Reducing optical and electrical losses in germanium via nanostructures and surface passivation

Germanium (Ge) offers several distinct advantages over silicon, including higher carrier mobility and narrower bandgap, making it an attractive substrate material for various optoelectronic ap-plications, such as near-infrared detectors, multijunction solar cells, and thermophotovoltaics. However, the full potential of Ge has not been realized yet due to challenges related to optical and electrical losses, namely high reflectivity and high recombination at the surfaces. First, this dissertation addresses the high reflectivity of Ge surfaces by developing nanostructures, which could enable minimal reflectance over a wide spectral range resulting in visibly pitch-black Ge (b-Ge). b-Ge processes for two different fabrication methods, inductively-coupled plasma reactive-ion etching (ICP-RIE) and metal-assisted chemical etching (MACE), are developed. ICP-RIE is shown to be capable of producing b-Ge with a reflectance below 1% across 400–1600 nm wavelength range, whereas the MACE b-Ge reflectance remains somewhat higher at an average of 9%. Nevertheless, the MACE process provides some advantages over ICP-RIE, such as being considerably lower cost, making it equally competitive. In order to address the second main challenge, i.e. the high surface recombination, an ALD Al2O3-based surface passivation process is developed. ALD Al2O3 is demonstrated to provide efficient passivation for polished Ge surfaces, achieving a surface recombination velocity (SRV) of 6.55 cm/s. This is obtained with optimized process parameters such as an HCl pre-treatment and a post-anneal in 400 °C for 30 mins. After more detailed characterization, it is identified that the low SRV is based on a strong field-effect rather than good chemical passivation. In order to improve the chemical passivation, an ultra-high vacuum anneal as an ALD pre-treatment is employed. Consequently, the chemical passivation is improved but as a trade-off, the field-effect is reduced leading to a similar level of overall passivation. Finally, the above results are combined targeting a simultaneous reduction of reflective and recombination losses. In the case of ICP-RIE fabricated b-Ge, some chemical residues are observed to deteriorate the surface passivation. Cyclical HCl and H2O2-based cleaning process is seen to remove the residues but simultaneously leads to changes in surface morphology and reflectance. Hence a trade-off between reflectance and surface passivation is necessary. As an example, without deteriorating the reflectance too much (< 2%), an SRV of 30 cm/s is obtained. In the case of MACE, achieving efficient surface passivation seems to be more complicated. The obtained results in this thesis give a good basis for designing high-efficiency optoelectronic devices.