H. MULLER : Simulation of Oriented Crystallization in Evaporated Sb Films

phys. stat. sol. (a) 70, 249 (1982)

Subject classification: 1.5 and 3; 21.7

249

Zentralinstitut fur Optik und Bpektroskopie der Akadernie der Wissenschaften der DDR, Bereich Strahlungsempfanger, Jenal)

Simulation of Oriented Crystallization in Evaporated Antimony Thin Films BY H. MULLER

Evaporated antimony films are normally characterized by a spherulitic growth process. Thereby the crystallization takes place spontaneously from an amorphous phase. The orientation of the film after the phase transition is caused by two different factors of influence. In the present paper this process is illustrated by geometrical model representations. The validity of the models involved is proved by comparison with real structures.

Antimon-Aufdampfschichten zeigen in der Regel spharolithisches Wachstum. Dabei erfolgt die Kristallisation spontan aus einer amorphen Phase. Die Orientierung der Sohichtpartikel nach dem Phasenubergang wird durch zwei unterschiedliche EinfluSfaktoren ausgelost. In der vorliegenden Arbeit wird dieser Vorgang anhand von Modellvorstellungen erliiutert, deren Gultigkeit durch Gegenuberstellung mit Realstrukturen bestitigt werden.

1. Introduction

During formation of antimony thin films on amorphous substrates first of all a lattice- like amorphous phase condenses [l] if the substrate temperature is below 420 K [2]. This intermediate phase remains stable up to a critical mass coverage [l, 31. Then a transition into the crystalline state occurs. This phase transition is dependent on the deposition conditions especially the deposition rate and the substrate temperature [4] and may be collective or individual [5 ] .

After a collective phase transition the film consists of a very large number of plate- like crystallites arranged in a spherulitic manner. The corresponding electron diffrac- tion pattern is expected to show well textured rings. But in most of all cases an ex- cellent spot pattern is found. This means the orientation of the crystallites is not only with the close-packed plane parallel to the substrate surface but also perfect in the azimuth.

In a previous published paper [6] models and real structures for external spherulite boundaries have been confronted. The aim of this paper is to simulate the oriented crystallization of spherulites.

2. Orientation Formation and Geometrical Models

The simulation is based on the following assumptions : The phase transition starts a t statistically distributed crystallization centres.

Around each centre a temperature field builds up. This field is caused by the heat of crystallization. On the one hand, orientation takes place due to the influence of nearest neighbours, on the other hand, by a central force originating from the tem- perature gradient, internal stress caused by density differences between amorphous and crystalline antimony 171, etc.

I) HumboldtstraDe 11, DDR-6900 Jena, GDR.

250 H. M~LLEB

Fig. 1. Models for orientation of antimony clusters at the phase transition amorphous crystal- line: a) strong osentation by nearest neighbours, b) strong orientation by & central force field, c) and d) combinations of orientations after a) and b) with different influence of both crystalli- zation types

Simulation of Oriented Crystalliza.tion in Evaporated Sb Thin Films 251

Antimony has an equilibrium rhombohedra1 structure which can also be described in the hexagonal system [S]. Therefore, we took hexagonal building elements in order to construct the film model.

Fig. 1 a shows a hexagonal crystallization centre with perfect oriented crystallites around. The result is an ideal (111)-plane with six-fold symmetry which can be ob- served often in real antimony films. Depending on the point of view it is possible to find within a 60"-segment parallel chains of crystallites both parallel and perpendicular to a radius. Similarly stripes can be found in nature as shown in Fig. 3a.

Pig. 2. Electron diffraction pattern corresponding to the models shown in Fig. 1

252 H. MULLER

Fig. 3. Electron micrographs of evaporated antimony films - spherulitically crystallized. a) C-Pt replica of a 500 nm thick film, b) and c) TEM micrograph of a 10 nm thick film? d) TEM micrograph of a 50 nm thick film, annealed a t 150 "C for 2 min

Simulation of Oriented Crystallization in Evaporated Sb Thin Films 253

The orientation of the crystallites in the model shown in Fig. 1 a is only caused by the influence of nearest neighbours. The corresponding diffraction pattern must be a perfect six-fold spot pattern which could be obtained from selected regions of a spherulite (Fig. 2a).

Fig. 1 b shows a model with orientation of the crystallites in the direction of a central force without any influence of nearest neighbours. The resulting structure consists of chains of crystallites with strong radial symmetry. Such structures can be found frequently in nature, too (Fig. 3). The corresponding diffraction pattern must be more or less well textured as shown in Fig. 2 b or 4a (according to the diameter and position of the diffraction area).

Fig. 1 c and 1 d show combinations of both models. I n this case the orientation does not take place in all possible, but only in some few radii. Thereby the adjoining clusters may be influenced regarding to their orientation. The radial symmetry is caused by the orientation in the central force field. The orientation by nearest neigh- bours produces an accumulation of crystallite chains parallel to this directions. The structure resulting after such a mixed crystallization is characterized by strip or wedge- shaped regions pointing to the crystallization centres.

Both models are basing on the same construction principle. In Fig. 1 c the influence of nearest neighbours is the dominating orientation mechanism and in Fig. I d the central force field dominates.

The angles between the wedges are 30" in Fig. l c and 10" in Fig. Id, respectively. It is possible to find corresponding diffraction patterns with the same angles between the diffraction spots (Fig. 2c and 2d).

It is not necessary to say that the models with reference to the angles are arbitrarily chosen. I n nature all angles are possible and there is no reason for uniformity [see Fig. 3).

3. Discussion

Electron diffraction with focus in the recording plane gives a beam diameter in the object plane of nearly 120 pm2). I n this case the number of crystallites contributing to the diffraction pattern is 5.6 x lo6. With focus in the object plane the diameter is nearly 2 pm. This means only 1.6 x 103 crystallites are forming the diffraction pattern.

Using a large focus the diffraction pattern is well textured since the single point reflexes are not resolved (Fig. 2 b and 4a). With focus in the object plane the resolution is increased noticeably and consequently the reflexes are less stroke-like and often divided in two, three, four, or more spots (Fig. 4 b to 4d).

By displacing the object during examination with 120 pi-focus the stroke-like reflexes appear to shift around the centre. Repeating this displacement with 2 pm- focus a circular arranged row of points on fixed places can be observed. The points disappear on one side of the "strokes" and new ones appear on the other side. Hence the shift of reflexes is only decided by the small resolution of Lebedew diffraction and the crystallites appear to be misoriented of f5 to 10" around the fibre axis. By using high resolution Kossel-Mollenstedt diffraction a superposition of two-, three-, or more six-fold spot diagrams can be observed. The number of superimposed diagrams and the angles between them strongly depend on the place and diameter of the selected area (Fig. 3 b and 3c).

2) The investigation is performed with an electron optical plant EF produced by VEB Carl Zeiss Jena.

254 H. M” ULLm

Fig. 4. Electron diffraction pattern from specimens comparable with Fig. 3 b t o 3d: a) obtained with focus in the recording plane, b) to d) obtained with focus in the object plane (a) and b) from the same object place)

4. Conclusions

The general interpretation that the orientation of crystals within one spherulite may be single-crystal-like does not correspond to reality. I n dependence on the diameter of selected area the diffraction pattern shows very different results (Fig. 4 a and 4b). So it is possible that the diffraction pattern shows an ideal single-crystalline orientation

Simulation of Oriented Crystallization in Evaporated Sb Thin Films 255

provided the focus is small enough. Such a pure spot pattern is not an argument for single-crystalline evaporated films. The models and the micrographs have shown that spherulitic antimony films are always highly ordered polycrystalline and only single- crystalline in strip- or wedge-shaped areas (Fig. 3d).

Acknowledgements

The author would like to express his thank to Prof. Dr. IT. Andra and Mr. N. Kaiser for critical inspection of the manuscript and for helpful discussions.

References [l] A. GOTZBERGER, Z. Phys. 142,182 (1952). [2] A. BARNA, P. B. BARNA, G. RADNOCZI, and I. RECHENBERG, Acta Phys. Acad. Sci. Hungar.

[3] M. HASHIMOTO, T. NIIZEKI, and K. KAMBE, Japan. J. appl. Phys. 19,21 (1980). [4] N. KAISER, Acta Phys. Acad. Sci. Hungar. 49, 255 (1980). [5] N. KAISER, H. MULLER, and C. GLOEDE, Thin Solid Films 85,293 (1981). [S] H. M ~ L E R , phys. stat. sol. (a) 66, 199 (1981). [7] H. HORIKOSHI and N. TAMURA, Japan, J. appl. Phys. 2,328 (1963). [S] GMELIN, Hdb. anorg. Chem., Vol. 18, Antimon B, Verlag Chemie, Weinheim 1949 (p. 203).

80, 821 (1975).

(Received December 17, 1981)