RESEARCH PAPER

Nanostructured TiOx film on Si substrate: roomtemperature formation of TiSix nanoclusters

Mirco Chiodi • Emanuele Cavaliere •

Iskandar Kholmanov • Monica de Simone •

Oumar Sakho • Cinzia Cepek • Luca Gavioli

Received: 29 July 2009 / Accepted: 27 December 2009

� Springer Science+Business Media B.V. 2010

Abstract We present a morphologic and spectro-

scopic study of cluster-assembled TiOx films depos-

ited by supersonic cluster beam source on clean

silicon substrates. Data show the formation of

nanometer—thick and uniform titanium silicides film

at room temperature (RT). Formation of such thick

TiSix film goes beyond the classical interfacial limit

set by the Ti/Si diffusion barrier. The enhancement of

Si diffusion through the TiOx film is explained as a

direct consequence of the porous film structure. Upon

ultra high vacuum annealing beyond 600 �C, TiSi2 is

formed and the oxygen present in the film is

completely desorbed. The morphology of the nano-

structured silicides is very stable for thermal

treatments in the RT—1000 �C range, with a slight

cluster size increase, resulting in a film roughness an

order of magnitude smaller than other TiOx/Si and

Ti/Si films in the same temperature range. The

present results might have a broad impact in the

development of new and simple TiSi synthesis

methods that favour their integration into

nanodevices.

Keywords TiSix � Cluster � Cluster-assembled

TiOx � Silicide � Nanostructure � Thin film �Synthesis

Introduction

Nanostructured systems are at the edge of material

physics research (Moriarty 2001; Hiramoto et al.

2006), with physical and chemical properties that

make them extremely promising for industrial appli-

cations such as protective coatings (Wang et al. 2008;

Chen 2005), microelectronics (Ho et al. 2008),

biomedicine (Ebron et al. 2006), catalysis (Fujishima

and Honda 1972; Linsebigler et al. 1995). In this

context, nanoscaled titanium silicides are currently

investigated as materials for next generation’s

devices, such as sensors, solar cells, logical circuit

devices, field emitters (Zhou et al. 2009; Lin et al.

2008; Zou et al. 2008; Chang et al. 2009), thanks to

the low resistivity, thermal stability and good adhe-

sion to silicon substrates (Zhang and Ostling 2003).

M. Chiodi (&) � E. Cavaliere � I. Kholmanov �L. Gavioli

Dipartimento di Matematica e Fisica, Universita Cattolica

del Sacro Cuore di Brescia, Via Musei 41, Brescia 25121,

Italy

e-mail: chiodi@dmf.unicatt.it

M. Chiodi � M. de Simone � O. Sakho �C. Cepek � L. Gavioli

CNR-INFM, Laboratorio Nazionale TASC, SS-14,

Km 163.5, Trieste 34012, Italy

I. Kholmanov

CNR-INFM SENSOR Lab, Via Valotti, 9, BRESCIA

25133, Italy

O. Sakho

Departement de Physique, Universite Cheikh Anta, DIOP,

Dakar, BP 5005, Senegal

123

J Nanopart Res

DOI 10.1007/s11051-009-9843-3

To understand and control the formation of sili-

cides, the study of reactivity processes occurring at

metal–silicon interfaces has crucial importance (Chen

2005), in particular if the metal film is deposited with

pre-assembled nanosized building blocks. In fact, the

variation of the physical and chemical properties

related to the nanostructured nature of the film might

significantly modify its reactivity (Moriarty 2001;

Hiramoto et al. 2006; Rao et al. 2002; Sun 2007).

Moreover, the surface to volume ratio (SVR) increases

dramatically as the size of the building blocks

diminishes, providing a higher number of active

surface sites, and facilitating the possible interactions

occurring at the interface (Rao et al. 2002; Sun 2007).

So far, the interface reactivity has been investi-

gated either in uniform two-dimensional TiSi films

(Zhang and Ostling 2003; Jeon et al. 1992; Ilango

et al. 2005a; Wang and Chen 1991; Butz et al. 1984),

or scarcely in one-dimensional nanowires, obtained

by various growth methods (Zhou et al. 2009; Xiang

et al. 2005; Stevens et al. 2003). Ultra high vacuum

(UHV) deposition of a titanium layer on clean (111) or

(001) silicon surfaces by sublimation or sputtering

actually leads to titanium silicide formation at room

temperature (RT) (Jeon et al. 1992; Wang and Chen

1991; Butz et al. 1984). However, the Ti/Si interaction

leads to a mixture of Ti5Si3 and TiSi layer at the

interface, acting as a barrier against Si diffusion and

therefore limiting the titanium silicide thickness to the

first four atomic layers at RT (Chambers et al. 1987;

Arranz and Palacio 2005). The barrier presence

requires a film annealing at high temperatures (typ-

ically [450 �C and [700 �C for TiSi2) to sustain an

extensive inter-diffusion of Si atoms in the Ti

overlayer (Wang and Chen 1991; Butz et al. 1984),

but the annealing results in a large increase of the film

roughness from few nanometer up to hundreds of

nanometer (Ilango et al. 2005a; Jeon et al. 1997;

Ilango et al. 2005b). These phenomena are strictly

connected with the structure and the growth morphol-

ogy of the deposited Ti film. As Jeon et al. (1997)

pointed out, even small morphological differences at

the Ti/Si interface change the formation temperature

of the silicide by more than 100 �C. Since the

diffusion barrier formation takes places at the Ti/Si

interface, a controlled modification of the deposited

layer morphology may enhance the Si inter-diffusion.

However, traditional Ti deposition techniques, like

magnetron sputtering and/or sublimation (Jeon et al.

1992; Wang and Chen 1991; Butz et al. 1984), do not

allow the control of the film morphology. This can be

accomplished with the deposition of Ti films in form

of clusters with nanometric dimensions, which max-

imize the SVR and are therefore good candidates for

enhanced Si diffusion and for determining the chem-

ical and physical properties of the film. However, no

study on such titanium silicides nanoclusters has been

reported so far.

In this study, we show that depositing cluster-

assembled TiOx films by supersonic cluster beam

deposition on clean silicon substrates under UHV

conditions leads to formation of nanmometer—thick

and uniform titanium silicides film at RT. Moreover,

we discuss the chemical composition and stability of

the system. A thorough spectroscopic study of the

system allows a detailed description of the chemical

changes occurring in the film at RT and as a function

of the annealing temperature. Several evidences of

the RT formation of TiSix extending much beyond

the classical interfacial limit are found. The latter

result is a direct consequence of the porous nano-

cluster morphology of the TiOx film, which allows

overcoming the Si diffusion barrier. The morphology

of such nanostructured silicides is very stable upon

temperature variations up to 1000 �C, presenting only

a slight variation of the film RMS roughness.

Experimental methods

Nanostructured titanium oxide films (&10 nm thick-

ness, determined as described below) were deposited

by supersonic cluster beam deposition using a pulsed

microplasma cluster source (Mazza et al. 2005). The

source produces a beam of nanoclusters (diameter in

the range of 2–10 nm) that are deposited on the

substrate directly into the UHV chamber, allowing

the growth of a reactive and highly porous film, with

a high SVR, as explained in details by Barborini et al.

(2003). TiOx films were deposited in UHV conditions

at RT on (001)- and (111)-oriented clean silicon

surfaces. Before depositions, the Si substrates were

cleaned by several thermal treatments up to 1000 �C

in UHV until no impurities were detected by Auger

electron spectroscopy (AES) and X-ray photoemis-

sion spectroscopy (XPS), and a sharp low energy

electron diffraction (LEED) pattern appeared

[(2 9 1) and (7 9 7) LEED pattern, respectively].

J Nanopart Res

123

The films were characterized in situ, just after

growth and/or thermal treatments, via AES, XPS and

ultra-violet photoemission spectroscopy (UPS). The

AES spectra were acquired in integral mode using an

Omicron Multiscan system, the XPS spectra using a

non-monochromatized Mg X-ray source (hm =

1253.6 eV), while the UPS bands were obtained

using a conventional He discharge lamp (hm =

21.2 eV). All spectra were acquired at RT in normal

emission geometry. The films were annealed at

temperatures up to 1000 �C for a period of 10 min.

The nominal film thickness and deposition rate were

measured in situ by a quartz microbalance. The film

morphology was also characterized ex situ using an

Atomic Force Microscope (AFM) (Solver-pro NT-

MDT), acquiring the data under nitrogen flow in

semicontact mode with nominal tip radius below

10 nm. The AFM was used to measure the actual film

thickness at the border of deposited area of the

sample surface, allowing a comparison of the

extrapolated thickness obtained with the microbal-

ance with the observed film thickness.

Results and discussion

In Fig. 1a, a representative AFM image of a 10-nm

thick film deposited at RT on either (001) or (111)

surface is presented. The film morphology is uniform

over wide areas (typically 300–400 lm) and porous, as

usual for cluster-assembled films (Mazza et al. 2005).

The clusters are well resolved and exhibit an almost

circular shape, with an average lateral size of

(10.7 ± 2.1) nm, which should be considered as an

upper limit since it corresponds to the curvature radius

of our AFM tip. The largest cluster size observed is

around 15 nm. Our measured nanoparticle size is

remarkably smaller than what is reported in literature

for other titanium and titanium oxide films obtained

with different deposition techniques. In particular, the

grain size of stoichiometric TiO2 films prepared by

either pulsed laser ablation or magnetron sputtering is

several tens of nanometers (Inoue et al. 2002; Hou et al.

2003) while it ranges from 10 to 100 nm for electron-

beam deposited Ti films on Si (Kuri et al. 2002). Note

also that the RMS roughness of our 10-nm thick film is

2.9 ± 0.1 nm, i.e. very similar to the roughness of

much thinner (*1 nm) films obtained with traditional

deposition techniques (Hou et al. 2003; Kuri et al.

2002). These data suggest that the deposition of titania

clusters by supersonic beam is much more effective in

obtaining a film with lower grain size when compared

to standard evaporation techniques. This might be

explained by a lower diffusion coefficient of the

nanoclusters on clean silicon surfaces with respect to

sublimated titanium adatoms.

Fig. 1 a AFM image of the titania film deposited on Si(001)

substrate at room temperature (area: 191 lm2) and b the

corresponding Ti L3M2,3V Auger line (circles). The solid linerepresents a numerical fit based upon the convolution of three

distinct peaks attributed to titanium atoms interacting with

oxygen (black peak), silicon (grey peak) and titanium (whitepeak)

J Nanopart Res

123

The chemical composition of the nanostructured

films deposited at RT have been analysed in situ

through AES, and the lineshape of the Ti L3M2,3V

transition centred around 415 eV is presented in the

lower panel of Fig. 1. The data have been background

subtracted following the procedure of Sickafus

(Sickafus and Kukla 1979), and then fitted with a

least square fitting procedure using three pseudo-

Voigt functions. The full width at half maximum

(FWHM) of each component has been kept fixed at

4.4 eV with a Lorentzian contribution of 2.1 eV.

Three distinct components can be safely identified at

416.2, 413.0 and 409.6 eV kinetic energy (KE),

respectively (see also the temperature dependence

behaviour presented below). The high KE peak is due

to Ti primarily involved in Ti–Ti metal-like bonds,

and the low KE component at 409.6 eV is due to Ti

atoms bonded to oxygen (Contarini et al. 2002). We

attribute the peak at 413 eV to the presence of Ti

atoms forming titanium silicide, as confirmed also by

the thermal evolution discussed below. The TiSi

presence is also confirmed by the XPS data taken at

RT on the same system (see Fig. 3), in which the

binding energy (BE) of the Si 2p peak is consistent

with the formation of TiSi, as discussed by Larciprete

et al. (2001) for Ti/Si(001). Neither AES (not shown)

nor XPS data indicate any presence of Si–O interac-

tion at RT. In fact the Si 2p spectrum (Fig. 3a) shows

no trace of the oxidized component at &104 eV BE

(Seo 2006), and the O 1s core level spectrum

(Fig. 3b) shows the component at 531.4 eV BE,

related to Ti–O interactions (McCurdy et al. 2004).

The percentage of the titanium silicide present in the

film can be estimated from the relative integral

intensities of the three components of the Ti L3M2,3V

Auger spectrum in Fig. 1b. Our results indicate that

25% is due to TiSi, 25% to TiOx and 50% originating

from metal-like Ti (see Fig. 1b).

Along with the Ti L3M2,3V Auger spectrum, the Si

L2,3VV at 92 eV KE and the O KLL at 505 eV KE

spectra have been collected (not shown). The com-

parison between the peak areas, corrected for ele-

mental sensitivity factors (Mroczkowski and

Lichtman 1985), allows us to tentatively identify the

as-deposited film composition. The RT film stoichi-

ometry can be estimated by considering the electron

inelastic mean free path (k) of a particular Auger peak

in a uniform film (Shimizu 1983). However, the

nanostructured nature and the high porosity of the

present film makes this point not trivial, because k is

unknown for such kind of film structure. The

estimated dimension of a single titania cluster is at

least 3 nm (Barborini et al. 2003), and the AFM data

show that in this 10-nm thick film more than 90% of

the substrate is covered by clusters at least 3 nm in

height. Therefore, we estimate the expected substrate

contribution to the Si Auger intensity not to be greater

than 10%. In pure, non-nanostructured titanium,

electrons with the kinetic energy of Si L2,3VV (Ek:

90 eV) have a k lower than 0.5 nm, and even shorter

in pure TiO2 (Lesiak et al. 2006; Fuentes et al. 2002).

By weighting the Si Auger line intensity with such

considerations, we obtain an upper limit for the Si

contribution to the film stoichiometry, which results to

be composed of silicon (47%) and oxygen (40%),

while the amount of titanium is around 13%. We have

no evidence of oxygen bonded to silicon; hence, the

discrepancy between the oxygen amount bonded to Ti

and the total oxygen observed in the film is likely due

to the extremely high reactivity of such nanoclusters.

This suggests that even under UHV conditions they

are acting as efficient getter of residual water. We also

note that some error in the total stoichiometry can be

induced by the sensitivity factors applied, that for

Auger integral intensities can change by a factor of 5

for oxygen (Mroczkowski and Lichtman 1985; Seah

and Gilmore 1998).

The percentage of silicon within the deposited film

results to be remarkably high, indicating a huge

diffusion of the Si atoms from the substrate through

the nanostructured film at RT. The enhancement of

the Si diffusion is most likely due to the porosity and

to the nanocluster structure of the film. In fact, this

avoids the formation of the silicide barrier usually

observed for uniform titanium films deposited on

silicon substrates (Chambers et al. 1987). Such a

large interfacial intermixing at RT has never been

reported before, neither for clean Ti/Si interface nor

in presence of interfacial oxygen (Chambers et al.

1987; Ilango et al. 2005a; Del Giudice et al. 1987;

Wan and Wu 1997). In both the cases the maximum

extent of the RT interdiffusion was limited to a few

angstroms.

A comprehensive spectroscopic analysis (XPS,

AES, UPS) has been carried out to investigate the

film stability and stoichiometry as a function of the

annealing temperature. Figure 2a shows the behav-

iour of Auger Ti L3M2,3V lineshape at selected

J Nanopart Res

123

annealing temperatures, together with the least square

fitting results. Figure 2b presents the corresponding

intensities of the three components employed in the

fit. The Si and O Auger lines are not plotted since

most of the information on the nanostructured film

evolution can be extracted from the analysis of the Ti

excitation line.

The attribution of the component at 413 eV KE to

the oxygen-bonded Ti (Ti–O) and the peak at

409.6 eV at the silicon-bonded Ti (Ti–Si) becomes

evident by observing that the Ti–O peak disappears as

the oxygen is desorbed from the system (above

700 �C, spectra not shown), while the Ti–Si peak

persists up to the highest annealing temperature

(1000 �C), and it is not observed on metallic Ti films.

Moreover, a 0.5 eV shift towards lower KE appears

between 600 and 700 eV for the Ti–Si-related com-

ponent. The peak intensity behaviour shown in

Fig. 2b confirms that above 600 �C the film undergoes

some changes in the chemical composition. Both the

metallic Ti and the Ti–Si-related components increase

in the relative intensity with temperature, suggesting

that the Ti bonded to the oxygen partly becomes

metallic Ti and partly bonds to silicon.

The energy shift of the Ti–Si component is

observed also for the XPS Si 2p core level peak,

centred at 99.3 eV BE (Fig. 3 a), and the O 1s spectra

(Fig. 3b) remain essentially unchanged in the RT–

500 �C annealing range. At 600 �C, a component

appears around 103.3 eV in the Si 2p, indicating the

formation of SiOx (Seo et al. 2006). This is confirmed

by the appearance of a new component in the O 1s

core level spectrum, centred at 532.9 eV (McCurdy

et al. 2004). The oxygen evolution from Ti–O

towards Si–O bonds has been reported and discussed

in details before (Nemanich et al. 1985), and a full

understanding of this process can be gained by

considering the ternary phase diagram. In fact,

calculations based on thermodynamic properties of

the three phases (Si–Ti–O) indicate that the only

stable oxide at 600 �C is SiO2 (Beyers 1984).

The film chemical composition changes as the

temperature exceeds 700 �C: the oxygen is com-

pletely removed from the system (Fig. 2b), due to the

sublimation of the SiO2 previously formed. At the

same time, the Si 2p peak shifts towards higher BE at

99.6 eV. This shift indicates the conversion of TiSi

into TiSi2 (Larciprete et al. 2001).

The thermal evolution of the Valence Band (VB)

photoemission data presented in Fig. 4 is consistent

with the contemporary presence of TiOx, metallic Ti,

and TiSi at RT. The presence of the broad feature

around 5.8 eV is due to the O 2p levels of TiOx (Le

Fevre et al. 2004), while the filled states in the

Fig. 2 a Ti L3M2,3V Auger line as a function of annealing

temperature. The solid line represents a numerical fit based

upon the convolution of three distinct peaks attributed to

titanium atoms interacting with oxygen (black peak), silicon

(grey peak) and titanium (white peak). b Temperature

evolution of the Auger intensity of the different components

as found from data fitting

J Nanopart Res

123

0–2 eV BE range with a quite sharp cut-off at the

Fermi edge are related to both the semi-metallic

character of the titanium silicides and to the non-

stoichiometric TiOx. When the temperature reaches

600 �C, a broad peak appears around 7.3 eV, origi-

nating from Si–O interaction (Kim et al. 2003).

Finally, at 800 and 1000 �C, all the oxygen-related

features disappear and the VB spectra present a quite

sharp component centred at 1 eV, due to the hybrid-

ization of Ti 3d and Si 3p states in TiSi2 (Butz et al.

1984; Arranz and Palacio 2005).

The AFM image of the film after the annealing

treatment in UHV at 1000 �C (Fig. 5) reveals that the

surface morphology suffered only marginal varia-

tions. The RMS roughness of the film (*4.5 nm)

remains in the same order of magnitude as before

annealing, while there is an increase of the average

clusters lateral size up to 22.6 ± 3.2 nm, indicating

the coalescence of the smaller clusters. This process

could also involve a partial dewetting of the film that

could explain the appearance of small regions in

which the silicon substrate is left uncovered (see

black areas in Fig. 5), and the increase of the Si 2p/Ti

3d XPS intensity ratio at this temperature (not

shown).

A comparison with previous works on Ti/Si

interfaces suggests that the morphology of our

nanostructured film is by far the most stable upon

annealing treatments. In fact, oxygen-free Ti/Si

interfaces heated in UHV beyond 500 �C show rough

surfaces, with grains lateral size of hundreds of

nanometres (Ohmi and Tung 1999). In TiSi2 films,

obtained by rapid thermal annealing in UHV beyond

800 �C, the average grain diameter is *110 nm, five

times larger than our result, while for TiSi2 films

synthesized with pulsed-laser irradiation the value

is *85 nm (Chen et al. 1999). Moreover, in partially

oxidized TiOx/Si interfaces (grown by means of RF

sputter deposition in vacuum conditions) Ilango et al.

(2005b) found that the RMS roughness suffers a steep

increase after a vacuum (P \ 10-7 Pa) annealing

treatment beyond 500�, reaching a value of *50 nm

Fig. 3 XPS Si 2p (a) and

O 1s (b) core level

photoemission spectra for a

TiOx film deposited on a

Si(111) substrate as a

function of the annealing

temperature. The spectra

are collected in normal

emission geometry are

normalized to the photon

flux and the incident photon

energy is 1253.6 eV

J Nanopart Res

123

at 800 �C. Moreover, the estimated average grain size

of similar films is *42 nm (Ilango et al. 2005a).

Such a film roughening upon annealing is observed

by Hou et al. (2003) in stoichiometric TiO2 films

grown in low-vacuum conditions as well; they

measure a RMS roughness varying between

18.9 and 43.5 nm at 900 and 1100 �C respectively,

with an average grain size ranging from 300 nm up to

1–3 lm. Hence, the previously investigated Ti–Si

systems exhibit a strong tendency towards roughen-

ing upon temperature increase, irrespectively of the

different deposition conditions. On the contrary, our

nanostructured film is by far the less affected by such

surface roughening, and this is likely to be related to

the cluster-assembled structure, which prevents an

extensive coalescence of the smaller clusters.

Conclusions

In conclusion, this work shows that titanium silicide

formation extending over several nanometer from the

Ti/Si interface is possible even at RT, by depositing

in UHV conditions a cluster-assembled TiOx film on

clean silicon surfaces. The porous structure of the

nanocluster film provides an effective Si mixing into

the TiOx film, strongly enhancing the silicide nucle-

ation also at RT. The nanostructured film morphology

exhibits a high stability against roughening upon

temperature variations up to 1000 �C.

The use of a nanostructured Ti film might open

new possibilities for the TiSi synthesis and integra-

tion on devices; although, further analysis is needed

to fully characterize the electronic properties and the

crystalline structure of such nanocluster films. More-

over, thanks to the morphological stability of the TiSi

nanoclusters, these structures could be good candi-

dates as nanotemplates for high temperature fabrica-

tion of nanowires and nanocables with controlled

dimensions.

Acknowledegments This work was supported by CARIPLO

foundation under the project ‘‘Sviluppo di Film Fotocatalitici

Nanostrutturati per la Conversione di Energia su

Micropiattaforme’’ and by the program PRIN 2006 of MIUR.

Fig. 4 Valence band photoemission spectra plotted with

respect to the Fermi level. Normalization coefficients to the

photon flux are reported on the right side along with the

annealing temperature. The spectra are collected in normal

emission geometry and the used photon energy is 21.2 eV

Fig. 5 AFM image of the titania film deposited on Si(001)

substrate after being annealed in UHV condition at 1000 �C

(area: 191 lm2)

J Nanopart Res

123

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