Structure of Polycrystalline Silicon Films by Glow-Discharge Decomposition using SiH 4 /H 2 /SiF 4 at Low Temperature
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D____ r DClllrVD
CUT
I
(cA) S_
------ .LRF POWER 20W
-
I
,
22.
_[SiF
W
4]
0 seCM
[H2] 20 scCm S
( D)-'
---
---- ----
0aa [siF4] 0.5 .. 2 sccm [H
....
(C)~~~~
r:.•[i0
1
Fig 1.
2,,....
(C)-[i'--1----C
S..... i _(D) 400
Fig. 1. Raman spectra for films A
•--H 1]20 ....
S 4..5?
--450
500
spctacorfim
550
m -
600
with different [H2]ad[i, values and rf pow er values of 5 and 20 W.
RAMAN SHIFT (cm -')
crystallization as low rf power conditions are used, but the SiF4 addition under high rf power supply appears to assist the crystallization as seen in Figs. (A) and (B). On the other hand, we find that an increase in [H2] acts to monotonically enhance the crystallization, independent of the rf power values, as seen in Figs. (C) and (D). Enhanced crystallization with adding SiF, under high rf power supply, as seen in Fig. (B), has also been found in a previous work [2]. In that work, we examined the structural changes of poly-Si films which were deposited at rf power of 20 W and by changing [SiF4] under [SiHl] = 0.15 or 1 sccm conditions in SiH,/SiF4 feed gases without H2 dilution. As a result, under [Sill4] =1 sccm, the crystallization was found to increase monotonically with increasing [SiF4 ]. We suggested that the enhanced crystallization with adding SiF, would be caused by the effects of a change in the surface morphology of the substrates at an initial stage of film deposition [2]. These results will be discussed again at a later stage. As seen in Fig. 1, the effects of H2 addition on the crystallinity are more sensitive for lower rf power. Furthermore, as seen in the results for two kinds of films with [H2] = 20 sccm, the H2 addition is also found to weaken the effects of adding SiF 4 on the crystallinity. This may be due to the formation of more stable H-F bonds in the gas phase [3,4], since the binding energy of H-F bonds, E(H-F), is the highest among those of various bonds related to film growth: E(H-F) = 5.8 eV, E(Si-F) = 5.6 eV, E(Si-Si) = 1.8 eV, E(Si-H) = 3.1 eV, and E(H-H) = 4.5 eV (ref 5). Thus, the increase in the density of H-related radicals may eliminate active F-related radicals in plasma. Based on the results shown in Fig. 1, the addition of SiF 4 or H 2 in Sill4 feed gases not only causes a different contribution to the change in p (crystallinity) as a macro property of the films, but also strongly affects the local structure of Si network in the films. Figure 2 shows (a) the'relative intensities (integrated area) of x-ray diffraction (XRD) spectra and (b) those of the XRD spectra for films with [H2] = 0 and 20 sccm and [SiF4] = 0 and 0.5 sccm, as a function of rf power. The relative intensity with different textures, as shown in Fig. 2, was normalized using the intensity of the corresponding XRD from Si powder. The difference in film thickness among samples was corrected using the x-ray absorption coefficient for Si. As shown in this diagram, the texture for films with [H2] = 0 and [SiF4] = 0.5 sccm under high rf power supply is found to be the strongest spectrum amon
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