【作者】 陈成；

【导师】 陈长清； 戴江南；

【作者基本信息】 华中科技大学 ， 光学工程， 2017， 博士

【摘要】 氧化锌(ZnO)材料为典型的第三代宽禁带半导体,拥有较宽禁带宽度(3.37 eV)及直接带隙的能带特点。ZnO具有高的激子束缚能,能替代GaN材料实现室温或者更高工作温度下高效的紫外激子发光。相比ZnO薄膜跟块体材料,纳米结构的ZnO显示出多变的形貌特征以及制备的多样性,呈现出不同的光电特性。通过将低维ZnO纳米结构材料应用于发光二极管中会极大的降低器件的制备成本。在稳定可靠的p型ZnO仍未实现的背景下,本文以ZnO基异质结紫外发光器件的开发为主要内容,通过将ZnO的异质结构作为切入点,在引入低成本的一维ZnO纳米棒、零维ZnO量子点的同时,系统研究了低维ZnO纳米结构的制备与物理特性,并结合其结构特点构建了差异化的器件结构,以实现ZnO高效的短波长紫外发射。(1)采用滴液-覆盖法在Si片上制备了 ZnO种子层,作为随后ZnO纳米棒生长的籽晶层,然后利用漂移法自组装了大面积单层密排的聚苯乙烯(PS)微球薄膜,结合纳米球刻蚀技术,利用TiO2胶体反向复制PS球自组装单层,获得了反蛋白石结构模板,并以此为模板通过水热法成功制备了高度垂直有序的一维ZnO纳米棒阵列,研究了水热生长过程中反应时间、反应浓度对ZnO纳米棒形貌的影响。通过对ZnO纳米棒的生长形貌的演化分析进而探讨了水热法生长ZnO纳米棒的生长机理。排列有序的ZnO纳米棒阵列在直径跟高度上有着高度的一致性,纳米棒沿[0001]方向生长,强而尖锐的带边发光峰及弱的深能级发光说明ZnO纳米棒阵列有良好的光学性能,同时Raman测试中B1(low)模的缺失显示出图案化ZnO纳米棒阵列有着良好的晶体质量。(2)通过在p-GaN上采用Ga液滴法外延生长GaN量子点,研究了不同的退火温度、生长过程以及降温过程中NH3流量的变化对GaN量子点的直径、高度以及密度的影响。由于nanodrill效应,这些GaN量子点在形成过程中出现纳米环的形貌。利用GaN量子点表面能高的特点,研究了以GaN量子点作为模板水热法生长一维ZnO纳米棒。实验结果表明,通过调节GaN量子点的密度可以实现调控ZnO纳米棒阵列的密度,同时证明GaN量子点确实对ZnO纳米棒起到了成核作用。另外,实验发现在具有纳米环形貌的GaN量子点上生长ZnO纳米棒会出现直径粗、细两种纳米棒,分别位于GaN量子点环中和环壁,以此探讨了 GaN量子点对水热生长ZnO纳米棒的成核机制。(3)利用Al掺杂ZnO(AZO)透明导电玻璃为衬底水热法制备了一维ZnO纳米棒。为避免带有结构缺陷的ZnO籽晶层对LED器件发光性能的影响以及充分发挥一维ZnO纳米棒的单晶优势,提出了一种新型的直接表面贴合结构在一维ZnO纳米棒顶端与p-GaN之间制备了高质量的n-ZnO/p-GaN异质结,实现了发光中心波长在390 nm的近紫外发射,器件的电致发光光谱中无任何缺陷相关的深能级可见光。通过对电致发光光谱的高斯拟合发现,该器件的近紫外发射源自于n-ZnO纳米棒、p-GaN以及两者的界面,同时结合能带结构图给出了器件的发光机制。(4)通过低温溶液法以NaOH和乙酸锌为原料制备了直径大小约7 nm的ZnO量子点,并引入MgO作为电子阻挡层,设计了 n-ZnO QDs/MgO/p-GaN全无机的LED。从器件的电、光学性能表征测试中发现,该器件在正向偏压(n-ZnO接负极)下不发光,只有在反向偏压下才有光发射出,而且随着反向偏压的增大,ZnO缺陷相关的深能级可见光被抑制住,当施加的反向电压达到25 V时,器件获得了纯的370 nm紫外发射,半高宽仅7.5 nm,并对器件反向偏压下的能带图进行了分析和探讨揭示了该现象的成因。为评价器件的稳定性,将器件放置空气环境中暴露一个月时间,实验结果发现,ZnO量子点的近带边发射减弱,而深能级可见光发射强度增强,这与量子点表面吸附的O2和OH-(水蒸气)等作为受主型和施主型表面态有关,使得表面无辐射复合和表面中介的深能级复合增加,进而导致器件的发光效率降低。(5)为提高LED的量子效率,将Ag纳米粒子引入ZnO量子点中,利用ZnO量子点的量子限制效应结合Ag纳米颗粒局域表面等离激元(LSP)两者的优势,通过Ag LSP和ZnO激子的共振耦合效应及球形金属纳米粒子LSP本身的强局域场各向同性散射效应,来改善器件的紫外光发射效率。利用溶剂热法成功制备了平均直径为40 nm,LSP共振中心峰位处于400 nm处的类球型的Ag纳米颗粒,通过优化介电层MgO薄膜的厚度,以平衡LSP近场耦合与非辐射共振能量转移,最终实现了 4.3倍近紫外电致发射增强,并提出了异质结界面处电荷传输模型以解释发光增强的原因。更多还原

【Abstract】 Zinc oxide(ZnO)material is a typical third-generation wide bandgap semiconductor with a wide and direct bandgap of 3.37 eV.ZnO has a high exciton binding energy and acts as an alternative to achieve efficient ultraviolet exciton luminescence using GaN material in the room temperature or higher operating temperature.Compared with ZnO thin films and bulk materials,ZnO nanostructures show a variety of morphological features and the diversity of preparation,showing different optical properties.By employing the low-dimensional ZnO nanostructured materials in light-emitting diodes field,it will be great to reduce the cost of preparation of the device.Because the stable and reliable p-type ZnO has not been made a breakthrough,the development of ZnO-based ultraviolet light-emitting devices is the main content.By introducing the low-cost one-dimensional ZnO nanorods and zero-dimensional ZnO quantum dots,the heterostructure of ZnO acts as the entry point.Meanwhile,the structure and properties of low-dimensional ZnO nanostructures are studied systematically.Combined with their structural characteristics,the different device structures are constructed to realize the efficient short-wavelength ultraviolet emission of ZnO.First of all,ZnO seed layer is produced on the Si substrate by using the liquid droplet overlay method for the subsequent growth of ZnO nanorods,a large-area close packed monolayer polystyrene(PS)microspheres thin film is prepared by a self-assembly drift method.Meanwhile,non-close packed PS template obtains by the reactive ion etch technology.Then TiO2 sol fills the non-close packed PS microspheres with the appropriate concentration,and toluene removals PS microspheres to obtain a large-area inverse opal template.The ZnO nanorods are grown by hydrothermal method using the inverse opal template.Finally,the one-dimensional ZnO nanorod arrays with vertical structure are fabricated.The effects of reaction time and reaction concentration on the morphology of ZnO nanorods are investigated.The growth mechanism of ZnO nanorods by hydrothermal method is discussed through analyzing the growth morphology evolution of ZnO nanorods.Patterned and ordered ZnO nanorods array is consistent in diameter and height,the nanorods grow along[0001]direction.Sharp and strong near band edge emission and weak deep level luminescence intensity of ZnO nanorods array indicates that the ZnO nanorods have the good optical properties.Meanwhile,B1(low)model has been absent in the Raman test,it shows that the patterned ZnO nanorod arrays have a good crystal quality.Secondly,GaN quantum dots are grown on p-GaN by Ga droplet method.The effects of different annealing temperature,NH3 flow rate in growth and cooling process on the diameter,height and density of GaN quantum dots are studied.Due to the nanodrill effect,GaN quantum dots appear the nonohole morphology in the growth process.Using the characteristics of high surface energy of GaN quantum dots,the growth of one-dimensional ZnO nanorods by hydrothermal method using GaN quantum dots as the template is studied.The experimental results show that the density of ZnO nanorods can be controlled by adjusting the density of GaN quantum dots,and it shows that the GaN quantum dots play a nucleation role in the growth of ZnO nanorods.In addition,we find that the growth of ZnO nanorods with nanohole morphology by using the GaN quantum dots as a template will appear on the two different locations,they are located on the quantum ring and ring on the wall,and we explore the mechanism of growth of ZnO nanorods using the GaN quantum dots as a templateThirdly,one-dimensional ZnO nanorods are prepared by Al-doped ZnO(AZO)transparent conductive glass.In order to avoid the effect of ZnO seed layer with crystal defects on the luminescent properties of LED devices and take full advantage of the single crystal properties of one-dimensional ZnO nanorods,a new direct surface bonding structure is proposed.The high-quality heterojunctions are fabricated between p-GaN and the n-ZnO nanorods,and the near-ultraviolet emission centered at 390 nm.There is no defect visible light in the electroluminescence spectrum of the device.It is found that the near-ultraviolet emission of the device is derived from the n-ZnO nanorods,p-GaN and the interface,the luminescence mechanism of the device is given by combining the energy band structure.Fourthly,well-distributed 0D ZnO QDs with an average diameter of about 7 nm are prepared via NaOH and acetate dehydrate through low-temperature solution method.MgO is intruduced as the electron blocking layer between the ZnO layer and the GaN layer so as to improve the recombination in ZnO,and we design all-inorganic the n-ZnO QDs/MgO/p-GaN LED.From the test of the optoelectronic performance of the devices,we find that the device in the forward bias(n-ZnO as negative)has no light emission,the EL spectra show that light emission can only be detected when the LEDs are applied with reverse bias.With the increase of reverse bias,deep level ZnO defect related visible light is suppressed.When the reverse voltage achieves to 25 V,the device obtain the pure 370 nm UV emission,the width at the half maximum is only 7.5 nm.We use the energy band diagram of the device under the reverse bias to explain the causes of this phenomenon.For the evaluation of the stability of the device,the device is placed in the ambient air to exposure for one month,the experimental results show that near band edge emission in ZnO decreases,while the deep level visible emission increases,and the adsorption of O2 and OH-(water vapor)on the surface of quantum dots as the acceptor and donor type affect the properties of the device,it increase the surface radiative recombination and intermediary deep level recombination,thus resulting in the decreased efficiency of the devices.At last,to further improve the quantum efficiency of heterojunction LEDs,the LSP of Ag nanoparticles are incorporated into the ZnO quantum dots.By combining the advantages of both quantum confinement effect and metal LSP,we look forward to improve of the emission efficiency of UV LED.A facile synthetic method to produce Ag nanoparticles with controllable shapes using Poly(vinylpyrrolidone)(PVP)as surfactant is introduced.The Ag nanoparticles with a diameter of about 40 nm have a LSP peak located at 400 nm By optimizing the MgO thickness,which can suppress the nonradiative Forster resonant energy transfer between Ag and ZnO,a 4.3-fold EL enhancement is achieved.We put forward the heterojunction interface charge transfer model to explain the luminescence enhancement.The emission enhancements mainly result from the increased spontaneous emission rate and internal quantum efficiency of ZnO quantum dots.更多还原