Raman activity, piezoelectric response, and carrier mobility in two-dimensional Janus TiGeZ3H (Z= N, P, As) semiconductors: A first-principles prediction

Abstract

In this article, we theoretically propose a series of TiGe$Z_3$H ($Z=$ N, P, As) monolayers and comprehensively investigate their structural, vibrational, piezoelectric, electronic, and transport properties using first-principles simulations. The structural stability of the suggested monolayers is verified by phonon dispersion analysis, ab-initio molecular dynamics calculations, and Born–Huang mechanical stability criteria. Based on the calculations for the mechanical response, it is shown that TiGeN${}_3$H is the stiffest material compared to the other two structures with Young’s modulus found to be 252.11 Nm⁻¹. Besides, we also examine the vibrational characteristics of TiGe$Z_3$H through the analysis of their phonon spectra and Raman active modes. Due to the broken vertical mirror symmetry, TiGe$Z_3$H monolayers possess both out-of-plane and in-plane piezoelectric responses, in particular, the out-of-plane piezoelectric coefficient of TiGeAs${}_3$H is computed to be up to $-0.42$ pm/V. Janus TiGe$Z_3$H monolayers are found to be indirect semiconductors with decreasing bandgap as $Z$ changes from N to As. Particularly, the Rashba spin splitting is found in TiGeAs${}_3$H when the spin–orbit coupling is taken into account. The calculations for the transport features indicate that while TiGeN${}_3$H monolayer exhibits low electron mobility, both TiGeP${}_3$H and TiGeAs${}_3$H have electron mobility ${\mu }_x$ over 400 cm${}^2$V⁻¹s⁻¹, which is suitable for applications in electronics.