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Growth and modification of thin SiGeC films at low substratetemperatures through UV laser assisted processing
E. Lopeza,*, S. Chiussia, J. Serraa, P. Gonzaleza, C. Serraa,b, U. Koscha,B. Leona, F. Fabbric, L. Fornarinic, S. Martelli1
aDpto. Fısica Aplicada, Universidade de Vigo, Lagoas Marcosende, E-36200 Vigo, SpainbCACTI, Universidade de Vigo, Lagoas Marcosende, E-36200 Vigo, Spain
cENEA Frascati, Via E. Fermi 27, I-00044 Frascati, Roma, Italy
Abstract
Enhancing the performance of solar cells, near infrared photo-detectors and microelectronic devices through band gap
engineering caused an increasing attention in processes for growing thin silicon germanium carbon (SiGeC) films in a wide
range of composition and crystalline structures. Moreover, the demand of using cheap substrates and the development of new
devices with advanced materials like ‘‘high-k dielectrics’’ and ‘‘organic materials’’ implies the need of new processes avoiding
high substrate temperatures that may decompose or alter the substrate materials, crystallise part of the heterostructures or
promote segregation effects.
Laser induced chemical vapour deposition (LCVD) and excimer laser assisted crystallisation (ELC) are such alternative and
relatively cheap ‘‘low thermal budget’’ techniques that, in addition, are compatible with conventional IC silicon technology. The
present study will show the possibility of tailoring the composition of amorphous SiGeC coatings through the adjustment of gas flow
rates in LCVD processes performed at substrate temperatures between 180 and 400 8C. The modification of an amorphous film
through a subsequent ELC process performed at room temperature is analysed through SEM and depth profile XPS in order to study
the effects of controlled laser radiation on it, as well as on a very thin underlaying interfacial SiO2 layer and on the Si(1 0 0) substrate.
# 2004 Elsevier B.V. All rights reserved.
Keywords: SiGeC; Excimer laser; Laser CVD; PLIE; Thin film processing
PACS: 42.62; 73.61J; 81.15Fg
1. Introduction
The SiGe system grown on Si(1 0 0) is the most
investigated heterostructure due to its promising
properties as active material or buffer layer in hetero-
junction bipolar transistor (HBT) [1], metal-oxide–
semiconductor field effect transistors (MOSFET),
photodiodes and near infrared photo-detectors.
Although Ge and Si share a common crystal structure,
the diamond cubic lattice, a lattice mismatch of 4.17%
between Si and Ge introduces a considerable strain [2]
in SiGe layers that limits both the thickness of defect
poor alloys as well as their germanium content. Above
a critical thickness strong driving forces are observed,
Applied Surface Science 234 (2004) 422–428
* Corresponding author. Tel.: þ34 986 812216;
fax: þ34 986 812201.
E-mail address: [email protected] (E. Lopez).1 Present address: Centro Sviluppo Materiali, Via di Castel
Romano 100, I-00128 Roma, Italy.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2004.05.053
relieving the elastic energy stored in the layer through
the generation of misfit dislocations [3]. In order to
control the strain, while maintaining the possibility of
bandgap engineering, the necessity of producing a
ternary alloy by adding other elements appeared. Only
a few years ago it was proved that carbon could be an
appropriate candidate provoking a strain relieve, a fine
adjustment of the band gap and, at the same time, a
strong impact on the electrical and optical properties
of the films. It has been shown that, additionally to an
efficient strain relieve [4] the incorporation of sub-
stitutional carbon can increase the band gap [5] and
block the diffusion of dopants such as boron through
the heterostructure [6].
Conventional physical and chemical techniques
such as solid phase epitaxy (SPE) [7], molecular beam
epitaxy (MBE) [4], rapid thermal chemical vapour
deposition (RT-CVD) [8] and ultra-high vacuum che-
mical vapour deposition (UHV-CVD) [9] have already
been employed to deposit SiGeC films, but the high
cost of these processes, their low efficiency for carbon
incorporation and, especially, the high substrate tem-
peratures needed for producing such heterostructures
force the development of cheaper alternative processes
with lower thermal budget. Among such alternative
methods, laser induced chemical vapour deposition
(LCVD) and laser assisted crystallisation techniques
have achieved a considerable success in the last
decades. Especially ArF-LCVD in parallel configura-
tion has proved to be a feasible ‘‘soft’’ deposition
technique that allows the use of temperature sensible
substrates like organic materials, high-k dielectrics or
multilayer structures, as well as of low cost substrates
such as glass or plastic involving an increase of the
technological potential of SiGeC containing hetero-
structures.
Laser assisted crystallisation techniques such as
excimer laser crystallisation (ELC) and pulsed laser
induced epitaxy (PLIE), based on fast melting-solidi-
fication cycles induced by laser radiation, have
recently been applied to the SiGeC system due to
their high capability for the preparation of SiGe alloys
[10–16]. In both techniques, ELC and PLIE, the
irradiation leads to the ultra-rapid melting of the sur-
face down to a certain depth and the subsequent re-
solidification of an alloy with high solid–liquid inter-
face velocities of several meters/second. The fluence
threshold for these 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.
The aim of this work is to show the results con-
cerning to the deposition of SiGeC films at different
substrate temperatures via laser induced chemical
vapour deposition (LCVD) in parallel configuration
and to investigate the crystallisation process as a
function of the number and energy density of the laser
pulses impinging on tailored a-SiGeC:H/SiO2 hetero-
structures. Therefore, a detailed study of the interface
and elements concentration in the films using X-ray
photoelectron spectroscopy (XPS) depth profile ana-
lysis has been combined with the observation of the
surface morphology by scanning electron microscopy
(SEM).
2. Experimental
SiGeC films were grown on Corning (7059) sub-
strates and on Si(1 0 0) wafers by ArF-LCVD in
parallel configuration (substrate above the beam) at
1.2 kPa of total pressure and various substrate tem-
peratures (180, 250, 320 and 400 8C). The experi-
mental set-up consisted of a hybrid self made stainless
steel UHV-HV chamber (base pressure of 0.03 mPa)
connected to a gas supply handing system that have
been described in a previous paper [17]. Disilane
(Si2H6) as Si, germane (GeH4) as Ge and ethylene
(C2H4) as C source were diluted in He and introduced
into the chamber through mass flow controllers. Two
different Si2H6 flows (0.5 and 1 sccm) were used while
GeH4 and C2H4 flows were kept constant at 1 sccm.
These precursors gases were photolitically decom-
posed by a 193 nm ArF-excimer laser (Lambda Physik
LPX 220i) radiation at constant laser power (5 W) and
a power density of 0.7 W/cm2.
The crystalline films have been achieved irradiating
30 nm thick a-Si0.63Ge0.30C0.07:H films grown on
native oxide covered Si(1 0 0) wafers at a relatively
low substrate temperature (250 8C) by ArF-LCVD
parallel configuration.
The crystallisation process was performed using
spatially homogenised pulses (Exitech EX-HS-
700D) of the same ArF-excimer laser, impinging
normal to the substrate. Different areas of 8.8 mm2
E. Lopez et al. / Applied Surface Science 234 (2004) 422–428 423
were treated in air and at room temperature with
different laser fluences (185–800 mJ/cm2) and various
number of pulses (1, 5 and 10).
The thickness of all the coatings was characterised
by profilometry (Dektak3ST-Veeco) and the composi-
tion of the films was determined through X-ray photo-
electron spectroscopy (XPS, ESCALAB 250iXL-VG
Scientific). Depth profile analysis of the composition
was achieved using alternating sputtering cycles (Arþ
ion beam) with X-ray photoelectron spectroscopy
surface analysis using monochromatic Al Ka radiation
at 1486.92 eV. Scanning electron microscopy (SEM;
XL30-PHILIPS) and tapping mode atomic force
microscopy (AFM, Discoverer-Topometrix) with typi-
cal tip-sample surface force varying from 10�11 to
10�6 N were applied to obtain information about
surface morphology.
3. Results and discussion
A study on the substrate temperature (Ts) depen-
dence of the growth rate was made for two different
precursor gas ratios with constant total pressure of
1.2 kPa. According to the results of a previous work
about LCVD of a-SiGeC:H coatings [17], in which we
observed that the slight variation of germane and
ethylene flows at a constant substrate temperature
of 250 8C leaded only to insignificant fluctuations
of the growth rate, we decided to study the substrate
temperature dependence using two gas mixtures with
identical GeH4 and C2H4 and different Si2H6 flows. As
it can be observed in Fig. 1, a higher Si2H6 flow
(Fig. 1a) as well as an increase of Ts (Fig. 1b) leads,
in principle, to a faster growth of adherent and uniform
coatings. A comparison with the temperature depen-
dence of the SiGe growth (Fig. 1b) was added in order
to see how the presence of ethylene in the mixture of
precursor gases rises considerably the growth rate of
the coatings. It has to be noted, that the samples
deposited at 180 8C, both for the SiGeC and the SiGe
system, showed abnormal very high growth rates. This
effect is often observed in CVD processes at low
substrate temperatures and is attributed to the fact
that such low substrate temperatures favour the for-
mation of more porous hydrogen rich material with
considerable amount of voids, drastically increasing
the film thickness.
AFM analysis revealed that the coatings have a root
mean square (rms) roughness varying between 2.6 and
13.3 nm. The highest value was found for the samples
deposited at 180 8C confirming the formation of more
porous material. However, an increase of the substrate
temperature above 300 8C also leads to a increase of
roughness as it can be observed in the AFM image
(Fig. 2) of the 400 8C sample. This effect can be
explained by the observed increase of the deposition
rate that might hinder the film forming gas phase
molecules to find an energetically favourable site
before being covered by a new particle.
Fig. 1. (a) Dependence of the deposition rate on the substrate temperature for two SiGeC series deposited with different Si2H6 ratios at
constant GeH4 and C2H4 flows of 1 sccm. (b) Comparison between SiGe and SiGeC growth rates obtained for the same flows of Si2H6 and
GeH4 (both 1 sccm) at different substrate temperatures.
424 E. Lopez et al. / Applied Surface Science 234 (2004) 422–428
The alloy composition was determined by XPS
through the study of the C 1s, Si 2p and Ge 3d
transitions. It has been observed that the C 1s peak
with a binding energy around 285 eV, considered to be
related to hydrocarbon impurities on the surface and
the graphite C 1s (284.4–284.8 eV) disappears after
sputtering an approximately 5 nm thick surface layer
with Ar. The presence of Si–C bonds has been con-
firmed by a peak at 283 eV that can be assigned to
carbon C 1s bonded to silicon (Si–C) and through the
deconvolution of the Si 2p transition containing the
contribution of both Si–C at 100.5 eV and of Si–Si
bonds at 99.3 eV. A small peak at 103 eV that fits with
Si from SiO2 disappears after the first 8 nm of Ar
sputtering and can be attributed to a native oxide cap-
layer. The Ge 3d transition (29 eV) was also analysed
and exhibits an almost insignificant small peak related
to GeO2 which is totally removed after 2 nm of
sputtering.
The intensity of the different peaks after sputtering
these surface impurities has been used for calculating
the stoichiometry of the samples. It has been noticed
that in the SiGeC system an increase of the tempera-
ture above 320 8C provokes an important increase of
Ge content in the film which should be caused by the
beginning of the pirolysis of GeH4. In contrast to this
Ge tendency, the C content diminished with increasing
Ts as confirmed studying in detail the C 1s transition.
As it can be seen in Fig. 3, for low substrate tempera-
tures of 180 and 250 8C the C 1s peak corresponding to
Si–C bonds appeared at 283.11 eV but for higher
substrate temperatures of 320 8C or 400 8C the carbon
signal moves to 283.0 eV or 282.75 eV respectively.
This displacement of the C 1s peak to lower binding
energy suggests that the samples grown at low sub-
strate temperature contain mainly substitutional car-
bon whereas an increase of the substrate temperature
involves a decrease of this element in substitutional
sites.
In spite of the fact that laser annealing of SiGeC on
air implies the exposure of the molten surface to the
atmosphere, thus a possible carbon loss due to oxida-
tion at high temperatures (>850 8C) [18], this study
has been realised in order to assess the possibility of
taking advantage of ultra-rapid melt/solidification
cycles for guarantying a low cost of the ELC process.
The possible segregation effects and the evolution of a
Fig. 2. AFM image of a SiGeC sample deposited at 400 8C with a rms value of 10 nm.
Fig. 3. C 1s photoelectron binding energy spectra of SiGeC films
grown at different substrate temperatures.
E. Lopez et al. / Applied Surface Science 234 (2004) 422–428 425
thin SiO2 interface between substrate and coating have
been studied performing XPS depth profile analysis of
30 nm thick a-Si0.63Ge0.30C0.07:H films irradiated with
10, 5 and 1 pulses of different energy densities and
contrasted with the profile of the original amorphous
one (Fig. 4).
Although the experimental results indicate that at
fluences of 185 mJ/cm2 the substrate does not melt, as
the absence of germanium and carbon diffusion into
the substrate suggests (Fig. 5a), the XPS depth profile
analysis reveals an increase of Ge and C segregation to
the surface and a reduction of their concentration in
the film with the number of pulses, suggesting the
formation of non-uniform Ge and C rich regions on the
surface of the coating and their oxidation or ablation
(estimated Ge ablation threshold is 60 mJ/cm2 [19])
by the subsequent laser pulses. A reason for this
element behaviour might be the phase separation
between Si–C and Si–Ge, which is also observed
during the conventional growth of films with consid-
erable carbon concentration (more than 2%) [20,21]
and can be attributed to the extremely low solubility of
C in Ge [22] as well as to the results of theoretical
studies predicting that no GeC bonds are formed in
SiGeC coatings [23]. The evolution of the SiO2 layer
in the interface between the SiGeC alloy and the
Si(1 0 0) wafer was also evaluated by XPS depth
profile analysis which reveals the diffusion of the
interfacial oxide layer towards the alloy surface using
a fluence of 185 mJ/cm2 (Fig. 5a). SEM and EDS
analyses confirmed the formation of Ge-rich islands
on the surface of the samples. Furthermore, SEM
analysis revealed that the size and shape of the islands
as well as their distribution changed with the number
of pulses and their energy density. With more pulses
and higher energy density more isolated small round
islands were found in contrast to interconnected ones
observed at lower fluences and less number of shots. It
is known that this effect can be assigned to an increase
of strain [24] that in our case may be originated by the
Fig. 4. XPS depth profile analysis of a 30 nm thick a-
Si0.63Ge0.30C0.07:H film deposited by ArF-LCVD in parallel
configuration.
Fig. 5. XPS depth profile analysis of samples irradiated on air with one pulse at (a) 185 and (b) 450 mJ/cm2 with the beam impinging normal
to the substrate.
426 E. Lopez et al. / Applied Surface Science 234 (2004) 422–428
higher Ge concentration on the surface caused by the
Ge segregation as laser fluence or number of pulses is
increased. Furthermore, the reduction of the Ge
islands size can also be provoked by their possible
partial ablation.
In contrast, no carbon rich regions have been
detected indicating a complete carbon loss after its
segregation to the surface. This effect can be easily
explained by the oxidation of carbon at the coating
surface due to processing in air that provokes both,
heating up of the surface as well as an increase of
reactive O and OH radical concentration due to
193 nm photolysis of O2 and H2O in the surrounding
atmosphere.
XPS depth profile analysis of coatings irradiated
with a laser fluence equal or exceeding 450 mJ/cm2
revealed again the segregation of Ge and C towards
the surface and a diminishing of the content of these
two elements in the film, but also a strong increase of
the alloy thickness indicating a considerable inter-
mixing of the coating with the substrate (Fig. 5b).
This fact is provoked by the laser induced melting of
both, coating and part of the Si(1 0 0) substrate, and
the subsequent intermixing of the elements in the
molten pool prior to the solidification of the alloy.
Moreover, the native interfacial SiO2 observed in the
amorphous film appears near the surface in the sam-
ple irradiated with one pulse at 450 mJ/cm2 and is
completely eliminated using higher fluences or num-
ber of pulses, evidencing the complete diffusion of
the oxide and the ‘‘cleaning’’ of the SiGeC/Si(1 0 0)
interface. On the other hand, SEM analysis indicated
that island formation can only be observed after the
first shot and that the successive laser pulses produce
a very smooth surface (rms < 5 nm), suggesting the
removal of superficial Ge-rich islands through laser
ablation.
4. Conclusion
SiGeC films were produced by LCVD using a
commercial ArF laser irradiating parallel to Corning
(7059) substrates and Si(1 0 0) wafers at different
substrate temperatures. The analysis of these films
revealed that an increase of the substrate temperature
provokes higher growth rates analogous to the SiGe
system, rougher surfaces, higher Ge content in the film
and a reduction of C content in substitutional sites.
Laser crystallisation has been used to make
Si(1�x�y)GexCy/Si(1 0 0) heterostructures using a-
Si0.63Ge0.30C0.07:H films on Si (1 0 0) obtained by
LCVD. The characterization through depth profile
XPS and SEM/EDS showed the segregation of Ge
and C, the loss of C at the coating surface due to
oxidation during processing on air, the formation of
Ge-rich island and the diffusion towards the surface of
interfacial SiO2 for a fluence of 185 mJ/cm2. Finally
the irradiation of the coatings with fluences above
450 mJ/cm2 clearly shows a considerable increase of
alloy thickness and the intermixing of the coating with
the substrate indicating that both, the coating and part
of the substrate have been molten during the laser
process. Using repetitive pulses at this relatively high
fluence allowed the complete removal of the Ge
islands as well as of the native SiO2 interface between
coating and Si(1 0 0) wafer.
Acknowledgements
This work has been partially supported by EU as
well as by Spanish contracts and grants HA1999-
0106, MAT2000-1050, XUGA32107BB92DOG211,
UV62903I5F4, PGIDT01PX130301PN and PR405-
A2001/35-0. The authors wish to thank A. Abalde
and J.B. Rodrıguez (Univ. Vigo) for their extensive
technical help and for fruitful discussions.
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