Surface segregation and interdiffusion of Ge on Si(001) studied by medium-energy ion scattering

Thin Solid Films 369 (2000) 112±115

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Surface segregation and interdiffusion of Ge on Si(001) studied by medium-energy ion scattering K. Sumitomo a,*, K. Shiraishi a, Y. Kobayashi a, T. Ito 1,b, T. Ogino a a

NTT Basic Research Laboratories, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan b NTT Photonics Laboratories, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan

Abstract The mechanism of the surface segregation and interdiffusion of Ge on Si(001) during epitaxial growth and annealing was studied using medium-energy ion scattering (MEIS). One monolayer of Ge was grown on Si(001) at 4508C and annealed at 7808C. The Ge was found to redistribute during the very early stages (within 1 min) of the annealing process and remained constant even after annealing at longer periods of time. Therefore, we conclude that the top few layers attain thermal equilibrium in a short period of time, such as less than 1 min. The coverage dependence of Ge distribution in the top few layers was also observed clearly from the surface of Ge deposited on Si(001) at 7808C. This study strongly suggests that the subsurface phenomena should be considered in order to more accurately describe the Si/Ge interface structure. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Surface segregation; Diffusion; Ion scattering

1. Introduction Ge/Si heteroepitaxial growth has been studied extensively because of its technological potential. Control of the interdiffusion and surface segregation of Ge is important for forming an abrupt interface. In the last decade, the interdiffusion and surface segregation in Si/Ge systems have been investigated both experimentally and theoretically [1±10]. Even at low temperatures where no diffusion of Ge in the Si bulk is expected, Ge has been found in the Si subsurface. Sasaki et al. [3] studied the initial growth process of Ge on a Si(001) surface using Auger electron diffraction (AED) and reported that Ge atoms distributed down to the ®fth layer following room temperature (RT) deposition and subsequent annealing. In a previous work, we analyzed the intermixing at the Ge/Si(001) interface during Ge growth at 4008C using surface energy spectroscopy of medium-energy ion scattering (MEIS) [9], and found that the deposited Ge distributed not only in the topmost layer but also in the second and third layers. We also reported a mechanism of Ge segregation during Si overlayer growth on Ge/Si(001), which takes into account the exchange of Si and Ge atoms in buried layers [10]. We found that the activation energy for the site exchange * Corresponding author. 1 Present address: Department of Physics Engineering, Mie University, Japan.

process at the subsurface is lower than the previously believed value and ¯ip-¯op exchange at the subsurface plays a more important role in the Ge segregation process. In this paper, we investigate the mechanism of interdiffusion between the Ge thin ®lm and Si(001) substrate using MEIS. We examined Ge redistribution during annealing and epitaxial growth at a much higher temperature than in our previous works [9,10]. We found that the distribution of Ge in the top few layers remains constant during annealing after the initial redistribution. Therefore, we should consider the top few layers as being in thermal equilibrium. The coverage dependence of Ge distribution in the top few layers was also observed. We have to consider the exchange phenomena in the subsurface in order to control the Si/Ge interface.

2. Experiment Sample preparation and MEIS measurement were carried out in an ultrahigh-vacuum (UHV) system, which is described in detail elsewhere [11]. The samples prepared in a molecular beam epitaxy (MBE) chamber can be transferred to the MEIS chamber without breaking the vacuum. Si(001) wafers with dimensions of 15 £ 16 mm 2 (Bdoped, r~1±10 V cm) were preoxidized chemically and then Si(001)-2 £ 1 clean surfaces were prepared in situ by heating to 9008C. Then 0.3±1.0 monolayers (ML) of Ge Ê /min onto the clean surfaces were deposited at a rate of 0.9 A

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(00)0084 7-6

K. Sumitomo et al. / Thin Solid Films 369 (2000) 112±115

Fig. 1. Ge peaks of MEIS energy spectra from the Ge(1 ML)/Si(001) surface before and after annealing at 7808C. The spectra were calculated for the model with Ge distributions of 8:0:0:0:0, 4:3:1:0:0, and 4:1:1:1:1.

using a Knudsen cell operated at 11808C. Substrate temperatures ranged from RT to 7808C and were monitored by a pyrometer and a WRe thermocouple welded to the sample holder. The thickness of Ge was controlled by its deposition time and con®rmed by the Ge peak intensity of the MEIS random spectra. The depth distributions in the top few layers at the surface were investigated using MEIS. Scattering geometry with glancing trajectory was adopted in order to improve the depth resolution. The probe was a H 1 ion beam and its energy was 99.5 keV. The energy and scattering angle of backscattered ions were analyzed with a two-dimensional detector [12], whose energy resolution DE/E is about 3 £ 10 23. After MEIS measurements, AFM ex situ observations were done using a Digital Instruments Nanoscope III. Three-dimensional islands were not observed. The surface Ê. was ¯at with atomic step spacing of about 3000 A

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lower temperature such as 4508C on the Si(001) surface are distributed not only in the topmost layer but also in the second and third layers, as reported in previous works. The experimental spectrum before annealing is in agreement with the spectrum calculated using the model with 4:3:1:0:0. After annealing at 7808C for 1 or 3 min, the shape of the Ge peak in the MEIS energy spectrum changed, indicating Ge redistribution in the top few layers. The total areas of Ge peaks before and after annealing are almost the same. This indicates that the Ge redistribution is restricted within the top few layers ± the annealing temperature of 7808C is high enough for such rearrangement. We can ignore the effect of the Ge desorption from the surface. The peak intensity around 96.7 keV decreased and the yield of the tail at lower energy (less than 96 keV) increased during the annealing process. This shows that interdiffusion between the Ge thin layer and Si substrate progressed and that some of Ge atoms moved toward the Si substrate. Here, one should note that the peak shapes after annealing for 1 and 3 min are almost the same. In order to investigate time dependence of the Ge distribution in detail during the annealing process, we plotted the energy losses of Ge peaks as a function of the exit angle measured from the surface (Fig. 2). We set the incident angle of the probe ion to normal incidence and collected the ions scattered from Ge atoms in the top few layers with a scattering angle range between 90 and 1158. We evaluated the surface energy loss as hDEi ˆ K…u; M 1 =M2 †E0 2 hEi 2 b

…1†

where K(u ,M1/M2) is the kinematic factor, kEl is the average detected energy, and b is a ®tting parameter that is independent of the exit angle. Fitting parameter b originates from the inelastic energy loss caused by the large-angle (90±1158) collision with Ge [13]. The energy losses as a function of the exit angle are ®t successfully by a simple relation with the time spent near the surface

3. Results Fig. 1 shows Ge peaks in the MEIS energy spectra from the Si(001) surface covered by 1 ML Ge. The Ge deposition temperature was 4508C and the annealing temperature was 7808C. The incident angle of the probe ions was set 28 off from normal incidence and the scattering plane was set 78 off from the [11Å0] plane. In this geometry, almost all Ge atoms distributed in the surface region (up to a depth of Ê ) can be seen from the probe ion. The exit about 50 A angle measured from the surface was set at 48. With this measurement condition, the depth resolution of the Ge distribution for the top few atomic layers is almost one atomic layer. The energy spectra calculated for using models with Ge distributions of 8:0:0:0:0, 4:3:1:0:0, and 4:1:1:1:1 (which were reported in previous works [3,9]) are also shown in Fig. 1. The Ge atoms deposited at a

Fig. 2. Energy losses of Ge peaks in the MEIS energy spectra as a function of exit angle c for the Ge(1 ML)/Si(001) surface before and after annealing for 1, 3, and 28 min at 7808C. The solid and dashed curves are the ®tted results obtained with Eq. (2).

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4. Discussion

Fig. 3. The Ge peaks of MEIS energy spectra for the Si(001) surface covered by Ge of 1 and 0.3 ML. Temperature of the substrate during Ge deposition was 7808C. These spectra are normalized by their Ge coverage to allow comparison with the other spectrum. The solid and dashed lines are best-®t calculations.

DE ˆ a…1 1 1=sinc†

…2†

where a is the ®tting parameter and c is the exit angle measured from the surface. The Ge distribution is directly re¯ected in ®tting parameter a and can be estimated quantitatively. This approach makes distribution analysis more accurate because it is independent of the total amount of Ge in the top few layers and the error of primary beam energy. Since the interdiffusion between the Ge thin layer and Si substrate progressed after the initial annealing, the ®tting factor a increased from 0.027 to 0.037. However, a was found to be independent of annealing time. The ®tting value after annealing for 28 min is almost same as that for 1 min. Next, we investigated the coverage dependence of the Ge distribution. Fig. 3 shows the Ge peak pro®les of MEIS energy spectra for the Si(001) surface covered by Ge of 1 and 0.3 ML. The substrate temperature during Ge deposition was 7808C. The spectra are normalized by the amount of Ge deposited on the surface to allow the comparison of the peak shape. The peak energy for the 0.3-ML Ge surface is higher than that for 1-ML Ge surface. The peak width is also sharper, although the shapes of the tails below 95.5 keV are almost the same. It is indicated that the diffusion for 0.3ML Ge surface was less advanced than for the 1-ML Ge surface. The Ge distribution in the top few layers is dependent of the amount of Ge. This is especially conspicuous in the top three or four layers, which correspond to the MEIS energy spectra in the 96±97-keV range. Together these layers can be regarded as a thermal equilibrium region. Fig. 4 shows the ratio of Ge distributed in the topmost surface to the total amount of deposited Ge. The ratio was estimated by the calculated spectra ®t with the experimental spectra. The error bar represents the error of least-square ®tting. As the Ge coverage increases, the ratio of Ge distributed in the topmost surface decreases.

After high-temperature annealing, the inter diffusion between the Ge thin ®lm and Si substrate progressed (Fig. 1). Such redistribution during annealing is in agreement with the work by Nakajima et al. [14]. However, the distribution of Ge remained constant after the initial redistribution (Fig. 2). Since the activation barrier for exchange between Ge atoms in the third layer and Si atoms in the fourth layer was too high to allow interdiffusion at 4508C, the Ge distribution was restricted to within the top three layers. During the annealing at 7808C, the probability of exchange between the fourth and ®fth layers (as well as between the ®fth and sixth layers) became high and the exchanges between these layers were enhanced. Since the exchange at the subsurface occurs frequently, we can regard the top few layers as being in thermal equilibrium during epitaxial growth of Si/Ge heterostructures. Since the activation barrier of Ge in the Si bulk is too high (4.7 eV [15]) even at such a high temperature, the Ge atoms redistributed in the top few layers maintain a stable distribution. In a previous work [10], we reported that Ge segregation occurs in the buried layer as well as in the topmost surface when a Si overlayer is grown on the Ge/Si(001) surface. The activation energy for site exchange process at the subsurface is lower than the previously believed value, and the second layer contributes to Ge segregation just as the topmost surface layer does. The Ge distributions during annealing and/or Si overlayer growth remain constant, although thickness in thermal equilibrium is dependent on the substrate temperature. When we discuss the mechanism of surface segregation and interdiffusion, we should consider the top few layers as being in thermal equilibrium. Ge has a lower surface energy than Si on the (001) surface [16]. However, during the deposition of Ge, some of the Ge diffuses into the subsurface in order to increase their entropy and decrease their strain due to lattice mismatch. Therefore, in the present work, the Ge diffusion for the 0.3-ML Ge surface was less advanced than that for the 1-ML Ge surface

Fig. 4. The ratio of Ge distributed in the topmost surface to the total amount of deposited Ge. The growth temperature was 7808C.

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(Figs. 3 and 4). These results are in qualitative agreement with the theoretical study by Yoshimoto and Tsukada [17]. We observed no coverage dependence when Ge was deposited on Si(001) at 4508C [9]. In the case of lower growth temperature, the thermal equilibrium region is thinner and the effect of the surface structure (e.g. the asymmetric dimer structure) is dominant. Unfortunately, we cannot distinguish between the Ge atoms in the up-dimer and down-dimer. Since high-temperature growth and/or annealing extended the thermal equilibrium region in this study, we could observe clear coverage dependence of Ge distribution. The mechanism of the Ge diffusion and redistribution in the equilibrium region will be described in detail using a theoretical approach [18]. 5. Conclusion We investigated the Ge distribution in the top few layers during epitaxial growth and annealing. Ge atoms deposited at 4508C not only distribute at the topmost surface but also in the second and third layers. After annealing at higher temperatures (~7808C), the interdiffusion between the Ge thin ®lm and Si substrate progressed. The redistribution of Ge took place during the very early stages of annealing and was completed within 1 min of the start of annealing. It remained unchanged even after annealing at longer periods of time. Therefore, we conclude that the top few layers attain thermal equilibrium in a short period of time, such as less than 1 min. The coverage dependence of Ge distribution in the top few layers was also observed clearly from the surface of Ge deposited on Si(001) at 7808C. Since the time required for the redistribution is short enough, we have to consider the exchange phenomena in the subsurface during epitaxial growth in order to control the Si/Ge interface. Understanding the exchange phenomena in the subsurface as well as in the topmost surface layer will be indispensable for fabricating Si/ Ge heterostructures with the desired concentration pro®les.

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Acknowledgements We thank Hiroki Hibino, Kuniyil Prabhakaran, and Hiroyuki Kageshima for many stimulating discussions. We would like to acknowledge the valuable discussions we had with Yoshiaki Kido and Atsushi Ikeda.

References [1] K. Nakagawa, M. Miyao, J. Appl. Phys. 69 (1991) 3058. [2] S. Fukatsu, K. Fujita, H. Yaguchi, Y. Shiraki, R. Ito, Appl. Phys. Lett. 59 (1991) 2103. [3] M. Sasaki, T. Abukawa, H.W. Yeom, et al., Surf. Sci. 82±83 (1994) 387. [4] G.G. Jernigan, P.E. Thompson, C.L. Silvestre, Appl. Phys. Lett. 69 (1996) 1894. [5] G.G. Jernigan, P.E. Thompson, C.L. Silvestre, Surf. Sci. 380 (1997) 417. [6] F. Liu, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 3156. [7] A.M. Lam, Y.-J. Zheng, J.R. Engstrom, Appl. Phys. Lett. 73 (1998) 2027. [8] D.J. Godbey, M.G. Ancona, Surf. Sci. 395 (1998) 60. [9] A. Ikeda, K. Sumitomo, T. Nishioka, T. Yasue, T. Koshikawa, Y. Kido, Surf. Sci. 385 (1997) 200. [10] K. Sumitomo, Y. Kobayashi, T. Ito, T. Ogino, Thin Solid Films 357 (1999) 76. [11] K. Sumitomo, T. Nishioka, N. Shimizu, Y. Shinoda, T. Ogino, J. Vac. Sci. Technol. A 13 (1995) 289. [12] R.M. Tromp, M. Copel, M.C. Reuter, M. Horn-von Hoegen, J. Speidell, R. Koudis, Rev. Sci. Instrum. 62 (1991) 2679. [13] K. Sumitomo, T. Nishioka, A. Ikeda, Y. Kido, Phys. Rev. B 56 (1997) 7011. [14] K. Nakajima, A. Konichi, K. Kimura, Phys. Rev. Lett. 83 (1999) 1802. [15] J. RaÈisaÈnen, J. Hirvonen, A. Anttila, Solid-State Electron. 24 (1981) 333. [16] P.C. Kelires, J. Tersoff, Phys. Rev. Lett. 63 (1989) 1164. [17] Y. Yoshimoto, M. Tsukada, Surf. Sci. 423 (1999) 32. [18] K. Shiraishi, K. Sumitomo, Y. Yoshimoto, et al., (2000) submitted for publication.