Ion-beam-produced structural defects in ZnO

We study the evolution of lattice defects in single-crystal ZnO bombarded with 60-keV Si-28 and 300-keV Au-197 ions at 77 and 300 K. To characterize ion-beam-produced structural defects, we use a combination of Rutherford backscattering/channeling (RBS/C) spectrometry, cross-sectional transmission electron microscopy (XTEM), x-ray photoelectron spectroscopy, and atomic force microscopy. Results show that ZnO exhibits strong dynamic annealing, and even high-dose bombardment with heavy (Au-197) ions at 77 K does not render ZnO amorphous. However, a crystalline-to-amorphous phase transition can be induced by irradiation with relatively light Si-28 ions. In this latter case, amorphization is attributed to strong chemical effects of Si atoms implanted into the ZnO lattice, resulting in the stabilization of an amorphous phase. High-dose heavy-ion bombardment also results in a strong stoichiometric imbalance (loss of O) in the near-surface region. A variation in irradiation temperature from 77 up to 300 K has a minor effect on the damage buildup behavior in ZnO bombarded with Au ions. Data analysis also shows that a variation in the density of collision cascades by increasing ion mass from Si-28 up to Au-197 has a negligible effect on the damage buildup behavior. For both light- (Si-28) and heavy- (Au-197) ion bombardment regimes, XTEM reveals that ion irradiation produces energetically favorable planar defects which are parallel to the basal plane of the wurtzite structure of ZnO. Interestingly, our RBS/C study also reveals the formation of a middle defect peak between the surface and bulk peaks of disorder in Au-implanted ZnO, but not in Si-bombarded samples. The formation of this middle peak, most likely to be related to complex defect agglomeration processes, is rather unexpected and, to our knowledge, has not been observed in any other material. Physical mechanisms of defect formation in ZnO under ion bombardment are discussed based on these experimental findings.