Fabrication of Polycrystalline Silicon on Glass From Fluorinated Precursors with the Aid of Atomic Hydrogen
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Poly-Si thin films at low substrate temperature by Plasma Enhanced Chemical Vapor Deposition (PECVD) using various source gases such as SiH 4 , SiH2 F2 and SiF4 as the analogous manner to the fabrication a-Si:H. [5].[6] Rather high FET mobility of over 40 cm 2 /Vs, so far, was practically obtained on Poly-Si deposited from fluorinated precursors by PECVD. [7] As far as mass production is concerned, PECVD offers marked advantages compared with LA, RTA and SPR. In this study, we investigated systematically chemical reactions of the fluorinated precursors on the vicinity of growing surface with in situ observation with spectroscopic ellipsometry(SE). A novel technique was consequently developed for the aim of making Poly-Si on glass using two step growth, i.e., (1) formation of seed crystals on glass by LL technique and (2) grain growth on the seeds. EXPERIMENT
Precursors given by SiF2 H and SiFH 2 were made resulting from decomposition of gaseous mixture of SiF4 and H2 in microwave (2.45 GHz) plasma.[6] Real time observation of the growing surface was performed 391
Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
with a spectroscopic phase modulated ellipsometer (UVISEL Jobin-Yvon). In the LL-technique, the source molecule, SiF4 , was intermittently supplied into the reactor, whereas hydrogen was fed at a constant flow rate. [8] A Schottky diode was fabricated by successive deposition of n+Poly-Si/ undoped Poly-Si( 1.4 rtm thick)/Pt, and photovoltaic effect was measured under light illumination (100 mW/cm 2 ). RESULTS AND DISCUSSION Selection of precursors Neutral radicals, SiH 2F and SiFH 2, are made resulting from the successive reduction of fluorinated fragments with atomic hydrogen. The mixing ratio of these species are, consequently, selected simply by changing the ratio of SiF 4 and H2.[6] Ratio of SiH2F, namely, increases with an increase in the ratio of H2 in respect to SiF 4 due to promotion of reducing reaction. Conspicuous differences in behavior on growing surface were found between them. Fig.l(left) shows the ellipsometric trajectories observed with light of 3.2 eV during the initial stage of growth on Si0 2 coated c-Si substrate from SiF 2H(A) and SiFH 2(B), respectively. 180
A
hv:32ev . .............. ii i........ .. ... ..i ......................... .... ........ . ..i ...................
. ...... ...... ..... ... .... ............ ........ ...... ......................... 140.. 16
B
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120
Amorphous o
A
A
.
.
B
100
crystal
80 6
10
14
18
22
26
Psi (deg.)
Fig.1 SE trajectories and schematic sketch of ultra-thin (40 nm thick) films grown in condition "A" and "B" The solid lines are the curves fitted with the trajectories observed under the assumption that the crystalline fraction in respect to the amorphous phase is linearly increased with deposition time. In Fig. 1 (right), schematic sketches are illustrated of fraction and size of crystals in an ultra-thin films of 40 nm thick made from these respective precursors, which are calculated from the spectra of p
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