Nitrogenated nanocrystalline diamond films: Thermal and optical properties

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Diamond & Related Materials 16 (2007) 2067 – 2073 www.elsevier.com/locate/diamond

Nitrogenated nanocrystalline diamond films: Thermal and optical properties V. Ralchenko a,⁎, S. Pimenov a , V. Konov a , A. Khomich b , A. Saveliev a , A. Popovich a , I. Vlasov a , E. Zavedeev a , A. Bozhko c , E. Loubnin d , R. Khmelnitskii e b

a A.M. Prokhorov General Physics Institute RAS, 38 Vavilov str. 119991 Moscow, Russia Institute of Radio Engineering and Electronics RAS, 1 Vvedenskogo Sq., 141190 Fryazino, Moscow region, Russia c Physics Department, M.V. Lomonosov Moscow State University, 119899 Moscow, Russia d Institute of Physical Chemistry RAS, 31 Leninskii prosp., 117915 Moscow Russia e P.N. Lebedev Physical Institute RAS, 53 Leninskii prosp. 119991 Moscow, Russia

Available online 24 May 2007

Abstract Ultrananocrystalline diamond films have been grown by microwave plasma CVD using CH4/H2/Ar mixtures with N2 added in plasma in amounts up to 25%. The films were characterized with AFM, Raman, XRD, and UV–IR optical absorption spectroscopy mainly focusing on optical and thermal properties. In comparison with polycrystalline CVD diamond the UNCD are very smooth (Ra b 10 nm), have low thermal conductivity (∼ 0.10 W/cm K), high optical absorption (∼ 103 cm− 1 at 500 nm) and high concentration of bonded hydrogen (∼ 9 at.%). The nitrogen presence in the plasma has a profound impact on UNCD structure and properties, particularly leading to a decrease in resistivity (by 12 orders of magnitude), thermal conductivity, Tauc band gap, optical transmission and H content. The UNCD demonstrated rather good thermal stability in vacuum: the diamond phase still was present in the films subjected to annealing to 1400 °C. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond films; Grain boundaries; Thermal conductivity; Optical absorption; Annealing

1. Introduction Polycrystalline diamond films are commonly produced by chemical vapor deposition (CVD) typically using CH4–H2 gas mixtures with a small (few percents) fraction of methane. Due to the big progress in the last two decades in the CVD technology the polycrystalline diamond films and wafers become now an engineering material with properties often approaching to those for the best natural single crystals [1,2]. Quite special type of diamond, ultrananocrystalline diamond (UNCD) films with grain size as small as 2–10 nm, can be grown in an CH4–H2 environment with a reduced (or even zero) H2 content with addition of Ar or other noble gases [3]. The UNCD deposition is carried out mostly in microwave plasma [3,4], but the DC plasma [5–7] can also be used for the synthesis. Under the specific growth conditions the rate of secondary nucleation of diamond is so high

⁎ Corresponding author. E-mail address: [email protected] (V. Ralchenko). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.05.005

that the grains keep a nanoscale size even at film thickness of a few or tens microns [3,7]. This differs the UNCD from those nanocrystalline diamond (NCD) films that are deposited in hydrogen-reach mixtures on a substrate with very high nucleation density (N1011 cm− 2), so the first layer is composed of nanocrystallites, which, however, evolve later to a textured, columnar structure typical for micro(poly-)crystalline films [11]. The UNCD films attract a growing interest as the material for applications in microelectromechanics, tribology, electronics, biology and medicine, due to possibility to form ultrathin, hermetic, superhard, low friction, low-field electron emission, biocompatible, chemically resistant coatings [7–10]. New activity in UNCD research has been triggered by the finding [12], that the UNCD films grown with nitrogen added in high amounts in gas phase become electroconductive (metallic-like conductivity is achievable). This makes the nitrogenated UNCD promising for applications, in particular, in electrochemistry and electronics [13]. Moreover, the films with high enough concentration of incorporated nitrogen (∼2 × 1020 cm− 3) show n-type conductivity [13,14]. Although the term “nitrogen-doped” is often used to refer to this sort of conductive UNCD films [12,15,16] the conductivity

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is not the result of doping in a common sense (when nitrogen atom occupies a substitutional position in diamond lattice with activation energy as high as 1.7 eV), but rather the nitrogen in the plasma increases sp 2 bonding at grain boundaries, introducing π and π⁎ states within the band gap as discussed in [13,17], so the conduction takes place mostly along grain boundaries. Thus the nitrogen atoms effect is indirect, yet, keeping this in mind, we will conditionally use for briefness hereafter the terms “nitrogen doping”, or more general, “nitrogenation” to designate the UNCD samples produced with N2 added in the gas phase. Besides the crucial impact on electronic properties the sp2 bonding stimulated by nitrogen presence in the process gas should essentially influence the optical absorption [18] and thermal conductivity in UNCD. Here we report on deposition and characterization of nitrogenated UNCD films focusing primarily on optical and thermal properties, which are scarcely presented in literature till now. 2. Experimental details The UNCD films were deposited in a microwave plasma CVD reactor (DF-100 model, 5 kW, 2.45 GHz) in Ar/CH4/H2/ N2 mixtures, using an approach similar to that reported by ANL group [12,19]. The percentages of CH4 (2%) and H2 (5%) in the gas mixture were constant, while nitrogen N2 was added in concentration varied from 0% to 25% by partly replacing Ar to keep the total gas flow constant at 500 sccm. Other deposition parameters were as follows: pressure 90 Torr, microwave power 2.3 kW, substrate temperature 800 °C, deposition time 40 min– 11 h. The substrate were mirror-polished commercial Si cut to 10 × 10 × 0.5 mm pieces, which were ultrasonically seeded with nanometric detonation diamond particles (nominal size 4–6 nm, however, larger agglomerates presented in the powder and suspension) to enhance nucleation density. For optical transmission/absorption measurements the UNCD films were grown on high-quality transparent polycrystalline CVD diamond (PCD) substrates grown in the same CVD reactor using a standard 2% CH4–98% H2 process gas composition. The 3.5 × 5.0 × 0.4 mm PCD plates were laser cut from bigger wafer and were polished on both sides to surface roughness Ra of about 10 nm. No seeding was used in this case since the UNCD layer formed epitaxially on the polycrystalline (gain size 60–90 μm) substrate. To measure the thickness of asgrown UNCD film a small portion of the PCD substrate was covered by thin thermally evaporated copper film prior to UNCD deposition. This prevented diamond nucleation on metallized surface, so after the film deposition run the copper was chemically removed, and the height of step formed (the film thickness) was measured with a white-light optical interferometer ZYGO (модель NewView5000). On Si substrates the film thickness was measured by viewing sample's cross-section with optical and electron scanning microscopy (SEM). Surface topography was characterized with an atomic force microscope Ultra Objective (Carl Zeiss). The Raman analysis of the carbon phases was performed with a Dilor XY Raman spectrometer using the 413.1 nm line of a Kr+ laser for excitation. Optical transmission/absorption measurements were

carried out using spectrophotometers Specord M400 and M80 from Carl Zeiss, Jena, in spectral ranges of 0.185–0.9 μm and 2.5–50 μm. X-ray diffraction measurements were performed with JEOL-JDX 10P instrument (CuKα-radiation source, 1.54 Å wavelength) in the diffraction angle 2θ interval from 15° to 130° with a step of 0.02°. Thermal diffusivity perpendicular to the film surface was measured at room temperature using a laser flash technique [20]. The front side of the specimen was irradiated with a Nd:YAG laser, the diffusivity value being calculated from the temperature rise kinetics on the rear side as monitored by a sensitive IR detector. This method requires samples in form of a membrane, which were prepared by selective-area etching Si substrate to leave a 5 mm diameter window of 10–11 μm thick free-standing UNCD film. A thin Ti film for absorption of the radiation was deposited onto both sides of the diamond membranes. To examine the thermal stability of the UNCD films one sample (5% N2 added in plasma) was sequentially annealed in an oven with graphite walls in vacuum of 10− 5 Torr, in steps from 750 to 1400 °C, for 1 h at each temperature. After each heat treatment the samples were oxidized in boiling H2SO4 and K2Cr2O7 acids to remove the graphite which could be formed on external surfaces during the annealing. This cleaning procedure has been proved to remove all residual surface graphite from natural diamonds annealed in the same conditions at 1650 °C, so the darkening (if observed) of the annealed UNCD sample was caused by a graphitization in bulk only, not on the film surface. As it was established earlier [21,22] the onset of darkening of highquality PCD films due to a-C and graphitic inclusions formation on grain boundaries and in defected nanodomains, takes place at annealing above 1300 °C. To improve the optical properties and thermal stability of our diamond substrates a 50 μm thick defected layer was mechanically removed from nucleation side of the PCD plates. The annealing of these plates (without UNCD deposited on top) showed that there were no observable changes in its optical properties up to 1500 °C. 3. Results 3.1. Topography and roughness The deposition rate was found to rise almost linearly from 0.4 to 1.8 μm/h when the N2 flux introduced into the reactor increased from zero to 25%. As a rule, 2 h deposition runs were done to prepare the samples for further analysis. The films on Si substrates showed low surface roughness (Ra = 16–50 nm) with a globular morphology as measured with AFM (Fig. 1). Even lower roughness Ra = 7–28 nm has been measured on UNCD on PCD substrates (see Table 1), while the globule size spanned from 50 nm to 1700 nm without any systematic variation with N2 content in gas. Higher Ra values for the films on Si compared to PCD substrates could be due to a seed agglomeration in the former case. 3.2. Raman spectra Raman spectra for “undoped” (0% N2) films and four samples produced at different N2 percentages are shown in Fig. 2. For the

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Fig. 1. AFM surface relief image of 0.8 µm thick undoped UNCD film grown on PCD substrate. Surface roughness Ra = 10.6 nm, image size is 10 × 10 μm2.

undoped film the following phases can be identified from specific features in the spectra: diamond with the peak at 1335.7 cm− 1; sp2-bonded carbon (disordered graphite) with D and G peaks at 1350 cm− 1 and 1550 cm− 1; and transpolyacetylene (t-PA) with two peaks at 1157 cm− 1 (C–H inplain bending mode) and 1539 cm− 1 (CC stretch) [23–25]. As the optical absorption in the films increased (see below), and the overall Raman signal intensity decreased with nitrogen content in plasma, the Raman spectra were normalized in such a way that the intensity of 1550 cm− 1 band for each sample was equal to that for the undoped sample 0% N2. With N2 addition the intensities of diamond and t-PA Raman peaks decrease relative to G band, indicating that the nitrogen stimulates an increase in sp2 phase content. This corroborates with observations by Birrell et al. [26] of an increase in thickness of grain boundaries (which contain sp2 carbon) in the UNCD films from 0.5 nm to 2 nm when the nitrogen content in Ar/CH4/H2 mixture changed from 0% to 20%. 3.3. XRD analysis X-ray diffraction analysis revealed crystalline diamond structure for all specimens examined, undoped as well as nitrogenated ones. The survey XRD pattern for one nitrogencontaining (25% N2) film is shown in Fig. 3. Three principal reflexes (111), (220) and (311) are present. A strong texture in [110] orientation, with a texture factor as high as 97% (compared to polycrystalline powder etalon) has been observed

Fig. 2. Raman spectra of UNCD films grown on Si substrates at different N2 content added in plasma.

for undoped film, while the texture gradually vanished with “doping” (N15 %N2). The grain size was estimated with Scherer's formula from the width of (111) reflex at high diffraction angles taking into account instrumental broadening. The crystallite size of 22 nm and 26 nm has been evaluated for undoped and 25% N2 film, respectively. These values are somewhat larger compared to minimum grain size of 2–5 nm in undoped UCND films reported by Gruen [3], but closer to the data of Birrell et al. [26] for nitrogenated samples. The reason could be that our material was grown with a small amount (5%) of hydrogen in the process gas, resulting in slightly larger grain size than in the UNCD produced in Ar/CH4 mixtures [3], and thus, the volume fraction of grain-to-grain boundary could also be different. However, in many aspects, for instance, in the nitrogen-induced

Table 1 Surface roughness Ra and Rrms, as measured with AFM at scan area 10 × 10 μm2 for UNCD films grown PCD substrates at different concentration of N2 added in plasma Concentration of N2, % Growth rate, μm/h Thickness, μm Ra, nm Rrms, nm 0 5 10 15 20 25

0.4 0.7 1.0 1.4 1.8 1.7

0.81 1.45 2.05 2.7 3.5 3.4

10,6 28.1 9.9 7.1 24.5 19,8

13,3 42.2 12.3 9.0 31.2 24,1

Fig. 3. XRD pattern for 10 μm thick UNCD film grown on Si substrate at 25% N2 added in gas.

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conductivity, our films demonstrate a similarity in behavior with those grown without H2 added [12,17]. Transmission electron microscopy would be helpful to get more information on grain size distribution and grain boundaries structure. 3.4. Thermal conductivity The measured thermal diffusivity (D) at room temperature (RT) for the UNCD films produced with different N2 amount added in plasma is shown in Fig. 4. The highest diffusivity D = 0.056–0.07 cm 2 /s is for undoped sample, gradually decreasing with N2 addition, down to 0.034 cm2/s for 25% N2 film. The thermal conductivity K was calculated as K = DρC, where ρ and C are the mass density and specific heat of the material, respectively. Assuming ρ = 3.51 g/cm3 and C = 0.511 J/g K corresponding to crystalline diamond (as no relevant data for UNCD were available for us) we find K = 0.10–0.14 W/cm K for undoped film, which is two orders of magnitude lower than that for single crystal diamond or PCD (10–20 W/cm K). The thermal conductivity reduces further for nitrogenated films, arriving to ∼ 0.06 W/cm K for 25% N2 sample. These K values are an order of magnitude higher compared to amorphous carbon, including diamond-like carbon films [27,28]. Shamsa et al. [27] found K = ∼ 0.01 W/cm K for tetrahedral ta-C:H films and 0.035 W/cm K for ta-C. The temperature dependence of thermal conductivity has been also studied for our undoped and nitrogenated UNCD films for T = 80–400 K using the 3ω method [28]. A monotonic increase in K with temperature was observed similar to that for disordered materials. The K(T) behavior has been described by a phonon-hopping model (PHM), that assumes that the phonon transport inside the grain of size d follows “bulk rules”, while the phonon transition through the grain boundary (“hopping”) is characterized by a “transparency” parameter t. The best-fit PHM curves for undoped UNCD (grain size d = 22 nm) and for nitrogenated UNCD (d = 26 nm) were obtained with t = 0.32 and t = 0.20, respectively. The smaller t value for the doped film

Fig. 5. Resistivity of UNCD films vs N2 content added in plasma.

could be related to an increase in grain boundary thickness with N2 added into plasma [26]. The phonon free pass (Lph) in the UNCD films is small, it does not exceed the diamond grain (nano)size, while for type IIa single crystal diamond Lph = 240 nm at RT. The actual Lph can be estimated from relationship K = 1 / 3CvL, where C is heat capacity and v = 13,430 m/s is average sound velocity for diamond. With experimental K values for the nanocrystalline films one obtain Lph = 0.7–1.7 nm, much smaller than the grain size, but comparable with grain boundary thickness. 3.5. Electrical conductivity The resistivity of the UNCD films on Si substrates was measured in two variants: (i) in sandwich geometry, the Si substrate being the back contact with a silver epoxy electrode deposited on top of the film; and (ii) in planar geometry with the electrodes on the film only. The data obtained were similar. The resistivity was found to drop from ∼ 1010 Ω cm for undoped sample down to 3 × 10- 2 Ω cm with the N2 content in plasma rising to 25% as shown in Fig. 5. This observation is in a good agreement with the results reported in [12]. The possibility to get high conductivity in UNCD simply by adding N2 in process gas is one of the most interesting properties of this material. One promising application of the nitrogenated nanocrystalline films is electrochemistry where very stable electrodes with a wide potential window are required [29]. A recent review on UNCD applications in electronics can be found in [13]. 3.6. IR spectra

Fig. 4. Thermal diffusivity of UNCD films at room temperature vs N2 concentration added in plasma.

The evolution of IR optical transmission spectra for UNCD (grown in 2 h runs) with the increase in N2 content reveals an increase in absorption (in parallel, the film thickness increases too) with doping level (Fig. 6). The strong bands between 1700 and 2650 cm− 1 and two wide bands between 3000 and 3800 cm− 1 are due to intrinsic two- and three-phonon diamond absorption in 0.4 mm thick CVD diamond substrate. A monotonically decreasing transmission vs wavenumber was observed for all samples.

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Fig. 6. IR transmittance spectra of UNCD films grown on PCD substrates at different N2 content in plasma.

The extinction coefficient k (k = αλ / 4π), where α is absorption coefficient, was calculated from the spectra. For the high-resistivity films (N2 = 0 ÷ 10%) the k value turned out to be almost constant (k ∼ 0.02–0.05) over the entire spectral range of 400–4000 cm− 1. Similarly, constant k values were reported by other authors for polycrystalline [30] and for thick undoped nanocrystalline [31] diamond films. In contrast, for highlyconductive samples (N2 = 15 ÷ 25%) the extinction coefficient k decreased with wavenumber, in particular, for 20% N2 sample k reduced monotonically from 0.4 (400 cm− 1) to 0.2 (4000 cm− 1). We found a substantial amount of hydrogen in the UNCD films. Absorption features due to the stretching modes of sp3and sp2-bonded CHx groups were observed between 2800 and 3150 cm− 1 in IR spectra (Fig. 6). The concentration of C–H bonds in the films has been estimated from the integral intensity of CHx stretching mode vibrations according to [32] (absorption background due to CHx groups in diamond substrate was subtracted). It was as high as 1.6 × 1022 cm− 3 in undoped UNCD, which is equal to 9.0 at.% (in the calculation the mass density 3.5 g/cm3 for diamond was taken for UNCD). The concentration of C–H bonds decreases down to ≈ 5 at.% with N2 addition in process gas, still being 2–3 orders of magnitude higher than the typical hydrogen impurity concentrations (30– 1000 ppm) in polycrystalline diamond films [2,33]. This is assumed to be a consequence of H localization on grain boundaries, which have an enormous area in the UNCD due to nanoscale grain size. Note that in nanodiamond films grown in DC plasma with CH4–H2 mixtures [34] even higher hydrogen content of 15–20 at.% (integral quantity measured with SIMS) has been observed, however those films seem to have the structure quite different from that considered here. The nitrogen incorporation induces a complex broad absorption band in the 600–1900 cm− 1 range (Fig. 6), where several stretching and bending mode vibrations with very different characteristics could contribute. The integral intensity of this band increases with N2 added in plasma. What is more interesting is that for “low-doped” samples (0–10% N2) the

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shape of the band with maximum at 1330 cm− 1 (similar to that observed in [32]) does not change essentially, but is modified dramatically with further N2 addition, namely it is split to three lines at ∼ 1550, 1230 and 760 cm− 1. The nature of this complex band possibly is directly or indirectly related to nitrogen impurity. Note that both the re-arrangement of absorption band at 1330 cm− 1 and the rapid increasing of conductivity (Fig. 5) take place at the same amounts of added N2 (N 10%). This may indicate some critical level of nitrogen content in plasma and in the UNCD, above which a sharp change in the film structure and properties occurs. Birrell et al. [15] found the nitrogen content in their UNCD to be low and constant while N2 in plasma rose from 3% to 8%, followed by a 5 time increase at 13% N2. We didn't observe an appearance of any specific lines in 3200–3300 cm− 1 range, where stretching bands NxHy are usually observed [35]. This may indicate that N and H atoms in nitrogenated UNCD films are located in different regions, so hydrogen does not passivate nitrogen. 3.7. UV–vis spectra The optical absorption of UNCD films at the UV–vis range is also increased by the addition of nitrogen gas to the plasma. Achatz et al. [18] attributed the enhanced absorption to transitions from π to π⁎ band within band gap, the nitrogen promoting an increasing in number of sp2-bonded carbon atoms, and an increase and broadening of the π and π⁎ bands. The strong absorption in our case (α = 4.5 ⁎ 10 3 cm − 1 , 2.2 ⁎ 104 cm− 1 and 7.3 ⁎ 104 cm− 1 at λ = 400 nm for 0, 5 and 25% N2-samples, respectively) can be related to an intergrain mixed phase located in-between nanodiamond crystallites (the sp2-bonded grain boundaries with thickness of a few interatomic distances for graphite are treated as a separate phase). Often the Tauc model is used as the most common definition of the optical band gap of amorphous and noncrystalline semiconductors [36]. The localized states are present throughout the band gap of the amorphous semiconductors because of

Fig. 7. Tauc plot (αhv)1/2 vs photon energy (E = hv) for UNCD films grown on PCD substrates at different N2 content in plasma; the straight lines shown for each spectrum were used to determine Eg.

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Fig. 8. Tauc optical band gap for UNCD films grown at different N2 amounts added in plasma.

topological disorder in the network. The Tauc model is applicable over a wide enough spectral range towards high energies assuming that the tail of localized states is small compared with the parabolic part of the band and can be neglected. According to Tauc the absorption coefficient α obeys the following relationship [37]: αhv = B(hv − Eg)2, where h is the Planck constant, v is the frequency of radiation, Eg the optical band gap and B is the disorder parameter [38]. The optical band gap Eg can be determined from the plot (αhv)1/2 vs photon energy E = hv, as the intercept of the extrapolated linear fit with the abscissa axis (Fig. 7). For linear extrapolation of Tauc curve we took the range of (αhv)1/2 between 200 and 300 cm− 1/2 eV1/2 which usually is used for hydrogenated carbon [39] and carbon nitride [40] films. The dependence of Eg on N2 percentage in plasma is shown in Fig. 8. The decrease of optical band gap from 2.2 eV to 0.2 eV with nitrogen addition can be related to an increase in size of sp2 clusters, because nitrogen can act as a bridging atom between clusters [41].

Fig. 10. Evolution of IR spectra of UNCD film (5% N2) subjected to vacuum annealing at different temperatures. The top spectrum is for PCD substrate before UNCD deposition. The CH-bonds content decreases with annealing temperature (inset).

3.8. Thermal stability The coarse-grain PCD films show thermal stability upon heating in vacuum up to 1050–1300 °C [21,22], graphitization of grain boundaries and defected domains begin at higher temperatures. There is only indirect evidence of similar high stability of UNCD films, as the diode based on nitrogenated UNCD retained its performance up to 1050 °C [42]. Annealing of our 5% N2 film at 800 °C did not cause any change in optical properties of the sample. Further heating to higher temperatures, from 900 to 1400 °C, resulted in a perceptible film darkening as seen in absorption increase both in UV–vis (Fig. 9) and IR (Fig. 10) spectral ranges. The concentration of C–H bonds decreased by one third (see inset in Fig. 10). Only a slight change in one-phonon diamond absorption band (800– 1400 cm− 1) could be observed. The Tauc optical gap shrinks from 0.6 to 0.25 eV (Fig. 9), probably reflecting the process of partial transformation of sp3 to sp2 phase on grain boundaries. Raman spectra excited in UV range (244 nm) confirmed the presence of diamond phase in 5% N2 film annealed even at 1400 °C, indicating the UNCD thermal stability is comparable with defected (black) PCD films, which are less stable than “white” high-quality PCD [21,22]. Preliminary measurements showed a drop in resistivity of the annealed film by several orders of magnitude. This opens a way to tune the electrical properties of nitrogenated UNCD simply by annealing as it was demonstrated earlier for CVD polycrystalline diamond [43]. 4. Conclusions

Fig. 9. Tauc UV–vis spectra of UNCD film (5% N2) grown on PCD substrate and annealed at different temperatures. The optical band gap decreases with Tann.

Ultrananocrystalline diamond films were grown on Si and transparent polished polycrystalline CVD diamond substrates by microwave plasma CVD system using CH4/H2/Ar mixtures with N2 added in plasma in amounts up to 25%. The films are quite smooth, with surface roughness Ra ∼ 10 nm. The UNCD films have thermal conductivity two orders of magnitude lower,

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and optical absorption 2–3 orders of magnitude higher, compared to polycrystalline films. The presence of N2 in the process gas strongly changes the structure and properties of UNCD. In comparison with undoped films the nitrogenated UNCD show: (i) resistivity decrease by 12 orders of magnitude, down to 10− 2 Ω cm; (ii) thermal conductivity decrease from 0.10 to 0.06 W/cm K at RT; (iii) UV-IR absorption increase, the optical band gap Eopt shrinking from 2.2 eV to 0.2 eV. The concentration of bonded hydrogen (C–H) is very high (9 at.%) in the undoped material, but reduces to ∼ 5 at.% for 25% N2 sample. All these trends reflect the processes of the film structure transformation, presumably on grain boundaries of diamond nanocrystallites. Annealing tests showed that, in spite of a certain optical degradation, the nitrogenated UNCD survives at heating in vacuum at least up to 1400 °C. Acknowledgements The work has been supported by Federal Program “Leading Scientific Schools”, contract no. 02.445.11.7353 (1007.2006.2) and by RFBR grants no. 05-02-19807 and 04-02-39027. References [1] B. Dischler, C. Wild (Eds.), Low-pressure Synthetic Diamond: Manufacturing and Applications, Springer, Berlin, 1998. [2] S.E. Coe, R.S. Sussmann, Diamond Relat. Mater. 9 (2000) 1726. [3] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [4] X. Xiao, J. Birrel, J.E. Gerbi, O. Auciello, J.A. Carlisle, J. Appl. Phys. 96 (2004) 2232. [5] V.I. Konov, A.A. Smolin, V.G. Ralchenko, S.M. Pimenov, E.D. Obraztsova, E.N. Loubnin, S.M. Metev, G. Sepold, Diamond Relat. Mater. 4 (1995) 1073. [6] S.M. Pimenov, V.G. Ralchenko, V.I. Konov, A.V. Khomich, E.V. Zavedeev, V.D. Frolov, in: I.A. Shcherbakov, et al., (Eds.), Advanced Laser Technologies 2004, Proc. SPIE, vol. 5850, 2005, p. 230. [7] L.C. Nistor, J. Van Landuyt, V.G. Ralchenko, E.D. Obraztsova, A.A. Smolin, Diamond Relat. Mater. 6 (1997) 159. [8] A.R. Krauss, O. Auciello, D.M. Gruen, A. Jayatissa, A. Sumant, J. Tucek, D.C. Mancini, N. Moldovan, A. Erdemir, D. Ersoy, M.N. Gardos, H.G. Busmann, E.M. Meyer, M.Q. Ding, Diamond Relat. Mater. 10 (2001) 1952. [9] A.R. Krauss, O. Auciello, M.Q. Ding, D.M. Gruen, Y. Huang, V.V. Zhirnov, E.I. Givargizov, A. Breskin, R. Chechen, E. Shefer, V. Konov, S. Pimenov, A. Karabutov, A. Rakhimov, N. Suetin, J. Appl. Phys. 89 (2001) 2958. [10] J.A. Carlisle, O. Auciello, Interface 12 (2003) 28. [11] J. Philip, P. Hess, T. Feygelson, J.E. Butler, S. Chattopadhyay, K.H. Chen, L.C. Chen, J. Appl. Phys. 93 (2003) 2164. [12] S. Bhattacharyya, O. Auciello, J. Birrell, J.A. Carlisle, L.A. Curtiss, A.N. Goyette, D.M. Gruen, A.R. Krauss, J. Schlueter, A. Sumant, P. Zapol, Appl. Phys. Lett. 79 (2001) 1441.

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