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Epitaxial growth of Er2O3 films on oxidized Si(111) and Si(001) substrates
Thin Solid Films 508 (2006) 86 – 89 www.elsevier.com/locate/tsf
Epitaxial growth of Er2O3 films on oxidized Si(111) and Si(001) substrates Y.Y. Zhu, R. Xu, S. Chen, Z.B. Fang, F. Xue, Y.L. Fan, X.J. Yang, Z.M. Jiang * Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai 200433, China Available online 8 November 2005
Abstract The crystalline structures and morphologies of Er2O3 films epitaxially grown on both oxidized and clean Si surfaces are investigated by X-ray diffraction, in situ reflection high energy electron diffraction and atomic force microscopy. Both crystallinity and surface roughness of Er2O3 films grown on oxidized Si substrates are improved compared to those grown on the corresponding clean ones, indicating that the oxidized Si surfaces are favorable to the epitaxial growth of Er2O3 films compared with the clean ones. At the same time, the oxidized Si surface can suppress the formation of Er silicide at the interface during the film growth, which is preferable for epitaxial Er2O3 films with smooth surfaces. Good epitaxial ˚ and good crystallinity as well, has been achieved on an oxidized Si(111) growth of Er2O3 films, with a surface mean roughness as small as 1.51 A surface by molecule beam epitaxy. D 2005 Elsevier B.V. All rights reserved. PACS: 81.15 Hi (Molecular beam epitaxy); 77.55+f (dielectric thin films); 68.35Ct (interface structure and roughness) Keywords: Erbium oxide; Epitaxial growth; Surface roughness
1. Introduction Oxides with a wide variety of physical properties have been an ongoing interest for many applications in electronic devices. In order to directly integrate these properties with semiconductor devices, it is generally necessary to epitaxially grow these oxides on semiconductor substrates in order to improve their electronic and optical properties [1]. Due to the sophisticated Si integrated circuit technology, the epitaxial growth of oxides on Si substrates is especially important. Well epitaxial oxide layers could serve as buffer layers for the subsequent epitaxial growth of ferroelectrics, high-Tc superconductors on Si substrates [2]. To be specific, oxide films with high dielectric constant (high j) such as HfO2, Y2O3 and Pr2O3, have also been studied as potential replacements of SiO2 layer in complementary metal –oxide –semiconductor (CMOS) device [1– 5]. Because of its high dielectric constant (¨ 13), wide band gap (¨ 5 eV) and relatively large conduction band offset, erbium oxide (Er2O3) may also be a promising candidate as suitable gate dielectric layer in CMOS device materials [6 – 8]. In addition, Er2O3 has a cubic fluorite-related bixbyite structure and a very closely matched lattice constant to silicon: ˚ and a(Si) 2 = 10.86 A ˚ [8]. lattice constant a(Er2O3) = 10.54 A * Corresponding author. Tel.: +86 21 65643827; fax: +86 21 65104949. E-mail address: [email protected] (Z.M. Jiang). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.389
Therefore, it is expected that Er2O3 could be well epitaxially grown on Si substrates. The epitaxial growth of Er2O3 films on Si(001) substrates has been reported for the first time elsewhere [8]. But, there have been no reports about the epitaxial growth of Er2O3 films on Si(111) substrates. Furthermore, most oxides epitaxially grown on Si substrates do not show smooth surfaces with the root-mean-square (rms) ˚ [9]. And the smooth surface roughness typically as large as 10 A of oxide films on Si is also most required for the alternatives in CMOS device. In order to solve this problem, many efforts have been paid to the thin film preparation methodology, and also Si surface treatments before growth [1,10]. In this work, both oxidized and clean Si(111) and Si(001) substrates are used to epitaxially grow Er2O3 films. Er2O3 film with smooth surface and interface, and good crystallinity as well, is obtained by epitaxial growth on oxidized Si(111) ˚ , to substrate. The surface mean roughness of this film is 1.51 A our knowledge, which is the smallest value reported so far. Effects of the surface Si dioxide layer on epitaxial growth of Er2O3 films are also briefly discussed. 2. Experimental Er2O3 films were deposited in a multi-chamber Si molecule beam epitaxy (MBE) system with a base pressure of about 3 10 9 Torr, by using a metallic erbium source.
Y.Y. Zhu et al. / Thin Solid Films 508 (2006) 86 – 89
1.5-in. p-type Si(111) and Si(001) wafers with resistivity of 2 10 V cm were used as substrates. Two kinds of Si surfaces, clean Si(111) and Si(001) surfaces and oxidized ones, were prepared for the growth of Er2O3 films. The clean and oxidized Si surfaces were prepared by the following sequences. The oxidized surfaces, with about 0.8-nm-thick SiO2 layers, were formed by wet chemical oxidization using Shiraki method. The clean surfaces were prepared by being dipped in dilute HF solution after cleaned by Shiraki method, and then by desorption of the adsorbed H atoms at high temperatures in vacuum. The 7 7 reconstruction patterns for clean Si(111) surfaces and the 2 1 reconstruction patterns for clean Si(001) surfaces were observed by in situ reflection high energy electron diffraction (RHEED). The growth was carried out at a temperature of 700 -C with oxygen partial pressure of about 7 10 6 Torr, which are the optimum conditions obtained previously for epitaxial growth of Er2O3 on Si(001) substrates [8]. The film structures were studied by X-ray diffraction (XRD) h – 2h scan and in situ RHEED. Film surface morphologies were obtained using atomic force microscopy (AFM). 3. Results and discussion Fig. 1 shows the h – 2h X-ray diffraction curves for two Er2O3 films grown on (a) oxidized and (b) clean Si(111) substrate surfaces, respectively. Except the Si(111) peak at 28.4- and Si(222) peak at 58.9-, in a wide angle range, other Er2O3(222)
(a) Si(111)
25
Er2O3(111)
30
35
Si(222) Er2O3(444)
(b)
25
30
35
ErSix(001) ErSix(002)
10
20
30
40
50
60
2θ Fig. 1. XRD patterns of the films grown on: a) oxidized, b) clean Si(111) surfaces at 700 -C in an oxygen ambient pressure of 7 10 6 Torr.
87
(a)
A
A B B B
(b)
Fig. 2. RHEED patterns of the films grown on: a) oxidized, b) clean Si(111) surfaces at 700 -C in an oxygen ambient pressure of 7 10 6 Torr. The incident electron beam is parallel to the Si [1¯10] axis.
strong peaks belong to the reflection of the Er2O3 (111) series planes of the crystalline Er2O3 films, indicating that two films grown on oxidized and clean Si(111) surfaces are both preferentially (111) oriented, i.e., Er2O3 (111)//Si(111). This orientation relationship agrees with most oxides with a cubic bixbyite structure epitaxially grown on Si(111), such as Y2O3(111)//Si(111), La2O3(111)//Si(111) and Pr2O3(111)// Si(111) [9– 12]. The lattice parameter, determined from the Er2O3 (111) peak position, agrees well with that of bulk Er2O3 of cubic bixbyite structure, which means that both Er2O3 films grown on oxidized and clean Si(111) substrates are totally ˚ . It is noteworthy relaxed at this film thickness of about 100 A that at both sides of Si(111) peak, many oscillation peaks are clearly observed, which are attributed to interference fringes of Er2O3 films. From oscillation periods of the interference fringes, the thicknesses of Er2O3 films are obtained to be ˚ and 91.1 A ˚ , respectively. The appearance of the 113.2 A interference fringes means that both surfaces and interfaces of Er2O3 films are smooth which will be further confirmed by RHEED and AFM observations afterward. Additionally, as shown in Fig. 1(b) two additional weak peaks can be observed for the film grown on clean Si(111) surface at 2h = 21.7- and 44.3-, which are attributed to the hexagonal (001) and (002) silicide peaks. No (001) peak and much weaker (002) silicide peak can be observed for the film grown on oxidized Si(111) surface, implying that silicide formation in Er2O3 film is much suppressed by the oxidized surface, which will be discussed in detail later. Fig. 2 shows RHEED patterns of Er2O3 films grown on oxidized and clean Si(111) substrate surfaces, respectively. The incident electron beam is parallel to the Si [1¯10] axis. Both patterns look streak-like, indicating that both films have single
88
Y.Y. Zhu et al. / Thin Solid Films 508 (2006) 86 – 89
Fig. 3. 0.8 0.8 Am2 AFM images of the films grown on: a) oxidized, b) clean Si(111) surfaces at 700 -C in an oxygen ambient pressure of 7 10
crystalline structures and smooth surfaces. It can be seen the RHEED patterns contain two types of streaks, bright streaks and weak streaks. The bright streaks, partially denoted by A, resulted from the fractional diffraction, which spacing corresponds to the distance of the adjacent atomic planes of Er2O3(110) in real-space. The weak streaks, partially denoted by B, resulted from the integer diffraction, which spacing corresponds to the distance of every two or four atomic planes of Er2O3(110) in real space. The reason that the fractional diffraction order is much brighter than the integer diffraction order is due to the crystalline Er2O3 unit cell structure, which composes four (110) planes in one axis of Er2O3 unit cell. This observation is well consistent with the RHEED patterns of Y2O3 films grown on Si(111) substrates [7]. In addition, there are no changes in the RHEED patterns when rotating the samples every 60- from the [1¯10] direction along the Si[111] direction, reflecting the sixfold symmetry along Er2O3[111] direction. No RHEED patterns of reconstructed Er2O3(111) surface is observed. Comparing Fig. 2(a) with Fig. 2(b), it can be found that the integer diffraction streaks from the film grown on oxidized Si(111) surface are clearer than that from the film grown on the clean one. This trend indicates that the crystallinity of the film is improved when the Er2O3 film is grown on the oxidized Si(111) surface, which agrees well with the results of the crystalline Y2O3 films grown on the clean and oxidized Si(111) substrates [10]. Possible explanations will be given below. Fig. 3 shows the AFM images of the Er2O3 films grown on (a) oxidized and (b) clean Si(111) substrates. The root-meansquare (rms) surface roughness values of these two films are ˚ and 2.63 A ˚ , respectively, indicating very smooth 1.51 A
6
Torr.
surfaces obtained, especially for the film grown on the oxidized Si(111) surface. There are only few data published on surface roughness of oxide films. For example, Cho et al. reported a 15 ˚ rms surface roughness of Y2O3 film obtained by an ionA assisted beam [9]. As shown in Fig. 3(a), Er2O3 film grown on the oxidized Si(111) surface has an extremely smooth surface, ˚ , which, to our knowledge, is with surface roughness of 1.51 A the smallest value reported so far. By comparing the surface roughness values of the two films, it can be deduced that the oxidized substrate could improve the surface roughness of Er2O3 films. The same case is obtained when the Er2O3 films were deposited on the oxidized and clean Si(001) surfaces [8]. Fig. 4 shows the AFM images of the Er2O3 films, grown on (a) oxidized and (b) clean Si(001) surfaces, with the thickness of ˚ . The rms roughness values of these two Er2O3 about 100 A ˚ and 7.45 A ˚ , respectively. Again, the Er2O3 films are 4.68 A films grown on the oxidized Si(001) surface is much smoother than that grown on the clean one. It should be noted that the surface roughness of the Er2O3 films grown on Si(001) substrates is much larger than those grown on Si(111) substrates, which may be due to the overall surface energy of the film [13]. The Er2O3 (111) plane has the lower surface energy than the (100) plane, which would result in a much smoother surface. The different surface roughness on oxidized and clean silicon surfaces could be explained in the view of the silicide formation. When the Er2O3 film is deposited on a clean Si substrate, silicide is inevitably formed by the direct reaction between Er and Si due to its high reaction kinetics, while less silicide is formed when the Er2O3 film is deposited on the oxidized Si surface, confirmed by the XRD results of Fig. 1.
Fig. 4. 4 4 Am2 AFM images of the films grown on: a) oxidized, b) clean Si(001) surfaces at 700 -C in an oxygen ambient pressure of 7 10
6
Torr.
Y.Y. Zhu et al. / Thin Solid Films 508 (2006) 86 – 89
From the literatures, we know that the silicide formation is strongly dependent on the growth conditions and the substrate treatment before growth [14]. Therefore, the oxidized Si surface plays the role of suppressing the formation of silicide under the same growth conditions. Many authors have reported that the surface of the film will become rough if the silicide is formed at the interface between the Si substrate and the asgrown oxide film at the initial growth stage [8,9,14], which is the reason why the relatively smoother Er2O3 film surface has obtained on the oxidized Si substrate than that on the clean Si. Moreover, the reaction between the silicon oxide and erbium atoms may provide sites for Er2O3 nucleation, resulting in a reactively better crystalline Er2O3 layer in comparison with that grown on a clean silicon substrate, which forms clear substreaks in the RHEED pattern of Fig. 2(b). However, an interfacial SiO2 layer may exist if the reaction between the silicon oxide and erbium atoms is insufficient, which will increase the equivalent oxide thickness of the film. Therefore, a moderate SiO2 thickness is critically important in order that SiO2 fully react with Er atoms to form Er2O3 with no SiO2 left. 4. Conclusions In conclusion, the epitaxial growth of Er2O3 films with good crystallinity and smooth surface has been achieved on oxidized Si(111) substrate by MBE at a growth temperature of 700 -C in an oxygen pressure of 7 10 6 Torr. The surface roughness and crystallinity of Er2O3 films grown on oxidized Si(111) and Si(001) surfaces are more improved than those grown on the clean Si surfaces, indicating that oxidized Si surfaces are preferable to clean ones for epitaxially growing Er2O3 films.
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Acknowledgments This work was supported by the special funds for Major State Basic Research Project No. G2001CB3095 of China, the special funds for the National Advanced Technology Research and Development, and Shanghai Science and Technology Commission. This work is also partially supported by NSFC under grant nos. 10321003 and 60425411. References [1] P. David, Norton, Mater. Sci. Eng. R43 (2004) 139. [2] P. J.P.Liu, E. Zaumseil, H.J. Bugiel, H.J. Osten, Appl. Phys. Lett. 79 (2001) 671. [3] A. Fissel, J. Dabrowski, H.J. Osten, J. Appl. Phys. 91 (2002) 8986. [4] A. Dimoulas, G. Vellianitis, A. Travlos, V. Ioannou-Sougleridis, A.G. Nassiopoulou, J. Appl. Phys. 92 (2002) 426. [5] V.V. Afanasev, A. Stesmans, Appl. Phys. Lett. 81 (2002) 1053. [6] V. Mikhelashvili, G. Eisenstein, J. Appl. Phys. 90 (2001) 5447. [7] V. Mikhelashvili, G. Eisenstein, F. Edlman, R. Bremer, N. Zakharov, P. Wemer, J. Appl. Phys. 95 (2004) 613. [8] R. Xu, Y.Y. Zhu, S. Chen, F. Xue, Y.L. Fan, X.J. Yang, Z.M. Jiang, J. Cryst. Growth 277 (2005) 496. [9] M.H. Cho, D.H. Ko, K. Jeong, I.W. Lyo, S.W. Whangbo, H.B. Kim, S.C. Choi, J.H. Song, S.J. Cho, C.N. Whang, J. Appl. Phys. 86 (1999) 198. [10] M.H. Cho, D.H. Ko, Y.K. Choi, I.W. Lyo, K. Jeong, T.G. Kim, J.H. Song, C.N. Whang, J. Appl. Phys. 89 (2001) 1647. [11] H.J. Osten, J.P. Liu, E. Bugiel, H.J. Mussig, P. Zaumseil, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 87 (2001) 297. [12] Guha Supratk, Nestor A. Bojarozuk, Vijay Narayanan, Appl. Phys. Lett. 80 (2002) 766. [13] L.J. Schowalter, R.W. Fathauer, R.P. Goehner, L.G. Turner, R.W. Deblos, J. Appl. Phys. 58 (1985) 302. [14] S.S. Lau, C.S. Pai, C.S. Wu, Appl. Phys. Lett. 41 (1982) 77.
Epitaxial growth of Er2O3 films on oxidized Si(111) and Si(001) substrates Y.Y. Zhu, R. Xu, S. Chen, Z.B. Fang, F. Xue, Y.L. Fan, X.J. Yang, Z.M. Jiang * Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai 200433, China Available online 8 November 2005
Abstract The crystalline structures and morphologies of Er2O3 films epitaxially grown on both oxidized and clean Si surfaces are investigated by X-ray diffraction, in situ reflection high energy electron diffraction and atomic force microscopy. Both crystallinity and surface roughness of Er2O3 films grown on oxidized Si substrates are improved compared to those grown on the corresponding clean ones, indicating that the oxidized Si surfaces are favorable to the epitaxial growth of Er2O3 films compared with the clean ones. At the same time, the oxidized Si surface can suppress the formation of Er silicide at the interface during the film growth, which is preferable for epitaxial Er2O3 films with smooth surfaces. Good epitaxial ˚ and good crystallinity as well, has been achieved on an oxidized Si(111) growth of Er2O3 films, with a surface mean roughness as small as 1.51 A surface by molecule beam epitaxy. D 2005 Elsevier B.V. All rights reserved. PACS: 81.15 Hi (Molecular beam epitaxy); 77.55+f (dielectric thin films); 68.35Ct (interface structure and roughness) Keywords: Erbium oxide; Epitaxial growth; Surface roughness
1. Introduction Oxides with a wide variety of physical properties have been an ongoing interest for many applications in electronic devices. In order to directly integrate these properties with semiconductor devices, it is generally necessary to epitaxially grow these oxides on semiconductor substrates in order to improve their electronic and optical properties [1]. Due to the sophisticated Si integrated circuit technology, the epitaxial growth of oxides on Si substrates is especially important. Well epitaxial oxide layers could serve as buffer layers for the subsequent epitaxial growth of ferroelectrics, high-Tc superconductors on Si substrates [2]. To be specific, oxide films with high dielectric constant (high j) such as HfO2, Y2O3 and Pr2O3, have also been studied as potential replacements of SiO2 layer in complementary metal –oxide –semiconductor (CMOS) device [1– 5]. Because of its high dielectric constant (¨ 13), wide band gap (¨ 5 eV) and relatively large conduction band offset, erbium oxide (Er2O3) may also be a promising candidate as suitable gate dielectric layer in CMOS device materials [6 – 8]. In addition, Er2O3 has a cubic fluorite-related bixbyite structure and a very closely matched lattice constant to silicon: ˚ and a(Si) 2 = 10.86 A ˚ [8]. lattice constant a(Er2O3) = 10.54 A * Corresponding author. Tel.: +86 21 65643827; fax: +86 21 65104949. E-mail address: [email protected] (Z.M. Jiang). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.389
Therefore, it is expected that Er2O3 could be well epitaxially grown on Si substrates. The epitaxial growth of Er2O3 films on Si(001) substrates has been reported for the first time elsewhere [8]. But, there have been no reports about the epitaxial growth of Er2O3 films on Si(111) substrates. Furthermore, most oxides epitaxially grown on Si substrates do not show smooth surfaces with the root-mean-square (rms) ˚ [9]. And the smooth surface roughness typically as large as 10 A of oxide films on Si is also most required for the alternatives in CMOS device. In order to solve this problem, many efforts have been paid to the thin film preparation methodology, and also Si surface treatments before growth [1,10]. In this work, both oxidized and clean Si(111) and Si(001) substrates are used to epitaxially grow Er2O3 films. Er2O3 film with smooth surface and interface, and good crystallinity as well, is obtained by epitaxial growth on oxidized Si(111) ˚ , to substrate. The surface mean roughness of this film is 1.51 A our knowledge, which is the smallest value reported so far. Effects of the surface Si dioxide layer on epitaxial growth of Er2O3 films are also briefly discussed. 2. Experimental Er2O3 films were deposited in a multi-chamber Si molecule beam epitaxy (MBE) system with a base pressure of about 3 10 9 Torr, by using a metallic erbium source.
Y.Y. Zhu et al. / Thin Solid Films 508 (2006) 86 – 89
1.5-in. p-type Si(111) and Si(001) wafers with resistivity of 2 10 V cm were used as substrates. Two kinds of Si surfaces, clean Si(111) and Si(001) surfaces and oxidized ones, were prepared for the growth of Er2O3 films. The clean and oxidized Si surfaces were prepared by the following sequences. The oxidized surfaces, with about 0.8-nm-thick SiO2 layers, were formed by wet chemical oxidization using Shiraki method. The clean surfaces were prepared by being dipped in dilute HF solution after cleaned by Shiraki method, and then by desorption of the adsorbed H atoms at high temperatures in vacuum. The 7 7 reconstruction patterns for clean Si(111) surfaces and the 2 1 reconstruction patterns for clean Si(001) surfaces were observed by in situ reflection high energy electron diffraction (RHEED). The growth was carried out at a temperature of 700 -C with oxygen partial pressure of about 7 10 6 Torr, which are the optimum conditions obtained previously for epitaxial growth of Er2O3 on Si(001) substrates [8]. The film structures were studied by X-ray diffraction (XRD) h – 2h scan and in situ RHEED. Film surface morphologies were obtained using atomic force microscopy (AFM). 3. Results and discussion Fig. 1 shows the h – 2h X-ray diffraction curves for two Er2O3 films grown on (a) oxidized and (b) clean Si(111) substrate surfaces, respectively. Except the Si(111) peak at 28.4- and Si(222) peak at 58.9-, in a wide angle range, other Er2O3(222)
(a) Si(111)
25
Er2O3(111)
30
35
Si(222) Er2O3(444)
(b)
25
30
35
ErSix(001) ErSix(002)
10
20
30
40
50
60
2θ Fig. 1. XRD patterns of the films grown on: a) oxidized, b) clean Si(111) surfaces at 700 -C in an oxygen ambient pressure of 7 10 6 Torr.
87
(a)
A
A B B B
(b)
Fig. 2. RHEED patterns of the films grown on: a) oxidized, b) clean Si(111) surfaces at 700 -C in an oxygen ambient pressure of 7 10 6 Torr. The incident electron beam is parallel to the Si [1¯10] axis.
strong peaks belong to the reflection of the Er2O3 (111) series planes of the crystalline Er2O3 films, indicating that two films grown on oxidized and clean Si(111) surfaces are both preferentially (111) oriented, i.e., Er2O3 (111)//Si(111). This orientation relationship agrees with most oxides with a cubic bixbyite structure epitaxially grown on Si(111), such as Y2O3(111)//Si(111), La2O3(111)//Si(111) and Pr2O3(111)// Si(111) [9– 12]. The lattice parameter, determined from the Er2O3 (111) peak position, agrees well with that of bulk Er2O3 of cubic bixbyite structure, which means that both Er2O3 films grown on oxidized and clean Si(111) substrates are totally ˚ . It is noteworthy relaxed at this film thickness of about 100 A that at both sides of Si(111) peak, many oscillation peaks are clearly observed, which are attributed to interference fringes of Er2O3 films. From oscillation periods of the interference fringes, the thicknesses of Er2O3 films are obtained to be ˚ and 91.1 A ˚ , respectively. The appearance of the 113.2 A interference fringes means that both surfaces and interfaces of Er2O3 films are smooth which will be further confirmed by RHEED and AFM observations afterward. Additionally, as shown in Fig. 1(b) two additional weak peaks can be observed for the film grown on clean Si(111) surface at 2h = 21.7- and 44.3-, which are attributed to the hexagonal (001) and (002) silicide peaks. No (001) peak and much weaker (002) silicide peak can be observed for the film grown on oxidized Si(111) surface, implying that silicide formation in Er2O3 film is much suppressed by the oxidized surface, which will be discussed in detail later. Fig. 2 shows RHEED patterns of Er2O3 films grown on oxidized and clean Si(111) substrate surfaces, respectively. The incident electron beam is parallel to the Si [1¯10] axis. Both patterns look streak-like, indicating that both films have single
88
Y.Y. Zhu et al. / Thin Solid Films 508 (2006) 86 – 89
Fig. 3. 0.8 0.8 Am2 AFM images of the films grown on: a) oxidized, b) clean Si(111) surfaces at 700 -C in an oxygen ambient pressure of 7 10
crystalline structures and smooth surfaces. It can be seen the RHEED patterns contain two types of streaks, bright streaks and weak streaks. The bright streaks, partially denoted by A, resulted from the fractional diffraction, which spacing corresponds to the distance of the adjacent atomic planes of Er2O3(110) in real-space. The weak streaks, partially denoted by B, resulted from the integer diffraction, which spacing corresponds to the distance of every two or four atomic planes of Er2O3(110) in real space. The reason that the fractional diffraction order is much brighter than the integer diffraction order is due to the crystalline Er2O3 unit cell structure, which composes four (110) planes in one axis of Er2O3 unit cell. This observation is well consistent with the RHEED patterns of Y2O3 films grown on Si(111) substrates [7]. In addition, there are no changes in the RHEED patterns when rotating the samples every 60- from the [1¯10] direction along the Si[111] direction, reflecting the sixfold symmetry along Er2O3[111] direction. No RHEED patterns of reconstructed Er2O3(111) surface is observed. Comparing Fig. 2(a) with Fig. 2(b), it can be found that the integer diffraction streaks from the film grown on oxidized Si(111) surface are clearer than that from the film grown on the clean one. This trend indicates that the crystallinity of the film is improved when the Er2O3 film is grown on the oxidized Si(111) surface, which agrees well with the results of the crystalline Y2O3 films grown on the clean and oxidized Si(111) substrates [10]. Possible explanations will be given below. Fig. 3 shows the AFM images of the Er2O3 films grown on (a) oxidized and (b) clean Si(111) substrates. The root-meansquare (rms) surface roughness values of these two films are ˚ and 2.63 A ˚ , respectively, indicating very smooth 1.51 A
6
Torr.
surfaces obtained, especially for the film grown on the oxidized Si(111) surface. There are only few data published on surface roughness of oxide films. For example, Cho et al. reported a 15 ˚ rms surface roughness of Y2O3 film obtained by an ionA assisted beam [9]. As shown in Fig. 3(a), Er2O3 film grown on the oxidized Si(111) surface has an extremely smooth surface, ˚ , which, to our knowledge, is with surface roughness of 1.51 A the smallest value reported so far. By comparing the surface roughness values of the two films, it can be deduced that the oxidized substrate could improve the surface roughness of Er2O3 films. The same case is obtained when the Er2O3 films were deposited on the oxidized and clean Si(001) surfaces [8]. Fig. 4 shows the AFM images of the Er2O3 films, grown on (a) oxidized and (b) clean Si(001) surfaces, with the thickness of ˚ . The rms roughness values of these two Er2O3 about 100 A ˚ and 7.45 A ˚ , respectively. Again, the Er2O3 films are 4.68 A films grown on the oxidized Si(001) surface is much smoother than that grown on the clean one. It should be noted that the surface roughness of the Er2O3 films grown on Si(001) substrates is much larger than those grown on Si(111) substrates, which may be due to the overall surface energy of the film [13]. The Er2O3 (111) plane has the lower surface energy than the (100) plane, which would result in a much smoother surface. The different surface roughness on oxidized and clean silicon surfaces could be explained in the view of the silicide formation. When the Er2O3 film is deposited on a clean Si substrate, silicide is inevitably formed by the direct reaction between Er and Si due to its high reaction kinetics, while less silicide is formed when the Er2O3 film is deposited on the oxidized Si surface, confirmed by the XRD results of Fig. 1.
Fig. 4. 4 4 Am2 AFM images of the films grown on: a) oxidized, b) clean Si(001) surfaces at 700 -C in an oxygen ambient pressure of 7 10
6
Torr.
Y.Y. Zhu et al. / Thin Solid Films 508 (2006) 86 – 89
From the literatures, we know that the silicide formation is strongly dependent on the growth conditions and the substrate treatment before growth [14]. Therefore, the oxidized Si surface plays the role of suppressing the formation of silicide under the same growth conditions. Many authors have reported that the surface of the film will become rough if the silicide is formed at the interface between the Si substrate and the asgrown oxide film at the initial growth stage [8,9,14], which is the reason why the relatively smoother Er2O3 film surface has obtained on the oxidized Si substrate than that on the clean Si. Moreover, the reaction between the silicon oxide and erbium atoms may provide sites for Er2O3 nucleation, resulting in a reactively better crystalline Er2O3 layer in comparison with that grown on a clean silicon substrate, which forms clear substreaks in the RHEED pattern of Fig. 2(b). However, an interfacial SiO2 layer may exist if the reaction between the silicon oxide and erbium atoms is insufficient, which will increase the equivalent oxide thickness of the film. Therefore, a moderate SiO2 thickness is critically important in order that SiO2 fully react with Er atoms to form Er2O3 with no SiO2 left. 4. Conclusions In conclusion, the epitaxial growth of Er2O3 films with good crystallinity and smooth surface has been achieved on oxidized Si(111) substrate by MBE at a growth temperature of 700 -C in an oxygen pressure of 7 10 6 Torr. The surface roughness and crystallinity of Er2O3 films grown on oxidized Si(111) and Si(001) surfaces are more improved than those grown on the clean Si surfaces, indicating that oxidized Si surfaces are preferable to clean ones for epitaxially growing Er2O3 films.
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Acknowledgments This work was supported by the special funds for Major State Basic Research Project No. G2001CB3095 of China, the special funds for the National Advanced Technology Research and Development, and Shanghai Science and Technology Commission. This work is also partially supported by NSFC under grant nos. 10321003 and 60425411. References [1] P. David, Norton, Mater. Sci. Eng. R43 (2004) 139. [2] P. J.P.Liu, E. Zaumseil, H.J. Bugiel, H.J. Osten, Appl. Phys. Lett. 79 (2001) 671. [3] A. Fissel, J. Dabrowski, H.J. Osten, J. Appl. Phys. 91 (2002) 8986. [4] A. Dimoulas, G. Vellianitis, A. Travlos, V. Ioannou-Sougleridis, A.G. Nassiopoulou, J. Appl. Phys. 92 (2002) 426. [5] V.V. Afanasev, A. Stesmans, Appl. Phys. Lett. 81 (2002) 1053. [6] V. Mikhelashvili, G. Eisenstein, J. Appl. Phys. 90 (2001) 5447. [7] V. Mikhelashvili, G. Eisenstein, F. Edlman, R. Bremer, N. Zakharov, P. Wemer, J. Appl. Phys. 95 (2004) 613. [8] R. Xu, Y.Y. Zhu, S. Chen, F. Xue, Y.L. Fan, X.J. Yang, Z.M. Jiang, J. Cryst. Growth 277 (2005) 496. [9] M.H. Cho, D.H. Ko, K. Jeong, I.W. Lyo, S.W. Whangbo, H.B. Kim, S.C. Choi, J.H. Song, S.J. Cho, C.N. Whang, J. Appl. Phys. 86 (1999) 198. [10] M.H. Cho, D.H. Ko, Y.K. Choi, I.W. Lyo, K. Jeong, T.G. Kim, J.H. Song, C.N. Whang, J. Appl. Phys. 89 (2001) 1647. [11] H.J. Osten, J.P. Liu, E. Bugiel, H.J. Mussig, P. Zaumseil, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 87 (2001) 297. [12] Guha Supratk, Nestor A. Bojarozuk, Vijay Narayanan, Appl. Phys. Lett. 80 (2002) 766. [13] L.J. Schowalter, R.W. Fathauer, R.P. Goehner, L.G. Turner, R.W. Deblos, J. Appl. Phys. 58 (1985) 302. [14] S.S. Lau, C.S. Pai, C.S. Wu, Appl. Phys. Lett. 41 (1982) 77.