Type-II Band Profile of GaAs/Si Hetero Junctions by Surface Activated Bonding for Hybrid Tandem Cells Naoteru Shigekawa a , Jianbo Liang a , Masashi Morimoto a , and Shota Nishida a a Department of Applied Physics and Electronics, Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan We fabricated p + -GaAs/n-Si and n + -GaAs/p-Si junctions by using surface activated bonding and measured their current-voltage and capacitance-voltage characteristics at room temperature. Their conduction band offset, which was extracted from the capacitance- voltage characteristics, was 0.57-0.84 eV. The results suggested that the band profiles of junctions had type-II features, which was likely to be preferable for fabricating low-resistance tunnelling junctions in hybrid tandem cells. The influence of possible interface states on the electrical properties of junctions was also discussed using the charge neutral level model. Introduction Tandem cells made of compound-semiconductor-based sub cells with different band gaps are promising as high-efficiency next-generation solar cells (1). From the practical viewpoints Si-based cells should be used as bottom cells instead of cells grown on GaAs or InP substrates. Given that the growth of III-V materials on Si substrates is still difficult technical issues because of the difference in thermal expansion coefficients and lattice constants (2) as well as the frequent occurrence of anti-phase domain (3), the hybrid approach employing the direct wafer bonding is attractive. The present authors reported on the characteristics of InGaP-on-Si and InGaP/GaAs-on-Si tandem cells (4,5) that had been fabricated by surface-activated bonding (SAB). The efficiency of the InGaP/GaAs/Si tandem cells was 24.4% (5). The key issue in fabricating such tandem cells lies in formation of low-parasitic-resistance tunnelling junctions made of III-V- materials, typically GaAs, and Si. The band profile of junctions, which dominantly determines the transport characteristics of carriers across the interfaces, must be clarified. Interface states are introduced at the bonding interfaces in junctions made by the direct wafer bonding. Their electrical characteristics were analysed by using the charge neutral level (CNL) model, in which interface states with energies lower (higher) than the energy of the CNL   are assumed to have donor-like (acceptor-like) features (6). The Fermi level at the interface relative to   determines the density of electrical charges at the interface   . The influence of the interface charges was investigated for bonding based Si/Si junctions (7). The present authors reported that traps at the interface played a major role in the carrier transport properties in SAB-based p-Si/n-Si junctions (8). In this work, we fabricated p + -GaAs/n-Si and n + -GaAs/p-Si hetero junctions by SAB. We measured their current-voltage (I-V) and capacitance-voltage (C-V) characteristics 10.1149/06405.0235ecst ©The Electrochemical Society ECS Transactions, 64 (5) 235-242 (2014) 235 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 116.81.239.11 Downloaded on 2014-10-12 to IP