Development of GaN HEMTs Fabricated on Silicon, Silicon-on-Insulator, and Engineered Substrates and the Heterogeneous Integration

GaN HEMT has attracted a lot of attention in recent years owing to its wide applications from the high-frequency power amplifier to the high voltage devices used in power electronic systems. Development of GaN HEMT on Si-based substrate is currently the main focus of the industry to reduce the cost as well as to integrate GaN with Si-based components. However, the direct growth of GaN on Si has the challenge of high defect density that compromises the performance, reliability, and yield. Defects are typically nucleated at the GaN/Si heterointerface due to both lattice and thermal mismatches between GaN and Si. In this article, we will review the current status of GaN on Si in terms of epitaxy and device performances in high frequency and high-power applications. Recently, different substrate structures including silicon-on-insulator (SOI) and engineered poly-AlN (QST®) are introduced to enhance the epitaxy quality by reducing the mismatches. We will discuss the development and potential benefit of these novel substrates. Moreover, SOI may provide a path to enable the integration of GaN with Si CMOS. Finally, the recent development of 3D hetero-integration technology to combine GaN technology and CMOS is also illustrated.

Abstract 14925: Mechanical Tension in Syndecan-1 is Regulated by Extracellular Mechanical Cues and Fluidic Shear Stress

Syndecan-1 (SDC1) is a transmembrane proteoglycan that mediates the shear stress-induced signaling and inflammatory phenotypes of endothelial cells. While SDC1 is known to be involved mechanically in regulating the behavior of cells, it remains unknown how SDC1 responds to extracellular mechanical cues on the molecular level. We designed a set of FRET-based SDC1 tension sensors including full-length SDC1 (SDC1TS), SDC1 with deleted ectodomain (ΔEcto), SDC1 with deleted glycosylation sites (ΔGAG) and SDC1 with a deleted cytoplasmic tail (ΔCyto). We transduced these constructs into ECs and validated the constructs by WB and confocal imaging. When cultured on glass-bottom coverslips, we found that the baseline tension in SDC1TS was significantly higher than that in Ecto and GAG mutants. When cultured on engineered substrates, we demonstrated that tension in SDC1 is modulated by micropatterned surfaces and nanotopographical cues. To study if SDC1 directly responds to substrate stiffness, transduced ECs were cultured on 0.2 kPa and 25 kPa substrates. We found that there was significantly decreased tension in the Ecto construct compared to the full-length SDC1TS construct on 0.2 kPa substrates but no significant changes were observed on the 25 kPa substrates. To examine the effect of substrate stiffness on the association of SDC1 with adhesion-related proteins, we performed IP and WB on ECs expressing HA-tagged SDC1 cultured on 0.2 kPa or 25 kPa substrates. We found there were increases in SDC1 binding to action and myosin IIb, while there were decreased in SDC1 binding to Src, PKA, and FAK in cells grown on stiff substrates versus on soft substrate. To study the effect of shear stress on the tension of SDC1, ECs expressing the constructs were cultured on static or under 12dyn/cm 2 fluidic shear stress. We found that a gradient developed in the tension of SDC1 in all four constructs. There was higher tension of SDC1 in the upstream region in comparison to the downstream region. IP and WB results showed that shear stress-induced SDC1 association with actin, Src, myosin IIb, cortactin, calmodulin, and integrin beta3. In summary, our results demonstrate that SDC1 is mechanically responsive to substrate mediated biophysical cues and shear stress.

Emergent collective organization of bone cells in complex curvature fields

Individual cells and multicellular systems have been shown to respond to cell-scale curvatures in their environments, guiding migration, orientation, and tissue formation. However, it remains unclear how cells collectively explore and pattern complex landscapes with curvature gradients across the Euclidean and non-Euclidean spectra, partly owing to fabrication limitations and the lack of formal geometric considerations. Here, we show that micro-engineered substrates with controlled curvature variations induce the collective spatiotemporal organization of preosteoblasts. By leveraging mathematical surface design and a high-resolution free-form fabrication process, we exposed cells to a broad yet controlled, heterogeneous spectrum of curvature fields. We quantified curvature-induced spatial patterning at different time points and found that cells generally prefer regions with at least one negative principal curvature. We also show that multicellular cooperation enables cells to venture into unfavourably-curved territories, bridging large portions of the substrates, and collectively aligning their stress fibres. We demonstrate that this behaviour is partly regulated by cellular contractility and extracellular matrix development, underscoring the mechanical nature of curvature guidance. Our findings offer unifying perspectives on cell-geometry interactions that could be harnessed in the design of micro-engineered biomaterials, for example, for tissue engineering applications.

Star-Shaped Fe3-xO4-Au Core-Shell Nanoparticles: From Synthesis to SERS Application

In this work, the preparation of magneto-plasmonic granular nanostructures and their evaluation as efficient substrates for magnetically assisted surface enhanced Raman spectroscopy (SERS) sensing are discussed. These nanostructures consist of star-shaped gold Au shell grown on iron oxide Fe3-xO4 multicores. They were prepared by seed-mediated growth of anisotropic, in shape gold nanosatellites attached to the surface of polyol-made iron oxide polycrystals. In practice, the 180 nm-sized spherical iron oxide particles were functionalized by (3-aminopropyl) triethoxysilane (APTES) to become positively charged and to interact, in solution, with negatively charged 2 nm-sized Au single crystals, leading to nanohybrids. These hybrids acted subsequently as nucleation platforms for the growth of a branched gold shell, when they were contacted to a fresh HAuCl4 gold salt aqueous solution, in the presence of hydroquinone, a reducing agent, for an optimized nominal weight ratio between both the starting hybrids and the gold salt. As expected, the resulting nanocomposites exhibit a high saturation magnetization at room temperature and a rough enough plasmonic surface, making them easily attracted by a lab. magnet, while exhibiting a great number of SERS hot spots. Preliminary SERS detection assays were successfully performed on diluted aqueous thiram solution (10−8 M), using these engineered substrates, highlighting their capability to be used as chemical trace sensors.