Showing papers by "Youdou Zheng published in 2015"

Electrically pumped random lasing based on an Au–ZnO nanowire Schottky junction

Abstract: Electrically pumped random lasing based on an Au–ZnO nanowire Schottky junction diode is demonstrated. The device exhibits typical Schottky diode current–voltage characteristics with a turn-on voltage of 0.7 V. Electroluminescence characterization shows good random lasing behavior and the output power is about 67 nW at a drive current of 100 mA. Excitonic recombination is responsible for lasing generation. Zn plasma is only observed under high applied bias, which can be distinguished from the random lasing spectral features near 380 nm. The laser diode based on the Schottky junction provides an alternative approach towards semiconductor random lasers.

Reducing contact resistance in ferroelectric organic transistors by buffering the semiconductor/dielectric interface

Abstract: The reduction of contact resistance in ferroelectric organic field-effect transistors (Fe-OFETs) by buffering the interfacial polarization fluctuation was reported. An ultrathin poly(methyl methacrylate) layer was inserted between the ferroelectric polymer and organic semiconductor layers. The contact resistance was significantly reduced to 55 kΩ cm. By contrast, Fe-OFETs without buffering exhibited a significantly larger contact resistance of 260 kΩ cm. Results showed that such an enhanced charge injection was attributed to the buffering effect at the semiconductor/ferroelectric interface, which narrowed the trap distribution of the organic semiconductor in the contact region. The presented work provided an efficient method of lowering the contact resistance in Fe-OFETs, which is beneficial for the further development of Fe-OFETs.

Optical polarization characteristics of c-plane InGaN/GaN asymmetric nanostructures

Abstract: Highly ordered c-plane InGaN/GaN elliptic nanorod (NR) and nano-grating (NG) arrays were fabricated by our developed soft UV-curing nanoimprint lithography on a wafer. The polarized photoluminescence emission from these elliptic NR and NG arrays has been investigated both theoretically and experimentally. Considerable in-plane optical anisotropy, with a polarization ratio of 15% and 71% and a peak shift of 5.2 meV and 28.1 meV, was discovered from these c-plane InGaN/GaN elliptic NR and NG arrays, respectively. The k·p perturbation theory was adopted to explore this situation, simulating the transitions from conduction subbands to valence subbands and their corresponding optical momentum matrix elements at/around Γ point under the in-plane asymmetric strain. The good agreements of observed and simulated results demonstrate that the in-plane strain asymmetry is the essential cause of the optical polarization in this case, revealing the great potential to utilize strain effect to control the polarization of...

High-temperature and reliability performance of 4H-SiC Schottky-barrier photodiodes for UV detection

Abstract: With its wide bandgap, excellent electrical properties, and relative mature crystal growth technique, 4H-SiC is an attractive candidate material for fabricating high performance ultraviolet photodetectors, which have potential to work in harsh environments. In this work, a vertical type 4H-SiC Schottky-barrier photodiode is designed and fabricated. The photodiode exhibits very low dark current and high quantum efficiency in the entire temperature range from 25 to 200 °C. A high Schottky barrier height of over 1.58 eV and an ideality factor as low as 1.074 at room temperature are deduced for the semitransparent Ni/4H-SiC Schottky metal contact. The high-temperature reliability characteristics are evaluated by high-temperature storage at 200 °C and high-temperature spike annealing up to 550 °C.

Passive Quenching Electronics for Geiger Mode 4H-SiC Avalanche Photodiodes

Abstract: We design and fabricate 4H-SiC UV avalanche photodiodes (APDs) with positive beveled mesa, which exhibit low leakage current and high avalanche gain when working in the Geiger mode. The single photon counting performance of the SiC APDs is studied by using a passive-quenching circuit. A new method to determine the exact breakdown voltage of the APD is proposed based on the initial emergence of photon count pulses. The photon count rate and dark count rate of the APD are also evaluated as a function of quenching resistance.

In-Situ Measurement of Junction Temperature and Light Intensity of Light Emitting Diodes With an Internal Sensor Unit

Abstract: This study designed and tested an innovative light-emitting diode (LED) chip with a built-in sensor Two electrically isolated units, the LED (for light emission) and the sensor (for monitoring junction temperature and light intensity), were integrated on a single chip The sensor unit determines the junction temperature by measuring the forward voltage; the light output power of the LED unit can be precisely extrapolated with a polynomial function based on the photocurrent and junction temperature This novel structure enables the in-situ real-time monitoring of the LED junction temperature and light output power, which allows a highly detailed and/or in-field LED reliability analysis and provides valuable feedback information for smart LED lighting systems

Asymmetric tunneling model of forward leakage current in GaN/InGaN light emitting diodes

Abstract: Through investigating the temperature dependent current-voltage (T-I-V) properties of GaN based blue and green LEDs in this study, we propose an asymmetric tunneling model to understand the leakage current below turn-on voltage (V < 3.2 V): At the forward bias within 1.5 V ∼ 2.1 V (region 1), the leakage current is main attributed to electrons tunneling from the conduction band of n-type GaN layer to the valence band of p-type GaN layer via defect states in space-charge region (SCR); While, at the forward bias within 2 V ∼ 2.4 V (region 2), heavy holes tunneling gradually becomes dominant at low temperature (T < 200K) as long as they can overcome the energy barrier height. The tunneling barrier for heavy holes is estimated to be lower than that for electrons, indicating the heavy holes might only tunnel to the defect states. This asymmetric tunneling model shows a novel carrier transport process, which provides better understanding of the leakage characteristics and is vital for future device improvements.

Annealing in tellurium-nitrogen co-doped ZnO films: The roles of intrinsic zinc defects

Abstract: In this article, the authors have conducted an extensive investigation on the roles of intrinsic zinc defects by annealing of a batch of Te-N co-doped ZnO films. The formation and annihilation of Zn interstitial (Zni) clusters have been found in samples with different annealing temperatures. Electrical and Raman measurements have shown that the Zni clusters are a significant compensation source to holes, and the Te co-doping has a notable effect on suppressing the Zni clusters. Meanwhile, shallow acceptors have been identified in photoluminescence spectra. The NO-Zn-Te complex, zinc vacancy (VZn)-NO complex, and VZn clusters are thought to be the candidates as the shallow acceptors. The evolution of shallow acceptors upon annealing temperature have been also studied. The clustering of VZn at high annealing temperature is proposed to be a possible candidate as a stable acceptor in ZnO.

Investigation of surface-plasmon coupled red light emitting InGaN/GaN multi-quantum well with Ag nanostructures coated on GaN surface

Abstract: Surface-plasmon (SP) coupled red light emitting InGaN/GaN multiple quantum well (MQW) structure is fabricated and investigated. The centre wavelength of 5-period InGaN/GaN MQW structure is about 620 nm. The intensity of photoluminescence (PL) for InGaN QW with naked Ag nano-structures (NS) is only slightly increased due to the oxidation of Ag NS as compared to that for the InGaN QW. However, InGaN QW with Ag NS/SiO2 structure can evidently enhance the emission efficiency due to the elimination of surface oxide layer of Ag NS. With increasing the laser excitation power, the PL intensity is enhanced by 25%–53% as compared to that for the SiO2 coating InGaN QW. The steady-state electric field distribution obtained by the three-dimensional finite-difference time-domain method is different for both structures. The proportion of the field distributed in the Ag NS for the GaN/Ag NS/SiO2 structure is smaller as compared to that for the GaN/naked Ag NS structure. As a result, the energy loss of localized SP modes ...

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

Abstract: 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.

Discrimination Voltage and Overdrive Bias Dependent Performance Evaluation of Passively Quenched SiC Single-Photon-Counting Avalanche Photodiodes

Abstract: In many critical civil and emerging military applications, low-level UV detection, sometimes at single photon level, is highly desired. In this work, a mesa-type 4H-SiC UV avalanche photodiode (APD) is designed and fabricated, which exhibits low leakage current and high avalanche gain. When studied by using a passive quenching circuit, the APD exhibits self-quenching characteristics due to its high differential resistance in the avalanche region. The single photon detection efficiency and dark count rate of the APD are evaluated as functions of discrimination voltage and over-drive voltage. The optimized operation conditions of the single photon counting APD are discussed.

Study on interface characteristics in amorphous indium–gallium–zinc oxide thin-film transistors by using low-frequency noise and temperature dependent mobility measurements

Abstract: In this work, the interface properties of amorphous indium–gallium–zinc oxide thin film transistors annealed at different temperatures ranging from 150 to 250 °C are studied by temperature dependent mobility and low-frequency noise (LFN) characterizations. The dominant scattering mechanism for carrier transport is found to be Coulomb scattering based on gate bias and temperature dependent mobility measurement. Meanwhile, as the annealing temperature increases, the dominant mechanism of LFN within the device channel varies from carrier number fluctuation to carrier mobility fluctuation. The border trap density as well as the distribution properties of charged border traps is deduced. The present results suggest that annealing at higher temperature has a more remarkable effect on removing deeper border traps than traps closer to the channel/dielectric interface.

Fabrication and characterization of highly oriented N-doped ZnO nanorods by selective area epitaxy

Abstract: High-quality nitrogen-doped ZnO nanorods have been selectively grown on patterned and bare ZnO templates by the combination of nanoimprint lithography and chemical vapor transport methods. The grown nanorods exhibited uniformity in size and orientation as well as controllable density and surface-to-volume ratio. The structural and optical properties of ZnO nanorods and the behaviour of N dopants have been investigated by means of the scanning electron microscope, photoluminescence (PL) spectra, and Raman scattering spectra. The additional vibration modes observed in Raman spectra of N-doped ZnO nanorods provided solid evidence of N incorporation in ZnO nanorods. The difference of excitonic emissions from ZnO nanorods with varied density and surface-to-volume ratio suggested the different spatial distribution of intrinsic defects. It was found that the defects giving rise to acceptor-bound exciton (A0X) emission were most likely to distribute in the sidewall surface with nonpolar characteristics, while the donor bound exciton (D0X) emission related defects distributed uniformly in the near top polar surface.

Influence of dislocations on indium diffusion in semi-polar InGaN/GaN heterostructures

Abstract: The spatial distribution of indium composition in InGaN/GaN heterostructure is a critical topic for modulating the wavelength of light emitting diodes. In this letter, semi-polar InGaN/GaN heterostructure stripes were fabricated on patterned GaN/Sapphire substrates by epitaxial lateral overgrowth (ELO), and the spatial distribution of indium composition in the InGaN layer was characterized by using cathodoluminescence. It is found that the indium composition is mainly controlled by the diffusion behaviors of metal atoms (In and Ga) on the surface. The diffusivity of metal atoms decreases sharply as migrating to the region with a high density of dislocations and other defects, which influences the distribution of indium composition evidently. Our work is beneficial for the understanding of ELO process and the further development of InGaN/GaN heterostructure based devices.

Bloch surface plasmon enhanced blue emission from InGaN/GaN light-emitting diode structures with Al-coated GaN nanorods

Abstract: InGaN/GaN light-emitting diode structures with Al-coated GaN nanorods were fabricated by using soft ultraviolet nanoimprint lithography. The intensity of light emission was found to be greatly enhanced due to the strong near-fields confined at the interface of Al/GaN and extended to the multiple quantum wells (MQWs) active region. The dynamics of carrier recombination and plasmon-enhanced Raman scattering were also investigated, providing a progressive view on the effective energy transfer between MQWs and surface plasmons.

Comparative study of the effect of H2 addition on ZnO films grown by different zinc and oxygen precursors

Abstract: In this study, the authors have comparatively studied the influence of H2 addition on the structures and properties of ZnO films grown by metal organic (MO) chemical vapor deposition with dimethyl zinc and diethyl zinc as zinc precursors and N2O and O2 as oxygen sources, respectively. Various characterization methods, like x-ray diffraction, Raman scattering, Hall effect, photoluminescence, and atomic force microscopy, have been utilized, showing that H2 has different effects on different MO precursors and oxidants. The H2 addition has significantly improved the crystal structural quality of ZnO thin films for the case of dimethyl zinc source, but an opposite effect has been found for the case of diethyl zinc. Moreover, the H2 addition can significantly improve the optical properties of the ZnO films, regardless of the zinc MO sources used, with the surface morphology improved too. The suppression of carbon-related contaminations depends on the use of different precursors and whether H2 is added. By analyzing the experimental results, we have given the effects of H2 on the decomposition of the discussed MO precursors and oxidants, the proposed mechanism could be used in understanding the experimental data.

Structural evolution of Ge-rich Si1−xGex films deposited by jet-ICPCVD

Abstract: Amorphous Ge-rich Si1−xGex films with local Ge-clustering were deposited by dual-source jet-type inductively coupled plasma chemical-vapor deposition (jet-ICPCVD). The structural evolution of the deposited films annealed at various temperatures (Ta) is investigated. Experimental results indicate that the crystallization occurs to form Ge and Si clusters as Ta = 500 °C. With raising Ta up to 900 °C, Ge clusters percolate together and Si diffuses and redistributes to form a Ge/SiGe core/shell structure, and some Ge atoms partially diffuse to the surface as a result of segregation. The present work will be helpful in understanding the structural evolution process of a hybrid SiGe films and beneficial for further optimizing the microstructure and properties.

Surface Morphology of AlN Nucleation Layer Grown on Si by MOCVD

Abstract: The AlN nucleation layer (NL) has been deposited on Si (111) substrate by metal-organic chemical vapor deposition (MOCVD). The result indicates that the growth mode of the AlN NL is in the form of 2-dimensional plane and 3-dimensional island. The proportion of 3-dimensional region increases gradually and the 2-dimensional region reduces correspondingly with the increase of growth time. The decrease of the coverage ratio of AlN grains in the 2-dimensional growth region is due to the effect of etching. AlN film with the single crystal orientation has been deposited on the optimized AlN NL.

Electrically pumped random lasing based on Au-ZnO nanowire Schottky junction

Abstract: Electrically pumped random lasing based on Au-ZnO nanowire Schottky junction diode is demonstrated. Good lasing behavior is achieved and excitonic recombination is responsible for lasing generation. It provides an alternative approach towards semiconductor random lasers.

Determination of Temperature-Dependent Stress State in Thin AlGaN Layer of AlGaN/GaN HEMT Heterostructures by Near-Resonant Raman Scattering

Abstract: The temperature-dependent stress state in the AlGaN barrier layer of AlGaN/GaN heterostructure grown on sapphire substrate was investigated by ultraviolet (UV) near-resonant Raman scattering. Strong scattering peak resulting from the A1(LO) phonon mode of AlGaN is observed under near-resonance condition, which allows for the accurate measurement of Raman shifts with temperature. The temperature-dependent stress in the AlGaN layer determined by the resonance Raman spectra is consistent with the theoretical calculation result, taking lattice mismatch and thermal mismatch into account together. This good agreement indicates that the UV near-resonant Raman scattering can be a direct and effective method to characterize the stress state in thin AlGaN barrier layer of AlGaN/GaN HEMT heterostructures.

First-Principles GGA+U Study of Intermediate-Band Characters from Zn1−xMxO (M = 3d Transition-Metal) Alloys Suitable for High Efficiency Solar Cell*

Abstract: The electronic structure characters are calculated for the Zn1−xMxO alloys with some Zn atoms in ZnO substituted by 3d transition-metal atoms (M), in order to find out which of these alloys could provide an intermediate band material used for fabricating high efficiency solar cell. Especially, among of these alloys, the electronic structure character and optical performance of Zn1−xCrxO alloys clearly show an intermediate band filled partially and isolated from the VB and the CB in energy band structure of ZnO host, and the intermediate band characters can be preserved with increasing Cr concentrations no more than 8.33% in Zn1−xCrxO alloys, at the same time, the ratio 0.52 of EgFC to EgVF in Zn1−xCrxO, (x = 4.16%) alloy is closest to the optimal ratio of 0.57. Besides, compared to the ZnO, the optical absorption does indicate a great improved absorption below the calculated band gap of the ZnO and an enhancement of the optical absorption in the whole solar spectral energy range.