Electrical properties and memory effects in self-oxidized MoOx/MoS2 transistor
| Autor: | Wang, Po-Sheng, 王柏勝 |
|---|---|
| Rok vydání: | 2017 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 105 Mechanically exfoliated molybdenum disulfide (MoS2) flakes are dispersed on silicon substrates capped with a 300-nm thick silicon dioxide layer. The standard electron beam lithography and thermal evaporation were used to pattern Au leads on the MoS2 flakes. The as-patterned MoS2 field effect transistors (FETs) were then annealed in a high vacuum to reduce the contact resistance. The MoS2 FETs demonstrates n-type semiconductor and possess good ohmic contact behaviors. The MoS2 FETs were exposed to ozone gas for self-limiting oxidation and the surface layer was converted to MoOx (x≤3). By controlling the ozone treatment, the FET devices changed from its natively n-type to an either ambipolar or a p-type semiconductor. The on-off ratio of transistors is up to 104. The variation of electron or hole dominated transport is owing to the change of the work function after oxidation. The oxidized MoOx has higher work function and gives efficient hole injections. Current-voltage (Ids -Vds) curves showed back-to-back Schottky diodes, which implies that the Schottky junction is formed between Au electrodes and MoOx. The Schottky barrier was inspected, showing a variation with bias and gating voltages. In the following discussion, we categorized oxidized MoS2 FETs into n-type, ambipolar, and p-type FETs. Electron transport of n-type FETs (MoS2) is well described by the theory of two-dimensional variable range hopping at temperatures in the range between 80 and 180 K. At higher temperatures from 180 to 240 K, it can be fitted using thermally activated transport. Nevertheless, electron transport of ambipolar FETs (mildly oxidized MoS2) can be separated into two regions of either electron or hole doping. The electron doping regime is well described by the theory of two-dimensional variable range hopping at temperatures from 80 to 180 K whereas, at higher temperatures from 180 to 240 K, it is better described by thermally activated transport. The hole doping regime is well described by the theory of two-dimensional variable range hopping from 220 to 280 K. At temperatures lower than 220 K, electron transport is dominated by tunneling. The hole transport of p-type FETs is well described by the theory of two-dimensional variable range hopping at temperatures from 120 to 200 K. On the other hand, we discovered a memory effect on p-type FETs. The state of the p-type FETs can be changed by a writing voltage. If the writing current are precisely controlled, the devices can be operated at multiple states and the states can be stored for a long time. After a simple reliability test, we observe reproducible and stable current states in the memory devices. |
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