Characterization Of Doping Behavior Of Cu In ZnO And Effect Of The Behavior On Properties Of ZnO Film

As the demands of electronics, information, precision measurement, aviationhigh-tech area for light-emitting devices continue to grow, the study of newbroadband gap semiconductor ZnO materials has become the focus in the world,especially the p-type ZnO materials. To control compensation defects is the keyfactor to get high crystal quality p-type ZnO.According to the theory calculation ofYan et al., the formation energies are very low for these group-IB elements on thesubstitutional sites, but rather high at the interstitial sites. Therefore, inquasi-equilibrium state, if we use IB group elements as p-type doping for ZnO films,it will favor us to control the stable doping state and growth condition during thep-type ZnO films growth. In the process of doping, most IB elements doped intoZnO films will take the substituent sites in ZnO lattice, avoiding the compensationeffect of interstitial atoms. Of IB group elements, Cu doping was selected in thispaper because of its low price. In addition, Cu has limited solid solubility in theZnO. It has more than one valence and the p-type ZnO can be obtained as long asthe good control its valence state is well controlled.Cu doped ZnO films were prepared by sol–gel method using spin coatingtechnique and vacuum, argon gas, air and oxygen after heat treatment technology ontoquartz substrates. These substrates were cleaned chemical method by using acetone，ethanol， ultrasonic cleaner and dried with nitrogen. zinc acetate dihydrate[Zn（CH₃COO）₂·2H₂O] was used as a precursor.2-methoxyethanol （C₃H₈O₂）,ethanolamine （C₂H₇NO, MEA）, copper acetate （Cu（CH₃COO）₂） were used as asolvent, stabilizer and dopant source, respectively. For Cu doped p-type ZnO thin filmpreparation and Cu doping in ZnO thin film behavior and its effects on electrical andoptical properties were studied，The results are as follows:1. For preparing Cu-doped ZnO thin film with Cu doping concentration （Cu/Znratio） of0-10mol%, when the nominal doping amount of Cu is3mol%, the film ismade of Cu and ZnO in Wurtzite structure, and when the nominal doping amount ofCu is5mol%, the film is made of Cu and ZnO:Cu. Cu exists in three chemical state inthe films, metal Cu, monovalent Cu and divalent Cu. Metal Cu is distributed on thesurface of the film or in the grain boundary of ZnO. Monovalent Cu and divalent Cuare doped into ZnO substituting for Zn. As nominal amount of doping Cu increases,the doping amount of metal Cu and divalent Cu increases, while the doping amount of monovalent Cu slightly decreases.2. Cu has a limited solid solution in ZnO.The doping Cu content in ZnO reducesas the annealing temperature rises, which results in Cu precipitation from ZnO:Cu.Between500oC and700oC, the precipitated Cu exists in metal Cu on the surface ofthin film or in the grain boundary of ZnO, and its precipitated amount increases as thetemperature rises. When the temperature is equal or greater than800oC, most of theprecipitated Cu sublimates, only a small amount of the metal Cu exists on the surfaceof the film or the grain boundary. When the light shines on the film, the metal Cunanoparticles existing on the surface of the film will occur to the surface plasmonresonance absorption with the absorption peak around2.0EV. The intensity ofabsorption peak is proportional to the quantity of metal Cu, and the absorption peakposition moves to the high energetic direction with an increase in size of the metal Cunanoparticles.3. The conductivity type of Cu-doped ZnO gradually changes from n-type top-type with the increase amount of doping Cu. Cu+Znis the acceptor in ZnO, whichleads to the change of ZnO conductivity from n-type to p-type. When the nominalamount of doping Cu is10mol%, the Cu-doped ZnO shows stable p-typeconductivity. It attributes to the acceptor Cu+Znand the increasing amount of VZnasthe content of doping Cu increases. The bandgap of ZnO decreases with theincreasing amount of doping Cu because the doping amount of Cu2+Znions increasesas the doping amount of Cu increases.