Estimation of the Crystallinity of P-type Hydrogenated Nanocrystalline Cubic Silicon Carbide by Conductive Atomic Force
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Estimation of the Crystallinity of P-type Hydrogenated Nanocrystalline Cubic Silicon Carbide by Conductive Atomic Force Microscopy ż
Daisuke Hamashita1, Yasuyoshi Kurokawa1, Makoto Konagai1,2 Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1-S9-9, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan 2 Photovoltaic Research Center (PVREC), Tokyo Institute of Technology, 2-12-1-S9-9, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan 1
ABSTRACT P-type hydrogenated nanocrystalline cubic silicon carbide is a promising material for the emitter of n-type crystalline silicon heterojunction solar cell due to its lower light absorption and wider bandgap of 2.2 eV. The electrical properties of hydrogenated nanocrystalline cubic silicon carbide can be influenced by its crystallinity. In this study, we propose the use of conductive atomic force microscopy (Conductive-AFM) to evaluate the crystalline volume fraction (fc) of pnc-3C-SiC:H thin films (20~30 nm) as a new method instead of Raman scattering spectroscopy, X-ray diffraction, and spectroscopic ellipsometry. INTRODUCTION Recently, n-type crystalline silicon heterojunction solar cells with a p-type hydrogenated amorphous silicon emitter can achieve the high efficiency of 23.7 % [1]. However, the p-type hydrogenated amorphous silicon emitter has high absorption coefficient, which leads to lower short circuit current density (Jsc). Further improvement of Jsc, a new heterojunction emitter with lower light absorption is required. P-type hydrogenated nanocrystalline cubic silicon carbide (nc3C-SiC:H) is a promising material for the emitter of n-type crystalline silicon heterojunction solar cell compared to conventional p-type hydrogenated amorphous silicon due to its lower light absorption and wider indirect bandgap of 2.2 eV [2]. Previously, we succeeded in depositing highly conductive p-type Al-doped nc-3C-SiC:H thin films by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at a low temperature of 390 oC [3,4]. In the case of nc-3C-SiC:H films, high crystalline volume fraction (fc) is important factor to improve the electrical and optical properties of these films [5,6]. For the application to the emitter of crystalline silicon heterojunction solar cells, the thickness of the emitter should be limited to less than 30 nm. However, it is difficult to estimate the fc of p-nc-3C-SiC:H thin films (the thickness is about 20~30 nm) since conventional method to determine the fc, including Raman scattering spectroscopy and X-ray diffraction, can be used for only thick films more than 100 nm. Therefore, we propose the use of conductive atomic force microscopy (Conductive-AFM) to evaluate the fc of p-nc-3C-SiC:H thin films as a new method. Conductive-AFM is now used as effective tool not only for observing surface structures but also for evaluating nano-scale electrical properties such as I-V characteristic of solar cells using a conducting cantilever [7,8]. Electrical properties of thin films less than 100 nm can be also measured by this method. In t
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