Journal of Magnetism and Magnetic Materials 343 (2013) 65–68

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Journal of Magnetism and Magnetic Materials

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Structural, morphological and magnetic properties of AlGaN thin filmsco-implanted with Cr and Sm ions

Xingguo Gao a,b, Chao Liu c,n, Chunhai Yin c, Lili Sun c, Dongyan Tao c, Cheng Yang b,Baoyuan Man b

a School of Science, Shandong Polytechnic University, Jinan 250353, Chinab College of Physics and Electronics, Shandong Normal University, Jinan 250014, Chinac Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

a r t i c l e i n f o

Article history:Received 3 January 2013Received in revised form18 April 2013Available online 2 May 2013

Keywords:AlGaNDiluted magnetic semiconductorIon implantationRoom-temperature ferromagnetism

53/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jmmm.2013.04.072

esponding author.ail addresses: cliu@semi.ac.cn (C. Liu), byman@

a b s t r a c t

Cr and Sm co-implanted AlGaN (AlGaN:Cr:Sm) films have been fabricated by metal organic chemicalvapor deposition, ion implantation and annealing. No secondary phase and metal-related peak can bedetected within the detection limit of X-ray diffraction measurement. The Raman analysis demonstratedthat the peak of E2 (H) phonon mode of sample B is much more narrow and sharp than that of sample A.The atomic force microscopy measurements indicated that the root mean square roughness for sample Aand sample B were 2.26 and 1.07 nm, respectively. According to superconducting quantum interferencedevice analysis, the AlGaN:Cr:Sm films exhibit room-temperature ferromagnetism and colossal magneticmoment effect. Moreover, the saturation magnetization of sample B is 9.75 μB/atom, which is muchhigher than that of sample A (1.86 μB/atom). Finally, the possible origin of the room-temperatureferromagnetism in AlGaN:Cr:Sm films was discussed.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Nitride based diluted magnetic semiconductors (DMSs) can befabricated by the doping of transition-metals (TM), such as Fe, Cr,Mn and Cu [1–3], or rare earth metals (RE), such as Sm, Gd and Eu[4–7]. In recent years, GaN based DMSs which might have a Curietemperature (TC) above room temperature, have attracted greatattention for their potential applications in new generations offast, low dissipation, nonvolatile magneto-electrical and magneto-optical devices [8–10]. Up to now, the magnetic properties ofAlGaN based DMSs have been rarely reported [11,12], although it isalso interesting to use TM doped AlGaN for possible applications inspintronic devices such as spin transistors or polarized lightemitters [13]. Moreover, the GaN/AlGaN heterostructure is thekey structure of GaN-based devices such as UV lasers [14], light-emitting diodes [15] and high electron mobility transistors (HEMT)[16]. If room-temperature ferromagnetism can be realized inAlGaN based DMSs, it is very possible to integrate magnetic,electrical and optical functionalities on a single chip. Furthermore,it has been reported that the magnetic properties of TM dopedGaN can be modified by co-doping of N ions [17], which enlightenus to think of another interesting and important topic, i.e.,whether the magnetic property of AlGaN films can be modified

ll rights reserved.

sdnu.edu.cn (B. Man).

effectively by co-doping of TM and RE ions. However, the magneticproperties of nitride co-doped with both TM and RE is not clear.Therefore, the experimental result about this topic is very mean-ingful for the design of novel DMSs with desired magnetic proper-ties. Also it is useful for theoretical study about the origin of theferromagnetism in TM and RE co-doped DMSs, and/or the inter-action between TM and RE ions.

In this paper, the structural, morphological and magneticproperties of AlGaN thin films co-implanted with Cr and Sm ionswere investigated systematically. Special attention was paid to theexpected 3d and 4f coupling between Cr3+ and Sm3+ ions in oursamples annealed in different temperatures.

2. Experiment

Unintentionally doped AlGaN films (with Al fraction of about50%) were grown by low-pressure metal organic chemical vapordeposition (MOCVD) on AlN buffered c-plane sapphire substratesat 1050 1C. The trimethyl aluminum, trimethyl gallium and ammo-nia were used as the sources of Al, Ga and N, respectively. Thethickness of the AlGaN films is about 1 μm. The Cr+ and Sm+ ionswere implanted sequentially into the AlGaN films at 400 1C withan energy and dose of 200 keV, 1.5�1015 cm−2, and 400 keV,1.0�1015 cm−2 respectively. To prevent channeling, the ion beamswere oriented 71 off perpendicular to the samples' surface.Subsequently, an annealing process was carried out at 800 1C

X. Gao et al. / Journal of Magnetism and Magnetic Materials 343 (2013) 65–6866

(sample A) and 900 1C (sample B) for 5 min in a flowing N2

atmosphere in a rapid thermal processor (RTP-300). Both sampleshave the same area of about 6�6 mm2. The distribution range ofthe implanted ions is about 150 nm and the total concentration ofboth Cr and Sm ions is about 1.7�1020/cm3, which can beobtained based on the well-known SRIM 2008 simulations [18].

The structure of the samples was investigated by means ofX-ray diffractometry (XRD, X'Pert Pro MPD). The Raman spectrawere acquired in a laser Raman spectrometer (Jy-HR800), usingthe 473 nm Nd:YAG laser as excitation source in backscatteringconfiguration. The surface morphology of the samples was studiedby using atomic force microscopy (AFM, Multimode 8). Magneti-zation measurements were carried out using a superconductingquantum interference device (SQUID, MPMS XL-7) with themagnetic field applied parallel to the samples' surface. Hallmeasurement showed that all of the samples are high-resistance films.

3. Results and discussions

The XRD patterns of sample A and sample B are shown in Fig. 1.Both the samples exhibit prominent diffraction peaks at about35.031 and 36.01, corresponding to the AlGaN (0002) and AlN(0002) crystal plane with wurtzite structure, respectively. There isa shoulder on the left of AlGaN (0002) peak, which is caused bythe incorporation of Cr3+ and Sm3+ ions. The diffraction peaklocating at 41.651 corresponds to the sapphire (0006) crystal plane.A weak peak at 32.431 can be attributed to the GaN (1010) peak.No secondary phase and metal-related peak can be detectedwithin the detection limit of XRD measurement. Moreover, thefull width at half maximum (FWHM) of AlGaN (0002) peak ofsample A and sample B is 338 and 277 arc-seconds, respectively,indicating that higher annealing temperature is helpful to recoverthe crystal degradation caused by ion implantation.

Fig. 1. Logarithmic scale XRD patterns of (a) sample A and (b) sample B.

Fig.2. Raman spectra of (a) sample A and sample B,

Fig. 2(a) gives the Raman spectra of sample A and sample B,obtained in the backscattering configuration with no polarizationdetection. The phonon modes marked with asterisks at 418, 576and 751 cm−1 were attributed to the sapphire substrate, and thephonon modes denoted by dotted lines at 248, 657 and 890 cm−1

came from AlN buffer layer [19–21]. For AlGaN films, the E2 (L), E2(H) and A1(LO) phonon modes are indicated by arrows, which arelocated at 160, 593 and 826 cm−1, respectively. Peng et al. [21]have reported the Raman spectra of AlGaN films with different Alfractions. Our results are consistent with their AlGaN film with Alfraction of 45.8%. From Fig. 2(b), it is found that the FWHM of E2(H) phonon mode of sample A and sample B is about 17 and8 cm−1, respectively, which reveals that sample B has a bettercrystal quality than sample A. This also indicates that higherannealing temperature is beneficial to improve the crystal quality.Moreover, it has been reported that the Al content of AlGaN alloyscan be determined by the following two equations [22]:

E2ðLÞ ¼ 142:8þ 43:5x−14:5xð1−xÞ ð1Þ

A1ðLOÞ ¼ 734þ 153xþ 75xð1−xÞ ð2Þ

where x is the Al content of AlGaN alloys; E2(L) and A1 (LO) are theRaman shift of E2(L) and A1 (LO) in cm−1, respectively. Thecalculated Al content is about 48%, which is consistent with thegrowth condition. In addition, the A1(LO) mode is not suited as ameasure for crystal quality since they may be strongly influencedby the presence of free charge carriers [23].

Fig. 3 (a) and (b) shows the AFM images of sample A andsample B, respectively. Both of the images were measured over anarea of 5�5 μm2. The root mean square (RMS) roughness values ofsample A and sample B are 2.26 and 1.07 nm, respectively. There-fore, sample B shows a smoother surface morphology than sampleA. We believe that smoother surface morphology can be obtainedby using higher annealing temperature due to the fact that highertemperature may facilitate the surface reconstruction of AlGaN:Cr:Sm films.

Fig. 4 shows the field dependent magnetization curves (M–H)for sample A and sample B. The measurements were carried out byusing SQUID at 300 K with the magnetic field parallel to thesamples' surface. The diamagnetic contribution from the sapphiresubstrate was subtracted from the magnetization measurements.As shown in Fig. 4, both sample A and sample B exhibit room-temperature ferromagnetism behavior. The coercive field (Hc),remnant magnetization (Mr), saturation magnetization (Ms) andsaturation magnetization per implanted atom (Ms/atom) of sampleA and sample B are listed in Table 1. The calculation of Ms/atomvalues is according to the formula m/(μBAD) [3], where m is themeasured magnetic moment; A is the implanted area; D is theimplantation dose of both Cr and Sm ions, and μB is the value of

and (b) comparative view of E2(H) peaks.

Fig. 3. AFM images of (a) sample A and (b) sample B.

Fig. 4. Magnetization–field curves (M–H) of sample A and sample B obtained at300 K. The inset shows the zoomed image for the corresponding M–H curves.

Table 1Ferromagnetic properties of sample A and sample B.

Hc (Oe) Mr (emu/cm3) Ms (emu/cm3) Ms/atom (μB/atom)

Sample A 58 0.38 2.89 1.86Sample B 63 0.75 15.07 9.75

X. Gao et al. / Journal of Magnetism and Magnetic Materials 343 (2013) 65–68 67

one Bohr magneton. Fig. 5 shows the temperature dependentmagnetization (ZFC/FC) curves obtained in an applied field of500 Oe for sample A and sample B. It was measured in thetemperature range between 5 and 300 K due to the temperaturelimit of the instrument. The difference between ZFC and FC curvesreveals that the samples do not contain any superparamagneticnanoclusters.

Up to now, there is no generally accepted model that candescribe the origin of the room-temperature ferromagnetism ofGaN-based DMSs systematically. Considering that our samples arehigh-resistance films, i.e., the charge carriers are highly localized,which precluded the possibility of carrier induced ferromagnetism[8,24]; All the possible Sm-related secondary phases, if any exists,are nonferromagnetic at room temperature [4]. And even CrN or/and Cr2N clusters exist, they would not contribute to the observedroom-temperature ferromagnetism, because CrN is antiferromag-netic and Cr2N is not ferromagnetic between 85 and 500 K [25]; Tothe best of our knowledge, the incorporation of aluminum intoGaN will not introduce any ferromagnetic phase. Therefore, wespeculate that the ferromagnetism is an intrinsic characteristic ofour samples, which may be explained by the bound magneticpolaron (BMP) theory [26,27]. Although the BMP model does notentail a high carrier density, it does require high-density spinpolarized impurities which mediate the exchange interactions

between BMPs. In our samples, the spin polarization comes fromCr3+ (Sm3+) ions due to its incompletely filled 3d3 (4f5) outer shell.Compared with Cr3+ 3d (Sm3+ 4f) wave function that characterizesthe extent of Cr3+ (Sm3+) local moment, the Bohr radius thatcharacterizes the localized carrier wave function is large enough tofacilitate the formation of Cr3+–BMP (Sm3+–BMP) by the exchangeinteraction between localized carriers and magnetic ions withinthe carrier orbit, by lowering the total energy of the system [28].The ferromagnetic ordering could be formed by either directoverlaps between BMPs or indirect BMP-magnetic impurities-BMP interactions [29].

Almost all the reportedMs/atom value of Cr doped nitride films,including AlGaN:Cr [30], GaN:Cr [31–33] and AlN:Cr [34], arebelow 1.0 μB/atom. However, the Ms/atom values of our sample Aand sample B is 1.86 and 9.75 μB/atom, respectively, are shown inTable 1. This is much greater than any of the theoretical value ofthe magnetic moment of Cr3+ and Sm3+ ions (0.77 μB for Cr3+ and0.85 μB for Sm3+ calculated by the formula M¼g[J(J+1)]½). That isto say the colossal magnetic moment effect exists in our samples.Although it has been reported in Gd or Sm doped GaN-based DMSs[4,5], to the best of our knowledge, it is the first time to report thecolossal magnetic moment effect in AlGaN-based DMS samples.We speculate that the colossal magnetic moment effect in oursamples may come from 3d and 4f coupling between Cr3+ and Sm3

+ ions and/or the interaction between BMPs in AlGaN lattice.Moreover, as shown in Fig. 4 and Table 1, the Ms/atom value ofsample B is much higher than that of sample A, indicating that theimpact of annealing temperature on the magnetic properties ofAlGaN:Cr:Sm films is much obvious at higher temperature. Thismay result from the higher cation substitution efficiency of Cr3+

and Sm3+ ions at higher annealing temperature, which inducedstronger magnetic coupling effect in sample B. However, the exactmagnetic interaction mechanism is still unclear, and deservesfurther theoretical calculations.

4. Conclusion

In summary, AlGaN:Cr:Sm films have been fabricated by MOCVD,ion implantation and annealing. No secondary phase and metal-related peak can be detected within the detection limit of XRDmeasurement. XRD, Raman and AFM analyses show that higherannealing temperature can facilitate the recovery of crystal degrada-tion introduced by ion implantation. According to the SQUID analysis,all the samples exhibit obvious room-temperature ferromagnetismand the colossal magnetic moment effect, especially in the sampleannealed at 900 1C. The origin of the ferromagnetism in AlGaN:Cr:Smfilms can be explained by BMP theory. The colossal magneticmoment effect may come from 3d and 4f coupling between Cr3+

and Sm3+ ions and/or the interaction between BMPs in AlGaN lattice.

Fig. 5. Temperature dependent magnetization (M–T) curves measured between 5 and 300 K for (a) sample A and (b) sample B. The solid lines were obtained by extrapolationmethod as a guide to the eye.

X. Gao et al. / Journal of Magnetism and Magnetic Materials 343 (2013) 65–6868

The ferromagnetic properties of AlGaN-based DMSs can be effec-tively modified by co-doping with Cr and Sm ions and subsequentannealing process, which is very meaningful for the design of novelAlGaN based DMSs with desired magnetic properties.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant nos. 60876004, 11274204).

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