  We investigate exciton bound-state formation and crystallization effects in
two-dimensional electron-hole bilayers. Performing unbiased path integral Monte
Carlo simulations all quantum and Coulomb correlation effects are treated on
first principles. We analyze diverse pair distribution functions in dependence
on the layer separation, particle density and hole-to-electron mass ratio and
derive a schematic phase diagram for the neutral mass-asymmetric bilayer
system. Our simulations reveal a great variety of possible phases namely an
exciton gas, an exciton crystal, an electron-hole liquid and a hole crystal
embedded in an electron gas.
