The influences of O/Zn ratio and growth temperature on carbon impurity incorporation in ZnO grown by metal-organic chemical vapor deposition

Abstract: The strong correlations between the O/Zn ratio and carbon impurity incorporation have been observed on the ZnO films grown using N2O or O2 as oxygen source in metal-organic chemical vapor deposition (MOCVD). From in-situ mass spectrometric measurements, the O/Zn ratio in the MOCVD reactor is found to decrease to a minimum value as the growth temperature increased till a critical growth temperature T c, and then increased above T c due to different dissociation rates of the oxygen and Zn sources. The strongest D and G modes, which are ascribed to carbon clusters sp2 related modes, have been observed in Raman scattering spectroscopy for the ZnO samples grown at T c, indicating the highest incorporation rate of carbon impurity in the samples grown at T c. Compared with O2, N2O has a low dissociation rate and that leads to a lower value of O/Zn ratio, resulting in much stronger D and G modes and higher incorporation rate of carbon impurities in the samples grown at T c. It is interesting to note that the lowest specific resistances from Hall effect measurements were also obtained on the samples grown at T c, indicating possible electrical contributions from the formation of carbon clusters, which should be highly conductive regions in ZnO. Furthermore, ionization or addition of H2 in the case of N2O can significantly enhance the dissociation of N2O, with film quality improved significantly. This study shows that a high O/Zn ratio is critical to suppress carbon impurity incorporation and to grow high quality ZnO by MOCVD, especially at low growth temperature. [ABSTRACT FROM AUTHOR]