Epitaxial growth and surface morphology of aluminum films deposited on mica studied by transmission electron microscopy and atomic force microscopy
Introduction
Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) provide clear surface images of deposited metal thin films in situ and non-destructively with a high spatial resolution on the nanometer and angstrom scale [1], [2], [3]. These techniques also enable us to evaluate the surface roughness of the metal films and characterize the surface morphology quantitatively. However, the finite tip size of STM and AFM may distort the profiles of finely-structured surfaces [4], [5], [6]. Since this tip artifact often occurs in the imaging of metal films with columnar structures, the images of the films must be carefully interpreted [7], [8], [9], [10].
Metal films prepared by physical vapor deposition under various conditions reveal characteristic microstructures and physical properties. The effect of the deposition conditions on the structures and the properties of the metal films has been explained by structure zone models [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The film structure is strongly dependent on the substrate temperature (T). A metal film deposited on a substrate with a temperature gradient has three characteristic structure zones with boundary temperatures T1 = 0.3Tm and T2 = 0.5Tm, where Tm (K) is the melting point of the metal [11]. Films in the first zone are characterized by the formation of tapered columns with domed tops separated by voids. These columns with the characteristic morphology are caused by self-shadowing during the deposition process, because the surface diffusion of adatoms is too small to diffuse into the shadowed regions at these temperatures. Films in the second zone have smooth matt surfaces. In this zone, the surface diffusion of adatoms is sufficient and leads to surface recrystallization. There occurs a coarsening of the internal structure and a change to a more clearly-defined columnar structure at around T1. The width of the crystals increases with temperature, and the resulting films have well-defined grain boundaries and a columnar orientation. Films in the third zone have a polyhedral surface structure and consist of equiaxed grains with bright surfaces. Bulk diffusion and recrystallization have a dominant influence on the film structure.
Mica is an easily-cleavable substrate, which yields an atomically-smooth surface, and is crystallographically stable up to 700 °C [24], [25]. Mica is a useful substrate for preparing metal films and observing the surface morphology. The elimination of surface contaminants resulting from cleavage in air [24] and the dehydration and depletion of water [25], [26], [27] following vacuum heat treatment were investigated. Capillary forces arise from the adsorbed water between the tip and the surface [28] and the condensation and evaporation of a monolayer of water on the atomically-smooth surface [29], [30] after cleavage in air were reported by AFM. The surface modification of mica with radio-frequency plasma discharges were investigated by AFM [31], [32]. Unstable steps much lower (0.1 nm) than the unit cell height (1.0 nm) of mica formed due to the presence of domains of residual K+ ions on the freshly-cleaved surfaces, and these were observed by AFM [33]. The surface chemical composition of the freshly-cleaved mica was characterized by X-ray photoelectron spectroscopy [34], [35], [36]. Mica, which was freshly cleaved in air and cleaned by use of an argon dc glow discharge and heat treatment in high vacuum, was used as a substrate in the present study.
The epitaxial growth and bulk structure of deposited Al films on various substrates were investigated by electron diffraction and electron microscopy [37], [38], [39], [40], [41], [42], [43], [44], [45]. The surface morphology of the deposited Al films was also studied by a replica technique [39], [40]. While these techniques are useful for characterizing many of the features of metal thin films, it is difficult to obtain surface morphology and roughness data by use of these traditional techniques [37], [38], [39], [40], [41], [42], [43], [44], [45]. Since the surface properties of thin films and surface-oxidized films of Al influence properties such as coating [46], friction [47], wear [48], light scattering [49], [50], hardness [51], electronic device performance [52], etc., it is important to obtain information on the surface morphology for controlling these film properties. The correlation between the epitaxy and the surface morphology of Al films is not clear. The motivation for the present study is to observe the epitaxial growth and bulk structure of Al films deposited on mica, and to relate the surface morphology of the films to the epitaxy.
Plasma oxidation of Al produces a dense self-limiting growth oxide film that is chemically similar to the alumina used in industry [53], [54], [55], [56], [57]. The oxidized Al films are useful for adsorption studies of various chemical species on alumina by inelastic electron tunneling spectroscopy [58], [59], [60], [61], [62], [63]. We have already reported the preparation and characterization of very smooth Al films on mica on the nanometer scale over a wide surface area [64], [65]. Such atomically-smooth films are important and useful in modern surface science techniques in order to study adsorption from submonolayer to multilayer coverage. The fabrication of atomically-smooth Al films has enabled us to not only observe the detailed morphology of tetracyanoquinodimethane (TCNQ) deposited from various solutions, but also to obtain the volumes of deposited TCNQ on the oxide surfaces [66], [67], [68], [69].
In the present paper, we report a thorough transmission electron microscopy (TEM), transmission electron diffraction (TED), and AFM study of the deposited Al films on mica in the wide temperature range from 16 °C (T/Tm = 0.31) to 550 °C (T/Tm = 0.88), where T and Tm are the temperatures (K) of the mica and the melting point (934 K) of Al, respectively. The surface morphology of these Al films was studied based on the structure zone model. The temperature range for the epitaxial growth of Al films on mica was determined. The correlation between the epitaxy and surface morphology of the films was investigated.
Section snippets
Experimental details
Aluminum films with thicknesses of 149–154 nm were prepared on mica substrates heated at various temperatures (Hakko HLF 1201) under high vacuum. The metal deposition was conducted in a metal bell-jar (40 cm in diameter and 30 cm in height) evaporator equipped with a liquid-nitrogen-trapped 6 in. diffusion-pump, as reported previously [64], [65], [66], [67], [68], [69], [70]. The details of the apparatus and procedures have been described in our previous paper [70]. The temperature of the mica
Bulk structure and epitaxial growth of Al films
Bright-field and dark-field TEM micrographs and TED patterns of the Al films prepared on mica at various temperatures up to 550 °C (T/Tm = 0.88) were taken in order to observe the bulk structures and epitaxial growth, where T and Tm are the temperatures (K) of the mica and the melting point (934 K) of Al, respectively. The typical bright-field TEM micrographs and TED patterns taken with two aperture settings (diameters of the observation areas: 0.5 and 8.6 μm) are shown in Fig. 1. The average
Conclusions
The bulk structure and surface morphology of Al films with a thickness of 150 nm prepared on mica at various temperatures under high vacuum by thermal evaporation were investigated. The films prepared at room temperatures consist of grains having a diameter of 90 ± 40 nm. The grains are preferentially oriented along the (111) planes parallel to the substrate and have a preferred in-plane orientation. This is mainly caused by the self-shadowing effect and the surface morphology is characterized by
Acknowledgements
We thank Miss. N. Sawamura, Mr. S. Higashi, and Mr. Y. Tanaka for their cooperation. The present study was partially supported by a Grant-in-Aid for Scientific Research (No.16550080) from the Ministry of Education, Culture, Sports, Science, and Technology, Iketani Science and Technology Foundation (No.0161022-A), and the Light Metal Educational Foundation.
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