Nanoscale characterisation of nitride semiconductor thin films using EBSD, ECCI, CL and EBIC.

本项目的目的是产生一个步骤的变化，在性能的扫描电子显微镜技术的电子背散射衍射（EBSD），电子通道对比度成像（ECCI），并利用这些相对较新的技术与阴极发光成像（CL）和电子束感应电流（EBIC）的特性，因此改善氮化物半导体薄膜。此类材料用于制造UV/蓝色激光二极管、UV/可见光LED和白色LED。氮化物激光二极管目前正在主导下一代DVD的发展以及打印和彩色复印的发展。氮化物LED的当前应用从街道照明延伸到移动的电话中的背光照明，再到交通灯。氮化物基LED的未来应用有望彻底改变家庭和办公室的照明。氮化物也被开发用于生产高频、大功率电子器件。EBSD是一种有吸引力的技术，用于询问材料的晶体学性质，因为它可以提供数十纳米分辨率的晶体取向、多型体和应变信息。在EBSD中，电子束入射到以通常70度的角度倾斜的样品上。撞击的电子通过高角度非弹性地散射，形成可以被衍射的发散电子源。对电子背散射衍射图样（EBSP）的形成的简单描述假定满足给定平面的布拉格条件的电子从该平面的上表面和下表面在衍射锥中发射。当这些锥体与荧光屏相交时，观察到菊池线。EBSP由大量重叠的菊池带组成，是晶体结构的二维投影。晶体的旋转将产生EBSP的旋转;晶体的倾斜将产生EBSP的移位，应变将产生EBSP的变形。从样品上的点的网格获取的EBSP将产生该样品中的倾斜、旋转或应变的图。在我们的研究过程中，我们的目标是获得更高水平的灵敏度（低于万分之一），以获得氮化物半导体中晶体织构和应变分布的详细高分辨率信息。ECCI可以用来揭示半导体薄膜中的单晶缺陷，如位错。在ECCI中，从适当取向的样品背散射的电子的强度取决于晶体中平面的相对取向。由于应变导致的晶体取向的变化或晶格常数的变化通过图像的灰度级的变化来揭示，所述图像是通过监测当电子束在样品上扫描时背散射电子的强度而构建的。由于晶格平面倾斜和它们产生的应变，缺陷被成像。我们的目标是开发ECCI技术，使其可以用于完全消除我们薄膜中存在的所有缺陷。我们计划应用这些强大的技术来优化氮化物半导体薄膜的质量，包括：（i）非极性和半极性氮化物薄膜，（ii）外延过生长氮化物薄膜和生长在天然衬底上的氮化物薄膜，（iii）高铝含量氮化物薄膜（包括生长在硅上的薄膜），（iv）高铟含量氮化物薄膜，和（v）氮化锌薄膜。这种新颖的氮化物半导体结构将允许生产更高功率的LED，并将其波长范围扩展到红色和红外线。他们还将为氮化物基器件开辟新的应用-例如太阳能电池。在整个项目中，我们还将与工业合作伙伴建立新仪器开发和商业化的最佳路线，以及氮化物薄膜生长工艺。

The aim of this project is to produce a step change in the performance of the scanning electron microscopy techniques of electron backscatter diffraction (EBSD), and electron channelling contrast imaging (ECCI), and exploit these relatively new techniques together with cathodoluminescence imaging (CL) and electron beam induced current (EBIC) for the characterisation and hence the improvement of nitride semiconductor thin films. Such materials are used in the manufacture of UV/blue laser diodes, UV/visible LEDs and white LEDs. Nitride laser diodes are presently dictating the development of next generation DVDs and developments in printing and colour copying. Present applications of nitride LEDs extend from street lighting, to back lighting in mobile phones, to traffic lights. Future use of nitride-based LEDs promises to revolutionise lighting in the home and office. Nitrides are also being developed for the production of high frequency, high power electronic devices.EBSD is an attractive technique with which to interrogate the crystallographic properties of materials because it can provide information on crystal orientation, polytype and strain with a resolution of tens of nanometres. In EBSD an electron beam is incident on a sample which is tilted at an angle of typically 70 degs. The impinging electrons are scattered inelastically through high angles forming a diverging source of electrons that can be diffracted. A simple description for the formation of an electron backscatter diffraction pattern (EBSP) presumes that electrons that satisfy the Bragg condition for a given plane emanate in diffraction cones from both the upper and lower surfaces of that plane. When these cones intersect a phosphor screen Kikuchi lines are observed. An EBSP consists of a large number of overlapping Kikuchi bands and is a 2-D projection of the crystal structure. Rotation of a crystal will produce a rotation of the EBSP; a tilt of a crystal will produce a shift in the EBSP, strain will produce distortion of the EBSP. EBSPs acquired from a mesh of points on a sample will produce a map of tilt, rotations or strain in that sample. In the course of our research we aim to attain much improved levels of sensitivity (less than 1 part in 10,000) to changes in tilt, rotation and strain to obtain detailed high resolution information on the crystallographic texture and strain distribution in nitride semiconductors.ECCI can be used to reveal single crystallographic defects such as dislocations in semiconductor thin films. In ECCI the intensity of electrons backscattered from a suitably oriented sample depends on the relative orientation of planes in a crystal. Changes in crystallographic orientation or changes in lattice constant due to strain are revealed by changes in grey scale of an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over the sample. Defects are imaged due to lattice plane tilting and strain they produce. Our aim is develop the ECCI technique so that it may be used to fully characterise all defects present in our films.We plan to apply these powerful techniques to optimise the quality of nitride semiconductor thin films, including: (i) non-polar and semi-polar nitride thin films, (ii) epitaxially overgrown nitride thin films and nitride thin films grown on native substrates, (iii) high aluminium content nitride thin films (including films grown on silicon), (iv) high indium content nitride thin films, and (v) zinc blende nitride thin films. Such novel nitride semiconductor structures will allow the production of higher power LEDs and extend their wavelength range to the red and infrared. They will also open up new applications for nitride-based devices - solar cells for example.Throughout the project we will also establish with industrial partners the best routes for development and commercialisation of new instrumentation, and nitride thin film growth processes.