4.1.1 Surface morphology
Figure 4-1 shows the SEM image of no thermal treatment IGZO sample. We can observe that lots of particles on IGZO thin film. From the view of CVD process, the precursor from the nozzle may be nucleation in the gas phase and then formation the particles fall on the thin film surface and then migrates on the surface, finally nucleation on the surface to form the IGZO thin films. Because of the sample had not be thermal treatment, the particles have not enough energy to migrate. Therefore, there are many particles on the thin film surface and the IGZO film is sparse.
Figure 4-2 and figure 4-3 show the top view of SEM images of IGZO thin films which represent the different annealing temperature in nitrogen and oxygen ambiance, respectively. We can observe that the IGZO thin molecules in the films. When the films are be thermal treated, the water molecules will be released from the IGZO films and the structure of films
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become compact. Evidently, the IGZO thin films have greater quality with thermal annealing.
We also use the AFM to analyze the surface morphology which is show in figure 4-4 and figure 4-5 represented the IGZO thin films of different annealing temperature. The results of AFM analysis were consistent with SEM analysis. In summary, postannealing treatment can produce IGZO thin films that have better quality and reduce surface roughness for achieving good transfer characteristics and stability of TFTs.
4.1.2 Structure properties
Figure 4-6 and figure 4-7 show the XRD patterns with different annealing temperature in nitrogen and oxygen ambiance, respectively. In the figure 4-6, there is no prominent peak in XRD pattern which means that the sample of IGZO thin film with no annealing has an amorphous phase. So far, we understand that the IGZO thin film deposited by APPJ system is amorphous state, because we prepared IGZO thin films by solution based precursor and the process in APPJ system under low temperature. After thermal treatment from 200°C to 500°C, we observe that the XRD patterns without significant change between no annealing sample. Even at a relatively high annealing temperature of 500°C, the IGZO thin films have an amorphous phase. In the figure 4-7, we also observe that the XRD patterns which annealing in oxygen ambiance have no obvious peak similar to the XRD patterns which annealing in nitrogen ambiance, However, the IGZO thin films have an amorphous phase in the
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different anneal ambiance. We successfully deposit IGZO thin films with amorphous state which conduce as active layer of TFT application.
The XRD results are in good agreement with the high-resolution TEM image of the sample annealed at 500°C shown in figure 4-8. In the case of the sample annealed at 600°C, it is observed that a very small amount of crystallization occurs with tiny nanoparticles being formed. In the figure 4-8 shows the nanocrystalline with a size less than 5 nm in the IGZO thin film. Because of the thermal treatment that provide thermal energy to change the phase, the partial structure of IGZO change phase from amorphous to nanocrystalline. However, the low density and small size of these crystals result in their producing no diffraction peaks in the XRD measurements.
In the figure 4-9, it shows the thickness of native oxide on silicon wafer. After the silicon wafer cleaned by RCA process, the wafer will temporarily expose to air environment when it prepared to deposit gate dielectric. The native oxide will add the thickness of gate dielectric and decrease the capacitance of gate dielectric. Because of the rough surface of native oxide, it maybe affects dielectric quality. The native oxide must be as thin as possible to reduce the influence on device property. In the figure 4-9, we can see the native oxide with a thickness of about 1.67 nm.
The IGZO thin films deposited by APPVCD are the most important key point of the electrical properties of devices. In APPJ system, we could control several conditions including scan times, carrier gas, main gas, hot plate temperature to influence the quality of IGZO. Electrical properties of devices including current on-off ratio, field effect mobility,
61 easy to understand that the IGZO deposited by APPJ under atmospheric, the atmosphere is formed by number of elements including nitrogen, oxygen, carbon, argon, hydrogen and so on. When depositing IGZO thin films, these elements maybe dissociated by plasma or thermal decomposition and then falling on the thin films. Furthermore, nitrogen of main gas accompany with precursor through plasma region, so nitrogen incorporation of IGZO effect on the stability and electrical performance of IGZO TFT [73]. Because the Ga Auger signal is similar to the signal of N1s in the IGZO films, nitrogen proportion in the films is not correct.
In the table 4-1 and table 4-2 shown the elements proportion of the IGZO thin films in different annealing ambiance respectively, but the
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nitrogen content should be amend. We observed that In, Ga, and Zn ratio is inconsistent with previous preparation of precursor. When the precursor was atomized which delivered into plasma zone by carrier gas, electron would collide with these molecules and break the chemical bond to generate free radicals. Because indium has relatively inertia among gallium and zinc, indium nitrate would be hard dissociated to radicals.
For this reason, the indium radicals were less than the others. Moreover, indium has lower reaction activity than gallium and zinc, even when they forming radicals, they would be recombination soon and take them away from substrate to air by main gas. In summary, indium radicals were difficult to react with gallium and zinc forming IGZO films when indium was dissociated by plasma.
4.1.3 Optical properties
The transmission of solution based IGZO thin films deposited on glass substrate with difference annealing temperature in the wavelength range from 300 to 900 nm is shown in figure 4-11 and 4-12. The spectrum shows that the IGZO thin film is highly transparent with above 80% transparency in the visible range. And we used the following functions to determine the optical band gap:
( ) (4.1)
( ) ( ) (4.2)
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First, we extract optical absorption coefficient (α) by transmittance (T), reflectance (R) and thickness of IGZO (d). Second, we could use the optical absorption coefficient (α) to determine the optical band gap (Eg).
The optical band gap can be determined by the extrapolation of the linear region from a plot of (α)2 vs. photon energy (hν) near the onset of the absorption edge to the photon energy axis. The optical band gap of the solution based IGZO thin film was measured to be 3.78 eV.
When the photon energy is more than 3.78 eV which the wavelength is less than 328 nm, the photon energy would be absorbed by electrons, and electrons transited form valance band to conduction band. Therefore, the high transmittance of IGZO thin film in visible range could be used extensively for display applications. figure 4-13 shows a photography image of 500°C annealed IGZO film on glass, clearly illustrating the transparency of the film.