稀土配合物光致发光和电致发光性能的研究

有机电致发光器件(organic light-emitting diodes, OLEDs)由于具有全色、低压直流驱动、视角广、高效率和易于制备大屏幕等优点，在平板显示领域具有广阔的应用前景，已引起世界范围内科技界和企业界的广泛重视。稀土配合物作为发光及载流子传输层材料，具有窄带发射和发射波长分布范围广的特点，有利于实现高的色纯度。然而，有机电致发光的机理还未完全弄清楚，稀土配合物电致发光器件距离实际应用还比较远，需要进一步进行研究。本论文研究了Tb~(3+)、Eu~(3+), Sm~(3+)配合物的光致发光和电致发光性质，制备了绿色、红色和橙色电致发光器件，并对稀土配合物的电致发光机理进行了探讨。对于试配合物，以acac为第一配体，研究了第二配体AAP和bath对配合物和器件发光性能的影响并对器件的优化进行了讨论；以Tb(acac)_3bath为发光中心，发现电致发光中~5D_4→~7F_5跃迁相对~5D_4→~7F_5跃迁的增强现象；以Tfacac为第一配体，首次讨论了F取代H对于配体的三重态能级的降低和对配合物以及电致发光器件发光强度的增强作用，对于提高稀土配合物电致发光的性能提供了参考依据。对于铺配合物，以Tfacac为第一配体，以phen和bipy为第二配体，制备了红色电致发光器件并进一步验证了F取代H对于配体的三重态能级的降低作用；以HTH为第一配体，国际上首次报道了室温下Eu~(3+)离子的~5D_1→~7F_J的跃迁，并且其强度依赖于驱动电压的强度，光致发光和电致发光的不同表明二者的发光机理有所不同，并对铺配合物电致发光机理进行了讨论；以HTH为第一配体，国际上首次报道了衫配合物的电致发光现象，并且最大得到了21 cd/m~2的发光强度；为了改善稀土β-二酮配合物光稳定性和热稳定性差的缺点，引入了稀土芳香梭酸配合物，并对配体进行了引入长链的化学修饰，提高了配合物在一般有机溶剂中的溶解度，用旋涂法制备的器件的最大发光亮度达到了174 cd/m~2.

OLED中空穴传输层NPB薄膜的结晶化行为研究

近年来，虽然有机电致发光材料和有机电致发光器件得到了广泛的研究，然而贯穿整个有机电致发光研究主线的稳定性问题仍是一个重要的议题。作为发光器件的主要组成部分，有机小分子薄膜的性能不仅由组成分子结构决定，而且还受到自身形态结构的制约。要解决有机材料的热稳定性问题，材料薄膜的形态结构是一个重要的影响因素。弄清有机小分子薄膜的形态结构和性能间的相互关系对于进一步提高器件性能（效率和寿命）是十分必要的。有机分子的热不稳定性表现在形态结构上主要是由非晶态到结晶态的转变，从而引起器件性能的不稳定。所以本文详细研究了一种典型的半晶性有机分子薄膜（同时也是最经常使用的有机小分子空穴传输材料之一）-NPB薄膜的结晶化相转变，分别从均相成核结晶化相转变和异相成核结晶化相转变两个角度来阐述。本文通过AFM、PLM、X射线衍射、DSC等实验方法和研究手段表征了NPB薄膜由非晶态到结晶态的相态转化，并从热力学上进行了分析。同时结合了实际OLED器件中的应用。本论文研究表明NPB分子是一种典型的半晶性小分子，非晶与结晶两种状态的并存导致热力学上的不稳定。本论文详细研究了NPB薄膜的均相成核结晶化相转变和异相成核结晶化相转变，并指出半晶性的NPB分子薄膜的均相成核结晶化存在一个临界结晶厚度，当薄膜厚度超过临界结晶厚度时才能发生结晶。并且临界结晶厚度的大小与基底温度有关。而NPB薄膜的异相成核结晶化转变则由于异核的引入结晶能的降低相对来说容易许多。本论文首次从理论角度归纳提出了均相成核结晶老化机制和异相成核结晶老化机制两种关系到OLED器件稳定性（老化问题）的机制。是为数不多的从薄膜形态角度来研究器件稳定性的一篇文章，对从材料化学角度深入理解OLED器件稳定性有着重要的意义。

LIQUID-PHASE EPITAXY GROWTH AND PROPERTIES OF GAINASSB/ALGAASSB/GASB HETEROSTRUCTURES

The GaInAsSb/AlGaAsSb/GaSb heterostructures were grown by the liquid phase epitaxy (LPE) technique. The materials were characterized by means of optical microscopy, electroprobe microanalysis (EPMA), double-crystal X-ray diffraction, capacitance-voltage (C-V) and Van der Pauw measurments, infrared absorption spectra, photoluminescence and laser Raman scattering. The results show that the materials have fine surface morphology, low lattice mismatch and good homogeneity. Room-temperature light-emitting diodes with an emission wavelength of 2.2-mu-m were obtained by using the GaInAsSb/AlGaAsSb DH structures.

Some properties of gallium nitride films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy

The Raman and photoreflectivity spectra of gallium nitride (GaN) films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy (GSMBE) have been investigated. The Raman spectra showed the presence of the E-2(high) mode and a shift in the wavenumber of this mode with respect to the GaN epilayer thickness. The Raman scattering results suggest the presence of stress due to lattice and thermal expansion misfit in the films, and also indicate that the buffer layer play an important role in the deposition of high quality GaN layers. The residual stress changes from tensile to compressive as the epilayer thickness increases. Samples subjected to anneal cycles showed an increase in the mobility due probably to stress relaxation as suggested by an observed shift in the E-2(high) mode in the Raman spectra after annealing.

Growth of GaN layers on GaAs and GaP (111) and (001) substrates by molecular beam epitaxy

Films of GaN have been grown using a modified MBE technique in which the active nitrogen is supplied from an RF plasma source. Wurtzite films grown on (001) oriented GaAs substrates show highly defective, ordered polycrystalline growth with a columnar structure, the (0001) planes of the layers being parallel to the (001) planes of the GaAs substrate. Films grown using a coincident As flux, however, have a single crystal zinc-blende growth mode. They have better structural and optical properties. To improve the properties of the wurtzite films we have studied the growth of such films on (111) oriented GaAs and GaP substrates. The improved structural properties of such films, assessed using X-ray and TEM method, correlate with better low-temperature FL.

Optical properties of AIInGaN quaternary alloys

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved photoluminescence (TRPL). The fast redshift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/tau)(beta)], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed quantum dots or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent 8 on the temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered quantum dots. Furthermore, the localized states are found to have OD density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature.

The growth and characterization of GaN grown on a gamma-Al2O3/(001) Si substrate by metalorganic vapor phase epitaxy

Wurtzite single crystal GaN films have been grown onto a gamma-Al2O3/Si(001) substrate in a horizontal-type low pressure MOVPE system. A thin gamma-Al2O3 layer is an intermediate layer for the growth of single crystal GaN on Si although it is only an oriented polycrystal film as shown by reflection high electron diffraction. Moreover, the oxide is not yet converted to a fully single crystal film, even at the stage of high temperature for the GaN layer as studied by transmission electron microscopy. Double crystal x-ray linewidth of (0002) peak of the 1.3 mu m sample is 54 arcmin and the films have heavy mosaic structures. A near band edge peaking at 3.4 eV at room temperature is observed by photoluminescence spectroscopy. Raman scattering does not detect any cubic phase coexistence.

Growth and characterization of GaN on LiGaO2 and LiAlO2

The nearly lattice-matched LiGaO2 and LiAlO2 substrates have been used for the growth of GaN by LP-MOVPE. GaN epilayers have been grown on the two substrates at very low input partial pressure of hydrogen and relatively low growth temperature. The difference in the growth rate, crystal and optical qualities of hexagonal GaN epilayers grown on LiAlO2 and LiGaO2 substrate with two polar domains are investigated. LiAlO2 and LiGaO2 single crystal with a single domain structure and an adequate surface plane are promising substrates for the growth of high quality of hexagonal GaN thin films.

Yellow luminescence mechanism and persistent photoconductivity in n-GaN single crystal films grown on alpha-Al2O3(0001) substrates by LP-MOCVD

Unintentionally doped and Si-doped single crystal n-GaN films have been grown on alpha-Al2O3 (0001) substrates by LP-MOCVD. Room temperature photoluminescence measurement showed that besides the bandedges, the spectrum of an undoped sample was a broad deep-level emission band peaking from 2.19 to 2.30eV, whereas the spectrum for a Si-doped sample was composed of a dominant peak of 2.19eV and a shoulder of 2.32eV. At different temperatures, photoconductance buildup and its decay were also observed for both samples.. The likely origins of persistent photoconductivity and yellow luminescence, which might be associated with deep defects inclusive of either Ga vacancy(V-Ga)/Ga vacancy complex induced by impurities or N antisite (N-Ga), will be proposed.

Design of high brightness cubic-GaN LEDs grown on GaAs substrate

The Principle of optical thin film was used to calculate the feasibility of improving the light extraction efficiency of GaN/GaAs optical devices by wafer-bonding technique. The calculated results show that the light extraction efficiency of bonded samples can be improved by 2.66 times than the as-grown GaN/GaAs samples when a thin Ni layer was used as adhesive layer and Ag layer as reflective layer. Full reflectance spectrum comparison shows that reflectivity for the incident light of 459.2 nm of the bonded samples was improved by 2.4 times than the as-grown samples, which is consistent with the calculated results.

AlGaInP LED with surface structure of two-dimensional photonic crystal

Some progress in the research of GaN based LED with photonic crystal structure has been made recently. Based on the photonic crystal's photonic band gap effect and photon grating diffraction principle, the extraction efficiency of LED with photonic crystal can be improved. In this paper, the restriction on AlGaInP LED's extraction efficiency is analyzed, and the photonic crystal is introduced in to the AlGaInP LED to improve the extraction efficiency. The theoretical analyses and the experiment results show that the output luminous intensity of LED with photonic crystal is improved by 16%, which results from some effect of the GaN based LED with photonic crystal.

Strain effects on optical polarisation properties in (11(2)over-bar2) plane GaN films

We present the theoretical results of the electronic band structure of wurtzite GaN films under biaxial strains in the (11 (2) over bar2)-plane The calculations are performed by the kappa p perturbation theory approach through using the effective-mass Hamiltonian for an arbitrary direction The results show that the transition energies decrease with the biaxial strains changing from -0 5% to 0 5% For films of (11 (2) over bar2)-plane, the strains are expected to be anisotropic in the growth plane Such anisotropic strains give rise to valence band mixing which results in dramatic change in optical polarisation property The strain can also result in optical polarisation switching phenomena Finally, we discuss the applications of these properties to the (11 (2) over bar2) plane GaN based light emitting diode and lase diode

Studying the Attribution of LiF in OLED by the C-V Characteristics

The organic light-emitting device (OLED) with simple structures of indium tin oxide (ITO)/tris(8-quinolinolato) aluminum (Alq(3))/LiF/Al and ITO/Alq(3)/Al was fabricated to analyze the contribution of LiF in OLED. We used the C-V characteristics to investigate the contribution of LiF in OLED and found that the capacitance of the above-mentioned structures was 12.5 nF and 77.5 nF, respectively. It is shown that the LiF layer affects the property of OLED resulting in the change of the capacitance of the device.

High PL-efficiency ZnO nanocrystallites/PPV composite nanofibers

In this paper, we report for the first time on the synthesis of ZnO nanocrystallites in conjugated polymer (PPV) nanofibers by the coupling of the in situ/blend methods and electrospinning. These composite nanofibers were characterized by fluorescence microscopy, atomic force microscope (AFM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectra, Fourier transform infrared (FT-IR) spectroscopy, and X-ray powder diffraction (XRD).