Andras Kovács1, Felix Predan2, Jens Ohlmann2, David Lackner2, Frank Dimroth2,Rafal E. Dunin-Borkowski1, Wolfgang Jäger3
1Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, 52425 Jülich, Germany
2Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany
3Institute for Materials Science, Christian-Albrechts-University of Kiel, 24143 Kiel, Germany
Multi-junction solar cells based on III-V semiconductors reach the highest conversion efficiencies and are currently used primarily in concentrator photovoltaic systems and for power generation on satellites or spacecraft. Several cells of different III-V compound semiconductor materials are generally combined to absorb a different wavelength range of the solar spectrum and to convert it into electric power. Fabrication of these cells by wafer bonding is of interest since efficiencies of up to 46 % have been obtained [1], and efficiencies of up to 50% are within reach. Fast atom beam activation is generally used as a pre-treatment to remove oxides and contamination before bonding [2]. Activation treatment and bond processing often result in the formation of amorphous interface layers with inadvertent impurities [3]. We have applied high-resolution imaging and spectroscopic transmission electron microscopy (TEM) techniques to investigate interface regions in as-bonded and thermally annealed (at temperatures of T ≤ 500 °C) GaSb/GaInAs and GaSb/GaInP layer systems. We used aberration-corrected high-resolution TEM, high-angle annular dark-field scanning (S)TEM, and energy-dispersive X-ray spectroscopy (XEDS) with an aberration-corrected probe to monitor elemental distributions with high precision and sub-nanometer spatial resolution.
For GaSb/GaInAs, we find that the crystal lattices are interconnected. The bond interfaces exhibit terraces, misfit dislocations, and nanometer-sized pores, as well as compositional fluctuations in the near-interface regions. For GaSb/GaInP, the interface regions are characterized by an amorphous interlayer that has a thickness of approximately 1.5 nm, with a minor enrichment of Ga. These phenomena are attributed to the wafer pre-treatment before bonding. Thermal annealing, which is often used as a method for improving the interface conductance, results in changes in structure and composition. The amorphous interlayers are reduced in thickness by recrystallization, resulting in a largely epitaxial interface after annealing at 500 °C. XEDS mapping reveals detectable amounts of In and P after annealing at temperatures T ≥ 225 °C, as well as small pores and In-rich crystalline precipitates for T ≥ 400 °C in the GaSb near-interface regions. These observations provide an understanding of the electrical properties of the interfaces [4] and can be compared with results obtained from GaAs/Si wafer-bond interfaces [3]. From a methodological point of view, our results show how aberration-corrected TEM can be used to contribute to the monitoring, control, and optimization of concepts for the fabrication of high-efficiency solar cells. An optimized 4-junction solar cell based on GaInP/AlGaAs//GaInAs/Ge currently has an efficiency of 38.5% under a concentration of 188 suns, while a GaInP/GaAs/GaInAs//GaSb cell has a first efficiency of 29.1% under 194 suns [1].
References
[1] F. Dimroth et al., IEEE Journal of Photovoltaics, 6, 343 (2016), https://doi.org/10.1109/JPHOTOV.2015.2501729
[2] S. Essig et al., J. Appl. Phys. 113, 203512 (2013), https://doi.org/10.1063/1.4807905
[3] D. Häussler, L. Houben et al., Ultramicroscopy, 134, 55 (2013), https://doi.org/10.1016/j.ultramic.2013.07.005
[4] F. Predan, A. Kovács et al., submitted to Journal of Applied Physics (2017).
