Optical and electron transport properties of rock-salt Sc1-xAlxN

Epitaxial single-crystal Sc1-xAlxN ternary alloy layers deposited by magnetron co-sputtering on MgO(001) substrates at 950 degrees C exhibit a solid solution rock-salt phase for x = 0-0.2 without decomposition. Optical absorption indicates a linear increase in the optical gap from 2.51 eV for ScN to 3.05 eV for Sc0.8Al0.2N and, after correction due to the Moss-Burstein shift, a direct X point interband transition energy E-g(X) = 2.15 + 2.75 x (eV). Correspondingly, the direct transition at the zone center increases with Al concentration according to E-g(Gamma) = 3.80 + 1.45 x (eV), as determined from a feature in the reflection spectra. All layers are degenerate n-type semiconductors with a room temperature mobility that decreases from 22 to 6.7 to 0.83cm(2)/V s as x increases from 0 to 0.11 to 0.20. The corresponding carrier densities are 9.2 x 10(20), 7.9 x 10(20), and 0.95 x 10(20) cm(-3) as determined from Hall measurements and consistent with optical free carrier absorption below photon energies of 1 eV. Temperature dependent transport measurements indicate metallic conduction for ScN, but weak localization that leads to a resistivity minimum at 85 and 210K for x = 0.051 and 0.15, respectively, and a negative temperature coefficient over the entire measured 4-300K range for Sc0.8Al0.2N. The decreasing mobility is attributed to alloy scattering at randomly distributed Al atoms on cation sites, which also cause the weak localization. The carrier density is primarily due to unintentional F doping from the Sc target and decreases strongly for x > 0.15, which is attributed to trapping in defect states due to the deterioration of the crystalline quality, as evidenced by the x-ray diffraction peak width that exhibits a minimum of 0.14 degrees for x = 0.11 but increases to 0.49 degrees for x = 0.20. This is consistent with asymmetric x-ray diffraction analyses, indicating a relaxed lattice constant that decreases from 4.511 +/- 0.005 to 4.411 +/- 0.004 angstrom for x = 0-0.2, and a biaxial in-plane compressive strain that decreases from -1.1% to -0.2% as x increases from 0 to 0.11, which is attributed to the higher Al adatom mobility, but increases again to -1.8% for x = 0.20, as x approaches the critical composition for phase separation, which causes structural instability and a higher defect density. (C) 2015 AIP Publishing LLC.