We demonstrate the formation of ferromagneticGa₁ˍₓMnₓAsfilms by Mn ion implantation into GaAs followed by pulsed-laser melting. Irradiation with a single excimer laser pulse results in the epitaxial regrowth of the implanted layer with Mn substitutional fraction up to 80% and effective Curie temperature up to 29 K for samples with a maximum Mn concentration of x≈0.03. A remanent magnetization persisting above 85 K has been observed for samples with x≈0.10, in which 40% of the Mn resides on substitutional lattice sites. We find that the ferromagnetism in Ga₁ˍₓMnₓAs is rather robust to the presence of structural defects.The work at the Lawrence Berkeley National Laboratory
was supported by the Director, Office of Science, Office of
Basic Energy Sciences, Division of Materials Sciences and
Engineering, of the U.S. Department of Energy under Contract
No. DE-AC03-76SF00098. The work at Harvard was
supported by NASA Grant No. NAG8-1680. One of
the authors ~M.A.S.! acknowledges support from an NSF
Graduate Research Fellowship

Aziz, M. J.

Dubon, O. D.

Monteiro, O.

Pillai, M. R.

Ridgway, M. C.

Scarpulla, M. A.

Yu, K. M.

Applied Physics Letters

English

The Australian National University

APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 8 24 FEBRUARY 2003Ferromagnetic Ga 1ÀxMnxAs produced by ion implantationand pulsed-laser meltingM. A. Scarpullaa) and O. D. Dubonb)Department of Materials Science & Engineering, University of California at Berkeley, Berkeley,California 94720 and Lawrence Berkeley National Laboratory, Berkeley, California 94720K. M. Yu and O. MonteiroLawrence Berkeley National Laboratory, Berkeley, California 94720M. R. Pillai and M. J. AzizDivision of Engineering & Applied Sciences, Harvard University, Cambridge, Massachusetts 02138M. C. RidgwayDepartment of Electronic Materials Engineering, Research School of Physical Sciences and Engineering,Australian National University, Canberra, Australia~Received 26 September 2002; accepted 3 January 2003!We demonstrate the formation of ferromagnetic Ga12xMnxAs films by Mn ion implantation intoGaAs followed by pulsed-laser melting. Irradiation with a single excimer laser pulse results in theepitaxial regrowth of the implanted layer with Mn substitutional fraction up to 80% and effectiveCurie temperature up to 29 K for samples with a maximum Mn concentration ofx'0.03. Aremanent magnetization persisting above 85 K has been observed for samples withx'0.10, inwhich 40% of the Mn resides on substitutional lattice sites. We find that the ferromagnetism inGa12xMnxAs is rather robust to the presence of structural defects. ©2003 American Institute ofPhysics. @DOI: 10.1063/1.1555260#rstivuntic-aityetofit;mgodith-eogeraooactorf aon-hastfr ofinairing-nthe, ash ays-beiondcetertheThe integration of diluted magnetic semiconducto~DMSs! into quantum structures may enable such innovatechnologies as spin-based optoelectronics and even qtum computing.1–3 The addition of a few atomic percent Mto GaAs results in the formation of ferromagneGa12xMnxAs, the most widely studied III–V DMS. In general, only limited success has been achieved in the preption of potentially useful DMSs because of the low solubilof magnetic species in semiconductor hosts. In the casGa12xMnxAs, the range of Mn compositions requiredachieve this phase~typically x<0.09! is some 2–3 orders omagnitude greater than the equilibrium solubility limtherefore, synthesis must be carried out far from equilibriuMn ion implantation followed by rapid thermal annealin~RTA! has been reported to lead to the formation of MnAsGaxMny precipitates,4–7 although one group has reportemagnetic circular dichroism signals consistent wGa12xMnxAs after RTA at 300 °C.8 Until now, ferromagneticGa12xMnxAs has been produced exclusively by lowtemperature molecular-beam epitaxy~LT-MBE!, in whichcase the formation of Mn-containing second phases is kincally suppressed.Here, we present an alternative, highly versatile methfor the formation of ferromagnetic Ga12xMnxAs using Mn1implantation into GaAs followed by pulsed-laser meltin~PLM!. The rapid melting and recrystallization associatwith laser processing leads to a high level of Mn incorpotion in the regrown layer, while suppressing the formationsecond phases. Compared to MBE, our synthesis appra!Electronic mail: mascarpulla@lbl.govb!Electronic mail: oddubon@socrates.berkeley.edu1250003-6951/2003/82(8)/1251/3/$20.00Downloaded 21 Feb 2003 to 131.243.163.5. Redistribution subject to Aean-ra-of.rti-dd-fchhas fewer restrictions on the choice of host semiconduand alloying species. Thus, it enables the investigation owide range of materials systems. The incorporation of cventional dopant species into semiconductors using PLMbeen studied extensively,9 but alloying with magnetic impu-rities at the atomic percent level has not been explored.Semi-insulating GaAs~001! wafers were implanted aroom temperature with Mn1 ions to 0.3431016 cm22 at 150keV and to 1.131016 cm22 at 300 keV. This combination oion doses and energies resulted in an amorphized layeabout 2500 Å with a relatively uniform Mn concentrationthe range of 1–1.5 at. %. Each sample was irradiated inwith a single pulse from a XeCl1 excimer laser@l5308 nm,full width at half maximum~FWHM!530 ns#. A multiprismhomogenizer was used to produce a uniform fluence rangbetween 0.40 and 0.61 J/cm2 over the sample area of approximately 5 mm35 mm. The 488-nm line of an argon-iolaser was used to monitor the time-resolved reflectivity ofsamples during the laser irradiation. The melt duration (tmelt)ranged between 250 and 400 ns for this range of fluencedetermined by a constant reflectivity signal consistent witlayer of molten GaAs. The threshold fluences at which crtalline and amorphous GaAs melt were determined to0.22 and 0.08 J/cm2, respectively.The structure of the Ga12xMnxAs layers was studied bychanneling Rutherford backscattering spectrometry~c-RBS!using a 2-MeV He1 beam and by x-ray diffractometry~XRD!. Particle-induced x-ray emission~PIXE! experimentswere also carried out during the c-RBS, and this combinatof data was used to determine the lattice location of Mn. Asuperconducting quantum interference device magnetomwas used to measure the in-plane magnetic behavior of1 © 2003 American Institute of PhysicsIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jspioaivpwaobes-tivth-onurss.-n ofth-lly,n.rofed.b-rys-theorthatsesrro-allouredpytedsti-ofde-x-i-aovKn.eVnan1252 Appl. Phys. Lett., Vol. 82, No. 8, 24 February 2003 Scarpulla et al.films along the@001# direction. Mn depth profiles with anestimated resolution of 50 Å were obtained by secondarymass spectrometry~SIMS! using a 5.5 keV Cs1 primary ionbeam. Electrochemical capacitance–voltage profiling wused to obtain the net concentration of electrically actacceptors~presumed to be substitutional Mn! as a function ofdepth. For each of the samples examined, the net acceconcentration was found to range between 1020 and1021 cm22 over the depth of the layer, and to closely follo~within the ;15% combined experimental uncertainty! heMn profile determined by SIMS.Figure 1~a! presents the film magnetization~M! versusapplied field~H! measured at 5 K for a sample irradiated0.41 J/cm2. Open, nearly square hysteresis loops wereserved at 5 K for all of the samples, with lower coercivity(Hc) and saturation (M sat) being observed for those samplirradiated at higher fluences. The main panel of Fig. 1~b!presents the remanent magnetization (Mr) versus temperature for this sample. An effective Curie temperature (TC) of29 K was determined by extrapolating the data with negacurvature down to zero magnetization. It is apparent frominset of Fig. 1~b!, which depicts the Mn concentration profiles determined by SIMS before and after laser irradiatithat significant Mn redistribution toward the surface occFIG. 1. ~a! Magnetization (M ) vs magnetic field (H) measured at 5 K for asample implanted at 150 and 300 keV and irradiated at 0.41 J/cm2. Thesaturation magnetization and coercivity are, respectively, 78 emu/g Mn;100 Oe. A diamagnetic background due to the substrate has been rem~b! Remanent magnetization~ ormalized to remanent magnetization at 5!vs temperature for the same sample; the inset shows Mn concentratiodepth before~dashed! and after~solid! laser melting for the same sampleDownloaded 21 Feb 2003 to 131.243.163.5. Redistribution subject to Ansetort-ee,sduring the PLM and rapid liquid-phase epitaxy proceThus, as the Curie temperature of Ga12xMnxAs has beenshown to depend on Mn concentration,2 the temperature dependence of film magnetization represents a superpositiocontributions arising from regions throughout the Mn depdistribution.Figure 2 presents@001# c-RBS data from a sample before ~as-implanted! and after laser irradiation at 0.41 J/cm2.c-RBS from the as-implanted sample reveals a;2500-Ålayer of amorphous GaAs~with yields similar to the spec-trum measured at random incidence!. After PLM, the chan-neled yield of this surface 2500-Å layer drops dramaticareflecting the epitaxial regrowth of the implanted regioWhile the yield from the regrown layer is slightly highethan that of the virgin wafer—indicating the presencestructural defects—a single crystal layer has been achievu–2u XRD yielded only GaAs substrate peaks. The asence of additional peaks discounts the presence of polyctallinity. High-resolution scans showed a shoulder on~004! peak at lower diffraction angles~larger lattice param-eter!, consistent with a layer of Ga12xMnxAs of varying Mncomposition.The fact that the films exhibit ferromagnetic behavithat disappears well below room temperature indicatesmagnetic contributions from ferromagnetic second phasuch as MnAs (TC5318 K!5 and GaxMny (TC5450–800K!4,6 compounds are not responsible for the observed femagnetic behavior. However, their presence in smamounts or as superparamagnetic precipitates belowXRD detection limit cannot be completely ruled out basupon this current work. A transmission electron microscostudy is underway to help clarify this issue.Simultaneous c-RBS and PIXE spectra were collecfrom the sample irradiated at 0.41 J/cm2 in both the@001#and@011# channeling directions. From these data, the subtutional fraction of Mn was determined to be 80% in boththe axial directions. Because the substitutional fractionstermined in the two directions were identical within the eperimental uncertainty of,5%, we postulate that nonsubstnded.vsFIG. 2. @001# c-RBS spectra from a sample implanted at 150 and 300 kand irradiated at 0.41 J/cm2. The channeled spectrum before irradiatiocoincides with the random spectrum to a depth of;2500 Å, indicating fullamorphization to this depth during ion implantation. The spectrum fromunimplanted GaAs wafer is presented for reference.IP license or copyright, see http://ojps.aip.org/aplo/aplcr.jspbo-pyfseerimoffluwrahle%ituernofndn,genourtseticothex-dal-cancesryofndon-as.toofF.G.ett.., C.s.nt1253Appl. Phys. Lett., Vol. 82, No. 8, 24 February 2003 Scarpulla et al.tutional Mn exists as clusters or precipitates too small todetected by XRD and not as interstitials in tetrahedralhexagonal sites.10 This is quite different from as-grown LTMBE samples, in which a large fraction of Mn atoms occuthese interstitial sites.10 The presence of Mn in other types ointerstitial sites cannot be ruled out based on the prechanneling measurements.In order to determine whether Curie temperatures nthe 110 K reported for MBE-grown films withx'0.052,3could be achieved using our synthesis approach, anotheof samples was prepared. An identical GaAs wafer wasplanted with Mn ions to a dose of 1.531016 cm2 using themetal-vapor vacuum arc~MEVVA ! technique11 at an accel-erating voltage of 35 kV and roughly equal fractionsMn11 and Mn21. These samples were laser melted at aence of 0.30 J/cm2 using a KrF1 laser ~l5248 nm,FWHM538 ns!. The lower ion energies used in MEVVAresult in shallower implanted layers, which can be regrowith higher crystalline quality.The remanent magnetization as a function of tempeture for a sample from this set is presented in Fig. 3. Tinset shows the Mn depth profile for the irradiated sampThe maximum Mn concentration of 431021 cm23, orx'0.10, is in excess of the concentration (x'0.05) requiredto achieve aTC of 110 K if all Mn atoms reside on Gasublattice. However, c-RBS and PIXE reveal that only 40of Mn atoms are substitutional, corresponding to a substtional Mn content ofx'0.04, which is consistent with thFIG. 3. Remanent magnetization vs temperature for a sample implausing MEVVA and irradiated with a KrF1 excimer laser at 0.3 J/cm2; theinset shows Mn concentration vs depth after laser melting.Downloaded 21 Feb 2003 to 131.243.163.5. Redistribution subject to Aerntarset--n-e.-observed effectiveTC of 86 K. This sample exhibits a greatecoercivity ~;150 Oe! and saturation magnetization~;110emu/g Mn! at 5 K than observed in samples of lower Mconcentration.In conclusion, we have demonstrated the formationferromagnetic films of Ga12xMnxAs using ion implantationfollowed by pulsed-laser melting. The films are epitaxial asingle-crystalline. Without significant process optimizatiowe have achieved Curie temperatures above liquid nitrotemperature. The observation of ferromagnetism inGa12xMnxAs layers, which contain both structural defecand Mn composition gradients, suggests that the magncoupling between Mn in this system is rather robust to bof these factors. This versatile synthesis approach opensciting opportunities for the study of a wide range of dilutemagnetic semiconductor materials, including quaternaryloys and multi-dopant systems. Moreover, this processalso be exploited to create all-planar spin-injection deviand circuits.The work at the Lawrence Berkeley National Laboratowas supported by the Director, Office of Science, OfficeBasic Energy Sciences, Division of Materials Sciences aEngineering, of the U.S. Department of Energy under Ctract No. DE-AC03-76SF00098. The work at Harvard wsupported by NASA Grant No. NAG8-1680. We thank AStacy and E. Carleton for the use of facilities. Thanks alsoY. Gao and W. Walukiewicz for helpful discussions. Onethe authors~M.A.S.! acknowledges support from an NSGraduate Research Fellowship.1G. A. Prinz, Science282, 1660~1998!.2H. Ohno, Science281, 951 ~1998!.3H. Ohno, J. Magn. Magn. Mater.200, 110 ~1999!, and references therein4J. Shi, J. M. Kikkawa, R. Proksch, T. Schaeffer, D. D. Awschalom,Medeiros-Ribeiro, and P. M. Petroff, Nature~London! 377, 707 ~1995!.5P. J. Wellmann, M. Garcia, J.-L. Feng, and P. M. Petroff, Appl. Phys. L71, 2532~1997!.6J. Shi, J. M. Kikkawa, D. D. Awschalom, G. Medeiros-Ribeiro, P. MPetroff, and K. Babcock, J. Appl. Phys.79, 5296~1996!.7J. De Boeck, R. Oesterholt, A. Van Esch, H. Bender, C. BruynseraedeVan Hoof, and G. Borghs, Appl. Phys. Lett.68, 2744~1996!.8K. Ando, A. Chiba, H. Tanoue, F. Kirino, and M. Tanaka, IEEE TranMagn.35, 3463~1999!.9See, for example,Laser Annealing of Semiconductors, edited by J. M.Poate and J. W. Mayer~Academic, New York, 1982!.10K. M. Yu, W. Walukiewicz, T. Wojtowicz, I. Kuryliszyn, X. Liu, Y. Sasaki,and J. K. Furdyna, Phys. Rev. B65, 201303~R! ~2002!.11I. G. Brown, Rev. Sci. Instrum.65, 3061~1994!.edIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

Ferromagnetic Ga₁ˍₓ Mnₓ As produced by ion implantation and pulsed-laser melting

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Ferromagnetic Ga₁ˍₓ Mnₓ As produced by ion implantation and pulsed-laser melting

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