Ion-Assisted Low-Temperature Silicon Epitaxy on Randomly Textured Seed Layers on Glass N.-P.
Harder,*,†
T.
Puzzer,†
P. I.
Widenborg,†
S.
Oelting,‡
and A. G.
Aberle†
Centre for Photovoltaic Engineering, University of New South Wales, Sydney NSW 2052, Australia, and ANTEC Solar GmbH, Arnstaedter Strasse 22, D-99334 Rudisleben, Germany Received June 12, 2003;
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 5 767-771
Revised Manuscript Received June 28, 2003
ABSTRACT: Ion-assisted deposition (IAD) of silicon at a substrate temperature of 630 °C was used for thickening a crystalline silicon seed layer made on glass by aluminum-induced crystallization (AIC) of amorphous silicon. The seed layer consists of randomly oriented crystal grains whose crystallographic orientations with respect to the substrate’s normal were determined with cross-sectional transmission electron microscopy (TEM) by analyzing their Kikuchi diffraction patterns. As shown by the TEM analysis, the IAD film grown on the AIC seed layer continues epitaxially the crystallographic structure of the seed layer. The crystallographic orientation of the seed layer does not appear to play the dominant role in determining the crystal quality of the subsequently grown IAD film. The fabrication of continuous AIC seed layers results in the production of island-like silicon protrusions on the surface of the seed layer. The impact of these “islands” on the subsequent IAD growth was investigated by comparison to the growth on similarly oriented island-free regions of the AIC seed layer. It is concluded that the IAD material grown on top of the AIC islands is of similar structural quality as the IAD material grown on island-free grains. 1. Introduction Forming device-grade crystalline silicon films on standard glass at low temperature (i.e., below the softening point of the glass) using simple and fast processes is one of the major challenges for low-cost thin-film solar cells. One strategy is the fabrication of fine-grained (microcrystalline) material andsto achieve the desired device-grade qualitysusing suitable means such as hydrogenation for passivating the many defects in the material. This approach has led to remarkable results over the last several years.1-3 However, we pursue an alternative strategy, the fabrication of largegrained (>1 µm) polycrystalline silicon (poly-Si) films on glass via epitaxial thickening of a large-grained polySi seed layer on glass. We believe that, due to the substantially lower density of structural defects and in combination with an efficient (postdeposition) hydrogen passivation scheme, large-grained poly-Si on glass has the potential for achieving higher electronic quality than fine-grained microcrystalline material. Efficient postdeposition hydrogen passivation of poly-Si on glass is, for example, used by the company Pacific Solar in their production process of thin-film poly-Si solar cells.4 Postdeposition hydrogenation has also been used for early, yet promising, results of solar cells fabricated on ceramic substrates from epitaxial thickening of a polySi seed layer by high-temperature chemical vapor deposition (CVD).5 The high substrate temperature required for CVD does not allow using cheap glass substrates for producing economically feasible thin-film solar cells. We therefore focus our research efforts on * To whom correspondence should be addressed. Nils-Peter Harder is now with Saint-Gobain Glass Deutschland, Research and Development Glass for Buildings (FEB), Glasstr.1, 52134 Herzogenrath, Germany. Tel.: +49 2406 911516. Fax: +49 2406 911276. E-mail:
[email protected]. † Centre for Photovoltaic Engineering, University of New South Wales. Phone: +61 2 9385 4018. Fax: +61 2 9662 4240. ‡ ANTEC Solar GmbH.
Figure 1. Focused ion beam picture of the surface of a seed layer fabricated by AIC on glass (after Al etching). Tilt: 45°. Multiply vertical distances with x2 for comparison with horizontal scale bar.
low-temperature Si growth methods and have recently reported on using low-temperature ion-assisted deposition (IAD)6 to epitaxially thicken7,8 seed layers made by aluminum-induced crystallization (AIC)9 on glass substrates. The AIC layer is heavily p-type due to aluminum doping in the order of 5 × 1018 cm-3.9 The desired doping profile of the IAD material can achieved in situ by coevaporation of dopants from effusion sources. It is well-known that the crystallographic orientation of the substrate plays an important role for the quality of the epitaxy, and we therefore investigate in this paper the influence of this parameter on the IAD growth on top of differently oriented grains of the seed layer. An additional aspect of the present investigation is the influence of the surface topography of the seed layer. As shown in Figure 1, seed layers produced by AIC contain a number of protrusions, or “islands”, of Si that
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remain on the surface of the continuous poly-Si film after Al removal.10 It was believed that the epitaxy reported in refs 7 and 8 required a smooth, flat seed layer. Here, we show that these islands do not present a barrier to epitaxial film growth. The structural quality of the Si grown on top of the islands is compared to that grown on other seed crystals having a similar crystal orientation. 2. Experiment 2.1 Sample Preparation. Polycrystalline, approximately 400-nm thick seed layers were prepared by AIC on Corning 1737F glass substrates.9,10 The AIC process is capable of producing a continuous silicon film of large crystal grains exceeding lateral dimensions of 20 µm.9 However, in this investigation we have used an AIC layer, which was not optimized with respect to the grain size. The approximate average grain size was 2 µm lateral dimension. After removal of the aluminum, the AIC layers were cleaned using argon plasma etching and subsequent chemical etching with a solution of HF/HNO3/H2O, followed by removal of the native oxide by HF vapor. The argon plasma is performed for removing contamination such as residues of oxidized aluminum by means of argon ion bombardment of the surface of the AIC seed layer. However, apart from removing contamination, the impact of the ions creates a defect-rich surface layer which has to be removed by the subsequent wet-chemical etch for exposing the high-quality crystal structure of the AIC seed layer. The typical island-like silicon protrusions at the surface of the AIC seed layer are not removed by this cleaning procedure. The results presented further below allow a comparison between the quality of the silicon growth on these protrusions and the silicon growth on the flat regions of the continuous AIC layer between the protrusions. The samples were then loaded into the IAD reactor and approximately 5 µm of silicon was deposited onto the AIC seed layer. The substrate temperature was 630 °C and the deposition rate about 0.1 µm/min. The first 50-100 nm were deposited using an elevated ion acceleration voltage of 100 V, while for the rest of the deposition process we used 20 V. The ion flux is estimated to be 1% of the total Si flux.6 2.2 Transmission Electron Microscopy (TEM). TEM images were taken from cross sections of the AIC/IAD sandwich structure, using a Philips CM200 microscope. Kikuchi diffraction patterns were obtained using a convergent beam and a spot size of 20 nm. This spot was subsequently moved along the seed layer and over each observed grain to check the uniformity of the grain orientation. The agreement between the crystal orientation of the IAD-grown crystals and the underlying AIC seed grains was verified by comparing their selected-area diffraction patterns as well as Kikuchi diffraction patterns. The crystal orientation with respect to the substrate’s normal was calculated from recorded Kikuchi diffraction pattern for each crystal grain of the seed layer. The local thickness of the TEM sample cross section was determined for each grain from its correlation with the dimensions of the reciprocal lattice spikes and thus from the size of the zeroth-order Laue zone.11
3. Results and Discussion The crystallographic orientations of the crystal grains that were investigated in this study are shown in Figure 2. For the purpose of comparison with the growth on the “island”-like grains, we focused our attention to grains of similar orientation as those found for the island-like (protrusion) grains. Thus, the grains shown in Figures 3-5 (their crystal orientations with respect to the normal of the substrate are shown in Figure 2) are not selected on the basis of their quality. Instead, the choice of the presented grains is entirely determined
Figure 2. Stereographic projection of 12 selected crystal orientations observed in the AIC layer.
by the crystal orientation of the underlying AIC seed. As a result, they represent very well the average quality of the IAD film grown on the AIC layer. Using their proximity to the (001), (011), and (112) orientation, the three groups of orientations shall subsequently be referred to as the “(001)” group, the (011) group and the (112) group. Cross sectional TEM images of the grains of these groups are shown in Figures 3-5 and are ordered by the local thickness of the TEM sample slice (given next to each image in nanometers). The contrast decreases for thicker samples where inelastic scattering becomes more noticeable. When assessing the quality of the samples, it must be considered that a TEM image is a two-dimensional projection of a three-dimensional slice of material. Thus, with increasing thickness of the TEM sample slice the two-dimensionally projected defect density appears to increase even when the volume defect density remains constant. It is immediately striking that the IAD film appears to have grown epitaxially on top of the islands of the AIC layer. Compared to the typical fine-grained microcrystalline structure (e.g., of IAD-grown Si on glass without a seed layer), the IAD material on top of the AIC islands appears to be of good structural quality, displaying a relatively low density of defects. Note that the corresponding TEM images of IAD material on AIC islands in Figure 3 and Figure 5 were obtained from rather thick regions of the TEM sample slice. The good crystal quality of the AIC islands itself10 provides an explanation for the quality of the IAD Si crystal growth on the islands. The TEM sample is thin, yet still three-dimensional, and therefore may contain a boundary region of two neighboring grains. As a result, some of the TEM images are a superposition of images of two grains. This, as well as increased strain fields at grain boundaries, can create the impression of defects in the bulk of the grain even in cases when the bulk of the grain is relatively free of defects. Strain fields seem to be present, e.g., in the vicinity of the black streaks of the grain shown in Figure 5c. Kikuchi diffraction performed on this region resulted in superimposed patterns from different crystal orienta-
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Figure 4. (011)-group of crystal grains, local thickness of TEM sample given next to each image, scale given implicitly by approximately 400-nm thick AIC seeding layer.
Figure 3. (001)-group of crystal grains, local thickness of TEM sample next to each image, scale given implicitly by approximately 400-nm thick AIC seeding layer.
tions. This seems to support the assumption that the black streaks and the strain fields stem from a grain boundary and not from defects within the bulk of the grain. Above the dark streaks is a region, which clearly has a comparatively low defect density. Bands with different crystal orientation (dark in the dark field image) than the bulk of the grain can also be seen in the grain shown in Figure 5b. However, there is only very little evidence for strain fields between the dark, parallel and very straight bands. Lattice imper-
fections such as stacking faults and twins form stressfree interfaces between regions of different orientations. Only few defects can be seen between the dark bands. The bands themselves, which are dark in Figure 5b and bright in Figure 6, also show a rather low density of defects. A TEM sample slice cut at an angle with respect to a vertical grain boundary may run through one grain at the bottom and another one at the top. This seems to be quite clearly the case in the 23-nm thick part of the TEM sample shown in Figure 4c. Fringes at the top of the bright area indicate that the thickness of the grain in diffraction condition is decreasing until it eventually vanishes and is replaced entirely by the other grain at the top of the sample slice. A similar situation, but not with the classic appearance of thickness fringes, seems to be the case in Figure 3a,c. With the exception of the grain shown in Figure 5a and considering the above-mentioned complications in interpreting cross-sectional TEM images of polycrystalline material, all images in Figures 3-5 appear to show
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Figure 6. Additional cross-sectional dark field TEM image of the grain shown in Figure 5b. The different diffraction condition gives rise to complementary contrast. Scale given implicitly by approximately 400-nm thick AIC layer.
Si AIC seed layers is dominated by other factors than the crystallographic orientation of the underlying seed grains. Surface roughness, surface-near defects in the seed layer, oxide and other contaminants at the surface are likely candidates for strongly influencing the crystal quality of the IAD-grown film. Inhomogeneous distribution of such imperfections across the surface of the seed layer may explain the existing variation of the crystal quality of the IAD-grown material on different grains of the seed layer. 4. Conclusions Figure 5. (112)-group of crystal grains, local thickness of TEM sample given next to each image, scale given implicitly by approximately 400-nm thick AIC seeding layer.
epitaxial growth on the seed layers. Some grains, in particular, those of the (112)-oriented group that grew on flat, island-free grains of the AIC layer, additionally contain regions with high defect density. However, the IAD grain of the (112)-oriented group that grew on top of the AIC island-like grain displays a remarkably low density of defects. Thus, within the range of investigated crystal orientations, it appears that the orientation of the seed grains in the AIC layer does not play the dominant role in determining the structural quality of the subsequently grown IAD silicon film. A recent investigation on the orientation dependence of epitaxial silicon growth by IAD12 on polished mono- and multicrystalline silicon wafer substrates shows a rather strong orientation dependence of the structural quality for deposition temperatures of 550 °C and below. For temperatures around 630 °C, as used here, this dependence is less pronounced.12 In contrast to the findings for polished Si wafer substrates, our present work shows that the quality of the Si crystal growth on poly-
Using transmission electron microscopy, we have investigated the crystal growth of silicon by IAD at 630 °C on a seed layer made by AIC. The typical island-like protrusions at the surface of the AIC layer were not removed prior to the deposition of silicon by IAD. We find that IAD on the AIC islands results in epitaxial growth of silicon, displaying a defect density that is very low in comparison to microcrystalline Si. A comparison between material grown by IAD on three crystallographically differently oriented groups of seed grains suggests that the crystallographic orientation of the grains in the AIC seed layer does not play the dominant role in determining the structural quality of the subsequently grown epitaxial film by IAD at 630 °C. Instead, the existing variation of the crystal quality of the IAD-grown material on different grains of the seed layer has do be due to other mechanisms. A potential explanation is residual contamination on the surface of the AIC seed layer or incomplete wet-chemical removal of the defect-rich surface layer after argon plasma etching. Epitaxial thickening of AIC seed layers by lowtemperature IAD can produce poly-Si on glass with a much lower density of structural defects than found in
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device-grade fine-grained microcrystalline silicon solar cells.13 We have furthermore shown that differently oriented seed grains or a nonflat topography of the seed layer do not lower the structural quality of the IADgrown material. On the basis of the assumption that structural defects as observable in TEM are a major contributor for producing electronic defects such as dangling bonds in poly crystalline thin-film silicon, the findings of this paper make the combination of AIC and IAD a promising candidate for the fabrication of efficient poly-Si thin-film solar cells on glass. Thereby, it is expected that postdeposition treatments such as hydrogenation are necessary to passivate the remaining defects as observed by TEM and other (electronic) defects unobservable by TEM. Acknowledgment. N.P.H. thankfully acknowledges the financial support of his Ph.D. research at UNSW by a scholarship from the “Evangelisches Studienwerk Villigst e.V.”, Germany. The work described here has been supported by the Australian Research Council within its Large-Grant-scheme. References (1) Vetterl, O.; Lambertz, A.; Dasgupta, A.; Finger, F.; Rech, B.; Kluth, O.; Wagner, H. Sol. Energy Mater. Sol. Cells 2001, 66, 345-351.
Crystal Growth & Design, Vol. 3, No. 5, 2003 771 (2) Meier, J.; Vallat-Sauvain, E.; Dubail, S.; Kroll, U.; Dubail, J.; Golay, S.; Feitknecht, L.; Torres, P.; Fay, S.; Fischer, D.; Shah, A. Sol. Energy Mater. Sol. Cells 2001, 66, 73-84. (3) Yamamoto, K.; Yoshimi, M.; Tawada, Y.; Okamoto, Y.; Nakajima, A. Sol. Energy Mater. Sol. Cells 2001, 66, 117125. (4) Basore, P. A., presented at the 3rd World Conference on Photovoltaic Energy Conversion in Osaka, May 2003. (5) Ornaghi, C.; Beaucarne, G.; Poortmans, J.; Nijs, J.; Mertens, R., presented at the European Materials Research Society Spring Meeting in Strasbourg, 2003. (6) Oelting, S.; Martini, D.; Bonnet, D., Proceedings of the 11th European Photovoltaic and Solar Energy Conference, 1992, p 491. (7) Harder, N.-P.; Xia, J. A.; Oelting, S.; Nast, O.; Widenborg, P.; Aberle, A. G. Proceedings of the 28th IEEE Photovoltaics Specialists Conference, 2000, p 351. (8) Aberle, A. G.; Harder, N.-P.; Oelting, S. J. Cryst. Growth 2001, 226, 209-214. (9) Nast, O.; Wenham, S. R. J. Appl. Phys. 2000, 88, 124-132. (10) Widenborg, P. I.; Aberle, A. G. J. Cryst. Growth 2002, 242, 270-282. (11) Pinsker, Z. G. Electron Diffraction; Butterworths: London, 1953; p 82. (12) Wagner, T. A.; Oberbeck, L.; Nerding, M.; Strunk, H. P.; Bergmann, R. B. Mater. Res. Soc. Symp. Proc. 2001, 664, A22.3.1-6. (13) Luysberg, M.; Scholten, C.; Houben, L.; Carius, R.; Finger, F.; Vetterl, O. Mater. Res. Soc. Symp. Proc. 2001, 664, A15.2.1-6.
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