Epitaxial Ti 2 GeC, Ti 3 GeC 2 , and Ti 4 GeC 3 MAX-phase thin films grown by magnetron sputtering
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Epitaxial Ti2GeC, Ti3GeC2, and Ti4GeC3 MAX-phase thin films grown by magnetron sputtering H. Högberg, P. Eklund, J. Emmerlich, J. Birch, and L. Hultman Linköping University, Department of Physics IFM, Thin Film Physics Division, SE-58183 Linköping, Sweden (Received 6 October 2004; accepted 21 December 2004)
We have grown single-crystal thin films of Ti2GeC and Ti3GeC2 and a new phase Ti4GeC3, as well as two new intergrown MAX-structures, Ti5Ge2C3 and Ti7Ge2C5. Epitaxial films were grown on Al2O3(0001) substrates at 1000 °C using direct current magnetron sputtering. X-ray diffraction shows that Ti–Ge–C MAX-phases require higher deposition temperatures in a narrower window than their Ti–Si–C correspondences do, while there are similarities in phase distribution. Nanoindentation reveals a Young’s modulus of 300 GPa, lower than that of Ti3SiC2. Four-point probe measurements yield resistivity values of 50–200 ⍀cm. The lowest value is obtained for phase-pure Ti3GeC2(0001) films.
The combined metallic and ceramic properties of the archetype MAX-phase Ti3SiC21 have resulted in intense research on these materials almost 40 years after their discovery and crystallographic determination.2,3 The properties of the approximately 50 known MAX-phases stem from the highly anisotropic hexagonal crystal structure, often described as a nanolaminate. The early transition metal (M) atoms and C or N (X) atoms form edgesharing octahedral MX layers interleaved by group 13– 15 element layers (A). Depending on the stacking sequence of the MX layers between neighboring Aelement layers, phases from any of the three known subgroups M2AX, M3AX2, and M4AX3 can form. Although the focus has been on Ti3SiC2, it is believed that the other relatively unexplored members share this set of attributes with possible extreme values in ductility or electrical conductivity. For instance, theoretical modeling of Ti3GeC2 has shown similarities in electronic structure to the closely related Ti3SiC2 but revealed a slightly increased density of states at the Fermi level for the Gecontaining MAX-phase, suggesting that Ti3GeC2 has better electrical properties than Ti3SiC2.4 Understanding of the origin of the physical properties of the known Ti3GeC2 and Ti2GeC phases and any related compounds, as well as fulfillment of their full potential, requires the synthesis of phase-pure material. This has been difficult
DOI: 10.1557/JMR.2005.0105 J. Mater. Res., Vol. 20, No. 4, Apr 2005
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to achieve by the presently used bulk sintering technique, suggesting the use of alternative synthesis routes. Further, the realization of a thin film deposition process for these compounds as demonstrated here has significant impact in applications such as wear resistant electrical contacts. In recent publications, we have demonstrated that epitaxial Ti–Si–C MAX-phase films can be deposited on MgO(111) and Al2O3(0001) substrates, using direct current (dc) magnetron sputtering.5–8 Besides growing single-crystal Ti3SiC2 films at temper
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