Varistor Properties And Control Of Doped ZnO Thin Films

Varistors,because of its typical nonlinear current-voltage（Ⅰ-Ⅴ）characteristics,have been widely used to protect circuits of integrated circuits and other electronic equipment from the damage caused by electrostatic discharge,surges and their transient currents（such as lightning strikes）.With the rapid development of the microelectronics industry,the market demand for miniaturized devices are growing rapidly and miniature electronic components that can work at low voltages,including varistors,are extensively needed.As a key element of protection circuit,thin layer varistors can not only realize the miniaturization and low voltage but also the surface installation,which is of great practical importance to VLSI.In this work,a series of ZnO-based thin films have been prepared by magnetron sputtering technology and the effects of heat treatment temperature,atmosphere,Bi---₂O₃ layer and light radiation on the nonlinear Ⅰ-Ⅴ properties were investigated.The main work and contents are as follows:（1）The multi-component metal oxides doped ZnO films were deposited on Si substrates by magnetron sputtering method with ZnO varistor target.The effects of heattreatment temperature,atmosphere and Bi---₂O₃ film on phase,microstructure and varistor characteristics were investigated.The results of XRD,SEM and Ⅰ-Ⅴ show that the multicomponent doped ZnO films have complete crystalline phase,compact surface,uniform grains and good nonlinear Ⅰ-Ⅴ property after the annealing at 600℉,and the maximum nonlinear coefficient,a,reach 9.23.Meanwhile,the nonlinear Ⅰ-Ⅴ property of the sample films sandwiched with Bi---₂O₃ films can be further improved and the corresponding αincreases to be 15.49.Based on the double Schottky barrier model（DSB）,it is found that the double Schottky barrier with NPN structure can be formed in the sample films and the barrier height of multicomponent is estimated to be about 0.661 eV from the Ⅰ-Ⅴ curve.The results show that the addition of Bi---₂O₃ layer will make more Bi elements segregate at the grain boundary of ZnO grains,and promote the increase of Schottky barrier height and then the nonlinear coefficient.The results from the Ⅰ-Ⅴ measurement of the sample under the radiation of 365nm light illuminate that the UV light can obviously affect the low voltage of the films.The threshold voltage Vb of ZnO based film decreases from 13.2V to 8 V while the Vb of the one sandwiched Bi---₂O₃ decreases from 17.1 Ⅴ to 13.8 Ⅴ.（2）The Al-doped ZnO（AZO）films were deposited on Si substrates by RF magnetron sputtering.The films were characterized by XRD,UV-Vis,PL and Hall effect by optimizing the preparation process（such as sputtering power and sputtering time）.The experimental results shows that the annealed AZO films crystallize along the c axis under the condition of sputtering power of 100W and sputtering time of 15 minutes.The grain size is about 20.9nm.The average transmittance of the films in the visible range is about 82.7%and the optical band gap is estimated to be about 3.266eV.The resistivity of the films is about 0.57×10^-3Ω·cm.The films exhibit n-type semiconductor.（3）Sandwich AZO-Bi---₂O₃-AZO films were prepared on the basis of AZO films.The ⅠⅤ property of the samples with different Bi---₂O₃ films thickness were investigated.It is found that the threshold voltage of AZO-Bi---₂O₃-AZO thin films is positively correlated with the thickness of Bi---₂O₃ layer and increases with the increase of the Bi---₂O₃ layer thickness while the corresponding crystal boundary barrier and nonlinear coefficient increases first and then decreases.When the sputtering time of Bi---₂O₃ layer is 15 min,the crystal boundary barrier of the sample films is about 0.637 eV,the nonlinear coefficient α is 10.64,the voltage is 7.1V and the leakage current is 6.648×10^-5A.The experiments show that a low voltage varistor film can be achieved by annealing the AZO-Bi---₂O₃-AZO（15/15/15min）laminated films at 500℃ in O₂ atmosphere.The nonlinear coefficient of the sample films,α,can reach=22.62 and the corresponding threshold voltage is around 4.7 V,indicating sandwich AZO-Bi---₂O₃AZO film can achieve good varistor behavior.Thus,our work provides an effective route to realize the miniaturization and low-voltage of varistors.