Public defence in Micro- and nanosciences, M.Sc.(Tech.) Joonas Isometsä

The title of the thesis: Reducing optical and electrical losses in germanium via nanostructures and surface passivation

Doctoral student: Joonas Isometsä
Opponent: Prof. Bart Macco, Eindhoven University of Technology, The Netherlands
Custos: Prof. Hele Savin, Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering

Semiconductor devices and their fabrication methods have improved drastically over the years, and, for example, the state-of-the-art silicon photodetectors are able to capture almost every photon within the limitations of silicon. As a result, the base substrate material has become the primary limiting factor for these devices. Therefore, alternative materials with promising properties, such as germanium (Ge), have received an increasing amount of attention. Ge offers several distinct advantages over silicon, including higher carrier mobility and a narrower bandgap, making it an attractive substrate material for various optoelectronic applications, 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 challenge of high reflectivity of Ge surfaces. The performance of traditional antireflective coatings was considered inadequate, and hence, an alternative reflectance reduction approach of creating nanoscale structures on the surface was used. Two different nanostructuring 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 Ge with a reflectance below 1% across the 400–1600 nm wavelength range, whereas the MACE 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. 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. Consequently, the surface passivation process was combined with the abovementioned nanostructuring process, resulting in both effective surface passivation and reflectance of below 2%. These results provide a good basis for designing high-efficiency optoelectronic devices.

Keywords: germanium, black germanium, surface passivation, atomic layer deposition

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact:

Doctoral theses in the School of Electrical Engineering: https://aaltodoc.aalto.fi/handle/123456789/53

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