Photonic crystals or nanostructures are regarded as promising in the field of optoelectronics since they could control and manipulate photons. Management of photons is of great importance also in solar cells. However, application of photonic nanostructures to solar cells has been considered to be not feasible. This is due to the fact that fabrication of photonic nanostructures generally requires advanced lithography techniques and is limited to a small-area. It would be ideal if one could fabricate large-area photonic nanostructures without any lithography techniques so that one can implement photonic nanostructures to solar cells.

We show that lateral modulation in strain in vertically aligned Ge quantum dots leads to modulation in the etching rate against a chemical solution. This could be utilized for fabrication of Si-based large-area photonic nanostructures coupled with Ge quantum dots without any lithography techniques.

We have grown 15-50 multiple layers of Ge/Si on Si(100) by gas-source molecular beam epitaxy.  The amount of Ge was chosen as ~8 monolayers (ML) to exceed the critical coverage of the growth mode changeover. The Si spacer thickness was carefully chosen so that Ge dots can be vertically aligned. Scanning electron microscopy and atomic force microscopy clarified that photonic nanostructures can be formed by a simple wet etching of the vertically aligned Ge quantum dots. The surface dip depth formed by the wet etching increases with decreasing Si spacer thickness. Furthermore, spatial uniformity of the nanostructures was found to be affected by the Si spacer thickness. By changing with the etchant, the geometry of the nanostructures can be widely changed.

The photonic nanostructures could change optical properties. In fact, we observed that photoluminescence intensity of Ge quantum dots is drastically increased by the presence of the photonic nanostructures. Furthermore, the conversion efficiency of the solar cell with photonic nanostructures was found to be slightly higher than that of the control Si solar cell with a flat surface.
It is therefore concluded that photonic nanostructures formed by our unique technology are promising for management of photons in large-area optical devices such as solar cells.

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