Showing papers in "Physica Status Solidi (a) in 2016"

Graphene growth on silicon carbide: A review

Abstract: Graphene has been widely heralded over the last decade as one of the most promising nanomaterials for integrated, miniaturized applications spanning from nanoelectronics, interconnections, thermal management, sensing, to optoelectronics. Graphene grown on silicon carbide is currently the most likely candidate to fulfill this promise. As a matter of fact, the capability to synthesize high-quality graphene over large areas using processes and substrates compatible as much as possible with the well-established semiconductor manufacturing technologies is one crucial requirement. We review here, the enormous scientific and technological advances achieved in terms of epitaxial growth of graphene from thermal decomposition of bulk silicon carbide and the fine control of the graphene electronic properties through intercalation. Finally, we discuss perspectives on epitaxial graphene growth from silicon carbide on silicon, a particularly challenging area that could result in maximum benefit for the integration of graphene with silicon technologies.

Cubic and orthorhombic SnS thin‐film absorbers for tin sulfide solar cells

Abstract: The tin sulfide solar cell has acquired prominence in recent years. We present the characteristics of two polymorphs of SnS and their perspectives in thin-film solar cells. Thin-film SnS with cubic crystalline structure (SnS-CUB) was obtained via two chemical routes. This semiconductor is distinct from the more common SnS thin films of orthorhombic crystalline structure (SnS-ORT), also obtained by chemical routes. The SnS-CUB reported here with a lattice constant a of 11.587 A replaces the zinc blende structure previously reported for this material with a of 5.783 A. Thin films of SnS-CUB have an optical bandgap (Eg) of 1.66–1.72 eV and electrical conductivity (σ) of 10−6 Ω−1 cm−1. These characteristics distinguish them from SnS-ORT presented here with an Eg of 1.1 eV and σ typically higher by two orders of magnitude. We discuss the uncertainties that have prevailed in the assignment of crystalline structure for SnS-CUB and SnS-ORT. The optical and electrical properties of these two polymorphs of SnS are contrasted in the context of light-generated current density in solar cells. We conclude that the two SnS polymorphs when considered together as optical absorbers offer wider prospects for tin sulfide thin-film solar cells.

Synthesis of wide bandgap Ga2O3 (Eg - 4.6-4.7 eV) thin films on sapphire by low pressure chemical vapor deposition

Abstract: This paper presents the synthesis of wide bandgap Ga2O3 thin films on differently oriented sapphire substrates by using low pressure chemical vapor deposition (LPCVD) technique. The effects of substrate orientation on the Ga2O3 thin film surface morphology, crystal orientation, growth rate, and optical properties were studied. The Ga2O3 thin films were synthesized on the c-plane (0001), a-plane (11−20), and r-plane (1−102) sapphire substrates using high purity metallic Ga and oxygen (O2) as source materials and argon (Ar) as carrier gas. The Ga2O3 thin films grown on the c-plane and a-plane sapphire substrates are composed of pure β-Ga2O3. A mixture of β-Ga2O3 and α-Ga2O3 phases is observed for the films grown on r-plane sapphire substrate. Well-distinct transmission, absorption, and reflectance edge at Eg ∼ 4.6–4.7 eV are visible for all the films in the optical spectra measured in the spectral range from 200 to 800 nm.

Permanent annihilation of thermally activated defects which limit the lifetime of float-zone silicon

Abstract: We have observed very large changes in the minority carrier lifetime when high purity float-zone (FZ) silicon wafers are subject to heat-treatments in the range of 200– 1100˚C. Recombination centres were found to become activated upon annealing at 450–700˚C, causing significant reductions in the bulk lifetime, detrimental for high efficiency solar cells and stable high powered devices. Photoluminescence imaging of wafers annealed at 500˚C revealed concentric circular patterns, with lower lifetimes occurring in the centre, and higher lifetimes around the periphery. Deep level transient spectroscopy measurements on samples extracted from the centre of an n-type FZ silicon wafer annealed at 500˚C revealed a large variety of defects with activation energies ranging between 0.16– 0.36eV. Our measurements indicate that vacancy related defects are causing the severe degradation in lifetime when FZ wafers are annealed at 450–700˚C. Upon annealing FZ silicon at temperatures >800°C, the lifetime is completely recovered, whereby the defect-rich regions vanish and do not reappear (permanently annihilated). Our results indicate that, in general, as-grown FZ silicon should not be assumed to be defect lean, nor can it be assumed that the bulk lifetime will remain stable during thermal processing, unless annealed at temperatures >1000°C.

Milliwatt-class GaN-based blue vertical-cavity surface-emitting lasers fabricated by epitaxial lateral overgrowth

Abstract: We have achieved continuous-wave (CW) operation of gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs) fabricated by epitaxial lateral overgrowth (ELO) using dielectric distributed Bragg reflectors (DBRs) as masks for selective growth. The GaN VCSELs exhibited CW operation at a wavelength of 453.9 nm, and the maximum output power was 1.1 mW, which is the highest value reported to date. GaN-based materials have presented challenges for obtaining DBRs with high reflectivity and a wide stopband, precise control of the cavity length and a lateral confinement structure to provide laser operation. The proposed VCSEL is immune to these concerns. Its two dielectric DBRs were obtained free from cracks. A high reflectance of more than 99.9% and a stopband with a width of 80–97 nm were obtained for both DBRs. The cavity length was controlled by epitaxial growth to as short as 4.5 µm. An ITO contact electrode on p-type GaN, which is required for a lateral confinement structure, showed electrical reliability under a high current density of 59.6 kA cm−2. The present data demonstrate that the fabrication process adopted here overcomes the shortcomings that have prevented the widespread use of GaN-based VCSELs.

Semi‐transparent NiO/ZnO UV photovoltaic cells

Abstract: Semi-transparent pn-heterojunctions were fabricated from pulsed laser deposited (PLD) n-type ZnO and DC magnetron sputtered p-type NiO, working as UV-active solar cells. The complete cell stack has an average transmission of 46% in the visible spectral range and an optical absorption edge at 380 nm. The diodes exhibit high current rectification of up to eight orders of magnitude at ±2 V. Upon illumination with a solar simulator, the devices show photovoltaic activity with open-circuit voltages of up to 520 mV, short-circuit current densities of 0.5 mAcm2, and a maximum external quantum efficiency of 55%. However, we observed rather low fill factors of the current–voltage characteristics of around 40%, resulting in total power conversion efficiencies of around 0.1% and efficiencies in the UV range of 3.1%. To identify possible loss mechanisms, the voltage-dependent efficiency of carrier collection was calculated. The data were fitted using a model that considers recombination losses at the NiO/ZnO interface as well as within the electric field region, yielding a high hole interface recombination velocity of 1×105cm s−1. We conclude that the carrier collection efficiency is strongly deteriorated by recombination losses at the NiO/ZnO interface, causing a strong bending of the jV characteristics under illumination and thereby low fill factors.

Preparation and characterization of methylammonium tin iodide layers as photovoltaic absorbers

Abstract: The preparation of absorber layers composed of methylammonium tin iodide (CH3NH3SnI3) in a two-step process was investigated. This material is designed as a less toxic alternative to CH3NH3PbI3 which is commonly used as active material in perovskite solar cells. Tin(II) iodide (SnI2) layers prepared by physical vapor deposition were converted to CH3NH3SnI3 by reaction with a spin-coated solution of methylammonium iodide (MAI). The perovskite particles formed in this process were over 200 nm in size and reached full surface coverage. A band gap of 1.23 eV was determined and the material, thus, absorbs over a broad part of the solar spectrum, broader even than CH3NH3PbI3. The chemical composition and solid state structure of the prepared films were analyzed by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. The films turned out to be remarkably stable, another key prerequisite for applications as absorber layers in perovskite solar cells.

Electrochemical processes and device improvement in conductive bridge RAM cells

Abstract: In this paper, we discuss the recent progress on the fundamental understandings of ECM/CBRAM cells but also on the improved device structures and reliability for high-density applications. The influences of the local chemical environment and the material selection/combination are highlighted, and the filament dynamics is described in a general framework that relates all the reported switching modes. Furthermore we also detail some correlation evidences between the filament shape and device electrical characteristics. Finally, we discuss technological challenges related to current-scaling and cell size-scaling. Large switching variability associated to low current needs to be mitigated by appropriate Write-verify methods, while cell scaling requires novel processing techniques such as Cu dry-etch.

Radiation effects on two-dimensional materials

Abstract: The effects of electromagnetic and particle irradiation on two-dimensional materials (2DMs) are discussed in this review. Radiation creates defects that impact the structure and electronic performance of materials. Determining the impact of these defects is important for developing 2DM-based devices for use in high-radiation environments, such as space or nuclear reactors. As such, most experimental studies have been focused on determining total ionizing dose damage to 2DMs and devices. Total dose experiments using X-rays, gamma rays, electrons, protons, and heavy ions are summarized in this review. We briefly discuss the possibility of investigating single event effects in 2DMs based on initial ion beam irradiation experiments and the development of 2DM-based integrated circuits. Additionally, beneficial uses of irradiation such as ion implantation to dope materials or electron-beam and helium-beam etching to shape materials have begun to be used on 2DMs and are reviewed as well. For non-ionizing radiation, such as low-energy photons, we review the literature on 2DM-based photo-detection from terahertz to UV. The majority of photo-detecting devices operate in the visible and UV range, and for this reason they are the focus of this review. However, we review the progress in developing 2DMs for detecting infrared and terahertz radiation.

A chemists view: Metal oxides with adaptive structures for thermoelectric applications

Abstract: Thermoelectric devices can help to tackle future challenges in the energy sector through the conversion of waste heat directly into usable electric energy. For a wide applicability low-cost materials with reasonable thermoelectric performances and cost-efficient preparation techniques are required. In this context metal oxides are an interesting class of materials because of their inherent high-temperature stability and relative high sustainability. Their thermoelectric performance, however, needs to be improved for wide application. Compounds with adaptive structures are a very interesting class of materials. A slight reduction of early transition metal oxides generates electrons as charge carriers and crystallographic shear planes as structure motif. The crystallographic shear planes lead to a reduction of intrinsic thermal conductivity. At the same time, the electronic transport properties can be tuned by the degree of reduction. So far only a few transition metal oxides with adaptive structures have been investigated with respect to their thermoelectric properties, leaving much room for improvement. This review gives an overview of thermoelectric oxides, highlights the structural aspects of the crystallographic shear planes and the resulting thermoelectric properties.

Phosphor-converted white light from blue-emitting InGaN microrod LEDs

Abstract: A uniform array of gallium nitride core-shell microrod (MR) light-emitting diode (LED) structures was grown by metalorganic vapor phase epitaxy. Defects and the quantum well (QW) luminescence in an individual rod were investigated by scanning tunneling electron microscopy (STEM) and STEM cathodoluminescence. Luminescence with different wavelength was detected from the quantum wells on the semipolar tip facets and the nonpolar sidewalls of the MRs. Furthermore, the MR array is processed into LED chips. The electro-optical characteristics of the devices are analyzed. Two separate emission bands are distinguished, which are attributed to the QWs on the semipolar tip facets and the nonpolar sidewalls, respectively. To obtain white LEDs, micrograin phosphors were developed which fit in between individual MRs. By using electrophoretic particle deposition, these phosphors are deposited onto the MR LED chips. Color coordinates, color temperature, and device efficiency are evaluated. Blue (top) and phosphor-converted white (bottom) microrod LEDs on 4″ wafer.

Calculating the thermal conductivity of the silicon clathrates using the quasi‐harmonic approximation

Abstract: authoren A model of the lattice thermal conductivity, based on the quasi-harmonic approximation, is validated on a dataset of 42 rock-salt and zinc-blende compounds. The model reliably reproduces experimental thermal conductivities ranging over several orders of magnitude. The good performance of the model is related to the definition of the mode-averaged Gruneisen parameter. The model is applied to the silicon-based clathrates and found to correctly reproduce the two orders of magnitude reduction of the lattice thermal conductivity compared to Silicon in the diamond structure. It is shown how the introduction of the clathrate structure and the guest atoms lead to a reduction of approximately one order of magnitude each. The clathrate structure is found to both increase the anharmonicity and decrease the average phonon velocity, whereas the reduction due to the guest atom is almost purely due to increased anharmonicity.

Magnetostriction investigation of soft magnetic microwires

Abstract: The values of the magnetostriction constant, λs, of Co-based glass-coated microwires have been investigated by using the small-angle magnetization rotation (SAMR) method. Performing the systematic measurements, we were able to choose the appropriate measurement conditions achieving a high-precision result. From the dependence of the evaluated magnetostriction coefficient on annealing conditions, we evaluated the influence of the internal stresses on the magnetostriction coefficient value and the influence of the heat treatment on the magnetostriction coefficient of nearly zero magnetostrictive Co-based microwires. We observed changes of the magnetostriction value and sign after annealing. The maximum on the dependence of the magnetostriction coefficient on annealing time is explained considering superposition of the stress relaxation and ordering and beginning of the crystallization process.

High breakdown voltage p–n diodes on GaN on sapphire by MOCVD

Abstract: In order to achieve high breakdown voltage in GaN vertical power devices, low threading dislocation density and low background carrier concentration is required. This work demonstrates a decrease in the background carrier concentration and threading dislocation density (TDD) with an increase in the thickness of un-intentionally doped (UID) GaN grown on sapphire. p–n diodes grown and fabricated on this epi, using 4.8 µm UID GaN, showed a breakdown voltage of 730 V, breakdown field of 1.75 MV cm−1 and an on-resistance of 5.1 m Ω cm2. The figure of merit (FOM), VBR2/RON, thus obtained is approximately 105 MW cm−2. This is the highest reported FOM value for p–n diodes on GaN on sapphire or Si. Lowering the carrier concentration and dislocation density is thus shown to be critical for achieving high breakdown voltages on GaN on sapphire.

Effect of Al doping on the structural and optical properties of electrodeposited SnS thin films

Abstract: Al-doped SnS thin films with different Al concentrations were deposited on fluorine doped tin oxide (FTO) substrate from aqueous solution containing 2 mM SnCl2 and 20 mM Na2S2O3 and various amounts of 5 mM AlCl3 solution. The pH, temperature, time, and deposition potential (E) of the solution were kept at 2.1, 60 °C, 30 min and −1 V, respectively. The deposited films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), photoluminescence (PL), and UV-Vis. XRD patterns obviously indicated that the synthesized Al-doped SnS were polycrystalline with orthorhombic structure and by increasing the amount of Al concentration, the crystallinity was increased. The FESEM images showed that the morphology of the nanostructures was changed with increasing Al content. PL and UV-Vis analysis were used to investigate the optical properties of materials. The PL spectra showed a red shift with increasing of Al concentration.

Fabrication of graphene-based 3D structures by stereolithography

Abstract: Stereolithographic fabrication of graphene-based 3D constructs was demonstrated. The photocrosslinkable resin/graphene oxide mixtures were fabricated to samples that closely followed the structure of the designed 3D model. After 3D printings, the graphene oxide/polymer composites were pyrolyzed to reduce graphene oxide to electrically conductive graphene and to decompose the polymer, resulting in 3D structures consisting entirely of graphene. The method presented in this paper proved to be successful for producing designed 3D structures but further optimization is needed for practical applications due to high shrinking and brittleness of the pyrolyzed 3D constructs. By pyrolyzing the polymer component only partly, electrical conductivities in the range of semiconductors were achieved.

Al‐, Ga‐, and In‐doped ZnO thin films via aerosol assisted CVD for use as transparent conducting oxides

Abstract: Al-, Ga-, and In-doped ZnO thin films were deposited on glass substrates by aerosol assisted chemical vapour deposition (AACVD) at a deposition temperature of 450 °C. The air-stable compound zinc acetylacetonate [Zn(acac)2] was used as a Zn source, whilst for the dopants of Al, Ga and In, the corresponding trichloride was used. Methanol solutions of the metal salts were used as precursor solutions and N2 carrier gas was used for the aerosol. Films were grown in approximately 30 min and were synthesised using dopant values of 5, 10, 15 and 20 mol.% (with respect to the Zn) in the precursor solution. XRD analysis showed that the films were wurtzite ZnO. XPS analysis confirmed the presence of the dopants in the films. Several of the films showed high transparency (>80%) in the visible range, and low resistivity (∼10−3 Ω cm).

Influence of AlN nucleation layer on vertical breakdown characteristics for GaN‐on‐Si

Abstract: A study of metal-organic chemical vapor deposition (MOCVD) grown AlN nucleation layer (NL) on breakdown characteristics for GaN-on-Si is presented. It is widely believed that AlN NL can act as an insulator because of its large band gap ∼6.2 eV. On contrary, this study of AlN NL/Si reveals conductive nature and shows high vertical leakage. The structural examinations along with electrical characterization show AlN NL/Si quality depends on the growth temperature. The surface morphology and presence of unintentional oxygen impurities govern the vertical leakage of AlN NL/Si. Interestingly, the AlN NL influences the growth of subsequent epitaxial layers as well as their vertical breakdown voltages (BVs). Further, it is found that AlGaN intermediate layer and multipairs of AlGaN/AlN strained layer superlattice (SLS) grown over AlN NL with better surface properties enhances the vertical BV. A high BV of 1.3 kV is achieved for SLS multipairs with a total thickness of 4.4 μm and the translated breakdown field strength is 2.8 MV cm−1 for MOCVD grown GaN-on-Si.

SciFab -a wafer-level heterointegrated InP DHBT/SiGe BiCMOS foundry process for mm-wave applications

Abstract: We present a wafer-level heterointegrated indium phosphide double heterobipolar transistor on silicon germanium bipolar-complementary metal oxide semiconductor (InP DHBT on SiGe BiCMOS) process which relies on adhesive wafer bonding. Subcircuits are co-designed in both technologies, SiGe BiCMOS and InP DHBT, with more than 300 GHz bandwidth microstrip interconnects. The 250 nm SiGe HBTs offer cutoff frequencies around 200 GHz, the 800 nm InP DHBTs exceed 350 GHz. Heterointegrated signal sources are demonstrated including a 328 GHz quadrupling source with dBm RF output power. A common design kit for full InP DHBT/SiGe BiCMOS co-design was set up. The technology is being opened to third-party customers through IHP's multi-purpose wafer foundry interface. Microphotograph of InP DHBT / SiGe BiCMOS wafer

Thermal conductivity of bulk boron nitride nanotube sheets and their epoxy-impregnated composites

Abstract: The thermal conductivity of bulk, self-supporting boron nitride nanotube (BNNT) sheets composed of nominally 100% BNNTs oriented randomly in-plane was measured by a steady-state, parallel thermal conductance method. The sheets were either collected directly during synthesis or produced by dispersion and filtration. Differences between the effective thermal conductivities of filtration-produced BNNT buckypaper (∼1.5 W m−1 K−1) and lower-density as-synthesized sheets (∼0.75 W m−1 K−1), which are both porous materials, were primarily due to their density. The measured results indicate similar thermal conductivity, in the range of 7–12 W m−1 K−1, for the BNNT network in these sheets. High BNNT-content composites (∼30 wt.% BNNTs) produced by epoxy impregnation of the porous BNNT network gave 2–3 W m−1 K−1, more than 10× the baseline epoxy. The combination of manufacturability, thermal conductivity, and electrical insulation offers exciting potential for electrically insulating, thermally conductive coatings and packaging. Thermal conductivity of free-standing BNNT buckypaper, buckypaper composites, and related materials at room temperature.

Evaporated and solution deposited planar Sb2S3 solar cells: A comparison and its significance

Abstract: Sb2S3 is an alternative emerging material for chalcogenide solar cells. In this paper, we compare planar solar cells in which the Sb2S3 absorber was deposited by either thermal evaporation or a solution based process. The planar inorganic solar cell with evaporated Sb2S3 has an efficiency of 1.7%. This is much higher than the efficiency of 0.8% for solution deposited Sb2S3, and the highest known reported efficiency for a planar Sb2S3 solar cell with a CuSCN hole conducting layer. The evaporated Sb2S3 film is sulfur-rich, which results in flattening of the film surface after annealing at 300 °C, thereby reducing the likelihood of contact between the Sb2S3 and gold contact. In addition, the crystal size of the evaporated Sb2S3 film is about 30% larger than that of the solution deposited Sb2S3 and has different preferential crystal planes. These features make evaporation a promising deposition method for flat solar cells made from sulfur-based semiconducting materials. Cross section image of the evaporated Sb2S3 solar cell with labels for each layer.

AlN growth on nano‐patterned sapphire: A route for cost efficient pseudo substrates for deep UV LEDs

Abstract: C-plane-oriented sapphire substrates that were patterned on the nanoscale were overgrown by AlN using metal-organic vapor phase epitaxy. The occurrence of undesired misaligned AlN growth was detected. We found that this misaligned growth can be overcome by a proper choice of growth temperature and V/III ratio. Up to 8 μm thick c-plane-oriented AlN with a coalesced surface was obtained. An effective dislocation reduction was found due to bending of threading dislocation lines toward free surfaces during lateral growth. The distribution of crystal defects suggests that step bunching in AlN is accompanied by dislocation accumulation. Furthermore, nearly defect-free AlN crystallites with a hexagonal shape and a size of about 2 μm were observed. Schematic cross-section representation of AlN grown on nano-patterned sapphire. Different AlN crystal orientations (arrows) and dislocations (solid black lines) are indicated.

Capacitance roll-off and frequency-dispersion capacitance–conductance phenomena in field plate and guard ring edge-terminated Ni/SiO2/4H-nSiC Schottky barrier diodes

Abstract: In this work, field plate and guard ring edge-terminated Ni/4H-nSiC Schottky barrier diodes (SBD) were fabricated using standard photolithography process. Strange peaks in capacitance–conductance curves, capacitance roll-off, and a high value of ideality factor (η = 1.3) in fabricated SBD were seen as a signature of interface trap states (Nss) at the residual oxide (2.2 nm)/4H-nSiC interface and series resistance (Rs). Schottky capacitance spectroscopic, High–low capacitance–voltage (C–V) and forward-bias current–voltage (I–V) techniques, in the frequency range from 100 Hz to 1 MHz, determines Nss of the order of 1012 cm−2 eV−1 and were found exponentially distributed in the bandgap of SiC. Using Hill–Coleman's method, the density Nss was calculated to be 1.15 × 1015 cm−2 eV−1 at 100 Hz and 7.81 × 1012 cm−2 eV−1 at 1 MHz, which explains the larger value of capacitance at low frequencies. Relaxation times and capture cross sections of Nss were also estimated. Calculated values of Nss were used in a Silvaco simulation that emphasize that bulk level defects present in the SiC also contributes in the experimentally observed strange peaks in C–V characteristics of fabricated SBD. At higher current levels, calculated values of Rs (V, f), confirm an increase of leakage current through residual oxide and describes the capacitance roll-off phenomena in the fabricated SBD.

A hybrid memristor-CMOS XOR gate for nonvolatile logic computation

Abstract: Memristor-based logic gates that can execute memory and logic operations are promising elements for building non-von Neumann computation architecture. In this paper, we proposed a fundamental hybrid memristor-CMOS nonvolatile XOR Boolean logic block with one memristor and four voltage-controlled switches, such as MOSFET or transmission gate. The Ag/Ag5In5Sb60Te30/Ta structure memristors with bipolar resistive switching behaviors were utilized to experimentally demonstrate the XOR function of the block. Further, with the XOR logic block as a basis, a full adder as an extensive circuit example was designed and verified by HSPICE simulation. Our work shows logic-in-memory capabilities with memristors, and provides an alternative way to implement nonvolatile logic operations and construct parallel computation systems.

Cubic-structured tin selenide thin film as a novel solar cell absorber

Abstract: Tin selenide thin film with a simple cubic crystalline structure (SnSe-CUB) of unit cell dimension a = 11.9632 A is obtained via chemical deposition on a tin sulfide (SnS-CUB) thin film base layer of simple cubic structure of a = 11.5873 A. The SnSe-CUB films obtained this way are thermally stable while heating to 300 °C. Its optical band gap is 1.4 eV. A thin film of 200 nm in thickness of this material in a solar cell may lead to a light generated current density of 23 mA cm−2 and a maximum of 29 mA cm−2. Thin film of SnSe-CUB possesses p-type electrical conductivity of 5 × 10−5 Ω−1 cm−1, which is three orders of magnitude lower than that of SnSe films of orthorhombic crystalline structure. Overall, these characteristics make SnSe-CUB thin film a novel solar cell absorber material.

Making one-dimensional electrical contacts to molybdenum disulfide-based heterostructures through plasma etching

Abstract: A “passivation first, metallization second” technique is developed for fabricating edge contacts to a multi-layer MoS2 sample encapsulated under an Al2O3 thin film. The in-time sealing of the newly exfoliated MoS2 under a dielectric ensures a complete isolation from the environment. CF4 plasma is used to open trenches in the passivation layer and to expose the atoms at the edges of MoS2. Edge contacts are next made to h-BN/MoS2/h-BN 3-level heterostructures, earlier assembled through a solvent-free 2D material transfer procedure. Both types of MoS2-based heterostructures are further fabricated into back-gated FETs and show n-type doping behavior. In particular, trends of field-effect mobility with respect to a varying drain voltage are analyzed based on the ID–VDS data measured from each device. The result verifies the effect of Schottky barrier on channel conduction, which is, only at the presence of a highly transparent contact interface, the field-effect mobility can manifest the intrinsic material property by staying constant against the changes in drain voltage. The wide applicability of the processing sequence makes edge contacts an appealing option to future nanoelectronics on 2D heterostructures.

Effects of silicon doping on the performance of tin oxide thin film transistors

Abstract: Thin film transistors (TFTs) with silicon-doped tin oxide (TSO) as channel layer were prepared by radio frequency magnetron sputtering. The decreased defect-state-related peak in photoluminescence (PL) excitation spectra and oxygen-vacancy-related O 1s peak in X-ray photoelectron spectroscopy (XPS) with increasing Si content, accompanied by the decreased off-state current and positive shift of turn-on voltage, confirms that silicon can be a good carrier suppressor. The optimum TFT performance after annealing at 300 °C was achieved at Si content 5.4 at.%, with saturation mobility of 5.3 cm2 V−1 s−1, turn-on voltage of −0.2 V, and on-off current ratio of 3.3 × 106, respectively. Increasing the annealing temperature is useful to improve the mobility, but the polycrystalline structure formed above 350 °C goes against the uniformity of oxide TFTs.

Optical vortex pulse illumination to create chiral monocrystalline silicon nanostructures

Abstract: We discovered that a nanosecond optical vortex pulse can create a chiral cone-shaped monocrystalline silicon (Si) nanostructure (chiral Si nanocone) by transferring its optical angular momentum to a monocrystalline Si substrate. The fabricated Si nanocone, with a length of 4.8 µm and a tip curvature of ∼110 nm, was fully monocrystalline, and it had a spiral structure with an 86 nm line width on a conical surface. Furthermore, its chirality was also determined directly from the handedness of the optical vortex pulse. Such chiral Si nanocones may enable the development of novel silicon photonic devices as well as ultra-highly efficient photovoltaic devices.

Design and fabrication of a 1.2 kV GaN-based MOS vertical transistor for single chip normally off operation

Abstract: We report a novel design to achieve normally off, high voltage power switch using an “all-GaN” vertical MOS-gate transistor (MOSVFET). In this structure, two MOS gates were formed vertically on the sidewalls of the pillar, whose opposite ends were connected to the source and drain of the device. The design space and associated performance based on 2D drift-diffusion model is discussed highlighting the trade-off between blocking voltage (Vbl), on resistance (Ron), and threshold voltage (Vth). An optimized device structure was proposed with more than 1.2 kV blocking capability and normally off behavior. With proper design of the drift region thickness and doping, the corresponding Ron was as low as 2.8 mΩ cm2. The role of key parameters such as bulk GaN mobility, channel mobility, gate to gate distance (Lgtg) and gate length (Lg) on the Ron, Vbl, leakage current, and Vth were also examined. The effect of mobility on the device performance and the role of the bulk GaN material was analyzed utilizing the model to create a comprehensive design space for achieving low-loss switching.

A study on the thermal sintering process of silver nanoparticle inkjet inks to achieve smooth and highly conducting silver layers

Abstract: Silver nanoparticle inkjet inks are commonly used to print electrically conductive patterns, such as sensors or electrodes in organic light emitting diodes (OLEDs) or organic photovoltaic devices (OPVs). After printing, a sintering step is required to transform the printed layer into an electrically conductive one. Gaining more insight into the occurring phenomena during this post-treatment step is necessary when applying different kinds of inkjet ink. Therefore, in this work the commercially available silver nanoparticle inkjet ink Metalon JS-B30G from Novacentrix is characterised during the different stages in the printing and thermal sintering sequence. The pre-printing and post-sintering characterisation proves that the inkjet ink used has got the right material parameters, such as viscosity and particle size. Silver layers with sheet resistances of 40 mΩ/sq were obtained with an average roughness lower than 10 nm. The experiments performed show the different stages during the thermal sintering procedure. Based on this, suitable thermal sintering parameters are defined leading to application of these conductive silver layers in OLEDs.