GaAs(N)Bi alloys have been proposed to be used in devices emitting or absorbing light in fields such as telecommunications and solar energy. Their various characteristics could decrease the cost and improve the efficiency of these technologies. One of the major areas of application are multijunction solar cells, where each subcell is used to absorb a different wavelength region of sunlight. Although the efficiency of multijunction solar cells in solar energy conversion is currently the highest of all, it is still far from the theoretical maximum. Increasing the efficiency above 50 % requires the development of materials that are structurally compatible, high quality and can absorb light at the suitable wavelengths.
While GaAs(N)Bi alloys can theoretically fulfill these goals, their performance is currently limited due to challenges related to the fabrication and optimization of the material quality. This material is especially sensitive to variations in the fabrication parameters of the molecular beam epitaxy (MBE) technology.
“Optimization of the MBE fabrication process is important in the research of many optoelectronic materials. This is emphasized for complex materials with many different elements. It is essential to investigate novel ways to improve the fabrication and characterization processes and to take into account the specific challenges related to the material that is being studied”, says Puustinen.
To address these issues, he has investigated the use of a special type of MBE fabrication process. This method drastically increased the amount of useful data from a single sample and eliminated the effect of unintentional variations in the fabrication conditions. To ensure the efficient analysis of large amounts of research data, he also developed Python-based computational tools.
In his research, Puustinen observed dramatic changes in the properties of GaAs(N)Bi alloys for very small changes in fabrication parameters. Collaboration with international research groups led to the discovery of various novel material properties providing insight on the formation and unique properties of these alloys. These properties can be useful in various novel applications. The application potential was investigated in practice by using the results of the research to develop solar cells achieving a band gap energy of 0.86 eV, demonstrating the applicability of this material for multijunction solar cells.
Public defence on Friday 24 November
The doctoral dissertation of MSc (Tech) Janne Puustinen in the field of semiconductor technology titled Epitaxy and Properties of GaAs(N)Bi Alloys will be publicly examined at the Faculty of Engineering and Natural Sciences at Tampere University at 12:00 on Friday 24th November at Hervanta Campus, Tietotalo, Auditorium TB109 (Korkeakoulunkatu 1, Tampere). The Opponent will be Prof. Thomas Tiedje from the University of Victoria, Canada. The custos will be Prof. Mircea Guina from the Optoelectronics Research Centre, Faculty of Engineering and Natural Sciences, Tampere University.