Microvoid formation in hydrogen-implanted ZnO probed by a slow positron beam

ZnO crystals were implanted with 20-80 keV hydrogen ions up to a total dose of 4.4x10(15) cm(-2). Positron lifetime and Doppler broadening of annihilation radiation measurements show introduction of zinc vacancy-related defects after implantation. These vacancies are found to be filled with hydrogen atoms. After isochronal annealing at 200-500 degrees C, the vacancies agglomerate into hydrogen bubbles. Further annealing at 600-700 degrees C causes release of hydrogen out of the bubbles, leaving a large amount of microvoids. These microvoids are annealed out at high temperature of 1000 degrees C. Raman spectroscopy for the implanted sample shows the enhancement of vibration modes at about 575 cm(-1), which indicates introduction of oxygen vacancies. These oxygen vacancies disappear at temperatures of 600-700 degrees C, which is supposed to contribute to the hydrogen bubble formation. Cathodoluminescence measurements reveal that hydrogen ions also passivate deep level emission centers before their release from the sample, leading to the improvement of the UV emission.