Studies On The Properties Of Morphology And Stress Of ZnO Films

ZnO has recently attracted great attention for its application as optoelectronic and photonic devices in the ultraviolet (UV) region. ZnO has a wide direct band gap of 3.37 eV and a relatively large exciton binding energy of 60 MeV. ZnO films have other several advantages due to the simple apparatus, high deposition rate, low substrate temperature and dense layer for motion. Many techniques such as vacuum evaporation, pulsed laser deposition.(PLD), sol-gel processing, and magnetron sputtering have been employed to prepared ZnO films.The physical properties of ZnO films are affected by the deposition parameters including the deposition methods, sputtering power, buffer, substrate temperature, working pressure, etc.It is well known that residual stress in the deposited ZnO films is unavoidable due to the growth process as well as difference in the thermal expansion coefficients and the lattice mismatch of the films and the substrates. The stresses in thin films will deform the unit cell and introduce unwanted anisotropy which have large effects on the physical properties of the films such as structures mechanical stability and electrical parameters. The energy gap, effective masses and exciton resonance energy are also strongly dependent upon the strain of the films. Under the common conditions, the growth of the thin films is a process of far-from-equilibrium, and the complicated dynamics of the growth leads film morphology to evolve in the rough surface structure of random self-affine fractality. From such fractal structure, described by the surface parameter of roughness exponentα, together with the evolutions of the familiar parameters of roughness w and autocorrelation lengthξ, the mechanism governing the thin film growth may be understood.Under such background, the stress in the ZnO films and the morphology of the ZnO films are investigated.There are three parts in the article. In the first part, the Ga doped ZnO(GZO) films deposited on quartz substrates by a laser deposition system are researched. The morphology evolution of the films and the dependence of the electrical and optical properties on the film thickness and morphology are studied. In the second part, we study the structure and stress properties of ZnO films on flexible substrates deposited by RF magnetron sputtering. In the last part, the stress and morphology of ZnO:Co films deposited on different substrates prepared by LMBE are investigated.The major research work and results of the first part are as follows:We analyze the morphology evolution of the Ga doped ZnO(GZO) films deposited on quartz substrates by a laser deposition system. The surface morphologies of the film samples grown with different times are measured by the atomic force microscope, and they are analyzed quantitatively by using the image data. In the initial stage of the growth time shorter than 8 min, our analysis show that the GZO surface morphologies are influenced by such factors as the random fluctuations, the smoothening effects in the deposition, the lateral strain and the substrate. The interface width w( t) and the lateral correlation lengthξ(t) at first decrease with deposition time t. For the growth time larger than 8 min, w( t) andξ(t) increase with time and it indicates the roughening of the surface and the surface morphology exhibits the fractal characteristics. By fitting data of the roughness w( t) versus deposition time t larger than 4min to the power-law function, we obtain the growth exponentβis 0.3; and by the height-height correlation functions of the samples to that of the self-affine fractal model, we obtain the value of roughness exponentαabout 0.84 for all samples with different growth time t.The thickness and morphology roughness evolution influence on the electrical resitivity and optical transmittance are studied. It is found that the electrical resitivity mainly depend on the film thickness when t is less than 8 min, while in the following stage after the film posses self-affine fractal characteristics, the morphology roughness evolution play a important role in the resitivity. The transmittance is found decrease with the increase of film thickness and it is also found to sensitive to the evolution of surface roughness. The lowest resitivity obtained was 4.85×10^-4Ω·cm with average optical transmittance of 85% in the 200nm thick film deposited for 10min.The major research work and results of the second part are as follows:1. ZnO films deposited on flexible substrate Teflon at different substrate temperature by radio frequency (RF) magnetron sputtering method are studied. The XRD analyses reveal that the crystalline quality of the films improved with the increase of the substrate temperature. The effect of the temperature on the residual stress and the room temperature photoluminescene (PL) properties of the ZnO films are discussed. The reduction of stress and defect as well as the stronger c-axis orientation in the films deposited at 200℃indicate a better overall film crystalline quality and result in the enhancement of PL properties in such film.2. The best (002) c-axis orientation and largest grain size were observed in the films deposited at higher Ar pressure of 4Pa. All the ZnO films exhibited compressive stress. At lower pressure range of 0.5Pa and 1.5Pa, the stress was weak and the stress increased with the increase of Ar pressure from 1.5Pa to 2Pa. With the increase of Ar pressure from 2Pa to 4Pa, the stress decreased abruptly. The intensity of the UV emission and visible luminescence is weak in the films deposited at lower pressure in compare to the films deposited at high pressure of 4Pa. The best (002) c-axis orientation and the deduction of the stress in the films deposited at 4Pa were believed to attribute to the more efficient UV PL emission in this film. Another possible reason for the strong UV emission in the film of 4 Pa may be ascribed to the larger grains in this film.3. The introduction of homo-buffer layer was proved to be of advantage for the enhancement of the quality of the ZnO films deposited on flexible Polytetrafluoroethylene (Teflon) substrates. With a buffer layer and annealed buffer layer, an increase in the intensity of the (002) peak together with a decrease in the full-width at half-maximum(FWHM) of (002) peak was obtained in the ZnO films. The PL investigation revealed that the employment of buffer layer and the anneal treatment of the buffer layer may contribute to the superior optical quality of the ZnO films deposited on Teflon.4. We find that the increase of RF power results in the change in the nature of stress and the difference of the grain size in the ZnO films on Teflon substrate. This indicates the possibility of the stress relaxation by increasing the RF power. Considering the (002) orientation and the mechanical stress, we suggest that the ZnO film deposited at RF power of 200W for 1 hour is optimal.The major research work and results of the third part are as follows: 1. The ZnO:Co films are deposited on sapphire substrates at different temperature.The XRD result reveal that when the substrate temperature increase from 300oC to 500oC, the FWHM of (002) peak of the ZnO film increase firstly, and then decrease, at last, it increase. The largest grain size is obtained in the film deposited at 400 oC. All the ZnO:Co films prepared by LMBE on sapphire substrates exhibit tensile stress,except the film deposited at 350oC.The interface width w is influenced obviously by the temperature.The autocorrelation lengthξdecrease at first and then increase as the temperature increase.2. When the temperature increase, the FWHM variation of (002) peak of the ZnO:Co films deposited on glass substrates are the same with the films on the sapphire substrate. The compression stress in the film is large at lower substrate(<350 oC). When the temperature increase to 400 oC,the compression stress decrease obviously ,which indicate the improvement of the mechanical property in the film. roughness exponentαalmost an constant as the temperature increase. The interface width w increases gradually when the temperature increases. The autocorrelation lengthξincreases when T increases from 300oC to 400oC, and after 400oC,ξalmost unchanged.3. When the temperature increase, the FWHM variation of (002) peak of the ZnO:Co films deposited on Si substrates are the same with the films on the sapphire substrate.The only C preferred orientation is found in the film deposited at 400oC. The stress property of ZnO:Co film deposited on the Si substrate is improved when the temperature is high than 400 oC. The roughness exponentαis influenced obviously by the temperature, but the autocorrelation lengthξhas slightly change as T increases. The interface width w increases gradually when the temperature increases, and decreases when the temperature is 500oC.