Stacking induced indirect-to-direct bandgap transition in layered group-IV monochalcogenides for ideal optoelectronics

Abstract

Ideal optoelectronic semiconductors, with suitable direct bandgaps, high optical absorptions, low carrier effective masses, high mobility, low cost, and high stability, have been long sought after during the past few decades. The recent emergence of layered systems opens an alternative door for this search with novel band structure engineering avenues beyond traditional strategies such as strain, alloying and atomic transmutation. However, ideal layered materials for optoelectronics are rarely reported. Here, we report successful non-traditional band structure engineering in naturally-existing layered group-IV monochalcogenides MX (M = Ge, and Sn; X = S, and Se) by finding that stacking can induce indirect-to-direct bandgap transition in MX bilayers due to interlayer interactions. We identify a family of MX bilayers with direct bandgaps ranging from 1.10 eV to 2.20 eV. While these bilayers have nearly similar energetic stabilities as the bulk-derived ground state bilayers, they show superb electronic properties such as high optical absorption, low exciton binding energies, small carrier effective masses and high mobility, making them ideal candidates with better properties than any currently existing materials for optoelectronic applications. Our work shows that layered systems can have additional and unique degrees of freedom beyond the traditional ones for band structure engineering, thus providing new opportunities for the optoelectronic community.