SiO2-coated Si Substrates Research Articles

Nanopatterning Induced Si Doping in Amorphous Ga2O3 for Enhanced Electrical Properties and Ultra-Fast Photodetection.

Ga2O3 has emerged as a promising material for the wide-bandgap industry aiming at devices beyond the limits of conventional silicon. Amorphous Ga2O3 is widely being used for flexible electronics, but suffers from very high resistivity. Conventional methods of doping like ion implantation require high temperatures post-processing, thereby limiting their use. Herein, an unconventional method of doping Ga2O3 films with Si, thereby enhancing its electrical properties, is reported. Ion-beam sputtering (500eV Ar+) is utilized to nanopattern SiO2-coated Si substrate leaving the topmost part rich in elemental Si. This helps in enhancing the carrier conduction by increasing n-type doping of the subsequently coated 5nm amorphous Ga2O3 films, corroborated by room-temperature resistivity measurement and valence band spectra, respectively, while the nanopatterns formed help in better light management. Finally, as proof of concept, metal-semiconductor-metal (MSM) photoconductor devices fabricated on doped, rippled films show superior properties with responsivity increasing from 6 to 433mAW-1 while having fast detection speeds of 861µs/710µs (rise/fall time) as opposed to non-rippled devices (377ms/392ms). The results demonstrate a facile, cost-effective, and large-area method to dope amorphous Ga2O3 films in a bottom-up approach which may be employed for increasing the electrical conductivity of other amorphous oxide semiconductors as well.

Field-free spin-orbit torque switching assisted by in-plane unconventional spin torque in ultrathin [Pt/Co

Electrical manipulation of magnetization without an external magnetic field is critical for the development of advanced non-volatile magnetic-memory technology that can achieve high memory density and low energy consumption. Several recent studies have revealed efficient out-of-plane spin-orbit torques (SOTs) in a variety of materials for field-free type-z SOT switching. Here, we report on the corresponding type-x configuration, showing significant in-plane unconventional spin polarizations from sputtered ultrathin [Pt/Co]N, which are either highly textured on single crystalline MgO substrates or randomly textured on SiO2 coated Si substrates. The unconventional spin currents generated in the low-dimensional Co films result from the strong orbital magnetic moment, which has been observed by X-ray magnetic circular dichroism (XMCD) measurement. The x-polarized spin torque efficiency reaches up to −0.083 and favors complete field-free switching of CoFeB magnetized along the in-plane charge current direction. Micromagnetic simulations additionally demonstrate its lower switching current than type-y switching, especially in narrow current pulses. Our work provides additional pathways for electrical manipulation of spintronic devices in the pursuit of high-speed, high-density, and low-energy non-volatile memory.

Augmentation of surface plasmon-enhanced second harmonic generation from Au nanoprisms on SiO2/Si: interference contribution.

We present an augmentation of Surface Plasmon (SP)-enhanced second harmonic generation (SHG) due to interference field enhancement in Au nanoprisms (AuNPs) on SiO2-coated Si substrates. The SiO2 spacer contributed for the optical interference and increased the coupling efficiency of the pump light with the SP polarization as well as a decoupling efficiency of the SHG waves from nonlinear polarization. The intensity of the SP-enhanced SHG signals increased 4.5-fold with respect to the AuNPs on the bare SiO2 substrate by setting the SiO2 spacer layer to the appropriate thickness. The numerical analysis revealed that the optimal SHG conversion was determined by the balance between the degree of the optical interference at the fundamental and SHG wavelengths.

Transparent elongation and compressive strain sensors based on aligned carbon nanowalls embedded in polyurethane

Highly extensible transparent composite materials comprised of maze-like vertically aligned carbon nanowalls embedded perpendicularly into a polyurethane film were used as strain tensors and tested by an electrical resistance method in the course of extension, extension/relaxation and compression/expansion cycles. The maze-like carbon nanowall networks with wall-to-wall average distances of 100, 200 and 300 nm were formed on SiO2-coated Si substrates by a plasma-enhanced chemical vapor deposition system. Afterwards, the nanowall network was embedded into a stretchable polyurethane matrix which enabled a high deformation of the composite. The measured extensibility of the composite was over 440 %, and its resistance increased with the extension. The sensitivity of the detection of extension, which was evaluated by the gauge factor, increased over 2000. These sensor properties can be readily tuned by varying distances of nanowalls within the network. Finally, thanks to their optical transparency in the visible light region and thermoelectric properties, these composites offer a wide range of further practical applications.

Facile synthesis of ZnO microrod photodetectors by solid-state reaction

In this study, ZnO microrods were directly synthesized via a single-step solid-state reaction using ZnO powders mixed with graphite without a catalyst under an air atmosphere, instead of using vacuum systems and flowing gases. The structure, growth mechanism, and electrical and optical properties of the microrods were investigated under varying growth conditions. The high-resolution transmission electron microscopy images confirmed the formation of single-crystal ZnO microrods. The photoluminescence spectra of the microrods showed green emission, suggesting the formation of non-stoichiometric ZnO microrods due to the formation of oxygen vacancies during sintering. The length and width of the ZnO microrods could be modulated by controlling the graphite content, sintering temperature, and residual time. A simple photodetector consisting of a single-crystal ZnO microrod on a SiO2-coated Si substrate was fabricated. The photodetector exhibited a linear current–voltage curve in visible light. The slope of the curve increased under ultraviolet (UV) irradiation, maintaining a linear shape of the curve. The curve reversibly returned to the initial shape in the absence of UV illumination. The UV on-off current ratio increased because of the decrease in the off-current by compensating for the oxygen vacancies through annealing in an oxygen atmosphere. Thus, in this study, we proposed a simple and efficient approach to fabricate single-crystal ZnO microrods for application in low-cost photodetectors.

A comprehensive study on the effects of gamma radiation on the physical properties of a two-dimensional WS2monolayer semiconductor

This article reports the effects of gamma radiation on the structural, optical and magnetic properties of monolayer tungsten disulfide (WS2) grown by a scalable van der Waals epitaxial (VdWE) process on a SiO2coated Si substrate.

Synthesis of highly conductive cobalt thin films by LCVD at atmospheric pressure

We have deposited very low resistant Co films on SiO2-coated Si substrates using UV pulsed laser pyrolytic decomposition of Co2(CO)8 with 355nm laser radiation at atmospheric pressure. Facile decomposition of the precursors and the use of Ar curtain enable the deposition of relatively pure Co (with O less than 7% and negligible C) at the power of 1.11–3.33W, and of pure Co at 6.67W. The resistivity decreases from 58 to 19µΩ-cm as the power increases from 2.22 to 3.33W, showing inverse-linear dependence on grain size. In addition, further increase of the power to 6.67W decreases the resistivity to 9µΩ-cm, due to both the growth of large grains with negligible contaminants, and the adverse effect of surface roughness. The effects of oxygen contaminants on the resistivity can be minimal, because of its presence in the form of oxide. These low resistant fine metal lines deposited by a direct-writing laser chemical vapor deposition technique at atmospheric pressure have been reported for the first time.

Influence of the oxide thickness of a SiO2/Si(001) substrate on the optical second harmonic intensity of few-layer MoSe2

The nonlinear optical properties of few-layer MoSe2 on a SiO2/Si substrate were investigated with our optical second harmonic generation (SHG) microscope. Few-layer flakes were mechanically exfoliated from a single crystal onto a 90- or 270-nm-thick SiO2-coated Si(001) substrate. The polar plot of the second-harmonic (SH) intensity from a mono- or trilayer MoSe2 flake as a function of the rotation angle of incident polarization shows a threefold symmetry, indicating that the isolated few-layer flakes retain their single crystallographic orientation. SHG spectra were found to depend strongly on the oxide thickness of the substrate (90 or 270 nm), which was interpreted using the interference among the multiply reflected SH light beams in the system. By taking this interference into account, a resonant peak may be identified at a two-photon energy of equal to or less than 2.9 eV in an SHG spectrum. The spatial resolution of the SHG microscope was estimated as 0.53 µm.

Electrical Characteristics of Horizontally and Vertically Oriented Few-Layer Graphene on Si-Based Dielectrics.

Horizontally and vertically oriented few-layer graphenes have been synthesized directly on SiO2 coated Si substrates via thermal and hot-filament chemical vapor deposition, respectively. The effect of the direction of mass flow on the fabrication of graphene film is analysed and a plausible mechanism is proposed. The graphene/p-Si heterojunction is fabricated and tested for its potential in optoelectronic devices. Rectification behaviour is observed at the interface of graphene/p-Si under dark conditions. A dark current of 1.1 mA with an ideality factor of 1.5, in addition of a high rectification ratio of 15.99 at ± 0.5 V is found for the vertically oriented graphene/p-Si heterojunction.

Fabrication of CMOS-compatible abrupt electronic switches based on vanadium dioxide

Vanadium dioxide thin films were deposited on amorphous SiO2-coated Si substrates, by reactive magnetron sputtering at 490°C of a pure vanadium target. The deposition parameters were optimized in order to maximize the resistivity modulation due to the metal–insulator transition. The best results were obtained working at an operating pressure of 2·10−3 mbar, a constant argon flow of 12.5 sccm and a starting oxygen flow of 2.41 sccm with a feedback control to keep constant the oxygen partial pressure in the sputtering chamber. This allowed to observe for the first time an electrically induced metal–insulator transition with a resistance ratio as high as 824.8 for sputtered VO2 on SiO2/Si substrates.

Low temperature carbon nanotube and hexagonal diamond deposition with photo-enhanced chemical vapor deposition

Deposition of carbon nanotube and hexagonal diamond thin films at low substrate temperature with photo-enhanced chemical vapor deposition is described here. Extensive experimentation is conducted to optimize the catalyst layer utilized for deposition by varying Al/Ni/Al metal layer thicknesses on SiO2 coated Si substrates. The coated substrates are annealed to transform the thin metal layers into nanoparticles. Suitable catalyst layer thicknesses obtained are 3/2/3, 5/1/5 and 5/3/5 nm for Al/Ni/Al sandwich metal layers. Suitable annealing conditions are in the range of 350---450 °C for substrate temperature and from 0.22 to 10 Torr for chamber pressure in ammonia ambient for 25 min. Carbon tetrachloride (CCl4) is used as a carbon precursor in this work. Argon to CCl4 flow ratio is varied in 1.5---19 range, chamber pressure is varied in 3---10 Torr range, and the substrate temperature is varied in 350---450 °C range. Carbon nanotubes (CNT) growth is observed at lower chamber pressure, lower partial pressure of CCl4, lower substrate temperature and for thin Ni catalyst layer. The optimal CNT deposition condition observed is 5 Torr total chamber pressure, 9:1 partial pressure ratio of Ar to CCl4, 400 °C substrate temperature and 5/1/5 nm thick Al/Ni/Al catalyst layers. The hexagonal diamond deposition is observed at a higher chamber pressure, higher partial pressure of CCl4, higher substrate temperature and for a thicker Ni catalyst layer. The optimal condition for hexagonal diamond deposition observed is 10 Torr total chamber pressure, 7:3 partial pressure ratio of Ar to CCl4, 450 °C substrate temperature and 5/3/5 nm thick Al/Ni/Al catalyst sandwich layers.

Area Selective Molecular Layer Deposition of Polyurea Films

Patterned organic thin films with submicrometer features are of great importance in applications such as nanoelectronics and optoelectronics. We present here a new approach for creating patterned organic films using area selective molecular layer deposition (MLD). MLD is a technique that allows for conformal deposition of nanoscale organic thin films with exceptional control over vertical thickness and composition. By expanding the technique to allow for area selective MLD, lateral patterning of the film can be achieved. In this work, polyurea thin films were deposited by alternating pulses of 1,4-phenylenediisocyanate (PDIC) and ethylenediamine (ED) in a layer-by-layer fashion with a linear growth rate of 5.3 Å/cycle. Studies were carried out to determine whether self-assembled monolayer (SAM) formed from octadecyltrichlorosilane (ODTS) could block MLD on silicon substrates. Results show that the MLD process is impeded by the SAM. To test lateral patterning in MLD, SAMs were patterned onto silicon substrates using two different approaches. In one approach, SiO2-coated Si(100) substrates were patterned with an ODTS SAM by soft lithography in a well-controlled environment. In the second approach, patterned ODTS SAM was formed on H-Si/SiO2 patterned wafers by employing the chemically selective adsorption of ODTS on SiO2 over H-Si. Auger electron spectroscopy results revealed that the polyurea film is deposited predominantly on the ODTS-free regions of both patterned substrates, indicating sufficient blocking of MLD by the ODTS SAM layer to replicate the pattern. The method we describe here offers a novel approach for fabricating high quality, three-dimensional organic structures.

Synergistic Interaction between Substrate and Seed Particles in Ultrathin Ultrananocrystalline Diamond Film Nucleation on SiO2 with Controlled Surface Termination

We have investigated the effect of surface modification on the dispersion of nanodiamond seeds on the SiO2-coated Si substrate, to enable the ultrathin ultrananocrystalline diamond (UNCD) coating on the substrate, by direct current plasma-assisted chemical vapor deposition (DC-PACVD) using hydrogen-rich chemistry. The ultrasonically dispersed seed density on the SiO2-coated Si wafer was so much lower than that on the pristine Si wafer that the void-free ultrathin UNCD coating was impossible. For surface modification, we have exposed the substrate to (1) the hydrogen/hydrocarbon plasma in the DC-PACVD chamber or (2) the hydrocarbon atmosphere in the hot filament chemical vapor deposition (HF-CVD) chamber, prior to the ultrasonic seeding. The exposure to hydrocarbon or to its plasma greatly reduced the seed density, while exposure to hydrogen plasma drastically enhanced it by a factor of 6, which enabled a void-free ultrathin UNCD coating as thin as 30 nm. The UNCD film and the substrate surface before and ...

Effects of post-annealing on thermoelectric properties of bismuth–tellurium thin films deposited by co-sputtering

This paper reports the microstructure evolution of Bi–Te thermoelectric films upon post-annealing and its effects on the thermoelectric properties. Bi–Te films with the composition of around 61 at.% Te and the thickness of 300 nm were deposited onto SiO 2-coated Si substrates by using bismuth and tellurium targets in a radio frequency (RF) magnetron sputtering system. We annealed the films at different temperatures (100, 150 and 200 °C) under N 2 ambient for 8 h, and characterized the crystallinity and morphology of the Bi–Te films. Microstructure characterization using X-ray diffraction and scanning electron microscopy disclosed that the post-annealing treatment entailed a drastic microstructural evolution by inducing the development of a strong texture of grains with their c-axis oriented normal to the substrate. In addition, we measured the electrical transport and thermoelectric properties of the films to reveal their close link with the microstructure changes. The electron mobility and Seebeck coefficient increase significantly, leading to a remarkable improvement in the power factor from 3.3 μW/K 2 cm for the as-deposited sample to 24.1 μW/K 2 cm for the 200 °C-annealed sample.

Effects of defects generated in ALD TiO2 films on electrical properties and interfacial reaction in TiO2/SiO2/Si system upon annealing in vacuum

Thin TiO2 layers grown at 130°C on SiO2-coated Si substrates by atomic layer deposition (ALD) using TTIP and H2O as precursors were annealed, and the effects of the annealing temperature on the resulting electrical properties of TiO2 and the interface properties between a Pt electrode and TiO2 were examined using transmission line model (TLM) structures. The as-deposited TiO2 thin film had an amorphous structure with OH groups and a high resistivity of 6×103Ω-cm. Vacuum annealing at 700 °C transformed the amorphous film into an anatase structure and reduced its resistivity to 0.04Ω-cm. In addition, the vacuum-annealing of the TiO2/SiO2 structure at 700°C produced free silicon at the TiO2-SiO2 interface as a result of the reaction between the Ti interstitials and SiO2. The SiO2 formed on the TiO2 surface caused a Schottky contact, which was characterized by the TLM method. The use of the TLM method enabled the accurate measurement of the resistivity of the vacuum-annealed TiO2 films and the characterization of the Schottky contacts of the metal electrode to the TiO2.

Doped Polycrystalline 3C-SiC Films Deposited by LPCVD for Radio-Frequency MEMS Applications

Polycrystalline 3C-SiC films are deposited on SiO2 coated Si substrates by low pressure chemical vapour deposition (LPCVD) with C3H8 and SiH4 as precursors. Controlled nitrogen doping is performed by adding NH3 during SiC growth to obtain the low resistivity 3C-SiC films. X-ray diffraction (XRD) patterns indicate that the deposited films are highly textured (111) orientation. The surface morphology and roughness are determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface features are spherulitic texture with average grain size of 100 nm, and the rms roughness is 20 nm (AFM 5 × 5 μm images). Polycrystalline 3C-SiC films with highly orientational texture and good surface morphology deposited on SiO2 coated Si substrates could be used to fabricate rf microelectromechanical systems (MEMS) devices such as SiC based filters.

Electrical Characterizations of InGaAs Nanowire-Top-Gate Field-Effect Transistors by Selective-Area Metal Organic Vapor Phase Epitaxy

Single InGaAs nanowire-top-gate metal–semiconductor field-effect transistors (MESFETs) were fabricated and characterized. Silicon-doped n-InGaAs nanowires (with a typical diameter of 100 nm) were grown by catalyst-free selective-area metal–organic vapor-phase epitaxy (SA-MOVPE). The FETs of single nanowires on SiO2-coated Si substrates were fabricated by defining metal contacts at both ends of the nanowires and the metal top gate between contacts. According to the measurements of drain current–voltage and gate transfer characteristics, the top-gate MESFETs exhibited significant enhancements in device performance characteristics compared with FETs under back-gate operation; that is, a peak transconductance of 33 mS/mm and a current on–off ratio of 103 were obtained. A possibility for further improvements in FET characteristics was also considered.

Low-temperature fabrication and random laser action of doped zinc oxide nanoneedles

Zinc oxide (ZnO) thin films grown on SiO 2-coated Si substrates by filtered cathodic vacuum arc (FCVA) at 200 °C was irradiated with Ar + ions with a simultaneous supply of Fe atoms or with N 2 + ions (without Fe) at room temperature to fabricate Fe- or N-doped ZnO nanoneedles, respectively. The morphological structure of the doped nanoneedles was almost identical with that of non-doped nanoneedles. The detailed analyses by energy dispersive X-ray spectroscopy combined with transmission electron microscope revealed that Fe was doped at and beneath the surface of the nanoneedles as well as at the needle tip. Random lasing was observed for both Fe- and N-doped ZnO nanoneedles thus fabricated, and the lasing peaks detected were redshifted compared with those attained for non-doped ZnO nanoneedles. Since the lasing wavelength, and hence the bandgap, was readily tunable, the combination of FCVA and ion-irradiation methods was concluded to be quite promising to fabricate 1D nanostructures for optoelectronics applications at low temperatures.

Study of poly(vinylidene fluoride-trifluoroethylene) as a potential organic high K gate dielectric

Spin cast films of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer deposited on bare Si and SiO2 coated Si substrates were vacuum annealed and treated in supercritical carbon dioxide (scCO2) and the dielectric properties of the films before and after treatments were studied using capacitance-voltage and conductance-voltage techniques on thin film capacitor structures. After annealing, the static dielectric constant (K) for P(VDF-TrFE) was found to be more than double that for SiO2 films and to increase with film thickness and annealing time, and reached a maximum of nearly 10 for thick films on SiO2 or Si substrates. Treating annealed P(VDF-TrFE) films in scCO2 initially decreased K and the refractive index n to as-deposited values that increased and stabilized after reannealing. The changes in P(VDF-TrFE) were attributed to densification and ordering. No systematic differences in interface charges and states were found between treated and untreated samples and with various substrates. The leakage current for P(VDF-TrFE) films was found to be higher than that for SiO2.

Selective growth of β-SiC whisker on a patterned Si (111) substrate for a field emission device

β-SiC whiskers were grown via a vapor-solid (VS) reaction on two types of substrates; bare Si (111) and SiO 2-coated Si (111). Different growth behaviors of β-SiC whiskers were observed for each substrate. β-SiC whiskers grew only on the bare Si substrate, not on the SiO 2-coated Si surface. Therefore, β-SiC whiskers could be selectively grown along the patterns on the SiO 2-coated Si substrate. The turn-on field and the maximum current density of the specimens using selectively grown whiskers along the patterns on the SiO 2-coated Si substrate were 2.0 V/μm and 1.01 mA/cm 2, respectively. The converted curve of I– V characteristics of β-SiC whiskers by the Fowler–Nordheim equation maintained a linear slope.