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Epitaxial Y2O3 film growth on an oxidized Si surface
Thin Solid Films 402 (2002) 38–42
Letter
Epitaxial Y2O3 film growth on an oxidized Si surface M.-H. Choa,*, D.-H. Kob, Y.K. Choia, I.W. Lyoa, K. Jeonga, C.N. Whanga a
Atomic-scale Surface Science Research Center and Department of Physics, Yonsei University, Seoul, 120-749 South Korea b Department of Ceramics Engineering, Yonsei University, Seoul, 120-749, South Korea Received 27 September 2000; received in revised form 30 August 2001; accepted 6 September 2001
Abstract The effects of the Si surface state on epitaxial growth of Y2 O3 layers were investigated by various measurement methods. The characterization using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS) and high-resolution transmission electron microscopy (HRTEM) shows excellent film crystallinity when grown on an oxidized Si surface. The crystalline structure of the film was influenced by the interfacial mosaic structure, which depended on whether the Si surface contained adsorbed O or not. The thin SiO2 layer of approximately 1.5 nm, provided favorable interfacial reaction sites for the nucleation of Y2O3, and still maintained the structural registry with the underlying Si substrate. In particular, the reaction between the Y and SiO2 layer resulted in coherent finite growth, whereas the direct interaction between Y and Si was hindered by the nucleation of Y2O3. The high-quality epitaxial layer with the minimum channel yield (xmin ), lower than 3%, could be grown on the oxidized Si surface. 䊚 2002 Elsevier Science B.V. All rights reserved. PACS: 68.10.Jy; 68.55.-a; 81.15.Hi; 61.72.-y Keywords: Oxides; Yttrium
Over the years, the growth of oxide films on Si has been an area of intense research interest due to the stringent requirements for thin insulating layers used in highly integrated circuits w1,2x. Examples of these include perovskite oxide films such as SrTiO3 and BaTiO3, and these have been applied to the application of memory devices w3,4x. Others include fluorite-type oxides such as CeO2, ZrO2, PrO2 and Y2O3, and these have been used as the epitaxial buffer layers for the growth of high temperature superconducting oxide films w5–9x. They are also attractive for various applications including storage capacitors and gate insulators in dynamic random access memory. Among the oxide materials, Y2O3 has attracted the utmost attention because of its wide energy band gap, high dielectric constant (13–17), good thermal stability (up to 23258C), and well-matched lattice constant with Si wa(Y2O3)s * Corresponding author. E-mail address: [email protected] (M.-H.-H. Cho).
1.06 nm, a(Si)=2s1.086 nmx. Despite intense efforts to grow the Y2O3 films epitaxially using various deposition methods w10–13x, only a few of these methods have reported the epitaxial growth of Y2O3 films on Si w12,13x. Even in these cases, the crystal quality was not good because of suspected growth kinetics of the film. Moreover, the complex crystal structure of Y2O3 and the complication of defects in the film attracted less attention to the growth system. In this paper, we focus on the effects that the surface state has on the crystalline structure of the Y2O3 nucleation layers. Our study revealed that the ordering of the atomic positions in Y2O3 films increases significantly as the Si surface becomes oxidized. As direct interaction between the deposited Y film and the substrate increased, a silicide layer appeared, and an island-like growth was enhanced. On the oxidized Si surface, Y2O3 film growth was obtained without the growth of a silicide layer. Moreover, we demonstrate the growth of high-quality epitaxial Y2O3 films on a wet chemically
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 6 2 5 - X
M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
oxidized Si(111) surface; Rutherford backscattering spectroscopy (RBS)ychanneling yield (xmin) equals 3%. The thickness of the oxidized layer is expected to be 1.5 nm, which hinders the epitaxial growth. A film with sharp and abrupt interfaces can be grown because the oxidized layer controls the reaction between Y, O and Si. It would provide fundamental information on the growth kinetics of the Y2O3 film on Si. The epitaxial Y2O3 films were grown using the deposition system consisting of a load-lock chamber, a growth chamber with reflection high-energy electron diffraction (RHEED)w13x. Metal Y in W crucible was evaporated using an electron beam. Si substrates with 48 vicinal (111) orientations were chemically cleaned using the RCA method w14,15x. The sequential cleaning solutions used were NH4OH–H2O2–H2O and H2SO4– H2O2–H2O. The RCA cleaning resulted in the formation of 4–6 monolayers of SiO2, removing organic and metallic residues on the surface at the same time w14,15x. The oxidized surface formed using the RCA method and the clean surface with the 7=7 structure were used. The base pressure of the growth chamber was set under 5=10y10 torr. The substrates were heated up to 5008C during the deposition of Y. At the same time, O2 gas was directly injected onto the wafer with a background partial pressure of O2 raised to 1=10y5 torr to grow Y2O3 film. The typical Y2O3 film growth rate was ˚ approximately 0.2 Ays. To investigate the growth of the epitaxial layer, in-situ RHEED, X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and high resolution transmission electron microscopy (HRTEM) were used. To observe the surface state of the oxidized Si, scanning tunneling microscopy (STM) was employed. The crystallinity of the films grown on the clean 7=7 surface at 6008C was investigated using the synchrotron XRD as shown in Fig. 1. The rocking curves near the Bragg peak in the surface normal direction of Y2O3 N111M have sharp and broad components. The sharp component has the full width at half maximum (FWHM) below 0.18, while the FWHM of the broad component was 1.88. The sharp component indicated that the aligned Y2O3 layer had a constrained orientation while the broad component represented the crystalline orientation that was slightly misaligned from the substrate normal direction. Thus, the increased intensity of the sharp component with the increase of oxygen partial pressure implies that the crystallinity of the films grown on the clean Si have improved after the Si surface was oxidized. The amount of supplied oxygen even with the pressure of 2=10y7 torr was sufficient to form a stoichiometric ˚ Y2O3 at the growth rate of 0.2 Ays. Therefore, the oxidized Si surface at the initial growth stage provided a preferable surface state to grow Y2O3 film. In order to determine the role of the oxidized surface,
39
Fig. 1. Transverse scan of X-ray scattering of Y2 O3 (111) films grown on clean Si (111) at the substrate temperature of 5008C and oxygen partial pressure of (a) 1=10y5 torr, (b) 1=10y6 torr, and (c) 2=10y7 torr.
we increased the thickness of the oxidized layer up to ;1.5 nm using the RCA cleaning method. Fig. 2 shows a contradicting result from a well-known report that the oxide layer acts as an obstacle to the crystallization of the Y2O3 film w16x. The crystallinity of the film grown on the oxidized Si with RCA cleaning, significantly increased even at a low substrate temperature of 5008C; i.e. the minimum channeling yield (xmin) below 3% in RBSychanneling was caused by the excellent crystallinity of the Y2O3 (111) film, which is equivalent to the bulk crystallinity. To our knowledge, this is the best crystallinity in that low temperature range ever reported to date. When Y2O3 film is grown on a clean Si (111) (7=7) surface under identical conditions, the xmin value rises to 90%, indicating a high degree of disorder. Moreover, AFM images on the right side of Fig. 2 illustrate the effect that the surface state has on the surface morphology of the films. The surface of the film grown on the oxidized Si shows only planar and flat surfaces, while the surface of the film on clean Si is composed of 3D islands. A significant change in morphology and a drastic increase in crystallinity suggest that there is a critical growth mechanism at the initial growth stage. We must ask what makes the epitaxial film have such ordered structure on the SiO2 layer without any lattice matching. To investigate the surface condition of the oxidized surface in detail, scanning tunneling microscopy (STM) was used. Fig. 3 shows the images of the
40
M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
surfaces after vacuum annealing at 5008C for 20 min, which is the same condition as the pre-deposition of Y2O3 film. No distinguishable structure was found on the pre-annealed oxidized Si surface because of the insulating layer of SiO2 formed by the RCA cleaning. However, the shapeless surface was changed to circular morphology of 5-nm average diameter as the substrate temperature was raised up to 5008C in UHV condition. Given the large bandgap of SiO2, the STM image as shown on Fig. 3 reflects bulk or interface defects rather than surface morphology. Although the Si–SiO2 interface region is largely disordered, our STM image clearly shows that after annealing, the region consists of randomly distributed ;50-nm size platelets, filling almost the entire surface. It may be thus assumed that the platelets are in fact, defect agglomerates at the interface with each defect in registry with the underlying substrate. If this is true, then it is likely that where the platelets are located, subsequent Y2O3 film can grow with high crystallinity in registry with the Si substrate. Another question is what causes improvements in crystallinity and structural changes in film grown on the oxidized Si? In order to investigate the role of the SiO2 layer at the initial growth stage when Y interacts with Si or O, only Y was first evaporated on the clean and oxidized Si (111) surfaces at the process pressure below 6=10y9 torr up to a thickness of 0.6 nm, and then oxygen was supplied at an oxygen partial pressure of 1=10y5 torr. The longitudinal scans of the Y2O3 (111) reflection in Fig. 4 show significant differences in interference fringes and in crystallinity along the film
Fig. 3. STM image of oxidized surface after annealing at 5008C for 20 min in UHV.
normal direction; i.e. interference fringes along the normal film (qz direction) only appear in the film grown on the oxidized surface, which indicates that the film grown on the oxidized surface has the typical signature of a structurally coherent finite size system. The crystallinity of the film grown on the oxidized Si is much higher than that on the clean Si in agreement with the result found in Fig. 2. The interfacial chemical interaction between Y and Si, such as silicide formation, is inevitable at the initial growth stage because of the high reactivity and large surface energy of Y w16,17x, leading to the low crystallinity of film grown on the unbuffered surface. The
Fig. 2. RBSychanneling data (left side) and AFM images (right side) of Y2 O3 films grown on (a) oxidized, and (b) clean Si surfaces. The films were grown at the substrate temperature of 5008C and the background oxygen partial pressure of 1=10y6 torr.
M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
41
Fig. 4. HRTEM images and longitudinal scans of Y2O3 (111) films grown by firstly evaporating Y of 0.6 nm and then supplying O to two surface states: (a) oxidized; and (b) clean Si.
yttrium silicide formation at the initial growth stage clearly disturbs the nucleation of the Y2O3 because of competing nucleation of Y2O3 and silicide. Therefore, this disturbance causes a downgrade in crystallinity and structural change of the film grown on the clean Si. It implies that the SiO2 layer works as a reaction barrier; the oxide layer prevents strong interaction between Y and the substrate. The HRTEM images of the two samples provide information on the reaction at the interface as shown in Fig. 4. The adsorbed silicide layer is clearly observed at the interface of the film grown on the clean Si when 0.6-nm-thick Y is deposited on the surface. Such defects as dislocations are generated and the structure of the film at the interface severely disordered. However, no free Y layer is formed and the sharp interface remains on the oxidized surface. These results illustrate the influence on nucleation of the silicide layer. Thus, the role of the SiO2 layer may be understood as a barrier to the yttrium silicide formation. Another key ingredient for the growth of crystalline Y2O3 is the reaction mechanism to nucleate the Y2O3 on the oxidized surface at the initial growth stage. An exothermal reaction at the interface w13x may supply the necessary step: 3SiO2q4Y™2Y2O3q3Si 3Žy908 kJykmol.q0 kJykmol™ 2Žy1758 kJykmol.q0 kJykmol.
The reaction mechanism enables the SiO2 layer to act as nucleation centers for further growth. Thus, the SiO2 layer performs a dual role, i.e. reaction control in the formation of a silicide layer and nucleation center for Y2O3. We demonstrated that it is possible to grow an epitaxial Y2O3 film of the highest quality reported to date on a wet chemically oxidized Si. The physical properties of the film grown on an oxidized surface reveal clear superiority over the film grown on a clean surface. This implies that Y2O3 formation is closely related with the reaction mechanism between SiO2 and Y, as well as the coherence growth resulting from the interfacial interaction. We have shown that the SiO2 buffer layer in the growth of high quality epitaxial film on oxidized Si, plays a critical role in blocking the formation of a silicide layer and at the same time, providing the nucleation sites for Y2O3. Acknowledgements This work was supported by the BK21 project and the KOSEF through the ASSRC at Yonsei University. References w1x L. Manchanda, M. Gurvitch, IEEE Electron. Device Lett. 9 (1988) 180. w2x C. Hu, IEDM Technical Digest, IEEE, New York, 1985, p. 368..
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M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
w3x D. Roy, S.B. Krupanidhi, Appl. Phys. Lett. 62 (1993) 1056. w4x S. Hontsu, H. Tabata, M. Nakamori, J. Ishii, T. Kawai, Jpn. J. Appl. Phys. 35 (1996) L774. w5x K. Harada, H. Nakanishi, H. Itozaki, S. Yazu, Jpn. J. Appl. Phys. 60 (1991) 934. w6x D.K. Fork, D.B. Fenner, T.H. Geballe, J. Appl. Phys. 68 (1990) 4316. w7x T. Inoue, Y. Yamamoto, S. Koyama, S. Suzuki, Y. Ueda, Appl. Phys. Lett. 56 (1990) 1332. w8x D.K. Fork, D.B. Fenner, G.A. Connell, J.M. Phillips, T.H. Gabelle, Appl. Phys. Lett. 57 (1990) 1137. w9x P. Leganeux, G. Garry, D. Dieumegard, C. Schwebel, C. Pellet, G. Gqutherin, J. Siejka, Appl. Phys. Lett. 53 (1988) 1506.
w10x T.h. Matthee, J. Wecker, H. Behner, G. Friedl, O. Eible, K. Samwer, Appl. Phys. Lett. 61 (1992) 1240. w11x M. Gurvitch, L. Manchanda, J.M. Gibson, Appl. Phys. Lett. 51 (1987) 919. w12x H. Fukumoto, T. Imura, Y. Osaka, Appl. Phys. Lett. 55 (1989) 360. w13x S.C. Choi, M.H. Cho, S.W. Whangbo, C.N. Whang, Appl. Phys. Lett. 71 (1997) 903. w14x W. Kern, RCA Rev. 31 (1970) 207. w15x W. Kern, RCA Rev. 31 (1970) 234. w16x J.A. Knapp, S.T. Picraux, Appl. Phys. Lett. 48 (1986) 466. w17x M.P. Siegal, F.H. Kaatz, W.R. Graham, J.J. Santiago, J. Van der Spiegel, J. Appl. Phys. 66 (7) (1989) 2999.
Letter
Epitaxial Y2O3 film growth on an oxidized Si surface M.-H. Choa,*, D.-H. Kob, Y.K. Choia, I.W. Lyoa, K. Jeonga, C.N. Whanga a
Atomic-scale Surface Science Research Center and Department of Physics, Yonsei University, Seoul, 120-749 South Korea b Department of Ceramics Engineering, Yonsei University, Seoul, 120-749, South Korea Received 27 September 2000; received in revised form 30 August 2001; accepted 6 September 2001
Abstract The effects of the Si surface state on epitaxial growth of Y2 O3 layers were investigated by various measurement methods. The characterization using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS) and high-resolution transmission electron microscopy (HRTEM) shows excellent film crystallinity when grown on an oxidized Si surface. The crystalline structure of the film was influenced by the interfacial mosaic structure, which depended on whether the Si surface contained adsorbed O or not. The thin SiO2 layer of approximately 1.5 nm, provided favorable interfacial reaction sites for the nucleation of Y2O3, and still maintained the structural registry with the underlying Si substrate. In particular, the reaction between the Y and SiO2 layer resulted in coherent finite growth, whereas the direct interaction between Y and Si was hindered by the nucleation of Y2O3. The high-quality epitaxial layer with the minimum channel yield (xmin ), lower than 3%, could be grown on the oxidized Si surface. 䊚 2002 Elsevier Science B.V. All rights reserved. PACS: 68.10.Jy; 68.55.-a; 81.15.Hi; 61.72.-y Keywords: Oxides; Yttrium
Over the years, the growth of oxide films on Si has been an area of intense research interest due to the stringent requirements for thin insulating layers used in highly integrated circuits w1,2x. Examples of these include perovskite oxide films such as SrTiO3 and BaTiO3, and these have been applied to the application of memory devices w3,4x. Others include fluorite-type oxides such as CeO2, ZrO2, PrO2 and Y2O3, and these have been used as the epitaxial buffer layers for the growth of high temperature superconducting oxide films w5–9x. They are also attractive for various applications including storage capacitors and gate insulators in dynamic random access memory. Among the oxide materials, Y2O3 has attracted the utmost attention because of its wide energy band gap, high dielectric constant (13–17), good thermal stability (up to 23258C), and well-matched lattice constant with Si wa(Y2O3)s * Corresponding author. E-mail address: [email protected] (M.-H.-H. Cho).
1.06 nm, a(Si)=2s1.086 nmx. Despite intense efforts to grow the Y2O3 films epitaxially using various deposition methods w10–13x, only a few of these methods have reported the epitaxial growth of Y2O3 films on Si w12,13x. Even in these cases, the crystal quality was not good because of suspected growth kinetics of the film. Moreover, the complex crystal structure of Y2O3 and the complication of defects in the film attracted less attention to the growth system. In this paper, we focus on the effects that the surface state has on the crystalline structure of the Y2O3 nucleation layers. Our study revealed that the ordering of the atomic positions in Y2O3 films increases significantly as the Si surface becomes oxidized. As direct interaction between the deposited Y film and the substrate increased, a silicide layer appeared, and an island-like growth was enhanced. On the oxidized Si surface, Y2O3 film growth was obtained without the growth of a silicide layer. Moreover, we demonstrate the growth of high-quality epitaxial Y2O3 films on a wet chemically
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 6 2 5 - X
M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
oxidized Si(111) surface; Rutherford backscattering spectroscopy (RBS)ychanneling yield (xmin) equals 3%. The thickness of the oxidized layer is expected to be 1.5 nm, which hinders the epitaxial growth. A film with sharp and abrupt interfaces can be grown because the oxidized layer controls the reaction between Y, O and Si. It would provide fundamental information on the growth kinetics of the Y2O3 film on Si. The epitaxial Y2O3 films were grown using the deposition system consisting of a load-lock chamber, a growth chamber with reflection high-energy electron diffraction (RHEED)w13x. Metal Y in W crucible was evaporated using an electron beam. Si substrates with 48 vicinal (111) orientations were chemically cleaned using the RCA method w14,15x. The sequential cleaning solutions used were NH4OH–H2O2–H2O and H2SO4– H2O2–H2O. The RCA cleaning resulted in the formation of 4–6 monolayers of SiO2, removing organic and metallic residues on the surface at the same time w14,15x. The oxidized surface formed using the RCA method and the clean surface with the 7=7 structure were used. The base pressure of the growth chamber was set under 5=10y10 torr. The substrates were heated up to 5008C during the deposition of Y. At the same time, O2 gas was directly injected onto the wafer with a background partial pressure of O2 raised to 1=10y5 torr to grow Y2O3 film. The typical Y2O3 film growth rate was ˚ approximately 0.2 Ays. To investigate the growth of the epitaxial layer, in-situ RHEED, X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and high resolution transmission electron microscopy (HRTEM) were used. To observe the surface state of the oxidized Si, scanning tunneling microscopy (STM) was employed. The crystallinity of the films grown on the clean 7=7 surface at 6008C was investigated using the synchrotron XRD as shown in Fig. 1. The rocking curves near the Bragg peak in the surface normal direction of Y2O3 N111M have sharp and broad components. The sharp component has the full width at half maximum (FWHM) below 0.18, while the FWHM of the broad component was 1.88. The sharp component indicated that the aligned Y2O3 layer had a constrained orientation while the broad component represented the crystalline orientation that was slightly misaligned from the substrate normal direction. Thus, the increased intensity of the sharp component with the increase of oxygen partial pressure implies that the crystallinity of the films grown on the clean Si have improved after the Si surface was oxidized. The amount of supplied oxygen even with the pressure of 2=10y7 torr was sufficient to form a stoichiometric ˚ Y2O3 at the growth rate of 0.2 Ays. Therefore, the oxidized Si surface at the initial growth stage provided a preferable surface state to grow Y2O3 film. In order to determine the role of the oxidized surface,
39
Fig. 1. Transverse scan of X-ray scattering of Y2 O3 (111) films grown on clean Si (111) at the substrate temperature of 5008C and oxygen partial pressure of (a) 1=10y5 torr, (b) 1=10y6 torr, and (c) 2=10y7 torr.
we increased the thickness of the oxidized layer up to ;1.5 nm using the RCA cleaning method. Fig. 2 shows a contradicting result from a well-known report that the oxide layer acts as an obstacle to the crystallization of the Y2O3 film w16x. The crystallinity of the film grown on the oxidized Si with RCA cleaning, significantly increased even at a low substrate temperature of 5008C; i.e. the minimum channeling yield (xmin) below 3% in RBSychanneling was caused by the excellent crystallinity of the Y2O3 (111) film, which is equivalent to the bulk crystallinity. To our knowledge, this is the best crystallinity in that low temperature range ever reported to date. When Y2O3 film is grown on a clean Si (111) (7=7) surface under identical conditions, the xmin value rises to 90%, indicating a high degree of disorder. Moreover, AFM images on the right side of Fig. 2 illustrate the effect that the surface state has on the surface morphology of the films. The surface of the film grown on the oxidized Si shows only planar and flat surfaces, while the surface of the film on clean Si is composed of 3D islands. A significant change in morphology and a drastic increase in crystallinity suggest that there is a critical growth mechanism at the initial growth stage. We must ask what makes the epitaxial film have such ordered structure on the SiO2 layer without any lattice matching. To investigate the surface condition of the oxidized surface in detail, scanning tunneling microscopy (STM) was used. Fig. 3 shows the images of the
40
M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
surfaces after vacuum annealing at 5008C for 20 min, which is the same condition as the pre-deposition of Y2O3 film. No distinguishable structure was found on the pre-annealed oxidized Si surface because of the insulating layer of SiO2 formed by the RCA cleaning. However, the shapeless surface was changed to circular morphology of 5-nm average diameter as the substrate temperature was raised up to 5008C in UHV condition. Given the large bandgap of SiO2, the STM image as shown on Fig. 3 reflects bulk or interface defects rather than surface morphology. Although the Si–SiO2 interface region is largely disordered, our STM image clearly shows that after annealing, the region consists of randomly distributed ;50-nm size platelets, filling almost the entire surface. It may be thus assumed that the platelets are in fact, defect agglomerates at the interface with each defect in registry with the underlying substrate. If this is true, then it is likely that where the platelets are located, subsequent Y2O3 film can grow with high crystallinity in registry with the Si substrate. Another question is what causes improvements in crystallinity and structural changes in film grown on the oxidized Si? In order to investigate the role of the SiO2 layer at the initial growth stage when Y interacts with Si or O, only Y was first evaporated on the clean and oxidized Si (111) surfaces at the process pressure below 6=10y9 torr up to a thickness of 0.6 nm, and then oxygen was supplied at an oxygen partial pressure of 1=10y5 torr. The longitudinal scans of the Y2O3 (111) reflection in Fig. 4 show significant differences in interference fringes and in crystallinity along the film
Fig. 3. STM image of oxidized surface after annealing at 5008C for 20 min in UHV.
normal direction; i.e. interference fringes along the normal film (qz direction) only appear in the film grown on the oxidized surface, which indicates that the film grown on the oxidized surface has the typical signature of a structurally coherent finite size system. The crystallinity of the film grown on the oxidized Si is much higher than that on the clean Si in agreement with the result found in Fig. 2. The interfacial chemical interaction between Y and Si, such as silicide formation, is inevitable at the initial growth stage because of the high reactivity and large surface energy of Y w16,17x, leading to the low crystallinity of film grown on the unbuffered surface. The
Fig. 2. RBSychanneling data (left side) and AFM images (right side) of Y2 O3 films grown on (a) oxidized, and (b) clean Si surfaces. The films were grown at the substrate temperature of 5008C and the background oxygen partial pressure of 1=10y6 torr.
M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
41
Fig. 4. HRTEM images and longitudinal scans of Y2O3 (111) films grown by firstly evaporating Y of 0.6 nm and then supplying O to two surface states: (a) oxidized; and (b) clean Si.
yttrium silicide formation at the initial growth stage clearly disturbs the nucleation of the Y2O3 because of competing nucleation of Y2O3 and silicide. Therefore, this disturbance causes a downgrade in crystallinity and structural change of the film grown on the clean Si. It implies that the SiO2 layer works as a reaction barrier; the oxide layer prevents strong interaction between Y and the substrate. The HRTEM images of the two samples provide information on the reaction at the interface as shown in Fig. 4. The adsorbed silicide layer is clearly observed at the interface of the film grown on the clean Si when 0.6-nm-thick Y is deposited on the surface. Such defects as dislocations are generated and the structure of the film at the interface severely disordered. However, no free Y layer is formed and the sharp interface remains on the oxidized surface. These results illustrate the influence on nucleation of the silicide layer. Thus, the role of the SiO2 layer may be understood as a barrier to the yttrium silicide formation. Another key ingredient for the growth of crystalline Y2O3 is the reaction mechanism to nucleate the Y2O3 on the oxidized surface at the initial growth stage. An exothermal reaction at the interface w13x may supply the necessary step: 3SiO2q4Y™2Y2O3q3Si 3Žy908 kJykmol.q0 kJykmol™ 2Žy1758 kJykmol.q0 kJykmol.
The reaction mechanism enables the SiO2 layer to act as nucleation centers for further growth. Thus, the SiO2 layer performs a dual role, i.e. reaction control in the formation of a silicide layer and nucleation center for Y2O3. We demonstrated that it is possible to grow an epitaxial Y2O3 film of the highest quality reported to date on a wet chemically oxidized Si. The physical properties of the film grown on an oxidized surface reveal clear superiority over the film grown on a clean surface. This implies that Y2O3 formation is closely related with the reaction mechanism between SiO2 and Y, as well as the coherence growth resulting from the interfacial interaction. We have shown that the SiO2 buffer layer in the growth of high quality epitaxial film on oxidized Si, plays a critical role in blocking the formation of a silicide layer and at the same time, providing the nucleation sites for Y2O3. Acknowledgements This work was supported by the BK21 project and the KOSEF through the ASSRC at Yonsei University. References w1x L. Manchanda, M. Gurvitch, IEEE Electron. Device Lett. 9 (1988) 180. w2x C. Hu, IEDM Technical Digest, IEEE, New York, 1985, p. 368..
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M.-H.-H. Cho et al. / Thin Solid Films 402 (2002) 38–42
w3x D. Roy, S.B. Krupanidhi, Appl. Phys. Lett. 62 (1993) 1056. w4x S. Hontsu, H. Tabata, M. Nakamori, J. Ishii, T. Kawai, Jpn. J. Appl. Phys. 35 (1996) L774. w5x K. Harada, H. Nakanishi, H. Itozaki, S. Yazu, Jpn. J. Appl. Phys. 60 (1991) 934. w6x D.K. Fork, D.B. Fenner, T.H. Geballe, J. Appl. Phys. 68 (1990) 4316. w7x T. Inoue, Y. Yamamoto, S. Koyama, S. Suzuki, Y. Ueda, Appl. Phys. Lett. 56 (1990) 1332. w8x D.K. Fork, D.B. Fenner, G.A. Connell, J.M. Phillips, T.H. Gabelle, Appl. Phys. Lett. 57 (1990) 1137. w9x P. Leganeux, G. Garry, D. Dieumegard, C. Schwebel, C. Pellet, G. Gqutherin, J. Siejka, Appl. Phys. Lett. 53 (1988) 1506.
w10x T.h. Matthee, J. Wecker, H. Behner, G. Friedl, O. Eible, K. Samwer, Appl. Phys. Lett. 61 (1992) 1240. w11x M. Gurvitch, L. Manchanda, J.M. Gibson, Appl. Phys. Lett. 51 (1987) 919. w12x H. Fukumoto, T. Imura, Y. Osaka, Appl. Phys. Lett. 55 (1989) 360. w13x S.C. Choi, M.H. Cho, S.W. Whangbo, C.N. Whang, Appl. Phys. Lett. 71 (1997) 903. w14x W. Kern, RCA Rev. 31 (1970) 207. w15x W. Kern, RCA Rev. 31 (1970) 234. w16x J.A. Knapp, S.T. Picraux, Appl. Phys. Lett. 48 (1986) 466. w17x M.P. Siegal, F.H. Kaatz, W.R. Graham, J.J. Santiago, J. Van der Spiegel, J. Appl. Phys. 66 (7) (1989) 2999.