Photoelectron spectroscopic investigation of the bias-enhanced nucleation
of polycrystalline diamond films
P. Reinke and P. Oelhafen
Universita ¨t Basel, Institut fu ¨r Physik, Klingelbergstrasse 82, 4056 Basel, Switzerland
Received 25 March 1997
In the present work we describe an investigation of the nucleation mechanism of polycrystalline diamond
films if the bias-enhanced-nucleation BEN method is used. Photoelectron spectroscopy with excitation ener-
gies in the ultraviolet ultraviolet photoelectron spectroscopy UPS and x-ray regime x-ray photoelectron
spectroscopy as well as electron energy loss spectroscopy are employed to monitor the nucleation process and
the subsequent diamond film growth. The deposition is performed in situ, thus avoiding surface contamination
with oxygen or hydrocarbons. The observation of the temporal evolution of composition and structure of the
deposited film and its interface with the underlying silicon substrate allow us to develop a qualitative model,
which describes the nucleation process. The BEN pretreatment leads, through the irradiation with low-energy
ions, to the codeposition of an amorphous carbon phase and the crystalline diamond phase. The presence of
both phases is readily apparent in the UPS analysis, which will prove to be an indispensible tool in the
structural characterization of the carbon phase present at the surface. There is no indication for the presence of
graphite or large graphitic clusters. A deconvolution of the C 1s and Si 2p core-level peaks does confirm the
presence of two carbon phases and the formation of a silicon carbide interface. With increasing deposition time
the contribution of diamond to the carbon film increases and upon switching to diamond growth conditions the
amorphous carbon phase is rapidly etched and only the diamond crystals remain and continue to grow. This
removal of the amorphous phase leads to a decrease in the overall carbon concentration at the surface by
18–30 % during the first 30 sec of the diamond growth period and was observed for a variety of pretreatment
conditions. A silicon carbide interfacial layer is formed early on during the BEN pretreatment and its thickness
is reduced considerably by etching during the diamond growth period. These results are summarized and
discussed in the framework of a qualitative model for the nucleation process. S0163-18299703628-X
INTRODUCTION
For a long time the extreme properties of diamond have
sparked interest in its use for technical applications apart
from the admiration of its beauty as a gemstone. But only the
development of techniques to synthesize diamond and poly-
crystalline diamond films has allowed us to take advantage
of the exceptional properties of this material. The advances
made in the last few years with respect to the texture control
and high rate deposition of diamond films provide the means
to produce, for example, diamond windows of high optical
quality. A prerequisite for the growth of a polycrystalline
diamond film is a pretreatment of the substrate in order to
provide a surface structure and composition which favors
diamond nucleation.
1
The most frequently applied methods
are an abrasion of the surface with a diamond paste, which is
a purely mechanical pretreatment, and the newly developed
bias-enhanced nucleation BEN.
2–4
The BEN method is ad-
vantageous in several respects. It is a nondestructive in situ
method which leads to a high nucleation density and favors
under certain deposition conditions the formation of nearly
heteroepitaxial diamond films on Si.
2,5,6
The BEN method
involves the irradiation of the substrate with low-energy ions
which is usually achieved through the application of a dc
voltage to the substrate. The estimates for the ion energies
range from 30 to 150 eV, which results in a rather shallow
penetration depth of at most 2–3 monolayers.
7–9
As an alter-
native for insulating substrates, such as ceramics or quartz,
the application of a rf bias voltage can be envisaged.
10
Apart
from the technical challenges of the BEN method the ques-
tions which have to be dealt with now concern the mecha-
nism or reaction pathways by which BEN proceeds. Or in
other words, why does the BEN pretreatment prior to the
diamond growth process itself provide such a favorable en-
vironment for the nucleation of diamond as opposed to the
diamond nucleation on an untreated Si or SiC substrate? This
question has been central to a number of recent publications
and different models have been proposed to describe the
nucleation mechanism in BEN.
3,4,11–14
The presence of a car-
bon phase different from diamond such as graphite,
14
tetra-
hedral amorphous carbon ta-C,
7
or sp
2
amorphous carbon
has been suggested to play an important role in promoting
diamond nucleation, especially during BEN. However, most
analytical methods frequently employed in the study of dia-
mond growth and nucleation are not sensitive to thin layers
of noncrystalline or graphitic material formed at the substrate
surface prior to or during the early stages of crystal growth.
It was already demonstrated by Belton and Schmieg
15
that
surface-sensitive methods such as x-ray photoelectron spec-
troscopy XPS or electron energy loss spectroscopy EELS
can be applied successfully to study diamond growth on dif-
ferent substrate materials. But Belton and Schmieg also
pointed out that XPS alone is not sufficient to characterize
the structure of the carbonaceous deposit and has to be
supplemented by other methods.
In order to contribute to the development of a better un-
derstanding of the nucleation process we combined the depo-
PHYSICAL REVIEW B 15 JULY 1997-II VOLUME 56, NUMBER 4
56 0163-1829/97/564/21838/$10.00 2183 © 1997 The American Physical Society