First-Principles Study of the Physical Properties of Novel Polytypes of Gallium Phosphide

The polytypism of solids has been established as one of the most effective approaches to alter their physical properties. In this first-principles-based study, we practiced the polytypism of gallium phosphide (GaP) to evolve its physical properties for advanced technological applications. In this regard, new polytypes of GaP in wurtzite (wz)-, sphalerite (sp)-, beryllium oxide (β-BeO)-, and silicon carbide (SiC)-like structures have been generated by applying compressive and tensile pressure on its ground-state zinc blende (zb) structure. Our analysis shows its transformation to a SiC-like structure at a small pressure of 0.60 GPa. On the other hand, transition to wz-, sp-, and β-BeO-like structures has been realized from the zb phase of GaP by applying moderate tensile stress of -0.07, -0.79, and -3.65 GPa, respectively. Calculations of the cohesive energies of the new polytypes are found comparable to those of the zb phase. Similarly, the phonon band structures calculated for these new polytypes have been found free from the imaginary frequencies, indicating their adequate dynamical stability. Like the zb phase, the sp phase exhibited an indirect band gap, whereas wz-GaP, β-BeO-GaP, and SiC-GaP demonstrated direct band gaps. The computed energy gaps for the zb phase, wz phase, sp phase, β-BeO phase, and SiC phase amount to 2.359, 2.528, 1.740, 1.814, and 1.984 eV, respectively. On the other hand, these polytypes exhibited a remarkable absorption of the visible and ultraviolet (UV) light of the order of ∼106 cm-1, which is useful for photovoltaic applications. Investigations of the refractive indices reveal their transparency for light photons having energy less than 7.5 eV. We also estimated plasmon excitation energies as 16.97, 17.075, 16.97, 16.90, and 16.91 eV, respectively, for the zb phase, wz phase, sp phase, β-BeO phase, and SiC phase. Our predictions indicate novel polytypes of GaP as promising candidates for use in electronic and optoelectronic devices.