By selective wet etching, freely suspended thin slabs can be obtained in epitaxial heterostructures. This technique is employed to create 2D photonic crystals for infrared applications in thin suspended AlGaAs membranes. The active areas contain regular arrays of dry etched nano-sized holes. For an appropriate selection of the slab thickness and the lattice parameter, the guided modes are explicitely computed by solving the Maxwell equations in a geomety containing a waveguide-embedded 2D photonic crystal. Presently, lattice parameters down to 400 nm can be realized, corresponding to a predicted photonic band gap for even modes centered around 1 micrometer.