Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors

In this study, we report on the formation of a single-crystalline Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a supercritical fluid-liquid-solid (SFLS) synthesized Ge nanowire and Ni metal contacts. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni(2)Ge by a thermal diffusion process. The maximum current density of the fully germanide Ni(2)Ge nanowires exceeds 3.5 × 10(7) A cm(-2), and the resistivity is about 88 μΩ cm. The in situ reaction examined by TEM shows atomically sharp interfaces for the Ni(2)Ge/Ge/Ni(2)Ge heterostructure. The interface epitaxial relationships are determined to be [Formula: see text] and [Formula: see text]. Back-gate field effect transistors (FETs) were also fabricated using this low resistivity Ni(2)Ge as source/drain contacts. Electrical measurements show a good p-type FET behavior with an on/off ratio over 10(3) and a one order of magnitude improvement in hole mobility from that of SFLS-synthesized Ge nanowire.     

Ultrashort channel silicon nanowire transistors with nickel silicide source/drain contacts

We demonstrate the shortest transistor channel length (17 nm) fabricated on a vapor-liquid-solid (VLS) grown silicon nanowire (NW) by a controlled reaction with Ni leads on an in situ transmission electron microscope (TEM) heating stage at a moderate temperature of 400 °C. NiSi(2) is the leading phase, and the silicide-silicon interface is an atomically sharp type-A interface. At such channel lengths, high maximum on-currents of 890 (μA/μm) and a maximum transconductance of 430 (μS/μm) were obtained, which pushes forward the performance of bottom-up Si NW Schottky barrier field-effect transistors (SB-FETs). Through accurate control over the silicidation reaction, we provide a systematic study of channel length dependent carrier transport in a large number of SB-FETs with channel lengths in the range of 17 nm to 3.6 μm. Our device results corroborate with our transport simulations and reveal a characteristic type of short channel effects in SB-FETs, both in on- and off-state, which is different from that in conventional MOSFETs, and that limits transport parameter extraction from SB-FETs using conventional field-effect transconductance measurements.     

Analysis of effective channel length variation for thin-film transistors with edge waviness in source/drain electrodes

We analyzed the effective channel length variation of hydrogenated amorphous silicon thin-film transistors (TFTs) that have wavy edge source/drain (S/D) electrodes. Edge waviness is frequently observed when narrow electrodes are fabricated by using printing methods. We used hydrogenated amorphous silicon (a-Si:H) TFTs and photolithographically patterned wavy edge S/D electrodes for accurate analysis. From a transmission line method (TLM), we successfully related the channel current variation to the variation of current transfer length (L_{T_wavy}) of the wavy edge S/D electrodes originated from current spreading and geometrical edge waviness effects which can be separately extracted.     

Formation of polycrystalline thin-film transistors with stacked poly-SiGe/poly-Si channel layer for low-voltage applications

A thin-film transistor (TFT) with polycrystalline SiGe/Si stacked channel layer has been proposed for low-voltage applications. For the stacked poly-SiGe/poly-Si channel layer, the resultant 1-μm TFT device can achieve an on/off current ratio above 7 orders and a relatively large on-state current at a low operating voltage, and also cause better transfer characteristics than both the conventional poly-Si and poly-SiGe channel layers. As compared to the poly-Si channel layer, the poly-SiGe channel layer may cause a larger on-state current at a small gate bias of 3 V, due to smaller difference between conduction band and intrinsic level. However, even at a small drain bias of 3 V, the poly-SiGe channel layer leads to an off-state leakage current of about 2 order larger than the poly-Si channel layer, since a smaller energy bandgap may cause more carrier field emission via trap states. As a result, when a poly-SiGe/poly-Si stacked channel layer is employed, the leakage current may be suppressed to a low level as that for the poly-Si channel layer, and the resultant on-state current at a low gate bias voltage can be close to a relatively high level as that for the poly-SiGe channel layer.     

Oxynitridation of Si(100) surface with thermally excited N2O gas

Silicon oxynitride films have been grown with thermally excited N₂O gas, which has a low toxicity in comparison with other oxynitridation agents. Dependences of reaction rates on excitation temperature and substrate temperature have been investigated by Auger electron and photoelectron spectroscopies. These results show that the thermal excitation of N₂O obviously promotes the oxynitridation of the silicon surface, especially the oxidation reaction. At higher substrate temperatures, the nitridation of the silicon surface increases and the oxidation is reduced. By mass analysis of the residual gas in the reaction chamber, it was also found that the thermal excitation of N₂O causes N₂O to be decomposed into N₂ and O. This is consistent with the obtained effect that the thermal excitation of N₂O promotes especially the oxidation reaction, because atomic oxygen (O) acts as a strong oxidant.     

The relationship between the reliability of transistors with 2D AlGaN/GaN channel and organization type of nanomaterial

The first experimental results demonstrating that the carrier mobility in the AlGaN/GaN 2D channel of transistor structures (AlGaN/GaN-HEMT) is correlated with the manner in which the nanomaterial is organized and also with the operation reliability of transistor parameters are presented. It is shown that improving the nature of organization of the nanomaterials in AlGaN/GaN-HEMT structures, evaluated by the multifractal parameter characterizing the extent to which a nanomaterial is disordered (local symmetry breaking) is accompanied by a significant, several-fold increase in the electron mobility in the 2D channel and in the reliability of parameters of transistors fabricated from these structures.     

Deep void formation mechanism in Si(100) during its carbonization reaction with C2H2

Deep silicon voids, located several hundreds of nm below the Si-SiC interface, were observed in Si(100) crystal after its reaction with C₂H₂ at 1200 °C. Interface voids are commonly observed during the reaction between Si and carbohydrides, but the proposed mechanisms for the interface voids cannot explain the appearance of the deep Si voids. Based on the experimental results, a formation mechanism for the deep voids is proposed in this letter. The proposed mechanism identifies carbon diffusion into the Si crystal as the initial cause and makes a relationship to the fast cooling during the employed rapid-thermal processing.     

Structural roughness and interface strain properties in Si/SiO2/Poly-Si1−x Ge x tri-layer system with ultrathin oxide

We have explored the microstructure and local interface strain in the poly-Si_1-xGe_x/SiO₂/Si tri-layer system with ultrathin oxides. High-resolution transmission electron microscopy (HRTEM) and high-resolution X-ray diffraction rocking curves (HR-RC) and two-dimensional reciprocal space mapping (2D-RSM) were the main characterization tools. The poly-Si_1-xGe_x/SiO₂/Si structures have x=0, 0.2, and 0.35 for ultrathin oxides (2.0–3.0 nm). The result shows that for the adopted growth process, the poly grain size depends very strongly on the Ge concentration, and it increases with increasing Ge mole fraction. In turn, this increase of the grain size in the poly-Si_1-xGe_x/SiO₂/Si reduces the strain in the film, which then affects the interface strain at the lower SiO₂/Si interface. In addition, the presence of defects at the SiO₂/Si interface was found to be greater for samples with no local interface strain.     

A facile template-free method for preparing bi-phase TiO2 nanowire arrays with high photocatalytic activity

Ordered bi-phase TiO₂ nanowire arrays were simply obtained by heat treating TiO₂ nanotube arrays prepared by a two-step anodization method. The nanowire arrays are composed of anatase and rutile phases with uniform diameters around 50 nm. The photocatalysis activities of TiO₂ nanowire arrays were characterized by quantifying the degradation of methyl orange solution. And the results indicated that the bi-phase nanowire arrays, especially obtained at 700 °C, showed much higher activity than that of P25 film or anatase TiO₂ nanotube array.     

纳米WO3/TiO2的制备及光催化甲烷和水的反应性能

采用溶胶-凝胶法制备了WO3/TiO2纳米复合催化剂,通过XRD、IR、UVDRS技术对材料的表面性质与构造、光响应性能进行了表征,并研究了该催化剂作用下光催化甲烷和水生成甲醇的反应性能;考察了煅烧温度、WO3掺杂量对光催化活性的影响,并研究了反应条件对甲醇产量的影响。结果表明:掺入WO3使TiO2光催化活性提高,扩大了光激发波长范围;催化剂煅烧温度为600℃、WO3摩尔分数为3%时,光催化性能最佳;在紫外光照射下,光激发WO3/TiO2表面产生光生空穴,催化甲烷转化为甲醇,甲烷转化率为16.2%,对甲醇的选择性达到76%。     

Integrated ferroelectric stacked mim capacitors with 100 nF/mm2and 90 V breakdown as replacement for discretes

This paper shows for the first time integrated thin film ferroelectric metal-insulator-metal capacitors on silicon with a record high capacitance density above 100 nF/mm² combined with a breakdown voltage of 90 V and a lifetime exceeding 10 years at 85degC and 5 V. The high capacitance density was obtained by a combination of material optimizations resulting in a dielectric constant of 1600, and stacking of capacitors. The reliability of these ferroelectric capacitors was studied in detail with accelerated lifetime testing. The high performance of the integrated capacitors in this paper shows great potential for applications demanding high capacitance densities combined with electrostatic discharge protection.     

Deposition of ZnO thin films on SiO2/Si substrate with Al2O3 buffer layer by radio frequency magnetron sputtering for high frequency surface acoustic wave devices

ZnO films with c-axis (0002) orientation have been grown on SiO₂/Si substrates with an Al₂O₃ buffer layer by radio frequency magnetron sputtering. Crystalline structures of the films were investigated by X-ray diffraction, atomic force microscopy and scanning electron microscopy. The center frequency of the surface acoustic wave (SAW) device with a 4.8 μm thick Al₂O₃ buffer layer was measured to be about 408 MHz, which was much higher than that (265 MHz) of ZnO/SiO₂/Si structure and approaches that (435 MHz) of ZnO/sapphire. It is a possible way as an alternative for the sapphire substrate for the high frequency SAW device applications, and is also useful to integrate the semiconductor and high frequency SAW devices on the same Si substrate.     

Direct measurement of the direction of interface motion in the oxidation of metals and covalent solids—Al(111) and Si(100) oxidation with O2 at 300 K

Four point probe measurements of the surface electrical resistance at an oxide film-metal interface and at an oxide-film semiconductor interface have shown with Å sensitivity that the direction of the buried interface motion during oxide film growth is opposite in the two cases in accordance with the Mott-Cabrera theory. During the formation of amorphous Al₂O₃ layers on Al(111) at 300 K, outward film growth occurs due to Al³⁺ ion transport from the metal into the growing oxide film. For the formation of amorphous SiO₂ layers on Si(100) at 300 K, oxygen transport occurs inwardly into the Si lattice as the oxide film forms.     

Experimental investigation on the effect of mixing length on the performance of two-phase ejector for CO2 refrigeration cycle with and without heat exchanger

In ejector system using the promising natural refrigerant CO₂, the mixing of high-speed two-phase primary flow and suction vapor is crucial in designing an efficient ejector. In this study, the effect of mixing length on ejector system performance was analyzed experimentally. The mixing lengths used were 5 mm, 15 mm, and 25 mm, with constant rectangular cross-section. The experiments were performed for both ejector and conventional expansion systems with and without internal heat exchanger (IHX) at different operating conditions. Based on the experimental results, mixing length had significant effect on entrainment ratio and on magnitude and profile of pressure recovery. The 5 mm and 15 mm types yielded the lowest and highest ejector efficiency and COP in all of the conditions used in this research, respectively. The use of IHX had net positive effect on system performance which verified the results of our previous study. A COP improvement of up to 26% over conventional system was obtained but improper sizing of mixing length lowered the COP by as much as 10%.     

Preparation and surface properties of mesoporous silica particles modified with poly(N-vinyl-2-pyrrolidone) as a potential adsorbent for bilirubin removal

The surface of silica particles was modified with polyvinyl pyrrolidone (PVP) through sol–gel process. The different experimental techniques, i.e., thermogravimetric analysis (TGA and DTG), nitrogen adsorption, scanning electron microscopy (SEM), laser diffraction analysis (LDA), fourier transform spectroscopy (FTIR) are used to characterize the pure non-functionalized and functionalized silicas containing different amount of PVP. It was shown that PVP-modified silica samples have well developed porous structure; the values of specific surface area for PVP-modified silicas are in the range of 140–264 m² g⁻¹. While the non-functionalized silica shows the low surface area (S_BET = 40 m² g⁻¹). The BJH analysis showed that PVP can be used as an effective agent to increase an average pore size and total pore volume. The results indicate that PVP functionalized silicas show a potential as effective adsorbents for bilirubin removal compared to other available adsorbents.