Synthesis of amorphous Si nanowires from solid Si sources using microwave irradiation

Si nanowires were synthesized from Si wafers and from thin Si films deposited on various substrates by microwave irradiation. The power and time were key determinants of the diameter and morphology of the synthesized Si nanowires. The nanowires had an amorphous structure due to the extremely high heating rate. Carbon coating of the Si nanowires was easily achieved by introducing acetylene after synthesizing the nanowires. Carbon-coated Si nanowires are potential candidates for use as the anode material in next generation Li-ion batteries.     

Si-based flexible memristive systems constructed using top-down methods

Si-based memristive systems consisting of Ag, amorphous Si, and heavily doped p-type Si nanowires were successfully constructed on plastic substrates through top-down methods, including the crystallographic wet etching of Si wafers, transfer onto plastic substrates, and thin film patterning. The memristive systems showed excellent memory characteristics and flexibility, such as intrinsic hysteric and rectifying behaviors, on/off resistance ratios of >1 × 10(5), and durability for up to 1000 bending cycles. The correlations between the Ag-filament-related nanostructures formed in amorphous Si and the resistance-switching behaviors were carefully examined with the tunneling current model, transmission electron microscopy, and secondary ion mass spectroscopy to explore the switching mechanism. Our study suggests the promising potential of the Si-based memristive systems for the development of next-generation flexible nonvolatile memory.     

Growth of GaAs Nanowires on Si Substrates Using a Molecular Beam Epitaxy

Au-catalyzed GaAs nanowires were grown on Si substrates by vapor-liquid-solid growth method using a molecular beam epitaxy (MBE). The MBE growth could produce controlled crystalline orientation and uniform diameter along the wire axis of the GaAs nanowires by adjusting growth conditions including growth temperature and V/III flux ratio. Growths of GaAslang001rang as well as GaAslang111rang nanowires were observed by transmission electron microscopy and scanning electron microscopy. Epitaxially grown GaAslang111rang nanowires on a Si(111) substrate were verified through x-ray diffraction out-of-plane 2thetas/omega-scans. A strong room-temperature photoluminescence (PL) was observed from the epitaxially grown GaAslang111rang nanowires on a Si(100) substrate. Results of low-temperature (10 K) PL measurements and current-sensing atomic force microscopy indicated that the GaAs nanowires on a Si substrate were unintentionally doped with Si     

Nanoheteroepitaxy of GaN on a nanopore array of Si(111) surface

Nanoheteroepitaxial (NHE) growth of GaN using AlN/AlGaN as a graded buffer layer by metalorganic chemical vapor deposition has been demonstrated on the nanoporous patterned Si(111) substrates. The nanopore array on Si(111) has been fabricated by using anodized aluminum oxide membrane as an induced couple plasma dry etching mask. The reduction of the threading dislocation density and relaxation of the tensile stress in NHE GaN are revealed by transmission electron microscopy (TEM), micro-Raman spectrum and photoluminescence spectrum, respectively. Cross-sectional TEM analysis shows that dislocations nucleated at the interface are forced to bend into (0001) basal plane. Red shift in the E₂ (TO) phonon peak of micro-Raman spectrum indicates the relaxation of tensile stress in the nanoheteroepitaxial lateral overgrowth of GaN. A single step ELO without mask on nanopatterned Si(111) substrates is a simple and promising way for the improvement of the quality of GaN on Si substrates.     

Si(001)衬底上方形3 C-SiC岛的LPCVD生长

Si衬底上SiC的异质外延生长深受关注,为了了解Si衬底上的成核及长大过程,采用PLCVD方法在Si(001)衬底上生长出了方形3C-SiC岛,利用Nomarski光学显微镜和扫描电子显微镜（SEM）观察了SiC岛的形状,尺寸,密度和界面形貌,结果表明,3C－SiC岛生长所需的Si原子来自反应气源,衬底上的Si原子不发生迁移或外扩散,气相中C原子浓度决定了SiC岛的生长过程。     

Fabrication of ordered arrays of InP microstructures by wet chemical etching with Au masks

Ordered arrays of InP microstructures have been fabricated on InP(001) substrates by wet chemical etching in aqueous HCl with patterned Au masks. The masks were produced by Au deposition through copper grids or a monolayer of polystyrene microspheres. Square InP mesas (20 x 20 microns) and pillars (approximately 100 nm in both diameter and height) were both produced and characterized by scanning electron microscopy and atomic force microscopy.     

Ordering of Ge islands on Si(001) substrates patterned by nanoindentation

Spatial organization of Ge islands, grown by physical vapor deposition, on prepatterned Si(001) substrates has been investigated. The substrates were patterned prior to Ge deposition by nanoindentation. Characterization of Ge dots is performed by atomic force microscopy and scanning electron microscopy. The nanoindents act as trapping sites, allowing ripening of Ge islands at those locations during subsequent deposition and diffusion of Ge on the surface. The results show that island ordering is intrinsically linked to the nucleation and growth at indented sites and it strongly depends on pattern parameters.     

Morphology- and orientation-controlled gallium arsenide nanowires on silicon substrates

GaAs nanowires were epitaxially grown on Si(001) and Si(111) substrates by using Au-catalyzed vapor-liquid-solid (VLS) growth in a solid source molecular beam epitaxy system. Scanning electron microscopy analysis revealed that almost all the GaAs nanowires were grown along <111> directions on both Si substrates for growth conditions investigated. The GaAs nanowires had a very uniform diameter along the growth direction. X-ray diffraction data and transmission electron microscopy analysis revealed that the GaAs<111> nanowires had a mixed crystal structure of the hexagonal wurtzite and the cubic zinc-blende. Current-voltage characteristics of junctions formed by the epitaxially grown GaAs nanowires and the Si substrate were investigated by using a current-sensing atomic force microscopy.     

Self-organized growth of Si/Silica/Er2Si2O7 core-shell nanowire heterostructures and their luminescence

Self-organized Si-Er heterostructure nanowires showed promising 1.54 microm Er(3+) optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor-liquid-solid mechanism in an Si-Er-Cl-H(2) system using an Au catalyst. Meanwhile, a single-crystalline Er(2)Si(2)O(7) shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 microm Er(3+) photoluminescence spectrum consisting of a series of very sharp peaks. The Er(3+) spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 micros. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.     

Synthesis of GaN nanowires by CVD method: effect of reaction temperature

GaN nanowires are fabricated on Si substrates by ammoniating Ga₂O₃/NiCl₂ thin films using chemical vapour deposition method. The influence of reaction temperature on microstructure, morphology and optical properties of GaN nanowires is characterised by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrophotometer, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence. The results demonstrate that the GaN nanowires are single crystalline and exhibit hexagonal wurtzite symmetry. The best crystalline quality was achieved for an reaction temperature of 1150°C for 15?min. The growth process follows vapour–liquid–solid mechanism and Ni plays an important role as the nucleation point and as a catalyst.     

EBL- and AFM-based techniques for nanowires fabrication on Si/SiGe

Two approaches for sub-100 nm patterning are applied to Si/SiGe samples.The first one combines electron beam lithography (EBL) and anisotropic wet etching to fabricate wires with triangular section whose top width is narrower than the beam size. Widths as small as 20 nm on silicon and 60 nm on Si/SiGe heterostructures are obtained.The second lithographic approach is based on the local anodization of an aluminum film induced by an atomic force scanning probe. Using atomic force microscopy (AFM) anodization and selective wet etching, aluminum and aluminum oxide nanostructures are obtained and used as masks for reactive ion etching (RIE). Sub-100 nm wide wires are fabricated on Si/SiGe substrates.     

Single crystalline PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices

We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heterostructure has an atomically sharp interface with epitaxial relationships of Si[110]//PtSi[010] and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities approximately 28.6 microOmega.cm and high breakdown current densities>1x10(8) A/cm2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/microm, field-effect hole mobility of 168 cm2/V.s, and on/off ratio>10(7), demonstrating the best performing device from intrinsic silicon nanowires.     

Patterning of various silicon structures via polymer lithography and catalytic chemical etching

We demonstrate a facile fabrication of a rich variety of silicon patterns with different length scales by combining polymer lithography and a metal-assisted chemical etching method. Several types of polymer patterns were fabricated on silicon substrates, and silver layers were deposited on the patterned silicon surfaces and used to etch the silicon beneath. Various silicon patterns including topographic lines, concentric rings, and square arrays were created at a micro-?and nanoscale after etching the silicon and subsequent removal of the patterned polymer masks. Alternatively, the arrays of sub-30?nm silicon nanowires were produced by a chemical etching of the silicon wafer which was covered with highly ordered polystyrene-block-polyvinylpyridine (PS-b-PVP) micellar films. In addition, silicon nanohole arrays were also generated by etching with hexagonally packed silver nanoparticles that were prepared using PS-b-PVP block copolymer templates.     

Synthesis,characterization and growth mechanism of ZnO nanowires on NiCl<Subscript>2</Subscript>-coated Si substrates

Well-crystallized ZnO nanowires have been successfully synthesized on NiCl₂-coated Si substrates via a carbon thermal reduction deposition process. The pre-deposited Ni nanoparticles by dipping the substrates into NiCl₂ solution can promote the formation of ZnO nuclei. The as-synthesized nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectrum. The results demonstrate that the as-fabricated nanowires with about 60 nm in diameter and several tens of micrometers in length are preferentially arranged along [0001] direction with (0002) as the dominate surface. Room temperature PL spectrum illustrates that the ZnO nanowires exist a UV emission peak and a green emission peak, and the peak centers locate at 387 and 510 nm. Finally, the growth mechanism of the nanowires is briefly discussed.     

Selective area growth of GaN nanowires using metalorganic chemical vapor deposition on nano-patterned Si(111) formed by the etching of nano-sized Au droplets

We report on the selective area growth of GaN nanowires (NWs) on nano-patterned Si(111) substrates by metalorganic chemical vapor deposition. The nano-patterns were fabricated by the oxidation of Si followed by the etching process of Au nano-droplets. The size of formed nano-pattern on Si(111) substrate was corresponding to the size of Au nano-droplet, and the diameter of GaN NWs grown was similar to the diameter of fabricated nano-pattern. The interesting phenomenon of using the nano-patterned Si(111) substrates is the formation of very clear substrate surface even after the growth of GaN NWs. However, in the case of GaN NWs grown using Au nano-droplets, there was several nanoparticles including GaN bulk grains on the Si(111) substrates. The smooth surface morphology of nano-patterned Si(111) substrates was attributed to the presence of SiO₂ layer which prevents the formation of unnecessary GaN particles during the GaN NW growth. Therefore, we believe that nano-patterning method of Si(111) which was obtained by the oxidation of Si(111) substrate and subsequent Au etching process can be utilized to grow high-quality GaN NWs and its related nano-device applications.     

Fabrication, structural and electrical characterization of VO2 nanowires

The structural and electrical properties of VO₂ nanowires synthesized on Si₃N₄/Si substrates or molybdenum grids by a catalyst-free vapour transport method were investigated. The grown VO₂ nanowires are single crystalline and rectangular-shaped with a preferential axial growth direction of [1 0 0], as examined with various structural analyses such as transmission electron microscopy, electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. In particular, it was found that growing VO₂ nanowires directly on Si₃N₄ deposited molybdenum transmission electron microscopy grids is advantageous for direct transmission electron microscopy and electron diffraction characterizations, because it does not involve a nanowire-detachment step from the substrates that may cause chemical residue contamination. In addition to structural analyses, VO₂ nanowires were also fabricated into field effect transistor devices to characterize their electrical properties. The transistor characteristics and metal-insulator transition effects of VO₂ nanowires were investigated.     

Observation of room-temperature negative differential resistance in Gd-doped Si nanowires on Si(110) surface

The massively parallel arrays of highly periodic Gd-doped Si nanowires (SiNWs) self-organized on Si(110)-16?×?2 surface were investigated by scanning tunneling microscopy and spectroscopy. These periodic Gd-doped SiNWs are atomically precise and show equal size, periodic positions, and high-integration densities. Surprisingly, the scanning tunneling spectroscopy results show that each metallic-like, Gd-doped SiNW exhibits room-temperature negative differential resistance (RT-NDR) behavior, which can be reproducible with various Gd dopings and is independent of the tips. Such massively parallel arrays of highly ordered and atomically identical Gd-doped SiNWs with one-dimensional laterally confined RT-NDR can be exploited in Si-based RT-NDR nanodevices.     

n-ZnO:Ga/i-ZnO/p-Si heterojunction light emitting diodes fabricated on patterned Si substrates

n-ZnO:Ga/i-ZnO/p-Si heterojunction light-emitting diodes were fabricated on patterned Si substrates with increased interface area where hole carriers were transported to the i-ZnO layer. The patterned Si substrates were prepared by electrochemical etching, and the n-type ZnO:Ga films were deposited by high-temperature sputtering. In the patterned LED, the lower breakdown and greater leakage current under a reverse bias was attributed to the formation of a high density of grain boundaries and random tilting of the c-axis. Compared to an LED without patterning, the patterned substrates resulted in approximately 75% improvement in the output power of visible emission, which was attributed to a 1.33-fold increase in the heterojunction area and the increase in grain boundary density due to grain tilting.     

Nucleation and growth of GaN nanowires on Si(111) performed by molecular beam epitaxy

GaN nanowires (NWs) have been grown on Si(111) substrates by plasma-assisted molecular beam epitaxy (PAMBE). The nucleation process of GaN-NWs has been investigated in terms of nucleation density and wire evolution with time for a given set of growth parameters. The wire density increases rapidly with time and then saturates. The growth period until the nucleation of new nanowires is terminated can be defined as the nucleation stage. Coalescence of closely spaced nanowires reduces the density for long deposition times. The average size of the well-nucleated NWs shows linear time dependence in the nucleation stage. High-resolution transmission electron microscopy measurements of alternating GaN and AlN layers give valuable information about the length and radial growth rates for GaN and AlN in NWs.     

The oxides growth during high-temperature oxidation of Si1-xGe(x) nanowires

Concerning the oxidation behavior of Si1-xGe(x) (x = 0.15, 0.3) nanowires at high temperature, Si1-xGe(x) nanowires were thermally oxidized for various lengths of time compared with Si nanowires, Si and Si1-xGe(x) thin films. The structural and compositional properties of the oxidized nanowires were characterized using several transmission electron microscopy (TEM) techniques including energy dispersive X-ray spectroscopy (EDS), which confirm that the oxidation rates of Si1-xGe(x) and Si (silicon) nanowires were saturated with increasing oxidation time due to retarding behavior, while the oxidation rate of Si1-xGe(x) nanowires were faster than that of Si nanowires. In addition, the differences in Ge (germanium) content and stress distribution contribute to the observed differences in oxidation behavior.