Study On The P-type Doping Of ZnO Thin Films Grown On Si Substrate And Related Physical Problems

ZnO is a Ⅱ-Ⅵ group compound semiconductor material with a direct wide bandgap. Its band gap is3.37eV at room temperature, and the exciton binding energy is60meV, such a high binding energy can achieve high efficiency at room temperature oreven higher temperatures exciton stimulated emission, therefore, ZnO materials havegreat potential applications in the field of short wavelength optoelectronic devices. Toobtain p and n type ZnO is an indispensable condition of its application to the field ofoptoelectronic devices, because of the non-symmetry of the doping of ZnO, the n-typeZnO is easily to get. However, the p-type ZnO is difficult to obtain because of itshighly self-compensation effect and lower accepter impurity doing concentration. Toobtain high-quality, and repeatable p-type ZnO has become the bottleneck problemsthat hinder its developments, and the international hot issues of ZnO in recent years,also.According to the theory calculation of Yan et al., when Ag doped into ZnO, itwould rather like to substitute Zn sites than exist in the interstitial sites, its acceptorlevel is at about0.4eV top of the VBM, and still not easily to obtain p-type ZnO.Persson found that the incorporation of S in ZnO will cause a strong VB offsetbowing whereas the CB is almost no change, so if mixed with the Ag in ZnO byadding S, the acceptor level of Ag may be shallow. As we all know, Si has beenwidely used in the electronics industry, and the technology is perfect and mature, ifwe apply Si and ZnO together, combined with their respective advantages, it will bevery meaningful. Based on above bases, we investigated p-type doping of ZnO thinfilms grown on Si substrates by magnetron sputtering technology, major researchesand results are as following: 1. Utilize RF magnetron sputtering technology combined with vacuum thermalannealing, we deposited Ag and S co-doped ZnO thin films on high resistance Si.Change substrate temperature from room temperature to150℃，and in a vacuumenvironment annealing temperature from350to400°C, under above experimentalconditions we obtained p-type ZnO thin films repeatedly. Approximately sevenmonths of following-up test, samples are stable p-type conductivity. Obtained at roomtemperature Ag-S doped ZnO thin films, its resistivity1.73cm, the holeconcentration of2.99×1016cm-3, the mobility of1.21×102cm2/Vs. ZnOhomojunction consist of obtained p-type ZnO thin film shows a significant rectifyingbehavior of pn junction, reveals that the p-type conductivity of the ZnO films isreliable.2. Ag-S co-doped ZnO thin films were prepared with changed substratetemperature from room temperature to150℃, after400℃vacuum annealing, theconductivity type of the samples experienced a p-type to n-type to a high resistancechanges. The growth of samples at room temperature in a vacuum environment, theannealing temperature from300℃to400℃, the sample conductivity type changedinto p-type from the p/n hybrid, with the annealing temperature continues to rise to450℃, the conductivity of the sample type changes from p-type to p/n hybrid.3. Under the same conditions, we prepared Ag mono-doped ZnO films on Sisubstrate, but we didn’t gain p-type ZnO. Combines with results of Ag and Sco-doped ZnO films, we point out doped S have the impact on the p-type doping ofZnO as follow: make Ag easily doped into ZnO films; cause VB bending, shallowacceptor levels. The p-type conductive of ZnO thin films originate from theacceptor-type defects formed by the doped Ag and S.4. Using radio frequency magnetron sputtering combined with vacuum thermalannealing, the p-type Ag-S co-doped ZnO thin films were grown on p-type Sisubstrates and SiO2generated by thermal oxidation of Si, and n-type ZnO thin filmson p-type Si. We found that the mobility of Ag-S co-doped p-type ZnO thin films ishigher than n-type and p-type ZnO films grown on SiO2. The high mobility has arelationship with the p-type conductivity of Ag-S co-doped ZnO thin films, we consider it can attributed to high mobility behavior of the two-dimension hole gasformed at the interface of p-type Ag-S codoped ZnO films and p-type Si substrates.