Every aspect of basic nanoscale science, as well as the commercial production of nanotechnologies, is dependent upon the capacity of instruments and methodologies to measure, sense, fabricate, and manipulate matter at the nanoscale. Microscopy has the advantage over other characterisation techniques, e.g., bulk spectroscopy or electrical testing, in that it is descriptive, producing images of objects that are directly related to their structure, morphology, and composition. Within MCAG we use advanced imaging and manipulation techniques to understand the fundamentals of growth, processing and integration of nanomaterials into functional devices with applications in electronics, photonics and life-sciences.
Observing processes “on site” as they are occurring and under changing external stimuli is the paramount goal of in-situ techniques. Various in situ and operandi techniques have emerged and are gaining importance in different areas of science and engineering. In the field of nanoscience and nanotechnology, there are only a handful of techniques that can merge extreme spatial resolution with the possibility of in-situ real-time detection. Among them, in situ electron microscopy is probably the most versatile and mature technique.
In MCAG we use an in-situ heating TEM stage to follow nanowire growth processes, such as re-crystallisation after ion-beam damage, reactive metal/Ge nanowire re-growth as well as monitoring thermal stability of nanowire silicides and germanides, metal/semiconductor nanoparticle sintering etc. Hence, we can obtain information about the kinetics of the associated processes to devise nanowire growth and processing conditions, solve contact formation and device reliability issues.
In-situ TEM heating stage for following re-crystallisation, reactive growth, sintering, etc.
We use electrical nanocontacting capabilities on TEM and SEM/FIB instruments to study processes such as electromigration induced mass transport (EIMT), resistive heating, and dielectric breakdown, addressing the reliability of nanowire interconnects. We have developed contacting and mechanical stressing strategies to understand the electrical properties of piezoelectric nanowires as well as to obtain piezoresistance coefficients of Si and Ge nanowires.
In-situ electrical/optical TEM stage for following nanowire devices as they operate.
In-situ electrical probing within SEM/FIB for following EBID, reliability of interconnects, etc.
The weak-beam dark field method (WBDF) is the one of the most powerful method available for studying the detailed geometry of individual lattice defects. It enables the positions of dislocation cores to be determined to an accuracy of better than 1 nm, and allows the geometry of dislocation interactions to be studied with greatly increased resolution compared with previous electron microscope methods. Within MCAG we used WBDF methods to study crystal defects and their distribution in III-Ns materials, in Ge and SiGe after ion beam damage and in grown Ge nanowires.
Weak-beam dark field technique applied in studying dislocations in III-Ns and Ge nanowires after ion irradiation.
Conventional electron microscopy produces images that are two dimesionsl (2D) representation of three dimensional (3D) objects. Electron nanotomography, also known as 3D electron imaging, enables researchers to visulaise the real 3D shape of nanomaterials. At MCAG we have developed serial sectioning SEM/FIB tomography for 3D visuslisation of materials and devices relevant to electronics, photonics and life sciences. Briefly, serial sectioning SEM/FIB nanotomography uses a finely controlled ion beam to mill away thin sections into a material’s volume, whilst high resolution SEM is used to image obtained section at an oblique angle. The set of images obtained can be used to reconstruct a 3D reperesntion of the whole volume of the milled material. The figure below shows an example of the application of this technique in solving performance issues with nanowire transistors produced by CMOS-compatible top down fabrication. This is of particular importnace to microelectronics fabrication as the latest 22-nm note devices have truly 3D topology.
3D imaging of CMOS fabricated nanowire FET devices for solving performance issues.
The ability to impart changes in the structure of crystalline materials by ion or electron manipulation down to single atoms holds great promise in creating new materials with functionalities that have never before been observed. In MCAG we use focused electron and ion beam to impart nanoscale alternations in nanowires. For example, ion beam induced changes in Ge nanowires were studied by innovative step-wise ion-beam irradiation/TEM imaging technique to visualise structural transformations in nanowires during ion-beam doping.
Step-wise Ga-ion irradiation and TEM imaging following ion beam doping in Ge nanowires.
Petkov, N. ‘In Situ Real-Time TEM Reveals Growth, Transformation and Function in One-Dimensional Nanoscale Materials: From a Nanotechnology Perspective’ in ISRN Nanotechnology. 2013, article ID 893060.
O’Regan, C.; Biswas, S.; O’Kelly, C.; Jung, S. J.; Boland, J. J.; Petkov, N.; Holmes, J. D. ‘Engineering the growth of germanium nanowires by tuning the supersaturation of Au/Ge binary alloy catalysts’ Chem. Mater. 2013, 25, 3096-3104.
Andzane, J.; Petkov, N.; Livshits, A. I.; Boland, J. J.; Holmes, J. D.; Erts, D. ‘Two-terminal nanoelectromechanical devices based on germanium nanowires’ Nano Lett. 2009, 9, 1824-1829.
Conroy, M.; Zubialevich, V. Z.; Li, H.; Petkov, N.; Holmes, J. D.; Parbrook, P. J. ‘Epitaxial lateral overgrowth of AlN on self-assembled patterned nanorods’ J. Mater. Chem. C 2015, 3 (2), 431-437.
Kelly, R. A.; Holmes, J. D.; Petkov, N. ‘Visualising discreet structural transformations in germanium nanowires during ion beam irradiation and subsequent annealing’ Nanoscale 2014, 6 (21), 12890-12897.
Phelan, R.; Holmes, J. D.; Pekov, N. ‘Application of serial sectioning FIB/SEM tomography in the comprehensive analysis of arrays of metal nanotubes’ J. Microscopy 2012, 246, 33-42.