Paper EN-ThP6
Epitaxial Growth of ZnInON Films for Piezo-Electric-Field Effect MQW Solar Cells

Thursday, November 1, 2012, 6:00 pm, Room Central Hall

Multi-quantum well (MQW) solar cells belong to the most promising “third generation photovoltaics” with ultra-high conversion efficiencies at low cost. In order to approach their theoretical maximum efficiency (>50%), a significant improvement in the extraction efficiency of photo-generated carriers is important. We have recently proposed piezo-electric-field effect (PEF) MQW solar cells that utilize novel oxynitride semiconductor ZnInON (ZION) films [1,2]. Our simulation predicts that the electron-hole recombination rate in ZION-QWs is noticeably low compared to those in conventional QWs with III-V materials such as GaAs and InGaN. Here we fabricate epitaxial ZION films on GaN and ZnO templates aiming at realization of PEF-MQW solar cells. Furthermore, we also demonstrate coherent growth of ZION films in order to study the piezo-eletrcic field in ZION-QWs which can prevent the recombination and thus enhance the extraction efficiency of photo-generated carriers.

Epitaxial ZION films were fabricated by RF magnetron sputtering, which is suitable for large-area and low-cost fabrication. Commercial GaN templates that were fabricated by metal-organic chemical vapor deposition (MOCVD) and sputtered ZnO templates were used as epitaxial substrates. The ZnO templates were fabricated by RF magnetron sputtering at 700^oC in Ar-O₂ atmosphere on ZnO buffer layers that were fabricated via nitrogen mediated crystallization (NMC) in N₂-Ar atmosphere at 700^oC [3,4]. On GaN and ZnO templates, ZION films were deposited in N₂-Ar atmosphere at the total pressure of 0.2-0.5 Pa. The applied RF power was 10-90 W and the deposition temperature was 400^oC. The film thickness was 30-700 nm.

X-ray diffraction measurements show that the ZION films have wurtzite crystal structure and the full width at half maximum (FWHM) of rocking curve from (002) plane for the films is noticeably small of 0.115°, indicating small fluctuation of the crystalline orientation. Furthermore, measurements of reciprocal lattice map of (105) plane suggest that the ZION films are grown coherently on GaN templates, indicating that a piezo-eletrcic field can be generated. From these results, we conclude that ZION films are full of promise for Piezo-Electric-Field effect MQW solar cells that can be fabricated at low cost.

[1] N. Itagaki, et al., Jpn. Patent Application No. 2012-49805 (2012) (in Japanese).

[2] N. Itagaki, et al., U.S. Patent Publication No. 2010/0109002 (2010).

[3] N. Itagaki, et al., Appl. Phys. Express 4 (2011) 011101.

[4] K. Kuwahara, et al., Thin Solid Films 520 (2012) 4674.