Materials Letters 64 (2010) 1031–1033

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Materials Letters

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Structural and magnetic properties of GaN:Sm:Eu films fabricated byco-implantation method

Lili Sun ⁎, Chao Liu, Jianming Li, Junxi Wang, Fawang Yan, Yiping Zeng, Jinmin LiInstitute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

⁎ Corresponding author.E-mail address: lilisun@semi.ac.cn (L. Sun).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.01.087

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 November 2009Accepted 31 January 2010Available online 4 February 2010

Keywords:Magnetic materialsSemiconductorsIon implantation

Diluted-magnetic GaN:Sm:Eu films have been fabricated by co-implantation of Sm and Eu ions into c-plane(0001) GaN films and a subsequent annealing process. The structural, morphological and magneticcharacteristics of the samples have been investigated by means of high-resolution X-ray diffraction(HRXRD), atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). TheXRD and AFM analyses show that the annealing process can effectively recover the crystalline degradationcaused by implantation. Compared with GaN:Sm films, more defects have been introduced into GaN:Sm:Eufilms due to the Eu implantation process. According to the SQUID analysis, GaN:Sm:Eu films exhibit clearroom-temperature ferromagnetism. Moreover, GaN:Sm:Eu films show a lower saturation magnetization(Ms) than GaN:Sm films.

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1. Introduction

GaN films doped with rare-earth metals (RE) have widelypotential applications in optical communications, optoelectronics,and flat panel displays due to their capability of emitting light withnarrow line width ranging from infrared (IR) to ultraviolet (UV) [1–5].Moreover, RE-doped GaN films [6–10] with room-temperatureferromagnetism have attracted great attention due to their potentialapplications in spintronic devices. Recently, it has been reported thatthe color emitting from RE-doped GaN films can be effectively tailoredby codoping method [5,11]. Therefore it is very meaningful to studythe magnetic properties of GaN films doped with several RE metalsdue to the fact that the magnetic and optical functionalities of RE-doped semiconductors may be realized on a single chip [10]. GaN filmscodopedwith REmetalsmay exhibit interestingmagnetic property dueto the magnetic coupling between different magnetic ions. However,there are few reports about the magnetic property of RE-codoped GaNfilms. In this paper, the structural and magnetic properties of GaN filmscodoped with Sm and Eu have been studied in detail.

2. Experiment

Unintentionally doped c-plane GaN films were grown by low-pressure metal organic chemical vapor deposition (MOCVD) onsapphire substrates. The thickness of the GaN film is about 2 μm (as

the as-grown sample). Sm and Eu ions were implanted into the GaNfilms at 400 °Cwith an energy of 400 keV and a dose of 4.6×1014 cm−2

and 4.6×1013 cm−2 (as the as-implanted sample), respectively. Asubsequent annealing process was carried out at 800 °C for 5 minunder the protection of flowing N2 (as the as-annealed sample). As areference, we have fabricated GaN:Sm films by implanting Sm ionsinto GaN films at 400 °C with an energy of 400 keV and a dose of4.6×1014 cm−2. The GaN:Sm films have also been annealed at 800 °Cfor 5 min under the same condition(as the reference sample).

The structural, morphological and magnetic characteristics of thesamples were studied by means of high-resolution X-ray diffractom-etry (HRXRD), atomic force microscopy (AFM, D3100), and super-conducting quantum interference device (SQUID, MPMS XL-7)magnetometer, respectively.

3. Results and discussion

The typical XRD patterns of the as-grown, as-implanted and as-annealed samples are shown in Fig. 1(a). All samples exhibit promi-nent diffraction peaks at 2θ=34.5° and 2θ=41.6°, corresponding tothe GaN(0002) and sapphire(0006) crystal plane, respectively. Nosecondary phase and metal-related peak can be detected within thesensitivity of XRD measurement. Moreover, the GaN(0002) peak ofthe as-implanted sample is much weaker than that of as-grownsample, which shows crystalline degradation in GaN films caused bythe implantation process. After annealing, the GaN(0002) peaksbecome stronger compared with that of as-implanted sample,indicating that annealing process can effectively recover the crystal-line degradation.

Fig. 1. XRD patterns of the (a) as-grown, as-implanted, and as-annealed samples. (b) Logarithmic scale GaN(0002) peaks of the as-grown, as-implanted, and as-annealed samples.(c) Logarithmic scale GaN(0002) peaks of the as-annealed and reference samples.

1032 L. Sun et al. / Materials Letters 64 (2010) 1031–1033

In order to show the influence of the implantation and annealingprocesses on the structure of GaN films clearly, GaN(0002) peaks ofthe as-grown, as-implanted, and as-annealed samples are shown inFig. 1(b). A clear broad feature appears at the left side of the GaN(0002) peak for both the as-implanted and as-annealed samples,which can be attributed to the impact of interstitial Ga and Nintroduced by ion bombardment [8,9], as well as other defects causedby ion implantation. As shown in Fig. 1(b), the broad feature of GaN(0002) peak for the as-implanted sample decreases after annealing,indicating that the concentration of defects caused by implantation insamples can be effectively reduced by annealing process.

In order to study the impact of additional Eu implantation on thestructural characteristics of GaN:Sm films, GaN(0002) peaks of the as-annealed and reference samples are shown in Fig. 1(c). The broadfeature of the GaN (0002) peak for the as-annealed sample is muchmore obvious than that of the reference sample, indicating that theadditional Eu implantation will introduce more Ga and N interstitialsto GaN:Sm films. Moreover, the peak position of the as-annealedsample shifts to lower angle compared with that of the referencesample, suggesting a slight expansion of the GaN lattice of the as-annealed sample due to the impact of its higher implanted magneticions and interstitial Ga and N atoms.

Using AFM images, we investigate morphological changes whichresulted from implantation and annealing. All the AFM images weremeasured over an area of 10×10 μm2. As shown in Fig. 2(a), the as-grown sample exhibits a relatively smooth surface morphology with aroot mean square (RMS) value of 0.58 nm. As can be seen in Fig. 2(b),the as-implanted sample shows a rough surface morphology with aRMS value of 1.44 nm. The rough surface can be attributed to thedamage caused by the implantation process. As displayed in Fig. 2(c),the as-annealed sample becomes smoother than the as-implanted

Fig. 2. AFM images of the (a) as-grown, (b)

sample, with a RMS value of 0.73 nm. This may be caused by thesurface reconstruction of the sample during the annealing process.

Fig. 3(a) and (b) show the magnetization–field curves (M–H) at300 K for the as-annealed and reference samples, respectively,subtracting the diamagnetic background of the GaN substrate. Thedown right insets in Fig. 3(a) and (b) show the zoomed image for thecorresponding M–H curve. All the samples have the same area ofabout 0.42 cm2. As shown in Fig. 3, both the as-annealed sample andreference sample exhibit a clear room-temperature ferromagnetism.The coercive field (Hc), remanent magnetization (Mr), saturationmagnetization (Ms), and average saturation magnetization permagnetic atom (Ms/atom) are listed in Table 1. Because Jiang et al.[8] and J. Hite et al. [10] have reported that no secondary phases willresult in room-temperature ferromagnetism in GaN:Sm and GaN:Eufilms due to their low Curie temperatures, we speculate that themechanism proposed by S. Dhar et al. [6,7] to explain theferromagnetism in GaN:Gd films may also be applicable to both thereference sample and as-annealed sample. For the reference sample,the room-temperature ferromagnetism are introduced by the intrin-sic GaN:Sm phase. For the as-annealed sample, the ferromagnetismare caused by both GaN:Sm and GaN:Eu phases.

Moreover, an unexpected result is that, as shown in Table 1, thesaturation magnetization (Ms) of the reference sample is 3.0×10−5

emu, much higher than that of the as-annealed sample (9.1×10−6

emu). Because the as-annealed sample containsmoremagnetic atomsthan the reference sample, the Ms value of the as-annealed sample isexpected to be higher. The lower Ms and Ms/atom values of the as-annealed sample than the reference sample may be caused by thefollowing two reasons. Firstly, this may be caused by the magneticcoupling between implanted Sm and Eu atoms, or between GaN:Smand GaN:Eu phases, which deserve further ab initio studies. Secondly,

as-implanted, (c) as-annealed samples.

Fig. 3. Magnetization–field curves (M–H) at 300 K for the (a) as-annealed sample and(b) reference sample. The down right inset shows the zoomed image for thecorresponding M–H curve.

Table 1Ferromagnetic properties of as-annealed and reference samples.

Hc (Oe) Mr (emu) Ms (emu) Ms/atom (μB/atom)

As-annealed sample 99.9 1.6×10−6 9.1×10−6 4.6Reference sample 64.9 3.3×10−6 3.0×10−5 16.7

1033L. Sun et al. / Materials Letters 64 (2010) 1031–1033

this may be resulted from the impact of defects. According to themodel proposed by S. Dhar [6,7], the Ms of GaN:Sm films containscontributions from both Sm atoms and the polarization of GaN matrixcaused by Sm atoms. M. A. Khaderbad et al. [9] have considered the

contributions from defects, and they have found that high densitydefects may introduce higher Ms values in GaN:Gd films due topolarization of interstitial defects. However, we should note that,when the density is high enough, the polarization of GaN matrixcaused by magnetic atoms may be reduced sharply, which will resultin a lower total Ms value in samples. Therefore, we speculate that thelower Ms of the as-annealed sample may come from its higher defectsdensity confirmed by the XRD analysis.

4. Conclusion

In conclusion, we have fabricated GaN:Sm:Eu films by co-implantation method. The XRD and AFM analyses show that theannealing process can effectively recover the crystalline degradationcaused by implantation. Compared with GaN:Sm films, more defectshave been introduced into GaN:Sm:Eu films due to the Eu implan-tation process. According to the SQUID analysis, both GaN:Sm:Eu andGaN:Sm films exhibit clear room-temperature ferromagnetism, whichmay come from the intrinsic ferromagnetic phases. Moreover, thelower Ms of GaN:Sm:Eu films than that of GaN:Sm films may becaused by the magnetic coupling between implanted Sm and Euatoms, the magnetic coupling between GaN:Sm and GaN:Eu phases,or the impact of high density of defects.

Acknowledgement

This work was supported by the Natural Science Foundation ofChina (grant no. 60876004).

References

[1] Lee DS, Steckl AJ. Appl Phys Lett 2002;80:728.[2] Citrin PH, Northrup PA, Birkhahn R, Steckl AJ. Appl Phys Lett 2000;76:2865.[3] Lee DS, Heikenfeld J, Steckl AJ, Hommerich U, Seo JT, Braud A, et al. Appl Phys Lett

2001;79:719.[4] Song SF, Chen WD, Zhang C, Bian L, Hsu CC, Ma B, et al. Appl Phys 2004;96:4930.[5] Zhang L, Liu FQ, Liu C. Appl Phys Lett 2007;91:143514.[6] Dhar S, Brandt O, Ramsteiner M, Sapega VF, Ploog KH. Phys Rev Lett 2005;94:

037205.[7] Dhar S, Pérez L, Brandt O, Trampert A, Ploog KH. Phys Rev B 2005;72:245203.[8] Jiang LJ, Wang XL, Xiao HL, Wang ZG, Feng C, Zhang ML, et al. Chin Phys Lett 2009;

vol. 26:077502 [No.7].[9] Khaderbad MA, Dhar S, Pérez L, Ploog KH, Melnikov A, Wieck AD. Appl Phys Lett

2007;91:072514.[10] Hite J, Thaler GT, Khanna R, Abernathy CR, Pearton SJ, Park JH, et al. Appl Phys Lett

2006;89:132119.[11] Lee DS, Heikenfeld J, Birkhahn R, Garter M, Lee BK, Steckl AJ. Appl Phys Lett

2000;76:1525.