Electron Beam Induced Current in Wide Bandgap Semiconductors using Scanning Transmission Electron Microscopy

Wide bandgap semiconductors are those with a larger bandgap than silicon; this property allows them to operate at higher voltages, higher driving frequencies, and higher operating temperatures. Gallium nitride (GaN) in particular is attractive for its high critical electric field and thus high breakdown strength allowing for the design of a thinner drift region for a given blocking voltage. It is for these same reasons that GaN is also more radiation resistant than Si, and thus is attractive for satellite or space applications. With the recent commercial availability of free standing GaN substrates, there are many fundamental properties of GaN-on-GaN devices that are still not understood. One of the main characterization techniques used to classify GaN device quality is the measurement of the minority carrier diffusion length via electron beam induced current (EBIC). One of the main limitations of the traditional scanning electron microscopy (SEM) EBIC technique is due to the size of the electron beam/specimen interaction volume at > 5 kV, as well as large collection losses due to carrier recombination at the surface at < 5 kV. This dissertation addresses the previous issues of SEM EBIC with a non-traditional bulk scanning transmission electron microscopy (STEM) EBIC technique which allows for high resolution measurements of the hole diffusion length in n-GaN/Ni Schottky diodes. A reproducible, non-invasive bulk STEM sample preparation technique for n-GaN/Ni Schottky diodes is developed for the use of collecting bulk STEM EBIC micrographs. Despite the large interaction volume in this system at 100-200 kV, quantitative bulk STEM EBIC imaging is possible due to the small STEM probe beam diameter and sustained collimation of the incident electron beam in the sample. Using a combination of experimental bulk STEM EBIC measurements, Monte Carlo simulations, and numerical simulations, a hole diffusion length of 250 ± 15 nm was determined for homoepitaxial n-GaN samples with a threading dislocation of approximately 10^6 cm^-2. In-situ reverse biasing measurements allowed for the measurement of depletion region growth with increasing bias. Furthermore, accumulated electron irradiation damage was studied at 200 kV. An accumulated dose of 24 x 10^16 electrons cm^-2 caused a 35% reduction in the minority carrier diffusion length which is attributed to knock-on damage of the N sublattice. Additionally, the design and development of a custom STEM holder for in-situ liquid cell electrochemical microscopy is discussed

Simultaneous Solid Electrolyte Deposition and Cathode Lithiation for Thin Film Batteries and Lithium Iontronic Devices

We show that the deposition of the solid-state electrolyte LiPON onto films of V2O5 leads to their uniform lithiation of up to 2.2 Li per V2O5, without affecting the Li concentration in the LiPON and its ionic conductivity. Our results indicate that Li incorporation occurs during LiPON deposition, in contrast to earlier mechanisms proposed to explain postdeposition Li transfer between LiPON and LiCoO2. We use our discovery to demonstrate symmetric thin film batteries with a capacity of >270 mAh/g, at a rate of 20C, and 1600 cycles with only 8.4% loss in capacity. We also show how autolithiation can simplify fabrication of Li iontronic transistors attractive for emerging neuromorphic computing applications. Our discovery that LiPON deposition results in autolithiation of the underlying insertion oxide has the potential to substantially simplify and enhance the fabrication process for thin film solid state Li ion batteries and emerging lithium iontronic neuromorphic computing devices