Preparation Of Phosphorus Doped P-type ZnO Thin Films And Research On Related Problems

ZnO, with a wide band gap of3.37eV and a large exciton binding energy of60meV at room temperature, is an II-VI direct band gap semiconductor. ZnO has almostthe same band gap with GaN, but its exciton binding energy is far larger than that ofGaN. Therefore, ZnO has been taken as a promising material to fabricate ultraviolet（UV） light emitting diodes （LEDs） and ultraviolet laser diodes （LDs）. At present,LEDs and LDs based on GaN have been successfully fabricated and commercialized,however, the optoelectronic application of ZnO has not been broken though. This isdue to that ZnO suffers notoriously the doping asymmetry problem, that is, it can beeasily doped n-type but not p-type. The lack of stable, reproducible p-type ZnO hasseriously hindered the application of ZnO in optoelectronic devices. In recent years,the research effort has been primarily focused on the fabrication of p-type ZnO, butmany fundamental physical problems such as the instability of p-type conductivity,the p-type conduction mechanism and the origin of n-type conductivity of undopedZnO have not been successfully solved. Therefore, in the future, p-type doping ofZnO will rely on an understanding of these fundamental problems.Aiming at the sectional fundamental problems in current ZnO, the major workand results are listed as follow:（1） Using radio frequency magnetron sputtering technique, in pure argonsputtering ambient, phosphorus-doped ZnO （PZO） thin films were fabricated onquartz substrates at a substrate temperature of500oC. Hall measurement shows that the as-grown PZO thin film exhibits good n-type conductivity with an electronconcentration up to1.19×10²⁰cm^-3. The as-grown PZO thin films were rapidlyannealed at different temperatures in the range of600–900oC in steps of100oC under10-4Pa for15min. As annealed in the temperature range from600oC to700oC, thePZO film still shows n-type conduction, but the electron concentration monotonicallydecreases. When the PZO film is annealed at800oC, it exhibits best p-typeconductivity with a hole concentration of3.81×1016cm^-3, a resistivity of64.2cmand a mobility of2.82cm²V^-1s^-1. To further increase annealing temperature to900oC,the sample still keeps p-type conductivity but its hole concentration decreases. Byanalyzing the XRD, XPS and PL results, the high electron concentration in as-grownn-type PZO film is due to that phosphorus atoms substitute Zn sites and form largenumbers of PZndonors. It is found that PZndonors gradually decrease with increasingannealing temperature, but the amount of VZnacceptor increases with increasingannealing temperature from600oC to800oC and then decrease above800oC, whichleads to that the change of the amount of PZn-2VZncomplex with the annealingtemperature is the same as the VZn. Therefore, as annealed in the temperature rangefrom600oC to700oC, the electron concentration monotonically decreases, and thesamples still show n-type conductivity due to that the PZndonors still dominate in the600oC and700oC-annealed samples. For the800oC-annealed PZO film, thePZn-2VZncomplex reaches to maximum and dominates in it, hence it converts top-type conductivity. To further increase the annealing temperature, the amount of thePZn-2VZncomplex rapidly decreases, and hence the hole concentration of the900oC-annealed sample decreases. Obviously, the PZn-2VZncomplex acceptors contributep-type conductivity of PZO film, and the conversion of conductivity is due to thechange of the amount of the PZnand PZn-2VZnwith annealing temperature.（2） Using radio frequency magnetron sputtering technique, in argon and oxygensputtering ambient, phosphorus-doped ZnO and MgZnO thin films were fabricated onquartz substrates at a substrate temperature of500oC. The as-grownphosphorus-doped ZnO and MgZnO thin films were rapidly annealed at800oC under10-4Pa for30min. Hall measurement show that the phosphorus-doped ZnO thin film is weak p-type, but the phosphorus-doped MgZnO thin film exhibits better p-typeconductivity with a hole concentration of1.75×10¹⁷cm^-3, a resistivity of21.1cmand a mobility of1.52cm²V^-1s^-1. By analyzing the XRD, XPS and PL results, it issuggested that incorporation of Mg in ZnO can increase the concentration of VZn,which not only make part of PZndonors convert to PZn-2VZncomplex and reduces thecompensation effect from PZndonor but also can compensation the native donors anddecrease the background electron concentration, leading to better p-type conductivityof the phosphorus-doped MgZnO film. To further confirm the experimental results,we calculate, based on the first-principles calculations, the formation energy ofnMgZn-VZncomplex with Mg content under the metal-rich and O-rich limit.Theoretical calculations suggest that, regardless of O-rich or Zn-rich conditions, theformation energy of nMgZn-VZncomplex decreases with the increasing Mg content,indicating that incorporation of Mg favors the formation of VZnand the formation ofVZnis favored at the higher Mg content, well supporting the experimental results. It issuggested from experiment and theoretical calculations that incorporation of Mg inZnO can reduce the self-compensation effect from PZn, which make thephosphorus-doped ZnO exhibit better p-type conductivity.（3） Using radio frequency magnetron sputtering technique, in pure argonsputtering ambient, undoped ZnO thin film was fabricated on quartz substrate at asubstrate temperature of500oC. Hall measurement show that the as-grown undopedZnO thin film exhibits n-type conductivity with a electron concentration of6.23×1016cm^-3, a resistivity of97.9cm and a mobility of1.03cm²V^-1s^-1. XPS and AES showthat the as-grown undoped ZnO thin film is extremely Zn-rich circumstance and largenumbers of Zniexist in sample. A dominant violet emission at2.980eV and a weakultraviolet emission at3.317eV are observed in its83K low-temperature PLspectrum. By analysis of Hall, PL, XPS and AES measurement results, it is suggestedthat the2.980eV emission band originates from the transition of electrons from Znidonor level to valence band and that the Znihas an ionization energy of about400meV below conduction band minimum （CBM） and is responsible for the n-typeconductivity of ZnO.