  We discuss neutral and charged complexes (biexciton and trion) formed by
indirect excitons in layered quasi-two-dimensional semiconductor
heterostructures. Indirect excitons -- long-lived neutral Coulomb-bound pairs
of electrons and holes of different layers -- have been known for semiconductor
coupled quantum wells and are recently reported for van der Waals
heterostructures such as bilayer graphene and transition metal dichalcogenides.
Using the configuration space approach, we derive the analytical expressions
for the trion and biexciton binding energies as functions of the interlayer
distance. The method captures essential kinematics of complex formation to
reveal significant binding energies, up to a few tens of meV for typical
interlayer distances ~3-5 A, with the trion binding energy always being greater
than that of the biexciton. Our results can contribute to the understanding of
more complex many-body phenomena such as exciton Bose-Einstein condensation and
Wigner-like electron-hole crystallization in layered semiconductor
heterostructures.
