Materials Letters 65 (2011) 667–669

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Materials Letters

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The impact of annealing temperature on the structural and magnetization propertiesof Sm implanted GaN films

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. Tel.: +86 1082304132; fax:E-mail address: lilisun@semi.ac.cn (L. Sun).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.11.017

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 September 2010Accepted 4 November 2010Available online 11 November 2010

Keywords:Diluted magnetic semiconductors (DMSs)Ion implantationRoom-temperature ferromagnetic properties

The impact of the annealing temperature on the structural and magnetic characteristics of GaN:Sm filmsfabricated by implantation method have been investigated in this paper. High-resolution X-ray diffraction(HRXRD) results indicate that the concentration of defects in samples decreases with increasing annealingtemperature. Superconducting quantum interference device (SQUID) results show that all samples exhibitroom-temperature ferromagnetic properties and colossal magnetic effect. Moreover, the average saturationmagnetization per Sm atom (Ms/Sm) of samples decreases sharply with increasing annealing temperature.The strong colossal magnetic effect (496.6 μB/Sm) of samples annealed at 700 °C may have close relation withits high concentration defects.

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1. Introduction

Diluted magnetic semiconductors (DMSs), fabricated by substi-tutionally incorporating magnetic transition metals and rare-earthmetals to the host compound semiconductors, have attracted greatattention due to their applications in spintronic devices[1,2]. Rare-earth metal Gd doped GaN films, have been found to have room-temperature ferromagnetism. Moreover, the average value ofeffective magnetic moment per Gd atom is as high as 4000 μB,much higher than its atomic moment of 8 μB [3,4]. Similar colossalmagnetic effect have also been observed in GaN:Eu films[5,6]. ThusGaN films doped with rare-earth metals are becoming more andmore important and promising for application in spintronics andoptoelectronics due to their room-temperature ferromagnetism,colossal magnetic effect, and interesting optical properties [5,7,8].Therefore, it is very meaningful to study the property of GaN filmsdoped with other rare-earth metals, such as Sm. Very recently, thestructural and magnetic properties of Sm implanted GaN films havebeen studied by changing the implantation dose with the same postannealing temperature of 850 °C [9]. However, there are twopending questions about the magnetic property of GaN:Sm films.Firstly, they have observed that a weak colossal magnetic effect withan average value of effective magnetic moment per Sm atom is80 μB, only about 50 times as large as the atomic moment of Sm

(1.55 μB). Whether GaN:Sm films can exhibit much stronger colossalmagnetic effect is still unknown. Secondly, diluted GaN crystallinematerials following the band structure dispersion [10] are promis-ing for the occurrence of spin-polarized defect states which areresponsible for the ferromagnetic features. Moreover, some exper-imental results have already shown that defects may play animportant role in the magnetic property of DMSs [11–15]. However,the impact of defects on the magnetic property, especially on thecolossal magnetic effect of GaN:Sm films is unclear. As we know,post annealing can effectively reduce the concentration of defects inimplanted GaN films [5]. Thus changing the post annealingtemperature is an effective way to study the impact of defects onthe magnetic property of GaN:Sm films. In this paper, the impact ofthe annealing temperature on the structural and magnetic proper-ties of GaN:Sm films has been studied systematically.

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.Then Sm ions were implanted into GaN films at 400 °C with an energyof 400 keV and a dose of 4.6×1014 cm−2. A subsequent thermalannealing was carried out for 5 min under the protection of flowingN2 at 700 °C (sample A), 800 °C (sample B), and 900 °C (sample C),respectively. The structural and magnetic characteristics of thesamples were studied by means of high-resolution X-ray diffractom-etry (HRXRD) and superconducting quantum interference device(SQUID, MPMS XL-7) magnetometer, respectively.

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668 L. Sun et al. / Materials Letters 65 (2011) 667–669

3. Results and discussion

3.1. XRD analysis

The typical XRD patterns of sample A, sample B, and sample C areshown in Fig. 1(a). All samples exhibit the prominent diffraction peaksat 2θ=34.5° and 2θ=72.8°, corresponding to the GaN (0002) andGaN (0004) crystal plane with a wurtzite structure, respectively. Thediffraction peaks locating at 2θ=41.67° correspond to the sapphire(0006) crystal plane. No secondary phase and metal-related peak canbe detected within the sensitivity of XRD measurement.

The GaN (0002) peaks of the sample A, sample B and sample C areshown in Fig. 1(b). A clear broad feature appears at the left side of theGaN (0002) peak for all the samples, which may be attributed tothe expansion of GaN lattice because of the Ga and N interstitialsintroduced by ion bombardment [5,9], as well as other defects causedby ion implantation. As shown in Fig. 1(b), the broad feature of GaN(0002) peak decreases with increasing annealing temperature,indicating that the concentration of defects caused by implantationin samples decreases with increasing annealing temperature.

Fig. 2 shows the magnetization-field curves (M–H) for sample A,sample B, and sample C at 300K, subtracting the diamagneticbackground of the GaN substrate. All the samples have a same areaabout 0.42 cm2. As shown in Fig. 2, all the samples exhibit a clearroom-temperature ferromagnetism behavior. Although no secondaryphases have been detected in the XRD measurement, we cannotpreclude the existence of second phases such as Ga–Sm and Sm–Nbelow the resolution limit of the XRD instrument. However, Jianget al., [9] reported that no Ga–Sm or Sm–N alloys with room-temperature ferromagnetism had been found. Therefore, the room-temperature ferromagnetism behavior of the samples probably come

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Fig. 1. XRD spectra of (a) sample A, sample B, and sample C; and (b) the GaN (0002)peaks of sample A, sample B, and sample C.

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Fig. 2.Magnetization-field curves (M–H)of (a) sampleA, (b) sampleB, and (c) sample C at300 K. The down right inset shows the zoomed image for the corresponding M–H curve.

from the GaN:Sm alloy based on a similar mechanism proposed by S.Dhar et al. in GaN:Gd films [3,4].

The coercive field (Hc), remanent magnetization (Mr), saturationmagnetization (Ms), and average saturation magnetization per Smatom (Ms/Sm) for sample A, sample B, and sample C are listed inTable 1. All the samples exhibit colossal magnetic effect. The Ms/Sm

Table 1Ferromagnetic properties of sample A, sample B, and sample C.

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

Sample A 11.3 1.2×10−5 8.9×10−4 496.6Sample B 64.9 3.3×10−6 3.0×10−5 16.7Sample C 78.7 1.3×10−6 1.2×10−5 6.7

669L. Sun et al. / Materials Letters 65 (2011) 667–669

value of sample A is 496.6 μB, which is about 320 times as large as theatomic moment of Sm (1.55 μB). The Ms/Sm value of sample B andsample C is 16.7 μB and 6.7 μB, respectively, which is much lower thanthat of sample A.With regards to colossal magnetic effect of rare-earthdoped GaN films, S. Dhar et Al. [3,4] have developed a phenomeno-logical model. According to this model, the Ms of GaN:Sm filmscontains contributions from both Sm atoms and the polarization of theGaN matrix caused by Sm atoms. The Ms of sample A, sample B andsample C should be similar due to the same number of Sm atoms in allthe samples, if the impact of defects on Ms is ignored. However, theexperimental results reveal that sample A exhibits a much higher Msvalue (496.6 μB) than that of sample B (16.7 μB) and sample C (6.7 μB),indicating that the effect of defects must be considered in GaN:Smfilms. As have already shown by the XRD results, the density of defectsin sample A is larger than that of defect in sample B and sample C, wededuce that high density of defects may enhance the colossalmagnetic effect of GaN:Sm films. We speculate that defects mayimpact Ms by the two following means. Firstly, defects may facilitatethe polarization of the GaN matrix by Sm atoms. Secondly, defects,such as interstitial Ga and N atoms may be polarized by Sm atoms.However, the exact explanation about the impact of defects on theproperty of GaN:Sm films is not clear, and may need further systemicab initio studies.

4. Conclusion

In conclusion, GaN:Sm films with room-temperature ferromagne-tism and colossal magnetic effect have been fabricated by theimplantation method. This paper mainly studies the impact of theannealing temperature on the magnetic property of GaN:Sm films.According to SQUID analysis, the average saturation magnetizationper Sm atom (Ms/Sm) of sample A (annealed at 700 °C), sample B

(annealed at 800 °C) and sample C (annealed at 900 °C) is 496.6 μB,16.7 μB, and 6.7 μB, respectively. We speculate that the strong colossalmagnetic effect of sample A may have close relation with its highconcentration defects. Our results suggest that, besides changing theannealing temperature, the magnetic property of GaN:Sm films maybe effectively controlled by other defect engineering methods.

Acknowledgement

This work was supported by the Natural Science Foundation ofChina (Grant No. 60876004).

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