Structural Transformation upon Nitrogen Doping of Ultrananocrystalline Diamond Films by Microwave Plasma CVD

The molecular properties and surface morphology of undoped and N-doped ultra-nanocrystalline diamond (UNCD) films deposited by microwave plasma CVD with addition of nitrogen are investigated with various spectroscopic techniques. The results of spatially resolved Raman scattering, ATR/FT-IR and XPS spectra show more amorphous and sp2/sp3 ratio characteristics in N-doped UNCD films. The surface morphology in AFM scans shows larger nanocrystalline diamond clusters in N-doped UNCD films. Incorporation of nitrogen into UNCD films has promoted an increase of amorphous sp2-bonded carbons in the grain boundaries and the size of nanocrystalline diamond grains that are well correlated to the reported enhancement of conductivity and structural changes of UNCD films.

Both theoretical calculations and experimental data showed that the preferential incorporation of nitrogen into the grain boundaries of N-doped UNCD films would form the larger clustering and disordering of graphite-like sp 2bonded carbons [5,6,11]. Recently, diamond nanowires consisting of diamond core sheathed with sp 2 -bonded carbon about 1 nm in thickness are observed in N-doped UNCD films with addition of 10% N 2 [11,12].
Raman spectroscopy has been used to characterize molecular properties of UNCD films. On the other hand, IR spectroscopy is known as a complementary technique to Raman scattering for the studies of structural vibrations, such as C-H bonds stretching (2750 ∼ 3300 cm −1 ) in sp 2 /sp 3 carbon-based materials [2,13]. In particular, attenuated total reflection Fourier transform infrared (ATR/FT-IR) spectroscopy is an effective technique for UNCD mirrorflat thin films samples due to the increasing optical path length by the multiple-reflected radiation in the total internal reflection element [14]. In this paper, we investigate and compare the spatial uniformity of molecular properties and surface morphology of MPCVD UNCD films processed with and without the addition of 20% nitrogen by using Raman and ATR/FT-IR spectroscopy as well as AFM for morphology images and XPS for chemical environment identifications. The correlations of spectroscopic results and the reported electrical conductivity and structural modifications are also discussed.

UNCD Films by MPCVD.
The UNCD films grown on Si with and without N 2 addition samples were prepared using 6 in CYRRANUS Innovative Plasma System (MPECVD, IPLAS GmbH) and provided by Dr. Gruen's group at Argonne National Lab, USA. The details of deposition parameters were the same as described in [12].
The UNCD films were deposited on a large 4 in Si wafer. The size, distribution, and uniformity of microwave plasma and substrate temperature can effectively affect the quality, property, and morphology of UNCD films. In particular, the UNCD film grown at the center of the Si wafer may be quite different from that at the edge. In this work, the UNCD film on Si wafer was divided into ten (10) intervals along the radius from the edge to the center, and labeled as #1 → #10.

Characterization Methods. Renishaw Raman Scattering
Noodles System 2000, equipped with a Leica microscope and a HeNe red laser (632.8 nm) as an excitation source, was used to examine the molecular properties of UNCD films. X-ray photoelectron spectroscopy (Mg-K α radiation of 1253.6 eV with a surface analysis depth of ∼2 nm) was used to prove the binding states of carbon 1s core level in UNCD films without Ar + sputtering pretreatment. ATR/FT-IR spectrometer (PIKE, MIRacle Single Reflection Horizontal ATR Accessory equipped with 1.8 mm round ZnSe crystal IRE plate) was used to diagnose the molecular vibrations of UNCD films in mid-IR (650∼4000 cm −1 ) range recorded with 500 scans and 1 cm −1 resolution. Atomic force microscopy (AFM, Quesant Q-scope 350) measurements were scanned for the surface morphology of UNCD films.

Results and Discussions
3.1. Raman Spectra of UNCD Films. The spectroscopic measurement was monitored at each interval position. Figure 1 shows Raman spectra of undoped and N-doped UNCD films. The top two (green and pink) and the bottom two (brown and blue) are recorded at the edge and at the center of substrate, respectively. The assignment and interpretation of Raman spectra of undoped and N-doped UNCD films deposited from 1%CH 4 /N 2 /Ar MPCVD have been extensively discussed recently [4,6,11,12]. The following spectral features are typically observed: (1) a broad band at 1332 cm −1 from nanodiamond crystallites, (2) the sp 2 -bonded carbon features around 1340∼1600 cm −1 (in particular at ∼1350 cm −1 and ∼1580 cm −1 for graphite D-& G-bands), and (3) The C-H vibration characteristics of short-chain conjugated t-PA in the grain boundaries at 1120 ∼1190 cm −1 together with a weak peak at 1460 cm −1 and the overtone features at 2245∼2810 cm −1 [3,9,10]. Moreover, the sp 3 /sp 2 ratio in N-doped UNCD films normally decreases as compared to the undoped UNCD films [4].
In the bottom two spectra of Figure 1, the differences in the Raman spectra between undoped and N-doped UNCD films taken at the center position are similar to those observed by Polyakov et al. [7]. However, Polyakov et al. did not report the Raman spectra measured at the edge position for UNCD films. When the top two spectra (edge) and bottom two spectra (center) in Figure 1 are compared, the following observations are noted: (1) a large increase (or almost inverse) in the I(G)/I(D) ratio, (2) the t-PA peak has transformed from a weak shoulder band at 1130 cm −1 to a well-defined peak at 1180 cm −1 and a large enhancement in overtone feature around 2270∼2600 cm −1 , and (3) a stronger Raman intensity in 1100∼1650 cm −1 for N-doped UNCD films is observed at the edge. These observed results strongly suggest that (1) the undoped UNCD films grown at the edge seem to contain a larger percentage of sp 2 -bonded carbons due to the smaller nanodiamond crystallite clusters (see Figure 4 of AFM images) as compared to that at the center, and (2) the N-doped UNCD films show an increase in clustering and disordering of aromatic sp 2 (π)-bonded carbons in the grain boundaries [5,6,11]. However, one should also note that a large increase in Raman intensity of the top two spectra, such as the peak at 1540 cm −1 , may in part due to the higher scattering cross-section of sp 2 sites for visible Raman transitions. The nanodiamond crystallinity for both undoped and Ndoped UNCD films has shown to enhance as going from the edge to the center of substrate. This is evidenced by the observation of an increase in Raman intensity ratio (sp 3 /sp 2 ) of diamond peak (shift from 1350 cm −1 to 1332 cm −1 ) relative to graphite G-peak (shift from 1540 cm −1 to 1560∼ 1590 cm −1 ). On the other hand, the spectral peaks of t-PA seem to disappear gradually from the edge to the center substrate position, while only a small shoulder around 1130 cm −1 and a broad band around 2600∼2700 cm −1 are observed, suggesting an improvement of sp 2 -bonded carbons phases in the grain boundaries of UNCD films. Meanwhile, the broad spectral background is also greatly reduced.
The spatial variation in compositional changes of the UNCD films with and without N-doping as shown in Figure 1 is possibly resulted from the varying alpha parameter (appearing of crystal facets) due to the inhomogeneous of plasma density distribution from the perimeter of the plasma ball to the center of substrate that leads to different gas phase composition, varying substrate temperature and surface chemistry for diamond film growth [15,16]. Figure 2 shows the intensity variation of spatially resolved Si Raman peak at 520 cm −1 taken along the radial Journal of Nanomaterials position of substrate from the edge to the center, for both UNCD films with (solid) and without (dash) N 2 doping. For comparison, the Si peak of undoped UNCD film is normalized by referring to that of N-doped UNCD film at the edge position, labeled #1. The Si peak is observed because the thicknesses of UNCD films are thin enough for being penetrated through by the excitation laser beam. The penetration depth of the visible light at an excitation wavelength of 632.8 nm is about 1∼2 μm [11]. Therefore, the change of Si peak intensity should inversely reflect the thickness uniformity of UNCD films over the Si substrate from the edge to the center. From the results illustrated in Figure 2, the films are thicker and smoother at the center (∼1 μm) than those at the edge. The oscillation of Si peak intensity for N-doped UNCD film indicates its thickness fluctuation as going from the edge to the center positions, which is much larger than that of undoped UNCD film. The actual thickness profile of the films along the substrate position can be obtained by the calibration of Si Raman peak intensity. Figure 3(a) shows ATR/FT-IR absorption spectra of undoped (Blue) and Ndoped (Red) UNCD films at the center position of substrate in the mid-IR range of 675∼4000 cm −1 . The absorption of water (3500∼3900 cm −1 and 1400∼1800 cm −1 ) and carbon dioxide (2350 cm −1 ) is noticed in the spectra [17]. The C-C and C-H rocking characteristics of undoped UNCD films are observed between 800∼1200 cm −1 and 600∼900 cm −1 , respectively, in the spectrum (Blue) [13].

ATR/FT-IR Spectra of UNCD Films.
For this study, the focus is on the transformation of sp 3 to sp 2 CH vibrations upon nitrogen doping that are located in the spectral ranges of 2600 cm 2 to 3200 cm 2 as shown in Figures 3(b), 3(c). Both N-doped Figure 3   peak assignments are summarized in Table 1. The distinctive differences between N-doped and undoped UNCD films at center are in the spectral range of 2800∼3100 cm −1 . The sp 2 -CH 2 asymmetric and sp 2 -CH stretching vibrations at 3078 cm −1 and 3039 cm −1 are observed in N-doped UNCD films [14,18,19]. The sp 3 -CH and sp 3 -CH 2 asymmetric stretching vibrations at 2895 cm −1 and 2923 cm −1 are shown for undoped UNCD films [13,14,18,19]. We have also analyzed the ATR/FT-IR absorption spectra in the same spectral ranges as in Figure 3(b) for undoped and N-doped UNCD films at the edge position of substrate, and the results are quite similar to those at the center. The only difference is that the CH vibrational features of UNCD films are stronger and better defined at the center than those at the edge position. These results are in agreement with the Raman spectroscopic observations above that the incorporation of nitrogen into UNCD films would lead to the formation of sp 2 -bonded carbons. Figure 4, the XPS C 1s spectra of undoped and N-doped UNCD films at the center position (labeled #10) of the substrate are deconvoluted by Gaussian multiple peaks fitting into two main components with the binding energies (BEs) of 284.0∼ 284.2 eV (for carbon sp 2 bonding) and 285.6∼285.9 eV (for carbon sp 3 bonding). In N-doped UNCD films, the following XPS spectral features are observed: (1) sp 2 peak shifts to lower BE by 0.2 eV, (2) sp 3 peak shifts to higher BE by 0.3 eV, and (3) both sp 2 and sp 3 peaks are broadened as compared to the undoped UNCD films. The observed spectral broadening and shifts of C 1s BE have been suggested due to the interactions of C with N [20,21]. The broadened C 1s peaks in XPS imply the increasing in the formation and disorderness of sp 2 -bonded carbon phase as resulted from the incorporation of nitrogen into UNCD films that have also been illustrated in Raman and ATR/FT-IR spectra above.

AFM Morphology of UNCD Films.
The AFM surface morphology of UNCD films was monitored for the spatial scanning areas with different film thickness. Figure 5 shows AFM surface morphology of undoped UNCD films (top four) and N-doped UNCD films (bottom four), both scanned at the edge (upper two labeled as (e)) and center (lower two labeled as (c)) positions of substrates. In Figure 5 (top), the undoped UNCD films were formed by coalescence of coin-like diamond crystal clusters. In a smaller scanning area (or high resolution with a scale bar 100 nm) of Figure 5 (top, upper-right), the sizes of diamond crystal clusters at the edge are 433 nm and 391 nm as marked by red and blue arrows. In a larger scanning area (or low resolution with a scale bar 1 μm) of Figure 5 (top, lowerleft), the sizes of diamond crystal clusters at the center are 817 nm, 701 nm, and 695 nm as marked by red, green, and blue arrows. In a smaller scanning area (or high resolution with a scale bar 100 nm) of Figure 5 (top, lower-right), the radius and central spacing of diamond crystal clusters at the center are about 471 nm and 527 nm as marked by green and blue arrows. Therefore, the sizes of diamond crystal clusters increase from 400∼500 nm (top, upper-left, edge) to 700∼ 800 nm (top, lower-left, center) along the radial position of substrate.
However in Figure 5 (bottom), the N-doped UNCD films were formed by coalescence of erythrocyte-like. In a larger scanning area (or low resolution with a scale bar 1 μm) of Figure 5 (bottom, left), the sizes of diamond crystal clusters at the edge are 794 nm, 915 nm, and 895 nm as marked by red, green, and blue arrows. The sizes of diamond crystal clusters at the center are 792 nm, 724 nm, and 781 nm as marked by red, green, and blue arrows. In a smaller scanning area (or high resolution with a scale bar 100 nm) of Figure 5 (bottom, upper), the widths of the thicker circumferential part of the erythrocyte-like diamond crystal cluster at the edge are about 348 nm and 189 nm as marked by green and blue arrows. Therefore, the diamond crystal clusters of 800∼1000 nm in size over the substrate, both at the edge and center positions, are more uniform than that of undoped UNCD films over the entire substrate areas.
It is worthy of mentioning that the morphology of N-doped UNCD film seems to show a small opening in each diamond crystal cluster at the center as compared to that at the edge as shown in Figure 5 (bottom, lower-left). However, the similar morphology without those openings  was observed in the AFM image scanned by using a new probe as shown in Figure 6(b), where Figure 6(a) is the same AFM image as Figure 5 (bottom, lower-left). Therefore, those openings are the artifacts coming from the convolution of the tip and sample surface.
In a smaller scanning area (or a higher resolution with a scale bar 100 nm) of Figure 5 (top, right two), the surface morphology of each diamond crystal cluster of undoped UNCD films was revoluted from the periodically well-  The observed AFM morphologies may suggest that when undoped UNCD films grow thicker at the center of substrate, the diamond crystal clusters as building blocks have transformed into a 3-dimensional growth, the larger diamond clusters have been grown from a plate-like at the edge into a ball-like at the center of substrate. However, as N-doped UNCD films become thicker, the diamond crystal clusters start with a similar size and then proceed to a preferentially growth in the direction perpendicular to the substrate surface. Figure 7 shows the mean surface roughness (Ra) of the UNCD films for a larger scanning area, 10 μm × 10 μm (pink: undoped UNCD and light green: N-doped UNCD, hollow markers) and a smaller scanning area, 1 μm × 1 μm (red: 6 Journal of Nanomaterials undoped UNCD and blue: N-doped UNCD, solid markers), from the edge (#1) to the center (#10) position as labeled in X-axis. The standard deviation error bars are indicated together with the mean Ra markers.
In pink markers, the Ra of the undoped UNCD film gradually decreases from 11 nm (edge, #1) to around 6 nm (center, #10), with the overall mean value of 7 nm (STED 1.4 nm). In red markers, on the other hand, the Ra of the undoped UNCD film displays a slight fluctuation with the overall mean Ra 2.5 nm (STED 0.9 nm).
This observation seems to associate with the numbers and size of diamond crystal clusters within the AFM scanning area. For example, in a (1 μm × 1 μm) viewing area, the Ra (2.5 nm) is quite uniform within the area down to 1∼4 diamond crystal clusters. For a larger viewing of undoped UNCD film, for example, 10 μm × 10 μm area covering about 100∼400 diamond crystal clusters, the Ra increases up to 7 nm. The corresponding standard deviation of the mean Ra also increases in 10 μm × 10 μm area as compared to 1 μm × 1 μm area. The Ra is 6 nm at the center (#10) with a cluster size of 800 nm and the Ra = 11 nm in the edge (#1), while the cluster size is 500 nm, as shown in Figure 5 (top).
Similarly, in light green and blue markers, the overall mean Ra of the N-doped UNCD films, both in 10 μm × 10 μm and 1 μm × 1 μm scanning areas, is 8 nm (STED 1.8 nm) and 3 nm (STED 1.1 nm), respectively. But the Ra of N-doped UNCD films seems to fluctuate larger than that of undoped UNCD films, suggesting the formation of a larger diamond crystal cluster size for the N-doped UNCD films.

Effect of N-Doping on UNCD Films.
Normally, UNCD films grown by MPCVD using CH 4 and Ar gas mixtures without the addition of N 2 gas should follow the C 2based growth mechanism [7], where the grown UNCD films should compose of nanocrystalline diamond grains and a structurally disordered mixture of sp 3 /sp 2 -bonded carbons in the grain boundaries. Upon introducing nitrogen into UNCD films, the most energetically favorable sites should be at the grain boundaries, where the lone pair electrons in nitrogen can facilitate the transformation of tetrahedral coordinated sites of sp 3 -like carbon configuration into a three-fold-coordinated site of sp 2 -like carbon arrangement with a perturbed N-doped characteristic at the grain boundaries [6]. Thus, the incorporation of nitrogen into UNCD films could lead to an increase in amorphous sp 2bonded (π-) carbon phase by the formation of conjugated networking of the chain-like sp 2 and sp 1 hybridized with Ndoped structures together with aromatic carbon clusters in the grain boundaries as evidenced by the results of Raman, ATR/FT-IR, and XPS spectral investigations.
The sp 3 nanodiamond grains of undoped UNCD films are electrically inactive based on the theoretical tightbonding calculations [8]. For N-doped UNCD film, the structural transformation has shown to lead to an increase of sp 2 -carbon phase at the grain boundaries as illustrated in the spectroscopic investigation above that can give the n-type electrical conductivity as resulted from the increase in the electron delocalization of π-π * transition states. This structural assessment is in agreement with the observed high conductivity (up to 150 Ω −1 cm −1 ) for the nitrogen containing UNCD films, via the enhanced sp 2 -carbon phase about ∼10% at the dense and connected grain boundaries [1,21].
AFM surface morphology pictures of N-doped UNCD films in Figure 5 show larger nanocrystalline diamond clusters and more wrinkled surface than those of undoped UNCD films. The increase of the overall grain boundary volume and grain size in N-doped UNCD films measured at the center, and the edge positions of substrate could offer a possible film growth mechanism that the N-doped molecular species may reduce secondary nucleation rate by blocking potential sites for C 2 addition through the absorption of these N-doped molecules and thus enhance the growth of diamond crystals [4,6].

Conclusions
The incorporation of nitrogen into UNCD films prepared by MPCVD using CH 4 /N 2 /Ar gas mixtures has shown to lead to lower sp 3 /sp 2 -bonded carbon characteristic as evidenced in the spectral features of sp 2 -bonded carbon phases in the grain boundaries of N-doped UNCD films. Higher amorphous and sp 2 carbon feature in thin UNCD film deposited at the edge position of substrate was observed. The increase in size of nanocrystalline diamond clusters leading to a larger fluctuation of surface roughness was also revealed by AFM surface morphology of N-doped UNCD films. The assessment of structural transformation of UNCD films upon nitrogen doping is in agreement with the previously proposed high electron transport via grain boundaries for Ndoped UNCD films.