Modified Structure Zone Model to Describe the Morphological Evolution of ZnO Thin Films Deposited by Reactive Sputtering Eugenia Mirica,† Glen Kowach,‡ and Henry Du*,†
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 1 157-159
Department of Chemical, Biochemical and Materials Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, and Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974 Received October 4, 2002;
Revised Manuscript Received July 2, 2003
ABSTRACT: The morphological evolution of zinc oxide (ZnO) thin films deposited by magnetron sputtering is described by the use of a Structure Zone Model. A modified Structure Zone Model was revealed, in which the boundaries between zones with specific features are shifted toward lower homologous temperatures (T/Tm) than in the classical models. The range of homologous temperatures for this study were 0.13 < T/Tm < 0.43. The promotion of the formation of high temperature structures at relatively low temperatures is a consequence of the energetic species generated during the sputtering process that bombard the growing film. The reduction of the shadowing effect, along with the substrate heating that increases the surface diffusion, led to the suppression of zone T. Two new subzones of zone II were identified, IIa and IIb. The film in subzone IIa displays pronounced faceting. The film in subzone IIb has a characteristic smooth surface due to enhanced surface diffusion at higher substrate temperatures during deposition, although pitted due to still incomplete surface diffusion. Introduction The Structure Zone Models have been extensively used to classify the morphology of thin films deposited by physical vapor deposition techniques. The analysis of the fundamental phenomena responsible for structure forming and evolution of the films and their dependence on the processing parameters enable the construction of Structure Zone Models. The initial model proposed by Movchan and Demchishin1 (MD) describes the morphology of evaporated metallic films in terms of reduced temperature T/Tm, where T is the deposition temperature and Tm the melting temperature of the deposited material, both in Kelvin. The diagram consists of three zones, each with its own characteristic structure. According to this diagram, the film microstructure and texture are controlled by the shadowing effect in zone I, and the film is columnar with tapered voids between columns. In zone II, surface diffusion is the leading process in morphological evolution, and the film consists of columnar grains with defined grain boundaries and increased grain width. In zone III, the film evolution is governed by bulk diffusion, and the structure consists of equiaxed grains. Thornton2, further extending the initial MD model, added the deposition pressure as a parameter on the Structure Zone Model to describe the effect of adatom mobility induced by energetic particle bombardment in a sputtering system. In Thornton’s model, an additional transition zone T between zones I and II is shown, in which the surface diffusion governs the films morphology as in zone II, and the films are smooth and dense, but still preserving a fibrous texture, as in zone I. The original MD and the modified Thornton’s model * Corresponding author. E-mail:
[email protected]. Tel: (201) 216-5464. Fax: (201) 216-8306. † Stevens Institute of Technology. ‡ Lucent Technologies.
are still widely used to roughly estimate thin film morphology deposited under specific conditions. The apparent universality of these two models seems to stem from their simplicity. However, certain morphological features could not be found in Thornton’s diagram, especially when additional factors affecting sputtering conditions are taken into account such as sputtering power or reactive sputtering (sputtering in the presence of a reactive gas). Messier et al.3 have further developed the Structure Zone Model by considering the evolution of film morphology and texture as a function of the substrate voltage bias and film thickness. They proposed a Structure Zone Model for thick films showing the effect of both bombardment- and thermalinduced mobility. With increasing bombardment and/ or temperature, the rate of evolution of dominant morphology (i.e., average size of morphological features) decreases. Also, the morphological evolution with increasing film thickness has been recognized in their study in a three-parameter evolutionary Structure Zone Model for the low mobility range (T/Tm < 0.5) in which the dominant surface morphology size is nearly linear with ∼3/4 power of the film thickness. Governor et al.4 proposed a zone model for the grain structure of coevaporated metallic films considering more the grain boundary diffusion than the surface diffusion to control the grain structure in zones T and II. They proposed a mechanism for the development of thin and thicker Ni films based on the dual process of grain growth and granular epitaxy, which, they claim, allows a new interpretation for the MD and Thornton models described previously. More recently, Barna and Adamik5 constructed a real structure zone model for polycrystalline metallic films accounting for the effect of impurities in the film as a new deposition parameter responsible for the structure evolution in the films. They proposed real structure zone models at low, medium, and high impurity content. The higher the imputities
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content, the more enhanced the tendency of imputities segregation at grain boundaries, and hence the smaller the diameter of the grains. Kelly and Arnell6 also published a structure zone model for Al, Zr, and W films deposited in a closed-field unbalanced magnetron sputtering system (CFUBMS), and the coating structures are described in terms of homologous temperature, bias voltage, and the ion-to-atom ratio incident at the substrate. They observed the same structures as in the Thornton model but obtained at homologous temperatures well below those required for sputtering systems. This model probably reflects accurately the parameters that control the structure of coatings deposited using CFUBMS systems. It seems important also because the ion energy and the ion flux can be considered separately. Studies of Yahashi et al.7 revealed a Structure Zone Model applied to Al deposited on SiO2 by the ionized cluster beam (ICB) method. The results are in general in accordance with MD and Thonton models. However, the effect of increasing acceleration voltage during ICB deposition has been recognized as determining the film structure. Ellmer8 proposed a complex structure-phase zone model for reactive sputtering of transparent Al-doped ZnO having as deposition parameters oxygen partial pressure and the deposition rate. The diagram is intended to relate the phase composition, the mechanical stress, and the transparency for establishing the technological process window for depositing transparent conductive ZnO. A zone model for ZnO deposited by combustion chemical deposition has been proposed by Polley and Carter.9 The microstructures of the deposited films were divided into three general regions: As amorphous, cauliflowerlike; Tscrystalline materials with small grains; and IIswell-defined columnar growth. The model is interesting, but it has a limited applicability. There are many factors influencing the Structure Zone Models and therefore microstructure: energetic particle bombardment, local defects on the substrate (leading to preferential nucleation or an enhanced shadowing effect), and the nature of the gaseous species (especially the bond strength). As shown earlier, Structure Zone Models for the metallic system have been intensely investigated. However, oxide systems demonstrate a morphological development that is by far more complicated than in metallic systems due to oxidation rates and to the partial pressure of O2, which affects especially the stoichiometry of the films, with implications on their physical properties. These factors are in addition to the well-known processes for the deposition of any material, for example, nucleation process, crystal growth, surface and bulk diffusion, etching rates, etc. The aim of this study is to develop a Structure Zone Model for ZnO thin films deposited by reactive sputtering using the information gained during the detailed investigation of morphological evolution of the films. A detailed description for the motivation of sputtered ZnO films having fiber texture can be found in the corresponding article by the authors.10 Experimental Procedures Thin films of zinc oxide (ZnO) were deposited on Si-based substrates covered with a (111) textured Pt film by reactive sputtering of a Zn target. The details of the sputtering
Mirica et al.
Figure 1. Comparison between various published Structure Zone Models and the modified Structure Zone Model derived in this study. Detail on the microstructure of ZnO films deposited in this study is presented on the bottom; the formation of high temperature structure at relatively low homologous temperatures has been promoted. parameters were described in the corresponding article.10 Most of the influential parameters were kept constant to highlight the effect of the substrate temperature during deposition. Substrate temperatures ranged from near room temperature (45 °C) to 700 °C.
Results and Discussion The ZnO films in this study were deposited at homologous temperature in the range of T/Tm ) 0.13 to 0.43. These values would correspond to structure zones 1 and T in Thornton’s Structure Zone Diagram. However, the evolution of the structural features of the ZnO thin films deposited in this study and presented in details in the corresponding paper revealed a modified Structure Zone Model.10 It has been shown in the corresponding paper10 that ZnO thin films deposited by reactive sputtering show different morphological features depending on the substrate temperature and substrate type. When deposited on (111) textured Pt and temperatures higher than 500 °C, the films show enhanced crystallinity and texture due to the nonoxide forming nature of the substrate and enhanced surface diffusion. The ZnO films deposited at low temperatures have a high density of stacking faults lying in planes parallel to the growth direction, whereas films deposited at higher temperatures show the presence of dislocations. The mechanistic model for morphological evolution of the ZnO films involves the interplay between atomic mobility, flux of incoming particles, and rates of nucleation and crystalline growth. The modified 2-D microstructure zone diagram for reactive sputtered ZnO thin films deposited on (111) textured Pt based on the results from this study is presented in Figure 1 against some of the celebrated diagrams cited in the literature. It can be seen that the boundaries between the investigated zones in this study are shifted toward lower substrate temperatures. Even more importantly, two new subzones of the structure zone II (IIa and IIb) with new features were revealed. The films in subzone IIa display pronounced faceting, which confers a very rough top surface. Subzone IIb
Evolution of ZnO Thin Films by Reactive Sputtering
features films with a smooth but pitted top surface due to incomplete surface diffusion, yet having a very prominent texture and high density. The pit density decreases when the surface temperature increases, as expected, since the adatom mobility increases with temperature. In this scenario, the atoms have greater kinetic energy to diffuse on the surface, and therefore, enhanced ability to fill the “valleys” created on the film surface between the columnar grains. It is worth noting that the temperature window for obtaining films with markedly different morphologies in this study is not more than 100 °C. As it can be seen, deposition under the conditions in this study has promoted the formation of a high temperature structure at relatively low homologous temperatures. The shift of boundaries between specific morphologies toward lower temperatures than in the Thornton diagram is a consequence of the energetic species generated during the sputtering process. The reduction of shadowing effect, along with the substrate heating that increased the surface diffusion, led to the suppression of zone T. ZnO films with features corresponding to zone III were not obtained in the deposition conditions used in this study. Conclusions A modified Structure Zone Model was revealed, in which the boundaries between zones with specific features are shifted toward a lower T/Tm than in the classical models. Moreover, two new subzones of zone II were identified (IIa and IIb). The film in subzone IIa
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displays a pronounced faceting, while the film in subzone IIb has a smooth but pitted surface due to incomplete surface diffusion, although the film texture is still preserved. Therefore, deposition in the conditions in this study has promoted the formation of high temperature structure at relatively low homologous temperatures. The shift of boundaries between specific morphologies toward lower temperatures is a consequence of the energetic species generated during the sputtering process. The reduction of shadowing effect, along with the substrate heating that increased the surface diffusion, led to the suppression of zone T. References (1) Movchan, B. A.; Demchishin, A. V. Phys. Met. Mettalogr. 1969, 28, 83. (2) Thornton, J. A. J. Vac. Sci. Technol. 1986, A4 (6), 3059. (3) Messier, R.; Giri, A. P.; Roy, R. A. J. Vac. Sci. Technol. 1984, A2 (2), 500. (4) Governor, C. R. M.; Hentzell, H. T. G.; Smith, D. A. Acta Metall. 1984, 32 (5), 773. (5) Barna, P. B.; Adamik, M. Thin Solid Films 1998, 317 (12), 27. (6) Kelly, P. J.; Arnell, R. D. J. Vac. Sci. Technol. 1998, A16 (15), 2858. (7) Yanashi, A.; Levenson, L. L.; Usui, H.; Yamada, I. Appl. Surf. Sci. 1989, 43, 37. (8) Ellmer, K. J. Phys. D: Appl. Phys. 2000, 33 (4), R17. (9) Polley, T. A.; Carter, W. B. Thin Solid Films 2001, 384, 177. (10) Mirica, E.; Kowach, G.; Evans, P.; Vaudin, M.; Du, H. Cryst. Growth Des. 2004, 1, 147-156.
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