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InSb(1 1 1)A surfaces
ARTICLE IN PRESS
Journal of Crystal Growth 301–302 (2007) 394–399 www.elsevier.com/locate/jcrysgro
Characterization of Au thin films deposited on a-Sn(1 1 1)-(3 3)/ InSb(1 1 1)A surfaces Y. Kasukabea,b,, X. Zhaoa, S. Nishidaa, Y. Fujinob a
Department of Metallurgy, Tohoku University, Aramaki-Aza-Aoba 02, Sendai 980-8579, Japan Center for International Exchange, Tohoku University, 41 Kawauchi, Sendai 980-8576, Japan
b
Available online 26 January 2007
Abstract For the purpose to elucidate the character of the interface between Au metal and a-Sn films, Au metal thin films deposited on a-Sn(1 1 1) surfaces grown on InSb(1 1 1)A substrates are examined by using reflection high-energy electron diffraction (RHEED). The (1 1 1) surface of the a-Sn film with only the 3 3 reconstruction grown at 50 1C is used as a well-defined a-Sn(1 1 1) substrate for the growth of Au films. NiAs-type AuSn films with a high crystallinity are grown in the 1.1-nm-thick Au-deposited thin films on the a-Sn(1 1 1) held at room temperature (RT). The NiAs-type AuSn films have a 2 2 reconstructed surface structure. The orientation relationships between the AuSn and the a-Sn films are (0 0 1) AuSn//(1 1 1) a-Sn and [2 1 0]AuSn//½2 1 1 a-Sn. On the other hand, at 60 1C, the AuSn with a high crystallinity grows in Au-deposited films of about 2.8 nm in thickness, although at RT the AuSn with a high crystallinity grows in Au-deposited films of about 1.1 nm in thickness as stated above. The difference of Au-deposited film thickness between them results from the difference of their interdiffusion lengths of Au and Sn atoms, which depend on the temperature. Taking the interdiffusion into account, the properties and the formation mechanism of interfaces between Au metal thin films as an electrode and the a-Sn/InSb semiconductor superlattice are discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 61.14.Hg; 61.82.Bg; 68.35.Fx; 68.65.Ac Keywords: A1. Crystal structure; A1. Reflection high-energy electron diffraction; A1. Surface structure; A3. Molecular beam epitaxy; B1. Alloys
1. Introduction The properties of the interfaces between electrode metal thin films and the semiconductor superlattices are quite interesting, due to their promising application as high electron mobility transistors and semiconductor laser devices. An Au metal film grown on a-Sn/InSb surfaces is one of such systems [1,2]. Understanding the properties of interface between the electrode Au metal thin film and the a-Sn/InSb semiconductor superlattice will play an important role in improving the reliability of semiconductor quantum devices (for example, infrared absorption devices). In order to examine the interface between the Au Corresponding author. Center for International Exchange, Tohoku University, 41 Kawauchi, Sendai 980-8576, Japan. Tel.: +81 22 795 7978; fax: +81 22 795 7978. E-mail address: [email protected] (Y. Kasukabe).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.218
thin film and the a-Sn/InSb semiconductor superlattice, atomically flat and clean surfaces of a-Sn thin films with a well-defined surface structure have to be prepared on InSb substrates that have a wider band gap than a-Sn. Although the bulk Sn is known to have a semiconducting a-phase with a diamond structure and an identically zero gap at temperature below 13.2 1C, and to have a metallic b-phase with a tetragonal structure above this temperature, Sn films grown on InSb substrates are reported to have an a-phase even above room temperature (RT) [2–7]. The (1 1 1) surface of the a-Sn film grown on an InSb(1 1 1)A-(2 2) was reported to show different surface reconstructions depending on the cleaning methods of the substrate [5]. That is, 3 3 and 2 2 reconstructions of the a-Sn(1 1 1) surface were observed on the cleaned InSb(1 1 1)A surface prepared by using the molecular beam epitaxy (MBE) method [3,5], while only a 1 1 reconstruction was observed in the case of cleaning by repeated sputtering
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and annealing [2,4,6,7]. It was reported that this difference cannot result from the diffusion of In atoms into the Sn overlayer, and that, taking into account that the 3 3 and 2 2 reconstructions observed at lower temperature transformed into a 1 1 structure at higher temperatures [5], one possible explanation is the disordering of the surface Sn atoms, which can be produced by the presence of only small 3 3 and 2 2 domains, whose phases are shifted [4]. These small Sn domains could be induced by the repetition of sputtering and annealing. It was also reported that the 2 2 reconstruction was observed in the transition processes from the 3 3 reconstruction of as-grown a-Sn film surface to the 1 1 structure during heating of the a-Sn film [5]. Thus, in this paper, the (1 1 1) surface of the a-Sn film with only the 3 3 reconstruction is used as a well-defined a-Sn(1 1 1) surface. The detailed growth processes of Au films on a well-defined a-Sn(1 1 1) surface with the 3 3 reconstruction prepared by using the MBE method have not yet been investigated. In this paper, Au metal thin films deposited on aSn(1 1 1) surfaces grown on InSb(1 1 1)A are examined by using reflection high-energy electron diffraction (RHEED) for the purpose to elucidate the structural properties of the interface between the Au metal and the a-Sn films. 2. Experimental procedure The experiment was performed in the MBE system equipped with RHEED, which was evacuated by a turbomolecular pump and an ion pump system. The ultimate pressure and the pressure during deposition of Sn and Au were, after activating the liquid N2-cooled baffles in the vicinity of the evaporation sources, 1 1010 and 6 1010–1 109 Torr, respectively. The substrates were chemically polished before being loaded into the evaporation chamber. First, the substrates were rinsed in trichloroethylene and ethanol, three times repeatedly, and were then etched in a lactic acid (LA)–HNO3 (10:1) solution for 1 h. Immediately after the chemical polishing, the substrate was mounted onto the molybdenum heating block attached to the sample holder in the MBE chamber. The first cleaning treatment of the substrate surface in the MBE system was performed by heating for 20 min at 460 1C with Sb4 beams [3,5,8], which were evaporated from an effusion cell of Ta, impinging onto the substrate at a rate of 3.8 1016 (atoms/(m2 s)). The second treatment was as follows: Molecular beams of In1 and Sb4 were impinged on InSb(1 1 1)A substrates at 330 1C with fluxes of In1 (4.8 1016 (atoms/(m2 s))) and Sb4 (5.5 1016 (atoms/ (m2 s))). The sources of In1 and Sb4 beams were effusion cells composed of high-purity Ta thin plates. Finally, the InSb substrate was annealed for 20 min at 400 1C. After the thermal cleaning treatments, the substrate was cooled down to RT at a rate of 3 1C/min. Atoms of Sn and Au were, respectively, deposited from each W-basket onto the substrate at a rate of 0.1 nm/min. The deposition rate and film thickness were monitored with a quartz crystal
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microbalance. All RHEED patterns were observed at an accelerating potential of 20 keV and were recorded by a CCD camera–video system. The substrate temperatures were measured by a chromel–alumel thermocouple and were calibrated by melting the InSb substrate at a temperature of 525 1C, corresponding to the bulk melting point. 3. Results and discussion It was confirmed by RHEED observations performed along ½2 1 1 and ½0 1 1 azimuths of the thermally cleaned InSb(1 1 1)A substrate prepared by a MBE method that the thermally cleaned InSb(1 1 1)A surface (lattice constant: a ¼ 0:64798 nm) has a 2 2 reconstructed structure [3,5,9–11]. Figs. 1(a) and (b) show typical RHEED patterns taken along the ½2 1 1 and ½0 1 1 azimuths of 2.0-nm-thick a-Sn(1 1 1) surface grown on InSb(1 1 1)A-(2 2) substrates at 50 1C by a MBE method, respectively. An analysis of fundamental reflections marked by large arrows and reflections marked by small arrows in Fig. 1 indicates that the a-Sn(1 1 1) surface (lattice constant: a ¼ 0:6489 nm) grown on InSb(1 1 1)A-(2 2) at 50 1C has a 3 3 reconstructed structure [3,5]. The results of RHEED observations of a-Sn(1 1 1) surface grown on InSb(1 1 1)A-(2 2) substrates at RT were similar to those of a-Sn(1 1 1) surface grown at 50 1C, although the diffraction pattern of the latter was sharper than that of the former, which meant that the crystallinity of a-Sn(1 1 1) surface grown at 50 1C was better than that grown at RT. Taking account of the better crystallinity of a-Sn(1 1 1) surface grown at 50 1C, and of the 3 3 reconstruction of as-grown a-Sn(1 1 1) surface mentioned in Section 1, the (1 1 1) surface of the a-Sn film grown at 50 1C with only the 3 3 reconstruction was used as a well-defined a-Sn(1 1 1) substrate for the growth of Au films. Fig. 2 shows a series of RHEED patterns of Audeposited thin films on a-Sn(1 1 1) held at RT as a function of deposited film thickness of Au, taken along the ½0 11 azimuth of a-Sn(1 1 1) surface; (a) as-grown a-Sn(1 1 1), (b) 0.2 nm, (c) 0.4 nm, (d) 1.2 nm, and (e) 3.0 nm. The 13 order reflections indicated by small arrows in Fig. 2(a) disappeared at the Au-deposited film thickness of 0.2 nm in Fig. 2(b), while the weak 12 order reflection indicated by a small arrow in Fig. 2(b) was observed. As can be seen from Figs. 2(c) and (d), the 12 order reflection indicated by a small arrow becomes stronger with the increase in the Au-deposited film thickness up to about 1.2 nm. In case of the thicker Au-deposited film than 1.2 nm, the 12 order reflection becomes weaker with the increase in the film thickness. At the Au-deposited film thickness of 3.0 nm in Fig. 2(e), it is difficult to observe the 12 order reflection and the fundamental reflections become weaker as indicated by a large arrow. These results mean that for the thicker Au-deposited film than about 1.2 nm, the disordered area increases gradually with the film thickness.
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Fig. 1. RHEED patterns taken along the (a) ½2 1 1 and (b) ½0 1 1 azimuths of a-Sn(1 1 1) surfaces grown on InSb(1 1 1)A-(2 2) substrates held at 50 1C. Large and small arrows indicate fundamental and 13 ordered reflections, respectively.
Judging from the results mentioned above, the crystallinity of the Au-deposited film surface at the deposited film thickness of about 1.2 nm was better than those at the other deposited film thicknesses. Figs. 3(a) and (b) show RHEED patterns of the 1.1-nm-thick Au-deposited thin film with a high crystallinity on a-Sn(1 1 1) held at RT, taken along the ½2 1 1 and ½0 1 1 azimuths of a-Sn(1 1 1) surface, respectively. An analysis of Fig. 3 indicates that NiAs-type AuSn (lattice constants: a ¼ 0:432 nm, c ¼ 0:552 nm) film surfaces are grown, and that the orientation relationships between the AuSn and the a-Sn films are (0 0 1)AuSn// (1 1 1) a-Sn and [2 1 0]AuSn//½2 1 1 a-Sn. Furthermore, it can be seen that reflections marked by large arrows in Figs. 3(a) and (b) denote 12 order reflections, whereas small arrows in Fig. 3(a) indicate 13 order reflections. In light of these facts, it is concluded that the 1.1-nm-thick Audeposited film has double domains, that is, a 2 2 reconstructed AuSn surface (domain) with the larger area and a 1 3 reconstructed surface (domain) with the smaller area. Taking into account of the relationship between the rod profile and intensity of RHEED patterns with coherent areas [12,13], it is considered that a 2 2 reconstructed AuSn domain has a width of more than 10 nm, and that a 1 3 reconstructed domain has a width of less than 5 nm. At the present stage, the origin of the 1 3 reconstructed surface cannot be asserted. However, the origin may be considered to be an Au-deposited structure on the InSb(1 1 1)A surface: RHEED observations of the 1.1nm-thick Au-deposited thin film on the InSb(1 1 1)A substrate held at RT indicate that the Au-deposited film on InSb(1 1 1)A surface can be considered to have a 1 3 structure. Therefore, it can be considered that depositions of Au atoms on a-Sn(1 1 1)/InSb(1 1 1)A produce AuSn domains with the larger area and those on Sn-free InSb(1 1 1)A produce domains with the smaller area, and that the adsorption of Au atoms on the Sn-free InSb(1 1 1)A surface, then, gives rise to the reflections of the 1 3 reconstruction in Fig. 3.
Figs. 4(a)–(c) show a series of RHEED patterns of Au-deposited thin films on a-Sn(1 1 1) held at 60 1C as a function of deposited film thickness of Au, taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface; (a) 0.4 nm, (b) 1.2 nm, and (c) 2.8 nm. The very weak 12 order reflections indicated by small arrows in Fig. 4 were observed at the Audeposited film thickness of 0.4 and 1.2 nm, whereas the 12 order reflection becomes stronger with the increase in the Au-deposited film thickness up to about 2.8 nm, which is different from the results at RT as shown in Fig. 2. In case of the thicker Au-deposited film than 3.2 nm, the 12 order reflection becomes weaker with the increase in the film thickness. Thus, at 60 1C the crystallinity of the Au-deposited film surface at the deposited film thickness of about 2.8 nm was better than those at the other deposited film thicknesses. Figs. 4(c) and (d) show RHEED patterns of the 2.8-nm-thick Au-deposited thin films with a high crystallinity on a-Sn(1 1 1) held at 60 1C, taken along the ½0 1 1 and ½2 1 1 azimuths of a-Sn(1 1 1) surface, respectively. An analysis of Figs. 4(c) and (d) indicates that NiAstype AuSn film surfaces are grown, and that reflections indicated by small arrows in Fig. 4 correspond to 12 order reflections of the AuSn (0 0 1) surface. At this temperature, 1 3 order reflections were not observed. Thus, it was concluded that 2.8-nm-thick Au-deposited film grown at 60 1C has a 2 2 reconstructed AuSn surface. Distances between lattice planes of Au-deposited films estimated from fundamental reflections taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface such as those in Fig. 2 are shown as a function of thickness of Au-deposited films in Fig. 5. The estimated distances of lattice planes of Audeposited films on a-Sn(1 1 1) surfaces held at RT and 60 1C are, respectively, represented by solid circles and open squares in Fig. 5. The thickness of Au-deposited film which shows the sharp reflection of a 2 2 reconstructed structure at each temperature, is indicated by an arrow; 1.0 nm for RT and 2.8 nm for 60 1C. The distance between (1 0 0) planes of the bulk NiAs-type AuSn is 0.374 nm,
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Fig. 2. A series of RHEED patterns of Au-deposited thin films on a-Sn(1 1 1) held at RT, taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface; deposited film thicknesses of Au are (a) 0 nm: as-grown a-Sn(1 1 1), (b) 0.2 nm, (c) 0.4 nm, (d) 1.2 nm, and (e) 3.0 nm. Large and small arrows indicate fundamental and ordered reflections, respectively.
which is indicated by a dotted line. In the early Au-deposition stage, the estimated distances of lattice planes are different from that of the bulk AuSn, which means that non-stoichiometric Au–Sn films without the growth of ordered surface are grown. It is considered that short-range-ordered crystallites of small AuSn, AuSn2 and/ or AuSn4 domains are included in the non-stoichiometric Au–Sn films. The formation of the AuSn induced by the interdiffusion of Au and Sn at RT does not conflict with the results obtained by Hugsted et al. [14]. At RT, the AuSn with a high crystallinity grows in Au-deposited films
of about 1.0 nm in thickness, induced by the interdiffusion of Au and Sn atoms, whereas at 60 1C the AuSn with a high crystallinity grows in Au-deposited films of about 2.8 nm in thickness. It is considered that this difference of Au-deposited film thickness for forming the AuSn with a high crystallinity results from the difference of the interdiffusion length of Au and Sn atoms, which depends on the substrate temperature. Since the interdiffusion length at 60 1C is larger than that at RT, a thicker Au-deposited film is needed for the formation of stoichiometric AuSn at the surface region at 60 1C; in case of the
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Fig. 3. RHEED patterns of the 1.1-nm-thick Au-deposited thin film on a-Sn(1 1 1) held at RT, taken along the (a) ½2 1 1 and (b) ½0 1 1 azimuths of aSn(1 1 1) surface, respectively. Large and small arrows indicate 12 ordered and 13 ordered reflections, respectively.
Fig. 4. A series of RHEED patterns of Au-deposited thin films on a-Sn(1 1 1) held at 60 1C; deposited film thicknesses of Au are (a) 0.4 nm, (b) 1.2 nm, and (c,d) 2.8 nm. RHEED patterns of (a)–(c) are taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface and (d) is taken along the ½2 1 1 azimuth. Large and small arrows indicate fundamental and 12 ordered reflections, respectively.
same Au-deposited film thickness, larger interdiffusion length results in the decrease in the number of Au atoms at the surface region. Preparation for further detailed crystallographic study of the AuSn film, a-Sn film and AuSn/aSn/InSb interfaces by cross-sectional transmission electron microscope observations is in progress.
4. Conclusions Thin Au metal films deposited on a-Sn(1 1 1) surfaces grown on InSb(1 1 1)A substrates were examined by using RHEED. The (1 1 1) surface of the a-Sn film with only the 3 3 reconstruction grown at 50 1C was used as a
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number of Au atoms at the surface region. Thus, it was found that, in order to elucidate the properties of interfaces between the electrode Au metal thin film and the a-Sn/InSb semiconductor superlattice, the interdiffusion in Au/Sn thin film couples have to be sufficiently paid attention to, and that the crystallinity of Ni-As type AuSn films depends on both the Au-deposited film thickness and the substrate temperature. Acknowledgements
Fig. 5. Distances between lattice planes of Au-deposited films estimated from fundamental reflections observed along the ½0 1 1 azimuth of a-Sn(1 1 1) surface such as those in Fig. 3, as a function of thickness of Au-deposited films. The distances of lattice planes of Au-deposited films on a-Sn(1 1 1) surface held at RT and 60 1C are represented by solid circles and open squares, respectively.
well-defined a-Sn(1 1 1) substrate for the growth of Au films. NiAs-type AuSn films were grown in the 1.1-nmthick Au-deposited thin films with a high crystallinity on a-Sn(1 1 1) held at RT. The NiAs-type AuSn films had a 2 2 reconstructed surface structure. The orientation relationships between the AuSn and the a-Sn were (0 0 1) AuSn//(1 1 1) a-Sn and [2 1 0] AuSn//½2 1 1 a-Sn. On the other hand, 2.8-nm-thick Au-deposited films grown at 60 1C had a 2 2 reconstructed AuSn surface with a high crystallinity. At RT, the AuSn grew in Au-deposited film of about 1.1 nm in thickness, induced by the interdiffusion between Au and Sn atoms, whereas at 60 1C the AuSn grew in the Au-deposited film of about 2.8 nm in thickness. This difference of Au-deposited film thickness for the growth of the AuSn with a high crystallinity results from the difference of the interdiffusion length of Au and Sn atoms. Since the interdiffusion length at 60 1C is larger than that at RT, a thicker Au-deposited film is needed for the formation of stoichiometric AuSn at the surface region, that is, in case of the same Au-deposited film thickness, larger interdiffusion length results in the decrease in the
The authors thank Messrs. T. Nakamura, D. Kikawa and H. Nagatani for their assistance in this work. This work was partially supported by a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. References [1] R.F.C. Farrow, D.S. Robertson, G.M. Williams, A.G. Cullis, G.R. Jones, I.M. Young, P.N.J. Dennis, J. Crystal Growth 54 (1981) 507. [2] I. Herna´dez-Caldero´n, H. Ho¨chst, Surf. Sci. 152/153 (1985) 1035. [3] Y. Kasukabe, M. Iwai, T. Osaka, Jpn. J. Appl. Phys. 27 (1988) 3234. [4] D. Kondo, K. Sakamoto, M. Shima, W. Takeyama, K. Nakamura, K. Ono, Y. Kasukabe, M. Oshima, Phys. Rev. B 70 (2004) 233314. [5] T. Osaka, H. Omi, K. Yamamoto, A. Ohtake, Phys. Rev. B 50 (1994) 7567. [6] E. Magnano, M. Pevetta, M. sancrotti, L. Casalis, P. Fantini, M.G. Betti, C. Mariani, Surf. Sci. 433–435 (1999) 387. [7] P. Fantini, S. Gardonio, P. Barbieri, U.D. Pennino, C. Mariani, M.G. Betti, E. Maganano, M. Sancrotti, Surf. Sci. 463 (2000) 174. [8] J. Mu¨hlbach, P. Phan, E. Recknagel, K. Sattler, Surf. Sci. 106 (1981) 18. [9] K. Oe, S. Ando, K. Sugiyama, Jpn. J. Appl. Phys. 19 (1980) L417. [10] A.J. Noreika, M.H. Francombe, C.E.C. Wood, J. Appl. Phys. 52 (1981) 7416. [11] T. Nakada, T. Osaka, Phys. Rev. Lett. 67 (1991) 2834. [12] D.E. Savage, M.G. Lagally, in: P.K. Larsen, P.J. Dobson (Eds.), Reflection High-Energy Electron Diffraction and Reflection Electron Imaging of Surfaces, Plenum Press, New York, 1988, p. 475. [13] J. Aarts, P.K. Larsen, in: P.K. Larsen, P.J. Dobson (Eds.), Reflection High-Energy Electron Diffraction and Reflection Electron Imaging of Surfaces, Plenum Press, New York, 1988, p. 449. [14] B. Hugsted, L. Buene, T. Finstad, O. Lønsjø, T. Olsen, Thin Solid films 98 (1982) 81.
Journal of Crystal Growth 301–302 (2007) 394–399 www.elsevier.com/locate/jcrysgro
Characterization of Au thin films deposited on a-Sn(1 1 1)-(3 3)/ InSb(1 1 1)A surfaces Y. Kasukabea,b,, X. Zhaoa, S. Nishidaa, Y. Fujinob a
Department of Metallurgy, Tohoku University, Aramaki-Aza-Aoba 02, Sendai 980-8579, Japan Center for International Exchange, Tohoku University, 41 Kawauchi, Sendai 980-8576, Japan
b
Available online 26 January 2007
Abstract For the purpose to elucidate the character of the interface between Au metal and a-Sn films, Au metal thin films deposited on a-Sn(1 1 1) surfaces grown on InSb(1 1 1)A substrates are examined by using reflection high-energy electron diffraction (RHEED). The (1 1 1) surface of the a-Sn film with only the 3 3 reconstruction grown at 50 1C is used as a well-defined a-Sn(1 1 1) substrate for the growth of Au films. NiAs-type AuSn films with a high crystallinity are grown in the 1.1-nm-thick Au-deposited thin films on the a-Sn(1 1 1) held at room temperature (RT). The NiAs-type AuSn films have a 2 2 reconstructed surface structure. The orientation relationships between the AuSn and the a-Sn films are (0 0 1) AuSn//(1 1 1) a-Sn and [2 1 0]AuSn//½2 1 1 a-Sn. On the other hand, at 60 1C, the AuSn with a high crystallinity grows in Au-deposited films of about 2.8 nm in thickness, although at RT the AuSn with a high crystallinity grows in Au-deposited films of about 1.1 nm in thickness as stated above. The difference of Au-deposited film thickness between them results from the difference of their interdiffusion lengths of Au and Sn atoms, which depend on the temperature. Taking the interdiffusion into account, the properties and the formation mechanism of interfaces between Au metal thin films as an electrode and the a-Sn/InSb semiconductor superlattice are discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 61.14.Hg; 61.82.Bg; 68.35.Fx; 68.65.Ac Keywords: A1. Crystal structure; A1. Reflection high-energy electron diffraction; A1. Surface structure; A3. Molecular beam epitaxy; B1. Alloys
1. Introduction The properties of the interfaces between electrode metal thin films and the semiconductor superlattices are quite interesting, due to their promising application as high electron mobility transistors and semiconductor laser devices. An Au metal film grown on a-Sn/InSb surfaces is one of such systems [1,2]. Understanding the properties of interface between the electrode Au metal thin film and the a-Sn/InSb semiconductor superlattice will play an important role in improving the reliability of semiconductor quantum devices (for example, infrared absorption devices). In order to examine the interface between the Au Corresponding author. Center for International Exchange, Tohoku University, 41 Kawauchi, Sendai 980-8576, Japan. Tel.: +81 22 795 7978; fax: +81 22 795 7978. E-mail address: [email protected] (Y. Kasukabe).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.218
thin film and the a-Sn/InSb semiconductor superlattice, atomically flat and clean surfaces of a-Sn thin films with a well-defined surface structure have to be prepared on InSb substrates that have a wider band gap than a-Sn. Although the bulk Sn is known to have a semiconducting a-phase with a diamond structure and an identically zero gap at temperature below 13.2 1C, and to have a metallic b-phase with a tetragonal structure above this temperature, Sn films grown on InSb substrates are reported to have an a-phase even above room temperature (RT) [2–7]. The (1 1 1) surface of the a-Sn film grown on an InSb(1 1 1)A-(2 2) was reported to show different surface reconstructions depending on the cleaning methods of the substrate [5]. That is, 3 3 and 2 2 reconstructions of the a-Sn(1 1 1) surface were observed on the cleaned InSb(1 1 1)A surface prepared by using the molecular beam epitaxy (MBE) method [3,5], while only a 1 1 reconstruction was observed in the case of cleaning by repeated sputtering
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and annealing [2,4,6,7]. It was reported that this difference cannot result from the diffusion of In atoms into the Sn overlayer, and that, taking into account that the 3 3 and 2 2 reconstructions observed at lower temperature transformed into a 1 1 structure at higher temperatures [5], one possible explanation is the disordering of the surface Sn atoms, which can be produced by the presence of only small 3 3 and 2 2 domains, whose phases are shifted [4]. These small Sn domains could be induced by the repetition of sputtering and annealing. It was also reported that the 2 2 reconstruction was observed in the transition processes from the 3 3 reconstruction of as-grown a-Sn film surface to the 1 1 structure during heating of the a-Sn film [5]. Thus, in this paper, the (1 1 1) surface of the a-Sn film with only the 3 3 reconstruction is used as a well-defined a-Sn(1 1 1) surface. The detailed growth processes of Au films on a well-defined a-Sn(1 1 1) surface with the 3 3 reconstruction prepared by using the MBE method have not yet been investigated. In this paper, Au metal thin films deposited on aSn(1 1 1) surfaces grown on InSb(1 1 1)A are examined by using reflection high-energy electron diffraction (RHEED) for the purpose to elucidate the structural properties of the interface between the Au metal and the a-Sn films. 2. Experimental procedure The experiment was performed in the MBE system equipped with RHEED, which was evacuated by a turbomolecular pump and an ion pump system. The ultimate pressure and the pressure during deposition of Sn and Au were, after activating the liquid N2-cooled baffles in the vicinity of the evaporation sources, 1 1010 and 6 1010–1 109 Torr, respectively. The substrates were chemically polished before being loaded into the evaporation chamber. First, the substrates were rinsed in trichloroethylene and ethanol, three times repeatedly, and were then etched in a lactic acid (LA)–HNO3 (10:1) solution for 1 h. Immediately after the chemical polishing, the substrate was mounted onto the molybdenum heating block attached to the sample holder in the MBE chamber. The first cleaning treatment of the substrate surface in the MBE system was performed by heating for 20 min at 460 1C with Sb4 beams [3,5,8], which were evaporated from an effusion cell of Ta, impinging onto the substrate at a rate of 3.8 1016 (atoms/(m2 s)). The second treatment was as follows: Molecular beams of In1 and Sb4 were impinged on InSb(1 1 1)A substrates at 330 1C with fluxes of In1 (4.8 1016 (atoms/(m2 s))) and Sb4 (5.5 1016 (atoms/ (m2 s))). The sources of In1 and Sb4 beams were effusion cells composed of high-purity Ta thin plates. Finally, the InSb substrate was annealed for 20 min at 400 1C. After the thermal cleaning treatments, the substrate was cooled down to RT at a rate of 3 1C/min. Atoms of Sn and Au were, respectively, deposited from each W-basket onto the substrate at a rate of 0.1 nm/min. The deposition rate and film thickness were monitored with a quartz crystal
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microbalance. All RHEED patterns were observed at an accelerating potential of 20 keV and were recorded by a CCD camera–video system. The substrate temperatures were measured by a chromel–alumel thermocouple and were calibrated by melting the InSb substrate at a temperature of 525 1C, corresponding to the bulk melting point. 3. Results and discussion It was confirmed by RHEED observations performed along ½2 1 1 and ½0 1 1 azimuths of the thermally cleaned InSb(1 1 1)A substrate prepared by a MBE method that the thermally cleaned InSb(1 1 1)A surface (lattice constant: a ¼ 0:64798 nm) has a 2 2 reconstructed structure [3,5,9–11]. Figs. 1(a) and (b) show typical RHEED patterns taken along the ½2 1 1 and ½0 1 1 azimuths of 2.0-nm-thick a-Sn(1 1 1) surface grown on InSb(1 1 1)A-(2 2) substrates at 50 1C by a MBE method, respectively. An analysis of fundamental reflections marked by large arrows and reflections marked by small arrows in Fig. 1 indicates that the a-Sn(1 1 1) surface (lattice constant: a ¼ 0:6489 nm) grown on InSb(1 1 1)A-(2 2) at 50 1C has a 3 3 reconstructed structure [3,5]. The results of RHEED observations of a-Sn(1 1 1) surface grown on InSb(1 1 1)A-(2 2) substrates at RT were similar to those of a-Sn(1 1 1) surface grown at 50 1C, although the diffraction pattern of the latter was sharper than that of the former, which meant that the crystallinity of a-Sn(1 1 1) surface grown at 50 1C was better than that grown at RT. Taking account of the better crystallinity of a-Sn(1 1 1) surface grown at 50 1C, and of the 3 3 reconstruction of as-grown a-Sn(1 1 1) surface mentioned in Section 1, the (1 1 1) surface of the a-Sn film grown at 50 1C with only the 3 3 reconstruction was used as a well-defined a-Sn(1 1 1) substrate for the growth of Au films. Fig. 2 shows a series of RHEED patterns of Audeposited thin films on a-Sn(1 1 1) held at RT as a function of deposited film thickness of Au, taken along the ½0 11 azimuth of a-Sn(1 1 1) surface; (a) as-grown a-Sn(1 1 1), (b) 0.2 nm, (c) 0.4 nm, (d) 1.2 nm, and (e) 3.0 nm. The 13 order reflections indicated by small arrows in Fig. 2(a) disappeared at the Au-deposited film thickness of 0.2 nm in Fig. 2(b), while the weak 12 order reflection indicated by a small arrow in Fig. 2(b) was observed. As can be seen from Figs. 2(c) and (d), the 12 order reflection indicated by a small arrow becomes stronger with the increase in the Au-deposited film thickness up to about 1.2 nm. In case of the thicker Au-deposited film than 1.2 nm, the 12 order reflection becomes weaker with the increase in the film thickness. At the Au-deposited film thickness of 3.0 nm in Fig. 2(e), it is difficult to observe the 12 order reflection and the fundamental reflections become weaker as indicated by a large arrow. These results mean that for the thicker Au-deposited film than about 1.2 nm, the disordered area increases gradually with the film thickness.
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Fig. 1. RHEED patterns taken along the (a) ½2 1 1 and (b) ½0 1 1 azimuths of a-Sn(1 1 1) surfaces grown on InSb(1 1 1)A-(2 2) substrates held at 50 1C. Large and small arrows indicate fundamental and 13 ordered reflections, respectively.
Judging from the results mentioned above, the crystallinity of the Au-deposited film surface at the deposited film thickness of about 1.2 nm was better than those at the other deposited film thicknesses. Figs. 3(a) and (b) show RHEED patterns of the 1.1-nm-thick Au-deposited thin film with a high crystallinity on a-Sn(1 1 1) held at RT, taken along the ½2 1 1 and ½0 1 1 azimuths of a-Sn(1 1 1) surface, respectively. An analysis of Fig. 3 indicates that NiAs-type AuSn (lattice constants: a ¼ 0:432 nm, c ¼ 0:552 nm) film surfaces are grown, and that the orientation relationships between the AuSn and the a-Sn films are (0 0 1)AuSn// (1 1 1) a-Sn and [2 1 0]AuSn//½2 1 1 a-Sn. Furthermore, it can be seen that reflections marked by large arrows in Figs. 3(a) and (b) denote 12 order reflections, whereas small arrows in Fig. 3(a) indicate 13 order reflections. In light of these facts, it is concluded that the 1.1-nm-thick Audeposited film has double domains, that is, a 2 2 reconstructed AuSn surface (domain) with the larger area and a 1 3 reconstructed surface (domain) with the smaller area. Taking into account of the relationship between the rod profile and intensity of RHEED patterns with coherent areas [12,13], it is considered that a 2 2 reconstructed AuSn domain has a width of more than 10 nm, and that a 1 3 reconstructed domain has a width of less than 5 nm. At the present stage, the origin of the 1 3 reconstructed surface cannot be asserted. However, the origin may be considered to be an Au-deposited structure on the InSb(1 1 1)A surface: RHEED observations of the 1.1nm-thick Au-deposited thin film on the InSb(1 1 1)A substrate held at RT indicate that the Au-deposited film on InSb(1 1 1)A surface can be considered to have a 1 3 structure. Therefore, it can be considered that depositions of Au atoms on a-Sn(1 1 1)/InSb(1 1 1)A produce AuSn domains with the larger area and those on Sn-free InSb(1 1 1)A produce domains with the smaller area, and that the adsorption of Au atoms on the Sn-free InSb(1 1 1)A surface, then, gives rise to the reflections of the 1 3 reconstruction in Fig. 3.
Figs. 4(a)–(c) show a series of RHEED patterns of Au-deposited thin films on a-Sn(1 1 1) held at 60 1C as a function of deposited film thickness of Au, taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface; (a) 0.4 nm, (b) 1.2 nm, and (c) 2.8 nm. The very weak 12 order reflections indicated by small arrows in Fig. 4 were observed at the Audeposited film thickness of 0.4 and 1.2 nm, whereas the 12 order reflection becomes stronger with the increase in the Au-deposited film thickness up to about 2.8 nm, which is different from the results at RT as shown in Fig. 2. In case of the thicker Au-deposited film than 3.2 nm, the 12 order reflection becomes weaker with the increase in the film thickness. Thus, at 60 1C the crystallinity of the Au-deposited film surface at the deposited film thickness of about 2.8 nm was better than those at the other deposited film thicknesses. Figs. 4(c) and (d) show RHEED patterns of the 2.8-nm-thick Au-deposited thin films with a high crystallinity on a-Sn(1 1 1) held at 60 1C, taken along the ½0 1 1 and ½2 1 1 azimuths of a-Sn(1 1 1) surface, respectively. An analysis of Figs. 4(c) and (d) indicates that NiAstype AuSn film surfaces are grown, and that reflections indicated by small arrows in Fig. 4 correspond to 12 order reflections of the AuSn (0 0 1) surface. At this temperature, 1 3 order reflections were not observed. Thus, it was concluded that 2.8-nm-thick Au-deposited film grown at 60 1C has a 2 2 reconstructed AuSn surface. Distances between lattice planes of Au-deposited films estimated from fundamental reflections taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface such as those in Fig. 2 are shown as a function of thickness of Au-deposited films in Fig. 5. The estimated distances of lattice planes of Audeposited films on a-Sn(1 1 1) surfaces held at RT and 60 1C are, respectively, represented by solid circles and open squares in Fig. 5. The thickness of Au-deposited film which shows the sharp reflection of a 2 2 reconstructed structure at each temperature, is indicated by an arrow; 1.0 nm for RT and 2.8 nm for 60 1C. The distance between (1 0 0) planes of the bulk NiAs-type AuSn is 0.374 nm,
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Fig. 2. A series of RHEED patterns of Au-deposited thin films on a-Sn(1 1 1) held at RT, taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface; deposited film thicknesses of Au are (a) 0 nm: as-grown a-Sn(1 1 1), (b) 0.2 nm, (c) 0.4 nm, (d) 1.2 nm, and (e) 3.0 nm. Large and small arrows indicate fundamental and ordered reflections, respectively.
which is indicated by a dotted line. In the early Au-deposition stage, the estimated distances of lattice planes are different from that of the bulk AuSn, which means that non-stoichiometric Au–Sn films without the growth of ordered surface are grown. It is considered that short-range-ordered crystallites of small AuSn, AuSn2 and/ or AuSn4 domains are included in the non-stoichiometric Au–Sn films. The formation of the AuSn induced by the interdiffusion of Au and Sn at RT does not conflict with the results obtained by Hugsted et al. [14]. At RT, the AuSn with a high crystallinity grows in Au-deposited films
of about 1.0 nm in thickness, induced by the interdiffusion of Au and Sn atoms, whereas at 60 1C the AuSn with a high crystallinity grows in Au-deposited films of about 2.8 nm in thickness. It is considered that this difference of Au-deposited film thickness for forming the AuSn with a high crystallinity results from the difference of the interdiffusion length of Au and Sn atoms, which depends on the substrate temperature. Since the interdiffusion length at 60 1C is larger than that at RT, a thicker Au-deposited film is needed for the formation of stoichiometric AuSn at the surface region at 60 1C; in case of the
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Fig. 3. RHEED patterns of the 1.1-nm-thick Au-deposited thin film on a-Sn(1 1 1) held at RT, taken along the (a) ½2 1 1 and (b) ½0 1 1 azimuths of aSn(1 1 1) surface, respectively. Large and small arrows indicate 12 ordered and 13 ordered reflections, respectively.
Fig. 4. A series of RHEED patterns of Au-deposited thin films on a-Sn(1 1 1) held at 60 1C; deposited film thicknesses of Au are (a) 0.4 nm, (b) 1.2 nm, and (c,d) 2.8 nm. RHEED patterns of (a)–(c) are taken along the ½0 1 1 azimuth of a-Sn(1 1 1) surface and (d) is taken along the ½2 1 1 azimuth. Large and small arrows indicate fundamental and 12 ordered reflections, respectively.
same Au-deposited film thickness, larger interdiffusion length results in the decrease in the number of Au atoms at the surface region. Preparation for further detailed crystallographic study of the AuSn film, a-Sn film and AuSn/aSn/InSb interfaces by cross-sectional transmission electron microscope observations is in progress.
4. Conclusions Thin Au metal films deposited on a-Sn(1 1 1) surfaces grown on InSb(1 1 1)A substrates were examined by using RHEED. The (1 1 1) surface of the a-Sn film with only the 3 3 reconstruction grown at 50 1C was used as a
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number of Au atoms at the surface region. Thus, it was found that, in order to elucidate the properties of interfaces between the electrode Au metal thin film and the a-Sn/InSb semiconductor superlattice, the interdiffusion in Au/Sn thin film couples have to be sufficiently paid attention to, and that the crystallinity of Ni-As type AuSn films depends on both the Au-deposited film thickness and the substrate temperature. Acknowledgements
Fig. 5. Distances between lattice planes of Au-deposited films estimated from fundamental reflections observed along the ½0 1 1 azimuth of a-Sn(1 1 1) surface such as those in Fig. 3, as a function of thickness of Au-deposited films. The distances of lattice planes of Au-deposited films on a-Sn(1 1 1) surface held at RT and 60 1C are represented by solid circles and open squares, respectively.
well-defined a-Sn(1 1 1) substrate for the growth of Au films. NiAs-type AuSn films were grown in the 1.1-nmthick Au-deposited thin films with a high crystallinity on a-Sn(1 1 1) held at RT. The NiAs-type AuSn films had a 2 2 reconstructed surface structure. The orientation relationships between the AuSn and the a-Sn were (0 0 1) AuSn//(1 1 1) a-Sn and [2 1 0] AuSn//½2 1 1 a-Sn. On the other hand, 2.8-nm-thick Au-deposited films grown at 60 1C had a 2 2 reconstructed AuSn surface with a high crystallinity. At RT, the AuSn grew in Au-deposited film of about 1.1 nm in thickness, induced by the interdiffusion between Au and Sn atoms, whereas at 60 1C the AuSn grew in the Au-deposited film of about 2.8 nm in thickness. This difference of Au-deposited film thickness for the growth of the AuSn with a high crystallinity results from the difference of the interdiffusion length of Au and Sn atoms. Since the interdiffusion length at 60 1C is larger than that at RT, a thicker Au-deposited film is needed for the formation of stoichiometric AuSn at the surface region, that is, in case of the same Au-deposited film thickness, larger interdiffusion length results in the decrease in the
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