Structural, optical properties of crystalline silicon (c-si) deposited. on porous aluminium by PECVD technique

Transcription

1 Structural, optical properties of crystalline silicon (c-si) deposited on porous aluminium by PECVD technique M. Ghrib 1, T. Abdellaoui 1, N. Khedher 1, M. Gaidi 1, M. Ben Salem 2, Hatem Ezzaouia 1, 1 Laboratoire de Photovoltaïque des Semi-conducteurs et des nano-structures (LPVSN), Technopôle de Borj-Cédria B.P. 95, 2050 Hammam-Lif, Tunisia. 2 Laboratoire de matériaux, de microscopie et de micro analyse L3M * Corresponding author: mondherghrib@yahoo.fr Abstract: In this paper we report detail investigation and correlation between microstructural and optical properties of amorphous silicon (c-si) deposited by plasma enhanced chemical vapor deposition (PECVD) on porous aluminum structure. The influence of the microstructure of c- Si thin films on their optical properties was investigated through an extensive characterization. The effect of anodisation conditions (current) on the microstructure of alumina surface layer and c-si films was systematically studied by transmission electron microscopy (TEM), atomic force microscopy (AFM) and Raman spectroscopy. The optical constants (n and k as a function of wavelength) of the films were obtained using variable angle spectroscopic ellipsometry in the UV-vis-NIR regions. The porous aluminum and silicon layer (P.A/S.L) was modeled as a mixture of void, crystalline silicon and aluminum using the Bruggeman effective medium approximation. Based on this full characterization, it is demonstrated that the optical characteristics of the films are directly correlated to their microstructural properties. In particular, the refractive index increases with film density, while losses decrease. Keywords: Porous aluminum, crystalline silicon (c-si); Pore size distribution; Spectroscopic ellipsometry.

2 1-Introduction In the field of photovoltaic conversion, silicon thin film technology appears as a promising approach, because it combines the advantages of the good control of silicon processing and those of thin layer there is the use of thin-layer crystalline silicon on low cost foreign substrates. However, reasonable efficiency values require quite large grains with respect to the film thickness as well as a narrow grain size distribution. The porous aluminum crystallization technique has been reported to lead to the formation of large grains crystalline silicon films on foreign substrates [1 4]. It is based on the overall layer exchange of adjacent Si and Porous aluminum layer (P.A.L) during the transformation of amorphous to crystalline Si. Thus the silane deposit by PECVD [5] on the layer of this porous matrix rests the synthesis of a material of great electronic quality and the crystalline silicon cells made on porous aluminum offer an exciting possibility of constructing and fabricating compact photovoltaic and thermal solar collectors by direct cell deposition on a suitable porous aluminum heatexchange plate. Those are required conditions for the formation of a good active quality layer down epitaxial on film PAL for the solar cell applications. Finally, a continuous film(p.a/s.l) is necessary for the formation of a back surface field as well as for reduction of back contact resistance. In the present work we demonstrate the capability of growing c-si films on porous alumina substrates. The optical properties of the deposited films are examined by ellipsometry and correlated to the film microstructure. Such an understanding is especially crucial for the development of effective photovoltaic cell. In this context, we have tailored the microstructure of the porous alumina layer and c-si films by varying the anodisation conditions, namely the current. This parameter is found to directly impact the surface roughness and film density which in turn influence the values and profiles of the refractive index and extinction coefficient.

3 A B C d=64,06nm d=66,06nm d=73,14nm Fig 1. TEM image of Porous Aluminum layer (P.A.L) as a function anodisation current (300,3500) ma A1 B1 C1 Figure 2 :AFM images of silicon deposited on Porous Aluminum Layer (P.A.L) has as a function the anodisation current (300mA fig A1), ( 350 ma fig B1 ), ( 400 ma fig C1 ).

4 tan ψ mA 1,8 2,0 2, mA - 1,8 2,0 2,2 tan ψ 3,0 2,5 2,0 1,5 0,5 350mA 1,8 2,0 2, mA 1,8 2,0 2,2 tan ψ 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 0,5 400mA 1,8 2,0 2, mA 1,8 2,0 2,2 Fig 3: SE measurements (dots) of the P.A.S.L obtained with (300,3500) ma/cm2 during 25min and the calculated spectra (line) based on the best fitted parameters.

5 Reference 1. H. Masuda, K. Fukuda, Science 268,146 (1995). 2. D. Al-Mawlawi, X.W. Liu, M. Moskovits, J. Mater. Res. 9, 1014(1994). 3. J.C. Hulteen, C.R. Martin, J. Mater. Chem. 7, 1075 (1997). 4. S. Dubois, J.-L. Duvail, L. Piraux, Actualite Chimique, Volume Avril (2000) SIMONNE, J. J., INFOS 83 Conf. Eindhoven (P.B.)(1983) State of the art in InP MIS Technology