Molecular beam epitaxy of the van der Waals heterostructure MoTe2 on MoS2: phase, thermal, and chemical stability

(Sub) monolayer MoTe2 is grown by molecular beam epitaxy on a bulk MoS2 substrate. The film morphology, the thermally induced transformation of structural and compositional phases, as well as the chemical stability upon exposure to atmosphere are investigated by scanning tunneling microscopy and photoemission spectroscopy. Predominantly, semiconducting alpha-MoTe2 islands are obtained under tellurium rich growth conditions and a substrate temperature of 200 degrees C. Under less tellurium-rich conditions, elongated and meandering MoTe2-x strands are formed rather than compact islands. Similarly, annealing of initial alpha-MoTe2 islands to above 500 degrees C causes the loss of tellurium and possibly transformation into the same MoTe2-x strands. Consequently, under vacuum conditions the the transformation of alpha-MoTe2 monolayers into the semimetallic beta-MoTe2 high temperature phase is accompanied by a loss of Te and formation of MoTe2-x phase. The obtained tellurium deficient MoTe2-x phase is almost metallic but a small band gap of a few tens meV remains. The as-grown alpha-MoTe2 islands exhibit a moire structure with similar to 2.6 nm periodicity. This periodicity implies a rotation of similar to 56 degrees between the MoTe2 and MoS2. We assign the observation of a specific rotation angle for the grown MoTe2 islands with respect to the MoS2 substrate to the lowest energy adsorption configuration for MoTe2 monolayers on MoS2 substrates. Exposure of the as grown films to atmosphere results in oxidation of the MoTe2 film. The oxidized film maintains the two-dimensional island morphology of the initial film and thus is a candidate for a 2D(amorphous) oxide layer on MoS2.