Daya Sagar Dhungana – STAR Fellow

Recently, I completed my PhD from LAAS-CNRS Toulouse, France focusing on the growth of InAs and Bi1-xSbx nanowires growth on silicon by Molecular Beam Epitaxy (MBE) at LAAS- CNRS, Toulouse, France. Prior to that, I hold an engineering and a master degree respectively in electronics and nanoscale engineering. So far, during the 5 years of stay in France for Master an PhD, I had the opportunity of 4 years of research in 3 different CNRS (National Centre for Scientific Research) labs. The start was with a four months internship in the city of Lyon in the field of electron microscopy, followed by a 6 months master thesis with microfluidics group in the same city respectively in the first and second year of my master degree. At the end of my master thesis, I was offered a PhD from LAAS-CNRS in the field of high electron mobility III-V nanowires. The PhD focused on developing new materials for our future transistors: vertical InAs nanowires on Si for high mobility 3D transistors and Bi1-xSbx nanostructures (nanowires, nanoflakes, and nanoribbons) for topological qubits.

My thesis contributes to the development of new surface treatments and the demonstration of full CMOS compatible InAs nanowires on Si(111) by MBE for the first time. The idea behind the full CMOS compatibility was the addition of a hydrogen treatment (gas & plasma) in the degassing system so that the degassing of HF-treated Si(111) wafers takes place under H2 environment before the growth. The InAs nanowires thus obtained have an extremely high aspect ratio (3 microns in length and 50 nm in length for 1-hour growth period), and even a transition from Vapor-Solid (VS) to Vapor-Solid-Liquid (VLS) growth mode was observed; something which is always difficult to achieve. The growth on unpatterned wafers was adapted successfully to the patterned Si wafers too where 85% of vertical yield was constantly achieved for holes larger than 100 nm. Similarly, the high vertical yield was repeated for InAsSb nanowires too. In addition, I developed an automated image and data processing software using two open source software: ImageJ and R, to facilitate the statistical analysis of these nanowires. The second aspect of the PhD was the integration of defect-free Bi1-xSbx nanostructures on Si(001), Si (111) and Si(110) substrates with controlled Sb composition in the range of 3D topological insulators (0.07<x<0.24). This was the first demonstration of the nanostructures of first experimentally confirmed 3D topological insulator. Furthermore, I presented effects of Bi flux, Sb- flux and growth temperature as well as the growth on the nanostructures and the Sb composition. Furthermore, and to my surprise, the work on Bi1-xSbx nanowires was funded as part of PIRE: HYBRID program ( and a new PhD student is continuing my work. During the three years of my PhD, I supervised 3 students from three different countries. Similarly, I presented my work in various national and international conferences; MRS-Fall Boston 2017 remains the most memorable one.