Solution processed zinc oxide thin film transistors (TFTs) were investigated for spacial identification of instability inducing electronic trap states by utilizing surface-to-active-channel distance dependent analysis. It is shown that the performance and stability of zinc oxide TFTs deposited by spray pyrolysis strongly depend on the surface-to-channel distance and herewith on the film thickness in the investigated regime from 1 nm to 30 nm. In thin layers, the charge transport process is dominated by the number of percolation paths and near channel trapping processes due to coulomb interactions with surface charges. This leads to a high thickness of 3 nm for the percolation threshold. As soon as a closed layer is formed and the charge separation of 7 nm between surface and active channel is exceeded, bulk properties become more dominant. A maximum linear mobility of 11cm2 V−1 s−1 and an on-set voltage of 2 V were obtained for a film thickness of 30 nm. An increase of the film thickness from 10 nm to 30 nm leads to a reduction in the trap rate by one order of magnitude from 4.3 × 108 cm−2 s−1 to 3.7 × 107 cm−2 s−1. Due to this, the bias stress stability and the long term storage stability were found to improve significantly.
A large lateral size and low dimensions are prerequisites for next generation electronics. Since the first single layer MoS transistor reported by Kis's group in 2011, layered transition metal dichalcogenides (TMDs) have been demonstrated to be the ideal candidate for next generation electronics. However, the development of large scale and low cost growth techniques is a crucial step towards TMDs' inclusion in modern electronics and photoelectronics. In this work we develop a cheap, wet chemical, and environment friendly deposition process for two dimensional MoS flakes with extended size. For our deposition process, ammonium tetrathiomolybdate (ATTM) dissolved in deionized water was used as precursor solution and was deposited on a SiO/Si substrate through a Langmuir-Blodgett like deposition process. To our knowledge, this is the first time MoS flakes have been grown in an aqueous solution. Large-sized MoS flakes exceeding 150 μm in lateral size were obtained after thermal decomposition. Thicknesses ranging from a monolayer to 5 monolayers were confirmed by AFM and Raman spectroscopy. Further investigations revealed that the quality of the produced flakes strongly depends on the post growth thermal treatment and its atmosphere. This simple and nontoxic deposition method is suitable for the preparation of large (hybrid) transition metal dichalcogenide nanostructures for applications in next generation electronics.
Zinc oxide thin film transistors (TFTs) deposited by continuous and pulsed spray pyrolysis were investigated to analyze process kinetics which make reduction of process temperature possible. Thus, fluid mechanics, chemical composition, electrical performance, and deposition and annealing temperature were systematically analyzed. It was found that ZnO layers continuously deposited at 360 °C contained zinc oxynitrides, CO3, and hydro carbonate groups from pyrolysis of basic zinc acetate. Statistically, every second wurtzite ZnO unit cell contained an impurity atom. The purity and performance of the ZnO-TFTs increased systematically with increasing deposition temperature due to an improved oxidation processes. At 500 °C the zinc to oxygen ratio exceeded a high value of 0.96. Additionally, the ZnO film was not found to be in a stabilized state after deposition even at high temperatures. Introducing additional subsequent annealing steps stabilizes the film and allows the reduction of the overall thermal stress to the substrate. Further improvement of device characteristics was obtained by pulsed deposition which allowed a more effective transport of the by-products and oxygen. A significant reduction of the deposition temperature by 140 °C was achieved compared to the same performance as in continuous deposition mode. The trap density close to the Fermi energy could be reduced by a factor of two to 4 × 1017 eV−1 cm−3 due to the optimized combustion process on the surface. The optimization of the deposition processes made the fabrication of TFTs with excellent performance possible. The mobility was high and exceeded 12 cm2/V s, the subthreshold slope was 0.3 V dec−1, and an on-set close to the ideal value of 0 V was achieved.
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