Low Temperature Gate Oxide ALD on Transition Metal Dichalcogenides
Transition Metal Dichalcogendes(TMDs) have attracted attention for future electronic devices due to their excellent electronic and optoelectronic properties. These devices require few nanometer thick and pin hole-free dielectric layers as gate insulators. However, due to the inert nature of TMDs such as MoS2 WSe2, the dielectric layer selectively nucleates on defect sites or step edges. In the conventional atomic layer deposition (ALD) process on the materials, such non-uniform oxides result in large leakage currents in TMD based device. Therefore, for successful integration into device, uniform and insulating gate oxides on TMDs should be prepared. In this work, Al2O3 was directly deposited on MoS2 surface by low temperature ALD with trimethylaluminum(TMA) and H2O without any seeding layer or surface treatments. Using short purge time between two precursor pulses at 50C, a CVD growth component was intentionally induced to provide more nucleation sites on the surface. The CVD growth component induces deposition of 1 nm Al2O3x particles on the surface which provide a uniform layer of nucleation centers. Before ALD, MoS2 samples were cleaned by mechanical exfoliation method. 50 cycles of Al2O3 ALD was deposited at 50C using 600ms of TMA and 50 ms of H2O pulse time with 500ms purge time between two pulses. The same ALD recipes were performed on SiGe substrates in order to compared the quality of the oxide. In order to obtain higher Cox value, 10 cycles of Al2O3 ALD was deposited at 50C for a ALD seeding layer and 40 cycles of HfO2 ALD was deposited with Tetrakis(dimethylamido)hafnium(TDMAH) and H2O at 250C. After ALD process, MOSCAP devices were fabricated to measure the capacitance and leakage current of the oxide. Non-contact mode AFM was performed to check the topography of the oxide and the results showed that uniform and pin hole-free oxide layer was formed on the surface. The leakage current of the oxide on MoS2 was as low as 10 -5 A/cm 2 which was comparable to the oxide on SiGe substrates.