Elsevier

Surface Science

Volume 524, Issues 1–3, 1 February 2003, Pages 148-156
Surface Science

Epitaxial titanium silicide islands and nanowires

https://doi.org/10.1016/S0039-6028(02)02506-2Get rights and content

Abstract

The growth of titanium silicide islands formed by reactive deposition of Ti on Si(1 1 1) at T∼850 °C has been studied using atomic force microscopy and transmission electron microscopy. The predominant shape is very long and narrow, and can be considered to be a nanowire (NW). Other flat-topped structures coexist with the NWs, including small equilateral triangles and large rectangular plates. Most NWs are oriented along Si〈220〉 directions, with typical dimensions 20 nm wide, 10 nm high and several microns long. A minority of NWs are oriented along Si〈224〉. These latter tend to break up into chains of small segments with regular size and spacing. Growth at lower temperature or higher deposition rate results in smaller and more numerous NWs. Length appears to be limited by intersection with other NWs oriented 120° apart. The junction between NWs appears to be incoherent in most cases. The triangular islands are positively identified as fully relaxed C54 TiSi2, while the chains are relaxed C49 TiSi2. The dominant NW structure is incommensurate and is tentatively identified as C49 TiSi2.

Introduction

Titanium silicide is widely used in the microelectronics industry, primarily because it forms metallurgically stable contacts to silicon and has low resistivity [1], [2]. As device dimensions shrink, it becomes important to make uniform, thin overlayers with sharp interfaces, particularly to implement shallow junctions. This is best achieved with an epitaxial overlayer, which offers the possibility of an atomically sharp interface over large regions. Epitaxial growth of TiSi2 films on Si has not been achieved, despite much effort. Epitaxy is not favored since the TiSi2 lattice symmetry and size are not closely matched with the Si substrate. This contrasts with the case for CoSi2, which has a CaF2-type lattice with 1.2% mismatch with the Si substrate. Hence, epitaxial CoSi2 layers are readily formed on both Si(1 1 1) and Si(0 0 1), using a variety of growth methods [3], [4].

Essentially no epitaxy has been reported for TiSi2 on Si(1 0 0), while limited epitaxy has been reported on Si(1 1 1). We discuss only the latter. Contact reaction (cold deposition of metal, followed by annealing) for UHV thin films (∼100 Å) results in C49 TiSi2 at 600 °C and C54 TiSi2 at 700 °C; both phases yield poor epitaxy with multiple orientations [1], [5], [6], [7]. Many researchers have studied the phase transformation from C49 to C54 TiSi2, and its dependence on film thickness, substrate orientation, metallic “impurities”, grain structure, etc. [8], [9], [10]. Jeon et al. [11] reported that the transformation temperature is significantly lowered in ultrathin films and suggested that the interfacial energy stabilizes C49 over C54. Conversely, a metastable C40 (Mo,Ti)Si2 ternary phase reportedly can serve as a template layer that favors C54, possibly suppressing C49 entirely in the contact reaction sequence [12].

Here we use reactive deposition to grow silicide overlayers. This process produces separated islands with no wetting layer. It is also expected to produce Si-rich phases and single-crystal epitaxial structures. We chose this growth method in order to emphasize or isolate the fundamental epitaxial behavior of the titanium silicide/silicon interface, without steric or kinetic constraints that are intrinsic to the contact reaction. We find that a variety of island shapes result, including a surprising and very interesting nanowire (NW) structure.

Spontaneous NW formation has been observed previously for rare-earth silicides deposited on Si(1 0 0) at 650 °C [13], [14], [15], [16]. This process is thought to result from an anisotropic lattice mismatch that favors long thin structures in the direction of small lattice mismatch. Self-assembled NWs offer the possibility of electrical interconnects on a scale that cannot be attained with conventional lithographic methods. They may also display novel electronic properties that could be exploited to make functional circuit elements, as recently demonstrated for carbon nanotubes or semiconducting NWs [17], [18], [19]. Our report here of NW formation in the Ti/Si(1 1 1) system provides an interesting variant to the rare-earth/Si(1 0 0) system. Growth on Si(1 1 1) presents a threefold symmetry which allows for intersecting NWs on a single terrace with no intervening steps, in contrast to the rare-earth NWs. Furthermore, a host of other materials properties may be explored with other silicides, such as step interactions, structural and chemical stability, and electronic properties. Elongated island shapes have been reported for Co/Si(1 0 0) [20], [21] and for annealed Ti/Si(1 1 1) [22], but these islands have unremarkable aspect ratios. Genuine NW formation has been reported for Gd/Si(1 1 1) [23] and for Pt/Si(1 0 0) [24].

In this paper, we report AFM observations of the size and shape of epitaxial islands observed during reactive epitaxy for the Ti/Si(1 1 1) system under various growth conditions. In related papers we report detailed structural determination using transmission electron diffraction and imaging [25] and in situ growth observations using LEEM [26].

Section snippets

Procedure

Si(1 1 1) samples (B-doped, ∼10 Ω cm) were cleaned by flashing multiple times at 1250 °C, using resistive heating. Titanium was deposited by sublimation from a 5 N-purity Ti wire. Coverage was determined in situ using a crystal thickness monitor and ex situ using AFM and Rutherford backscattering on select samples, with an accuracy of ∼20%. Coverage values are given in monolayer (ML) units, where 1ML=7.8×1014 Ti atoms/cm2. Temperature was determined using an optical pyrometer (emissivity=0.4),

Atomic force microscopy

Fig. 1 shows AFM images of the basic island types found on the surface following deposition of 1–2 ML Ti on Si(1 1 1) at 850 °C. The predominant structure type is the NW, with typical dimensions: 20 nm wide, 5–10 nm high and ∼1 μm long, as shown in panel a. Most NWs are oriented along the three equivalent Si〈220〉 directions with equal probability. From observations of hundreds of NWs, grown under various conditions on many different samples, we found the distribution of NW orientations to be: 80%

Discussion and conclusion

We have shown that reactive deposition of Ti on Si(1 1 1) at temperatures in the range 750–850 °C produces a variety of epitaxial single-crystal silicide island structures. It is interesting that both C54 and C49 TiSi2 islands are formed, with multiple shapes and orientations. Such structural variants might be expected (and are observed) for contact reactions in “thin films” (∼100 Å), due to kinetic and stearic constraints [29], [30], [31]. For example, Catana et al. [29] report 12 different

Acknowledgements

This work was supported by NSF grants DMR9981779 and DMR9632635 (ASU MRSEC). We acknowledge use of facilities in the Center of High Resolution Electron Microscopy.

References (36)

  • R.T. Tung

    Mater. Chem. Phys.

    (1992)
  • K. Ezoe et al.

    Appl. Surf. Sci.

    (1998)
  • K.L. Kavanagh et al.

    J. Cryst. Growth

    (1997)
  • K.N. Tu et al.
  • A.H. Reader et al.

    Rep. Prog. Phys.

    (1993)
  • H. von Kaenel

    Mater. Sci. Rep.

    (1992)
  • M.H. Wang et al.

    J. Appl. Phys.

    (1992)
  • L.J. Chen et al.

    Mater. Res. Soc. Symp. Proc.

    (1986)
  • K.H. Kim et al.

    J. Appl. Phys.

    (1992)
  • Z. Ma et al.

    Phys. Rev. B

    (1994)
  • R.W. Mann et al.

    Mater. Res. Soc. Symp.

    (1996)
  • H. Jeon et al.

    Mater. Res. Soc. Symp. Proc.

    (1993)
  • H. Jeon et al.

    Mater. Res. Soc. Symp. Proc.

    (1991)
  • A. Mouroux et al.

    Phys. Rev. B

    (1999)
  • C. Preinesberger et al.

    J. Phys. D: Appl. Phys.

    (1998)
  • C. Preinesberger et al.

    J. Appl. Phys.

    (2002)
  • Y. Chen et al.

    Appl. Phys. Lett.

    (2000)
  • J. Nogami et al.

    Phys. Rev. B

    (2001)
  • Cited by (50)

    • Temperature dependence on p-Cu <inf>2</inf> O thin film electrochemically deposited onto copper substrate

      2014, Applied Surface Science
      Citation Excerpt :

      This implies that films deposited at lower temperatures lead to obtain a (1 1 1) preferential orientated Cu2O film. In contrast, those Cu2O films electrodeposited in an environment with faster nucleation and growth rates are in favor of developing films into a relatively random manner [20]. In addition to the success of preparing Cu2O films under a relatively lower temperature, Fig. 1 also conveys a clear idea that it is possible to grow Cu2O films electrochemically at lower electronegative potentials, such as at −0.300 V vs. the SCE (equal to −0.058 V vs. the NHE).

    • Self-organized growth of higher manganese silicide nanowires on Si(1 1 1), (1 1 0) and (0 0 1) surfaces

      2011, Acta Materialia
      Citation Excerpt :

      In contrast, silicide NWs can be formed on the Si(1 1 0), (0 0 1) and (1 1 1) surfaces by using reactive epitaxy (deposition on a heated Si substrate) at a temperature in the range of ∼400–700 °C and an Mn deposition rate below ∼1.2 ML min−1. This could be due to the fact that the reactive deposition most probably involves diffusion of monomers on the surface, which favors the production of Si-rich phases with a single-crystal structure [15]. When the growth temperature is higher than ∼700 °C, no silicide islands are found on the Si surfaces due to the long diffusion distance of the Mn or Si adatoms on the surface or re-evaporation of the silicide islands.

    • Endotaxial silicide nanowires: A review

      2011, Thin Solid Films
      Citation Excerpt :

      No structural information was given, but it is reasonable to expect an endotaxial structure with habit planes on Si{111}, since the lattice mismatch in this configuration would be small (+ 1.8%). The Ti/Si(111) system forms multiple island types, as shown in Fig. 6 [34]. One island type (Fig. 6a, d) has the shape of long, thin NWs oriented along Si <220>, and is identified as C49-TiSi2; a second type (Fig. 6b) has the shape of compact rod structures oriented along Si <224>, and is identified as B27-TiSi [35].

    • Kinetic frustration of Ostwald ripening in Ge/Si(100) hut ensembles

      2009, Solid State Communications
      Citation Excerpt :

      Our kinetic stability model is general, and should apply to any system with a constant density of replenishable nucleation centers producing facets that do not grow much in size. This is clearly satisfied for Ge/Si(100) hut clusters and should be satisfied for, e.g., metal silicide nanowires [32]. Whether or not similar arguments could apply to more complex, multifaceted structures such as domes or the Ge pyramid analogs found in InAs/GaAs(100) that are bound by {137} facets [33] is not clear and invites further experimental and theoretical investigation.

    • Growth of Ag nanowires on Au-pre-facetted 4° vicinal Si(0 0 1)

      2008, Surface Science
      Citation Excerpt :

      The results of these studies generally fall into two categories. In the first category are nanowires that form a silicide during reaction deposition epitaxy, including certain rare earth metals [2–8] and transition metals [9–12]. In the second category are nanowires that grow while maintaining an abrupt interface with the substrate and without intermixing or compound formation.

    View all citing articles on Scopus
    View full text