Carrier gas and VI/II ratio effects on carbon clusters incorporation into ZnO films grown by MOCVD

Undoped ZnO films were deposited by atmospheric metal-organic chemical vapor deposition (MOCVD) on (0001) ZnO substrate. The films were grown at various partial pressure ratios of oxygen and zinc precursors (VI/II) using either N2 or H2 as carrier gas. Micro-Raman scattering was employed to study the effects of carrier gas, VI/II ratio and annealing on carbon impurity incorporation into the ZnO films. Besides the well known phonon modes of ZnO, Raman spectra of the samples grown with N2 carrier gas show two additional broad peaks, which are ascribed to carbon sp2 clusters related modes, spreading in the frequency range 1300–1600 cm−1 and dominate the Raman spectrum of the sample grown under oxygen deficiency (VI/II=0.25). In addition, a band centered at ∼520 cm−1, considered as some defects related local vibrations, appears in the samples grown with N2 as carrier gas and its intensity increases when the VI/II ratio decreases. The average cluster size, estimated from the intensity ratio of D over G bands of the carbon sp2 clusters, ranges from 16.5 to 19.4 Å. However, in all the samples grown with H2 as carrier gas, the bands related to carbon sp2 clusters and defects, are largely suppressed and the second-order-Raman scattering band (1050–1200 cm−1) is clearly observed in addition to the bulk ZnO lattice modes. After annealing the samples at 900 °C in oxygen ambient, the crystal quality has been improved for all the samples but the carbon related bands, formed in the as-deposited films grown with the N2 carrier gas, were only weakened.