High-Quality Polycrystalline Silicon Thin Film Prepared by a Solid Phase Crystallization Method
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Crystal growth layer (Undoped a-Si)
Crystal growth laye f(P-do ed a- i) Substrate Fig. 1 Our SPC method.
Table 1. Deposition conditions of Si films.
layer Crystal
growth layer
Flow rate (sccm) H2 100%SiH 4 0.1%PH 3/H 40-300 1-10 3-10 10-50
-
-
RF power Pressure Temperature (°C) (Pa) (W) 10-100 550-650 20-50 80
50-100
550-600
50 40 0~-U,
"
"30
S20 =3 "75
>
0
10
100 200 300 400 500 600 700 Deposition temperature (°C) Fig. 2 Deposition temperature dependence on the volume fraction of crystallites. Open symbols are quoted from Ref.7. Solid symbols indicate our experimental results.
896
layer are as important as those of the crystal growth layer. Thus, in the present work, we have focused on the film structure of the nucleation layer and obtained a silicon film with a unique structure. In this paper, we will discuss the features and growth mechanism of the unique film, and then, will describe its application to the fabrication of a high-quality poly-Si thin film. EXPERIMENTAL PROCEDURE Nucleation layers were deposited by plasma-enhanced (PE) CVD from silane (SiH 4 ) gas diluted with hydrogen on flat or textured quartz substrates. The degree of roughness for the textured substrate is about lop m. Crystal growth layers with a thickness of 5pm were subsequently deposited by PECVD from Sil 4 gas onto the nucleation layers. The deposition conditions are summarized in Table 1. Sample annealing for SPC was performed in a vacuum at 6001C for 10-600 min. The poly-Si and microcrystalline silicon (pc-Si) formation or morphology was studied using various experimental techniques, such as scanning electron microscopy (SEM) after Secco etching, transmission electron microscopy (TEM), transmission electron diffraction (TED) pattern measurements, and X-ray diffraction (XD) spectroscopy. The films were also analyzed by Halleffect measurements with the magnetic field intensity of 5000 Gauss at 300K. RESULTS AND DISCUSSION A silicon film including single crystalline grains as the nucleation layer Figure 2 shows the deposition temperature dependence on the volume fraction of crystallites for pc-Si films fabricated by PECVD from Sil 4 diluted with hydrogen. The volume fraction is
estimated by XD spectroscopy. The open symbols represent the work done by Matsuda 7 which
shows that a crystalline-to-amorphous transition region exists at -5001C, and that the deposited films are amorphous above the temperature. From the viewpoint of obtaining poly-Si with a larger grain size, a film with a smaller proportion of crystallites to amorphous tissue is preferable to a nucleation layer, because the density of the nuclei can be smaller. Thus, we experimented at the temperature region above 500'C. Our experimental results are indicated by the solid symbols in Fig. 2. As can be seen, it seems that no crystallite exists in the obtained films. However, the results of SEM photographs are different. In Fig. 3, SEM photographs of a conventional pc-Si film and a silicon film deposited at 550"C are shown. Our film features relatively large
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