Complementary study of the photoluminescence and electrical properties of ZnO films grown on 4H-SiC substrates

We have studied the photoluminescence and electrical properties of ZnO films grown epitaxially by atmospheric pressure MOCVD on 4H-SiC substrates. The dominating D°X line on the low temperature PL spectrum is attributed to the emission of an exciton bound to the neutral donor. The intensity of this line correlates with the electrical properties of the films: the decrease of D°X intensity occurs simultaneously with the increase of the carrier׳s mobility. This we explain as donor activation providing free electrons to the conduction band. Based on the comparison of the calculated value of donor binding energy, the literature data and complementary SIMS analysis a suggested donor impurity is aluminum (Al). The exciton localization energy is 16.3 meV, and agrees well with localization energy of 15.3 meV for Al impurity reported by other authors (e.g. Ref. [33]). The thermal activation energy ED=22 meV, determined from the Hall data and is in agreement with the optical activation energy ~20 meV, which is derived from the temperature-dependent PL study. The calculated value of the donor binding energy of 54.3 eV is in agreement with the ionization energy of 53 meV mentioned in earlier reports for Al in ZnO films. Our results prove that the commonly observed line at ~3.3599 eV on low temperature PL spectra of ZnO is a neutral donor bound exciton emission due to the Al impurity.