Research On Surface Characterization By Wavelet And Fractal Analysis And The Stress Properties Of Doped ZnO Thin Films

Transparent conductive thin film is an important photoelectric functional material and is widely used in electronics,optoelectronics,machinery,and other fields due to the high conductivity and transmittance in the visible range.Surface morphology and residual stress of the thin film are closely related to the growth process and then affect the physical properties of thin film and its performance in devices.In this dissertation,to study the relationship between surface morphology and residual stress,the single-layer ZnO-based films deposited under different preparation conditions were regarded as the research object,the wavelet transform,and fractal analysis were applied,respectively.The main specific contents and results are as follows:First,the surface morphology of Ga-doped ZnO(GZO)thin films prepared under different deposition times was studied by wavelet transform.The Multi-resolution signal decomposition(MRSD)is taken as a kind of algorithm to carry out discrete wavelet transform(DWT).DWT with Daubechie's wavelet of order 10(db 10)is applied to obtain the information nature of non-stationary surface profiles and the 2D-AFM images.It can be seen that with the addition of deposition time,the intensity of diffraction peak(002)gradually increases,the size and height of cluster on film surfaces are also linearly related to the deposition times.The results of the six-level MRSD decomposition express that the approximate component and part of the detailed components resemble a sinusoidal curve.It means that no matter how the deposition time changes,the GZO thin films own the same structure as the background,and the roughness of thin films is mainly composed of the local surface morphology.Second,according to the results of MRSD decomposition,and the self-affine feature of the surface morphology,the thin film can be analyzed by fractal geometry.The traditional analysis of surface roughness is compiling the distribution of height information of an image,for instance,root-mean-square(RMS)which is not inadequate to provide a complete description.As a key index for measuring fractality,fractal dimension is applied to measure the irregularity of surface morphology.Based on the results of decomposed profiles,a simplified thin film-substrate model was introduced.The analysis results show that the fractal dimension is closely connected to the underside diameter and the distance between adjacent grains.The global morphology is the basic structure of thin films,and the local morphology is affected by various rough mechanisms.The fractal dimension of global morphology is smaller than the local morphology.Third,the relationship between RMS roughness and stress of GZO films with different deposition times was studied,and no obvious correlation was found.The fractal analysis shows that the fractal dimension increases with the decrease of stress.And then the GZO thin films annealed under different thermal annealing temperatures were studied.The film morphologies were analyzed by the grayscale fractal study,which could offer full perspectives about the cluster change(like cluster size,cluster amount,cluster height,and so forth)of thin films.Consequently,the change of cluster amount and size on/in films could be obtained by the fractal spectra.The higher annealing temperature will lead to higher atomic mobility,which is good for the formation of bigger size clusters but is a disadvantage to decrease the in-plane stress.Remarkably,the thin film annealed at suitable temperature exhibits the smallest value of in-plane stress and the most average and suitable size of clusters.Fourth,the microstructure,residual stress,surface morphology,and photoelectric characteristics of GZO films prepared under different sputtering pressure were characterized.The thin film prepared under 1.0 Pa shows the smallest residual stress with a uniform cluster height and size.The resistivity and fractal dimension show the same trend with the increase of sputtering pressure.The further study shows that the height of clusters has a dominant effect on the resistivity,and the change of the resistivity with the morphology can be characterized by the grayscale fractal spectrum.The electron effective mass of GZO thin films is determined using electrical and spectroscopic ellipsometry measurements.The self-contradictory results of electrical transport behavior and the small deviation of optical band gap shift from the fixed electron effective mass indicate that the accurate electron effective mass has a more significant influence on the analysis of electrical transport behavior than the optical band gap shift.The anomalous shift of optical band gap is hard to be explained only by the accurate effective mass and BursteinMoss(BM)effect,band gap renormalization(BGR)effect,and so on.And then the Firstprinciples Calculation was applied to simulate the shift of the band gap of ZnO system under different strains,the results show that the change of Zn-3d and O-2p under varied strains results in the shift of band gap.