Article pubs.acs.org/crystal
Orientation Control of Large-Grained Si Films on Insulators by Thickness-Modulated Al-Induced Crystallization Ryohei Numata,† Kaoru Toko,*,† Noriyuki Saitoh,‡ Noriko Yoshizawa,‡ Noritaka Usami,§ and Takashi Suemasu† †
Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Electron Microscope Facility, IBEC Innovation Platform, AIST, 16-1 Onogawa, Tsukuba 305-8569, Japan § Institute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan ‡
ABSTRACT: The thickness-modulated Al-induced crystallization technique enables us to control the orientation of polycrystalline Si films on insulators. The amorphous Si film on a SiO2 substrate was crystallized at a low temperature (425 °C) through the layer exchange between Si and catalytic Al layers. Electron backscattering diffraction (EBSD) measurements revealed that the crystal orientation of the grown Si layer varied significantly depending on the Al/Si thickness: The [111]-orientation fraction reached 97% for the 50nm-thick sample and the [100]-orientation fraction 88% for the 200-nm-thick sample. This mechanism was discussed in terms of the heterogeneous nucleation energy. The 50-nm-thick sample provided the maximum Si grain-size as large as 68-μm diameters. These large-grained Si films with controlled-orientations are promising epitaxial templates for advanced Si-based thin-film solar cells. orientations of grown Si films are dramatically controlled to either [100] or [111] by modulating the Al/Si thickness in the inverted-ALILE process. Moreover, the maximum grain size of the grown Si films reaches as large as 68-μm diameters.
I. INTRODUCTION The low-temperature formation of polycrystalline Silicon (polySi) thin films on glass substrates is essential for thin-film solar cells and thin-film transistors. Laser annealing techniques are sound methods to form high-quality Si films in low temperature processes.1−3 However, solar-cell applications require more simple techniques providing large-area poly-Si films on glass substrates. Moreover, the crystal orientation of the Si layer is important: [100] is essential for texturing antireflection structures and [111] is useful as epitaxial templates for aligned nanowires and for advanced solar-cell materials.4−6 In line with this, metal-induced crystallization (MIC) has been studied for large-area formations of poly-Si films in low temperature processes (200 nm) Al/Si layers. Meanwhile, the TD orientation maps in Figures 3f−3j suggest that the grain sizes are as large as 30-μm diameters for all the samples except the 100-nm-thick sample. The EBSD analysis derived the area-fraction of [100] and [111] orientations from the ND-orientation maps shown in Figure 3a−3e. The result is illustrated in Figure 4a as a function of the Al/Si thickness. Here, by definition, the [111] and [100] fractions contain the orientations tilted within 15° from the exact [111] and [100] orientations. The [111] fraction increases with decreasing the Al/Si thickness; in contrast, the [100] fraction increases with increasing the Al/Si thickness. Note that the [111] fraction reaches as high as 97% at the 50nm thickness, and the [100] fraction reaches 88% at the 200nm thickness. From the TD orientation maps in Figures 3f−3j, the EBSD analysis calculated the grain sizes of the poly-Si layers. Figure 4b displays the average and the maximum grain diameters as a function of the Al/Si thickness. The average grain sizes reach approximately 30-μm diameters for all the samples except the 100-nm-thick sample. The 50-nm-thick sample provides the largest grains: The average grain size reaches 32 μm and the maximum grain size as large as 68 μm. These orientation fractions and grain sizes are the highest ever reported for the poly-Si layers on amorphous substrates.2,11,27−30 Now we discuss the mechanism of the Al/Si thickness dependent ALILE process. Some papers on the ALILE describe that the Si nucleation heterogeneously occurs at the interface between Al and interfacial diffusion-controlling layers (SiO2 layers in this experiment).11−15 In particular, Sarikov et al. theoretically explain that the crystal orientation of the nuclei is very sensitive to the interfacial energy between the Al and the interfacial layer, and changes to either [100] or [111] orientations.17 During the annealing for the ALILE, the interfacial SiO2 layer is stressed by the Al and Si layers because of the difference in the thermal-expansion coefficients (Si, 2.6 × 10−6 K−1; Al, 2.3 × 10−5 K−1; SiO2, 5.0 × 10−7 K−1). This stress on the interfacial SiO2 layer varies with the Al/Si thickness. Therefore, the preferential orientation depends on the Al/Si thickness, as experimentally shown in Figure 4a. For the 100nm-thick sample, both [100] and [111] oriented nuclei can occur. This increases the nucleation density and makes the grain size small compared to the other samples. Since the growth morphology will also depend on the growth temperature or on the interfacial-layer thickness,16−20 further investigations are required for establishing the completed growth model. Figure 5a shows a typical bright-field TEM cross-section of the grown 50-nm-thick sample. STEM-EDX line-scan analyses (spatial resolution = 10 nm) were performed along arrowed lines in Figure 5a, and the results were respectively shown in Figure 5b and 5c. These data prove that the top layer successfully becomes Si by the inverted-ALILE process. The Si layer shown in Figure 5a has no dislocations or stacking faults. As indicated in Figure 5b, the Al layer, which should be present between Si and SiO2 substrates, has almost dropped out in the TEM sample preparation (mechanical polishing and ion
Figure 1. (a) Schematic structure of the sample preparation procedure for the inverted-ALILE of a-Si films on SiO2 glass substrates. Normarski optical micrographs of the surface of the sample with 50nm thickness annealed at 425 °C for (b) 0 h, (c) 2 h, and (d) 10 h.
Si atoms diffuse into the Al layer, grow laterally, and cover the whole surface of the sample during the annealing. This growth characteristic is almost the same as the conventional ALILE.11−20 Figure 2a and 2b shows SEM-EDX spectra respectively obtained from the sample surfaces before and after annealing
Figure 2. SEM-EDX spectra of the sample surface (a) before and (b) after annealing for 10 h at 425 °C. (c) XRD patterns of the sample before and after annealing for 10 h at 425 °C. Here, the thicknesses of Si and Al layers are 50 nm.
(425 °C, 10 h). Here, the electron acceleration voltage was as low as 2.5 keV for the selective detection of elements near the sample surface. Figure 2a indicates that the Al layer is present on the sample surface before annealing. On the other hand, Figure 2b proves that the top layer changes to Si after annealing. The XRD patterns in Figure 2c show the appearance of a sharp peak after annealing. The peak at around 28° corresponds to the Si [111] orientation, and any other peak is absent in the measured 2θ range (15−80°). These results indicate that the Si layer crystallized through the layer exchange process and is preferentially oriented to [111]. The precise orientation fraction will be estimated later using EBSD measurements. The above results demonstrated a successful inverted-ALILE process. The crystal orientations and the grain sizes of the grown Si layers were evaluated using EBSD measurements. Figures 3 1768
dx.doi.org/10.1021/cg4000878 | Cryst. Growth Des. 2013, 13, 1767−1770
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Figure 3. EBSD images of the grown Si layers surfaces with various thicknesses (50−400 nm) along (a−e) the normal direction (ND) and (f−j) the transverse (in-plane) direction (TD) to the sample surfaces. The insertions are a color key corresponding to crystal orientations and a schematic image indicating ND and TD.
Figure 5. Characterization of the cross-section structure of the grown Si layer with 50-nm thickness. (a) Bright-field TEM image with a low magnification. (b, c) Elementary-composition profiles along the arrowed lines in panel a analyzed by STEM-EDX. (d) SAED pattern obtained at the area shown in panel a. (e) Lattice image of the grown Si layer. (f) Bright-field TEM image of an area where plane defects are present. (g) SAED pattern obtained at the area including plane defects shown in panel f.
Figure 4. Al/Si thickness dependences of (a) [111] and [100] orientation fractions along the ND and (b) grain sizes of the grown Si layers. Both the crystal orientation and the grain size were derived from the EBSD analysis.
silicon, and the streaks at the diffraction spots along Si ⟨111⟩ are the result of the planar defects in Si (111) planes. These defects are parallel to the substrates, and thus no defects appear on the Si surface. Therefore, this Si layer is promising epitaxial template for low-defect Si light-absorbing layers as well as advanced materials.
milling). Meanwhile, some pillars consisting of Si and Al are present between Si and SiO2 substrates as indicated in Figure 5c. These Si−Al mixed layers are found at the sample surface grown by the normal ALILE process.11,14 A selected area electron diffraction (SAED) pattern in Figure 5d shows that the ⟨111⟩ direction is perpendicular to the substrate. In addition, a lattice image in Figure 5e clearly exhibits Si (111) planes which are parallel to the substrate. These results agree with the EBSD measurement. In the TEM-observed area (∼10 μm width), we found only one area (∼700 nm width) where the Si layer includes planar defects as shown in Figure 5f. The SAED pattern in Figure 5g indicates some twinned structures of
IV. CONCLUSIONS We investigated the low-temperature (425 °C) inverted-ALILE of a-Si focusing on the Al/Si thickness dependence. The crystal orientations of grown poly-Si on Al layers were controlled to either [111] or [100] by modulating the Al/Si thickness. 1769
dx.doi.org/10.1021/cg4000878 | Cryst. Growth Des. 2013, 13, 1767−1770
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(21) Toko, K.; Kurosawa, M.; Saitoh, N.; Yoshizawa, N.; Usami, N.; Miyao, M.; Suemasu, T. Appl. Phys. Lett. 2012, 101, 072106. (22) Gordon, I.; Carnel, L.; Gestel, V. D.; Beaucarne, G.; Poortmans, J. Thin Solid Films 2008, 516, 6984−6988. (23) Wu, B. R.; Lo, S. Y.; Wuu, D. S.; Ou, S. L.; Mao, H. Y.; Wang, J. H.; Horng, R. H. Thin Solid Films 2012, 520, 5860−5866. (24) Becker, C.; Conrad, E.; Dogan, P.; Fenske, F.; Gorka, B.; Hänel, T.; Lee, K. Y.; Rau, B.; Ruske, F.; Weber, T. Sol. Energy Mater. Sol. Cells 2009, 93, 855−858. (25) Lee, K. Y.; Muske, M.; Gordon, I.; Berginski, M.; D’Haen, J.; Hüpkes, J.; Gall, S.; Rech, B. Thin Solid Films 2008, 516, 6869−6872. (26) Hu, S.; Marshall, A. F.; McIntyre, P. C. Appl. Phys. Lett. 2010, 97, 082104. (27) Ekanayake, G.; Quinn, T.; Reehal, H. S. J. Cryst. Growth 2006, 293, 351−358. (28) Jaeger, C.; Matsui, T.; Takeuchi, M.; Karasawa, M.; Kondo, M.; Stutzmann, M. Jpn. J. Appl. Phys. 2010, 49, 112301. (29) Kuraseko, H.; Orita, N.; Koaizawa, H.; Kondo, M. Appl. Phys. Express 2009, 2, 015501. (30) Geis, M. W.; Flanders, D. C.; Smith, H. I. Appl. Phys. Lett. 1979, 35, 71−74.
Namely, the [111]-orientation fraction reached as high as 97% for the 50-nm-thick sample; in contrast, the [100]-orientation fraction reached 88% for the 200-nm-thick sample. Moreover, the 50-nm-thick sample provided the maximum grain diameter as large as 68 μm. These large-grained poly-Si films with controlled-orientations promise to be favorable epitaxial templates for advanced materials, nanowires, and Si-based thin-film solar cells with texturing antireflection structures.
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AUTHOR INFORMATION
Corresponding Author
*Address: Kaoru Toko Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennohdai, Tsukuba, Ibaraki 305-8573, Japan. Phone: +81-29-853-5472. Fax: +81-29-853-5205. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sport, Science, and Technology in Japan. This work was performed under the Inter-University Cooperative Research Program of the Advanced Research Center of Metallic Glasses, Tohoku University.
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REFERENCES
(1) Hatano, M.; Moon, S.; Lee, M.; Suzuki, K.; Grigoropoulos, C. P. J. Appl. Phys. 2000, 87, 36−43. (2) Lee, B. M.; Kuranaga, T.; Munetoh, S.; Motooka, T. J. Appl. Phys. 2007, 101, 054316. (3) Sugawara, Y.; Uraoka, Y.; Yano, H.; Hatayama, T.; Fuyuki, T.; Nakamura, T.; Toda, S.; Koaizawa, H.; Mimura, A.; Suzuki, K. Appl. Phys. Lett. 2007, 91, 203518. (4) Campbell, P.; Wenham, S. R.; Green, A. M. Sol. Energy Mater. Sol. Cells 1993, 31, 133. (5) Schmidt, V.; Senz, S.; Gösele, U. Nano Lett. 2005, 5, 931−935. (6) Tsukada, D.; Matsumoto, Y.; Sasaki, R.; Takeishi, M.; Saito, T.; Usami, N.; Suemasu, T. Appl. Phys. Express 2009, 2, 051601. (7) Hayzelden, C.; Batstone, J. L. J. Appl. Phys. 1993, 73, 8279−8289. (8) Yoon, S. Y.; Oh, J. Y.; Kim, C. O.; Jang, J. J. Appl. Phys. 1998, 84, 6463−6465. (9) Muramatsu, S. I.; Minagawa, Y.; Oka, F.; Sasaki, T.; Yazawa, Y. Sol. Energy Mater. Sol. Cells 2002, 74, 275−281. (10) Toko, K.; Kanno, H.; Kenjo, A.; Sadoh, T.; Asano, T.; Miyao, M. Appl. Phys. Lett. 2007, 91, 042111. (11) Nast, O.; Hartmann, A. J. J. Appl. Phys. 2000, 88, 716−724. (12) Widenborg, P.; Aberle, A. G. J. Cryst. Growth 2002, 242, 270− 282. (13) Sugimoto, Y.; Takata, N.; Hirota, T.; Ikeda, K.; Yoshida, F.; Nakashima, H.; Nakashima, H. Jpn. J. Appl. Phys. 2005, 44, 4770− 4775. (14) Wang, J. Y.; Wang, Z. M.; Mittemeijer, E. J. J. Appl. Phys. 2007, 102, 113523. (15) Birajdar, B. I.; Antesberger, T.; Butz, B.; Stutzmann, M.; Spiecker, E. Scr. Mater. 2012, 66, 550−553. (16) Kurosawa, M.; Kawabata, N.; Sadoh, T.; Miyao, M. Appl. Phys. Lett. 2009, 95, 132103. (17) Sarikov, A.; Schneider, J.; Berghold, J.; Muske, M.; Sieber, I.; Gall, S.; Fuhs, W. J. Appl. Phys. 2010, 107, 114318. (18) Jung, M.; Okada, A.; Saito, T.; Suemasu, T.; Usami, N. Appl. Phys. Express 2010, 3, 095803. (19) Kurosawa, M.; Toko, K.; Kawabata, N.; Sadoh, T.; Miyao, M. Solid-State Electron. 2011, 60, 7−12. (20) Okada, A.; Toko, K.; Hara, K. O.; Usami, N.; Suemasu, T. J. Cryst. Growth 2012, 356, 65−68. 1770
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