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Crystallization and decomposition of amorphous silicon-aluminium films
Journal of Non-Crystalline Solids 17 (1975) 359-368 © North-Holland Publishing Company
CRYSTALLIZATION SILICON-ALUM1NIUM
AND DECOMPOSITION
OF AMORPHOUS
FILMS
Uwe K(}STER and Petra WEISS Institut far Werkstoffe, Ruhr-Universitiit Bochum, D-463 Bochum, Federal Republic of Germany Received 2 August 1974
The crystallization and decomposition of vacuum-deposited amorphous silicon-aluminium films have been examined by means of transmission electron microscopy. Depending on the aluminium concentration, the transformation of the metastable amorphous phase into the stable phases of aluminium and silicon proceeds by different reactions such as pre-crystallization of aluminium, polymorphous transformation into supersaturated crystalline solid solutions or eutectic decomposition. The temperature dependence of the eutectic crystallization was measured. The results are discussed in terms of the thermodynamics of amorphous-to-crystalline transformation.
1. Introduction In recent years, amorphous semiconductors have attracted an increasing scientific and technological interest, especially since they have been used in switching devices. Numerous papers on structural and electronic properties or on crystallization of such amorphous materials have been published. Most investigations on crystallization and decomposition behaviour of amorphous alloys, however, are based on differential thermal analysis (DTA) and give little information on the micromechanisms of these reactions. The purpose of this paper is to examine crystallization and decomposition reactions in a very simple amorphous alloy by means of hot-stage transmission electron microscopy. Silicon and aluminium form a simple eutectic system. Whereas aluminium crystallizes even at a substrate temperature of 20 K during evaporation [1 ], silicon-rich alloys can be obtained amorphous over a large concentration range by co-evaporation [2]. The structure of amorphous silicon has been the subject o f many recent studies [3]. The currently favoured structural model is the continuous random network that was first proposed by Polk [4]. Crystallization in amorphous silicon occurs in the 4 5 0 - 7 0 0 0 C temperature range [ 5 - 7 ] . To our knowledge there exists neither a study o f the morphology of crystallization of amorphous silicon nor an investigation of the crystallization and decomposition of amorphous S i - A t alloys.
360
U. KOster, P. Weiss / Amorphous Si-AI films
2. Experimental Thin silicon-aluminium films were prepared by co-evaporation of silicon (Balzers 99.999%) and aluminium (Balzers 99.999%) from two Balzers electron-beam guns. The evaporation was carried out under a pressure of about 10 - 7 tort. The films were deposited onto freshly cleaved NaC1 substrates, onto quartz glass or onto soda glass substrates covered with a thin layer of a wetting agent (Lensodel R). The substrates were kept at room temperature throughout the evaporation. The films had thicknesses in the range between 80 and 200 nm. The deposition rates were in the range between 0.5 and 1 nm/sec. The compositions of the films were determined after crystallization of tile stable phases aluminium and silicon by electron microscopy from estimates of the volume fractions of the two phases. The accuracy of this method was about + 3 at%. After evaporation, the films were removed from the substrate by dissolving the salt or the wetting agent in distilled water and floating them onto a copper or platinium electron microscope grid. The structures of tile films and their annealing behaviour were investigated by hot-stage transmission electron microscopy in a Philips EM 300 electron microscope operating at 100 kV. No influence of the preparation methods was observed on the crystallization and decomposition behaviour of the films. Films annealed immediately after evaporation in a vacuum which was always better than 10 - 6 torr show the same crystallization kinetics and morphologies as films annealed after preparation in the electron microscope. 3. Results After deposition, silicon-aluminium films containing up to 40 at% A1 were found to be amorphous. The structure and electron diffraction pattern of such films are shown in fig. 1. Depending on aluminium concentration, the radius of the broad rings in the electron diffraction pattern changed as shown in table 1. Films containing more aluminium consist of small aluminium crystallites embedded in the amorphous matrix (fig. 2). On annealing, the following reactions have been observed: (1) polymorphous crystallization: a*'SixAll - x ~ c * ' S i x A l l - x , x >~ 0.8; (2) eutectic crystallization: a-SixA11 - x ~ c-Si + c-A1, 0.7~ x ~ 0.8; Table 1 Diffraction-ring diameters for Si and Si-A1 films.
dl (A) d2 (A) d3 (A)
Si [8]
Si
Si--20 at% AI
Si-40 at% A1
3.13 1.76 1.19
3.13 1.75 -
3.14 1.77 1.1.4
3.14 1.81 -
* a = amorphous, c = crystalline.
U. Koster, P. Weiss/Amorphous Si-Al films
361
Fig. 1. Amorphous Si-20 at% AI film.
Fig. 2. AI spheres embedded in an amorphous Si-A1 matrix (Si-60 at% AI).
(3) pre-crystallization of aluminium (followed by eutectic crystallization): a ' S i x A l l - x -+ c-A1 + a-SiyA11 _ y , y -~..x, 0.6 ~ 0.7; SiyA11 _y -+ c-Si + c-Al. In Si-A1 alloys with aluminium concentrations higher than 40 at%, the pre-crystallization of aluminium occurs during evaporation and preparation at room temperature. During the annealing of these alloys, the eutectic crystallization was observed.
362
U. KOster, P. Weiss/Amorphous Si-Al films
Crystallization of pure silicon was observed at a temperature of about 700°C (fig. 3). With increasing A1 content, crystallization occurs at lower temperatures and the morphology of crystallization changes. Whereas in pure silicon relatively
Fig. 3. Crystallization of amorphous Si (1 min, 700°C).
Fig. 4. Polymorphous crystallization in Si-15 at% A1; dark field image with (111)Si (1 rain, 500°C).
363
U. Koster, P. Weiss/Amorphous Si-Al films
Fig. 5. "Spherulitic" crystallization in Si -30 at% A1 (1 rain, 300°C).
(a)
Fig. 6. "Spherulites" in Si-50 at% AI (polarized light, polarizer and analyzer nearly crossed).
(b)
Fig. 7. "Spherulitic" crystallization in Si-20 at% A1 (isochronal up to 450°C); (a) reaction front, (b) electron diffraction pattern; AI spots are indicated by arrows. large crystals with a high density o f dislocations are formed, amorphous films with 15 at% A1 crystallize at 500°C into a fine-grained supersaturated solid solution. with a diamond structure. A decomposition reaction of this metastable solid solution was not observed, even at the highest test temperature o f 600°C. This polymorphous crystallization into the fine-grained polycrystalline solid solution occurs by an autocatalytic reaction (fig. 4), in which the new grains crystallize at the surfaces o f the already existing grains. Amorphous sflicon--aluminium films with
364
U. KOster, P. Weiss/Amorphous Si-Al films
Fig. 8. Pre-crystallization of A1 spheres followed by "spherulitic" crystallization (Si-40 at% A1; 1 min, 200°C).
about 30 at% A1 crystallize by an eutectic reaction (fig. 5) with "spherulitic" morphology which can be observed by microscopy with polarized light (fig. 6). In a small concentration range, polymorphic and eutectic crystallization can be observed in the same film: polymorphic crystallization occurs at high annealing temperatures, whereas at relatively low temperatures the eutectic reaction is observed. In this alloy, the morphology of the "spherulites" is characterized by a strong texture of
t
120t"
-°/, AI
~ -
25
2/.
1/"0~
160"
A
2,3
2,2
180"
200'
S
~
"/
21 I
220"
2,0
2~0,
1'0-3
110-z
I0-I
I
(;rowth rQte u C# m/toni
Fig. 9. Growth rates of "spherulites" in amorphous Si-A1 alloys.
~0
,~,
10 2
300' t 10 5
I 10 6
I 10 7 rote
Fig. 10. Nucleation rates of "spherulites" in amorphous Si-A1 alloys.
N/cm 2 mini
Nuctecttion
1,8
280
2,1
2,2
1,9
i 10/.
.,~.Si-/.0 At.--*/.A[
250"
•
2,1.
2,0
I 103
~
i-.50 At.-°/.AI
'~
2 sI
230
210
190
"1 17o÷
~ ~ 15Ol~
-3°I
(.~
I
(3
3
366
u. KOster, P. Weiss/Amorphous Si-Al films
the silicon crystallites as shown in fig. 7. With increasing A1 concentration, this texture becomes weaker. In alloys with A1 concentrations higher than 40 at% homogeneous crystallization of small A1 crystallites has been observed prior to the eutectic reaction (fig. 8). In figs. 9 and 10 the growth and nucleation rates of these "spherulites" are given as a function of the Al concentration in the film. With increasing A1 content, the number of nuclei increases rapidly, the growth rate, however, increases relatively slowly. Therefore, in Si-A1 films with low A1 content larger "spherulites" were observed than in films containing more aluminium.
4. Discussion The observed reactions can be understood using a hypothetical diagram of free energy versus aluminium concentration as shown in fig. 1 l. According to this diagram, the following reactions are possible, i.e. by these reactions the free energy of the system will be reduced: (a) Pure silicon or silicon-rich amorphous films can crystallize by a polymorphous reaction, i.e. without any change in concentration. This reaction has been observed in alloys with an A1 concentration up to 20 at%. Supersaturation of such a value cannot be obtained in crystalline silicon by quenching a solid solution from higher temperatures, because the solid solibility of aluminium in crystalline silicon is smaller than 1 at% [9]. The same behaviour, however, hasbeen reported in crystallization of amorphous Ge-A1 [10] or G e - S n [11,12] alloys.
c -Si
Si
0
30
0
70
90
AI
concentrotfon
Fig. 11. Hypothetical plot of free energy for the various phases versus aluminium concentration in Si-Al. The observed crystallization reactions are indicated by arrows: (1) polymorphous transformation into a supersaturated Si solid solution, (2) eutectic crystallization of Si and AI, (3) pre-crystallization of A1 spheres.
U. Koster, P. Weiss/Amorphous Si-Al films
367
(b) Pre-crystallization of silicon is a possible reaction, due to tile reduction in free energy. However, this reaction has not been observed. In contrast to a polymorphous reaction, long-range diffusion is necessary for such pre-crystallization. Assuming the activation energy for diffusion in amorphous silicon-rich St-A1 alloys is of the same order of magnitude as in crystalline silicon, i.e. the diffusivity of silicon atoms is too low, it is understandable that this reaction does not occur prior to polymorphous or eutectic crystallization. (c) By pre-crystallization of Al, the homogeneous amorphous alloy transforms into a metastable equilibrium mixture consisting of small A1 crystallites embedded in an amorphous matrix. In St-A1 alloys with an aluminium concentration higher than 30 at% this reaction was observed to always be the first one. In an alloy with 40 at% A1 the activation energy for pre-crystallization of A1 was found to be about 48 kcal/mole. At higher temperatures, the metastable state transforms into the stable one by the eutectic reaction. (d) Eutectic crystallization can occur over the whole concentration range between the two stable phases of silicon and aluminium and possesses the highest driving force compared with the polymorphous crystallization or pre-crystallization. However, only in the concentration range from 20 to 30 at% A1 was this reaction observed to be the first. This may be so due to the difficulties in nucleation for the two phases in the eutectic reaction. At the moment, we are not able to give a quantitative prediction as to which of the possible reactions will occur first. The kinetics of the eutectic reaction have been investigated. Both, growth and nucleation of the "spherulites", can be described by an equation of Arrhenius type (see figs. 9 and 10). The activation energy for growth is 28.5 kcal/mole and is nearly independent of alloy composition. This value is only a little less than the activation energy for diffusion of Si in crystalline aluminium (30.5 kcal/mole) [13]. The pre-exponential factor increases slightly with A1 concentration. The activation energy for nucleation was found to be about 39 kcal/mole in alloys (e.g. Si-50 at% A1) consisting of small A1 crystallites embedded in an amorphous matrix and about 54 kcal/mole in alloys (e.g. Si-30 at% AI) without precrystallization of aluminium. The pre-exponential factor increases considerably with aluminium content. In an alloy with 40 at% A1 in which pre-crystallization occurs during annealing, the activation energy for nucleation of "spherulites" was about 52 kcal/mole. This value is a little higher than the activation energy found for precrystallization of A1 in this alloy (48 kcal/mole). The value for pre-crystallization, however, is not sufficiently accurate to allow us to draw any safe conclusions about the mechanism of transformation. There is indeed some evidence that the nucleation of the "spherulites" is controlled by pre-crystallization of A1. In Ge-A1 alloys [10], the eutectic reaction is characterized by the appearance of an aluminium film which separates the Ge crystallites and the amorphous phase. In this case, growth of the "spherulites" occurs only by diffusion of Ge through this aluminium film. In Si-A1 alloys such A1 films have not been observed by electron
368
U. KOster, P. Weiss/Amorphous Si-Al films
microscopy. However, in this alloy, it is very difficult to distinguish between small aluminium and silicon crystallites because of the more similar atomic number contrast of these elements compared with aluminium and germanium. Only the activation energy for growth of the "spherulites" indicates a mechanism like that in Ge-A1 alloys. At very high growth rates, a deviation from the Arrhenius plot to higher rates was observed. This may be due to the liberation of the heat of crystallization during transformation of a large mass in a relatively short time. This causes an increase in the temperature at the reaction front, resulting in higher growth rates than expected.
Acknowledgements The authors thank Professor E. Hornbogen for his interest in the work and his continuing support. The financial help of the Landesamt fiir Forschung des Landes Nordrhein-Westfalen is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [ 13 ]
H. Billow and W. Buckel, Z. Phys. 145 (1956) 141. P. Weiss and U. Kbster, unpublished results. S.C. M6ss and D. Adler, Comments Solid State Physics 5 (1973) 47. D.E. Polk, J. Non-Crystalline Solids 5 (1971) 365. H. Richter and G. Breitling, Z. Naturf. 13a (1958) 988. M.H. Brodsky, R.S. Title, K. Weiser and G.D. Petit, Phys. Rev. B1 (1970) 2632. N.A. Blum and C. Feldman, J. Non-Crystalline Solids 11 (1972) 242. G. Hass, Z. anorg. Chemie 257 (1948) 166. R.P. Elliot, Constitution of Binary Alloys, first supplement (McGraw-Hill,New York, 1965). U. K6ster, Acta Met. 20 (1972) 1361. R.J. Temkin and W. Paul, in: Amorphous and Liquid Semiconductors, ed. J. Stuke (Taylor and Francis, London, 1973) pp. 1193 ff. U. KiSster, to be published. D. Altenpohl, Aluminium and Aluminiumlegierungen (Springer-Verlag,Berlin, 1965).
CRYSTALLIZATION SILICON-ALUM1NIUM
AND DECOMPOSITION
OF AMORPHOUS
FILMS
Uwe K(}STER and Petra WEISS Institut far Werkstoffe, Ruhr-Universitiit Bochum, D-463 Bochum, Federal Republic of Germany Received 2 August 1974
The crystallization and decomposition of vacuum-deposited amorphous silicon-aluminium films have been examined by means of transmission electron microscopy. Depending on the aluminium concentration, the transformation of the metastable amorphous phase into the stable phases of aluminium and silicon proceeds by different reactions such as pre-crystallization of aluminium, polymorphous transformation into supersaturated crystalline solid solutions or eutectic decomposition. The temperature dependence of the eutectic crystallization was measured. The results are discussed in terms of the thermodynamics of amorphous-to-crystalline transformation.
1. Introduction In recent years, amorphous semiconductors have attracted an increasing scientific and technological interest, especially since they have been used in switching devices. Numerous papers on structural and electronic properties or on crystallization of such amorphous materials have been published. Most investigations on crystallization and decomposition behaviour of amorphous alloys, however, are based on differential thermal analysis (DTA) and give little information on the micromechanisms of these reactions. The purpose of this paper is to examine crystallization and decomposition reactions in a very simple amorphous alloy by means of hot-stage transmission electron microscopy. Silicon and aluminium form a simple eutectic system. Whereas aluminium crystallizes even at a substrate temperature of 20 K during evaporation [1 ], silicon-rich alloys can be obtained amorphous over a large concentration range by co-evaporation [2]. The structure of amorphous silicon has been the subject o f many recent studies [3]. The currently favoured structural model is the continuous random network that was first proposed by Polk [4]. Crystallization in amorphous silicon occurs in the 4 5 0 - 7 0 0 0 C temperature range [ 5 - 7 ] . To our knowledge there exists neither a study o f the morphology of crystallization of amorphous silicon nor an investigation of the crystallization and decomposition of amorphous S i - A t alloys.
360
U. KOster, P. Weiss / Amorphous Si-AI films
2. Experimental Thin silicon-aluminium films were prepared by co-evaporation of silicon (Balzers 99.999%) and aluminium (Balzers 99.999%) from two Balzers electron-beam guns. The evaporation was carried out under a pressure of about 10 - 7 tort. The films were deposited onto freshly cleaved NaC1 substrates, onto quartz glass or onto soda glass substrates covered with a thin layer of a wetting agent (Lensodel R). The substrates were kept at room temperature throughout the evaporation. The films had thicknesses in the range between 80 and 200 nm. The deposition rates were in the range between 0.5 and 1 nm/sec. The compositions of the films were determined after crystallization of tile stable phases aluminium and silicon by electron microscopy from estimates of the volume fractions of the two phases. The accuracy of this method was about + 3 at%. After evaporation, the films were removed from the substrate by dissolving the salt or the wetting agent in distilled water and floating them onto a copper or platinium electron microscope grid. The structures of tile films and their annealing behaviour were investigated by hot-stage transmission electron microscopy in a Philips EM 300 electron microscope operating at 100 kV. No influence of the preparation methods was observed on the crystallization and decomposition behaviour of the films. Films annealed immediately after evaporation in a vacuum which was always better than 10 - 6 torr show the same crystallization kinetics and morphologies as films annealed after preparation in the electron microscope. 3. Results After deposition, silicon-aluminium films containing up to 40 at% A1 were found to be amorphous. The structure and electron diffraction pattern of such films are shown in fig. 1. Depending on aluminium concentration, the radius of the broad rings in the electron diffraction pattern changed as shown in table 1. Films containing more aluminium consist of small aluminium crystallites embedded in the amorphous matrix (fig. 2). On annealing, the following reactions have been observed: (1) polymorphous crystallization: a*'SixAll - x ~ c * ' S i x A l l - x , x >~ 0.8; (2) eutectic crystallization: a-SixA11 - x ~ c-Si + c-A1, 0.7~ x ~ 0.8; Table 1 Diffraction-ring diameters for Si and Si-A1 films.
dl (A) d2 (A) d3 (A)
Si [8]
Si
Si--20 at% AI
Si-40 at% A1
3.13 1.76 1.19
3.13 1.75 -
3.14 1.77 1.1.4
3.14 1.81 -
* a = amorphous, c = crystalline.
U. Koster, P. Weiss/Amorphous Si-Al films
361
Fig. 1. Amorphous Si-20 at% AI film.
Fig. 2. AI spheres embedded in an amorphous Si-A1 matrix (Si-60 at% AI).
(3) pre-crystallization of aluminium (followed by eutectic crystallization): a ' S i x A l l - x -+ c-A1 + a-SiyA11 _ y , y -~..x, 0.6 ~ 0.7; SiyA11 _y -+ c-Si + c-Al. In Si-A1 alloys with aluminium concentrations higher than 40 at%, the pre-crystallization of aluminium occurs during evaporation and preparation at room temperature. During the annealing of these alloys, the eutectic crystallization was observed.
362
U. KOster, P. Weiss/Amorphous Si-Al films
Crystallization of pure silicon was observed at a temperature of about 700°C (fig. 3). With increasing A1 content, crystallization occurs at lower temperatures and the morphology of crystallization changes. Whereas in pure silicon relatively
Fig. 3. Crystallization of amorphous Si (1 min, 700°C).
Fig. 4. Polymorphous crystallization in Si-15 at% A1; dark field image with (111)Si (1 rain, 500°C).
363
U. Koster, P. Weiss/Amorphous Si-Al films
Fig. 5. "Spherulitic" crystallization in Si -30 at% A1 (1 rain, 300°C).
(a)
Fig. 6. "Spherulites" in Si-50 at% AI (polarized light, polarizer and analyzer nearly crossed).
(b)
Fig. 7. "Spherulitic" crystallization in Si-20 at% A1 (isochronal up to 450°C); (a) reaction front, (b) electron diffraction pattern; AI spots are indicated by arrows. large crystals with a high density o f dislocations are formed, amorphous films with 15 at% A1 crystallize at 500°C into a fine-grained supersaturated solid solution. with a diamond structure. A decomposition reaction of this metastable solid solution was not observed, even at the highest test temperature o f 600°C. This polymorphous crystallization into the fine-grained polycrystalline solid solution occurs by an autocatalytic reaction (fig. 4), in which the new grains crystallize at the surfaces o f the already existing grains. Amorphous sflicon--aluminium films with
364
U. KOster, P. Weiss/Amorphous Si-Al films
Fig. 8. Pre-crystallization of A1 spheres followed by "spherulitic" crystallization (Si-40 at% A1; 1 min, 200°C).
about 30 at% A1 crystallize by an eutectic reaction (fig. 5) with "spherulitic" morphology which can be observed by microscopy with polarized light (fig. 6). In a small concentration range, polymorphic and eutectic crystallization can be observed in the same film: polymorphic crystallization occurs at high annealing temperatures, whereas at relatively low temperatures the eutectic reaction is observed. In this alloy, the morphology of the "spherulites" is characterized by a strong texture of
t
120t"
-°/, AI
~ -
25
2/.
1/"0~
160"
A
2,3
2,2
180"
200'
S
~
"/
21 I
220"
2,0
2~0,
1'0-3
110-z
I0-I
I
(;rowth rQte u C# m/toni
Fig. 9. Growth rates of "spherulites" in amorphous Si-A1 alloys.
~0
,~,
10 2
300' t 10 5
I 10 6
I 10 7 rote
Fig. 10. Nucleation rates of "spherulites" in amorphous Si-A1 alloys.
N/cm 2 mini
Nuctecttion
1,8
280
2,1
2,2
1,9
i 10/.
.,~.Si-/.0 At.--*/.A[
250"
•
2,1.
2,0
I 103
~
i-.50 At.-°/.AI
'~
2 sI
230
210
190
"1 17o÷
~ ~ 15Ol~
-3°I
(.~
I
(3
3
366
u. KOster, P. Weiss/Amorphous Si-Al films
the silicon crystallites as shown in fig. 7. With increasing A1 concentration, this texture becomes weaker. In alloys with A1 concentrations higher than 40 at% homogeneous crystallization of small A1 crystallites has been observed prior to the eutectic reaction (fig. 8). In figs. 9 and 10 the growth and nucleation rates of these "spherulites" are given as a function of the Al concentration in the film. With increasing A1 content, the number of nuclei increases rapidly, the growth rate, however, increases relatively slowly. Therefore, in Si-A1 films with low A1 content larger "spherulites" were observed than in films containing more aluminium.
4. Discussion The observed reactions can be understood using a hypothetical diagram of free energy versus aluminium concentration as shown in fig. 1 l. According to this diagram, the following reactions are possible, i.e. by these reactions the free energy of the system will be reduced: (a) Pure silicon or silicon-rich amorphous films can crystallize by a polymorphous reaction, i.e. without any change in concentration. This reaction has been observed in alloys with an A1 concentration up to 20 at%. Supersaturation of such a value cannot be obtained in crystalline silicon by quenching a solid solution from higher temperatures, because the solid solibility of aluminium in crystalline silicon is smaller than 1 at% [9]. The same behaviour, however, hasbeen reported in crystallization of amorphous Ge-A1 [10] or G e - S n [11,12] alloys.
c -Si
Si
0
30
0
70
90
AI
concentrotfon
Fig. 11. Hypothetical plot of free energy for the various phases versus aluminium concentration in Si-Al. The observed crystallization reactions are indicated by arrows: (1) polymorphous transformation into a supersaturated Si solid solution, (2) eutectic crystallization of Si and AI, (3) pre-crystallization of A1 spheres.
U. Koster, P. Weiss/Amorphous Si-Al films
367
(b) Pre-crystallization of silicon is a possible reaction, due to tile reduction in free energy. However, this reaction has not been observed. In contrast to a polymorphous reaction, long-range diffusion is necessary for such pre-crystallization. Assuming the activation energy for diffusion in amorphous silicon-rich St-A1 alloys is of the same order of magnitude as in crystalline silicon, i.e. the diffusivity of silicon atoms is too low, it is understandable that this reaction does not occur prior to polymorphous or eutectic crystallization. (c) By pre-crystallization of Al, the homogeneous amorphous alloy transforms into a metastable equilibrium mixture consisting of small A1 crystallites embedded in an amorphous matrix. In St-A1 alloys with an aluminium concentration higher than 30 at% this reaction was observed to always be the first one. In an alloy with 40 at% A1 the activation energy for pre-crystallization of A1 was found to be about 48 kcal/mole. At higher temperatures, the metastable state transforms into the stable one by the eutectic reaction. (d) Eutectic crystallization can occur over the whole concentration range between the two stable phases of silicon and aluminium and possesses the highest driving force compared with the polymorphous crystallization or pre-crystallization. However, only in the concentration range from 20 to 30 at% A1 was this reaction observed to be the first. This may be so due to the difficulties in nucleation for the two phases in the eutectic reaction. At the moment, we are not able to give a quantitative prediction as to which of the possible reactions will occur first. The kinetics of the eutectic reaction have been investigated. Both, growth and nucleation of the "spherulites", can be described by an equation of Arrhenius type (see figs. 9 and 10). The activation energy for growth is 28.5 kcal/mole and is nearly independent of alloy composition. This value is only a little less than the activation energy for diffusion of Si in crystalline aluminium (30.5 kcal/mole) [13]. The pre-exponential factor increases slightly with A1 concentration. The activation energy for nucleation was found to be about 39 kcal/mole in alloys (e.g. Si-50 at% A1) consisting of small A1 crystallites embedded in an amorphous matrix and about 54 kcal/mole in alloys (e.g. Si-30 at% AI) without precrystallization of aluminium. The pre-exponential factor increases considerably with aluminium content. In an alloy with 40 at% A1 in which pre-crystallization occurs during annealing, the activation energy for nucleation of "spherulites" was about 52 kcal/mole. This value is a little higher than the activation energy found for precrystallization of A1 in this alloy (48 kcal/mole). The value for pre-crystallization, however, is not sufficiently accurate to allow us to draw any safe conclusions about the mechanism of transformation. There is indeed some evidence that the nucleation of the "spherulites" is controlled by pre-crystallization of A1. In Ge-A1 alloys [10], the eutectic reaction is characterized by the appearance of an aluminium film which separates the Ge crystallites and the amorphous phase. In this case, growth of the "spherulites" occurs only by diffusion of Ge through this aluminium film. In Si-A1 alloys such A1 films have not been observed by electron
368
U. KOster, P. Weiss/Amorphous Si-Al films
microscopy. However, in this alloy, it is very difficult to distinguish between small aluminium and silicon crystallites because of the more similar atomic number contrast of these elements compared with aluminium and germanium. Only the activation energy for growth of the "spherulites" indicates a mechanism like that in Ge-A1 alloys. At very high growth rates, a deviation from the Arrhenius plot to higher rates was observed. This may be due to the liberation of the heat of crystallization during transformation of a large mass in a relatively short time. This causes an increase in the temperature at the reaction front, resulting in higher growth rates than expected.
Acknowledgements The authors thank Professor E. Hornbogen for his interest in the work and his continuing support. The financial help of the Landesamt fiir Forschung des Landes Nordrhein-Westfalen is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [ 13 ]
H. Billow and W. Buckel, Z. Phys. 145 (1956) 141. P. Weiss and U. Kbster, unpublished results. S.C. M6ss and D. Adler, Comments Solid State Physics 5 (1973) 47. D.E. Polk, J. Non-Crystalline Solids 5 (1971) 365. H. Richter and G. Breitling, Z. Naturf. 13a (1958) 988. M.H. Brodsky, R.S. Title, K. Weiser and G.D. Petit, Phys. Rev. B1 (1970) 2632. N.A. Blum and C. Feldman, J. Non-Crystalline Solids 11 (1972) 242. G. Hass, Z. anorg. Chemie 257 (1948) 166. R.P. Elliot, Constitution of Binary Alloys, first supplement (McGraw-Hill,New York, 1965). U. K6ster, Acta Met. 20 (1972) 1361. R.J. Temkin and W. Paul, in: Amorphous and Liquid Semiconductors, ed. J. Stuke (Taylor and Francis, London, 1973) pp. 1193 ff. U. KiSster, to be published. D. Altenpohl, Aluminium and Aluminiumlegierungen (Springer-Verlag,Berlin, 1965).