Study on the characteristics of ZnO films fabricated by AP-MOCVD

• Main Authors: Yen-Chin Huang, 黃彥欽
• Other Authors: Wu-Yih Uen
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/64203145048136347332

博士 === 中原大學 === 電子工程研究所 === 98 === This thesis, focuses on the characterizations of undoped, n- and p- type zinc oxide (ZnO) semiconductors fabricated by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). During the AP-MOCVD process, diethylzinc (DEZn) and water were used as the reactant gases for obtaining Zn and O elements, respectively. First, undoped ZnO films on Si (100) substrate prepared at various deposition temperatures ranging from 300 to 600C were examined and compared. X-ray diffraction analyses and scanning electron microscopy images indicated that the deposition temperature influenced greatly the microstructures of ZnO films. Two morphological transitions occurred at 400 and 600C, respectively, due to the change of dominant grain orientation. The preferential grain orientation evolved from (002), (101), and then to (100) phases, respectively, for the films deposited at 300, 400, and 600C. Both the Hall and photoluminescence (PL) measurements performed at room temperature demonstrated a good-quality film could be obtained with the deposition temperature of about 400C. Next, we also examined and compared undoped ZnO films fabricated on both Si (111) and Si (100) substrates under the same growth conditions. X-ray diffraction (XRD) analyses indicated that the preferred grain orientation of ZnO film grown on Si (111) was (002), while that of the film grown on Si (100) was (101). Cross-sectional transmission electron microscopy (TEM) observations demonstrated that a more homogeneous growth of ZnO film with grains in a columnar shape proceeded on the Si (111) substrate. Moreover, the 13 K PL spectrum of ZnO film deposited on Si (111) was found to be dominated by an emission line at 3.360 eV, differently that of the film deposited on Si (100) was dominated by a line at 3.320 eV. The former is ascribed to the recombination of neutral donor-bound excitons (D0X), while the latter is ascribed to the donor to acceptor pair (DAP) transition. The electrical and optical properties of undoped ZnO films fabricated under various gas flow ratios of [H2O]/[DEZn] (VI/II ratio) ranging from 0.55 to 2.74 were systematically examined. Hall effect measurements exhibited an evident effect of the VI/II ratio on the conduction type of the intrinsic films. An n-type film was fabricated at the VI/II ratio = 0.55; however, p-type ZnO films with the hole concentration of the order of 1017 cm-3 could be achieved at VI/II ratios higher than 1.0. In particular, a highest mobility of 91.6 cm2/V s and a lowest resistivity of 0.369 Ω-cm have been achieved for the specimen fabricated at the VI/II ratio = 1.10. Moreover, room temperature PL measurements demonstrated an interstitial Zn (Zni) donor defect related emission at 2.9 eV for the n-type film, while a Zn vacancy (VZn) acceptor defect related one at 3.09 eV for the p-type films. For n-type ZnO, Ga-doped zinc oxide (ZnO:Ga) films were deposited on glass substrate by AP-MOCVD using triethyl gallium (TEG) as an n-type dopant gas. The structural, electrical and optical properties of ZnO:Ga films obtained at various flow rates of TEG ranging from 1.5 to 10 sccm were investigated. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga-doping plays an important role in forming microstructures in ZnO films. A smooth surface with a predominant orientation of (101) was obtained for the ZnO:Ga film grown at a flow rate of TEG = 7.5 sccm. Moreover, a lowest resistivity of 3.6×10-4 Ω-cm and a highest mobility of 30.4 cm2/V-s were presented by the same sample, as evaluated by Hall measurement. Otherwise, as the flow rate of TEG was increased, the average transmittance of ZnO:Ga films increased from 75% to more than 85% in the wavelength range of 400 - 800 nm, simultaneously with a blue-shift in the absorption edge. On the other hand, we also examined the structural, electrical and optical properties of ZnO:Ga films on p-Si(111) substrates obtained by varying the flow rate of TEG from 2 to 12 sccm. A flat surface with a predominant orientation (101) was obtained for the ZnO:Ga film fabricated at a flow rate of TEG = 10 sccm. This film also revealed a lowest resistivity of 4.54×10-4 Ω-cm, as measured using the van der Pauw method. Moreover, low temperature PL emission recorded at 12 K demonstrated the Burstein Moss shift of PL line from 3.365 to 3.403 eV and a line broadening from 100 to 165 meV as the TEG flow rate varied from 2 to 10 sccm. Finally, effects of annealing on the structural, electrical, and optical properties of ZnO films fabricated on semi-insulating GaAs (100) substrate by AP-MOCVD were investigated. The annealing temperature has been varied from 500 to 650oC. X-ray diffraction analyses and scanning electron microscopy images indicated that As atoms interdiffused from the GaAs substrate into the ZnO films to form Zn3As2 phase and influenced greatly the microstructures of ZnO film. Hall effect measurements exhibited p-type conductivity for ZnO films with the hole carrier concentration in the range of 1018-1019 cm-3. Moreover, both room temperature PL measurements (RT-PL) and low temperature PL measurements conducted at 10 K (LT-PL) exhibited the superior quality of the film annealed at 600C. In particular, the LT-PL demonstrated the luminescence spectra from this film was dominated by the neutral acceptor bound exciton emission at 3.334-3.339 eV. Besides, the ionization energy of acceptor impurity was calculated to be 133-146 meV, approximating to the reported energy level presented by the AsZn-2VZn complex, about 150 meV off the top of valence band. There was also a compound of Zn3As2 produced by thermal annealing. However, the ionization energy of it was reported to be about 930 meV above the top of valence band and therefore would not be possible to play a part in the p-type conductivity of films fabricated. Conclusively, the main doping mechanism of the p-type films fabricated should be the formation of AsZn-2VZn complex defects.