Towards the synthesis of MAX-phase functional coatings by

pulsed laser deposition

Christian Lange a,*, Michel W. Barsoum b, Peter Schaaf a

a University of Gottingen, II. Physikalisches Institut, Friedrich-Hund-Platz 1, 37077 Gottingen, Germanyb Drexel University, Department of Materials Engineering, Philadelphia, PA 19104, USA

Received 18 May 2007; received in revised form 23 July 2007; accepted 23 July 2007

Available online 27 July 2007

Abstract

Pulsed laser deposition with a Nd:YAG laser was used to grow thin films from a pre-synthesized Ti3SiC2 MAX-phase formulated ablation target

on oxidized Si(1 0 0) and MgO(1 0 0) substrates. The depositions were carried out in a substrate temperature range from 300 to 900 K, and the

pressure in the deposition chamber ranged from vacuum (10�5 Pa) to 0.05 Pa Argon background pressure. The properties of the films have been

investigated by Rutherford backscattering spectrometry for film thickness and stoichiometric composition and X-ray diffraction for the

crystallinity of the films. The silicon content of the films varied with the energy density of the laser beam. To suppress especially the silicon

re-sputtering from the substrate, the energy of the incoming particles must be below a threshold of 20 eV. Therefore, the energy density of the laser

beam must not be too high. At constant deposition energy density the film thickness depends strongly on the background pressure. The X-ray

diffraction measurements show patterns that are typical of amorphous films, i.e. no Ti3SiC2 related reflections were found. Only a very weak

TiC(2 0 0) reflection was seen, indicating the presence of a small amount of crystalline TiC.

# 2007 Elsevier B.V. All rights reserved.

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Applied Surface Science 254 (2007) 1232–1235

Keywords: MAX phases; Pulsed laser deposition; Functional coatings; Functional ceramics

1. Introduction

The Mn + 1AXn (MAX) phases are a group of ternary

carbides or nitrides with M being an early transition metal

(mainly of the groups IVB and VB), A being an A group

element (mostly IIIA and IVA) and X being either carbon or

nitrogen. The prototypic compound Ti3SiC2 had already been

synthesized in the 1960s [1], but not until 1996 its remarkable

properties have been discovered [2]. MAX phases combine

metallic properties such as good electrical and thermal

conductivity with ceramic properties like thermal stability

and resistance against oxidation [3]. These properties give rise

to a potential application of MAX phases in thin films—for

instance as wear and corrosion resistant coatings of electrical

contacts. The growth of crystalline MAX-phase thin films

however is not trivial. The deposition of such films by

magnetron sputtering has been reported [4–6]. One work

* Corresponding author. Tel.: +49 551 39 7649; fax: +49 551 39 4493.

E-mail address: clange1@uni-goettingen.de (C. Lange).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.07.156

concerning the pulsed laser deposition of MAX-phase thin

films has been published by Hu et al. [7] but was argued over in

the scientific community [8,9].

In this paper, the growth of Ti/Si/C thin films from a pre-

synthesized MAX-phase Ti3SiC2 ablation target by pulsed laser

deposition (PLD) is reported. The stoichiometric compositions,

film thicknesses and their crystallinity are discussed in relation

to various deposition parameters, i.e. laser energy density,

substrate temperature and background pressure during deposi-

tion.

2. Experimental details

The Ti/Si/C films were deposited from a pre-synthesized

Ti3SiC2 MAX-phase target [2] by pulsed laser deposition with a

Quantel Brilliant pulsed Nd:YAG laser (Quantel S.A., Les Ulis,

France; wavelength l = 1064 nm; pulse duration t = 6 ns;

repetition rate f = 20 Hz). The energy density of the laser beam

was set to 4 or 8 J/cm2, respectively, and the deposition time for

all experiments was 15 min. The Si(1 0 0) single crystal

substrates were oxidized for 24 h at 1300 K in air to obtain an

Fig. 2. SEM cross sectional image of a 750 nm thick Ti/Si/C layer on an

MgO(1 0 0) substrate. The higher thickness compared to the other films is due to

the longer deposition time of 30 min.

C. Lange et al. / Applied Surface Science 254 (2007) 1232–1235 1233

amorphous substrate surface. During deposition the substrates

were heated by a resistance heater from the reverse side to

temperatures ranging from 300 to 950 K. The temperature was

measured by a NiCr–Ni (type K) thermocouple. Prior to the

film growth, the deposition chamber was evacuated to 10�5 Pa.

The films were either grown at this base pressure or at an

increased background pressure of 0.01–0.05 Pa Argon

(99.999%, Messer GmbH, Sulzbach, Germany) to a total film

thickness of 150–350 nm. Rutherford backscattering spectro-

metry (RBS) analysis was performed at the IONAS accelerator

facility at Gottingen University with 900 keV He2+ ions at

normal incidence [10]. The backscattered particles were

detected at an angle of 1658. WinDF and RUMP software

packages were used for the analysis of the RBS spectra [11,12].

A Scintag X2 diffractometer (Scintag Inc., Cupertino, USA)

utilizing Cu Ka radiation was used for X-ray diffraction

measurements. The scans were performed in standard Bragg-

Brentano 2-theta measurements from 58 to 908 with 0.028increments, using time steps of 0.5 s.

3. Results and discussion

Scanning electron micrographs of the deposited layers in

Figs. 1 and 2 show crack free, homogeneous films with very few

droplets. The existing droplets are 200 nm or smaller in size and

their density is about 2.5 � 105 cm�2. Fig. 2 shows a layer

grown on an MgO(1 0 0) substrate. This layer was deposited for

30 min and is therefore – with a thickness of 750 nm – thicker

than the other films discussed here.

3.1. Film thickness and stoichiometry

RBS analysis of the films deposited at 8 J/cm2 in vacuum

shows that the films are silicon deficient compared to the

desired Ti:Si:C = 3:1:2 composition. The RBS spectra of these

films grown at different temperatures are given in Fig. 3. All

films are about 150 nm thick and they contain an average of

7(2) at.% silicon, whereas the desired silicon content is 17 at.%.

Fig. 1. SEM surface image of a deposited Ti/Si/C layer. Droplets with a

diameter of 200 nm and below are marked with white circles.

Since the ablation target is a pre-synthesized MAX phase with

the desired chemical composition, the loss of Si during

deposition is most probably due to re-sputtering effects from

the substrate. In order to quantify these events, a calculation

with the SRIM simulation software [13] was performed,

concerning the re-sputter yields of the various elements as a

function of the energy of the incoming particles. The sputter

yields for each element in the film (Ti, Si and C) were

calculated for each incoming element (Ti, Si and C) and then

normalized to the stoichiometric composition Ti3SiC2 of the

ablation target material. The total sputter yield SY(element) for

each substrate element is therefore given by the equation:

SYelement ¼ 16ð3SYelement;Ti þ SYelement;Si þ 2SYelement;CÞ;

where for instance, SYelement,Ti is the sputter yield for the

substrate element when only Ti particles are incoming. Fig. 4

shows the results of these simulations. In order to effectively

suppress re-sputtering, the energy of the incoming particles must

not exceed 20 eV. To reduce this energy, the energy density of the

laser beam was reduced to 4 J/cm2 and the background pressure

was increased to 0.01–0.05 Pa Argon. Consequently, both film

thickness and Si content increased (Fig. 5). Although the deposi-

tion time was 15 min for both laser energy densities (4 and 8 J/

cm2), the layers deposited at 4 J/cm2 are thicker (225–350 nm,

Fig. 6.) and the silicon content is raised to 12(1) at.%.

Fig. 3. RBS spectra of the Ti/Si/C films deposited at an energy density of 8 J/

cm2. The deposition temperature is indicated on the graph.

Fig. 4. Sputtering yields (SY) of the different MAX-phase elements as a

function of the energy of the incoming particles as described in the text.

The connecting lines are a guide to the eye.

Fig. 5. RBS spectra of the Ti/Si/C films deposited at room temperature with an

energy density of 4 J/cm2. The Ar background pressure is indicated on the

graph.

Fig. 7. X-ray diffraction pattern of a film deposited at room temperature. A

weak TiC(2 0 0) reflection can be seen. The sharp reflection is a substrate

artefact.

C. Lange et al. / Applied Surface Science 254 (2007) 1232–12351234

3.2. Crystallinity of the films

XRD measurements show no Ti3SiC2-phase related reflec-

tions over the whole deposition temperature range. Instead a

Fig. 6. Film thicknesses for films deposited at room temperature with 4 J/cm2 as a

function of the Ar background pressure as determined from RBS measurements.

typical XRD-amorphous pattern is seen. The layers grown at

room temperature and 700 K show a very weak reflection at

41.68 that could be attributed to a small amount of crystalline

TiC(2 0 0). The films deposited at 850 and 950 K do not show

this reflection. In Fig. 7, the diffraction pattern of a film

deposited at room temperature is given. The sharp reflection at

32.98 is a substrate artefact.

4. Summary

Thin films from a pre-synthesized Ti3SiC2 MAX-phase

formulated target were grown by means of PLD. It was shown

that, due to re-sputtering effects, the silicon content of the films

decreases with increasing energy density of the laser beam.

While growing the films at a constant energy density, the film

thickness depends on the applied pressure during deposition.

Even at substrate temperatures as high as 950 K, no MAX-

phase related X-ray reflections were observed. This is in

agreement with findings of Palmquist et al. who reported a

minimum MAX-phase-formation temperature of about 1000 K

in thin films [4]. At lower deposition temperatures, i.e. room

temperature and 750 K a weak peak at 41.68 hints to a small

amount of crystalline TiC. The silicon deficiency at the lower

laser energy densities is still another problem. Further

investigations to clarify whether this is a diffusion phenomenon

or a loss due to relatively volatile silicon compounds – such as

SiO – are currently in progress.

Acknowledgements

The authors wish to thank S. Muller and M. Hahn for the

assistance at the scanning electron microscope. We would also

like to thank T. Scabarozi for the XRD spectra. This work was

partially funded by an NSF grant (DMR 0503711) to Drexel

University and by the Deutsche Forschungsgemeinschaft (DFG

grant Scha 632/10).

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