NANO LETTERS
Optical Properties of Rectangular Cross-sectional ZnS Nanowires
2004 Vol. 4, No. 9 1663-1668
Qihua Xiong,†,‡ G. Chen,† J. D. Acord,‡ X. Liu,† J. J. Zengel,‡ H. R. Gutierrez,† J. M. Redwing,‡ L. C. Lew Yan Voon,§ B. Lassen,§ and P. C. Eklund*,†,‡ Department of Physics, Department of Materials Science and Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 Received June 1, 2004; Revised Manuscript Received July 6, 2004
ABSTRACT ZnS nanowaveguides with rectangular cross-section (∼50 × 50 nm2 ) and tens of microns in length have been synthesized by pulsed laser vaporization of ZnS/10% Au targets in a flow of Ar/5% H2. The highly crystalline filaments exhibit the wurtzite structure, growing mainly along the [001] or [100] directions. Photoluminescence at room temperature shows strong near-edge luminescence doublets (∼3.75 eV and 3.68 eV) and a weak defect luminescence structure attributed to stoichiometric defects and possibly to Au impurities. Optical absorption (OA) at room temperature shows a strong broadening of the fundamental direct absorption edge identified with stoichoimetric defects. Two peaks (3.75 and 3.85 eV) in the OA are also observed. We believe that the structure in the photoluminescence and optical absorption (3.68, 3.75, 3.85 eV) are from direct transitions between the conduction band and the spin−orbit/crystal field splitting of the valance bands. Theoretical results are also presented that show the size-dependence of the band gap in ZnS nanowires.
Wide band-gap semiconducting nanowires (e.g., ZnO, GaN) have attracted considerable attention recently because of their promising applications in optoelectronics and UV nanolasers.1-4 Optically and electrically driven lasing has been demonstrated in ZnO2,4 and GaN nanowires,3 respectively. One essential requirement to obtain nanowire lasers is that photonic confinement must be achieved. This requires a smooth surface along the wire and, ideally, cleaved end surfaces which can work as efficient, partially transmitting mirrors that define the optical cavity.3 It is technically important to explore methods to grow highly crystalline nanowires with smooth surfaces and controlled cross sections. Yang et al. reported epitaxial growth of hexagonal cross section ZnO nanowires that grow perpendicular to sapphire substrates. They used a vapor transport and condensation technique.5 Using a simple thermal evaporation method, Pan et al. grew semiconducting nanobelts or nanoribbons (ZnO, SnO2, In2O3, and CdO), i.e., the width-to-thickness ratio g10.6 Most recently, Kuykendall et al. reported GaN nanowires with a triangular cross section grown by a metallorganic chemical vapor deposition route.7 It seems that nanowires may be grown with a variety of cross-sectional configurations depending on the details of the growth conditions. Fundamental understanding of how to control specific cross-sectional configurations is still lacking. * Corresponding author. E-mail:
[email protected]. † Department of Physics, PSU. ‡ Department of Materials Science and Engineering, PSU. § Worcester Polytechnic Institute. 10.1021/nl049169r CCC: $27.50 Published on Web 07/31/2004
© 2004 American Chemical Society
ZnS is another important direct band-gap semiconductor that has been investigated extensively because of its potential optical applications.8 The cubic ZnS phase has Eg ) 3.68 eV.9 Considerable effort has been devoted recently into controlling the size and shape of ZnS nanofilaments so that size-dependent and shape-dependent properties can be studied, e.g., ZnS nanobelts with wurtzite structure were grown by thermal evaporation methods.10,11 These nanobelts were several tens or hundreds of nanometers in width and several microns in length.10,11 Using the same technique,10,11 Wang et al. were able to grow micron-long, cylindrical ZnS nanowires with diameter d ) 30-60 nm.12 Most recently, Zapien et al.13 demonstrated room-temperature laser action in ZnS nanoribbons with an optical pumping threshold φ ∼ 40-60 kW/cm2. Further detailed investigations of the optical properties of nanowires in general, and ZnS nanowires in particular, are needed to appreciate their possible applications in nanooptoelectronics and nanolasers. For example, how do the morphology, doping, and cross-sectional area affect the photoluminescence, optical band gap, and the laser efficiency? In this paper, we present a route to grow highquality, rectangular cross-sectional wurtzite ZnS nanowaveguides ∼50 × 50 nm2 × 10 µm in length using pulsed laser vaporization (PLV). Photoluminescence and optical absorption experiments have been carried out to study the optical properties of these “waveguides”. We also present results of theoretical calculations on the nanowire diameter dependence of the fundamental band gap.
Figure 1. FESEM image of ZnS nanowires dispersed on a piece of Si wafer. The inset at the upper left corner shows a nanowire growth tip with higher magnification. The inset to the bottom right corner shows a thickness distribution obtained by AFM.
Figure 2. X-ray diffraction spectrum of ZnS nanowires using a Cu KR radiation. The peaks are identified with a wurtzite 2H ZnS structure with lattice constants a ) 3.83 Å and c ) 6.26 Å, which are consistent with powder diffraction data (10-434, a ) 3.82 Å, c ) 6.26 Å).
The ZnS nanowires were grown by PLV.14 A heated target of well-mixed ZnS (Alfa Aesar) and 10 at. % Au powder (Alfa Aesar) was ablated by a Nd:YAG laser. The nanowire growth was found to proceed via the vapor-liquid-solid (VLS) growth mechanism.15 A detailed description of our apparatus is available elsewhere.16 Briefly, the target was placed in a double-quartz tube system centered in a tube furnace. A carrier gas of 100 sccm Ar (5%H2) was introduced between the inner and outer quartz tubes, allowing the gas to be preheated before entering the reaction zone (pressure ∼ 225 Torr). The target was centered in the 1-meter-long furnace that was operated at 950 °C. The as-grown nanowires were collected from the inner quartz tube down stream by ∼40 cm from the target. The ZnS nanowires appeared as a white powder. Field emission scanning electron microscopy (FE-SEM, JEOL 6700F), X-ray diffraction (XRD, Phillips X-Pert MPD Cu KR), transmission electron microscopy (TEM, Philips 420, JEOL 2010F), and atomic force microscopy (AFM, Digital Instruments, Multimode) have been used to characterize the nanowires. The nanowires were first dispersed into 1664
Figure 3. (a) Bright-field TEM image of ZnS nanowires. Faceted tips (solid arrows) at one end of the nanowires and abrupt ends (hollow arrows) at the other ends are quite prominent. Inset at the right corner is the width distribution calculated from several TEM images, showing a most abundant population around 45 nm. (b) Higher magnification TEM image of an individual nanowire shows parallelepiped feature.
ethanol by ultrasonication. Then a few drops of this suspension were deposited either onto Si substrates, copper grids, or freshly cleaved mica substrates, respectively, for optical studies, FE-SEM, TEM, and AFM; the solvent was then allowed to vaporize under ambient conditions. To have direct visualization of the cross sections of our wires, we used an ultramicrotome technique to prepare specimens for TEM observations. Detailed information on sample preparation can be found in ref 17. Photoluminescence (PL) of the ZnS nanowires was taken at room temperature and excited by a pulsed Nd:YAG 4th harmonic (266 nm) laser (Nanolase, MicroChip NanoUV 266 nm) with the beam at 45° to the surface of the supporting substrate. The pulse width and repetition rate are, respectively, about 0.5 ns and 10 kHz. The average power was ∼5 mW and the laser spot was ∼4 mm2. Optical absorption experiments (Pelkin Elmer UV-vis Lambda 900) were carried out on a thin film of ZnS nanowires deposited on sapphire substrates. For PL experiments, Si/SiOx substrates were used. Figure 1 shows a FE-SEM image (15 kV) of ZnS nanowires deposited on a Si substrate. The nanowires are flat with abrupt ends; they were found to be tens of microns in length. At one end of some of the nanowires, a solidified Nano Lett., Vol. 4, No. 9, 2004
Figure 4. (a-d) TEM and HRTEM images of the cross section of the ZnS nanowires. Most of the wires have rectangular or nearly square cross sections (a and b). Some of the nanowires also have hexagonal cross sections (c and d). (b) and (d) are HRTEM images taken at corners of (a) and (c), respectively.
metal particle was observed (inset to Figure 1, upper left corner). We take this as proof of the VLS growth mechanism.14 The nanowires are clearly electron transparent, i.e., the electron beam can easily penetrate through the nanowires. AFM height measurements of many isolated nanowires were obtained to determine the thickness distribution. The average thickness of the wires ht relative to the mica was found to be ht ∼ 49 ( 13.6 nm, calculated from ∼30 separated wires; 13.6 nm is the standard deviation. The bottom right inset to Figure 1 shows the thickness distribution of our wires. A more detailed explanation of our sample preparation procedure for AFM can be found in ref 18. Figure 2 shows an X-ray powder diffraction spectrum of the ZnS nanowires using Cu KR radiation. All the peaks are identified with the wurtzite 2H polytype of ZnS, except the three peaks from Au. The ZnS lattice constants obtained are a ) 3.83 Å, c ) 6.26 Å, in good agreement with powder XRD data for the bulk (10-434, a ) 3.82 Å, c ) 6.26 Å). Figure 3 shows two bright-field TEM images of our ZnS nanowires. From several such TEM images, the lateral dimension (width) distribution was determined as shown in the inset to Figure 3a. We find that the most abundant nanowires have a width of w j ∼ 55 nm, while a few wires are as wide as 200 nm. The good agreement between the thickness distribution by AFM and width distribution by TEM suggests that the majority of the filaments have nearly square cross section. If they were ribbons we would expect the ribbons would lie flat on mica rather than on edge. In this case, we would expect ht , w j . However, we observe ht Nano Lett., Vol. 4, No. 9, 2004
∼w j , suggesting that nearly square cross-sectional wires were grown. This is further confirmed by cross-sectional TEM observations (Figure 4a). Two other interesting features were observed in TEM. First, most of the metallic particles at the growth tip of the nanowires are observed to be polyhedrons (solid arrows in Figure 3a), instead of the usual spheres we have seen in other nanowires grown by PLV. Second, the other free end of the nanowires (hollow arrows in Figure 3a) appears cleaved in many cases, and the cleaved surfaces are almost perpendicular to the wire axis, consistent with observations by FE-SEM (Figure 1). The stripe contrast in the TEM image can be identified with well-known “bend contours”.19 A magnified TEM image of a nanowire end is shown in Figure 3b and exhibits a rectangular cross section and a well-cleaved end. The cleavage probably occurs in the ultrasonic processing that is used to achieve a metastable suspension in ethanol for preparing TEM grids, etc. The wellcleaved ends of nanowires introduced by ultrasonication have also been observed in CdS nanowires.3 From our SEM and TEM results, we believe that the small and intermediate cross-sectional nanowires follow the VLS growth mechanism. The larger cross-sectional wires may stem from a thermal evaporation growth similar to earlier observations.6,10,11 However, it should be noted that the width distribution (Figure 3a) is continuous. Therefore, we tentatively invoke a 2nd growth mechanism only because we have not found evidence of ∼100 nm diameter metallic particles in the TEM images of our samples. For the second growth mechanism (thermal), we believe the furnace drives the 1665
Figure 6. EDX spectra from the tip (up panel) and the body (bottom panel) of an individual nanowire. The tip consists of mainly Au and Zn, little S was detected. So the solidified liquid droplet is Au-Zn alloy; while the body contains mainly Zn and S, no other impurities were detected. Copper peaks are from the grid.
Figure 5. (a) HRTEM image of a ZnS naowire growing along [001] direction. The spacing between adjacent lattice fringes is 0.319 nm. The inset shows a selective-area electron diffraction pattern from the same nanowire. There is no appreciable amorphous outer coating on the nanowire. (b) Some wires growing along [100] were also observed. The lattice spacing between fringes is 0.335 nm and the inset shows a corresponding FFT of the image.
decomposition of the target and the carrier gas simply moves the vapor to a cooler region in the furnace, where the growth takes place. Figure 4 a-d shows TEM and HRTEM images of the cross sections of our ZnS nanowires. Most of the wires have a rectangular or nearly square cross section (Figure 4a), while a few of the wires also have a hexagonal cross section (Figure 4c). Figure 4b,d shows lattice images taken at the corner of the cross sections shown in (a) and (c), respectively. High-resolution TEM (HRTEM), selective-area diffraction (SAD), and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the structure and composition of our nanowires. Figure 5 shows two HRTEM images of ZnS nanowires growing along the 〈001〉 (Figure 5a) or 〈100〉 directions (Figure 5b). Our nanowires are highly crystalline, as indicated by the lattice fringes and SAD pattern. The distances between lattice fringes (0.335 nm for 〈100〉 and 0.319 nm for 〈001〉 planes) were measured and they are consistent with the d spacing values listed in the XRD powder diffraction file (10-434). The two insets to Figures 5a and 5b show the Fourier transform of the HRTEM image and SAD pattern from the same nanowire. Figure 6 shows two EDX spectra from the tip (top panel) and the body 1666
(bottom panel) of our nanowire. The tip consists mainly of Au and Zn. Very little S was detected in the tip, suggesting that the solidified liquid droplet is a Au-Zn alloy. While the nanofilament body contains only Zn and S, no other impurities (e.g., Au) were detected in the filament by EDX. It may be that the S diffuses out of the Au-Zn particle after the nanowire growth stops. Optical absorption (OA) and photoluminescence (PL) are two closely related methods to probe the band structure (e.g., fundamental edges, excitons, surface, and impurity states) of semiconductor materials.20 OA and PL analyses yield information about how the optical properties of semiconducting nanowires can be tailored by intentional doping or by surface modifications. Two emission bands in PL of ZnS nanowires12 and nanobelts10 were reported in the previous literature. However, the explanations presented for the peaks are slightly different. On the one hand, Wang et al.12 attributed the emission band around 450 nm to surface states, while Zhu et al.10 attributed it to defect-related emission. Another stronger emission band was reported between 500 and 600 nm, depending on the specific impurity species, e.g., Au impurities form luminescence centers around 520 nm,12 while Mn2+ impurities introduce emission around 600 nm.10 A luminescence study on ZnS colloidal nanocrystals attributed visible emission (∼590 nm) to Mn2+ impurities and the structures between 415 and 440 nm to vacancies and interstitials.21 Tran et al.22 studied the photoluminescence of ZnS epilayers on GaAs substrates systematically from 1.6 to 320 K. They observed free exciton photoluminescence at room temperature. The fundamental band gap was determined to be 3.723 eV at room temperature, and the exciton binding energy is about 40 meV.22 As shown in Figure 7a, a strong luminescence doublet (3.68 and 3.75 eV) and weaker defect luminescence structure Nano Lett., Vol. 4, No. 9, 2004
Figure 7. (a) PL spectrum of ZnS nanowires taken at roomtemperature excited by a 266 nm UV laser. Two strong emission bands near band edge and four weak defect luminescence bands were observed. Blue curves are Lorentzian line shape analyses after a least-squares multi-Lorentzian fit (red curves). (b) Optical density plot versus photon energy of ZnS nanowires. The inset shows a zoomed view at the vicinity of the fundamental absorption edge.
at 2.44, 2.66, 2.86, 3.06 eV were observed for our nanowires. The defect luminescence spectrum is multiplied by a factor of 40 in the figure for clarity. These features were fitted with multiple Lorentzians, as shown by the blue curves in the figure. The 2.44 eV (∼510 nm) emission band might be due to Au impurities,12 although no EDX evidence for Au in the nanowire body was found. The other three weak emission bands are probably due to stoichiometric vacancies or interstitial impurities.21 The position of the structure (in eV) in Figure 7a we observed is slightly different from that reported for vacancies and interstitials in nanoparticles.21 Figure 7b shows the optical density vs photon energy for our ZnS nanowires. The inset to Figure 7b gives a magnified view of the optical density near the position of the band edge in the bulk. First of all, even though the nanowire PL is in good agreement with that of the bulk, the absorption edge is very broad. We take this broadness to indicate strong band tailing stemming from the stoichiometric defects. Two peaks in the optical density are observed at 3.75 and 3.85 eV. These positions are close in energy to the structure obtained from our PL measurements. Nano Lett., Vol. 4, No. 9, 2004
Wurtzite ZnS (in the bulk) is a direct gap semiconductor with Eg ) 3.7 eV at the center of Brillouin zone.9 The topmost valance band is split into three bands due to the crystal field and spin-orbit coupling. Exciton states associated with electron-hole pairs in each of these valance bands involved are usually denoted as A, B, C excitons (LandoltBornstein new series III 41B, p 1309). The spin-orbit coupling and crystal-field splitting energy given in ref 9 are 89 and 57 meV, respectively. The three features of 3.68, 3.75, 3.85 eV observed in PL and OA are tentatively assigned to the A, B, C excitons. The energy splittings we obtained are approximately 100 and 70 meV. The reason we could not observe structure at 3.68 eV in the optical absorption or structure at 3.85 eV in PL is probably that these features are masked by the strong band tailing, while the 3.85 eV exciton state may decay rapidly to lower energy exciton states, i.e., 3.75 eV and 3.68 eV. Time-resolved photoluminescence would be important to support this assignment. Taking the exciton binding energy of 40 meV into account, the fundamental band gap we observed in w-ZnS nanowires is ∼3.72 eV, and the structures at 3.75 and 3.85 eV are identified with B and C exciton states. Even though the large exciton binding energy (40 meV) is much higher than thermal energy at room temperature (∼26 meV), the band edge photoluminescence of ZnS at room temperature was observed only in very high quality single crystals.22 The observation of strong band edge luminescence indicates that our nanowires are highly crystalline. At photon energy g 5.0 eV, we observe a slight decrease in absorption, associated with diminished high energy oscillator strength.20 We attempted to observe laser action in our ZnS nanowires using a pulsed Nd:YAG laser at 266 nm. Unfortunately, we must not have exceeded the lasing threshold in our experiments, as there were no signs of lasing as has been observed in GaN,1 ZnO,2and CdS3 nanowires. Our approximate power density was ∼500 mW/cm2, much lower than the threshold reported for ZnS nanowires.13 In anticipation of the growth of nanowires with much smaller cross sections, we have used a band-structure model to study how the band gap changes with the linear dimension for a square nanowire. The theory employed is the six-band k.p model for the valence band, as originally proposed by Mireles and Ulloa,23 and a one-band model for the conduction band. We have adapted them for a free-standing nanowire, i.e., we have assumed no confinement along the wire axis and infinite confinement inside the transverse boundaries of the nanowire. The valence-band parameters for ZnS were obtained from Xia,24 and the band gap and conduction effective mass from Landolt-Bornstein.9 The confinement energies for various [100] and [001] nanowires, as a function of the linear dimension w, are plotted in Figure 8; the band gaps of the nanowires can then be obtained by adding the bulk band gap to the two confinement energies. The calculation verifies that, for the square nanowire with linear dimension of about 50 nm, the quantum confinement energy is negligible (less than 2 meV), and that the analysis of the optical data in terms of the bulk band structure is appropriate. As expected, the band gap energy increases with decreasing 1667
two exciton energy states of 3.75 and 3.85 eV are due to deeper valence bands split by spin-orbit and crystal field effects. Photoluminescence also shows weak defect luminescence due to stoichiometric defects and interstitial impurities. We cannot ambiguously identify Au as a luminescent center in the wires. Acknowledgment. This work was supported by NSFNIRT program (Nanotechnology and Interdisciplinary Research Initiative), grant DMR-0304178, Upenn MRSEC and NSF-DMR (LYV). We thank Dr. Jinguo Wang for helpful discussion on TEM. References
Figure 8. Variation of conduction (top, dotted curve) and valence (bottom) confinement energies as a function of the size of square [100] (solid curve) and [001] (dashed curve) nanowires.
lateral dimension w; the PL spectrum should therefore be blue-shifted (e.g., by more than 100 meV for w < 10 nm). We note, nevertheless, that the band gap increase remains small until w e 20 nm or less. Due to the hexagonal symmetry, nanowires with [100] and [001] growth directions are expected to have different electronic and optical properties. The difference in the valence band energies is clear in Figure 8. The conduction energies are the same because the conduction effective mass is isotropic. We find that the band gap is indeed different, but the difference is at most about 12 meV for the 5 nm wire. If such wires form a thin film of entangled wires as for the current samples, the effect of the different types of nanowires should be observed as a small additional broadening of the PL line. We also carried out systematic room-temperature Raman scattering studies of these nanowaveguide filaments in air and different dielectric surrounding media in which the filaments were imbedded. We observed strong first-order longitudinal optic (LO), transverse optic (TO), and surface optic (SO) phonon modes and second-order combinations and overtones. Detailed discussion on the Raman assignments and symmetry breaking mechanism leading to an SO scattering is given in ref 25. In summary, highly crystalline ZnS nanowaveguides, i.e., rectangular cross-sectional nanowires have been synthesized by pulsed laser vaporization. The growth mechanism was attributed to a vapor-solid-liquid mechanism. The wires grow mainly along the 〈100〉 or 〈001〉 directions. The nanowires have a nearly square cross section ∼50 × 50 nm2. Photoluminescence and optical absorption investigations revealed that the fundamental energy gap of 3.72 eV and
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(1) Johnson, J. C.; Choi, H. J.; Knutsen, K. P.; Shaller, R. D.; Yang, P. D.; Saykally, R. J. Nature Materials 2002, 1, 106. (2) Johnson, J. C.; Yan, H. Q.; Yang, P. D.; Saykally, R. J. J. Phys. Chem. B 2003, 107(34), 8816. (3) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421(6920), 241. (4) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292(5523), 1897. (5) Yang, P. Y.; Mao, S.; Russo, R,.; Johnson, J.; Saykally, R.; Morris, N.; Pharm, J.; Morris, R.; Pham, N.; He, R.; Choi, H.-J. AdV. Funct. Mater. 2002, 12(5), 323. (6) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291(5510), 1947. (7) Kuykendall, T.; Pauzauskie, P.; Lee, S. K.; Zhang, Y.; Goldberger, J. Yang, P. Nano Lett. 2003, 3(8), 1063. (8) Zhou, J.; Zhou, Y.; Buddhudu, S.; Ng, S. L.; Lam, Y. L.; Kam, C. H. Appl. Phys. Lett. 2000, 76(24), 3513. Huang, J. M.; Yang, Y.; Xue, S. H.; Yang, B.; Liu, S. Y.; Shen, J. C. Appl. Phys. Lett. 1997, 70(18), 2335. (9) Landolt, H.; Bo¨rnstein, R. Numerical data and functional relationships in science and technology, New Series ed.; Springer-Verlag: Berlin, 1999. (10) Zhu, Y. C.; Bando, Y.; Xue, D. F. Appl. Phys. Lett. 2003, 82(11), 1769. (11) Li, Q.; Wang, C. R. Appl. Phys. Lett. 2003, 83(2), 359. (12) Wang, Y. W.; Zhang, L. D.; Liang, C. H.; Wang, G. Z.; Peng, X. S. Chem. Phys. Lett. 2002, 357(3-4), 314. (13) Zapien, J. A.; Jiang, Y.; Meng, X. M.; Chen, W.; Au, F. C. K.; Lifshitz, Y.; Lee, S. T. Appl. Phys. Lett. 2004, 84(7), 1189. (14) Morales, A. M.; Lieber, C. M. Science 1998, 279(5348), 208. (15) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4(5), 89. (16) Xiong, Q.; Gupta, R.; Adu, K. W.; Dickey, e. C.; Lian, g. D.; tham, D.; Fischer, J. E.; Eklund, P. C. J. Nanosci. Nanotechnol. 2003, 3, 335. (17) Teo, B. K.; Sun, X. H.; Hung, T. F.; Meng, X. M.; Wong, N. B.; Lee, S. T. Nano Lett. 2003, 3(12), 1735. (18) Gupta, R.; Xiong, Q.; Adu, C. K.; Kim, U. J.; Eklund, P. C. Nano Lett. 2003, 3(5), 627. (19) Williams, D. B.; Carter, C. B. Transmission electron microscopy: a textbook for materials science; Plenum Press: New York, 1996. (20) Pankove, J. I. Optical processes in semiconductors; Prentice-Hall: Englewood Cliffs, NJ, 1971. (21) Denzler, D.; Olschewski, M.; Sattler, K. J. Appl. Phys. 1998, 84(5), 2841. (22) Tran, T. K.; Park, W.; Tong, W.; Kyi, M. M.; Wagner, B. K.; Summers, C. J. J. Appl. Phys. 1997, 81(6), 2803. (23) Mireles, F.; Ulloa, S. E. Phys. ReV. B 1999, 60(19), 13659. (24) Xia, J. B.; Li, J. B. Phys. ReV. B 1999, 60(16), 11540. (25) Xiong, Q.; Wang, J.; Reese, O.; Lew Yan Voon, L. C.; Eklund, P. C.; in preparation.
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