  Transition metal dichalcogenide (TMD) heterobilayers provide a versatile
platform to explore unique excitonic physics via properties of the constituent
TMDs and external stimuli. Interlayer excitons (IXs) can form in TMD
heterobilayers as delocalized or localized states. However, the localization of
IX in different types of potential traps, the emergence of biexcitons in the
high-excitation regime, and the impact of potential traps on biexciton
formation have remained elusive. In our work, we observe two types of potential
traps in a MoSe$_2$/WSe$_2$ heterobilayer, which result in significantly
different emission behavior of IXs at different temperatures. We identify the
origin of these traps as localized defect states and the moir{\'e} potential of
the TMD heterobilayer. Furthermore, with strong excitation intensity, a
superlinear emission behavior indicates the emergence of interlayer biexcitons,
whose formation peaks at a specific temperature. Our work elucidates the
different excitation and temperature regimes required for the formation of both
localized and delocalized IX and biexcitons, and, thus, contributes to a better
understanding and application of the rich exciton physics in TMD
heterostructures.
