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Formation of lateral nanowires by Ge deposition on Si(111) at high temperatures
Author’s Accepted Manuscript Formation of lateral nanowires by Ge deposition on Si(111) at high temperatures A.А. Shklyaev, А.V. Latyshev
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S0022-0248(16)30042-2 http://dx.doi.org/10.1016/j.jcrysgro.2016.02.015 CRYS23202
To appear in: Journal of Crystal Growth Received date: 18 December 2015 Revised date: 9 February 2016 Accepted date: 10 February 2016 Cite this article as: A.А. Shklyaev and А.V. Latyshev, Formation of lateral nanowires by Ge deposition on Si(111) at high temperatures, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.02.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation of lateral nanowires by Ge deposition on Si(111) at high temperatures A. А. Shklyaeva,b,* and А. V. Latysheva,b a
A.V. Rzhanov Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia
b
Novosibirsk State University, Novosibirsk 630090, Russia
*
E-mail: [email protected]
Abstract The lattice strain is the main factor which governs the surface morphology formation during the Si/Ge heteroepitaxial growth. Its influence becomes significantly weakened at high growth temperatures due to strong Si-Ge intermixing, giving an advantage of other factors which may produce unusual effects. We observed the formation of lateral SiGe nanowires (NWs) after Ge deposition on Si(111) at 830 – 860 ºС using solid-source molecular beam epitaxy. An additional factor to the strain minimization is associated with an energy barrier for the misfit dislocation network introduction at the NW/substrate interface, which causes the NWs to be straight. However, the requirement to attain a certain SiGe composition provides the formation of winding NWs, reflecting the subtle aspects of the growth process.
Keywords: A1. Surface processes, A1. Segregation, A1. Interfaces, A3. Molecular beam epitaxy, B1. Germanium silicon alloys.
1. Introduction The development of different techniques for semiconductor nanowires (NWs) fabrication is important, since they are required for applications in various modern devices [1,2]. From the viewpoint of growth mechanisms, the NW formation is attractive as a self-organized process. The Ge and SiGe NW formation is commonly initiated by metal nanoparticles which serve as catalyst being prior deposited on substrates by different methods [3-7]. The NW growth usually occurs in the directions out of the substrate plane by means of the vapor-liquid-solid process. Ge NWs can also be grown laterally on Si(100) [8,9]. Since such grown NWs contain metal 1
nanoparticles, producing the depletion region in NWs [10], their possible application for interconnections in microelectronic devices is limited. Without catalyst nanoparticles, the lateral Ge NW growth was observed on vicinal Si(100) surfaces, such as Si(1 1 10), in particular, by means of a self-assembled process using a solid-source molecular beam epitaxy (MBE) [11,13]. It has been recently found that Ge NWs can be obtained on Si(100) substrates by thermal annealing of supersaturated Ge wetting layers at appropriate temperatures in the range of 550-650 ºС [14]. In contrast, Ge from the supersaturated Ge wetting layers on Si(111) is used for supporting the three-dimensional (3D) islands growth [15,16]. The island formation is accompanied by Si-Ge intermixing at temperatures above 450 ºС [17,18]. The Ge deposition on Si(111) at about 800 ºС results in the large-sized 3D structures formation [19,20] through the process which was classified as dewetting [20]. The 800 ºС-grown 3D structures are composed of SiGe with the amount of Si up to 70% [20]. We report here that long NWs can be formed during Ge deposition on Si(111) at temperatures in the range from about 830 to 860 ºС by means of the MBE method. The NWs grow laterally on flat surface areas between the large 3D structures. We describe their shapes and growth directions, and discuss the conditions of and reasons for their formation.
2. Experimental details The experiments were carried out in an ultrahigh-vacuum chamber with the base pressure of about 1 × 10-10 Torr. A 10 × 2 × 0.3 mm3 sample was cut from an n-type Si(111) wafer with the resistivity of 5–10 Ω cm. Clean Si surfaces were prepared by flash direct-current heating at 1250 °C. The sample temperature was measured using an IMPAC IGA 12 pyrometer. A Knudsen cell with a BN crucible was used for Ge deposition at the rate from 0.1 to 1.0 nm/min, which was calibrated with the STM for the Ge wetting layer growth on the Si(111) surface. The Ge deposition was performed on samples at temperatures in the range from 830 to 860 °C and for different Ge coverages up to 10 nm. The chamber was equipped with a scanning tunneling microscope (STM) manufactured by Omicron. After the samples removal from the growth chamber, their scanning electron microscope (SEM) images were obtained using a Pioneer microscope manufactured by Raith. The use of SEM allows imaging the surface areas in a wide range of their sizes and with a velocity which is much faster than in the methods, such as STM and atomic force microscopy. This permits obtaining a comprehensive picture of the grown 3D structure distributions in size and shape, and over the sample surface.
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3. Experimental results and discussion Ge deposition on Si(111) at temperatures from 830 to 860 °C leads to the formation of four types of 3D structures, as shown in Fig. 1. One of them is the groups of very large 3D islands. Their observation was previously described for the Ge growth on Si(111) at about 800 °C [19,20]. However, the large 3D islands grown at 830-860 °C are different from them in shape. The 800 °C-grown 3D structures had a dendritic-like shape composed of long quasi-parallel ridges, when they were situated sufficiently far from others. Otherwise, they formed nets of ridges by means of their intersections [20]. The 830-860 °C-grown large 3D islands look like as a result of the ridges decomposition into island groups (Fig. 1).
Fig. 1. (a) SEM and (b) STM images of the surfaces prepared by 10 nm Ge deposition on Si(111) at 860 °C.
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In the areas between the large 3D structures, the much smaller structures with flat (111) surfaces on their top are observed. These are disk-like islands and two types of NWs: straight and winding (Fig. 2). From the shape of small-sized structures, it can be suggested that their formation stars with the disk-like island nucleation. Then some of them grow anisotropically. The straight NWs grew in the directions, such as <2 -1 -1>, <-2 1 1> and <0 -1 1>, as shown in Fig. 2(b). The growth front in <2 -1 -1> and <-2 1 1> directions is perpendicular to close-packed atomic rows on the Si(111) surface. The growth direction of the large 3D ridges at 800 °C was more selective, being only <-2 1 1> [19,20].
Fig. 2. (a) and (b) SEM images of two different surface areas of the sample prepared by 8 nm Ge deposition on Si(111) at 850 °C. The growth directions for straight NWs are indicated in (b). The map of stereographic projections of the NWs and the height profile for their crosssections show that there is no probable preferential facets which would determine their sidewalls shape (Fig. 3). The narrow part of the line on the map, representing the small-angle inclinations, indicates that the upper parts of the NWs are smooth. At the same time, the broadening of the line representing the relatively large inclination angles of 20-30º shows that the corresponding
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sidewalls are uneven. This surface roughness looks as wavy edges along the NW footprints in the STM and SEM images (Figs. 2 and 3). It is known that 3D structures of Si, Ge and SiGe grown on Si(111) are usually covered with facets, such as {111}, {311}, {23 15 3} and {23 20 4} [21-23]. However, only {311} facets are evidently involved into the NW shape formation. Namely, the {311} facets form the growth front for the NW growth in <2 -1 -1> directions. They can also determine sidewalls at NW bases for the NWs that grow in <0 1 -1> directions. The NWs are flat with the (111) facet on their top. The (111) facet covers more than 80% of the total NW surface, providing the minimization of the NW surface energy. As for {23 15 3} and {23 20 4} facets, their orientation does not permit them to be located on smooth sidewalls of straight NWs of all three growth directions. However, the observed wavy edges of NW sidewalls with the relatively big inclination angles could indicate that these facets are involved into the shape formation of NWs at their bases. The STM images show that there are no trenches in the Si substrate along the NW perimeter, as they commonly appear around 3D SiGe structures during Ge deposition on Si(111) at the temperatures above 450 °C [24-26]. This indicates the absence of an appreciable lattice strain between the NWs and the substrate.
Fig. 3. (a) A SEM image of the straight NWs grown on Si(111) at 840 °C by 5 nm Ge deposition. (b) A cross-section profile of a straight NW. (c) The map of stereographic projections obtained from an STM image of one long straight NW.
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The STM data show that the NWs are broadened and slightly thickened at their ends (Fig. 4). The long straight NWs may have the width at their ends up to about two times bigger than that in their initial growth place. This means that parts of NWs become reduced after their growth. This may occur through the NW atom diffusion along them and the atom intermixing between NWs and substrates. At the same time, the NWs become wider during their growth, since long straight NWs are usually broader than short ones.
Fig. 4. (a) A STM image of the surfaces with the straight NWs prepared by 10 nm Ge deposition on Si(111) at 860 °C. (b) The width and thickness of the longest NW, shown in (a), as a function of the distance from its initial growth place to its end.
The winding NWs were arranged adjacent to the straight NWs on flat surface areas between the large 3D structures. Their number was several times bigger than that of the straight NWs (Fig. 1 and 5). The width of the winding NWs was not uniform along their length. It contained local broadenings. They appear in the places which are favorable for NW branching. The specific aspect of the winding NW growth consists in their exclusive propagation in the vicinity of the previously grown NWs and disk-like small islands. This feature points out that the winding NW growth requires a limited amount of deposited Ge adatoms which can diffuse to the NWs from the surrounding areas. It is probable that the winding NWs contain a smaller Ge content in their SiGe composition than the straight NWs which grow towards the open surface areas, i.e. the areas without the presence of any 3D structures. This is consistent with the fact that the straight NWs look brighter in the SEM images in comparison with the winding NWs [Figs 1(a) and 5(a)]. 6
Fig. 5. (a) and (b) SEM images of the NWs grown by 5 nm Ge deposition and (c) the STM image of the surface morphology after 10 nm Ge deposition on Si(111) at 840 °С. Another specific feature of the winding NW growth consists in the fact that they never create closed surface areas. The closed areas may accumulate and, thereby, contain an elevated concentration of Ge adatoms. This circumstance is consistent with the above discussed suggestion that the winding NWs can grow only in the conditions of a limited Ge adatom concentration. This feature is also valid for the straight NWs which stop their growth as their ends approach other NWs. The NW growth possesses the features that are the same as those for the endotaxial growth. In both cases, the intermixing of a deposited material with a substrate takes place. The endotaxial growth forms structures with a homogeneous chemical composition and a sharp interface with the substrate [27,28]. The SiGe structures grown on Si(111) at high temperatures also have a sharp interface with the Si substrate [20]. However, the chemical composition of the SiGe layers is usually inhomogeneous and varies as a function of the distance from the interface [18,29]. The last feature makes the SiGe structure formation on Si(111) different from the endotaxial growth. As for the SiGe NWs, they are formed at very high temperatures of ~850 °C and are rather thin, 15-20 nm, therefore, they may have a homogeneous chemical composition. If so, the NW formation can be referred to as the endotaxial growth. The lateral NW formation has been recently found on Si(100) during annealing of supersaturated Ge wetting layers at about 600 °C [14]. It was concluded that the NWs shape is determined by the domination of the longitudinal growth rate of the {501} facets located on the NW sidewalls [30,31]. In our case, the straight NWs have thickening ends whose shape is formed by several small-sized facets, and the large-sized (111) facet forms the NWs top. Since there are no data that the (111) surface plane is characterized by a preferable lateral growth 7
direction, the NW growth cannot be attributed to dominating elongation of certain faceting planes. The small disk-like islands appear on Si(111) at the beginning of Ge deposition. Some of them attain the anisotropic growth and coalesce with the neighbor islands during the further Ge deposition, developing into the winding NWs. This occurs instead of the possible isotropic growth of the disk-like islands. Let us consider the reasons for such behavior. When the area sizes of disks and NWs at the interface with the substrate are equal, NWs have a longer perimeter than disks. This indicates that the lattice strain in the local areas along perimeters is not an important factor determining the shape formation despite the fact that the strain is usually the strongest at the perimeters [32,33]. The similar situation is for the surface energies, since NWs have larger surface area sizes than disks, as was shown comparing ridges and individual islands [33]. It was theoretically proved that islands whose lateral size is larger than a critical one have a tendency to elongation for a better lattice strain reduction [34]. Moreover, there is another factor that provides the stain relaxation. This is the introduction of misfit dislocations at the interface between a grown 3D structure and a substrate. The dislocations form a network in the case of Ge layers on Si(111) [35-37]. The crystallographic orientations of the straight NWs is in agreement with the orientations of misfit dislocation at the Ge/Si(111) interface [35]. It is evident that disklike islands require the introduction of a larger number of dislocations than that of NWs due to the short distance in the direction across the NWs for the dislocation network formation. The distance between dislocations in their network is about 10 nm for Ge layers grown on Si(111) at relatively low temperatures of 400-450 °C, when the Si-Ge intermixing does not occur [24-26] and the Ge/Si lattice mismatch is 0.04%. Since the 3D SiGe structures grown at 800 °C contain only 30% of Ge [Ref. 20] and the rate of Si-Ge intermixing strongly increases with temperature [38], these allow us to assume that the Ge content in the NWs grown at about 850 °C is substantially smaller than 30%. The lattice strain in NWs with such a SiGe composition is significantly reduced in comparison with that of the pure Ge layers, and, hence, the distance between dislocations should increase, becoming comparable to the width of the NWs. Thus, only a few dislocations can be formed in the direction along the NWs or they may not appear at all. 4. Conclusion The different flat 3D SiGe structures were observed after Ge depositions on Si(111) at temperatures around 850 °C. Their formation proceeds through the nucleation of islands which then develop into long straight and winding NWs. The NWs are formed in spite of their larger surface area size, in comparison with disk-like islands of the same base size, and longer perimeters at which the lattice strain is known to be the strongest. The general reason for the NW formation is the minimization of the strain energy by means of elongation instead of flat 3D 8
structures isotropic-like growth, as has been theoretically demonstrated by Tersoff and Tromp [34]. Additional factors that govern the NW shape formation are different for the straight and winding NWs. The crystallographic orientations of NWs on the Si(111) surface and their probable SiGe composition suggest that the energy barrier for the dislocation net work formation at the NW/substrate interface is responsible for the straight shape of the NWs and for their certain width. As for the winding NWs, however, they grow in the direction in which the Ge adatom concentration is favorable for the NW formation with a required SiGe composition.
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[19] J.M. MacLeod, J.A. Lipton-Duffin, U. Lanke, S.G. Urquhart, F. Rosei, Shape transition in very large germanium islands on Si(111), Appl. Phys. Lett. 94 (2009) 103109. [20] A. Shklyaev, L. Bolotov, V. Poborchii, T. Tada, Properties of three-dimensional structures prepared by Ge dewetting from Si(111) at high temperatures, J. Appl. Phys. 117 (2015) 205303. [21] B. Voigtländer, A. Zinner, Simultaneous molecular beam epitaxy growth and scanning tunneling microscopy imaging during Ge/Si epitaxy, Appl. Phys. Lett. 63 (1993) 3055-3057. [22] Z. Gai, R.G. Zhao, X. Li, W.S. Yang, Faceting and nanoscale faceting of Ge(hhl) surfaces around (113), Phys. Rev. B 58 (1998) 4572. [23] A.A. Shklyaev, K.N. Romanyuk, S.S. Kosolobov, Surface morphology of Ge layers epitaxially grown on bare and oxidized Si(001) and Si(111) substrates, Surf. Sci. 625 (2014) 50-56. [24] T.I. Kamins, E.C. Carr, R.S. Williams, S.J. Rosner, Deposition of three-dimensional Ge islands on Si(001) by chemical vapor deposition at atmospheric and reduced pressures, J. Appl. Phys. 81 (1997) 211-219. [25] F. Boscherini, G. Capellini, L. Di Gaspare, M.De Seta, F. Rosei, A. Sgarlata, N. Motta, S. Mobilio, Ge–Si intermixing in Ge quantum dots on Si, Thin Solid Films 380 (2000) 173-175. [26] A.A. Shklyaev, K.N. Romanyuk, A.V. Latyshev, Epitaxial Ge Growth on Si(111) Covered with Ultrathin SiO2 Films, JSEMAT 3 (2013) 195-204. [27] P.A. Bennett, Z. He, D.J. Smith, F.M. Ross, Endotaxial silicide nanowires: A review, Thin Solid Films 519 (2011) 8434–8440. [28] Z.-P. Li, E. Tok, Y. Foo, Shape transition of endotaxial islands growth from kinetically constrained to equilibrium regimes, Mater. Res. Bull. 48 (2013) 2998–3008. [29] N. Tanaka, S.-P. Cho, A.A. Shklyaev, J. Yamasaki, E. Okunishi, M. Ichikawa, Spherical aberration corrected STEM studies of Ge nanodots grown on Si(0 0 1) surfaces with an ultrathin SiO2 coverage, Appl. Surf. Sci. 254 (2008) 7569–7572 [30] J.J. Zhang, G. Katsaros, F. Montalenti, D. Scopece, R.O. Rezaev, C. Mickel, B. Rellinghaus, L. Miglio, S. De Franceschi, A. Rastelli, O.G. Schmidt, Monolithic growth of ultrathin Ge nanowires on Si(001), Phys. Rev. Lett. 109 (2012) 085502. [31] K.A. Lozovoy, A.P. Kokhanenko, A.V. Voitsekhovskii, Influence of Edge Energy on Modeling the Growth Kinetics of Quantum Dots, Cryst. Growth Des. 15 (2015) 1055-1059. 11
[32] J. Fuhr, P. Müller, Strain distribution due to surface domains: a self-consistent approach with respect to surface elasticity, Beilstein J. Nanotechnol. 6 (2015) 321-326. [33] A.A. Shklyaev, K.E. Ponomarev, Strain-induced Ge segregation on Si at high temperatures, J. Cryst. Growth 413 (2015) 94-99. [34] J. Tersoff, R.M. Tromp, Shape transition in growth of strained islands: spontaneous formation of quantum wires, Phys. Rev. Lett., 70 (1993) 2782-2785. [35] M. Horn-von Hoegen, A. Al-Falou, H. Pietsch, B.H. Muller, M. Henzler, Formation of interfacial dislocation network in surfactant mediated growth of Ge on Si(111) investigated by SPA-LEED: Part I, Surf. Sci. 298 (1993) 29-42. [36] B. Voigtländer, N. Theuerkauf, Ordered growth of Ge islands above a misfit dislocation network in a Ge layer on Si(111), Surf. Sci. 461 (2000) L575-L580. [37] A.S. Ilin, E.M. Trukhanov, S.A. Teys, A.K. Gutakovskii, Analysis of the dislocation structure at the Ge/Si(111) heterointerface, J. Surf. Invest.: X_Ray, Synchrotron Neutron Tech. 8 (2014) 787-793. [38] A.A. Shklyaev, A.E. Budazhapova, Ge deposition on Si(100) in the conditions close to dynamic equilibrium between islands growth and their decay, Appl. Surf. Sci. 360 (2016) 1023-1029.
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Figure captions Fig. 1. (a) SEM and (b) STM images of the surfaces prepared by 10 nm Ge deposition on Si(111) at 860 °C. Fig. 2. (a) and (b) SEM images of two different surface areas of the sample prepared by 8 nm Ge deposition on Si(111) at 850 °C. The growth directions for straight NWs are indicated in (b). Fig. 3. (a) A SEM image of the straight NWs grown on Si(111) at 840 °C by 5 nm Ge deposition. (b) A cross-section profile of a straight NW. (c) The map of stereographic projections obtained from an STM image of one long straight NW. Fig. 4. (a) A STM image of the surfaces with the straight NWs prepared by 10 nm Ge deposition on Si(111) at 860 °C. (b) The width and thickness of the longest NW, shown in (a), as a function of the distance from its initial growth place to its end. Fig. 5. (a) and (b) SEM images of the NWs grown by 5 nm Ge deposition and (c) the STM image of the surface morphology after 10 nm Ge deposition on Si(111) at 840 °С.
Highlights - SiGe lateral nanowires (NWs) are grown by Ge deposition on Si(111) at 830 – 860 ºС. - NW shapes are straight or winding. - NWs do not connect each other in order to form enclosed areas - It is suggested that NW formation takes place to avoid the introduction of misfit dislocations.
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PII: DOI: Reference:
S0022-0248(16)30042-2 http://dx.doi.org/10.1016/j.jcrysgro.2016.02.015 CRYS23202
To appear in: Journal of Crystal Growth Received date: 18 December 2015 Revised date: 9 February 2016 Accepted date: 10 February 2016 Cite this article as: A.А. Shklyaev and А.V. Latyshev, Formation of lateral nanowires by Ge deposition on Si(111) at high temperatures, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.02.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation of lateral nanowires by Ge deposition on Si(111) at high temperatures A. А. Shklyaeva,b,* and А. V. Latysheva,b a
A.V. Rzhanov Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia
b
Novosibirsk State University, Novosibirsk 630090, Russia
*
E-mail: [email protected]
Abstract The lattice strain is the main factor which governs the surface morphology formation during the Si/Ge heteroepitaxial growth. Its influence becomes significantly weakened at high growth temperatures due to strong Si-Ge intermixing, giving an advantage of other factors which may produce unusual effects. We observed the formation of lateral SiGe nanowires (NWs) after Ge deposition on Si(111) at 830 – 860 ºС using solid-source molecular beam epitaxy. An additional factor to the strain minimization is associated with an energy barrier for the misfit dislocation network introduction at the NW/substrate interface, which causes the NWs to be straight. However, the requirement to attain a certain SiGe composition provides the formation of winding NWs, reflecting the subtle aspects of the growth process.
Keywords: A1. Surface processes, A1. Segregation, A1. Interfaces, A3. Molecular beam epitaxy, B1. Germanium silicon alloys.
1. Introduction The development of different techniques for semiconductor nanowires (NWs) fabrication is important, since they are required for applications in various modern devices [1,2]. From the viewpoint of growth mechanisms, the NW formation is attractive as a self-organized process. The Ge and SiGe NW formation is commonly initiated by metal nanoparticles which serve as catalyst being prior deposited on substrates by different methods [3-7]. The NW growth usually occurs in the directions out of the substrate plane by means of the vapor-liquid-solid process. Ge NWs can also be grown laterally on Si(100) [8,9]. Since such grown NWs contain metal 1
nanoparticles, producing the depletion region in NWs [10], their possible application for interconnections in microelectronic devices is limited. Without catalyst nanoparticles, the lateral Ge NW growth was observed on vicinal Si(100) surfaces, such as Si(1 1 10), in particular, by means of a self-assembled process using a solid-source molecular beam epitaxy (MBE) [11,13]. It has been recently found that Ge NWs can be obtained on Si(100) substrates by thermal annealing of supersaturated Ge wetting layers at appropriate temperatures in the range of 550-650 ºС [14]. In contrast, Ge from the supersaturated Ge wetting layers on Si(111) is used for supporting the three-dimensional (3D) islands growth [15,16]. The island formation is accompanied by Si-Ge intermixing at temperatures above 450 ºС [17,18]. The Ge deposition on Si(111) at about 800 ºС results in the large-sized 3D structures formation [19,20] through the process which was classified as dewetting [20]. The 800 ºС-grown 3D structures are composed of SiGe with the amount of Si up to 70% [20]. We report here that long NWs can be formed during Ge deposition on Si(111) at temperatures in the range from about 830 to 860 ºС by means of the MBE method. The NWs grow laterally on flat surface areas between the large 3D structures. We describe their shapes and growth directions, and discuss the conditions of and reasons for their formation.
2. Experimental details The experiments were carried out in an ultrahigh-vacuum chamber with the base pressure of about 1 × 10-10 Torr. A 10 × 2 × 0.3 mm3 sample was cut from an n-type Si(111) wafer with the resistivity of 5–10 Ω cm. Clean Si surfaces were prepared by flash direct-current heating at 1250 °C. The sample temperature was measured using an IMPAC IGA 12 pyrometer. A Knudsen cell with a BN crucible was used for Ge deposition at the rate from 0.1 to 1.0 nm/min, which was calibrated with the STM for the Ge wetting layer growth on the Si(111) surface. The Ge deposition was performed on samples at temperatures in the range from 830 to 860 °C and for different Ge coverages up to 10 nm. The chamber was equipped with a scanning tunneling microscope (STM) manufactured by Omicron. After the samples removal from the growth chamber, their scanning electron microscope (SEM) images were obtained using a Pioneer microscope manufactured by Raith. The use of SEM allows imaging the surface areas in a wide range of their sizes and with a velocity which is much faster than in the methods, such as STM and atomic force microscopy. This permits obtaining a comprehensive picture of the grown 3D structure distributions in size and shape, and over the sample surface.
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3. Experimental results and discussion Ge deposition on Si(111) at temperatures from 830 to 860 °C leads to the formation of four types of 3D structures, as shown in Fig. 1. One of them is the groups of very large 3D islands. Their observation was previously described for the Ge growth on Si(111) at about 800 °C [19,20]. However, the large 3D islands grown at 830-860 °C are different from them in shape. The 800 °C-grown 3D structures had a dendritic-like shape composed of long quasi-parallel ridges, when they were situated sufficiently far from others. Otherwise, they formed nets of ridges by means of their intersections [20]. The 830-860 °C-grown large 3D islands look like as a result of the ridges decomposition into island groups (Fig. 1).
Fig. 1. (a) SEM and (b) STM images of the surfaces prepared by 10 nm Ge deposition on Si(111) at 860 °C.
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In the areas between the large 3D structures, the much smaller structures with flat (111) surfaces on their top are observed. These are disk-like islands and two types of NWs: straight and winding (Fig. 2). From the shape of small-sized structures, it can be suggested that their formation stars with the disk-like island nucleation. Then some of them grow anisotropically. The straight NWs grew in the directions, such as <2 -1 -1>, <-2 1 1> and <0 -1 1>, as shown in Fig. 2(b). The growth front in <2 -1 -1> and <-2 1 1> directions is perpendicular to close-packed atomic rows on the Si(111) surface. The growth direction of the large 3D ridges at 800 °C was more selective, being only <-2 1 1> [19,20].
Fig. 2. (a) and (b) SEM images of two different surface areas of the sample prepared by 8 nm Ge deposition on Si(111) at 850 °C. The growth directions for straight NWs are indicated in (b). The map of stereographic projections of the NWs and the height profile for their crosssections show that there is no probable preferential facets which would determine their sidewalls shape (Fig. 3). The narrow part of the line on the map, representing the small-angle inclinations, indicates that the upper parts of the NWs are smooth. At the same time, the broadening of the line representing the relatively large inclination angles of 20-30º shows that the corresponding
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sidewalls are uneven. This surface roughness looks as wavy edges along the NW footprints in the STM and SEM images (Figs. 2 and 3). It is known that 3D structures of Si, Ge and SiGe grown on Si(111) are usually covered with facets, such as {111}, {311}, {23 15 3} and {23 20 4} [21-23]. However, only {311} facets are evidently involved into the NW shape formation. Namely, the {311} facets form the growth front for the NW growth in <2 -1 -1> directions. They can also determine sidewalls at NW bases for the NWs that grow in <0 1 -1> directions. The NWs are flat with the (111) facet on their top. The (111) facet covers more than 80% of the total NW surface, providing the minimization of the NW surface energy. As for {23 15 3} and {23 20 4} facets, their orientation does not permit them to be located on smooth sidewalls of straight NWs of all three growth directions. However, the observed wavy edges of NW sidewalls with the relatively big inclination angles could indicate that these facets are involved into the shape formation of NWs at their bases. The STM images show that there are no trenches in the Si substrate along the NW perimeter, as they commonly appear around 3D SiGe structures during Ge deposition on Si(111) at the temperatures above 450 °C [24-26]. This indicates the absence of an appreciable lattice strain between the NWs and the substrate.
Fig. 3. (a) A SEM image of the straight NWs grown on Si(111) at 840 °C by 5 nm Ge deposition. (b) A cross-section profile of a straight NW. (c) The map of stereographic projections obtained from an STM image of one long straight NW.
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The STM data show that the NWs are broadened and slightly thickened at their ends (Fig. 4). The long straight NWs may have the width at their ends up to about two times bigger than that in their initial growth place. This means that parts of NWs become reduced after their growth. This may occur through the NW atom diffusion along them and the atom intermixing between NWs and substrates. At the same time, the NWs become wider during their growth, since long straight NWs are usually broader than short ones.
Fig. 4. (a) A STM image of the surfaces with the straight NWs prepared by 10 nm Ge deposition on Si(111) at 860 °C. (b) The width and thickness of the longest NW, shown in (a), as a function of the distance from its initial growth place to its end.
The winding NWs were arranged adjacent to the straight NWs on flat surface areas between the large 3D structures. Their number was several times bigger than that of the straight NWs (Fig. 1 and 5). The width of the winding NWs was not uniform along their length. It contained local broadenings. They appear in the places which are favorable for NW branching. The specific aspect of the winding NW growth consists in their exclusive propagation in the vicinity of the previously grown NWs and disk-like small islands. This feature points out that the winding NW growth requires a limited amount of deposited Ge adatoms which can diffuse to the NWs from the surrounding areas. It is probable that the winding NWs contain a smaller Ge content in their SiGe composition than the straight NWs which grow towards the open surface areas, i.e. the areas without the presence of any 3D structures. This is consistent with the fact that the straight NWs look brighter in the SEM images in comparison with the winding NWs [Figs 1(a) and 5(a)]. 6
Fig. 5. (a) and (b) SEM images of the NWs grown by 5 nm Ge deposition and (c) the STM image of the surface morphology after 10 nm Ge deposition on Si(111) at 840 °С. Another specific feature of the winding NW growth consists in the fact that they never create closed surface areas. The closed areas may accumulate and, thereby, contain an elevated concentration of Ge adatoms. This circumstance is consistent with the above discussed suggestion that the winding NWs can grow only in the conditions of a limited Ge adatom concentration. This feature is also valid for the straight NWs which stop their growth as their ends approach other NWs. The NW growth possesses the features that are the same as those for the endotaxial growth. In both cases, the intermixing of a deposited material with a substrate takes place. The endotaxial growth forms structures with a homogeneous chemical composition and a sharp interface with the substrate [27,28]. The SiGe structures grown on Si(111) at high temperatures also have a sharp interface with the Si substrate [20]. However, the chemical composition of the SiGe layers is usually inhomogeneous and varies as a function of the distance from the interface [18,29]. The last feature makes the SiGe structure formation on Si(111) different from the endotaxial growth. As for the SiGe NWs, they are formed at very high temperatures of ~850 °C and are rather thin, 15-20 nm, therefore, they may have a homogeneous chemical composition. If so, the NW formation can be referred to as the endotaxial growth. The lateral NW formation has been recently found on Si(100) during annealing of supersaturated Ge wetting layers at about 600 °C [14]. It was concluded that the NWs shape is determined by the domination of the longitudinal growth rate of the {501} facets located on the NW sidewalls [30,31]. In our case, the straight NWs have thickening ends whose shape is formed by several small-sized facets, and the large-sized (111) facet forms the NWs top. Since there are no data that the (111) surface plane is characterized by a preferable lateral growth 7
direction, the NW growth cannot be attributed to dominating elongation of certain faceting planes. The small disk-like islands appear on Si(111) at the beginning of Ge deposition. Some of them attain the anisotropic growth and coalesce with the neighbor islands during the further Ge deposition, developing into the winding NWs. This occurs instead of the possible isotropic growth of the disk-like islands. Let us consider the reasons for such behavior. When the area sizes of disks and NWs at the interface with the substrate are equal, NWs have a longer perimeter than disks. This indicates that the lattice strain in the local areas along perimeters is not an important factor determining the shape formation despite the fact that the strain is usually the strongest at the perimeters [32,33]. The similar situation is for the surface energies, since NWs have larger surface area sizes than disks, as was shown comparing ridges and individual islands [33]. It was theoretically proved that islands whose lateral size is larger than a critical one have a tendency to elongation for a better lattice strain reduction [34]. Moreover, there is another factor that provides the stain relaxation. This is the introduction of misfit dislocations at the interface between a grown 3D structure and a substrate. The dislocations form a network in the case of Ge layers on Si(111) [35-37]. The crystallographic orientations of the straight NWs is in agreement with the orientations of misfit dislocation at the Ge/Si(111) interface [35]. It is evident that disklike islands require the introduction of a larger number of dislocations than that of NWs due to the short distance in the direction across the NWs for the dislocation network formation. The distance between dislocations in their network is about 10 nm for Ge layers grown on Si(111) at relatively low temperatures of 400-450 °C, when the Si-Ge intermixing does not occur [24-26] and the Ge/Si lattice mismatch is 0.04%. Since the 3D SiGe structures grown at 800 °C contain only 30% of Ge [Ref. 20] and the rate of Si-Ge intermixing strongly increases with temperature [38], these allow us to assume that the Ge content in the NWs grown at about 850 °C is substantially smaller than 30%. The lattice strain in NWs with such a SiGe composition is significantly reduced in comparison with that of the pure Ge layers, and, hence, the distance between dislocations should increase, becoming comparable to the width of the NWs. Thus, only a few dislocations can be formed in the direction along the NWs or they may not appear at all. 4. Conclusion The different flat 3D SiGe structures were observed after Ge depositions on Si(111) at temperatures around 850 °C. Their formation proceeds through the nucleation of islands which then develop into long straight and winding NWs. The NWs are formed in spite of their larger surface area size, in comparison with disk-like islands of the same base size, and longer perimeters at which the lattice strain is known to be the strongest. The general reason for the NW formation is the minimization of the strain energy by means of elongation instead of flat 3D 8
structures isotropic-like growth, as has been theoretically demonstrated by Tersoff and Tromp [34]. Additional factors that govern the NW shape formation are different for the straight and winding NWs. The crystallographic orientations of NWs on the Si(111) surface and their probable SiGe composition suggest that the energy barrier for the dislocation net work formation at the NW/substrate interface is responsible for the straight shape of the NWs and for their certain width. As for the winding NWs, however, they grow in the direction in which the Ge adatom concentration is favorable for the NW formation with a required SiGe composition.
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Figure captions Fig. 1. (a) SEM and (b) STM images of the surfaces prepared by 10 nm Ge deposition on Si(111) at 860 °C. Fig. 2. (a) and (b) SEM images of two different surface areas of the sample prepared by 8 nm Ge deposition on Si(111) at 850 °C. The growth directions for straight NWs are indicated in (b). Fig. 3. (a) A SEM image of the straight NWs grown on Si(111) at 840 °C by 5 nm Ge deposition. (b) A cross-section profile of a straight NW. (c) The map of stereographic projections obtained from an STM image of one long straight NW. Fig. 4. (a) A STM image of the surfaces with the straight NWs prepared by 10 nm Ge deposition on Si(111) at 860 °C. (b) The width and thickness of the longest NW, shown in (a), as a function of the distance from its initial growth place to its end. Fig. 5. (a) and (b) SEM images of the NWs grown by 5 nm Ge deposition and (c) the STM image of the surface morphology after 10 nm Ge deposition on Si(111) at 840 °С.
Highlights - SiGe lateral nanowires (NWs) are grown by Ge deposition on Si(111) at 830 – 860 ºС. - NW shapes are straight or winding. - NWs do not connect each other in order to form enclosed areas - It is suggested that NW formation takes place to avoid the introduction of misfit dislocations.
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