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Enhancement of Curie temperature in Ga1

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Journal of Crystal Growth 269 (2004) 298–303

Journal of Crystal Growth 269 (2004) 298–303

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  • 1. ARTICLE IN PRESS Journal of Crystal Growth 269 (2004) 298–303Enhancement of Curie temperature in Ga1ÀxMnxAs epilayers grown on cross-hatched InyGa1ÀyAs buffer layers O. Maksimov, B.L. Sheu, G. Xiang, N. Keim, P. Schiffer, N. Samarth* Department of Physics and Materials Research Institute, Pennsylvania State University, 104 Davey Lab, University Park, PA 16802, USA Received 12 April 2004; accepted 25 May 2004 Available online 2 July 2004 Communicated by H. OhnoAbstract Relaxed InyGa1ÀyAs epilayers grown on (0 0 1) GaAs are known to exhibit a cross-hatched surface with ridges %running along the [1 1 0] and ½1 1 0Š directions. We find that Ga1ÀxMnxAs epilayers grown on such buffer layers canhave as-grown Curie temperatures (TC ) that are higher than the as-grown 110 K value typical of Ga1ÀxMnxAs/GaAsheterostructures. Further, low-temperature annealing leads to only modest additional increases in TC ; contrasting withthe behavior in Ga1ÀxMnxAs/GaAs where TC typically increases significantly upon annealing. Our observationssuggest that the initial concentration of Mn interstitials in as-grown Ga1ÀxMnxAs /InyGa1ÀyAs heterostructures issmaller than that in as-grown Ga1ÀxMnxAs/GaAs heterostructures. We propose that strain-dependent diffusion maydrive Mn interstitials from the bulk of the growing crystal to more benign locations on the ridged surface, providing apossible route towards defect-engineering in these materials.r 2004 Elsevier B.V. All rights reserved.PACS: 75.50.Pp; 61.82.Fk; 71.20.NrKeywords: A3. Molecular beam epitaxy; B1. GaMnAs; B2. Ferromagnetic semiconductors There is wide interest in the diluted magnetic transition temperature (TC ) ranging up to B110 Ksemiconductor Ga1ÀxMnxAs for exploring proof- for as-grown samples. Post-growth annealing atof-concept applications in semiconductor spintro- low temperatures (180 C oTanneal o260 C) cannics [1]. Ga1ÀxMnxAs is grown by low-tempera- increase TC up to B160 K [2–4]. The enhancementture molecular beam epitaxy (LT-MBE), usually of TC is attributed to the migration of Mnon GaAs (1 0 0) substrates, and exhibits ferromag- interstitials (MnI) from the bulk of the sample tonetism for 0:02oxo0:09 with the ferromagnetic the surface; this reduces the compensation of exchange-mediating holes by these interstitial *Corresponding author. Tel.: +1-8148630136; fax: +1- defects and hence increases TC [5,6]. This hypoth-2534848667. esis is also supported by scanning tunneling E-mail address: nsamarth@psu.edu (N. Samarth). microscopy of the Ga1ÀxMnxAs surface [7] and0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jcrysgro.2004.05.091
  • 2. ARTICLE IN PRESS O. Maksimov et al. / Journal of Crystal Growth 269 (2004) 298–303 299by the capping-induced suppression of the anneal- ridges and/or dislocations occurs during theing effect [8]. A number of attempts have been MBE growth.made to enhance TC by combining the low- Samples are grown on epi-ready semi-insulatingtemperature annealing process with specific sample (Fe-doped) GaAs (1 0 0) substrates bonded withdesign such as Be modulation doping [9], N doping indium to Mo blocks. The growth is performed in[10], or growth of Mn d-doped GaAs layers [11]. an applied EPI 930 MBE system equipped with In,Here, we explore a promising alternate route Ga, Mn and As effusion cells. The substrates aretoward the control of MnI defects in Ga1ÀxMnxAs deoxidized using standard protocol, by heating toBy carrying out epitaxial growth on strain-relaxed B580 C with an As flux impinging on the surface.In0.12Ga0.88As templates with a rippled surface A thick (100 nm) GaAs buffer layer is first grownmorphology. This appears to inhibit the formation after the deoxidization. The samples are thenof MnI defects in the bulk of the crystal during cooled to B500 C for the growth of a 750 nmepitaxy, suggesting that a combination of inho- In0.12Ga0.88As buffer layer. Following this, themogeneous strain fields and misfit dislocation growth is interrupted, the substrate temperature isnetworks may provide a means of gettering MnI decreased to B250 C, and a Ga1ÀxMnxAs epi-interstitials to benign locations. Our results suggest layer is deposited. The growth is performed underthat it may be profitable to explore approaches to the group V rich conditions with a As:Ga beamdefect-engineering in these ferromagnetic semicon- equivalent pressure ratio of B12:1. The growthductors in a manner akin to that exploited in more rate is B300 nm/h and the Mo block is rotated at astandard semiconductors [12,13]. rate of 12 rpm for compositional uniformity. The We focus on Ga1ÀxMnxAs epilayers grown by growth mode and surface reconstruction areLT-MBE on a relaxed In0.12Ga0.88As buffer layer monitored in situ by reflection high-energy elec-that is itself deposited on (1 0 0) GaAs. The lattice tron diffraction (RHEED) at 12 keV. The RHEEDmismatch between the relaxed In0.12Ga0.88As pattern shows a (1  2) reconstruction during thebuffer layer and GaAs (Da=ao ) is B0.9%. Hence, Ga1ÀxMnxAs growth and there is no obviousthe Ga1ÀxMnxAs epilayers deposited on top of indication of large-scale second phase formationsuch a buffer layer are subject to a significant in- (e.g. hexagonal MnAs precipitates). After removalplane tensile strain compared to those grown on from the MBE chamber, we anneal pieces cleavedGaAs. Although earlier work has studied the out of each wafer at 250 C for 2 h in a highinfluence of such ‘‘strain-engineering’’ on the purity nitrogen gas (99.999%) flowing at a ratemagnetic anisotropy of Ga1ÀxMnxAs [14], we are un- of 1.5 ft3/h.aware of any attempts to understand the The Ga1ÀxMnxAs composition is measured byinfluence of annealing on such samples. We electron probe microanalysis; the thickness (esti-demonstrate that the Curie temperature of as- mated from RHEED oscillation measurements) isgrown Ga1ÀxMnxAs epilayers on InyGa1ÀyAs can verified by cross-sectional scanning electron mi-be consistently higher than in samples grown on croscopy. Surface morphology is investigatedGaAs, with 105 KoTC o125 K: Furthermore, in using a tapping mode AFM. Structural propertiesas-grown samples with such high values of TC ; the are determined using X-ray diffraction. Magneticordering temperature only shows a modest in- properties are measured using a Quantum Designcrease upon annealing (reaching between 125 and superconducting quantum interference device145 K). This behavior suggests that as-grown (SQUID) magnetometer.epilayers of Ga1ÀxMnxAs grown on In0.12Ga0.88As Fig. 1A shows a y22y scan for a symmetrichave a smaller fraction of MnI defects than (0 0 4) reflection from a thick Ga0.94Mn0.06Asepilayers grown on GaAs. Atomic force micro- epilayer on a GaAs buffer. The lattice mismatchscopy studies of our samples show a cross-hatched between the Ga0.94Mn0.06As and GaAs is Da=ao(CH) pattern with ridges running along [1 1 0] and B0.52%, similar to the data reported by other %½1 1 0Š directions. This observation suggests that groups [15]. Fig. 1B shows a (0 0 4) reflection fromstrain-driven segregation of MnI towards the a thick Ga0.94Mn0.06As epilayer on a relaxed
  • 3. ARTICLE IN PRESS300 O. Maksimov et al. / Journal of Crystal Growth 269 (2004) 298–303 Fig. 2. An AFM surface image of a Ga0.94Mn0.06As film grown on a In0.12Ga0.88As buffer. higher than those running along the latter; the spacing between the ridges is B1 mm. AFM measurements show that the CH pattern forms during the InyGa1ÀyAs growth, as reported inFig. 1. y22y scans for the symmetric (0 0 4) Bragg reflections other studies of relaxed InyGa1ÀyAs epilayersfrom Ga0.94Mn0.06As epilayers grown on GaAs (A) and grown on GaAs substrates [16]. The ridges areIn0.12Ga0.88As (B) buffer layers. known to originate in the formation of misfit dislocations during the InyGa1ÀyAs growth [17] and the preferable striation along the ½1 1 0Š%In0.12Ga0.88As buffer layer. The lattice mismatch direction has been attributed to the anisotropybetween the In0.12Ga0.88As and GaAs is Da=ao in surface diffusion length of In atoms [18]. WeB0.89%. Since the lattice constant of In0.12- find that the ridges are the most pronouncedGa0.88As is larger than that of Ga0.94Mn0.06As, for the thin Ga1ÀxMnxAs epilayers and thethe latter is under in-plane tensile strain and the surface becomes smoother for thicker epilayers.position of the Ga0.94Mn0.06As peak shifts toward For example, the root mean square roughnessa larger angle (seen as a shoulder on the substrate decreases from 6.1 nm for a 30-nm thick Ga0.94peak). We note that the In0.12Ga0.88As buffer layer Mn0.06As to 3.1 nm for a 240-nm thick Ga0.94relaxes by the formation of dislocations. Since the Mn0.06As.Ga1ÀxMnxAs films are grown directly on this Fig. 3 shows the SQUID magnetization data asInyGa1ÀyAs without additional buffer layers, we a function of temperature for 30 nm thickexpect that the misfit dislocations do not terminate Ga1ÀxMnxAs epilayers grown on In1ÀyGa1ÀyAsat the interface and propagate throughout the (A, B) and GaAs (C) buffer layers. The TC ofGa1ÀxMnxAs. X-ray diffraction measurements sample A (030716B) is not affected by annealingindicate that the Ga1ÀxMnxAs epilayers are and stays at B125 K, the TC of sample Bcoherently strained by the relaxed In0.12Ga0.88As (030716C) increases from 105 to 145 K, and thebuffer layer. TC of sample C (030623A) increases from 86 to Fig. 2 shows an AFM surface image of a 138 K. Table 1 summarizes the values of TCGa0.94Mn0.06As epilayer deposited on the In0.12- for a set of samples grown under similar condi-Ga0.88As buffer. The grown surface shows a CH tions. For Ga1ÀxMnxAs grown on In1ÀyGa1ÀyAs, %pattern along the ½1 1 0Š and [1 1 0] directions, with we routinely obtain as-grown TC in the rangethe ridges running along the former 2–3 times 105–125 K, and annealing boosts this up to the
  • 4. ARTICLE IN PRESS O. Maksimov et al. / Journal of Crystal Growth 269 (2004) 298–303 301 range 125–145 K. We find that for samples of Ga1ÀxMnxAs grown on GaAs during the same time frame, the TC is lower (B80 K), while the annealing effect is far more pronounced, enhan- cing TC by as much as 70%. The statistical validity of this statement may be generalized by examining data published by several groups, clearly showing that in as-grown samples of Ga1ÀxMnxAs on GaAs, TC never exceeds 110 K, and is typically below 100 K [2–6,19,20,21]. We caution that Table 1 shows significant variation in the TC for as-grown and annealed samples grown on In1ÀyGa1ÀyAs buffers under nominally identical conditions: specifically, although several samples show an as-grown TC > 110 K with little annealing effect, other samples have an as-grown TC o110 K with a very pro- nounced annealing effect. We do not find any obvious correlations of the observed behavior with physical parameters such as strain, surface rough- ness or defect density which are roughly similar for all these samples. A plausible reason for theFig. 3. Temperature dependence of the remnant magnetization observed sample-to-sample variations is the im-measured with out-of-plane magnetic field of 50 G for two 30- precise control over the substrate temperature; thisnm thick GaxMn1ÀxAs epilayers grown on an In0.12Ga0.88As is a common problem in MBE systems such asbuffer layer: (A) 030716B with both as-grown and annealed ours that rely on a radiatively coupled thermo-TC ¼ 125 K and (B) 030716C with as grown and annealed TC ¼105 and 145 K, respectively. (C) Shows similar data for a 30-nm couple located behind the sample-mounting block.thick GaxMn1ÀxAs epilayer grown on GaAs (030623A) with as- Towards the end of this paper, we speculategrown and annealed TC ¼ 86 and 138 K, respectively. Open further about physical reasons for the sensitivitysymbols correspond to the as-grown data; solid symbols of the annealing effect to variations in thecorrespond to the data after annealing. substrate temperature.Table 1Curie temperature for Ga1ÀxMnxAs epilayers of varying thickness grown on both In0.12Ga0.88As buffer layers and GaAs buffer layersunder nominally identical conditionsSample Composition Buffer Thickness (nm) TC as-grown (K) TC annealed (K)030320A Ga0.94Mn0.06As InGaAs 75 120 120030619A Ga0.95Mn0.05As GaAs 25 87 123030623A Ga0.94Mn0.06As GaAs 30 86 138030623B Ga0.94Mn0.06As GaAs 30 77 112030701A Ga0.94Mn0.06As InGaAs 20 110 112030701B Ga0.94Mn0.06As InGaAs 40 120 132030703B Ga0.95Mn0.05As InGaAs 30 112 135030703C Ga0.95Mn0.05As InGaAs 30 100 135030714B Ga0.94Mn0.06As GaAs 20 70 110030716A Ga0.94Mn0.06As InGaAs 30 105 135030716B Ga0.94Mn0.06As InGaAs 30 125 125030716C Ga0.93Mn0.07As InGaAs 30 105 145030718B Ga0.94Mn0.06As InGaAs 30 105 140
  • 5. ARTICLE IN PRESS302 O. Maksimov et al. / Journal of Crystal Growth 269 (2004) 298–303 As we mentioned in the introductory paragraph, buffer layers that have a cross-hatched surfaceMnI defects play a critical role in limiting TC in morphology. While the TC of as-grown Ga1ÀxMn-Ga1ÀxMnxAs by compensating holes. Low-tem- xAs/InyGa1ÀyAs samples is typically higher thanperature annealing of the Ga1ÀxMnxAs alloy that of Ga1ÀxMnxAs/GaAs samples, a smallerincreases TC through the removal of MnI from increase in TC is observed upon annealing. Wethe bulk of the crystal to the surface. The propose that the segregation of MnI defects occursdifferences that we observe in the TC of Ga1Àx during the MBE growth because of the ridgeMnxAs/GaAs and Ga1ÀxMnxAs/InyGa1ÀyAs sam- morphology of the relaxed InyGa1ÀyAs bufferples suggest that in the latter case a smaller layer and is responsible for our observations. Ourfraction of MnI defects is formed in the bulk of observations suggest possible pathways to thethe crystal during sample growth. While we do not controlled defect-engineering of Mn interstitialshave the necessary measurements to further in Ga1ÀxMnxAs.substantiate the microscopic details, we propose We thank J. Van Vechten for motivating thetwo speculative scenarios that could account for experiments described in this manuscript. We areour observations. It is known that the lattice strain also grateful to Khalid Eid and Keh Chiang Kuis not uniform across the surface of relaxed for additional sample growth, and to M. S.InyGa1ÀyAs films: it is lower at the top of the Angelone for help with the XRD and EPMAridges and higher in the valleys [16]. Thus, it may measurements. Authors (O. M, G. X, and N. S)be energetically more favorable for the highly were supported by ONR Grants N00014-99-1-mobile MnI defects to segregate to the top of the 0071 and N00014-02-10996, and DARPA/ONRridges in a manner similar to the accumulation of N00014-99-1-1093. Authors (B.L.S and P.S) wereInAs islands on the top of the ridges [16] and the supported by DARPA N00014-00-1-0591 andalignment of InAs quantum dots along the [1 1 0] NSF DMR-0101318. N.K. also acknowledges thedirections [22]. Alternatively, MnI defects may be support of the Penn State Physics REU site DMR-gettered by the misfit dislocation network that 0097769.straddles the surface ridges: for instance, it isknown that the misfit dislocation density is highestin the trough regions of CH samples [23],providing energetically favorable locations for Referencesthe interstitials. [1] H. Ohno, in: D.D. Awschalom, D. Loss, N. Samarth This model also allows us to speculate about the (Eds.), Semiconductor Spintronics and Quantum Informa-origin of the inconsistencies in the annealing tion, Springer, Berlin, 2002, pp. 1–30.behavior of samples grown under nominally [2] K.W. Edmonds, K.Y. Wang, A.C. Neumann, N.R.S.identical conditions. Variations in substrate tem- Farley, B.L. Gallaher, C.T. Foxon, Appl. Phys. Lett. 81 (2002) 4991.perature invariably influence both the surface [3] K.C. Ku, S.J. Potashnik, R.F. Wang, S.H. Chun, P.mobility and the Mn solubility: the former Schiffer, N. Samarth, M.J. Seong, A. Mascarehnas, E.decreases at lower substrate temperatures, inhibit- Johnston-Halperin, R.C. Myers, A.C. Gossard, D.D.ing the effective gettering of MnI defects, while the Awschalom, Appl. Phys. Lett. 82 (2003) 2302.latter decreases at higher substrate temperatures, [4] D. Chiba, K. Takamura, F. Matsukura, H. Ohno, Appl.possibly enhancing the formation of MnI defects. Phys. Lett. 82 (2003) 3020. [5] K.M. Yu, W. Walukiewicz, T. Woitowicz, I. Kiryliszyn, X.Both these effects can increase the concentration Liu, Y. Sasaki, J.K. Furdyna, Phys. Rev. B 65 (2002)of interstitials if the substrate temperature is either 201303 (R).above or below the optimal temperature range; as [6] K.W. Edmonds, P. Boguslawski, K.Y. Wang,a consequence, non-optimal substrate temperature R.P. Campion, S.N. Novikov, N.R.S. Farley, B.L.can result in a lower as-grown TC with a Gallagher, C.T. Foxon, M. Sawicki, T. Dietl, M. Buongiorno Nardelli, J. Bernholc, Phys. Rev. Lett. 92pronounced enhancement of TC after annealing. (2004) 037201. In conclusion, we have grown Ga1ÀxMnxAs [7] A. Mikkelsen, J. Gustafson, J. Sadowski, J.N. Andersen,epilayers by LT-MBE on relaxed In0.12Ga0.88As J. Kanski, E. Lundgren, Appl. Surf. Sci. 222 (2004) 23.
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