Theoretical Investigation Of Structural And Electronical Properties Of Be,Mg Doping ZnO-based Materials

ZnO is a semiconductor with direct band gap of 3.37 eV and high excitonbinding energy?60 meV?at room temperature.These excellent optical propertiesmake it a promising material for the application to the ultraviolet?UV?regionoptoelectronic devices,including the light emitting diodes?LEDs?and the UVdetectors.Among these concerned devices,the detectors operating in the solar-blindregion from 4.4eV?280nm?to 5.6 eV?220nm?have been paid much attention recently.Clearly,the band gap of ZnO does not match this region,and thus it is necessary totune the band gap of ZnO to the desired energy region.For this purpose,alloying ZnOwith other ?-? oxide semiconductors such as MgO,BeO,etc.has been proposed.Unfortunately,it is still an issue to obtain high crystal quality ZnO based alloy withthe band gap turned in the expected range.Additionally,the p-type doping is achallenging issue for optoelectronic applications of ZnO.In this dissertation,wesystematically investigate the physical properties of wide band gap Be,Mg dopingZnO based materials as well as the p-type conductivity of ZnO.This dissertationcontains six chapters.In the first chapter,we review the researches on ZnO based solar-blind regiondetector materials as well as the p-type doping of ZnO.In the second chapter,the cluster expansion?CE?approach and the "specialquasirandom structures"?SQSs?approach that used in our calculations are brieflyintroduced.In the third chapter,phase transition of Be_xM_gyZn_1-x-yO alloy is investigated bytheoretically calculations and experiments.We find that as the content of Be increases,the maximum content of Mg in wurtzite Be_xM_gyZn_1-x-y alloy increases accordingly.And the feature in the Be-and Mg-dependent band gap of Be_xM_gyZn_1-x-yO at thetransition point is obtained.Moreover,based on our theoretical advices,the desirablewide-gap materials have been synthesized in experiment now.Additionally,weextensively study the structural evolution of the segregation phases in Be_xM_gyZn_1-x-yOalloy at finite temperatures by using the first-principles calculations combined withthe cluster expansion approach.And we find that,the segregation behavior ofBe_xM_gyZn_1-x-yO alloy is dominated by Be content.Among the segregated phases,Be1/3Zn₂/3O with direct wide-gap?4.88 eV?is a promising candidate for solar-blinddetectors.In the fourth chapter,we present the systematical investigations on variousphysical properties of impurities in Be,Mg doping ZnO.By performingfirst-principles calculations,it is found that an interstitial Be?Be1?atom preferablymigrates in a basal plane.During the migration,such a Be1 atom favorably bonds toa substitutional Be?BeZn?atom,forming a new defect complex?2Be?Zn,showing atrend of aggregation of Be atoms in ZnO.Furthermore,the stability of the defectcomplex?2Be?Zn can be weakened by substitutional Mg?MgZn?.Therefore,the Mgimpurities in Be-doped ZnO might suppress the aggregation of Be,so as tosignificantly improve the effect of the doped Be on modulating the band gap of ZnO.Meanwhile,the origin for the enhancement of exciton binding energy of Be dopingZnO is systematically studied by first-principle calculations.Basing on theinvestigation of the electronic structure of Be-doped ZnO,the enhancement of excitonbinding energy is revealed mainly from the long-distance perturbation of Be.In the fifth chapter,we reveal the mechanism for the formation of the shallowacceptor NO-V_Zn and?NO-nH?-V_Zn?n= 1-2?in bulk and the growth process ofN-doped ZnO.By performing first-principles calculations,we demonstrate that thep-type NO-V_Zn pair is easy' to form in bulk N-doped ZnO through the chargetransfer and acceptor-acceptor level repulsion.More importantly,the NO-V_Zn paircan be further stabilized through hydrogenation,forming the more stable shallowacceptor complexes?NO-nH?-V_Zn?n=1-2?with ionization energy of less than 162meV,in good agreement with experiment.Additionally,it is found that althoughNO-V_Zn may be formed at the topmost layer at the surface,it cannot be kineticallystable when it is underneath the topmost layer.However,the hydrogenated NO-V_Zncomplex.NO-H-V_Zn can not only be generated at the topmost layer easily,butalso sink into deeper atomic layers successfully.Our findings shed a light onexperimentally improving the quality of N-doped ZnO via introducing properconcentration of H into the system.In the sixth chapter.we systematical study the Be impurities assisting N dopingduring the groxwth process of ZnO.We reveal that,the Be-N dimmers and the Beimpurities at stair of surface is useful to enhance the N doping during the growthprocess of ZnO.