Strain-Free Layered Semiconductors for 2D Transistors with On-State Current Density Exceeding 1.3 mA ?m-1

High-mobility and air-stable two-dimensional (2D)Bi2O2Se semiconductor holds promise as an alternative fast channelmaterial for next-generation transistors. However, one of the keychallenges remaining in 2D Bi2O2Se is to prepare high-quality crystalsto fabricate the high-performance transistors with a high on-statecurrent density. Here, we present the free-standing growth of strain-free 2D Bi2O2Se crystals. An ultrahigh Hall mobility of 160 000 cm2V-1s-1is measured in strain-free Bi2O2Se crystals at 2 K, whichenables the observation of Shubnikov-de Haas quantum oscillationsand shows substantially higher (>4 times) mobility over previous in-plane 2D crystals. The fabricated 2D transistors feature an on-offcurrent ratio of similar to 106and a record-high on-state current density of similar to 1.33 mA mu m-1, which is comparable to that of commercial Siand Ge n-typefield-effect transistors (FETs) for similar channel length. Strain-free 2D Bi2O2Se provides a promising materialplatform for studying novel quantum phenomena and exploration of high-performance low-power electronics