Nondislocation Origin of GaAs Nanoindentation Pop-In Event

The present Letter demonstrates a pop-in event that is caused by a nanoindentation-induced phase transformation in GaAs, and not accompanied by any dislocation nucleation. Our computer simulations reveal the appearance of the new phase, documented by the structural correlation functions and visualization of the atomic positions. This challenges the orthodox view that the initial pop-in event reflects nucleation of dislocations or their movement, and has a bearing on materials where dislocation activity is not present.

Figure 1

The variation in potential energy $E$ of a GaAs crystal deformed in the course of consecutive depth increments $\Delta h$ of penetration into the (001) plane (a), and the nanoindentation load-depth $F–h$ data (b) obtained by the derivation of the $E(h)$ relationship [$F(h)=dE/dh$]. The singularity $S$ at the depth $h_{\mathrm{pop}}=3.12\AA$ marks transition from a parabolic (green) to linear (red) $E(h)$ relationship (a) and denotes the starting point of GaAs pop-in (b) that occurs under the force mean value $F_{\mathrm{pop}}=140\mathrm{nN}$.

Figure 2

The RDF (a),(c) and BADF (b),(d) functions determined for the entire deformed area of the GaAs crystal (the indentation depth $h_{\max }=12.12\AA$) (a),(b) as well as a restricted volume ($U$ domain) located directly under the indenter (c),(d). The former (a),(c) define the predominant Ga-As distance, while the latter (b),(d) the bonding angle. Green lines mark the locations of the peaks for an undistorted GaAs zinc-blende structure, while the BADF-peak, in the position of 90°, detected in a deformed solid, is typical a GaAs rocksalt lattice. The marked peak observed for an interatomic distance of 3.3 Å (c) and, more importantly, the preferred concentration of bond angles around 90° (d) shows that, in the highly stressed $U$ domain, one is dealing with a new phase of GaAs.

Figure 3

Direct visualization of: the atomic positions determined for a nanoindented ($h_{\max }=12.12\AA$) GaAs crystal viewed along the [111] direction (a), the orientation of the GaAs rocksalt unit cell with respect to the zinc-blende lattice (b), and the GaAs rocksaltlike structure generated under the acting indenter (c). The ${\mathrm{GaAs}}^{\prime }$ ordering resembles that of a GaAs rocksalt structure (b). The $[\overline{1}\overline{1}\overline{1}]$, $[1\overline{1}0]$, and $[\overline{1}\overline{1}2]$ directions of the zinc-blende configuration (b) define the principal vectors $\overrightarrow{a}$, $\overrightarrow{b}$, and $\overrightarrow{c}$ of a rocksalt unit cell, respectively. The ${\overrightarrow{c}}^{\prime }$ vector coincides with the $[\overline{1}\overline{1}1]$ direction for a rocksaltlike arrangement (c) and forms the angle of 70.5° with the direction, in accord with the BADF analysis [Figs. 2b, 2d]. IM defines an intermediate region with a mixed arrangement of Ga (magenta) and As (yellow) atoms.

Figure 4

Depth dependence of a number of bond angles, between 80° and 95°, characteristic of a GaAs rocksalt structure. The obtained relationship reflects the depth-resolution of the total potential energy $E(h)$ [Fig. 1a], supporting our concept of GaAs pop-in behavior as having its origin in phase transformation.