Yilun Wang

Current transport mechanism of Au∕Ni∕GaN Schottky diodes at high temperatures

Current transport mechanism in Au∕Ni∕GaN Schottky diodes has been investigated using current-voltage characterization technique between 27 and 350°C. It is found that the ideality factor n of the diode decreases with increasing temperature when the temperature is lower than 230°C, and then increases with increasing temperature when the temperature is higher than 230°C. The corresponding Schottky barrier height (SBH) increases all through the temperature range. Thermionic-emission model with a Gaussian distribution of SBHs is thought to be responsible for the electrical behavior at temperatures lower than 230°C, while the generation-recombination (GR) process takes place in at temperatures above 230°C. The effective Richardson constant is determined to be 24.08Acm−2K−2, in excellent agreement with the theoretical value. The extrapolated activation energy of the GR process is determined to be 1.157eV. Based on the cathodoluminescence measurements, it is suggested that the deep level defects inducing yellow ...

The In-Plane Anisotropy of WTe2 Investigated by Angle-Dependent and Polarized Raman Spectroscopy

Tungsten ditelluride (WTe2) is a semi-metallic layered transition metal dichalcogenide with a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various materials properties. We have systemically studied the in-plane anisotropy of Raman modes in few-layer and bulk WTe2 by angle-dependent and polarized Raman spectroscopy (ADPRS). Ten Raman modes are clearly resolved. Their intensities show periodic variation with sample rotating. We identify the symmetries of the detected modes by quantitatively analyzing the ADPRS results based on the symmetry selection rules. Material absorption effect on the phonon modes with high vibration frequencies is investigated by considering complex Raman tensor elements. We also provide a rapid and nondestructive method to identify the crystallographic orientation of WTe2. The crystallographic orientation is further confirmed by the quantitative atomic-resolution force image. Finally, we find that the atomic vibrational tendency and complexity of detected modes are also reflected in the shrinkage degree defined based on ADPRS, which is confirmed by corresponding density functional calculation. Our work provides a deep understanding of the interaction between WTe2 and light, which will benefit in future studies about the anisotropic physical properties of WTe2 and other in-plane anisotropic materials.

Study of the structural damage in the "0001… GaN epilayer processed by laser lift-off techniques

The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.

Ultrasensitive Near-Infrared Photodetectors Based on a Graphene–MoTe2–Graphene Vertical van der Waals Heterostructure

Graphene and other layered materials, such as transition metal dichalcogenides, have rapidly established themselves as exceptional building blocks for optoelectronic applications because of their unique properties and atomically thin nature. The ability to stack them into van der Waals (vdWs) heterostructures with new functionality has opened a new platform for fundamental research and device applications. Nevertheless, near-infrared (NIR) photodetectors based on layered semiconductors are rarely realized. In this work, we fabricate a graphene-MoTe2-graphene vertical vdWs heterostructure on a SiO2/p+-Si substrate by a facile and reliable site-controllable transfer method and apply it for photodetection from the visible to NIR wavelength range. Compared to the layered semiconductor photodetectors reported thus far, the graphene-MoTe2-graphene photodetector has a superior performance, including high photoresponsivity (∼110 mA W-1 at 1064 nm and 205 mA W-1 at 473 nm), high external quantum efficiency (EQE; ∼12.9% at 1064 nm and ∼53.8% at 473 nm), rapid response and recovery processes (a rise time of 24 μs and a fall time of 46 μs under 1064 nm illumination), and free from an external source-drain power supply. We have employed scanning photocurrent microscopy to investigate the photocurrent generation in this heterostructure under various back-gate voltages and found that the two Schottky barriers between the graphenes and MoTe2 play an important role in the photocurrent generation. In addition, the vdWs heterostructure has a uniform photoresponsive area. The photoresponsivity and EQE of the photodetector can be modulated by the back-gate (p+-Si) voltage. We compared the responsivities of thin and thick flakes and found that the responsivity had a strong dependence on the thickness. The heterostructure has promising applications in future novel optoelectronic devices, enabling next-generation high-responsivity, high-speed, flexible, and transparent NIR devices.

Efficient silicon quantum dots light emitting diodes with an inverted device structure

We use silicon quantum dots (SiQDs) with an average diameter of 2.6 ± 0.5 nm as the light emitting material and fabricate inverted structure light emitting diodes (SiQD-LEDs) with bottom cathodes. ZnO nanoparticles with high electron mobility, a deep valence band edge, and robust features to resist dissolving by the SiQD solvent were used as the electron transport layer. 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) with high hole transport mobility and a high lowest unoccupied molecular orbital level was used as the hole transport layer. Poly(ethylene imine) (PEI) modified indium-tin oxide (ITO) was used as the low work function (∼3.1 eV) cathode and MoO3/Al as the high work function anode. Electroluminescence of the SiQD-LEDs is mainly from the SiQDs with a peak located at ∼700 nm. The maximum external quantum efficiencies of the SiQD-LEDs are 2.7%.

Effect of excess PbBr2 on photoluminescence spectra of CH3NH3PbBr3 perovskite particles at room temperature

The organic-inorganic halide perovskites have promising applications in light-emitting devices besides solar cells. We here prepare CH3NH3PbBr3 perovskite particles on SiO2 substrates and find that the photoluminescence (PL) spectrum of the particles at room temperature has two peaks, locating at 529 and 549 nm, respectively, much different from that of the corresponding films prepared on the oxygen plasma-cleaned SiO2 substrates, which has a single peak. The double peaks have different temperature-dependence behaviors. By the x-ray diffraction and energy dispersive x-ray spectroscopy analyses, excess PbBr2 is detected inside the particles. We deduce that such excess PbBr2 has introduced shallow level defects. It is concluded that band-to-band recombination and these defects result in the double-peaked feature of the PL spectra of CH3NH3PbBr3 particles at room temperature.

Flexible solar cells based on CdSe nanobelt/graphene Schottky junctions

Flexible solar cells have attracted intense interest since they have potential for the construction of portable and wearable power sources. We fabricated CdSe nanobelt (NB)/graphene Schottky junction flexible solar cells on polyethylene terephthalate (PET) substrates for the first time. The solar cells have an excellent rectification behavior in the dark with a typical on/off current ratio of about 2 × 105. Under air mass (AM) 1.5 global (1.5G) illumination, the solar cells exhibit good photovoltaic (PV) behavior, with a typical open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) of about 0.31 V, 4.73 mA cm−2, and 36.14%, respectively. The corresponding energy conversion efficiency (η) is about 0.53%. Under bending conditions, the performance of the solar cells does not change obviously. We attribute the satisfactory performance of the flexible solar cells to the ingenious nanomaterials and Schottky junction device structures employed. Our work shows that the semiconductor NBs (NWs) and graphene are promising building blocks for future flexible devices and the CdSe NB/graphene Schottky junction solar cells have potential applications in flexible nano-optoelectronic systems.

Origin of Improved Optical Quality of Monolayer Molybdenum Disulfide Grown on Hexagonal Boron Nitride Substrate

Monolayer MoS2 is synthesized on hexagonal boron nitride (h-BN) flakes with a simple, high-yield method. Monolayer MoS2 on h-BN exhibits improved optical quality. Combining the theoretical and experimental analysis, it is concluded that the enhanced photoluminescence and Raman intensities of monolayer MoS2 probably originate from the relatively weak doping effect from the h-BN substrate rather than the optical interference effect.

CdSe Nanowire-Based Flexible Devices: Schottky Diodes, Metal-Semiconductor Field-Effect Transistors, and Inverters.

Novel CdSe nanowire (NW)-based flexible devices, including Schottky diodes, metal-semiconductor field-effect transistors (MESFETs), and inverters, have been fabricated and investigated. The turn-on voltage of a typical Schottky diode is about 0.7 V, and the rectification ratio is larger than 1 × 10(7). The threshold voltage, on/off current ratio, subthreshold swing, and peak transconductance of a typical MESFET are about -0.3 V, 4 × 10(5), 78 mV/dec, and 2.7 μS, respectively. The inverter, constructed with two MESFETs, exhibits clear inverting behavior with the gain to be about 28, 34, and 38, at the supply voltages (V(DD)) of 3, 5, and 7 V, respectively. The inverter also shows good dynamic behavior. The rising and falling times of the output signals are about 0.18 and 0.09 ms, respectively, under 1000 Hz square wave signals input. The performances of the flexible devices are stable and reliable under different bending conditions. Our work demonstrates these flexible NW-based Schottky diodes, MESFETs, and inverters are promising candidate components for future portable transparent nanoelectronic devices.

Large-Scale Synthesis and Systematic Photoluminescence Properties of Monolayer MoS2 on Fused Silica

Monolayer MoS2, with fascinating mechanical, electrical, and optical properties, has generated enormous scientific curiosity and industrial interest. Controllable and scalable synthesis of monolayer MoS2 on various desired substrates has significant meaning in both basic scientific research and device application. Recent years have witnessed many advances in the direct synthesis of single-crystalline MoS2 flakes or their polycrystalline aggregates on numerous diverse substrates, such as SiO2-Si, mica, sapphire, h-BN, and SrTiO3, etc. In this work, we used the dual-temperature-zone atmospheric-pressure chemical vapor deposition method to directly synthesize large-scale monolayer MoS2 on fused silica, the most ordinary transparent insulating material in daily life. We systematically investigated the photoluminescence (PL) properties of monolayer MoS2 on fused silica and SiO2-Si substrates, which have different thermal conductivity coefficients and thermal expansion coefficients. We found that there exists a stronger strain on monolayer MoS2 grown on fused silica, and the strain becomes more obvious as temperature decreases. Moreover, the monolayer MoS2 grown on fused silica exhibits the unique trait of a fractal shape with tortuous edges and has stronger adsorbability. The monolayer MoS2 grown on fused silica may find application in sensing, energy storage, and transparent optoelectronics, etc.