DEFECTS IN ZINC OXIDE NANOCRYSTALS
Teklemichael, Samuel Tesfai
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Defects in ZnO nanocrystals were investigated. While ZnO has potential for optoelectronic applications, the lack reliable p-type doping remains a major challenge. We provide evidence that ZnO nanocrystals contain uncompensated acceptors. IR absorption peaks at liquid-helium temperatures suggest a hydrogenic acceptor with a hole binding energy of 0.4 0.5 eV. Electron paramagnetic resonance (EPR) measurements in the dark showed a resonance at g = 2.003, characteristic of acceptors that involve a zinc vacancy. An EPR resonance due to vacancy hydrogen complexes was observed after exposure to light.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 the acceptor level observed in the IR. 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. IR absorption peaks at liquid-helium temperatures, which correspond to electronic transitions of the acceptor, disappeared after exposure to formic acid (HCOOH) vapor. This observation is consistent with electrical compensation of the acceptor by the formate ion. The energy level of the formate ion is estimated to be ~ 0.9 eV above the valence-band maximum. Room temperature IR measurements show that the formate species are adsorbed on the surface of ZnO nanocrystals. A broad PL peak centered at 3.2 eV, for samples exposed to HCOOH, is attributed to an exciton bound to a formate species. High field EPR measurements of ZnO nanocrystals showed new resonances, in addition to the previously observed line at g = 2.003. We propose a model of two systems with an axial center of g = 2.003274 and g = 2.007471 along with an isotropic center at g = 2.005254. Samples exposed to HCOOH did not show a complete disappearance of the EPR resonances. Instead, a decrease in intensity is observed.