Photo-oxidized HfS2 - An embeddable and writable high-k dielectric for flexible Van der Waals nano-electronics

Scaling improvements in conventional semiconductor devices have been facilitated by incorporating high-k dielectric materials such as HfO₂[1]. This has led to increased device density and lower drive voltages. Similarly, the rapid technological progress of flexible Van der Waals (VdW) heterostructure devices based on two-dimensional (2D) materials, will also likely require comparable scaling to reduce device drive voltages to a comparable level (~1 V). Typically, flexible VdW heterostructures rely on high purity hexagonal boron nitride dielectrics (hBN) which has a dielectric constant k ~ 4, 4-5 times smaller than HfO₂[2]. To overcome these limitations, we developed a novel technology to incorporate the high-k dielectric HfO_x within VdW heterostructures through photo-oxidation of the 2D semiconductor HfS₂ via laser irradiation. The resultant oxide is found to have a dielectric constant k ~ 15. This oxide can be selectively written even once embedded within complex multi-layer VdW heterostructures, and under metallic contacts. This avoids the need for expensive sputtering and atomic layer deposition techniques, which are known to badly damage 2D materials. We exploit this technology to demonstrate several different VdW devices to suit various functionalities, including: flexible field effect transistors (FETs), resistive switching memory elements (ReRAMs) and light emitting and ultra-fast light detecting quantum well structures. All these devices show performances equal or superior to state-of-the-art VdW devices, with FETs displaying on/off ratios of 10⁴ and subthreshold swings of 60 mV/dec.