- Email: [email protected]
Si structure
Journal of Magnetism and Magnetic Materials 148 (1995) 83-84
~
Journalof magnetism magneilc mateflals
ELSEVIER
Positron annihilation and NMR studies of thin multilayered C o / S t structure V.V. Kotov, S.P. Likhtorovich, A.I. Matvienko, M.M. Nishchenko, A.N. Pogorily * Institute of Metal Physics, Vernadsky blvd. 36, Kiev, 252142 Ukraine
Abstract Multilayered strtictures consisting o f ten C o / S t pairs (20 and 80 nm thick, respectively) on St(100) substrates have been studied by m e a n s o f positron annihilation and N M R techniques in the process o f isochronal annealing. Two stages o f defect-structure evolution have been found and accounted for the defect annealing i.~ C o / S t film (below 400°C) and for the defect formation under the ot ~ 13 phase transition in Co layers at 470°C.
The electronic properties o f thin m e t a l - s e m i c o n d u c t o r films are affected by dopants and structural defects both at the interface and itx the layers' bulk. T h e growing requirements to the quality o f the multilayered ( M L ) structures necessitates investigation and control of their structural perfection. In the present paper we have studied the defect annealing in the M L C o / S t film deposited onto 300 ~ m thick St(100) substrate (heated to 100°(2) b y means o f thermal evaporation in v a c u u m o f ~ 10 - 4 Pa. T h e M L structure consisted o f alternating Co and Si layers 20 and 80 nm thick, respectively. The total thickness o f ten such m e t a l semiconductor pairs was 1 ~ m . Studies were carried out b y means o f positron annihilation and N M R techniques in the process o f isochronal (0.5 h) annealing in a v a c u u m o f 10 - 5 Pa in the temperature range 2 0 0 - 6 5 0 ° C . The angular distributions o f annihilation photons ( A D A P ) have been measured at room temperature b y means o f a long-slit spectrometer with an angular step o f 0.5 mrad. The standard deviation o f the resolution function ( a p p r o x i m a t e d by a Gaussian curve) was 0.55 mrad and the nurnber o f counts accumulated per spectrum amounted to ~ 1 0 6. The A D A P spectra for the M L film have been obtained b y elimination o f the substrate contribution in a m a n n e r analogous to Ref. [1]. The annihilation S parameter has been calculated as the ratio o f the A D A P intensity near the m a x i m u m (in the interval 6.5 mrad wide) to the total area o f the spectrum. In general, the S parameter reacts on the changes o f the s a m p l e ' s defect structure in such a way that an increase o f defect concentration in a sample results in an increase of S and vice versa [2].
The N M R spectra were measured at r o o m temperature. T h e spin-echo m e t h o d was applied and the spin-echo amplitudes were reduced to zero interval between the exciting pulses. The oJ 2 and to 3 corrections o f the N M R spectra w e r e not carried out. On the basis o f the annihilation S parameter b e h a v i o u r (see Fig. 1) w e can divide the anmealing range into two stages. The decrease o f S at the first stage (200-460°(2) indicates that defects anneal out. The isolated monovacancies in Si are k n o w n to anneal well b e l o w r o o m temperature [3]. The vaeancy-o~:ygen ( V - O ) and v a c a n c y - i m p u r i t y pairs are less mobile. E.g., the ( V - P ) complexes disappear after annealing at 200°C [4] and the decrease o f ( V - O ) concentration begins above 350°C and terminates at about 500°C [5]. Another complex, V - O 2, which is an electron a c c e p t e r and hence can trap positrons, anneals out at 450°C [3]. This temperature coincides with a m i n i m u m o f the S p a r a m e t e r dependence on temperature, visible in Fig. 1. The sharp increase o f S after the annealing o f C o / S t film at 5 0 0 - 5 5 0 ° C is attributed to the a --~ I~ phase transformation in Co layers, which i s manifested in the N M R spectra. A s seen from Fig. 2, the N M R spectrum for the
NO
400
-r ('c)
* Corresponding author. [email protected].
Fax:
+7-044-4441020;
emaii:
eoo
Fig. 1. Plot of annihilation parameter S vs. annealing temperature for the ML C o / S i film. S l is the S value for the initial (as-deposited) state.
0304.8853/95/$09.50 © 1995 Elsevier Science, B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 1 5 9 - X
84
V. V. Kotov et a L / Journal o f Magnetism and Magnetic Materials 148 (1995) 83-84
~0 [ "
16~/ 200
220 w/2n (MHz)
210
Fig. 2. Yariation of the N M R spectra with annealing temperature of the ML Co/Si film. The reference spectrum for the well-annealed Co powder is shown at the bottom.
initial state of the sample is a strongly broadened line of the low-temperature a phase (hcp) centered at 217 MHz. The broadening is an indication of heterogeneous field distribution at 59(::o nuclei due to the presence of the stacking faults and other defects. At low annealing temperatures (200-400°C) the number of defects in the Co layers decreases and the NMR spectra become structured and asymmetric since the stacking faults still remain [6]. The NMR peak with lower magnetic field intensity at S9Co ( o J / 2 ~ ffi (211 + 0 . 5 ) MHz) which is characteristic o f the high-temperature [3 phase (fcc) appears after the annealing of ML C o / S i film at 500°C (Fig. 2). This phase persists after cooling of the sample down to room temperatare. The intensity of the NMR peak corresponding to the phase increases with annealing temperature while that of
the low-temperature a phase decreases respectively. These results are consistent with ReL [7] where the temperature of a --, ~ phase transition on heating Co is indicated to be 470°C. It should be noted that the NMR frequencies of the fcc and hep phases for thin Co layers are shifted downwards by --, 1.5 MHz relative to those for the bulk Co (Fig. 2 at the bottom). This deviation is most likely caused by the Si penetration into the Co layers during the ML-structure fabrication and results in a reduced magnetization. The appearance of a 'valley' in the NMR spectra between the fcc and hcp lines after annealing at 500°C and above may be accounted for the annealing of stacking faults in Co [6]. However, their contribution to the S parameter variation is much less than the effect of phase transformation [8]. References
[1] Y.A. Novikov, A.V. Rakov and V.P. Shantarovich, Poverkhnost 6 (1991) 36. [2] Positrons in solids, ed. P. Hautojarvi (Springer "Vedag, Berlin, 1979). [3] V.S. Vavilov, V.F. Kiseliov and V.N. Mukasbev, Defects in silicon and on its surface (Nauka, Moscow, 1990) (in Russian). [4] X.T. Meng, A.K. Liolios, M. Chardalas, Sp. Dedoussis, C.A. Eleftheriadis and Stef. Charalambous, Phys. Lett. A 157 (1991) 73. [5] N. Fukuoka and J.W. Cleland, Rad. Eft. 51 (1980) 215. [6] N.A. Lesnik and A.N. Pogorily, Phys. Met. Metallogr. 34 (1972) 67 (in Russian). [7] R.M. Bozort, Ferromagnetism (New York, 1951). [8] A.V. Chemyashevskii, B.I. Nikolin and V.S. Mikhalenkov, Phys. Met. 9 (1990) 434.
~
Journalof magnetism magneilc mateflals
ELSEVIER
Positron annihilation and NMR studies of thin multilayered C o / S t structure V.V. Kotov, S.P. Likhtorovich, A.I. Matvienko, M.M. Nishchenko, A.N. Pogorily * Institute of Metal Physics, Vernadsky blvd. 36, Kiev, 252142 Ukraine
Abstract Multilayered strtictures consisting o f ten C o / S t pairs (20 and 80 nm thick, respectively) on St(100) substrates have been studied by m e a n s o f positron annihilation and N M R techniques in the process o f isochronal annealing. Two stages o f defect-structure evolution have been found and accounted for the defect annealing i.~ C o / S t film (below 400°C) and for the defect formation under the ot ~ 13 phase transition in Co layers at 470°C.
The electronic properties o f thin m e t a l - s e m i c o n d u c t o r films are affected by dopants and structural defects both at the interface and itx the layers' bulk. T h e growing requirements to the quality o f the multilayered ( M L ) structures necessitates investigation and control of their structural perfection. In the present paper we have studied the defect annealing in the M L C o / S t film deposited onto 300 ~ m thick St(100) substrate (heated to 100°(2) b y means o f thermal evaporation in v a c u u m o f ~ 10 - 4 Pa. T h e M L structure consisted o f alternating Co and Si layers 20 and 80 nm thick, respectively. The total thickness o f ten such m e t a l semiconductor pairs was 1 ~ m . Studies were carried out b y means o f positron annihilation and N M R techniques in the process o f isochronal (0.5 h) annealing in a v a c u u m o f 10 - 5 Pa in the temperature range 2 0 0 - 6 5 0 ° C . The angular distributions o f annihilation photons ( A D A P ) have been measured at room temperature b y means o f a long-slit spectrometer with an angular step o f 0.5 mrad. The standard deviation o f the resolution function ( a p p r o x i m a t e d by a Gaussian curve) was 0.55 mrad and the nurnber o f counts accumulated per spectrum amounted to ~ 1 0 6. The A D A P spectra for the M L film have been obtained b y elimination o f the substrate contribution in a m a n n e r analogous to Ref. [1]. The annihilation S parameter has been calculated as the ratio o f the A D A P intensity near the m a x i m u m (in the interval 6.5 mrad wide) to the total area o f the spectrum. In general, the S parameter reacts on the changes o f the s a m p l e ' s defect structure in such a way that an increase o f defect concentration in a sample results in an increase of S and vice versa [2].
The N M R spectra were measured at r o o m temperature. T h e spin-echo m e t h o d was applied and the spin-echo amplitudes were reduced to zero interval between the exciting pulses. The oJ 2 and to 3 corrections o f the N M R spectra w e r e not carried out. On the basis o f the annihilation S parameter b e h a v i o u r (see Fig. 1) w e can divide the anmealing range into two stages. The decrease o f S at the first stage (200-460°(2) indicates that defects anneal out. The isolated monovacancies in Si are k n o w n to anneal well b e l o w r o o m temperature [3]. The vaeancy-o~:ygen ( V - O ) and v a c a n c y - i m p u r i t y pairs are less mobile. E.g., the ( V - P ) complexes disappear after annealing at 200°C [4] and the decrease o f ( V - O ) concentration begins above 350°C and terminates at about 500°C [5]. Another complex, V - O 2, which is an electron a c c e p t e r and hence can trap positrons, anneals out at 450°C [3]. This temperature coincides with a m i n i m u m o f the S p a r a m e t e r dependence on temperature, visible in Fig. 1. The sharp increase o f S after the annealing o f C o / S t film at 5 0 0 - 5 5 0 ° C is attributed to the a --~ I~ phase transformation in Co layers, which i s manifested in the N M R spectra. A s seen from Fig. 2, the N M R spectrum for the
NO
400
-r ('c)
* Corresponding author. [email protected].
Fax:
+7-044-4441020;
emaii:
eoo
Fig. 1. Plot of annihilation parameter S vs. annealing temperature for the ML C o / S i film. S l is the S value for the initial (as-deposited) state.
0304.8853/95/$09.50 © 1995 Elsevier Science, B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 1 5 9 - X
84
V. V. Kotov et a L / Journal o f Magnetism and Magnetic Materials 148 (1995) 83-84
~0 [ "
16~/ 200
220 w/2n (MHz)
210
Fig. 2. Yariation of the N M R spectra with annealing temperature of the ML Co/Si film. The reference spectrum for the well-annealed Co powder is shown at the bottom.
initial state of the sample is a strongly broadened line of the low-temperature a phase (hcp) centered at 217 MHz. The broadening is an indication of heterogeneous field distribution at 59(::o nuclei due to the presence of the stacking faults and other defects. At low annealing temperatures (200-400°C) the number of defects in the Co layers decreases and the NMR spectra become structured and asymmetric since the stacking faults still remain [6]. The NMR peak with lower magnetic field intensity at S9Co ( o J / 2 ~ ffi (211 + 0 . 5 ) MHz) which is characteristic o f the high-temperature [3 phase (fcc) appears after the annealing of ML C o / S i film at 500°C (Fig. 2). This phase persists after cooling of the sample down to room temperatare. The intensity of the NMR peak corresponding to the phase increases with annealing temperature while that of
the low-temperature a phase decreases respectively. These results are consistent with ReL [7] where the temperature of a --, ~ phase transition on heating Co is indicated to be 470°C. It should be noted that the NMR frequencies of the fcc and hep phases for thin Co layers are shifted downwards by --, 1.5 MHz relative to those for the bulk Co (Fig. 2 at the bottom). This deviation is most likely caused by the Si penetration into the Co layers during the ML-structure fabrication and results in a reduced magnetization. The appearance of a 'valley' in the NMR spectra between the fcc and hcp lines after annealing at 500°C and above may be accounted for the annealing of stacking faults in Co [6]. However, their contribution to the S parameter variation is much less than the effect of phase transformation [8]. References
[1] Y.A. Novikov, A.V. Rakov and V.P. Shantarovich, Poverkhnost 6 (1991) 36. [2] Positrons in solids, ed. P. Hautojarvi (Springer "Vedag, Berlin, 1979). [3] V.S. Vavilov, V.F. Kiseliov and V.N. Mukasbev, Defects in silicon and on its surface (Nauka, Moscow, 1990) (in Russian). [4] X.T. Meng, A.K. Liolios, M. Chardalas, Sp. Dedoussis, C.A. Eleftheriadis and Stef. Charalambous, Phys. Lett. A 157 (1991) 73. [5] N. Fukuoka and J.W. Cleland, Rad. Eft. 51 (1980) 215. [6] N.A. Lesnik and A.N. Pogorily, Phys. Met. Metallogr. 34 (1972) 67 (in Russian). [7] R.M. Bozort, Ferromagnetism (New York, 1951). [8] A.V. Chemyashevskii, B.I. Nikolin and V.S. Mikhalenkov, Phys. Met. 9 (1990) 434.