Deformation under nanoindents in Si, Ge, and GaAs examined through transmission electron microscopy
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Cross sections through nanoindents on Si, Ge, and GaAs {001} were examined through transmission electron microscopy. A focused ion beam workstation was used to machine electron transparent windows through the indents. In both Si and Ge there was a transformed zone immediately under the indent composed of amorphous material and a mixture of face-centered-cubic and body-centered cubic crystals. Cracking and dislocation generation were also observed around the transformed zone. In GaAs the dominant deformation mechanism was twinning on the {111} planes. The hardness of these materials is discussed in light of these observations and their macroscopic material properties such as phase transformation pressure.
Nanoindentation is a very convenient way to study the resistance to local deformation in a hard or brittle material, such as may occur during abrasion or wear.1 However, obtaining a full understanding of the processes that occur requires the use of techniques such as transmission electron microscopy (TEM), which can image deformation induced defects such as dislocations, twins, and phase transformations at an atomic scale. In the past the application of TEM has generally been limited to samples deformed homogeneously because of the difficulty in preparing an electron-transparent specimen from a particular site in the specimen. Now, with a focused ion beam (FIB) microscope, it is possible to make a thin section through a specified local region such as a nanoindent.2 In the FIB a beam of Ga ions sputters material away, and the secondary electrons emitted during this process are used to image the specimen. In this way an electron-transparent cross section of uniform thickness can be machined through the indent,3 as described in more detail below. In this paper we examine nanoindents in three undoped semiconductor single crystals: Si, Ge, and GaAs. The indentation behavior of Si has been widely studied because of interest in the high-pressure phases that are induced.4 While high-pressure studies show that similar behavior is expected in Ge, III-V semiconductors, such as GaAs, do not display the same range of phase transformations.5 A line of indents were made in each specimen (indented in the {001} surface) with a Nanotest 600 (Micro Materials Ltd., Wrexham, UK) machine with a Berkovich diamond indenter under an applied load of 50 mN. To aid sample preparation some larger indents were also J. Mater. Res., Vol. 16, No. 12, Dec 2001
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made in a line either side of the 50 mN indents to act as easily visible markers. Some specimens were made with the smaller indents spaced at intervals of 30 m (to ensure the indents did not interact with each other) and other specimens were made with an indent spacing of 5 m (to allow several indents to be examined in the same thin window in the TEM). The specimens were then ground to approximately 50 m and attached to Cu grid before thinning in a FIB. A schematic diagram of the specimen is shown in Fig. 1. The indented surface was
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