Band Alignment Study for CVD Grown Vertically Stacked Different Configurations of Monolayer MoS₂-ReS₂ Photodetector

This work demonstrates different configurations of vertically stacked two step CVD (chemical vapour deposition) grown monolayers of MoS₂-ReS₂ and stacking significance in tailoring band type and hence charge migration phenomenon upon visible illumination. The MoS₂/ReS₂ heterostructure is popularly known for its high stability, greater light matter interaction due to high absorption coefficient, strong in-plane carrier mobility of MoS₂,direct bandgap of ReS₂ irrespective of the number of layers ,tunable transport mechanism upon different stacking configurations and greater responsivity of ReS₂ due to its high density of states .To better understand the issue of stacking for different configurations i.e. ReS₂-MoS₂(MoS₂ on top) or MoS₂-ReS₂ (ReS₂ on top), the optical properties are examined by micro-photoluminescence wherein it was interestingly observed a change in band type in accordance with stacking order for different stacking configurations i.e. Type-I for ReS₂ on top and Type-II for MoS₂ on top. Furthermore, the morphology was examined to confirm the stacking of vertical heterostructure by HR-TEM which clearly reveals the formation of ReS₂ on MoS₂ and vice-versa. Photodetection experiment was conducted for devices having different stacking configurations and it was noted that ReS₂ on top (Type-I) and MoS₂ on top (Type-II) exhibits responsivity of 400 A/W and 72 A/W respectively under visible illumination. It was clearly observed that the responsivity of Type-I band alignment is much higher than that of Type-II band alignment. The increment in the responsivity value when ReS₂ is on top (Type-I band alignment) is due to the effective separation of photogenerated carriers to MoS₂ and high symmetric crystallographic orientation that was confirmed by low temperature PL studies and extracted band structure. It is noted that suitable stacking plays a vital role in deciding suitable band alignment and accordingly the desired responsivity. The insightful study of such 2D vertically stacked heterostructure configurations are important to conveniently decide the best version of device in developing high responsive photodetectors.