From monolayer to thin films: engineered bandgap in CVD grown Bi₂Se_(3−x)S_x topological insulator alloys

Topological insulators, a class of materials possessing bulk bandgap and metallic surface states with a topological nontrivial symmetry, are considered promising candidates for emerging quantum and optoelectronic applications. However, achieving scalable growth and control over the parameters including thickness, carrier density, bulk bandgap, and defect density remains a challenge in realizing such applications. In this work, we show the scalable growth of topological insulator alloys Bi₂Se_(3−x)S_x and demonstrate composition-tunable bandgap, using chemical vapor deposition (CVD). A bandgap increase of up to ∼40% at a sulfur concentration of ∼15% is demonstrated. Correspondingly, the real part (n) of the refractive index is reduced in the alloy by ∼25% relative to that of Bi₂Se₃. Additionally, electronic transport measurements indicate a bulk p-type doping and field-effect tunable metallic surface states of the alloy. This work paves the way for the controlled growth of topological insulators, free from surface-state pinning, suitable for quantum optoelectronics and spintronics applications.