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Solid xenon bubbles in Fe and Mo thin films
Materials Letters North-Holland
MALETTSRS
17 ( 1993) 37-41
Solid xenon bubbles in Fe and MO thin films J.F. Dinhut and M.F. Denanot Lahoratoire Received
de M&allurgie
Physique,
ITRA 131 CNRS, 40, Avenue du Recteur Pineau, 86022 Poitiers, France
2 April 1993
TEM observations of Xe-implanted iron and molybdenum thin films were carried out. Solid Xe bubbles are detected at room temperature. Based on the measured lattice parameters and the atomic volume values, the fee structure is shown to be the most probable structure for the solid bubbles in both materials. Moreover, in MO an fee phase is also formed by Xe-ion implantation with a lattice parameter a=0.42 nm. The appearance of the fee MO phase greatly reduces the lattice parameter mismatch between Xe bubbles and the bee MO matrix.
1. Introduction The ion-implantation technique is extensively used in order to improve the surface properties of various materials. and especially hardness, wear resistance, corrosion [ 11. It has been demonstrated that implantation of inert-gas ions into fee and hcp metals resulted in the formation of solid fee or hcp bubbles [ 2,3 1. In all cases, a simple epitaxial relationship exists between the close-packed planes of the matrix and those of the bubbles, in spite of a possible mismatch of the lattice parameters. Because of the penetration depth, ion implantation is well adapted to the thin film technology, and in a practical way the inert gas ions are the most commonly used. In this Letter we investigate Xebubbles formation by TEM in bee Fe and MO films. A comparison will be made with previous results on the formation of solid krypton bubbles in bulk molybdenum [ 41.
drogen and nitrogen atoms were believed to be trapThe ped by means of titanium sublimator. evaporation rate was monitored by a calibrated quartz oscillator and fixed at 0.5 rim/s.. The samples were deposited on microscope grids covered with amorphous carbon film, and the thicknesses of evaporated Fe and MO films for TEM observations were limited at 60 nm. In order to obtain homogeneous deposition, the substrates were always rotating ( 1 rev/s) during evaporation. With this procedure, specimens were directly observable in the electron microscope without any polishing technique. Ion implantations were carried out at 300 K using very low currents in order to avoid beam heating of the targets. The pressure in the implantation chamber was about 5 x 1O-6 Pa. The characteristic parameters (table 1) were calculated using the TRIM 91 computer code [ 5 1.
3. Results 2. Experimental Fe and MO thin films were evaporated using a conventional electron-gun technique at room temperature. Using a vacuum system consisting of ionic and cryogenic pumps, the pressure before deposition was lower than 5x lo-’ Pa. Thus, the residual atmosphere was free of active gas and carbon, oxygen, hy0167-577x/93/$
06.00 0 1993 Elsevier
Science Publishers
Fig. 1a shows the selected area diffraction pattern taken from an as-evaporated iron thin film. From the successive Debye-Scherrer rings a bee structure can be clearly identified. From fig. lb, a mean grain size of about 10 nm is statistically deduced, according to X-ray diffraction measurements. The same parameters are determined after a 2 X lO”j ions/cm2 implantation. The diffraction rings
B.V. All rights reserved.
37
Volume 17, number Table I Ion-implantation
1,2
MATERI.~LS
LETTERS
July 1993
parameters
Xe-Fe Xe+Mo
Energy
Piojected
(keV)
(nm)
range
(nm)
Dose (ions/cm’)
300 330
41 44
18 18
2x 10’6 1x10’6
Straggling
220 bee 110
xIelll 110 bee it *on i
220
c
b 1OOnm Fig. 1. Electron diffraction
38
patterns
and bright fields of iron thin films: (a,b) as-evaporated,
d (c,d) after 2x 1016 Xe ion/cm* implantation.
Volume 17, number I,2
are more dotted because of the grain growth, and two additional rings can be detected (fig. lc). It seems natural to attribute these two rings to solid Xe bubbles, and in this case we have to keep in mind that Xe implantation introduces a large structural evolution since the mean grain size is now 50 nm (fig. Id). Assuming that the bubbles had a bee structure, via the measured ( 110) diameters, the Xe lattice parameter is 0.47 nm. This assumption leads to an atomic volume value Va’,,=50.6 x lop3 nm’lat, to be compared with those of the fee bubbles observed in bulk fee matrices (33.2 x 1O-’ nm’/at from ref. [ 4 1, 43.2x lO-3 nm”/at from ref. [7] for Kr and 59.6x lo-’ nm3/at for Xe bubbles). However, the diameter of the following (200) ring is not consistent with the measured one (fig. 1c). AS
bee +
in the bulk MO matrix [4], this left the more probable situation that the Xe bubbles had an fee structure as generally observed in all others materials. In this case, the central ring is due to the ( 1 1 1) reflection, leading to a lattice parameter of solid Xe of 0.56 nm, and an atomic volume of 44X 10W3 nm’/at. Nevertheless, in the fee hypothesis, the (200) reflection is never detected; Evans and Mazey [ 41, in bulk MO with large grains, on the basis that the Xe bubbles close-packed ( 1 11) planes must be parallel to the one of the six close-packed { 1101 planes of the bee matrix, assume that the (200) diffraction spots cannot be formed because of the infinite number of possible rotations within the matching close-packed planes. This explanation seems no longer valid in the case of rings due to small randomly oriented grains.
fee
110
2oonm
220
MO i 200
2
111
F
111
--rC-
Xe
b
July 1993
MATERIALS LETTERS
t--i,
d
Fig. 2. Electron diffraction patterns and bright fields of molybdenum thin films: (a,b) as-evaporated, (c,d) after 10lh Xe ion/cm2 implantation.
39
Volume 17, number 1,2
The second additional ring in fig. lc is inside the ( 110) diffraction ring of the bee iron matrix. Thus it is difficult to conclude if this (220) ring of the fee structure exists because of the large intensity of the matrix rings. It is possible that the central ring is the only one due to Xe bubbles, the others being too weak to be observed. In order to avoid the overlap of the various rings coming from xenon and iron structures, a 1016 ions/ cm2 Xe implantation has been performed in a MO thin film, under the same experimental conditions. The as-evaporated structure is deduced from the diffraction pattern (fig. 2) and the classical bee structure of MO can be clearly identified. The lattice parameter obtained (0.31 nm) is comparable with the bulk one and the mean grain size is small (about 5 nm). TEM observations after Xe implantation ( lOI ions/cm*, 330 keV; see other parameters in table 1) are shown in fig. 2. Three different structures are then detected: besides the bee MO structure already observed in the as-evaporated state, a typical fee structure also appears. Moreover, a central ring characteristic of inert gas bubbles is now detected. In order to prevent secondary phenomenon during implantation, especially sputtering of aluminium holders, an X-ray analysis has been performed using a classical Link device. Fig. 3 shows the composition of the as-implanted film. Obviously, the help of a, supplementary fee component is not necessary; only MO and Xe are present in the film. Usual MO oxides and carbides do not correspond to the observed supplementary fee structure. In all cases such pollution is not expected in the experimental conditions described above and cannot explain the extra rings of fig. 2c. The lattice parameter of this new fee structure is found to be 0.42 nm, corresponding to the fee MO phase observed by Aggarwal and Goswami [6]. It seems reasonable to think that in addition with bee MO and solid Xe bubbles, an amount of fee molybdenum is also observed. A more probable reason could be due to the fact that the fee MO-phase is the more convenient situation (the less energetic one) to make a buffer zone between the bee MO film and the fee Xe bubbles. However, it is somewhat surprising that the similar fee phase is not detected after Xe implantation 40
July 1993
MATERIALS LETTERS
4
2
6
8
10
Fig. 3. X-ray analysis of molybdenum thin films after lOI Xe ion/cm* implantation.
I
-0 0
Xe fee sites
0
MO fee sites
/ / 0’
0
Fig. 4. Coexistence of fee Xe bubbles and fee MO.
of iron films where the fee structure can be obtained in an easier way. Assuming fee solid Xe bubbles, the central ( 111) ring leads to a lattice parameter of about 0.6 nm. In this case, there will be no mismatch between the { IOO} fee molybdenum planes and the { 100) planes of solid Xe (fig. 4). This suitable situation could be the reason why an fee MO phase appears under Xe implantation, instead of the usual hypothesis where
Volume 17, number I ,2
July 1993
MATERIALS LETTERS
close-packed fee Xe { 111) planes are along the closepacked bee { 110) planes, as probably observed in iron films. The appearance of the fee iron as a buffer be-
sure in Xe bubbles
tween bee iron matrix and fee Xe bubbles being in this case less probable because of the large mismatch of the lattice parameters of the two fee structures. The atomic volume for fee Xe bubbles in MO is 54x lo-’ nm3/at, close to the value of 59.6x 1O-3 nm3/at reported in ref. [ 71 for fee Xe bubbles in fee bulk matrices. From the Ronchi equation [ 81, the internal pressure in Xe bubbles in MO films is close to 1.6 GPa instead of 6 GPa in Fe films.
Acknowledgement
4. Conclusion In both iron and molybdenum thin films, Xe-ion implantations performed at room temperature produce: - fee Xe solid bubbles with lattice parameters a, ~0.56 nm and az= 0.6 nm in iron and molybdenum respectively; - the appearance of the fee MO phase in order to reduce the lattice parameter mismatch between Xe bubbles and bee MO matrix. Thus, the internal pres-
decreases
in molybdenum
films.
The authors would like to thank Professor C. Templier for many valuable discussions.
References [ 11 P.J. Burnett and T.F. Page, Radiat. Effects 97 ( 1986) i 23. [2] C. Templier, C. Jaouen, J.P. Riviere, J. Delafond and J. Grilhe, Compt. Rend. Acad. Sci. (Paris) 299 ( 1984) 613. [3] A. von Felde, J. Fink, Th. Miiller-Heinzerling, J. Pfuger, B. Scheerer, G. Linker and D. Kaletta, Phys. Rev. Letters 53 (1984) 92. [ 41 J.H. Evans and D.J. Mazey, Scripta Metall. 19 ( 1985) 62 1. [ 51J.F. Ziegler, J.P. Biersack and V. Littmark, The stopping and range of ions in solids (Pergamon Press, Oxford, 1988 ). [6] P.S. Aggatwal and A. Goswami, Proc. Phys. Sot. 70 ( 1957) 708. [ 71 C. Tempiier, in: Fundamental aspects of inert gases in solids, eds. SE. Donnelly and J.H. Evans (Plenum Press, New York, 1990) p. 117. [ 8] C. Ronchi, J. Nucl. Mater. 96 ( 198 1) I 34.
41
MALETTSRS
17 ( 1993) 37-41
Solid xenon bubbles in Fe and MO thin films J.F. Dinhut and M.F. Denanot Lahoratoire Received
de M&allurgie
Physique,
ITRA 131 CNRS, 40, Avenue du Recteur Pineau, 86022 Poitiers, France
2 April 1993
TEM observations of Xe-implanted iron and molybdenum thin films were carried out. Solid Xe bubbles are detected at room temperature. Based on the measured lattice parameters and the atomic volume values, the fee structure is shown to be the most probable structure for the solid bubbles in both materials. Moreover, in MO an fee phase is also formed by Xe-ion implantation with a lattice parameter a=0.42 nm. The appearance of the fee MO phase greatly reduces the lattice parameter mismatch between Xe bubbles and the bee MO matrix.
1. Introduction The ion-implantation technique is extensively used in order to improve the surface properties of various materials. and especially hardness, wear resistance, corrosion [ 11. It has been demonstrated that implantation of inert-gas ions into fee and hcp metals resulted in the formation of solid fee or hcp bubbles [ 2,3 1. In all cases, a simple epitaxial relationship exists between the close-packed planes of the matrix and those of the bubbles, in spite of a possible mismatch of the lattice parameters. Because of the penetration depth, ion implantation is well adapted to the thin film technology, and in a practical way the inert gas ions are the most commonly used. In this Letter we investigate Xebubbles formation by TEM in bee Fe and MO films. A comparison will be made with previous results on the formation of solid krypton bubbles in bulk molybdenum [ 41.
drogen and nitrogen atoms were believed to be trapThe ped by means of titanium sublimator. evaporation rate was monitored by a calibrated quartz oscillator and fixed at 0.5 rim/s.. The samples were deposited on microscope grids covered with amorphous carbon film, and the thicknesses of evaporated Fe and MO films for TEM observations were limited at 60 nm. In order to obtain homogeneous deposition, the substrates were always rotating ( 1 rev/s) during evaporation. With this procedure, specimens were directly observable in the electron microscope without any polishing technique. Ion implantations were carried out at 300 K using very low currents in order to avoid beam heating of the targets. The pressure in the implantation chamber was about 5 x 1O-6 Pa. The characteristic parameters (table 1) were calculated using the TRIM 91 computer code [ 5 1.
3. Results 2. Experimental Fe and MO thin films were evaporated using a conventional electron-gun technique at room temperature. Using a vacuum system consisting of ionic and cryogenic pumps, the pressure before deposition was lower than 5x lo-’ Pa. Thus, the residual atmosphere was free of active gas and carbon, oxygen, hy0167-577x/93/$
06.00 0 1993 Elsevier
Science Publishers
Fig. 1a shows the selected area diffraction pattern taken from an as-evaporated iron thin film. From the successive Debye-Scherrer rings a bee structure can be clearly identified. From fig. lb, a mean grain size of about 10 nm is statistically deduced, according to X-ray diffraction measurements. The same parameters are determined after a 2 X lO”j ions/cm2 implantation. The diffraction rings
B.V. All rights reserved.
37
Volume 17, number Table I Ion-implantation
1,2
MATERI.~LS
LETTERS
July 1993
parameters
Xe-Fe Xe+Mo
Energy
Piojected
(keV)
(nm)
range
(nm)
Dose (ions/cm’)
300 330
41 44
18 18
2x 10’6 1x10’6
Straggling
220 bee 110
xIelll 110 bee it *on i
220
c
b 1OOnm Fig. 1. Electron diffraction
38
patterns
and bright fields of iron thin films: (a,b) as-evaporated,
d (c,d) after 2x 1016 Xe ion/cm* implantation.
Volume 17, number I,2
are more dotted because of the grain growth, and two additional rings can be detected (fig. lc). It seems natural to attribute these two rings to solid Xe bubbles, and in this case we have to keep in mind that Xe implantation introduces a large structural evolution since the mean grain size is now 50 nm (fig. Id). Assuming that the bubbles had a bee structure, via the measured ( 110) diameters, the Xe lattice parameter is 0.47 nm. This assumption leads to an atomic volume value Va’,,=50.6 x lop3 nm’lat, to be compared with those of the fee bubbles observed in bulk fee matrices (33.2 x 1O-’ nm’/at from ref. [ 4 1, 43.2x lO-3 nm”/at from ref. [7] for Kr and 59.6x lo-’ nm3/at for Xe bubbles). However, the diameter of the following (200) ring is not consistent with the measured one (fig. 1c). AS
bee +
in the bulk MO matrix [4], this left the more probable situation that the Xe bubbles had an fee structure as generally observed in all others materials. In this case, the central ring is due to the ( 1 1 1) reflection, leading to a lattice parameter of solid Xe of 0.56 nm, and an atomic volume of 44X 10W3 nm’/at. Nevertheless, in the fee hypothesis, the (200) reflection is never detected; Evans and Mazey [ 41, in bulk MO with large grains, on the basis that the Xe bubbles close-packed ( 1 11) planes must be parallel to the one of the six close-packed { 1101 planes of the bee matrix, assume that the (200) diffraction spots cannot be formed because of the infinite number of possible rotations within the matching close-packed planes. This explanation seems no longer valid in the case of rings due to small randomly oriented grains.
fee
110
2oonm
220
MO i 200
2
111
F
111
--rC-
Xe
b
July 1993
MATERIALS LETTERS
t--i,
d
Fig. 2. Electron diffraction patterns and bright fields of molybdenum thin films: (a,b) as-evaporated, (c,d) after 10lh Xe ion/cm2 implantation.
39
Volume 17, number 1,2
The second additional ring in fig. lc is inside the ( 110) diffraction ring of the bee iron matrix. Thus it is difficult to conclude if this (220) ring of the fee structure exists because of the large intensity of the matrix rings. It is possible that the central ring is the only one due to Xe bubbles, the others being too weak to be observed. In order to avoid the overlap of the various rings coming from xenon and iron structures, a 1016 ions/ cm2 Xe implantation has been performed in a MO thin film, under the same experimental conditions. The as-evaporated structure is deduced from the diffraction pattern (fig. 2) and the classical bee structure of MO can be clearly identified. The lattice parameter obtained (0.31 nm) is comparable with the bulk one and the mean grain size is small (about 5 nm). TEM observations after Xe implantation ( lOI ions/cm*, 330 keV; see other parameters in table 1) are shown in fig. 2. Three different structures are then detected: besides the bee MO structure already observed in the as-evaporated state, a typical fee structure also appears. Moreover, a central ring characteristic of inert gas bubbles is now detected. In order to prevent secondary phenomenon during implantation, especially sputtering of aluminium holders, an X-ray analysis has been performed using a classical Link device. Fig. 3 shows the composition of the as-implanted film. Obviously, the help of a, supplementary fee component is not necessary; only MO and Xe are present in the film. Usual MO oxides and carbides do not correspond to the observed supplementary fee structure. In all cases such pollution is not expected in the experimental conditions described above and cannot explain the extra rings of fig. 2c. The lattice parameter of this new fee structure is found to be 0.42 nm, corresponding to the fee MO phase observed by Aggarwal and Goswami [6]. It seems reasonable to think that in addition with bee MO and solid Xe bubbles, an amount of fee molybdenum is also observed. A more probable reason could be due to the fact that the fee MO-phase is the more convenient situation (the less energetic one) to make a buffer zone between the bee MO film and the fee Xe bubbles. However, it is somewhat surprising that the similar fee phase is not detected after Xe implantation 40
July 1993
MATERIALS LETTERS
4
2
6
8
10
Fig. 3. X-ray analysis of molybdenum thin films after lOI Xe ion/cm* implantation.
I
-0 0
Xe fee sites
0
MO fee sites
/ / 0’
0
Fig. 4. Coexistence of fee Xe bubbles and fee MO.
of iron films where the fee structure can be obtained in an easier way. Assuming fee solid Xe bubbles, the central ( 111) ring leads to a lattice parameter of about 0.6 nm. In this case, there will be no mismatch between the { IOO} fee molybdenum planes and the { 100) planes of solid Xe (fig. 4). This suitable situation could be the reason why an fee MO phase appears under Xe implantation, instead of the usual hypothesis where
Volume 17, number I ,2
July 1993
MATERIALS LETTERS
close-packed fee Xe { 111) planes are along the closepacked bee { 110) planes, as probably observed in iron films. The appearance of the fee iron as a buffer be-
sure in Xe bubbles
tween bee iron matrix and fee Xe bubbles being in this case less probable because of the large mismatch of the lattice parameters of the two fee structures. The atomic volume for fee Xe bubbles in MO is 54x lo-’ nm3/at, close to the value of 59.6x 1O-3 nm3/at reported in ref. [ 71 for fee Xe bubbles in fee bulk matrices. From the Ronchi equation [ 81, the internal pressure in Xe bubbles in MO films is close to 1.6 GPa instead of 6 GPa in Fe films.
Acknowledgement
4. Conclusion In both iron and molybdenum thin films, Xe-ion implantations performed at room temperature produce: - fee Xe solid bubbles with lattice parameters a, ~0.56 nm and az= 0.6 nm in iron and molybdenum respectively; - the appearance of the fee MO phase in order to reduce the lattice parameter mismatch between Xe bubbles and bee MO matrix. Thus, the internal pres-
decreases
in molybdenum
films.
The authors would like to thank Professor C. Templier for many valuable discussions.
References [ 11 P.J. Burnett and T.F. Page, Radiat. Effects 97 ( 1986) i 23. [2] C. Templier, C. Jaouen, J.P. Riviere, J. Delafond and J. Grilhe, Compt. Rend. Acad. Sci. (Paris) 299 ( 1984) 613. [3] A. von Felde, J. Fink, Th. Miiller-Heinzerling, J. Pfuger, B. Scheerer, G. Linker and D. Kaletta, Phys. Rev. Letters 53 (1984) 92. [ 41 J.H. Evans and D.J. Mazey, Scripta Metall. 19 ( 1985) 62 1. [ 51J.F. Ziegler, J.P. Biersack and V. Littmark, The stopping and range of ions in solids (Pergamon Press, Oxford, 1988 ). [6] P.S. Aggatwal and A. Goswami, Proc. Phys. Sot. 70 ( 1957) 708. [ 71 C. Tempiier, in: Fundamental aspects of inert gases in solids, eds. SE. Donnelly and J.H. Evans (Plenum Press, New York, 1990) p. 117. [ 8] C. Ronchi, J. Nucl. Mater. 96 ( 198 1) I 34.
41