GaN基稀磁材料中磁纳米晶-半导体纳米复合体微观结构与磁、光、电性能研究

Prompted by theoretical predictions of room-temperature ferromagnetism，a great deal of effort has been put into the investigation of the mechanisms behind the ferromagnetism of GaN based diluted magnetic semiconductors （DMS） in the past couple of decades. Indeed, a ferromagnetic response persisting up to above room temperature has been reported for GaN based DMS. However, despite the apparent agreement between promising experimental findings and the computation expectations, the origin of ferromagnetism in DMS has started to emerge as the most unsettled question in today's materials science and engineering. Especially the application of state-of-the-art nanocharacterization tools appears to reveal that some condensed magnetic semiconductor (CMS) nanocrystals might be responsible for the ferromagnetism. The kind of ferromagnetic metal/semiconductor nanocomposites offer a spectrum of so far unexplored possibilities in various fields of materials science and device physics. Hence, it will be an unexpectedly challenging issues how to visualize and control the magnetic ion aggregation to develop methods of producing lateral and vertical distributions, as well as different shapes of magnetic nanocrystals. .In this project, a series of codoping of GaN based DMS with shallow nonmagnetic impurities will be used to affect the self-assembly of magnetic crystals during the growth, and therefore modifies both the global and local magnetic behavior of the materials. Also the p-d Zener model of ferromagnetism driven by the exchange interaction between band carriers and localized spins will be further reconsidered from the level of electron state controlling by the codoping. The state-of-the-art nanocharacterization technique of atomic mumber (Z)-contrast scanning transmission electron microscopy (STEM) will be used to comprehensively investigate the incoherent image of crystals with atomic number for the ferromagnetic metal/semiconductor nanocomposites at atomic resolution. Finally，some optic-electronic-magnetic multi-physical fields coupling experiments, such as magneto-optical Kerr rotation under the application of electronical field, optic-electronic response under an application of magnetic field, and even magnetic-electronic coupling under the illumination of laser etc., will be carried out to reveal the potential new physic mechanisms in the ferromagnetic metal/semiconductor nanocomposites.

自GaN基稀磁半导体材料存在室温铁磁性理论预言以来，一直备受关注，然而至今人们对其磁性起源及调控仍存在广泛争议。最新研究发现这类材料中因掺杂离子聚集而形成磁纳米晶被认为是这类材料磁性主要来源。如何认识掺杂磁性离子纳米尺度上的聚集机理并调控由此而形成的磁纳米晶的分布、形状、大小，将是GaN基稀磁材料走向最终器件应用过程中必须解决的重要科学问题之一。本项目将通过非磁性离子共掺设计，从电子态调控层面重新认识GaN基稀磁材料中基于载流子诱导的p-d Zener铁磁性产生相关机制；采用具有原子序数衬度的扫描透射电子显微术（STEM），对实验上最近发现的磁纳米晶（铁磁金属）-半导体纳米复合体的微观结构进行进一步的系统表征和分析，最后从多参量耦合角度，探索和分析这种纳米复合体中电场调控下的磁光克尔旋转、磁场调控下的光电响应、光场调控下磁电耦合等潜在物理机制。

稀磁半导体的研究对自旋电子学的发展具有重要的意义，但近年来其发展遇到了巨大的阻碍，特别是关于半导体中的磁性是样品的本征特性还是来源于其中的磁性离子聚集存在较大的争议。本项目在此背景下，期望通过对这类材料中存在的微观结构进行系统研究，探索新物理、新现象。项目执行过程中，主要依托HVPE生长技术，系统研究了Fe掺GaN的生长技术，获得了位错密度低于106cm-2的Fe掺GaN单晶；同时对获得的Fe掺GaN单晶的微结构、光谱特性、电学特性、磁学特性进行了系统表征。本项目主要研究成果有以下几个方面：1）、通过生长界面的微纳结构和对背景载流子浓度的控制，实现了不同Fe掺浓度的GaN单晶的生长；2）、通过HRTEM观察到了HVPE生长的Fe掺GaN中在微观尺度上存在一种不稳定的Moiré条纹结构，随着电子辐照时间增加，这种Moiré条纹结构会逐渐变小直至消失，我们认为这种微观结构由Fe离子掺入GaN中替代Ga格点引入的晶格畸变和[Fe2+-VN]复合体微观结构形成共同导致的；3）通过TRPL技术进一步研究了不同Fe掺浓度的GaN单晶中的Fe离子电荷转移过程和Fe离子相关束缚激子的动力学过程，观察到一种偏离Rashba-Gurgenishvili理论的激子行为；它被认为随着Fe掺浓度增加，Fe离子状态将倾向于以具有更慢的非辐射复合寿命的Fe2+离子形式存在，并易于与氮空位[Fe2+-VN]复合体结构；4）结合磁性测量，证实[Fe2+-VN]复合体结构的形成，将有利于Fe掺GaN长程磁有序的形成，因此我们建议Fe掺GaN生长过程中，如能对氮空位形成密度进行有效调控，将是一条提高GaN基稀磁材料磁性的重要途径；5）研究了Fe掺GaN的电学特性的影响因素，发现其电学性质不仅与Fe离子浓度相关，亦与非故意掺杂的Si离子背景浓度密切相关；6）建立了一种研究Fe掺GaN单晶的声子极化激元特性的分析模型；7）本项目亦针对AlN微观结构和声子特性进行了研究，如发展了用于准声子分析的Loudon公式。这些结果不仅对进一步认识和分析GaN基稀磁材料的微观机制具有重要意义，同时本项目的实施，也进一步促进了高质量Fe掺GaN单晶的生长技术的发展。