Narrow fluence window of hydrogen-implantation-induced exfoliation in ZnO

Zinc oxide (ZnO) bulk crystals were implanted by 100 keV H+2 ions with various fluences in the range of 5 × 1016 to 3 × 1017 cm−2 and subsequently annealed at temperatures up to 800 °C in order to observe the formation of surface blisters or exfoliation. The ZnO crystals implanted up to a fluence of 2.2 × 1017 cm−2 did not show any surface blistering/exfoliation even after post-implantation annealing, while those crystals implanted at or above a fluence of 2.8 × 1017 cm−2 already exhibited exfoliated surfaces in the as-implanted state. The ZnO crystals implanted with a fluence of 2.5 × 1017 cm−2 showed surface exfoliation only after post-implantation annealing. Cross-sectional transmission electron microscopic images showed that a large number of nanovoids were formed within the implanted zone of ZnO. These nanovoids served as precursors for the formation of microcracks leading to the exfoliation of ZnO layers.