  We investigate numerically different phases that can occur at half filling in
the lowest and the first excited Landau levels in wide-well twodimensional
electron systems exposed to a perpendicular magnetic field. Within a
twocomponent model that takes into account only the two lowest electronic
subbands of the quantum well, we derive a phase diagram that compares favorably
with an experimental one by Shabani et al. [Phys. Rev. B 88, 245413 (2013)]. In
addition to the compressible composite-fermion Fermi liquid in narrow wells
with a substantial subband gap and the incompressible twocomponent (331)
Halperin state, we identify in the lowest Landau level a rectangular Wigner
crystal occupying the second subband. This crystal may be the origin of the
experimentally observed insulating phase in the limit of wide wells and high
electronic densities. In the second Landau level, the incompressible Pfaffian
state, which occurs in narrow wells and large subband gaps, is also separated
by an intermediate region from a large-well limit in which a similar
rectangular Wigner crystal in the excited subband is the ground state, as for
the lowest Landau level. However, the intermediate region is characterized by
an incompressible state that consists of two four-flux Pfaffians in each of the
components.
