Crystallization of amorphous silicon thin films deposited by PECVD on nickel-metalized porous silicon
© Ben Slama et al.; licensee Springer. 2012
Received: 30 April 2012
Accepted: 9 August 2012
Published: 17 August 2012
Porous silicon layers were elaborated by electrochemical etching of heavily doped p-type silicon substrates. Metallization of porous silicon was carried out by immersion of substrates in diluted aqueous solution of nickel. Amorphous silicon thin films were deposited by plasma-enhanced chemical vapor deposition on metalized porous layers. Deposited amorphous thin films were crystallized under vacuum at 750°C. Obtained results from structural, optical, and electrical characterizations show that thermal annealing of amorphous silicon deposited on Ni-metalized porous silicon leads to an enhancement in the crystalline quality and physical properties of the silicon thin films. The improvement in the quality of the film is due to the crystallization of the amorphous film during annealing. This simple and easy method can be used to produce silicon thin films with high quality suitable for thin film solar cell applications.
The use of porous silicon as intermediate layer for silicon thin film solar cells has attracted many research groups in the last decade [1–3]. A porous silicon (PS) layer with double porosity is generally used. The upper PS layer with low porosity serves as a seeding layer for epitaxial growth. The second layer with high porosity is used to (1) prevent pore filling during silicon deposition, (2) act as a gettering barrier, and (3) make easy the separation process of the device. The use of porous silicon as a substrate for silicon thin film deposition has many advantages if compared with foreigner substrates such as glass or ceramics: porous silicon can act as a barrier that prevents the diffusion of impurities from the substrate to the film. It can also support high temperatures required for solar cell processing (doping, metallization, etc.). The elaborated solar cell can be transferred to a low cost substrate (glass, ceramic, plastic, etc.). On the other hand, the silicon substrate can be used several times after the transfer of the solar cell.
The major problem for such structure, if the porous layer is not removed, is its high series resistance. One possible way to overcome this problem is the metallization of the intermediate PS layer. There are many techniques for the metallization of porous silicon such as vapor deposition, sputtering, and electrodeposition. Among these techniques, immersion plating is the simplest and the most economical method [4, 5]. In this work, we present a study on the metallization of porous silicon by immersion in Ni solution and the use of the obtained material as a substrate for the deposition and crystallization of amorphous silicon thin films intended for thin film solar cell application. The presence of Ni on the porous structure walls will play a crucial role both in the crystallization process and the back metallic contact quality that will be realized after the transfer process of the thin silicon solar cells.
Porous silicon was elaborated by electrochemical anodization of heavily boron-doped p-type silicon substrate in HF solution. Two current densities were successively used. A low density of 5 mA/cm2 was used to form an upper thin porous layer with low porosity of about 30% and a higher density of 50 mA/cm2 to form a deeper layer with high porosity of about 60%. Metallization of the porous layer was carried out by simple immersion in diluted nickel solution. Amorphous silicon (a-Si) thin films were deposited on metalized porous silicon substrates using plasma-enhanced chemical vapor deposition (PECVD) by decomposition of SiH4 (3 sccm) electronic grade mixed with H2 (150 sccm) at a reaction pressure of 133.3 Pa. The substrate temperature and the RF power were fixed to 300°C and 60 W, respectively. Obtained films were thermally annealed under vacuum at 750°C for 2 h. Structural and optical properties of crystallized silicon thin films were systematically analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and UV–vis spectrophotometer.
Results and discussion
Sheet resistance and resistivity of a-Si thin film before and after annealing
Sheet resistanceRs (kΩ)
Resistivity ρ (Ω cm)
Obtained results from structural, optical, and electrical characterizations show that thermal annealing of amorphous silicon deposited on Ni metalized porous silicon leads to an enhancement in the crystalline quality and physical properties of the silicon thin films. The improvement in the quality of the film is due to the crystallization of the amorphous film during annealing. In this case, crystallization is controlled by the presence of silicon crystallites of the porous layer (solid phase crystallization) that act as nucleation centers and the diffusion of nickel in silicon film (metal-induced crystallization) to form nickel silicides (Ni2Si, NiSi, NiSi2, etc.). The presence of these silicides, having a generally low resistivity, at the interface Si/PS is very important for the realization of metallic contacts with low resistance. This simple and easy method can be used to produce silicon thin films with high quality suitable for thin film solar cell applications.
Deposition and crystallization of amorphous silicon thin films on Ni metalized porous silicon layer with double porosity were studied. Results show that amorphous silicon thin films were fully crystallized and preferentially <400 > oriented. This method seems to be very interesting for the production of high quality silicon thin films that can be used for the production of efficient and freestanding thin solar cells.
This work was supported by the Ministry of High Education and Scientific Research of Tunisia.
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