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Acceptor behavior of N2O in MOCVD-grown ZnO thin-film transistors
|Ogweon Seo , Haemi Kim , Junho Yun , Jungyol Jo|
Ajou University, wonchondong, Suwon 443-749, Korea, South
ZnO grown by metalorganic chemical vapor deposition (MOCVD) usually shows high density of n-type defects, and we showed that it could be improved by employing growth interruptions . In this work, we studied the effect of nitrogen doping in ZnO films grown by MOCVD. Nitrogen doping was achieved by introducing N2O during growth. Nitrogen concentration was investigated by x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). We observed that N2O doping shifted threshold voltage of thin-film transistor (TFT) by a large amount to positive direction. We explain that this positive shift is evidence of acceptor behavior of N2O. Our MOCVD system has a horizontal reactor operating at atmospheric pressure, using diethylzinc (DEZ) and oxygen for sources. XPS results showed a broad peak at 399.8 eV (N-H bonding), and the amount of nitrogen was estimated to be 0.47%. The presence of nitrogen was also confirmed by SIMS. We fabricated TFT structure (channel length= 15 μm, width= 500 μm, gate oxide thickness= 110 nm) by evaporating Al through a shadow mask. Current-voltage characteristics of two consecutively grown TFT’s were compared. The difference between the two TFT’s is the presence of N2O doping. Before annealing N2O did not cause much difference. After vacuum annealing at 450 deg C, threshold voltage of N2O doped TFT was shifted by +70 V. In contrast, the shift in TFT without N2O was only +30 V. We explain the large shift as a result of hydrogen escape during vacuum annealing. It is well known that N2O induces high concentration of hydrogen, which is n-type dopant. Acceptor behavior of nitrogen would be shielded by the hydrogen. During vacuum annealing, the excess hydrogen will escape, and underlying p-type dopants would show up. We conclude that N2O can be used as p-type dopants in MOCVD grown ZnO TFT's. . J. Jo, O. Seo, H. Choi, and B. Lee, Appl. Phys. Express 1 (2008) 041202.
Presentation: Oral at E-MRS Fall Meeting 2009, Symposium C, by Jungyol Jo
See On-line Journal of E-MRS Fall Meeting 2009
Submitted: 2009-05-16 10:33 Revised: 2009-06-07 00:48