Yingmin Luo

Cu related doublets green band emission in ZnO:Cu thin films

Cu-doped ZnO (ZnO:Cu) thin films were grown on Si (111) substrate by low-pressure metal-organic chemical vapor deposition equipment. The crystal structures and optical properties of as-grown sample were examined. X-ray diffraction patterns indicated a lattice relaxation after the Cu doping. The incorporation of Cu atoms into ZnO film and its existence in a bivalent state were demonstrated by x-ray photoelectron spectroscopy measurements. Low-temperature photoluminescence was carried out at temperature of 11.4 K for both unintentionally doped and Cu-doped ZnO films. A characteristic green-luminescence with fine structure consisted of doublets emission peak was observed, which was believed to be associated with Cu doping. A theoretical model based on hydrogen analog has been proposed to explain this phenomenon. It provides new information about the detailed role of Cu in ZnO thin films.

Flexible piezoelectric nanogenerator based on Cu2O–ZnO p–n junction for energy harvesting

In this study, a nanogenerator based on Cu2O–ZnO p–n junction has been fabricated on Cu wire substrates for harvesting mechanical energy from the environment. The flexible nanogenerator is composed of a Cu substrate, a Cu2O layer, ZnO nanorods and an outer Au-coated paper electrode; the Cu2O layer was obtained by oxidizing Cu wires directly and the ZnO nanorods were grown on the Cu2O layer using a low-temperature hydrothermal method. The existence of the Cu2O–ZnO p–n junction makes a contribution towards reducing the number of excess electrons in the ZnO, which facilitates in improving the output signal and also overcomes short circuits. An Au-coated paper electrode can involve more nanorods in the power generation process. The DC output voltage was up to 42 mV and the maximum output current density was 400 nA, which are approximately a 13-fold higher voltage and a one order of magnitude larger current in comparison to devices without a Cu2O layer, respectively. This study may provide important insight into a facile fabrication method for low-cost and high-performance energy harvesting devices.

Fabrication of p-NiO/n-ZnO heterojunction devices for ultraviolet photodetectors via thermal oxidation and hydrothermal growth processes

Abstract In this research work, a p-NiO/n-ZnO heterostructure was fabricated using thermal oxidation and hydrothermal growth processes. The p-NiO films were oxidized at different temperatures. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and UV–visible spectral analysis were used to characterize the p-NiO/n-ZnO heterostructure. The results indicated that the NiO films oxidized at higher temperature have wider optical band gap and lower defect density. In particular, by comparing the photoresponse properties of the UV photodetectors oxidized at different temperatures we suggest that the oxidation temperatures have a great influence on the photoresponse time. The defect density of NiO film decreases with increasing oxidation temperature. And the defect density affects the photoresponse characteristics that the decay time decreases with the decreasing of defect density as the NiO oxidation temperature increases. This work could serve as a valuable guideline for designing and improving the p-NiO/n-ZnO UV photodetectors in a low-cost and large-scale way.

Enhanced performance of wearable piezoelectric nanogenerator fabricated by two-step hydrothermal process

A simple two-step hydrothermal process was proposed for enhancing the performance of the nanogenerator on flexible and wearable terylene-fabric substrate. With this method, a significant enhancement in output voltage of the nanogenerator from ∼10 mV to 7 V was achieved, comparing with the one by conventional one-step process. In addition, another advantage with the devices synthesized by two-step hydrothermal process was that their output voltages are only sensitive to strain rather than strain rate. The devices with a high output voltage have the ability to power common electric devices and will have important applications in flexible electronics and wearable devices.

Negative differential resistance in low Al-composition p-GaN/Mg-doped Al0.15Ga0.85N/n+-GaN hetero-junction grown by metal-organic chemical vapor deposition on sapphire substrate

Negative differential resistance (NDR) behavior was observed in low Al-composition p-GaN/Mg-doped-Al0.15Ga0.85N/n+-GaN hetero-junction grown by metal-organic chemical vapor deposition on sapphire substrate. The energy band and free carrier concentration of hetero-junction were studied by the model of the self-consistent solution of Schrodinger-Poisson equations combined with polarization engineering theory. At the forward bias of 0.95 V, the NDR effect has a high peak-to-valley current ratio of ∼9 with a peak current of 22.4 mA (∼current density of 11.4 A/cm2). An interesting phenomenon of NDR disappearance after consecutive scans and recurrence after electrical treatment was observed, which was associated with Poole-Frenkel effect.

Low Al-composition p-GaN/Mg-doped Al0.25Ga0.75N/n+-GaN polarization-induced backward tunneling junction grown by metal-organic chemical vapor deposition on sapphire substrate

Low Al-composition p-GaN/Mg-doped Al0.25Ga0.75N/n+-GaN polarization-induced backward tunneling junction (PIBTJ) was grown by metal-organic chemical vapor deposition on sapphire substrate. A self-consistent solution of Poisson-Schrödinger equations combined with polarization-induced theory was used to model PIBTJ structure, energy band diagrams and free carrier concentrations distribution. The PIBTJ displays reliable and reproducible backward tunneling with a current density of 3 A/cm2 at the reverse bias of −1 V. The absence of negative differential resistance behavior of PIBTJ at forward bias can mainly be attributed to the hole compensation centers, including C, H and O impurities, accumulated at the p-GaN/Mg-doped AlGaN heterointerface.

Improvement of the quality of GaN epilayer by combining a SiNx interlayer and changed GaN growth mode

GaN epilayers with porous SiNx interlayer and changed growth modes were grown by metal–organic chemical vapor deposition on c-plane sapphire substrates. Comparing with GaN epilayer grown by ordinary method, the crystalline qualities were significantly improved. The improvement was attributed to the reduction of the density of threading dislocations causing by over-growth process combining with delayed coalescence of individual GaN islands. The influence of the deposition and annealing of nucleation layer on the GaN regrowth was also discussed.

Vertically conducting deep-ultraviolet light-emitting diodes with interband tunneling junction grown on 6H-SiC substrate

Vertically conducting deep-ultraviolet (DUV) light-emitting diodes (LEDs) with a polarization-induced backward-tunneling junction (PIBTJ) were grown by metal–organic chemical vapor deposition (MOCVD) on 6H-SiC substrates. A self-consistent solution of Poisson–Schrodinger equations combined with polarization-induced theory was applied to simulate the PIBTJ structure, energy band diagrams, and free-carrier concentration distribution. AlN and graded AlxGa1−xN interlayers were introduced between the PIBTJ and multiple quantum well layers to avoid cracking of the n-Al0.5Ga0.5N top layer. At a driving current of 20 mA, an intense DUV emission at ~288 nm and a weak shoulder at ~386 nm were observed from the AlGaN top layer side. This demonstrates that the PIBTJ can be used to fabricate vertically conducting DUV LED on SiC substrates.

Piezoelectric nanogenerator with 3D-ZnO micro-thornyballs prepared by chemical vapour deposition

Piezoelectric nanogenerators have been intensively developed in terms of their materials and applications; however, only modest structural progress has been made due to limitations in the growth mechanisms of nano-materials. In this work, a piezoelectric nanogenerator based on ZnO micro-thornyballs (ZMTBs) was introduced. ZMTBs were synthesized by chemical vapor deposition method without the presence of any seed layers or substrates. Electrical characterization was subsequently performed to reveal the characteristics of the contacts formed between the ZnO micro-thornyballs and the copper electrode. The electric output ability of the ZNTTs nanogenerators has been studied in reference to the experiment and the numerically calculation.

The nonlinear energy spectrum dependence of the optical conductivity in graphene

In this article, we present a theoretical study of the optical spectrum induced by electron-electron interaction in graphene, which can be compared with the case in a conventional two-dimensional electron gas (2DEG). It is found that in sharp contrast to the conventional 2DEG, two intra- and inter-band transition channels for optical transition via absorption scattering can be observed. The optical conductivity depends on the electron density, the linearity of the energy spectrum, and the broadening width. The dependence of the optical spectrum on radiation frequency and broadening width is discussed. Analytical and numerical results showed that the inter-band transition contribution to optical conductivity has a turning point at two times the Fermi energy where the Fermi energy position can be tuned by the gate voltage. And the intra-band contribution depends on nonlinear energy dispersion and broadening width obviously.