Defects in undoped pulsed laser deposition grown ZnO

Abstract

Zinc oxide (ZnO) has received great research interests in the past decade. Its outstanding physical properties and potential applications in optoelectronics make ZnO an important semiconducting material. It is essential to study the defects in ZnO for the realization of device application since defects determine the material’s electrical and optical properties. We report our results on studying defects in undoped pulsed laser deposition (PLD) grown ZnO thin film using Raman spectroscopy, photoluminescence (PL), positron annihilation spectroscopy (PAS), and secondary ion mass spectroscopy (SIMS). The influence of the growth oxygen pressure, and post-growth annealing are systemically studied. The samples are grown at the substrate temperature of 300 oC. Two defect-related Raman peaks namely at 560cm-1 and 584cm-1 are observed in the samples. Stoichiometric study shows that these two peaks are suppressed by oxygen-rich growth environment. The 584cm-1 peak anneals out at 600-750 oC. This peak has position and annealing temperature coinciding well with the values reported for oxygen vacancy. The 560cm-1 anneals out at a higher temperature between 800oC and 900oC. The 560cm-1 has been previously assigned to Zinc interstitial. However because of the low migration barrier, this assignment is suspicious as it has a high annealing temperature. The S-W parameter plot of the PAS measurements shows that zinc vacancyrelated exists in samples. Thermal induced microstructure change of the zinc vacancy-related defect is observed after the 750 oC annealing. Hall measurement results show that the sample grown without oxygen has electron concentration of 4E19 cm-3 and mobility of 10 cm2 V-1s-1. After the 750 oC annealing, its electron concentration drops to 5E18 cm-3 and mobility increases to 60 cm2V-1s-1. The increase of the mobility is associated to the improvement of the crystalline quality as evidenced by the X-ray diffraction measurement, as well as the annealing out of the oxygen vacancy which would limit the carrier mobility through vacancy scattering. The significant drop of the carrier concentration is attributed to the removal of hydrogen, which is clearly shown by the SIMS study on the samples. No defect emission is found in the 10 K PL spectrum of the as-grown sample grown in vacuum. Green luminescence (GL) with peak position at 2.45 eV is induced after the 900 oC annealing. The GL has a fine structure with subsequent peaks separated by 70meV. We have also carried out 10 K PL measurement on the Cu-doped (1%) ZnO sample to study for the correlation between the GL and the Cu impurity. Similar findings are observed, i.e. GL is induced after the annealing but not observed in the as-grown sample. Moreover, the characteristic fine structure and the intensity of the GL are very similar to those observed in the annealed undoped sample.