Microstructure and optoelectronic properties of p-type CuAlO2 thin films

• Main Authors: Ruei-Sung Yu, 游瑞松
• Other Authors: Fuh-Sheng Shieu
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/80237923531802459082

博士 === 中興大學 === 材料工程學系所 === 95 === The p-type CuAlO2 films were prepared by dc reactive sputtering technique and annealed under controlled Ar atmosphere. The structural, optical, and electrical properties of the p-type CuAlO2 films were studied. The annealed Cu-Al-O films showed marked structural changes and differing optoelectronic properties with varying annealing temperature. Results of X-ray diffraction demonstrated that CuO and CuAl2O4 were the intermediate reaction phases. XRD also showed that phase grown above 800℃ annealing temperature was pure CuAlO2 phase. Cross-sectional high-resolution transmission electron microscope revealed that the crystallization behavior of the Cu-Al-O films belonged to an outward model. The optimum properties of delafossite structure CuAlO2 film was attained after annealing at 800℃. The surface morphology of CuAlO2 had a cell-like surface appearance and the grain sizes were approximately 20 to 100 nm. The optical direct band gap of the CuAlO2 film was estimated to be 3.11 eV. Hall effect measurements revealed that the CuAlO2 film belonged to the p-type conduction category, with a carrier concentration of 4.81×10e16 cm-3 and the conductivity of 3.8×10e-2 (Ωcm)-1. The optoelectronic properties of the Cu-Al-O system are dominated by the delafossite CuAlO2. This study reports a novel technique for increasing the carrier concentration and the conductivity of the p-type CuAlO2 through doping the material with nitrogen. The X-ray photoelectron spectroscopy and the optical band gap analyses suggested that the nitrogen atoms occupying the interstitial sites of the delafossite structure provided the p-type CuAlO2 with an impurity energy level in the energy gap. It was also found that the N-doped CuAlO2 film had its optimum conduction properties when the dopant level reached 1.1 at. %. Here, the carrier concentration was raised from 4.81×10e16 in the undoped film to 2.13×10e17 cm-3 in the doped film, and the corresponding the film’s conductivity was increased from 3.8×10e-2 to 5.4×10e-2 (Ωcm)-1, as compared with the undoped CuAlO2 film. The anisotropic relaxation behavior of the compressive residual stress of delafossite CuAlO2 was identified to take place on silicon substrate, on which the N-doped CuAlO2 film was grown. Experimental results suggest that in order to release the internal compressive residual stress of the CuAlO2 film, CuO hillocks would be favored to grow on the film surface. It was also proposed that because of the structural anisotropic nature associated with the delafossite CuAlO2, the compressive residual stress was released first by breaking the O-Cu-O bonds of the dumbbell layers and subsequently by the diffusion of Cu and O atoms along the a-axis direction on the close-packed Cu layers, suggesting that the c-axis direction across the AlO6 octahedral layers has a greater resistance to compressive residual stress.