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

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.