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Observations on sputtered epitaxial Au3Cd films
Mat. Res. Bull. Vol. 13, pp. 723-727, 1978. Pergamon P r e s s , Inc. Printed in the United States.
OBSERVATIONS ON SPUTTERED EPITAXIAL AU3Cd FILMS
T. E. Huson and C. M. Wayman Department of Metallurgy and Mining Engineering and Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, Illinois 61801
(Received May 30, 1978; Communicated by J. J. Gilman)
ABSTRACT [001} epitaxial films of Au3Cd were deposited on heated NaCI following an appreciable loss of Cd dOring sputtering. After removal from the substrates the films were observed by transmission electron microscopy and diffraction to undergo an order-disorder reaction at 300°C although the corresponding reaction in bulk materials occurs at 412°C. The disorder-reaction was observed to occur within I0 days after the films were removed from the substrates, but attachment to the substrates inhibited ordering indefinitely. The Au3Cd superlattice was verified by electron diffraction and lattice imaging.
Introduction Numerous studies on the gold-rich portion of the Au-Cd system have identified three phases, ~, ~i, ~2 which are f.c.c., f.c.t., and h.c.p., respectively (1-6). The ~i phase is ordered and corresponds to the stoichiometric composition Au3Cd , with fundamental lattice parameters af = 4.109 and cf = 4.135A at 25 at.% Cd. The unit cell is composed of four fundamental unit cells aligned in a one-dimensional antiphase structure with dimensions a = af and c = 4cf. The fundamental unit cells in an antiphase domain period, M ffi 2, are shown in Fig. i, where Mf is the antiphase length. No previous work involving electron microscopy of Au3Cd has been reported, on specimens either thinned from the bulk or epitaxially deposited. Thus the present work is a contribution in this respect. Procedure Au3Cd specimens were prepared by the RF sputtering of a target consisting of Au-47.5 at.% Cd onto heated (275-325°C) NaCI substrates with an [001} cleavage surface. Following sputtering, the films were annealed on the substrates for 1-3 hours. After cooling to room temperature the films were floated off the substrates in water and scooped onto copper grids for examination by electron microscopy at 125KV. The film composition was ascertained by the appearance of superlattice reflections, which are found only over a narrow composition range near 25.0 at.% Cd. Considering the target material
723
724
T . E . HUSON, et al.
o To".r.
~
.
I' Figure i.
"i" . " M
.
o
.
'.
• "['1~
~1
One-dimensional long period superlattice with antiphase domain width M.
000
Figure 3.
o
Vol. 13, No. 7
•
= Fundamental Reflections
•
= Superlattice Reflections
080
Schematic of Fig. 2 showing reflections used for dark field imaging.
Vol. 13, No. 7
EPITAXIAL AU3Cd FILMS
and the final film composition, it is evident that a change of Cd content of nearly 25 at.% between the target and the substrate occurred.
Figure 2.
Transmission electron diffraction pattern of ordered, epitaxial Au3Cd film.
Figure 4.
Dark field image taken from three beams
Figure 5.
Dark field image taken from five beams shown in Fig. 3.
725
726
T . E . HUSON, et al.
Vol. 13, No. 7
Results The ~l-~lorder-disorder reaction was examined by transmission electron diffraction using a heating stage. The disordering of an initially ordered specimen was found to occur at 300°C and to be reversible upon cooling. This reversibility was lost, however, if specimens were heated above 325°C because of a change in composition through Cd evaporation. In contrast, the ~I-~ reaction occurs at 412°C in bulk specimens. Some specimens did not show ordering immediately after removal from the substrate although others prepared under identical conditions were ordered. Subsequent examination of the non-ordered specimens kept at room temperature for 7-10 days exhibited a gradual increase in the intensity of superlattice reflections. Specimens in contact with the NaCI substrates remained disordered until they were removed, after which ordering gradually occurred. This indicates that the substrate exerts a constraint on the interchange between atoms required for ordering. Fig. 2 shows the observed reciprocal lattice section of an ordered film in (001) orientation. This diffraction pattern is consistent with expectations if the superperiod extends in three directions. The lattice parameter of this film, as measured from the diffraction pattern (the effects of tetrag0~ality being small were ignored), was 4.17~ giving a superperlod length of 16.7A, compared to the value 16.54~ for bulk specimens. Dark field imaging using the groups of three and five superlatt~ce reflections ~ndicated in Fig. 3 showed fringes of spacing approximately 20A (at "A") and 31A (at "B") for both the three- and five-spot cases, Figs. 4 and 5, respectively. The smallest distance between superlattice spots corresponds inversely to the superperlod length (7) and hence the observed fringes should correspond to the superlattice dimensions. The ~rlnge spacing predicted from the electron di~fractlon lattice parameter, 16.TA, compares reasonably with the observed 20A fringes. The image obtained from five superlattice reflections, Fig. 5, involves two possible diffraction vectsrs at 45 ° angles with magnitudes in the ratio V2 which would explain th~ 31A fringes in Fig. 5 lying at 45 ° relative to the 20~ fringes. The 31A fringes arising from the three-spot images, Fig. 4 at "B", may result from double diffraction, too faint to be noted in the observed diffraction patterns. This would account for the faintness of the fringes and their appearance in only one direction, as compared to the fringe patterns resulting from the dark field images using five superlattice reflections. Acknowledgements This work was supported by the National Science Foundation through the Materials Research Laboratory at the University of Illinois. References i.
E. A. Owen and E. A. Roberts, J. Inst. Met., 66, 389 (1940).
2.
P. Saldau, Intern. Z. Metallog., ~, 3 (1915).
3.
W. Koster and A. Schneider, Z. Metall., 32, 156 (1940).
4.
A. Bystrom and K. E. Almln, Acta Chem. Scand.,!,
5.
K. Schubert et al., Z. Metall., 46, 692 (1956).
6.
H. Hirabayashi and S. Ogawa, Acta Met., 9, 264 (1961).
7.
H. Sato and R. S. Toth, Phys. Rev., 127, 469 (1962).
76 (1947).
OBSERVATIONS ON SPUTTERED EPITAXIAL AU3Cd FILMS
T. E. Huson and C. M. Wayman Department of Metallurgy and Mining Engineering and Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, Illinois 61801
(Received May 30, 1978; Communicated by J. J. Gilman)
ABSTRACT [001} epitaxial films of Au3Cd were deposited on heated NaCI following an appreciable loss of Cd dOring sputtering. After removal from the substrates the films were observed by transmission electron microscopy and diffraction to undergo an order-disorder reaction at 300°C although the corresponding reaction in bulk materials occurs at 412°C. The disorder-reaction was observed to occur within I0 days after the films were removed from the substrates, but attachment to the substrates inhibited ordering indefinitely. The Au3Cd superlattice was verified by electron diffraction and lattice imaging.
Introduction Numerous studies on the gold-rich portion of the Au-Cd system have identified three phases, ~, ~i, ~2 which are f.c.c., f.c.t., and h.c.p., respectively (1-6). The ~i phase is ordered and corresponds to the stoichiometric composition Au3Cd , with fundamental lattice parameters af = 4.109 and cf = 4.135A at 25 at.% Cd. The unit cell is composed of four fundamental unit cells aligned in a one-dimensional antiphase structure with dimensions a = af and c = 4cf. The fundamental unit cells in an antiphase domain period, M ffi 2, are shown in Fig. i, where Mf is the antiphase length. No previous work involving electron microscopy of Au3Cd has been reported, on specimens either thinned from the bulk or epitaxially deposited. Thus the present work is a contribution in this respect. Procedure Au3Cd specimens were prepared by the RF sputtering of a target consisting of Au-47.5 at.% Cd onto heated (275-325°C) NaCI substrates with an [001} cleavage surface. Following sputtering, the films were annealed on the substrates for 1-3 hours. After cooling to room temperature the films were floated off the substrates in water and scooped onto copper grids for examination by electron microscopy at 125KV. The film composition was ascertained by the appearance of superlattice reflections, which are found only over a narrow composition range near 25.0 at.% Cd. Considering the target material
723
724
T . E . HUSON, et al.
o To".r.
~
.
I' Figure i.
"i" . " M
.
o
.
'.
• "['1~
~1
One-dimensional long period superlattice with antiphase domain width M.
000
Figure 3.
o
Vol. 13, No. 7
•
= Fundamental Reflections
•
= Superlattice Reflections
080
Schematic of Fig. 2 showing reflections used for dark field imaging.
Vol. 13, No. 7
EPITAXIAL AU3Cd FILMS
and the final film composition, it is evident that a change of Cd content of nearly 25 at.% between the target and the substrate occurred.
Figure 2.
Transmission electron diffraction pattern of ordered, epitaxial Au3Cd film.
Figure 4.
Dark field image taken from three beams
Figure 5.
Dark field image taken from five beams shown in Fig. 3.
725
726
T . E . HUSON, et al.
Vol. 13, No. 7
Results The ~l-~lorder-disorder reaction was examined by transmission electron diffraction using a heating stage. The disordering of an initially ordered specimen was found to occur at 300°C and to be reversible upon cooling. This reversibility was lost, however, if specimens were heated above 325°C because of a change in composition through Cd evaporation. In contrast, the ~I-~ reaction occurs at 412°C in bulk specimens. Some specimens did not show ordering immediately after removal from the substrate although others prepared under identical conditions were ordered. Subsequent examination of the non-ordered specimens kept at room temperature for 7-10 days exhibited a gradual increase in the intensity of superlattice reflections. Specimens in contact with the NaCI substrates remained disordered until they were removed, after which ordering gradually occurred. This indicates that the substrate exerts a constraint on the interchange between atoms required for ordering. Fig. 2 shows the observed reciprocal lattice section of an ordered film in (001) orientation. This diffraction pattern is consistent with expectations if the superperiod extends in three directions. The lattice parameter of this film, as measured from the diffraction pattern (the effects of tetrag0~ality being small were ignored), was 4.17~ giving a superperlod length of 16.7A, compared to the value 16.54~ for bulk specimens. Dark field imaging using the groups of three and five superlatt~ce reflections ~ndicated in Fig. 3 showed fringes of spacing approximately 20A (at "A") and 31A (at "B") for both the three- and five-spot cases, Figs. 4 and 5, respectively. The smallest distance between superlattice spots corresponds inversely to the superperlod length (7) and hence the observed fringes should correspond to the superlattice dimensions. The ~rlnge spacing predicted from the electron di~fractlon lattice parameter, 16.TA, compares reasonably with the observed 20A fringes. The image obtained from five superlattice reflections, Fig. 5, involves two possible diffraction vectsrs at 45 ° angles with magnitudes in the ratio V2 which would explain th~ 31A fringes in Fig. 5 lying at 45 ° relative to the 20~ fringes. The 31A fringes arising from the three-spot images, Fig. 4 at "B", may result from double diffraction, too faint to be noted in the observed diffraction patterns. This would account for the faintness of the fringes and their appearance in only one direction, as compared to the fringe patterns resulting from the dark field images using five superlattice reflections. Acknowledgements This work was supported by the National Science Foundation through the Materials Research Laboratory at the University of Illinois. References i.
E. A. Owen and E. A. Roberts, J. Inst. Met., 66, 389 (1940).
2.
P. Saldau, Intern. Z. Metallog., ~, 3 (1915).
3.
W. Koster and A. Schneider, Z. Metall., 32, 156 (1940).
4.
A. Bystrom and K. E. Almln, Acta Chem. Scand.,!,
5.
K. Schubert et al., Z. Metall., 46, 692 (1956).
6.
H. Hirabayashi and S. Ogawa, Acta Met., 9, 264 (1961).
7.
H. Sato and R. S. Toth, Phys. Rev., 127, 469 (1962).
76 (1947).