Anomalous behavior of the optical band gap of nanocrystalline zinc oxide thin films

The optical band gap of ZnO films on fused silica in the carrier concentration regime of 10¹⁸−10²⁰/cm³ is reported. Contrary to theoretical predictions there is an anomalous increase in the band gap of ZnO films at a carrier concentration of 5 × 10¹⁸/cm³, followed by an abrupt decrease at a critical concentration of 3−4 × 10¹⁹/cm³ before the optical band gap rises again. Similar observations have been made before, but an explanation of these observations was lacking. We propose a model based on the existence of potential barriers at the grain boundaries, causing quantum confinement of the electrons in the small grains realized in these films. Quantum confinement leads to the initial rise in the optical band gap. On increasing the carrier concentration to the critical value, the potentials at the grain boundaries collapse, leading to the abrupt decrease in the optical band gap. Above this carrier concentration the films behave according to existing many-body theories.