PhD Portrait: Défi Junior Jubgang Fandio
In high school, Défi is not particularly good at science subjects such as physics and math. In fact, he is, in his own words, “average at everything”. However, knowing this, his parents advise him to pursue a degree in physics. Applying for his license (the bachelor’s equivalent) and his master’s degree at the University of Buea in Cameroon, Défi has to choose three subjects he would like to study. His choices: physics, mathematics and electronics. He is finally selected for his first choice, physics.
During his studies, Défi meets his department head, Alain Moise Dikandé, who himself has completed his PhD in physics at the Université de Sherbrooke, under the direction of Prof. Claude Bourbonnais. Prof. Dikandé encourages Défi to follow the same path as him to pursue his graduate studies. Défi approaches several professors in the Physics Department at UdeS, and Prof. Denis Morris agrees to integrate him into his team.
According to Prod. Dikandé, Défi is in the right field: “As the supervisor of Défi Junior’s master’s research, I remember him as a discreet, studious and trustworthy young man. Moreover, as a representative of the teachers in the Department of Physics at the University of Buea, I can say that we have always believed in his potential.”
His research project
Défi’s PhD project, conducted under the supervision of Prof. Denis Morris, focused on the study of electronic properties and charge dynamics in various semiconductor nanostructures using terahertz spectroscopy. With his research team, Défi focused on three families of innovative nanostructures: graphenized mesoporous silicon nanocomposites, silicon nanowires and InGaAs layers.
By studying graphenized mesoporous silicon nanocomposites developed by the teams of Prof. Richard Arès and Prof. Abderraouf Boucherif, Défi and his collaborators have demonstrated that the graphene coating inside these nanocomposites significantly reduces the lifetime of the phototransporters, while preserving the effective mobility of the charges in them. In other words, this graphene coating provides the nanocomposites with interesting properties for the fabrication of pulsed terahertz radiation emission devices.
With the second family of semiconductor nanostructures, silicon nanowires, Défi’s research team studied the impact of the nanowire substrate as well as the influence of doping on their electronic transport. Its major discovery shows that the lifetime of the phototransporters decreases with doping, but remains relatively high compared to the state of the art.
As for the third family, the InGaAs layers, Défi and his team studied them for photoswitching applications. Their goal was to find the best combination of good resistivity, relatively short lifetime and increased mobility of the photocarriers. The solution they propose lies in the partial recrystallization of the amorphized InGaAs layers by ion implantation.
“Our studies have shown that, for layers implanted and then annealed at temperatures below 500°C, we were able to obtain quite good resistivity, and sub-picosecond lifetimes of the photoportals, which is very interesting. But the major improvement is that we obtained an effective mobility of 2150 cm2/(V.s), which is relatively high for layers with disorder. I may be optimistic, but I think that these structures are already ready for the fabrication of photocommutators, devices that can generate terahertz pulses. I think this is the major contribution of our study on this last family of nanomaterials,” explains Défi.
As of today, Défi is appearing in two publications on graphenized mesoporous silicon nanocomposites. His work on the study of charge transport in InGaAs layers has also led to a scientific publication. Additional work on silicon nanowires is still to be done and may, at best, lead to a scientific publication.
Prof. Morris praises the quality and scope of the experimental work carried out by Défi Junior to complete this doctoral thesis. “His work could guide the development of certain processes for the fabrication of photoconductive materials, of interest for fast optoelectronics and nanoelectronics. The basic idea, which can be used here, is to take advantage of point defects or to minimize their influence on the transport of charges in these photoconductive materials. Défi has shown great rigor and perseverance in achieving the objectives of his research project and I am very proud of the work he has accomplished. The algorithms he has developed to extract the dielectric or conductive properties of various types of materials are and will continue to be extremely valuable tools for my team members.”
Défi has already started a postdoctoral fellowship in the group of Prof. Jean-Michel Ménard, at the University of Ottawa. He is working on a project on ultra-sensitive detection of terahertz radiation.
In the longer term, he plans to settle in Canada and start an academic career, possibly becoming a professor.
He would like to express his acknowledgements to his family, his colleagues, the professors of the Department of Physics, 3iT and IQ, as well as a special mention to his thesis supervisor Prof. Denis Morris, who has significantly marked his career and who is a source of inspiration for him. He attributes his success to his faith in Jesus Christ, which helped him persevere in his studies while maintaining a social balance.