Research

My work is focussed on the structural changes that occur in a range of semiconductors during mechanical deformation.   My PhD studies "Nanoindentation-induced Deformation of Semiconductors" focused on bulk deformation and I am currently investigating deformation in thin film semiconductor structures including SiGe, and InP/InGaAs.

Below is a summary of the techniques I use, the materials I have studied to date, and a brief summary of my main research outcomes.

Techniques
Indentation - spherical indentation  has been favored since the stress field is more uniformly distributed under the indenter and the brittle semiconductors are prone to cracking.

Characterization techniques used include:

Atomic force microscopy
Raman microspectroscopy
Cross-section transmission electron microscopy (XTEM)
Focused ion-beam milling

In addition, novel in-situ electrical characterization technique, developed as a part of this work, has enabled the direct correlation of structural (phase) changes with features in the nanoindentation load-unload curves. 

Outcomes
The results obtained have revealed a rich array of deformation processes across the range of semiconductors examined, including several phase transformations (in Si) and twinning (Ge), dislocation slip and cracking depending on loading conditions and the material under study.  For the compound semiconductors, the response of the cubic materials (InP and GaAs) to nanoindentation is to plastically deform via the initiation and propagation of (dislocation) slip along the {111} planes. At higher loads, sub-surface cracking caused by dislocation pile-up was also revealed.  In contrast, no cracking was found after nanoindentation loading of the hexagonal materials GaN and ZnO.  XTEM of these materials revealed that the prime deformation mechanism in both GaN and ZnO is the nucleation of slip on both the basal and pyramidal planes.

The evolution of complex deformation behavior in Si during nanoindentation has been studied in detail.  On loading, diamond-cubic Si (Si-I) does undergo slip but also transforms to a metallic (Si-II) phase.  On unloading, Si-II transforms to a number of less electrically conducting phases of Si.  It is suggested that, although crystalline Si-III and Si-XII are the preferred low pressure phases during pressure release, amorphous Si is often obtained during fast unloading rates predominantly as a result of a high kinetic barrier to nucleation of the crystalline phases. Such structural changes have been correlated with fine details of spherical indentation load-unload curves to provide a comprehensive picture of the complex deformation mechanisms in Si.