Alexander Zintler (Karlsruhe / DE), Robert Winkler (Darmstadt / DE), Stefan Petzold (Darmstadt / DE), Sankaramangalam Ulhas Sharath (Darmstadt / DE), Enrico Bruder (Darmstadt / DE), Nico Kaiser (Darmstadt / DE), Lambert Alff (Darmstadt / DE), Leopoldo Molina-Luna (Darmstadt / DE)
Abstract text (incl. figure legends and references)
Titanium nitride (TiN) thin films are intensively studied in their application at room temperature (RT) as electrode materials in oxide electronics[1] and at low temperatures (LT) for microwave resonators and superconducting (SC) Josephson junctions.[2, 3] In these applications, low surface roughness, high electrical conductivity and high critical temperatures (TC) for SC applications are essential.[4]
Here, TiN1-x films of 25 nm thickness have been deposited by reactive molecular beam epitaxy (RMBE) on c-cut sapphire substrates. For the sample series, increasing nitrogen deficiency was introduced by lowering the substrate temperature and increasing the Ti deposition rate. In their application as bottom electrodes in oxide electronic valence change memories (VCM) the nitrogen deficiency is discussed to enable electrical device performance.[5]
Besides characterization of the RT and LT electrical conductivity, a scale bridging set of characterization methods has been employed to create a global (mm scale) down to atomistic (Å scale) understanding of the microstructure and resulting grain boundaries. Starting from X-ray diffraction (XRD) over backscatter electron (BSE) imaging and ion channeling contrast imaging (iCCI) and atomic resolution high-angle annular dark-field (HAADF) STEM imaging, a comprehensive analysis has been carried out.
All samples showed highly textured out-of-plane growth (TiN (111)||Al2O3(001)) and two options for in-plane orientations, separated by 60°, for the TiN grains. iCC imaging revealed near identical grain sizes for all samples. For the low film thickness of 25 nm, the lateral grain size of over 100 nm indicated quasi-epitaxial growth introduced by domain matching epitaxy. The grain boundaries present are mainly twin boundaries of the <112> type. For the most deficient TiN0.7 sample, these twin boundaries emit <112> {111} stacking faults, resulting in a low transparency for electrical transport of the extended grain boundary. Critical temperatures of up to 4.9 K are reported for the first time for TiN films of low thickness on c-cut sapphire.
Figure 1: (a,b) BSE images of the surfaces of the TiN (A) and TiN0.7 (B) thin film showing GBs in BSE contrast only for TiN0.7. (c,d) iCC images for the same samples, indication the two in-plane oriented grain types (60° in-plane rotation) and near identical grain sizes for both samples. (e,f) HAADF-STEM images of twin boundaries of the same samples, the TiN0.7 GB shows an extended GB structure resulting from the emission of stacking faults into the neighboring grain. (g) Temperature dependent resistivity measurements showing the anomalous increase in resistivity for the TiN0.7 sample and SC transition temperatures of the 25 nm TiN1-x films.
[1] Sharath SU, Vogel S, et al., Adv. Funct. Mater. 31 (2017) 10.1002/adfm.201700432
[2] Gao R, Yu W, et al., Phys. Rev. Materials 3 (2022) 10.1103/PhysRevMaterials.6.036202
[3] Krockenberger Y, Karimoto S, et al., Journal of Applied Physics 8 (2012) 10.1063/1.4759019
[4] Zintler A, Eilhardt R, et al.,(2022) 10.1021/acsomega.1c05505
[5] Niu G, Calka P, et al., 3 (2019) 10.1080/21663831.2018.1561535