We fabricate ultrathin HfO2 gate stacks of very high permittivity by atomic layer deposition (ALD) and oxygen-controlled cap post-deposition annealing. The HfO2 layer is directly deposited on a wettability-controlled Si surface by ALD. To enhance permittivity, a cubic crystallographic phase is generated in ALD-HfO2 by short-time annealing with a Ti capping layer. The Ti layer absorbs residual oxygen in the HfO2 layer, which suppresses the growth of the interfacial SiO2 layer. The dielectric constant of ALD-HfO2 is increased to ∼40, and a gate stack of extremely scaled equivalent oxide thickness (∼0.2 nm) is obtained.
We fabricate p- and n-channel Si tunnel field-effect transistors (TFETs) with an epitaxially grown tunnel junction. In a novel source/drain-first and tunnel-junction-last fabrication process, a thin epitaxial undoped Si channel (epichannel) is deposited on a preferentially fabricated p- or n-type source area. The epichannel sandwiched by a gate insulator and a highly doped source well acts as a parallel-plate tunnel capacitor, which effectively multiplies drain current with an enlarged tunnel area. On the basis of its simple structure and easy fabrication, symmetric n- and p-transistor and complementary metal oxide semiconductor inverter operations were successfully demonstrated.
In this study, we successfully introduced an atomic-layer-deposited (ALD) titanium nitride (TiN) gate grown with a tetrakis(dimethylamino)titanium (TDMAT) precursor into fin-type metal–oxide–semiconductor field-effect transistor (FinFET) fabrication for the first time, and comparatively investigated the electrical characteristics, including mobility and threshold voltage (Vth) variation, of the fabricated ALD and physical-vapor-deposited (PVD)-TiN gate FinFETs. The ALD-TiN gate FinFETs showed superior conformality to the PVD-TiN gate FinFETs. The electron mobilities of the ALD- and PVD-TiN gate FinFETs were comparable in the small Lg region. It was also confirmed that the ALD-TiN gate FinFETs showed a smaller Vth variation than the PVD-TiN gate FinFETs.
Placing a sensor close to the target at the nano-level is a central challenge in quantum sensing. We demonstrate high-spatial-resolution magnetic field imaging with a boron vacancy (V$_\text{B}^-$) defects array in hexagonal boron nitride with a few 10 nm thickness. V$_\text{B}^-$ sensor spots with a size of (100 nm)$^2$ are arranged periodically with nanoscale precision using a helium ion microscope and attached tightly to a gold wire. The sensor array allows us to visualize the magnetic field induced by the current in the wire with a spatial resolution beyond the diffraction limit. Each sensor exhibits a practical sensitivity of $73.6~μ\text{T/Hz}^{0.5}$, suitable for quantum materials research. Our technique of arranging V$_\text{B}^-$ quantum sensors periodically and tightly on measurement targets will maximize their potential.
Electrical performances of ultra-short channel MOSFETs were investigated on SOI substrates. The channel length was scaled to 3 nm using the anisotropic wet etching technique. A difficulty of junction technology was solved by fabrication of Junctionless-FET, which consists of uniform high concentration dopants through the body of device. Superior Junctionless-FET performance was confirmed when the channel thickness and gate dielectric film thickness were scaled close to 1 nm. Experimental and simulation studies suggest that variation of performance originates from atomic-scale fluctuation in device structures.
The performance of parallel electric field tunnel field-effect transistors (TFETs), in which band-to-band tunneling (BTBT) was initiated in-line to the gate electric field, was evaluated. The TFET was fabricated by inserting a parallel-plate tunnel capacitor between heavily doped source wells and gate insulators. Analysis using a distributed-element circuit model indicated there should be a limit of the drain current caused by the self-voltage-drop effect in the ultrathin channel layer. We also propose a scheme to improve the performance of the TFETs by modification of the gate and channel configurations.
We have fabricated Si-on-insulator (SOI) layers with a thickness h1 of a few nanometers and examined them by Raman spectroscopy with 363.8 nm excitation. We have found that phonon and electron confinement play important roles in SOI with h1 < 10 nm. We have confirmed that the first-order longitudinal optical phonon Raman band displays size-induced major homogeneous broadening due to phonon lifetime reduction as well as minor inhomogeneous broadening due to wave vector relaxation (WVR), both kinds of broadening being independent of temperature. Due to WVR, transverse acoustic (TA) phonons become Raman-active and give rise to a broad band in the range of 100–200 cm−1. Another broad band appeared at 200–400 cm−1 in the spectrum of SOI is attributed to the superposition of 1st order Raman scattering on longitudinal acoustic phonons and 2nd order scattering on TA phonons. Suppression of resonance-assisted 2-nd order Raman bands in SOI spectra is explained by the electron-confinement-induced direct band gap enlargement compared to bulk Si, which is confirmed by SOI reflection spectra.
Short-channel effects of tunnel field-effect transistors (FETs) are investigated in detail using simulations of a nonlocal band-to-band tunneling model. Discussion is limited to silicon. Several simulation scenarios were considered to address different effects, such as source overlap and drain offset effects. Adopting the drain offset to suppress the drain leakage current suppressed the short channel effects. The physical mechanism underlying the short-channel behavior of the tunnel FETs (TFETs) was very different from that of metal–oxide–semiconductor FETs (MOSFETs). The minimal gate lengths that do not lose on-state current by one order are shown to be 3 nm for single-gate structures and 2 nm for double gate structures, as determined from the drain offset structure.
Evaluation of lubrication in articular cartilage has been developed with various techniques in order to obtain reasonable explanations about this superb mechanism. However, a definitive theory about this mechanism of lubrication is not clear until present time. One of the most relevant restrictions for the comprehension of this mechanism is the difficulty to detect and to evaluate the surface layer of articular cartilage. Acquisition of images from the conventional techniques are not sufficient for the analysis of the surface layer conditions because the most of methods require the dehydration of specimens which change their physiological state. In addition, methods that allow the evaluation of articular cartilage in the aqueous environment do not provide sufficient and clear data for a definitive evaluation. In this work, we present the application of Surface Plasmon Resonance (SPR) principle in the evaluation of articular cartilage surface. In our apparatus, it is possible to acquire images of articular cartilage surface during the lubrication experiments and in the aqueous environment. Variations in the reflectance were used in order to identify the substances present in the surface layer, since the intensity of light depends on the chemical characteristics of the substances located on the articular cartilage surface. The correlation of friction behavior with the possible substances present in the surface layer of articular cartilage can contribute to clarify the mechanism of lubrication of articular cartilage.
The molecular arrangement of copper phthalocyanine (CuPc) crystals on hydrogen-terminated Si(111) surfaces was investigated by using both scanning probe microscopy and x-ray diffraction in terms of influence of the surface roughness. On a rough surface with a root-mean-square roughness of 0.20 nm, the molecules were stacked so as to form an α crystal where the molecular column was parallel to the surface. On the other hand, a new crystal form with its column exactly perpendicular to the Si(111) plane was grown on a atomically flat surface. In this case, the molecules were stacked perpendicular to the substrate with the underlying molecules situated directly below. These molecular arrangements were independent of the growth temperature in the range of 60–180 °C. On the atomically flat surfaces, the strong interactive force between the surface and the planar CuPc molecule may result in the new growth behavior.
