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Optical Properties of Nanoporous Germanium Thin Films

In the present article we report enhanced light absorption, tunable size-dependent blue shift, and efficient electron–hole pairs generation in Ge nanoporous films (np-Ge) grown on Si.

Authors

Daniela Cavalcoli, Giuliana Impellizzeri, Lucia Romano, Maria Miritello, Maria Grazia Grimaldi and Beatrice Fraboni

Abstract

The enhancement of solar energy conversion efficiency at reasonable prices is a fundamental issue in the development of photovoltaics. This efficiency increases if the absorption in the material of photons from the whole solar spectrum is maximized: efficient photon management and light trapping are perfect tools to achieve this target. In the present article we report enhanced light absorption, tunable size-dependent blue shift, and efficient electron–hole pairs generation in Ge nanoporous films (np-Ge) grown on Si.

The Ge films are grown by sputtering and molecular beam epitaxy;  subsequently, the nanoporous structure is obtained by Ge+ self-implantation. We show, by surface photovoltage spectroscopy measurements, blue shift of the optical energy gap and strong signal enhancement effects in the np-Ge films. The blue shift is related to quantum confinement effects at the wall separating the pore in the structure, the signal enhancement to multiple light-scattering events, which result in enhanced absorption.

All these characteristics are highly stable with time. Therefore these nanoporous Ge layers can efficiently absorb sun-light and can generate free carriers that can easily circulate and thus could be collected generating a photocurrent. All these processes occur at tunable energies, thus an optimum match with the solar spectrum can be achieved.  Therefore, the present results demonstrate that this material shows excellent properties in view of photovoltaic applications.

The figure shows a cross-section Scanning Electron micrograph (left) of the ion-implanted nano-porous Ge film, the red line schematically represents the light path in the layer. Visible light interacts with the porous Ge layer and creates electron-hole pairs that can be collected by the structure.

Electron –hole pairs are generated at different wavelengths due to the quantum confinement which occurs due to the small dimension (less than 10 nm) of the walls separating the pores.

Authors from UNIBO-DIFA: Daniela Cavalcoli, Beatrice Fraboni

ACS Appl. Mater. Interfaces, 2015, 7 (31), pp 16992–16998

http://pubs.acs.org/doi/abs/10.1021/acsami.5b02089