Width dependence of annealing effects in „Ga,Mn…As nanowiresB. L. Sheu, K. F. Eid, O. Maksimov, N. Samarth, and P. Schiffe...
in-plane field. The results are shown in the inset of Fig. 2͑a͒,where the as-grown and annealed piece both show similar TCo...
ditions such as higher annealing temperatures and longer an-nealing times.6,7In Fig. 3, we show the annealing time depende...
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Width dependence of annealing effects in GaMnAs nanowires


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DOI: 10.1063/1.2150809

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Width dependence of annealing effects in GaMnAs nanowires

  1. 1. Width dependence of annealing effects in „Ga,Mn…As nanowiresB. L. Sheu, K. F. Eid, O. Maksimov, N. Samarth, and P. Schiffera͒Department of Physics and Materials Research Institute, Pennsylvania State University, University Park,Pennsylvania 16802͑Presented on 1 November 2005; published online 17 April 2006͒We study the time dependence of annealing on a series of GaAs-capped ͑Ga,Mn͒As nanowires ofvarying widths. For different annealing times, our measurements indicate that decreasing the wirewidth monotonically increases the Curie temperature enhancement associated with annealing, aswell as the drop in resistivity. These results are consistent with the lateral diffusion of interstitial Mnions, which constitute an important source of defects in these materials. Furthermore, the thinnerwires show a higher rate of change of conductivity with annealing time, suggesting a more efficientremoval of Mn interstitials in thinner wires. © 2006 American Institute of Physics.͓DOI: 10.1063/1.2150809͔An ongoing interest in spin-based devices has motivatedresearch into magnetoelectronic materials over the pastdecade.1,2Among them, III–V magnetic semiconductors suchas ͑Ga,Mn͒As are extremely attractive due to their compat-ibility with current semiconductor technology. These materi-als display “carrier-mediated ferromagnetism,” so that themagnetic properties of the system can be manipulated byprecisely controlling the electronic properties.3–5Previousstudies have shown that the Curie temperature ͑TC͒ of͑Ga,Mn͒As can be increased substantially by performingpost-growth annealing.6,7Recent work has shown that TC canreach above 150 K,8,9and a recent report of TCϳ250 K in͑Ga,Mn͒As-based heterostructures10even hints at the possi-bility of room temperature ferromagnetism.In situ resistance monitoring during annealing,9as wellas studies of annealing effects on ͑Ga,Mn͒As under differentconditions11,12or with GaAs or As capping,13,14all suggestthat the removal of Mn interstitials ͑MnI͒ from ͑Ga,Mn͒Asplays an important role in the enhancement of TC upon an-nealing. This can be easily understood, since the MnI bothintroduce disorder and compensate the holes which mediateferromagnetic exchange. Understanding the rate-limiting fac-tors of the MnI removal in ͑Ga,Mn͒As will be particularlyimportant for designing heterostructure devices, since MnIcannot diffuse to a free surface if the ͑Ga,Mn͒As layer isburied within a heterostructure. One route to effective an-nealing of ͑Ga,Mn͒As in heterostructures is to lithographi-cally reduce the lateral dimensions of the heterostructure andthus allow MnI to diffuse out laterally.15Here, we report adetailed study of time-dependent annealing effects in a seriesof wires of different widths fabricated from GaAs-capped͑Ga,Mn͒As. The annealing effects on Curie temperature andon resistivity increase substantially as the wire width is de-creased, although we are not able to obtain a clear agreementwith a diffusion model as was possible for epilayers of dif-ferent thicknesses.9Our samples were fabricated by electron beam lithogra-phy and dry etching of a GaAs/͑Ga,Mn͒As͑50 nm͒/GaAs͑10 nm͒ heterostructure grown by molecular beam ep-itaxy ͑MBE͒ on semi-insulating GaAs ͑001͒ wafers; detailsof the MBE growth and post-growth processing are de-scribed elsewhere.7,8,13,15Figure 1͑a͒ shows a field emissionscanning electron microscopy image of the designed layoutfor four probe resistance measurement of a 70 nm wide wire.Figure 1͑b͒ shows the more detailed features of a 220 nmwide single wire under higher magnification. All other wireswere patterned on the same wafer along ͓110͔ direction ofthe GaAs substrate for consistency, although no effect ofcrystalline anisotropy in resistivity is expected based uponearlier studies.15Magnetization versus temperature measurements for as-grown and annealed ͑190 °C, 5 h, N2 gas flowϳ1.5 scfh͒macroscopic pieces of our sample were performed in a com-mercial superconducting quantum interference device͑SQUID͒ magnetometer. Samples were precooled in 1 Tmagnetic field and measured while warming in a 0.005 Ta͒Electronic mail: schiffer@phys.psu.eduFIG. 1. FESEM images of the nanowires. ͑a͒ The layout of a four proberesistance measurement for a 70 nm wide single wire patterned onGaAs/͑Ga,Mn͒As͑50 nm͒/GaAs͑10 nm͒. ͑b͒ A 220 nm wide single wireunder higher magnification. All the wires in the series of study are patternedalong ͓110͔ direction of the same GaAs wafer.JOURNAL OF APPLIED PHYSICS 99, 08D501 ͑2006͒0021-8979/2006/99͑8͒/08D501/3/$23.00 © 2006 American Institute of Physics99, 08D501-1Downloaded 17 Apr 2006 to Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
  2. 2. in-plane field. The results are shown in the inset of Fig. 2͑a͒,where the as-grown and annealed piece both show similar TCof ϳ66 K, identifying suppressed effect of annealing due toGaAs capping.13An alternative method of determining TC of͑Ga,Mn͒As is through the peak of the temperature-dependent resistivity, where the peak temperature is usuallywithin 10 K of the estimated TC from SQUIDmeasurements.16,17The equivalence is demonstrated in Fig.2͑a͒ by resistivity versus temperature data from 1 mmϫ2 mm mesas patterned on the same as-grown and annealedpieces.We fabricated wires of widths 1 ␮m, 420, 220, and120 nm for four probe resistivity measurement. We measuredthe temperature-dependent resistivity, ␳͑T͒, of all the wiresand then annealed for an hour at 190 °C in flowing N2, andthen measured again, and then annealed again for anotherhour. This cycle was repeated for 6 h of total annealing time,but the samples degraded during a longer anneal which wasattempted after 6 h, and became unmeasurable. Typical dataare shown in Figs. 2͑b͒ and 2͑c͒ for the wires as-grown andannealed for 6 h total.18Compared to the results for the mac-roscopic pieces of sample ͓Fig. 2͑a͔͒, there is a significanteffect of annealing on the nanowires. The increase in TC afterannealing is almost 45 K for the 120 nm wire, indicating theeffectiveness of outdiffusion of MnI towards the lateral freesurfaces. Though we have no explanation for the slightlyincreased resistivity in our set of annealed wires compared tothe unpatterned as-grown sample, similar effects due to re-construction of defect structures have been observed for con-FIG. 3. ͑a͒ Curie temperature, ͑b͒ conductivity at T=300 K, and ͑c͒ rate ofchange of conductivity ͑measured at T=300 K͒ for the different wires as afunction of total anneal time. Each of these wires is measured as-grown andafter every additional hour of annealing at 190 °C up to 6 h. The ratechange of conductivity was calculated based on the neighboring conductiv-ity values of the corresponding annealing time.FIG. 2. ͑a͒ Resistivity vs temperature ͑measured in van der Pauw geometry͒of as-grown and annealed 1 mmϫ2 mm mesa patterned onGaAs/͑Ga,Mn͒As͑50 nm͒/GaAs͑10 nm͒ heterostructures. The inset showsmagnetization vs temperature for the same sample measured in a magneticfield of 50 Oe in-plane while warming after field cooling in a 1 T field. Thevalues of TC obtained from both measurements are consistent. ͑b͒ Tempera-ture dependence of ͑four-probe͒ resistivity for unannealed wires, patternedfrom the same sample. ͑c͒ The temperature dependence of the resistivity ofthe same wires after six 1 h anneals.08D501-2 Sheu et al. J. Appl. Phys. 99, 08D501 ͑2006͒Downloaded 17 Apr 2006 to Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
  3. 3. ditions such as higher annealing temperatures and longer an-nealing times.6,7In Fig. 3, we show the annealing time dependence of TCand resistivity at 300 K. We note that after 6 h of annealing,the increase in TC for the 120 nm wire is three times of the1 ␮m wire; at the same time, the resistivity at the ␳͑T͒ peakis reduced to half that of the 1 ␮m wire, confirming thestrong correlation between the enhancement of TC and de-crease of resistivity, which was observed in uncappedepilayers.7Figure 3͑c͒ shows the rate change of conductivity versusannealing time, indicating the MnI diffuse out faster in athinner wire in the initial stages of annealing, and the rate ofMnI removal decreases with annealing time. This is some-what different from the results of in situ resistance monitor-ing in uncapped epilayers, in which the thinner layers had aslower rate of change of resistivity.9Due to the limited timeinterval of our measurements, we cannot rigorously test ourdata against the one-dimensional diffusion model used in thatstudy. On the other hand, the higher rate of change of theconductivity in thinner wires, suggests that a more complexmodel would be required, perhaps due to the more complexgeometry presented by the wires. Another possibility is thatdiffusion is different in the thinner epilayers studied previ-ously or different in the lateral as opposed to the growthdirection ͑although different lateral directions appear to beequivalent͒.15In summary, we studied lateral diffusion of Mn intersti-tials in a post-growth annealed series of GaAs-capped͑Ga,Mn͒As wires of different widths. For different anneal-ing time periods, our measurements indicate that the nar-rower wires are fabricated, the more significant are the ef-fects of Curie temperature enhancement, accompanied by alarger resistivity decrease and higher rate change of conduc-tivity.This research has been supported by Grant Nos. ONRN0014-05-1-0107, DARPA/ONR N00014-99-1093, -00-1-0951, University of California-Santa Barbara SubcontractKK4131, and NSF DMR-0305238 and DMR-0401486. Thiswork was performed in part at the Penn State Nanofabrica-tion Facility, a member of the NSF National NanofabricationUsers Network.1S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. vonMolnár, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Science294, 1488 ͑2001͒.2A. H. MacDonald, P. Schiffer, and N. Samarth, Nat. Mater. 4, 195 ͑2005͒.3T. Dietl, H. Ohno, F. Matsukra, J. Cibert, and D. Ferrand, Science 287,1019 ͑2000͒.4H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, and Y.Iye, Appl. Phys. Lett. 69, 363 ͑1996͒.5A. M. Nazmul, S. Sugahara, and M. Tanaka, Phys. Rev. B 67, 241308͑R͒͑2003͒; S. Lee et al., J. Appl. Phys. 93, 8307 ͑2003͒.6T. Hayashi, Y. Hashimoto, S. Katsumoto, and Y. Iyea, Appl. Phys. Lett.78, 1691 ͑2001͒.7S. J. Potashnik, K. C. Ku, S. H. Chun, J. J. Berry, N. Samarth, and P.Schiffer, Appl. Phys. Lett. 79, 1495 ͑2001͒.8K. C. Ku et al., Appl. Phys. Lett. 82, 2302 ͑2003͒.9K. W. Edmonds et al., Phys. Rev. Lett. 92, 037201 ͑2004͒.10A. M. Nazmul, T. Amemiya, Y. Shuto, S. Sugahara, and M. Tanaka, Phys.Rev. Lett. 95, 017201 ͑2005͒.11R. Mathieu et al., Phys. Rev. B 68, 184421 ͑2003͒.12M. Malfait, J. Vanacken, V. V. Moshchalkov, W. Van Roy, and G. Borghs,Appl. Phys. Lett. 86, 132501 ͑2005͒.13M. B. Stone, K. C. Ku, S. J. Potashnik, B. L. Sheu, N. Samarth, and P.Schiffer, Appl. Phys. Lett. 83, 4568 ͑2003͒.14M. Adell et al., Appl. Phys. Lett. 86, 112501 ͑2005͒.15K. F. Eid, B. L. Sheu, O. Maksimov, M. B. Stone, P. Schiffer, and N.Samarth, Appl. Phys. Lett. 86, 152505 ͑2005͒.16H. Ohno, in Semiconductor Spintronics and Quantum Computation, editedby D. D. Awschalom, D. Loss, and N. Samarth ͑Springer, Berlin, 2002͒,p. 1.17N. Samarth, in Solid State Physics, edited by H. Ehrenreich and F.Spaepen ͑Elsevier/Academic, San Diego 2004͒, Vol. 58.18The monotonically decreasing resistivity with wire width in the as-grownsamples is possibly due to local heating during the dry etching processresulting in some annealing of the wires.08D501-3 Sheu et al. J. Appl. Phys. 99, 08D501 ͑2006͒Downloaded 17 Apr 2006 to Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp