# Time-Resolved Observation of Deposition Process of Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films in Pulsed Laser Deposition

Authors: Kenji Hanada; Takashi Nishiyama; Tsuyoshi Yoshitake; Kunihito Nagayama
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

Optical emission spectroscopy was used to study pulsed laser ablation of graphite in a hydrogen atmosphere wherein ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon  (a-C:H)  composite films were grown on heated substrates. Time-resolved photographs of a plume that expanded from a laser-irradiation spot toward a substrate were taken using a high-speed ICCD camera equipped with narrow-bandpass filters. While the emissions from C atoms andC2 dimers lasted above the laser-irradiation spot on the target, the emission from  C+ ions lasted above the substrate surface for approximately 7 microseconds, although the emission lifetime of species is generally approximately 10 nanoseconds. This implies that C+ ions actively collided with each other above the substrate surface for such a long time. We believe that the keys to UNCD growth in PLD are the supply of highly energetic carbon species at a high density to the substrate and existence of atomic hydrogen during the growth.


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## Body

## 1. Introduction

Ultrananocrystalline  diamond
(UNCD)  films are  new candidates for hard coatings, semiconducting devices, and microelectromechanical
systems (MEMSs). UNCD
is obviously distinguished from nanocrystalline diamond (NCD). The grain sizes
of NCD and UNCD have been quoted as less than 100 nm and 2–15 nm, respectively
[1]. UNCD films are structurally
characterized by the existence of grain boundaries with volume fractions more
than 10% for the total and coexistence of a great proportion of amorphous
carbon in the films [1],  which induces an
extremely large optical absorption [2]. In addition, it is possible for a
highly nitrogenated film to possess a high n-type
conductivity [3, 4].The UNCD films have mostly been
fabricated by chemical vapor deposition (CVD) [5].
It is well known that C2 dimers have an important role in the growth
of UNCD films by CVD
[6]. Recently, we have succeeded in
growing UNCD films, wherein 5-nm diamond crystallites are embedded in a
hydrogenated amorphous carbon (a-C:H)
matrix, in a hydrogen
atmosphere by pulsed laser deposition (PLD) with a graphite target
[7, 8].
They can be grown at low substrate temperatures and high deposition rates. In
addition, while CVD necessitates the pretreatment of substrates on the surface
of which diamond crystallites are seeded using diamond powders
[6, 9], pulsed
laser deposition (PLD) does not. In other words, PLD enables not only the
growth of UNCD films but also the nucleation of diamond. Since the deposition
conditions in PLD are completely different from those in CVD, there must be a
difference in the UNCD growth mechanism.In this paper, we discuss the
UNCD growth mechanism in PLD on the basis of previous reports and the experimental
results of this study. In this study, we deposited UNCD films, and by using a
high-speed intensified charge-coupled (ICCD) camera, we observed the ablation
process of graphite under the deposition conditions of these films. Further, we
evaluated the structure of the deposited films.


## 2. Experimental Section

Optical emission spectroscopy was used to study the laser ablation
process. Observations  were made under the optimum deposition condition of the
UNCD/a-C:H films. The film
preparation conditions have been detailed in our previous papers
[2, 10].
Optical emission spectra of a plume were measured in the wavelength region of
200 to 800 nm by using a multichannel spectrometer (Ocean Photonics, USB4000). Time-resolved
plume images were recorded using an ICCD camera system (C7972-11) capable of recording
12-bit images and equipped with an 18-mm diameter image intensifier and a 1024×1024  pixel CCD array. In order to observe the behavior of C+ ions,
C atoms, and C2 dimers separately, narrow-bandpass filters
transparent to the wavelengths of 394, 505, and 515 nm were placed between the
vacuum chamber and the lens. The wavelengths of 394, 505, and 515 nm correspond
to those of the representative emission lines of C+ ions, C atoms,
and C2 dimers, respectively. The images were taken from laser-irradiation until the plume disappeared, at an interval of 100 nm and an exposure
time of 20 ns.


## 3. Results and Discussion

A typical optical emission spectrum
of the plume is shown in Figure1. The spectrum is continuous at wavelengths
between 300 and 700 nm, and the intense peaks that originate from C2 Swan bands, C atoms, and C+ ions were broadened. This is evidently due to
Bremsstrahlung radiation. High-density plasma that is similar to previously reported
ablation plumes was formed [11]. The emission intensity was markedly large in the
area surrounding the laser irradiation spot on the target. The spectrum shown
in Figure 1 predominantly comprises the emissions from that area.Figure 1
Optical emission spectrum of the entire plume.Figure2 shows the
time-resolved plume images captured using the high-speed ICCD camera without a bandpass
filter. The plume gradually expanded with time. It is well known that a plume
expands behind a traveling shock wave in this pressure range [11]. Although the emission lifetimes of species are
approximately 10 nanoseconds in general [11], the emission lasted up to 20 μs. This indicates that the plume has a
sufficiently high density for the species to collide with each other. The plume
front area located behind the shock wave markedly emitted since the species ejected
from the target caught up with the shock wave and gathered inside this area
[12]. The front edge of the plume reached the
substrate, which was 15 mm away from the target, at delay times between 1.0 and
1.5 μs. After that, the emission above the substrate
surface further lasted for several microseconds. This evidently implies that
high densities of species are present above the substrate surface for such a
long time. This situation must have a significant role in UNCD growth.Figure 2
Time-resolved photographs taken without using a filter.In order to observe the
behavior ofC+ ions, C atoms, and C2 dimers in the plume
separately, time-resolved images were taken using narrow-bandpass filters. Typical
time-resolved images for C+ ions, C atoms, and C2 dimers
are shown in Figures 3(a), 3(b),
and 3(c), respectively. While the emissions from
C atoms and C2 dimers lasted around the target, the emission from C+ ions was observed for 7 μs above the substrate surface. The emission
observed above the substrate surface in the images captured without using the
filters is evidently attributable to C+ ions. It was evidently shown
that high-energy C+ ions are present in high density above the substrate
surface.Time-resolved photographs captured using (a) 394-nm
(C+ ion), (b) 505-nm (C atom), and (c) 515-nm (C2 dimer)
bandpass filters.
(a)(b)(c)In Figure4, the plume’s front-edge
position from the  target surface is plotted as a function of time. The
front-edge positions were carefully determined from the images that were
digitally analyzed using image processing software. The plume expansion was
fitted with the drag model
[12], and the expansion velocity at the
substrate was estimated to be 14 km/s for C+ ions. For the other
species, the expansion velocities were impossible to be estimated since their
emissions disappeared from the front edge of the plume.Figure 4
Change in the front-edge position of the plume
with time. Here, the laser-irradiation spot is defined as the origin.On the basis of previous
reports and the experimental results of this study, the UNCD growth mechanism
can be considered to be as follows. (i) The root-mean-square roughness of the
film surface was estimated to be 0.6 nm by using atomic force microscopy (AFM),
as shown in Figure5. This value is extremely small as compared to that of the
UNCD films prepared by CVD [13].
This implies that UNCD crystallites are formed at a subsurface rather than the film
surface. It has been reported that diamond nucleation occurs when highly
energetic C+ ions are implanted into amorphous carbon
[14]. This does not
contradict the fact that the pretreatment of substrates is not required for PLD,
while it is indispensable for CVD. We believe that the supply of highly
energetic C+ ions at a high density to the substrate has an
important role in UNCD growth in PLD. Here we must emphasize that this does not
have the predominant role on the UNCD growth because the UNCD crystallites do
not grow in vacuum wherein the velocities of species must be greater than those
in 53.3 Pa hydrogen atmospheres. Although a 53.3 Pa hydrogen atmosphere reduces
the mean-free-paths and velocities of species, it is indispensable for UNCD
growth in PLD. (ii) It has been theoretically predicted that carbon clusters
whose dangling bonds are terminated by atomic hydrogen and whose sizes are
smaller than 3 nm will energetically prefer a tetrahedral structure (diamond)
to a hexagonal one (graphite) [15]. The UNCD
crystallites grown by PLD have an average size of 5 nm. This size is close to
the predicted value of 3 nm. Since UNCD growth by PLD requires a hydrogen
atmosphere, the existence of atomic hydrogen during UNCD film deposition might contribute
to UNCD growth in the mechanism mentioned above. (iii) It has been reported
that diamond nucleation is induced under supersaturated conditions
[16].
In other words, UNCD crystallites can be formed under supersaturated conditions
without pretreating the substrates. Supersaturated conditions might be realized
by the presence of high-density C+ ions on the substrate surface. The
probable mechanisms mentioned above are summarized in Figure 6. We believe that
these mechanisms (i), (ii), and (iii) interactively contribute to UNCD growth
in PLD.Figure 5
AFM image of the UNCD/a-C:H film deposited on Si.Figure 6
Schematic diagram of probable growth mechanisms of
UNCD crystallites in the film, based on previous reports and our experimental
results.


## 4. Conclusions

Optical emission spectroscopy
was used to study pulsed laser ablation of graphite in a hydrogen atmosphere
wherein UNCD films were grown. Time-resolved photographs were taken using a
high-speed ICCD camera equipped with 394-, 505-, and 515-nm filters. Emissions
from C atoms andC2
dimers lasted around the laser-irradiation spot
on the target. On the other hand, emission from C+ ions lasted above
the target surface for approximately 7 microseconds, although the emission lifetime is
approximately 10 nanoseconds. This implies that C+ ions actively collide with
each other above the substrate surface for such a long time. We believe that the
keys to UNCD growth in PLD are the supply of energetic carbon species at a high
density to the substrate and existence of atomic hydrogen during the growth.



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*Source: 901241-2009-01-04.xml*