PhD defense: Gunjan Piyush Nagda

Planar semiconductor nanowires on high-index substrates

Engineering materials especially at the nanoscale plays an important role in the development of quantum technologies. To that effect, in-plane selective area growth (SAG) of III-V nanowires (NWs) is a scalable and versatile materials platform that holds promise for nanoelectronics and photonics applications. The potential of SAG as a scalable platform for large-scale quantum circuits depends, however, on the structural and electrical reproducibility of SAG structures. 

This thesis presents the study of SAG of GaAs and InAs/GaAs NW arrays using molecular beam epitaxy (MBE). For GaAs homo-epitaxy, the effect of in-plane misalignment as a result of the tolerances associated with substrate fabrication is studied via atomic force microscopy (AFM). Further, the possibility of engineering the cross-sectional shapes of GaAs NWs by choice of substrate in-plane and out-of-plane orientations is presented. Novel high-index substrates such as (211) and (311) have been used which open the possibility to obtain NWs with desired combinations of facets.  

Finally, the structural variability of InAs/GaAs NWs grown on (311)A substrates across very large arrays has been quantified with statistical significance. The NW morphologies obtained via AFM are supplemented with electron microscopy methods to analyze the crystal quality. The observed structural reproducibility is substantiated by electrical transport measurements on NW based field effect transistors in a large-scale multiplexer/de-multiplexer set-up operating at cryogenic temperatures. Using this set-up, NWs with varying SAG mask dimensions are studied and the resulting transport characteristics are found to be directly correlated with the structural and crystal quality of the NWs.