Transmission Theoretical Simulation And Experiment Research Of ZnO/Cu (Ag, Al)/ZnO Multilayers

Transparent conductive oxide thin films have high visible light transmittance, UV cut-off, high infrared reflectance and low resistance rate. So they are widely applied in solar cells and liquid crystal displays as transparent electrodes, gas sensors, electroluminescent devices and some other fields. Now, with the replacement of display devices, LCD popularize rapidly, that results in the increasing demand of transparent electrodes, and in the meantime the solar cell is booming. So the demand of transparent conductive thin films is rapidly expanding.Indium tin oxides are applied the most widely currently for their excellent electrical and optical properties. But indium is expensive because of its limited global reserves. It is necessary to find some cheap alternative materials with superior performances. So the ZnO-based transparent conductive thin films become a current research focus for ZnO is rich in reserves, much cheaper, non-toxic and has excellent photoelectric properties.ZnO is a wide band gap semiconductor material and pure ZnO is dielectric. ZnO can get high electrical conductivity when properly doped. M.L.Huberman and J.Maserjian thought metal layer could be considered as dopant to ZnO thin films. So the electrical conductivity of the multilayer can be controlled by the controlling of the thickness of metal layer. In this structure, the outside ZnO film over the metal layer plays a protective role as well as anti-reflect the light reflected from the surface of Cu coating. And ZnO/metal/ZnO can allow the transmission of a certain wavelength range of light after design, which makes it be able to be applied as a filter. Therefore, ZnO/metal/ZnO multilayer structure can be simply prepared, doped under control, and has good conductivity, transmittance, can be served as filter and so on. The conductivity of Cu and Ag are similar, but Cu is much cheaper than Ag, so ZnO/Cu/ZnO has more practical value than Ag. Therefore the optical and electrical properties of ZnO/Cu/ZnO are primarily studied. In the first part of this paper, the transmittance of ZnO/metal/ZnO multilayer is simulated by Matlab based on Max Born's principles of optics. The characteristic matrix of the stratified medium relates the x- and y-components of the electric (or magnetic) vectors in the plane z=0 to the components in an arbitrary plane z=constant. For several layers of different medium, the characteristic matrix can be calculated by multiplying these respective matrixes. Through the simulation by Matlab, we find the transmittance of ZnO/metal/ZnO will increase at first and then decrease with the increasing thickness of ZnO from 30nm to 80nm; the transmittance will decrease monotonically with the increasing thickness of Cu coating; we will get the maximum transmittance when the thicknesses of the two ZnO layers are approximately the same; the average transmittance will decrease when there are more layers.In the second part, ZnO thin films are prepared by radio frequency magnetron sputtering. We can demonstrate by XRD that all the samples are ZnO thin films. All the films shows a single peak, as showed in figure1, the associated 2θ(=34.05) is very close to the (002) peak of ZnO polycrystalline powder whose peak appears at 2θ=34.291. The tiny difference between the XRD peak of the samples and that of the ZnO polycrystalline powder appears due to the process of the preparation.After the determination of ZnO,the deposition conditions such as sputtering power, pressure, argon oxygen flow ratio and annealing temperature are studied. Rigaku D/max-rA X-ray diffraction instrument, Hitachi S-4800 scanning electron microscope and AvaSpec-2048 dual-channel fiber optical spectrometer are used to characterize the structure, morphology and transmission properties.The crystallinity of ZnO thin films is better when the sputtering power is higher, when the sputtering pressure is lower, and when argon oxygen flow ratio is at a lower value. So the optimized sputtering condition is that sputtering power 100W, sputtering pressure 0.6Pa, and argon oxygen flow ratio 4:1.In the third part, the transmittance and sheet resistance are measured to characterize the transmission and electrical conductivity properties of ZnO/Cu/ZnO. RTS-9 dual four-probe electrical measurement instrument is used to measure the sheet resistance of the samples.The thickness of ZnO thin films can be controlled through changing the deposition time. The deposition times of the ZnO layers are 5min, 7min, 9min, 11min and 13min respectively. The transmittance of the multilayer will increase if the thickness of ZnO layer is increasing as shown in figure2. The relationship between sheet resistance and ZnO deposition time is given in figure3. The sheet resistance shows an upward tendency with the increasing deposition time of ZnO. When the deposition time of ZnO is less than 10 minutes, the sheet resistance is relatively small and changes slowly. When the deposition time of ZnO layer is short, there are channels among the external ZnO particles, and the Cu particles can defuse into the ZnO layer and the potential barrier will be much smaller for electronics to get through, so when the top ZnO layers are thinner, the electrical conductivity will be lower.When the deposition times of the Cu layers are 50s, 70s, 105s, and 104s respectively, the transmittance of these multilayers will show a decreasing tendency with the increasing thickness of Cu layers as shown in figure4. The relationship between sheet resistance and Cu deposition time is given in figure5. The sheet resistance will decrease with the increasing deposition time of Cu. When the deposition time of Cu is longer than 100 seconds, the sheet resistance is relatively small and changes slowly. That is because when the thickness of Cu layer reaches a certain value the Cu particles will found an intact film, and electronics will move directionally along the plane, while the thickness will do less to the sheet resistance.The influence of argon oxygen flow ratio when depositing ZnO layers to the transmission and electrical conductivity properties of ZnO/Cu/ZnO is also discussed. When argon oxygen flow ratio is 3:1, the multilayer gains the minimum sheet resistance. That is different to the optimized argon oxygen flow ratio 4:1 when depositing ZnO single layers. It means that ZnO/metal/ZnO multilayer doesn't show the best properties when ZnO layers are the most compact.ZnO layers of all the above samples are deposited at a sputtering power of 100W. It's found that when the deposition rate is high and the migration rate is low, holes are easily formed in the external ZnO layers after analyzing the conductive mechanism of ZnO/metal/ZnO. So the external ZnO layers of some samples are prepared at a sputtering power of 70W, and lower sheet resistance is obtained.Annealing after deposition makes the ZnO thin films much more compaes it more difficult for electronics to get through the potential barrier. And annealing also makes Cu particles agglomerate to lager particles, so it will break the integrality of the Cu layer. As a result, the conductivity will decrease after annealing.Finally, we get the best transmittance of 83% and the lowest sheet resistance of 9.7Ω/σwhen the sputtering powers of the basic ZnO layer and the top ZnO layer are 100W and 70W respectively, while the deposition time of the two ZnO layers are 11min and 13min respectively, and the sputtering pressure is 0.6Pa when the argon oxygen flow ratio maintains 4:1and 3:1 for the basic and top ZnO layer respectively.