We investigate DC characteristics of a two-dimensional electron gas (2DEG) in an undoped Si/SiGe heterostructure and its temperature dependence. An insulated-gate field-effect transistor was fabricated, and transfer characteristics were measured at 4 K–300 K. At low temperatures (T < 45 K), source electrons are injected into the buried 2DEG channel first and drain current increases with the gate voltage. By increasing the gate voltage further, the current saturates followed by a negative transconductance observed, which can be attributed to electron tunneling from the buried channel to the surface channel. Finally, the drain current is saturated again at large gate biases due to parallel conduction of buried and surface channels. By increasing the temperature, an abrupt increase in threshold voltage is observed at T ∼ 45 K and it is speculated that negatively charged impurities at the Al2O3/Si interface are responsible for the threshold voltage shift. At T > 45 K, the current saturation and negative transconductance disappear and the device acts as a normal transistor.
A demonstration of 2D hole gases in GeSn/Ge heterostructures with a mobility as high as 20 000 cm2 V−1 s−1 is given. Both the Shubnikov–de Haas oscillations and integer quantum Hall effect are observed, indicating high sample quality. The Rashba spin‐orbit coupling (SOC) is investigated via magneto‐transport. Further, a transition from weak localization to weak anti‐localization is observed, which shows the tunability of the SOC strength by gating. The magneto‐transport data are fitted to the Hikami–Larkin–Nagaoka formula. The phase‐coherence and spin‐relaxation times, as well as spin‐splitting energy and Rashba coefficient of the k‐cubic term, are extracted. The analysis reveals that the effects of strain and confinement potential at a high fraction of Sn suppress the Rashba SOC caused by the GeSn/Ge heterostructures.
In the large full-thickness mouse skin regeneration model, wound-induced hair neogenesis (WIHN) occurs in the wound center. This implies a spatial regulation of hair regeneration. The role of mechanotransduction during tissue regeneration is poorly understood. Here, we created wounds with equal area but different shapes to understand if perturbing mechanical forces change the area and quantity of de novo hair regeneration. Atomic force microscopy of wound stiffness demonstrated a stiffness gradient across the wound with the wound center softer than the margin. Reducing mechanotransduction signals using FAK or myosin II inhibitors significantly increased WIHN and, conversely, enhancing these signals with an actin stabilizer reduced WIHN. Here, α-SMA was downregulated in FAK inhibitor-treated wounds and lowered wound stiffness. Wound center epithelial cells exhibited a spherical morphology relative to wound margin cells. Differential gene expression analysis of FAK inhibitor-treated wound RNAseq data showed that cytoskeleton-, integrin-, and matrix-associated genes were downregulated, while hair follicular neogenesis, cell proliferation, and cell signaling genes were upregulated. Immunohistochemistry staining showed that FAK inhibition increased pSTAT3 nuclear staining in the regenerative wound center, implying enhanced signaling for hair follicular neogenesis. These findings suggest that controlling wound stiffness modulates tissue regeneration encompassing epithelial competence, tissue patterning, and regeneration during wound healing.
distribution. Novel devices with different operational principles for a sharper SS than 60 mV per decade have been extensively investigated, such as impact ionization MOS (I-MOS) transistors, [1] negative-capacitance field-effect transistors (NC-FETs), [2] or tunnel field-effect transistors (TFETs). [3] While an SS of sub-60 mV per decade has been demonstrated for those devices, the issues of the required high gate biases for I-MOS transistors or the hysteresis for NC-FETs are critical issues for the reliability of integrated circuits. For TFETs, whose device structures are similar to conventional CMOS transistors, a steep SS is achieved by band-toband tunneling (BTBT) processes between the source and channel regions. The major issue for TFETs is their low drive current. For group-IV materials such as silicon (Si) or germanium (Ge), the device performance of TFETs is poor due to their large bandgap energies and effective masses of carriers, and the requirement of phonon participation for the momentum reservation in the BTBT process. [4,5] For III-V materials, while the tunneling current is high because of their direct-bandgap characteristics, [6,7] the poor quality of oxide interface could lead to the instability of device operations. Moreover, the compatibility with the Si-based VLSI technology is still a critical issue. [8] Recently, germanium-tin (GeSn) has attracted much attention for electronic, optoelectronic, and spintronic device applications owing to its direct-bandgap characteristics, [9] high carrier mobility, [10] strong spin-orbit coupling (SOC) effects, [11] and compatibility with the VLSI technology. Simulations on GeSn TFETs showed strong current enhancement than Si-or Gebased devices owing to the direct BTBT process and the smaller bandgap and effective carrier masses in GeSn. [12,13] To justify the BTBT process and calibrate the tunneling rates, Esaki diodes with negative differential resistance (NDR) are used to characterize the peak current density. Thus far, there is no NDR demonstrated in any GeSn-based Esaki diodes at room temperature. To observe NDR, the Fermi levels must be higher or lower than the conduction band minimum or the valence band maximum at the n-type or p-type regions, respectively. If the doping levels or the activation rates of dopants are not high enough, NDR will not be observed. Moreover, if there exists a large amount of defect states in the bandgap, electrons can tunnel across the junction via those states with the assistance of thermal processes, Tunnel field-effect transistors (TFETs) are a promising candidate for lowpower applications owing to their steep subthreshold swing of sub-60 mV per decade. For silicon-or germanium-based TFETs, the drive current is low due to the indirect band-to-band tunneling (BTBT) process. Direct-bandgap germanium-tin (GeSn) can boost the TFET performance since phonon participation is not required during the tunneling process. Esaki diodes with negative differential resistance (NDR) are used to characterize the BTBT properties and calibr...
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