NANO LETTERS
Zn1-xMnxTe Diluted Magnetic Semiconductor Nanowires Grown by Molecular Beam Epitaxy
2008 Vol. 8, No. 11 4061-4065
Wojciech Zaleszczyk,*,† Elz˙bieta Janik,† Adam Presz,‡ Piotr Dłuz˙ewski,† Sławomir Kret,† Wojciech Szuszkiewicz,† Jean-Franc¸ois Morhange,§ Elz˙bieta Dynowska,† Holm Kirmse,| Wolfgang Neumann,| Aleksy Petroutchik,† Lech T. Baczewski,† Grzegorz Karczewski,† and Tomasz Wojtowicz† Institute of Physics, Polish Academy of Sciences, Al. Lotniko´w 32/46, 02-668 Warsaw, Poland, Institute of High Pressure Physics (UNIPRESS), Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142 Warsaw, Poland, Institut des Nanosciences de Paris, UniVersite´ Paris VI et Paris VII, CNRS UMR 7588, 140 rue de Lourmel, Paris 75015, France, Humboldt-UniVersita¨t zu Berlin, Institut fu¨r Physik, Newtonstrasse 15, D-12489 Berlin, Germany Received August 12, 2008
ABSTRACT It is shown that the growth of II-VI diluted magnetic semiconductor nanowires is possible by the catalytically enhanced molecular beam epitaxy (MBE). Zn1-xMnxTe NWs with manganese content up to x ) 0.60 were produced by this method. X-ray diffraction, Raman spectroscopy, and temperature dependent photoluminescence measurements confirm the incorporation of Mn2+ ions in the cation substitutional sites of the ZnTe matrix of the NWs.
Diluted magnetic semiconductors (DMSs) are classical semiconductors, such as CdTe, ZnSe or GaAs, in which a fraction of the metal sites is replaced by magnetic ions (e.g., Mn, Fe, or Co). Because of the exchange interaction between these magnetic ions and quasi-free carriers, the DMSs represent a unique class of materials in which both charge and spin states of carriers can be manipulated by an external magnetic field.1 DMSs have recently become the focus of renewed interest due to the occurrence of Zener-type ferromagnetism in these materials.2 The very pronounced spin properties of DMSs make them the materials of choice for future spintronic applications.3 DMSs have usually been investigated in the form of bulk crystals (3D), epitaxial layers (2D), or quantum dots (0D). However, in the past several years, many research groups have also begun studies of one-dimensional DMS nanowires (NWs).4-6 This interest is primarily motivated by the fact that DMS NWs combine two new degrees of freedom, the giant spin effects characteristic of DMSs and “bottom-up” fabrication scheme, both of which hold out the promise of * Corresponding author. E-mail:
[email protected]. † Institute of Physics, Polish Academy of Sciences. ‡ Institute of High Pressure Physics, Polish Academy of Sciences. § Institut des Nanosciences de Paris, Universite ´ Paris VI et Paris VII, CNRS UMR 7588. | Humboldt-Universita ¨ t zu Berlin, Institut fu¨r Physik. 10.1021/nl802449g CCC: $40.75 Published on Web 10/09/2008
2008 American Chemical Society
extending the capabilities of electronic devices beyond what is possible by the simple “top-down” scaling down of electronic devices.7 Indeed, in the area of II-VI DMSs, there already are several reports on Mn-doped NWs and nanorods, mostly based on sulfide compounds6,8-10 and zinc oxide,11-13 that have been obtained by chemical “bottom-up” methods. In this letter, we show that high quality self-standing II-VI DMS NWs can also be fabricated by molecular beam epitaxy (MBE), the method that has already been proven to be the most powerful technique for making 0D and 2D nanostructures made of DMSs (both in II-VI and III-V family). One should additionally note, as will be shown below, that this approach also provides the ability of reliable control of the composition of NW material, which constitutes an added advantage for planning and designing spintronic nanodevices based on such DMS NWs. To demonstrate these features, we will use Zn1-xMnxTe as the NW material, although the same procedures can naturally be extended to other II-Mn-VI alloys. The case of Zn1-xMnxTe is, however, particularly well suited for the present purpose because this material is one of the most thoroughly understood DMS systems.1,14-22 In addition, carrier-induced ferromagnetism has already been observed in nitrogen-doped epilayers of Zn1-xMnxTe.23 It is therefore likely that Zn1-xMnxTe NWs with appropriate Mn concentrations and doping levels can
be regarded as potential building blocks of prototype spintronic nanodevices. The present work on Zn1-xMnxTe NWs was preceded by detailed studies of growth and physical properties of pure ZnTe NWs produced by MBE on oriented GaAs substrates with various crystallographic orientations.24,25 The growth of Zn1-xMnxTe NWs described in this letter was carried out on (100) and (110)-oriented epi-ready GaAs substrates covered with uniform Au/Ga eutectic nanodroplets 40 to 50 nm in diameter. The process of nanodroplet formation was demonstrated earlier.24,25 The growth temperature was optimized to obtain reproducible NWs that are homogeneous in shape and that have a minimum number of stacking faults. Best results were obtained at a growth temperature in the range 470-500 °C. Zn1-xMnxTe NWs discussed in this letter (x ) 0.17, 0.20, 0.30, 0.49, and 0.60) were grown for 30 min at a substrate temperature of 490 °C. The morphology of the as-grown Zn1-xMnxTe NWs was characterized by field-emission scanning electron microscopy (FE-SEM Leo 540). Structural characterization of NWs was performed by X-ray diffraction (XRD) using characteristic Fe: KR (λ ) 1.9373 Å) and Kβ (1.75661 Å) radiation. Optical properties of Zn1-xMnxTe NWs were studied by Raman scattering (RS) and photoluminescence (PL) was performed in both macro- and micromodes. The spectra of as-grown Zn1-xMnxTe NWs on GaAs substrate, as well as of NWs removed mechanically from the substrate and deposited on Si wafers, were measured at temperatures from 5.5 to 295 K. Several Ar+ and Kr+ laser lines were used for Raman scattering in order to determine the best wavelength for the resonant excitation. The 442 nm line of the He-Cd laser was used for photoluminescence excitation. For the lowtemperature measurements, samples were placed on a coldfinger of a continuous flow helium cryostat. In the micro mode, both Raman (at room temperature only) and PL spectra of a small number of single Zn1-xMnxTe NWs were acquired using the ×100 (Raman) or ×50 (PL) objective of a microscope coupled to the spectrometer, which ensured a spot diameter of the order of 1-2 µm. All Raman scattering measurements were performed in a quasibackscattering geometry using a Jobin-Yvon U1000 spectrometer equipped with holographic gratings, an S20 photomultiplier, and a photon counting system. The PL measurements were performed using a Jobin-Yvon SPEX 270 M spectrometer with a CCD camera as a detector. Representative FE-SEM images of the as-grown samples of Zn1-xMnxTe NWs are shown in Figure 1. As in the case of ZnTe NWs,24,25 the NWs containing Mn preferentially grow along the 〈111〉 directions of the GaAs substrate, with 〈111〉B preferred over 〈111〉A, independent of the orientation of the substrate surface. All NWs are tapered, with a small droplet of the catalyst on the tip, confirming that the growth mechanism is catalytic. Figure 1b shows that the NWs have a typical length of at least 1 µm and the top diameter of about 30 nm, while the base diameter is larger, up to about 90 nm. The inset of Figure 1b also shows that the crosssection of the Zn1-xMnxTe NWs is triangular. 4062
Figure 1. FE-SEM images of Zn0.8Mn0.2Te nanowires grown on (110)-oriented GaAs substrates. Panel (a) represents top view, while panel (b) is the side view (90°). The inset to the panel (b) shows triangular cross-section of NWs (one of the NWs got accidentally broken). The approximate viewing direction and schematic projections of NWs grown along 〈111〉-type crystallographic directions of the substrate onto the plane of the pictures for (a,b) are given.
It is of course of critical importance to establish that Mn ions have indeed been incorporated substitutionally into the crystalline lattice of the NWs. We use three complimentary experimental techniques to demonstrate this: XRD, Raman scattering, and photoluminescence. Because the Mn tetrahedral radius is larger than that of Zn,26 the substitution of Mn at the Zn positions in the ZnTe lattice should lead to an increase of the lattice parameter, as is observed in Zn1-xMnxTe crystals in both bulk and layer form. The systematic increase of the lattice parameter in our Zn1-xMnxTe NWs was indeed established by X-ray diffraction experiments. As is well-known, these measurements can also be used for a quantitative determination of the Mn content in Zn1-xMnxTe NWs.27 Assuming that the Vegard rule is also obeyed in the case of NWs, we can use the bulk crystal relation between lattice parameter a(x) and Mn concentration x, which for Zn1-xMnxTe is given by: a(x) ) 6.1035 + 0.2337x. The lattice parameter in our NW samples was determined to vary from 6.110 to 6.2430 Å. Using this, the Mn content in our series of Zn1-xMnxTe NWs was found to vary from 0 up to x ) 0.60. An independent confirmation of the presence of Mn2+ ions located at the substitutional metal sites in Zn1-xMnxTe NWs can be obtained from photoluminescence (PL) measurements. A scanning electron microphotograph of a typical specimen used in these studies is shown in Figure 2a, together with the schematic image of a 2 µm laser light spot used for optical excitation of the nanowires in the micro-PL mode. We estimate that in the micro-PL mode, we measure a PL signal originating from about 10 NWs, while in the macroPL mode, the contributing number of NWs is about 400 times larger. The spectra obtained in the macro-PL mode (thin upper line) and in the micro-PL mode (thick lower noisy line) are compared in Figure 2b. Although low temperature PL measurements do not reveal any optical signal in the excitonic region of energies (above 2.4 eV), they do show a Nano Lett., Vol. 8, No. 11, 2008
Figure 2. (a) FE-SEM images of Zn0.8Mn0.2Te nanowires removed from (001)-oriented GaAs substrates and deposited on Si wafer. Typical diameter of the area studied by microphotoluminescence (2 µm) is marked by the circle. (b) Macrophotoluminescence (upper thin line) and microphotoluminescence (lower thick noisy line) related to intraion optical transitions between 3d electronic levels of Mn2+ perturbed by the crystal field. Spectra were excited by a 442 nm laser line at 11 and 5.5 K, respectively. (c) Temperature variation of the PL peak position determined in the macro mode.
broad PL band at about 2.0 eV. This is a well-known PL band characteristic of all wide-gap II-VI DMSs containing Mn and is attributed to intra-atomic 4T1 f 6A1 transitions within the 3d5 electron shell of Mn2+ions. In zinc-blende crystals, the free-ion terms of Mn2+ ions occupying the substitutional lattice sites (Td site symmetry) split due to the cubic crystal field, allowing the 4T1 f 6A1 optical transitions. The crystal field splitting depends on the temperature both because of lattice dilation and lattice harmonic vibrations. As a result, the peak energy of the Mn2+ PL band exhibits characteristic nonmonotonic temperature dependence, with a minimum at about 50 K. Such temperature dependence was observed in Cd1-xMnxTe DMS crystals and has been described in terms of crystal field theory by Biernacki et al.,28 who showed that the PL energy scales with thermal expansion of the lattice and that the minimum in the PL position is caused by the negative expansivity around 30-80 K observed for semiconductor crystals with zinc-blende structure.29 We investigated the temperature dependence of the Mn2+ PL for our Zn1-xMnxTe NWs in the macro mode, and we found a similar nonmonotonic behavior of the PL, as shown in Figure 2c. Thus the PL results serve both to confirm the presence of Mn ions in the NWs and also to provide strong evidence that the ions are located at substitutional sites of the Zn1-xMnxTe lattice. Additonally, resonant Raman scattering (RS) was studied on Zn1-xMnxTe NW specimens removed from GaAs substrate and placed on Si wafers. These measurements provide further proof of substitutional incorporation of Mn ions into the NWs. Figure 3 shows the compositional evolution of the room temperature Raman spectrum in the range 185-210 Nano Lett., Vol. 8, No. 11, 2008
Figure 3. Raman scattering spectra of single Zn1-xMnxTe NWs removed from (110)-oriented GaAs substrates and deposited on Si wafer. The inset shows a typical Raman scattering spectrum of as grown Zn1-xMnxTe nanowires (x ) 0.3) on (110)-oriented GaAs taken in the frequency range from 0 to 500 cm-1. All data were taken at T ) 295 K using different laser excitation lines in the vicinity of resonance excitation. The dashed curves are guides for the eye, illustrating the composition dependence of the LO1 and LO2 phonon modes.
cm-1. For pure ZnTe NWs (x ) 0), one observes a single Raman line at 206 cm-1 resulting from the scattering on the longitudinal optical (LO1) phonons. With increasing Mn content, this dominant Raman line shifts to higher frequencies and, in addition, a new Raman line appears on the low frequency side of the main line, shifting to lower frequencies with increasing Mn content. Qualitatively, this is very similar to what is observed for the LO phonon modes in bulk Zn1-xMnxTe mixed crystal30 and represents a behavior intermediate between the classical one- and two-mode behavior. We interpret the high frequency mode as the LO1 phonon mode and the lower frequency satellite as one of the two peaks split off from the Mn impurity mode in ZnTe. Taking into account the temperature difference between the results in ref 30 (80 K) and our data (295 K), the frequency variation of this mode with Mn concentration leads us to identify this feature as the LO2 mode. The close similarity of the LO phonon behavior observed in bulk crystals and in the NWs thus serves as evidence that the NWs consist of good quality Zn1-xMnxTe mixed crystals. It is important to emphasize that the spectral width of the LO1 and LO2 lines observed in NWs is similar to that observed in Zn1-xMnxTe both in single bulk crystals and in epilayer form, thus demonstrating the good electronic quality of the NWs. The resonant Raman conditions also allow the observation of the 2LO2 and 2LO1 phonon replicas at higher frequencies, confirming again the high electronic quality of the Zn1-xMnxTe NWs. These replicas are visible in the inset of Figure 3, presenting a Raman spectrum for as-grown Zn0.7Mn0.3Te NWs in a wider frequency range. The three additional peaks in the spectral range between 90 and 150 cm-1 are due to the Raman scattering by phonons in small precipitates containing trigonal Te, which are commonly 4063
Figure 4. Dependence of Raman spectra of ZnTe and Zn0.7Mn0.3Te NWs on excitation wavelength. For each series, the spectra have been normalized to the intensity of the A1 phonon line of tellurium (125 cm-1). Spectra a, b, c were obtained for incident lines 520.8, 530.9, 568.2 nm, respectively, and spectra A, B, C for 501.7, 514.5, 530.9 nm, respectively. All data were taken at room temperature.
observed Te-based semiconductors, both bulk and grown by MBE.31 Figure 4 compares Raman spectra of Zn0.7Mn0.3Te NWs to the spectra of pure ZnTe NWs for various excitation wavelengths. For each sample, the spectra are normalized using the intensity of the strongest Te peak (A1) as the reference. As can be seen, in the case of pure ZnTe, the highest intensity is obtained when using the 530.9 nm laser line (∼2.33 eV), while for Zn1-xMnxTe NWs, the strongest intensity occurs for 514.5 nm (∼2.41 eV). Although the limited number of discrete laser lines that we had at our disposal did not allow us to determine precisely the resonant energy in each case, it is clear that those energies are different for ZnTe and Zn1-xMnxTe NWs. This means that the energy gaps of both materials are different and provides another strong proof that Mn atoms occupy substitutional sites in the ZnTe lattice. For pure ZnTe NWs, the resonance near 2.33 eV is reasonably close to the room temperature energy gap of bulk ZnTe (2.28 eV), while in the case of Zn1-xMnxTe, the observed value of 2.41 eV would correspond to the gap of a mixed crystal with x ) 0.30.32 This value of x is in perfect agreement with the value of x determined by XRD measurements carried out on the same nanowires. The agreement of these two independent experimental methods indicates that the composition of the ternary NWs is quite homogeneous. A good homogeneity of the NWs was further confirmed by electron energy loss spectroscopy (EELS), which will be discussed in detail in a separate paper. The background of the Raman spectra in Figure 4 depends on the energy of the excitation, having a maximum near the resonance. It is thus tempting to attribute it to weak luminescence (weaker than the Raman lines) observable only when the excitation is resonant (hot luminescence). The presence of resonance in Raman scattering from the NWs 4064
for laser energy near the band gap of Zn1-xMnxTe with x corresponding to that determined by X-ray diffraction on these NWs, together with the observation of the resonant enhancement of the PL background, is an indication of excellent electronic properties of the NW material, and it is of course reassuring that the Raman features are fully consistent with the XRD data obtained on the same NWs. In conclusion, we have shown that molecular beam epitaxy, a method widely used for the growth of DMS-based nanostructures such as 2D quantum wells, superlattices, and 0D quantum dots, can also be successful in producing selfstanding DMS NWs if appropriate nanosize particles are used as catalysts. This was demonstrated by using the particular example of Zn1-xMnxTe, one of the best known and most thoroughly studied DMS materials, that has also been shown to exhibit low temperature carrier-induced ferromagnetism. We showed that Zn1-xMnxTe NWs with manganese concentration up to x ) 0.6 can be epitaxially grown on (100)- and (110)-oriented GaAs substrates with the use of Au nanocatalysts. It was found that the Zn1-xMnxTe NWs form in the zinc-blende structure, with an average diameter of about 60 nm and a typical length of 1 µm. We used four characterization techniques of NWs (XRD, PL, RS, and EELS) to confirm that the incorporation of Mn2+ ions into the NWs is homogeneous and occurs substitutionally at the Zn sites of the ZnTe matrix. We believe that the successful fabrication of DMS NWs opens new perspectives for application of DMS nanostructures as building blocks for future nanospintronics devices assembled with bottom-up approach to nanotechnology. Acknowledgment. This research was partially supported by the Ministry of Science and Higher Education (Poland) through grant nos. N507 030 31/0735, N515 015 32/0997, and by the Foundation for Polish Science through Subsidy 12/2007. References (1) Furdyna, J. K. J. Appl. Phys. 1988, 64, R29-R64. (2) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (3) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (4) Kulkarni, J. S.; Kazakova, O.; Holmes, J. D. Appl. Phys. A: Mater. Sci. Process. 2006, A85, 277. (5) Sadowski, J.; Dluzewski, P.; Kret, S.; Janik, E.; Lusakowska, E.; Kanski, J.; Presz, A.; Terki, F.; Charar, S.; Tang, D. Nano Lett. 2007, 7, 2724. (6) Radovanovic, P. V.; Barrelet, C. J.; Gradecak, S.; Fang, Q.; Lieber, C. M. Nano Lett. 2005, 5, 1407. (7) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (8) Na, C. W.; Han, D. S.; Kim, D. S.; Kang, Y. J.; Lee, J. Y.; Park, J.; Oh, D. K.; Kim, K. S.; Kim, D. J. Phys. Chem. B 2006, 110, 6699. (9) Yuan, H. J.; Yan, X. Q.; Zhang, Z. X.; Liu, D. F.; Zhou, Z. P.; Cao, L.; Wang, J. X.; Gao, Y.; Song, L.; Liu, L. F.; Zhao, X. W.; Dou, X. Y.; Zhou, W. Y.; Xie, S. S. J. Cryst. Growth 2004, 271, 403. (10) Jian, P. G.; Jin, W.; Hao, X. Z.; Xun, W.; Qing, P.; Ya, D. L. AdV. Funct. Mater. 2005, 15, 303. (11) Baik, J. M.; Kang, T. W.; Lee, J. L. Nanotechnology 2007, 18, 095703. (12) Liu, J. J.; Yu, M. H.; Zhou, W. L. Appl. Phys. Lett. 2005, 87, 172505. (13) Chang, Y. Q.; Wang, D. B.; Luo, X. H.; Xu, X. Y.; Chen, X. H.; Li, L.; Chen, C. P.; Wang, R. M.; Xu, J.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 4020. (14) Giebultowicz, T. M.; Rhyne, J. J.; Furdyna, J. K. J. Appl. Phys. 1987, 61, 3537. Nano Lett., Vol. 8, No. 11, 2008
(15) Debowska, D.; Kisiel, A.; Rodzik, A.; Antonangeli, F.; Zema, N.; Piacentini, M.; Giriat, W. Solid State Commun. 1989, 70, 699. (16) Shand, P. M.; Christianson, A. D.; Martinson, L. S.; Schweitzer, J. W.; Pekarek, T. M.; Miotkowski, I.; Crooker, B. C. J. Appl. Phys. 1996, 79, 6164. (17) Happo, N.; Sato, H.; Mihara, T.; Mimura, K.; Hosokawa, S.; Ueda, Y.; Taniguchi, M. J. Phys.: Condens. Matter 1996, 8, 4315. (18) Shih, Y. T.; Chiang, W. C.; Yang, C. S.; Kuo, M. C.; Chou, W. C. J. Appl. Phys. 2002, 92, 2446. (19) Masterson, H. J.; Lunney, J. G.; Coey, J. M. D. J. Appl. Phys. 1997, 81, 799. (20) Stachow-Wojcik, A.; Mac, W.; Twardowski, A.; Karczewski, G.; Janik, E.; Wojtowicz, T.; Kossut, J.; Dynowska, E. Phys. Status Solidi A 2000, 177, 555. (21) Imamura, M.; Okada, A.; Yamaguchi, T. J. Appl. Phys. 2006, 99, 8M706. (22) Han, S. W.; Kang, H. S.; Jiang, B. Z.; Seo, S. Y.; Park, H. W.; Ri, H. C. J. Magn. Magn. Mater. 2007, 310, 2114. (23) Ferrand, D.; Cibert, J.; Wasiela, A.; Bourgognon, C.; Tatarenko, S.; Fishman, G.; Andrearczyk, T.; Jaroszynski, J.; Kolesnik, S.; Dietl, T.; Barbara, B.; Dufeu, D. Phys. ReV. B 2001, 63, 085201.
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(24) Janik, E.; Sadowski, J.; Dluzewski, P.; Kret, S.; Baczewski, L. T.; Petroutchik, A.; Lusakowska, E.; Wrobel, J.; Zaleszczyk, W.; Karczewski, G.; Wojtowicz, T.; Presz, A. Appl. Phys. Lett. 2006, 89, 133114. (25) Janik, E.; Dluzewski, P.; Kret, S.; Presz, A.; Kirmse, H.; Neumann, W.; Zaleszczyk, W.; Baczewski, L. T.; Petroutchik, A.; Dynowska, E.; Sadowski, J.; Caliebe, W.; Karczewski, G.; Wojtowicz, T. Nanotechnology 2007, 18, 475606. (26) Furdyna, J. K.; Kossut, J. Superlattices Microstruct. 1986, 2, 89. (27) Furdyna, J. K.; Giriat, W.; Mitchell, D. F.; Sproule, G. I. J. Solid State Chem. 1983, 46, 349. (28) Biernacki, S.; Kutrowski, M.; Karczewski, G.; Wojtowicz, T.; Kossut, J. Semicond. Sci. Technol. 1996, 11, 48. (29) Biernacki, S.; Scheffler, M. Phys. ReV. Lett. 1989, 63, 290. (30) Peterson, D. L.; Petrou, A.; Giriat, W.; Ramdas, A. K.; Rodriguez, S. Phys. ReV. B 1986, 33, 1160. (31) Shin, S. H.; Bajaj, J.; Moudy, L. A.; Cheung, D. T. Appl. Phys. Lett. 1983, 43, 68. (32) Liu, X.; Bindley, U.; Sasaki, Y.; Furdyna, J. K. J. Appl. Phys. 2002, 91, 2859.
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