Shurong Dong

Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p–n Heterojunctions

After a decade of intensive research on two-dimensional (2D) materials inspired by the discovery of graphene, the field of 2D electronics has reached a stage with booming materials and device architectures. However, the efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design. In this review, we investigate the experimental efforts in interfacing 2D layers with 3D materials and analyze the properties of the heterojunctions formed between them. The contact resistivity of metal on graphene and related 2D materials deserves special attention, while the Schottky junctions formed between metal/2D semiconductor or graphene/3D semiconductor call for careful reconsideration of the physical models describing the junction behavior. The combination of 2D and 3D semiconductors presents a form of p-n junctions that have just marked their debut. For each type of the heterojunctions, the potential applications are reviewed briefly.

Asymmetric electrochemical capacitors with high energy and power density based on graphene/CoAl-LDH and activated carbon electrodes

Asymmetric electrochemical capacitors (ECs) consisting of composites of reduced graphene oxide (RGO) and CoAl layered double hydroxide (LDH) as positive electrode, activated carbon (AC) as negative electrode (RGO/LDH//AC) and 6 M KOH solution as electrolyte were achieved. RGO/LDH was synthesized by a facile solution method in one step under mild conditions. Owing to its wide operating voltage window of 0–1.75 V and the utilization of RGO, the RGO/LDH//AC capacitor exhibited excellent electrochemical properties with a maximum energy density of 35.5 W h kg−1, much higher than those of LDH//AC (25.1 W h kg−1) and AC//AC (6.3 W h kg−1) electrochemical capacitors. Moreover, the devices also showed a high power density of 8.75 kW kg−1 and long cycle life of ∼90% retention after 6000 cycles at the current density of 4 A g−1. Such high-performance asymmetric electrochemical capacitors offer great promise in the application of energy storage systems.

Flexible surface acoustic wave resonators built on disposable plastic film for electronics and lab-on-a-chip applications

Flexible electronics are a very promising technology for various applications. Several types of flexible devices have been developed, but there has been limited research on flexible electromechanical systems (MEMS). Surface acoustic wave (SAW) devices are not only an essential electronic device, but also are the building blocks for sensors and MEMS. Here we report a method of making flexible SAW devices using ZnO nanocrystals deposited on a cheap and bendable plastic film. The flexible SAW devices exhibit two wave modes - the Rayleigh and Lamb waves with resonant frequencies of 198.1 MHz and 447.0 MHz respectively, and signal amplitudes of 18 dB. The flexible devices have a high temperature coefficient of frequency, and are thus useful as sensitive temperature sensors. Moreover, strong acoustic streaming with a velocity of 3.4 cm/s and particle concentration using the SAW have been achieved, demonstrating the great potential for applications in electronics and MEMS.

Bipolar resistive switching characteristics of low temperature grown ZnO thin films by plasma-enhanced atomic layer deposition

ZnO films deposited by plasma-enhanced atomic layer deposition (PEALD) have been used to investigate resistive memory behavior. The bipolar resistance switching properties were observed in the Al/PEALD-ZnO/Pt devices. The resistance ratio for the high and low resistance states (HRS/LRS) is more than 103, better than ZnO devices deposited by other methods. The dominant conduction mechanisms of HRS and LRS are trap-controlled space charge limited current and Ohmic behavior, respectively. The resistive switching behavior is induced upon the formation/disruption of conducting filaments. This study demonstrated that the PEALD-ZnO films have better properties for the application in 3D resistance random access memory.

High sensitivity humidity sensors using flexible surface acoustic wave devices made on nanocrystalline ZnO/polyimide substrates

Flexible ZnO/polyimide surface acoustic wave (SAW) based humidity sensors were fabricated and their performances were investigated. ZnO films deposited on polyimide substrates are (0002) oriented columnar nanocrystals with grain sizes of 50–60 nm. SAW devices with different wavelengths showed high transmission performance, and the sensors exhibited a good repeatability in response to the cyclic change in humidity from ∼5% relative humidity (RH) to ∼87% RH. The responses of the sensors to RH change increase with the increase in humidity. The sensitivity increases with the decrease in wavelength owing to the improved characteristics of the sensors with shorter wavelengths. A high sensitivity of 34.7 kHz/10% RH has been obtained from a SAW sensor with no surface treatment, demonstrating that the flexible SAW humidity sensors are very promising for applications in flexible sensors and microsystems.

Silicon-Controlled Rectifier Stacking Structure for High-Voltage ESD Protection Applications

Latchup immunity is a challenging issue for the design of power supply clamps used in high-voltage electrostatic discharge (ESD) protection applications. While silicon-controlled rectifiers (SCRs) are highly robust ESD devices, they are traditionally not suited for high-voltage ESD due to their inherent low holding voltage and, thus, vulnerability to latchup. In this letter, a novel SCR stacking structure with an extremely high holding voltage, very small snapback, and acceptable failure current has been developed. The new and existing high holding voltage ESD devices are also compared to demonstrate the advancement of this work.

Crystalline structure effect on the performance of flexible ZnO/polyimide surface acoustic wave devices

This paper reports the fabrication of flexible surface acoustic wave (SAW) devices on ZnO/polyimide substrates and investigation of the effects of the deposition conditions, crystal quality, and film thickness of the ZnO films on the performance of the SAW devices. The deposition pressure has a significant effect on the crystal quality of the ZnO film, and which in turn affects the transmission of the SAW devices strongly. The device performance improves greatly and is mainly attributed to the better crystal quality of the film deposited at high pressure. The performance of the SAW devices also improves significantly with increase in ZnO film thickness, owing to the reduced defects and improved piezoelectric effect for the films with large grain sizes and better crystallinity as the film thickness increases. Flexible SAW devices with a resonant frequency of 153 MHz, a phase velocity of 1836 m/s, and a coupling coefficient of 0.79% were obtained on the ZnO film of 4 μm thickness, demonstrated its great pot...

High Holding Voltage SCR-LDMOS Stacking Structure With Ring-Resistance-Triggered Technique

A novel ring-resistance-triggered stacked SCR-laterally diffused MOSs has been successfully verified in a 0.35 μm, 30-V/5-V bipolar CMOS DMOS process to solve the coupling of trigger voltage and holding voltage in stacking structures. The holding voltage of the proposed structure can be modulated by varying stacking numbers, and a high holding voltage of 22 V has been achieved using six stacks. On the other side, the trigger voltage almost keeps constant at ~ 53 V and a high failure current of 3.5 A has been achieved.

Film bulk acoustic resonator pressure sensor with self temperature reference

A novel film bulk acoustic resonator (FBAR) with two resonant frequencies which have opposite reactions to temperature changes has been designed. The two resonant modes respond differently to changes in temperature and pressure, with the frequency shift being linearly correlated with temperature and pressure changes. By utilizing the FBARs sealed back trench as a cavity, an on-chip single FBAR sensor suitable for measuring pressure and temperature simultaneously is proposed and demonstrated. The experimental results show that the pressure coefficient of frequency for the lower frequency peak of the FBAR sensors is approximately −17.4 ppm kPa−1, while that for the second peak is approximately −6.1 ppm kPa−1, both of them being much more sensitive than other existing pressure sensors. This dual mode on-chip pressure sensor is simple in structure and operation, can be fabricated at very low cost, and yet requires no specific package, therefore has great potential for applications.

Transparent Surface Acoustic Wave Devices on ZnO/Glass Using Al-Doped ZnO as the Electrode

This letter reports the design of transparent surface acoustic wave (SAW) resonators using aluminum-doped ZnO (AZO) as the transparent electrode on ZnO/glass substrates. SAW devices with a resonant frequency up to 204.4 MHz and signal amplitude up to 25 dB were obtained with transparency above 80%. An AZO layer of more than 250 nm is needed to minimize the series resistance of the electrode to achieve reasonable performance. Temperature sensing showed that they have a temperature coefficient of frequency of ~50 ppm/K. This demonstrates the great potential of transparent SAW devices for widespread applications, as such devices are one the of building blocks of electronics, sensors, and microsystems.