Electrical resistivity of Ti_n+1AC_n (A = Si, Ge, Sn, n = 1–3) thin films

Abstract

We have investigated the electrical resistivity of (0001)-oriented Tin+1ACn (A = Si, Ge, Sn, n = 1–3) thin films deposited by magnetron sputtering onto Al2O3(0001) substrates at temperatures ranging from 500 to 950 °C. Four-point-probe measurements show that all films are good conductors with resistivity values of ∼21–51 μΩ cm for Ti–Si–C films, ∼15–50 μΩ cm for Ti–Ge–C films, and ∼46 μΩ cm for Ti2SnC. We find a general trend of decreasing resistivity with decreasing n for the Ti–Si–C and Ti–Ge–C systems due to the increased metallicity obtained with increasing density of A-element layers. We also show that crystalline quality and competitive growth of impurity phases affect the measured resistivity values. The effect of a given impurity phase largely depends on its location in the sample. Specifically, a TiCx layer in the center of the film constricts the current flow and results in an increased measured resistivity value. However, TiCx transition or seed layers at the substrate–film interface as well as surface segregation of Ge and Ti5Ge3Cx (for Ti–Ge–C) have only little effect on the measured resistivity values. For the Ti–Sn–C system, the resistivity is mainly influenced by the segregation of metallic Sn, yielding a wide spread in the measured values ranging from 20–46 μΩ cm, in the order of increased film purity.

Epitaxial growth and electrical transport properties of Cr2GeC thin films

“…These observations can be explained as a consequence of the fast growth and Ge-diffusion rates along the basal planes. The phenomenon of surface Ge segregation is similar to what has been observed for Ti 2 GeC by Emmerlich et al 26 ; the effect is even more pronounced for Sn, a larger group-14 element than Ge, as observed 26 for Ti 2 SnC. In contrast, the Ti-Si-C based MAX phases appear less sensitive; here, Si will typically float on top of the growing film during the initial stages of growth, but once sufficient supersaturation has been achieved, Ti 3 SiC 2 (or Ti 4 SiC 3 ) nucleates rather than segregating Si.…”

“…for Ti 3 SiC 2 contacts to SiC. [20][21][22] As thin films, the close relatives of Cr 2 GeC, Ti 2 GeC, [23][24][25][26] V 2 GeC, 27,28 and Cr 2 AlC [29][30][31][32][33][34] have all been investigated. From a potential technological viewpoint, the latter two are particularly interesting since they can be grown at a relatively low substrate temperature (450 • C), 27,29 which is essential for deposition onto technologically relevant substrates, such as steels.…”

Sputter deposition from a Ti2AlC target: Process characterization and conditions for growth of Ti2AlC

“…The strong temperature dependence of the Al content is not surprising given the high vapor pressure of metallic Al. Similar A-element deficiency has been reported for the Ti-Sn-C, V-Ge-C, and Ti-Si-C systems [26,17,12], where the evaporation tendency depends on the vapor pressure of each A-element. This shows the importance of the A-element properties for growth of MAX-phases.…”

The M+1AX phases: Materials science and thin-film processing

“…The results of measurements of electrical properties depend strongly on not only the presence, but also the distribution, of impurity phases. A clever example of a controlled measurement of this effect for a MAX phase is an investigation [272] of the effect of a deliberately inserted TiC layer in the bulk of a Ti 3 SiC 2 epitaxial film; this greatly increases measured resistivity values compared to a pure Ti 3 SiC 2 film. Yet, many experimental publications on electrical properties of MAX phases contain limited, if any, information on impurities and minority phases, a problem that often makes it difficult to assess the validity of the results.…”