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Microstructure dependence of ZnO:Al films on the deposition conditions and the surface morphology of silicon substrate
ARTICLE IN PRESS
Journal of Crystal Growth 289 (2006) 464–471 www.elsevier.com/locate/jcrysgro
Microstructure dependence of ZnO:Al films on the deposition conditions and the surface morphology of silicon substrate X. Jianga,, C.L. Jiab, R.J. Honga a
Institute of Materials Engineering, University of Siegen, Paul-Bonatz-Str. 9-11, 57068 Siegen, Germany b Institut fu¨r Festko¨rperforschung, Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany Received 20 October 2005; received in revised form 30 November 2005; accepted 1 December 2005 Available online 23 January 2006 Communicated by R. Kern
Abstract The microstructure of ZnO:Al films prepared under different conditions on silicon wafers was investigated by means of diffraction analysis and high-resolution transmission electron microscopy. It was found that the structure feature of the films is strongly dependent on the preparation conditions. The films obtained in a transition mode (higher oxygen partial pressure) show a structure of welldeveloped columnar grains with a defined texture with c-axis vertical to the substrate surface that can be in different orientations. These grains nucleate directly on the amorphous surface of the substrate and grow with a nearly unchanged lateral dimension through the full film thickness. The films obtained in a metallic mode (lower oxygen partial pressure) consist of randomly oriented grains. A thin nanocrystalline layer was observed close to the interface. Some grains with columnar feature are found to develop during the later stages of the film growth. The dependence of the film structure on the growth conditions is discussed in terms of surface energy of the sample system. r 2006 Elsevier B.V. All rights reserved. PACS: 68.37.Lp; 73.61.Ga; 81.15.Cd Keywords: A3. Physical vapor deposition processes; B1. Oxides; B1. Zinc compounds
1. Introduction ZnO thin films have received more attention in different fields of research due to the potential applications in photoelectronics, surface acoustic wave (SAW) devices as well as in solar cell technology [1–5]. In order to obtain desirable properties for different applications, ZnO thin films were prepared with and without doping on different substrates by various deposition techniques [6–13]. In general, the films on some oxide single crystalline substrates such as sapphire (Al2O3) show an epitaxial growth [14,15], while on most traditional substrates of semiconductors like silicon and GaAs the films are textural. Details of the film structure strongly depend on the deposition techniques, Corresponding author. Tel.:+49 271 7402966; fax: +49 271 7402442.
E-mail address: [email protected] (X. Jiang). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.067
processing conditions as well as the substrates [16,17]. On the silicon substrate, it was found that the concentration of dopant Al has a strong effect on the microstructure evolution of the films prepared by reactive DC magnetron co-sputtering technique [18]. In many cases, the films are required to be grown on a patterned substrate in order to meet the requirements from the fabrication technology of the devices or to obtain a special property. Therefore, the microstructure of the ZnO films developed on patterned substrates is also an interesting and important topic of research. Recently, the reactive mid-frequency magnetron sputtering (RMFMS) technique was developed suitably for reactive sputtering from metallic targets and for large area deposition. Promising results for ZnO:Al films are obtained. The films deposited at a substrate temperature of 200 1C with a deposition rate up to 9 nm/s exhibited low
ARTICLE IN PRESS X. Jiang et al. / Journal of Crystal Growth 289 (2006) 464–471
resistivity of 300 mO cm. Furthermore, they are more stable in reducing ambient than other transparent conductive oxide materials [19] and their surface topography can be modified by chemical etching [20], which makes them very attractive for photovoltaic applications. In the present work, we report the effects of the deposition conditions and the substrate surface on the microstructure of the Al-doped ZnO thin films prepared by RMFMS technique on silicon substrates. Detailed investigations of the microstructure of the films were performed by means of conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The evolution of the film microstructure is discussed in terms of a surface energycontrolled step-flow mechanism and the process modes.
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3. Results 3.1. Effect of the process modes Fig. 1 shows cross-sectional overviews of samples A (a) and B (b). The difference in grain morphology and configuration is very evident from the two images. In image (a) we see a clear columnar structure of the ZnO:Al film in the sample A. The boundaries between these grain columns look sharp and straight. No evident change occurs in the lateral dimension of individual column through the
2. Experimental details The ZnO:Al thin films were prepared by reactive mid frequency magnetron sputtering technique in a batch coater (Balzers-Pfeiffer, PLS 580) equipped with PK500 cathodes (Leybold, TwinMags) with Zn–Al (Al: 1.5 wt%) alloy targets. The reactive sputtering processes were carried out in both the transition mode and metallic mode in which a closed loop control system was established for the adjustment of discharge power according to reactive gas partial pressure detected by a l-sensor. Details of this process control are discussed in the text and other deposition conditions are summarized in Table 1. Si wafers were used as substrates. The samples A and B were deposited on flat (0 0 1) silicon substrates in transition mode and in metallic mode, respectively. In the sample C the ZnO:Al film was deposited on a patterned silicon substrate with surface parallel to either (0 0 1) or {1 1 1} crystallographic planes. Both cross-sectional and plan-view specimens were prepared for TEM and HRTEM investigations by a traditional procedure that includes grinding, dimpling and finally ion-milling. The TEM and HRTEM investigations were performed on a JEOL 4000EX microscope operated at 400 kV.
Table 1 Parameters for reactive MF magnetron sputtering (RMFMS) of ZnO:Al layers in transition and metallic modes Sample
A, C
B
Process mode O2 partial pressure (mTorr) Total gas pressure (mTorr) MF power (kW) Target voltage (V) Target/substrate distance (mm) Substrate temperature (1C) Deposition rate (nm/s) Deposition time (s)
Transition mode 0.25 1.45 3.5–5.5 360 90 150 3–4 170
Metallic mode 0.15 1.35 4.5 430 90 150 3–4 120
Fig. 1. Cross-sectional overview of sample A (a) and sample B (b).
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Fig. 2. Electron diffraction patterns from sample A (a) and sample B (b).
film thickness. In contrast, the grains in the sample B (Fig. 1b) show a less dominant columnar feature. The continuity of the grain columns through the film thickness is in most cases disturbed by secondary nucleation of grains during the film growth. In this sample a column may include several grains with different orientations. The orientations of the columnar grains in the two samples were investigated by selected-area electron diffraction analysis. Fig. 2 shows the electron diffraction patterns (EDPs) recorded with electron beam parallel to the [1 1 0] zone axis of silicon from cross-sectional specimens of (a) the sample A and (b) the sample B. Each of the patterns includes the crystallographic information of both the film and the substrate obtained using a selected-area aperture covering the film and part of the substrate. In both the EDPs the reflection spots of single crystal silicon are
identical and some of the reflection spots are indexed. The reflection patterns of the films are different. The EDP in Fig. 2(a) consists of reflection spots of the film A that are identified as those belonging to different zone axes with an included common direction row of the (0 0 0 l) reflection spots parallel to the (0 0 l) spot row of silicon. A (0 0 0 4) common reflection spot is indexed. Further investigation by moving the aperture along the interface between the film and the substrate to different areas has led to similar results. This indicates that the columnar grains in film A are generally oriented with their c-plane parallel to the surface plane of the substrate. We note that the (0 0 0 l) spots of these columnar grains are elongated, indicating a small deviation of some grains from the exact parallel orientation relationship. In the EDP (b) of film B we see, instead of spots, arcs consisting of spots. These arcs demonstrate a large angle deviation of the c-plane from the substrate surface. The diffraction analysis of the plan-view specimens also showed a (0 0 0 1) oriented feature for the grains in the film A and a configuration of randomly oriented grains in the film B. Fig. 3 shows lattice images of the film A (a) and the film B (b) in a cross-sectional view. The two images were recorded with the electron beam parallel to the [1 1 0] zone axis of the silicon substrate. Both images show the typical features of the grains along the interface area in the two samples, respectively. A thin amorphous layer is clearly seen along the interfaces. This layer covers the crystal surface of silicon substrate with a homogeneous thickness. The difference in the microstructure, i.e. the orientation of the ZnO grains, between the two films close to the interfaces can also be recognized although the structural feature of the substrate surface looks the same for the two samples. The white arrows denote the [0 0 0 1] direction of the grains and the black arrows mark the grain boundaries. In image (a) three ZnO grains are presented to have approximately a same c-plane orientation, while in image (b) two grains of the three grains show a large deviation of their c-plane from the surface plane of the substrate. In the film A the majority of the grains are found to nucleate directly on the substrate surface with their c-plane parallel to the surface plane of the substrate. During growth these grains keep their orientation unchanged up to the film surface. Fig. 4 shows an image of a columnar grain. As can be seen in the image, this grain does not only keep its orientation during growth, but also keeps its lateral dimension. The grain boundaries with the adjacent grains are marked with two arrows. In the film B small grains with random orientation are often found close to the interface and are overgrown by other grains with increase in the film thickness. Lattice defects and lattice distortion are commonly observed in the grains and at the grain boundaries in both the samples. The most common defects are the stacking faults or inversion domain boundaries occurring in the c-plane [21–23]. One of the stacking faults is indicated by a thick arrow in Fig. 4.
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Fig. 3. Lattice images of the interface area between the film and the substrate in sample A (a) and sample B (b). The images were recorded with the electron beam parallel to the [1 1 0] of silicon.
We also found that the film evolution is rather interesting, featuring a cone-shaped grain surface. The SEM micrographs of the film shown in Fig. 5 clearly reveal a regular cone-shaped grain surface and columnar structure. The sample was prepared under the similar deposition conditions used for the sample A. The grain columns are tightly packed and grow through the entire film. The columns are vertical to the substrate, showing preferred c-axis orientated growth. 3.2. Effect of surface morphology of the substrate In order to investigate the effect of the substrate surface morphology and surface orientation on the microstructure of the film and on the grain orientation, we prepared a silicon substrate with a pattern of many pits on the surface. The pit is surrounded by four {1 1 1} plane walls as shown schematically in Fig. 6. Fig. 7a shows a low magnification image of the film across an upper step edge area of a pit. The inset schematically marks the position of the pit. The initial surface area of the film on the edge was removed by
Fig. 4. A lattice image of a columnar grain in sample A. The vertical arrows mark the grain boundaries in the film and the lateral arrow denotes a stacking fault.
the ion-milling thinning procedure. In the area adjacent to the substrate we see the textural feature of the grains that changes its direction across the step-edge to maintain the
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Fig. 7. (a) A low magnification image of the ZnO film deposited on the patterned silicon substrate shown in Fig. 6. The inset shows the surface position from which the image has been taken. (b) A diffraction pattern from the pit wall area. (c) A diffraction pattern from the film area on the flat surface of silicon. Fig. 5. SEM micrographs of ZnO:Al film. The upper image shows the surface morphology, whereas the lower image shows the cross-sectional microstructure of the film.
Fig. 6. Schematic drawing of the pits patterned on the silicon surface.
normal direction to the substrate surface. The orientation relation between the grains and the substrate can be clearly seen in the diffraction patterns from different film areas on the flat surface and on the pit wall surface. Figs. 7b and c
are the diffraction patterns from a film area on the pit wall and an area on the flat surface, respectively. In pattern (c) we see a same configuration of the reflection spots of the film and the substrate as that of Fig. 2a. The grains of the film on the flat surface are oriented with their c-plane parallel to the surface plane of substrate. This is expected since we used the same conditions for deposition of films A and C. In the pattern (b), however, the sets of reflection spots from different grains rotate by 54.71 around the [1 1 0] zone axis of silicon. The (0 0 0 l) spots are in this pattern in line with the ðl ¯l lÞ spots of silicon. Therefore, the film grains of the film part on the pit wall exhibit in general an arrangement with the c-plane parallel to the ð1 1¯ 1Þ plane of silicon, i.e. the surface plane of the pit wall. Fig. 8 shows a lattice image of the step-edge area viewed along the [1 1 0] direction of silicon. From this lattice image we see again an amorphous layer that looks thicker on the pit wall than on the flat crystal surface. The flat surface plane is parallel to the (0 0 1) plane and the pit wall surface is in the ð1 1¯ 1Þ plane of silicon. Three distinct parts of the film are divided by two arrows with respect to the c-plane
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Fig. 8. Lattice image of a step-edge area of a pit. The white lines mark the (0 0 0 1) plane of ZnO in the film, and the (0 0 1) and the (1 1 1) planes in silicon substrate. Two arrows denote the grain boundaries in the film that occur across the step-edge of the pit.
fringes marked by white lines, a part on the flat surface, a part on the pit wall surface and a part on the step edge. It is very evident that the c-plane of the film parts on the two types of surface is parallel to the surface plane. The part on the step-edge involves two sets of lattice planes penetrating into each other, as shown by the crossing lines. This indicates that the two grains represented by the two sets of lattice planes overlap in the viewing direction. Both the two sets of lattice planes exhibit a small angle deviation from their neighbouring parts. 4. Discussion The lattice images of sample A provide us with a strong evidence that strictly c-axis-oriented ZnO:Al crystals can be grown on a silicon wafer in spite of the amorphous native surface SiO2 layer, i.e. without influence of epitaxial growth. This leads to the conclusion that the film crystal orientation is not affected by the crystallographic structure of substrate, but a result of a self-ordering effect caused by the minimization of the crystal surface free energy as well as the interaction between the deposit material and the substrate surface. Such kind of crystallization phenomena is often called self-texture. The mechanism of self-texture control is based on the equilibrium state of the deposition. The conditions at the sputtered target are mainly determined by the power density and the partial pressure of the reactive gas. Generally, the reactive sputtering can be carried out in both metallic mode and oxide mode (Fig. 9). The metallic mode is characterized by a clean (metallic) target, understoichiometric film and high deposition rates. The oxide mode, on the other hand, is characterized by highly transparent films, but poisoned target and therefore low deposition rates. However, it is especially difficult in the
Fig. 9. Typical S-curve showing the dependence of reactive gas partial pressure po2 on discharge power P for the deposition of ZnO:Al films (ptot ¼ 400 mPa).
case of Al-doped ZnO films to hit the narrow process window for optimum TCO layer performance, because if the oxygen partial pressure is too high the dopant is oxidized and at too low oxygen partial pressure the films become substoichiometric, which results in absorbing layers [24]. To meet both the requirements for high deposition rates and highly transparent films, the deposition process has to be stabilized in the transition region during reactive sputtering, which is called transition mode. In our sputtering system, reactive gas partial pressure is taken as a controlling variable. The discharge power is adjusted according to the reactive gas partial pressure detected by a fast l-probe. By using a closed loop control, a set point (process point) ramp resulting in an S-shaped curve (so-called S curve, see Fig. 9) allowed us to stabilize the complete transition mode with gradually oxidized targets. The structure and orientation of our films A and B were found strongly dependent on the process mode. The c-axisoriented ZnO:Al film in sample A was obtained in transition mode that should be closer to a more equilibrium state. By reducing the oxygen partial pressure and adjusting the power density ZnO films were also deposited in a metallic mode. The ZnO:Al film of sample B was prepared under such a mode. The HRTEM image of sample B shows a random grain orientation at the beginning of the film formation. The (0 0 0 1) crystal planes of the ZnO crystallites are no more c-axis oriented and the film shows a more interrupted columnar film structure. The change of the film structure is believed to be related to the lower O2 partial pressure. At low O2 partial pressure, the oxygen content is too small to oxidize all of the
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Fig. 10. Crystal forms of a deposit on a substrate showing the influences of the surface energy and separation work.
sputtered metal atoms, but high enough to avoid the deposition of metallic film with a small oxygen contamination. O-deficient ZnO:Al films are formed, resulting in poor columnar structure with more defects in the film. In general, a textured film growth can, according to the established mechanisms, be realized either by an evolution competition process of the statistically oriented grains or by particle flux-determined selective growth process. In the former case, the so-called van de Drift mechanism, only grains with their fastest crystal growth direction parallel to the film growth direction can survive while other grains will be overgrown. In the latter case the grains with certain special orientation will be allowed/supported to grow by the ion impact while growth of other grains will be suppressed, due to a selective ion etching or re-sputtering process. In our case, the van de Drift mechanism can be ruled out due to the fact that all the initial ZnO crystallites are highly oriented, forming a columnar structure and no crystallites were overgrown during the growth process. In contrast to an ion beam sputtering no strictly oriented ion beam flux is available in the MF magnetron sputtering process, especially if no substrate bias potential is applied as in our experiment. Therefore a different mechanism must be considered to interpret the observed phenomenon. According to the generalized Wulff theorem, the thermodynamic equilibrium shape of a crystal is determined by the surface and, in the case of a heterogeneous crystal nucleation, by the interface energies [25]: Dm sj si sa b ¼ ¼ ¼ ... ¼ , 2u hab hj hi where Dm is the change in chemical potential during crystallization, u is the volume of one growth unit in the crystal, sa is the surface energy of the crystal face a, h is the distance from the Wulff point to the subscripted plane, as shown in Fig. 10, and b is the separation work of film crystal from substrate. It is known from the Wulff theorem that the dominant crystal faces are those with smallest surface energy. The crystal faces with the lowest free energy will have the tendency to grow parallel to the substrate (see Fig. 10). Since the (0 0 0 1) faces of ZnO have the lowest free energy (1.6 J/m2 for (0 0 0 1), 2.0 J/m2 for ð1 1 2¯ 0Þ, and 3.4 J/m2 for ð1 0 1¯ 0Þ, respectively) [26], they will try to grow parallel to the silicon surface if the process parameters are adjusted so that the deposition occurs at the thermodynamic equili-
brium. Growth on the faces of lowest energy requires the formation of two-dimensional nuclei of new layers or defects like screw dislocations. The steps (edges of the nuclei) flow parallel to the surface resulting in a net crystal growth in the c-axis direction. In fact, the self-texturing of a film may lead to a similar film structure as by particle flux-determined selective growth process. The difference in the two processes is however that in the latter case the columnar axis of film grains are parallel with ion flux while the self-texture is independent of the ion flux direction. Our HRTEM investigation of sample C rules out the ion flux effect. The self-texturing process can provide us with a method to design and manufacture a film microstructure which is of crucial importance for specific electronic and optical applications. Using a specially patterned substrate we can define the grain size, surface morphology and roughness of the deposited films for photovoltaic technology. The c-axis orientation of the film crystallites can be controlled on local areas on a micrometer scale which will have potential application in the micro-system techniques due to the isotropic piezoelectric properties of ZnO. 5. Conclusions The evident structure differences of ZnO:Al films prepared under different conditions on silicon wafers were revealed by the HRTEM investigation of the interfaces. A self-texture of the film grains with well-defined columnar morphology was obtained in transition mode. The growth of c-axis oriented grains is favored already in the nucleation stage due to the minimization of surface energy. The growth is strongly affected by the MF reactive magnetron sputtering modes of Zn–Al targets. On a patterned substrate the individual grains grow with their c-plane following the substrate surface planes that are in different orientation. In contrast, the films prepared in the metallic mode show a structure of randomly oriented nanoscale grains in the area close to the interface. Some columnar grains of the film are developed during the later stages of the film growth. Acknowledgement The authors would like to thank Dr. B. Szyszka for fruitful discussion. References [1] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 51 (1980) 3113. [2] C. Beneking, B. Rech, S. Wieder, O. Kluth, H. Wagner, W. Frammelsberger, R. Gever, P. Lechner, H. Schade, Thin Solid Films 351 (1999) 241. [3] Y. Akao, T. Haranoh, in: H. Pulker, H. Schmidt, M.A. Aegerter (Eds.), Proceedings of the Second International Conference on
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[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Coatings on Glass (ICCG), Elsevier Science, Amsterdam, 1999, p. 338. I. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. G. Granqvist, Appl. Phys. A 57 (1993) 19. D.L. Raimondi, E. Kay, J. Vac. Sci. Technol. 7 (1970) 96. F.S. Hickernell, J. Appl. Phys. 44 (1973) 1061. S. Ja¨ger, B. Szyszka, J. Szczyrbowski, G. Bra¨uer, Surf. Coat. Technol. 98 (1998) 1304. K. Machida, M. Shibutani, Y. Murayama, T.R. Matsumoto, IECE 62 (1979) 358. J. Exarhos, S.K. Sharma, Thin Solid Films 270 (1995) 27. J.S. Zabinski, J. Corneille, S.V. Prasad, N.T. McDevitt, J.B. Bultmann, J. Mater. Sci. 32 (1997) 313. T. Shiosaki, S. Ohnishi, A. Kawabata, J. Appl. Phys. 50 (1979) 3113. C. Galli, J.E. Cooker, Appl. Phys. Lett. 16 (1970) 439. V. Srikant, V. Sergo, D.R. Clarke, J. Am. Ceram. Soc. 78 (1995) 1931. C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, M. Wraback, H. Shen, J. Appl. Phys. 85 (1999) 2595.
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[16] Y. Yoshino, K. Inoue, M. Takeuchi, T. Makino, Y. Katayama, T. Hata, Vacuum 59 (2000) 403. [17] L. Sagalowicz, G.R. Fox, M.A. Dubois, C.A.P. Muller, P. Muralt, N. Setter, J. Euro. Ceram. Soc. 19 (1999) 1427. [18] I. Sieber, N. Wanderka, I. Urban, I. Do¨rfel, E. Schierhorn, F. Fenske, W. Fuhs, Thin Solid Films 330 (1998) 108. [19] T. Minami, H. Sato, H. Nanto, S. Takata, Thin Solid Films 176 (1989) 277. [20] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schope, C. Beneking, H. Wagner, A. Loffl, H.W. Schock, Thin Solid Films 351 (1999) 247. [21] J.C. Kim, E. Goo, J. Am. Ceram. Soc. 73 (1990) 877. [22] M.A. McCoy, R.W. Grimes, W.E. Lee, J. Mater. Res. 11 (1996) 2009. [23] L. Sagalowicz, G.R. Fox, J. Mater. Res. 14 (1998) 1876. [24] N. Malkomes, M. Vergo¨hl, B. Szyszka, J. Vac. Sci. Technol. A 19 (2001) 414. [25] R. Kern, G. Le Lay, J.J. Metois, in: E. Kaldis (Ed.), Current Topics in Material Science, Amsterdam, 1979. p. 138. [26] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Crystal Growth 130 (1993) 269.
Journal of Crystal Growth 289 (2006) 464–471 www.elsevier.com/locate/jcrysgro
Microstructure dependence of ZnO:Al films on the deposition conditions and the surface morphology of silicon substrate X. Jianga,, C.L. Jiab, R.J. Honga a
Institute of Materials Engineering, University of Siegen, Paul-Bonatz-Str. 9-11, 57068 Siegen, Germany b Institut fu¨r Festko¨rperforschung, Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany Received 20 October 2005; received in revised form 30 November 2005; accepted 1 December 2005 Available online 23 January 2006 Communicated by R. Kern
Abstract The microstructure of ZnO:Al films prepared under different conditions on silicon wafers was investigated by means of diffraction analysis and high-resolution transmission electron microscopy. It was found that the structure feature of the films is strongly dependent on the preparation conditions. The films obtained in a transition mode (higher oxygen partial pressure) show a structure of welldeveloped columnar grains with a defined texture with c-axis vertical to the substrate surface that can be in different orientations. These grains nucleate directly on the amorphous surface of the substrate and grow with a nearly unchanged lateral dimension through the full film thickness. The films obtained in a metallic mode (lower oxygen partial pressure) consist of randomly oriented grains. A thin nanocrystalline layer was observed close to the interface. Some grains with columnar feature are found to develop during the later stages of the film growth. The dependence of the film structure on the growth conditions is discussed in terms of surface energy of the sample system. r 2006 Elsevier B.V. All rights reserved. PACS: 68.37.Lp; 73.61.Ga; 81.15.Cd Keywords: A3. Physical vapor deposition processes; B1. Oxides; B1. Zinc compounds
1. Introduction ZnO thin films have received more attention in different fields of research due to the potential applications in photoelectronics, surface acoustic wave (SAW) devices as well as in solar cell technology [1–5]. In order to obtain desirable properties for different applications, ZnO thin films were prepared with and without doping on different substrates by various deposition techniques [6–13]. In general, the films on some oxide single crystalline substrates such as sapphire (Al2O3) show an epitaxial growth [14,15], while on most traditional substrates of semiconductors like silicon and GaAs the films are textural. Details of the film structure strongly depend on the deposition techniques, Corresponding author. Tel.:+49 271 7402966; fax: +49 271 7402442.
E-mail address: [email protected] (X. Jiang). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.067
processing conditions as well as the substrates [16,17]. On the silicon substrate, it was found that the concentration of dopant Al has a strong effect on the microstructure evolution of the films prepared by reactive DC magnetron co-sputtering technique [18]. In many cases, the films are required to be grown on a patterned substrate in order to meet the requirements from the fabrication technology of the devices or to obtain a special property. Therefore, the microstructure of the ZnO films developed on patterned substrates is also an interesting and important topic of research. Recently, the reactive mid-frequency magnetron sputtering (RMFMS) technique was developed suitably for reactive sputtering from metallic targets and for large area deposition. Promising results for ZnO:Al films are obtained. The films deposited at a substrate temperature of 200 1C with a deposition rate up to 9 nm/s exhibited low
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resistivity of 300 mO cm. Furthermore, they are more stable in reducing ambient than other transparent conductive oxide materials [19] and their surface topography can be modified by chemical etching [20], which makes them very attractive for photovoltaic applications. In the present work, we report the effects of the deposition conditions and the substrate surface on the microstructure of the Al-doped ZnO thin films prepared by RMFMS technique on silicon substrates. Detailed investigations of the microstructure of the films were performed by means of conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The evolution of the film microstructure is discussed in terms of a surface energycontrolled step-flow mechanism and the process modes.
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3. Results 3.1. Effect of the process modes Fig. 1 shows cross-sectional overviews of samples A (a) and B (b). The difference in grain morphology and configuration is very evident from the two images. In image (a) we see a clear columnar structure of the ZnO:Al film in the sample A. The boundaries between these grain columns look sharp and straight. No evident change occurs in the lateral dimension of individual column through the
2. Experimental details The ZnO:Al thin films were prepared by reactive mid frequency magnetron sputtering technique in a batch coater (Balzers-Pfeiffer, PLS 580) equipped with PK500 cathodes (Leybold, TwinMags) with Zn–Al (Al: 1.5 wt%) alloy targets. The reactive sputtering processes were carried out in both the transition mode and metallic mode in which a closed loop control system was established for the adjustment of discharge power according to reactive gas partial pressure detected by a l-sensor. Details of this process control are discussed in the text and other deposition conditions are summarized in Table 1. Si wafers were used as substrates. The samples A and B were deposited on flat (0 0 1) silicon substrates in transition mode and in metallic mode, respectively. In the sample C the ZnO:Al film was deposited on a patterned silicon substrate with surface parallel to either (0 0 1) or {1 1 1} crystallographic planes. Both cross-sectional and plan-view specimens were prepared for TEM and HRTEM investigations by a traditional procedure that includes grinding, dimpling and finally ion-milling. The TEM and HRTEM investigations were performed on a JEOL 4000EX microscope operated at 400 kV.
Table 1 Parameters for reactive MF magnetron sputtering (RMFMS) of ZnO:Al layers in transition and metallic modes Sample
A, C
B
Process mode O2 partial pressure (mTorr) Total gas pressure (mTorr) MF power (kW) Target voltage (V) Target/substrate distance (mm) Substrate temperature (1C) Deposition rate (nm/s) Deposition time (s)
Transition mode 0.25 1.45 3.5–5.5 360 90 150 3–4 170
Metallic mode 0.15 1.35 4.5 430 90 150 3–4 120
Fig. 1. Cross-sectional overview of sample A (a) and sample B (b).
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Fig. 2. Electron diffraction patterns from sample A (a) and sample B (b).
film thickness. In contrast, the grains in the sample B (Fig. 1b) show a less dominant columnar feature. The continuity of the grain columns through the film thickness is in most cases disturbed by secondary nucleation of grains during the film growth. In this sample a column may include several grains with different orientations. The orientations of the columnar grains in the two samples were investigated by selected-area electron diffraction analysis. Fig. 2 shows the electron diffraction patterns (EDPs) recorded with electron beam parallel to the [1 1 0] zone axis of silicon from cross-sectional specimens of (a) the sample A and (b) the sample B. Each of the patterns includes the crystallographic information of both the film and the substrate obtained using a selected-area aperture covering the film and part of the substrate. In both the EDPs the reflection spots of single crystal silicon are
identical and some of the reflection spots are indexed. The reflection patterns of the films are different. The EDP in Fig. 2(a) consists of reflection spots of the film A that are identified as those belonging to different zone axes with an included common direction row of the (0 0 0 l) reflection spots parallel to the (0 0 l) spot row of silicon. A (0 0 0 4) common reflection spot is indexed. Further investigation by moving the aperture along the interface between the film and the substrate to different areas has led to similar results. This indicates that the columnar grains in film A are generally oriented with their c-plane parallel to the surface plane of the substrate. We note that the (0 0 0 l) spots of these columnar grains are elongated, indicating a small deviation of some grains from the exact parallel orientation relationship. In the EDP (b) of film B we see, instead of spots, arcs consisting of spots. These arcs demonstrate a large angle deviation of the c-plane from the substrate surface. The diffraction analysis of the plan-view specimens also showed a (0 0 0 1) oriented feature for the grains in the film A and a configuration of randomly oriented grains in the film B. Fig. 3 shows lattice images of the film A (a) and the film B (b) in a cross-sectional view. The two images were recorded with the electron beam parallel to the [1 1 0] zone axis of the silicon substrate. Both images show the typical features of the grains along the interface area in the two samples, respectively. A thin amorphous layer is clearly seen along the interfaces. This layer covers the crystal surface of silicon substrate with a homogeneous thickness. The difference in the microstructure, i.e. the orientation of the ZnO grains, between the two films close to the interfaces can also be recognized although the structural feature of the substrate surface looks the same for the two samples. The white arrows denote the [0 0 0 1] direction of the grains and the black arrows mark the grain boundaries. In image (a) three ZnO grains are presented to have approximately a same c-plane orientation, while in image (b) two grains of the three grains show a large deviation of their c-plane from the surface plane of the substrate. In the film A the majority of the grains are found to nucleate directly on the substrate surface with their c-plane parallel to the surface plane of the substrate. During growth these grains keep their orientation unchanged up to the film surface. Fig. 4 shows an image of a columnar grain. As can be seen in the image, this grain does not only keep its orientation during growth, but also keeps its lateral dimension. The grain boundaries with the adjacent grains are marked with two arrows. In the film B small grains with random orientation are often found close to the interface and are overgrown by other grains with increase in the film thickness. Lattice defects and lattice distortion are commonly observed in the grains and at the grain boundaries in both the samples. The most common defects are the stacking faults or inversion domain boundaries occurring in the c-plane [21–23]. One of the stacking faults is indicated by a thick arrow in Fig. 4.
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Fig. 3. Lattice images of the interface area between the film and the substrate in sample A (a) and sample B (b). The images were recorded with the electron beam parallel to the [1 1 0] of silicon.
We also found that the film evolution is rather interesting, featuring a cone-shaped grain surface. The SEM micrographs of the film shown in Fig. 5 clearly reveal a regular cone-shaped grain surface and columnar structure. The sample was prepared under the similar deposition conditions used for the sample A. The grain columns are tightly packed and grow through the entire film. The columns are vertical to the substrate, showing preferred c-axis orientated growth. 3.2. Effect of surface morphology of the substrate In order to investigate the effect of the substrate surface morphology and surface orientation on the microstructure of the film and on the grain orientation, we prepared a silicon substrate with a pattern of many pits on the surface. The pit is surrounded by four {1 1 1} plane walls as shown schematically in Fig. 6. Fig. 7a shows a low magnification image of the film across an upper step edge area of a pit. The inset schematically marks the position of the pit. The initial surface area of the film on the edge was removed by
Fig. 4. A lattice image of a columnar grain in sample A. The vertical arrows mark the grain boundaries in the film and the lateral arrow denotes a stacking fault.
the ion-milling thinning procedure. In the area adjacent to the substrate we see the textural feature of the grains that changes its direction across the step-edge to maintain the
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Fig. 7. (a) A low magnification image of the ZnO film deposited on the patterned silicon substrate shown in Fig. 6. The inset shows the surface position from which the image has been taken. (b) A diffraction pattern from the pit wall area. (c) A diffraction pattern from the film area on the flat surface of silicon. Fig. 5. SEM micrographs of ZnO:Al film. The upper image shows the surface morphology, whereas the lower image shows the cross-sectional microstructure of the film.
Fig. 6. Schematic drawing of the pits patterned on the silicon surface.
normal direction to the substrate surface. The orientation relation between the grains and the substrate can be clearly seen in the diffraction patterns from different film areas on the flat surface and on the pit wall surface. Figs. 7b and c
are the diffraction patterns from a film area on the pit wall and an area on the flat surface, respectively. In pattern (c) we see a same configuration of the reflection spots of the film and the substrate as that of Fig. 2a. The grains of the film on the flat surface are oriented with their c-plane parallel to the surface plane of substrate. This is expected since we used the same conditions for deposition of films A and C. In the pattern (b), however, the sets of reflection spots from different grains rotate by 54.71 around the [1 1 0] zone axis of silicon. The (0 0 0 l) spots are in this pattern in line with the ðl ¯l lÞ spots of silicon. Therefore, the film grains of the film part on the pit wall exhibit in general an arrangement with the c-plane parallel to the ð1 1¯ 1Þ plane of silicon, i.e. the surface plane of the pit wall. Fig. 8 shows a lattice image of the step-edge area viewed along the [1 1 0] direction of silicon. From this lattice image we see again an amorphous layer that looks thicker on the pit wall than on the flat crystal surface. The flat surface plane is parallel to the (0 0 1) plane and the pit wall surface is in the ð1 1¯ 1Þ plane of silicon. Three distinct parts of the film are divided by two arrows with respect to the c-plane
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Fig. 8. Lattice image of a step-edge area of a pit. The white lines mark the (0 0 0 1) plane of ZnO in the film, and the (0 0 1) and the (1 1 1) planes in silicon substrate. Two arrows denote the grain boundaries in the film that occur across the step-edge of the pit.
fringes marked by white lines, a part on the flat surface, a part on the pit wall surface and a part on the step edge. It is very evident that the c-plane of the film parts on the two types of surface is parallel to the surface plane. The part on the step-edge involves two sets of lattice planes penetrating into each other, as shown by the crossing lines. This indicates that the two grains represented by the two sets of lattice planes overlap in the viewing direction. Both the two sets of lattice planes exhibit a small angle deviation from their neighbouring parts. 4. Discussion The lattice images of sample A provide us with a strong evidence that strictly c-axis-oriented ZnO:Al crystals can be grown on a silicon wafer in spite of the amorphous native surface SiO2 layer, i.e. without influence of epitaxial growth. This leads to the conclusion that the film crystal orientation is not affected by the crystallographic structure of substrate, but a result of a self-ordering effect caused by the minimization of the crystal surface free energy as well as the interaction between the deposit material and the substrate surface. Such kind of crystallization phenomena is often called self-texture. The mechanism of self-texture control is based on the equilibrium state of the deposition. The conditions at the sputtered target are mainly determined by the power density and the partial pressure of the reactive gas. Generally, the reactive sputtering can be carried out in both metallic mode and oxide mode (Fig. 9). The metallic mode is characterized by a clean (metallic) target, understoichiometric film and high deposition rates. The oxide mode, on the other hand, is characterized by highly transparent films, but poisoned target and therefore low deposition rates. However, it is especially difficult in the
Fig. 9. Typical S-curve showing the dependence of reactive gas partial pressure po2 on discharge power P for the deposition of ZnO:Al films (ptot ¼ 400 mPa).
case of Al-doped ZnO films to hit the narrow process window for optimum TCO layer performance, because if the oxygen partial pressure is too high the dopant is oxidized and at too low oxygen partial pressure the films become substoichiometric, which results in absorbing layers [24]. To meet both the requirements for high deposition rates and highly transparent films, the deposition process has to be stabilized in the transition region during reactive sputtering, which is called transition mode. In our sputtering system, reactive gas partial pressure is taken as a controlling variable. The discharge power is adjusted according to the reactive gas partial pressure detected by a fast l-probe. By using a closed loop control, a set point (process point) ramp resulting in an S-shaped curve (so-called S curve, see Fig. 9) allowed us to stabilize the complete transition mode with gradually oxidized targets. The structure and orientation of our films A and B were found strongly dependent on the process mode. The c-axisoriented ZnO:Al film in sample A was obtained in transition mode that should be closer to a more equilibrium state. By reducing the oxygen partial pressure and adjusting the power density ZnO films were also deposited in a metallic mode. The ZnO:Al film of sample B was prepared under such a mode. The HRTEM image of sample B shows a random grain orientation at the beginning of the film formation. The (0 0 0 1) crystal planes of the ZnO crystallites are no more c-axis oriented and the film shows a more interrupted columnar film structure. The change of the film structure is believed to be related to the lower O2 partial pressure. At low O2 partial pressure, the oxygen content is too small to oxidize all of the
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Fig. 10. Crystal forms of a deposit on a substrate showing the influences of the surface energy and separation work.
sputtered metal atoms, but high enough to avoid the deposition of metallic film with a small oxygen contamination. O-deficient ZnO:Al films are formed, resulting in poor columnar structure with more defects in the film. In general, a textured film growth can, according to the established mechanisms, be realized either by an evolution competition process of the statistically oriented grains or by particle flux-determined selective growth process. In the former case, the so-called van de Drift mechanism, only grains with their fastest crystal growth direction parallel to the film growth direction can survive while other grains will be overgrown. In the latter case the grains with certain special orientation will be allowed/supported to grow by the ion impact while growth of other grains will be suppressed, due to a selective ion etching or re-sputtering process. In our case, the van de Drift mechanism can be ruled out due to the fact that all the initial ZnO crystallites are highly oriented, forming a columnar structure and no crystallites were overgrown during the growth process. In contrast to an ion beam sputtering no strictly oriented ion beam flux is available in the MF magnetron sputtering process, especially if no substrate bias potential is applied as in our experiment. Therefore a different mechanism must be considered to interpret the observed phenomenon. According to the generalized Wulff theorem, the thermodynamic equilibrium shape of a crystal is determined by the surface and, in the case of a heterogeneous crystal nucleation, by the interface energies [25]: Dm sj si sa b ¼ ¼ ¼ ... ¼ , 2u hab hj hi where Dm is the change in chemical potential during crystallization, u is the volume of one growth unit in the crystal, sa is the surface energy of the crystal face a, h is the distance from the Wulff point to the subscripted plane, as shown in Fig. 10, and b is the separation work of film crystal from substrate. It is known from the Wulff theorem that the dominant crystal faces are those with smallest surface energy. The crystal faces with the lowest free energy will have the tendency to grow parallel to the substrate (see Fig. 10). Since the (0 0 0 1) faces of ZnO have the lowest free energy (1.6 J/m2 for (0 0 0 1), 2.0 J/m2 for ð1 1 2¯ 0Þ, and 3.4 J/m2 for ð1 0 1¯ 0Þ, respectively) [26], they will try to grow parallel to the silicon surface if the process parameters are adjusted so that the deposition occurs at the thermodynamic equili-
brium. Growth on the faces of lowest energy requires the formation of two-dimensional nuclei of new layers or defects like screw dislocations. The steps (edges of the nuclei) flow parallel to the surface resulting in a net crystal growth in the c-axis direction. In fact, the self-texturing of a film may lead to a similar film structure as by particle flux-determined selective growth process. The difference in the two processes is however that in the latter case the columnar axis of film grains are parallel with ion flux while the self-texture is independent of the ion flux direction. Our HRTEM investigation of sample C rules out the ion flux effect. The self-texturing process can provide us with a method to design and manufacture a film microstructure which is of crucial importance for specific electronic and optical applications. Using a specially patterned substrate we can define the grain size, surface morphology and roughness of the deposited films for photovoltaic technology. The c-axis orientation of the film crystallites can be controlled on local areas on a micrometer scale which will have potential application in the micro-system techniques due to the isotropic piezoelectric properties of ZnO. 5. Conclusions The evident structure differences of ZnO:Al films prepared under different conditions on silicon wafers were revealed by the HRTEM investigation of the interfaces. A self-texture of the film grains with well-defined columnar morphology was obtained in transition mode. The growth of c-axis oriented grains is favored already in the nucleation stage due to the minimization of surface energy. The growth is strongly affected by the MF reactive magnetron sputtering modes of Zn–Al targets. On a patterned substrate the individual grains grow with their c-plane following the substrate surface planes that are in different orientation. In contrast, the films prepared in the metallic mode show a structure of randomly oriented nanoscale grains in the area close to the interface. Some columnar grains of the film are developed during the later stages of the film growth. Acknowledgement The authors would like to thank Dr. B. Szyszka for fruitful discussion. References [1] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 51 (1980) 3113. [2] C. Beneking, B. Rech, S. Wieder, O. Kluth, H. Wagner, W. Frammelsberger, R. Gever, P. Lechner, H. Schade, Thin Solid Films 351 (1999) 241. [3] Y. Akao, T. Haranoh, in: H. Pulker, H. Schmidt, M.A. Aegerter (Eds.), Proceedings of the Second International Conference on
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