Ferromagnetism in cobalt-doped n - Ga NS. Dhara, B. Sundaravel, K. G. M. Nair, R. Kesavamoorthy, M. C. Valsakumar, T. V. Chandrasekhar Rao, L. C.

Chen, and K. H. Chen

Citation: Applied Physics Letters 88, 173110 (2006); doi: 10.1063/1.2194347 View online: http://dx.doi.org/10.1063/1.2194347 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in First-principles investigations of Co- and Fe-doped Sn O 2 J. Appl. Phys. 101, 09H104 (2007); 10.1063/1.2709740 Electrical transport properties of ferromagnetic Ga x Cr 1 x N thin films Appl. Phys. Lett. 89, 142105 (2006); 10.1063/1.2357603 The role of Cr substitution on the ferromagnetic properties of Ga 1 x Cr x N Appl. Phys. Lett. 86, 012504 (2005); 10.1063/1.1843276 Dependence of ferromagnetic properties on carrier transfer at Ga Mn N Ga N : Mg interface Appl. Phys. Lett. 85, 3809 (2004); 10.1063/1.1810216 Effect of additional nonmagnetic acceptor doping on the resistivity peak and the Curie temperature of Ga 1x Mn xAs epitaxial layers Appl. Phys. Lett. 82, 1206 (2003); 10.1063/1.1554482

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APPLIED PHYSICS LETTERS 88, 173110 �2006�

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Ferromagnetism in cobalt-doped n-GaNS. Dhara,a� B. Sundaravel, K. G. M. Nair, R. Kesavamoorthy, and M. C. ValsakumarMaterials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

T. V. Chandrasekhar RaoTechnical Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

L. C. Chen and K. H. Chenb�

Centre for Condensed Matter Sciences, National Taiwan University, Taipei-106, Taiwan

�Received 7 January 2006; accepted 26 February 2006; published online 26 April 2006�

Ferromagnetic ordering is reported in the postannealed samples of Co doped n-GaN formed by Co+

implantation. A maximum Curie temperature �250 K is recorded for the sample with 8 at. % Co.Particle induced x-ray emission–channeling study confirmed the substitutional Co in Ga lattice site.Local atomic arrangement around magnetic impurities is also analyzed using Raman study. Adisordered model with carrier mediated coupling of localized magnetic moments is maderesponsible for the observed ferromagnetic ordering. © 2006 American Institute of Physics.�DOI: 10.1063/1.2194347�

Since the prediction of finding ferromagnetism in dilutemagnetic semiconductors �DMSs� at room temperature byDietl et al.,1 there is a tremendous advancement in this areaof research and the possibility of practical application ofspintronic �semiconductor spin transfer electronics� deviceshas also got enhanced. Theoretical studies based on meanfield theory �MFT� in p-type semiconductors predicted thepossibility of room temperature ferromagnetism only in Mn-doped GaN and ZnO,1 and with refined indirect exchangeinteraction in InN and AlN host semiconductors.2 Eventhough the theoretical predictions of TC based on MFT fitwell with the experimental values for �Ga,Mn�As system, thesame model always underestimate the TC value for Mn-doped III–V and II–VI compound semiconductors.3 In an-other approach, based on Korringa-Kohn-Rostoker-coherentpotential approximation �KKR-CPA�, ferromagnetic orderingwith various transition metal dopants is tested for both GaNand ZnO.4,5 In the context of the present letter, we will re-strict our discussion on GaN system only. While a generalagreement between experiment and theory is being found forMn �Ref. 6� and Cr �Refs. 7 and 8� dopants in both n- andp-type GaN prominent exceptions are dopants such as Fe�Refs. 9 and 10� and Co.8 As a matter of fact, Fe-doped GaNsystem shows ferromagnetism close to room temperature��250 K�.9 Also there is a report of ferromagnetic orderingin Co-doped p-GaN:Mg system formed by Co+ implantationon epitaxial �epi-� GaN and subsequent annealing for shortduration.8 However, the experimental claim was later on es-timated to be ferrimagnetic ordering using full potential lin-earized augmented plane wave calculation.11 Until now nodefinite value of TC, where normally the field cooled �FC�and zero field cooled �ZFC� curves open in the temperaturedependence plot of the magnetization �M-T�, is reported inCo-doped GaN system. Lack of detailed study of the Co-doped GaN system and early experimental result of ferro-

a�Author to whom correspondence should be addressed; electronic mail:dhara@igcar.ernet.in

b�Also at: Institute of Atomic and Molecular Sciences, Academia Sinica,

Taipei 106, Taiwan.

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magnetic ordering motivated us for further investigation inthis system.

In the present study, we report on Co-doped GaN systemin detail for the presence of strong ferromagnetic orderingand TC extending close to room temperature in postannealedsamples of Co+ implanted n-GaN. A disordered model,where localized magnetic moments coupling with carriersinteract with each other, is adopted to explain ferromagneticordering.

Co-doped samples are prepared using implantation tech-nique in metal-organic chemical vapor deposited n-GaN lay-ers grown on �0001� c-Al2O3 and subsequent annealing treat-ment. A 400 keV Co+ is implanted at 2�10−7 mbar for ionfluences of 3�1016, 5�1016, and 7�1016 cm−2, which cor-respond to Co concentrations of �5, 8, and 11 at. %, respec-tively, as calculated from the SRIM code.12 The range andthickness of implanted ions are calculated to be approxi-mately 175 and 67 nm, respectively.12 Implantation studies

FIG. 1. �Color online� PIXE-channeling study showing Co K� �PIXE� yieldand Ga RBS yield vs tilt angle for the sample with 8 at. % Co. Inset at thecenter shows channeling �Ga RBS� yield vs tilt angle for the pristine epi-GaN samples with �min�2.7%. Inset at the right bottom shows the typical

polarized Raman spectra of the sample with 8 at. % Co.

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are performed at an elevated temperature of 620 K to avoidamorphization in the as-grown samples. A 1.7 MV Tandetronaccelerator �High Voltage Engineering Europa, The Nether-lands� is used for the implantation study. High temperatureannealing at 973 K is performed for a short duration of7 min in ultrahigh pure �UHP� N2 ambient to avoid growthof secondary phases in the implanted samples. Cobalt sitelocation is studied by particle induced x-ray emission�PIXE�-channeling measurement using 2 MeV He+ in the

FIG. 2. �Color online� Field cooled �FC� and zero field cooled �ZFC� tem-perature dependence of magnetization at different applied fields for postan-nealed samples of 400 keV Co+ implanted with a fluence of �a� 3�1016 cm−2 �5 at. % Co�, �b� 5�1016 cm−2 �8 at. % Co�, and �c� 7�1016 cm−2 �11 at. % Co� with insets showing corresponding �M vs tem-perature plots.

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1.7 MV Tandetron accelerator. Separate techniques of Ruth-

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erford backscattering spectroscopy �RBS� for Ga and PIXEfor Co K� yield in the channeling direction are employed, asmass numbers of Ga and Co are close and RBS yield fromimplanted Co overriding on substrate Ga signal will be dif-ficult to resolve. Local atomic arrangement around magneticimpurities is also analyzed using Raman study with an exci-tation of 488 nm line of Ar+ laser in the backscattered con-figuration. Magnetic study is performed using Quantum De-sign superconducting quantum interference device �SQUID�magnetometer �model MPMS� with samples stuck inpolyamide tubes. Variable temperature measurements aredone in FC and ZFC modes, while M-H loops are recordedafter cooling the sample under zero field to the requisitetemperature.

Figure 1 shows a typical PIXE-channeling study of thepostannealed sample with 8 at. % Co. The channeling studyof the pristine epi-GaN sample is also shown in the inset.Reasonable good crystallinity in the pristine epi-GaN samplewas observed �center inset Fig. 1� with minimum yields��min� of 2.7% measured from the ratio of the RBS yield forGa in the channeled and random directions. Similar shape ofthe curves and dip at the same tilt angle �Fig. 1� for the GaRBS yield and Co K� yield of the postannealed sample in-dicate that the majority of the Co prefers to occupy Ga sitesin the GaN lattice.13 High �min values for 8 at. % Co-dopedsample indicate that the lattice defects have not been com-pletely annealed out in the short annealing duration. For an

FIG. 3. �Color online� Field dependent magnetization curves at 5 K for thepostannealed Co-doped samples �a� 5 at. %, and �b� 8 at. %.

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���1−�min�Co��� / �1−�min�Ga��,13 where �min�Co� and�min�Ga� are the minimum yields for the corresponding ele-ments, we find about �77% of substitutionality for Co in Galattice. Rest of the nonsubstitutional Co atoms locates closeto Ga sites, avoiding any major clustering, as indicated withsimilar angular distribution of Co and Ga �Fig. 1�. However,we cannot stress much on the estimate as it is calculatedfrom �min derived from two different studies of RBS andPIXE with different parameters, namely, sensitivities, impactparameters for interaction processes, and analyzing depth. Aclear difference exists between normalized RBS and PIXEyields even if they come from the same depth. Unfortunately,we cannot compare the results of �min derived from anysingle technique, as because in the case of PIXE signal fromCo comes from a narrow width of depth and from a wideregion for Ga in the entire ion range. At the same time, asmentioned earlier, in the case of RBS alone we cannot re-solve Co and Ga, so we chose the signal only for Ga. In thepolarized Raman study, shown typically for the 8 at. % Co-doped sample �right bottom inset in Fig. 1�, the peak around670 cm−1 is missing in the horizontally �Z�X ,Y�−Z� polar-ized configuration with respect to that for the peak at710 cm−1, which is observed for both horizontally and verti-cally �Z�X ,X�−Z� polarized configuration. Thus the peakaround 670 cm−1 corresponds to local vibrational mode�LVM� of magnetic dopant at Ga site. Similar value of LVMis reported for Mn doping at Ga site in GaN.14 The peak at710 cm−1 may correspond to the disorder assisted mode. Thecontribution from this peak diminishes along with the peak at670 cm−1 for cobalt concentration above 8 at. % observed inour unpolarized Raman scattering study �not shown in fig-ure� indicating long range order disappearing with increasingfluence.14

Figure 2 shows the FC and ZFC M-T curves for Co-doped n-GaN between 5 and 300 K with different appliedmagnetic fields. The ferromagnetic behavior is observed be-low about TC�160 K for the 5 at. % Co-doped sample �Fig.2�a�� with an opening of FC and ZFC curves in the M-T plotwith an applied field of 1 kOe and clearly observed from the�M plot with temperature �inset Fig. 2�a��. The ferromag-netic behavior is observed in the M-T plot with an appliedfield of 1 kOe till TC�250 K for the 8 at. % Co-dopedsample �Fig. 2�b�� as also clearly indicated in the �M plotwith temperature �inset Fig. 2�b��. In sharp contrast, thesample with 11 at. % Co shows behavior typical of a spinglass at �100 K �Fig. 2�c�� with very little opening of �Mabove 100 K �inset Fig. 2�c��. Field dependent magnetization�M-H� loops were plotted at 5 K for postannealed samplesup to 8 at. % Co �Fig. 3�, showing saturation magnetizationclose to the bulk value of �1.7�B. The poor hysteresis in theM-H loops may be attributed to the thin layer ��67 nm� ofmaterials prepared in the ion beam implantation technique.

The ferromagnetic ordering found in the Co-doped

n-GaN is something unique in the sense that, so far, the

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predicted ferromagnetic coupling in GaN system by MFT�Refs. 1 and 2� or KKR-CPA �Ref. 4� models is only for thep-type GaN. Moreover, an n-type DMS have certain advan-tages with the performance of devices such as resonant tun-nel diode which should be superior for n-type conduction.Recently few reports of ferromagnetic ordering in n-GaN isalso appearing in the literature. One of the most interestingreport is by Thaler et al.,6 where Mn-doped n-GaN showedferromagnetic behavior at room temperature with convexshaped �opposite to normal ferromagnetic Cuire-Weiss con-cave shape� M-T curves similar to our study. The M-T curvesin our study are consistent with the disorder model of Berciuand Bhatt,15 and most recently with the percolation model ofbound magnetic polaron by Kaminski and Das Sarma.16

In conclusion, a strong ferromagnetic ordering with TCextending �250 K is observed in the dilute magnetic semi-conductors of Co-doped n-GaN. A disordered model withn-type carrier mediated coupling of localized magnetic im-purities is proposed to be suitable for explaining the ferro-magnetic ordering in the Co-doped n-GaN dilute magneticsemiconductors.

The authors thank A. K. Grover and S. Ramakrishnan ofTata Institute of Fundmental Research, Mumbai, India. Theauthors also acknowledge B. K. Panigrahi and C. S. Sundarof MSD, IGCAR for their role in persuing this work.

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