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Nanostructure and Strain Field in Vertically Aligned Nano-Islands for Si/Ge 2D Photonic Nanocrystals. Takanori Kiguchi 1, Yusuke Hoshi 1, Takeshi Tayagaki 2.3, and Noritaka Usami 1 1 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan 2 Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan 3 PRESTO, Japan Science and Technology Agency, 5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan

ABSTRACT The local strain field and the intermixing of a Ge nano-islands (NIs)/Si spacer stacked structure in a novel solar cell with a p-i-n type Si single crystal with two-dimensional photonic nanocrystals connecting to the vertically aligned NIs were analyzed using electron microscopy. High-angle annular dark field-scanning transmission electron microscope (HAADF-STEM) images show intermixing between Ge and Si clearly and reveal that the surface segregation of Ge becomes advanced. The average composition of the NIs is Ge0.42Si0.58, which is almost constant in a row of vertically aligned NIs. The local strain analysis results obtained from the high-resolution transmission electron microscope (HRTEM) images show that the strain state is partially relaxed after the elastic relaxation of NIs.

INTRODUCTION Energy issues are recognized as extremely important and emergent on a global scale. Solar light is an attractive energy source because it is sustainable and clean. However, the conversion efficiency of solar cells is limited by many factors: band gap loss, optical loss, limitation of the build-in potential, recombination loss, and Joule heat loss. Recently, we attempted to improve the band gap loss, optical loss, and recombination loss using a novel solar cell with a p-i-n type Si single crystal with two-dimensional photonic nanocrystals connecting to the vertically aligned Ge nano-islands (NIs) in the i-Si layer [1]. The solar cell has unique characteristics: (1) light-trapping by photonic crystals, (2) carrier separation by the type-II superlattice band alignment at Ge/Si interface, and (3) expansion of the absorption wavelength by Ge. We attained higher absolute conversion efficiency of 0.7% on average, and 1.7% at maximum compared with conventional CZ Si solar cells [1]. The NI structure is well known to form in Stranski–Krastanov growth mode. The Ge layers stack over the critical thickness, 3 ML, and the growth mode is changed from the 2D layer-by-layer growth into 3D island growth. In the Ge/Si heteroepitaxial system, coherent Stranski–Krastanov growth occurs; then no misfit dislocation is introduced until the 3D island is small [2]. Instead, elastic relaxation occurs and the strained state is partially relaxed without misfit dislocation. This characteristic is advantageous

for solar cells because the recombination center is not created by dislocation. In 3D island growth of Ge layer, surface segregation of Ge atoms takes place. Then intermixing between Ge and Si occurs. The strained state of the Ge layer is relaxed by the elastic relaxation of t