Effect of Oxygen Adsorbates on Terahertz Emission Properties of Various Semiconductor Surfaces Covered with Graphene

We have studied coherent terahertz (THz) emission from graphene-coated surfaces of three different semiconductors-InP, GaAs, and InAs-to provide insight into the influence of O-2 adsorption on charge states and dynamics at the graphene/semiconductor interface. The amplitude of emitted THz radiation from graphene-coated InP was found to change significantly upon desorption of O-2 molecules by thermal annealing, while THz emission from bare InP was nearly uninfluenced by O-2 desorption. In contrast, the amount of change in the amplitude of emitted THz radiation due to O-2 desorption was essentially the same for graphene-coated GaAs and bare GaAs. However, in InAs, neither graphene coating nor O-2 adsorption/desorption affected the properties of its THz emission. These results can be explained in terms of the effects of adsorbed O-2 molecules on the different THz generation mechanisms in these semiconductors. Furthermore, these observations suggest that THz emission from graphene-coated semiconductors can be used for probing surface chemical reactions (e.g., oxidation) as well as for developing O-2 gas sensor devices.