Showing papers by "Ali Javey published in 2013"

User-interactive electronic skin for instantaneous pressure visualization

Abstract: Electr onic skin (e-skin) presents a network of mechanically flexible sensors that can conformally wrap irregular surfaces and spatially map and quantify various stimuli 1‐12 . Previous works on e-skin have focused on the optimization of pressure sensors interfaced with an electronic readout, whereas user interfaces based on a human-readable output were not explored. Here, we report the first user-interactive e-skin that not only spatially maps the applied pressure but also provides an instantaneous visual response through a built-in active-matrix organic light-emitting diode display with red, green and blue pixels. In this system, organic light-emitting diodes (OLEDs) are turned on locally where the surface is touched, and the intensity of the emitted light quantifies the magnitude of the applied pressure. This work represents a system-on-plastic 4,13‐17 demonstration where three distinct electronic components— thin-film transistor, pressure sensor and OLED arrays—are monolithically integrated over large areas on a single plastic substrate. The reported e-skin may find a wide range of applications in interactive input/control devices, smart wallpapers, robotics and medical/health monitoring devices.

Degenerate n-doping of few-layer transition metal dichalcogenides by potassium.

Abstract: We report here the first degenerate n-doping of few-layer MoS2 and WSe2 semiconductors by surface charge transfer using potassium. High-electron sheet densities of ~1.0 × 10(13) cm(-2) and 2.5 × 10(12) cm(-2) for MoS2 and WSe2 are obtained, respectively. In addition, top-gated WSe2 and MoS2 n-FETs with selective K doping at the metal source/drain contacts are fabricated and shown to exhibit low contact resistances. Uniquely, WSe2 n-FETs are reported for the first time, exhibiting an electron mobility of ~110 cm(2)/V·s, which is comparable to the hole mobility of previously reported p-FETs using the same material. Ab initio simulations were performed to understand K doping of MoS2 and WSe2 in comparison with graphene. The results here demonstrate the need of degenerate doping of few-layer chalcogenides to improve the contact resistances and further realize high performance and complementary channel electronics.

Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on Flexible Substrates

Abstract: Fully printed transistors are a key component of ubiquitous flexible electronics. In this work, the advantages of an inverse gravure printing technique and the solution processing of semiconductor-enriched single-walled carbon nanotubes (SWNTs) are combined to fabricate fully printed thin-film transistors on mechanically flexible substrates. The fully printed transistors are configured in a top-gate device geometry and utilize silver metal electrodes and an inorganic/organic high-κ (∼17) gate dielectric. The devices exhibit excellent performance for a fully printed process, with mobility and on/off current ratio of up to ∼9 cm2/(V s) and 105, respectively. Extreme bendability is observed, without measurable change in the electrical performance down to a small radius of curvature of 1 mm. Given the high performance of the transistors, our high-throughput printing process serves as an enabling nanomanufacturing scheme for a wide range of large-area electronic applications based on carbon nanotube networks.

Carbon nanotube electronics – moving forward

Abstract: Single-walled carbon nanotubes (SWNTs) possess fascinating electrical properties and offer new entries into a wide range of novel electronic applications that are unattainable with conventional Si-based devices. The field initially focused on the use of individual or parallel arrays of nanotubes as the channel material for ultra-scaled nanoelectronic devices. However, the challenge in the deterministic assembly has proven to be a major technological barrier. In recent years, solution deposition of semiconductor-enriched SWNT networks has been actively explored for high performance and uniform thin-film transistors (TFTs) on mechanically rigid and flexible substrates. This presents a unique niche for nanotube electronics by overcoming their limitations and taking full advantage of their superb chemical and physical properties. This review focuses on the large-area processing and electronic properties of SWNT TFTs. A wide range of applications in conformal integrated circuits, radio-frequency electronics, artificial skin sensors, and displays are discussed – with emphasis on large-area systems where nm-scale accuracy in the assembly of nanotubes is not required. The demonstrations show SWNTs' immense promise as a low-cost and scalable TFT technology for nonconventional electronic systems with excellent device performances.

Ballistic InAs nanowire transistors.

Abstract: Ballistic transport of electrons at room temperature in top-gated InAs nanowire (NW) transistors is experimentally observed and theoretically examined. From length dependent studies, the low-field mean free path is directly extracted as ∼150 nm. The mean free path is found to be independent of temperature due to the dominant role of surface roughness scattering. The mean free path was also theoretically assessed by a method that combines Fermi’s golden rule and a numerical Schrodinger–Poisson simulation to determine the surface scattering potential with the theoretical calculations being consistent with experiments. Near ballistic transport (∼80% of the ballistic limit) is demonstrated experimentally for transistors with a channel length of ∼60 nm, owing to the long mean free path of electrons in InAs NWs.

Reactive Sputtering of Bismuth Vanadate Photoanodes for Solar Water Splitting

Abstract: Bismuth vanadate (BiVO4) has attracted increasing attention as a photoanode for photoelectrochemical (PEC) water splitting. It has a band gap in the visible light range (2.4–2.5 eV) and a valence band position suitable for driving water oxidation under illumination. While a number of methods have been used to make BiVO4 photoanodes, scalable thin film deposition has remained relatively underexplored. Here, we report the synthesis of BiVO4 thin films by reactive sputtering. The use of separate Bi and V sputtering targets allows control of the Bi/V ratio in the film. Under optimized, slightly V-rich conditions, monoclinic phase BiVO4 with photoactivity for water oxidation is obtained. The highest photocurrents, ca. 1 mA cm–2 at the reversible O2/H2O potential with simulated AM 1.5G illumination, are obtained with bilayer WO3/BiVO4, where the WO3 serves as a hole-blocking layer.

Amorphous Si thin film based photocathodes with high photovoltage for efficient hydrogen production.

Abstract: An amorphous Si thin film with TiO2 encapsulation layer is demonstrated as a highly promising and stable photocathode for solar hydrogen production. With platinum as prototypical cocatalyst, a photocurrent onset potential of 0.93 V vs RHE and saturation photocurrent of 11.6 mA/cm2 are measured. Importantly, the a-Si photocathodes exhibit impressive photocurrent of ∼6.1 mA/cm2 at a large positive bias of 0.8 V vs RHE, which is the highest for all reported photocathodes at such positive potential. Ni–Mo alloy is demonstrated as an alternative low-cost catalyst with onset potential and saturation current similar to those obtained with platinum. This low-cost photocathode with high photovoltage and current is a highly promising photocathode for solar hydrogen production.

Uncovering the intrinsic size dependence of hydriding phase transformations in nanocrystals

Abstract: Although quantitative understanding of nanocrystal phase transformations is important for efficient energy conversion and catalysis, difficulties in directly monitoring nanoscale systems in reactive environments remain. Direct quantification of hydriding transformations in palladium nanocrystals now clearly reveals that the transformation rates are governed by nanocrystal dimensions.

Carbon Nanotube Active-Matrix Backplanes for Mechanically Flexible Visible Light and X-ray Imagers

Abstract: We report visible light and X-ray imagers on lightweight and mechanically flexible plastic substrates. The process involves solution processing of organic photodetectors on top of an active-matrix backplane consisting of carbon nanotube thin-film transistors. The system takes advantage of the high mobility of nanotube transistors for low operating voltages and efficient light absorption of organic bulk-heterojunctions for high imaging sensitivity. With this highly scalable process scheme, 18 × 18 pixel-array flexible imagers (physical size of 2 cm × 1.5 cm) with high performance are successfully demonstrated. In addition, as the absorption peak of the adopted organic photodiodes covers the green band of the light spectrum, X-ray imaging is readily demonstrated by placing a scintillator film on top of the flexible imagers.

Quantum of optical absorption in two-dimensional semiconductors

Abstract: The optical absorption properties of free-standing InAs nanomembranes of thicknesses ranging from 3 nm to 19 nm are investigated by Fourier transform infrared spectroscopy. Stepwise absorption at room temperature is observed, arising from the interband transitions between the subbands of 2D InAs nanomembranes. Interestingly, the absorptance associated with each step is measured to be ∼1.6%, independent of thickness of the membranes. The experimental results are consistent with the theoretically predicted absorptance quantum, A Q = πα/n c for each set of interband transitions in a 2D semiconductor, where α is the fine structure constant and n c is an optical local field correction factor. Absorptance quantization appears to be universal in 2D systems including III–V quantum wells and graphene.

Short-Channel Transistors Constructed with Solution-Processed Carbon Nanotubes

Abstract: We develop short-channel transistors using solution-processed single-walled carbon nanotubes (SWNTs) to evaluate the feasibility of those SWNTs for high-performance applications. Our results show that even though the intrinsic field-effect mobility is lower than the mobility of CVD nanotubes, the electrical contact between the nanotube and metal electrodes is not significantly affected. It is this contact resistance which often limits the performance of ultrascaled transistors. Moreover, we found that the contact resistance is lowered by the introduction of oxygen treatment. Therefore, high-performance solution-processed nanotube transistors with a 15 nm channel length were obtained by combining a top-gate structure and gate insulators made of a high-dielectric-constant ZrO2 film. The combination of these elements yields a performance comparable to that obtained with CVD nanotube transistors, which indicates the potential for using solution-processed SWNTs for future aggressively scaled transistor technology.

Near-ideal electrical properties of InAs/WSe2 van der Waals heterojunction diodes

Abstract: Here, we present the fabrication and electrical analysis of InAs/WSe2 van der Waals heterojunction diodes formed by the transfer of ultrathin membranes of one material upon another. Notably, InAs and WSe2 are two materials with completely different crystal structures, which heterojunction is inconceivable with traditional epitaxial growth techniques. Clear rectification from the n-InAs/p-WSe2 junction (forward/reverse current ratio >106) is observed. A low reverse bias current <10−12A/μm2 and ideality factor of ∼1.1 were achieved, suggesting near-ideal electrically active interfaces.

A direct thin-film path towards low-cost large-area III-V photovoltaics

Abstract: III-V photovoltaics (PVs) have demonstrated the highest power conversion efficiencies for both single- and multi-junction cells. However, expensive epitaxial growth substrates, low precursor utilization rates, long growth times, and large equipment investments restrict applications to concentrated and space photovoltaics (PVs). Here, we demonstrate the first vapor-liquid-solid (VLS) growth of high-quality III-V thin-films on metal foils as a promising platform for large-area terrestrial PVs overcoming the above obstacles. We demonstrate 1–3 mm thick InP thin-films on Mo foils with ultra-large grain size up to 100 mm, which is ,100 times larger than those obtained by conventional growth processes. The films exhibit electron mobilities as high as 500 cm 2 /V-s and minority carrier lifetimes as long as 2.5 ns. Furthermore, under 1-sun equivalent illumination, photoluminescence efficiency measurements indicate that an open circuit voltage of up to 930 mV can be achieved, only 40 mV lower than measured on a single crystal reference wafer.

Spin-On Organic Polymer Dopants for Silicon

Abstract: We introduce a new class of spin-on dopants composed of organic, dopant-containing polymers. These new dopants offer a hybrid between conventional inorganic spin-on dopants and a recently developed organic monolayer doping technique that affords unprecedented control and uniformity of doping profiles. We demonstrate the ability of polymer film doping to achieve both p-type and n-type silicon by using boron- and phosphorus-containing polymer films. Different doping mechanisms are observed for boron and phosphorus doping, which could be related to the specific chemistries of the polymers. Thus, there is an opportunity to further control doping in the future by tuning the polymer chemistry.

High quality interfaces of InAs-on-insulator field-effect transistors with ZrO2 gate dielectrics

Abstract: Interface quality of InAs-on-insulator (XOI) field-effect transistors (FETs) with a ZrO2 gate dielectric is examined as a function of various chemical treatments With a forming gas anneal, InAs XOI FETs exhibit a low subthreshold swing of ∼72 mV/dec with an interface trap density of ∼15 × 1012 states/cm2 eV—both of which are comparable to the best reported epitaxially grown III-V devices on III-V substrates Importantly, the results indicate that the surface properties of InAs are preserved during the layer transfer process, thereby, enabling the realization of high performance III-V FETs on Si substrates using the XOI configuration

Surface Charge Transfer Doping of III–V Nanostructures

Abstract: Surface charge transfer is presented as an effective doping technique for III–V nanostructures. We generalize that the technique is applicable to nanoscale semiconductors in the limit where carriers are quantum confined. As a proof-of-concept, potassium surface charge transfer doping is carried out for one-dimensional (1D) and two-dimensional (2D) InAs on Si/SiO2 substrates. Experiments and simulations show that equivalent dopant areal dose of up to ∼2 × 1012 cm–2 is obtained, which is sufficient for degenerate doping of InAs nanostructures. This work presents a new pathway for controllable doping of inorganic semiconductors with limits fundamentally different from those of substitutional doping.

Influence of catalyst choices on transport behaviors of InAs NWs for high-performance nanoscale transistors

Abstract: The influence of the catalyst materials on the electron transport behaviors of InAs nanowires (NWs) grown by a conventional vapor transport technique is investigated. Utilizing the NW field-effect transistor (FET) device structure, ∼20% and ∼80% of Au-catalyzed InAs NWs exhibit strong and weak gate dependence characteristics, respectively. In contrast, ∼98% of Ni-catalyzed InAs NWs demonstrate a uniform n-type behavior with strong gate dependence, resulting in an average OFF current of ∼10−10 A and a high ION/IOFF ratio of >104. The non-uniform device performance of Au-catalyzed NWs is mainly attributed to the non-stoichiometric composition of the NWs grown from a different segregation behavior as compared to the Ni case, which is further supported by the in situ TEM studies. These distinct electrical characteristics associated with different catalysts were further investigated by the first principles calculation. Moreover, top-gated and large-scale parallel-array FETs were fabricated with Ni-catalyzed NWs by contact printing and channel metallization techniques, which yield excellent electrical performance. The results shed light on the direct correlation of the device performance with the catalyst choice.

Two-dimensional to three-dimensional tunneling in InAs/AlSb/GaSb quantum well heterojunctions

Abstract: We examine room temperature band-to-band tunneling in 2D InAs/3D GaSb heterostructures. Specifically, multi-subband, gate-controlled negative differential resistance is observed in InAs/AlSb/GaSb junctions. Due to spatial confinement in the 10 nm-thick InAs layer, tunneling contributions from two distinct subbands are observed as sharp steps in the current-voltage characteristics. It is shown that the relative position of the steps can be controlled via external gate bias. Additionally, the extracted separation in the subband energy agrees well with the calculated values. This is the first demonstration of a gate controlled tunneling diode with multiple subband contributions.

Carbon nanotube network thin-film transistors on flexible/stretchable substrates

Abstract: This disclosure provides systems, methods, and apparatus for flexible thin-film transistors. In one aspect, a device includes a polymer substrate, a gate electrode disposed on the polymer substrate, a dielectric layer disposed on the gate electrode and on exposed portions of the polymer substrate, a carbon nanotube network disposed on the dielectric layer, and a source electrode and a drain electrode disposed on the carbon nanotube network.

Be Critical but Fair

Abstract: Author(s): Parak, Wolfgang J; Chan, Warren CW; Hafner, Jason H; Hammond, Paula T; Hersam, Mark C; Javey, Ali; Khademhosseini, Ali; Kotov, Nicholas A; Mulvaney, Paul; Nel, Andre E; Nordlander, Peter J; Penner, Reginald M; Rogach, Andrey L; Schaak, Raymond E; Stevens, Molly M; Wee, Andrew TS; Willson, C Grant; Weiss, Paul S

MORPHOLOGICAL AND SPATIAL CONTROL OF InP CRYSTAL GROWTH USING CLOSED-SPACED SUBLIMATION

Abstract: A new solar cell comprising a substrate, a VIB metal thin film deposited on the substrate, and a polycrystalline III-V semiconductor thin film deposited on the VIB metal thin film. A method of making a solar cell comprising providing a substrate, depositing a VIB metal thin film on the substrate, and depositing a polycrystalline III-V semiconductor thin film on the VIB metal thin film. In one embodiment, a polycrystalline III-V semiconductor thin film comprising Indium Phosphide (InP) is deposited on a VIB metal thin film comprising Molybdenum (Mo) by Metal Organic Chemical Vapor Deposition (MOCVD). In another embodiment, growth of Indium phosphide (InP) crystals directly on metal foils is described using a method comprising a closed-spaced sublimation (CSS). In another embodiment, both InP nanowires and polycrystalline films were obtained by tuning growth conditions. In another embodiment, utilizing a silicon dioxide mask, selective nucleation of InP on metal substrates was obtained.

High optical quality polycrystalline indium phosphide grown on metal substrates by mocvd

Abstract: A new solar cell is disclosed wherein the solar cell comprises a substrate, a VIB metal thin film deposited on the substrate, and a polycrystalline III-V semiconductor thin film deposited on the VIB metal thin film. A method of making the solar cell is described comprising providing a substrate, depositing a VIB metal thin film on the substrate, and depositing a polycrystalline III-V semiconductor thin film on the VIB metal thin film. In one embodiment a polycrystalline III-V semiconductor thin film comprising Indium Phosphide (InP) is deposited on a VIB metal thin film comprising Molybdenum (Mo) by Metal Organic Chemical Vapor Deposition (MOCVD).

Carbon Nanotube Electronics — Moving Forward

Abstract: Single-walled carbon nanotubes (SWNTs) possess fascinating electrical properties and offer new entries into a wide range of novel electronic applications that are unattainable with conventional Si-based devices. The field initially focused on the use of individual or parallel arrays of nanotubes as the channel material for ultra-scaled nanoelectronic devices. However, the challenge in the deterministic assembly has proven to be a major technological barrier. In recent years, solution deposition of semiconductor-enriched SWNT networks has been actively explored for high performance and uniform thin-film transistors (TFTs) on mechanically rigid and flexible substrates. This presents a unique niche for nanotube electronics by overcoming their limitations and taking full advantage of their superb chemical and physical properties. This review focuses on the large-area processing and electronic properties of SWNT TFTs. A wide range of applications in conformal integrated circuits, radio-frequency electronics, artificial skin sensors, and displays are discussed – with emphasis on large-area systems where nm-scale accuracy in the assembly of nanotubes is not required. The demonstrations show SWNTs' immense promise as a low-cost and scalable TFT technology for nonconventional electronic systems with excellent device performances.

Quantum membranes: A new materials platform for future electronics

Abstract: Over the past several years, the inherent scaling limitations of electron devices have fueled the exploration of high carrier mobility semiconductors as a Si replacement to further enhance the device performance. To ensure effective gate control, semiconductors with ultrathin body thickness are needed. At such regime, strong quantum confinement usually comes into play; therefore, we call these ultrathin compound semiconductor membranes as quantum membranes (QMs). Compound quantum membranes heterogeneously integrated on Si substrates have been studied by us, combining the high mobility of compound semiconductors and the well-established Si technology.