Investigating the structural properties of nitride-based semiconductors in the scanning electron microscope

具有高结构质量的氮化物薄膜对于优化下一代氮化物半导体器件的性能至关重要。其中包括基于algan的紫外线（UV）发光二极管（led），可用于广泛的应用，包括消毒和治疗皮肤病；基于algan的高迁移率电子晶体管，将促进紧凑型电源、微波发射器和电动汽车的生产；以及基于ingan的绿色激光器，可用于小型投影仪和激光显示器。高结构质量薄膜的生产需要理解和减少/消除结构缺陷，如晶界、螺纹位错和堆叠缺陷，这些缺陷限制了器件的性能，例如，导致led产生热而不是光，诱导电短路并限制器件的使用寿命。在这个博士项目中，Dale将应用电子背散射衍射（EBSD）和电子通道对比成像（ECCI）的扫描电子显微镜（SEM）技术来理解和优化我们来自世界各地学术界和工业界的合作者生产的材料的纳米结构。他将通过参与开发和应用新的先进sem和数据分析软件，对材料进行表征，并推进ECCI和EBSD技术的应用。当放置样品使一个或多个平面处于或接近于入射电子束的布拉格角时，可以产生ECCI显微照片。由于局部应变引起的晶体取向或晶格常数的任何偏差，都会在所得的ECCI显微照片中产生对比度的变化。取向和应变的极小变化是可检测的，例如，揭示低角度倾斜和旋转边界和原子步骤，并使扩展缺陷，如位错和层错能够成像。在EBSD中，样品与入射电子束的法线倾斜约70度。碰撞电子通过高角度散射，形成可衍射的发散电子源。由此产生的电子背散射衍射图（EBSP）由大量重叠带组成，称为菊池带，它们与晶体结构的二维投影密切相关。例如，晶体取向的变化可以通过从样品上的点网格获取ebsp来绘制。EBSD是一种成熟的织构分析和量化晶界和晶相的技术。基于互相关分析的EBSPs的引入也使得相对应变、晶格倾斜和扭曲以及晶体极性的测量成为可能。斯特拉斯克莱德大学的研究人员率先将这些技术应用于氮化薄膜的表征，目前正在与学术和工业研究人员合作，以支持新型氮化材料的开发。目前的合作者包括：Peter Parbrook教授，爱尔兰科克大学学院廷德尔国家研究所；Michael Kneissl教授，德国柏林工业大学Sylvia Hagedorn博士，德国柏林费迪南-布劳恩研究所；Ferdinand Scholz教授，德国乌尔姆大学Philippe venngu<e:1>博士，法国瓦尔邦纳CRHEA-CNRS；英国谢菲尔德大学王涛教授；英国巴斯大学Philip Shields博士；David Wallis教授，英国卡迪夫大学和剑桥大学；IQE欧洲有限公司；欧司朗光电半导体，德国雷根斯堡。我们还与国家物理实验室的Ken Mingard博士合作；Philippe venngu<e:1>博士，法国瓦尔邦纳CRHEA-CNRS；法国TESCAN和德国汉诺威激光中心（Laser Zentrum Hannover）的Aimo Winkelmann教授就EBSD和ECCI技术的发展进行了合作。

Nitride thin films exhibiting high structural quality are crucial for optimising the performance of next generation nitride semiconductor-based devices. These include AlGaN-based ultraviolet (UV) light emitting diodes (LEDs) which can be used for a wide range of applications including sterilisation and treatment of skin disease; AlGaN-based high mobility electron transistors which will facilitate the production of compact power supplies, microwave transmitters and electric cars; and InGaN-based green lasers which can be used in compact projectors and laser displays. The production of high structural quality thin films requires the understanding and reduction/elimination of structural defects such as grain boundaries, threading dislocations and stacking faults, which limit device performance, e.g., lead to the generation of heat rather than light in LEDs, induce electrical shorts and limit device lifetime.In this PhD project Dale will apply the scanning electron microscope (SEM) techniques of electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) to both understand and optimise the nanostructure of materials produced by our collaborators from both academia and industry from across the world. He will both characterise materials and advance the applications of the ECCI and EBSD techniques through engagement in the development and application of new advanced SEMs and data analysis software.ECCI micrographs may be produced when a sample is placed so that a plane or planes are at, or close to, the Bragg angle with respect to the incident electron beam. Any deviation in crystallographic orientation or in lattice constant due to local strain, will produce a variation in contrast in the resultant ECCI micrograph. Extremely small changes in orientation and strain are detectable, revealing, for example, low angle tilt and rotation boundaries and atomic steps and enabling extended defects such as dislocations and stacking faults to be imaged.In EBSD the sample is tilted at around 70 degrees to the normal of the incident electron beam. The impinging electrons are scattered through high angles forming a diverging source of electrons which can be diffracted. The resultant electron backscatter diffraction pattern (EBSP) consists of a large number of overlapping bands, known as Kikuchi bands, which are closely related to a 2-D projection of the crystal structure. Changes in crystal orientation for example, can be mapped by acquiring EBSPs from a mesh of points on a sample. EBSD is a well-established technique for texture analysis and for quantifying grain boundaries and crystal phases.The introduction of cross-correlation based analysis of EBSPs has also made possible measurements of relative strain, lattice tilts and twists and crystal polarity.Strathclyde researchers have pioneered the application and combination of these techniques for the characterisation of nitride thin films and are presently collaborating with both academic and industrial researchers to support the development of novel nitride materials. Present collaborators include: Prof. Peter Parbrook, Tyndall National Institute, University College Cork, Ireland; Prof. Michael Kneissl, TU Berlin, Berlin, Germany; Dr Sylvia Hagedorn, Ferdinand-Braun-Institut, Berlin, Germany; Prof. Ferdinand Scholz, Ulm University, Ulm, Germany; Dr Philippe Vennéguès, CRHEA-CNRS, Valbonne, France; Prof. Tao Wang, University of Sheffield, UK; Dr Philip Shields, University of Bath, UK; Prof. David Wallis, Universities of Cardiff and Cambridge, UK; IQE Europe Ltd; and OSRAM Opto Semiconductors, Regensburg, Germany.We are also collaborating with Dr Ken Mingard at NPL; Dr Philippe Vennéguès, CRHEA-CNRS, Valbonne, France; TESCAN, France and Prof Aimo Winkelmann at Laser Zentrum Hannover e.V., Hannover, Germany on the development of the EBSD and ECCI techniques.