Polishing and etching damages of ZnO single crystals studied using time-resolved photoluminescence spectroscopy

Controlled thinning of wide bandgap semiconductors by chemo-mechanical polishing (CMP) and/or reactive ion etching (RIE) has been one of the versatile methods for various optoelectronic applications. The influences of CMP and subsequent wet chemical etching, as well as independent RIE, on the room-temperature photoluminescence lifetime for the near-band-edge emission [ t PL NBE ( RT )] of O-polarity c-plane ZnO single crystals were examined by using time-resolved photoluminescence measurements. t PL NBE ( RT ) decreased from a nanosecond range to a few picoseconds (ps) by a conventional CMP, indicating a generation of high-concentration midgap recombination centers, such as nonradiative recombination centers and deep radiative recombination centers. t PL NBE ( RT ) was progressively regained up to 600 ps by a subsequent etching using HCl aqueous solution. However, the recovery saturated at the etching depth of about 200 nm and t PL NBE ( RT ) was not restored even after etching by 350 nm. The results indicate the introduction of certain structural deformations during the CMP. Because x-ray diffraction measurement revealed the presence of incoherent surface domains right after the CMP and the HCl etching gave rise to inhomogeneously etched canyons, nonradiative recombination centers, such as dislocations and vacancy clusters, are likely generated by mechanical shear stresses. t PL NBE ( RT ) also decreased by the RIE. However, the degradation was less significant than the case for the CMP, because RIE scarcely gives mechanical stresses. Interestingly, t PL NBE ( RT ) for the samples etched under higher plasma power was longer than the lower power cases. From the results of x-ray photoelectron spectroscopy measurements, unintentionally deposited oxide films containing Si are proposed to act as an attenuating layer for the introduction of nonradiative recombination centers.