Structural phase transitions in Au thin films on Si (1 1 0): An in situ temperature dependent transmission electron microscopy study

Applied Surface Science 256 (2009) 567–571

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Structural phase transitions in Au thin films on Si (1 1 0): An in situ temperature dependent transmission electron microscopy study Umananda M. Bhatta, J.K. Dash, A. Rath, P.V. Satyam * Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India

A R T I C L E I N F O

A B S T R A C T

Article history: Available online 19 August 2009

We present a review on the formation of gold silicide nanostructures using in situ temperature dependent transmission electron microscopy (TEM) measurements. Thin Au films of two thicknesses (2.0 nm and 5.0 nm) were deposited on Si (1 1 0) substrate under ultra-high vacuum (UHV) conditions in a molecular beam epitaxy (MBE) system. Also a 2.0 nm thick Au film was deposited under high vacuum condition (with the native oxide at the interface of Au and Si) using thermal evaporation. In situ TEM measurements (for planar samples) were made at various temperatures (from room temperature, RT to 950 8C). We show that, in the presence of native oxide (UHV-MBE) at the interface, high aspect ratio (15.0) aligned gold silicide nanorods were observed. For the films that were grown with UHV conditions, a small aspect ratio (1.38) nanogold silicide was observed. For 5.0 nm thick gold thin film, thicker and lesser aspect ratio silicides were observed. Selected area diffraction pattern taken at RT after the sample for the case of 5.0 nm Au on Si (1 1 0)-MBE was annealed at 475 8C show the signature of gold silicide formation. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Nanosilicides In situ electron microscopy Au on Si (1 1 0)

1. Introduction Au–Si system has been widely used in many technological disciplines, mainly the integrated circuit fabrication. As the present trend of miniaturization of the microelectronics is continuing, understanding inter-diffusion on the nanometer scale at the interface in all possible systems and conditions has become very important. The mechanism of these interactions, the range of temperatures required and the resulting composition of Au–Si alloy depends on whether these gold nanoislands have the luxury of interacting directly with the substrate or they will have to deal with some kind of obstacle, like a native oxide layer, before they could get access to the pure silicon. Either of the processes may even give rise to various kinds of self-assembled nano-/microstructures depending on the symmetry of the substrate. Systematic study of such processes has tremendous implication on modern day semiconductor technology. In the same context, several groups studied the processes going on at the metal/Si interfaces at low temperatures [1–8]. The inter-diffusion, reaction at surface and interfaces in Au–Si system was studied under various conditions of surface treatment. Previous results for gold films deposition on Br passivated Si

* Corresponding author at: Institute of Physics, Experimental Physics, Sachivalaya Marg, Bhubaneswar 751005, India. Tel.: +91 674 2301058; fax: +91 674 2300142. E-mail address: [email protected] (P.V. Satyam). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.08.054

substrates showed that the native oxide acts like a diffusion barrier [9,10]. Vacuum annealing of thick and continuous gold films deposited on passivated silicon surface showed the formation gold silicides (Au4Si) around 363 8C. For Au thick films on Br-passivated Si (1 1 1), a shape transition from triangle to trapezoid was observed at the eutectic annealing point (viz. 363 8C) [9]. But for the similar conditions on Si (1 1 0) substrate, a very high aspect ratio of gold silicides were observed [10]. Shape transition has long been regarded as a major mechanism of strain relief along with the formation of dislocations. Tersoff and Tromp have given an analytical theory and showed that strained epitaxial islands, as they grow in size, may undergo a shape transition [11]. Recently, we showed the formation of well-aligned low aspect ratio (1.38) gold silicide nanostructures that were grown in UHV conditions at 600–700 8C in the absence of native oxide at the interface [12] following a similar lattice matching theory along the easy axis of the Si (1 1 0) substrate. Few decades back, Wagner and Ellis showed the growth of vertically aligned crystalline silicon nanowires (SiNW) using vapor–liquid–solid (VLS) mechanism in which gold silicide alloys have been used as catalysts for the growth [13]. SiNW growth process involves many parameters, such as, a proper orientated substrate, surface condition, deposition of metal catalyst, formation of eutectic droplet and semiconductor deposition under conditions of chemical vapor deposition (CVD) or molecular beam epitaxy (MBE), partial pressure of oxygen, growth temperature, etc. Systematic studies on the above parameters’ effect on nucleation and growth kinetics would enhance our understanding

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on VLS growth mechanism [14–18]. Systematic in situ transmission electron microscopy measurements by Kodambaka et al. showed that the presence of oxygen is important for the growth of long untapered Si nanowires [17]. According to these reports, presence of oxygen reduces the diffusion of gold away from the catalyst droplets, allowing the droplet volume to remain constant to permit growth of untapered wires. Similarly presence of a native oxide layer also contributes to the formation of in plane aligned nanostructures on a properly oriented substrate. Fujita et al. studied the void formation during thermal decomposition of ultrathin oxide layer on a Si (1 1 0) surface using scanning tunneling microscopy followed by selective epitaxial growth of Si crystals in those voids [19]. Recently we reported formation of long aspect ratio gold silicide nanorods in presence of native oxide at the interface of gold nanostructures and silicon substrate [20]. SAD shows that all the microrods aligned along [1 1¯ 0] direction. As the Au–Si nanoalloy plays a vital catalyst role in nanowire growth, or magnetic particle growth, a proper understanding during the initial nucleation and growth of these would enable us to engineer the structures, which in turn helps for many applications in nanotechnology [21]. In this paper, we review some of these results on the growth of low and high aspect ratio gold silicide nanorods for 2.0 nm Au film grown in UHV (without native oxide at the interface) and non-UHV (with native oxide interface) during the in situ heating treatment in the TEM measurements. We also report the growth of gold silicide in case of thicker gold layer of 5.0 nm thickness on Si (1 1 0).

thickness of 2.0 nm from cross-sectional TEM measurements. For UHV system the native oxide layer has been removed in a two step process: first degassing has been carried out at 600 8C for 12 h followed by flashing for 30 s at 1250 8C. RHEED measured showed 16  2 reconstruction. We have also grown 5.0 nm thick gold epitaxially by evaporating Au from a Knudsen cell on a similar ultra-clean surface. Deposition rate was kept constant at 0.14 nm min 1. During the growth, chamber vacuum was 5.0  10 10 mbar. The thickness monitor was calibrated with Rutherford backscattering spectrometry (RBS) measurements. After the deposition, both systems were processed for planar TEM sample preparation. A disk of 3 mm diameter was cut using ultrasonic disc-cutter followed by lapping until it reached 100 mm thick. Using a dimple grinder, sample was further thinned down to 25 mm thick at the center of the sample. Finally, electron transparency was achieved through low energy Ar ion milling. TEM measurements were done with 200 keV electrons (2010, JEOL HRTEM). The samples were loaded in a (GATAN 628UHR) single tilt heating holder. The heating stage has a tantalum furnace that can be used to heat the specimen up to 1000 8C. The temperature is measured by a Pt/Pt–Rh thermocouple and is accurate within a couple of degrees. The holder has a water cooling system to avoid over heating of the sample surroundings and the specimen chamber, while keeping only the sample at a specified temperature. Real time measurements were carried out using a CCD camera (GATAN 832) in which real time movies can also be recorded at a grab rate of 25 frames per second.

2. Experimental

3. Results and discussion

About 2.0 nm thick gold films were deposited in two conditions: one under high vacuum (4  10 6 mbar) conditions on ntype Si (1 1 0) substrates and in another case under ultra-high vacuum (UHV: 2.8  10 10 mbar) conditions. For growing gold thin films on an atomically clean Si (1 1 0) surfaces a custom-made, compact MBE system has been used. More about the MBE system has been explained elsewhere [22]. For the high vacuum deposition, native oxide was kept intact and found to have a

Fig. 1 schematically depicts effect of temperature on the Au/Si (1 1 0) systems with and without a native oxide layer. In both cases aligned nanostructures were found following the symmetry of the substrate. In the first case, without an oxide layer, low aspect ratio nanorods were formed at around 700 8C [12]. Where as the presence of an oxide layer present the process was delayed and aligned nanostructures of much higher aspect ratios (as high as 15) were formed at much higher temperatures (around 850 8C) [20].

Fig. 1. Growth of varying aspect ratio gold silicides from gold nanostructures deposited on Si (1 1 0) surfaces (with and without oxide).

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Fig. 2. As-deposited MBE sample showing (a) 2.0 nm connected gold nanostructures, (b) corresponding SAD pattern showing Si and Au reflections, (c) corresponding high resolution image of one of the islands, (d) 5.0 nm connected gold nanostructures, (e) corresponding SAD pattern showing Si and Au reflections and (f) corresponding high resolution image of one of the islands.

Fig. 3. Bright field transmission electron micrographs depicting real time morphological changes during the in situ heating. (a), (b), (c), (d), (e) and (f) show the morphology at 325 8C, 350 8C, 363 8C, 400 8C, 600 8C and 700 8C, respectively. With increasing temperature growth of rod-like nanostructures can be seen aligned in one particular direction [12].

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Fig. 4. Transmission electron micrographs depicting sequential changes in the morphology with increasing temperature (from 320 8C to 850 8C).

Fig. 2(a) and (d) depicts the bright field TEM for 2.0 nm and 5.0 nm thick Au films deposited on Si (1 1 0) substrate, respectively. Connected gold nanostructures were seen in both the cases. 2.0 nm film gives a 48% gold coverage while for 5.0 nm it shoots up to about 70%. The selected area diffraction (SAD) in both the cases shows the presence of polycrystalline gold and the single crystalline silicon background (Fig. 2(b) and (e)). Fig. 2(c) and (f) shows the corresponding HRTEM images from some of the typical gold islands. The in situ heating experiments on the 2.0 nm epitaxially grown gold thin film on similar substrate (Si (1 1 0)) were carried out at various temperatures [12]. Planar TEM sample was heated up to 700 8C in several steps, at a rate of 7 8C min 1. Even at 200 8C morphology was similar to as-deposited system. Fig. 3 depicts bright field images at various temperatures (325 8C, 350 8C, 363 8C, 400 8C, 600 8C and 700 8C). Isolated nanostructures transformed their shape with increasing temperature to form well-aligned nanorod-like structures at 700 8C (Fig. 3(f)). Starting from 325 8C

(Fig. 3(a)) the aspect ratio kept on increasing from 1.04 to a maximum of 1.38 at 700 8C. Real time selected area diffractions (keeping the samples at these temperatures) were taken at various temperatures to observe crystalline structural changes, if any. But, at higher temperatures we did not observe any reflections other than silicon. In a similar system after heating up to 600 8C and cooling down to RT, extra reflections other than silicon and gold were observed. Detailed analysis showed the presence of Au5Si2 phase of gold silicide. Formation of this particular phase has been explained on the basis of lattice matching along [1 1 0] direction [12]. Similarly, the second system with a 5.0 nm Au film was also subjected to similar heat treatment to see the formation of aligned nanostructures as well as silicide formation, if any. No significant changes in morphology (through TEM imaging) or in crystallinity (through SAD) until 200 8C temperature were observed. Fig. 4 depicts the bright field images at RT after annealing the system at various temperatures (ranging from 200 8C to 850 8C). After

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degrees of ordering of super lattices of one fundamental structure. Because, the fundamental lines are similar for all reported phases except Au5Si2. If we observe Fig. 5 carefully, Au (1 1 1) and Au (2 0 0) reflections are accompanied by a diffused amorphous ring which give a slightly higher spacing than corresponding spot. Although difference in the spacing of spot and the corresponding amorphous ring comes within the error bar (0.005 nm), looking at the pattern presence of amorphous gold silicide also cannot be ruled out. As in our case, lack of symmetry further adds to the difficulty of assigning one definite phase to the obtained diffraction pattern. 4. Conclusions We have reported in situ temperature dependent TEM studies on nano-Au on Si (1 1 0) substrate. In this review, we have reported the formation of two kinds of nanosilicide rods (aspects ratios 1.38 and 15.0) based on the interface native oxide. While for thick Au film case also silicide were formed but the alignment was not as good as lower thickness. Fig. 5. Selected area diffraction taken at room temperature after the sample was heated up to 475 8C and cooled down to RT showing mixed phase of gold silicide.

annealing up to 320 8C connected nanostructures were more or less isolated (Fig. 4(a)). With increasing temperature, isolation of these irregular nanostructures continued. Even though we see some kind of alignment at some places in the region of interest, this is still very early stage to see any kind of large scale alignment. Increase in temperature did not yield any large scale alignment unlike in 2.0 nm case so that appreciable change in aspect ratios could be measured (Fig. 4(b)–(e)). But still the nanostructures were stable at 800 8C unlike in 2.0 nm case where in we saw these nanostructures start desorbing. Here above 800 8C these irregular nanostructures indeed melt and coalesce to form a bigger microrod at 850 8C (Fig. 4(f)) which is aligned in the same direction as it was the case with smaller nanostructures at lower temperatures. The size of microrod shown in Fig. 4(f) is 2.16 mm  0.5 mm. Selected area diffractions were also taken at RT after annealing the system at these different intervals of temperatures. Apart from single crystalline silicon background and un-reacted polycrystalline gold reflections, SAD shows multiple scattering as due to some kind of superstructure formation. Since these reflections come from several number of nanostructures (even after using minimum aperture) detecting and indexing all these spots looks impractical. Fig. 5 gives a representative SAD pattern at RT after cooling down the sample from 475 8C. Apart from Si and Au reflections we found a few extra spots which were found to be matching with at least four reported phases of gold silicide. Spot nos. 1–4 corresponds to a d-spacing of about 0.27 nm, 0.18 nm, 0.31 nm and 0.23 nm, respectively. While comparing them with the reported phases these spacings match with several phases like Au2Si, Au3Si, Au7Si and Au4Si. Many diffraction patterns of gold silicides have been presented over the years [23–28]. Hultman et al. rightly pointed out that when comparing the observed data with reported phases of gold silicides, surprisingly there is a good agreement between the d values for at least 8–12 strongest reflections of all the alloys [29]. Looking at the constancy of lattice spacings of strongest lines, they doubt whether these diffraction patterns arise from different phases or they represent various

Acknowledgements The financial support from the Department of Atomic Energy, Government of India, (project no.: 11-R&D-IOP-5.09-0100) is acknowledged. PVS would like to thank Prof. C.P. Liu for the stimulus discussions and financial support for the visit to NCKU. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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