Integration of High-<i>k</i> Material and Hexagonal Boron Nitride for Top-Gate WS₂ Field-Effect Transistors

For the development of integrated circuits of two-dimensional (2D) semiconductors, such as MoS2 and WS2, effective deposition of high-k materials on these 2D materials is important for fabrication of top-gate field-effect transistors (FETs). However, the atomically flat and inert surface of the 2D materials makes it difficult to deposit uniform high-k materials without deteriorating the 2D materials. Here, we have developed a method to deposit high-k materials on semiconducting WS2 monolayers with the aid of a large-area few-layer hexagonal boron nitride (hBN) sheet as a buffer layer. The few-layer hBN was treated with ozone for the formation of oxygen functional groups, followed by transfer onto monolayer WS2. This functionalized boron nitride acts as a seed layer for the uniform deposition of a dielectric HfO2 layer, resulting in high breakdown voltage, a low leakage current, and preventing damage of the WS2 channel during the deposition. The top-gate WS2-FETs using the HfO2/BN stack as a gate insulator exhibited a high on/off ratio of >107 and a low subthreshold slope value of 110 mV/dec. Our method of utilizing functionalized BN as a buffer layer offers a promising route to deposit high-k materials for the development of 2D electronic devices.