2:30 PM - EN11.06.04
Atomistic Structure of Si/GaAs Heterointerfaces Fabricated by Surface Activated Bonding Revealed by STEM Combined with Low-Temperature FIB
Yutaka Ohno1,Yasuo Shimizu1,Yasuyoshi Nagai1,Ryotaro Aso2,Naoto Kamiuchi2,Hideto Yoshida2,Jianbo Liang3,Naoteru Shigekawa3
Tohoku University1,Osaka University2,Osaka City University3
Tandem solar cells consisting of Si and III-V compounds are one of the promising candidates for next-generation terrestrial photovoltaic systems. Surface-activated bonding (SAB) at room temperature (RT), in which surfaces of substrates are activated before bonding by creating dangling bonds under an energetic particle bombardment in a high vacuum, is applied to form Si/GaAs heterointerfaces with a low interface electrical resistance , and high-efficiency InGaP/GaAs/Si triple-junction cells are demonstrated . Even though the interface electrical resistance (~10-1 Ωcm2) is low enough for solar cells, it is still higher than the ideal one at defect-free heterointerfaces (~10-4 Ωcm2), presumably due to the defects introduced during SAB processes. The interface resistance varies depending on SAB conditions including energetic atom irradiation and post-bonding annealing . In order to understand the origin of the resistance, atomistic structure of the hetero-interface, depending on SAB conditions, have been examined by cross-sectional transmission electron microscopy (X-TEM).
In general, X-TEM specimens with heterointerfaces, in which the bonding materials are different in etching rate, are fabricated with milling techniques using energetic ions such as focused ion beam (FIB). We have clarified that the structural and compositional properties of semiconductor homointerfaces fabricated by SAB are modified during FIB processes operated at RT, especially for wide-gap materials, and such a modification can be suppressed by FIB processes operated at -150 oC . In the present work, we have therefore examined the atomic arrangement and composition at Si/GaAs heterointerfaces fabricated by SAB using X-TEM specimens fabricated by FIB milling operated at -150 oC.
Si/GaAs heterointerfaces were fabricated at RT under a SAB condition , with the substrates of B-doped (100) p-Si (with a carrier concentration of 2x1014 cm-3) and Si-doped (100) n-GaAs (2x1016 cm-3). X-TEM specimens with an as-bonded heterointerface were prepared at -150 oC by using a FIB system (FEI, Helios NanoLab600i) with a cold stage customized for the FIB system (IZUMI-TECH, IZU-TSCS004) . The specimens were examined by high-angle annular dark-field (HAADF) and energy dispersive x-ray spectroscopy (EDX) analyses under scanning TEM (STEM) with a JEOL JEM-ARM200F analytical microscope.
HAADF-STEM-EDX revealed that, an amorphous layer, about 4 nm in thickness, is introduced along the as-bonded heterointerface. Most of the amorphous layer are composed of Si, and no amorphous GaAs is apparently observed, as proposed . Atomic intermixing across the heterointerface, within a range of a few nm, is observed. Both Ga and As atoms penetrate into the amorphous Si layer, and the amount of Ga atoms in the layer is about two times larger in comparison with As atoms. Those excess Ga atoms, acting as p-type dopant, can improve the electrical property of the amorphous p-Si layer, that is damaged during the SAB processes. Similarly, the electrical property of the n-GaAs bonding surface can be improved via the penetration of Si atoms. Those self-restoration processes may assist the formation of low-resistance Si/GaAs heterointerfaces by SAB. We will also discuss the effect of post-bonding annealing.
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 Y. Ohno, H. Yoshida, N. Kamiuchi, R. Aso, S. Takeda, Y. Shimizu, Y. Nagai, J. Liang, and N. Shigekawa, submitted to Jpn. J. Appl. Phys.
 Y. Ohno, H. Yoshida, S. Takeda, J. Liang, and N. Shigekawa, Jpn. J. Appl. Phys. 57, 02BA01 (2018).