Single crystal field-effect transistors based on layered semiconductors

Abstract

With parylene as a gate-dielectric material, I was able to successfully produce<br />FETs based on a variety of organic as well as transition metal dichalcogenide<br />semiconductors. The results of this work are briely summarized below.<br />By employing single crystals, the device performance, including the<br />charge carrier mobility, the field effect threshold, and the subthreshold<br />slope, have been significantly improved. The FET characteristics are<br />no longer limited by the disorder common for thin films. To limit the concentration of impurities, it is preferable to grow crystals<br />by physical vapor transport since crystals grown from solution tend to<br />incorporate the solvents into the intra-molecular position in the weakly<br />bonded Van der Waals network of molecules. However, organic materials especially are known for undergoing several<br />disproportionation reactions during the sublimation process. The products<br />of these reactions may become embedded into the host crystal. To<br />limit the formation of impurities and therefore improve the electronic<br />charge transport in the crystal the sublimation temperature and the<br />presence of oxygen during the growth process should be reduced.<br />If the structure of a molecules is conjugated, impurities themselves can<br />show field effect activity. In general, an important criterion for the<br />choice of potential organic semiconductor material seems to be to pick<br />molecules that consist of an alternating sequence of single and double<br />bonds which allow charge transport though the molecules. So far, of all the organic semiconductor materials, rubrene exhibits the best device performance. A mobility anisotropy and an increase of mobility with cooling indicating intrinsic charge transport was observed only for rubrene single crystal FETs. Modifying tetracene molecules allows changing the herring bone packing, thus affecting the electronic transport properties of the system. The pi- stacking structure was obtained when two hydrogen atoms of tetracene were substituted by chlorine, and a mobility exceeding that of tetracene was observed along the stack direction. Overall, the pi-orbital overlap in the crystal plays a crucial role for the device performance of the semiconductor material. The perylene-TCNQ charge-transfer salt presents a different approach,where the combination of two different molecules in a crystal produces a partial charge transfer from one molecule type to the other. Here, the molecules alternate in stacks forming a quasi-one-dimensional semiconductor material. This arrangement leads to a small band gap system where n-type field effect activity is observed. Layered transition metal dichalcogenides are interesting alternative to organic semiconductors. Similar to organic materials their surface is Van der Waals determined. Therefore, an intrinsic low density of trap states at semiconductor/dielectic interface is observed. The WSe2-based devices with their high carrier mobilities and ambipolar operation are especially promising.