Epitaxial titanium silicide islands and nanowires
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 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 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 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.
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