Intrinsic and Extrinsic Donors and Acceptors in Zinc Oxide Crystals

技术说明本项目解决与氧化锌(ZnO)中的缺陷和杂质相关的基础材料科学问题。这项研究的总体目标是全面了解氧化锌中施主和受主的性质和行为，从而实现对氧化锌晶体生长和掺杂的改进。我们将利用电子顺磁共振、光致发光、光吸收和霍尔效应等多种技术来研究块状氧化锌晶体的光学、电学和磁性。氧化锌是一种宽禁带半导体，具有作为多功能、低成本紫外光发射体和探测器的良好前景。它也是新兴的室温自旋电子学领域的主要参与者。它具有60 meV的大激子结合能，可以很容易地生长成大的单晶，适合作为同质外延薄膜生长的衬底。然而，在氧化锌能够充分发挥其潜力之前，必须更好地了解浅施主和受主。制备n型氧化锌很容易，但很难制备p型材料。在p掺杂方面的重大进展取决于将不想要的施主的浓度降到最低，同时结合隔离的受主，从而产生所需的电学行为。这就是研究在没有故意掺杂的情况下发生在氧化锌中的施主的动机(例如，自然缺陷，如氧空位或锌间隙，或者可能是起始材料中的微量杂质)。众所周知，在锌蒸气中，温度在1100℃左右的温度下，退火易使氧化锌的电导率增加。方法将是确定增加是由氧空位或锌间隙造成的。一组平行的实验将探索在真空、氮气或其他还原气氛中退火氧化锌是否可能增加材料中锌间隙的数量，因为由于氧气的去除，在表面形成了富锌层。在该项目的第二部分，高能电子和质子辐照将产生大量的氧空位和锌空位。电子顺磁共振提供了对这些缺陷的明确识别，并允许建立其相关的吸收带和发光特征。然后，这些信息将被用来监测通过不同技术生长并在不同退火热条件下生长的氧化锌的这些缺陷。该项目的第三和第四部分涉及氮和锂等受体的电子结构以及氢在氧化锌中的作用(即评估它是浅施主还是仅仅是受体的钝化剂)。该项目的最后两部分讨论了孤立的过渡金属离子在氧化锌中的性质(例如，Mn2+、Co2+、V2+、Ni3+、Fe3+和Cu2+)以及非结构化绿色发光带的起源。通过这种方法，该项目希望确定控制氧化锌大块晶体和薄膜导电性的基本机制。非技术解释该项目涉及与电子和光子学有很强技术相关性的基础材料研究，并有效地将研究和教育结合在一起。项目活动包括在该地区的非博士机构(马歇尔大学、弗罗斯特堡州立大学、西弗吉尼亚大学理工学院、西弗吉尼亚卫斯理学院、费尔蒙州立学院和惠灵耶稣会大学)进行校园访问和演讲，将基于氧化锌的研究实例(包括演示)纳入西弗吉尼亚大学的本科生固态物理课程，并从代表性不足的群体中挑选一名本科生物理或工程学专业来参与该项目(在学年和夏季)。

TECHNICAL EXPLANATION This project addresses fundamental materials science issues associated with defects and impurities in zinc oxide (ZnO). The overall goal of the research is to obtain a complete understanding of the nature and behavior of donors and acceptors in ZnO, so that improvements in the growth and doping of ZnO crystals can be realized. Optical, electrical, and magnetic properties of bulk ZnO crystals will be studied using a variety of techniques including electron paramagnetic resonance, photoluminescence, optical absorption, and Hall effect. ZnO is a wide-band-gap semiconductor with excellent prospects to be a versatile and low-cost ultraviolet light emitter and detector. It is also a major participant in the emerging field of room-temperature spintronics. It has a large exciton binding energy of 60 meV and can easily be grown as large single crystals suitable for use as substrates in homoepitaxial film growth. However, before ZnO is able to reach its full potential, a better understanding of the shallow donors and acceptors must be developed. It is easy to produce n-type ZnO, but very difficult to produce p-type material. Significant progress in p-doping depends on minimizing the concentrations of unwanted donors and, at the same time, incorporating isolated acceptors that result in the desired electrical behavior. This is the motivation for investigation of donors that occur in ZnO without deliberate doping (e.g., native defects such as oxygen vacancies or zinc interstitials, or perhaps trace impurities in the starting materials). It is well known that the conductivity of ZnO is easily increased by annealing crystals in zinc vapor at temperatures near 1100C. The approach will be to determine whether the increase is caused by oxygen vacancies or zinc interstitials. A parallel set of experiments will explore the possibility that annealing ZnO in vacuum, nitrogen, or other reducing atmospheres increases the number of zinc interstitials in the material when a zinc-rich layer forms at the surface due to the removal of oxygen. In a second part of the project, large concentrations of oxygen vacancies and zinc vacancies will be produced by irradiation with high-energy electrons and protons. Electron paramagnetic resonance provides clear identification of such defects, and allows their associated absorption bands and luminescence features to be established. This information will then be used to monitor these defects in ZnO grown by different techniques and subjected to various anneal conditions. The third and fourth parts of the project address the electronic structure of acceptors such as nitrogen and lithium and the role of hydrogen in ZnO (i.e., to assess whether it is a shallow donor or simply a passivator of acceptors). The final two parts of the project address the properties of isolated transition-metal ions in ZnO (e.g., Mn2+, Co2+, V2+, Ni3+, Fe3+ and Cu2+) and the origin of an unstructured green luminescence band. Through this approach the project expects to identify the fundamental mechanisms that control the electrical conductivity of ZnO bulk crystals and thin films. NON-TECHNICAL EXPLANATIONThe project addresses fundamental materials research with strong technological relevance to electronics and photonics, and effectively integrates research and education. Project activities include campus visits and presentations at non-Ph.D. institutions in the region (Marshall University, Frostburg State University, West Virginia University Institute of Technology, West Virginia Wesleyan College, Fairmont State College, and Wheeling Jesuit University), the integration of ZnO-based research examples (including demonstrations) into the undergraduate solid-state physics course at West Virginia University, and the selection of an undergraduate physics or engineering major from an underrepresented group to work on the project (during the academic year and in the summer).