Raman spectroscopy and lattice dynamics calculations of tetragonally-structured single crystal zinc phosphide (ZnP) nanowires.

Earth-abundant and low-cost semiconductors, such as zinc phosphide (ZnP), are promising candidates for the next generation photovoltaic applications. However, synthesis on commercially available substrates, which favors the formation of defects, and controllable doping are challenging drawbacks that restrain device performance. Better assessment of relevant properties such as structure, crystal quality and defects will allow faster advancement of ZnP, and in this sense, Raman spectroscopy can play an invaluable role. In order to provide a complete Raman spectrum reference of ZnP, this work presents a comprehensive analysis of vibrational properties of tetragonally-structured ZnP (space group P4/nmc) nanowires, from both experimental and theoretical perspectives. Low-temperature, high-resolution Raman polarization measurements have been performed on single-crystalline nanowires. Different polarization configurations have allowed selective enhancement of A, B and E Raman modes, while B modes were identified from complementary unpolarized Raman measurements. Simultaneous deconvolution of all Raman spectra with Lorentzian curves has allowed identification of 33 peaks which have been assigned to 34 (8 A + 9 B + 3 B + 14 E) out of the 39 theoretically predicted eigenmodes. The experimental results are in good agreement with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Three separate regions were observed in the phonon dispersion diagram: (i) low-frequency region (<210 cm) which is dominated by Zn-related vibrations, (ii) intermediate region (210-225 cm) which represents a true phonon gap with no observed vibrations, and (iii) high-frequency region (>225 cm) which is attributed to primarily P-related vibrations. The analysis of vibrational patterns has shown that non-degenerate modes involve mostly atomic motion along the long crystal axis (c-axis), while degenerate modes correspond primarily to in-plane vibrations, perpendicular to the long c-axis. These results provide a detailed reference for identification of the tetragonal ZnP phase and can be used for building Raman based methodologies for effective defect screening of bulk materials and films, which might contain structural inhomogeneities.