Raman activities and high electron mobility of piezoelectric semiconductors GaSi$X_2$ ($X$ = N, P, and As) toward flexible nanoelectronic devices

Abstract Inspired by the urgent demand for novel multifunctional materials in advanced technology applications, herein, first-principles calculations are utilized to construct new two-dimensional GaSi$X_2$ ($X$ = N, P, As) monolayers. After optimizing the GaSi$X_2$ crystal structures, their stabilities, Raman responses, piezoelectricity, and electronic/transport characteristics are systematically studied. The results from phonon dispersions, {\it ab initio} molecular dynamics simulations, elastic constants, and cohesive energies confirm the good dynamic, thermal, energetic, and mechanical stabilities of the proposed GaSi$X_2$ monolayers. Based on the Heyd--Scuseria--Ernzerhof approach, the GaSi$X_2$ exhibit semiconductor behaviors with favorable bandgaps for absorption of the visible spectrum of 1.82 for GaSiP$_2$ and 1.43 eV for GaSiAs$_2$ monolayer. Besides, the GaSi$X_2$ monolayers also show piezoelectric effects, the high in-plane piezoelectric coefficients $d_{11}$ are found for the GaSiP$_2$ ($-1.16$ pm/V) and GaSiAs$_2$ ($-1.26$ pm/V). The carrier mobilities of the GaSi$X_2$ are estimated by the deformation potential method for their transport properties. Along two in-plane transport directions, the carrier mobilities of the GaSi$X_2$ are anisotropy for holes and electrons. The GaSiP$_2$ and GaSiAs$_2$ show large electron mobilities in the $x$ axis of 2816.99 and 1211.41~cm$^2$V$^{-1}$s$^{-1}$, respectively. Thus, the GaSiP$_2$ and GaSiAs$_2$ monolayers have not only high and anisotropic electron mobilities but also favorable bandgaps and large piezoelectric coefficients, indicating potential applications of the GaSiP$_2$ and GaSiAs$_2$ in electronic, photovoltaic, and piezoelectric devices. The findings in this study may provide insights into the GaSi$X_2$ monolayers for multifunctional applications.