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Nano-Voids in Strained Silicon for Plasmonics

Final Report Summary - NOVOSIP (Nano-Voids in Strained Silicon for Plasmonics)

The EU-funded project No 911932 'Nano-voids in strained silicon for plasmonics' (NoVoSiP, Return Phase) is devoted to strain-enhanced self-organization of nano-voids and nano-dots in Si/SiGe/Si and Si/SiSn/Si layered structures.
Voided structures in strained Si offer an opportunity to exploit their unique properties in photovoltaic (PV) applications. Through irradiation with light ions, strained Si — accomplished by putting of the silicon-germanium (SiGe) or silicon-tin (SiSn) and Si layers over a substrate of silicon — results in nanodot and nanovoid formation. Their unusual electronic and optical properties have not been yet experimentally studied in solar cell configurations.
The project NoVoSiP is aimed to investigate the structural and optical properties of crystalline silicon for possible improvement of light harvesting in silicon-based photovoltaic devices. The NoVoSiP project offered a unique PV device configuration to study void-containing Si/SiGe-strained structures. The nanodots and nanovoids are placed in a highly doped emitter layer close to the p-n-junction to extend near-field effects to the depletion region. These effects would then give rise to carrier multiplication and increase light scattering due to far-field effects, both being promising for enhancing sunlight absorption.
We used different methods to produce and investigate the structural, optical and electronic properties of the periodically formed strained layers. The plan was to use the findings to prepare plasmonic structures for Si-based PV devices, solid-state gas sensors and optoelectronic devices. In particular, molecular beam epitaxy, magnetron sputtering and chemical vapour depositions are used for production of strained Si/SiGe/Si or Si/SiSn/Si structures. Ion implantation and hydrogen plasma treatment are used for injection of dopant atoms as well as point defects in the strained layers. The nano-voids and nanodots are subsequently investigated by transmission-electron microscopy (TEM), dopant depth profiling by Rutherford backscattering/channeling (RBS/Ch), secondary-ion mass spectroscopy (SIMS), and the optical properties by Raman scattering, reflectance/transmittance, and photocurrent measurements.
The following main results have been achieved during the NoVoSiP project:
 Through high-temperature ion irradiation of the Si/SiSn/Si structure, we obtained - to - phase transformation in the tin (Sn) precipitates and self-assembling of spherically shaped nanovoids and strained Sn precipitates. These structures hold great promise for optoelectronic applications such as light-emitting or PV devices.
 Segregation of carbon atoms and formation of carbon nano-flakes has been observed and studied in strained multilayered Si/SiGe/Si structures after MBE growth, carbon ion implantation, and thermal treatment. The flake crystalline structure allows the crystals to absorb all light, potentially allowing a very high energy conversion rate.
 Carbon related suppression of tin precipitation in supersaturated SiSn layers is found after carbon implantation and thermal treatment. The prospects for carbon assisted growth of unique heteroepitaxial SiSn/Si and GeSn/Si structures are argued.
 A new concept for self-assembling of metallic nano-shells and nano-particles in the strained Si/SiGe heterostructures is proposed and developed. The concept utilizes strain-induced formation of nano-voids followed by segregation and gettering of gold atoms. Optical measurements of the layers with void-related Au nano-shells show increase of reflectivity and absorption in the infrared spectral range of 800 – 1800 nm. The metallic nano-shells are tested to be effective in solar cell structures for demonstration of plasmonic effects.
 The concept of plasmonic based SnO2 sensors is developed. Gas sensing effect is demonstrated in the SnO2/Ag nano-composite layers with Kirkendall voids.
High efficiencies in addition to low costs are the main design challenges in fabricating solar cells. With experimental investigation of plasmonic structures in strained silicon multi-layers, NoVoSiP paved the way to enhancing light harvesting in solar cells.
Main results of the investigations are presented at the Project website: