Deposition of n-type diamondlike carbon by using the layer-by-layer technique and its electron emission properties

Article abstract of DOI:10.1063/1.119076

We have studied the electron emission behavior of the diamondlike carbon (DLC) films by plasma enhanced chemical vapor deposition using a layer-by-layer deposition, in which the deposition of a thin layer of DLC and a CF₄ plasma exposure on its surface were carried out alternatively. The electron emission current increases with CF₄ plasma exposure time. The increase in emission current appears to be due to the n-type behavior of the DLC.

Full text of DOI:10.1063/1.119076

Deposition of n-type diamondlike carbon by using the layer-by-layer technique and its
electron emission properties
Kyu Chang Park, Jong Hyun Moon, Suk Jae Chung, Jin Jang, Myung Hwan Oh, and W. I. Milne
Citation: Applied Physics Letters 70, 1381 (1997); doi: 10.1063/1.119076
View online: http://dx.doi.org/10.1063/1.119076
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/70/11?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Field emission properties of phosphorus doped microwave plasma chemical vapor deposition diamond films by
ion implantation
J. Vac. Sci. Technol. B 21, 603 (2003); 10.1116/1.1531170
Electron field emission properties of nanodiamonds synthesized by the chemical vapor deposition process
J. Vac. Sci. Technol. B 19, 975 (2001); 10.1116/1.1349207
Microstructure and its effect on field electron emission of grain-size-controlled nanocrystalline diamond films
J. Appl. Phys. 88, 2967 (2000); 10.1063/1.1287602
Preparation of N-doped hydrogen-free diamondlike carbon and its application to field emitters
J. Vac. Sci. Technol. B 17, 241 (1999); 10.1116/1.590506
Role of the hydrogen plasma treatment in layer-by-layer deposition of microcrystalline silicon
Appl. Phys. Lett. 71, 3403 (1997); 10.1063/1.120324
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
155.33.16.124 On: Wed, 26 Nov 2014 02:36:57

Deposition of n-type diamondlike carbon by using the layer-by-layer
technique and its electron emission properties
Kyu Chang Park, Jong Hyun Moon, Suk Jae Chung, and Jin Janga)
Department of Physics, Kyung Hee University, Dongdaemoon-ku, Seoul 130-701, Korea
Myung Hwan Oh
Korea Institute of Science and Technology, Seoul, Korea
W. I. Milne
University of Cambridge, Cambridge CB2 1PZ, United Kingdom
͑Received 24 September 1996; accepted for publication 10 January 1997͒
We have studied the electron emission behavior of the diamondlike carbon ͑DLC͒ films by plasma
enhanced chemical vapor deposition using a layer-by-layer deposition, in which the deposition of a
thin layer of DLC and a CF4 plasma exposure on its surface were carried out alternatively. The
electron emission current increases with CF4 plasma exposure time. The increase in emission
current appears to be due to the n-type behavior of the DLC. © 1997 American Institute of
Physics. ͓S0003-6951͑97͒01811-1͔
Tetrahedral amorphous carbon ͑ta-C͒ films with no in-
corporated hydrogen are of considerable interest, because of
their high hardness and substitutional doping capability com-
pared to diamondlike carbon ͑DLC͒.1–3 DLC deposited by
plasma enhanced chemical vapor deposition ͑PECVD͒ has
typically higher than 30 at.% hydrogen incorporation.4,5 The
incorporated hydrogen reduces film hardness. The ta-C can
be obtained by a filtered vacuum arc deposition6. Mckenzie
et al., deposited ta-C with ϳ85% sp3 fraction by a filtered
vacuum arc deposition.2
In our previous work, a layer-by-layer deposition
method, that is, the deposition of a thin DLC layer and sub-
sequent exposure of its surface to a CF4 plasma, was applied
to deposit the DLC films with various hydrogen contents.7
The thin DLC layer was exposed to a CF4 plasma be-
cause the CF4 plasma removes weak bonds such as amor-
phous graphite and hydrocarbon phases.8 The CF4 plasma
exposure time and each layer thickness are important depo-
sition variables to control the hydrogen content in the DLC.
In the present work, we fixed each layer thickness of 5 nm
and varied the CF4 plasma exposure time and studied its
electron emission properties as well as the current–voltage
characteristics. The electron emission current of the DLC
increases and the turn-on field decreases with increasing
CF4 exposure time.
Table I resulted in a 5 nm thick DLC layer. We deposited
about 200 nm thick DLC films in order to measure the ab-
sorption of C–Hn stretch modes by FTIR ͑Fourier transform
infrared͒ spectrophotomer. The absorption coefficients corre-
sponding to C–Hn (nϭ1,2,3) stretch modes were obtained,
and the hydrogen content was obtained from the integration
of the absorption coefficients.9
The electron emission currents were measured in a
vacuum of 1ϫ10Ϫ7 Torr, between metal and DLC plate with
an area of 0.9 cm2 and 50 ␮m spacing.
Figure 1 shows the absorption coefficients of C–Hn
stretching modes for the DLC films with CF4 plasma expo-
sure times of ͑a͒ 0 s, ͑b͒ 50 s, ͑c͒ 100 s, and ͑d͒ 120 s,
deposited using a layer-by-layer technique. We could not see
the absorption of the C–Hn modes when the exposure time
was 120 s. The IR absorption peaks at 2925 cmϪ1, 2960
cmϪ1, and 2850 cmϪ1 corresponding, respectively to sp3
asymmetry CH2, sp3 asymmetry CH3, and sp3 symmetry
CH2 stretch vibrations can be seen.10 The absorption band
for sp2 stretching modes appears at Ͼ3000 cmϪ1, for ex-
ample CH olefinic and CH aromatic rings appear at 3000
cmϪ1 and 3050 cmϪ1, respectively. In general, overall
C–Hn absorption coefficients decrease with increasing CF4
plasma exposure time. However, as can be seen from Figs.
1͑a͒ through 1͑d͒, most of the hydrogen atoms bonded to
sp2 C–Hn are diffused out first and then hydrogen atoms
bonded to sp3 C–Hn are desorbed.
We used a conventional PECVD system, in which the rf
power was applied to the substrate holder. CH4 /H2/He and
CF4/He were introduced for the deposition of DLC and sur-
face treatment, respectively. Glass plates and silicon wafers
were used as the substrates for film deposition.
The hydrogen content in the DLC films, obtained from
TABLE I. Layer-by-layer deposition conditions for the DLC films.
Table I depicts the layer-by-layer deposition conditions
for the DLC films. The flow rates of He, H2 , and CH4 were
fixed at 20 sccm, 6 sccm, and 3 sccm, respectively, for DLC
deposition, and the flow rate of CF4 was fixed at 20 sccm for
plasma treatment. The self-bias voltage of the substrate
holder was found to be Ϫ80 V at a fixed rf power of 100 W,
and it depended strongly on the gas pressure and on rf power
used. The 95 s growth under the deposition mode shown in
Condition
Deposition
CF4 plasma exposure
rf power͑W͒
Pressure͑mbar͒
Flow rate͑sccm͒
He
H2
CH4
CF4
Sub. temp.͑K͒
Time͑s͒
100
0.2
100
0.26
20
6
3
0
300
95
20
0
0
20
300
0–160
a͒
Electronic mail: jjang@nms.kyunghee.ac.kr
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
Appl. Phys. Lett. 70 (11), 17 March 1997 0003-6951/97/70(11)/1381/3/$10.00 © 1997 American Institute of Physics 1381
155.33.16.124 On: Wed, 26 Nov 2014 02:36:57

TABLE II. The relation between emission current at 20 V/␮m and CF4
plasma exposure time on a thin DLC layer.
Exposure time͑s͒
IE ͑A; at 20 V/␮m͒
3ϫ10Ϫ8
50
3.5ϫ10Ϫ8
1.4ϫ10Ϫ4
3.8ϫ10Ϫ4
100
120
140
Table II. It is noted that the emission current is the highest
when CF4 exposure time is 140 s.
Figure 3 shows the current voltage characteristics of ͑a͒
conventional DLC, ͑b͒ 100 s and ͑c͒ 120 s CF4 plasma ex-
posed DLC layers deposited on heavily doped p-type Si.
Here bias voltage is applied to the Si substrate. The nearly
symmetric behavior of conventional DLC is due to the
p-type behavior, whereas the diode-like behaviors of 100 s
and 120 s CF4 plasma exposed DLCs are due to the n-type
DLC.
It is known that the conventional DLC and ta-C show
p-type behavior,12 but the DLC deposited by a layer-by-layer
deposition technique is n type. The position of the Fermi
level will be determined by density of states distribution and
the nature of the density of gap states13 because the conduc-
tion band tails are acceptor-like states and the valence band
tails are donor-like states. For example, the reduction of con-
duction band tails would result in a shift in the Fermi level
toward the conduction band. The density of gap states of the
DLC can be modified by CF4 plasma exposure, resulting in
the shift of the Fermi level toward the conduction band. It is
noted that F atoms in the hydrogen-free DLC is less than 1
at.% from XPS ͑x-ray photoelectron spectroscopy͒ measure-
ments. However, the F atoms can play an important role to
change the density of states in the gap if there are some F
atoms inside.
FIG. 1. Absorption coefficients of layer-by-layer deposited DLC films
with various CF4 plasma exposure times of ͑a͒ 0 s, ͑b͒ 50 s, ͑c͒ 100 s, and
͑d͒ 120 s.
the integration of absorption coefficients of C–Hn stretching
modes appeared between 2800 cmϪ1 and 3100 cmϪ1, de-
creases from 41.5 at.% to 0.6 at.% with increasing CF4
plasma exposure time from 0 to 140 s. The exposure of F
radicals on the thin DLC layer gives rise to the desorption of
H atoms from thin DLC layer because the bond strength of
F–H ͑8.67 eV͒ is higher than that of C–H ͑5.90 eV͒.11
Figure 2 shows the current (I) - electric field (E) char-
acteristics of the 90 nm thick DLC films, showing typical
electron emission behavior. The emission currents at Eϭ20
V/␮m increase with CF4 plasma exposure time as depicted in
FIG. 3. The current–voltage characteristics of ͑a͒ the conventional DLC and
͑b͒ and ͑C͒ layer-by-layer deposited DLC films on heavily doped p-type Si.
FIG. 2. I-E characteristics of the DLC films with various CF4 plasma ex-
posure times.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
1382 Appl. Phys. Lett., Vol. 70, No. 11, 17 March 1997 Park et al.
155.33.16.124 On: Wed, 26 Nov 2014 02:36:57

In summary, we have studied the electron emission cur-
rents from the DLC films deposited using a layer-by-layer
technique. The emission current increases with CF4 plasma
exposure time on the DLC surface. The increase in the emis-
sion current appears to be due to the n-type behavior of the
hydrogen-free DLC.
4 E. H. A. Dekempeneer, R. Jacobs, J. Smeets, J. Meneve, L. Eersels, B.
Blanpain, J. Roos, and D. J. Oostra, Thin Solid Films 217, 56 ͑1992͒.
5 C. Wild and P. Koidl, Appl. Phys. Lett. 51, 1506 ͑1987͒.
6 S. Aisenberg and R. Chabot, J. Appl. Phys. 42, 2953 ͑1971͒.
7 K. C. Park, J. H. Moon, M. H. Oh, and J. Jang, Appl. Phys. Lett. 68, 3594
͑1996͒.
8 Y. Shimada, N. Mutsukura, and Y. Machi, J. Appl. Phys. 71, 4019 ͑1992͒.
9 K. Nakazawa, S. Ueda, M. Kumeda, A. Morimoto, and T. Shimizu, Jpn. J.
Appl. Phys. 21, L176 ͑1982͒.
This work was supported by University Basic Research
Program by Ministry of Information and Communication.
10 N. Mutsukura, S. Inoue, and Y. Machi, J. Appl. Phys. 71, 43 ͑1992͒.
11 J. A. Kerr, CRC Handbook of Chemistry and Physics, 74th ed. ͑CRC,
London͒, p. 9–123.
12 V. S. Veerasamy, G. A. J. Amaratunga, C. A. Davis, A. E. Timbs, W. I.
Mline, and D. R. Mckenzie, J. Phys., Condens. Matter. 5, L169 ͑1993͒.
13 V. S. Veerasamy, G. A. J. Amaratunga, C. A. Davis, W. I. Mline, and P.
Hewitt, Solid-State Electron. 37, 319 ͑1994͒.
1 P. J. Fallon, V. S. Veerasamy, C. A. Davis, J. Robertson, G. A. J. Ama-
ratunga, J. Koskinen, and W. I. Milne, Phys. Rev. B 48, 4777 ͑1993͒.
2 D. R. Mckenzie, D. A. Muller, E. Kravtchin-skaia, D. Segal, and D. J. H.
Cockayne, Thin Solid Films 206, 198 ͑1991͒.
3 K. K. Chan, S. R. P. Silva, and G. A. J. Amaratunga Thin Solid Films 212,
232 ͑1992͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
Appl. Phys. Lett., Vol. 70, No. 11, 17 March 1997 Park et al. 1383
155.33.16.124 On: Wed, 26 Nov 2014 02:36:57