Advanced Electron Microscopy

Measured In distribution in quantum dot

The EPSRC-funded field emission gun transmission electron microscope (FEGTEM) is a UK North-Eastern facility. In addition to providing high resolution imaging with an ability to resolve better than 0.19nm, it gives state-of-the-art nanoscale analytical capabilities covering energy dispersive X-ray microanalysis, electron energy loss spectroscopy and imaging, electron holography and high angle annular dark-field imaging. The applications of the facility have been wide-ranging.

One very important area relates to the SiGe/Si HMOS interuniversity programme, with Warwick, Newcastle, Southampton, Cambridge and Glasgow universities, Imperial College, Mitel Semiconductor, QinetiQ and Daimler-Chrysler. Here, the FEGTEM studies have provided the nanoscale distribution of Ge in SiGe MOSFET channels, together with a quantification of the `snow-ploughing´ of Ge during surface oxidation. Additional DARPA-sponsored studies with Leeds, Cambridge and Heriot-Watt universities and QuinetiQ have given detailed structural and compositional analyses of SiGe/Si multilayers prepared for the development of terahertz lasers.

Work on the characterization of low energy As+ ion implants in Si with Applied Materials have provided detailed shallow As dopant profiles for direct correlation with SIMS measurements which give distorted distributions in the near surface regions. Also, exploitation of nanometre probe convergent beam electron diffraction on the EU STREAM project with Bologna university and 5 other partners has given direct measurements of strain in ULSI shallow trench isolated MOS devices.

Extensive studies of III-V materials have been carried out. Investigations of the composition gradients within InGaAs quantum dots on GaAs (with Bonn University) have demonstrated that the larger Group III atomic species (In) accumulates at the top of each dot (see illustration), with a corresponding effect upon dot electronic behaviour. Indeed, this work has also shown for the first time that the Stranski-Krastanow epitaxial islanding transition is controlled by elemental segregation in the initially-deposited flat wetting layer. This new understanding of a process which has been a mystery for 60 years offers the prospect of enhanced control of quantum dot properties leading to optimisation of advanced device performance. Further specific work on III-nitride materials is revealing new features of layer nucleation processes and is covered elsewhere in this report.