ZnO外延薄膜的p型掺杂及自补偿机理研究

As an important wide bandgap semiconductor, ZnO has been studied extensively due to its potential applications in next-generation short-wavelength optoelectronic devices. However, a formidable challenge in growth of reliable and repeatable p-type ZnO materials severely hampered its progress in optoelectronic device applications. It has been well established that either dopant-induced defects such as Li interstitial, or native defects like Zn interstitial and oxygen vacancy, will result in a strong self-compensation effect and make it difficult to realize stable and reproducible p-type conductivity in ZnO. However, the nature and interaction of various defects as well as the mechanism of their compenstation effects on acceptors haven't been clealy understood and solved yet. Therefore, it is very essential to explore these critical issues and provide a scientific guidance for further research in ZnO-based materials and devices. In this project, we will carry out the invesigation of p-type doping and self-compensation effect in high-quality ZnO single crystalline films prepared by radio-frequency plasma-assisted molecular beam epitaxy technique, which can exclude the unnecessary influence from impurities in hydrothermally grown ZnO bulk crystals. Some advanced analysis methods will be applied to carefully determine the physical properties, including secondary ion mass spectroscopy, photoluminescence, Raman scattering spectroscopy, Rutherford back scattering spectroscopy, Hall effect measurement and positron annihilation spectroscopy. We will firstly focus on verifing the speces and conducting natures of different native defects and dopants-induced defects, then try to figure out their correlations and the mechanism of self-compensation effects in ZnO, and finally develop effective routes to solve these problems and obtain stable p-type ZnO films with low conductivities. As the first experimental testing of doping effects in ZnO, Na-H and Li-H will be adopted as codoping sources, and the role of H element in elevating the Fermi level and promoting the substitution of Na and Li for Zn will be studied in detail. The strong self-compensation effect in highly doped ZnO is hence expected to be surpressed due to the incorporation of extra H donors. As and Sb have been proposed to occupy Zn lattice sites and form special acceptor-type complexes which contribute to p-type conductivities, but the microstructures and defect correlations haven't been well exploited experimentally so far. We will perform a systematic investigation on these important topics and try to find new approaches to optimize the doping efficiencies.

ZnO是一种非常有应用前景的短波长光电子材料，但是p型掺杂难题严重地阻碍了它在光电子器件方面的应用和发展，其中关键的科学问题- - 缺陷之间的相互作用以及缺陷对受主的补偿机理等物理难题尚未得到解决，澄清这一点对于ZnO研究的顺利开展非常重要。本项目拟利用射频等离子体辅助分子束外延技术，结合二次离子质谱、光致发光谱、拉曼散射光谱、卢瑟福背散射谱、霍尔测试、正电子湮灭谱等丰富的表征测试手段，在获得高质量ZnO单晶薄膜的基础上,采用Na-H/Li-H共掺、As/Sb单掺两种技术方案，系统开展ZnO的p型掺杂、缺陷控制及自补偿机理研究。通过理论计算和实验相结合的方式，深入探讨ZnO单晶薄膜中本征缺陷以及p型掺杂引入缺陷的类型、形成机制以及电子结构，研究缺陷之间的相互作用以及自补偿效应产生的机制，探索抑制自补偿的有效办法和实验手段，优化掺杂方案，获得国际公认的、可重复可检验的器件质量p型ZnO薄膜。

项目组完全按照计划书开展工作，在以下几个方面均获得了重要进展：.（1）发展了一种研究半导体材料本征点缺陷的方法—同位素示踪技术，即通过监控同位素原子在同位素异质结界面处的扩散过程，得到原子自扩散行为的变化，从而反映出点缺陷的能量学特性。系统研究了ZnO中重要点缺陷的能量学特性。证明了VO是ZnO中主要的点缺陷，它导致了非化学计量比。研究结果还表明VO为浅施主，是ZnO中n型导电性的主要来源；通过深入研究杂质或本征缺陷引起的费米能级钉扎对Zn原子扩散行为的重要影响，澄清了Ib族元素----Cu在进行p型掺杂时引起的自补偿效应及其内在机制。这一工作对深入理解本征点缺陷的行为特征，揭示其自补偿机理，解决p型掺杂核心科学问题具有重要的意义，是近年来国际氧化锌材料基础研究领域中的一个重大进展，而且该研究方法可以推广到其他氧化物材料的本征点缺陷研究中。.（2）深入探索与研究了费米能级的调节对Na单掺及Na-H和Na-F共掺引入的缺陷种类及p型掺杂自补偿机理的重要影响。我们发现，相比H而言，F杂质可以更为有效地将费米能级提高到导带底附近，从而使得薄膜中的Na原子浓度大幅度提高；但是高温退火后F和Na会同时逸出，说明两者之间的库仑相互作用很强，如何打断它们之间的键合是实现薄膜中高替位Na浓度、获得有效p型导电的核心问题。利用富氧条件及共掺工艺增加了NaZn固溶度，抑制了Nai自补偿的产生，获得了p型ZnO薄膜，其空穴浓度为2.6x1016cm-3，迁移率为4.5cm2/Vs。.项目在研期间共发表SCI论文20篇(其中2篇Phys. Rev. B、3篇Sci. Rep.、2篇Appl. Phys. Lett.)，新申请专利11项，获授权专利12项，取得了多项重要的研究成果，为推动ZnO基薄膜从基础研究走向真正的器件应用奠定了良好的基础。