Ying-Jay Yang

Ultrahigh photocurrent gain in m-axial GaN nanowires

An ultrahigh photocurrent gain has been found in the ultraviolet-absorbed GaN nanowires with m-directional long axis grown by chemical vapor deposition. The quantitative results have shown the gain values at 5.0×104–1.9×105 of the GaN nanowires with diameters from 40to135nm are near three orders of magnitude higher than the values of 5.2×101–1.6×102 estimated from the thin film counterparts. The intensity-dependent gain study has shown that the gain value is very sensitive to the excitation intensity following an inverse power law and no gain saturation observed in this investigated intensity range from 0.75to250W∕m2. This behavior has strongly suggested a surface-dominant rather than trap-dominant high gain mechanism in this one-dimensional nanostructure. The strong carrier localization effect induced by the surface electric field in the GaN nanowires is also discussed.

Electroluminescence from ZnO/Si-Nanotips Light-Emitting Diodes

A new and general approach to achieving efficient electrically driven light emission from a Si-based nano p-n junction array is introduced. A wafer-scale array of p-type silicon nanotips were formed by a single-step self-masked dry etching process, which is compatible with current semiconductor technologies. On top of the silicon nanotip array, a layer of n-type ZnO film was grown by pulsed laser deposition. Both the narrow line width of 10 nm in cathodoluminescence spectra and the appearance of multiphonon Raman spectra up to the fourth order indicate the excellent quality of the ZnO film. The turn-on voltage of our ZnO/Si nanotip array is found to be approximately 2.4 V, which is 2 times smaller than its thin film counterpart. Moreover, electroluminescence (EL) from our ZnO/Si nanotips array light-emitting diode (LED) has been demonstrated. Our results could open up new possibilities to integrate silicon-based optoelectronic devices, such as highly efficient LEDs, with standard Si ultralarge-scale integrated technology.

Efficient Light Harvesting by Photon Downconversion and Light Trapping in Hybrid ZnS Nanoparticles/Si Nanotips Solar Cells

A hybrid colloidal ZnS nanoparticles/Si nanotips p-n active layer has been demonstrated to have promising potential for efficient solar spectrum utilization in crystalline silicon-based solar cells. The hybrid solar cell shows an enhancement of 20% in the short-circuit current and approximately 10% in power conversion efficiency compared to its counterpart without integrating ZnS nanoparticles. The enhancement has been investigated by external quantum efficiency, photoluminescence excitation spectrum, photoluminescence, and reflectance to distinct the role of ZnS quantum dots for light harvesting. It is concluded that ZnS nanoparticles not only act as frequency downconversion centers in the ultraviolet region but also serve as antireflection coating for light trapping in the measured spectral regime. Our approach is ready to be extended to many other material systems for the creation of highly efficient photovoltaic devices.

Enhanced and partially polarized output of a light-emitting diode with its InGaN/GaN quantum well coupled with surface plasmons on a metal grating

The enhanced and partially polarized output of a green light-emitting diode (LED), in which its InGaN/GaN quantum well (QW) couples with surface plasmons (SPs) on a surface Ag grating structure, is demonstrated. Compared with a LED sample without (flat) Ag coating, the total output intensity of an LED of SP-QW coupling can be enhanced by ∼59% (∼200)% when the grating period and groove depth are 500 and 30 nm, respectively. Also, a bottom-emission polarization ratio of 1.7 can be obtained under the condition of 15 nm in groove depth.

p-Si nanowires/SiO2/n-ZnO heterojunction photodiodes

Influence of a SiO2 ultrathin film on n-ZnO/p-silicon nanowires photodiodes has been investigated. With a SiO2 thin layer, the diode characteristics can be significantly improved, which exhibits high responsivity under a reverse bias. Based on the electron conversion efficiency measurement, we show that the ultrathin SiO2 layer with positive fixed charges not only acts as a hole blocking layer but also helps the photogenerated electrons to tunnel through the barrier. In addition, the SiO2 layer can effectively passivate the defects generated by wet etching process. It is expected that our approach can be extended to many other nanoscale heterojunction devices.

High-gain photoconductivity in semiconducting InN nanowires

We report on the photoconductivity study of the individual infrared-absorbing indium nitride (InN) nanowires. Temperature-dependent dark conductivity measurement indicates the semiconducting transport behavior of these InN nanowires. An enhanced photosensitivity from 0.3 to 14 is observed by lowering the temperature from 300to10K. A calculated ultrahigh photoconductive gain at around 8×107 at room temperature is obtained from the low-bandgap nitride nanowire under 808nm excitation.

Polarization dependent coupling of surface plasmon on a one-dimensional Ag grating with an InGaN∕GaN dual-quantum-well structure

The authors report the observation of a polarization-dependent surface plasmon (SP) feature on a one-dimensional Ag-grating structure through the SP coupling with an InGaN∕GaN dual-quantum-well structure closely below the metal grating. Polarized photon output is observed because only the momentum matching condition of the SP mode propagating in the direction perpendicular to the grating grooves can be reached through the diffraction of the fabricated grating and, thus, the SP radiation efficiency is significantly enhanced only in this polarization. The dispersion curve of the observed SP mode shows a group velocity of 2.4×108m∕s, which manifests the SP characteristics in the air/Ag∕GaN grating structure.

Oxide-Relief and Zn-Diffusion 850-nm Vertical-Cavity Surface-Emitting Lasers With Extremely Low Energy-to-Data-Rate Ratios for 40 Gbit/s Operations

We demonstrate novel structures of a vertical-cavity surface-emitting laser (VCSEL) for high-speed (~40 Gbit/s) operation with ultralow power consumption performance. Downscaling the size of oxide aperture of VCSELs is one of the most effective ways to reduce the power consumption during high-speed operation. However, such miniaturized oxide apertures (~2 μm diameter) in VCSELs will result in a large differential resistance, optical single-mode output, and a small maximum output power (<; 1 mW). These characteristics seriously limit the maximum electrical-to-optical (E-O) bandwidth and device reliability. By the use of the oxide-relief and Zn-diffusion techniques in our demonstrated 850-nm VCSELs, we can not only release the burden imposed on downscaling the current-confined aperture for high speed with low-power consumption performance, but can also manipulate the number of optical modes inside the cavity to maximize the E-O bandwidth and product of bit-rate transmission distance in an OM4 fiber. State-of-the-art dynamic performances at both room temperature and 85 °C operations can be achieved by the use of our device. These include extremely high D-factors (~13.5 GHz/mA1/2), as well as record-low energy-to-data ratios (EDR: 140 fJ/bit) at 34 Gbit/s operation, and error-free transmission over a 0.8-km OM4 multimode fiber with a record-low energy-to-data distance ratio (EDDR: 175.5 fJ/bit.km) at 25 Gbit/s.

Transferable and Flexible Label‐Like Macromolecular Memory on Arbitrary Substrates with High Performance and a Facile Methodology

A newly designed transferable and flexible label-like organic memory based on a graphene electrode behaves like a sticker, and can be readily placed on desired substrates or devices for diversified purposes. The memory label reveals excellent performance despite its physical presentation. This may greatly extend the memory applications in various advanced electronics and provide a simple scheme to integrate with other electronics.

Size-dependent photoconductivity and dark conductivity of m-axial GaN nanowires with small critical diameter

The size effects on both the photoconductivity and dark conductivity have been observed in m-axial GaN nanowires grown by chemical vapor deposition (CVD). For these nanowires with diameters at 50–130 nm, the products of carrier lifetime (τ) and mobility (μ) derived from the photocurrent measurements are typically at (2–8)×10−1 cm2/V, which are over two orders of magnitude higher than the maximal reported values [τμ=(1–5)×10−4 cm2/V] for their thin film counterparts. A significant decrease of τμ value at diameter below the critical values (dcrt) at 30–40 nm is observed. Similar size dependence is also found from the dark conductivity study. The temperature-dependent measurements further indicate two different thermal activation mechanisms in GaN nanowires with sizes above and below the dcrt. These results suggest a surface-dominant transport property in GaN nanowires both in dark and under light illumination due to the presence of surface depletion and band bending. Probable reasons leading to the smaller ...