Flexible high-speed thin-film transistors and circuits based on large-area grown two-dimensional transition metal dichalcogenides

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The aim of the proposal is to develop high-speed devices and circuits based on large-area grown transition metal dichalcogenides (TMDs) on flexible substrates. TMDs are ideal candidates on flexible substrates, since they have superior mechanical properties, the important device parameters of TMDs based thin-film transistors (TFTs) (e.g. mobility, on/off ratio) are in a desirable range and a high-frequency operation in the sub GHz range can be achieved. Large area uniform growth of the TMDs is imperative for scalable integration of circuits. Thus, growth processes of TMDs by metalorganic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) will be developed within this project which also includes the relevant precursor chemistry combinations to obtain high quality and large area growth with controlled thickness. Particularly the low processing temperatures for ALD grown TMDs films will enable a bottom-up approach of TMDs based devices and circuits on flexible substrates without time and cost intensive TMDs transfer. n- and p-type TFTs will be prepared and gradually improved in terms of a low contact resistance, a pinhole-free high-k dielectric layer, an optimum transistor design and a suitable ALD/PEALD processed passivation layer for an enhanced reliability and stability. Especially for p-type TFTs, a central challenge is the reduction of the Schottky barrier between the electrodes and TMDs for an effective injection of holes. For a perfect band alignment three tasks must be handled: 1. choice of a suitable buffer layer to minimize Fermi-level pinning, 2. thickness engineering of the TMDs and 3. choice of a metal with an appropriate work function. By nanoscaling the TFTs we expect to achieve cut off frequencies in the low GHz range. The threshold voltage of the high-performance n- and p-type TMD TFTs will be tuned close to zero and subsequent inverters are prepared ideally based on one TMD material e.g. MoS2 or WSe2. As a proof of scalable integration on large area grown TMDs films multistage ring oscillators will be realized and characterized. Such ring oscillators are the key components in emerging technologies such as radio frequency identification as well as wireless sensor networks and short-range communication devices. Finally the bending behavior of the devices and circuits will be evaluated. This research project is expected to lead to new paradigms in terms of large scale synthesis of TMDs via vapor phase approaches and their application for flexible bendable electronics for wireless communication systems.