Large-area Uniformity Research Articles

Flexible Oxide Thin-Film Transistors Fabricated at Room Temperature By Introducing Polymer Dielectric and Microwave Annealing

Amorphous oxide semiconductors (AOSs) have attracted considerable attention as an active layer material for thin-film transistors (TFTs), due to their many advantages such as high field effect mobility and large area uniformity compared to amorphous Si counterparts. In particular, amorphous indium gallium zinc oxide (a-IGZO) have widely studied for high performance TFTs which require high mobility, high on/off ratio, low leakage current and room temperature fabrication. Even if, a-IGZO TFTs can be deposited at room temperature, additional thermal treatment is still required in order to obtain satisfactory electrical characteristic and stability.Organic materials can be used for dielectrics of flexible TFTs because of their flexible mechanical properties. Generally, organic dielectric films are fabricated by solution process like spin-coating. Spin coating process can be easy and cheap method, however, the quality of films is not as good as the one with vacuum process because of the residual impurities and thermal damage to the substrate during post-coating bake. Poly(chloro para-xylylene) (Parylene C) can be fabricated via pyrolysis-CVD process at room temperature, which lead to unique characteristics of pinhole-free nature. Also, electrical properties of parylene C films are quite enough to utilize as dielectric layer of TFTs. Moreover, parylene C can act as passivation layer due to their remarkable chemical resistance.In this work, microwave annealing and parylene C are adopted to IGZO TFTs to reduce overall process temperature. Microwave can selectively heat target material by conversion of electromagnetic energy to thermal energy inside of the target material. Finally, flexible IGZO TFTs were realized on PET substrate owing to low process temperature of microwave annealing and parylene C deposition.Transfer and output characteristics were measured in order to investigate the possibility of parylene C as an insulator on oxide TFTs. The device exhibit typical output characteristics with a clear pinch-off, current saturation and no current crowding. When the applied gate voltage bias was 20 V, the saturation drain current was around 230 μA at drain voltage of 20 V. From transfer curve, fully room temperature fabricated IGZO TFTs exhibit mobility, Vth, S.S and hysteresis of 12.9 cm2/Vs, 0.796 V, 0.651 V/dec and 0.62 V. Flexible IGZO TFTs were fabricated using PET substrate which shows low glass transition temperature. The device was well operated which indicate microwave annealing and parylene C deposition process is suitable for low glass transition temperature substrate. The electrical stability of the IGZO TFTs with parylene C gate dielectric under bending stress was evaluated. After bending 15,000 times, switching characteristics was still remained. Mobility and S.S were hardly deteriorated, however, Vth shows small negative shift of near -1 V. In this work, IGZO TFTs were fabricated at room temperature by introducing microwave annealing and room temperature deposited gate dielectric, parylene C. By combining microwave annealing and room temperature fabricated parylene C, the process temperature was decreased enough to utilize flexible substrate with low glass transition temperature. Fabricated flexible IGZO TFTs was shown to withstand harsh 15,000 bending test. Figure 1

Direct Writing Large-Area Multi-Layer Ultrasmooth Films by an All-Solution Process: Toward High-Performance QLEDs.

With increasing the film area/layer, deteriorating in both smoothness and uniformity of thin-films frequently happen, which remains a barrier for making large-area quantum dot light-emitting diodes (QLEDs) by solution processes. Here, we demonstrated a facile all-solution process guided by the conical fiber array to write multi-layer ultrasmooth thin-films directly in centimeter scale. The side-by-side fibrous array helps to align surface tensions at the tri-phase contact line to facilitate large-area homogeneous deposition, which was verified by theoretical simulation. The Laplace pressure along individual conical fiber contributes to the steady liquid transfer. Thin-films with small roughness (<2.03 nm) and large-area (2×2 cm2 ) uniformity were prepared sequentially on the target substrate, leading to large-area high-performance QLEDs. The result offers new insights for fabricating large-area high-performance thin-film devices.

Controllable deposition of organic metal halide perovskite films with wafer-scale uniformity by single source flash evaporation

Conventional solution-processing techniques such as the spin-coating method have been used successfully to reveal excellent properties of organic–inorganic halide perovskites (OHPs) for optoelectronic devices such as solar cell and light-emitting diode, but it is essential to explore other deposition techniques compatible with large-scale production. Single-source flash evaporation technique, in which a single source of materials of interest is rapidly heated to be deposited in a few seconds, is one of the candidate techniques for large-scale thin film deposition of OHPs. In this work, we investigated the reliability and controllability of the single-source flash evaporation technique for methylammonium lead iodide (MAPbI3) perovskite. In-depth statistical analysis was employed to demonstrate that the MAPbI3 films prepared via the flash evaporation have an ultrasmooth surface and uniform thickness throughout the 4-inch wafer scale. We also show that the thickness and grain size of the MAPbI3 film can be controlled by adjusting the amount of the source and number of deposition steps. Finally, the excellent large-area uniformity of the physical properties of the deposited thin films can be transferred to the uniformity in the device performance of MAPbI3 photodetectors prepared by flash evaporation which exhibited the responsivity of 51 mA/W and detectivity of 9.55 × 1010 Jones.

New Growth Frontier: Superclean Graphene.

The last 10 years have witnessed significant progress in chemical vapor deposition (CVD) growth of graphene films. However, major hurdles remain in achieving the excellent quality and scalability of CVD graphene needed for industrial production and applications. Early efforts were mainly focused on increasing the single-crystalline domain size, large-area uniformity, growth rate, and controllability of layer thickness and on decreasing the defect concentrations. An important recent advance was the discovery of the inevitable contamination phenomenon of CVD graphene film during high-temperature growth processes and the superclean growth technique, which is closely related to the surface defects and to the peeling-off and transfer quality. Superclean graphene represents a new frontier in CVD graphene research. In this Perspective, we aim to provide comprehensive understanding of the intrinsic growth contamination and the experimental solution of making superclean graphene and to provide an outlook for future commercial production of high-quality CVD graphene films.

How to address the issue of uniformity in large area capacitively coupled plasmas? A modeling investigation

Recently, the electrical asymmetry effect (EAE) has been proven effective not only in separate control of the ion energy and ion flux, but also in modulation of the plasma radial uniformity. Since no major change in reactor constructions is required, this method is promising and of significant importance in the plasma industry. In this work, a two-dimensional electromagnetic fluid model is employed to investigate the influence of the EAE on the plasma density distribution under various voltages (i.e., the peak-to-peak voltage V0 equals to 33 V, 56 V and 100 V) and pressures (i.e., 50 mTorr, 200 mTorr and 750 mTorr), in H2 capacitively coupled plasma (CCP) discharges sustained by multiple consecutive harmonics. The results indicate that at V0 = 33 V or 750 mTorr, the plasma density is characterized by a broad maximum at the radial center, and the effectiveness of the EAE on the modulation of the plasma radial uniformity is limited. When V0 becomes higher, the plasma distribution varies obviously by adjusting the phase shift of the fundamental frequency θ1. For instance, when θ1 increases from 0 to 2π at V0 = 56 V, the plasma density shifts from uniform over center-high to uniform. Moreover, at V0 = 100 V, the best uniformity is observed at θ1 = π. When the pressure decreases to 50 mTorr, although the uniformity varies significantly with θ1, the plasma density stays low, which may not be welcomed in the plasma processing. Different influences of the EAE on the plasma radial uniformity can be understood by examining the electron heating induced by the sheath expansion and the electric field reversal under various discharge conditions. The results obtained in this work could help us to have a better understanding of the EAE on the plasma radial uniformity, which would be useful to improve the plasma processing in large area CCP reactors.

DNA-directed nanofabrication of high-performance carbon nanotube field-effect transistors.

Biofabricated semiconductor arrays exhibit smaller channel pitches than those created using existing lithographic methods. However, the metal ions within biolattices and the submicrometer dimensions of typical biotemplates result in both poor transport performance and a lack of large-area array uniformity. Using DNA-templated parallel carbon nanotube (CNT) arrays as model systems, we developed a rinsing-after-fixing approach to improve the key transport performance metrics by more than a factor of 10 compared with those of previous biotemplated field-effect transistors. We also used spatially confined placement of assembled CNT arrays within polymethyl methacrylate cavities to demonstrate centimeter-scale alignment. At the interface of high-performance electronics and biomolecular self-assembly, such approaches may enable the production of scalable biotemplated electronics that are sensitive to local biological environments.

Polycrystalline lead selenide prepared by an oxygen ion beam induction

Manipulating optical and electrical properties is promising for applications such as photodetectors and solar cells. Comparing with chemical preparation methods, tuning optical and optoelectronic properties by physical vapor phase methods has extra merits such as large area uniformity and CMOS compatibility. An in situ oxygen ion beam preparation method with an ion beam modulation is demonstrated for lead selenide. Polycrystalline lead selenide thin films with tunable optical band gap and low structural disorder have been fabricated using an orthogonal experimental scheme. The morphological evolution between three specific microstructures, mainly governed by ion energy and annealing temperature, has been observed. The structural characterizations indicate the presence of some oxides phases and crystallite size of hundreds of nanometers. The more suitable optical band gap within 0.75–1.1 eV was obtained at room temperature for absorption of mid-infrared photons. The ultra-low Urbach energies suggest a structurally well-ordered microstructure. The dependences of the structural, electrical and optical properties on the preparation parameters have been revealed by range analysis, indicating the main role of ion energy in the manipulations of these properties. Ion energy and beam current were revealed to be more sensitive in affecting optoelectronic properties, and the manipulation effects were demonstrated to be long wavelength sensitive. These results provide an attractive approach to manipulate and optimize the structural, electrical and optical properties of polycrystalline lead chalcogenides in mid-infrared optics and optoelectronics.

A photoelectrochemical biosensor based on the direct electron transfer to galactose oxidase

A novel photoelectrochemical biosensor for quantitative detection of galactose was successfully developed by immobilizing galactose oxidase on TiO2 nanorod arrays modified F-doped tin oxide (FTO) electrode. Specifically, the direct electron transfer to galactose oxidase was achieved. TiO2 nanorod arrays directly grew on FTO electrode from peroxotitanium complex precursor under low-temperature and normal-pressure conditions. The large surface area and good uniformity of TiO2 nanorod arrays facilitated direct electron transfer between the electrode surface and galactose oxidase. The photocurrent spectra of as-prepared biosensor showed an obvious photocurrent response under UV light irradiation. The generated photocurrent of the stable platform was significantly enhanced after the addition of galactose in solution and the photocurrent intensity possessed linear relationship with the galactose concentration. Moreover, CaCl2, uric acid and ascorbic acid had no interference with the detection of galactose on the working electrode. This sensor can be reused and measure the concentration of galactose in lactose-free milk.

Exceptionally Uniform and Scalable Multilayer MoS2 Phototransistor Array Based on Large-Scale MoS2 Grown by RF Sputtering, Electron Beam Irradiation, and Sulfurization.

Two-dimensional molybdenum disulfide (MoS2) has emerged as a promising material for optoelectronic applications because of its superior electrical and optical properties. However, the difficulty in synthesizing large-scale MoS2 films has been recognized as a bottleneck in uniform and reproducible device fabrication and performance. Here, we proposed a radio-frequency magnetron sputter system, and post-treatments of electron beam irradiation and sulfurization to obtain large-scale continuous and high-quality multilayer MoS2 films. Large-area uniformity was confirmed by no deviation of electrical performance in fabricated MoS2 thin-film transistors (TFTs) with an average on/off ratio of 103 and a transconductance of 0.67 nS. Especially, the photoresponsivity of our MoS2 TFT reached 3.7 A W-1, which is a dramatic improvement over that of a previously reported multilayer MoS2 TFT (0.1 A W-1) because of the photogating effect induced by the formation of trap states in the band gap. Finally, we organized a 4 × 4 MoS2 phototransistor array with high photosensitivity, linearity, and uniformity for light detection, which demonstrates the great potential of 2D MoS2 for future-oriented optoelectronic devices.

Wearable and Ultrasensitive Strain Sensor Based on High-Quality GaN pn Junction Microwire Arrays.

With the rapid growth in wearable electronics sensing devices, flexible sensing devices that monitor the human body have shown great promise in personalized healthcare. In the study, high-quality GaN pn junction microwire arrays with different aspect ratios and large-area uniformity are fabricated through an easy, repeatable fabrication process. The piezoelectric coefficient (d33 ) of GaN pn junction microwire arrays increases from 7.23 to 14.46 pm V-1 with the increasing of the aspect ratio, which is several times higher than that of GaN bulk material. Furthermore, flexible ultrasensitive strain sensor based on GaN microwires with the highest d33 is demonstrated to achieve the maximum open circuit voltage of 10.4 V, and presents excellent durability with stable output signals over 10 000 cycles with a response time of 50 ms. As a flexible and wearable sensor attached to the human skin, the GaN microwire pn junction arrays with such a high degree of uniformity can precisely monitor subtle human pulse and motions, which show great promise in future personalized healthcare.

Realization of Nanolene: A Planar Array of Perfectly Aligned, Air-Suspended Nanowires.

Air suspension and alignment are fundamental requirements to make the best use of nanowires' unique properties; however, satisfying both requirements is very challenging due to the mechanical instability of air-suspended nanowires. Here, a perfectly aligned air-suspended nanowire array called "nanolene" is demonstrated, which has a high mechanical stability owing to a C-channel-shaped cross-section of the nanowires. The excellent mechanical stability is provided through geometrical modeling and finite element method simulations. The C-channel cross-section can be realized by top-down fabrication procedures, resulting in reliable demonstrations of the nanolenes with various materials and geometric parameters. The fabrication process provides large-area uniformity; therefore, nanolene can be considered as a 2D planar platform for 1D nanowire arrays. Thanks to the high mechanical stability of the proposed nanolene, perfectly aligned air-suspended nanowire arrays with an unprecedented length of 1 mm (aspect ratio ≈5100) are demonstrated. Since the nanolene can be used in an energy-efficient nanoheater, two energy-stringent sensors, namely, an air-flow sensor and a carbon monoxide gas sensor, are demonstrated. In particular, the gas sensor achieves sub-10 mW operations, which is a requirement for application in mobile phones. The proposed nanolene will pave the way to accelerate nanowire research and industrialization by providing reliable, high-performance nanowire devices.

Significant Performance Enhancement of Very Thin InGaZnO Thin-Film Transistors by a Self-Assembled Monolayer Treatment

The use of amorphous InGaZnO (IGZO) has become more and more popular especially in display technologies because of its high mobility, excellent large area uniformity, and low-temperature processabi...

Oxygen incorporated solution-processed high-κ La2O3 dielectrics with large-area uniformity, low leakage and high breakdown field comparable with ALD deposited films

Oxygen incorporated solution-processed high-κ La2O3 dielectrics exhibit large-area uniformity, low leakage and high breakdown field comparable with ALD deposited films.

Light response behaviors of amorphous In–Ga–Zn–O thin-film transistors via in situ interfacial hydrogen doping modulation

Thin-film transistors (TFTs) based on amorphous In–Ga–Zn–O (a-IGZO) channels present high mobility, large-area uniformity, mechanical flexibility and photosensitivity, and thus have extensive applicability in photodetectors, wearable devices, etc. However, pure a-IGZO based photosensors only exhibit a UV light response with limited sensitivity performance. By utilizing in situ interfacial hydrogen doping, it is demonstrated that the a-IGZO TFTs with the Al2O3 dielectric deposited by plasma-enhanced atomic layer deposition at room temperature (RT) have excellent photosensing performance, such as a photoresponsivity of over 6 × 105 A W−1 and a light to dark current ratio up to 107. This is attributed to spontaneous interfacial hydrogen doping into the a-IGZO channel during sputtering deposition of a-IGZO on hydrogen-rich Al2O3 films, thus generating subgap states in the band gap of IGZO. Further, color pattern imaging was achieved by employing an array of the color distinguishable devices, and flexibility was demonstrated by fabricating the TFTs onto polymer substrates. Moreover, it is also found that both the RT and 150 °C Al2O3 a-IGZO TFTs exhibit typical light-stimulated synaptic behaviors, including excitatory post-synaptic current and pair-pules facilitation, etc., and the memory time of the synaptic devices can be easily modulated by the degree of the interfacial hydrogen doping.

Direct growth of large area uniform bi-layer graphene films on silicon substrates by chemical vapor deposition

Direct growth of large area uniform high quality graphene films on dielectric or semiconductor substrate is very important for graphene application in electronic devices. In this paper, a simple method is introduced to directly grow uniform and large area bi-layer graphene films on silicon substrates by chemical vapor deposition. Raman spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy and electrical transport measurements are employed to characterize and evaluate the quality of the as-synthesized graphene films. Raman mapping indicates that the graphene film on silicon substrate is large area uniformity. The graphene-based electronic device is fabricated and the measured mobility of the directly grown graphene film is 343.3 cm2/(V.S). These as-synthesized graphene films may have potential applications in conducted films, electronic devices, etc.

Chemical vapor deposition synthesis of sp2-BN film on 2-inch Si substrates for bipolar resistive random access memory device

Resistive random access memory (RRAM) devices with Au/sp2-BN/p-Si sandwich structure have been successfully fabricated. Single precursor ammonia borane (NH3BH3) chemical vapor deposition (CVD) was used to directly grow amorphous sp2-BN film on a 2-inch commercial p-type Si substrate without any extra catalyst. The sp2-BN-based RRAM device exhibit reproducible switching endurance (>1000 cycles), long retention time (>1 × 104 s), capability to operate under appropriate voltages (between 2.225 V and 1.475 V for SET process) as well as large area uniformity (2 inch). The resistance value of low resistance state (LRS) is as high as 3 × 103 Ω and the typical VSET/ISET and VRESET/IRESET are 4.292 × 10−4 A @ 1.925 V and 6.143 × 10−4 A @ −0.862 V, respectively. A boron vacancies (VB) induced mechanism similar to valence change memory (VCM) has been used to explain the parameters of noble Au/sp2-BN/Si RRAM device such as, RESET stop voltage, temperature and device area dependence of resistances (both low & high states). This work will propel the combination of low power consumption in sp2-BN based RRAM with Si based electronics devices.

Direct growth of large area uniform double layer graphene films on MgO(100) substrates by chemical vapor deposition

Direct growth of large area uniform high quality graphene films on insulating substrates is very important for graphene-based devices. Here we report a facile approach to directly fabricate uniform and large area bi-layer graphene films on MgO(100) substrates by chemical vapor deposition, without the aid of any metal catalysts. Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy are used to characterize the synthesized graphene films. Raman mapping indicates that the graphene films are large area uniformity. The mobility of these directly grown graphene films is 130 cm2/(VS) and they may have potential applications in electronic devices, sensors, conductive films, etc.

Uniformity of GaAs solar cells grown in a kinetically-limited regime by dynamic hydride vapor phase epitaxy

We demonstrate solar cell devices grown on 50 mm diameter GaAs substrates by dynamic hydride vapor phase epitaxy (D-HVPE). In contrast to our prior D-HVPE devices, grown at 650 °C in a transport-limited regime, these devices were grown at 700 °C in a kinetically-limited growth regime in which the growth rate uniformity is controlled by thermally-activated surface reactions. These devices exhibit open-circuit voltages up to 1.07 V, nearly identical performance to the devices grown at lower temperature in a different growth regime. We evaluate the uniformity of device performance and find only a 1% variation in absolute, device efficiency across the majority of the wafer in devices without anti-reflection coating. We analyze the GaAs and GaInP thickness uniformity and GaInP compositional uniformity, using high-tresolution x-ray diffraction mapping, and find that any non-uniformity in device efficiency is not related to variations in these structural parameters. We combine three-dimensional computational fluid dynamics modeling of our reactor with a kinetic model for GaAs growth, and use the combined model to predict spatial GaAs growth rate over a 50 mm wafer area. We compare these predictions with experimental data from our D-HVPE reactor and find excellent agreement. We use the model to the gain insight into the specific mechanisms that control GaAs spatial uniformity in the kinetically-limitedgrowth regime. These results suggest a large parameter window for the growth of high-performance optoelectronic devices by D-HVPE, possibly with large area uniformity.

Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation.

When employing self-assembled monolayers (SAMs) for tuning surface and interface properties, organic molecules that enable strong binding to the substrate, large-area structural uniformity, precise alignment of functional groups, and control of their density are highly desirable. To achieve these goals, tripod systems bearing multiple bonding sites have been developed as an alternative to conventional monodentate systems. Bonding of all three sites has, however, hardly been achieved, with the consequence that structural uniformity and orientational order in tripodal SAMs are usually quite poor. To overcome that problem, we designed 1,8,13-trimercaptomethyltriptycene (T1) and 1,8,13-trimercaptotriptycene (T2) as potential tripodal SAM precursors and investigated their adsorption behavior on Au(111) combining several advanced experimental techniques and state-of-the-art theoretical simulations. Both SAMs adopt dense, nested hexagonal structures but differ in their adsorption configurations and structural uniformity. While the T2-based SAM exhibits a low degree of order and noticeable deviation from the desired tripodal anchoring, all three anchoring groups of T1 are equally bonded to the surface as thiolates, resulting in an almost upright orientation of the benzene rings and large-area structural uniformity. These superior properties are attributed to the effect of conformationally flexible methylene linkers at the anchoring groups, absent in the case of T2. Both SAMs display interesting electronic properties, and, bearing in mind that the triptycene framework can be functionalized by tail groups in various positions and with high degree of alignment, especially T1 appears as an ideal docking platform for complex and highly functional molecular films.

Scalable fabrication of geometry-tunable self-aligned superlattice photonic crystals for spectrum-programmable light trapping

Superlattice photonic crystals (SPhCs) possess considerable potentials as building blocks for constructing high-performance devices because of their great flexibilities in optical manipulation. From the prospective of practical applications, scalable fabrication of SPhCs with large-area uniformity and precise geometrical controllability has been considered as one prerequisite but still remains a challenge. In this work, we developed an anodic aluminum oxide template-guided approach to realize Ni SPhCs with the maximum area (~500 mm2) ever reported. By virtue of the dual-pore self-alignment effect arising from the periodic anodization electric fields, uniform structures over large areas were obtained for Ni SPhCs. Meanwhile, the geometrical parameters for every array of nanopores in terms of pore depth, size, and morphology can be independently controlled due to the sequential pore-opening. Based on the experimental observation about the geometrical dependence of the light absorption for Ni dual-pore SPhCs, we further fabricated Ni SPhCs with simultaneously-shaped nanopores and nanoconcaves, which not only simplified the fabrication process but also achieved omnidirectional stably-strong (~95%) light absorption spectra. Optical simulations elucidated that surface plasmon resonance and cavity resonance are responsible for the strong light trapping. Notably, the fabrication technique is applicable to Ni SPhCs with different periodicities, leading to spectrally programmable light absorption spectra. With Ni SPhCs as solar absorber, the water evaporation efficiency of a solar steam generation system and the open circuit voltage of a solar thermoelectric generator demonstrated 2.3 and 2.5 times improvement, respectively.