| ZnO is a direct band gap semiconductor, which has a band-gap of3.37eV atroom temperature and the binding energy of exciton up to60meV. Besides, ZnO hasgood electromechanical coupling properties, a rich source of inexpensive，moreimportant, it is environmentally friendly. All the advantages mentioned above makeZnO one of the most potential materials used in optoelectronic devices, and ZnO isconsidered to be an ideal material after GaN in fabrication of ultraviolet light-emittingdiodes (LEDs), laser diodes (LDs), ultraviolet detector and spin electronic devices. Asa prerequisite for application of ZnO-based optoelectronic material, stable andefficient p-type ZnO has been the research hotspot and difficulty. In recent years,element doping method has been widely used in fabrication of p-type ZnO thin film,although some initial results has been achieved using this method, but there are still alot of fundamental physics problems to be solved. For example: chemical state andsolubility of the doping element in ZnO; variation of chemical state with elementdoping content and annealing temperature; p-type conduction mechanism of theelement doping ZnO films.Aiming at the fundamental problems in the research field of ZnO-basedmaterial, our paper carried out the research work of p-type ZnO films, and achievedthe following results:1. Cu-doped ZnO films (ZnO:Cu) with different Cu content were fabricated onquartz substrates using radio frequency (RF) magnetron sputtering and post annealingtechniques. XRD measurements show that the c-axis lattice constant of the ZnO:Cufilm first decreases and then increases with increasing Cu content, indicating that thedoping state of Cu in the ZnO:Cu film transforms with the change of Cu content.Auger electron spectroscopy (AES) was used to distinguish Cu+from Cu0+Cu2+, AES spectra fitting results of the ZnO:Cu films with different Cu content reveal that, withthe increasing Cu content, the peak intensity of Cu+increases gradually, whileCu0+Cu2+peak intensity decreases gradually, indicating that with the increase of Cucontent, which is consistent with the variation of the c-axis lattice constant of ZnO:Cuwith increasing Cu content. The change of relative content of Cu+and Cu2+alsoresponsible for the conversion of ZnO:Cu conductivity from n-type to p-type. In orderto further study the effect of post annealing on doping state of Cu in the ZnO:Cu films,ZnO:Cu were post annealed at500-800℃, respectively. X-ray photoelectronspectroscopy (XPS) fitting results of Cu2p show that all the ZnO:Cu films has twoXPS peaks, corresponding to Cu2p3/2and Cu2p1/2. With increasing annealingtemperature, the binding energy of the Cu2p3/2moves towards high energy direction,meanwhile the full width at half maximum (FWHM) of the Cu2p3/2peak broadens,once the annealing temperature reaches800℃, besides Cu2p3/2and Cu2p1/2peaks,two additional peaks corresponding to CuO shake-up lines can be observed. XPSresults suggest that with the increase of annealing temperature there exists transitionfrom Cu0+Cu+to Cu2+in the ZnO:Cu film, the relative content of Cu2+increases.2. Au-doped ZnO films (ZnO:Au) with different Au content were fabricated onquartz substrates by RF magnetron sputtering and post annealing techniques. XRDmeasurement results show that the (002) diffraction angle variation tendency with theannealing temperature of all ZnO:Au samples is almost the same, suggesting that it isthe thermal stress induced by post annealing responsible for the change of (002)diffraction angles, excluding the effect of Au content, hence, it is considered that thesolubility of Au in ZnO films under annealing temperature of350℃is about1.00at.%. Moreover, it is deduced that the existence of Au can effectively release the stressand improve crystal quality of ZnO, even at room temperature. It is found from XPSresults that Au exists in ZnO:Au in chemical states of positive univalence thatsubstitute Zn (AuZn+) and metallic Au (Au0) in the grain boundaries of ZnO:Au. TheAuZn+act as acceptor, and its content decreases with increasing annealing temperature,leading to the conductivity conversion of ZnO:Au from p-type to n-type as theannealing temperature increases. The optical band-gap of the ZnO:Au shows red-shiftcompared to ZnO, which is attributed to the shift up of O2p level induced by strongp-d coupling between O and Au. Due to the existence of nonradiative recombinationcenter formed by AuZn+or its related defects, room temperature photoluminescence (PL) quenching occurs in the ZnO:Au films annealed at temperature below800℃, byanalyzing electron transfer, charge distribution and recombination process in ZnO:Au,we gave the possible physical mechanism of the PL quenching phenomena in ZnO:Au.3. Using RF magnetron sputtering, undoped ZnO, Ag-doped ZnO (ZnO:Ag) andAg-S co-doped ZnO films (ZnO:(Ag, S)) were fabricated on single crystal Si (111)substrates and post annealed at450℃under vacuum. XPS results reveal that, Agexist with chemical states of Ag+that substitute for Zn (AgZn+) and metallic Ag (Ag0)in the ZnO:Ag, while for ZnO:(Ag, S), all the Ag element exist in the film withchemical state of AgZn+, no Ag0peak was detected. Further calculation results showthat, there is68.2%Ag0in ZnO:Ag, while for ZnO:(Ag, S), there is39.3%AgZn+bondwith S2-and60.7%AgZn1+bond with O2-. In addition, the content of Ag in ZnO:Agand ZnO:(Ag, S) film is2.8%and7.5%, respectively, indicating that theincorporation of S can significantly increase the solubility of AgZn+in ZnO. Hall effectmeasurements show that ZnO:(Ag, S) film has stable p-type conduction, I-Vcharacteristic curves further confirmed its reliability. It is deduced from PLmeasurement that ionization energy of the AgZn+-SOcomplex acceptor is lower thanthat of the AgZn+. The stable p-type conduction of the ZnO:(Ag, S) is mainly attributedto increase of the AgZn+doping content with incorporation of S and formation ofshallower AgZn+-SOcomplex acceptor. The existing erperimental results show thatAg-S co-doping is likely to be the effective method to realize the p-type ZnO. |
PDF Full Text Request