate may include a plurality of triangular nanoholes, for example triangles 704+, arranged in a regular grid as described with reference to Fig. 7A, wherein the sides of the triangular nanoholes may be arrange parallel to the crystal directions of <110>.

For example, first side 7064 of nanohole 7044 may be parallel to a first

[011] direction, second side 7064 of the nanohole 7044 may be arranged parallel to a second direction [011] and third side 706; of the nanohole 7044 may be arranged parallel to the third direction [011] of the substrate. For this particular shape and orientation, an expected shape of a small diamond grain grown out from the holes can be determined. Fig. 7B illustrates such grain including a 111 triangular (flat) top surface, three (slanted) side surfaces (101)(011)(110) bordering the sides of the triangular top surface and three (slanted) side surfaces (001)(010)(100) arranged at the corners of the triangular top surface. Similar to the patterns of array of holes shown in Fig.7, for a (111) oriented substrate, in an embodiment, the edges of the triangular holes may be arranged along the orientations of the (111) plane as shown by lines 7104.3.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

CONCLUSIONS

A method of forming monocrystalline diamond on a substrate, comprising: forming at least one inorganic insulating layer over a top layer of the substrate, the top layer of the substrate approximately corresponding to the crystalline structure of diamond; forming nanoholes, preferably a two-dimensional grid of nanoholes, in the at least one insulating layer, wherein the bottom of each nanohole exposes at least part of the top layer of the substrate; and, heteroepitaxially growing monocrystalline diamond granules over the nanoholes using a chemical vapor deposition technique, preferably a microwave plasma-enhanced chemical vapor deposition (MWCVD) technique; .

The method according to claim 1, wherein the substrate comprises an epitaxial base layer, wherein the top layer of the epitaxial base layer forms the top layer of the substrate, wherein the epitaxial base layer preferably comprises an iridium layer.

The method according to any one of claims 1 or 2, further comprising: hetero-epitaxially growing a monocrystalline diamond layer, preferably a monocrystalline diamond thin film layer having a thickness between 100 nm and 1 mm on top of the monocrystalline diamond granules using a chemical vapor deposition technique, preferably a MWCVD technique.

The method according to claim 3, wherein the method further comprises: removing the inorganic insulating layer before the monocrystalline diamond layer is grown over the crystalline diamond granules, wherein the inorganic insulating layer is preferably removed using a selective etching technique.

The method according to any of claims 1-3, wherein the epitaxial base layer has direct contact with the substrate, preferably an MgO substrate.

The method according to any of claims 1-3, wherein at least one buffer layer is provided between the epitaxial base layer, wherein the substrate is preferably a Si or SiC substrate.

The method according to claim 6, wherein the at least one buffer layer comprises an SrTiO:3 layer and/or a YSZ layer.

The method according to any one of claims 1 to 7, wherein the inorganic insulating layer comprises an oxide layer, such as a SiO: or an Al:O 3 layer, or a nitride layer, such as a SisN 4 layer or an oxynitride layer; and/or, wherein the thickness of the inorganic insulating layer is selected between 10 nm and 10 micrometers, preferably between 20 nm and 5 micrometers, with a larger preference between 40 nm and 2 micrometers.

The method according to any of claims 1-8, wherein the nanoholes form a lattice of nanoholes over the substrate, wherein the distance between two adjacent holes is selected between 20 nm and 1000 nm.

The method according to any of claims 1-9, wherein the cross-sectional shape of the nanoholes is a rectangle, preferably a square.

The method of any one of claims 1-10, wherein the sides of rectangular nanoholes are aligned with crystalline orientations of the substrate and/or the epitaxial base layer.

The method according to any one of claims 1 to 11, wherein the horizontal cross-sectional sides of the nanoholes have a width selected between 10 and 500 nm and/or wherein the nanoholes have a depth between 10 nm and 10 micrometers, preferably 20 nm and 5 micrometers, more preferably between 50 nm and 1 micrometer.

The method of any one of claims 1 to 12, wherein the nanoholes form a two-dimensional grid of nanoholes, wherein the horizontal cross-sectional sides of the nanoholes, for example square nanoholes or triangular nanoholes, are aligned with the crystallographic orientation of the substrate plane.

The method according to any of claims 1 to 13, wherein the substrate temperature during growth is between 600 and 900 °C, the microwave power is selected between 100 W and

2000 W, the total pressure is between 1 and 50 Torr and/or the molar ratio of CH; and Hs: between 0.1% and 10%.

15. The method according to any of claims 1-14, wherein during the growth of the monocrystalline diamond granules based on a MWCVD technique, a direct voltage bias is applied to improve nucleation of diamond in the nanoholes, whereby the voltage applied to the substrate is selected in the range between -50V and -500V.