Showing papers by "Hoi Sing Kwok published in 2020"

Normally Transparent Tribo-Induced Smart Window

Abstract: Self-powered smart windows are desirable with the expectations of their energy-saving, weather-independent, user-controllable, and miniature performance. Recently developed solar- or thermal-powere...

Investigation of Electrical Properties and Reliability of GaN-Based Micro-LEDs

Abstract: In this paper, we report high-performance Micro-LEDs on sapphire substrates, with pixel size scaling to 20 µm and an ultra-high current density of 9902 A/cm2. The forward voltages (VF) of the devices ranged from 2.32 V to 2.39 V under an injection current density of 10 A/cm2. The size and structure-dependent effects were subsequently investigated to optimize the device design. The reliability of Micro-LED devices was evaluated under long-aging, high-temperature, and high-humidity conditions. It was found that Micro-LED devices can maintain comparable performance with an emission wavelength of about 445 nm and a full width at half maximum (FWHM) of 22 nm under extreme environments. Following this, specific analysis with four detailed factors of forward voltage, forward current, slope, and leakage current was carried out in order to show the influence of the different environments on different aspects of the devices.

High-Brightness InGaN/GaN Micro-LEDs With Secondary Peak Effect for Displays

Abstract: The green InGaN/GaN micro light-emitting diode (Micro-LED) is a significant component in micro-display whereas lack of size dependence and high injected current analysis. In this letter, different size Micro-LEDs were fabricated and the electro-optical characteristics were measured. The size-dependence phenomenon for EQE “efficiency droop”, luminous flux and brightness were observed and discussed. All sizes Micro-LEDs exhibited an extremely high brightness as 37.5k and 67.6k nits at 1 A/cm2, 2.89M and 3.81M nits at 300 A/cm2 for 25 and $200~\mu \text{m}$ respectively. In addition, the elevating current densities from 160 to 6400 A/cm2 were injected into $25~\mu \text{m}$ Micro-LED and the electroluminescence (EL) spectra transformation of different current densities were depicted. The green to blue shift and secondary peak effect, which promise an opportunity to modulate the emission wavelength for higher quality and lower the barrier of mass transfer technology, were observed and analyzed through the spectra.

Introducing Ion Migration and Light-Induced Secondary Ion Redistribution for Phase-Stable and High-Efficiency Inorganic Perovskite Solar Cells

Abstract: Inorganic halide perovskites have been demonstrated as a promising alternative for light absorption due to their improved thermal stability compared with organic-inorganic halide perovskites. Howev...

Low Leakage Current Vertical Thin-Film Transistors With InSnO-Stabilized ZnO Channel

Abstract: Vertical channel InSnO (ITO)-stabilized ZnO thin-film transistors (TFTs) with a channel length of 300 nm were developed and fabricated. These TFT devices exhibited low off-state leakage current and gate leakage current coming into the scale of 10−14 A. In addition, a high on/off current ratio was achieved, up to 107, with a drain voltage of 0.01 V. The effects of the overlap area between the source and drain region on the electrical performance of the proposed vertical TFTs were investigated. The results showed that the vertical channel ITO-stabilized ZnO TFTs with a smaller overlap exhibited a better switching speed, whether in DC or AC mode. The proposed vertical channel ITO-stabilized ZnO TFTs may be promising for application in flexible and high-density electronics.

Resilience of Fluorinated Indium-Gallium-Zinc Oxide Thin-Film Transistor Against Hydrogen-Induced Degradation

Abstract: Thin-film transistors (TFTs) based on amorphous indium-gallium-zinc oxide (IGZO) with or without plasma fluorination treatment were fabricated and the sensitivity of their characteristics to hydrogen exposure was compared. Consistent with the lower hydrogen content revealed using secondary ion-mass spectrometry, TFTs built with fluorinated IGZO were shown to exhibit improved intrinsic resilience against hydrogen-induced degradation. Further enhanced by the incorporation of aluminum oxide as a hydrogen diffusion-barrier, such resilience is beneficial to the integration of fluorinated IGZO TFTs with hydrogen-containing devices, such as photodiodes based on amorphous hydrogenated silicon and TFTs based on low-temperature polycrystalline silicon.

Low-driving-voltage, polarizer-free, scattering-controllable liquid crystal device based on randomly patterned photo-alignment

Abstract: In this work, a polarizer-free, electrically tunable liquid crystal device is presented. This device is based on randomly patterned photo-aligned boundaries generating light scattering in an adjacent liquid crystal film. Its transparent on state requires only 7.5Vrms driving voltage. Switching from a 49.5% hazy off state into the transparent on state occurs at 1.25 V, with only 1.2% residual haze. The effect exhibits fast rise and decay times of 0.3 ms and 7.2 ms, respectively. Thanks to its field-effect nature and the surface preparation process, zero ohmic low power consumption, fast response times, and steep transmission-voltage characteristics result. Applications are smart windows, light shutters, and/or transparent displays, as well as complementary metal-oxide-semiconductor driven multiplexed displays.

A Novel Envelope Detector Based on Unipolar Metal-Oxide TFTs

Abstract: The lack of an envelope detector (ED) hinders the realization of bidirectional communication between radio frequency identification (RFID)/near field communication (NFC) tags and readers in metal-oxide thin film transistor (TFT) technology. To solve this, we present a novel ED implementation based on unipolar metal-oxide TFTs. The ED is fully integrated by n-type TFTs and has a novel topology. It can detect an amplitude shift keying (ASK) signal with a minimum modulation depth of 15% and a maximum envelope rate of 8kbit/s, and its minimum supply voltage reaches 2.5 V. These characteristics are greatly improved compared with those of previous works based on complementary organic or hybrid organic/metal-oxide TFTs.

Extracting the Critical Breakdown Electrical Field of Amorphous Indium-Gallium-Zinc-Oxide From the Avalanche Breakdown of n-Indium-Gallium-Zinc-Oxide/p + -Nickel-Oxide Heterojunction Diode

Abstract: A heterojunction diode between n-type amorphous indium-gallium-zinc-oxide (a-IGZO) and p-type nickel oxide is experimentally demonstrated with self-aligned junction termination. The diode has an abrupt, non-destructive breakdown behavior, and the avalanche breakdown is confirmed by the positive temperature coefficient of the breakdown voltage. The room-temperature breakdown voltage of the device is 33 V, and the extracted critical breakdown electrical field ( ${E}_{CR}$ ) is 2.7 MV/cm. The ${E}_{CR}$ is one order of magnitude higher than that of single-crystal silicon, making a-IGZO promising for power electronics and other high-voltage applications.

Low voltage tunable liquid crystal Fibonacci grating

Abstract: In this article, we disclose low voltage tunable liquid crystal Fibonacci gratings (FbG). Photoalignment technique has been used to create two domains of Fibonacci structure using a photomask. Swit...

Voltage-controlled liquid crystal Pancharatnam-Berry phase lens with broadband operation and high photo-stability.

Abstract: Pancharatnam-Berry phase optical elements (PBOEs) have received much attention due to their ability to generate complex structured light or to manipulate the shape of a light beam. This work demonstrates a tunable liquid crystal (LC) Pancharatnam-Berry (LCPB) lens using a simple and cost-effective PB phase hologram optical setup and thermal polymerization to form an irreversible photo-patterning alignment layer. The LCPB lens with high photo-stability supports ultra-broadband operation and provides a diffraction efficiency of ∼90% throughout the visible spectral range, achieved by applying the appropriate voltages. The LCPB lens functions as a convex or a concave lens, depending on the handedness of the circularly polarized incident light, so its image reduction and magnification functions are demonstrated, and its photo-stability is characterized. The fabrication of the proposed LC PBOEs is simpler and more cost-effective than previous methods, and the irreversible photo-patterning alignment layer that is formed by thermal polymerization allows larger operational bandwidths, supporting new applications.

A High Gain Low-Noise Amplifier Based on ITO-Stabilized ZnO Thin-Film Transistors

Abstract: A novel high gain low-noise analog amplifier based on n-type indium tin oxide (ITO)-stabilized ZnO thin-film transistors (TFTs) was studied in this work. Amplifiers with capacitor bootstrap structure are proposed. The effect of unequal numbers of bootstrap structure, various capacitance, and different circuit topology on the gain and noise of circuits were investigated. The input referred noise of five kinds of amplifier circuits, Amp1, Amp2 (C444, C333, C422), and Amp3 were measured to be 101.1, 44, 47.3, 43.5, and $38.3~\mu \text{V}_{rms}$ , respectively, and the gains are 19.5, 28.8, 26.8, 25.7, and 32 dB, respectively. It is found that as the numbers of bootstrap structure increase in a small range, the input referred noise decreases, and the circuit gain increases. The effects of the capacitance on the gain or noise can be negligible. And also, the gain of the circuit with cascode structure is higher than that with common source. Results show that the noise is much lower than that of a-InGaZnO (IGZO) TFTs technology, which is widely used in TFT circuits design. The proposed high gain and low-noise analog amplifier with the ITO-stabilized ZnO TFTs technology has great potential in the biomedical fields like electro cardiogram (ECG) and pulse detection, as well as wearable electronic applications.

Ferroelectric liquid crystals for fast switchable circular Dammann grating [Invited]

Abstract: Diffractive optical elements attract a considerable amount of attention, mainly due to their potential applications in imaging coding, optical sensing, etc. Application of ferroelectric liquid crystals (FLCs) with photo-alignment technology in diffractive optical elements results in a high efficiency and a fast response time. In this study we demonstrate a circular Dammann grating (CDG) with a diffraction efficiency of 84.5%. The achieved response time of 64 μs is approximately two orders of magnitude faster than the existing response time of nematic liquid crystal devices. By applying a low electric field (V = 6 V) to the FLC CDG, it is switched between the eight-order diffractive state and the transmissive diffraction-free state.

Ferroelectric liquid crystal devices on strengthened photoalignment films

Abstract: Stable ferroelectric liquid crystal (FLC) elements have been realised by optically stabilised alignment film and thermally stabilised alignment film. The two methods of fabricated FLC elements have...

A Timing Model for the Optimal Design of a Prototype Active-Matrix Display

Abstract: Describing the dependence of both signal propagation delay and pixel charging time on display specifications and device parameters, a timing model and a combined coordinate system for its convenient visualization are developed for an active-matrix display. Presented also is an algorithm developed to optimally determine the width of the interconnects and that of the address transistor for maximizing the aperture ratio of a pixel. The model could be applied to search for optimal viable designs under different panel specifications and technology constraints.

Dimension Scaling Effects on Conduction and Low Frequency Noise Characteristics of ITO-Stabilized ZnO Thin Film Transistors

Abstract: Conduction characteristics and low frequency noises in ITO-stabilized ZnO thin film transistors (TFTs) with constant channel width (W=50 μm) and different channel lengths (L=5, 10, 25, 50 and 100 μm) are measured and analysis. Firstly, dependences of threshold voltage and field effect mobility on channel length are examined. With decreasing channel length, the threshold voltage shifts to the negative direction which may attribute to carrier diffusion from source/drain to the intrinsic device channel. In addition, significant degradation of field effect mobility is observed in short channel device which are induced by the presence of series contact resistance. The value of contact resistance has also been extracted. Secondly, the drain current noise power spectral densities (S ID ) are measured at varied effective gate voltages. The measured noises follow a 1/f γ and y is about -1.1. Moreover, the normalized noises (S ID /I ds ) are dependent linearly on the inverse of channel length and the slope is about -0.75. Finally, the dominant mechanism of low frequency noise in ITO-stabilized ZnO thin film transistor are examined. The slope of normalized noise against effective gate voltage are extracted, which are varied from -1.03 to -1.84 with decreasing channel length and thus it indicates that ITO-stabilized ZnO TFTs varied from bulk dominated devices to interface dominated devices. By using of ΔN-Δμ model, the normalized noises are simulated by considering of contact resistance. Several noise parameters (flat-band voltage noise spectral density, etc) are extracted and analysis.

Systematic Defect Manipulation in Metal Oxide Semiconductors towards High-Performance Thin-Film Transistors

Abstract: The manipulation of donor-like defects, including generation, suppression and diffusion, in metal oxide (MO) thin-film transistors (TFTs) has been reviewed systematically. Based on this, the defect-populated source/drain (S/D) regions and defect-free channel region are realized for high-performance TFTs, and even the location of self-aligned (SA) homojunctions can be accurately controlled.

High-efficient liquid crystal polarization gratings for autostereoscopic 3D display

Abstract: There are many implementations of naked-eye 3D display technology. Diffraction-based multiview 3D display is one of the most promising research directions. Several viewpoints are built where human can get different image in their eyes to get stereoscopic perception. The core element in this solution is grating, which diffracts the light to a specific direction, usually the +1 or -1 level. Compared to binary grating, a novel geometric phase element, liquid crystal polarization grating (LCPG) has many advantages in acting as the core element. First, LCPG shows periodically cycloidal texture and can diffract with nearly 100% efficiency which minimize the high order and concentrate the energy on ±1 order. Second, its polarization-sensitive character can be used for making a polarization-dependent display. What’s more, the grating is spin coated on the substrate and appears a super thin film for less than 10μm. Photo alignment technology is used for fabricating LCPGs. We proposed an interference-free and single exposure method using birefringent prism and wave plate. The fabricated LCPGs appear >99% efficiency and has potential to expand the effective waveband. This method is effective, robust and low-cost, which has potential for commercial applications.

Reversely-Synchronized-Stress-Induced Degradation in Polycrystalline Silicon Thin-Film Transistors and Its Suppression by a Bridged-Grain Structure

Abstract: In this letter, a reversely synchronized stress (RSS) is proposed to simulate the working condition of switching thin-film transistors (TFTs) in active-matrix displays The reliability of polycrystalline silicon (poly-Si) TFTs under RSS is characterized and investigated RSS brings huge device degradation A dynamic hot carrier (HC) effect, dependent on transition edges of RSS, dominates the degradation Combined with a transient simulation, the degradation mechanism under RSS in poly-Si TFTs is discussed and developed To suppress RSS-induced HC degradation, a bridged-grain (BG) structure is employed in the active layer of poly-Si TFTs Via BG lines’ reducing the lateral electric field in the channel at source/drain sides, the reliability of BG TFTs under RSS is significantly improved