V Mortet
email: mortetv@fzu.cz

A Taylor

Z Vlčková Živcová

D Machon

O Frank

P Hubík

D Tremouilles

L Kavan

Analysis of heavily boron-doped diamond Raman spectrum

Lattice disorder, electronic Raman scattering, and Fano interaction effects are at the genesis of the Raman spectrum of heavily boron-doped diamond. However, no accurate unified description of this spectrum has been reported yet. In this work, we propose a novel analysis of the Raman spectrum of boron-doped diamond based on classical models of electronic Raman scattering and Fano effect. This new analysis shows that the Raman spectrum of boron-doped diamond results from the combination of electronic Raman scattering and its interaction, i.e. Fano effect, with the diamond phonon density of states and it confirms the 500 cm -1 and 1200 cm -1 bands originate from the phonon density of states.

Introduction

Raman spectroscopy is a fast and nondestructive technique widely used for the characterization of diamonds. Intensities, position and full width at half maximum of Raman peaks are representative of the quality and composition of diamond layers. For instance, heavily borondoped diamond with metallic electrical conductivity has a characteristic Raman signature with two broad bands at ca. 500 cm -1 (B1) and ca. 1200 cm -1 (B2) along with a Fano-shaped zone-center phonon (ZCP) peak below 1332 cm -1 . It is conjectured the Fano-shape results from interaction between discrete diamond zone-center phonon (ZCP) and electronic Raman scattering effects. The majority of studies on the Fano-shaped diamond peak reports a negative value for the Fano asymmetric parameter (q) [START_REF] Szirmai | A detailed analysis of the Raman spectra in superconducting boron doped nanocrystalline diamond[END_REF][START_REF] Ushizawa | Boron concentration dependence of Raman spectra on {100} and {111} facets of B-doped CVD diamond[END_REF][START_REF] Bustarret | Optical and electronic properties of heavily boron-doped homo-epitaxial diamond[END_REF][START_REF] Pruvost | Analysis of the Fano in diamond[END_REF][START_REF] Piccirillo | The temperature dependence of the infrared absorption and Raman spectra due to boron in diamond[END_REF][START_REF] Gonon | Characterization of heavily B-doped polycrystalline diamond films using Raman spectroscopy and electron spin resonance[END_REF][START_REF] Gheeraert | Effect of boron incorporation on the "quality" of MPCVD diamond films[END_REF]. Other authors reported positive values of the Fano asymmetric parameter in layers [START_REF] Ushizawa | Boron concentration dependence of Raman spectra on {100} and {111} facets of B-doped CVD diamond[END_REF][START_REF] Locher | Optical and electrical characterization of boron-doped diamond films[END_REF][START_REF] Utyuzh | Effect of boron impurity on the Raman spectrum of synthetic diamond[END_REF] with boron concentrations close to the metal-insulator transition (Mott transition). The origin of other Raman peaks is subject to discussion. The B2 band had previously been attributed to vibration modes of boron atoms [START_REF] Werner | Charge transport in heavily B-doped polycrystalline diamond films[END_REF]. However, substitutional isotopic studies have, unambiguously, demonstrated its vibration origin to be related to carbon atoms [START_REF] Szirmai | A detailed analysis of the Raman spectra in superconducting boron doped nanocrystalline diamond[END_REF][START_REF] Dubrovinskaia | Large carbon-isotope shift of TC in boron-doped diamond[END_REF][START_REF] Achatz | Isotopic substitution of boron and carbon in superconducting diamond epilayers grown by MPCVD[END_REF]. It is generally agreed that this band corresponds to a maximum of the diamond phonon density of states (PDoS) [START_REF] Sidorov | Superconductivity in diamond[END_REF][START_REF] Vlasov | On the origin of the Raman scattering in heavily boron-doped diamond[END_REF]. The appearance of this Raman forbidden band is due to the relaxation of the Raman wave vector conservation rule in materials with high lattice disorder [START_REF] Gheeraert | Effect of boron incorporation on the "quality" of MPCVD diamond films[END_REF][START_REF] Tiong | Effects of as+ ion implantation on the Raman spectra of GaAs: "spatial correlation" interpretation[END_REF], i.e., the high concentration of substitutional boron atoms in the diamond lattice. However, this interpretation is not fully satisfactory due to the mismatch between the experimental peak position, the known optical maxima of diamond's PDoS [START_REF] Zaitsev | Optical Properties of Diamond: a Data Handbook[END_REF] and contradictory reports of the peak shape: Szirmai et al. presented this peak as symmetrical with a Lorentzian shape [START_REF] Szirmai | A detailed analysis of the Raman spectra in superconducting boron doped nanocrystalline diamond[END_REF], whereas Bustarret et al. depicted it as an asymmetric Fano-shaped peak [START_REF] Bustarret | Optical and electronic properties of heavily boron-doped homo-epitaxial diamond[END_REF][START_REF] Ghodbane | Specific features of 325 nm Raman excitation of heavily boron doped polycrystalline diamond films[END_REF]. The broad asymmetric band B1 is empirically modeled with the sum of a Lorentzian and a Gaussian [START_REF] Bernard | About the origin of the low wave number structures of the Raman spectra of heavily boron doped diamond films[END_REF] or a Lorentzian component [START_REF] Szirmai | A detailed analysis of the Raman spectra in superconducting boron doped nanocrystalline diamond[END_REF]. It is very sensitive to the boron concentration [START_REF] Bernard | Non-destructive determination of the boron concentration of heavily doped metallic diamond thin films from Raman spectroscopy[END_REF][START_REF] Mortet | Insight into boron-doped diamond Raman spectra characteristic features[END_REF] and it was attributed to vibration modes of boron dimers [START_REF] Bernard | About the origin of the low wave number structures of the Raman spectra of heavily boron doped diamond films[END_REF][START_REF] Bourgeois | Impurity dimers in su-B-doped experiment and first-principles calculations[END_REF]. However, substitutional isotopic studies have failed to unambiguously confirm this assignment [START_REF] Achatz | Isotopic substitution of boron and carbon in superconducting diamond epilayers grown by MPCVD[END_REF][START_REF] Vlasov | On the origin of the Raman scattering in heavily boron-doped diamond[END_REF]. The purpose of this article is to address these contradictions. Based on classic models of electronic Raman scattering, Fano effect, and analytical methodology we clarify the origin of the Raman spectrum of the boron-doped diamond.

Experimental

In order to remove any added complication due to the presence of sp 2 carbon phase Raman features, which are present in polycrystalline diamond layers, this study has been carried out on epitaxial borondoped diamond layers. The studied epitaxial layers were grown on high pressure and high-temperature diamond substrates in a 1.5 kW resonance cavity microwave plasma enhanced chemical vapor deposition system, AX5010 from Seki Diamond Systems, with two different boron concentrations. Sample S1 was grown with a boron to carbon ratio, in the gas phase, of 4000 ppm on a (100) oriented substrate using deposition conditions reported in ref. [START_REF] Mortet | Substrate processing optimization for fabrication of smooth boron doped epitaxial diamond layer by PE CVD[END_REF], whereas sample S2 was grown with a higher boron to carbon ratio of 40,000 ppm on a (111) oriented substrate using deposition conditions reported in ref. [START_REF] Mortet | Insight into boron-doped diamond Raman spectra characteristic features[END_REF]. Raman spectra were measured using a LabrRAM HR spectrometer from Horiba Jobin-Yvon interfaced to an Olympus microscope with a 100× objective and a 50 μm confocal aperture, and a HeNe laser 633 nm (1.96 eV) with 8 mW power. The spectrometer was calibrated using the silicon F 1g peak at 520.2 cm -1 and its spectral response was calibrated using a certified HL-2000 Tungsten Halogen Light Source from Ocean Optics. Raw Raman spectra were corrected for instrumental response, temperature effect and the frequency factor [START_REF] Carles | Plasmon-resonant Raman spectroscopy in metallic nanoparticles: surface-enhanced scattering by electronic excitations[END_REF][START_REF] Brooker | Assessment of correction procedures for reduction of Raman spectra[END_REF][START_REF] Shuker | Raman-scattering selection-rule breaking and the density of states in amorphous materials[END_REF]. The temperature correction is particularly important in order to observe the broad electronic Raman scattering response. Corrected spectra or reduced Raman (I r (ω)) spectra are calculated using Eq. ( 1) with I(ω) the experimental spectrum,K(ω) the spectrometer response; ω L the absolute frequency of laser excitation line, ω the Raman shift, and 1 thermal population factor with n(ω, T) the Bose-Einstein distribution.

= -+

--

I ω I ω K ω ω ω ω n ω T ( ) ( ). [ ( ). ( ) . . ( ( , ) 1)] r L 4 1 1
(1)

Results and discussion

Normalized raw and reduced Raman spectrum of both samples are shown in Fig. 1 and Fig. 2, respectively. The raw spectra are characteristic of boron-doped diamond with metallic electrical conductivity with low (sample S1) and high boron concentration (sample S2) [START_REF] Ghodbane | Specific features of 325 nm Raman excitation of heavily boron doped polycrystalline diamond films[END_REF][START_REF] Bernard | Non-destructive determination of the boron concentration of heavily doped metallic diamond thin films from Raman spectroscopy[END_REF]. In contrary to sample S2, sample S1 exhibits no measurable band at ca. 500 cm -1 . Both reduced spectra retain the characteristic band of boron-doped diamond. They share a broad asymmetric background with a maximum between 1000 and 2000 cm -1 attributed to electronic Raman scattering, a broadband B2 merged with the narrow diamond line, and a broad dip at 1350 cm -1 attributed to Fano antiresonance. The large width of this dip cannot originate only from the antiresonance of the Fano shaped narrow diamond line. We attribute this dip to a combination of the Fano antiresonance of the B2 band and the diamond line. This assumption is supported by the clear asymmetry of the B2 band, which is characteristic for Fano effect, and is consistent with the work of Bustarret et al. [START_REF] Bustarret | Optical and electronic properties of heavily boron-doped homo-epitaxial diamond[END_REF]. The two Raman spectra exhibit a different curvature at low wavenumbers, i.e., < 500 cm -1 . The concavity of S1 spectrum is consistent with electronic Raman scattering whereas the convexity of S2 sample may be attributed to a Fano antiresonance of the B1 band. This assumption is supported by the well-known asymmetry of this band. Based on these observations, we constructed a fitting function to describe the Raman spectrum of boron-doped diamond assuming the interaction of the observed Raman lines with electronic Raman scattering applies to the diamond ZCP phonon line and all other observed bands using well-established model of electronic Raman scattering and Fano effect. This hypothesis is legitimized by the asymmetric nature of the different bands and the broad range of electronic Raman scattering.

To our knowledge and in contrary to metals [START_REF] Akemann | Electronic Raman scattering at disordered noble-and alkalimetal surfaces[END_REF][START_REF] Ponosov | Measurements of Raman scattering by electrons in metals: the effects of electron-phonon coupling[END_REF] and heavily doped silicon [START_REF] Ipatova | Electron light scattering from doped silicon[END_REF], electronic Raman scattering has not been investigated in boron-doped diamond. In this work, the normalized electronic Raman scattering I e (ω) is modeled using the Eq. ( 2) used for Raman scattering by intra-band excitations and where ω e is the position of the maximum of the electronic Raman scattering spectrum. This equation was shown to satisfactorily describe electronic Raman scattering in conventional elemental metals [START_REF] Ponosov | Measurements of Raman scattering by electrons in metals: the effects of electron-phonon coupling[END_REF] and highly doped semiconductors [START_REF] Contreras | Raman scattering by intervalley carrier-density fluctuations in n-type Si: Intervalley and intravalley mechanisms[END_REF]. 3) and ( 4)) with q j the Fano asymmetric parameter, ω j the center position and Γ j the full width at half maximum of the Lorentzian shaped Raman lines.

= + I ω ω ω ω ω ( ) 2. ( / ) 1 ( / )
= + + + F ω q q ε ε ( ) 1 1 ( ) 1 j j j j j 2 2 2 (3) with
= - ε ω ω ω ( ) Γ j j j (4)
Based on the assumptions above, the reduced Raman spectrum of boron-doped diamond is fitted using Eq. ( 5) representing the sum of the different weighted light scattering events: pure electronic Raman scattering and its interaction with the different localized Raman scatterings. A e is the weighted factor of pure electronic Raman scattering and A j are the weighting factor of the different Fano-shaped peaks.

∑ = ⎡ ⎣ ⎢ + ⎤ ⎦ ⎥ I ω I ω A A F ω ( ) ( ). . ( ) r e e j j j (5)
The Raman spectra of diamond samples were fitted with Eq. ( 5) using the non-linear fitting tool of Origin 2016 © software. The fitting function and its different components are reported in Fig. 2, and the fitting parameter is reported in Table 1. fitting function satisfactorily the band, the diamond lines, and antiresonance dip as well as the broad B1 band. It particularly shows that the broad antiresonance dip at ca. 1350 cm -1 is a combination of the diamond ZCP and the B2 bands antiresonances. The Fano asymmetric parameter values of the B2 band are remarkably close to the ones of the ZCP line, which are consistent with values reported in the literature [START_REF] Szirmai | A detailed analysis of the Raman spectra in superconducting boron doped nanocrystalline diamond[END_REF][START_REF] Ushizawa | Boron concentration dependence of Raman spectra on {100} and {111} facets of B-doped CVD diamond[END_REF][START_REF] Bustarret | Optical and electronic properties of heavily boron-doped homo-epitaxial diamond[END_REF][START_REF] Pruvost | Analysis of the Fano in diamond[END_REF][START_REF] Piccirillo | The temperature dependence of the infrared absorption and Raman spectra due to boron in diamond[END_REF][START_REF] Gonon | Characterization of heavily B-doped polycrystalline diamond films using Raman spectroscopy and electron spin resonance[END_REF] and the position and the width of the diamond line are also consistent with our previous report [START_REF] Mortet | Insight into boron-doped diamond Raman spectra characteristic features[END_REF]. The fitting peak position of the B2 band (1250 cm -1 ) corresponds to the reported maxima of diamond PDoS [START_REF] Zaitsev | Optical Properties of Diamond: a Data Handbook[END_REF] which supports our analysis and confirms the PDoS origin of this band. Similarly, the fitting peak position (427 cm -1 ) and width (90 cm -1 ) of the B1 band are comparable with the reported values of the acoustic maximum of PDoS peak measured by Raman spectroscopy on not electrically conductive diamond nanoparticles and disordered diamond [START_REF] Prawer | The Raman spectrum of nanocrystalline diamond[END_REF][START_REF] Orwa | Raman investigation of damage caused by deep ion implantation in diamond[END_REF]. The assignment of the B1 band to a maximum of the acoustic branch of the PDoS is consistent with was already hypothesized by several other authors [START_REF] Gheeraert | Effect of boron incorporation on the "quality" of MPCVD diamond films[END_REF][START_REF] Prawer | Raman spectroscopy of diamond and doped diamond[END_REF]. This analysis indicates that the difference between the apparent position of the B1 band and the position of the maximum of PDoS in disordered diamond is not only due to phonon confinement but it is also due its interaction with electronic Raman scattering, i.e. Fano effect, conformably with the ZCP line [START_REF] Mortet | Insight into boron-doped diamond Raman spectra characteristic features[END_REF]. This analysis is also analogous to Yogi et al. work on silicon nanostructures who report the observation of a size-dependent position asymmetric peak due to acoustic phonons by Raman spectroscopy due to the quantum confinement effect and their interaction with intraband quasi-continuum [START_REF] Yogi | Fano scattering: manifestation of acoustic phonons at the nanoscale[END_REF].

Conclusion

In summary, we have shown the Raman spectrum of highly borondoped diamond can be satisfactorily fitted using the combination of well-established electronic Raman scattering and Fano effect models. This analysis shows that not only the diamond ZCP band is Fano-shaped but also the 500 cm -1 and 1200 cm -1 bands. As a result, their apparent position is shifted from their normal position, which corresponds to the acoustic and optical maxima of the phonon density of states in disordered diamond.

Fig. 1 .

 1 Fig. 1. Normalized raw Raman spectra of boron-doped diamond samples grown at B/C ratios of 4000 ppm (S1) and 40,000 ppm (S2). The spectra of sample S2 is shifted by +0.4 units for improved readability.

Fig. 2 .

 2 Fig.2. Experimental reduced and normalized Raman spectra (solid black line) of boron-doped diamond samples grown at B/C ratios of 4000 ppm (S1) and 40,000 ppm (S2), mathematical fittings (magenta solid line) with the contribution of the different Fano shape peaks (red, green and blue lines) and pure electronic Raman scattering broadband (cyan solid line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1

 1 Fitting parameter values of boron-doped diamond layer Raman spectra. Indices 1, 2, and 3 correspond to the B1 band, B2 band, and ZCP line, respectively.

	Parameter	Sample S1 (B/C = 4000 ppm)	Sample S2 (B/C = 40,000 ppm)
	ω e	1400	1400
	A e	0.365	0.005
	A 1	-	0.88
	q 1	-	1.83
	ω 1	-	427.2
	Γ 1	-	90
	A 2	0.45	1.3
	q 2	-1.75	-2.1
	ω 2	1250.9	1249.5
	Γ 2	40.7	65
	A 3	1.1	0.95
	q 3	-2.5	-1.5
	ω 3	1332	1323.3
	Γ 3	4.3	13.9

Acknowledgment

This work was financially supported by project 13-31783S of Czech Science Foundation, the French-Czech Project Barrande 35785SC -7AMB16FR004 of the Czech Ministry of Education, Youth and Sports and the J.E. Purkyně fellowship awarded to V. Mortet by the Czech Academy of Sciences.


M Davydova
email: davydova@fzu.cz

A Taylor

P Hubík

L Fekete

L Klimša

David Trémouilles

A Soltani

Vincent Mortet

Characteristics of zirconium and niobium contacts on boron-doped diamond

Keywords: Ohmic contact, boron-doped diamond, circular transmission line model, zirconium, niobium, titanium

Characteristics of zirconium and niobium contacts on boron-doped diamond

Introduction

Diamond is a semiconductor with unique properties [START_REF] Kalish | Diamond as a unique high-tech electronic material: difficulties and prospects[END_REF], which has been foreseen as an advantageous replacement for silicon in the fabrication of high frequency and high power devices such as high voltage Schottky diodes [START_REF] Traoré | Zr/oxidized diamond interface for high power Schottky diodes[END_REF][START_REF] Teraji | Electric field breakdown of lateral schottky diodes of diamond[END_REF] and more recently MOSFETs [START_REF] Jingu | Ultrashallow TiC source/drain contacts in diamond MOSFETs formed by hydrogenation-last approach[END_REF] and other electronic devices [START_REF] Davydova | Sensing of phosgene by a porouslike nanocrystalline diamond layer with buried metallic electrodes[END_REF][START_REF] Umezawa | High temperature application of diamond power device[END_REF][START_REF] Davydova | Catalyst-free site-specific surface modifications of nanocrystalline diamond films via microchannel cantilever spotting[END_REF][START_REF] Wang | Diamond based fieldeffect transistors of Zr gate with sinx dielectric layers[END_REF]. Stable ohmic contacts with a low specific contact resistance are crucial to limit electric losses in the operation of electronic power devices. Numerous factors, such as the nature of the metal [START_REF] Das | A review of the electrical characteristics of metal contacts on diamond[END_REF][START_REF] Kawaguchi | Dependence of contact resistance on metal electronegativity for B-doped diamond films[END_REF][START_REF] Koide | Carrier transport mechanisms through the metal /p-type diamond semiconductor interface[END_REF][START_REF] Werner | How to fabricate low-resistance metal-diamond contacts[END_REF][START_REF] Zhen | Ohmic contacts to boron-doped diamond[END_REF][START_REF] Johnston | High temperature contacts to chemically vapour deposited diamond films-reliability issues[END_REF][START_REF] Doneddu | SNOM Investigation of Surface Morphology Changes during Cr/Au Contact Fabrication on Single-Crystal CVD Diamond[END_REF][START_REF] Cheng | Effect of diamond facet on the electrical properties of Pt/BF +2 ion implanted polycrystalline diamond contacts[END_REF], the doping level [START_REF] Werner | How to fabricate low-resistance metal-diamond contacts[END_REF], the surface pre-treatment [START_REF] Das | A review of the electrical characteristics of metal contacts on diamond[END_REF][START_REF] Grot | The Effect of Surface Treatment on the Electrical Properties of Metal Contacts to Boron-Doped[END_REF][START_REF] Prins | Preparation of ohmic contacts to semiconducting diamond[END_REF][START_REF] Tachibana | Correlation of the Electrical-Properties of Metal Contacts on Diamond Films With the Chemical Nature of the Metal-Diamond Interface .2. Titanium Contacts -a Carbide-Forming Metal[END_REF], the crystalline orientation [START_REF] Cheng | Effect of diamond facet on the electrical properties of Pt/BF +2 ion implanted polycrystalline diamond contacts[END_REF], and the annealing time and temperature [START_REF] Werner | How to fabricate low-resistance metal-diamond contacts[END_REF][START_REF] Evans | Diamond-metal contacts: interface barriers and real-time characterization[END_REF][START_REF] Chen | Deposition of high-quality zinc oxide thin films on diamond substrates for high-frequency surface acoustic wave filter applications[END_REF][START_REF] Nakanishi | Formation of ohmic contacts to ptype diamond using carbide forming metals[END_REF] are important in the formation of ohmic or Schottky contacts. The ohmic behavior of the contacts is generally associated related to carbide formation [START_REF] Koide | Carrier transport mechanisms through the metal /p-type diamond semiconductor interface[END_REF][START_REF] Werner | How to fabricate low-resistance metal-diamond contacts[END_REF][START_REF] Zhen | Ohmic contacts to boron-doped diamond[END_REF][START_REF] Wang | Ohmic contacts on diamond by rf sputtering and Ti-Au metallization[END_REF][START_REF] Vijoen | Reaction between diamond and titanium for ohmic contact and metallization adhesion layers[END_REF][START_REF] Evans | Diamond-metal contacts: interface barriers and real-time characterization[END_REF]. A large variety of carbide forming metals have already been studied for the fabrication of ohmic contacts on boron (B)doped diamond [START_REF] Kawaguchi | Dependence of contact resistance on metal electronegativity for B-doped diamond films[END_REF][START_REF] Koide | Carrier transport mechanisms through the metal /p-type diamond semiconductor interface[END_REF][START_REF] Kono | Direct determination of the barrier height of Ti-based ohmic contact on p-type diamond (001)[END_REF][START_REF] Koné | An assessment of contact metallization for high power and high temperature diamond Schottky devices[END_REF][START_REF] Suzuki | Electrical characterization of diamond PiN diodes for high voltage applications[END_REF][START_REF] Braunstein | Effective p-type doping of diamond by boron ion implantation[END_REF][START_REF] Mortet | Titanium nitride grown by sputtering for contacts on boron-doped diamond[END_REF]. Titanium (Ti) is generally preferred for this purpose.

With the aim of studying impurity impact ionization and avalanche effects in B-doped diamond in high electric fields [START_REF] Mortet | Impurity impact ionization avalanche in p-type diamond[END_REF][START_REF] Mortet | Peculiarities of high electric field conduction in p-type diamond[END_REF], we searched carbide forming refractory metals with a high chemical affinity to carbon, similar to titanium, capable of forming good ohmic contacts, which could withstand high current densities and high temperatures. Niobium (Nb), tantalum (Ta) and zirconium (Zr) are refractory metals with a Gibbs free energy for carbide formation close to that of Ti [START_REF] Shatynski | The thermochemistry of transition metal carbides[END_REF][START_REF] Shiomi | Electrical Characteristics of Metal Contacts To Boron-Doped Diamond Epitaxial Film[END_REF]. While Ta has been already studied, the formation of ohmic contacts on B-doped diamond using Nb or Zr has not, according to our knowledge, been reported in the literature. On the contrary, Zr has been reported to form Schottky contacts [START_REF] Traoré | Zr/oxidized diamond interface for high power Schottky diodes[END_REF][START_REF] Baumann | Comparison of electron affinity and Schottky barrier height of zirconium and copper-diamond interfaces[END_REF]. In this work, we studied the formation of ohmic contacts on B-doped diamond using Zr and Nb metals in comparison to the well-studied Ti and Ta metals. Epitaxial B-doped diamond layers were grown by microwave plasma enhanced chemical vapor deposition with various B concentrations. Metal contacts were patterned by evaporation and standard lithography processes. The current-voltage (I-V) characteristics of different metal contacts were measured at room temperature, after annealing at various temperatures, and for different B concentrations. The diamond layers' resistivity, contact resistance, and specific contact resistance were determined using the circular Transmission Line Model (c-TLM) [START_REF] Klootwijk | Merits and limitations of circular TLM structures for contact resistance determination for novel III-V HBTs[END_REF][START_REF] Lester | Nonalloyed Ti/Al Ohmic contacts to ntype GaN using hightemperature premetallization anneal[END_REF].

Material and methods

B-doped epitaxial diamond layers were grown in a microwave plasma enhanced chemical vapor deposition (MW PECVD) AX5010 reactor using a methane, hydrogen and trimethylborane gas mixture. The boron to carbon ratio (B/C ratio) in the gas phase has been varied from 500 ppm to 20,000 ppm to obtain B-doped epitaxial diamond layers between a few 10 19 cm -3 to a few 10 21 cm -3 , i.e. from the semiconductor conduction region below metallic transition up to highly doped diamond with metallic conduction. B-doped epitaxial diamond layers, with B/C ratios of up to 6000 ppm, were grown on 330.8 mm 3 (100)-oriented highpressure high-temperature (HPHT) grown synthetic single crystal diamond substrates using the process reported in Ref. [START_REF] Mortet | substrate processing optimization for fabrication of smooth boron doped epitaxial diamond layer by PECVD[END_REF]. Layers grown using this process exhibit nearly defect-free growth with a very low roughness of 0.2 nm and 1 nm over areas of 55 μm 2 and 9090 μm 2 , respectively (see Figure 1). Above the threshold of B/C 6000 ppm, epitaxial diamond growth is no longer sustained on (100)-oriented substrates. Hence B-doped epitaxial diamond layers with B/C = 20,000 ppm were grown on 220.8 mm 3 (111)-oriented HPHT grown synthetic single crystal diamond substrates using deposition conditions reported in Table 1. In contrary to B-doped epitaxial diamond layers grown on (100)-oriented substrates, layers grown on (111)-oriented substrates were not etched in an oxygen-hydrogen plasma before diamond deposition. Prior to Before and after MW PECVD deposition, samples were cleaned in a hot admixture of sulfuric acid and potassium nitrate for 10 min. The samples were then rinsed with hot deionized water, acetone and isopropyl alcohol in an ultrasonic bath for 10 min and finally dried with clean compressed air. Several layers were grown with the same B/C ratio to enable the study of the properties of contacts fabricated with the four different metals. Thicknesses of the B-doped epitaxial diamond layers, used to compute the layer resistivity, was determined from double mass measurement using a high precision ME5 microbalance (Sartorius).

Circular Transmission Line Model test structures were fabricated by photolithography and metal evaporation. c-TLM structures were preferred as they are easily realized without the need of a MESA etch to prevent current spreading in linear TLM structures. The electrodes were patterned by the lift-off technique using a positive photoresist ma-P 1210 from Microresist technology exposed with a mask-less laser lithography system: Microwriter ML from the Lot Oriel company. Details of the lithography process are described in ref. [START_REF] Nicolas | Development of 100 ns wide blumlein transmission line pulse generator for study of high electric field properties in boron-doped diamond[END_REF]. The spacing between the inner circular electrodes (150 μm in diameter) and the outer electrodes varies between 10 and 60 μm (see Figure 1). Metals were deposited by e-beam evaporation in an Edwards Auto 500 vacuum coater. Zr, Nb, Ta and Ti 10 nm thick contacts were capped using a 90 nm thick gold layer with a 10 nm thick platinum inter-diffusion layer. The I-V characteristics of fabricated c-TLM structures were characterized using a computer controlled Agilent 4142B modular DC source/monitor and a probe station Suss Microtech PA-200. Electrical characteristics were measured at room temperature for as-deposited and consecutively annealed contacts with increasing temperature: 200 °C, 400 °C, 500 °C, 600 °C and 700 °C. Samples were annealed at low pressure in a rapid thermal annealing system, AS-One from Annealsys for 20 min in a flow of nitrogen.

The resistance of c-TLM structures has been calculated using linear least squares fitting I-V characteristics. Due to the circular geometry, the non-linear variation of the resistance as a function of the gap spacing is linearized using a correction factor (see Figure 2). The correction factor (c) is a function of the inner electrode radius (R) and the inter-electrode spacing (s) as described in equation [START_REF] Kalish | Diamond as a unique high-tech electronic material: difficulties and prospects[END_REF].

𝑐 = 𝑅 𝑠 𝑙𝑛 [𝑅+𝑠] 𝑅 (1)
Linear least squares fitting of corrected resistance enables the determination of the contact resistance (Rc), transfer length (LT), and the diamond sheet resistance (Rsh) from the Y-axis intercept, the X-axis intercept and the slope, respectively [START_REF] Klootwijk | Merits and limitations of circular TLM structures for contact resistance determination for novel III-V HBTs[END_REF].

Results and discussion

The resistivity of B-doped epitaxial diamond layers extracted from c-TLM as a function of the B/C ratio is reported in Fig. 3. Based on the ref. [START_REF] Mortet | Characterization of boron doped diamond epilayers grown in a NIRIM type reactor[END_REF][START_REF] Mortet | Properties of boron-doped epitaxial diamond layers grown on (110) oriented single crystal substrates[END_REF], one can cautiously assume conservation of the B/C ratio from the gas phase to the solid phase during deposition to estimate boron incorporation into grown layers. This assumption is supported by the quantitative agreement of measured values with values from the literature [START_REF] Lagrange | Activation energy in low compensated homoepitaxial boron-doped diamond films[END_REF][START_REF] Nebel | Properties, growth and applications of diamond[END_REF]. The resistivity drops from a few Ω.cm in the region of incomplete ionization, i.e. when B/C = 500 ppm, to a few 10s of mΩ.cm for B/C ≥ 2000 ppm in the region of metallic conduction. The higher resistivity values of epitaxial layers prepared with B/C = 20,000 ppm are attributed to the known poorer conduction properties of B-doped epitaxial diamond layers grown on (111)-oriented substrates [START_REF] Mortet | Characterization of boron doped diamond epilayers grown in a NIRIM type reactor[END_REF]. As deposited Ta, Nb and Zr contacts fabricated on B-doped epitaxial diamond layers grown with a B/C ratio of 500 ppm exhibit symmetric but non-ohmic I-V characteristics, which is in contrary to Ti contacts (not shown). Nb and Zr form ohmic contacts only after high temperature annealing (≥ 400°C). The evolution of I-V characteristics with annealing temperature of layers grown at high B concentration (B/C = 20,000 ppm) is shown in Fig. 4. Remarkably, as deposited Ta also formed ohmic contact on highly doped layers (B/C = 20,000 ppm) with a ca. 210 -4 Ω.cm 2 specific contact resistance. The total resistance of Ti contacts suddenly increases after high temperature annealing (≥ 600 °C). This increase in resistance indicates a degradation of the specific contact resistance. The variations of the calculated specific contact resistance as a function of the annealing temperature and the B concentration for the different fabricated metal contacts are reported in Fig. 5 and Fig. 6. Fig. 6 shows that a clear decrease in the specific contact resistance with the rising B concentration is observed for all metals. The specific contact resistance drops by almost four orders of magnitude from a few Ω.cm 2 to a few 10 -4 Ω.cm 2 and below as the B concentration increases by a factor of 40 from 500 ppm to 20,000 ppm. The lowest contact resistance of 410 -5 Ω.cm 2 was obtained for an asdeposited Ti contact on a layer grown with B/C = 20,000 ppm. Note that all contacts on heavily B-doped epitaxial diamond layers, i.e. above the metallic transition [START_REF] Venkatesan | Evaluation of ohmic contacts formed by B+ implantation and Ti-Au metallization on diamond[END_REF], exhibited low specific contact resistance values of about 10 -4 Ω.cm 2 after optimal annealing. The same decrease in specific contact resistance of Ti and Mo contacts on polycrystalline B-doped diamond was reported by Nakanishi et al. who also reported a linear correlation between specific contact resistance and the resistivity of the doped layer [START_REF] Nakanishi | Formation of ohmic contacts to ptype diamond using carbide forming metals[END_REF]. The strong influence of acceptor concentration (Na) is associated to tunneling transport at the contact interface [START_REF] Nakanishi | Formation of ohmic contacts to ptype diamond using carbide forming metals[END_REF] and the Na - 1/2 decrease of the Schottky contact depletion width [START_REF] Sze | Physics of Semiconductor Devices[END_REF]. The specific contact resistance of Ti and Ta contacts decreases steadily with annealing temperature between 200°C and 500°C. However, B-doped epitaxial diamond layers grown at a high B/C ratio, e.g. of 20,000 ppm, show that a rapid increase in the specific contact resistance is observed after annealing at higher temperatures, which indicates attributed to a degradation of the ohmic contacts. This effect is also observed in Nb contacts. Similar degradation of ohmic contacts observed for Ti and Mo contacts on B-doped diamond upon annealing was reported by Johnston et al. [START_REF] Johnston | High temperature contacts to chemically vapour deposited diamond films-reliability issues[END_REF]. Whilst less pronounced, degradation of Ti, Ta and Nb contacts is also observed on layers with lower B concentrations. In contrary to this, the On the contrary, Zr metal zirconium contacts show a steady improvement of ρc values with increasing temperature up to 700°C. The higher This observed better thermal stability of Zr contacts suggests that they might have a longer lifetime before failure than conventionally used Ti contacts. The optimal specific resistance values of Ti, Nb, Zr, and Ta contacts for the various B concentrations studied in this work and their respective annealing temperature are summarized in Table 2. These results clearly show that Nb and Zr metals form ohmic contacts with specific contact resistance comparable to the ordinarily used Ti or Ta metals. Specific resistances reported in this work are larger than reported values from the literature [START_REF] Nakanishi | Formation of ohmic contacts to ptype diamond using carbide forming metals[END_REF]. This difference is attributed to the absence of an amorphisation process step in the preparation of the metal contacts [START_REF] Cheng | Effect of diamond facet on the electrical properties of Pt/BF +2 ion implanted polycrystalline diamond contacts[END_REF][START_REF] Tachibana | Correlation of the Electrical-Properties of Metal Contacts on Diamond Films With the Chemical Nature of the Metal-Diamond Interface .2. Titanium Contacts -a Carbide-Forming Metal[END_REF][START_REF] Brandes | Ion implantation and anneal to produce low resistance metal-diamond contacts[END_REF][START_REF] Geis | Electrical, crystallographic, and optical properties of ArF laser modified diamond surfaces[END_REF].

Conclusion

The properties of Ti, Ta, Zr and Nb metal contacts on B-doped epitaxial diamond layers have been measured and compared using circular transmission line model for various B concentrations and different consecutive annealing temperatures. Doped diamond layer resistivity values determined from c-TLM are consistent with data from the literature. Formation of ohmic contact using Zr and Nb has been demonstrated with specific contact resistance comparable to the ones of Ti and Ta after high-temperature annealing. The specific contact resistance varies significantly with B concentration, nature of the metal contact, and annealing temperature. Finally, we observed Zr contacts on highly boron doped diamond exhibit better stability upon high-temperature annealing compared to other studied metallic contacts, especially titanium contacts. This result suggests that Zr contacts might advantageously replace conventional Ti contacts for a long lifetime and high-temperature applications of diamond devices. Comparison between literature (red dash line) [START_REF] Lagrange | Activation energy in low compensated homoepitaxial boron-doped diamond films[END_REF][START_REF] Nebel | Properties, growth and applications of diamond[END_REF] and experimental data from this study (the boron concentration ([B]) of the upper x-axis is calculated was determined assuming conservation of B/C ratio from the gas phase to the solid phase during diamond deposition [START_REF] Mortet | Properties of boron-doped epitaxial diamond layers grown on (110) oriented single crystal substrates[END_REF][START_REF] Teraji | Highly efficient doping of boron into highquality homoepitaxial diamond films[END_REF]) . Table 2 Optimal specific contact resistance of the Ti, Nb, Zr and Ta contacts for various B concentrations after annealing at optimal temperatures. ) of upper x-axis is calculated assuming conservation of B/C ratio from the gas phase to the solid phase during diamond deposition [START_REF] Mortet | Properties of boron-doped epitaxial diamond layers grown on (110) oriented single crystal substrates[END_REF][START_REF] Teraji | Highly efficient doping of boron into highquality homoepitaxial diamond films[END_REF]).

Figure 1 :

 1 Figure 1: Scanning electron microscopy images of (a) B-doped epitaxial diamond layer on (100)-oriented HPHT substrate and (b) c-TLM structures on a Bboron-doped epitaxial diamond layer deposited on a (111)-oriented HPHT substrate. 9090 μm 2 AFM topography image of a Bboron-doped epitaxial diamond layer grown on a (100)-oriented substrate (c).

Figure 2 :

 2 Figure 2: Total resistance vs electrodes' gap spacing before (squares) and after (red circles) correction for the circular geometry.

Figure 3 :

 3 Figure 3: Room-temperature resistivity of (100) and (111) B-doped epitaxial diamond layers as a function of the boron to carbon ratio B concentration in the gas phase with respect to the carbon source. Comparison between literature (red dash line)[START_REF] Lagrange | Activation energy in low compensated homoepitaxial boron-doped diamond films[END_REF][START_REF] Nebel | Properties, growth and applications of diamond[END_REF] and experimental data from this study (the boron concentration ([B]) of the upper x-axis is calculated was determined assuming conservation of B/C ratio from the gas phase to the solid phase during diamond deposition[START_REF] Mortet | Properties of boron-doped epitaxial diamond layers grown on (110) oriented single crystal substrates[END_REF][START_REF] Teraji | Highly efficient doping of boron into highquality homoepitaxial diamond films[END_REF]) .

Figure 4 :

 4 Figure 4: I-V characteristic of Ti, Nb, Zr and Ta contacts on B-doped diamond annealed at several temperatures up to 700°C.

Figure 5 :

 5 Figure 5: Specific contact resistance as a function of annealing temperature for Ti, Nb, Zr, and Ta contacts to various B-doped epitaxial diamond layers.

Figure 6 :

 6 Figure 6: Specific contact resistances of Ti, Nb, Zr, and Ta contacts annealed at different temperatures as a function of B/C ratio during deposition and boron concentration [B] (the boron concentration ([B]) of upper x-axis is calculated assuming conservation of B/C ratio from the gas phase to the solid phase during diamond deposition[START_REF] Mortet | Properties of boron-doped epitaxial diamond layers grown on (110) oriented single crystal substrates[END_REF][START_REF] Teraji | Highly efficient doping of boron into highquality homoepitaxial diamond films[END_REF]).

Table 1 :

 1 Diamond deposition conditions

Acknowledgements

This work was financially supported by the project 17-05259S of Czech Science Foundation, the French-Czech Project Barrande 35785SC -7AMB16FR004 of the Czech Ministry of Education, Youth and Sports, the MEYS CR FUNBIO CZ.2.16/3.1.00/21568 (SEM purchase), LO1409, LM2015088 projects (SEM maintenance) and the J.E. Purkyně fellowship awarded to V. Mortet by the Czech Academy of Sciences.


V Mortet

A Taylor

M Davydova

L Fekete

Z Vlčková Živcová

L Klimša

N Lambert

P Hubík

D Trémouilles

A Soltani

Comparison of ohmic contact formation of titanium and zirconium on boron doped diamond

With a high affinity to carbon comparable to titanium and an electrically conductive carbide, zirconium has potential to form ohmic contact on boron doped diamond. In this work, formation of ohmic contacts on boron doped diamond using zirconium is studied in comparison to titanium. Boron doped diamond epitaxial layers have been grown by microwave plasma enhanced chemical vapour deposition with various B/C ratio. Circular Transmission Line Model structures were fabricated using standard micro-fabrication technologies. Specific contact resistance of fabricated contacts was determined for different boron concentrations and for various annealing temperatures. Ohmic contact using zirconium are form after annealing at 400 °C. Specific contact resistance steadily decreases with high temperature annealing down to a value of ca. 1 mΩ.cm 2 after annealing at 700 °C for highly boron doped diamond. In comparison, titanium contact fabricated on highly doped diamond appears not stable under high temperature annealing.

INTRODUCTION

Its extraordinary properties have made diamond a promising material for fabrication of novel power electronic devices. Stable ohmic contacts with a low specific contact resistance are crucial in the fabrication of power electronic devices. Properties of ohmic contacts are determined by many parameters: the crystalline orientation, the nature of the metal, the doping level, the surface pre-treatment and the annealing process [1,2]. Many carbide forming metals have been studied for the formation of ohmic contact on doped diamond. Despite its possible use for Schottky contact formation [3], the high affinity to carbon of zirconium to form electrically conductive carbide [4] highlights its potential for ohmic contact formation. In this study, we compare the formation and the properties of ohmic contact on boron-doped diamond (BDD) using zirconium and titanium. Epitaxial BDD layers were grown by microwave plasma enhanced chemical vapour deposition (MW-PECVD) with various boron concentrations. The morphology and crystal structure of the diamond layers were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. Metals contacts were fabricated using standard micro-fabrication technologies. Specific contact resistance was investigated using the circular Transmission Line Model (c-TLM) [START_REF] Klootwijk | Merits and limitations of circular TLM structures for contact resistance determination for novel III-V HBTs[END_REF] after consecutive annealing processes.

EXPERIMENTAL DETAILS

Epitaxial BDD layers were grown on synthetic diamond crystals in an AX5010 MW-PECVD reactor using a methane, hydrogen and trimethylborane gas mixture. The boron to carbon ratios (B/C) was varied from 500 ppm up to 20,000 ppm (see Table I). All layers were grown on (100) oriented substrates apart from layers with B/C = 20,000 ppm which were grown on (111) oriented substrates. Deposition process parameters are reported in Table II. Prior to and after deposition, diamond samples were cleaned in a hot mixture of H2SO4 and KNO3, rinsed in DI-water, ultrasonically cleaned in acetone and isopropyl alcohol and finally dried using filtered compressed air.

The morphology and structure of deposited layers was characterized by using a Tescan FERA3 SEM, AFM, and Raman spectroscopy. AFM measurements were carried out on an ambient Dimension Icon (Bruker) in Peak Force Tapping mode with ScanAsyst Air tips (Bruker). Raman spectroscopy measurements were carried out using a Labram HR spectrometer (Horiba Jobin-Yvon) interfaced to an Olympus microscope (objective 100x) equipped with a 50 μm confocal hole and a He-Ne laser 633 nm with 8mW power. c-TLM structures, i.e. circular electrodes (150 μm in diameter) with the outer electrodes varying between 10 and 60 μm, were patterned by the lift-off technique using a positive photoresist ma-P 1210 (Micro Resist technologies). The resit was exposed using a laser writer Microwriter ML (Lot Oriel). 10 nm thick titanium and zirconium contacts were deposited by electron beam evaporation (Edward Auto 500) and protected from oxidation by a 90 nm thick gold layer with a 10 nm thick platinum inter-diffusion layer. Contacts were consecutively annealed at 200 °C, 400 °C, 500 °C, 600 °C and 700 °C in nitrogen at low pressure for 20 min in a rapid thermal annealing system (AS-One from Annealsys). From room temperature up to 200 °C, the resistance of fabricated c-TLM structures was determined from linear least squares fitting of their I-V characteristic measured using a computer controlled Agilent 4142B modular DC source/monitor and a probe station Suss Microtech PA-20. Specific contact resistance and diamond resistivity were determined using c-TLM, i.e. from linear least squares fitting of total resistance corrected for circular geometry vs electrodes gap spacing [START_REF] Klootwijk | Merits and limitations of circular TLM structures for contact resistance determination for novel III-V HBTs[END_REF].

RESULTS AND DISCUSSION

Figure 1 shows the surface morphology of fabricated epitaxial layers as observed by scanning electron microscopy and atomic force microscopy. The surface of epitaxial layers is very smooth and free of frequently observed four-fold symmetry hillocks or nonepitaxial grains. The surface morphology was further characterized by atomic force microscopy. The surface of (100) boron doped epitaxial layers is very smooth with root mean square roughness below 1 nm, typically 0.5 nm on a 25 μm 2 area. The surface of (111) is rougher with an average roughness of 10.8 nm and a dendritic like surface morphology. Except for the layer grown with a B/C ratio of 500 ppm, Raman spectra of epitaxial layers exhibit characteristic peaks of highly boron doped diamond with metallic conductivity (see figure 1e), i.e. a red shifted diamond peak position with a Fano shape, additional bands, at ca. 500 cm -1 and ca. 1230 cm -1 . Small structures at 610 cm -1 and 1050 cm -1 attributed to boron-carbon vibrations and carbon vibrations [START_REF] Mortet | Refined analysis of boron doped diamond Raman spectrum[END_REF] are observed on layers grown on (100) oriented substrates with lower boron concentrations. While the origin of the wide band at 500 cm -1 remains an open question, the peak at 1230 cm -1 has been recently attributed to a Fano-shaped diamond maximum phonon density of states [START_REF] Mortet | Refined analysis of boron doped diamond Raman spectrum[END_REF]. Figure 2 compares the current voltage (I-V) characteristics measured at room temperature of a 35 μm gap c-TLM test structure of titanium and zirconium contacts on doped diamond layers deposited with various B/C ratio after different annealing processes: after metal deposition, i.e. no annealing (Fig. 2a), after annealing at 400 °C (Fig. 2b) and after annealing at 700 °C (Fig. 2c). As measured characteristics are symmetric, to ease comparison of the two metals' contact characteristics: zirconium contact characteristics on the negative half side and titanium contact characteristics on the positive half side are displayed. Noticeably, I-V characteristics of titanium contacts are ohmic for as deposited contacts and annealed contacts for all doped layers studied in this work. In contrary, I-V characteristics of as deposited zirconium contacts are not linear and annealing at 400 °C is necessary to obtain ohmic contacts. Taken into account comparable thickness of all studied layers, the observed higher total resistance, i.e. lower slope, of the c-TLM test structure fabricated on the doped diamond layer grown at B/C = 20,000 ppm compared to the one grown at B/C = 6,000 ppm is due to the generally observed higher resistivity of boron doped layers grown on (111) oriented substrates [START_REF] Mortet | [END_REF]. Average resistivity of doped diamond layers calculated using the circular Transmission Line Model are reported in table I. The evolution of the calculated specific contact resistance of titanium and zirconium contacts as a function of measurement temperature, annealing temperature and B/C ratio is reported in figure 2d-e. These figures clearly show that specific contact resistance scales with boron doping. High temperature annealing further decreases the specific contact resistance of zirconium contact whereas it increases the one of titanium contact for B/C ratio higher than 6,000 ppm. This instability of titanium contact, already reported, seems to affect only highly doped diamond layers [8]. The larger specific contact resistances compared to values from the [9] might be attributed to the absence of an amorphisation process prior to the metal deposition in the contact fabrication process.

CONCLUSION

Zirconium contacts on boron doped homo-epitaxial diamond layers were studied for different boron doping and annealing temperatures in comparison with titanium contacts. Zirconium ohmic contacts are formed after annealing at temperatures above 400 °C. Resistivity values determined from c-TLM are consistent with data from the literature. Calculated specific contact resistance of zirconium contacts are higher than titanium contacts, however, zirconium contacts show superior thermal stability following high temperature annealing (> 600 °C) for highly boron doped diamond layers (B/C ≥ 6,000 ppm).

AKNOWLEDGEMENT

This work was financially supported by the project 17-05259S of Czech Science Foundation, the French-Czech Project Barrande 35785SC -7AMB16FR004 of the Czech Ministry of Education, Youth and Sports, the MEYS CR FUNBIO CZ.2.16/3.1.00/21568 (SEM purchase), LO1409, LM2015088 projects (SEM maintenance) and the J.E. Purkyně fellowship awarded to V. Mortet by the Czech Academy of Sciences.

Figure 1 .

 1 Figure 1. Scanning electron microscopy (a, c) and atomic force microscopy images (b, d) of boron doped diamond epitaxial layers grown on (100) oriented (a, b) and (111) oriented (c,d) HPHT diamond substrates. (e) Comparison of Raman spectra of epitaxial boron doped diamond layer grown at B/C ratio of 6,000 ppm and 20,000 ppm.

Figure 2 .

 2 Figure 2. Room temperature I-V characteristics of c-TLM structure with 35 μm gap of titanium (solid curve) and zirconium (dash curve) contacts as a function of B/C ratio after different thermal treatment: (a) as deposited, (b) after 400°C annealing and (c) after 700°C annealing. Evolution of the titanium (d) and zirconium (e) specific contact resistance as a function of annealing temperature for B/C ratio of 500 ppm (black), 6,000 ppm (red), 20,000 ppm (green) and the contact resistance measurement's temperature: 25 °C (squares), 120 °C (circles) and 200 °C (triangle). (f) Comparison of the specific contact resistance of titanium and zirconium contacts as a function of B/C between annealing at 400 °C and 700 °C.

Table I .

 I Specific contact resistance (c) of zirconium (right) and titanium (left) contacts, and boron doped diamond layers' resistivity as function of B/C ration in gas phase during MW-PECVD deposition

	B/C ratio	500 ppm 2,000 ppm 6,000 ppm 20,000 ppm	Units
	Resistivity	6.5	13.5	12	84	Ω.cm
	c (400 °C) 996 / 1.7	54 / 1.0	38 / 0.37	2 / 0.09	mΩ.cm 2
	c (700 °C) 267 / 1.7	1.1 / 0.8	1.5 / 0.7	0.7 / 3.7	mΩ.cm 2

Table II. Epitaxial boron doped diamond deposition conditions Substrate

orientation Microwave power(W) Pressure (mbar) CH4 concentration Substrate temperature Deposition time

	(100)	700	100	1 %	1100 °C	1 h
	(111)	575	60	0.1 %	1000 °C	1 h