magnetism in gan layers implanted by la, gd, dy and lu
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Thin Solid Films 519 (2011) 6120–6125
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Thin Solid Films
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Magnetism in GaN layers implanted by La, Gd, Dy and Lu
Z. Sofer a,⁎, D. Sedmidubský a, M. Moram b, A. Macková c,d, M. Maryško e, J. Hejtmánek e, C. Buchal f,H. Hardtdegen f, M. Václavů g, V. Peřina c, R. Groetzschel h, M. Mikulics f
a Dept. of Inorganic Chemistry, Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republicb University of Cambridge, Dept. of Materials Science & Metallurgy, Pembroke Street, Cambridge CB2 3QZ, United Kingdomc Nuclear Physics Institute of the ASCR, v.v.i., 25068 Řež, Czech Republicd Department of Physics, Faculty of Science, J. E. Purkinje University, České Mládeže 8, 400 96 Ústi nad Labem, Czech Republice Institute of Physics of ASCR, v.v.i, Cukrovarnická 10, 162 00 Prague 6, Czech Republicf Institute of Bio- and Nanosystems, Forschungszentrum Jülich, 52425 Jülich, Germanyg Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague, Czech Republich Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf e.V., P.O. Box 51 01 19, 01314 Dresden, Germany
⁎ Corresponding author at: Institute of Chemical Techganic Chemistry, Technická 5, 166 28, Prague 6, Czech Refax: +420 220444411.
E-mail address: [email protected] (Z. Sofer).
0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.04.110
a b s t r a c t
a r t i c l e i n f oArticle history:Received 30 October 2010Received in revised form 5 April 2011Accepted 18 April 2011Available online 28 April 2011
Keywords:Magnetic semiconductorsIII–V semiconductorsIon implantationRare earthX-ray diffractionRutherford backscattering spectroscopySecondary ion mass spectrometryMagnetic properties of thin films
We present a complex study of rare earth elements implanted GaN layers grown by low pressure metal-organic vapor phase epitaxy on c-plane sapphire substrates. Gd, Dy, La and Lu ions were implanted withenergies of 200 keV and doses ranging from 5×1013 to 4×1017 atoms.cm−2. The chemical composition andconcentration profiles of ion-implanted layers were studied by secondary ion mass spectrometry andRutherford back scattering. The structural properties of the layers were characterized by Rutherford backscattering/channeling and X-ray diffraction reciprocal space mapping. Gd implanted layers exhibitferromagnetic behavior persisting up to~720 K. Since the ferromagnetic behavior was not observed in thecase of La and Lu implanted layers, it cannot be attributed to the structural damage of the layer. Based on thefact that the samples are electrically conducting we conclude that the ferromagnetism can be associated withdoped electrons mediating the ferromagnetic interaction between local moments on Gd and Dy.
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1. Introduction
The present generation of semiconductor electronic and photonicdevices is based on the transport of charged electrons and holes.However, the field of semiconductor spintronics seeks to exploit thespin of charge carriers in a future generation of transistors and lasers,which are going to be employed in ultra-low power high-speedmemory and photonic devices. The practical developments of thesespin-based devices depend on the availability of materials with amagnetic ordering temperature (TC) above room temperature [1].
In the last decade research on dilute magnetic semiconductors(DMS) was mainly concerned with transition metal doped galliumarsenide [2]. These narrow band-gap materials exhibit ferromagneticbehavior far below room temperature. The highest TC observed forGaxMn1−xAs has reached about 170 K [2]. Such a low TC is probablydue to a long but weak interaction betweenMn atoms beingmediatedby itinerant charge carriers. For wide band-gap materials doped by
transition metals, a TC over room temperature has been predicted [1].This has been confirmed for GaxMn1−xN and GaxCr1−xN, where a TCabove 300 K has been reported [3,4]. Rare-earth (RE) doping of GaNhas also attracted a recent attention in the search for ferromagneticsemiconductors with a Curie temperature above 300 K [5,6]. Dopingwith RE elements could be a promising alternative to transitionmetalsas RE atoms have partially filled f-orbitals with magnetic moments,similar to the case of transition metals with partially filled d-orbitals.Nonetheless a significant difference is that most RE atoms are sup-posed to be stabilized in GaN structure as trivalent ions. Therefore thesubstitution should not lead to generation of mobile charge carrierswhich canmediate the magnetic coupling as is thought to occur in thecase of the widely studied DMS GaAs:Mn.
In this work we present a complex study of La, Gd, Dy and Lu ionimplanted GaN layers. Some results on Gd implanted layers have beenalready reported in the previous article [7]. GaN layers were grown bylow pressure metal organic vapor phase epitaxy (MOVPE) on c-planesapphire substrates. The chemical composition of the layers and theconcentration profiles of implanted ions were measured by Rutherfordbackscattering (RBS) and secondary ionmass spectrometry (SIMS). Thestructural properties of the layerswere characterizedbyRBS/channelingand by high-resolution X-ray diffraction (HRXRD) reciprocal space
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mapping. The magnetic behavior of the implanted and unimplantedlayers was investigated in the temperature region between 4.5 and800 K.
2. Experiment
GaN layers with a thickness of about 3 μm were grown by lowpressure MOVPE on c-plane sapphire substrates. Trimethylgalliumand ammonia were used as gallium and nitrogen precursors, res-pectively, while hydrogen was used as a carrier gas. These GaN layerswere implanted with La, Gd, Dy and Lu ions accelerated by 200 keV.The implantation angle was 7° to prevent channeling effects. Allimplantation experiments were carried out at room temperature. Theimplanted doses are summarized in Table 1. The layers were sub-sequently annealed in a high purity nitrogen atmosphere in order toreduce the implantation damage. The annealing was performed at900 °C and a pressure of 100 kPa for 5 to 15 min.
The compositionof the layers and the concentrationdepthprofiles ofrare earths were measured by RBS. The element contents weredetermined using 2.2 MeV He+ ions as projectiles. The He+ ionsscattered under the laboratory scattering angle of 170° were detectedwithULTRA-ORTEC silicon detector. The RBS datawere evaluated by thecomputer code GISA 3 [8]. The concentration profiles of the implantedions were compared with the simulated data provided by a programSRIM 2008. The structural changes (structural damage, amount of theintroduced defects), which occurred during the ion implantation andsubsequent annealing, were examined by the RBS/channeling mea-surements using a beam of 1.8 MeV He+ ions from the Van de Graaffaccelerator in Forschungszentrum Dresden — Rossendorf, Germany.
The X-ray diffraction analysis was carried out using a PanalyticalX-Pert ProMRDdiffractometerwith amirror, a four-bounce asymmetricGe (220) monochromator and a triple-bounce analyzer. XRD ω-scans,ω–2θ scans and reciprocal space maps [9] were used to investigate thestrain state within each sample.
The secondary ionmass spectroscopy (SIMS) analysiswas performedwith a Perkin Elmer PHI 600 series instrument. A Cs+ primary ion beamfocused to diameter of ~30 μm with energy of 3.0 keV was used. Theangle of incidence was 60° relative to the sample normal, the beamcurrent was ~18 nA and the beam was raster-scanned across a samplearea of 400×1000 μm. The secondary ion signal was detected by aquadrupole mass analyzer. The sample charging was compensated byusing low energy electron bombardment. The magnetization curveswere measured using a squid magnetometer MPMS-5S (QuantumDesign) in the temperature range 4.5–330 K. The conventional pro-cedure of measurement in a constant magnetic field and varyingtemperature could not be employed in this case due to diamagneticsignal of the substrate being of comparable magnitude as the intrinsicferromagnetism of the implanted layers. Hence, the magnetization was
Table 1Summarized concentrations of rare earth metals implanted in GaN.
Sample Implanted dose T.C. RBS T.C. MAG[1015 atoms.cm−2] [1015 atoms.cm−2] [1015 atoms.
Gd-5 10 12 17 (10 K)Gd-6 50 7 11Gd-7 100 15 8Gd-8 400 27 34Dy-1 0.05 – 0.5Dy-2 0.1 – 0.8Dy-3 0.5 – 1.6Dy-4 1 – 3.0Dy-5 5 4 2.7Lu-1 10 15 –
La-1 10 –
Column description: T.C. RBS. — total concentration determined by RBS, T.C. MAG — totalmeasured at 5 K, C.FER — concentration of ferromagnetic ions, S.M.M. — saturated ferromagassociated with Gd and Dy ions the effective magnetic moments μeff (Gd)=7.9μB and μeff (
recordedvs.magneticfieldvarying from0 to5 T at constant temperatureand the resulting magnetization curves were corrected by subtractingthe diamagnetic addenda of the GaN substrate. The high temperaturemagnetic measurements up to 800 K were carried out by means of aDSM10 magnetometer-susceptometer (Manics) based on the determi-nation of the force acting on the sample in a non-homogeneous staticmagnetic field. The variation of the ferromagnetic moment withincreasing temperature was evaluated from the behavior of thesusceptibility.
3. Results and discussion
RBS gives information about the concentration depth profile ofimplanted rare earth and the integral amount of the implantedelement. We compared the concentration depth profiles determinedby RBS and the predicted concentration depth profiles simulated bySRIM 2008, but the simulations of concentration profiles by SRIM2008 were not in agreement with the implantation profiles measuredby RBS. A substantial amount of Gd is accumulated in the upper 30 nmthick layer (see Fig. 1). The RBS concentration profiles are shiftedcloser to the surface in comparison with the simulations of SRIM2008. In contrast to the Gd depth profiles, Dy implanted layer usingfluences up to 1×1015 atoms.cm−2 exhibits a typical Gaussianconcentration profile with a concentration maximum at a depth ofabout 40 nm as was predicted by SRIM 2008. A small surfaceaccumulation of Dy was observed due to diffusion during theannealing procedure. A slower decrease in the Dy concentrationwas observed due to the crater effect during SIMS concentrationprofiling. For implantation doses over 5×1016 atoms.cm-2, we alsoobserved disagreement between the concentration profiles calculatedby SRIM and measured by RBS. This disagreement is caused bysputtering effects, which always take place during the implantationstep [10]. For very high implantation doses at lower implantationenergy we can observe something like a “saturated implantationprofile” when the implanted dose and sputtered material are inequilibrium. This effect occurs for implantation doses higher than1×1017 atoms.cm−2 at 200 keV in the case of heavy ions like rareearth elements. The SRIM 2008 code does not take into account thestructural changes in the GaN matrix caused by ion irradiation, or thedynamic changes in thickness and composition occurring duringimplantation, such as atom sputtering and density enhancements viaimpurity atom implantation. The disagreement is observed also whencomparing the calculated integral amount of the implanted element andthe integral amount measured by RBS. This disagreement becomessignificant for doses higher than 5×1016 atoms.cm-2, due to thesputtering effect. The implanted fluences and the integral amountsmeasured by RBS are summarized in Table 1.
C. PAR C. FER S.M.M.cm−2] [1015 atoms.cm−2] [1015 atoms.cm−2] [μB.at−1]
16 (10 K) 0.6 0.3610 0.6 0.628 0.1 0.0732 2.3 0.640.2 0.3 64.70.5 0.3 12.91.0 0.6 1.082.4 0.6 0.012.3 0.4 0.03– – 0.01– – 0.03
concentration determined magnetically, C. PAR — concentration of paramagnetic ionsnetic moment measured at 300 K after corrections. For the analysis of paramagnetismDy)=10.6μB were considered.
Fig. 1. Influence of Gd-5 sample annealing on Gd concentration profile.
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The influence of annealing time on the concentration profiles of Gdis shown in Fig. 1. We can identify a significant change of concentrationprofile due to the annealing procedure. Annealing is accompanied byrare earth diffusion to the surface, so the surface concentration isincreased. The same behavior was observed in the case of Dy and Lu.The concentration profiles of Gd and Dy implanted layers annealedfor 15 min at 900 °C are shown in Fig. 2. The concentration profilesmeasured by RBS and SIMS were in good agreement. For the con-centration profiles measurement of La, Lu and Gd RBS was used andSIMS was used for Dy. The implanted and measured doses are sum-marized in Table 1. The asymmetrical shape of the Gd and Dy peaks inthe RBS spectra is an indication of surface morphology degradation dueto the sputtering effect of high implantation doses.
Data obtained by RBS/channeling illustrate how ion implantationinduced a considerable damage of the surface layers of the GaN crystalimplanted Gd with fluences 1×1016 atoms.cm−2 (Fig. 3a). After thepost-implantation annealing, a partial reconstruction of the surfacelayer occurred which can be deduced from the shape of the surfacepeak indicating the amount of the introduced disorder in theimplanted layer. In the as-implanted GaN aligned spectrawe observeda surface peak which is comparable with the random spectra yield,(indicating that the crystal structure had been rendered nearlyamorphous), while the aligned spectra of the annealed samplesexhibit a decrease of the yield at the surface peak. This indicates apartial structural reconstruction. For the complete structure recovery
Fig. 2. Concentration profiles of Gd, Lu and Dy ions implanted in GaN measured by RBSand SIMS. All samples were annealed for 15 min.
it is necessary to use temperatures over 1400 °C and under extremelyhigh nitrogen pressure to avoid GaN decomposition [11]. We can alsoobserve a decrease in the thickness of the damaged layer. The similarresult is observed in the case of Lu implanted GaN (Fig. 3b). Thethickness of the modified layer with defects introduced by Lu and Gdion implantation is similar and the amount of disordered atoms in themodified layer after Lu implantation in GaN is comparable to therandom spectra, as in the case of Gd implanted GaN. The differencerepresents the amount of disorder remaining after the annealingprocedure. In case of Lu implanted GaN, we observed a morepronounced crystal restoration than in GaN implanted by Gd. Thesurface peak maximum and the thickness of the Lu implanted layerdecrease more significantly after the annealing procedure, comparedto the Gd implanted layer.
Fig. 4 shows typical HRXRD reciprocal space maps of the (002)reflection for a GaN layer both before and after Gd implantation andannealing (this sample is representative, as all implanted samplesshowed similar features in reciprocal spacemaps). Themap for the Gdimplanted layer shows that this sample contains a crystalline regionwith a larger out-of-plane lattice parameter, which is ‘strained’ to themain non-implanted layer in-plane. A gradient in the out-of-planelattice parameter can be seen, in which most of the strained layer hasa lattice parameter close to that of the main non-implanted GaN layer.This indicates that a compressive residual stress gradient is presentperpendicular to the film surface. This stress gradient is consistentwith the RBS data showing a gradient in Gd concentration. Althoughthere was a moderate increase in the amount of diffuse backgroundscattering (consistent with an increase in the concentration of pointdefects and/or the presence of an amorphous surface layer), theoverall diffracted intensities remained high and the lateral full widthat half maximum of the (002) peaks did not increase significantly,suggesting that much of the structural damage due to ion implanta-tion was recovered after annealing (note that the inclined streak seenin both maps is an unavoidable measurement artifact [9]).
The paramagnetic (PM) and ferromagnetic (FM) components of theimplanted layers were evaluated after the careful subtraction of thediamagnetic contribution from the sapphire substrate. At roomtemperature the magnetic moment corrected for the substrate can beattributed to a FM component characterized by the saturated FMmoment shown in Table 1. At the lowest temperatures (T=4.5–10 K)the prevailing part of the observed moment arises from the PMcontribution. In order to evaluate the PM component more precisely itwas necessary to subtract a FM contribution taken from the measure-ment at room temperature. The PM magnetization curves obtained inthis way were then evaluated using a Brillouin fit enabling thedetermination of the saturated PM moment, which yields thecorresponding concentration of the paramagnetic ions (Table 1). Takinginto account the delicate measurement of a small magnetic momentsuperimposed on a large diamagnetic background and thus aninevitably large experimental error, a nice agreement between thechemically and magnetically determined Gd and Dy concentrations isobserved, at least for doses higher than 5×1015 atoms.cm−2. Surpris-ingly, for low implantation doses a relatively larger part of theimplanted ions is associated with a FM contribution while higherimplantation results in a predominantly PM behavior. Moreover, verylow concentrations of Dy (less than 5×1015 atoms.cm−2) lead tohigher saturated magnetic moments than those corresponding to thetotal angular momentum of free Dy3+ ion (J=15/2). This effect issimilar to the behavior of the giantmagneticmoment reported for GaN:Gd [5], although it does not reach those colossal values as observed inthe super-diluted GaN:Gd system [5].
The magnetization curves for selected samples are demonstratedin Fig. 5, which show evidence for both purely FM behavior at 150 Kand low temperature paramagnetism at 5 K Finally, we haveattempted to determine the Curie temperature of implanted layersusing a Faraday balance at elevated temperatures, see Fig. 6. Let us
Fig. 3a. RBS/channeling spectra of GaN implanted by 200 keV Gd ions with a fluence of 1×1016 atoms.cm−2. Comparison of the aligned spectra of as implanted sample and samplesannealed at 900 °C for 5 and 15 min. Implanted layer thickness is indicated.
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note that the extreme experimental difficulties associated with thehigh temperature measurement lead to a large experimental error,which precludes both quantitative analysis and accurate determina-tion of TC. Nonetheless, repeated comparative experiments suggestthat all Gd implanted layers possess TC~720 K, while a slightly lowerTC~500–550 K is observed for Dy implanted films.
As seen from the saturated magnetic moments given in Table 1,samples implanted by non-magnetic ions La3+ (4f0) and Lu3+ (4f14)reveal only negligible traces of ferromagnetism corresponding to 0.01and 0.03 μB per one atom of Lu and La, respectively, which can be simply
Fig. 3b. RBS/channeling spectra of GaN implanted by 200 keV Lu ions with fluences of 1×10annealed at 900 °C for 5 min. Implanted layer thickness is indicated.
attributed to impurities. Similarly, the PM moment determined from aBrillouin fit reached only 0.09 μB per implanted atom for both samples.Hence, we can definitely exclude the possibility that ferromagnetism inour samples was caused merely by defect formation due to bombard-ment by any (including non-magnetic) ions, as suggested byDubroca etal. [12]. The observed ferromagnetism in our Gd and Dy implantedlayers is thus unambiguously caused by exchange interactions betweenmagnetic moments localized on rare earth ions.
After the magnetic characterization the selected samples (Gd-6,Dy-5) were subjected to preliminary transport measurements. The
16 atoms.cm−2. Comparison of the aligned spectra of as implanted sample and sample
Fig. 4. Reciprocal spacemap of the GaN 002 reflection for sample (a) the GaN layer before implantation and (b) the GaN layer after Gd-implanted layer, showing a significant in-planecompressive stress gradient. The data are displayed using a logarithmic intensity scale.
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ohmic contacts based on evaporated Ti/Al/Ti/Au pads with subse-quent fast annealing in H2 up to 740 °C for 1 min were applied. Thesheet ohmic resistance of both samples amounted to Rsheet~500 Ω at300 K, which is very close to the values obtained for epitaxial layersbefore implantation. Based on Hall data the sheet concentration of n-type charge carriers was estimated as ~5×1013 cm−2 which results,supposing the carrier transport within the whole thickness of the4 μm epitaxial layer, in n~2×1017 cm−3 and μe~300 cm2.V−1.s−1,the values corresponding to a non-implanted layer.
Thus in order to discuss the ferromagnetic behavior we should notleave aside the fact that the GaN layers posses n-type charge carrierseither in the conduction band (with a long electron spin lifetime [13])or those trapped on donor levels. A simple scenario attempting toexplain the long range ferromagnetic order in such very diluted GaN:Gd, Dy layers is based on FM interaction between well separated REions mediated by either conduction electrons (Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction) or bound magnetic polaronsformed around donor impurities [14]. In this respect the coexistenceof paramagnetism and ferromagnetism (see Table 1) can be likelyassociated with an inhomogeneous distribution of RE-ions within thegiven wurtzite structure leading to a formation of FM clusters appear-ing in a paramagnetic phase. The large magnetic moments observedfor highly dilute Dy samples exceeding 3–10 times in magnitude thevalues corresponding to free Dy3+ ions can be hardly interpreted in
Fig. 5. The magnetization curves for Gd, Dy and Lu implanted layers. The data arecorrected for the underlying temperature independent diamagnetism experimentallydetermined on virgin GaN epitaxial layers.
terms of spin polarized charge carriers, unless a substantiallyenhanced doping level (10–100 times higher than in the parentGaN) brought about by ion bombardment is considered.
4. Conclusion
We present a complex study of La, Gd, Dy and Lu implanted GaNlayers grown by low pressure MOVPE on c-plane sapphire substrates.La, Gd, Dy and Lu ions were implanted with energy of 200 keV anddoses ranging from 5×1013 to 4×1017 atoms.cm−2. The concentra-tion profiles measured by RBS and SIMS show a strong accumulationof implanted ions close to the surface due to diffusion during post-implantation annealing. RBS/channeling and HRXRD reciprocal spacemapping showed structural damage due to the implantation. Thelayer structure was partially recovered by post-implantation anneal-ing. The layers implanted with Gd and Dy exhibit a ferromagneticbehavior persisting up to ~720 K. The ferromagnetic behavior was notobserved on the layers implanted with La and Lu. On the basis of thisexperimental evidence we can attribute this effect to local momentsof Gd and Dy ions respectively. Based on the fact that the samplespossess n-type conductivity we speculate that the ferromagnetismcan be associated with mobile electrons mediating the ferromagneticinteraction between the local moments on Gd and Dy, a mechanismsimilar to RKKY interaction. An alternative scenario is based on a
Fig. 6. Temperature evolution of the magnetic moment for sample Gd-6.
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formation of bound magnetic polarons around donor impurities.However, due to likely inhomogeneous distribution of magnetic ionsferromagnetic domains satisfying the percolation limit for theexchange interaction are formed inside a paramagnetic matrix corres-ponding to regions with lowered concentration of magnetic dopants.
Acknowledgment
The authors would like to thank A. Dahmen for the excellenttechnical support. This project was supported by the Czech ScienceFoundation, project no. 104/09/0621 and 106/09/0125 and theMinistryof Education of Czech Republic, project no. MSM6046137302.
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