Exposing Dynamical Phase Transitions and Electro-Thermal Transport in TiTe₂ Thin Films

Sputtered polycrystalline titanium ditelluride (TiTe₂) has recently been introduced as a thermal barrier layer in phase change heterostructures (PCHs) to realize energy-efficient and neuro-inspired phase change memory (PCM) [1-3]. PCHs such as TiTe₂/Sb₂Te₃ rely on the high melting temperature and predicted low thermal conductivity of ultrathin (few-nanometer) polycrystalline TiTe₂ layers [1,2]. The thermal properties of such TiTe₂ thermal barrier layers play a vital role within the thermally-driven phase change in PCH. Thus, the characterization of thermal transport across sputtered thin TiTe₂ films is critical for the adoption and optimization of PCH in PCM technology.<br/>In this work, we use time-domain thermoreflectance to uncover the thermal transport properties of sputtered TiTe_2. We explore the temperature (<i>T</i>) and the thickness dependence of sputtered TiTe₂ thin films deposited at varying temperatures. Additionally, our <i>T</i>-dependent Raman and X-ray diffraction (XRD) measurements shed further insight into the micro-structural evolution of TiTe₂ films with temperature.<br/>We sputter TiTe₂ thin films of 10 nm, 20 nm, 50 nm, and 120 nm in thickness at room temperature (RT) and at 230^oC on a silicon substrate. All films were capped in situ with 10 nm TiN to protect the TiTe₂ from surface oxidation. XRD spectra confirm both the amorphous and polycrystalline states of the TiTe₂ films deposited at RT and 230^oC, respectively. Our measured intrinsic thermal conductivity (<i>k</i>) of the amorphous TiTe₂ is ~0.1 W/m/K at RT, ~5x lower than the <i>k</i> of commonly used phase change material (Sb₂Te₃) in PCH. The RT thermal conductivities of amorphous TiTe₂ do not show a thickness dependence and hold constant at ~0.1 W/m/K, even for a 120 nm thick amorphous film. However, we measure a significantly higher <i>k</i> of ~1.95 W/m/K for TiTe₂(polycrystalline) films deposited at 230^oC, regardless of thickness. Such an increase in <i>k</i> for the polycrystalline sputtered TiTe₂ contrasts with earlier predictions in literature [5] and thus warrants careful consideration for PCH-based PCM design.<br/>We also measure the <i>T</i>-dependent <i>k</i> of the amorphous 120 nm thick TiTe₂ film. We observed ~15x increase in the thermal conductivity upon in-situ annealing at 180^oC from its RT value, pointing to the possibility of a phase transition from an amorphous state. The <i>k</i> was then found to decrease to ~10x its RT value at 220^oC and continued unchanged to 250^oC. Interestingly, there is a 3x increase in <i>k</i> with polycrystalline films deposited at 230^oC compared to amorphous films deposited at RT and annealed to the same <i>T</i>. We expect -dependent in-plane and cross-plane electrical resistivity measurements to provide further insight into such a trend in the measured <i>k</i>.<br/>The unique thermal transport and phase transitions within our sputtered TiTe₂films are further elucidated by the dynamical analysis of the crystal structure. Our -dependent Raman spectroscopy on the amorphous 120 nm films reveal the formation of in-plane/cross-plane Te bonds beginning at 125^oC, with a minimum of cross-plane Raman peak intensity near 230^oC [6]. We suspect a possible dynamic disparity in Te cross- and in-plane bond arrangement facilitated by the evolution of van der Waal gaps between adjacent sheets of Te and Ti [7].<br/>In summary, our detailed thermal measurements and material analysis of sputtered TiTe₂ thin films reveal an unexplored phase transition that has a dramatic effect on the thermal transport. These results are expected to enable accurate modeling and optimization of thermal barrier PCH based on TiTe₂.<br/>F. Rao et al., <i>Nat. Com.,</i> <b>6</b>, (2015)<br/>K. Ding et al., <i>Sci. Rep.</i>, <b>6</b>, (2016)<br/>K. Ding et al., <i>Science</i> <b>366</b>, (2019)<br/>H. Kwon et al., <i>Nan. Lett.,</i> <b>21</b>, (2021)<br/>C. Chiritescu et al., <i>J. Appl. Phys.</i>, <b>106</b>, 073503 (2009)<br/>J. M. Khan et al., <i>ECS Trans.</i>, <b>33</b>, 13, (2010)<br/>D. K. G. de Boer et al, <i>Phys. Rev. B</i>, <b>29</b>, 12 (1984)