Low Temperature ALD of High-K Oxides on 2D Materials
2D materials have attracted attention for future electronic devices due to their excellent electronic and optoelectronic properties. These devices require ultra-thin and pin hole-free dielectric layers as gate insulators. However, due to the inert nature of 2D semiconductors such as graphene and Transition Metal Chalcogenides (TMDs), the dielectric layer selectively nucleates on defect sites or step edges. In the conventional atomic layer deposition (ALD) process on graphene or other 2D semiconductors, such non-uniform oxides result in large leakage currents in 2D semiconductor based device. Therefore, for successful integration into device, uniform and insulating gate oxides on 2D semiconductors should be prepared.
In this work, Al2O3 was directly deposited on HOPG and MoS2 surface by low temperature ALD with trimethylaluminum(TMA) and H2O without any seeding layer or surface treatments. Before ALD, HOPG and MoS2 samples were cleaned by mechanical exfoliation method. 50 cycles of ALD were employed, each cycle consisting of a sequence of 600 ms TMA pulse, 500 ms Ar purge, 50 ms H2O pulse, and 500 ms Ar purge. 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 1-2 nm Al2Ox particles on the surface which provide a uniform layer of nucleation centers. In order to obtain higher Cox value, bilayer ALD with Al2O3 and HfO2 was employed. For initial nucleation on the surface, 10 cycles Al2O3 film was deposited at 50C. Then 40 cycles of HfO2 was deposited with TDMAH (Tetrakis(dimethylamido)hafnium) and H2O at 200 C. In the same ALD recipes were performed on Si0.7Ge0.3(001) substrates in order to compared the quality of the oxide.
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. Electrical characterization showed that insulating Al2O3 oxide with Cmax of (1.2 micro F/cm2) was deposited on and HOPG substrates with low temperature ALD. The leakage current of the Al2O3 on HOPG was as low as 10(-5) A/cm2 which was comparable to the oxide on Si0.7Ge0.3(001) substrates. In case of the bilayer ALD, Cmax value was increase to 2 micro F/cm2 and the Cmax of HOPG and MoS2 substrates is close to the value of Si0.7Ge0.3(001).