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Direct observation of crystalline to amorphous transition by ion implantation
Nuclear Instruments North-Holland
and Methods
in Physics Research
283
B58 (1991) 283-286
Letter to the Editor
Direct observation of crystalline to amorphous transition by ion implantation B. Rauschenbach Zentralinstitut ftir Kernforschung Rossendorf, Institut ftir Ionenstrahlphysik und Materialforschung, 8051 Dresden, P. 0. B. 19, Germany Received
7 November
1990 and in revised form 15 February
1991
The amorpbization of aluminium implanted iron is studied by high resolution electron microscopy - in particular on the ion fluence and the implantation temperature. Examples are given of the implantation induced amorphous
its dependence structure. The
formation and growth kinetics of amorphous clusters formed are discussed.
High-fluence ion implantation is a versatible technique to form amorphous metal layers by disorder production and chemical stabilization of this disorder. However, the mechnisms which lead to the crystallineamorphous transition are still unclear, although this transition process has been extensively studied (e.g. refs. [l-3]). In this contribution results are presented on the direct observation of the crystalline-amorphous transition using electron microscopy. Aluminium, which is known to be a glass former (see e.g. ref. [4]), was implanted in pure iron at different temperatures. The targets were prepared from high purity polycrystalline iron. Iron discs with a thickness of 1 mm and a diameter of 3 mm were used. After mechanical polish-
Fig. 1. Transmission
0168-583X/91/$03.50
electron
diffraction
patterns
ing all samples were electropolished ensuring damage free surfaces prior to implantation. The iron samples were implanted with 100 keV Al+ ions at the temperature of liquid nitrogen (90 K), at room temperature (300 K) and at a higher temperature (420 K) in the fluence range between 1 and 10 X 1016 ions/cm2. A cooling or a heating system in the isotope separator chamber was used in order to implant at different temperatures and the residual gas pressure in the target chamber during implantation was about 10e5 Pa. The ion current density was less than 3.5 PA/cm’. The mean projected range of the Al+ ions is 66 nm and the standard deviation is 3.5 nm. Planar sections were thinned for electron microscopi-
from iron before, (a), and after, (b), implantation room temperature.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
with 10 x 1016 Al+ ions/cm2
at
284
3. ~a~chenbaeh
/ Ion ~rn~~ffntation ~rno~phigat~~n of Fe
cat studies by grinding, dimpling and ion milling from the non-implanted side until perforation. The implanted surface was also ion milled slightly so as to examine the interior of the film. Ion milling was carried out in a cold stage and with low argon ion beam conditions (5 keV, 0.2 mA gun current) so as to minimize potential heating and ion bombardment damage to the sample. The implantation induced amorphization was not influenced by Ar ion milling at low temperature because changes of morphology (size and number of amorphized regions) were not observed by variation of ion milling conditions. High-resolution electron microscopy was carried out in a 200 keV microscope equipped with a high-resolution pole piece (point-to point resolution = 0.2 nm). The selected area diffraction pattern of iron before implantation in fig. la reveals the monocrystalline structure. The diffraction pattern after implantation with 10 X 1Ol6 Al+ ions/cm* at room temperature in fig. lb shows diffuse rings from the amorphous phase. For fluences of 8 - 10 X 1016 Al+ ions/cm2 or greater, amorphization of the layers was complete with no indication of reflection from the bee Fe matrix after implantation at 90 K, 300 K and 420 K, as in fig. lb. The
bee reflections were seen along with the diffuse rings for fluences between 4 and 8 x 1016Al+ ions/cm2. These samples exhibit the coexistence of both the amorphous and the crystalline phase and they demonstrate the crystalline-amorphous transition region. Figs. 2a, b and c are TEM micrographs showing the typical microstructure of iron samples after Al+ ion implantation. The low-magnification image (fig. 2c) of iron after implantation with 6.5 X lOI Al+ ions/cm’ at 420 K shows an area with amorphous regions (black regions, see also details, on the right-hand side) as well as crystalline regions. The density of the amorphous regions (clusters) is so high that the clusters partially overlap. High-magnification atomic structure imaging (fig. 2a and b) obtained from a sufficiently thin region of iron shows the amorphous regions directly. We can thus directly distinguish between the amorphous and crystalline regions as a function of ion fluence and/or temperature during implantation. In this way, the average diameter of the amorphous clusters d, may be measured directly. The iron atom columns are black and the optimum (Scherzer) defocus is 450 A. Fig. 2a shows an iron layer after implantation at room temperature. An amorphous region is formed. This region is surrounded by a nearly
Fig. 2. Electron micrographs of iron after implantation with Al + ions at different temperatures: (a) high-resolution micrograph after implantation with 5.5 x lOI Al+ ions/cm* at room temperature; (b) high-resolution micrograph after implantation with 7.2 X lOI Al+ ions/cm2 at 80 K; and (c) low-resolution micrograph after implantation with 6.5 X lOI Al+ ions/cm* at 420 K.
B. Rawchenbach / Ion implantation amorphigation of Fe AL 3
CONCENTRATION 5
ht -% 6
1 7
8
9
Fig. 3. Average diameter and volume of the implantation induced amorphous regions vs the Al+ ion fluence after imptantation at different temperatures.
perfect atomic structure of iron. After impl~tation at lower temperature (80 K) and higher Al+ ion fluence (7.2 x 1016 Al+ ions/cm2) the amorphous regions are increased in size and the crystalline iron matrix surrounded by a strongly damaged region (see fig. 2b). The presence of such damage may be considered as a preliminary step of transition in the amorphous state, The dependence of the average diameter d, or volume V, (a spherical shape is assumed) of the amorphous clusters on Aif ion fluence and Al ~on~n~ation (calibrated by using absolute backscattering standards) is shown in fig. 3. A significant feature is the fact that an amorphous cluster is formed when a sufficient size (with a critical diameter) is reached, i.e. a critical concentration of Al+ atoms is necessary to stabilize the glass structure. From the high-resolution TEM images, the onset of amorphiration (the existence of amorphous clusters) is observed for a critical diameter d, = 12 A (V, = 0.9 x 10m2’ cm3), d,=15.5A(~=l.9x10-21cm3)andd,=24A(‘V,= 7.2 x 10m2’ ems) after implantation at 90 K, 300 K and 420 K respectively. That means that such a region is rendered amorphous as soon as the critical concentration of 3.8 at.%, 3.9 at.% and 4.4 at.% respectively is reached. Similar results were reported by Linker et al. [2,5] and by Benyagoub et al. [LB]. At higher Al+ ion fluences (concentrations) a continuous increase of average diameter or volume of the clusters is observed for fluences up to 7.5 - 8 X 1016Al+ ions/cm2, i.e. for about 8.5 at.% Al. At Al ~ncentrat~ons more than 10 at.% Al, the statistically distributed amorphous regions form an extended amorphous layer. The kinetic evaluation of the average diameter of amorphous dusters in fig. 3 shows a characteristic error function-like concentration dependence. The results also show that the average diameter of amorphous clusters does depend on implantation temperature, but the slope of fluence-diameter curves is nearly independent of implantation temperature. These experimental results are interpreted as follows: (1) An amorphous cluster with a minimum diameter is
285
formed whenever a critical Al impurity concentration is reached locally which acts as disorder stabilizer. Likely, the formation of amorphous clusters connects with local lattice re~~gements f7]. According to Linker f2], the amorphization process leads to a continuous accumulation of strain and a spontaneous transition into the amorphous state. (2) The ~~-resolution micrographs (e.g. fig. 2b) illustrate that an amorphous cluster is surrounded by a less damaged region. It must be assumed that the growth of the amorphous cluster during ion irradiation is caused at the expense of this disordered region (see also remark 89 in ref. (61). (3) By assu~ng that the main contrast mecha~sm of low-magnification imaging is the diffraction contrast, i.e. the contrast arises from mass-thickness difference, then the density of amorphous clusters (dark regions in fig. 2c) is larger than the density of the crystalline target material. The experimental results by Seidel et al. [5] have also shown that the atomic density is higher than that of the target for concentrations of the implanted impurities up to 8 at.% approximately. According to Egami and Aur [8], the composition of the amorphous clusters should be equal to the average composition of the implanted layer after complete amorphization. (4) The variation of the average diameter of amorphous clusters (see fig. 3) results from a process depending on the implantation temperature. By implantation, the Al atoms and therefore the critical local Al concentration are distributed statistically. At low temperature (80 K), the implanted atoms are nearly immobile and amorphous clusters are stable when the critical local Al ~ncentration is reached. This concentration is larger than the average concentration in the host lattice. At higher temperatures, the implanted atoms become mobile and a diffusion (short-range migration) of Al atoms away from the amorphous cluster appears necessary. The driving force for this process is the concentration gradient between the amo~hous cluster and the host lattice. Under this condition, an amorphous cluster is stable when the number of stabilized atoms in the cluster is higher than the number of stabilized atoms in a cluster formed at low-temperature implantation, i.e. the average diameter or volume increases with increasing implantation temperature. But it must be emphasized, that the number of clusters per unit area is decreased by the increase of temperature during implantation. For a physical inte~retatio~ of the observed behavior we are able to consider the increase in the critical size of the zones with increasing temperature as a size vs free energy effect. (5) The growth kinetics of amorphous clusters are similar for used temperatures. The shape of these curves (see fig. 3) illustrates that the average diameter of the stable amorphous clusters is governed by ion fluence and implantation temperature [9].
286
B. Rauxhenbach
/ Ion implantation amorphigation of Fe
Acknowledgements The author would like to thank J. SchGneich for performing the implantations and is grateful to Dr. G. Linker for his comments and for discussions.
References [l] C. Cohen, A. Benyagoub, H. Bemas, J. Chaumont and L. Thorn& Phys. Rev. B31 (1985) 5. [2] G. Linker, Solid State Commun. 57 (1986) 773.
[3] B. Rauschenbach and V. Heera, Phys. Status Solidi Al00 (1987) 423. [4] F.E. Luborsky, (ed.), Amorphous Metallic Alloys (Butterworths, London, 1983). [5] A. Seidel, S. Massing, B. Strehlau and G. Linker, Mater. Sci. Eng. All5 (1989) 139 and 2. Phys. B74 (1989) 267. [6] A. Benyagoub and L. Thome, Phys. Rev. 38 (1988) 10205. [7] T. Egami and Y. Waseda, J. Non-Cryst. Solids 64 (1984) [8] T. Egami and S. Aur, J. Non-Cryst. Solids 89 (1987) 60. [9] V. Heera and B. Rauschenbach, Radiat. Eff. 91 (1986) 71.
and Methods
in Physics Research
283
B58 (1991) 283-286
Letter to the Editor
Direct observation of crystalline to amorphous transition by ion implantation B. Rauschenbach Zentralinstitut ftir Kernforschung Rossendorf, Institut ftir Ionenstrahlphysik und Materialforschung, 8051 Dresden, P. 0. B. 19, Germany Received
7 November
1990 and in revised form 15 February
1991
The amorpbization of aluminium implanted iron is studied by high resolution electron microscopy - in particular on the ion fluence and the implantation temperature. Examples are given of the implantation induced amorphous
its dependence structure. The
formation and growth kinetics of amorphous clusters formed are discussed.
High-fluence ion implantation is a versatible technique to form amorphous metal layers by disorder production and chemical stabilization of this disorder. However, the mechnisms which lead to the crystallineamorphous transition are still unclear, although this transition process has been extensively studied (e.g. refs. [l-3]). In this contribution results are presented on the direct observation of the crystalline-amorphous transition using electron microscopy. Aluminium, which is known to be a glass former (see e.g. ref. [4]), was implanted in pure iron at different temperatures. The targets were prepared from high purity polycrystalline iron. Iron discs with a thickness of 1 mm and a diameter of 3 mm were used. After mechanical polish-
Fig. 1. Transmission
0168-583X/91/$03.50
electron
diffraction
patterns
ing all samples were electropolished ensuring damage free surfaces prior to implantation. The iron samples were implanted with 100 keV Al+ ions at the temperature of liquid nitrogen (90 K), at room temperature (300 K) and at a higher temperature (420 K) in the fluence range between 1 and 10 X 1016 ions/cm2. A cooling or a heating system in the isotope separator chamber was used in order to implant at different temperatures and the residual gas pressure in the target chamber during implantation was about 10e5 Pa. The ion current density was less than 3.5 PA/cm’. The mean projected range of the Al+ ions is 66 nm and the standard deviation is 3.5 nm. Planar sections were thinned for electron microscopi-
from iron before, (a), and after, (b), implantation room temperature.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
with 10 x 1016 Al+ ions/cm2
at
284
3. ~a~chenbaeh
/ Ion ~rn~~ffntation ~rno~phigat~~n of Fe
cat studies by grinding, dimpling and ion milling from the non-implanted side until perforation. The implanted surface was also ion milled slightly so as to examine the interior of the film. Ion milling was carried out in a cold stage and with low argon ion beam conditions (5 keV, 0.2 mA gun current) so as to minimize potential heating and ion bombardment damage to the sample. The implantation induced amorphization was not influenced by Ar ion milling at low temperature because changes of morphology (size and number of amorphized regions) were not observed by variation of ion milling conditions. High-resolution electron microscopy was carried out in a 200 keV microscope equipped with a high-resolution pole piece (point-to point resolution = 0.2 nm). The selected area diffraction pattern of iron before implantation in fig. la reveals the monocrystalline structure. The diffraction pattern after implantation with 10 X 1Ol6 Al+ ions/cm* at room temperature in fig. lb shows diffuse rings from the amorphous phase. For fluences of 8 - 10 X 1016 Al+ ions/cm2 or greater, amorphization of the layers was complete with no indication of reflection from the bee Fe matrix after implantation at 90 K, 300 K and 420 K, as in fig. lb. The
bee reflections were seen along with the diffuse rings for fluences between 4 and 8 x 1016Al+ ions/cm2. These samples exhibit the coexistence of both the amorphous and the crystalline phase and they demonstrate the crystalline-amorphous transition region. Figs. 2a, b and c are TEM micrographs showing the typical microstructure of iron samples after Al+ ion implantation. The low-magnification image (fig. 2c) of iron after implantation with 6.5 X lOI Al+ ions/cm’ at 420 K shows an area with amorphous regions (black regions, see also details, on the right-hand side) as well as crystalline regions. The density of the amorphous regions (clusters) is so high that the clusters partially overlap. High-magnification atomic structure imaging (fig. 2a and b) obtained from a sufficiently thin region of iron shows the amorphous regions directly. We can thus directly distinguish between the amorphous and crystalline regions as a function of ion fluence and/or temperature during implantation. In this way, the average diameter of the amorphous clusters d, may be measured directly. The iron atom columns are black and the optimum (Scherzer) defocus is 450 A. Fig. 2a shows an iron layer after implantation at room temperature. An amorphous region is formed. This region is surrounded by a nearly
Fig. 2. Electron micrographs of iron after implantation with Al + ions at different temperatures: (a) high-resolution micrograph after implantation with 5.5 x lOI Al+ ions/cm* at room temperature; (b) high-resolution micrograph after implantation with 7.2 X lOI Al+ ions/cm2 at 80 K; and (c) low-resolution micrograph after implantation with 6.5 X lOI Al+ ions/cm* at 420 K.
B. Rawchenbach / Ion implantation amorphigation of Fe AL 3
CONCENTRATION 5
ht -% 6
1 7
8
9
Fig. 3. Average diameter and volume of the implantation induced amorphous regions vs the Al+ ion fluence after imptantation at different temperatures.
perfect atomic structure of iron. After impl~tation at lower temperature (80 K) and higher Al+ ion fluence (7.2 x 1016 Al+ ions/cm2) the amorphous regions are increased in size and the crystalline iron matrix surrounded by a strongly damaged region (see fig. 2b). The presence of such damage may be considered as a preliminary step of transition in the amorphous state, The dependence of the average diameter d, or volume V, (a spherical shape is assumed) of the amorphous clusters on Aif ion fluence and Al ~on~n~ation (calibrated by using absolute backscattering standards) is shown in fig. 3. A significant feature is the fact that an amorphous cluster is formed when a sufficient size (with a critical diameter) is reached, i.e. a critical concentration of Al+ atoms is necessary to stabilize the glass structure. From the high-resolution TEM images, the onset of amorphiration (the existence of amorphous clusters) is observed for a critical diameter d, = 12 A (V, = 0.9 x 10m2’ cm3), d,=15.5A(~=l.9x10-21cm3)andd,=24A(‘V,= 7.2 x 10m2’ ems) after implantation at 90 K, 300 K and 420 K respectively. That means that such a region is rendered amorphous as soon as the critical concentration of 3.8 at.%, 3.9 at.% and 4.4 at.% respectively is reached. Similar results were reported by Linker et al. [2,5] and by Benyagoub et al. [LB]. At higher Al+ ion fluences (concentrations) a continuous increase of average diameter or volume of the clusters is observed for fluences up to 7.5 - 8 X 1016Al+ ions/cm2, i.e. for about 8.5 at.% Al. At Al ~ncentrat~ons more than 10 at.% Al, the statistically distributed amorphous regions form an extended amorphous layer. The kinetic evaluation of the average diameter of amorphous dusters in fig. 3 shows a characteristic error function-like concentration dependence. The results also show that the average diameter of amorphous clusters does depend on implantation temperature, but the slope of fluence-diameter curves is nearly independent of implantation temperature. These experimental results are interpreted as follows: (1) An amorphous cluster with a minimum diameter is
285
formed whenever a critical Al impurity concentration is reached locally which acts as disorder stabilizer. Likely, the formation of amorphous clusters connects with local lattice re~~gements f7]. According to Linker f2], the amorphization process leads to a continuous accumulation of strain and a spontaneous transition into the amorphous state. (2) The ~~-resolution micrographs (e.g. fig. 2b) illustrate that an amorphous cluster is surrounded by a less damaged region. It must be assumed that the growth of the amorphous cluster during ion irradiation is caused at the expense of this disordered region (see also remark 89 in ref. (61). (3) By assu~ng that the main contrast mecha~sm of low-magnification imaging is the diffraction contrast, i.e. the contrast arises from mass-thickness difference, then the density of amorphous clusters (dark regions in fig. 2c) is larger than the density of the crystalline target material. The experimental results by Seidel et al. [5] have also shown that the atomic density is higher than that of the target for concentrations of the implanted impurities up to 8 at.% approximately. According to Egami and Aur [8], the composition of the amorphous clusters should be equal to the average composition of the implanted layer after complete amorphization. (4) The variation of the average diameter of amorphous clusters (see fig. 3) results from a process depending on the implantation temperature. By implantation, the Al atoms and therefore the critical local Al concentration are distributed statistically. At low temperature (80 K), the implanted atoms are nearly immobile and amorphous clusters are stable when the critical local Al ~ncentration is reached. This concentration is larger than the average concentration in the host lattice. At higher temperatures, the implanted atoms become mobile and a diffusion (short-range migration) of Al atoms away from the amorphous cluster appears necessary. The driving force for this process is the concentration gradient between the amo~hous cluster and the host lattice. Under this condition, an amorphous cluster is stable when the number of stabilized atoms in the cluster is higher than the number of stabilized atoms in a cluster formed at low-temperature implantation, i.e. the average diameter or volume increases with increasing implantation temperature. But it must be emphasized, that the number of clusters per unit area is decreased by the increase of temperature during implantation. For a physical inte~retatio~ of the observed behavior we are able to consider the increase in the critical size of the zones with increasing temperature as a size vs free energy effect. (5) The growth kinetics of amorphous clusters are similar for used temperatures. The shape of these curves (see fig. 3) illustrates that the average diameter of the stable amorphous clusters is governed by ion fluence and implantation temperature [9].
286
B. Rauxhenbach
/ Ion implantation amorphigation of Fe
Acknowledgements The author would like to thank J. SchGneich for performing the implantations and is grateful to Dr. G. Linker for his comments and for discussions.
References [l] C. Cohen, A. Benyagoub, H. Bemas, J. Chaumont and L. Thorn& Phys. Rev. B31 (1985) 5. [2] G. Linker, Solid State Commun. 57 (1986) 773.
[3] B. Rauschenbach and V. Heera, Phys. Status Solidi Al00 (1987) 423. [4] F.E. Luborsky, (ed.), Amorphous Metallic Alloys (Butterworths, London, 1983). [5] A. Seidel, S. Massing, B. Strehlau and G. Linker, Mater. Sci. Eng. All5 (1989) 139 and 2. Phys. B74 (1989) 267. [6] A. Benyagoub and L. Thome, Phys. Rev. 38 (1988) 10205. [7] T. Egami and Y. Waseda, J. Non-Cryst. Solids 64 (1984) [8] T. Egami and S. Aur, J. Non-Cryst. Solids 89 (1987) 60. [9] V. Heera and B. Rauschenbach, Radiat. Eff. 91 (1986) 71.