Vibrational modes in epitaxial Ti1-xScxN(001) layers: An ab initio calculation and Raman spectroscopy study - art. no. 174302

Ti1-xScxN alloys were used to investigate the effects of carrier density an d the density of states (DOS) at the Fermi level E-f on vibrational modes, phonon anomalies, and superconducting transition temperatures T-c. Single-c rystal Ti1-xScxN layers, 2000 Angstrom thick, with compositions spanning th e entire alloy range (0 less than or equal tox less than or equal to1), wer e grown on MgO(001) by ultrahigh vacuum reactive magnetron sputter depositi on at 750 degreesC. The Raman spectrum of pure TiN is dominated by two-phon on features while Ti1-xScxN spectra with 0.12 less than or equal tox less t han or equal to0.81 exhibit primarily one-phonon peaks. The acoustic peak s oftens from 310 cm(-1) with x=0.12 to similar or equal to 200 cm(-1) with x = 0.81, due to the corresponding increase in the lattice constant. All all oy layers exhibit an optical double-peak feature related to local vibration al modes at Ti and Sc sites. The two optical peaks shift from 560 and 640 c m(-1) for Ti0.88Sc0.12N to similar or equal to 480 and 570 cm(-1) for Ti0.1 9Sc0.81N with an intensity ratio which is proportional to (1 - x)/x. T-c fo r TiN is 5.4 K and decreases steeply with increasing x to T-c = 4.5, 3.4, a nd 1.9 K with x = 0.06, 0.12, and 0.18, respectively. The decrease is due t o a reduced DOS at Ef in combination with a weaker effective coupling at hi gher x. The strong coupling for TiN is related to an anomaly in the longitu dinal acoustic branch, which is absent for ScN, in the calculated phonon di spersion curves. Experimental evidence for the vanishing phonon anomaly is supplied by a broadening in the acoustic Raman peak with increasing x.