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2 Aville online t Current Applied Physics 8 (2008) Ni doped ZnO thin films for diluted mgnetic semiconductor mterils E. Liu, *, P. Xio, J.S. Chen,c, B.C. Lim,L.Li School of Mechnicl nd Aerospce Engineering, Nnyng Technologicl University, 50 Nnyng Avenue, Singpore 78, Singpore Dt Storge Institute, DSI Building, 5 Engineering Drive 1, Singpore 11708, Singpore c Deprtment of Mterils Science nd Engineering, Ntionl University of Singpore, Singpore 11757, Singpore Aville online 25 Octoer 2007 Astrct Ni doped ZnO (Zn 1 x Ni x O) thin films were grown on qurtz sustrtes vi mgnetron sputtering deposition process with the Ni concentrtions of 5, 10 nd 20 t.% in the films. The effects of Ni doping level nd post nneling on the structurl nd mgnetic properties of Zn 1 x Ni x O films were investigted y mens of X-ry diffrction (XRD), lternting grdient mgnetometer (AGM) nd photoluminescence (PL). A higher mgnetic moment ws cquired from the nneled Zn 1 x Ni x O film doped with 5 t.% Ni, which ws ttriuted to etter preferred orienttion from primry phse Ni 2+ :ZnO in the film. A reltively more pronounced ZnO(002) pek oserved from the Zn 1 x Ni x O film doped with 5 t.% Ni indicted good crystllinity of the film, which ws ttriuted to lower level of Ni content in the film s well s the Ni 2+ ions sustituted for the Zn 2+ ions to form Ni 2+ :ZnO. A slight shift in ZnO(002) pek position for the 5 nd 10 t.% Ni doped ZnO films could e due to the distortion of the ZnO lttice cused y the Ni 2+ ion sustituents for the Zn 2+ ions. Ó 2007 Elsevier B.V. All rights reserved. PACS: Pp; Ss; I; m Keywords: Ni doped ZnO thin film; Diluted mgnetic semiconductor; Mgnetron sputtering 1. Introduction Recently, much ttention nd interest hve een focused on the development of spintronics technologies. The use of crrier spins, in ddition to crrier chrges, of spintronics looks promising in the field of mgnetic recording medi [1]. The dvntges of diluted mgnetic semiconductor (DMS) sed spin-electronic devices include enling of instnt-on computer, incresed integrtion density, higher dt processing speed, low electricl energy demnd nd comptiility of their friction processes with those currently used in industry. An oxide sed DMS employs nonmgnetic oxide semiconductor mteril like ZnO or TiO 2 s the host which is doped with trnsition metls (TM) such s Co, Ni, Fe nd Mn. The interfce etween mgnetic nd nonmgnetic * Corresponding uthor. Tel.: ; fx: E-mil ddress: mejliu@ntu.edu.sg (E. Liu). mterils llows the injection of electron spin-polrized crriers into the semiconductor heterostructures nd the friction of vriety of device structures tht utilize the spins of electrons. For DMS, its Curie temperture ove room temperture is desired. It ws found tht the mgneto-opticl effect in DMS ws directly relted to the interctions etween the d electrons of trnsition metl ions nd the s nd p electrons of host semiconductors [2]. Room temperture ferromgnetic mgnetiztion ehvior of TM doped ZnO hs een studied in terms of type nd concentrtion of trnsition metl, post nneling condition nd type of co-dopnt [ 7]. Ni doped ZnO nnoprticles crystllized with wurtzite structure showed ferromgnetic ehvior t room temperture [8], which ws ttriuted to the ZnO nnocrystls implnted with Ni nnoprticles []. As-grown Zn 1 x Ni x O nnocrystls exhiited room temperture ferromgnetic ehviors with Ni concentrtion of out 1 t.% [10]. A slight decrese in the lttice constnts of Ni doped ZnO /$ - see front mtter Ó 2007 Elsevier B.V. All rights reserved. doi:10.101/j.cp

3 E. Liu et l. / Current Applied Physics 8 (2008) nnoprticles ws lso found when Ni ws doped into ZnO [8]. It ws reported tht Ni dopnt could sustitute for Zn up to 5 t.% for Zn 1 x Ni x O powders prepred y sol gel method [11]. The evolution of secondry phses (e.g. NiO) in TM doped ZnO long with the single phse Zn 1 x TM x O ws strongly relted to growth temperture nd doping concentrtion. Secondry phses formed in DMS mterils reduced the mgnetiztion of single phses Zn 1 x TM x O [], illustrting tht room temperture ferromgnetism ws n intrinsic property of Zn 1 x TM x O thin films [1]. For DMS mterils, the mjor issues re concerned with the low soluility of trnsition metls in these mterils nd the origin of their ferromgnetism. This pper studies the effects of Ni concentrtion nd post nneling on the structurl nd mgnetic properties of Zn 1 x Ni x O films produced with mgnetron co-sputtering deposition. 2. Experimentl detils Ni doped ZnO (Zn 1 x Ni x O) thin films of out 100 nm thick were grown on qurtz sustrtes (1 cm 1 cm) vi mgnetron co-sputtering deposition process with the Ni concentrtions of x = 5, 10 nd 20 t.% in the films. A DC power ws pplied to the Ni trget nd n RF power ws pplied to the ZnO trget during the co-sputtering deposition. The Ni trget used hd purity of.% nd the ZnO trget hd purity of.% (Kurt J Lesker). Both Ni nd ZnO trgets were three inches in dimeter. The RF sputter power pplied to the ZnO trget ws kept t 50 W nd the deposition rte of ZnO ws out 2.5 nm/min. The DC sputter powers pplied to the Ni trget were 11, 22 nd 50 W corresponding to the respective Ni deposition rtes of out 0.8, 1. nd.7 nm/min for the respective Ni concentrtions of 5, 10 nd 20 t.% in the Zn 1 x Ni x O films. The gs used ws mixture of 40 sccm Ar nd 4 sccm O 2 nd the working gs pressure ws 10 mtorr. The sustrte surfces were pre-clened with Ar + ion omrdment under n Ar gs pressure of out 1 mtorr in the deposition chmer efore film deposition. The Zn 1 x Ni x O coted smples were post-nneled y rpid therml processing (RTP) t out 800 C in Ar gs environment under the condition: rmp up: 2 min, holding: 10 min nd cooling: min. Both the s-grown nd the post-nneled Zn 1 x Ni x O films coted smples were mgnetized using lternting grdient mgnetometer (AGM) (MicroMg TM, 200). In the AGM mesurement, smple ws first psted onto the proe with dhesive grese nd then the proe ws inserted into the rcket. The proe ws lowered to the position in etween two sensing elements with prllel lignment. The mximum pplied field ws 10 koe t field increment of 50 Oe. The mgnetic moments (lb/ tom) of the Zn 1 x Ni x O films were mesured ginst oth in-plne nd out-of-plne mgnetic fields of 10 koe. The structure of Zn 1 x Ni x O films ws chrcterized y mens of high resolution X-ry diffrction (HR-XRD) (X pert PRO) with Cu K node operted t the wvelength k nm. The scn rnge ws 0 0 with step of 0.05 every 0 s. The nd structures of Zn 1 x Ni x O films were mesured with photoluminescence (PL) with lser operted t the wvelength k 25 nm, power 2 mw nd scn rnge nm. The slit width ws 0.2 mm nd the grting ws 00 g/mm.. Results nd discussion From Fig. 1, greter mgnetic moments of the Zn 1 x Ni x O thin films with lower Ni concentrtions (e.g. 5 t.% Ni) re oserved, especilly under the higher mgnetic fields pplied. Fig. 1 shows significnt increse in mgnetiztion of ll the Zn 1 x Ni x O films nneled t 800 C in the Ar gs environment. The higher mgnetic moments of the nneled films cn e ttriuted to the primry phse Ni 2+ :ZnO in the films. From the XRD spectr (Fig. 2), etter preferred orienttion of the Ni 2+ :ZnO Mgnetic Moment (μ B /tom) Mgnetic Moment (μ B /tom) As-deposited x x x10 1.0x Ar-nneled t 800º C Applied Field (Oe) x x x10 1.0x10 4 Applied Field (Oe) Fig. 1. Mgnetiztion of Zn 1 x Ni x O thin films of different Ni concentrtions: () s-grown nd () nneled in Ar t 800 C.

4 410 E. Liu et l. / Current Applied Physics 8 (2008) ZnO(002) Ni (111) Ni (200) θ (Degree) θ (degree) Fig. 2. XRD spectr of Zn 1 x Ni x O films of different Ni concentrtions: () s-grown nd () nneled in Ar t 800 C Wvelength (nm) Wvelength (nm) Fig.. PL spectr of Zn 1 x Ni x O thin films of different Ni concentrtions: () s-grown nd () nneled in Ar t 800 C.

5 E. Liu et l. / Current Applied Physics 8 (2008) phse in the Zn 0.5 Ni 0.05 O film my e responsile for its higher mgnetic moment cquired. From the PL spectr (Fig. ), prominent pek within the rnge k = nm is oserved for ll the smples, which is corresponding to n ultrviolet (UV) ner-ndedge (NBE) emission of ZnO s the nd gp of ZnO is out.1. ev corresponding to k = nm. The pek indictes the presence of the ZnO compound in the Zn 1 x Ni x O films. However, no other peks re found for ll the smples possily due to the defects nd poor crystllinity in the films. The PL peks re consistently stronger in the Ar-nneled Zn 1 x Ni x O films. The films nneled in inert environment tend to hve less oxygen incorported, which reduces the possiility of forming the secondry phses (e.g., NiO) in the films. Another oservtion from the dt shown in Fig. is tht the NBE emission of ZnO is generlly more pronounced for the Zn 1 x Ni x O films with lower Ni concentrtions. For the Zn 0.80 Ni 0.20 O film, the NBE pek is the lowest. The excess Ni exceeding the soluility limit in the Zn 1 x Ni x O films could induce higher density of defects which ct s non-rditive recomintion centres nd thus reducing the intensity of the emitted light. As such, the Zn 0.5 Ni 0.05 O film hs the strongest NBE emission of ZnO. From the XRD spectr shown in Fig. 2, the Zn 0.5 Ni 0.05 O film hs the ZnO(0 02) orienttion, indicting good crystllinity of the film. For the film nneled t 800 C, the ZnO(0 0 2) pek disppers proly due to the desorption of oxygen in the ZnO lttices (Fig. 2). For the Zn 0.5 Ni 0.05 O nd Zn 0.0 Ni 0.10 O films (Fig. 2), the reltively more pronounced ZnO(0 0 2) peks oserved my e ttriuted to the lower levels of Ni content in the films nd lso ttriuted to the Ni 2+ ions which hve sustituted for the Zn 2+ ions to form the primry phse Ni 2+ :ZnO. There is slight shift in ZnO(0 02) pek position for the Zn 0.5 Ni 0.05 O nd Zn 0.0 Ni 0.10 O films, which could e due to the distortion of the ZnO lttices cused y the Ni 2+ ion sustituents for the Zn 2+ ions. For the Zn 0.80 Ni 0.20 O film, other orienttions such s (100) nd (101) re lso oserved (Fig. 2). When nneled in the Ar environment (Fig. 2), the excessive Ni over the soluility limit of the Zn 1 x Ni x O films hs resulted in the secondry phses such s NiO nd metllic Ni. 4. Conclusion The effects of Ni concentrtion nd post-nneling on the structure nd mgnetiztion of the Zn 1 x Ni x O thin films were investigted. The Zn 0.5 Ni 0.05 O film possessed etter crystllinity thn the films with higher Ni contents nd its more pronounced ZnO(0 0 2) pek ws ttriuted to lower level of Ni content in the film s well s the Ni 2+ ions sustituted for the Zn 2+ ions to form primry phse Ni 2+ :ZnO. A slight shift in ZnO(0 02) pek position for the Zn 0.5 Ni 0.05 OndZn 0.0 Ni 0.10 O films could e due to the distortion of the ZnO lttice cused y the Ni 2+ ion sustituents for the Zn 2+ ions. A higher mgnetic moment of the nneled Zn 0.5 Ni 0.05 O film ws ttriuted to etter preferred orienttion from the Ni 2+ :ZnO phse in the film. The sustitution of Zn 2+ in ZnO y Ni 2+ to form Ni 2+ :ZnO ws elieved to e the key mechnism for the room temperture ferromgnetism of the Zn 1 x Ni x O films. References [1] S.J. Perton, C.R. Aernthy, D.P. Norton, A.F. Herd, Y.D. Prk, L.A. Boter, J.D. Budi, Mter. Sci. Eng. R 40 (200) 17. [2] K. Ando, H. Sito, V. Zyets, M.C. Denth, J. Phys.-Cond. Mter. 1 (2004) S5541. [] C. Sudkr, P. Khrel, G. Lwes, R. Surynrynn, R. Nik, V.M. Nik, J. Phys.-Cond Mter. 1 (2007) 022. [4] X.-C. Liu, E.-W. Shi, Z.-Z. Chen, H.-W. Zhng, L.-X. Song, H. Wng, S.-D. Yo, J. Cryst. Growth 2 (200) 15. [5] K. Nielsen, S. Buer, M. Luee, S.T.B. Goennenwein, M. Opel, J. Simon, W. Mder, R. Gross, Phys. Sttus Solid I A-Appl. Mter. Sci. 20 (200) 581. [] T. Zhng, L.-X. Song, Z.-Z. Chen, E.-W. Shi, L.-X. Cho, H.-W. Zhng, Appl. Phys. Lett. 8 (200) [7] C. Liu, F. Yun, B. Xio, S.J. Cho, Y.T. Moon, H. Morkoc, M. Aouzid, R. Rutern, K.M. Yu, J. Appl. Phys. 7 (2005) 107. [8] C.J. Cong, J.H. Hong, Q.Y. Liu, L. Lio, K.L. Zhng, Solid Stte Commun. 18 (200) 511. [] S.Q. Zhou, K. Potzger, G.F. Zhng, F. Eichhorn, W. Skorup, M. Helm, J. Fssender, J. Appl. Phys. 100 (200) [10] H. Wng, Y. Chen, H.B. Wng, C. Zhng, F.J. Yng, J.X. Dun, C.P. Yng, Y.M. Xu, M.J. Zhou, Q. Li, Appl. Phys. Lett. 0 (2007) [11] B.B. Li, X.Q. Xiu, R. Zhng, Z.K. To, L. Chen, Z.L. Xie, Y.D. Zheng, Z. Xie, Mter. Sci. Semicond. Proc. (200) 141. [] S.K. Mndl, A.K. Ds, T.K. Nth, D. Krmkr, Appl. Phys. Lett. 8 (200) [1] X. Liu, F. Lin, L. Sun, W. Cheng, X. M, W. Shi, Appl. Phys. Lett. 88 (200)