Subsurface deformations induced by a Vickers indenter in GaAs/AlGaAs superlattice

Abstract

During the past century, indentation mechanics has been extensively explored. This effort led to the development of instrumented microand nanoindentation techniques that have proved to be of first importance in materials science research. As reviewed recently by Tabor [1], indentation mechanics straightforwardly used today was developed mainly in view of experimental results found on metals [2–4]. Therefore, the use of indentation theory for other materials (for instance ceramics) assumes that plastic-flow characteristics are not much affected. Furthermore, Chaudhri showed using pearlite grains as markers that the measured plastic strains in heavily work-hardened mild steel deformed by indenters of various geometries were much higher than previously thought [5]. In a more recent work, the same author used the hardness-strain relation to map the strain field in high-conductivity copper deformed by spherical indenters [6]. He showed then that the strain-contour geometry was more complex than what had been observed previously in other metals [2]. Determination of the strain field under a Vickers indenter in semiconductors was studied recently in a GaAs/AlAs superlattice where interfaces were used as markers [7]. However, severe delamination occurred at the interfaces. In this paper, we have deformed a GaAs/AlGaAs superlattice by a Vickers indenter and have used focused-ion beam (FIB) technique to prepare cross-sectional thin foils through the center of the indent site for transmission electron microscopy (TEM) observation. These techniques let us get information on the plastic and brittle deformations underneath the indent site and allow us to determine the strain-field map. Superlattices were grown on (001) surfaces of GaAs single crystals by metal-organic vapor-phase epitaxy at 650 ◦C. GaAs and AlGaAs layers (30 of each) were grown alternatively to form a structure about 7.6 μm thick (period 254 nm). The composition of the Alx Ga1−x As (x = 0.85) alloy was chosen so that the misfit with GaAs was only e = 0.13% and the structure was almost lattice matched. The samples were deformed by a Vickers diamond pyramid at room temperature under 500 mN for 30 s. They were set in the indenting machine in such a way that the diagonals of the indentations were along a 〈110〉 direction. The central zone of the indent site was protected from the gallium FIB by a tungsten thin film. Two trenches were milled with a 30 kV FIB in such a way that a thin wall was left containing the selected zone that was transparent to the electron beam as described elsewhere [8]. The periodical structure of the superlattice can be observed in Fig. 1. AlGaAs layers appear in clear contrast while GaAs layers appear in dark contrast. At the bottom of the superlattice, the structure was started by an AlGaAs layer and a GaAs layer about 510 and 235 nm thick respectively. It should be noted that during the milling two layers on top of the structure on the left-hand side of the indent were lost despite the tungsten protection. The indent site is situated on the top of the median crack (observed perpendicularly to the indented surface). Curvature of the surface at the indent site and of layers underneath is obvious and is the result of the plastic deformation generated by the indenter. The curvature of the layers was observed to decrease with depth. For instance, it is about 158◦ for the third GaAs layer and about 176◦ for the 41st layer below. Interestingly the value found in the vicinity of the indent site (158◦) differs from the Vickers pyramid angle (148◦) because of the elastic recovery of the sample while the indenter was unloaded [8]. Using a higher-magnification image just beneath the indent site (Fig. 2), converging slip bands could be observed. They are inclined to the indented surface by about 54◦ and correspond to the gliding of dislocations in the {111} slip planes. On a higher contrast image (Fig. 3) slip bands in {111} planes diverging from the indent site could also be observed. Such a plastic-flow geometry is now well established for bulk semiconductors and was shown to operate here [8–11]. The slip bands correspond to the gliding of dislocations with Burgers vectors inclined to the indented surface that generates deformation of the layers on their path [8–10]. This finally results in the curvature of the superlattice that was described above. At the intersection of the converging slip bands (underneath the indent site and along the load axis direction), we observed a median crack that crossed the layers. This crack was propagated straight into the GaAs substrate on 4 μm. A median-radial crack system is commonly observed in brittle material under sharp contact [12]. Here the crack was initiated at