MOCVD生长的GaN_Mg外延膜的光电性质_英文_

第29卷　第1期2008年1月半　导　体　学　报J OU RNAL OF SEMICONDUCTORSVol.29　No.1J an.,20083Project supported by t he National Natural Science Foundation of China (Nos.60506001,60576003,60476021) Corresponding aut hor.Email :wangll @　Received 31May 2007,revised manuscript received 16August 2007Ζ2008Chinese Institute of ElectronicsOptical and E lectrical Properties of G a N :Mg G row n by MOCV D 3Wang Lili ,Zhang Shuming ,Yang Hui ,and Liang J unwu(I nstit ute of Semiconductors ,Chi nese A cadem y of S ciences ,B ei j ing 100083,China )Abstract :Mg 2doped GaN layers p rep ared by metalorganic che mical vap or dep osition were a nnealed at temp eratures be 2twee n 550and 950℃.Room te mperature (R T )Hall a nd p hot oluminesce nce (PL )spectroscop y measure ments were per 2f or med on t he as 2grow n a nd a nnealed samples.Af ter a nnealing at 850℃,a high hole conce nt ration of 8×1017cm -3a nd a resistivity of 018Ω・cm are obtained.Two domina nt def ect 2related PL emission ba nds in GaN :Mg are investigated ;t he blue ba nd is ce ntered at 218eV (BL )a nd t he ult raviolet e mission ba nd is around 3127eV (U VL ).The relative intensity of BL t o U VL increases af ter a nnealing at 550℃,but decreases w he n t he annealing temp erature is raised f rom 650t o 850℃,a nd finally increases sharply w hen t he a nnealing te mperature is raised t o 950℃.The hole concent ration increases wit h in 2creased Mg doping ,a nd decreases f or higher Mg doping conce nt rations.These results indicate t hat t he difficulties in achie 2ving high hole conce nt ration of 1018cm -3app ear t o be related not only t o hydrogen p assivation ,but also t o self 2comp ensa 2tion.K ey w ords :Hall eff ect ;p hot oluminescence ;p 2GaN PACC :7280;7850G ;7855C LC number :TN 30412+3 Document code :A Article ID :025324177(2008)01200292041　IntroductionWell cont rolled p 2type doping is one of t he fore 2mo st obstacles in t he progress of device develop ment of Ⅲ2nit rides for fabricating visible and ult raviolet light 2emitting devices.Low energy elect ron 2beam irradia 2tion [1]or t hermal annealing in N 2at mosp here [2]is nec 2essary to obtain significant hole concentrations for GaN grown by metal 2organic chemical vapor deposition (MOCVD ).Alt hough t he underlying mechanism for t hese activation p rocesses is not yet f ully understood ,it is widely believed t hat hydrogen imp urities created in t he growt h passivate Mg acceptors by forming Mg 2H neut ral co mplexes.Observation of a local vibrational mode (LVM )for t he Mg 2N 2H complex by inf rared (IR )absorption [3]and Raman scattering [4]in as 2grown samples gives convincing evidence for t his hypot hesis.Therefore ,a po st growt h t reat ment is required to acti 2vate t he Mg acceptors t hrough dissociation of Mg 2H complexes.However ,t here is st rong evidence t hat p 2type do 2ping of GaN is a more complex p rocess and Mg 2H com 2plexes may not be t he only passivating centers [4,5].Theoretical calculations predict t hat nitrogen vacancies are t he dominant point defect s in p 2type GaN because of t heir low formation energy [6,7].Moreover ,due to t he high Mg ionization energy ,high doping concentrations in t he 1019cm -3range are required to achieve hole con 2cent rations in t he low 1017cm -3range.Saarinen et al .applied positron annihilation spectroscopy (PAS )to i 2dentify Mg G a 2V N complexes as important compensating centers in Mg 2doped GaN layers [5].Despite t hat ,we still lack an understanding of t he defect s responsible for determining t he electrical and optical p roperties of Mg 2doped GaN layers.In t his work ,we perform Hall and p hotolumines 2cence (PL )spect ro scopy measurement s of Mg 2doped GaN :Mg layers prepared by MOCVD.Two clo sely connected effect s are observed.First ,a R T PL band peaked around 218eV dominates t he PL spect ra.Sec 2ond ,t he hole concent ration as a f unction of Mg concen 2t ration reaches a maximum value.The stability of de 2fect s in heavily Mg 2doped GaN layers is also investiga 2ted.The role t hat Fermi level plays on defect stability in p 2type GaN :Mg layers is taken into account.2　Experimentp 2type GaN :Mg layers were grown on sapp hire substrate using a horizontal MOCVD reactor.Trimet h 2yl 2gallium (TM Ga )and ammonia (N H 3)were used as Ga and N precursors ,respectively ,and bis 2cyclopenta 2dienyl magnesium (Cp 2Mg )as t he p 2type dopingsource.First ,a 4μm semi 2insulating GaN (n <5×1016cm -3)layer was grown on sapp hire followed by a 115μm GaN :Mg layer at 1040℃.The mole ratio of Cp 2Mg to TM Ga ([Cp 2Mg ]/[TM Ga ])varied be 2半　导　体　学　报第29卷Fig.1　(a)PL spectra for GaN:Mg layer before and after annea2 ling at different temperatures;(b)Relative intensity of BL to UVL as a f unction of annealing temperaturetween01004and01015.To achieve p2type characteris2 tics in t he GaN:Mg layer,rapid t hermal annealing (R TA)was carried out under various conditions.A one2step R TA p rocess was performed o n t he samples p repared wit h a[Cp2Mg]/[TM Ga]ratio of010065at temperat ures of550,650,750,850,and950℃in N2at2 mo sp here for20min.The samples were st udied by PL and R T Hall measurement s in t he as2grown co ndition and after each annealing step.Photoluminescence was excited wit h t he325nm line of a He2Cd laser(10mW). 3　R esults and discussionFigure1(a)shows t he typical PL spectra at295K of GaN:Mg layers before and after annealing at differ2 ent temperat ures.The peak centered at～315eV corre2 sponds to t he band edge emission of t he underlying GaN layer.An apparent intensity ultravioletemissionFig.2　Hole concentration and resistivity in GaN:Mg layers as determined by Hall measurement versus the annealing tempera2 tureband around312eV(UVL)is observed in t he as2 grown sample,which was previously att ributed to do2 nor2acceptor pair(DA P)emission involving a shallow do nor of unknown origin and a Mg acceptor[8].After annealing,t he intensity of t his DA P emission band is decreased by one order of magnit ude and t he peak posi2 tion gradually shift s to3127eV(UVL).The shift in t he peak position may be explained by t he reduction of potential fluct uations[9]as a result of screening from t he carriers.A t hird band wit h a peak around218eV (BL)is observed and t he intensity of t his peak changes significantly after annealing.To clearly demo nstrate t he change of t he intensity of t he observed peaks,Fig2 ure1(b)shows t he relative intensity of BL to UVL (BL intensity/UVL intensity)as a f unction of annea2 ling temperat ure(T).BL intensity/UVL intensity in2 creases after annealing at550℃,but decreases when t he annealing temperat ure is elevated from650to 850℃,and finally increases sharply when t he annealing temperat ure is raised to950℃.The observed change in t he PL spect ra upo n an2 nealing can be attributed to changes in t he defect con2 cent rations.Prior to annealing,shallow donors are p resent in high concent rations,leading to UVL.Upon annealing at550℃in nit rogen,t he shallow donors are eliminated and t he intensity of UVL decreases sharply. Since t he UVL is quenched by annealing at a relatively low temperat ure of550℃,it must involve a donor t hat has a high diff usivity.One possibility is hydrogen, which has a high diff usivity in GaN and has been p re2 dicted to be a donor[10].IR absorption[3]and Raman scattering data[4]show clear evidence t hat H2related complexes are present in as2grown GaN:Mg layers. The BL t hat becomes dominant upon annealing has been p reviously att ributed to deep donor shallow ac2 ceptor(DDA P)emission,where,in t his case,t he do nor is deep and t he acceptor is shallow[9].Figure2shows t he hole concent ration and resis2 tivity in GaN:Mg layers annealed at different tempera203第1期Wang Lili et al.:　Optical and Electrical Properties of G aN:Mg Grown byMOCVDFig.3　Dependence of the intensity of the PL bands(a),and the hole concentration of G aN:Mg layers on[Cp2Mg]/[TM Ga]ratio (b)t ures.The layers show poor p2type conductivity before annealing.Thermal annealing f rom550to950℃for 20min has no obvious effect on our GaN:Mg layers wit h high crystal quality,as demonst rated by X2ray dif2 f raction.After annealing at550℃,t he hole concent ration and t he resistivity change little.As t he annealing tem2 perat ure increases f rom550to850℃,t he hole concen2 t ration continuously increases from1016cm-3to8×1017 cm-3and t he resistivity decreases from9to018Ω・cm.However,t he hole concentration drop s dramatical2 ly after t he annealing temperat ure is raised to950℃, and t he resistivity increases to4Ω・cm.Figure3(a)shows t he dependence of t he intensity of t he PL bands,and Figure3(b)shows t he hole con2 cent ration of GaN:Mg layers on t he[Cp2Mg]/[TM2 Ga]ratio.The p2type GaN:Mg layers under investiga2 tion were annealed at750℃for20min.The increment of t he[Cp2Mg]/[TM Ga]ratio corresponds to t he in2 crease in Mg doping concent ration[11].The data in Fig.3(b)show t hat initially t he hole concentration increases as Mg doping increases,but de2 creases later for higher Mg doping concent rations.The resistivity shows t he oppo site t rend.The difficulties in achieving high hole concent ratio n above8×1017cm-3 and low resistivity below018Ω・cm appear to be p ri2 marily related to self2compensation of Mg[12].As indi2 cated in Fig13(a),for moderately Mg2doped p2type GaN,t he UVL is seen as a weak peak,compared to the Fig.4　Schematic energy band diagram to illustrate the peaks ob2 served in Mg2doped GaN　D indicates the donor state,and DD in2 dicates the deep donor state.BL.For heavily Mg2doped GaN,however,t he UVL quenches,and t he BL dominant s t he spectra.It seems t hat t he intensity of t he BL,is not directly related to t he p2type conduction mechanism[13],but increases as t he Mg doping concent ration increases.This band has t herefore been assigned to t he recombination involving isolated Mg acceptors and deep donors presumably in2 duced by self2compensation.A candidate for t he deep do nor is Mg G a associated wit h a nit rogen vacancy (V N),which is t he only native defect wit h a relevant concentration in p2GaN[7],and has t he oppo site charge of Mg G a.Thus,neut ral Mg G a2V N complexes are expec2 ted to form.At a growt h temperat ure around1000℃t he constit uent s are oppo sitely charged and V N is mo2 bile.After t hermal annealing at550～850℃t he Mg G a2 V N complexes dissociate and V N migrates to t he sur2 face.As a result,t he hole concent ration increases,while t he intensity of BL decreases,as shown in Figs.1and 2.The Mg G a2V N complexes are observed in as2grown MOCVD GaN:Mg,but not in t he material grown by MB E[5].The relevant difference is related to t he p res2 ence of hydrogen in t he growt h environment.The as2 grown MOCVD GaN:Mg commonly shows poor p2type conductivity because Mg acceptors are passivated by hydrogen.A post growt h t hermal anneal is required to activate t he Mg acceptors and obtain significant hole concentration,as illust rated in Fig12.Hydrogen is ab2 sent in t he MB E growt h and t he as2grown material al2 ready possesses well p2type conductivity.These result s suggest t hat t he Mg G a2V N complexes are stable at t he growt h temperat ure around1000℃only if t he Fermi level is close to t he midgap;ot herwise t he complexes dissociate.Thereby,as shown in Figs.1and2,when t he annealing temperat ure is raised f rom850to950℃, large amount s of V N are expected to form.As a conse2 quence,t he net hole concentration drop s,and t he Fermi level moves away from t he maximum of t he valence band,which result s in t he formation of Mg G a2V N com2 plexes again.Based on t he analysis above,t he dramatic increase in t he intensity of BL in Fig.1is understanda213半　导　体　学　报第29卷ble.In summary,Figure4shows a schematic energy band diagram to illust rate t he DA P peak(UVL)and DDA P peak(BL)observed in Mg2doped GaN layers. The shallow donor respo nsible for UVL is att ributed to hydrogen,whereas t he deep donor defect responsible for BL is attributed to nit rogen vacancy complexes as2 sociated wit h Mg acceptors.4　ConclusionAfter annealing at850℃,a high hole concent ra2 tion of8×1017cm-3of and a resistivity of018Ω・cm are obtained.PL and Hall result s on Mg2doped GaN layers indicate t hat t he difficulties in achieving higher hole concent ration appear to be related to self2co mpen2 sation and hydrogen passivation.The observed depend2 ence of t he relative intensity of BL to UVL on annea2 ling temperat ures can be explained in terms of hydro2 gen donors.Upon annealing,t he hydrogen related DA P defect s dissociate and t he associated luminescence band quenches,whereas Mg acceptors are activated and hole concentration increases.There is evidence t hat deep do2 nors,which are assigned to Mg G a2V N complexes,are formed as t he Mg doping concent ration increases.The Fermi level po sition may be a critical factor in t he sta2 bilization of Mg G a2V N co mplexes.When t he Fermi level po sition is near t he maximum of t he valence band,re2 sulting from high hole concent ration,Mg G a2V N comple2 xes are not stable,and dissociate after annealing be2 tween550and850℃.R eferences[1]　Amono H,K ito M,Hiramat su K,et al.P2type conduction in Mg2doped GaN treated wit h L EEBI.Jpn J Appl Phys Part2,1989,28:L2112[2]　Nakamura S,Iwasa N,Mukai T,et al.Thermal annealing effect s onp2type Mg2doped GaN films.Jpn J Appl Phys Part1,1992,31:L139[3]　G tz W,Johnson N,Bour D,et al.Local vibrational modes of t heMg2H acceptor complex in GaN.Appl Phys Lett,1996,75:1383 [4]　Reboredo F A,Pantelides S T.Novel defect complexes and t heirrole in t he p2type doping of GaN.Appl Phys Lett,1999,82:1887 [5]　Hautakangas S,Oila J,Alatalo M,et al.Vacancy defect s as com2pensating centers in Mg2doped GaN.Phys Rev Lett,2003,90:137402[6]　Neugebauer J,Van de Walle C G.Theory of point defect s and com2plexes in GaN.Mater Res Soc Symp Proc,1996,395:645[7]　Neugebauer J,Van de Walle C G.Atomic geometry and electronicstruct ure of native defect s in GaN.Phys Rev B,1994,50:8067 [8]　Viswanat h A K,Shin E,Lee J I,et al.Magnesium acceptor levels inGaN studied by photoluminescence.J Appl Phys,1998,83:2272 [9]　Reshchikov M,Y i G C,Wessels B W.Behavior of2.82and3.22eVphotoluminescence bands in Mg2doped GaN at different tempera2 tures and excitation densities.Phys Rev B,1999,59:13176 [10]　Van de Walle C G.Interactions of hydrogen wit h native defect s inGaN.Phys Rev B,1997,56:R10020[11]　Sugiura L,Suzuki M,Nishio J.P2type conduction in as2grown Mg2doped GaN grown by metalorganic chemical vapor deposition.ApplPhys Lett,1998,72:1748[12]　Obloh H,Bachem K H,Kauf mann U,et al.Self2compensation inMg doped p2type GaN grown by MOCVD.J Cryst Growt h,1998, 195:270[13]　Nakamura S,Iwasa N,Senoh M,et al.Hole compensation mecha2nism of p2type gan films.Jpn J Appl Phys,1992,31:1258MOCV D生长的G a N:Mg外延膜的光电性质3王莉莉 张书明　杨　辉　梁骏吾(中国科学院半导体研究所,北京　100083)摘要:用MOCVD技术生长GaN:Mg外延膜,在550～950℃温度范围内,对样品进行热退火,并进行室温Hall、光致发光谱(PL)测试.Hall测试结果表明,850℃退火后空穴浓度达到8×1017cm-3以上,电阻率降到018Ω・cm以下.室温PL谱有两个缺陷相关发光峰,位于218eV的蓝光峰(BL)以及3127eV附近的紫外峰(U VL).蓝光峰对紫外峰的相对强度(BL/U VL)在550℃退火后升高,之后随着退火温度的升高(650～850℃)而下降,继续提高退火温度至950℃,BL/U VL急剧上升.空穴浓度先随着Mg掺杂浓度的增加而升高;但继续增加Mg掺杂浓度,空穴浓度反而下降.这些结果表明要实现空穴浓度达1018cm-3,不仅要考虑H的钝化作用,还要考虑Mg受主的自补偿效应.关键词:霍尔效应;光致发光;p型GaNPACC:7280;7850G;7855中图分类号:T N30412+3 文献标识码:A 文章编号:025324177(2008)01200292043国家自然科学基金资助项目(批准号:60506001,60576003,60476021)通信作者.Email:wangll@　2007205231收到,2007208216定稿Ζ2008中国电子学会23。