The properties of diamond like carbon films grown by RF flow discharge 30 kHz plasma using methane are reported. The Cls XPS line shape of films showed localized hybrid carbon bonds as low as 40 to as high as 95 percent. Infrared spectroscopy and N(15) nuclear reaction profiling data indicated 35 to 42 percent hydrogen, depending inversely on deposition temperature. The deposition rate of films on Si falls off exponentially with substrate temperature, and nucleation does not occur above 200 C on GaAs and InP. Optical data of the films showed bandgap values of 2.0 to 2.4 eV increasing monotonically with CH4 flow rate

Alterovitz, S. A.

Lanford, W. A.

Liu, D. C.

Pouch, J. J.

Warner, J. D.

English

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NASA Technical Memorandum 86870
(NASA--rm-86870)	 FLASISA DEECSIZEU	 N85 -1C844
DiatlUNDLIKL CANEGN CN GdAs ANC InP (NASA)
15 p HC AU2/hf A01	 CSCL 20i.
Uncld s
'	 G3/76 24199
Plasma Deposited "Diamondlike"
Carbon on GaAs and InP
J. D. Warner, J. J. Pouch, S. A. Alterovitz,
and D. C. Liu
Lewis Research Center
Cleveland, Ohio
and	 rti	 ;;^,v 104^^
W. A. Lanford
SONY-Albany	
00 
Albany, New York
Prepared for the
Thirty-first National Symposium of the American Vacuum Society
Reno, Nevada, December 3-7, 1984 	
.1
r
LN
I. INTRODUCTION
There has been considerable interest in the properties of carbon f
prepared by ion-beam sputtering 1-6 and rf plasma decom;:sition of hydrocarbon
gases. 
3-5 
Information on film hardness and bonding arrangements (relative to
diamond) can be found in the literature. 2-6 Recently, some optical and
electrical characteristics of carbon films grown on Si, which may be useful for
semiconductor applications fabrication, have been reported. 1,4,8,9
We have utilized a rf plasma formed from methane gas to deposit the carbon
films. The physical and chemical properties of these films have been determined
using AES, XPS, ellipsometry, nuclear reaction techniques, IR and UV-vis-NIR
spectrometries. Our results provide information on the growth parameters needed
to achieve carbon films with a particular bandgap and hydrogen content., In
addition, we demonstrate the importance of the growth temperature on the
deposition rate and composition.
II. EXPERIMENT
a. Growth and Sample Preparation
Carbon films were prepared by subjecting the substrates to a rf plasma
discharge generated at 30 kHz. We used a Technics PD-II deposition system. All
samples were first cleaned in acetone and ethanol baths, and rinsed in deionized
water. The samples were then placed in the rf chamber; the background pressure
was typically 20 mtorr. Methane gas (99.97 percent pure) was used to flush the
chamber several times prior to each run. The power and flow rate settings were
varied from 30 to 240 W and from 30 to 90 sccm, respectively; substrate
temperatures ranged from ambient (-23° C) to 250° C. We noted that a minimum
pressure was required to maintain a stable plasma and it scaled directly with rf
power.
2
i
i
s	 •.
ZWe report the measured quantities in flow rates with units of standard
cubic centimeters (sccm) rather than pressure since it was the flow rate which
was controlled during deposition. It was found that the pressure varies 	
S'
Linearly with flow rate (f), in the region 30 to 90 sccm. To convert flow rate
from sccm to pressure in mtorr we used the formula
P - 3.5f + 70	 (1)
	
hk	 where P is the pressure in mtorr. Carbon films were grown on GaAs, InP, Si
wafers, quartz, and Multiple Internal Reflection (MIR) plates of Ge and Si.
All samples were stored either under vacuum or in a dry box.
b. AES/ESCA
The composition analyses experiments were performed using a PHI AES/ESCA
	
a	 system, interfaced with a MACS (Multiple-Technique Analytical Computer
System). The sample chamber was evacuated to about 10-10 torr prior to the
	
H	 Auger electron profiling and X-ray photoemission measurements. The sputter
profiling was done with 2 or 3 KeV Ar ions at 25 mA. The XPS measurements
were carried out using a 10 KeV Al Ka x-ray source.
c. IR
Determination of the relative amount of hydrogen in the film was
performed using a ratio recording infrared spectrometer with a data station
for data acquisition and analysis of the peak area. The area under the
absorption peaks at 2921, 1444, and 1374 cm-1 were used. These measurements
were taken in the spectrometer using Ge and Si MIR plates with a 30 0
 bevel on
the two ends. In order to reduce the edge effects and assure the uniformity
of the film across the MIR plates, an aluminum plate of approximately the same
thickness as the MIR plates was placed around the plates during deposition.
These peaks gave the main vibrational and bending modes of the C-Hx (x = 1
to 3) bondings.
2
tin
d. Optical Absorption
i'~	
Optical absorption measurea^ents were done on carbon films deposited on
fused quartz plates to determine the bandgap. The films were measured in a
UV-vis-NIR spectrometer with the sample beam perpendicular to the film. The
wavelength was scanned from 185 to 836 nm with a bandwidth of 1 nm and a scan
rate of 5 nm/sec while the background was automatically subtrated.
e. Ellipsometry
Optical properties such as the refractive index and the thickness of the
r
films were determined using a semi-automatic Gaertner L119X research
ellipsometer in a rotating analyzer mode. Data acquisition and analysis were
done using a computer. At each wavelength, up to nine angles of incidence
were used, as needed. The light sources were a He-Ne laser and a 100 W Hg arc
source. The photomultiplier readings were averaged over at least 50
rotations, with a measurement every 5 6
 of the analyzer, at each wavelength and
angle of incidence.
The thickness and the complex refractive index at 632.8 nm were measured
systematically on a large number of samples. On each sample, data was
obtained at angles of incidence in the range of 60 0 to 780 . From these
measurements * and a were calculated by Fourier analysis. In some
samples several wavelengths were used. Then the refractive index and the
z
	
thickness were calculated from a multiple angle and wavelength (MAW) program
developed at the University of Nebraska. 10 The optical data for the
substrate were taken from literature. 11 A three-phase model was used, i.e.,
ambient, film, and substrate. This model was found very adequate for our
films.
af	 ^
f. Nuclear Reaction
Determination of the absolute amount of hydrogen was performed at the
Suny-Albany nuclear accelerator. This method makes use of the 4.43 NeV Y-rays
emitted from the H I (Nis ,aY)C12 . reaction. This reaction has an
appreciable cross section for N 15 nucleon only ions at 6.385 NeV. The
method of calculating the hydrogen concentration from this reaction is given
in Ref. 12.
III. RESULTS
AES and XPS measurements indicated that the films contained only carbon;
no other element was observed to the detection limits (0.1 at. ) of the
instrument. Figure 1 shows a typical AES profile of carbon films on InP and
GaAs. Oxygen was present only on the surface of the films for the GaAs
samples whereas for the InP there was a few percent of oxygen at the
carbon-InP interface. This suggests that the methane plasma removes all of
the oxygen from the GaAs surfaces and most of it from InP surfaces.
Our analysis of the percent diamondlikeness of our films comes from the
measurement of the asymmetry in the carbon is XPS lineshape due to delocalized
,r-bonds. 13 To quantify this measurement, we compare the relative asymmetry
in the C is spectrum of our samples to that of Highly Oriented Pyrolytic
Graphite ('HOPG). We define the percent diamondlikeness (DL) to be
DL = (1 - al /a2 ) x 100	 (2)
where al is the area of the tail relative to the total area of the
symmetric peak for the sample, and a 2
 is the area of the tail relative to
the total area of the symmetric peak for HOPG. The measurement value for
':	 c
{
a2
 is 0.25. In Figure 2, we show a comparison of the XPS peaks of HOPG, a
plasma deposited carbon film, and a Gaussian curve fitted to the low energy
side of the peak for the carbon film.
The DL calculated for the various carbon films ranged from 40 to 97
percent. There is a general trend of decreasing DL values with increasing
power/flow rate values.
The XPS evaluation of the bonding of the carbon to GaAs and InP were
performed with films approximately 200 A thick. The results for a typical
film on InP is shown in Fig. 3. The interface was extending approximately
from 5 to 20 min ESCA profiling time. Data were taken at 8 and 14 min. All
XPS peaks are broadened and shifted in binding energy for material at the
interface, as compared to the bulk. This includes the C Is, the In 3d and
P 2p peaks, indicating some bonding between the carbon and the InP. Similar
behavior was found at the carbon-GaAs interface.
Using a rf power of 100 N and a flow rate of 50 sccm with Si and Ge
substrates it was found that the carbon growth rate and hydrogen concentration
decreases with increase in substrate temperature as measured by the N15
nuclear reaction. Figure 4 shows for carbon on Si that the deposition rate
falls off exponentially with temperature whereas the hydrogen concentration
decreases linearly with temperature. Deposition rates were determined from
measurements of film thickness by a standard profilometry technique. For InP
and GaAs, it was found that the carbon growth rate decreases with increasing
temperature and above 200° C nucleation does not occur.
The carbon films on Si and Ge MIR plates show a decrease in the intensity
of the infrared absorptive peaks, with increasing temperature. The IR data
show peaks around 2921 cm-I for C-Hx
 (x - 1 to 3) stretzhing modes, peaks
at 1445 cm-1 and 1370 cm-1 for the C-H2 and C-H3
 bending modes, and a
broad peak at 1610 cm-1 which is probably associated with amorphous carbon
5
I-
nodes. The absorption intensities of the C-H x
 peaks can be used to
determine the hydrogen concentration once the thickness and optical absorption
cross section of the C-H x
 peaks are known. The optical absorption cross
section for these films will be reported elsewhere. 14 Figure 5 illustrates
the sensitivity of the IR optical absorption intensity of the C-H x
 peaks due
to changes in Ge substrate temperature.
The ellipsometric measurements of index of refraction (n) and extinction
coefficient (k), for films produced at rf powers of 50 and 240 W and methane
flow rates of 30 to 90 sccm, together with deposition rates from the
ellipsometric measured thickness, are given in Table I. The growth rate
increased for both rf power and methane flow rate. The index of refraction
decreased slightly with flow rate and increased with power. The extinction
coefficient at 6328 A was nonzero only at the lowest flow rate whereas at
shorter wavelengths for the 50 W and 90 sccm samples, k was measurable but is
still gvite small.
The change in the index of refraction in samples prepared at different
power levels could be due to changes in concentration of voids from sample to
sample. To check this we used the effective medium approximation 15 to
calculate the concentration of the voids and the required AES profiling time
for several samples. Assuming that the 240 W, 50 sccm sample has no voids we
calculate the percent voids (PV) in other samples. For example, we found PV
= 45 for the 50 W, 90 sccm sample. This value leads to calculated AES
profiling time of 108 min as compared with the experimental value 105 min. We
thus obtained a ratio of 3.5 for the profiling time, as compared to a factor
of 2..0 for the thickness ratio.
The optical bandgap results for various power flow rate ratios are given
in figure 6. They range from 2.07 to 2.38 eV and decrease with increase in
6
ice►;.3. _ ,F^ .^ ^^ .
1
e
We would like to thank John A. Woollam and collaborators at University
Nebraska-Lincoln for fruitful discussions and technical help.
1
^] iE^	 iMx Yi^	 ^. ^^ ^r*^ .^IT:.^ 1^Y'. wy} jaifip
Y
I
r
power/flow rate for a given power. This tendency is the same as for the
percentage diamondlike behavior shorn in figure 3.
IV. SUMMARY
We have demonstrated that reasonably high purity and quality carbon film
can be produced on GaAs and InP over a wide range of rf power and methane flow
rate. These films show no impurities other than hydrogen. We found a
tendency for the carbon films to be more diamond at lower ratios of rf power
to methane flow rates. We were able to control the bandgap and growth rate of
the films through reproducible settings of power and flow rate. We have shown
the exponential decrease of the growth rate and a decrease of the hydrogen
concentration with increase in substrate temperature during deposition on Si
and Ge substrates. Also, we observe a critical temperature (200' Q above
which no nucleation of carbon occurs on GaAs and InP.
More work is needed on carbon films prepared by rf glow discharge to
fully determine the variation of the bandgap and growth rate during deposition
with the parameters rf power, methane flow rate, and temperature In addition,
the relationship between the bandgap, index of refraction, and the amount of
either voids or hydrogen in the films should be investigated.
ACKNOWLEDGMENTS
i
t
a
t.
w;
i
s
REFERENCES
1. A. A. Khan, J. A. Woollam, Y. Chung, and B. Banks, IEEE Electron Device
Letters, 4, 146 (1983).
2. C. Weissmantel, K. Bewilogua, D. Dietrick, H. J. Erler, H. J. Hinneberg,
S. Klose, W. Nowick and G. Reisse, Thin Solid Films, 72, 19-31, (1980).
3. C. W. Weissmantel, G. Reisse, H. J. Erler, F. Henny, K. Bewilogua, U.
Ebersback, and C. Shurer, Thin Solid Films, 63, 315-325 (1979).
4. D. Mathine, R. 0. Dillon, A. A. Khan, G. Bu-Abbud, J. A. Woollam, D. C.
Liu, B. Banks, and S. Domitz, J. Vac. Sci. Technol., A2, 365-366 (1984).
S.	 T. J. Moravec and T. W. Orent, J. Vac. Sci. Technol. 18, 226-228 (1981).
6. C. Weismantel, K. Bewilogua, K. Brewer, D. Dietrick, U. Ebersback, H. J.
Erler, B. Rau, and G. Reisse, Thin Solid Films, 96, 31-44 (1982).
7. S. Aisenberg and R. Chabot, J. Appl. Phys., 42, 2953-2958 (1971).
8. R. 0. Dillon, J. A. Woollam, and V. Katkanant, Phys. Rev. B29, 3482-3489
(1984).
9. Y. Ichinose and F. Shimokawa, in Extended Abstracts and Program. 16th
Biennial Conference on Carbon, (American Carbon. Society, University Park,
Penn, 1983).
10. G. H. Bu-Abbud, S. A. Alterovitz, N. M. Bashara, and J. A. Woollam, J.
Vac. Sci. Technol. Al, 619-620 (1983).
11. D. E. Aspnes, and A. A. Studna, Phys. Rev. B27, 985-1009 (1983).
12. T. T. P. Cheung, J. Appl. Phys., 53, 6857-6862 (1982).
13. W. A. Lanford, and M. J. Rand, J. Appl. Phys. 49, 2473-2477 (1978).
14. J. D. Warner, W. A. Lanford, J. J. Pouch, and S. A. Alterovitz, to be
published.
15. D. E. Aspnes, Thin Solid Films, 89, 249-262 (1982).
8
ORIGINAL
OF POOR QUALITY
TABLE I. - ELLIPSOMETRY DATA OF CARBON FILMS AT 6328 A
Sample Deposition
rate,
A/min
Index of
refraction.
n
Extinction
coefficient,
kPower, Flow rate.
M cm
SO 30 79.5 1.58±0.02 -0.01
50 50 83.3 1.71±.02 0
50 70 96.7 1.67±.01 0
50 90 100.0 1.59±.02 0
240 50 197 2.15±.04 -.015
240 70 263 2.08t.09 0
240 90 308 1.811.01 0
s!
1
a	 li
1r'
Afi
f.
*f4ORIGENALOF P002	 A
In 3d
	
6	 P
z
C Is
4
S	 P
z 2
15
0
/-0
-	
'0
	
0	 8	 16	 24	 32	 40
SPUTTERING TIME, min
4
C Is
r-a 3
As 3
	
la C 2	 Ga d
3d
ols
	
), 	 Ols0 lb1	 1
	
0	 10	 20	 30	 40	 50
SPUTTERING TIME, min
Figure 1. - Auger electron spectroscopy IAES) profiles
of carbon films on InP (curve A) and GaAs icurve B).
using 25 mA. 3 keV Ar ions.
8
gri 6
4C
gg
2 jA
B C
0	
C1	
I
M	 -290	 -298	 -286	 -284	 -282	 -280
BINDING ENERGY, eV
Figure 2. - X-ray photoemission spectroscopy (XPS) of the
Intensity of photoemission electrons versus binding energy.
(A) - highly oriented pyrolitic graphite 1B1- carbon film on
InP, (C► - fitted gausslon curve to the lower energy side of
curve IBL
V,
-280
ORIGINAL PAGE IF,
OF POOR QUALITY
PURE
CARBON
AFTER 14 min
OF PROFILING
'— AFTER 8 min
OF PROFILING
I	 1	 I	 I
-296	 -292	 -288	 -284
BINDING ENERGY, eV
(a) C is peaks,
AFTER 8 min
OF PROFILING
z
> 4
/-AFTER
	 min 3FI
OF PROFILING
m
m
2W
Z
1
InP SUBSTRATE :-,
458 -454 -450 -446 -442 -438
1
BINDING ENERGY, eV
(b) In 3d peaks.
8
AFTER 8 min	 r AFTER 14 min
OF PROFILING	 i OF PROFILING
4
W InP SUBSTRATE-/
z ^.
-144	 -140	 -136	 -132	 -128	 -124
BINDING ENERGY, eV k
(c) P 2p peaks.
Figure 3. - XPS intensity versus binding energy at
different depth of profiling time for carbon film of
thickness of 200 A on InP.
3
i
i
k
1
8
;i 6
z
m
i; 4
.o
W.
S
_W
Z 2
NO-
6
k¢ q
1
J1
3200	 3000	 2800	 2600
WAVENUMSER, cm-1
Figure 5. - IR absorption Intensity versus wavenumber for carbon
on Ge MIR (multiply internal reflection) plates for 7 deposition
temperatures at 100 W, 50 sccm, and 30 min.
NH
Z
}
d'
C
CD
C
S
i--
HZ
OHd
ON
m
<C
y
ORIGIN, AL
Of POOR QUALI fi i
11 . r	 .,/ 77'17
z 0 9x1022
E
.3 1.5 8
B
lu1.0 7
^
--►
0
0 5 6
0
5
50	 100	 150	 200	 250	 300
TEMPERATURE, °C
Figurek - Logarithmic plot of deposition rate versus tem-
periture (A) and hydrogen concentration pH (atoms per
cml versus temperature 10) for carbon on a SI substrate
with growth parameters: 100 W. 50 sccm, and 30 min.
DEPOSITION
TEMPERATURE,
oC
Lk
i
ORIGINAL PAGE M1
OF POOR QUALI 'F'Y
}
j
i
^I
a
24
2, 3
d
a
0 2.2za
m
z 
	
zo I	 I	 I	 I	 i
	2 	 3	 4	 5	 6
RATIO OF rf POWER TO FLOW RATE, Wlsccm
Figure 6. - Optical bandgap versus the ratio of rf power
(W) and flow rate (sccm) for different power levels.


Plasma deposited diamondlike carbon on GaAs and InP

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