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 TiGeZ_3H (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_3H 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 TiGeZ_3H through the analysis of their phonon spectra and Raman active modes. Due to the broken vertical mirror symmetry, TiGeZ_3H monolayers possess both out-of-plane and in-plane piezoelectric responses, in particular, the out-of-plane piezoelectric coefficient of TiGeAs_3H is computed to be up to .0.42 pm/V. Janus TiGeZ_3H 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_3H when the spin–orbit coupling is taken into account. The calculations for the transport features indicate that while TiGeN_3H monolayer exhibits low electron mobility, both TiGeP_3H and TiGeAs_3H have electron mobility μ_x over 400 cm^2V^-1s^-1, which is suitable for applications in electronics.