Resonant Raman data on ferromagnetic GaMnAs reveal the existence of a new kind of defect: insulating nanoislands consisting of substitutional Mn-Ga acceptors surrounded by interstitial Mn-I donors. As indicated by the observation of a sharp 1S(3/2)-> 2S(3/2) Raman transition at similar to 703 cm(-1), the acceptor-bound holes inside the islands are isolated from the metallic surroundings. Instead, Mn-bound excitons do couple to the ferromagnetic environment, as shown by the presence of associated Raman magnon side bands. This leads to an estimate of 5-10 nm for the nanoisland radius. The islands disappear after annealing due to the removal of the Mn-I ions

Fahy, Stephen B.

Furdyna, J. K.

Jacobs, P. W.

Liu, X.

Merlin, Roberto

Ren, Y. H.

Sapega, V. F.

Wang, D. M.

Physical Review Letters

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Title Observation of insulating nanoislands in ferromagnetic GaMnAsAuthor(s) Wang, D. M.; Ren, Y. H.; Jacobs, P. W.; Fahy, Stephen B.; Liu, X.;Furdyna, J. K.; Sapega, V. F.; Merlin, RobertoPublication date 2009Original citation Wang, D. M., Ren, Y. H., Jacobs, P. W., Fahy, S., Liu, X., Furdyna, J.K., Sapega, V. F. and Merlin, R. (2009) 'Observation of insulatingnanoislands in ferromagnetic GaMnAs', Physical Review Letters,102(25), 256401 (4pp). doi: 10.1103/PhysRevLett.102.256401Type of publication Article (peer-reviewed)Link to publisher'sversionhttps://journals.aps.org/prl/abstract/10.1103/PhysRevLett.102.256401http://dx.doi.org/10.1103/PhysRevLett.102.256401Access to the full text of the published version may require asubscription.Rights © 2009, American Physical SocietyItem downloadedfromhttp://hdl.handle.net/10468/4641Downloaded on 2018-08-23T19:40:59ZObservation of Insulating Nanoislands in Ferromagnetic GaMnAsD.M. Wang,1 Y.H. Ren,1,* P.W. Jacobs,1 S. Fahy,2 X. Liu,3 J. K. Furdyna,3 V. F. Sapega,4 and R. Merlin11Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040, USA2Department of Physics and Tyndall National Institute, University College, Cork, Ireland3Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA4Ioffe Physico-Technical Institute, Russian Academy of Sciences, 19402 St. Petersburg, Russia(Received 5 January 2009; revised manuscript received 4 May 2009; published 22 June 2009)Resonant Raman data on ferromagnetic GaMnAs reveal the existence of a new kind of defect:insulating nanoislands consisting of substitutional MnGa acceptors surrounded by interstitial MnI donors.As indicated by the observation of a sharp 1S3=2 ! 2S3=2 Raman transition at 703 cm1, the acceptor-bound holes inside the islands are isolated from the metallic surroundings. Instead, Mn-bound excitons docouple to the ferromagnetic environment, as shown by the presence of associated Raman magnon sidebands. This leads to an estimate of 5–10 nm for the nanoisland radius. The islands disappear afterannealing due to the removal of the MnI ions.DOI: 10.1103/PhysRevLett.102.256401 PACS numbers: 71.55.i, 75.30.Hx, 75.50.Pp, 78.30.jThe III-Mn-V diluted magnetic semiconductor (DMS)and, in particular, GaMnAs have attracted much attentionrecently due to their potential applications in spintronics[1]. It is now widely accepted that the ferromagnetism inthese systems arises from the hole-mediated interactionbetween local magnetic moments. While the majority ofthe Mn ions in GaMnAs exists in the form of substitutionalMnGa acceptors, a small amount also occurs as interstitialMnI donors. In addition to reducing the hole density, theinterstitials form antiferromagnetic MnGa-MnI pairs andMnGa-MnI-MnGa complexes, which reduce the number ofmagnetically activeMnGa ions [2,3]. A recent hot-electronphotoluminescence (PL) study indicates that the holesfromMnGa form a detached impurity band for a wide rangeof manganese concentrations [4]. The MnI ions can beremoved by low-temperature annealing [2,3,5].In this Letter, we report on a resonant Raman scattering(RRS) study of GaMnAs. Consistent with recent theoreti-cal analyses [2] as well as with the hot-PL results [4],which show evidence of the coexistence of paramagnetic(PM) and ferromagnetic (FM) regions below the Curietemperature, TC, and infrared absorption work [6], whichsuggests the existence of Mn acceptors isolated from theFM environment, our results conclusively show the occur-rence of a new type of defect: paramagnetic nanoislandscontaining magnetically-inactive MnGa acceptors sur-rounded by MnI donors. The RRS technique has beenused successfully to probe the 1S3=2 levels of Mn acceptorsin insulating GaAs [7]. Our study focuses on the 1S3=2 !2S3=2 transition of isolated neutralMnGa acceptors in DMSGaMnAs, which reveals itself as a Raman line at703 cm1. In addition, we identified several Raman side-bands as due to the interaction between magnons from theFM environment and MnGa-bound excitons. These obser-vations indicate that the distance between theMnGa accep-tor and the FM surroundings is5–10 nm. We find that thehole-bound transitions disappear after the sample is an-nealed, and we interpret this as due to the outdiffusion ofMnI ions [2].We studied several FM GaMnAs films and a lightly-doped PM GaAs:Mn sample. The FM samples were grownon semi-insulating (001) GaAs substrates by low-temperature molecular-beam epitaxy (MBE). Two as-grown FM films are 300-nm thick with nominal Mn con-centrations 1.4% and 2.6%, and the same Curie tempera-ture of 44 K. The third as-grown FM film is 120-nm thickwith 9% Mn and TC ¼ 60 K. A small portion of thissample was annealed for one hour in a nitrogen gas atmo-sphere at 270 C. The PM sample, grown on (111) GaAs,has a concentration of 0.04% [7]. Samples were placedinside a liquid-helium flow magnetic cryostat for lowtemperature measurements. We used a cw Ti:sapphire laserwhose wavelength L could be tuned from 750 nm to835 nm. RRS spectra were obtained with a Dilor XYsystem. The laser power density on the samples was8 mW=cm2. The experiments in a magnetic field werecarried out in a near-backscattering Faraday geometry withfields oriented along z^, the direction perpendicular to thefilms’ surface. The Raman geometry is described usingstandard notation: zðþ; Þ z denotes the back-scattering configuration where the incident and scatteredbeams are represented, respectively, by zðþ andÞz.Here þ and  stand for right and left circular polar-izations, whereas z and z indicate the directions of thecorresponding wave vectors.Figure 1(a) shows RRS spectra excited at different laserwavelengths. The feature indicated by the dashed line,which appears on the high-energy side of the PL peak, isascribed to the 1S3=2 ! 2S3=2 Mn acceptor transition (wenote that, due to exchange coupling between the j ¼ 3=2hole and the S ¼ 5=2 Mn2þ core, the acceptor ground stateis actually a F ¼ 1 multiplet [7,8]). Our assignment issupported by the facts that this peak is only observed insamples containing Mn and that its position, 703 cm1PRL 102, 256401 (2009) P HY S I CA L R EV I EW LE T T E R Sweek ending26 JUNE 20090031-9007=09=102(25)=256401(4) 256401-1  2009 The American Physical Society(87.8 meV), agrees extremely well with the measureddifference between the 1S3=2 and 2S3=2 binding energiesof Mn-bound holes in GaAs [8]. Furthermore, the703 cm1 line appears only in a narrow range of scatteredphoton energies, centered at1:41 eV, which is very closeto that of the dominant e A0 PL in GaAs:Mn arisingfrom the recombination of electrons with Mn-bound holes[9]. These observations are consistent with a double-resonant Raman process involving excitons bound to Mn-acceptors, denoted by A0X [7], for which the cross sectioncan be written approximately asd / ½ðEI  EA0XÞ2 þ 21 ½ðES  EA0X þ1S  2SÞ2 þ 21:Here EI (ES) is the incident (scattered) photon energy,EA0Xand @1 are the energy and the lifetime of the exciton, and1Sð2SÞ is the binding energy of the 1S3=2 (2S3=2) accep-tor state. In the Raman process, the exciton created by theincident photon interacts with the bound hole, originally inthe 1S3=2 state, and promotes it to the 2S3=2 level before itrecombines [10]. The scattering is strongly enhanced dueto both the incident and scattered resonances and, thus, it isextremely sensitive for investigating the spectrum of theimpurities.A comparison between RRS spectra of the PM and the2.6% FM samples is shown in Fig. 1(b). Both samplesexhibit relatively sharp peaks with very similar energy,linewidth ( 6 cm1) and resonant behavior. These re-sults strongly suggest that the feature observed in the FMsample is due to isolated acceptors, as opposed to transi-tions involving the 1S3=2 and 2S3=2 impurity bands[4,11,12], which should be significantly broader than thoseseen for the low concentration PM specimen. We believethat the presence of neutral Mn acceptors is closely relatedto the existence of MnI in the FM samples. The interstitialions, acting as double donors, can interact with neighbor-ing MnGa ions to form Mn complexes [2,3], in the neigh-borhood of which, the density of free holes is reducedfacilitating the existence of solitary MnGa acceptors. Onthe other hand, the MnI ions can be partly removed byannealing, which causes their outdiffusion to the surface[2,3]. The effect of low-temperature annealing on the1S3=2 ! 2S3=2 transition of the 9% sample is shown inFig. 1(c). Clearly, the Raman peak disappears after anneal-ing. This can be explained by the increase in the holedensity, which opens the initially isolated MnGa ions tobecome interactive with the FM environment. Previousmeasurements have shown that annealing at 250 C for30 minutes can increase the hole concentration by10%–30% and lead to an enhancement of TC by a factorof 1:7 [13]. Also, isolated Mn ions located outside theferromagnetic environment have been reported to exist inthe DMS InMnSb [14].Results in an applied magnetic field, of magnitude B,reveal a strong dependence of the scattering on the polar-ization of the incident light; see Fig. 2(a). As shown inFig. 2(b) at 2.6 K, the 1S3=2 ! 2S3=2 intensity increases(decreases) with increasing field if the exciting light is (þ) polarized. Referring to the simplified, single-particleRRS process depicted in Fig. 2(c), such a behavior can beaccounted for by the Zeeman splitting of the ground F ¼ 1multiplet, for which the gyromagnetic factor is g 2:75[7,8], and thermal population effects. At sufficiently lowtemperatures, the bound holes reside for the most part inthe lowest-lying state, mF ¼ 1, involving mostly mj ¼1=2, 3=2 states from the GaAs valence band [15]. Tominimize the Coulomb repulsion between the two holesin A0X, the Raman process is dominated by transitionsassociated with states that have the same magnetic quan-tum number. These excitations are allowed mainly in oneof the two circular polarizations because of the predomi-nance of positive values of mj. Conversely, when thethermal energy is much larger than the Zeeman splitting,the mF populations become comparable and, as illustratedby the 7 K data in Fig. 2(b), the difference in peak heightsbecomes negligible. These considerations also explain why-1690 700 710 as grownannealed(c)Raman IntensityRaman Shift (cm-1)690 700 7102.6 %0.04 %(b)   FIG. 1. (a) Data for the 1.4% Mn sample. The dashed lineindicates the position of the 1S3=2 ! 2S3=2 acceptor transition.Numbers on the left indicate the laser wavelength, L, in nm.The broad peak, PL, is due to e A0 photoluminescence.(b) Data at two, PM (0.04%) and FM (2.6%), Mn concentrationsobtained with L ¼ 826:5 nm. (c) Comparison between the as-grown and the annealed 9%Mn sample after a linear backgroundsubtraction (L ¼ 827:3 nm).PRL 102, 256401 (2009) P HY S I CA L R EV I EW LE T T E R Sweek ending26 JUNE 2009256401-2the scattering does not depend on the incident light polar-ization at B ¼ 0; see Fig. 2(a). The theoretical curves inFig. 2(b) were obtained using the optical selection rules forzinc blende semiconductors and the acceptor wave func-tions listed in [15].Figure 3 shows the temperature dependence and selec-tion rules of the acceptor transition in the 2.6% sample atB ¼ 6:9 T. At large fields, the Raman feature splits intoseveral peaks. In addition to the central band, labeled ‘‘0’’,there are three sidebands marked ‘‘1’’, ‘‘1’’ and ‘‘2’’(these sidebands were not seen in the PM sample and arenot resolved in the spectra of Fig. 2(a) at B ¼ 3 T). Theseparation between neighboring peaks is nearly the same,6:6 cm1. The data of Fig. 4 shows that the splitting isproportional to the field and, thus, that it can be describedby the Zeeman coupling energy EZ ¼ gBB. The ex-periments give g ¼ 2:1 0:3, a value that agrees well withg ¼ 2:02 for 3d Mn electrons, but differs significantlyfrom that of Mn-bound holes with g 2:75 [7,15].Figure 3(a) shows that, as the temperature increases, thesidebands 1 and 2 become broader and seemingly weakerwhile 1 becomes stronger. Figure 3(b) shows spectrameasured at different geometries at 6 K. Consistent withthe results of Fig. 2, the scattering is negligible forþ-polarized excitation. From the figure, one sees thatthe 1 and 1 sidebands are strongest when the scatteredlight is, respectively, left () and right (þ) circularlypolarized. We emphasize that, unlike the FM films, nosidebands were observed in the PM sample using thesame experimental conditions. From the Mn concentrationin the PM sample, we estimate that the average number ofMn ions within a sphere of radius10 nm, equal to that ofthe acceptor-bound exciton [7], is 14. Therefore, weconclude that the satellite lines appearing in the FM sampleare not due to the spin-flip of a neighboring Mn ion (other-wise, these lines should also appear in the PM sample). Thepresence of sidebands in the FM but not in the PM samplestrongly points to a connection with magnons and, accord-ingly, we assign the peak ‘‘1’’ to the 1S3=2 ! 2S3=2transition with one magnon emitted, and peaks ‘‘1’’ and‘‘2’’ to 1S3=2 ! 2S3=2 excitation followed by the absorp-tion of one and two magnons, respectively. These assign-ments are consistent with the temperature-dependentresults and the properties of GaMnAs magnons [16].First, we note that the measured g is consistent with ourinterpretation because the magnon g factor is nearly thesame as that of Mn 3d electrons [16]. Second, the fact thatthe intensity of peak 1 is significantly smaller at 1.9 Ksupports our assignment since magnons are thermally ex-cited and, thus, fewer magnons can be emitted at lowtemperatures. Finally, the observed dominance of one ofthe two circular polarizations in the scattering with mag-non emission (sideband1) and absorption (sideband 1) isalso consistent with our interpretation since the angularmomentum of the scattered photon must compensate forthe change in the total spin of the magnetic system.The occurrence of magnon sidebands indicates thatsome of the states involved in the RRS process interactwith the FM environment. For an isolated acceptor, the− − -− + -− − -+ + -+ − -FIG. 3. RRS spectra of the sample with 2.6% Mn showing(a) the temperature dependence and (b) the scattering selectionrules. The PL signal has been subtracted. Peak labeled 0 is the1S3=2 ! 2S3=2 hole transition whereas 1, 1, and 2 are magnonsidebands.+--13/23/2F-101FIG. 2 (color online). (a) RRS of GaMnAs (1.4% Mn) at 2.6 Kshowing the dependence of the acceptor scattering on the inci-dent photon polarization; L ¼ 826:5 nm. (b) Magnetic fielddependence of the Raman intensity, in arbitrary units. Filledand open circles (squares) indicate ðþÞ polarized incidentlight at, respectively, 7 K and 2.6 K. The curves are fromtheoretical expressions described in the text. (c) Diagram show-ing the RRS process at zero field and in an applied magneticfield. The dashed line indicates states in the vicinity of thebottom of the GaAs conduction band. The dominant single-particle transitions are indicated by arrows.PRL 102, 256401 (2009) P HY S I CA L R EV I EW LE T T E R Sweek ending26 JUNE 2009256401-3Bohr radius of the 1S3=2 and 2S3=2 states are, respectively,1 nm and 5 nm [7]. Since the linewidths of the1S3=2 ! 2S3=2 transition in the FM and PM samples arethe same within experimental uncertainty, as shown inFig. 1(b), it is apparent that there is little interactionbetween the 1S3=2 and 2S3=2 levels of the MnGa acceptorsin the islands and the FM surroundings. Therefore, thesidebands can only be the result of interactions betweenthe FM magnons and the intermediate resonant state in theRaman process, i.e., MnGa-bound excitons. It follows thatthe radius of the PM islands must be somewhere betweenthe Bohr radius of the 2S3=2 holes (5 nm) and that of theMnGa bound excitons (10 nm) [7]. We note that inhomoge-neous structures of sizes similar to those of the nanoislandshave been observed in GaMnAs using cross-sectional scan-ning tunneling microscopy [17].An idealized picture of the PM nanoislands is shown inFig. 5. The islands are insulating regions with a smallconcentration of holes immersed in the ferromagnetichost containing a much larger hole concentration. Theirexistence reflects indirectly the presence of MnI ionswhich, in itself, results from fluctuations favored by thenonequilibrium nature of low-temperature MBE [18]. TheMnGa concentration inside the islands is lower than that inthe film, while the MnI concentration is much higher.Acting as donors, the MnI ions significantly reduce thedensity of holes inside the island where the remainingneutral MnGa acceptors cannot interact with each other orwith the ferromagnetic metallic surroundings [19]. The PMregions disappear after annealing, which removesMnI ionsfrom the islands to the surface, as indicated by the dis-appearance of the acceptor transition in the annealed sam-ple in Fig. 1(c).In summary, the 1S3=2 ! 2S3=2 transition of holes boundto neutral Mn acceptors and associated magnon sidebandswere observed in FM GaMnAs using RRS. The resultsshow that insulating nanoislands of 5–10 nm radius existin as-grown samples, but disappear after low-temperatureannealing due to the removal of interstitial MnI ions.*Present address: Department of Physics and Astronomy,Hunter College of the City University of New York, 695Park Avenue, NY, NY 10021, USA.[1] H. Ohno, Science 281, 951 (1998); K. S. Burch, D. D.Awschalom, and D.N. Basov, J. Magn. Magn. Mater. 320,3207 (2008).[2] K.W. Edmonds et al., Phys. Rev. Lett. 92, 037201 (2004).[3] K.M. Yu et al., Phys. Rev. B 65, 201303 (2002).[4] V. F. Sapega, M. Moreno, M. Ramsteiner, L. Da¨weritz, andK.H. Ploog, Phys. Rev. Lett. 94, 137401 (2005).[5] B. S. Sørensen, P. E. Lindelof, J. Sadowski, R. Mathieu,and P. Svedlindh, Appl. Phys. Lett. 82, 2287 (2003).[6] Y. Nagai and K. Nagasaka, Infrared Phys. Technol. 48, 1(2006).[7] V. F. Sapega, T. Ruf, and M. Cardona, Phys. Status SolidiB 226, 339 (2001).[8] M. Linnarsson et al., Phys. Rev. B 55, 6938 (1997).[9] L. Montelius, S. Nilsson, and L. Samuelson, Phys. Rev. B40, 5598 (1989).[10] Such resonant processes are referred to as selective-excitation PL. See, e.g., J. Wagner and M. Ramsteiner,Appl. Phys. Lett. 49, 1369 (1986), and references therein.[11] E. J. Singley et al., Phys. Rev. B 68, 165204 (2003).[12] Y. J. Zhao, W. T. Geng, K. T. Park, and A. J. Freeman,Phys. Rev. B 64, 035207 (2001).[13] W. Limmer et al., Phys. Rev. B 71, 205213 (2005).[14] M. Csontos et al., Phys. Rev. Lett. 95, 227203 (2005).[15] J. Schneider, U. Kaufmann, W. Wilkening, M. Baeumler,and F. Ko¨hl, Phys. Rev. Lett. 59, 240 (1987).[16] D.M. Wang et al., Phys. Rev. B 75, 233308 (2007); Y.Hashimoto, S. Kobayashi, and H. Munekata, Phys. Rev.Lett. 100, 067202 (2008); C. Bihler, W. Schoch, W.Limmer, S. T. B. Goennenwein, and M. S. Brandt, Phys.Rev. B 79, 045205 (2009).[17] G. Mahieu et al., Appl. Phys. Lett. 82, 712 (2003).[18] S. C. Erwin and A.G. Petukhov, Phys. Rev. Lett. 89,227201 (2002)[19] F. Ma´ca and J. Masˇek, Phys. Rev. B 65, 235209 (2002).FIG. 5. Schematic diagram of a PM nanoisland. Interstitial(substitutional) ions are indicated by open (closed) circles. Theisland radius is 5–10 nm. The MnGa spins are oriented ran-domly inside the island, but along the same direction outside.FIG. 4. Magnetic field dependence of the spectral position ofthe Raman features. Results are shown for the PM and the 2.6%Mn FM sample. The geometry is zð; Þz.PRL 102, 256401 (2009) P HY S I CA L R EV I EW LE T T E R Sweek ending26 JUNE 2009256401-4

Observation of insulating nanoislands in ferromagnetic GaMnAs

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Observation of insulating nanoislands in ferromagnetic GaMnAs

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