Luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates

In this study, we report comparative luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). High-density InGaN quantum dots (QDs) are formed on GaN templates by decreasing the growth temperature and increasing the adatom hopping-barrier through surface passivation. Atomic force microscopy (AFM) has been employed to estimate the size and height of these dots. Photoluminescence (PL) spectra recorded from (1120) InGaN QDs/(1102) sapphire show much stronger emission intensity compared to spectra recorded from (0001) InGaN QDs/(0001) sapphire. Due to the absence of strong spontaneous polarization and piezoelectric field, such (1150) InGaN QDs in the active layers would lead to high efficiency light emitting devices. (c) 2005 Elsevier B.V. All rights reserved.

Study of persistent photoconductivity and subband electronic properties of the two-dimensional electron gas in modulation doped GaAs/AlGaAs structure

We obtained the high mobility Of mu(2K) = 1.78 x 10(6) cm(2)/V . s in Si-doped GaAs/AlGaAs two-dimensional electron gas (2DEG) structures. After the sample was illuminated by a light-emitting diode in magnetic fields up to 6 T at T = 2K, we did observe the persistent photoconductivity effect and the electron density increased obviously. The electronic properties of 2DEG have been studied by Quantum-Hall-effect and Shubnikov-de Haas (SdH) oscillation measurements. We found that the electron concentrations of two subbands increase simultaneity with the increasing total electron concentration, and the electron mobility also increases obviously after being illuminated. At the same time, we also found that the electronic quantum lifetime becomes shorter, and a theoretical explunation is given through the widths of integral quantum Hall plateaus.

Amorphous silicon nanoparticles in compound films grown on cold substrates for high-efficiency photoluminescence

Silicon nanoparticles have been fabricated in both oxide and nitride matrices by using plasma-enhanced chemical vapour deposition, for which a low substrate temperature down to 50 degreesC turns out to be most favourable. High-rate deposition onto such a cold substrate results in the formation of nanoscaled silicon particles, which have revealed an amorphous nature under transmission electron microscope (TEM) examination. The particle size can be readily controlled below 3.0 nm, and the number density amounts to over 10(12) cm(-2), as calculated from the TEM micrographs. Strong photoluminescence in the whole visible light range has been observed in the as-deposited Si-in-SiOx and Si-in-SiNx thin films. Without altering the size or structure of the particles, a post-annealing at 300 degreesC for 2 min raised the photoluminescence efficiency to a level comparable to the achievements with nanocrystalline Si-in-SiO2 samples prepared at high temperature. This low-temperature procedure for fabricating light-emitting silicon structures opens up the possibility of manufacturing integrated silicon-based optoelectronics.

Size evolution and optical properties of self-assembled InAs quantum dots on different matrix

InAs quantum dots have been grown by solid source molecular beam epitaxy on different matrix to investigate the effect on the structure and optical properties. High density of 1.02 x 10(11) cm(-2) of InAs islands on In0.15Ga0.85As and In0.15Al0.85As underlying layer has been achieved. Atomic force microscopy and photoluminescence spectra show the size evolution of InAs islands on In0.15Ga0.85As underlying layer. A strong 1.3 mum photoluminescence from InAs islands on In0.15Ga0.85As underlying layer and with InGaAs strain-reduced layer has been obtained. Single-mirror light emitting diode structures with InAs quantum dots capped by InGaAs grown on InGaAs layer as active layer were fabricated and the corresponding radiative efficiency was deduced to be as high as 20.5%. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2003 Elsevier B.V. All rights reserved.

Progress of Si-based nanocrystalline luminescent materials

Si-based nanomaterials are some new photoeletronic and informational materials developed rapidly in recent years, and they have potential applications in the light emitting devices, e. g. Si light emitting diode, Si laser and integrated Si-based photoelectronics. Among them are nano-scale porous silicon (ps), Si nanocrystalline embedded SiO2 (SiOx, x < 2.0) matrices, Si nanoquantum dot and Si/SiO2 superlattice, etc. At present, there are various indications that if these materials can achieve efficient and stable luminescence, which are photoluminescence (PL) and electroluminescence (EL), it is possible for them to lead to a new informational revolution in the early days of the 21st century. In this article, we will mainly review the progress of study on Si-based nanomaterials in the past ten years. The involved contents are the fabricated methods, structural characterizations and light emitting properties. Finally, we predicate the developed tendency of this field in the following ten years.

Ultraviolet-blue electroluminescence from Gd3Ga5O12 : Ag

UV-blue light was obtained from a thin-film electroluminescence device using Gd3Ga5O12:Ag as a light-emitting layer, which was deposited by using electron-beam evaporation. The crystal composition and structure of Gd3Ga5O12:Ag were studied by x-ray powder diffraction, The Gd3Ga5O12:Ag has a photoluminescence emission which peaked at around 397 and 467 nm, which were attributed to the oxide vacancies and Ag+, respectively. The brightness of 32 cd/m(2) was obtained when an alternating voltage of 130 V at 1 kHz was applied. (C) 2000 American Institute of Physics. [S0003-6951(00)05031-2].

Blue-violet photoluminescence from large-scale highly aligned boron carbonitride nanofibers

We report on the strong blue-violet photoluminescence (PL) at room temperature from the large-scale highly aligned boron carbonitride (BCN) nanofibers synthesized by bias-assisted hot filament chemical vapor deposition. The photoluminescence peak wavelength shifts in the range of 470-390 nm by changing the chemical composition of the BCN nanofibers, which shows an interesting blue and violet-light-emitting material with adjustable optical properties. The mechanism for the shift of the PL peaks at room temperature is also discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)04427-2].

Characterization of hot-electron effects on flicker noise in III-V nitride based heterojunctions

We report experiments on hot-electron stressing in commercial III-V nitride based heterojunction fight-emitting diodes. Stressing currents ranging from 100 mA to 200 mA were used. Degradations in the device properties were investigated through detailed studies of the I-V characteristics, electroluminescence, Deep-Level Transient Fourier Spectroscopy and flicker noise. Our experimental data demonstrated significant distortions in the I-V characteristics. The room temperature electroluminescence of the devices exhibited 25% decrement in the peak emission intensity. Concentration of the deep-levels was examined by measuring the Deep-Level Transient Fourier Spectroscopy, which indicated an increase in the density of deep-traps from 2.7 x 10(13) cm(-3) to 4.21 x 10(13) cm(-3) at E-1 = E-C - 1.1eV. The result is consistent with our study of 1/f noise, which exhibited up to three orders of magnitude increase in the voltage noise power spectra. Our experiments show large increase in both the interface traps and deep-levels resulted from hot-carrier stressing.

Defect influence on luminescence efficiency of GaN-based LEDs

Wafers with normal light-emitting diode structure were grown by metal organic chemical vapor deposition system. The pressure and temperature were varied during growth of buffer layer in order to grow different types of epilayers. The cathodoluminescence results show that the interface distortion of quantum well plays an important role in radiant efficiency. The electroluminescence detections indicate that the dislocations also influence the external quantum efficiency by lowering the electron injection efficiency. (c) 2006 Elsevier Ltd. All rights reserved.

有机白光电致发光器件的研究

有机电致发光显示作为最有前景的平板显示技术已经得到了越来越多的关注，而实现全色显示是其发展的必然趋势。在实现全色显示的几种方法中，利用白光电致发光器件加彩色滤光膜技术具有可实现高分辨率和简单加工的优势，而且可应用液晶显示器的彩色滤光膜技术，所以可能成为未来全彩色化的主流技术。由于白光有机电致发光器件在全色显示以及照明等方面具有的潜在应用价值，从而引起了人们越来越多的研究兴趣，近十年的研究也不断地取得新的突破。然而从目前的研究来看，白光器件距离大规模的商业化应用还有一定的距离，存在的问题突出地表现在低的发光效率和亮度以及器件的稳定性等方面。在本文中，我们系统的研究了两种类型的有机白光器件。采用染料掺杂的方法，通过多层结构对载流子的注入和传输以及激子复合区的调控实现了白光有机发光器件，这种白光有机电致发光器件具有高效率、高亮度、良好光谱稳定性的特点。制备的器件最高发光亮度为17100ed／mZ，电流效率和功率效率分别为13．1cd／A和5．51耐W。对于荧光有机小分子材料制备的白光器件来说，这是到目前为止报道的较为理想的实验结果。研究了利用叠层结构制备白光有机电致发光器件的新方法，采用电荷生成层将几个发光单元串接起来，各个发光单元之间互不影响，由几个发光单元分别发出的不同颜色的光叠加，最终形成白光发射。这种叠层结构的器件，具有高效率、光谱稳定性好的优点，制备的器件最高亮度loZooed／m2，最大电流效率10．7cd／A，最大功率效率为2．5Inm/W。这是目前首次用叠层器件结构制备出的白光有机发光器件的最好结果。

有机电磷光器件的研究

自1987年C.W.Tang等人首次报道以Alq_3为发光材料的多层结构有机电致发光器件以来，由于其简单工艺、低成本、主动发光、快速响应以及大面积和柔性可弯曲显示等特点，使其在未来的平板显示领域显示了诱人的应用前景，其研究倍受关注。近些年来，有机发光二极管己经开始商品化。其中，三重态电磷光有机发光材料以其在效率，亮度等方面的优秀性能表现，成为了该领域重要的研究课题。在众多的电磷光材料中，重金属铱的配合物的很多优点引起了广泛的研究。铱配合物电磷光不仅具备了磷光材料理论上可能达到了100％内量子效率的优势，还以其独特的金属一配体电荷跃迁发光性质实现了可见光范围内的全色发光。本论文主要研究了有机电磷光铱配合物的电致发光性能，从不同分子结构铱配合物在不同掺杂浓度下的优化、器件结构设计、母体材料选择以及电磷光发光器件的瞬态特性等方面进行了详细研究，开发出了高效率纯红光有机电磷光发光器件，实现了高效率单层电磷光聚合物发光二极管，用稀土配合物做电磷光掺杂的母体在一定程度上解决了电磷光器件在高电流密度下快速退化问题，对有机电磷光器件的三重态一三重态湮灭和激子衰减动力学过程有了初步认识。（1）研究了几种新型红光铱配合物的电致发光性能：①开发出了以毗嗦为配体的铱配合物，发现其发光光谱（光致和电致）均比哇琳为配体的铱配合物有大幅度的红移，如此制备出了主峰在677nm，色坐标为（0.71，0.28）的深红色有机电磷光器件，器件的外量子效率达到了5.5％以上；②比较了以嗯哇和噬哇为配体的配合物的电致发光性能，发现在结构相近的情况下，电负性越强的原子导致发射光谱蓝移，而在配体上引入高电负性O原子在一定程度上提高了器件的电致发光性能。（2）在高效有机发光器件的制备和结构优化等方面开展的一系列工作得到了如下结论：①不同取代基团配体铱配合物依赖于掺杂浓度，通过优化器件可以获得最好的电致发光性能。②用旋涂方法，通过对电子和空穴注入和传输的调控制备出了高效率单层有机电磷光发光器件，最大发光效率达到了25.2cd／A，由于减少了电子和空穴在磷光分子的直接俘获，避免了空间电荷积累引起的空间电场问题，使器件的电致发光性能得到了显著地提高。③研究了用稀土试络合物作为主体材料，掺杂一种铱配合物的电磷光有机发光二极管的电致发光性能，发现在较高的电流密度下，器件的电致发光效率仍然保持较好的稳定性，有效地降低了三重态一三重态湮灭引起的退化问题。（3）我们用瞬态电致发光方法详细研究了电磷光有机发光器件的三重态一三重态湮灭和激子衰减动力学过程，确定了不同取代基团苯基噬哇铱配合物的三重态激子的寿命τ、三重态一三重态湮灭常数K和饱和电流以及它们与器件电致发光效率的关系，这方面的研究对澄清有机发光器件的工作原理和退化机制，进一步改善器件性能具有重要意义。

Hydrogen related defects in InP

Local vibrational modes(LVMs) in tenths of InP samples reveal clearly existence of complexes related to hydrogen. Complexes of vacancy at indium site with one to four hydrogen atom(s) and isolated hydrogen or hydrogen dimers and complexes of hydrogen with various impurities and intrinsic defects are investigated by FTIR. Especially hydrogen related complexes between various transition metals and hydrogen or hydrogen related complexes between hydrogen with point defects. New LVMs related to hydrogen will be reported in this paper. Dynamical formation mechanism of defects in the annealed nominally undoped semiinsulating InP obtained by high pressure, high temperature annealing of ultra purity materials is proposed. Hydrogen can acts as actuator for antistructure defects production. Structural, electronic and vibrational properties of LVMs related to hydrogen as well as their temperature effects are discussed.

Hole-transporting polymers containing carbazol-3,9-diyl and 1,4-phenylene fragments in the main chain

New carbazole-based polymers, which contain various content of electro-active fragments in the main chain connected via alkylene spacers, have been synthesized by Ni(0)-catalyzed Yamamoto-type aryl-aryl coupling reactions. These compounds represent amorphous materials of high thermal stability with glass-transition temperatures of 139-151 degrees C and thermal decomposition starting at temperatures above 400 degrees C. UV-vis absorption and photoluminescence emission spectra of the materials confirmed that the conjugated segments in the macromolecules are rather short.