| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 98 === NiO has the rock salt structure of p-type semiconductors. Its electrical conductivity has made NiO useful in a wide range of applications, such as transparent conductive films, solar cells, electrochromic devices, and gas sensors. Although a number of reports have discussed the properties of sputtered NiO films for various parameters, some electrical properties of sputtered NiO are still unclear.
The dominant point defects in sputtered NiO films are unknown. According to defects chemistry, the major carriers of a p-type NiO semiconductor come from the vacancies of metal ions or the interstitial oxygen ions formed in NiO crystal. It is unclear which one is dominant. Sputtered NiO film experiences electrical aging. The resistivity of the sputtered film increases with time exposed to air. Electrical aging is not common in n-type semiconductors and it has been rarely discussed in p-type semiconductors. However, it greatly affects the longevity and applications of devices. Therefore, the study of its mechanism and suppression is very important. Finally, we discuss some reduction properties of NiO film annealed in a vacuum environment. Although reduction has been previously reported, its mechanism is still ambiguous.
To investigate the electrical conduction mechanism, an RF sputter and a NiO target were used to deposit NiO films. The composition of double layer films were adjusted by changing the working gas and the in-depth composition were measured by SIMS and XPS. The results show that the degree of non-stoichiometry of NiO films was determined by Ni content. Also, the coordination numbers of the annealed NiO films were measured by X-ray absorption spectroscopy to confirm the in-depth composition data. The results show that non-stoichiometry NiO contains more oxygen atoms than stoichiometric NiO. When the composition was made close to that of stoichiometry via the annealing process, the first coordination number shell does not change while the second coordination number shell increased, which implies that the composition change results from a change in Ni atoms. It was concluded that Ni vacancies are the dominant point defects that result in the electrical conductivity of NiO films.
To investigate electrical aging, stability tests were performed in various dry atmospheres (H2, CO, O2, CO2, N2 and Ar) and in a humid Ar environment. The stability was determined by measuring the resistance change with time. The results show that electrical aging was caused by the adsorption of reduction gas and water vapor, which injected electrons into the NiO film. These electrons then counteracted with the redundant electric holes and attenuated the charge carriers in the NiO film. This reaction lowered the carrier concentration; the electrical conductivity of NiO film subsequently decayed with time.
Electrical aging is due to gas adsorption; which greatly affected by film structure. In this study, the substrate temperature and Li doping were used to the suppress aging. The results show that an increase of substrate temperature changes the crystal structure. The preferred orientation changes from polar (111) into non-polar (200); this change starts from the top surface to the substrate. The dangling bonds on (111) surface increase the aging phenomena. The formation of non-polar (200) can decrease the aging rate of sputtered NiO films.
For Li doping, the concentration of Li in the thin films was adjusted by placing 0~15 Li2O disks on the target surface. The Li concentration in the films varied from 0 to 16.29 at.%, as determined by WDS and ICP-MS. The results show that the doped Li ions occupy crystal defect sites such as vacancies or segregate on the film surface. Initially, doped Li occupied the Ni vacancies in the film, decreasing electrical conductivity. When the Li concentration was further increased, some Li segregated on the film surface and formed bulges at high Li concentrations. These Li-rich oxides which covered the film surface served as partitions between the film and moisture from the atmosphere. Also, the doping process decreased the (111) peak, which means that it suppresses the formation of the (111) plane. As a result, the Li-doped NiO films show a relatively high arrestment to electrical resistance aging.
The effects of annealing temperature, substrate material, and gas atmosphere on reduction properties were studied. The results show that reduction is also related to the non-stoichiometry of the sputtered NiO films. The reduction occurs from the film surfaces and the reduction depth is about 100 nm.
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