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Thin Solid Films 508

UV-laser-assisted processing of thin silicon–germanium–carbon films

E. Lopez, S. Chiussi *, J. Serra, P. Gonzalez, B. Leon

Dpto. Fısica Aplicada, Universidade de Vigo, E.T.S.I Industriales, Rua Maxwell s/n Campus Universitario Lagoas Marcosende, E-36310 Vigo, Spain

Available online 28 November 2005

Abstract

The aim of enhancing the performance of solar cells and microelectronic devices through band-gap engineering as well as the demand of

using cheap organic substrates or advanced materials like ‘‘high-k dielectrics’’ for nanoelectronic based technologies has caused an increasing

attention in alternative processes for growing thin silicon–germanium–carbon alloys. Laser-induced chemical vapour deposition (LCVD),

excimer laser-assisted crystallisation (ELC) and pulsed laser-induced epitaxy (PLIE) are such alternative techniques that are relatively cheap

and capable to provide films in a wide range of composition and crystalline structure. This contribution shows some typical results achieved

combining these laser-assisted techniques, thus demonstrating the potential of integrated laser-assisted ‘‘single-chamber’’ processes. The growth

of amorphous hydrogenated SiGeC coatings at 250 -C with, on nanometer-scale, well-tailored thickness on large area as well as on small

selected areas using LCVD is presented and discussed. The subsequent irradiation of these films in a single-chamber process for the

dehydrogenation of the amorphous material, the modification of these coatings to polycrystalline or nanocrystalline films via ELC or to

heteroepitaxial alloys on Si(100) wafers through PLIE, is also shown and evaluated. The selected samples that will be shown have been

extensively studied through Fourier transform infrared and X-ray photoelectron spectroscopy, time of flight secondary ion mass spectrometry,

Rutherford backscattering, X-ray diffraction, atomic force microscopy and scanning electron as well as conventional and high-resolution

transmission electron microscopy.

D 2005 Elsevier B.V. All rights reserved.

Keywords: SiGeC; Excimer laser; LCVD; ELC; PLIE

1. Introduction

The increasing success of group IV semiconductors is

connected to the development of band-gap engineering in these

last 10 years and the extensive research on the SiGe system as a

new material with narrower band gap than pure Si. As a

consequence of developing new different electronic and

optoelectronic devices, the incorporation of another isovalent

element, like carbon, has appeared as a new alternative for

achieving larger band gap material [1] and strain compensation

in SiGe alloys. However, the fact that C can occupy

substitutional or interstitial sites as well as its low solubility

in Si and even lower in Ge [2] limits the amount of C that can

be incorporated in the alloy.

Various conventional deposition techniques have already

been extensively used to deposit the ternary SiGeC alloys but

exhibited several problems like high processing costs, low

0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2005.07.351

* Corresponding author. Tel.: +34 986 812216; fax: +34 986 812201.

E-mail address: schiussi@uvigo.es (S. Chiussi).

efficiency of carbon incorporation or high substrate tempera-

tures required [3–6]. As alternative low thermal budget

processing routes, laser-assisted techniques such as ArF-

excimer laser-induced chemical vapour deposition (ArF-

LCVD), excimer laser-assisted crystallisation (ELC) and pulsed

laser-induced epitaxy (PLIE) have been studied demonstrating a

considerable success for producing these alloys [7–16]. The

possibility of using temperature-sensible substrates based on

high-k dielectrics or multilayer structures, as well as of low-cost

substrates such as glass or organic materials, increases the

technological potential of SiGeC-containing heterostructures

produced by those alternative techniques.

In this paper, we aim to demonstrate the feasibility of the

laser-assisted techniques for both obtaining amorphous and

crystalline SiGeC alloys with laser-assisted single-chamber

processing. The combination of ArF-LCVD in parallel

configuration with crystallisation processes such as ELC or

PLIE allowed us to produce hydrogenated amorphous samples

and crystallise them to nanocrystalline, polycrystalline or

heteroepitaxial material.

(2006) 48 – 52

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E. Lopez et al. / Thin Solid Films 508 (2006) 48–52 49

2. Experimental

Amorphous hydrogenated SiGeC (a-SiGeC:H) films were

deposited at a low substrate temperature of 250 -C on Corning

(7059) and on Si (100) substrates by ArF-LCVD in parallel

configuration (substrate above the beam). The experimental set-

up used to produce these films consisted of a hybrid self-made

stainless steel UHV-HV chamber connected to a gas supply

system that has been described in a previous paper [11].

Different flow rates of the precursor gases Si2H6 (0–2 sccm),

GeH4 (0–2 sccm) and C2H4 (0–10 sccm), diluted in He, were

introduced in the near vicinity of the 250 -C hot substrate

maintaining a constant total pressure of 1.2 kPa. The gases were

photolytically decomposed by the 193-nm ArF-Excimer laser

(Lambda Physik LPX 220i) radiation at a power density of 0.7

W/cm2. Additional studies of the influence of different substrate

temperatures Ts (180, 250, 320 and 400 -C) on the film

properties were obtained using 1 sccm of each precursor gas.

The crystalline films have been achieved irradiating 30-nm-

thick a-Si0.63Ge0.30C0.07:H films grown on both Si (100) and

native oxide-covered Si (100) wafers at 250 -C and 1.2 kPa by

ArF-LCVD in parallel configuration. ELC and PLIE were

performed using spatially homogenised pulses (Excitech

Limited Beam homogenizer EX-HS-700D) of the above-

mentioned ArF-excimer laser impinging normal to the sub-

Photolysisof Si2H6 , GeH4 and C2H4

ArF-Laser radiation(λ(λ = 193 nm)

Growth of a thina-SiGeC:H coating

Substrate atTs=250°C

(a)

LCVDLCVD

(c)

Fig. 1. Scheme of the ArF-LCVD in parallel configuration (a) and views of a uniform

the marked area) (b) and in selected regions such as 100�100-Am2 squares (c) obt

stencils.

strate. Different areas of 8.8 mm2 were treated in ambient

atmosphere and at room temperature with different laser

fluences and various numbers of pulses.

The characterisation of the film thickness was carried out by

profilometry (Dektak3ST-Veeco) and atomic force microscopy

(AFM, Discoverer-Topometrix) measurements. Fourier trans-

form infrared (FTIR, MB100-Bomem) and X-ray photoelec-

tron spectroscopy (XPS, ESCALAB 250iXL-VG Scientific)

were used to determine the composition of the coatings.

Scanning electron microscopy (SEM, XL30-PHILIPS) and

atomic force microscopy were applied to obtain information

about surface morphology. The film crystallinity has been

studied by Raman spectroscopy (RFS 100-Bruker), X-ray

diffraction (XRD, Geigerflex-Rigaku) and transmission elec-

tron microscopy (TEM, CM20-PHILIPS). High-resolution

TEM images have been achieved with a JEOL 2011 (200

kV) in collaboration with another research group [17].

3. Results and discussion

ArF-LCVD in parallel configuration has demonstrated to be

a very efficient technique for depositing uniform a-SiGeC:H

films [8,11]. In this geometry, the laser causes the decompo-

sition of the precursor gases (Si2H6, GeH4, C2H4) near the

substrate surface without irradiating it (Fig. 1a), thus avoiding

(b)

(d)

Si1�x�yGexCy coating deposited on a 3WSi (100) wafer with a TEM mesh (in

ained using TEM meshes as masks or even smaller ones (c) using special SiN

Fig. 2. Ternary representation of the Si, Ge and C content of the Si1�x�yGexCy

alloys, determined by XPS. All samples have been deposited at 250 -C and 1.2

kPa by ArF-LCVD in parallel configuration.

E. Lopez et al. / Thin Solid Films 508 (2006) 48–5250

pyrolitic processes due to additional substrate heating and the

possible modification of the growing film or the underlying

substrate. Although photolytic-induced reactions at 193 nm are

referred in the literature [18], the amount of secondary gas

phase reactions, which depend on the collisions between gas

molecules, is clearly influenced by several processing para-

meters affecting, in consequence, the film growth.

In fact, the growth rate of adherent Si1�x�yGexCy films

deposited by ArF-LCVD at 250 -C and 1.2 kPa showed to be

strongly influenced by some processing parameters such as

substrate temperature, total pressure and reactant gas mixture,

as it has been studied in previous papers [11,14,15].

As it can be observed in Fig. 1b–d, ArF-LCVD offers the

possibility of depositing uniformly on large areas through

widening the laser beam as well as on small regions by using

masks or stencils. It has to be mentioned that the obtained films

are considerably smooth (RMS<2 nm) as determined by AFM

analysis.

The evaluation of the XPS peaks (C1s, Si2p and Ge3d

transitions) [11] and the FTIR bands [14,15] allow us to

determine the stoichiometry and types of bonding existing in

the films, thus confirming the possibility of tailoring the

composition in a wide range (Fig. 2). In all Si-containing

samples, we observed Si–Si (BE=99.3 eV) bonds after

ArF-Laser radiation( λ= 193 nm, ∼ 20 ns,

top-flat profile)

Dehydrogenation

of a-SiGeC and

Crystallisation

to poly-c-SiGe

Ts= 25°C

ELCELC(a) (b) a-S

Fig. 3. (a) Scheme of the dehydrogenation and ELC process, (b) high-resolution cro

(100) using 10 pulses with 120 mJ/cm2 laser fluence.

sputtering a 5-nm-thick surface native oxide layer with Ar.

The presence of Si–C bonds in SiC and SiGeC samples was

confirmed both by XPS (BE=283, 100.5 eV) and FTIR

techniques (680 cm�1) [19]. While the Si content varied from

77% to 96% in SiC, from 45% to 95% in SiGe and from 63%

to 94% in SiGeC films, we found that the C content varied

from 4% to 23% in SiC and from 1% to 7% in SiGeC samples.

The fact that Si is predominantly incorporated in the films can

be explained by the absorption coefficient of the different

precursor gases. For the 193-nm radiation of the ArF laser, the

absorption is approximately 100 times higher for Si2H6 than for

GeH4 or C2H4 [18,20].

TOF-SIMS and XPS depth profile analysis revealed also the

homogeneous distribution in depth of Si, Ge and C for

representative a-SiGeC:H samples [11,13], thus confirming

the composition homogeneity of the films deposited by this

method.

Additionally, FTIR spectroscopy showed the presence of H

in the films, especially in those deposited at 180 -C or 250 -C,and its tendency to selectively bond to Si or C rather than to

Ge. This is a well-known effect, often called as ‘‘preferential

attachment’’ [21,22], caused by the smaller binding energy of

H–Ge compared to H–Si or C–H. In fact, bands assigned to

the stretching (2000–2100 cm�1), bending (840–885 cm�1)

and wagging (640 cm�1) modes of an SiHx or an (SiH2)ngroup [19,23] could be observed in all amorphous Si-contain-

ing films. Moreover, the spectra of a-SiGeC:H and a-SiC:H

samples showed the Si–CHx stretching (800 cm�1) and C–H

wagging (980 cm�1) vibrational modes. However, weaker

bands related to Ge–H were also detected in SiGeC and SiGe

samples at 1880 cm�1 (stretching) and 775 cm�1 (rocking),

suggesting a possible saturation of Si-, Ge- and C-dangling

bonds.

The dehydrogenation of this amorphous material, its

modification to polycrystalline, nanocrystalline or heteroepi-

taxial films, has been performed in a single-chamber process

subsequently after the LCVD process using laser-assisted

crystallisation techniques, such as ELC or PLIE.

In ELC (Fig. 3a) and PLIE (Fig. 4a), the laser radiation leads

to the ultra-rapid melting of the surface down to a certain depth

followed by the re-solidification of the alloy with high solid–

Si(100)

a-SiGeC

c-SiGeC grainsiGeC

10nm

ss TEM image of SiGeC nanocrystals in an amorphous a-SiGeC:H matrix on Si

Meltingof film + Si(100)

Mixingof Ge and Si the

molten pool

Crystallisationusing Si(100) as seed

Ts= 25°C

Si (100)

ArF-Laser radiation( λ= 193 nm, ∼ 20 ns,

top-flat profile)

PLIEPLIE(a) (b)

Si(100)

[100]

[011]

Si

SiGeC

5 nm

Fig. 4. (a) Scheme of the PLIE process, (b) high resolution cross TEM image of a heteroepitaxial SiGeC film on Si (100) using 10 pulses with 450 mJ/cm2 laser

fluence.

E. Lopez et al. / Thin Solid Films 508 (2006) 48–52 51

liquid interface velocities of several meters per second. The

fluence threshold for this processes, the melt duration and

depth of the molten zone depend on the wavelength of the laser

beam, the duration and shape of the pulses and the optical and

thermal characteristics of the irradiated surface.

ELC has been used for crystallising a-SiGe:H films

deposited on Corning glass (7059) [9,10] and Si(100) wafers

through ArF-LCVD. A ‘‘step-by-step’’ process was required for

avoiding the formation of micro-holes caused by an explosive

dehydrogenation of the films. It has been observed that a

successive increase of the laser fluence to values higher than 80

mJ/cm2 allow the dehydrogenation and the subsequent crystal-

lisation leading to polycrystalline SiGe films with grains of

10–20 nm. In the particular case of low fluences (120 mJ/cm2),

the partial crystallisation to nanocrystalline material in an

amorphous matrix has also been achieved as it can clearly be

observed in high-resolution TEM images (Fig. 3b) of samples

obtained after irradiating a-SiGeC:H films on Si (100) wafers

[17].

The formation of epitaxial Si1�x�yGexCy/Si(100) hetero-

structures could be obtained combining ArF-LCVD in parallel

configuration and PLIE. Various a-Si0.63Ge0.30C0.07:H films

produced at 250 -C and 1.2 kPa by ArF-LCVD on clean Si

(100) [13] and also on Si (100) with a thin SiO2 film on its

surface [12] were irradiated in air. The characterization

through depth profile XPS and SEM/EDS showed that

fluences lower than 185 mJ/cm2 provoke only partially

epitaxial alloys and (i) segregation of Ge and C, (ii) the loss

of C at the coating surface due to oxidation during processing

on air, (iii) the formation of Ge-rich islands [13,17] and (iv)

the diffusion of O towards the surface for the samples with an

interfacial SiO2 layer. However, the irradiation of the coatings

with fluences above or equal to 450 mJ/cm2 clearly shows a

considerable increase of the alloy thickness indicating that

both the coating and part of the substrate have been molten

during the laser process and an intermixing of the coating

with the substrate has occurred. The rapid solidification of the

molten pool taking the Si(100) wafer as crystal seed produces

well-ordered heteroepitaxial SiGeC/Si(100) structures as it

can be observed in HRTEM images (Fig. 4b) and deduced

from electron diffraction [17] and RBS channelling analysis

[24].

4. Conclusion

Amorphous hydrogenated SiGeC films with different stoi-

chiometry were synthesised by LCVD on large areas as well as

on selected regions using an ArF laser irradiating parallel to

Corning (7059) and Si (100) substrates at 250 -C. Both the

uniform composition of the films (Si, Ge, C and H) and the

presence of Si–C bonds were confirmed by XPS, FTIR and

TOF-SIMS. These smooth surface samples were very homo-

geneous in depth and suitable for subsequent crystallisation

through ELC or PLIE. The laser-assisted crystallisation

techniques allowed us to perform fast single-chamber processes

obtaining an efficient dehydrogenation of the films, partially or

fully crystallised nanocrystalline material as well as hetero-

epitaxial structures on Si(100), thus demonstrating the high

potential of processes combining such laser-assisted techniques.

Acknowledgements

This work has been partially supported by the European,

Spanish and Galician contracts and grants FL24756,

MAT2000-1050, MAT2003-04908, UV62903I5F4,

PGIDT01PX130301PN and PR405A2001/35-0. The authors

thank U. Kosch, F. Gontad, J.B. Vazquez, S. Martelli, F.

Fabbri, L. Fornarini and, in particular, N. Frangis and C. Serra

(CACTI) for their extensive technical help and for fruitful

discussions.

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