Fangfang Ren

Electrical instability of amorphous indium-gallium-zinc oxide thin film transistors under monochromatic light illumination

This work was supported by the State Key Program for Basic Research of China under Grant Nos. 2010CB327504, 2011CB922100, 2011CB301900; the National Natural Science Foundation of China under Grant Nos. 60825401, 60936004, 11104130, BK2011556, and BK2011050.

Enhanced bias stress stability of a-InGaZnO thin film transistors by inserting an ultra-thin interfacial InGaZnO:N layer

Amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs) having an ultra-thin nitrogenated a-IGZO (a-IGZO:N) layer sandwiched at the channel/gate dielectric interface are fabricated. It is found that the device shows enhanced bias stress stability with significantly reduced threshold voltage drift under positive gate bias stress. Based on x-ray photoelectron spectroscopy measurement, the concentration of oxygen vacancies within the a-IGZO:N layer is suppressed due to the formation of N-Ga bonds. Meanwhile, low frequency noise analysis indicates that the average trap density near the channel/dielectric interface continuously drops as the nitrogen content within the a-IGZO:N layer increases. The improved interface quality upon nitrogen doping agrees with the enhanced bias stress stability of the a-IGZO TFTs.

Large-scale fabrication and luminescence properties of GaN nanostructures by a soft UV-curing nanoimprint lithography

GaN nanorods with a period of 400 nm and diameter of 200 nm, and nano-gratings with a period of 400 nm and gap width of 100 nm are fabricated on wafers by a soft UV-curing nanoimprint lithography. These nanostructures show high periodicity and good morphology. The photoluminescence (PL) spectra exhibit that the integral PL intensity of GaN nanorods is enhanced as much as 2.5 times, compared to that of as-grown GaN films. According to finite-difference time-domain simulations and cathodoluminescence mappings, it is concluded that the enhancement for nanorods is due to the improvements of both spontaneous emission rate and light extraction efficiency caused by periodic GaN structures on the surface. By identifying the Raman shift of E1(TO) and E2(H) modes of GaN films with nano-gratings and nanorods, the normal-plane strain ε(zz) is determined. The PL emission energy is found to be proportional to the ε(zz), whose linear proportionality factor is calculated to be -27 meV GPa(-1).

High-Temperature Single Photon Detection Performance of 4H-SiC Avalanche Photodiodes

In this letter, the high-temperature performance of 4H-SiC avalanche photodiodes (APDs) working in Geiger mode is studied for the first time. At unity gain bias, the maximum quantum efficiency of the APD increases from 53.4% at 290 nm to 63.3% at 295 nm as temperature rises from room temperature to 150 °C. Meanwhile, the dark current of the APD before breakdown increases by more than two to three orders of magnitude. At a fixed gain of 1.3 × 106, the single photon counting efficiency at 280 nm only slightly drops from 6.17% to 6% in the same temperature range, whereas the dark count rate increases from 22 to 80 KHz. This letter indicates that SiC APDs have the potential to work in a high-temperature harsh environment with single photon counting capability.

Large-Swing a-IGZO Inverter With a Depletion Load Induced by Laser Annealing

In this letter, a high-performance inverter based on amorphous indium-gallium-zinc oxide thin-film transistors (TFTs) is fabricated, which consists of an enhancement mode driver and a depletion mode load. By applying an area-selective laser annealing technique, the threshold voltage (Vth) of the load TFT could be tuned from positive to negative gate voltage. Based on X-ray photoelectron spectroscopy analyses, the negative Vth shift can be attributed to the increase of oxygen vacancy concentration within the device channel upon laser annealing. Meanwhile, compared with conventional inverters with an enhancement mode load, the proposed inverter shows much improved switching characteristics, including output swing range close to 100% full swing as well as an enhanced output voltage gain from -1.5 to -20.5 V/V.

Second-order surface-plasmon assisted responsivity enhancement in germanium nano-photodetectors with bull’s eye antennas

The enhancement of photo-response in nanometer-scale germanium photodetectors through bulls eye antennas capable of supporting 2nd-order Bloch surface plasmon modes is demonstrated in theory and experiment. A detailed numerical investigation reveals that the presence of surface wave and its constructive interference with the directly incident light are incorporated into the main mechanisms for enhancing transmission through the central nanoaperture. With a grating period of 1500 nm, the area-normalized responsivity can be enhanced up to 3.8 times at 2 V bias for a 780 nm laser. It provides an easier fabrication path for ultra-short wavelength operations especially in devices using optically denser materials.

High quantum efficiency back-illuminated AlGaN-based solar-blind ultraviolet p-i-n photodetectors

AlGaN-based back-illuminated solar-blind ultraviolet (UV) p—i—n photodetectors (PDs) with high quantum efficiency are fabricated on sapphire substrates. To improve the overall performance of the PD, a series of structural design considerations and growth procedures are implemented in the epitaxy process. A distinct wavelength-selective photo-response peak of the PD is obtained in the solar-blind region. When operating in photovoltaic mode, the PD exhibits a solar-blind/UV rejection ratio of up to 4 orders of magnitude and a peak responsivity of ~113.5 mA/W at 270 nm, which corresponds to an external quantum efficiency of ~52%. Under a reverse bias of −5 V, the PD shows a low dark current of ~1.8 pA and an enhanced peak quantum efficiency of ~64%. The thermal noise limited detectivity is estimated to be ~ 3.3 × 1013 cmHz1/2W−1

Electrically tunable terahertz metamaterials with embedded large-area transparent thin-film transistor arrays

Engineering metamaterials with tunable resonances are of great importance for improving the functionality and flexibility of terahertz (THz) systems. An ongoing challenge in THz science and technology is to create large-area active metamaterials as building blocks to enable efficient and precise control of THz signals. Here, an active metamaterial device based on enhancement-mode transparent amorphous oxide thin-film transistor arrays for THz modulation is demonstrated. Analytical modelling based on full-wave techniques and multipole theory exhibits excellent consistent with the experimental observations and reveals that the intrinsic resonance mode at 0.75 THz is dominated by an electric response. The resonant behavior can be effectively tuned by controlling the channel conductivity through an external bias. Such metal/oxide thin-film transistor based controllable metamaterials are energy saving, low cost, large area and ready for mass-production, which are expected to be widely used in future THz imaging, sensing, communications and other applications.

Demonstration of an AlGaN-based solar-blind high-voltage photoconductive switch

A solar-blind photoconductive semiconductor switch (PCSS) is first fabricated on high-resistivity Al0.4Ga0.6N layer grown on sapphire substrate. The PCSS exhibits a cutoff wavelength of ∼280 nm and a dark resistivity of ∼1012 Ω cm. A maximum blocking voltage of more than 950 V is obtained, corresponding to a breakdown electric-field of >1.35 MV/cm for the active AlGaN layer. When excited by a 266 nm ultraviolet pulsed laser, the PCSS under 500 V bias could produce a peak photocurrent density of 11.5 kA/cm2 within a rise time of ∼15 ns. The fall time of the photocurrent pulse is mainly RC time limited.

Split Bull’s Eye Antenna for High-Speed Photodetector in the Range of Visible to Mid-Infrared

Split bulls eye (SBE) antenna was developed to efficiently concentrate electromagnetic energy into an ultra-small area, and thereby enhanced the responsivity of high-speed near-infrared photodetectors. Here, we extend the investigation over a wider spectral range, and analyze the enhancement mechanisms based on finite-difference time-domain simulations. The SBE antenna exhibits far greater transmission enhancement than bulls eye antenna, especially at mid-infrared wavelengths, substantially due to the change in transmission mode from evanescent to propagating and also the induced Fabry-Perot resonances in the central slit. Our results illustrate the SBE antenna is a promising candidate for high-speed photodetection ranging from visible to mid-infrared.