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CuTi layers are co-sputter deposited on 20-nm-SiO2/Si(001) wafers at 350 ℃ to quantify their stability in direct contact with a dielectric and to explore the potential of CuTi as barrier- and liner-free interconnect metal. X-ray diffraction pole figures indicate a preferred 001 out-of-plane crystalline orientation and Rutherford backscattering confirms a stoichiometric composition. Vacuum annealing tests at 450 ℃ of CuTi layers indicate considerably higher thermal stability than for pure Cu layers, including negligible dewetting observed by scanning electron microscopy and negligible intermixing with the oxide substrate quantified by photoelectron spectroscopy. Four-point bend tests show a 25% higher interfacial toughness for CuTi/SiO2 than Cu/SiO2 interfaces. CuTi/SiO2 samples also exhibit a 300-times longer failure time than Cu/SiO2 during time-dependent dielectric breakdown tests using an externally applied 3 MV/cm electric field. The higher stability of CuTi in comparison to Cu is attributed to a higher cohesive energy in combination with an atomically thin self-limiting Ti oxide layer at the CuTi/SiO2 interface. 

Rocksalt structure nitrides emerge as a promising class of semiconductors for high-temperature thermoelectric and plasmonic applications. Controlling the bandgap and strain is essential for the development of a wide variety of electronic devices. Here we use (Ti 0.5 Mg 0.5 ) 1−x Al x N as a model system to explore and demonstrate the tunability of both the bandgap and the strain state in rocksalt structure nitrides, employing a combined experimental and computational approach. (Ti 0.5 Mg 0.5 ) 1−x Al x N layers with x ≤ 0.44 deposited on MgO(001) substrates by reactive co-sputtering at 700 °C are epitaxial single crystals with a solid-solution B1 rocksalt structure. The lattice mismatch with the substrate decreases with increasing x , leading to a transition in the strain-state from partially relaxed (74% and 38% for x = 0 and 0.09) to fully strained for x ≥ 0.22. First-principles calculations employing 64-atom Special Quasirandom Structures (SQS) indicate that the lattice constant decreases linearly with x according to a = (4.308 − 0.234 x ) Å for 0 ≤ x ≤ 1. In contrast, the measured relaxed lattice parameter a o = (4.269 − 0.131 x ) Å is linear only for x ≤ 0.33, its composition dependence is less pronounced, and x > 0.44 leads to the nucleation of secondary phases. The fundamental (indirect) bandgap predicted using the same SQS supercells and the HSE06 functional increases from 1.0 to 2.6 eV for x = 0–0.75. In contrast, the onset of the measured optical absorption due to interband transitions increases only from 2.3 to 2.6 eV for x = 0–0.44, suggesting that the addition of Al in the solid solution relaxes the electron momentum conservation and causes a shift from direct to indirect gap transitions. The resistivity increases from 9.0 to 708 μΩ m at 77 K and from 6.8 to 89 μΩ m at 295 K with increasing x = 0–0.44, indicating an increasing carrier localization associated with a randomization of cation site occupation and the increasing bandgap which also causes a 33% reduction in the optical carrier concentration. The overall results demonstrate bandgap and strain engineering in rocksalt nitride semiconductors and show that, in contrast to conventional covalent semiconductors, the random cation site occupation strongly affects optical transitions.