Fabrication And Optical Properties Study Of ZnO/ZnS:Mn NRs Arrays On P-GaN

ZnO and ZnS, as wide band semiconductors of II-VI group compounds, have been widelyapplied to LED, display, photocatalysis and so on, because of their outstanding optical andelectrical properties. Many novel nano-structures of ZnO or ZnS, such as nano-wire, nano-rod,nano-belt, nano-tube and ZnO/ZnS core/shell or shell/core nano-structure, have beenfabricated, as nano-technologies develop fast. Those fabricated novel nano-structures aredistinct from their bulk properties in small volume fraction and emission light tuning, whicharouse people’s interesting.Both of these two semiconductors exit in two crystal structures: zinc-blend and wurtzite.Stable ZnO synthesized at low temperature exhibits wurtzite structure, while ZnS showsstable zinc-blend structure. Point defects including oxygen vacancy (Vo) and interstitial zinc(IZn) contribute to wide green emission centered at570nm, meanwhile ultraviolet lightcentered at380nm originated from free-exciton can also be obtained. In contrast, ZnS hasmore complex emission spectroscopy which contains UV emission centered at350nmoriginated from near band edge, blue emission from composition of electron of sulfur vacancywith hole of valence band, and green emission from composition of electron of sulfur vacancywith hole of interstitial sulfur. Furthermore, small changes in crystal structure and surface orinterface can sensitively lead to change of emission band. Interstitial sulfur, which can captureholes, contributes to green emission, meanwhile it also restrain blue emission. That is to say,green emission has reciprocal relationship with blue emission.To addition, doped ZnS has superior properties in fluorescence, especially does ZnS:Mnhave in yellow-orange fluorescence emission, and it is used popularly in alternatingcurrent(AC) electroluminescence(EL). In ZnS:Mn ACEL, ZnS:Mn film is sandwichedbetween insulating layers, and then hot electrons driven by AC may impact Mn2+luminescence centers so that yellow-orange emission can be achieved. Recently,electroluminescence of ZnS:Mn nanocrystals(NCs) driven by direct current(DC) has beenreported.Applications of materials will be introduced briefly after brief description of materials. Atpresent, people are still interested in white emission which can be carried out by three ways.First, mixture of red, green and blue light from single color LED performs white color.Second, white nanocrystals excited by UV LED let out white emission. Third, green and redemission of phosphors excited by blue LED mixed with blue emission make white emissionrealization.Theoretically, white emission can be obtained in DC p-GaN/ZnO/ZnS:Mn double heterostructures. Emission of Mn2+excited by hot electron and that of sulfur defects may exitin ZnS:Mn. White emission could be achieved by tuning intensity ratio of green emission ofsulfur, yellow-orange emission of Mn2+and blue emission of Mg acceptor in p-GaN.Considering introduction above, researches in this work list as follow:(1)ZnO NRs arrays were grown on c-plane sapphire, and then those NRs were caped byZnS:Mn shell by RF sputter, with which we fabricated ZnO/ZnS:Mn core/shell NRs arrays,whose interface structure and optical properties were measured. ZnS:Mn layer with structureof zinc-blend together with ZnO NRs of hexagonal wurtzite structure formed a type II bandalignment that makes holes of ZnO exhausted and transferred to ZnS:Mn shell. At the sametime, holes are trapped by interstitial sulfurs formed as a result of interface stress, andelectrons captured by sulfur vacancy together with holes emit green light centered at530nm.In addition, Mn2+was not excited because the large size of crystal grain could make outplenty of surface states, therefore, little yellow-orange emission could be measured.(2)ZnO/ZnS:Mn core/shell NRs arrays were grown on p-GaN/Al2O3(0001) substrateswith two-step process as the same above, and DCEL of p-GaN/ZnO/ZnS:Mn as grown weremeasured and discussed deeply. In contrast to p-GaN/ZnO, p-GaN/ZnO caped with ZnS:Mnlet out green emission obviously. Electron-hole pairs combine firstly at interface ofZnO/ZnS:Mn under small DC that emit green emission centered at550nm, which containsemission of interstitial sulfur centered at530nm and that of oxygen vacancy centered at565nm. As increasing the current, more and more electrons can path through to the interfaceof p-GaN/ZnO, and blue emission increase notably, which makes the CIE ofp-GaN/ZnO/ZnS:Mn moving from green to white.