【作者】 曾昱嘉；

【导师】 叶志镇；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2008， 博士

【摘要】 氧化锌（ZnO）是一种新型的宽禁带化合物半导体材料,具有直接带隙能带结构,室温禁带宽度为3.37eV,对应于近紫外光波段。另外,ZnO还具有60 meV的高激子束缚能,其激子在室温下可以稳定存在,易于实现室温或更高温度下激子-激子碰撞的受激辐射。因此,ZnO是制备半导体紫外LEDs（Light-EmittingDiodes发光二极管）和LDs（Laser Diodes激光器）的理想材料。除了光学性能的优势,ZnO还具有无毒、热稳定性好、抗辐射性强、原材料丰富、外延薄膜容易生长、带隙宽度调节的合金体系（ZnMgO和ZnCdO）完备和体单晶易得等优点。另一方面,ZnO还具有丰富的纳米结构,包括纳米线、纳米管、纳米带、纳米环、纳米点等。当材料尺寸降低到纳米尺度时,可能出现许多与体材料不同的新颖光电性能。因此,各种ZnO纳米结构在纳米电子、纳米光电子、生物医药、气敏传感器等领域有望得到广泛的应用。要实现ZnO在光电领域的广泛应用,首先必须获得性能良好的n型和p型材料。然而,ZnO具有强烈的掺杂单极性,天然为n型,通过掺杂人们已经获得了高质量的n型ZnO。但是ZnO的p型掺杂却异常困难,在取得一定进展的同时仍存在诸多问题,并且对于何种元素为ZnO最佳受主元素的问题还不甚明了。这是目前制约ZnO材料实际应用的最大瓶颈,也是ZnO研究中面临的主要挑战。本文研究重点在于寻找合适的ZnO p型掺杂元素与掺杂技术,并深入探索p型掺杂机理。另外,本文也制备了ZnO纳米线和纳米点,并初步实现ZnO纳米材料的可控生长。主要工作包括以下内容:1.在之前制备Al-N共掺p型ZnO薄膜的基础上,研究生长温度和Al施主含量两个重要参数对薄膜p型导电性能的影响,提出一个共掺法生长p型ZnO的模型。2.采用磁控溅射法Li掺杂技术实现ZnO薄膜的p型转变,系统研究了实现p型ZnO的生长窗口和掺杂机理。研究表明,只有在富氧与合适Li含量的条件下,才能最大程度地促进Li_Zn受主形成,并抑制Li_i和Li_Zn-Li_i等缺陷复合体的补偿作用,从而获得良好的p型电导性能。3.采用氧等离子体辅助MOCVD（Metal Organic Chemical Vapor Deposition金属有机物化学气相沉积）法生长非故意掺杂的p型ZnO薄膜。氧等离子体不仅能提高氧的化学势,降低锌空位等受主型缺陷的形成能,并且可以有效地提高ZnO薄膜中的氧含量,从而降低氧空位施主浓度。这两方面最终导致本征p型电导的形成。4.采用射频等离子体辅助MOCVD法生长p型ZnO:N薄膜,揭示了p型性能对生长温度的依赖性。研究发现,p型ZnO:N中具有双受主行为,等离子体辅助生长将引入锌空位受主,它和N受主一起对ZnO:N的p型电导起作用。作者研究了p型ZnO:N薄膜的紫外光电导行为,提出一个表面吸附与光脱附模型,合理地解释了所观察到的紫外光电导现象。采用一种非等离子体N掺杂技术实现ZnO的p型转变,并制备了ZnO同质结LEDs的原型器件。5.采用MOCVD法在硅衬底上无催化生长ZnO纳米线阵列,引入低温形核层能为纳米线提供良好的生长核心,从而导致整齐阵列的生长。通过控制反应物气体流量,可以实现ZnO纳米线的尺寸的裁剪。6.采用MOCVD法在硅和蓝宝石衬底上可控生长ZnO纳米点,纳米点尺寸为5～10nm。研究发现,通过控制生长温度可以很好地调节ZnO纳米点的生长密度。作者报导了ZnMgO纳米点的生长,通过控制Mg含量,可以自由地调节ZnMgO纳米点中自由激子（FX）发射峰位。另外,通过在纳米点中分别引入Ga施主和N受主,可以实现ZnO纳米点费米能级与电阻率的调节。最后,本文介绍了ZnO/MgO准核壳结构纳米点的生长与表征,MgO壳层有效地钝化了ZnO纳米点活泼的表面,从而显著提高了纳米点的光致发光强度。作者在ZnO/MgO纳米点中观察到量子尺寸效应:FX发射峰位随着ZnO尺寸的减小而蓝移。并且由于量子约束效应,ZnO/MgO纳米点激子束缚能增大到118meV,约为体材料的两倍。更多还原

【Abstract】 With a large exciton binding energy of 60 meV and a wide bandgap of 3.37 eV at room temperature, ZnO is considered as a promising material for short-wavelength optoelectronic devices, such as light-emitting diodes （LEDs） and laser diodes （LDs）. In addition, ZnO has many other advantages, including the availability of large-area single crystal, the amenability to wet chemical etching, the high radiation resistance, the relatively low costs, and the availability of alloys system （ZnMgO and ZnCdO） for bandgap engineering, etc. On the other hand, by virtue of numerous unique properties expected in the low-dimensional system, ZnO nanometer-scale materials, such as nanowires, nanotubes, nanobelts, and nanodots promise to be important in the next-generation electronic, optoelectronic, and medical applications.Due to the asymmetric doping limitations, n-type ZnO, with a high electron concentration, has been well prepared. The realization of p-type ZnO, however, has proven difficult and thought to be the bottleneck in the development of ZnO-based devices. Also, the optimal choice of acceptor species in ZnO remains to be determined. In this regard, the work was focused on the growth and characterization of p-type ZnO thin films in this dissertation, in an attempt to better understand the p-type doping mechanism. In addition, the controllable growth of ZnO nanowires and nanodots were reported. The work included:1. Al-N codoped p-type ZnO thin films were prepared by dc reactive magnetron sputtering. The effects of growth temperature and Al content on properties of Al-N codoped ZnO were discussed.2. The author demonstrated the reproducible growth of Li-doped p-type ZnO thin films by dc reactive magnetron sputtering. Under the oxygen rich condition and a proper Li content, the formation of Li_Zn acceptor could be enhanced, resulting in low-resistivity, p-type ZnO thin films. In addition, the doping mechanism for the p-type ZnO.Li was proposed tentatively.3. Intrinsic p-type ZnO thin films were grown by plasma-assisted metalorganic chemical vapor deposition （MOCVD）. The increment of the oxygen concentration in the intrinsic p-type ZnO, compared with the intrinsic n-type layer, was well confirmed by secondary ion mass spectroscopy. The origin of intrinsic p-type behavior was ascribed to the formation of zinc vacancy and some complex acceptor center.4. N-doped, p-type ZnO thin films were grown by plasma-assisted MOCVD by using NO plasma. The intrinsic zinc vacancy and extrinsic nitrogen acceptor, identified by low-temperature photoluminescence （PL）, contributed to the p-type conductivity in the ZnO:N simultaneously. Also, the author demonstrated comparative study on ultraviolet photoconductivity of p-type ZnO:N thin films and n-type ZnO epilayer. The surface adsorption and photodesorption process, combined with a competition between holes and electrons, in the p-type ZnO:N was proposed, providing qualitative agreement with the observed behaviors. Furthermore, the author developed a plasma-free MOCVD method to grow reproducible N-doped, p-type ZnO thin films. The typical rectifying I-V characteristics and room-temperature electroluminescence from the ZnO homojunction LEDs were observed.5. Well-aligned ZnO nano wires were grown on silicon substrates by MOCVD without catalysts. The mechanism of the catalyst-free growth of ZnO nanowires on silicon substrates was discussed. By modulating the flux of the reactant, the diameter of ZnO nanowires could be well tailored.6. ZnO and ZnMgO nanodots （NDs）, with the diameter of 5～10 nm, were grown on silicon and sapphire substrates by MOCVD. The density of the NDs was shown to be temperature-dependent. By adjusting the Mg content, a tailored optical bandgap was achieved for the ZnMgO NDs. Also, the Ga donor and N acceptor were introduced into ZnO NDs, respectively. The author demonstrated, with a combination of valence band x-ray photoelectron spectroscopy and scanning tunneling microscopy, that the electrical properties as well as Fermi level of the ZnO NDs could be well tuned by the donor/acceptor doping. Finally, ZnO/MgO quasi core-shell NDs were grown by a MOCVD method. The free exciton （FX） emission from the NDs showed remarkable enhancement after the growth of MgO shell layer. Also, the FX emission exhibited an evident blueshift with the decreased ZnO size due to the quantum confinement effect. An exciton binding energy of 118 meV was obtained from temperature-dependent PL, which was almost two times as that of the bulk materials.更多还原