Center-of-mass quantization of excitons in PbI2 thin films grown by vacuum deposition

We have investigated excitonic properties of PbI2 thin films grown on a (001) NaCl substrate by a conventional vacuum deposition technique. The x-ray-diffraction patterns indicate that the crystal structure of the PbI2 thin films is just oriented along the [001] crystal axis. The surface morphology observed by an atomic force microscope shows that the surface roughness is within 1 nm. These results of the structural analysis demonstrate that the PbI2 thin films are suitable for the investigation of quantization effects. The absorption spectra of 8-, 10-, and 15-nm-thick films clearly exhibit oscillatory structures in the energy region higher than the fundamental exciton transition up to similar to 110 meV. This fact indicates the nice realization of the center-of-mass quantization of the exciton; however, the usually used model that is the wave-vector quantization of a parabolic exciton band could not be reasonably applied to explain the quantized exciton energies. The absorption spectra are quantitatively analyzed on the basis of a tight-binding exciton model for the energy dispersion using a nonlocal theory of linear optical response.