Effects of crystalline nanoparticle incorporation on growth, structure, and properties of microcrystalline silicon films deposited by plasma chemical vapor deposition
Introduction
Tandem Si thin film solar cells are fabricated using microcrystalline silicon (μc-Si:H) cells as the bottom cells and amorphous silicon (a-Si:H) cells as the top cells, in order to obtain a high efficiency with a low manufacturing cost [1], [2], [3], [4], [5]. The cell performance strongly depends on structure of μc-Si films such as the crystallinity and crystalline orientation [6]. Recent studies of correlation between solar cell performance and film microstructure revealed that a boundary phase of a-Si and μc-Si is a favorable structure for high performance of Si thin film solar cells with high stability [7], [8], [9], [10], [11], [12]. Further optimization of the devices requires to find the key to material property control.
During film deposition, Si nanoparticles generated in silane discharges can be incorporated into films, and such incorporation may change considerably their electronic and optical properties depending on the size, structure, and volume fraction of nanoparticles incorporated into the films. In fact we found that incorporation of amorphous Si nanoparticles into a-Si:H films during deposition leads to a higher SiH2 bonds in films and degrades stability of films against light irradiation [13], [14], [15]. Thus, it is important to control the formation and incorporation of nanoparticles to obtain highly stable a-Si:H films. In the case of microcrystalline thin film, on the other hand, the effects of incorporation of crystalline nanoparticles into films during their deposition on growth, structure and properties of the films are not fully studied yet. So, we investigated such effects with a multi-hollow discharge plasma chemical vapor deposition (CVD) reactor with which films with and without nanoparticles can be deposited at the same time. Here we report clear experimental results showing that incorporation of small amount of crystalline nanoparticles strongly affects growth, structure and properties of microcrystalline Si films.
Section snippets
Experimental details
Si films were deposited using a multi-hollow discharge plasma CVD reactor by which incorporation of nanoparticles into films could be significantly suppressed in the upstream region by setting the gas flow velocity high enough to drive nanoparticles toward the downstream region as shown in Fig. 1 [13], [14]. Multi-hollow electrodes had 24 holes of 5 mm in diameter, in which discharges were sustained. VHF power of 180 W (60 MHz) was applied between one powered electrode and a pair of grounded
Results and discussion
Fig. 2 shows a TEM image and its diffraction pattern of nanoparticles trapped on a stainless mesh placed in the downstream region. They are crystalline and 5–10 nm in size [17]. Fig. 3 shows spatial profiles of deposition rate and crystallinity (Xc) of films deposited on the substrates set parallel to gas flow [18], [19]. The deposition rate is nearly the same for the upstream and downstream films, indicating that volume fraction of crystalline nanoparticles incorporated into films is less than
Conclusions
Effects of crystalline nanoparticle incorporation on their growth, structure and properties during film deposition have been studied using multi-hollow discharge plasma CVD. Films with nanoparticles show lower (220) orientation ratio than those without nanoparticles, whereas both films have nearly the same film thickness and crystallinity. Films with nanoparticles have inverted conical growth, while films without nanoparticles have columnar grain growth. Nucleation density of the films with
Acknowledgment
This work was partly supported by JST‐CREST.
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