Letter pubs.acs.org/NanoLett
High Quality GaAs Nanowires Grown on Glass Substrates Veer Dhaka,*,† Tuomas Haggren,† Henri Jussila,† Hua Jiang,‡ Esko Kauppinen,‡ Teppo Huhtio,† Markku Sopanen,† and Harri Lipsanen† †
Department of Micro- and Nanosciences, Micronova, Aalto University, P.O. Box 13500, FI-00076, Finland Department of Applied Physics and Nanomicroscopy Center, Aalto University, P.O. Box 15100, FI-00076 Finland
‡
ABSTRACT: We report for the first time the growth of GaAs nanowires directly on lowcost glass substrates using atmospheric pressure metal organic vapor phase epitaxy via a vapor−liquid−solid mechanism with gold as catalyst. Substrates used in this work were of float glass type typically seen in household window glasses. Growth of GaAs nanowires on glass were investigated for growth temperatures between 410 and 580 °C. Perfectly cylindrical nontapered nanowires with a growth rate of ∼33 nm/s were observed at growth temperatures of 450 and 470 °C, whereas highly tapered pillar-like wires were observed at 580 °C. Nanowires grew horizontally on the glass surface at 410 °C with a tendency to grow in vertically from the substrate as the growth temperature was increased. X-ray diffraction and transmission electron microscopy revealed that the nanowires have a perfect zinc blende structure with no planar structural defects or stacking faults. Strong photoluminescence emission was observed both at low temperature and room temperature indicating a high optical quality of GaAs nanowires. Growth comparison on impurity free fused silica substrate suggests unintentional doping of the nanowires from the glass substrate. KEYWORDS: GaAs nanowires, glass substrate, fused silica, zinc blende structure, photoluminescence
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to reduce the number of recombination centers.10−13 Second, for optimum device operation, NWs must have a single crystal phase structure, i.e., either wurtzite (WZ) or zinc blende (ZB) but often NWs grown using MOVPE or molecular beam epitaxy (MBE) contain stacking faults with a polytypism structure.14−16 Fabrication of NWs free of structural defects is still a challenging task. Yet another important factor in mass scale production of devices is the cost of substrates. Traditionally, epitaxial growth of NWs so far is predominantly studied on semiconductor substrates such as GaAs and Si. Owing to their unique properties discussed earlier, NWs are potential candidates for growth on alternative nonsemiconductor substrates such as glass, metals, and polymers. Glass is mostly transparent, widely available, and much cheaper than Si. However, growth on a glass surface has many challenges, two of them being the amorphous state of the substrate and the effect of the impurities. In this letter, we report the growth of GaAs NWs using atmospheric pressure MOVPE on inexpensive glass substrates. To the best of our knowledge, there is no report of epitaxial GaAs NWs grown directly on glass substrates. We demonstrate that GaAs NWs grown on glass substrates show unique enhanced optical and structural properties. Namely, GaAs NWs grown on glass exhibit strong photoluminescence emission at low and room temperature and can be grown free of structural defects with a pure ZB structure.
emiconductor nanowires (NWs) are promising components for future generation optoelectronic devices and systems such as solar cells,1 lasers,2 light-emitting diodes,3 photodetectors,4 biosensors,5 and transistors.6 Group III−V semiconductors are materials of choice to fabricate such devices because they offer excellent optical and electrical properties including a direct band gap and high electron mobility, and most importantly, they can be grown via industrial mass scale production epitaxial growth techniques such as metal organic vapor phase epitaxy (MOVPE).7 Integration of bulk III−V semiconductors on relatively cost-efficient silicon technology remains challenging owing to traditional difficulties such as lattice mismatch, different thermal expansion coefficients, and formation of antiphase boundaries.8 NWs, the growth model of which is understood to follow vapor−liquid−solid9 (VLS) mechanism can overcome these difficulties because of their nanoscale radial dimensions and therefore can readily be integrated with Si microelectronics technology. Among III−V semiconductors for future NW based devices, GaAs NWs is a widely studied research topic worldwide for their potential application in high efficiency solar cells, similar to bulk GaAs. Success of III−V NW based devices depends on the ability to fabricate NWs with good control over surface properties and crystal structure quality. First, due to a large surface-to-volume ratio, the properties of NWs are extremely sensitive to surface electronic states.10 These surface states may act as nonradiative charge carrier traps degrading the optical quality, which may adversely affect optoelectronic device performance. In particular, GaAs NWs are known to contain a high density of surface defects and are consequently generally poor emitters of light at room temperature.10−13 Often, surface passivation is required © 2012 American Chemical Society
Received: December 7, 2011 Revised: March 7, 2012 Published: March 20, 2012 1912
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470 °C, NWs are nearly perfectly cylindrical in shape without tapering. This is attributed to negligible diffusion of Ga adatoms from the amorphous glass substrate to the NW side facets indicating the growth is mostly axial. The average lengths of GaAs NWs grown at 410−500 °C is about ∼6−10 μm and their diameters vary from about 40 to 60 nm. Longest NWs with a length of about 10 μm were observed at 470 °C yielding an average growth rate of ∼33 nm/s. The average areal density of NWs is estimated to vary between 107−109 NWs/cm2 and is a function of growth temperature. The density of NWs was even and uniform throughout the glass substrate between growth temperatures of 450 and 500 °C, respectively. Nevertheless, at all growth temperatures, a very high density of NWs was seen on the glass compared to the growth on Si substrate with similar growth conditions. This could be attributed to a more efficient adhesion of Au nanoparticles to the glass surface than on Si. At a similar nominal growth temperature of 450 °C with our MOVPE system, the growth rate of GaAs NWs on glass substrate was ∼3 times lower and the areal density of NWs was ∼4 times higher when compared to the growth on Si substrate with similar Au nanoparticle size. One contribution (albeit small) to this comparatively slow growth rate could be the shorter diffusion length of Ga adatoms on glass resulting in fewer Ga adatoms from the amorphous glass substrate reaching the Au nanoparticle. However, a shorter diffusion length of Ga adatoms on glass has no contribution to the growth rate once the length of the NW exceeds the Ga adatom diffusion length17 (approximately a few nanometers). The other major factor that is likely to contribute toward the slow growth rate is the small wire-to-wire spacing (due to a high density of NWs) on glass substrate that could lead to materials competition and a reduction in growth rate of NWs as reported by Borgström et al.18 Interestingly, unlike the growth on Si, no NWs growth was seen on the glass substrate without the pregrowth annealing step in MOVPE reactor. With an increase in growth temperature, NWs show more radial growth or tapering (Figure 2d−f). NWs grown at 500 °C show little tapering, whereas the ones grown at 580 °C are highly tapered with a pillar-like structure. Furthermore, a decrease in the length and an increase in the diameter at the base of NWs were observed (Figure 2d−f) at higher growth temperatures. In particular, the length of the NWs decreased from 10 μm at 470 °C to only about 4 μm at 580 °C. Tapering demonstrates that the radial growth changes from thermally activated at low temperature to a diffusion limited process at high temperature,17,19 and a decrease in the NW length with an increase in diameter confirms that the growth of the NWs is fed by diffusion of adatoms from the surface of the substrate and/or from the side walls to the Au droplet at the top of the NWs.17−19 We suggest an optimum growth temperature window between 450 and 500 °C, beyond which the NW length decreases substantially with an increase in diameter. Growth of GaAs NWs was also investigated on fused silica (fused quartz) substrates. Fused-silica is a type of amorphous glass with very high purity and with no impurities. Figure 3 shows SEM image of GaAs NWs grown on a 5 mm × 5 mm fused silica substrate at 470 °C. The average length of GaAs NWs grown on fused silica was about ∼7 μm and their diameters varying from about 40 to 60 nm. NWs on fused silica substrate were ∼3 μm shorter in length than on glass at a similar growth temperature. The areal density of the GaAs NWs on fused-silca was slightly higher compared to the growth on float type glass, and the NWs were uniform throughout the
In this study, GaAs NWs were fabricated in a horizontal flow atmospheric pressure MOVPE system using trimethylgallium (TMG) and tertiarybutylarsine (TBA) as precursors for gallium (Ga) and arsenic (As) sources, respectively. Hydrogen was used as a carrier gas, and the total reactor gas flow rate was ∼5 slm. Unpolished float type glass microscopic slides from Sigma Aldrich were used as the substrates. In addition, growth of GaAs NWs was also compared on a few fused silica (fused quartz) substrates from UQG optics, U.K. Float glass or sodalime glass is also known as window glass (typical composition of float glass was 72.6% SiO2, 0.8% B2O3, 1.7% Al2O3, 4.6% CaO, 3.6% MgO, and 15.2% Na2O). Glass samples of about 8 mm × 8 mm in dimensions were soaked in an ultrasonic bath with isopropanol and acetone for 2 min each, followed by 5 min DI water flow immersion. After cleaning, glass substrates were treated with poly-L-lysine (PLL) solution for about 1 min, followed by 2 min deposition of 40 nm diameter colloidal gold (Au) nanoparticles solution. Prior to growth, the glass substrate was annealed in situ at 650 °C for 10 min under hydrogen ambient to desorb surface contaminants. MOVPE growth was started by switching on TMG and TBA sources simultaneously for 5 min at growth temperatures ranging from 410 to 580 °C. The nominal V/III ratio during the growth was ∼25. The growth temperatures reported in this work are thermocouple readings and could be much higher than the real glass substrate surface temperature as glass is a poor conductor of heat. Figure 1 shows from left to right a picture of the bare glass substrate and the substrates with NWs grown at temperatures
Figure 1. Image of glass substrates prior to growth (left) and after the growth of GaAs NWs at 470 °C (middle) and 580 °C (right), respectively.
of 470 and 580 °C, respectively. In contrast to the bare glass substrate, the color of the substrate surface containing NWs varies from golden to blackish with increasing growth temperatures. These differences in visual appearance are related to the length and tapering of NWs at particular growth temperatures. Structural analysis such as length, diameter and shape of the NWs were determined by scanning electron microscopy (SEM) (Zeiss Supra system operating at 3 kV) measurements. Figure 2a−f shows top view SEM images of GaAs NWs on glass substrates at growth temperatures between 410 and 580 °C and at a constant V/III ratio of ∼25. These SEM pictures were sampled from low density growth areas for simplicity. As seen in Figure 2, NWs on the glass substrates show a large growth temperature window. At the growth temperature of 410 °C, NWs show a worm-like growth pattern at the base, and not all but many are cylindrical in shape with an average length of ∼6 μm. This worm-like growth ceases when the growth temperature reaches 450 °C. At the growth temperatures of 450 and 1913
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Figure 2. Top view SEM images of GaAs NWs grown on glass substrates at (a) 410, (b) 450, (c) 470, (d) 500, (e) 530, and (f) 580 °C. The scale bar corresponds to 2 μm scale.
temperatures. Also, we have tried three different glass surfaces with varying surface smoothness. Areal density and optical properties of NWs are better on smooth glass surfaces compared to the rough ones. The surface of the float glass used in this work is often smooth since during glass formation the molecules of the supercooled liquid are not forced to dispose in rigid crystal geometries but can follow surface tension, which imposes a microscopically smooth surface. Float glass is also known as the standard window glass. There is a scope for more experimental work to optimize growth related parameters to realize more vertical NWs on glass. High resolution transmission electron microscopy (TEM) measurements were carried out with a JEOL 2200FS double aberration corrected FEG microscope operated at 200 kV. To prepare TEM samples, NWs were transferred to holey carbon grids by gently rubbing the grid against the glass substrate containing the NWs. Figure 5 a−c shows high resolution TEM images from the area marked in the rectangle in a GaAs NW (Figure 5a) grown on a glass substrate at 470 °C. As seen in Figure 5a, the examined NW length is 5.2 μm with a diameter of about 59 nm. The Au nanoparticle is clearly visible at the end of the NW suggesting a VLS growth mechanism. An electron diffraction pattern which was taken on the NW along the [0− 11] direction is shown in Figure 5d.The diffraction pattern is indexed based on a fcc cubic ZB structure with a lattice constant of 0.53 nm. The growth direction of the wire is determined to be . Systematic TEM investigations along the whole wire length revealed a pure cubic ZB crystal structure, free of stacking faults, grain boundaries, and misfit dislocations. No polytypism of crystal structure was observed. Similar TEM results were obtained on two other different GaAs NWs grown on glass substrate. On the contrary, TEM results of
Figure 3. Top view SEM image of GaAs NWs grown on fused silica substrate at 470 °C. The scale bar corresponds to 2 μm.
substrate. We will discuss the significance of growth on fused silica substrates in a later section. Figure 4a−d shows tilted SEM images of NWs grown at different temperatures on glass substrates. We observed an interesting trend in NWs ability to grow in vertical direction relative to the substrate with increasing growth temperature. At 410 °C, NWs seems to grow only in the horizontal direction parallel to the glass substrate with no NWs seen in the vertical direction at different tilted angles. As seen in Figure 4, increasingly more NWs are systematically aligned in the vertical direction with increasing growth temperature. At 500 and 530 °C most nanowires are in the vertical direction. In summary, no particular growth direction can be attributed to NWs on glass but many are aligned in vertical direction at higher growth 1914
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Figure 4. Tilted view SEM images of the GaAs NWs grown on glass substrate at different growth temperatures (a) 410 °C, 40° tilt angle from top, (b) 450 °C, 90° tilt angle from top, (c) 500 °C, 70° tilt angle from top, and (d) 530 °C, 70° tilt view from top.
measurements using Cu Kα radiation were performed. Compared to the TEM measurements, XRD reveals information from a large area of the substrate. Figure 7 shows the measured 2θ/ω curve of the GaAs NWs grown on glass substrate. Three diffraction peaks were observed at 2θ angle of 27.30°, 45.41°, and 53.73°, which are indexed to GaAs ZB (111), (220), and (311) planes, respectively. The measured XRD curve supports the TEM results and implies that the GaAs NWs grown on glass have a single phase ZB crystal structure. In addition, it can be concluded that the ZB crystal structure of NWs is uniform along the substrate. Photoluminescence (PL) measurements were performed in order to determine the optical quality of NWs on glass. The GaAs NW samples were excited by a 532 nm laser. A closed state cryostat which can reach 15 K was used for low temperature measurements. Typical excitation density at the sample was 500 Wcm−2 with a laser spot size of ∼100 μm. A liquid nitrogen cooled germanium detector and standard lockin techniques were used for signal detection and data capture. Figure 8a shows low temperature (15 K) PL spectra from GaAs NWs on glass. A strong and a single PL position peak at 811 nm was observed at 15 K. The PL peak at 811 nm is about 10 nm shifted to lower wavelength than is expected from bulk ZB GaAs (∼820 nm) at 15 K.12 Quantum confinement effects are ruled out as the NWs diameter (∼50 nm) is well above the Bohr radius of bulk GaAs (∼14 nm).23 Further, laser heating effect24 could also shifts the PL, but only to higher wavelengths. We have carefully probed the heating effect at different laser powers and no shift in PL peak was observed. At the moment,
GaAs NWs grown on Si with similar growth conditions reveal stacking faults (not shown here) with a polytypism structure. As discussed earlier, the growth rate of GaAs NWs on glass is three times lower than on Si. One may speculate here that the slow growth rate on glass is associated with higher structural quality with no stacking faults.20−22 To determine the relative composition of the GaAs NW grown on glass substrate, energy-dispersive X-ray (EDX) measurements were performed using the convergent electron beam in TEM. As shown in Figure 6, EDX performed on the spot marked in red within the Au tip on the top of the NW displays only the Au signature with no other impurity presence visible. The Cu signal seen in the spectra arises from the TEM grid. Similarly, EDX on the spot marked in black within the body of the NW reveals only Ga and As signatures with nearly equal intensities (Ga0.5As0.5). Clearly, no impurity signatures were detected in EDX spectra. However, as glass contains many impurities mentioned previously, we suspect strongly the unintentional doping of the GaAs NWs from the glass substrate and that the low impurity(s) concentrations were not detected by the EDX system. In particular, sodium (Na) and Si atoms are mobile at the growth temperatures reported in this work and could incorporate in NWs during the growth and/or some impurity(s); especially, Na could dramatically change the VLS mechanism resulting in single phase purity. Additional sensitive experiments are required to ascertain the role of impurity(s) on the growth and structure of the NWs grown on glass substrate. To further examine the structural properties of GaAs NWs grown on the glass substrate, X-ray diffraction (XRD) 1915
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Figure 5. (a) TEM image of a GaAs NW grown on glass substrate and (b and c) HRTEM images taken from the area marked by a rectangle in image a. Arrow indicates the growth direction of the GaAs NW. The NW examined here is free of structural defects. Bottom right, image d is one of the many electron diffraction patterns repeated throughout the NW length and indicates a single phase ZB structure.
the origin of the blue shift in PL peak is unclear but as discussed earlier, the unintentional doping of the GaAs NWs from the glass substrate could result in shifts and broadening (fwhm ∼70 nm) of the PL spectra.
Figure 7. XRD diffraction curve obtained from GaAs NWs grown on glass substrate.
To probe further the role of impurities from the substrate, we compared the PL measurement results of GaAs NWs grown on float type glass substrate containing impurities with the growth on fused silica substrate with no impurities at growth temperatures of 470 (Figure 3) and 500 °C, respectively. Interestingly, no PL was detected from the GaAs NWs grown on fused-silica substrates at 15 K even though their density was higher than the NWs grown on glass and Si substrates. Optically, NWs grown on fused silica substrate were similar to the ones grown on the Si substrate. From PL measurements
Figure 6. Energy-dispersive X-ray (EDX) spectra collected from an individual GaAs NW. The inset is a TEM image of GaAs NW with 40 nm diameter, and the EDX spectra were collected from different positions along the NW as marked with an X. The Cu signal is arising from the TEM grid. 1916
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NWs grown on glass show strong photoluminescence emission at both low and room temperature indicating a high optical quality of the NWs. Single phase purity and high optical quality is possibly related to the unintentional doping of the NWs from the glass substrate. On the basis of this report, we are hopeful that NWs grown on glass holds promising potential of device integration on alternative inexpensive large-area substrates such as glass in future.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially carried out within the framework of NANORDSUN project No. 10048 under Nordic Innovation Centre (NICe), Norway.
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Figure 8. PL spectra recorded from GaAs NWs on glass substrates (a) low temperature (15 K) PL emission from the sample grown at 470 °C, and (b) room temperature (293 K) PL emission from the samples grown at different growth temperatures. The sample grown at 470 °C is illustrated in blue color and the dashed line in panel a is a guide to the eyes.
REFERENCES
(1) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455−459. (2) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897−1899. (3) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241−245. (4) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455−1457. (5) Hahm, J.-I.; Lieber, C. M. Nano Lett. 2004, 4, 51−54. (6) Thelander, C.; Fröberg, L. E.; Rehnstedt, C.; Samuelson, L.; Wernersson, L. E. IEEE Electron Device Lett. 2008, 29, 206−208. (7) Seifert, W.; Borgström, M.; Deppert, K.; Dick, K. A.; Johansson, J.; Larsson, M. W.; Mårtensson, T.; Sköld, N.; Svensson, C. P. T; Wacaser, B. A.; Wallenberg, L. R.; Samuelson, L. J. Cryst. Growth 2004, 272, 211−220. (8) Yi, S. S.; Girolami, G.; Amano, J.; Islam, M. S.; Sharma, S.; Kamins, T. I.; Kimukin, I. Appl. Phys. Lett. 2006, 89, 133121. (9) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (10) Demichel, O.; Heiss, M.; Bleuse, J.; Mariette, H.; i Morral, A. F. Appl. Phys. Lett. 2010, 97, 201907. (11) Titova, L. V.; Hoang, T. B.; Jackson, H. E.; Smith, L. M.; Yarrison- Rice, J. M.; Kim, Y.; Joyce, H. J.; Tan, H. H.; Jagadish, C. Appl. Phys. Lett. 2006, 89, 173126. (12) Hoang, T. B.; Jackson, H. E.; Smith, L. M.; Yarrison- Rice, J. M.; Kim, Y.; Joyce, H. J.; Tan, H. H.; Jagadish, C. Appl. Phys. Lett. 2006, 89, 173126. (13) Rosini, M.; Magri, R. ACS Nano 2010, 4, 6021−6031. (14) Hiruma, K.; Yazawa, M.; Haraguchi, K; Ogawa, K.; Katsuyama, T.; Koguchi, M.; Kakibayashi, H. J. Appl. Phys. 1993, 74, 3162. (15) Ohlsson, B. J.; Bjork, M. T.; Magnusson, M. H.; deppert, K.; Samuelson, L.; Wallenberg, L. R. Appl. Phys. Let. 2001, 79, 3335. (16) Wacaser, B. A.; Deppert, K.; Karlsson, L. S.; Samuelson, L.; seifert, W. J. Cryst. Growth 2006, 287, 504−508. (17) Verheijen, M. A.; Imminik, G.; Smet, T. D.; Borgström, M. T.; Bakkers, P. A. M. J. Am. Chem. soc. 2006, 128 (4), 1353−1359. (18) Borgström, M. T.; Immink, G.; Ketelaars, B.; Algra, R.; Bakkers, E. P. A. M. Nat. Nanotechnol. 2007, 2, 541−544. (19) Dubrovskii, V. G.; Sibirev, N. V. J. Cryst. Growth 2007, 304, 504−513. (20) Hall, R, N. Phys. Rev. 1952, 88, 139−139. (21) Hayakawa, Y.; Saitou, Y.; Sugimoto, Y.; Kumagawa, M. J. Electron. Mater. 1990, 19, 145−149. (22) Takahashi, K.; Moriizumi, T. Jpn. J. Appl. Phys. 1966, 5, 657− 662.
comparisons on glass and fused silica, we can conclude that it is the unintentional doping from one or more impurities from the glass, which is strangely making the GaAs NWs good emitters of light. It is likely that some unknown impurity(s) might possibly change the VLS mechanism resulting in single crystal phase and high optical quality of the NWs. This demands additional experimental and theoretical study and is a topic for furthur investigation. Figure 8b shows room temperature (RT) spectra of GaAs NWs grown at different growth temperatures. Strong RT PL was observed from all samples except the one grown at 410 °C, indicating high optical quality of NWs on glass. RT PL peak of the 470 °C grown sample at 870 nm corresponds well with the bulk ZB GaAs emission. Also, different PL peaks positions, asymmetry and broadening of the NW PL spectra were observed for the samples grown at different growth temperatures. These PL shifts at RT could be attributed partially to the tapering and more vertical NWs at different growth temperatures and to the unintentional doping of the GaAs NWs from the glass substrate. In addition, since the laser spot size is about 100 μm, therefore the observed PL signal is originating from a large number of NWs with slightly varying diameters and lengths, and small compositional fluctuations due to NW shadowing effect could also broaden and shift the emission spectra (albeit small). We could not detect any PL at RT from the NWs grown on Si and fused-silica substrates with similar growth parameters. As discussed earlier, unintentional impurity doping of the GaAs NWs from the glass substrate is possibly responsible for higher structural and optical quality of GaAs NWs on glass substrate. In conclusion, we have demonstrated the MOVPE growth of GaAs NWs on inexpensive glass substrates with promising results. The NWs grown on glass have a pure single phase ZB structure and are free of stacking faults. Furthermore, GaAs 1917
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(23) Zhang, G. Q.; Tateno, K.; Sanad, H.; Tawara, T.; Gotoh, H.; Nakano, H. Appl. Phys. Lett. 2009, 95, 123104. (24) Heiss, M. Phys. Rev. B 2011, 83, 045303.
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