Intrinsic and Extrinsic Donors and Acceptors in Zinc Oxide Crystals

本项目研究与氧化锌（ZnO）缺陷和杂质相关的基础材料科学问题。本研究的总体目标是全面了解ZnO中供体和受体的性质和行为，从而实现ZnO晶体生长和掺杂的改进。使用各种技术，包括电子顺磁共振、光致发光、光吸收和霍尔效应，将研究体ZnO晶体的光学、电学和磁性能。ZnO是一种宽禁带半导体，是一种多功能、低成本的紫外光发射体和探测器。它也是室温自旋电子学新兴领域的主要参与者。它具有60 meV的激子结合能，可以很容易地生长成大的单晶，适合用作同外延薄膜生长的衬底。然而，在ZnO能够充分发挥其潜力之前，必须更好地了解浅层供体和受体。制备n型ZnO材料比较容易，而制备p型ZnO材料则比较困难。p掺杂的重大进展取决于最小化不需要的供体浓度，同时结合分离的受体，从而产生期望的电行为。这就是研究在没有故意掺杂的情况下ZnO中出现的供体的动机（例如，天然缺陷，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，在接近1100C的温度下，在锌蒸气中退火晶体可以很容易地提高ZnO的电导率。该方法将确定增加是由氧空位还是锌间隙引起的。一组平行的实验将探索在真空、氮气或其他还原性气氛中退火ZnO的可能性，当由于氧的去除而在表面形成富锌层时，会增加材料中锌间隙的数量。在该项目的第二部分，将通过高能电子和质子辐照产生高浓度的氧空位和锌空位。电子顺磁共振提供了这类缺陷的清晰识别，并允许建立其相关的吸收带和发光特征。这些信息将用于监测通过不同技术生长的氧化锌中的缺陷，并受各种退火条件的影响。该项目的第三和第四部分讨论了氮和锂等受体的电子结构以及氢在ZnO中的作用（即，评估它是浅供体还是仅仅是受体的钝化剂）。项目的最后两部分解决了ZnO中分离过渡金属离子（例如Mn2+， Co2+, V2+, Ni3+， Fe3+和Cu2+）的性质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ZnO体晶体和薄膜电导率的基本机制。本项目主要从事与电子、光子学相关的基础材料研究，有效地将科研与教育结合起来。项目活动包括校园访问和非博士演讲。该地区的机构（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、费尔蒙特州立学院和惠灵耶稣会大学），将基于zno的研究实例（包括演示）整合到西弗吉尼亚大学的本科固态物理课程中，从代表性不足的群体中选择物理学或工程学专业的本科生参与项目（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).