Preparation And Characterization Of Nb-doped ZnO-based TCO Films By PLD

Zinc Oxide is a kind ofâ…¡-â…£group compound semiconductor material with a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature(RT).As an intrinsic n-type semiconductor having a hexagonal wurtzite structure,ZnO could realize a lower resistivity via impurity-doping,such as Al,Ga and Zr,while still keeping a high transmittance in the visible region.In comparison with other host oxides of TCOs,for example ITO,ZnO has been widely studied due to its non-toxicity,low cost,good thermal stability,and easy fabrication. In recent years,large-screen,high-definition liquid crystal displays(LCDs)have developed rapidly,and the demand is increasing continuously.While the global lack of energy,fuel and the request of natural environmental protection,provide solar cells a great development prospects.So it is really a big chance for ZnO-based TCO films to step on the stage of industrial application.In this thesis,Nb-doped ZnO-based(NZO)TCO films have been prepared on glass and quartz substrates by pulsed laser deposition(PLD)using high-purity Nb₂O₅-ZnO ceramic targets with different Nb contents.Though the comparative study of changing the substrate temperature,growth pressure and other parameters on the effect of crystallinity,surface morphology,electrical and optical properties,suitable process parameters for growing NZO films have been found and optimized.And a good repeatability has been realized.All the NZO films grown in the substrate temperature region between 100℃and 500℃are of acceptable crystallinity with a single-phase wurtzite structure and a high preferential c-axis orientation.And the crystallinity of NZO films increases evidently as the temperature increases,suggesting that the crystallinity is improved at a high growth temperature.Furthermore,the NZO films grown above 300℃have reasonably good crystallinity,with a smooth surface,small roughness and no obvious grain boundary.In the low temperature region(below 350℃),the resistivity of the film decreases with the increasing substrate temperature,and the mobility shows the opposite trend, while the electron concentration slightly changes.However,with further increasing the substrate temperature,a rapid increase in the resistivity appears,which is ascribed to the large reduction in the electron concentration.All the NZO films exhibit a high transmittance(above 85%)in the visible region, regardless of the deposition temperature,having sharp absorption edge in the UV range,which shifts to the longer wavelength side as the substrate temperature increases.Oxygen pressure is also an important parameter,which affect the deposition of NZO films.However its effect is slight comparing with the substrate temperature. Generally speaking,0.1 Pa is the best condition.The resistivity of the NZO films is found to initially decrease with increasing Nb content up to 1 at%,and then gradually increases with a further increase in the Nb content,while the carrier concertration shows the opposite trend.This decrease in mobility with increasing Nb content is associated with the enhancement of scattering possibility in the NZO films.The optical properties are intensively related to the Nb contents,ie carrier concentration.The obtained film deposited in the optimized condition exhibited quite good optical and electrical properties with a high transmittance above 85%,a resistivity of less than 6×10^-4Ωcm,a hall mobility of greater than 20 cm²/Vs,and an electron concentration of about 5×10²⁰cm^-3.Guided by x-ray photoemission spectroscopy analysis,Nb_Zn³⁺is believed to be the very possible donor in the Nb-doped ZnO films. A near-band emission peak at 3.3 eV is seen in the RT photoluminescence spectrum. No obvious deep-level defect peak is observed,indicating NZO films process a good crystallinity.