成果報告書詳細
管理番号20130000000221
タイトル*平成24年度中間年報 新エネルギー技術研究開発 革新的太陽光発電技術研究開発(革新型太陽電池国際研究拠点整備事業)高度秩序構造を有する薄膜多接合太陽電池の研究開発(擬単結晶固相成長技術)
公開日2013/10/25
報告書年度2012 - 2012
委託先名東海大学
プロジェクト番号P07015
部署名新エネルギー部
和文要約
英文要約Title: Development of semi-crystalline Si-related materials by solid phase crystallization(FY2008-2014) FY2012 Annual Report

Infrared-sensitive photovoltaic films are necessary for bottom cells of multi-junction cells for obtaining extremely high efficiency thin film solar cells. We have investigated crystalline germanium (Ge) films as an infrared-sensitive material. We pursue the epitaxial-like growth of crystalline Ge layers on single crystalline Si substrates due to solid phase crystallization (SPC). In this report, details of crystalline structure and effects of impurities were investigated on the SPC growth.
A-Ge films were prepared on n-type c-Si substrates with a (100) orientation by Knudsen cell (K-cell) evaporation and electron beam (EB) evaporation. The SPC was done at 400°C to 475°C in Ar atmosphere. The crystal orientation was evaluated by X-ray diffraction (XRD). The surface of crystallized Ge was observed by scanning electron microscope (SEM) and current mode atomic force microscope (C-AFM).
The epitaxial-like growth of Ge films is successfully achieved by SPC on (100) Si substrates as seed layers. But the rocking curve of XRD measurements shows much wider peaks compared with single crystalline Ge wafers, and the SEM image shows a microstructure of about 50 nm. So the epitaxial-like Ge films probably have micro-domain structure with ambiguity of orientation. The epitaxial growth is probably conducted in very small areas due to much nucleation on c-Si substrates. The C-AFM images show small and high-density conductive spots which have similar scales to the crystalline domains. It is suggested that boundaries of the grains cause low resistivity parts due to their defective structure. For device application, we need to avoid influence of the micro-domain structure. Controlling the nucleation site is a subject in the next phase.
Impurities such as Nitrogen and Oxygen are easily incorporated in a-Ge films while keeping them in the air and the incorporation is more serious in a-Ge prepared by the K-cell evaporation than that by the EB evaporation, probably because of lower density in a-Ge by the K-cell evaporation. The impurities in a-Ge disturb the epitaxial growth and cause random crystallization. It must be avoided to expose the a-Ge films to the air. We found that the impurity incorporation is sufficiently suppressed by covering a-Ge with a-Si cap layers. Nitrogen and Oxygen concentrations after air-exposure are successfully reduced by about two orders of magnitude, and the preferential growth is promoted following Si substrates.
These results give us an effective research direction to achieve the goal of our project. Photovoltaic crystalline Ge films will be pursued by optimizing the SPC with the controlled nucleation and the suppression of impurity incorporation.
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