- 05 September 2006 -

Research: Nanoindentation of GaN-on-sapphire for improved optoelectronics

In a recent issue of Materials Chemistry and Physics journal* a ‘pop-in’ phenomenon during nanoindentation in epitaxial GaN thin films on c-plane sapphire substrates was reported.



Amplitude mode AFM image showing that the residual impression of the indentation for the penetration depth of 125 nm.

Successful fabrication of optoelectronics based on thin films requires an understanding of the mechanical properties of this material as well as its optical and electrical properties. This is of special concern since heteroepitaxy using typical substrates (e.g., sapphire) involves a high lattice mismatch.

Also, compared to bulk single crystals, the deformation properties of thin films can differ because they strongly correlate to the geometrical dimensions and the materials’ defect-structure.

GaN thin films on sapphire substrates have a large lattice mismatch (about 14.5%) causing in-plane tensile strain in the GaN layers. It can also result in wafer bowing when cooled down from the growth temperature. As misfit dislocations at the interface degrade device performance (carrier mobility, luminescence efficiency, etc.), it is of interest to study the mechanical properties and a very good way to achieve this is through the nanoindentation technique.

The new work was reported by R. Navamathavana, and co-authors, from Nanophotonic Semiconductors Laboratory, Gwangju Institute of Science and Technology, and Chemical Metrology and Materials Evaluation Division, Korea Research Institute of Standards and Science, Republic of Korea.

There have been some reports on the nanoindentation studies of bulk single crystal and epitaxial thin films of GaN by different groups. During indentation loading of GaN film, a discontinuity (so called ‘pop-in’ or “burst”) in the load–displacement curve has been observed by some workers. Twinning, slip band movement and dislocation nucleation and mechanisms have been proposed to explain this ‘pop-in’ event. However, the occurrence of ‘pop-in’ event observed in some of the published work is still a matter of debate and remains unclear. So, Navamathavana report how this ‘pop-in’ phenomenon occurred in heterostructures and the possible mechanism.

The new studies involved undoped and doped epitaxial GaN thin films with a range of thicknesses (1–4-microns) grown on c-plane sapphire substrates by MOCVD. Multiple discontinuities – the so-called ‘pop-in’ events, were observed in the load–indentation depth curve irrespective of the thickness as well as the doping condition.

AFM studies on the residual indentation impression revealed no micro-cracks even after the indentation beyond the critical depth. The authors consider the physical mechanism responsible for the ‘pop-in’ is explained by the interaction of the deformed region, produced by the indenter tip, with the pre-existing threading dislocation in the epitaxial GaN thin films.

The work was supported by a grant from the program on the National Research Laboratory for Nanophotonic Semiconductors and ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier R&D Programs’ of the Ministry of Science and Technology, Korea.

*Vol 99, Issues 2-3, 10 October 2006, pp 410-413.

n_mathavan@yahoo.com