Mechanical properties of As-grown and ion-beam-modified GaN films

The deformation behavior of as-grown and ion-beam-modified wurtzite GaN films is studied by nanoindentation with a spherical indenter. Atomic force microscopy (AFM) and cathodoluminescence are used to characterize the deformation mode. No systematic dependence of the mechanical properties on the film thickness (at least for thicknesses from 1.8 to 4 μm) as well as on doping type is observed. Results strongly suggest that (i) slips is the major contributor to the plastic deformation of crystalline GaN and (ii) slip nucleation (rather than a phase transformation) is responsible for "pop-in" events observed during loading. Indentation with an ∼ 4.2 μm radius spherical indenter at maximum loads up to 900 mN does not produce any cracking visible by AFM in crystalline GaN. Instead, under such loads, indentation results in a pronounced elevation of the material around the impression. Implantation disorder dramatically changes the deformation behavior of GaN. In particular, implantation-produced defects in crystalline GaN suppress (i) "pop-in" events during loading, (ii) slip bands observed by AFM, and (iii) the plastic component of deformation. GaN amorphized by ion bombardment exhibits plastic flow even for very low loads. The values of hardness and elastic modulus of amorphous GaN are dramatically reduced compared to those of as-grown GaN.