Indentation crystallization and phase transformation of amorphous germanium
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Indentation crystallization and phase transformation of amorphous germanium G. Patriarche1, E. Le Bourhis2, M. M. Khayyat3, and M. M. Chaudhri3 Laboratoire de Photonique et de Nanostructures, CNRS UPR 20, Route de Nozay, 91460 Marcoussis, France. 2 Université de Poitiers, Laboratoire de Métallurgie Physique, UMR 6630 CNRS, SP2MI,Téléport 2, Av M&P Curie, BP 30179, 86962 Futuroscope-Chasseneuil Cedex, France. 3 Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK. 1
ABSTRACT It has been known for about 15 years that when a Vickers indenter is loaded on to a crystalline semiconductor, such as silicon, a semiconductor to metallic phase transition occurs during indenter loading and on removal of the indenter the material within the residual indentation becomes amorphous. Here we report on a completely opposite effect: when a Berkovich or Vickers diamond indenter is loaded on to a submicrometre thick film of amorphous germanium, it densifies, crystallizes and undergoes structural phase transitions. These observations are based on transmission electron microscopy and Raman scattering investigations. It has also been shown that the indentation-induced crystallization and phase transitions occur close to the indenter tip, where the plastic strains are the highest. INTRODUCTION Silicon and germanium are of considerable technological and scientific interest. When subjected to sufficiently high hydrostatic and non hydrostatic pressures these materials undergo structural phase transitions [1-4]. It has also been known for several years that when a Vickers diamond indenter is loaded on to a silicon crystal, there is a phase transition to the metallic phase (Si-II) of silicon within the plastically deformed volume around the indenter tip. On the removal of the indenter from the specimen, the deformed silicon does not revert to the original facecentred cubic phase (Si-I), but amorphous silicon has been shown to form by some investigators [5, 6] and a mixture of amorphous and crystalline silicon by others [7]. In the case of germanium crystals, there is still a controversy as to whether there is an indentation-induced phase transition in this material [8, 9]. As regards the amorphous films of silicon and germanium, very little work has been carried on their response to indentation-induced stresses and strains [10-13]. In a recent experimental study [10], it was shown that when a 0.5 µm thick film of amorphous silicon deposited on a 2 mm thick sapphire substrate was indented at room temperature with a Vickers diamond under a load of 137 mN, the amorphous silicon was found to have transformed to Si-I (face centred cubic) and Si-III (body centred cubic) phases. Using the techniques of nanoindentation and transmission electron microscopy, we show that a 0.3 µm thick amorphous germanium film deposited on a GaAs substrate densifies and transforms to the crystalline simple tetragonal and cubic phases of germanium when it is indented with a Berkovich or Vickers diamond
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