成果報告書詳細
管理番号20130000000799
タイトル*平成24年度中間年報 太陽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) 高効率集光型太陽電池セル、モジュール及びシステムの開発(日EU 共同開発) 新材料・新構造太陽電池の創製及び評価解析技術の開発(WP1)
公開日2014/5/9
報告書年度2012 - 2012
委託先名豊田工業大学
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:R & D on Innovative Solar Cells (International Research Center for Innovative Solar Cell Program) "Ultra-high efficiency concentration photovoltaic’s (CPV) cells, modules and systems (WP 1:New materials and device characterization)" (FY2011-FY2014) FY2012 Annual Report

1. III-V-N novel materials
(1) Development of III-V-N tandem cells
The N-H complex in GaAsN which can be a donor or acceptor depending on its structure has the preferential N-H bond direction. The two-fold rotational symmetries in (001) plane were observed for the IR absorption peak intensities of the N-H local vibration modes (LVM) at 2952 and 960 cm-1. The directions of both the N-H bonds tended to align along [1-10]. The peak intensities of the N-H LVM at 2952 and 960 cm-1 did not show any correlations each other when those were controlled by changing the growth condition. It means that those modes belong to different kinds of N-H complexes. Therefore, two kinds of N-H complexes tended to align along [1-10] in (001) plane.
Changes of N-H species in GaAsN induced by band-gap photoexcitation were studied by microsecond time-resolved IR absorption spectroscopy. When the sample was excited by nanosecond laser pulses (650 nm, 10 Hz, 6 ns duration), the peak intensity at 962 cm-1 decreased in intensity and a new band appered at 990 cm-1. Since vibrational frequency is very sensitive to the change of charge densities of the chemical bonds, this frequency shift would be due to the trapping of photogenerated electrons or holes.

2. III-V on Si structure
(1) Development of III-V on Si tandem structure
Formation and evolution of rotational twin domains (TW) during molecular beam epitaxial growth of GaAs on Si (111) substrates were studied by in-situ X-ray diffraction (XRD) technique. At the growth condition at low TW formation ratio, volume ratio of TW to normally grown domain (ND) decreased drastically at low film thickness region (~4 monolayer). In that case, mean domain size of TW is smaller than that of ND. By In pre-evaporation on Si (111) before GaAs growth, mean island sizes became small and TW ratio decreased compared with GaAs growth on the substrate directory. These results can be understand by Ostwalt ripening processes, which is the processes that small domains vanish while large ones grew. To enhance this phenomena, GaAs films on Si (111) substrate were annealed in vacuum chamber and measured TW and ND amounts by XRD. TW peak decreased while ND peak increased or kept constant. These results allows us to understand TW formation mechanisms and to reduce TW formation.

(2) Novel buffer structure for III-V on Si
We investigated reduction of the residual stress in GaAs film on Si substrate. Due to the large mismatch of thermal expansion coefficient between GaAs and Si, extremely large tensile stress remains in the GaAs after cooling down. In order to introduce compressive stress into the film, we used III-V semiconductors as a cap layer which lattice constant is larger than GaAs, and clarified the effect of InGaAs graded-layer. We confirmed wafer warpage of GaAs on Si drastically decreased by the introduction of InGaAs graded cap, which indicated that residual stress in GaAs layer was effectively reduced.
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