Effects of Composition on the Microstructures and Optical Properties of Hydrogenated Amorphous Silicon Carbide Films Pre
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the range between 350 and 2600 nm. From this information, the optical band gap was obtained by applying the Tauc plot, and the index of refraction can be derived, as well. Meanwhile, FTIR spectra were obtained by employing a Bomem model DA8 FTIR spectrophotometer. In addition, a detailed description regarding how to obtain both the absorption coefficients and the number of C-H bonds that associate with each vibration mode can be found in Ref .19. On the other hand, the total hydrogen content was obtained by assuming that all of the hydrogens were bonded to C. Therefore, the hydrogen concentration obtained was in the low end of the hydrogen content existed within the film. At last, JEOL JSM 5410 SEM was employed to investigate the surface morphology, and JEOL 200CX STEM was employed to study the crystal structure. RESULTS AND DISCUSSION Composition analysis The relative ratio of C to Si in the thin films was investigated by XPS. The relationship between the flux ratio of methane to mixed reactive gases, Y% (Y% = CH4 flux/(CH 4+SiH 4) total flux), and the carbon content in the thin films, X% (X% = number of C atom/total number of (Si+C) atoms), was shown in Fig. 1. In Fig. I, three regions were able to be divided along the curve: silicon-rich region (Y = 0, 14.5, 29.1), stoichiometric region (Y - 43.6, 56.4, 70.9) and carbon-rich region (Y = 85.5, 92.7, 100). The dotted line is the "X% = Y%" line. It can be seen that in silicon-rich region, the carbon content ( X%) in the films is larger than that of the corresponding 900 methane ratio (Y%) in the reactive gas; whereas, in 0 carbon-rich region, the carbon content in the films is 70 smaller than that of the methane ratio. This phenome. . 60 non implies that the films tend to form stoichiometric so 40 composition and both carbon and silicon atoms tend to 40/bond to different kind of atoms. 30 /FTIR was employed to analyze the hydrogen content 20in thin films. The hydrogen content in the thin films 10 " increases with increase in the ratio of methane(Y%) as 0 10 20 30 40 50 60 70 80 90100 shown in Fig. 2. The dotted curve "Z%" is the ratio of Y% the number of hydrogen atoms bonded to carbon atoms (nCH) to the total number of bonded hydrogen atoms (Z% Fig. 1. The relationship between = nCH/(nCH+ nsiH)* 100%). The number of hydrogen methane flux ratio (Y%) and carbon atoms bonded to C and the number of hydrogen atoms content in films (X%) bonded to Si are shown in Fig. 3 respectively. Legend Title
.-
E
80
H%
40_ -Total
40
40
SiHn
w
30
0
S20 -
40 40
•.
=10
20
010
CD
0
CHn
Z%
EC,30 -90
0 7--_
50
50
100
50
.40
60
80
0
100
Fig. 2. Methane flux ratio (Y%) vs. total hydrogen content(nH/l10") and the ratio of hydrocarbon bonding(Z%)
,/
20
0' -
0
- 20
-30
10
20
20 -10
40
60
80
0
100
Fig. 3. Methane flux ratio (Y%) vs. hydrogen content in silicon-hydrogen bonding mode(nSfH,) or in hydrocarbon bonding mode(ncHn)
In Fig. 2, the hydrogen concentration is relatively low in silicon-rich region, and it appears to level off in stoichiometr
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