Effect of Si doping on cubic GaN films grown on GaAs(100)

Epitaxial layers of cubic GaN have been grown by metalorganic vapor-phase epitaxy (MOVPE) with Si-doping carrier concentration ranging from 3 x 10(18) to 2.4 x 10(20)/cm(3). Si-doping decreased the yellow emission of GaN. However, the heavily doped n-type material has been found to induce phase transformation. As the Si-doping concentration increases, the hexagonal GaN nanoparticles increase. Judged from the linewidth of X-ray rocking curve, Si-doping increases the density of dislocations and stacking faults. Based on these observations, a model is proposed to interpret the phase transformation induced by the generated microdefects, such as dislocations and precipitates, and induced stacking faults under heavy Si-doping. (C) 1999 Elsevier Science B.V. All rights reserved.

Ge related defect energy and microcavity effect in GaN epitaxial layer

Using solid-phase regrowth technique, Pd/Ge contact has been made on the GaN layer, and very good ohmic behavior was observed for the contact. The Photoluminescence (PL) spectra for different structures formed by the Pd/Ge contact, GaN layer, sapphire substrate, and mirror were studied, and a defect-assisted transition was found at 450 nm related to Ge impurity. The results show that the microcavity effect strongly influences the PL spectra of the band-gap and defect-assisted transitions.

Properties of GaN epilayers with various growth conditions grown by gas source molecular beam epitaxy

GaN epilayers on sapphire (0001) substrates were grown by the gas source molecular beam epitaxy (GSMBE) method using ammonia (NH,) gas as the nitrogen source. Properties of gallium nitride (GaN) epilayers grown under various growth conditions were investigated. The growth rate is up to 0.6 mu m/h in our experiments. Cathodoluminescence, photoluminescence and Hall measurements were used to characterize the films. It was shown that the growth parameters have a significant influence on the GaN properties. The yellow luminescence was enhanced at higher growth temperature. And a blue emission which maybe related to defects or impurity was observed. Although the emission at 3.31 eV can be suppressed by a low-temperature buffer layer, a high-quality GaN epilayer can be obtained without the buffer layer. (C) 1998 Elsevier Science B.V. All rights reserved.

Dependence of photoluminescence induced by carbon contamination on GeSi structure

We studied the dependence of photoluminescence induced by carbon contamination on the Ge/GeSi structure. It is found that a carbon and silicon defect complex may be formed in a special structure by opening the in situ high-energy electron diffraction test during growth. There is an important difference in the dependence of photoluminescence on the temperature between the defect complex in our samples and in bulk Si. where the impurity-active center is generated by high-energy electron (about several MeV) irradiation. The quenching temperature of the photoluminescence from the impurity-active center is higher in our Ge/GeSi structure than in bulk Si. The defect complex may serve as an impurity-active center for a possible application in making Si-based light-emitting diodes whose wavelength is around 1.3 mu m in the window of optical communication. (C) 1998 Elsevier Science B.V. All rights reserved.

Influence of rapid thermal annealing on the optical properties of gallium nitride grown by gas-source molecular-beam epitaxy

Raman scattering, photoluminescence (PL), and nuclear reaction analysis (MA) have been employed to investigate the effects of rapid thermal annealing (RTA) on GaN films grown on sapphire (0001) substrates by gas-source molecular-beam epitaxy, The Raman spectra showed the presence of the E-2 (high) mode of GaN and shift of this mode from 572 to 568 cm(-1) caused by annealing. The results showed that RTA has a significant effect on the strain relaxation caused by the lattice and thermal expansion misfit between the GaN epilayer and the substrate. The PL peak exhibited a blueshift in its energy position and a decrease in the full width at half maximum after annealing, indicating an improvement in the optical quality of the film. Furthermore, a green luminescence appeared after annealing and increased in intensity with increasing annealing time. This effect was attributed to H concentration variation in the GaN film, which was measured by NRA. A high H concentration exists in as-grown GaN, which can neutralize the deep level, and the H-bonded complex dissociates during RTA, This leads to the appearance of a luminescent peak in the PL spectrum. (C) 1998 American Institute of Physics.

Rapid thermal annealing processing of GaN epilayer on sapphire(0 0 0 1)

The effect of rapid thermal annealing (RTA) in a Nz ambient up to 900 degrees C has been investigated for GaN films grown on sapphire(0 0 0 1) substrates. Raman spectra, X-ray diffractometry and Hall-effect studies were performed for this purpose. The Raman spectra show the presence of the E-2 (high) mode and a shift in the wave number of this mode with respect to the annealing processing. This result suggests the presence and relaxation of residual stress due to thermal expansion misfit in the films which are confirmed by X-ray measurements and the structure quality of GaN epilayer was improved. Furthermore, the electron mobility increased at room temperature with respect to decrease of background electron concentration after RTA. (C) 1998 Elsevier Science B.V. All rights reserved.

Wurtzite GaN epitaxial growth on a Si(001) substrate using gamma-Al2O3 as an intermediate layer

Wurtzite GaN films have been grown on (001) Si substrates using gamma-Al2O3 as an intermediate layer by low pressure (similar to 76 Torr) metalorganic chemical vapor deposition. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements revealed that the thin gamma-Al2O3 layer of "compliant" character was an effective intermediate layer for the GaN film grown epitaxially on Si. The narrowest linewidth of the x-ray rocking curve for (0002) diffraction of the 1.3 mu m GaN sample was 54 arcmin. The orientation relationship of GaN/gamma-Al2O3/Si was (0001) GaN parallel to(001) gamma-Al2O3 parallel to(001) Si, [11-20] GaN parallel to[110] gamma-Al2O3 parallel to[110] Si. The photoluminescence measurement for GaN at room temperature exhibited a near band-edge peak of 365 nm (3.4 eV). (C) 1998 American Institute of Physics.

High-efficiency GaN-based blue LEDs grown on nano-patterned sapphire substrates for solid-state lighting - art. no. 684103

Nano-patterning sapphire substrates technique has been developed for nitrides light-emitting diodes (LEDs) growths. It is expected that the strain induced by the lattice misfits between the GaN epilayers and the sapphire substrates can be effectively accommodated via the nano-trenches. The GaN epilayers grown on the nano-patterned sapphire substrates by a low-pressure metal organic chemical vapor deposition (MOCVD) are characterized by means of scanning electron microscopy (SEM), high-resolution x-ray diffraction (HRXRD) and photoluminescence (PL) techniques. In comparison with the planar sapphire substrate, about 46% increment in device performance is measured for the InGaN/GaN blue LEDs grown on the nano-patterned sapphire substrates.

Combined transparent electrodes for high power GaN-based LEDs with long life time

In this paper fabrication of high power light emitting diodes (LEDs) with combined transparent electrodes on both P-GaN and N-GaN have been demonstrated. Simulation and experimental results show that comparing with traditional metal N electrodes the efficacy of LEDs with transparent N electrode is increased by more than 10% and it is easier in process than the other techniques. Further more, combining the transparent electrodes with dielectric anti-reflection film, the extraction efficiency can be improved by 5%. At the same time, the transparent electrodes were protected by the dielectric film and the reliability of LEDs can be improved.

Optical properties and electrical bistability of CdS nanoparticles synthesized in dodecanethiol

We reported the synthesis of CdS semiconductor nanoparticles using a simple one-pot reaction by thermolysis of cadmium acetylacetonate in dodecanethiol. Optical measurements of the as-obtained CdS nanoparticles revealed that their optical properties were closely related to surface effects. Based upon the cocktail of poly (N-vinylcarbazole) (PVK) and CdS nanoparticles, a bistable device was fabricated by a simple solution processing technique. Such a device exhibited a remarkable electrical bistability, which was attributed to the electric field-assisted charge transfer between PVK and the CdS nanoparticles capped by dodecaethiol. The conduction mechanism changed from an injection-controlled current to a bulk-controlled one during switching from OFF-state to ON-state.

Electroluminescence behavior of ZnO/Si heterojunctions: Energy band alignment and interfacial microstructure

n-ZnO/p-Si heterojunction light-emitting diodes (LEDs) show weak defect-related electroluminescence (EL). In order to analyze the origin of the weak EL, the energy band alignment and interfacial microstructure of ZnO/Si heterojunction are investigated by x-ray photoelectron spectroscopy. The valence band offset (VBO) is determined to be 3.15 +/- 0.15 eV and conduction band offset is -0.90 +/- 0.15 eV, showing a type-II band alignment. The higher VBO means a high potential barrier for holes injected from Si into ZnO, and hence, charge carrier recombination takes place mainly on the Si side rather than the ZnO layer. It is also found that a 2.1 nm thick SiOx interfacial layer is formed at the ZnO/Si interface. The unavoidable SiOx interfacial layer provides to a large number of nonradiative centers at the ZnO/Si interface and gives rise to poor crystallinity in the ZnO films. The weak EL from the n-ZnO/p-Si LEDs can be ascribed to the high ZnO/Si VBO and existence of the SiOx interfacial layer.

Broadband external cavity tunable quantum dot lasers with low injection current density

Broadband grating-coupled external cavity laser, based on InAs/GaAs quantum dots, is achieved. The device has a wavelength tuning range from 1141.6 nm to 1251.7 nm under a low continuous-wave injection current density (458 A/cm(2)). The tunable bandwidth covers consecutively the light emissions from both the ground state and the 1st excited state of quantum dots. The effects of cavity length and antireflection facet coating on device performance are studied. It is shown that antireflection facet coating expands the tuning bandwidth up to similar to 150 nm, accompanied by an evident increase in threshold current density, which is attributed to the reduced interaction between the light field and the quantum dots in the active region of the device.

One-pot synthesis and self-assembly of colloidal copper(I) sulfide nanocrystals

A simple one-pot method is developed to prepare size-and shape-controlled copper(I) sulfide (Cu2S) nanocrystals by thermolysis of a mixed solution of copper acetylacetonate, dodecanethiol and oleylamine at a relatively high temperature. The crystal structure, chemical composition and morphology of the as-obtained products are characterized by powder x-ray diffraction (PXRD), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The morphology and size of the Cu2S nanocrystals can be easily controlled by adjusting the reaction parameters. The Cu2S nanocrystals evolve from spherical to disk-like with increasing reaction temperature. The spherical Cu2S nanocrystals have a high tendency to self-assemble into close-packed superlattice structures. The shape of the Cu2S nanodisks changes from cylinder to hexagonal prism with prolonged reaction time, accompanied by the diameter and thickness increasing. More interestingly, the nanodisks are inclined to self-assemble into face-to-face stacking chains with different lengths and orientations. This one-pot approach may extend to synthesis of other metal sulfide nanocrystals with different shapes and sizes.

高性能聚合物白光电致发光器件研究

聚合物白光电致发光器件可以采用旋涂、喷墨打印等方法成膜，具有工艺简单、成本低等优点，在全色平板显示和照明光源等方面具有广阔的应用前景，吸引了广泛的关注。聚合物白光电致发光器件同小分子发光器件相比发光效率仍然很低，如何提高聚合物白光器件的效率是急需解决的问题。而另一个评价白光器件性能的重要参数就是显色指数(color rendering index , CRI)。目前所报道的有机白光器件的显色指数普遍低于80，如何提高聚合物白光器件的显色指数也是我们研究的重点。本论文在载流子的注入平衡控制、白光聚合物薄膜的聚集态调控改善其光电性能和设计新型器件结构提高白光器件效率等方面进行了系统、深入的研究，在提高聚合物白光电致发光器件的效率和色纯度方面取得了较大的研究进展，为发展高质量的聚合物白光电致发光器件提供了理论基础和实施途径。 1. 首次将醇溶性含磷酸脂基团的聚芴衍生物作为电子注入材料应用到聚合物电致发光器件中，并采用高功函数铝作为阴极获得了高效率的聚合物电致发光。醇溶性含磷酸酯聚芴界面材料不但实现了多层聚合物结构，而且磷酸酯与金属铝的特殊作用有利于高效率电子注入。我们成功地将其应用到红、绿、蓝三基色聚合物发光器件中，与传统的Ca/Al电极聚合物器件相比，这种新型界面材料的使用有效降低了器件的工作电压，发光效率提高了25 %以上。 2. 利用物理掺杂的方法通过选用白光聚合物(由蓝光和橙光组成)作为主体、高效率红光磷光染料作为客体实现了包含红、绿、蓝三基色的高显色指数白光聚合物电致发光，色坐标为 (0.34，0.35)，显色指数达到 92。通过在发光层中引入电子传输材料实现了载流子的传输平衡，提高了白光器件的发光效率，发光效率达到5.3 cd/A。 3. 采用含苯并噻二唑基元的聚芴衍生物作为白光发光层(由蓝光和橙光组成)，利用热退火及溶剂处理等方式控制白光聚合物薄膜聚集态结构，有效调控了白光聚合物薄膜的载流子传输平衡和白光的色纯度。利用热处理或溶剂处理提高了薄膜的空穴迁移率，平衡了载流子传输，使发光效率由原来的7.3 cd/A提高到10 cd/A以上。同过热处理及溶剂处理后的白光聚合物薄膜中出现了高发光效率的聚芴结晶相，提高了蓝光部分的发光效率和色纯度，进而提高了白光的色纯度。 4. 利用含苯并噻二唑基元的聚芴衍生物白光聚合物和含磷酸酯醇溶性蓝光聚芴构筑了高效率双发光层聚合物白光电致发光器件。通过控制白光聚合物薄膜的聚集态结构提高空穴迁移率和引入醇溶性蓝光聚芴界面层改善电子注入实现了载流子传输平衡，同时将激子复合区域限制在两个发光层之间，避免了由于电极造成的激子淬灭，器件的发光效率达到17 cd/A和11 lm/W，是目前国际上报道的非掺杂型聚合物白光电致发光的最高值。 5. 利用高色纯度的蓝光聚芴、绿光和红光磷光染料实现了包含红绿蓝三基色的高显色指数白光；通过器件结构的设计提高器件中三线态激子的利用效率和抑制磷光染料中三线态激子向聚芴低三线态能级的能量转移，实现了高效率、高显色指数聚合物白光电致发光。 其中红光和绿光发光层由高三线态能级的聚乙烯咔唑掺杂绿光和红光磷光染料组成，蓝光发光层采用含磷酸脂基团聚芴，在两个发光层间引入高三线态能级激子限制层，有效抑制了磷光染料三线态激子被低三线态能级聚芴的淬灭，实现了高发光效率的包含红、绿、蓝三基色的高显色指数聚合物电致白光。器件最高效率可达到10.4 cd/A， 色坐标为 (0.36, 0.39)，显色指数达到91。

垂直结构金属基极有机晶体管及其在有机发光驱动中的应用

近几十年来，有机半导体材料作为新一代的信息功能材料正以其光电性能优异、生产成本低廉、加工工艺简单、选材范围宽广、机械性能柔软等显著的优点，吸引了世界范围内的目光，成为越来越多研究机构竞相研究和开发的对象，被广泛应用于发光二极管、薄膜晶体管、太阳能电池、存储器等光电子器件中。这些有机半导体器件的应用前景十分广阔，其巨大的商业价值极大地推动了有机半导体器件的发展。 本论文主要制备了N型金属基极有机晶体管，并对其性能进行了研究和分析，并在此基础上，研究了其在有机发光驱动中的应用。 (1) 用N型有机半导体材料Alq3、F16CuPc和BAlq3作发射极层，Au作为基极，N型硅作为收集极层，Al作为发射极接触电极成功地制备出了一系列N型无机/有机杂化金属基极晶体管，这些器件都表现出了良好的共基极增益特性，最大共基极增益达到了0.991，接近理想值1。在此基础上，通过在发射极层和发射极电极之间引入V2O5界面修饰层，还实现了具有良好共发射极特性的N型无机/有机杂化金属基极晶体管。研究发现，V2O5界面修饰层的引入明显地减小了共基极漏电流，使器件的共基极特性得到了进一步的改善，同时也使器件表现了共发射极特性，实现了电流的放大，我们已经把共基极特性的改善和共发射极特性的实现归功于界面修饰层的引入提高了电子注入的结果。 (2) 根据金属与半导体的接触理论,设计制备出了带有Au/Al双层金属基极的N型无机/有机杂化金属基极晶体管。由于Al和Alq3之间好的接触特性和有效的从Al到Alq3的空穴阻挡特性以及Au和Si之间良好的肖特基接触特性，大大降低了器件的漏电流，使器件在低的电压下表现了优异的共基极和共发射极特性，共基极增益达到了近似理想值1，最大共发射机增益达到了4000，克服了单层金属基极晶体管难实现共发射极特性的问题，为实现高性能金属基极晶体管提供了新的思路。 (3) 利用异质结的概念，设计制备出了带有BAlq3/Alq3异质结结构的N型无机/有机杂化金属基极晶体管，该器件同样表现了优异的共基极和共发射极特性。研究发现，同Alq3单发射极层结构的金属基极晶体管相比，BAlq3/Alq3异质结发射极层的使用进一步降低了器件的漏电流，使器件在相同的电压下表现了更高的输出电流和更高的共发射极增益，为进一步实现高性能金属基极晶体管提供了新的方法。 (4) 用有机半导体材料取代无机高掺杂硅作为收集极层，制备出了带有Al单层金属基极和Au/Al双层金属基极的N型垂直结构全有机金属基极晶体管，该器件表现出了良好的共基极特性和共发射极特性。研究表明，全有机金属基极晶体管表现了和无机/有机混合型金属基极晶体管相似的特性，其从本质上说也是一种渗透型金属基极晶体管。 (5) 实现了金属基极有机晶体管驱动有机发光二极管的集成器件。利用金属基极有机晶体管的共发射极电流放大特性，在基极输入电流IB量级比较低（uA）的情况下，得到了较大量级（mA）的输出电流IC，从而实现了对白光有机发光二极管的驱动，在基极输入电流IB为1×10-5A时有机发光二极管的亮度达到了1279 cd/m2。