Scanning tunneling microscopic and spectroscopic characterization of diamond film prepared by capacitively coupled radio frequency CH3OH plasma with OH radical injection

Article abstract of DOI:10.1063/1.118971

Electronic structure at a surface of diamond film synthesized by a capacitively coupled radio frequency CH₃OH plasma chemical vapor deposition with OH radical injection has been investigated. The electronic structure was characterized by using scanning tunneling microscopy and scanning tunneling spectroscopy in an ultrahigh vacuum condition. As a result, the electronic structure was identified as an n-type electronic structure at the surface, which was considerably different from those of boron-doped diamond films and amorphous carbon films.

Full text of DOI:10.1063/1.118971

Scanning tunneling microscopic and spectroscopic characterization of diamond film
prepared by capacitively coupled radio frequency CH 3 OH plasma with OH radical
injection
Masafumi Ito, Kazuya Murata, Koukichi Aiso, Masaru Hori, Toshio Goto, and Mineo Hiramatsu
Citation: Applied Physics Letters 70, 2141 (1997); doi: 10.1063/1.118971
View online: http://dx.doi.org/10.1063/1.118971
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/70/16?ver=pdfcov
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Scanning tunneling microscopic and spectroscopic characterization
of diamond film prepared by capacitively coupled radio frequency
CH3OH plasma with OH radical injection
Masafumi Ito,a) Kazuya Murata, Koukichi Aiso, Masaru Hori, and Toshio Goto
Department of Quantum Engineering, Nagoya University, Nagoya, 464-01, Japan
Mineo Hiramatsu
Department of Electrical and Electronic Engineering, Meijo University, Nagoya, 468, Japan
͑Received 14 October 1996; accepted for publication 18 February 1997͒
Electronic structure at a surface of diamond film synthesized by a capacitively coupled radio
frequency CH3OH plasma chemical vapor deposition with OH radical injection has been
investigated. The electronic structure was characterized by using scanning tunneling microscopy
and scanning tunneling spectroscopy in an ultrahigh vacuum condition. As a result, the electronic
structure was identified as an n-type electronic structure at the surface, which was considerably
different from those of boron-doped diamond films and amorphous carbon films. © 1997
American Institute of Physics. ͓S0003-6951͑97͒01916-5͔
Recently, diamond thin films have attracted much atten-
tion since the invention of the method of gas-phase synthesis
for their applications to electronic devices. Many methods
for gas-phase synthesis have been reported, e.g., hot fila-
ment,1 microwave plasma,2 magnetomicrowave plasma,3 and
so on. Diamond films have been successfully synthesized by
using these methods. In these methods, CH4, CH3OH, or CO
with high H2 dilution has been employed. It is said that the H
radical plays an important role for the selective etching of
the nondiamond phase in these plasmas.
investigated in the film prepared by RF CH3OH plasma CVD
with OH radical injection.
In this letter, STM and STS characterization of the dia-
mond film surface are carried out by using a commercial
UHV-STM ͓OMICRON UHV-STM/AFM ͑atomic force mi-
croscopy͔͒ to investigate the electronic structure of films pre-
pared by the capacitively coupled RF CH3OH plasma CVD
with OH radical injection. The surface electronic structure is
compared with those of amorphous carbon films and boron-
doped diamond films.
On the other hand, the synthesis of diamond films in a
capacitively coupled radio frequency ͑RF͒ ͑13.56 MHz͒
plasma chemical vapor deposition ͑CVD͒ was considerably
difficult because the shortage of H radicals resulted from the
low dissociation of H2. Therefore, we have proposed a new
method for diamond film synthesis using a capacitively
coupled RF CH3OH plasma CVD with H and OH radical
injection because the OH radical was considered to be more
effective than the H radical to etch the nondiamond phase.
The H and OH radicals were generated by a H2 and H2O
microwave plasma outside the capacitively coupled RF
CH3OH plasma reactor and were injected into the reactor. By
using this method, the diamond thin films have been success-
fully synthesized in a capacitively coupled RF plasma
reactor.4–6 This fact indicates the possibility of depositing
diamond thin films over a wide area with a good uniformity
because a capacitively coupled plasma configuration can be
easily scaled up for a wide area formation.
Recently, diamond films with the hydrogen-terminated
surface and boron-doped diamond films have been studied
by scanning tunneling microscopy ͑STM͒ and scanning tun-
neling spectroscopy ͑STS͒.7,8 It has been reported that the
nondoped diamond with the hydrogen-terminated surface has
a p-type electronic structure at the surface and that annealing
at 200 °C in an ultrahigh vacuum ͑UHV͒ condition is neces-
sary for the desorption of H2O in STS analysis on the dia-
mond surface.7 However, the electronic structure at the sur-
face, as well as the initial stage of nucleation, has never been
To obtain STS spectra and STM images, all samples
were annealed at 200 °C in UHV ͑Ͻ1ϫ10Ϫ7 Pa͒ and tung-
sten tips as STM/STS probes were cleaned by field evapora-
tion by a field ion microscope.9 Typical sample voltages and
constant tunneling currents used for imaging were 3 V and 1
nA, respectively. The STS curves, namely, current–voltage
(I–V) curves were obtained at a particular sample area by
interrupting the feedback and measuring the tunneling cur-
rent as a function of sample voltage for a fixed tip–sample
separation. To obtain the surface state density of films from
the I–V curves, we used expression d͑ln I͒/d͑ln V͒, which is
called normalized conductivity.10 The pressure for STM/STS
was 2ϫ10Ϫ8 Pa.
Figure 1͑a͒ shows a STM image of the diamond film that
was prepared by RF CH3OH plasma CVD with OH radical
injection. The film was confirmed to have a diamond phase
by x-ray diffraction and Raman spectroscopy.5 The CVD
condition was as follows: partial pressures of
CH3OH/H2O:0.67 Pa/12.7 Pa, RF power: 100 W, microwave
power ͑for H and OH radical injection͒: 100 W, substrate
temperature: 600 °C, substrate dc bias: ϩ5 V, and deposition
time: 3 h.6 All films were formed on p-type ͑100͒ Si sub-
strates. Other details concerning the experiment are de-
scribed in Refs. 5 and 6. The thicknesses of films were about
80 nm. As shown in Fig. 1͑a͒, the surface mainly consists of
grains with triangular facets whose size is about 600 nm. A
I–V curve and a normalized conductivity of the diamond
film are plotted by filled circles in Fig. 2.
Figure 1͑b͒ shows a surface image of amorphous carbon
film by STM. A I–V curve and a normalized conductivity of
a͒
Electronic mail: itou@nuee.nagoya-u.ac.jp
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FIG. 3. I–V characteristic of boron-doped diamond film and logarithmic
derivatives d ln ͑I͒/d ln ͑V͒ of the I/V curve.
FIG. 1. ͑a͒ STM image of a diamond film prepared by RF CH3OH plasma
CVD with OH radical injection. ͑b͒ STM image of an amorphous carbon
film.
rent at negative voltage applied to the sample is larger than
that at positive voltage. The I–V curve of the amorphous
carbon is symmetrical against the origin and is considerably
different from that of the diamond film. A Schottky barrier
can be formed at the interface between the amorphous car-
bon and the p-type Si substrate. However, the effect of the
Schottky barrier was not observed in the I–V curve of amor-
phous carbon shown in Fig. 2 in this study. Therefore, the
I–V curves of the diamond shown in Fig. 2 is considered to
be unaffected by the electronic characteristics at the interface
between the diamond and substrate.
the amorphous carbon film are shown by open rhombuses in
Fig. 2. The conditions for amorphous carbon film formation
were as follows: partial pressures of CH4 /H2:1.33 Pa/12.0
Pa, RF power: 100 W, microwave power ͑for H radical in-
jection͒: 100 W, substrate temperature: 600 °C, substrate dc
bias: ϩ5 V, and deposition time: 3 h.6 The surface of the
amorphous carbon film consists of many spherical particles
with about a 20 nm diam and the grains with triangular facets
shown in Fig. 1͑a͒ were not observed at all.
Moreover, to confirm that the STS characterization with
annealing samples at 200 °C in UHV gives a reliable elec-
tronic structure of the film, we measured an I–V curve of
boron-doped diamond films, which were prepared by a con-
ventional microwave plasma of CO with high dilution of
H2 including 10 ppm B2H6 and had a p-type electronic struc-
ture. The result is shown in Fig. 3. The tunneling current at
positive voltage is larger than that at negative voltage, which
indicates that the diamond film has a p-type electronic
structure.11 This result confirms that the STS characterization
with annealing samples at 200 °C in UHV is reliable. The
above facts indicate that the diamond film prepared by RF
CH3OH plasma with OH radical injection certainly has a
n-type electronic structure.
The I–V curve shown in Fig. 2 of the diamond film is
asymmetrical against the origin, namely, the tunneling cur-
From Fig. 1͑a͒, it is considered that there is no amor-
phous carbon layer at the diamond surface because no
spherical particles are seen. Moreover, any diffraction pat-
terns from graphitic carbon were not observed by x-ray dif-
fraction. Even if there might be a thin graphitic carbon layer
on the top of the diamond surface, the layer cannot have
n-type electronic conductivity. Therefore, it is considered
that there are no graphitic layers on the top of the surface and
the n-type electronic structure originates from the diamond
film.
FIG. 2. ͑a͒ I–V characteristic of the diamond film with OH radical injection
and the amorphous film shown in Fig. 1. ͑b͒ Logarithmic derivatives
d ln ͑I͒/d ln ͑V͒ of the curves shown in ͑a͒. Filled circles and open rhom-
buses correspond to the diamond and the amorphous film, respectively.
It is considered that O atoms and/or OH radicals play
important roles for synthesizing diamond film with a
n-type electronic structure at the surface. However, further
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2142 Appl. Phys. Lett., Vol. 70, No. 16, 21 April 1997 Ito et al.
128.138.73.68 On: Mon, 22 Dec 2014 20:10:35

investigation is necessary for elucidation of the roles. From
the normalized conductivity plots shown in Fig. 2, the band
gaps of the diamond and amorphous films are evaluated to be
around 3 and 0.7 eV, respectively. The band gap of the dia-
mond film is smaller by 2 eV than that of boron-doped dia-
mond film. The distortion of crystalline due to O atoms in-
cluded in the diamond film is considered to be a factor in
reducing the band gap. In general, however, large amounts of
O atoms cannot be incorporated into the diamond lattice.
Therefore, the main factor in reducing the band gap is con-
sidered to be the presence of adsorbed OH, ϭO, –O–
bridges in the top carbon layers of the diamond crystallites.
In conclusion, we have investigated the electronic struc-
ture at the surface of films prepared by RF CH3OH plasma
CVD with OH radical injection. The electronic structure was
considerably different from those of boron-doped diamond
films and amorphous carbon films. The structure was identi-
fied as an n-type electronic structure at the surface by using
STS.
The authors would like to thank Professor Kawarada
͑Waseda University͒ for offering boron-doped diamond
films.
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