Xiuhua Xie

Realization of a self-powered ZnO MSM UV photodetector with high responsivity using an asymmetric pair of Au electrodes

We demonstrate a novel type of ZnO self-powered photodetector based on the asymmetric metal-semiconductor-metal (MSM) structure: one Au interdigitated electrode with wide fingers and the other one with narrow fingers. These ZnO photodetectors exhibit attractive photovoltaic characteristics at 0 V bias. More interestingly, with increasing the asymmetric ratio (the width of wide fingers : the width of narrow fingers) of the interdigitated electrodes, the responsivity of the ZnO self-powered UV photodetectors was enhanced obviously, reaching as high as 20 mA W−1 when the asymmetric ratio was 20 : 1. A physical model based on band energy theory was developed to illustrate the origin of the photoresponse at 0 V in our device. Our findings provide a new route to realizing self-powered photodetectors.

Tunable enhancement of exciton emission from MgZnO by hybridized quadrupole plasmons in Ag nanoparticle aggregation

Mg0.2Zn0.8O/metal nanoparticle systems have been fabricated and investigated. The photoluminescence results indicate that Al and Au nanoparticles could slightly enhance the near-band-edge (NBE) emission from Mg0.2Zn0.8O. In contrast, a giant and tunable NBE emission enhancement could be induced by Ag nanoparticles based on the coupling interaction between the hybridized quadrupole plasmon in Ag nanoparticle aggregation and the excitons of Mg0.2Zn0.8O. Interestingly, the intensity and position of the narrow quadrupole resonance could be controlled by tuning the interspace gap and size of Ag nanoparticles, which was clearly demonstrated both experimentally and theoretically. Our findings may pave the way for further development of high-efficiency UV light-emitting devices.

Enhanced solar-blind responsivity of photodetectors based on cubic MgZnO films via gallium doping

We report on gallium (Ga) doped cubic MgZnO films, which have been grown by metal organic chemical vapor deposition. It was demonstrated that Ga doping improves the n-type conduction of the cubic MgZnO films. A two-orders of magnitude enhancement in lateral n-type conduction have been achieved for the cubic MgZnO films. The responsivity of the cubic MgZnO-based photodetector has been also enhanced. Depletion region electric field intensity enhanced model was adopted to explain the improvement of quantum efficiency in Ga doped MgZnO-based detectors.

Enhanced Responsivity of Photodetectors Realized via Impact Ionization

To increase the responsivity is one of the vital issues for a photodetector. By employing ZnO as a representative material of ultraviolet photodetectors and Si as a representative material of visible photodetectors, an impact ionization process, in which additional carriers can be generated in an insulating layer at a relatively large electric field, has been employed to increase the responsivity of a semiconductor photodetector. It is found that the responsivity of the photodetectors can be enhanced by tens of times via this impact ionization process. The results reported in this paper provide a general route to enhance the responsivity of a photodetector, thus may represent a step towards high-performance photodetectors.

Piezophototronic‐Effect‐Enhanced Electrically Pumped Lasing

This study demonstrates significant improvements of ZnO nanowire lasers by the piezophototronic effect. The laser output power can be enhanced by a factor of 4.96, and the threshold voltage can be decreased from 48 to 20 V by applying pressure. The mechanism of the improved performance can be attributed to the enhanced carrier injection and recombination due to the piezophototronic effect.

Electrical Characteristics of Superconducting Nanowire Single Photon Detector

By changing the bias scheme of a superconducting nanowire single photon detector (SNSPD), the current-voltage (I-V) characteristics of SNSPDs are extensively studied. Using a quasi-constant-voltage bias, the SNSPD nonlatching property is observed in the low voltage range. When the bias current surpasses the critical current, a thermal relaxation oscillation occurs because of the hotspots periodic appearance, growth, and disappearance. This produces a smooth transition in the I- V curve instead of latching. The oscillation frequency is proportional to the voltage of the SNSPD. A linear dependence of the oscillation frequency on the SNSPD voltage is derived from the circuit model of SNSPD electronics, which fits well with the experimental results. The kinetic inductance of the SNSPD is extracted from the fitting as the sole parameter. In the high-voltage range, the I-V curve is deduced from Skocpol, Beasley, and Tinkhams self-heating hotspot model, which is consistent with the experimental result. By measuring the return current, we obtained a heat transfer coefficient (α) of 0.7-2 × 105 W · m2 · K1 between the nanowire and the substrate.

Superconducting Nanowire Single-Photon Detector With a System Detection Efficiency Over 80% at 940-nm Wavelength

Implementations of quantum information require single-photon detectors (SPDs) with high detection efficiency (DE) at a wavelength of 940 nm, which is a challenge for the available semiconducting SPDs. Superconducting nanowire SPDs (SNSPDs) are capable of detecting visible and near-infrared single photons with high DE. However, these detection capabilities place stringent design requirements on the cavity and nanowire geometry structures. We design, fabricate, and measure SNSPDs with high DE optimized for the 940-nm wavelength. The NbN SNSPDs were fabricated on 1-D photonic crystals for high optical absorptance. By tuning the filling factor of the nanowire through numerical simulations and experiments, we were able to obtain an SNSPD (7 nm thick, 125 nm width, and 0.57 filling factor, as well as active area of 18 * 18 μm) with a saturated system DE of 83.6 ± 3.7%, at a dark count rate of 10 Hz, and a low polarization dependence of 1.17 ± 0.02. To our best knowledge, this is the highest value reported for NbN SNSPDs at 940-nm wavelength. The availability of an SNSPD with high system DE at 940 nm may have a profound impact in the field of photonic quantum technologies, such as multiphoton entanglement.

Performance improvement of a ZnMgO ultraviolet detector by chemical treatment with hydrogen peroxide

Herein, ZnMgO thin film ultraviolet (UV) photodetectors based on metal-semiconductor-metal structure were fabricated on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The effect of H2O2 solution treatment on the properties of the ZnMgO thin film and its UV photodetectors was investigated. After immersing the ZnMgO UV photodetector in a H2O2 solution at 100 degrees C for 3 min, the dark current of the device was reduced by more than one order of magnitude under 1 V bias, whereas the responsivity was slightly decreased. More interestingly, the response speed became much quicker and insensitive to the atmosphere after the treatment of the photodetector with H2O2 solution, which can be attributed to the reduction in the oxygen vacancy defects. Our findings may provide a promising approach for improving the performance of photodetectors.

Transparent ultraviolet photovoltaic cells

Photovoltaic cells have been fabricated from p-GaN/MgO/n-ZnO structures. The photovoltaic cells are transparent to visible light and can transform ultraviolet irradiation into electrical signals. The efficiency of the photovoltaic cells is 0.025% under simulated AM 1.5 illumination conditions, while it can reach 0.46% under UV illumination. By connecting several such photovoltaic cells in a series, light-emitting devices can be lighting. The photovoltaic cells reported in this Letter may promise the applications in glass of buildings to prevent UV irradiation and produce power for household appliances in the future.

Superconducting nanowire single-photon detectors at a wavelength of 940 nm

In this article, we demonstrate a superconducting nanowire single photon detector (SNSPD) at a wavelength of 2000 nm. We use numerical simulations to optimize the parameters of the device structure. Guided by the obtained numerical results, we fabricated and measured the device that operates at 2000 nm at 2.3K. The results indicate that our device responses well in 2000nm with a dark count rate of 100Hz. The recovering time is 100ns.