Thin Solid Films 311 Ž1997. 207–211

Concentration and thermal release of hydrogen in amorphous siliconcarbide films prepared by rf sputtering
Y. Suzaki a,) , T. Shikama a , S. Yoshioka a , K. Yoshii b, K. Yasutake bab

Research Laboratory for AdÕanced Technology, Takamatsu National College of Technology, 355 Chokushi, Takamatsu, Kagawa 761 Japan
Precision Science and Technology DiÕision, Graduate School of Engineering, Osaka UniÕersity, 2-1 Yamada-oka, Suita, Osaka 565, Japan
Received 5 November 1996; accepted 18 June 1997

Abstract
Concentration and thermal release of hydrogen in hydrogenated amorphous SiC Ža-SiC:H. films were studied. The films were preparedonto Si Ž111. wafers at room temperature by rf planer magnetron sputtering in a gas mixture of argon at partial pressures of 0.33 Pa andhydrogen from 0.065 to 1.3 Pa. The IR measurements conducted on the films annealed at various temperatures for 3600 s suggested thatthe hydrogen was released from Si–H and C–H bonds in the films at the temperatures above 600 and 850 K, respectively. In-situ
isochronal annealing for 300 s at various temperatures from 323 to 1123 K in the ERDA ŽElastic Recoil Detection Analysis. system wascarried out for the specimen having the hydrogen concentration of 7.1 = 10 27 atomsrm3. It was revealed that three types of hydrogen
exist in the films; hydrogen bonded to Si or C atoms and unbonded hydrogen, with the concentrations of 2.8 = 10 27, 1.9 = 10 27 and
2.4 = 10 27 atomsrm3, respectively. The concentration of unbonded hydrogen decreases with increase of the hydrogen partial pressure.
q 1997 Elsevier Science S.A.
Keywords: Amorphous materials; Annealing; Hydrogen; Silicon carbide

1. Introduction
Hydrogenated amorphous silicon carbide Ža-SiC:H.
films are used in the amorphous silicon solar cell w1,2x, thetandem solar cell w3x and the electroluminescent device w4x.
The structure, optical and electrical properties of the films
depend on the deposition conditions w5,6x. Especially, thehydrogen atoms in the films give much influence on someelectronic and optical properties w7,8x. Therefore, the measurement of hydrogen concentration and bonding structure
in a-SiC:H films and their thermal stability are important
for the devices. Hydrogen concentration is usually indirectly estimated by infrared ŽIR. absorption measurements
w9x except for the unbonded hydrogen.
In this paper, we studied the variation of hydrogen ina-SiC:H film prepared by rf sputtering method in a gas
mixture of argon and hydrogen by using IR spectroscopy
and elastic recoil detection analysis ŽERDA. w10,11x. In
addition, we have carried out in-situ characterization of)Corresponding author.

0040-6090r97r$17.00 q 1997 Elsevier Science S.A. All rights reserved.
PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 3 7 1 - 4hydrogen depth profiles during isochronal annealing of thespecimen to obtain the activation energy for detrapping ofhydrogen.
2. Experimental details
A-SiC:H films were prepared by diode-type rf planar
magnetron sputtering on to Si Ž111. wafers at room temperature. A sintered silicon carbide disk 75 mm in diameter was sputtered at a power of 210 W in a gas mixture ofargon at partial pressure Ž PAr . of 0.33 Pa, and hydrogenŽ P H . from 0.065 to 1.3 Pa, where the background pres2
sure before sputtering was 6.5 = 10y5 Pa. The distancefrom the cathode to the substrate was 50 mm. Under theseconditions, the deposition rate obtained was about 0.5 nm
sy1 . The component ratio of carbon to silicon determinedby Auger electron spectroscopy was nearly unity.
IR spectra of as-deposited and annealed films weremeasured using a double beam IR spectrophotometerŽHitachi Perkin-Elmer 225.. The annealing temperature
was in the range of 373–1373 K for 3600 s in vacuum of
5 = 10y4 Pa.

208

Y. Suzaki et al.r Thin Solid Films 311 (1997) 207–211

Fig. 1. Hydrogen depth profile in a-SiC:H film 154-nm-thick prepared at
H 2 partial pressure of 0.33 Pa obtained by ERDA.

Fig. 3. Annealing temperature dependence of the integrated intensityŽ HŽ a Ž v .r v .d v ., which is due to the vibrations of Si–H bonds.

In ERDA w12,13x, the recoiled hydrogen from elastic
collisions with the bombarding 6.06 MeV 19 F 3q ionstraverses through the 0.5 = 2 mm2 collimator and is at thefixed forward angle of 408 with an energy below 0.4 MeV.
A 6.2-m m-thick Mylar absorber was used as a particle
filter to shield the silicon surface barrier detector ŽSSBD.
from the forward scattered analysis beam and other recoiled heavy ion beams. The typical beam current of19 3q
F
on the samples was 5–10 particle nanoampere
ŽpnA.. The depth scale applied to the sample was obtainedusing Bragg’s law for stopping power based on the elemental values tabled by Ziegler w14,15x and Andersen andZiegler w16x. In-situ isochronal annealing for 300 s atvarious temperatures from 323 to 1123 K was carried out
in the ERDA system.

cated by broken lines. The depth profile of hydrogenconcentration in the film was uniform at 7.1 = 10 27
atomsrm3.

3. Results and discussion

3.2. IR absorption results
Fig. 2 shows IR absorption coefficient a Ž v . vs.
wavenumber for as-deposited a-SiC:H film prepared at
PH 2 of 0.33 Pa. This figure shows broad absorption bands
at around 800, 2100 and 2800 cmy1 , which are due to thestretching mode vibrations of Si–C, Si–H and C–H bonds,
respectively w8x. In addition, a shoulder on the absorption
band is shown at around 1000 cmy1 , which is due to therocking and wagging mode vibrations of C–H bonds. The
concentration of C–H bonds is estimated by using theabsorption spectra at around 1000 cmy1 which was separated from the absorption at around 800 cmy1 .
Using the value of a Ž v ., the concentration of bonds inthe film, NIR is given by w17x

3.1. Hydrogen depth profile by ERDA

aŽ v.

A hydrogen depth profile in the 154-nm-thick a-SiC:Hfilm prepared at PH 2 of 0.33 Pa obtained by ERDA isshown in Fig. 1. The surface of the a-SiC:H film and theinterface between the film and silicon substrate are indi-dv
Ž 1.
v
where A s is a conversion factor which depends on thebonding state, which will be estimated later, and theintegration is over the absorption band. Thus the integrated

Fig. 2. Typical IR absorption spectrum of a-SiC:H film prepared at H 2
partial pressure of 0.33 Pa.

Fig. 4. Annealing temperature dependence of the integrated intensityŽ HŽ a Ž v .r v .d v ., which is due to the vibrations of C–H bonds.

NIR s A s H

Y. Suzaki et al.r Thin Solid Films 311 (1997) 207–211

209

intensity Ž HŽ a Ž v .rv .d v . is directly proportional to theconcentration of bonds in the films w8,18x.
Figs. 3 and 4 show changes of the integrated intensitiesŽ HŽ a Ž v .rv .d v ., which are due to the vibrations of Si–Hand C–H bonds, with increase of annealing temperature
for the samples prepared in the same condition in Fig. 2.
These figures show the integrated intensities decrease withincrease of annealing temperature from 600 to 1000 K andfrom 850 to 1100 K for Si–H and C–H, respectively.
These results mean that the hydrogen which was bonded toSi or C atoms is released at different ranges of temperatures.
3.3. In-situ isochronal annealing in the ERDA system andthe kinematical analysis of hydrogen release
In-situ isochronal annealing for 300 s at various temperatures from 323 to 1123 K in the ERDA system wascarried out in order to measure a precise transition ofhydrogen density in the film directly. Fig. 5 shows thevariation of hydrogen depth profiles during the isochronalannealing. The surface of the film and the interface between the film and Si substrate are indicated by brokenlines. In this figure, it is seen that the hydrogen does not
diffuse into the Si substrate, but is released from surface ofthe sample with increasing temperature. At 1073 K, hydrogen is no longer detected. The average hydrogen concentrations deduced from Fig. 5 are plotted against annealing
temperatures in Fig. 6. In Fig. 6, open circles and solidlines show the experimental results and the calculated,respectively. The hydrogen concentration decreases withFig. 6. Change in hydrogen concentration in a-SiC:H film with theannealing temperature, obtained from the hydrogen depth profiles seen inFig. 5. Open circles and solid line show the experimental results and thecalculated, respectively.

increase of annealing temperature. Three characteristic regions are seen where the hydrogen concentration decreases
drastically at the annealing temperatures around 350, 700
and 950 K. These temperatures are in good agreement with
the integrated intensities of IR absorption bands as shownin Figs. 3 and 4. Thus, it can be said that three types ofhydrogen bonds are distributed in the films; the first type
is the unbonded, which corresponds to the release around350 K, the second and third types are the hydrogen bonded
to Si or C atom, which correspond to the release attemperatures around 700 or 950 K, respectively.
The solid line in Fig. 6 was calculated under theassumptions w19x that the detrapping is a rate limiting
process and that the release process obeys the first-orderkinetics; the hydrogen atoms escape from the surface ofthe film, and diffusion is independent of the hydrogenconcentration. In this case, the activation energy is expressed by a generalized form of the Arrhenius equation
w20x,N s N0 exp ytn expFig. 5. Change in the hydrogen depth profile of a-SiC:H film prepared at
H 2 partial pressure of 0.33 Pa during in-situ isochronal annealing for 300
s at the temperatures from 323 to 1123 K in ERDA system.

žyEDkT/Ž 2.

where N is the hydrogen concentration in the film afterannealing for a time t at a temperature T, N0 is the initialhydrogen concentration, ED is the activation energy fordetrapping, n is the detrapping attempt frequency and k isthe Boltzmann constant. Since three types of hydrogenbonds are present in these films, three kinds of the initialhydrogen concentration N1 , N2 and N3 , are to be considered. The experimental results in Fig. 6 were fitted by Eq.
Ž2. where the activation energy E D and the detrapping
attempt frequency n are obtained through the relation of1rT vs. ln ylnŽ NrN0 .4 in the three characteristic re-210

Y. Suzaki et al.r Thin Solid Films 311 (1997) 207–211

Fig. 8. Hydrogen concentrations in a-SiC:H films prepared under various
H 2 partial pressures P H 2 ranging from 0.065 to 1.3 Pa. Open circlesindicate total hydrogen concentrations as measured by ERDA, and bar
charts indicate the concentrations of Si–H and C–H bonds which wereestimated from the IR absorption spectra, using the conversion factors
estimated from the N2 and N3 values ŽSection 3.3.. Hatched region thenindicates unbonded hydrogen concentrations.

Fig. 7. Changes in the concentrations of three types of hydrogen in thefilm; Ža. the unbonded hydrogen, Žb. the hydrogen bonded to Si atom andŽc. that to C atom. N1 , N2 and N3 indicate the initial hydrogen concentrations of such three types of hydrogen.

gions. The initial concentrations N1 , N2 and N3 , weredetermined from iterative calculation. The initial hydrogenconcentrations N1 , N2 and N3 thus determined based on
Eq. Ž2. are 2.4 = 10 27 , 2.8 = 10 27 and 1.9 = 10 27
atomsrm3, respectively, and the activation energies are
0.079 eV Ž1.27 = 10y2 0 J., 0.44 eV Ž7.05 = 10y2 0 J. and1.3 eV Ž2.08 = 10y1 9 J., respectively. Fig. 7a, b and cshow the calculated concentrations of those three types ofhydrogen as a function of annealing temperature. We
determined the conversion factors A s for Si–H and C–Hbonds in a-SiC:H film based on Eq. Ž1. using the values ofHŽ a Ž v .rv .d v shown in Figs. 3 and 4 and N2 , N3 . The
conversion factors A s determined were 3.2 = 10 23 and
6.4 = 10 22 my2 for Si–H and C–H, respectively.

the unbonded hydrogen concentration has peaked in therange of P H 2 from 0.13 to 0.33 Pa.
The radicals in the plasma during sample preparation
were observed by optical emission measurements w21x. Fig.
9a, b shows the optical emission intensity of radicals of Ža.
Ar ) Ž415 nm. and Žb. H a) Ž652 nm. and H b) Ž486 nm. forvarious hydrogen partial pressures P H 2 . In Fig. 9, it is seen

3.4. Concentrations of three types of hydrogen in a-SiC:Hfilms prepared at Õarious H2 partial pressures
Fig. 8 shows the change of hydrogen concentration ina-SiC:H films with various hydrogen partial pressures P H 2
ranging from 0.065 to 1.3 Pa. In this figure, open circles
indicate total hydrogen concentrations as measured byERDA, and bar charts indicate the concentrations of Si–Hand C–H bonds which were estimated from the IR absorption spectra, using the conversion factors estimated fromthe N2 and N3 values ŽSection 3.3.. Hatched region thenindicates unbonded hydrogen concentrations. It is seen thatFig. 9. The optical emmission intensity of radicals of Ža. Ar ) Ž415 nm.
and Žb. H a) Ž652 nm. and H b) Ž486 nm. for various hydrogen partialpressures PH 2 .

Y. Suzaki et al.r Thin Solid Films 311 (1997) 207–211

that the intensities of H a) and H b) increase and the intensity of Ar ) decreases with increase of PH 2 , respectively.
The unbonded hydrogen concentration decreases in accordance with the increase of hydrogen radicals ŽH a) and H b) .
in the plasma.

211

Acknowledgements
We would like to acknowledge Professor K. Oura,
Osaka University, for his kind cooperation with the ERDA
measurements.
References

4. Summary and conclusions
Concentration and thermal release of hydrogen in thehydrogenated amorphous SiC Ža-SiC:H. films were studied
using IR spectroscopy and elastic recoil detection analysis
ŽERDA.. The results obtained are as follows:Ž1. Hydrogen depth profiles of as-deposited films by theERDA method showed that the total hydrogen concentrations in the films ranged from 5.3 to 7.1 = 10 27 atomsrm3.
Ž2. The IR measurements conducted on the films annealed at various temperatures for 3600 s suggested thatthe hydrogen was released from Si–H and C–H bonds inthe films at the temperatures from 600 to 1000 K and from
850 to 1100 K, respectively.
Ž3. In situ isochronal annealing for 300 s at varioustemperatures from 323 to 1123 K in the ERDA system wascarried out for the film having the hydrogen concentrationof 7.1 = 10 27 atomsrm3. Three types of hydrogen bonds
exist in the film; the first type is the unbonded, the second
and third types are the hydrogen bonded to Si or C atom.
The initial concentrations and the activation energies forthermal release of three types of hydrogen were determined to be 2.4 = 10 27 , 2.8 = 10 27 and 1.9 = 10 27
atomsrm3, and 0.079 eV Ž1.27 = 10y2 0 J., 0.44 eV Ž7.05= 10y2 0 J. and 1.3 eV Ž2.08 = 10y1 9 J., respectively.
Ž4. The conversion factors A s for the Si–H and C–Hbonds in a-SiC:H film were 3.2 = 10 23 and 6.4 = 10 22
my2 , respectively. The unbonded hydrogen concentration
decreased in accordance with the increase of hydrogenradicals ŽH a) and H b) . in the plasma.

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