# Masters defense: Alec Paul Romagosa

Title: Ballistic Electron Emission Microscopy in a 4-Probe STM System - Standardizing Techniques and Analysis to Explore its Potential for Buried Interface Characterization

Abstract:

This project standardizes the process of ballistic electron emission microscopy (BEEM) using a 4-probe STM system for multiple probe transport measurement on advanced nanostructures manufactured \textit{in-situ} in the attached ultra-high vacuum molecular beam epitaxy (UHV-MBE) system. Specifically, this provides a path to developing a system for producing the semiconductor-superconductor hybrid nanowires expected to host the computationally valuable phenomena known as Majorana zero modes (MZMs), and to this end, a wide variety of systems and statistical methods were tested.    The first sample attempted consisted of an Au thin film grown on a GaAs(100) substrate, with ballistically transported current measured through the backside plate used for staging the sample. Though this sample did not yield the response expected of the Schottky barrier which typically forms at Au/GaAs interfaces, it provided the basis for a further multiple-probe technique using an additional probe connected to a circular Au/Ti/GaAs deposition region, with this extra Ti layer creating an ohmic contact with a resistor-like I-V curve.   To optimize Schottky barrier height extraction and precisely measure spatial shifts, various models and optimization techniques were investigated and compared to literature values. One such model, which gave a highly precise fit in tests, was used to extract Schottky barrier heights and associated goodness-of-fit statistics over a $61 \times 61$ point scan, to be compared with an analogous 3600 nm$^2$ area on a topographic STM scan. As a final test, BEEM measurement was performed on a confined 2-dimensional electron gas (2DEG) system, composed of an $\text{In}_{0.8} \text{Al}_{0.2} \text{As}/\text{InAs}$ thin-film heterostructure with a patterned Al deposition acting as the BEEM contact layer. The 2DEG confinement layer, was composed on InAs in an attempt to analyze the barrier height of the wide-gap semiconducting alloy surface layer. The result was next compared to simulations of similar systems using self-consistent Schr\"odinger-Poisson simulation and $\textbf{k} \cdot \textbf{p}$ perturbation theory, though no barrier height was found.

Zoom:

https://ucph-ku.zoom.us/j/65098640587?pwd=YUZaR2dSTmlWZ1hZQi9PbytJajAzUT09

Meeting ID: 650 9864 0587

Passcode: 318463