Growth of nanocrystalline Pd films on Si (1 1 1)

Niraj Joshi, D.K. Aswal*, A.K. Debnath, S.K. Gupta, J.V. YakhmiTechnical Physics and Prototype Engineering Division, Bhabha Atomic Research Center, Mumbai 400085, India

Received 29 October 2003; received in revised form 29 October 2003; accepted 8 January 2004

Abstract

Nanocrystalline thin films of Pd metal has been deposited using inert gas condensation technique on (1 1 1) oriented Si

substrates. The film morphology of Pd films grown under different argon gas pressures has been investigated using atomic force

microscope (AFM), in contact mode. The results show that the film morphology depends strongly on argon pressure and the

lowest grain size of �20 nm is obtained at a pressure of 10�2 Torr. The films are found to grow with (1 1 1) orientation. X-ray

photoelectron spectroscopic studies show that grown films are always metallic.

# 2004 Elsevier B.V. All rights reserved.

PACS: 81.15.Ef; 81.07.Bc; 68.37.Ps; 79.60.–i

Keywords: Nanocrystalline films; Vacuum deposition; AFM; XRD; XPS

1. Introduction

Growth of nanocrystalline metal films has been a

subject of extensive experimental and theoretical stu-

dies in the recent years [1]. Palladium, in particular, is

technologically important because of its unusual prop-

erty of being able to absorb hydrogen upto 900 times

its own volume at room temperature. This makes Pd an

efficient storage medium for hydrogen. Hydrogen

absorption by Pd leads to the formation of PdHx

and, this process is driven by the surface adsorption

of hydrogen at grain boundaries of Pd [2]. Thus, for

maximal surface adsorption of hydrogen, the grain

size of Pd should be in the nanoscale range.

The nanocrystalline films are usually produced by

inert gas condensation technique. In this technique,

the size of the grains can be controlled by variation

of the substrate temperature, which is usually kept at

liquid nitrogen temperature, and type and pressure

of inert condensing gas, i.e. He, Ar, Ne, etc. In

addition, the texture of grains could be influenced by

the orientation of the substrates. The nanocrystalline

films have been studied by different techniques,

such as X-ray and neutron scattering, Mossbauer,

Raman and positron annihilation spectroscopies,

and transmission and electron microscopies [3].

In addition, scanning tunneling microscope (STM)

and atomic force microscope (AFM) are capable of

structural observations over the wide range of the

length scale important to nanocrystalline samples

[4].

In this paper, we present growth of naocrystalline

Pd films on Si(1 1 1) substrates by inert gas condensa-

tion technique. The grown films are characterized by

AFM, X-ray diffraction (XRD) and X-ray photoelec-

tron spectroscopy (XPS) measurements.

Applied Surface Science 228 (2004) 302–305

* Corresponding author. Tel.: þ91-222-550-3134;

fax: þ91-222-550-3134.

E-mail address: dkaswal@apsara.barc.ernet.in (D.K. Aswal).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2004.01.017

2. Experimental details

Pd films on (1 1 1) Si substrates were deposited in a

vacuum system (base vacuum �10�6 Torr). Prior to

the deposition, Si substrates were cleaned in dil HF

solution to remove the native SiO2 layer. In each

experiment, Pd (99.99%) wire of 0.8 mg was ther-

mally evaporating using tungsten filament. The source

to substrate distance was kept fixed at 10 cm in all the

experiments. During deposition, the pressure of inert

gas (Ar) was varied between 0.1 and 10�5 Torr, while

the substrates were kept at liquid nitrogen tempera-

ture. The thickness of the films was measured by a

quartz crystal thickness monitor.

The surface morphology of Pd films were recorded

under ambient conditions using a atomic force micro-

scope (model-SPM Solver P47) in contact mode.

Rectangular cantilever of Si3N4 having force constant

of 3 N/m was employed for the measurements. The

orientation of films was measured by XRD using

Cu Ka radiation. The oxidation states of Pd films

was examined using the X-photoelectron spectro-

scopy.

3. Results and discussions

The argon pressure maintained during depositions

is found to have a profound influence on the film

morphology as well as grain size. Fig. 1 depicts

500 nm � 500 nm AFM scans of Pd films grown under

different argon pressures. At low pressures, e.g.

10�5 Torr, the grown Pd crystallites were elongated

with an average length of 120–140 nm and an aspect

ratio of 4:1. At a higher pressure e.g. 10�3 Torr, the

grain size reduced upto �20 nm, and the crystallites

have nearly a hexagonal shape. At a still high pressure,

i.e. 10�2 Torr, the grains get agglomerated. However,

each agglomerated grain (about 150 nm in size) con-

sists of several smaller grains of about 20 nm size. At

very high pressures, i.e. 0.1 Torr, large clusters of

about 500 nm were observed. The pressure depen-

dence of the grain size of Pd films determined from the

AFM analyses is shown in Fig. 2. It is noted that the

minimum grain size is observed at a pressure of

10�2 Torr.

In Fig. 3, we show the XRD patterns recorded for the

Pd films grown under different pressures of argon gas.

It is seen that all the films grow with (1 1 1) crystal-

lographic orientation. However, fitting the (1 1 1) peak

with Gaussian line shape indicates that the full-width-

half-maximum (FWHM) increases monotonically with

Fig. 1. 500 nm � 500 nm AFM images for Pd films on Si(1 1 1) grown under different partial pressure of Ar; (a) 10�5 Torr, (b) 10�3 Torr and

(c) 10�2 Torr.

10-5 10-4 10-3 10-2 10-110

100

1000

Gra

in s

ize

(nm

)

Pressure (Torr)

Fig. 2. Grains size of Pd films, determined from AFM analyses, as

a function of the argon pressure.

N. Joshi et al. / Applied Surface Science 228 (2004) 302–305 303

increasing argon pressure. The crystallite size (D) has

been calculated using Scherrer equation

D ¼ 0:9lbcosðyÞ

where l is the wavelength of the X-ray (1.54056 for

Cu Ka1), b the corrected FWHM of (1 1 1) peak, and ythe peak position. Since in this case, Pd films are (1 1 1)

oriented, therefore the calculated D represents the

crystallite size in the columnar direction, i.e. normal

to the surface of the substrate. The calculated crystallite

size as a function of argon pressure is plotted in Fig. 4.

For comparison the thickness of the film, measured

using quartz crystal thickness monitor, as a function of

argon pressure is also plotted in Fig. 4. The decrease in

film thickness with increasing pressure is expected as at

higher pressures the mean-free-path of the evaporated

atoms decreases. From Fig. 4, it is evident that for argon

pressures <10�2 Torr, the crystallite size (normal to the

substrate plane) is larger then the film thickness. This is

because, volume of film being same at a particular

pressure, the coverage of film on substrate is not full, as

the crystallites grow via 3D nucleation and growth

mechanism leading to formation of voids and grain

boundaries. At higher pressures, the crystallite size

reduces and packing density increases.

Fig. 5 shows a typical core level Pd-3d XPS spec-

trum of a Pd film grown under 10�2 Torr argon

pressure. The peak positions of 3d5/2 and 3d3/2 are

at 335.9 and 341.1 eV, respectively. Such binding

values are similar to that reported for Pd metal in

literature [5]. Thus, the XPS data confirms that the

grown films correspond to metallic Pd.

4. Conclusions

We have grown nanocrystalline Pd thin films by

inert gas condensation technique. The morphology of

the films is found to be influenced by the argon

pressure employed during the deposition. As the argon

pressure increases, the grain size first decreases and

then increases at very high pressure. The lowest grain

size of �20 nm is obtained at a pressure of 10�2 Torr.

The crystallite size normal to the substrate plane has

been computed using the width of (1 1 1) XRD peak.

The metallic character of the grown nanocrystalline Pd

36 38 40 42 44

Inte

nsity

(ar

b. u

nits

)

2 theta (degree)

(111

)

10-4 Torr

10-3 Torr

10-2 Torr

10-1 Torr

10-5 Torr

Fig. 3. XRD patterns recorded for Pd films grown under different

argon pressures. Thin solid curves are Gaussion fit to the (1 1 1)

peaks.

10-5 10-4 10-3 10-2 10-1

20

40

60

80

100

120

140

Cry

stal

lite

size

/thic

knes

s(nm

)

Argon pressure (Torr)

Thickness using quartz crystal monitor Crystallite size normal to substrate plane

Fig. 4. The crystalline size of the film normal to substrate plane

(calculated using Scherrer equation) as a function of argon

pressure. For comparison, the thickness of Pd films is also plotted.

330 335 340 345 350

3d3/2

341.1eV

3d5/2

335.9eV

Inte

nsity

(ar

b. u

nits

))

B.E.(eV)

Fig. 5. The core level Pd-3d XPS spectrum for film grown under

10�2 Torr argon pressure.

304 N. Joshi et al. / Applied Surface Science 228 (2004) 302–305

thin films is confirmed by X-ray photoelectron spec-

troscopy.

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