Preparation And Properties Of P-type Transparent Conducting Oxide CuAlO₂ Thin Films

Based upon the theory of "the chemical modulation of the valence band", was firstly discovered as a p-type transparent conducting oxide (TCO), which has the especial optical and electrical properties. The discovery of p-type TCO made possible the fabrication of transparent oxide optoelectronic devices such as transparent p-n junction diodes and transistors using an appropriate combination of p-and n-type TCO films, which also led TCO material to the frontier of transparent oxide semiconductor (TOS). However, the preparation of CuAlO₂ film with excellent properties always is a very difficult case. Up to now, several techniques, such as pulse laser deposition, plasma enhanced chemical vapor deposition and sputtering, have been used to synthesize CuAlO₂ films, which are very different on the structure and optical/electrical properties. Considering issues about the present developing state of CuAlO₂ film and the superiority of sputtering in the industrialization, the dissertation is aimed at fabricating high-quality p-type CuAlO₂ films on quartz glass and Si substrates using rf magnetron sputtering, the main research processes achieved have been summed up as following:1. The optimized parameters have been obtained successfully to deposit Cu-Al-O films on quartz glass and Si substrates. The determination of chemical composition and valence has been solved. CuAlO₂ phase is found the dominator in Cu-Al-O films and a little Cu₂O also exists. The transmittance of Cu-Al-0 film with the thickness around 300 nm is about 60⁷0 % in the visible light range. The direct and indirect band gaps of the films are estimated by the linear fitting around 3.52 and 1.83 eV, respectively. The minimal resistivity of Cu-Al-0 film is 2.2×10²Î©cm at room temperature. Temperature dependence of the conductivity is in accord with the Arrhenius rule, indicating that the film is of semiconducting thermal-activation type in the near room temperature range.2. Actively utilized the anisotropic conductivity property (σ_ab>>σ_c) of CuAlO₂, the decrease of the resisitivity around three orders of magnitude for CuAlO₂ film is realized by the optimized annealing technique (in N₂ ambient at 900℃for 5 h). For the annealed CuAlO₂ film, the transmittance is around 60 % in the visible light range and above 80 % in the near infrared light. It is found that there are four direct band gaps around -3.00, -3.15, -3.50 and -3.75eV, respectively, estimated from the transmittance and reflectance data of the annealed films. They might be related with the different direct transitions in the Brillouin zone. The excellent ohmic contact is obtained between Ag electrodes and the annealed films, and the specific contact resistance can be decreased to 0.32Ωcm-2. The minimal resistivity of the annealed CuAlO₂ film is 37Ωcm, which is lower three orders of magnitude than that of the as-deposited one. Temperature dependence of conductivity for p-type CuAlO₂ film can be described by a thermal-activation theory when the temperature is above 190 K, but below 185 K a two-dimension variable-range hopping mechanism becomes dominant.3. For the native p-type CuAlO₂ film, adequate rich-oxygen is in favor of the improvement of the conductivity. However, the effect of the rich oxygen atoms on the structural properties for annealed CuAlO₂ film is also very obvious. The annealed films deposited at different oxygen parital pressures were characterized by XRD, Raman and AFM. It is found that the film prepared at 20 % oxygen partial pressure exhibits the excellent structure property. When the oxygen partial pressure is above 20 %, the anisotropic expansion behavior of CuAlO₂ will be obviously aggravated due to the excess oxygen atoms in interstitial position, which results in some microscopic cavities appeared in the surface of the films to release the internal stress during the annealing treatment. Moreover, the cavities become large with increasing oxygen concentration, which gradually degenerate the CuAlO₂ film to the amorphous state up to 60 % oxygen partial pressure.4. The p-CuAlO₂/n-Si heterojunction has been firstly prepared through sputtering CuAlO₂ film on low resistance Si substrate. Based upon the linear I-V characterizations of Ag/Si/Ag and Ag/CuAlO₂/Ag, the p-CuAlO₂/n-Si junction is detected a good rectifying property with the cut-in voltage of 0.5 V. Because the carrier concentration of n-Si is larger 3-4 orders of magnitude than that of p-CuAlO₂, the heterojunction can be fitted by the theory of p-n⁺ one-sided step junction. It is found that the effect of the interface state and series resistance is not neglected for the rectifying property of the junction. Moreover, the series resistance is fitted around 13Ωfor p-CuAlO₂/n-Si heterojunction.5. N doped CuA102 films have been prepared successfully by sputtering under the mix ambience of N₂O, O₂ and Ar. N concentration detected by AES is 5.9 at.% for the film doped with 15 % N₂O specific flux, indicating that acceptor impurities N are doped into CuAlO₂ films indeed. The optimal N doped film shows the minimal resistivity of 10Ωcm and the maximal hole concentration of 10¹⁶cm^-3, which are decreased and increased one order of magnitude comparing with the undoped CuAlO₂ thin film, respectively. The transmittance of N doped CuAlO₂ films are 60-70 % in the visible light range and above 85 % in the near infrared light. A blue-shift of optical absorption edge is observed for N doped CuAlO₂ film, which might be due to the Burstein-Moss effect.