Violet-blue LEDs based on p-GaN/n-ZnO nanorods and their stability

In this paper, we report a fabrication, characterization and stability study of p-GaN/n-ZnO nanorod heterojunction light-emitting devices (LEDs). The LEDs were assembled from arrays of n-ZnO vertical nanorods epitaxially grown on p-GaN. LEDs showed bright electroluminescence in blue (440 nm), although weaker violet (372 nm) and green-yellow (550 nm) spectral components were also observed. The device characteristics are generally stable and reproducible. The LEDs have a low turn-on voltage (～5 V). The electroluminescence (EL) is intense enough to be noticed by the naked eye, at an injection current as low as ～ 40 μA (2.1 × 10(-2) A cm(-2) at 7 V bias). Analysis of the materials, electrical and EL investigations point to the role of a high quality of p-n nano-heterojunction which facilitates a large rectification ratio (320) and a stable reverse current of 2.8 μA (1.4 × 10(-3) A cm(-2) at 5 V). Stability of EL characteristics was investigated in detail. EL intensity showed systematic degradation over a short duration when the LED was bias-stressed at 30 V. At smaller bias (<20 V) LEDs tend to show a stable and repeatable EL characteristic. Thus a simple low temperature solution growth method was successfully exploited to realize nanorod/film heterojunction LED devices with predictable characteristics.     

GaN纳米线材料的特性和制备技术

GaN是一种具有优越热稳定性和化学性质的宽禁带半导体材料，这种材料及相关器件可以工作在高温、高辐射等恶劣环境中，并可用于大功率微波器件．最近几年，由于GaN蓝光二极管的成功研制，使GaN成为了化合物半导体领域中最热门的研究课题．简要介绍了GaN纳米线材料的制备技术；综述了GaN纳米线材料的制备结果和特性．用CVD法研制的GaN纳米线的直径已经达到5～12nm，长度达到几百个微米．纳米线具有GaN的六方纤锌矿结构，其PL谱具有宽的发射峰，谱峰中心在420nm．GaN纳米线已经在肖特基二极管的研制中得到应用．     

Synthesis and electroluminescence properties of novel silicon-based copolymers containing oxadiazole and fluorene units for PLED

Novel silicon-based copolymers containing an electron-deficient oxadiazole unit and a fluorine unit have been successfully synthesized through the Heck reaction. They are soluble in common organic solvents such as THF, CHCl₃, etc. Their UV–visible absorption spectra exhibit a strong maximum band at the range of 355–381 nm in thin film. Upon a photoexcitation of 350 nm, their photoluminescence spectra show a strong maximum band around 455–475 nm in thin film. The multi-layered light-emitting diodes (LEDs) of Al(200 nm)/Ca(50 nm)/EL polymer(80 nm)/PEDOT(50 nm)/ITO were fabricated. J–V curves show the turn-on voltage in the range of 4.4–7 V. These LEDs emit the white emissive color, due to the combination of a blue electroluminescent (EL) color and a red EL color arising from the formation of a certain charge complex.     

Semiconductor Nanowire Light‐Emitting Diodes Grown on Metal: A Direction Toward Large‐Scale Fabrication of Nanowire Devices

Bottom‐up nanowires are attractive for realizing semiconductor devices with extreme heterostructures because strain relaxation through the nanowire sidewalls allows the combination of highly lattice mismatched materials without creating dislocations. The resulting nanowires are used to fabricate light‐emitting diodes (LEDs), lasers, solar cells, and sensors. However, expensive single crystalline substrates are commonly used as substrates for nanowire heterostructures as well as for epitaxial devices, which limits the manufacturability of nanowire devices. Here, nanowire LEDs directly grown and electrically integrated on metal are demonstrated. Optical and structural measurements reveal high‐quality, vertically aligned GaN nanowires on molybdenum and titanium films. Transmission electron microscopy confirms the composition variation in the polarization‐graded AlGaN nanowire LEDs. Blue to green electroluminescence is observed from InGaN quantum well active regions, while GaN active regions exhibit ultraviolet emission. These results demonstrate a pathway for large‐scale fabrication of solid state lighting and optoelectronics on metal foils or sheets.     

Large‐Scale Horizontally Aligned ZnO Microrod Arrays with Controlled Orientation,Periodic Distribution as Building Blocks for Chip‐in Piezo‐Phototronic LEDs

A novel method of fabricating large‐scale horizontally aligned ZnO microrod arrays with controlled orientation and periodic distribution via combing technology is introduced. Horizontally aligned ZnO microrod arrays with uniform orientation and periodic distribution can be realized based on the conventional bottom‐up method prepared vertically aligned ZnO microrod matrix via the combing method. When the combing parameters are changed, the orientation of horizontally aligned ZnO microrod arrays can be adjusted (θ = 90° or 45°) in a plane and a misalignment angle of the microrods (0.3° to 2.3°) with low‐growth density can be obtained. To explore the potential applications based on the vertically and horizontally aligned ZnO microrods on p‐GaN layer, piezo‐phototronic devices such as heterojunction LEDs are built. Electroluminescence (EL) emission patterns can be adjusted for the vertically and horizontally aligned ZnO microrods/p‐GaN heterojunction LEDs by applying forward bias. Moreover, the emission color from UV‐blue to yellow‐green can be tuned by investigating the piezoelectric properties of the materials. The EL emission mechanisms of the LEDs are discussed in terms of band diagrams of the heterojunctions and carrier recombination processes.     

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales. Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN. From the macroscopic point of view, the devices emit light in the green spectral range (around 550 nm) under electrical injection. At 100 mA DC current, a 1 mm2 chip that integrates around 10(7) nanowires emits an output power on the order of 10 μW. However, the emission of the nanowire-based LED shows a spotty and polychromatic emission. By using a confocal microscope, we have been able to improve the spatial resolution of the optical characterizations down to the submicrometre scale that can be assessed to a single nanowire. Detailed μ-electroluminescent characterization (emission wavelength and output power) over a representative number of single nanowires provides new insights into the vertically integrated nanowire-based LED operation. By combining both μ-electroluminescent and μ-photoluminescent excitation, we have experimentally shown that electrical injection failure is the major source of losses in these nanowire-based LEDs.     

Transport properties of hybrid nanoparticle-nanowire systems and their application to gas sensing

Experimental data and theoretical modelling of the I-V characteristics of a gas sensor constructed from a mat of Au nanoparticle-coated GaN nanowires are presented. The principal mechanism for the response of the gas sensor to methane is explained in terms of the formation of a depletion layer within the nanowires due to the presence of the gold nanoparticles. The depth of the depletion layer is modulated by the potential induced by the physisorption of gas molecules onto the Au nanoparticles. A statistical model of the temperature-dependent I-V characteristics of bare and Au nanoparticle-decorated mats of GaN nanowires based on Poisson's equation has been used to determine the depth of the depletion layers of the nanowires. The room-temperature carrier concentration for the GaN nanowires was determined to be approximately 2.2 × 10(17)?cm(-3). The induced potential due to methane physisorption onto the Au nanoparticles that decorate the GaN nanowires was determined to be approximately -37?mV.     

Blueshift of electroluminescence from single n-InP nanowire/p-Si heterojunctions due to the Burstein-Moss effect

Single-crystalline n-type InP nanowires (NWs) with different electron concentrations were synthesized on Si substrates via the vapor phase transport method. The electrical properties of the InP nanowires were investigated by fabricating and measuring single NW field-effect transistors (FETs). Single InP NW/p(+)-Si heterojunctions were fabricated, and electroluminescence (EL) spectra from them were studied. It was found that both the photoluminescence (PL) spectra of the InP NWs and the EL spectra of the heterojunctions blueshift from 920 to 775?nm when the electron concentrations of the InP NWs increase from 2 × 10(17) to 1.4 × 10(19)?cm(-3). The blueshifts can be attributed to the Burstein-Moss effect rather than the quantum confinement effect in the InP NWs. The large blueshifts observed in this study indicate a potential application of InP NWs in nano-multicolour displays.     

MBE grown InGaN quantum dots and quantum wells: effects of in-plane localization

InGaN/GaN heterostructure samples were grown by molecular beam epitaxy using ammonia as a nitrogen precursor. The growth of InGaN/GaN self-assembled quantum dots was monitored in situ by reflection high energy electron diffraction intensity oscillations. Atomic force microscopy scans showed a very high density of InGaN islands, 1×10¹¹ cm⁻², well above the dislocation density. This could explain the increased radiative efficiency of these samples compared to homogeneous quantum wells. Light emitting diodes (LEDs) with InGaN active layers buried in GaN were realized. Electroluminescence and photocurrent spectra of these LEDs evidence a strong Stokes shift that can be attributed to high localization of carriers in InGaN layers.     

Investigation on light emission in light-emitting diodes constructed with n-ZnO and p-Si nanowires

The light emission was investigated in light-emitting diodes (LEDs) constructed with n-ZnO and p-Si nanowires (NWs). ZnO NWs were synthesized by thermal chemical vapor deposition and Si NWs were formed by crystallographic wet etching of a Si wafer. The LEDs were fabricated using the NWs via dielectrophoresis (DEP) and direct transfer methods. The DEP method enabled to align the ZnO NW at the position that led to p-n heterojunction diodes by crossing with the transferred Si NW. The I-V curve of the p-n heterojunction diode showed the well-defined current-rectifying characteristic, with a turn-on voltage of 3 V. The electroluminescence spectrum in the dark showed the strong emission at approximately 385 nm and the broad emission centered at approximately 510 nm, at a forward bias of 30 V. Under the illumination of 325-nm-wavelength light, the luminescence intensity at 385 nm was dramatically enhanced, compared to that in the dark, probably due to the electric-field-induced enhancement of luminescence.     

Electroluminescence and rectifying properties of heterojunction LEDs based on ZnO nanorods

n-ZnO NR/p-Si and n-ZnO NR/p-PEDOT/PSS heterojunction light-emitting diodes (LEDs) have been fabricated with ZnO nanorods (NRs) grown by a low-temperature method as well as by employing pulsed laser deposition (PLD). The low-temperature method involves growing the ZnO nanorods by the reaction of water with zinc metal. The current-voltage (I-V) characteristics of the heterojunctions show good rectifying diode characteristics. The electroluminescence (EL) spectra of the nanorods show an emission band at around 390?nm and defect related bands in the 400-550?nm region. Room-temperature electroluminescence is detected under forward bias for both the heterostructures. With the low-temperature grown nanorods, the defect related bands in the 400-550?nm range are more intense in the EL spectra, whereas with the PLD grown nanorods, only the 390?nm band is prominent.     

Room-temperature red emission from light-emitting diodes with Eu-doped GaN grown by organometallic vapor phase epitaxy

We obtained room-temperature red emission from GaN-based light-emitting diodes (LEDs) using a Eu-doped GaN (GaN:Eu) as an active layer. The bright emission was observed under normal lighting condition, which is associated with the intra-4f shell transition of Eu³⁺ ions. The LED properties depends on the growth condition of GaN:Eu layer. Since the high-quality GaN can be grown at higher growth pressure, the intense electroluminescence (EL) was observed in the LED with a GaN:Eu active layer grown at atmospheric pressure, which is due to the enhancement of the energy transfer efficiency from the GaN host material to the Eu ions. At a d.c. current of 20 mA, the light output power and external quantum efficiency were 17 μW and 0.04%, respectively. These results indicate the feasibility of GaN:Eu to realize a GaN-based red emitter for fabrication of nitride-based monolithic optical devices.     

MBE n-ZnO/MOCVD p-GaN heterojunction light-emitting diode

The growth, fabrication, and subsequent electroluminescence (EL) characterization of an n-ZnO/p-GaN heterojunction light-emitting diode prepared on c-Al₂O₃ substrate are presented. The diode-like I-V characteristics and room temperature EL spectrum with an intense broadband emission in the yellow-green spectral region has been observed with forward bias applied. Photoluminescence (PL) and Raman spectra of the n-ZnO and p-GaN films were also measured. By comparing PL and EL spectra, it was concluded that the deep-level defect-related emission mainly originated from the GaN epitaxial layer.     

Catalyst-free ZnO nanowires grown on a-plane GaN

ZnO nanowires were grown on a-plane GaN templates by chemical vapor deposition (CVD) without employing a catalyst. The a-plane GaN templates were pre-deposited on an r-plane sapphire substrate by metal-organic CVD. The resulting ZnO nanowires grow in angles off- related to the GaN basal plane. X-ray diffraction (XRD) spectra showed that the ZnO layer was grown with a heteroepitaxial relationship of (110)_ZnO||(110)_GaN. Photoluminescence spectra measured at 17 K exhibited near-band-edge emission at 372 nm with a full width at half maximum of 10 nm. The growth mechanism on a-GaN was the Volmer-Weber (VW) mode and differed from the Stranski-Krastanow (SK) mode observed for growth on c-GaN. This difference results from the higher interfacial free-energy on the a-plane between ZnO and GaN than that on the c-plane orientation.     

ZnO/GaN heterostructure for LED applications

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of Zn:In onto GaN:Mg/GaN structures MOCVD-grown on Al₂O₃ substrates has been observed. The white light is produced by superposition of two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al₂O₃ structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN is constructed and a qualitative explanation of the EL is given. Conditions of ZnO deposition strongly affects the properties of the recombination emission and predetermines the EL spectrum of the LED structure if it does not have high quantum efficiency (more than 1%) such as in commercial LEDs.     

Crabwise ZnO Nanowire UV Photodetector Prepared on ZnO : Ga/Glass Template

Vertical well-aligned and crabwise ZnO nanowires were prepared on patterned ZnO:Ga/glass substrates by reactive evaporation method under different growth conditions. The average length and diameter of vertical well-aligned ZnO nanowires were around 1 mum and 50-100 nm, respectively. In contrast, the average length and diameter of crabwise ZnO nanowires were around 5 mum and 30 nm, respectively. Upon illumination with UV light (lambda = 362 nm), it was found that measured responsivities were 0.015 and 0.03 A/W for the crabwise ZnO nanowire photodetector biased at 10 and 15 V, respectively. Furthermore, a rejection ratio of approximately 10 was obtained for the crabwise ZnO nanowire photodetector with an applied bias of 10 V.     

Investigation of low-temperature electroluminescence of InGaN/GaN based nanorod light emitting arrays

For InGaN/GaN based nanorod devices using a top-down etching process, the optical output power is affected by non-radiative recombination due to sidewall defects (which decrease light output efficiency) and the mitigated quantum confined Stark effect (QCSE) due to strain relaxation (which increases internal quantum efficiency). Therefore, the exploration of low-temperature optical behaviors of nanorod light emitting diodes (LEDs) will help identify the correlation between these two factors. In this work, low-temperature electroluminescent (EL) spectra of InGaN/GaN nanorod arrays were explored and compared with those of planar LEDs. The nanorod LED exhibits a much higher optical output percentage increase when the temperature decreases. The increase is mainly attributed to the increased carriers in the quantum wells for radiative recombination. Also, due to a better spatial overlap of electrons and holes in the quantum wells, the increased number of carriers can be more efficiently recombined in the nanorod device. Next, while the nanorod array shows nearly constant peak energy in the EL spectra at various injection currents at the temperature of 300 K, a blue shift has been observed at 190 K. The results suggest that with less non-radiative recombination and thus more carriers in the quantum wells, carrier screening and band filling still prevail in the partially strain relaxed nanorods. Moreover, when the temperature drops to 77 K, the blue shift of both nanorod and planar devices disappears and the optical output power decreases since there are fewer carriers in the quantum wells for radiative recombination.     

锗纳米镶嵌薄膜的电致发光及其机制

采和射频磁控溅射技术,在Ge纳米镍嵌薄膜的基础上制备出电致发光器件。器件的结构为半透明Au膜／Ge纳米镍嵌薄膜／p-Si基片。当正向邻居坟大于6V时,用肉眼可以观察到可见的电致发光,但在反向偏压下探测不到光发射。所测电致发光谱中只有一个发光峰,峰位在510nm（2．4eV,绿光）,并且随着正向偏坟的升高,峰位不发生移动;对于不同温度退火的样品,峰位也保持不变。根据分析结果讨论了可能的电致发光机制。     

Photo- and electroluminescence properties of fluorene-based copolymers containing electron- or hole-transporting unit

We report the photophysical and electroluminescence (EL) properties of two fluorene-based copolymers, poly{[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl]-alt-[6,6′- bis(3-phenylquinoxaline)-2,2′-diyl]} (Qx-PF) and poly{[9,9-bis(2- ethylhexyl)fluorene-2,7-diyl]-alt-[N,N′-diphenyl-N,N′-bis(4-phenyl)-1,1′-biphenyl-4,4′-diamine]} (TPD-PF). The two copolymers in thin films show blue emission approximately 429-452 nm with relatively narrow bandwidth upon photoexcitation. Electroluminescence has been demonstrated using TPD-PF as the active polymer in the light-emitting electrochemical cell (LEC) with a turn-on voltage at 2.8 V and an EL efficiency of 0.002 cd/A. Due to the improved electron-transporting property, the Qx-PF-based LEC achieves the EL efficiency of 0.07 cd/A, 35 times higher than that of the TPD-PF-based device. Compared to the photoluminescence spectra, EL spectra show enhanced excimer emission, which is primarily related to self-heating of the devices during operation. The main process involved in the decrease of the light intensity during device operation is the electrochemical degradation of the polymer blend.     

Catalyst-free synthesis of well-aligned ZnO nanowires on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates

Well-aligned ZnO nanowires have been synthesized vertically on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates, using a catalyst-free carbon thermal-reduction vapor phase deposition method for the first time. The as-synthesized nanowires are single crystalline wurtzite structure, and have a growth direction of [0001]. Each nanowire has a smooth surface, and uniform diameter along the growth direction. The average diameter and length of these nanowires are 120-150 nm, and 3-10 )m, respectively. We suggest that the growth mechanism follow a self-catalyzing growth model. Excitonic emission peaked around 385 nm dominates the room-temperature photoluminescence spectra of these nanowires. The room-temperature photoluminescence and Raman scattering spectra show that these nanowires have good optical quality with very less structural defects.