Public defence in Advanced Materials and Photonics, M.Sc.(Tech.) Antti Myllynen

The title of the thesis: Diffusion-Driven Charge Transport in III-V Optoelectronic Devices 

Doctoral student: Antti Myllynen
Opponent: Prof. Magnus Borgström, Lund University, Sweden
Custos: Prof. Markku Sopanen, Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering

Optoelectronics that seamlessly convert electrical energy to optical energy and vice versa have significantly impacted our everyday lives through, e.g., the ubiquitous light-emitting diodes (LEDs) and increasingly important solar cells. While these technologies have seen recent advancements, their core design principles have remained static: the active region (AR), where energy conversion mainly occurs, is placed between n- and p-doped high bandgap materials. This has led to limitations such as resistive losses, poor current spreading, and contact shading. 

This doctoral thesis explores a new approach based on diffusion-driven charge transport (DDCT) that can allow a paradigm shift in the design of optoelectronic devices based on III–V compound semiconductor materials. These DDCT devices utilize diffusion currents that remove the need for the AR to be placed between n- and p-doped materials. This thesis focuses on two things: (1) identifying the advantages, requirements, and limitations of gallium arsenide (GaAs) based DDCT structures with device simulations and (2) developing a fabrication process for device demonstration. 

The simulation results suggest that GaAs-based DDCT structures can enable efficient, nearly resistance-free back-contacted LEDs with fully exposed front surfaces, optimizing light extraction. Furthermore, the results show how the reciprocal nature of the structure allows efficient GaAs solar cells resembling the interdigitated back-contact (IBC) design that has previously provided high-efficiency silicon solar cells. Laterally-doped DDCT devices were successfully demonstrated with a selective-area diffusion doping method utilizing redistribution of dopant atoms incorporated in the device structure at high temperatures. Demonstrated devices exhibit promising current-voltage and optical characteristics, showing clear evidence of lateral current spreading that is an indication of DDCT. 

Overall, DDCT holds the promise to enable highly efficient large-area devices appealing to several applications and technological areas The results of this doctoral thesis mark a significant step forward toward the next generation of DDCT devices, provide a new understanding of the internal processes of laterally-doped optoelectronic devices, and contribute to the continuing advancement of solid-state physics.

Keywords: III–V, light-emitting diodes, solar cells, numerical simulations, lateral doping

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

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