Topological Insulator Material for High Frequency Generation

Abstract

Despite decades of progress with traditional III-V semiconductors like Gallium Arsenide (GaAs), the development of compact terahertz sources remains constrained by their inherent limitations. While GaAs has been the cornerstone for devices like Gunn diodes, it often requires cryogenic cooling for efficient terahertz operation and faces challenges in power output at these frequencies. In contrast, topological insulators such as Bi₂Se₃ offer a promising alternative. These quantum materials possess unique, protected surface states that are inherently robust against scattering and could potentially enable lower-loss, higher-efficiency devices. However, the bulk transport properties of these topological materials have remained largely overlooked in favor of their surface phenomena. In this work, we experimentally demonstrate for the first time that bulk of Bi₂Se₃ itself can be engineered to exhibit reproducible Negative Differential Resistance (NDR) at 300K, marking a significant step toward expanding the materials base for high-frequency electronics. Vertical Bi₂Se₃ heterostructures were fabricated following systematic optimization of thickness, carrier density, and contact engineering. An optimized 25/50/25 nm alloyed-intrinsic-alloyed structure was developed, which exhibited robust NDR at room temperature. Pulsed current-voltage measurements revealed clear NDR with a Peak- to-Valley Current Ratio (PVCR) value up to ~2. The observed transport behaviour aligns with a bulk transferred-electron mechanism, the fundamental principle behind Gunn diodes. In this mechanism, a strong electric field causes charge carriers (electrons) to gain enough energy to scatter from a high-mobility, low-energy conduction band valley (the Γ valley) into lower- mobility, higher-energy satellite valleys. This transfer to a state of higher effective mass causes a drop in average electron velocity, resulting in the observed decrease in current with increasing voltage leading to a negative differential resistance. This distinguishes our results from the surface state conduction typically associated with Bi₂Se₃. Importantly, based on the device geometry and active-layer thickness, the optimized structures indicate a potential transit-time frequency in the 0.1-0.3 THz range. This highlights the feasibility of Bi₂Se₃ heterostructures for room-temperature oscillators operating well into the sub-terahertz band. By combining structural optimization with contact engineering, this work not only establishes a new class of material platform for terahertz NDR devices but also provides a clear design framework linking layer thickness, carrier density, and contact interfaces to device performance. These results show that bulk Bi₂Se₃ can enable compact on-chip oscillators and advance practical terahertz technologies based on topological insulator heterostructures.