Future studies

There are several issues arising from this thesis that could form the basis of future studies.

 Firstly, in this work it has been shown that nanoindentation-induced phase transformations in a-Ge occurs but a-Ge appears to transform differently varying film thickness. Tip geometry may play an important role as has been found for dc- Ge. The deformation of thin a-Ge films clearly and consistently shows two pathways and it would be interesting to examine such behaviour (in a-Ge) under different tip geometries and size of spherical indenters.

 It is also important to follow up on the suggestion that the transformation from (β- Sn) to dc-Ge in the extruded region triggers explosive crystallization. This could be done using different thicknesses of a-Ge (since there is expected to be a thickness dependence). Also different tip geometries and loading/unloading rates could assist in probing this phenomenon.

 The occurrence or absence of a pop-out event requires further study.

 Indenting dc-Ge at low temperature may lead to phase transformations favoured over deformation by slip and twinning. In such cases it would be interesting to compare dc-Ge and a-Ge behaviour.

 Investigating the temperature dependence of deformation in Ge (amorphous and crystalline forms) would be illuminating, similar to the studies carried out for Si [21].

 Finally, it would also be of interest to study III-V covalent and other semiconductors in both crystalline and amorphous forms to see if their deformation behaviour is similar to that of dc-Ge and a-Ge in the current study.

[1] G. Gotz, “Explosive crystallization processes in silicon,” Appl. Phys. A Solids Surfaces, vol. 40, no. 1, pp. 29–36, 1986.

[2] T. Takamori, R. Messier, and R. Roy, “Phenomenology of the ???explosive??? crystallization of sputtered non-crystalline germanium films,” J. Mater. Sci., vol. 8, no. 12, pp. 1809–1816, 1973.

[3] T. Takamori, “New Noncrystalline Germanium which Crystallizes

``Explosively’' at Room Temperature,” Appl. Phys. Lett., vol. 20, no. 5, p. 201, 1972.

[4] H. J. Leamy, “Explosive crystallization of amorphous germanium,” Appl. Phys. Lett., vol. 38, no. 1981, p. 137, 1981.

[5] M. Mulato, D. Toet, G. Aichmayr, P. V. Santos, and I. Chambouleyron, “Laser crystallization and structuring of amorphous germanium,” Appl. Phys. Lett., vol. 70, no. 26, p. 3570, 1997.

[6] D. J. Oliver, J. E. Bradby, S. Ruffell, J. S. Williams, and P. Munroe,

“Nanoindentation-induced phase transformation in relaxed and unrelaxed ion- implanted amorphous germanium,” J. Appl. Phys., vol. 106, no. 9, 2009. [7] A. Kailer, K. G. Nickel, and Y. G. Gogotsi, “Raman microspectroscopy of

nanocrystalline and amorphous phases in hardness indentations,” J. Raman Spectrosc., vol. 30, no. 10, pp. 939–946, 1999.

[8] D. J. Oliver, J. E. Bradby, J. S. Williams, M. V. Swain, and P. Munroe, “Rate- dependent phase transformations in nanoindented germanium,” J. Appl. Phys., vol. 105, no. 12, 2009.

[9] Y. G. Gogotsi, A. Kailer, and K. G. Nickel, “Pressure-induced phase transformations in diamond,” J. Appl. Phys., vol. 84, no. 3, p. 1299, 1998. [10] J. Jang, M. J. Lance, S. Wen, and G. M. Pharr, “Evidence for nanoindentation-

induced phase transformations in germanium,” Appl. Phys. Lett., vol. 86, no. 13, p. 131907, 2005.

[11] D. J. Oliver, J. E. Bradby, J. S. Williams, M. V Swain, and P. P. Munroe, “Thickness-dependent phase transformation in nanoindented germanium thin films.,” Nanotechnology, vol. 19, no. 47, p. 475709, 2008.

[12] J. S. Williams, B. Haber, S. Deshmukh, B. C. Johnson, B. D. Malone, M. L. Cohen, and J. E. Bradby, “Hexagonal germanium formed via a pressure-induced phase transformation of amorphous germanium under controlled

nanoindentation,” Phys. Status Solidi - Rapid Res. Lett., vol. 7, no. 5, pp. 355– 359, 2013.

[13] B. Haberl, J. E. Bradby, M. V. Swain, J. S. Williams, and P. Munroe, “Phase transformations induced in relaxed amorphous silicon by indentation at room temperature,” Appl. Phys. Lett., vol. 85, no. 23, p. 5559, 2004.

[14] B. Haberl, A. C. Y. Liu, J. E. Bradby, S. Ruffell, J. S. Williams, and P. Munroe, “Structural characterization of pressure-induced amorphous silicon,” Phys. Rev. B, vol. 79, no. 15, p. 155209, Apr. 2009.

[15] Y. G. Gogotsi, V. Domnich, S. N. Dub, A. Kailer, and K. G. Nickel, “Cyclic Nanoindentation and Raman Microspectroscopy Study of Phase Transformations in Semiconductors,” J. Mater. Res., vol. 15, no. 04, pp. 871–879, 2000.

[16] B. C. Johnson, B. Haberl, S. Deshmukh, B. D. Malone, M. L. Cohen, J. C. McCallum, J. S. Williams, and J. E. Bradby, “Evidence for the R8 phase of germanium,” Phys. Rev. Lett., vol. 110, no. 8, 2013.

[17] J.-I. Jang, M. J. Lance, S. Wen, J. J. Huening, R. J. Nemanich, and G. M. Pharr, “Micro-Raman mapping and analysis of indentation-induced phase

transformations in germanium,” in Materials Research Society Symposium Proceedings, 2005, vol. 841, pp. 291–296.

[18] Y. G. Gogotsi, V. Domnich, S. N. Dub, A. Kailer, and K. G. Nickel, “Cyclic Nanoindentation and Raman Microspectroscopy Study of Phase Transformations in Semiconductors,” J. Mater. Res., vol. 15, no. 04, pp. 871–879, Jan. 2011. [19] J. Il Jang, M. J. Lance, S. Wen, and G. M. Pharr, “Evidence for nanoindentation-

induced phase transformations in germanium,” Appl. Phys. Lett., vol. 86, no. 13, pp. 1–3, 2005.

[20] G. Patriarche, E. Le Bourhis, M. M. O. Khayyat, and M. M. Chaudhri, “Indentation-induced crystallization and phase transformation of amorphous germanium,” J. Appl. Phys., vol. 96, no. 3, pp. 1464–1468, 2004.

[21] M. S. R. N. Kiran, B. Haberl, J. S. Williams, and J. E. Bradby, “Temperature dependent deformation mechanisms in pure amorphous silicon,” J. Appl. Phys., vol. 115, no. 11, p. 113511, Mar. 2014.