Low-temperature synthesis of 2D anisotropic MoTe2 using a high-pressure soft sputtering technique

Abstract

We demonstrate a high-pressure soft sputtering technique that can grow large area 1T′ phase MoTe₂ sheets on HOPG and Al₂O₃ substrates at temperatures as low as 300 °C. The results show that a single Mo/Te co-sputtering step on heated substrates produces highly defected films as a result of the low Te sticking coefficient. The stoichiometry is significantly improved when a 2-step technique is used, which first co-sputters Mo and Te onto an unheated substrate and then anneals the deposited material to crystalize it into 1T′ phase MoTe₂. A MoTe_2−x 1T′ film with the lowest Te vacancy content (x = 0.14) was synthesized using a 300 °C annealing step, but a higher processing temperature was prohibited due to MoTe₂ decomposition with an activation energy of 80.7 kJ mol⁻¹. However, additional ex situ thermal processing at ∼1 torr tellurium pressure can further reduce the Te-vacancy (V_Te) concentration, resulting in an improvement in the composition from MoTe_1.86 to MoTe_1.9. Hall measurements indicate that the films produced with the 2-step in situ process are n-type with a carrier concentration of 4.6 × 10¹⁴ cm⁻² per layer, presumably from the large V_Te concentration stabilizing the 1T′ over the 2H phase. Our findings (a) demonstrate that large scale synthesis of tellurium based vdW materials is possible using industrial growth and processing techniques and (b) accentuate the challenges in producing stoichiometric MoTe₂ thin films.