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

Authors: Chien-Chung Teng; Shin-Min Song; Chien-Min Sung; Chhiu-Tsu Lin
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

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 andsp2/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.


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## Body

## 1. Introduction

Ultrananocrystalline diamond (UNCD) films have become very attractive materials for microelectronics applications
since UNCD films with and without nitrogen doping have recently been shown to
have mildn- and p-type semiconductor characteristics [1–8]. UNCD films
are usually prepared by  1%CH4/Ar
microwave plasma CVD (MPCVD) [9]. Typically, UNCD films compose of 3~8 nm
polycrystalline nanodiamond grains, containing the mixtures of  sp2/sp3-bonded
carbon (trans-polyacetylene (t-PA) like, and graphite-like sp2-bonded
CH groups and sp3-bonded CH2 groups), with boundaries of
~0.5 nm in width [3, 10–12].Both
theoretical calculations and experimental data showed that the preferential
incorporation of nitrogen into the grain boundaries ofN-doped UNCD films would
form the larger clustering and disordering of graphite-like sp2-bonded
carbons	[5, 6, 11]. Recently, diamond nanowires consisting of diamond core
sheathed with sp2-bonded carbon about 1 nm in thickness are observed
in N-doped UNCD films with addition of   10%N2 [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 (2750~3300cm−1) in sp2/sp3 carbon-based materials [2, 13]. In particular, attenuated total reflection
Fourier transform infrared (ATR/FT-IR) spectroscopy is an effective technique
for UNCD mirror-flat 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.


## 2. Experimental

### 2.1. UNCD Films by MPCVD

The UNCD films grown on Si with and withoutN2 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.


### 2.2. 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-Kα 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.


## 2.1. UNCD Films by MPCVD

The UNCD films grown on Si with and withoutN2 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.


## 2.2. 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-Kα 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.


## 3. Results and Discussions

### 3.1. Raman Spectra of UNCD Films

The spectroscopic measurement was
monitored at each interval position. Figure1 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% CH4/N2/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 sp2-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 sp3/sp2 ratio in N-doped UNCD films normally decreases
as compared to the undoped UNCD films [4].Figure 1
Raman spectra of UNCD films
taken along the radius from the edge to the center position, labeled as#1→#10.In the bottom two spectra of Figure1, 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
sp2-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 sp2 (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
sp2 sites for visible Raman transitions.The nanodiamond crystallinity for both undoped and N-doped 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 (sp3/sp2)
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 sp2-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 Figure1 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].Figure2 shows the intensity variation of spatially resolved Si Raman peak at 520 cm−1 taken along the radial position of substrate from the edge to the center, for both UNCD films with (solid) and without (dash) N2 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 2
Intensity variation of spatially resolved Si Raman peak at 520cm−1 of UNCD film
from the edge to the center position of substrates.


### 3.2. ATR/FT-IR Spectra of UNCD Films

Figure3(a) shows ATR/FT-IR absorption spectra of undoped (Blue) and N-doped (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].(a) ATR/FT-IR spectra of undoped (blue) and N-doped (red) UNCD films by MWCVD at the center position (labeled#10). (b), (c) Deconvolution (green) of ATR/FTIR spectra of N-doped (top) and undoped (bottom) UNCD films at the center position in CH vibrational region of 2600~3200 cm−1 after baseline correction and by Gaussian fitting (red).
(a)(b)(c)Figure 4
XPSC 1s spectra of undoped (bottom)
and N-doped (top) UNCD films with Gauss fits of sp2 & sp3 peaks.For this study, the focus is on the transformation ofsp3 to sp2 CH
vibrations upon nitrogen doping that are located in the spectral ranges of 2600 cm2 to 3200 cm2 as shown in Figures 3(b), 3(c). Both N-doped Figure
3(b) and undoped Figure 3(c) spectra are weak, broad, and featureless, so a
detailed deconvolution technique of Gaussian
fitting and baseline correction of ATR/FT-IR spectra is required to analyze the
UNCD films at center of substrate. The tentative 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 sp2-CH2 asymmetric and sp2-CH stretching
vibrations at 3078 cm−1 and 3039 cm−1 are observed in
N-doped UNCD films [14, 18, 19]. The sp3-CH and sp3-CH2 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 sp2-bonded
carbons.Table 1
Spectral band assignment ofCHx stretching vibration modes of UNCD
films at the center position, where minor peaks are shown in parenthesis.
Assignment in [13, 14, 16, 17]/(cm−1)Assignment in experimentsN-doped UNCD filmsUndoped UNCD filmssp2-CH2(sym.)/29803004(2988)sp2-CH2(asym.)/30803078(3088)sp2-CH/30253039(3033)sp3-CH/2900~2920(2902)2895sp3-CH2(sym.)/2850—2861sp3-CH2(asym.)/2920—2923


### 3.3. X-Ray Photoelectron Spectroscopy

In Figure4, 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 sp2 bonding) and 285.6~285.9 eV (for carbon sp3 bonding). In N-doped UNCD films, the following XPS spectral features are observed: (1) sp2 peak shifts to lower BE by 0.2 eV, (2) sp3 peak shifts to higher BE by 0.3 eV, and (3) both sp2 and sp3 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 sp2-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.


### 3.4. AFM Morphology of UNCD Films

The AFM surface morphology of UNCD films was monitored for the spatial scanning areas
with different film thickness. Figure5 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.AFM surface morphology of
undoped (top four) and N-doped (bottom four) UNCD films, scanned at edge (upper two labeled as (e)) and center (lower two labeled as (c)) in different areas.In Figure5
(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, lower-left), 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 Figure5 (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 Figure5 (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.Figure 6
AFM surface
morphology of N-doped UNCD films scanned at center by
different probes (a) and (b).In a smaller scanning area (or a higher resolution with a scale bar 100 nm) of Figure5
(top, right two), the surface morphology of each diamond crystal cluster of undoped UNCD
films was revoluted from the periodically well-aligned ridges of 70~80 nm in
spacing (two ridges of 155 nm as indicated by green arrows) at the edge (top,
upper-right) into a ballic surface profile at the center (top, lower-right). On
the other hand, in Figure 5 (bottom, right two), the surface morphology of each diamond crystal
cluster of N-doped UNCD films was revoluted from a flat surface
at the edge (bottom, upper-right) into a bump-like profile at the center
(bottom, lower-right).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.Figure7 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: 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.Figure 7
AFM mean surface roughness(Ra),
with standard deviation error bar, of undoped and N-doped UNCD films scanned at different film areas.In pink markers, theRa 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 (1μm × 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 meanRa 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.


### 3.5. Effect of N-Doping on UNCD Films

Normally, UNCD films grown by MPCVD usingCH4 and Ar gas mixtures without the addition of N2 gas should follow the C2-based growth mechanism [7], where the grown
UNCD films should compose of nanocrystalline diamond grains and a structurally
disordered mixture of sp3/sp2-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 sp3-like
carbon configuration into a three-fold-coordinated site of sp2-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 sp2-bonded
(π-) carbon phase by
the formation of conjugated networking of the chain-like sp2 and sp1 hybridized with N-doped structures together with aromatic carbon clusters in the grain
boundaries as evidenced by the results of Raman, ATR/FT-IR, and XPS spectral investigations.Thesp3 nanodiamond grains of undoped UNCD films are electrically inactive based on the
theoretical tight-bonding calculations [8]. For N-doped UNCD film, the structural
transformation has shown to lead to an increase of sp2-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−1cm−1) for the nitrogen containing UNCD films, via the enhanced sp2-carbon
phase about ~10% at the dense and connected grain boundaries [1, 21].AFM surface morphology pictures of N-doped UNCD films in Figure5 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 C2 addition through the absorption of these
N-doped molecules and thus enhance the growth of diamond crystals [4, 6].


## 3.1. Raman Spectra of UNCD Films

The spectroscopic measurement was
monitored at each interval position. Figure1 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% CH4/N2/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 sp2-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 sp3/sp2 ratio in N-doped UNCD films normally decreases
as compared to the undoped UNCD films [4].Figure 1
Raman spectra of UNCD films
taken along the radius from the edge to the center position, labeled as#1→#10.In the bottom two spectra of Figure1, 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
sp2-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 sp2 (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
sp2 sites for visible Raman transitions.The nanodiamond crystallinity for both undoped and N-doped 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 (sp3/sp2)
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 sp2-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 Figure1 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].Figure2 shows the intensity variation of spatially resolved Si Raman peak at 520 cm−1 taken along the radial position of substrate from the edge to the center, for both UNCD films with (solid) and without (dash) N2 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 2
Intensity variation of spatially resolved Si Raman peak at 520cm−1 of UNCD film
from the edge to the center position of substrates.


## 3.2. ATR/FT-IR Spectra of UNCD Films

Figure3(a) shows ATR/FT-IR absorption spectra of undoped (Blue) and N-doped (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].(a) ATR/FT-IR spectra of undoped (blue) and N-doped (red) UNCD films by MWCVD at the center position (labeled#10). (b), (c) Deconvolution (green) of ATR/FTIR spectra of N-doped (top) and undoped (bottom) UNCD films at the center position in CH vibrational region of 2600~3200 cm−1 after baseline correction and by Gaussian fitting (red).
(a)(b)(c)Figure 4
XPSC 1s spectra of undoped (bottom)
and N-doped (top) UNCD films with Gauss fits of sp2 & sp3 peaks.For this study, the focus is on the transformation ofsp3 to sp2 CH
vibrations upon nitrogen doping that are located in the spectral ranges of 2600 cm2 to 3200 cm2 as shown in Figures 3(b), 3(c). Both N-doped Figure
3(b) and undoped Figure 3(c) spectra are weak, broad, and featureless, so a
detailed deconvolution technique of Gaussian
fitting and baseline correction of ATR/FT-IR spectra is required to analyze the
UNCD films at center of substrate. The tentative 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 sp2-CH2 asymmetric and sp2-CH stretching
vibrations at 3078 cm−1 and 3039 cm−1 are observed in
N-doped UNCD films [14, 18, 19]. The sp3-CH and sp3-CH2 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 sp2-bonded
carbons.Table 1
Spectral band assignment ofCHx stretching vibration modes of UNCD
films at the center position, where minor peaks are shown in parenthesis.
Assignment in [13, 14, 16, 17]/(cm−1)Assignment in experimentsN-doped UNCD filmsUndoped UNCD filmssp2-CH2(sym.)/29803004(2988)sp2-CH2(asym.)/30803078(3088)sp2-CH/30253039(3033)sp3-CH/2900~2920(2902)2895sp3-CH2(sym.)/2850—2861sp3-CH2(asym.)/2920—2923


## 3.3. X-Ray Photoelectron Spectroscopy

In Figure4, 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 sp2 bonding) and 285.6~285.9 eV (for carbon sp3 bonding). In N-doped UNCD films, the following XPS spectral features are observed: (1) sp2 peak shifts to lower BE by 0.2 eV, (2) sp3 peak shifts to higher BE by 0.3 eV, and (3) both sp2 and sp3 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 sp2-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.


## 3.4. AFM Morphology of UNCD Films

The AFM surface morphology of UNCD films was monitored for the spatial scanning areas
with different film thickness. Figure5 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.AFM surface morphology of
undoped (top four) and N-doped (bottom four) UNCD films, scanned at edge (upper two labeled as (e)) and center (lower two labeled as (c)) in different areas.In Figure5
(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, lower-left), 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 Figure5 (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 Figure5 (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.Figure 6
AFM surface
morphology of N-doped UNCD films scanned at center by
different probes (a) and (b).In a smaller scanning area (or a higher resolution with a scale bar 100 nm) of Figure5
(top, right two), the surface morphology of each diamond crystal cluster of undoped UNCD
films was revoluted from the periodically well-aligned ridges of 70~80 nm in
spacing (two ridges of 155 nm as indicated by green arrows) at the edge (top,
upper-right) into a ballic surface profile at the center (top, lower-right). On
the other hand, in Figure 5 (bottom, right two), the surface morphology of each diamond crystal
cluster of N-doped UNCD films was revoluted from a flat surface
at the edge (bottom, upper-right) into a bump-like profile at the center
(bottom, lower-right).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.Figure7 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: 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.Figure 7
AFM mean surface roughness(Ra),
with standard deviation error bar, of undoped and N-doped UNCD films scanned at different film areas.In pink markers, theRa 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 (1μm × 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 meanRa 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.


## 3.5. Effect of N-Doping on UNCD Films

Normally, UNCD films grown by MPCVD usingCH4 and Ar gas mixtures without the addition of N2 gas should follow the C2-based growth mechanism [7], where the grown
UNCD films should compose of nanocrystalline diamond grains and a structurally
disordered mixture of sp3/sp2-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 sp3-like
carbon configuration into a three-fold-coordinated site of sp2-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 sp2-bonded
(π-) carbon phase by
the formation of conjugated networking of the chain-like sp2 and sp1 hybridized with N-doped structures together with aromatic carbon clusters in the grain
boundaries as evidenced by the results of Raman, ATR/FT-IR, and XPS spectral investigations.Thesp3 nanodiamond grains of undoped UNCD films are electrically inactive based on the
theoretical tight-bonding calculations [8]. For N-doped UNCD film, the structural
transformation has shown to lead to an increase of sp2-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−1cm−1) for the nitrogen containing UNCD films, via the enhanced sp2-carbon
phase about ~10% at the dense and connected grain boundaries [1, 21].AFM surface morphology pictures of N-doped UNCD films in Figure5 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 C2 addition through the absorption of these
N-doped molecules and thus enhance the growth of diamond crystals [4, 6].


## 4. Conclusions

The incorporation of nitrogen into UNCD films prepared by MPCVD usingCH4/N2/Ar gas mixtures
has shown to lead to lower sp3/sp2-bonded carbon
characteristic as evidenced in the spectral features of sp2-bonded
carbon phases in the grain boundaries of N-doped UNCD films. Higher amorphous and sp2 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
N-doped UNCD films.



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*Source: 621208-2009-04-29.xml*