Acceptor and surface states of ZnO nanocrystals: a unified model

Semiconductor nanocrystals have the potential for a range of applications in optoelectronics and nonlinear optics. As the surface-to-volume ratio increases, surface emission processes become more important. Using infrared (IR) and photoluminescence (PL) spectroscopy, we have developed a unified model for the acceptor and intragap surface states of ZnO nanocrystals. A PL peak was observed at 2.97 eV, in agreement with an acceptor level previously observed in the IR (Teklemichael et al 2011 Appl. Phys. Lett. 98 232112). The temperature dependence of the IR absorption peaks, which correspond to a hole binding energy of 0.46 eV, showed an ionization activation energy of only 0.08 eV. This activation energy is attributed to thermal excitation of the hole to surface states 0.38 eV above the valence band maximum. Therefore, while the acceptor is deep with respect to the bulk valence band, it is shallow with respect to surface states. A strong red PL emission centered at 1.84 eV, with an excitation onset of 3.0 eV, is attributed to surface recombination.