- Email: [email protected]
Solid phase reaction and epitaxy of Gd-silicide films on Si substrates
Applied Surface Science 70/71 (1993) 466-469 North-Holland
applied surface s c i e n c e
Solid phase reaction and epitaxy of Gd-silicide films on Si substrates G. Molnfir, G. P e t 6 a n d E. Z s o l d o s KFKI-Research Institute for Materials Science, P.O. Box 49, H-1525 Budapest, Hungary Received 31 August 1992; accepted for publication 11 October 1992
The solid phase reaction of e-beam evaporated Gd thin film and Si substrate was investigated by X-ray diffraction and scanning electron microscopy. A description has been developed for the kinetics of the reaction, which showed sequential, selective growth of two equilibrium phases. By strict control of the growth parameters it was possible to reach hexagonal GdSil. 7 epitaxy on (111) and orthorhombic GdSi 2 epitaxy on (100) Si substrates. In the lateral growth mode with a Ti step, Gd-Si showed a rapid and nonuniform reaction.
1. Introduction The rare-earth (RE) silicides have attracted attention because of their unusual properties of formation and electrical conduction. They form the lowest Schottky barrier on n-type silicon ( 0.4 eV) [1]. Their formation temperature is very low (300-350°C). To avoid the oxidation of the extremely reactive R E thin films, in situ annealing is recommended [2]. Previous studies reported that the growth of silicide is nonuniform [3] and the dominant diffusing component is silicon [4]. An illustrative example of rare-earths is gadolinium. Our group proposed that the silicide formation begins with silicon diffusion along the grain boundaries of Gd and the reaction proceeds at the G d - S i interface and at the grain boundaries of Gd simultaneously [5]. It was shown that the G d - S i system has two silicide phases [2], in contrast with other RE-silicides, which have only one hexagonal A1B 2 type phase. Knapp and Picraux investigated the epitaxy of rare-earth silicides but they could not find epitaxial Gd-silicide [6]. Later we prepared orthorhombic GdSi 2 epitaxy on (100) and hexagonal GdSil. 7 epitaxy on (111) Si [81.
This paper will demonstrate the evolution of the G d - S i solid phase reaction, epitaxial growth and an experiment on the gadolinium-silicide lateral growth.
2. Experimental We used p-type (111) and (100) oriented silicon wafers. Immediately before loading into the oil free evaporation system, the samples were dipped into diluted HF. After evacuation for 10 hours the base pressure was 1 x 10 -6 Pa. Prior to evaporation the Si wafers were annealed in situ for 5 rain at 800°C. Gd ingots of 99.9% purity were evaporated using an electron gun at 3 x 10 - 6 Pa at an evaporation rate of 0.5-1.0 nms -t. The film thickness of deposited Gd was measured by a vibrating quartz. The samples were annealed in situ under ( 3 - 5 ) × 10 - 6 Pa pressure. Samples were heated by radiation via resistively heated tungsten spirals. The temperature were monitored by Ni-NiCr thermocouples of small heat capacity. A series of samples were made on (111) Si with 100 nm gadolinium thickness to control the
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
467
G. Molndr et al. / Gd-silicide films on Si substrates
reaction kinetics of the silicide formation. The wafers were annealed for 5 min at 320, 350, 400, 450 and 600°C. To grow epitaxial layers samples were prepared on (111) and (100) silicon with 50 nm Gd thickness, 1.5 min at 350°C and 5 min at 500°C heat treatments, respectively. To study the lateral growth of Gd-silicides; 40 nm titanium was deposited onto a Si wafer• Then the Ti was totally removed from one half of the wafer by chemical etching, which resulted a sharp Ti step on the Si wafer. Then 100 nm Gd was evaporated onto both half of the wafer and annealed at 350°C for 1 hour. The silicide phases formed were identified by X-ray diffraction. The lateral growth of silicide phases was investigated by a J E O L scanning electron microscope.
3. Results and discussion 3.1. Silicide phase formation
The X-ray diffraction patterns of samples initially with 100 nm Gd contain lines of hexagonal GdSil. 7 and orthorhombic GdSi 2. These two phases are mixed inside the layer. The determination of the quantity of phases is a very difficult problem, because these compound structures moreover contain a lot of vacancies mainly in the Si sublattice [9]. The exact connection between the diffraction peak intensities and phase quantities is unknown, but the changing of the selected peak intensity ratios contains relevant information. The selected lines are the (001) line (d = 0.147 nm) for hexagonal GdSil. 7 and the (112) line (d = 0.263 nm) for orthorhombic GdSi 2. The systematical changing of the peak intensity ratios as a function of annealing t e m p e r a t u r e can be seen on fig. 1. This figure shows the steep rising of the G d S i 2 / G d S i l . 7 intensity ratio. The above results can be explained with the help of the G 6 s e l e - T u model [10]. This model is an appropriate description for the G d - S i reaction. The first phase appearing is hexagonal GdSil. 7 and it grows until all Gd metal is reacted, then
3J I / /
=..2!
/ / /
0
/ t I i 300
i f
t~
i 400
i 500
i
•
600 TEME
[°C ]
Fig. 1. X-ray diffraction intensity ratios of selected lines (IGdsi2//IGdsil.7) as a function of temperature. The Gd film thickness was 100 nm and the annealing time was 5 rain.
the second the orthorhombic GdSi 2 will form (fig. 2). The important parameters, time and temperature of the annealing and the film thickness determine together the reaction product. Two of these parameters should be kept constant and one may vary for the description of the formation process. This varying p a r a m e t e r could not be the time in the case of conventional heat treatment, because of the very short annealing times in comparison with the t e m p e r a t u r e rise time (1 min). The approximate time dependence of the reaction could be given indirectly via varying the t e m p e r a t u r e (fig. 1) or the film thickness [2]. 3.2. Epitaxy o f the silicides
The knowledge of the formation process gives a chance for us to try preparing epitaxial gadolin-
<= 0 t~
TIME
t2
Fig. 2. Model for evolution of the Gd silicide phases as a function of time. t 1 signs when all Gd metal transformed to GdSil.7, t 2 signs when all GdSil.7 transformed to the second (GdSi 2) phase.
468
G. Molndr et al. / Gd-silicide films on Si substrates
3b. Thicker films cannot be grown epitaxially because of the lattice misfit.
a, epitaxial GdSil, 7
3.3. Lateral growth O3
In lateral geometry the diffusion length is extended. The 40 nm titanium step on the (111) oriented Si wafer prevents direct silicon diffusion into the upper gadolinium layer on one half of the wafer (fig. 4), because at 350°C Ti does not form any silicide, while Gd reacts totally. The silicon diffuses and reacts only in lateral mode over the Ti step. As it can be seen from the SEM picture (fig. 4) the silicide formation takes place
Z
rr
03 Z I
b, epitaxial GdSi2
UJ Z
o
200
400
60 °
80 ° 20--
Fig. 3. X-ray diffraction pattern of epitaxial (a) hexagonal GdSil.7 on (111), (b) orthorhombic GdSi 2 on the (100) Si substrate.
ium silicide films. The lattice parameters of the first forming hexagonal GdSil. 7 are a = 0.388 nm, c = 0.417 nm and for (111) Si a = 0.383 nm. The misfit is better than 1%. From the previous description of the reaction we estimated the values of the growth parameters to prepare clean GdSil. 7 phase (fig. 2, point tl). These parameters are for a 50 nm thick film; 350°C, 1.5 min annealing. The X-ray diffraction pattern of the sample can be seen on fig. 3a. Longer annealing destroys the epitaxy, because at the interface appears the second orthorhombic phase. The lattice parameters of orthorhombic GdSi 2 are a = 0.401 nm, b = 0.409 nm, c = 1.344 nm and those of Si (100) are a = b = c = 0.543 nm. These values are too far apart from each other for an epitaxial growth. Taking the half diagonal of the Si lattice (x/2a/2 = 0.384 nm), it is possible to grow epitaxial GdSi 2 on (100) Si, because the misfit using this 45 ° rotated geometry is about 4%. The X-ray diffraction pattern of the sample with 50 nm evaporated Gd on (100) Si and annealed at 500°C for 5 min can be seen on fig.
Gd-
si l i c i d e
Gd on
on
i
Ti s t e p
Si
t ! Q
.~t~cide
•
G6- j s Ti
H
Gd
step
Si
II
Fig. 4. SEM photograph of the ]ateral Od silicide growth with the schematics of the geometry.
G. Molndr et al. / Gd-silicide films on Si substrates
in a lateral region of ~ 2 0 /zm after 1 hour annealing. The silicide-gadolinium interface is rough. On the Ti step mainly the hexagonal GdSil. 7 phase is present according to X-ray diffraction. The silicide lifts up at the Ti step edge by 40 nm (the thickness of the Ti step) and this height gradually decreases to the height of unreacted Gd. In the lateral growth area a periodic structure of the Gd-silicide can be observed. This structure consists of semicircular (with 5 - 8 /zm diameter) groups of silicide grains about 10 /zm apart from each other periodically along the edge of the Ti step in the lateral growth region (fig. 4). This semicircular grain structure could be caused by a kind of nucleation effect at the beginning of the lateral growth. The reason of the unexpected periodicity is unknown and could induce more refined experiments. The result of this experiment shows that the silicide formation is very rapid and produces complicated interfaces.
4. Conclusion The solid phase, thin film reaction of G d - S i diffusion couples shows sequential, selective growth of two equilibrium phases. The G/Ssele-Tu model is applicable to describe the formation process. With the help of this description and by the strict control of growth p a r a m e t e r s it was possible to reach hexagonal GdSiL7 epitaxy on
469
(111) and orthorhombic GdSi 2 epitaxy on (100) silicon. The lateral growth of Gd-silicides showed a rapid, nonuniform reaction.
Acknowledgements The authors would like to thank Szvetlana Sfindor for the SEM pictures. This work was partially supported by Hungarian Academy of Science O T K A G r a n t No. 2963.
References [1] K.N. Tu, R.D. Thompson and B.Y. Tsaur, Appl. Phys. Lett. 38 (1981) 626. [2] G. Moln~r, I. Ger6cs, G. Pet6, E. Zsoldos, E. J~iroli and J. Gyulai, J. Appl. Phys. 64 (1988) 6746. [3] B.Y. Tsaur and L.S. Hung, Appl. Phys. Lett. 37 (1980) 922. [4] J.E. Baglin, F.M. d'Heurle and C.S. Petersson, J. Appl. Phys. 52 (1981) 2841. [5] G. Moln~r, G. Pet6, E. K6tai, E. Zsoldos, J. Gyulai and L. Guczi, Surf. Interf. Anal. 19 (1992) 469. [6] J.A. Knapp and S.T. Picraux, Appl. Phys. Lett. 48 (1986) 466. [7] I. Ger6cs, G, Moln~r, E. Jfiroli, E. Zsoldos, G. Pet6, J. Gyulai and E. Bugiel, Appl. Phys. Lett. 51 (1987) 2144. [8] G. Moln~r, I. Ger6cs, G. Pet6, E. Zsoldos, J. Gyulai and E. Bugiel, Appl. Phys. Lett. 58 (1991) 249. [9] J.A. Perri, I. Binder and B. Post, J. Phys. Chem. 63 (1959) 616. [10] U. Gfisele and K.N. Tu, J. Appl. Phys. 53 (1982) 3252.
applied surface s c i e n c e
Solid phase reaction and epitaxy of Gd-silicide films on Si substrates G. Molnfir, G. P e t 6 a n d E. Z s o l d o s KFKI-Research Institute for Materials Science, P.O. Box 49, H-1525 Budapest, Hungary Received 31 August 1992; accepted for publication 11 October 1992
The solid phase reaction of e-beam evaporated Gd thin film and Si substrate was investigated by X-ray diffraction and scanning electron microscopy. A description has been developed for the kinetics of the reaction, which showed sequential, selective growth of two equilibrium phases. By strict control of the growth parameters it was possible to reach hexagonal GdSil. 7 epitaxy on (111) and orthorhombic GdSi 2 epitaxy on (100) Si substrates. In the lateral growth mode with a Ti step, Gd-Si showed a rapid and nonuniform reaction.
1. Introduction The rare-earth (RE) silicides have attracted attention because of their unusual properties of formation and electrical conduction. They form the lowest Schottky barrier on n-type silicon ( 0.4 eV) [1]. Their formation temperature is very low (300-350°C). To avoid the oxidation of the extremely reactive R E thin films, in situ annealing is recommended [2]. Previous studies reported that the growth of silicide is nonuniform [3] and the dominant diffusing component is silicon [4]. An illustrative example of rare-earths is gadolinium. Our group proposed that the silicide formation begins with silicon diffusion along the grain boundaries of Gd and the reaction proceeds at the G d - S i interface and at the grain boundaries of Gd simultaneously [5]. It was shown that the G d - S i system has two silicide phases [2], in contrast with other RE-silicides, which have only one hexagonal A1B 2 type phase. Knapp and Picraux investigated the epitaxy of rare-earth silicides but they could not find epitaxial Gd-silicide [6]. Later we prepared orthorhombic GdSi 2 epitaxy on (100) and hexagonal GdSil. 7 epitaxy on (111) Si [81.
This paper will demonstrate the evolution of the G d - S i solid phase reaction, epitaxial growth and an experiment on the gadolinium-silicide lateral growth.
2. Experimental We used p-type (111) and (100) oriented silicon wafers. Immediately before loading into the oil free evaporation system, the samples were dipped into diluted HF. After evacuation for 10 hours the base pressure was 1 x 10 -6 Pa. Prior to evaporation the Si wafers were annealed in situ for 5 rain at 800°C. Gd ingots of 99.9% purity were evaporated using an electron gun at 3 x 10 - 6 Pa at an evaporation rate of 0.5-1.0 nms -t. The film thickness of deposited Gd was measured by a vibrating quartz. The samples were annealed in situ under ( 3 - 5 ) × 10 - 6 Pa pressure. Samples were heated by radiation via resistively heated tungsten spirals. The temperature were monitored by Ni-NiCr thermocouples of small heat capacity. A series of samples were made on (111) Si with 100 nm gadolinium thickness to control the
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
467
G. Molndr et al. / Gd-silicide films on Si substrates
reaction kinetics of the silicide formation. The wafers were annealed for 5 min at 320, 350, 400, 450 and 600°C. To grow epitaxial layers samples were prepared on (111) and (100) silicon with 50 nm Gd thickness, 1.5 min at 350°C and 5 min at 500°C heat treatments, respectively. To study the lateral growth of Gd-silicides; 40 nm titanium was deposited onto a Si wafer• Then the Ti was totally removed from one half of the wafer by chemical etching, which resulted a sharp Ti step on the Si wafer. Then 100 nm Gd was evaporated onto both half of the wafer and annealed at 350°C for 1 hour. The silicide phases formed were identified by X-ray diffraction. The lateral growth of silicide phases was investigated by a J E O L scanning electron microscope.
3. Results and discussion 3.1. Silicide phase formation
The X-ray diffraction patterns of samples initially with 100 nm Gd contain lines of hexagonal GdSil. 7 and orthorhombic GdSi 2. These two phases are mixed inside the layer. The determination of the quantity of phases is a very difficult problem, because these compound structures moreover contain a lot of vacancies mainly in the Si sublattice [9]. The exact connection between the diffraction peak intensities and phase quantities is unknown, but the changing of the selected peak intensity ratios contains relevant information. The selected lines are the (001) line (d = 0.147 nm) for hexagonal GdSil. 7 and the (112) line (d = 0.263 nm) for orthorhombic GdSi 2. The systematical changing of the peak intensity ratios as a function of annealing t e m p e r a t u r e can be seen on fig. 1. This figure shows the steep rising of the G d S i 2 / G d S i l . 7 intensity ratio. The above results can be explained with the help of the G 6 s e l e - T u model [10]. This model is an appropriate description for the G d - S i reaction. The first phase appearing is hexagonal GdSil. 7 and it grows until all Gd metal is reacted, then
3J I / /
=..2!
/ / /
0
/ t I i 300
i f
t~
i 400
i 500
i
•
600 TEME
[°C ]
Fig. 1. X-ray diffraction intensity ratios of selected lines (IGdsi2//IGdsil.7) as a function of temperature. The Gd film thickness was 100 nm and the annealing time was 5 rain.
the second the orthorhombic GdSi 2 will form (fig. 2). The important parameters, time and temperature of the annealing and the film thickness determine together the reaction product. Two of these parameters should be kept constant and one may vary for the description of the formation process. This varying p a r a m e t e r could not be the time in the case of conventional heat treatment, because of the very short annealing times in comparison with the t e m p e r a t u r e rise time (1 min). The approximate time dependence of the reaction could be given indirectly via varying the t e m p e r a t u r e (fig. 1) or the film thickness [2]. 3.2. Epitaxy o f the silicides
The knowledge of the formation process gives a chance for us to try preparing epitaxial gadolin-
<= 0 t~
TIME
t2
Fig. 2. Model for evolution of the Gd silicide phases as a function of time. t 1 signs when all Gd metal transformed to GdSil.7, t 2 signs when all GdSil.7 transformed to the second (GdSi 2) phase.
468
G. Molndr et al. / Gd-silicide films on Si substrates
3b. Thicker films cannot be grown epitaxially because of the lattice misfit.
a, epitaxial GdSil, 7
3.3. Lateral growth O3
In lateral geometry the diffusion length is extended. The 40 nm titanium step on the (111) oriented Si wafer prevents direct silicon diffusion into the upper gadolinium layer on one half of the wafer (fig. 4), because at 350°C Ti does not form any silicide, while Gd reacts totally. The silicon diffuses and reacts only in lateral mode over the Ti step. As it can be seen from the SEM picture (fig. 4) the silicide formation takes place
Z
rr
03 Z I
b, epitaxial GdSi2
UJ Z
o
200
400
60 °
80 ° 20--
Fig. 3. X-ray diffraction pattern of epitaxial (a) hexagonal GdSil.7 on (111), (b) orthorhombic GdSi 2 on the (100) Si substrate.
ium silicide films. The lattice parameters of the first forming hexagonal GdSil. 7 are a = 0.388 nm, c = 0.417 nm and for (111) Si a = 0.383 nm. The misfit is better than 1%. From the previous description of the reaction we estimated the values of the growth parameters to prepare clean GdSil. 7 phase (fig. 2, point tl). These parameters are for a 50 nm thick film; 350°C, 1.5 min annealing. The X-ray diffraction pattern of the sample can be seen on fig. 3a. Longer annealing destroys the epitaxy, because at the interface appears the second orthorhombic phase. The lattice parameters of orthorhombic GdSi 2 are a = 0.401 nm, b = 0.409 nm, c = 1.344 nm and those of Si (100) are a = b = c = 0.543 nm. These values are too far apart from each other for an epitaxial growth. Taking the half diagonal of the Si lattice (x/2a/2 = 0.384 nm), it is possible to grow epitaxial GdSi 2 on (100) Si, because the misfit using this 45 ° rotated geometry is about 4%. The X-ray diffraction pattern of the sample with 50 nm evaporated Gd on (100) Si and annealed at 500°C for 5 min can be seen on fig.
Gd-
si l i c i d e
Gd on
on
i
Ti s t e p
Si
t ! Q
.~t~cide
•
G6- j s Ti
H
Gd
step
Si
II
Fig. 4. SEM photograph of the ]ateral Od silicide growth with the schematics of the geometry.
G. Molndr et al. / Gd-silicide films on Si substrates
in a lateral region of ~ 2 0 /zm after 1 hour annealing. The silicide-gadolinium interface is rough. On the Ti step mainly the hexagonal GdSil. 7 phase is present according to X-ray diffraction. The silicide lifts up at the Ti step edge by 40 nm (the thickness of the Ti step) and this height gradually decreases to the height of unreacted Gd. In the lateral growth area a periodic structure of the Gd-silicide can be observed. This structure consists of semicircular (with 5 - 8 /zm diameter) groups of silicide grains about 10 /zm apart from each other periodically along the edge of the Ti step in the lateral growth region (fig. 4). This semicircular grain structure could be caused by a kind of nucleation effect at the beginning of the lateral growth. The reason of the unexpected periodicity is unknown and could induce more refined experiments. The result of this experiment shows that the silicide formation is very rapid and produces complicated interfaces.
4. Conclusion The solid phase, thin film reaction of G d - S i diffusion couples shows sequential, selective growth of two equilibrium phases. The G/Ssele-Tu model is applicable to describe the formation process. With the help of this description and by the strict control of growth p a r a m e t e r s it was possible to reach hexagonal GdSiL7 epitaxy on
469
(111) and orthorhombic GdSi 2 epitaxy on (100) silicon. The lateral growth of Gd-silicides showed a rapid, nonuniform reaction.
Acknowledgements The authors would like to thank Szvetlana Sfindor for the SEM pictures. This work was partially supported by Hungarian Academy of Science O T K A G r a n t No. 2963.
References [1] K.N. Tu, R.D. Thompson and B.Y. Tsaur, Appl. Phys. Lett. 38 (1981) 626. [2] G. Moln~r, I. Ger6cs, G. Pet6, E. Zsoldos, E. J~iroli and J. Gyulai, J. Appl. Phys. 64 (1988) 6746. [3] B.Y. Tsaur and L.S. Hung, Appl. Phys. Lett. 37 (1980) 922. [4] J.E. Baglin, F.M. d'Heurle and C.S. Petersson, J. Appl. Phys. 52 (1981) 2841. [5] G. Moln~r, G. Pet6, E. K6tai, E. Zsoldos, J. Gyulai and L. Guczi, Surf. Interf. Anal. 19 (1992) 469. [6] J.A. Knapp and S.T. Picraux, Appl. Phys. Lett. 48 (1986) 466. [7] I. Ger6cs, G, Moln~r, E. Jfiroli, E. Zsoldos, G. Pet6, J. Gyulai and E. Bugiel, Appl. Phys. Lett. 51 (1987) 2144. [8] G. Moln~r, I. Ger6cs, G. Pet6, E. Zsoldos, J. Gyulai and E. Bugiel, Appl. Phys. Lett. 58 (1991) 249. [9] J.A. Perri, I. Binder and B. Post, J. Phys. Chem. 63 (1959) 616. [10] U. Gfisele and K.N. Tu, J. Appl. Phys. 53 (1982) 3252.