ZnO and Zn1-xMnxO Minicrystal by CVT and Temporal Process of Photoelectrons

JOURNAL OF RARE EARTHSVol.24 , Spec. Issue , Mar.2006 ,p. 37ZnO and Zn_x Mn, 0 Minicrystal by CVT and Temporal Process ofPhotoelectronsDong Guoyi(董国义),Lin Lin(林琳),Zhang Xiaojun(张晓军), Wei Zhiren(韦志仁)*( College of Physics Science and Technology , Hebei University , Baoding 071002 , China )Abstract :The ZnO and Zn-. Mn, O minicrystal were synthesized by chemical vapor transport( CVT ). The electron trapstructure( donor level ) and process on the temporal behavior of photoelectrons of materials were respectively studied bythermo-luminescence and microwave absorption dielectric spectrometry. There are two peaks in the thermo-luminescencespectra in pure ZnO ,oneis -183 C and the other is - 127 C , which shows two kinds of electron trap energy level pro-duced by the intrinsic defects in ZnO ; but obtain very low thermo-luminescence that only equals to ten percent of pure ZnOin Zn, .，Mn,O ,which shows that its intensity of electron trap is less. The studies of microwave absorption dielectric spetrometry show that conduction band photoelectrons are two-step exponential decay process in ZnO , the lifetime of rapidprocess is 83 ns , while slow process is 828 ns ,the reason of delay is relaxation effects of electron trap to conduction bandphotoelctrons. The intensity of electron Itrap is less in Zn, _。Mn, O minicrystal , the relaxation efects of conduction bandphotoelectrons from electron trap is lttle , so electrons disappeared quickly at conduction band , and the decay process ofphotoelectrons is only 10 ~ 20 ns.Key words :Zn0 ; chemical vapor transport ; thermo-luminescence ; photoelectronCLC number :0471.4 ;TN304 .Document code : AArticle ID : 1002 -0721( 2006 )- 0037 - 04ZnO attracts a wide interest as a promising mate-reaction with the free energy near zero and the equilib-rial for applications in blue and ultraviolet optoelec-rium constant near unity in an easily achievable tem-tronic devices and in future spintronics devices ( afterperature range , so that the reverse reaction can takedoping with magnetic impurities ) 1~7 ]. Melting pointplace. The CVT is a simple and not costly efficientof ZnO is close to 2000 C , and therefore obtainingtechnique suitable for the growth of large single crys-crystals by liquid phase epitaxy is practically impossi-tals when prolong the growth time. Dietl et al21 coble. It is also not possible to use physical vapor trans-cluded theoretically that obtained room-temperatureport method , because the vapor pressure of ZnO isferromagnetic in Mn, Zn_, 0 when the doping concen-very low( in the temperature range 900 ~ 1200 C ).tration of Mn is less than 5% and the hole density upFor this reason , chemical vapor transport( CVT)is ato 3.5 X 1020 hole per cm3 ; then ZnO diluted magneticsuitable method for high-purity and high-quality ZnOsemiconductors have drawn extraordinary atten-crystal ;furthermore increase crystal growth rates effec-But the reports about study doped Mn totively and obtain bigger crystal 6-10]. Useful methodsmaterial semiconductor properties is very few. So it isof ZnO growth are based on chemical transport. Irquite significant to study localization energy levelspite of a progress in the technology of ZnO growthformed by Mn ion and effect to movement of carrier.there is still a need of improvement in this field.In this paper , the Zny-x Mn, O minicrystals were ob-In the present study , the chemical vapor transporttained by doped manganese dioxide ( MnO2 ) in ZnO.( CVT ) of ZnO crystals using ammonium bromideScanning electron microscopy ( SEM ) , microwave ab-( NH,Br ) as a gaseous transporting agent has beensorption dielectric spectrum and thermo-luminescencecarried out in a closed system. Ammonium bromidespectrum have been used to characterize the propertieshas been chosen because it presents with ZnO a simpleof the crystals. .中国煤化工MYHCNMHGFoundation item : Project supported by the National Natural Science Foundation of China ( 50472037 ) ; the Natural Science Foundation of HebeiProvince ( E20040001 17 and F2004000130 ) ; the Science and Technology Research Program from the Bureau of Science andTechnology of Hebei Province( 2002158 )Biography P28$ Guoyi( 1964 - ) , Male , ProfessorCorrespondanguthor ( E-mail : weizhiren-1 @ 163. com )JOURNAL oF RARE EARTHS , Vol.24 ,Spec. Isuse ,Mar. 2006382 Results and Discussion1 ExperimentalAbout 30 g analytical Zn0 and 8 g ammoniumFig. 1 shows scanning electron micrograph of ZnObronide( NH,Br ) were mised equally , then the mix-cerystal material synthesized by CVT. The crystals haveture was put into silica crucible ,1 ml pure waler alsoZnO polarity growth charateristic obviously ，mostlyadded into silica erucible , after that put it in a silicaexposed eylinder m {10 10}, aper-facep {1011},ampoules ,then sealed heated about Ihat 960 C ,atnegative pole-face 0 {000 1 } and positive pole-facelast took it out and cooling naturally at room tempera-Zn {0001 } Fig. 2 shows scanning electron micro-tureMoreover in 10 g ZnO material doped 200 mggraph of 2Zn-. Mn,0 erystal material synthesied bymanganese dioxide ( MnO2 ) and 3 g ammonium bro-CVT ,keep morphological character of pure ZnO crys-mide( NH,Br ),then mixed them equality and addedtal ,mosly expoed erystal plane eylinder m {10 10},1ml water at one side , also sealed heated 1 hat 960taper-facep {101 }, negative pole-faceO f0001}C and cooled naturally at room temperature , synthe-and positive pole-face Zn {0001 }sized Zn, _Mn,0 semiconductor crystal power.Fig. 3( a) is the process on the temporal behaviorThe measurement of process on the temporal be-of polecrons of Zn0 erystal syuhesired by CVT.havior of photoletron was adopted by the technologyFig. 3( b ) is logarithmic curve made by density signalsof microwave absorption dielectrice spectrometry ,meas-of photoelectrons and time relationship. Compareduring principle and methods can refer to litera-with crystal synthesized by hydrothermal method ,mi-ture16J7]. When YAG pulse laser iradiated samples，crowave absorption delectrice spectrometry signal inthe eletrons in the valence band excited to the con-tensity of Zn0 crystal power synthesized by CVT is veryduction band and the migration of electrons in the con-one order of magnitude. The decay process of photoe-duction band ocurred in the microwave field , the mi-lectrons are composed of rapid and slow two-step expo-gaion of electrons inluenced the dstibution of thenentia decay process , the lifetime of rapid process ismicrowave field strengh. From measured the changes83ns ,while slow process is 828 ns. All above ilul-of the mierowave field strength , the density of photoe-trate that there are a lot of intrinsic defects in erstallectrons can also be measured indirectly.and form shallow eletron trap of higher density.(b)!10 μmFig. 1 SEM pttemns of Zn0 crystal materil syntheired by CVT中国煤化工CNMHG 10 um10umCHFig.2 SEM ptterms of ZnO doped Mn crystal material synthesized by CVTDong G Y et al. ZnO and ZnMnO Minicrystal by CVT and Temporal Process of Photoelectrons390.3{(a).1 (683 ns-20.2-3628 ns0.0.0010002000 300060100 2000 3000Time/nsTimc/nsFig.3 Process on temporal behavior of photoelectrons of ZnO crystal synthesized by CVI( a ) ;Logarithmic curve made by density sig-nals of photoelectrons and time relationship( b )Fig.4 is the process on the temporal behavior ofof pure ZnO ,one is -183 9C and the other is - 127photoelectrons of Zn_x Mn, 0 crystal synthesized byC , which shows that two kinds of electron trap energyCVT. From picture we can observe that the decaylevel produced by the intrinsic defects in ZnO. Fig. 5process of photoelectrons mostly is rapid process and( 2 ) isthethermo-luminescence spectrum of Zn -xthe duration only 10 ns , while the signals of slow de-Mn, 0 crystal synthesized by CVT. The signals of ther-mo-luminescence are weak and without obvious peakcay process are very feeble ，which illustrate thwhich shows that its intensity of electron trap is lessdoped Mn2 + has the effect of restrain shallow electronTherefore ,it ilumninate that Mn iron can be doped intotrap formation.ZnO erystal lattice in CVT reaction process and evenFig.5( 1 ) is the thermo-luminescence spectum ofchange the property of semiconductors.ZnO crystal synthesized by CVT. From picture we can3 Conclusionobserve two peaks in the thermo-luminescence spectraThere are two kinds of shallow electron trap ener-gy level in ZnO crystal synthesized by CVT. Thesshallow electron trap energy level prolong the relaxa-0.25-tion time of conduction band photoelectrons to severalhundred nanoseconds. Mn iron can be doped into ZnOcrystal lttice via CVT method and influence the prop-0.05-erty of semiconductors. Mn2+ have the effect of re-strain shallow electron trap formation and shorten the-0.05-relaxation time of conduction band photoelectrons to a-0 200 400 600 800 1000 .bout 10 ns.Fig.4 Process on temporal behavior of photoelectrons of 2ny-x.References :Mn, O crystal synthesized by CVT( left )[1]MatsumotoYJ,MurakamiM,ShonoTJ,etal.Room-temperature ferromagnetism in transparent transi-3Stion metal-doped titanium dioxide[ J ] Science ,2001291 :854.[2] Dietl T , Ohno H , Matsukura F ,et al. Zener modeldescription of ferromagnetism in zinc-blende magneticsemiconductors[J ] Science ,2000 ,287 :1019.(2[ 3] Sharma P ,Gupta A ,RaoK V ,et al. Ferromagnetismabove room temperature in bulk and transparent thin-200 -150 .100 -50 0 50 10中国煤化工Nature Materials ,2003 ,2MHCNMH GJ Detl I Utlute. 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