  A rigorous description of volume and surface spontaneous parametric
down-conversion in 1D nonlinear layered structures is developed considering
exact continuity relations for the fields' amplitudes at the boundaries. The
nonlinear process is described by the quantum momentum operator that provides
the Heisenberg equations which solution is continuous at the boundaries. The
transfer-matrix formalism is applied. The volume and surface contributions are
clearly identified. Numerical analysis of a structure composed of 20
alternating GaN/AlN layers is given as an example.
