Electroluminescence from ZnO nanowire-based p-GaN/n-ZnO heterojunction light-emitting diodes

Vertically aligned ZnO nanowires were successfully grown on the sapphire substrate by nanoparticle-assisted pulsed laser deposition (NAPLD), which were employed in fabricating the ZnO nanowire-based heterojunction structures. p-GaN/n-ZnO heterojunction light-emitting diodes (LEDs) with embedded ZnO nanowires were obtained by fabricating p-GaN:Mg film/ZnO nanowire/n-ZnO film structures. The current–voltage measurements showed a typical diode characteristic with a threshold voltage of about 2.5 V. Electroluminescence (EL) emission having the wavelength of about 380 nm was observed under forward bias in the heterojunction diodes and was intensified by increasing the applied voltage up to 30 V.     

n-ZnO/i-MgO/p-GaN异质结发光二极管

用等离子体辅助分子束外延的方法生长了n-ZnO/i-MgO/p-GaN异质结发光二极管。I-V测量表明其具有典型的二极管整流特性。电致发光峰位于382nm,通过与n型ZnO和p型GaN的光致发光谱比较,其发光峰位与线形都与ZnO的自由激子发射一致,表明该电致发光来自于ZnO的自由激子发射。通过Anderson模型比较了n-ZnO/i-MgO/p-GaN和n-ZnO/p-GaN异质结的能带示意图,证明了由于MgO层的插入抑制了ZnO向GaN层中的电子注入,且有利于空穴向ZnO层注入,从而实现了ZnO层中的电注入发光。     

Influence of growth temperature and post-annealing on an n-ZnO/p-GaN heterojunction diode

We report on an n-ZnO/p-GaN heterojunction diode fabricated from zinc oxide (ZnO) films at various growth temperatures (450, 500, 550, and 600 °C) by RF sputtering. The films were subsequently annealed at 700 °C in N₂ ambient. To investigate the influence of the growth temperature of n-ZnO films, the microstructural, optical, and electrical properties were measured using scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL), and Hall measurements. The XRD pattern showed the preferred orientation along the c-axis (002) regardless of growth temperature. The PL spectra showed a dominant sharp near-band-edge (NBE) emission. Current–voltage (I–V) curves showed excellent rectification behavior. The turn-on voltage of the diode was observed to be 3.2 V for the films produced at 500 °C. The ideality factor of ZnO film was observed to be 1.37, which showed the best performance of the diode.     

ZnMgO/n-ZnO/ZnMgO/p-GaN异质结LED的紫外电致发光

利用等离子体辅助分子束外延( P-MBE)技术制备了ZnMgO/n-ZnO/ZnMgO/p-GaN异质结LED.Ni/Au电极与p-GaN、In电极与ZnMgO之间都形成了良好的欧姆接触.在ZnMgO/n-ZnO/ZnMgO/p-GaN异质结器件中观察到了明显的整流特性.异质结的电致发光强度随着注入电流的增大而逐渐增强...     

Electroluminescence of an n-ZnO/p-GaN Heterojunction under Forward and Reverse Biases

Electroluminescent characteristics of n-ZnO/p-GaN heterojunctions under forward and reverse biases are studied. Emissions at 389nm and 57Ohm are observed under forward bias. An unusual emission at 390ram appears under reverse bias, and is attributed to the recombination in the p-GaN side of the heterojunction. The yellow emission peaked at 57Ohm is suppressed under reverse bias. The light intensity exponentially depends on the reverse current. The emission under reverse bias is correlated to tunnelling carrier transport in the heterostructure. Our results also support that the well-known yellow band of GaN comes from the transitions between some near-conduction-band-edge states and deep localized acceptor states.     

A new approach to white light emitting diodes of p-GaN/i-ZnO/n-ZnO heterojunctions

A simple method to fabricate one-chip white light emitting diodes (LEDs) is proposed. A series of p-GaN/i-ZnO/n-ZnO heterojunctions fabricated by pulsed laser deposition exhibit simultaneous Mg-related p-GaN (blue-violet) and deep-level i-ZnO (yellow) emissions under forward bias. Current–voltage measurements show a typical rectifying characteristic. Yellow emission intensity, and consequently the ratio of yellow to blue-violet emission, are tunable through control of the i-ZnO layer thickness. Heterojunctions with a 20 nm-thick i-ZnO layer exhibit white electroluminescence. PACS  78.60.Fi; 73.40.Lq; 85.60.Jb     

Improvement in electroluminescence performance of n-ZnO/Ga_2O_3 /p-GaN heterojunction light-emitting diodes

n-ZnO/p-GaN heterojunction light-emitting diodes with and without a Ga2O3 interlayer are fabricated. The electroluminescence （EL） spectrum of the n-ZnO/p-GaN displays a single blue emission at 430 nm originating from GaN, while the n-ZnO/Ga2O3/p-GaN exhibits a broad emission peak from ultraviolet to visible. The broadened EL spectra of n-ZnO/Ga2O3/p-GaN are probably ascribed to the radiative recombination in both the p-GaN and n-ZnO, due to the larger electron barrier （ΔEC=1.85 eV） at n-ZnO/Ga2O3 interface and the much smaller hole barrier （ΔEV=0.20 eV） at Ga2O3/p-GaN interface.     

Effect of ambient temperature on electrical properties of nanostructure n-ZnO/p-Si heterojunction diode

The nanostructure n-ZnO/p-Si heterojunction diode was fabricated by sol–gel method. The structural and morphological properties of the nanostructure ZnO film have been investigated. The X-ray diffraction spectra indicated that the films are of polycrystalline nature. The scanning electron microscopy images indicate that the surface morphology of ZnO film is almost homogeneous and the ZnO film is consisted of the circular formed with coming together of the nanoparticles. The electrical characterization of nanostructure n-ZnO/p-Si heterojunction diode has been investigated by current–voltage characteristics. The ideality factor (n) of the diode was found for different ambient temperatures and the obtained 6.40 value for 296 K is higher than unity due to the interface states between the two semiconductor materials and series resistance. The values of n increased with decreasing ambient temperature. The reverse current of the diode increased with illumination intensity of 100 mW cm⁻² and the diode gave a maximum open circuit voltage V_oc of 0.19 V and short-circuits current I_sc of 8.03 × 10⁻⁸ A.     

Low-voltage blue light emission from n-ZnO/p-GaN heterojunction formed by RF magnetron sputtering method

High quality n-ZnO/p-GaN heterojunction was fabricated by growing highly crystalline ZnO epitaxial films on commercial p-type GaN substrates via radio frequency (RF) magnetron sputtering. Low-voltage blue light emitting diode with a turn-on voltage of ∼2.5 V from the n-ZnO/p-GaN heterojunction was demonstrated. The diode gives a bright blue light emission located at ∼460 nm and a low threshold voltage of 2.7 V for emission. Based on the results of the photoluminescence (PL) and electroluminescence (EL) spectra, the origins of the EL emissions were studied in the light of energy band diagrams of ZnO–GaN heterojunction, and may attribute to the radiative recombination of the holes in p-GaN and the electrons injected from n-ZnO, which almost happened on the side of p-GaN layer. These results may have important implications for developing short wavelength optoelectronic devices.     

GaN基紫光LED的高反射率p型欧姆接触

研究用于GaN基大功率倒装焊(Flip-chip)紫光LED(UV-LED)的高反射率p型欧姆接触的电学和光学性能。用磁控溅射的方法在GaN基LED外延片表面沉积了不同厚度Ag,Al,Au和Pd四种金属,测量了样品的反射率和透射率。结合同步辐射高强度X射线衍射和AFM对金属薄膜的晶体结构进行分析,并对表面形貌进行了观测,对由金属薄膜构成的多层膜结构及其对光反射率的作用机理进行了研究。测量结果表明,在入射光波长为400nm时,Ni/Au/Ag和Ni/Au/Al电极的反射率比Ni/Au的反射率提高了三倍。同时与p-GaN有良好的欧姆接触特性。     

Growth of ZnO nanorods by aqueous solution method with electrodeposited ZnO seed layers

ZnO nanorod arrays on ZnO-coated seed layers were fabricated by aqueous solution method using zinc nitrate and hexamethylenetetramine at low temperature. The seed layers were coated on ITO substrates by electrochemical deposition technique, and their textures were dominated by controlling the deposition parameters, such as deposition potential and electrolyte concentration. The effects of the electrodeposited seed layers and the growing parameters on the structures and properties of ZnO nanorod arrays were primarily discussed. The orientation and morphology of both the seed layer and successive nanorods were analyzed by using X-ray diffraction (XRD), SEM and TEM. The results show that the seed layer deposited at −700 mV has evenly distributed crystallites and (0 0 2) preferred orientation; the density of resultant nanorods is high and ZnO nanorods stand completely perpendicular onto substrates. Meanwhile, the size of nanorods quite also depends on the growth solution, and the higher concentration of growth solution primary leads to a large diameter of the ZnO nanorods.     

Influence of p-GaN annealing on the optical and electrical properties of InGaN/GaN MQW LEDs

Optical and electrical properties of InGaN/GaN multiple quantum wells (MQWs) light emitting diodes (LEDs) annealed in pure O₂ ambient (500 °C) and pure N₂ ambient (800 °C) were systematically investigated. The temperature-dependent photoluminescence measurements showed that high-temperature thermal annealing in N₂ ambient can induce indium clusters in InGaN MQWs. Although the deep traps induced by indium clusters can act as localized centers for carriers, there are many more dislocations out of the trap centers due to high-temperature annealing. As a result, the radiative efficiency of the sample annealed in N₂ ambient was lower than that annealed in O₂ ambient at room temperature. Electrical measurements demonstrated that the LEDs annealed in O₂ ambient were featured by a lower forward voltage and there was an increase of ~41% in wall-plug efficiency at 20 mA in comparison with the LEDs annealed in N₂ ambient. It is thus concluded that activation of the Mg-doped p-GaN layer should be carried out at a low-temperature O₂ ambient so as to obtain LEDs with better performance.     

Improved Light Extraction of GaN-based LEDs with Nano-roughened p-GaN Surfaces

p-GaN surfaces axe nano-roughened by plasma etching to improve the optical performance of GaN-based light emitting diodes （LEDs）. The nano-roughened GaN present a relaxation of stress. The light extraction of the LEDs with nano-roughened surfaces is greatly improved when compared with that of the conventional LEDs without nano-roughening. PL-mapping intensities of the nano-roughened LED epi-wafers for different roughening times present two to ten orders of enhancement. The light output powers are also higher for the nano-roughened LED devices, This improvement is attributed to that nano-roughened surfaces can provide photons multiple chances to escape from the LED surfaces,     

Electroluminescence of the p-ZnO:As/n-ZnO LEDs grown on ITO glass coated with GaAs interlayer

In this paper, we proposed a new p-type ZnO doping method with metal organic chemical vapor deposition (MOCVD) technology by inserting a GaAs interlayer between substrate and ZnO epitaxial layer. The doping concentration of p-type ZnO film is able to be controlled by adjusting the thickness of the GaAs interlayer. With this method, we fabricated n-ZnO/p-ZnO:As homojunction light-emitting diode (LED) on ITO-glass substrate pre-coated with 20 nm GaAs interlayer. The device exhibits a typical rectifying behavior by current-voltage (I-V) measurement. When the device is forward biased, UV-vis electroluminescence (EL) emissions can be observed clearly.     

High efficiency and enhanced ESD properties of UV LEDs by inserting p-GaN/p-AlGaN superlattice

Significantly improved electrostatic discharge(ESD)properties of InGaN/GaN-based UV light-emitting diode(LED)with inserting p-GaN/p-AlGaN superlattice(p-SLs)layers(instead of p-AlGaN single layer)between multiple quantum wells and Mg-doped GaN layer are reported.The pass yield of the LEDs increased from 73.53%to 93.81%under negative 2000 V ESD pulses.In addition,the light output power(LOP)and efficiency droop at high injection current were also improved.The mechanism of the enhanced ESD properties was then investigated.After excluding the effect of capacitance modulation,high-resolution X-ray diffraction(XRD)and atomic force microscope(AFM)measurements demonstrated that the dominant mechanism of the enhanced ESD properties is the material quality improved by p-SLs,which indicated less leakage paths,rather than the current spreading improved by p-SLs.     

表面化学处理和退火对p-GaN/ZnO:Ga接触特性的影响

ZnO∶Ga(GZO)透明电极沉积在p-GaN表面,用作透明电流扩展层。直接沉积在p-GaN上的p-GaN/GZO存在较大的势垒,容易形成肖特基接触,而良好的欧姆接触对功率LED器件至关重要。为了降低接触势垒,采用盐酸和氢氧化钠溶液对GaN表面进行去氧化层处理,并对p-GaN/GZO进行退火处理,研究表面处理和退火对p-GaN/GZO接触特性的影响。研究表明:碱性溶液处理有利于降低接触势垒;退火处理后,接触势垒略有增加。     

Comparative study of ethanol sensor based on gold nanoparticles: ZnO nanostructure and gold: ZnO nanostructure

Gold colloid:ZnO nanostructures were prepared from Zn powder by using thermal oxidation technique on alumina substrates, then it was impregnated by gold colloid for comparative study. The gold colloid is the solution prepared by chemical reduction technique; it appeared red color for gold nanoparticle solution and yellow color for gold solution. The heating temperature and sintering time of thermal oxidation were 700 °C and 24 h, respectively under oxygen atmosphere. The structural characteristics of gold colloid:ZnO nanostructures and pure ZnO nanostructures were studied using filed emission scanning electron microscope (FE-SEM). From FE-SEM images, the diameter and length of gold colloid:ZnO nanostructures and ZnO nanostructures were in the ranges of 100-500 nm and 2.0-7.0 μm, respectively. The ethanol sensing characteristics of gold colloid:ZnO nanostructures and ZnO nanostructures were observed from the resistance alteration under ethanol vapor atmosphere at concentrations of 50, 100, 200, 500, and 1000 ppm with the operating temperature of 260-360 °C. It was found that the sensitivity of sensor depends on the operating temperature and ethanol vapor concentrations. The sensitivity of gold colloid:ZnO nanostructures were improved with comparative pure ZnO nanostructures, while the optimum operating temperature was 300 °C. The mechanism analysis of sensor revealed that the oxygen species on the surface was O²⁻.     

Enhanced ultraviolet photoresponse based on ZnO nanocrystals/Pt bilayer nanostructure

The development of solution strategies for Zinc oxide(Zn O) quantum dots provides a pathway to utilizing Zn O nanocrystal thin films in optoelectronic devices.In this work,quasi-spherical Zn O quantum dots with a diameter of 5 nm are synthesized by using ethanol as a solvent.Zn O nanocrystal thin film is obtained by spin-coating Zn O quantum dots on a Au interdigital electrode(IDE)/Al2O3 substrate and annealing at different temperatures in order to yield the optimal photosensitive on/off ratio of Zn O.For further enhancing the responsivity,ion sputtering is utilized to deposit Pt nanoparticles on the surface of Zn O nanocrystal thin film,the responsivity of the Zn O/Pt bilayer nanostructure increases from 0.07 A/W to 54 A/W,showing that the metal/inorganic nanocrystal bilayer nanostructure can be used to improve the performance of optoelectronic devices.The excellent properties of Zn O/Pt bilayer nanostructure have important applications in future electronic and optoelectronic devices.     

Electroluminescence from ZnO nanowires with a p-ZnO film/n-ZnO nanowire homojunction

ZnO homojunction light-emitting diodes based on ZnO nanowires were fabricated on Si(100) substrates. An N–In codoped p-type ZnO film grown by ultrasonic spray pyrolysis and an unintentionally doped n-type ZnO nanowire quasi-array grown by an easy low-temperature hydrothermal method were employed to form the homojunction diode. Under a forward bias larger than 8 V, electroluminescence, which was composed of an ultraviolet peak centered at 387 nm and a green band around 540 nm, was observed. The electroluminescence emission was contributed by the ZnO nanowires. The results reported here suggest that ZnO-based ultraviolet light-emitting devices could be realized at low cost. PACS 42.72.Bj; 73.40.Lq; 85.60.Jb     

The effect of a surface layer capping p-InGaN/p-GaN superlattices on the contact to p-GaN

In this study, the influence of the surface layer (p-InGaN or p-GaN) capping p-InGaN/p-GaN superlattices (SLs) on the contact to p-type GaN was investigated. It was found that the specific contact resistance (_ρc)$\left({\rho }_c\right)$ to p-type GaN is lower when using p-InGaN as the surface layer. The lowest value of _ρc${\rho }_c$ was 1.99×10⁻⁴ Ω cm² at room temperature. It was also found that low temperature growth of the p-GaN layers in the SLs is beneficial for lowering the ohmic contact resistance. Unlike Ni/Au deposited directly on p-GaN (without the strained p-InGaN/p-GaN SLs), Ni/Au deposited on p-InGaN/p-GaN SLs produces ohmic behavior even before annealing.