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

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.