Inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4–6 × 1013 protons/cm2. We also characterize the specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%–55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (VGa-related), carbon impurities (C-related), and gallium interstitials (Gai). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ∼500 nm, which suggests mobile Gai. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Gai.

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Gallium nitride is a promising wide bandgap material for demanding applications such as hightemperature and high-power electronics, as well as for space applications where its higher resistance to high fluxes of proton and electron radiation compared to Si represents an important advantage [1]. Electron irradiation of GaN is believed to create both N and Ga vacancies, as well as to induce threading dislocation glide. Typically, the creation or modification of structural defects in GaN are studied in a scanning electron microscope (SEM) retrofitted with cathodoluminescence (CL) or electron beam induced current (EBIC) tools, which identify locations of high defect concentration as dark regions due to increased (non-radiative) recombination rates. Unfortunately, the spatial resolution of these methods is limited due to the relatively large interaction volumes generated by primary electrons. While scanning transmission electron microscopy (STEM) enables the requisite spatial resolution necessary to understand the mechanisms of how defects are created and/or propagated under electron beam irradiation, it is still rarely combined with CL or EBIC, which can identify electronically active defects. Here we report the first STEM based EBIC characterization of Schottky diodes consisting of Ni contacts to free-standing hydride vapor phase epitaxy (HVPE) grown GaN with a threading dislocation density of ~10 5 /cm 2 .Our sample consists of a high purity single crystal n-type GaN substrate (275 µm thick) grown by HVPE with a nickel Schottky contact as well as an indium ohmic contact. Specimen preparation for this sample was unconventional because we did not thin the sample as is typically done in TEM sample preparation. As can be seen from Figure 1a), the Schottky contact remains intact after mounting the sample to the Nanofactory holder. The Ni contact can be seen in both the optical micrograph as well as the energy dispersive x-ray spectroscopy (EDS) map in Figure 1b) and c). The bright field STEM image in the inset of Figure 2a) shows the outline of the GaN and Au wire but does not show any features, as expected due to the specimen thickness. However, the EBIC image only shows areas where charge carriers are measurably excited and separated, i.e. the Ni pad, Ag epoxy, and Au wire. Furthermore, there is no difference in the EBIC signal on or off the GaN, which indicates this is a true EBIC signal and not just a specimen current from e.g. secondary electrons.In order to quantitatively interpret EBIC contrast, consideration of the beam/sample interaction is especially critical. During EBIC, the high energy electron beam excites electron-hole pairs in the material and, in the presence of an electric field, these charge carriers will separate and generate a current detectable with an ammeter [2]. In STEM, the interaction volume of the beam with a thick sample can be extremely large and thus decrease the resolution of the signal. We have begun Monte Carlo simulations to better understand our specific samples and plan to thin the ...

BackgroundInflammatory tumor micro-environments contain cells of various types and sub-types. The composition and spatial location of the cell populations reflects the host reaction to the inflammatory stimulus and increasingly is understood to influence responsiveness to tumor immunotherapies. Multiplexed imaging technologies allow identification of cell types and states within the spatial context of tissue architecture. We present here a prototype workflow that combines rapid high-resolution, whole-slide highly multiplexed immunofluorescence imaging with advanced image analysis tools for 1) segmenting tissues, cells, and quantifying cellular phenotypes based on multiple markers and 2) determining regional densities and proximity between cells. We apply the workflow to comparative assessment of three lymphoid tissues: tonsil (follicular hyperplasia); lymph node (quiescence); lymphoma (architectural effacement).MethodsFormalin-fixed, paraffin-embedded 5 micron sections of tonsil, lymph node and chronic lymphocytic leukemia/small lymphocytic lymphoma were deparaffinized, subjected to alkaline pH epitope retrieval, and then manually stained with a 17-plex panel including CD45 (leukocytes); CD20 (B cells); CD3d, CD4, CD8 (T cells); FOXP3 (T reg cells); CD68, CD163 (macrophages); CD45RO (activated cells); PD-L1, PD-1 (checkpoint markers); CD31 (vascular and lymphatic endothelial cells); cytokeratin, E-cadherin (epithelial cells); PCNA, Ki-67 (proliferating cells); and a nuclear dye. Stained slides were coverslipped and imaged on the Orion Instrument (RareCyte) generating .ome tiff image files. The HighPlex FL module of the HALO image analysis platform from Indica Labs with embedded HALO AI performed nuclear and cell segmentation, nuclear phenotyping, and user-defined thresholds were applied to each of the biomarkers to define positivity for the appropriate subcellular localization (nuclear, cytoplasmic, and/or membrane) for phenotypic analysis. H & E images from either the same or serial sections were integrated with the multiplex images using the HALO Serial Stain module.ResultsRegional masks that were defined by predominance of B-cells (CD20) or T-cells (CD3d) matched known lymphoid micro-anatomy of follicles and inter-follicular cortex respectively. Within the regions, populations and sub-populations of B-cells, T-cells, macrophages and vessels were measured, and their densities calculated and compared between tissues. Rare cell types of potential importance in immuno-oncology were investigated. The results demonstrate differences between the tissues at a phenotypic level that correspond to the morphologic differences seen by light microscopy.ConclusionsOrion imaging combined with HALO image analysis provides a powerful and intuitive workflow for visualization and quantification of distinct microenvironment populations for use in translational and clinical research.