【作者】 胡军；

【导师】 潘必才；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2009， 博士

【摘要】 ZnO为直接带隙的宽禁带半导体,室温下禁带宽度为3.37 eV;特别的,其激子束缚能高达60 meV。因此,ZnO在室温或更高温度下的紫外发光器件的应用中有极大的前景。但是,这首先要求有高质量的n型和p型ZnO。然而,尽管实验上很容易获得高质量的n型ZnO,但p型ZnO却很难被制备出来。近年来,人们从理论和实验上对如何在ZnO中进行有效的p型掺杂进行了大量的探索,取得了丰硕的成果,同时也提出了很多问题。本论文就ZnO中的本征缺陷性质、受主杂质与H的复合体的性质以及p型ZnO的导电性质进行了深入研究。本论文共分为五章。在第一章,我们总结了近年来关于ZnO的本征缺陷、ZnO中H元素的性质和ZnO中p型掺杂的研究。在第二章,我们介绍了理论计算中广泛使用的密度泛函理论,并且介绍了计算原子最佳迁移路径及其迁移势垒的方法。第三章讨论了ZnO中本征缺陷的电子结构、I族杂质缺陷的电子结构以及几种重要的本征缺陷的光吸收和振动性质。通过B3LYP方法对缺陷能级的准确计算,我们发现空位缺陷(V_O和V_Zn)和八面体填隙缺陷(O_i,o和Zn_i,o)都可能是实验观测到的绿光峰的来源。因此以前的研究仅仅把绿光峰归因于某一种缺陷是不对的。另外,O_i,o和Zn_i,o可能分别对黄光和红光有贡献。对I族杂质缺陷的B3LYP计算表明,实验上在掺Li和Na的ZnO中观测到的发光峰可能来自相关的复合缺陷。在ZnO中掺Ag会导致很多缺陷能级,其中一部分可能对实验观测到的3.17 eV的发光峰有贡献。我们研究了几种最重要的本征缺陷(V_O、H_O和V_Zn)的光吸收和振动性质,找到了每种缺陷的特征光吸收峰和振动模式,这些发现可用于指认这些缺陷。在第四章中,我们研究了ZnO中的几种重要的杂质（N、Li和Cu）与H的复合体的性质。我们首先研究了N_O-H、Li_Zn-H和Cu_Zn-H复合体的形成和离解。通过计算H原子从不同方向到达基态构型的最低能量路径,得到了H原子在迁移过程中所需克服的能量势垒。我们发现,这三种复合体都可以在200 K以下的低温形成,并在室温下稳定存在。N_O-H、Li_Zn-H和Cu_Zn-H复合体的离解所需要的激活能分别为1.25-1.48、1.10-1.35和1.42-1.63 eV,对应的激活温度约分别为480-570、420-520和540-620 K。另外,我们对这些复合体的电子结构、缺陷能级、振动性质以及光学性质进行了研究,深入讨论了H原子在这些复合体中所起的作用。在第五章中,我们对p型掺杂ZnO导电性质的不稳定性和低效率的问题进行了研究。通过分析掺N的ZnO中的几种主要缺陷,我们提出了一种全新的微观模型,很好地解释了光照诱导的N掺杂ZnO导电性质转变的现象。我们发现,H_O（即被V_O捕获的H原子）在这种现象中扮演了重要的角色。由此,我们推测如果能控制V_O的浓度,使其很低,则会避免H_O量存在,从而能增强掺N的ZnO的p型导电性的稳定性。另一方面我们基于对在N掺杂的p型ZnO中观测到的实验现象的分析,提出了受主缺陷不完全离化的观点,成功地解释了相关实验现象。通过分析电子结构和自旋密度在空间上的分布,我们发现非局域的缺陷态波函数是上述现象的物理根源。我们将这一观点应用于掺P和As的ZnO中,也解释了相关实验现象。另外,我们还预测,掺Ag的ZnO中,受主能级也将介于0.1-0.2 eV之间,Ag_Zn为浅受主。更多还原

【Abstract】 ZnO is a wide-band-gap semiconductor, with band gap of 3.37 eV at room temperature （RT）. Particularly, its exciton binding energy is as large as 60 meV, which promises its applications at RT or even higher temperature in ultraviolet optoelectronic devices. However, its applications depend on high-quality n-type and p-type ZnO. Although high-quality n-type ZnO can be obtained easily, p-type ZnO is difficult to be produced. Recent years, many researches have been conducted to explore the dopability of p-type ZnO in both theoretical and experimental studies, which give many valuable results and simultaneously bring up many questions. In this dissertation, we investigate properties of native defects and acceptor-H complexes, as well as the conductivity of p-type ZnO. This dissertation consists of five chapters.In the first chapter, we review the researches on native defects and H as well as p-type doping in ZnO.In the second chapter, we introduce density functional theory which is widely used in theoretical studies, and then describe CI-NEB method which is usually used to compute the economical diffusion paths and the related energy barrier for the diffusion of an atom in bulk material.In the third chapter, we present our results on electronic structures of native defects and group-I impurities in ZnO, and optical and vibrational properties of some important native defects. By using B3LYP calculations, the energy levels arising from native defects of ZnO are accurately determined, from which we conclude that vacancies (V_P and V_Zn) and octahedral interstitials (O_i,o and Zn_i,o) may contribute to green photoluminescence （PL） observed in experimental investigations, which implies that it is improper to attribute the observed green PL to a single type of native defects as done in previous reports. Furthermore, we find O_i,o and Zn_i,o may contribute to yellow and red PL. For the group-I impurities, we find that the PL in Lior Na-doped ZnO may derive from defect complexes; while for Ag-doped ZnO, there are many defect energy levels induced by the doped Ag atom, most of which may contribute to the PL at 3.17 eV observed in experiments. We studied optical and vibrational properties of some important native defects (V_O,H_O and V_Zn,and find some typical optical absorption peaks and vibrational modes, which can be used to identify these defects. In the fourth chapter, we report the properties of N_O-H,Li_Zn-H and Cu_Zn-H complexes in ZnO. We firstly calculated the formation and dissociation of these complexes in ZnO. By computing the minimum energy paths of H diffusion, we predicted the energy barrier of the diffusions. We find that, these complexes can form below 200 K and be stable under RT. However, the dissociations of these complexes require activation energies of 1.25-1.48, 1.10-1.35 and 1.42-1.63 eV,respectively, which correspond to activation temperatures of 480-570, 420-520 and 540-620 K. Moreover, we investigated the electronic structures, defect energy levels, vibrational and optical properties of these complexes, which give further understandings of these defects in ZnO.In the fifth chapter, we investigate the instability and low efficiency of the conductivity in p-type ZnO. From analysis of possible defects in N-doped ZnO, we propose a new microscopic model to interpret the mechanism of the conductivity conversion induced by Blue/UV photo irradiation in N-doped ZnO. We find that, H_O （H is captured by V_O） plays an important role in this phenomenon. Accordingly, we predict that, if the concentration of V_O can be reduced so that the concentration of H_O will be sufficiently low, the stability of the conductivity of N-doped ZnO will beenhanced. On the other hand, we analyze some phenomena observed in N-doped ZnO from experimental studies, and speculate that the shallow acceptor levels are derived from incomplete ionization of the acceptors （N_O） in this material. From analysis of electronic structures and distributions of spin densities in N-doped ZnO, we find that the delocalized wavefunctions of defect states are the origin of incomplete ionization. Furthermore, we find that in P- and As-doped ZnO, the shallow acceptor levels are derived from incomplete ionization too. In addition, based on the proposed concept of incomplete ionization, we predicted that in Ag-doped ZnO, the acceptor levels are also shallow, being within 0.1-0.2 eV.更多还原