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Applied Surface Science 248 (2005) 108–112

Influence of the substrate temperature on the

structure of Ge containing thin films produced by

ArF laser induced chemical vapour deposition

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

Dpto. Fısica Aplicada, Universidad de Vigo, Lagoas-Marcosende 9, 36200 Vigo, Spain

Abstract

Ge, SiGe and SiGeC films were grown on Si(1 0 0) and Corning glass (7059) substrates by ArF-excimer laser induced

chemical vapour deposition in parallel configuration. Different substrates temperatures ranging from 180 to 400 8C, for a fixed

reactant gas composition, were used at constant total pressure and laser power. The analysis of the films showed the existence of

a relationship between the substrate temperature and the deposition rate as well as to the film properties. A comparison among

the pure, binary and ternary Ge containing system was performed to study the influence of the presence of different gases in the

reactant mixture. Structural properties of the deposited films were investigated by Raman and Fourier transform infrared

spectroscopy. Their surface morphology was evaluated by scanning electron microscopy and atomic force microscopy (AFM).

X-ray photoelectron spectroscopy (XPS) revealed the composition of the alloys and X-ray diffraction experiments demonstrated

the polycrystallinity of some pure Ge films.

# 2005 Elsevier B.V. All rights reserved.

PACS: 42.70; 81.15; 61.43 D; 33.80.G; 61.16C; 61.10; 33.60

Keywords: LCVD; Ge containing films; Thin films; FTIR; XRD

1. Introduction

Pure Si, Ge and C films have been studied

extensively for microelectronic and optoelectronic

devices. The possibility of producing new materials

with designed optical and electrical properties by

* Corresponding author. Tel.: +34 986812216;

fax: +34 986812201.

E-mail address: fani@uvigo.es (E. Lopez).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.03.091

alloying these group IV semiconductors has been the

recent goal in the development of silicon-based

devices for several applications, such as solar cells,

photodetectors, thin film transistors (TFTs) and flat

panel displays [1]. Other novel devices, such as

resonant tunnelling diodes (RTDs) [2] or long

wavelength detectors have been achieved using SiGe

alloys. Alloying Si with Ge or C allows decreasing or

increasing the optical gap of Si, respectively, and at the

same time, changes the lattice constant and creates a

.

E. Lopez et al. / Applied Surface Science 248 (2005) 108–112 109

tetragonal distortion of the cubic symmetry. Recent

studies revealed that forming the ternary alloy does not

only allow band gap engineering [3] but also alleviates

the stress found in epitaxial SiGe/Si heterostructures

by strain compensation [4]. However, obtaining high

quality SiGeC layers is quite complex for several

reasons, such as the extremely low solubility of C in Si

(10�4 at.% at the melting point of Si, 1420 8C) and in

Ge [5], the C tendency to form interstitial defect

complexes due to its small size, or the possible

presence of stable SiC precipitate, favoured at high

temperatures. Consequently, producing SiGeC films

with well-defined properties is strongly influenced by

the composition, structure and surface morphology,

features dependent on by the growth process.

Laser induced chemical vapor deposition (LCVD)

has proved to be a deposition method feasible for a

large variety of group IV semiconductors [6–12].

This method not only allows tailoring film properties

by controlling the precursor gas flows and substrate

temperature, but also by using various irradiation

geometries and laser fluences. Among the possible

geometries for ArF-LCVD the parallel configuration

is the most suitable for depositing uniform amor-

phous films [6,7]. In this geometry, the laser causes

the decomposition of the precursor gases near the

substrate surface without irradiating it, thus, avoid-

ing the substrate heating and the possible modifica-

tion of the growing film and the underlaying

substrate [13].

This work investigated the film properties depen-

dence on deposition parameters, such as the type of

reactant gas mixture and substrate temperature for

various Ge containing films deposited by ArF induced

chemical vapor deposition.

2. Experimental details

Various adherent Ge containing films were grown

on Si(1 0 0) and on Corning glass (7059) substrates by

ArF-LCVD in parallel configuration at a distance of

2.7 mm between the laser beam and the substrate. The

experiments were carried out in a hybrid self-made

stainless steel HV/UHV chamber at a base pressure of

0.03 mPa and connected to a gas supply handing

system described previously in detail [6]. The

depositions were performed for various substrate

temperatures Ts (180, 250, 320 and 400 8C) at a

constant total pressure of 1.2 kPa.

The SiGe and SiGeC films were grown, using a

mixture of 1 sccm of disilane (Si2H6), 1 sccm of

germane (GeH4) and 0 or 1 sccm of ethylene (C2H4)

diluted in He. These precursor gases enter through the

chamber by an adjustable nozzle in the near vicinity of

the substrate and were photolytically decomposed by

193 nm ArF-Excimer laser (Lambda Physik LPX

220i) radiation at 0.7 W cm�2 in the case of SiGe and

SiGeC films. Depositing pure Ge films with appro-

piate growth rates required the use of higher germane

flow (2 sccm) and laser power density (4.5 W cm�2).

OtherpureGefilmswerealsodepositedat5.3 kPafor

various Ts, in order to evaluate the influence of the total

pressure on the deposition rate and surface morphology.

The thickness of the coatings was characterised by

profilometry (Dektak3ST-Veeco) and the composition

of the films was determined by X-ray photoelectron

spectroscopy (XPS; Escalab 250iXL-VG Scientific)

using monochromatic Al Ka radiation at 1486.92 eV.

The surface morphology of the samples was studied by

atomic force microscopy (AFM, Discoverer-Topome-

trix) in contact mode and the surface roughness was

evaluated by the root mean square (rms) from the

AFM profiles.

X-ray diffraction was performed on a conventional

u–2u diffractometer (Rigaku, Geigerflex) in reflection

geometry and the diffraction patterns were acquired in

u–2u coupled mode using Cu Ka radiation

(l = 0.154 nm).

3. Results

The growth rate of homogeneous adherent Ge

containing films by ArF-LCVD was determined by

measuring the thickness of the films and the deposition

time. It is strongly influenced by some process

parameters, such as substrate temperature, total

pressure and reactant gas mixture. However, the

independence from the nature of the substrate was

demonstrated as no structural or compositional

differences were observed in Si(1 0 0) or Corning

glass (7059) substrates. A study of the growth rate

dependence on the substrate temperature Ts was

carried out for Ge, SiGe and SiGeC at constant total

pressure of 1.2 kPa (Fig. 1a). According to the results,

E. Lopez et al. / Applied Surface Science 248 (2005) 108–112110

the presence of a silicon source raises the growth rate

of the coatings, and even more if ethylene is added in

the mixture of precursor gases.

Surface roughness, studied by AFM analysis

(Fig. 1a), was affected in a different way by Ts for

SiGe and SiGeC alloys: an increase of Ts from 250 to

400 8C led to smoother surfaces for SiGe and rougher

ones for SiGeC films.

Abnormal high growth rates and surface roughness

were observed in SiGe and SiGeC samples deposited

at 180 8C. This may be attributed to the formation of

more porous hydrogen rich material with considerable

amount of voids especially for Si containing films, as

we verified by FTIR. This phenomenon increases

Fig. 1. RMS values and deposition rate dependence on substrate

temperature for pure Ge (*), SiGe (&) and SiGeC (~) films

deposited at 1.2 kPa (a) and for other pure Ge films deposited at

5.3 kPa (b).

drastically the film thickness and, consequently, the

deposition rate and roughness.

The composition of the coatings was investigated

by XPS. We observed that in SiGe and SiGeC films Si

was mainly incorporated (>60 at.%) in all the range of

Ts, since Si2H6 has higher absorption coefficient at

193 nm than GeH4 or C2H4.

SiGe samples were Ge-richer than SiGeC samples

deposited at the same conditions of Ts and pressure

(P = 1.2 kPa). In SiGe films, the highest Ge content was

found in the samples deposited at 320 8C (39.8%),

favoured possibly by the thermal decomposition of

GeH4 starting at 280 8C. While the beginning of Si2H6

thermal decomposition at 380 8C leads to a slight

decrease of Ge content (35.2%) in the SiGe film

deposited at 400 8C, this diminishing was not observed

in SiGeC films. In fact, the Ge content was 8.2% at

Ts = 320 8C and 11.4% at Ts = 400 8C. Additionally, in

SiGeC films we found higher values of C content for

180 8C (6.2%) and 250 8C (4.2%) than for 320 8C(1.7%) and 400 8C (2.5%). Thus, the presence of a C

source makes the deposition process more complex.

FTIR spectroscopy was performed to obtain

information concerning the bonding between the major

constituting elements: Si, Ge, H in SiGe samples, and

Si, Ge, C and H in SiGeC samples. Typical infrared

spectra of SiGeC films at different Ts are depicted in

Fig. 2, exhibiting various vibrational modes.

The modes between 1900 and 2000 cm�1 have been

associated to Si–H stretching vibrations in a SiHx group,

asSiHinvoidsor ina (SiH2)n groupappearat2100 cm�1

[14]. Near 1800 cm�1, Ge–H stretching modes emerge

with lower intensity. The band around 1000 cm�1

corresponds to the overlapping of the Ge–O (980 cm�1)

andSi–O(1040 cm�1)stretchingmodes,moreintenseat

lower Ts. The features near 840 and 885 cm�1 are

bending modes assigned to (SiH2)n or SiH2 species

[14,16] and at 640 cm�1 we observed SiH and SiH2

group rocking or wagging vibrations [15,17,18].

In SiGeC films (Fig. 2) appeared a peak at

680 cm�1, which may be assigned to Si–C stretching

mode [18,19]. More bands related to C–H vibrations in

CH3 and CH2 groups at 780 cm�1 (rocking or

wagging), 1250–1400 cm�1 (bending) and 2900–

3000 cm�1 (stretching) [15,17–19] were found.

FTIR results reveal a tendency of H atoms to

selectively bond to Si or C rather than to Ge, often

called ‘‘preferential attachment’’ [20,21], that can be

E. Lopez et al. / Applied Surface Science 248 (2005) 108–112 111

Fig. 2. FTIR spectra of SiGeC samples deposited using 1 sccm of

each precursor gas at 180 (a); 250 (b); and 320 8C (c) keeping the

total pressure constant at 1.2 kPa. Stretching (s), bending (b),

wagging (w) and rocking (r) vibrations are shown.

Fig. 3. (a) AFM surface morphology of a pure Ge film grown on a

Corning glass (7059) substrate using 2 sccm of GeH4 diluted in He at

400 8C and 5.3 kPa. (b) Raman spectra of pure Ge films grown on

Corning glass (7059) substrates at 5.3 kPa and 250, 320 and 400 8C.

caused by the smaller binding energy of H–Ge as

compared to those of H–Si or C–H. On the other hand,

Ge and C seem to be independently incorporated into

the film [21,22] as Ge–C bond vibrations (560 cm�1)

were not detected by FTIR at all studied Ts. This fact

was also verified by XPS.

A variation of the Ts for a particular gas flow

mixture affected the FTIR spectrum (Fig. 2). Hydro-

gen related peaks, especially the band situated around

2000 cm�1, and the Si–O stretching mode peak

decrease in intensity suggesting that low Ts favour

the incorporation of oxygen and hydrogen into the

films preferentially bonded to silicon.

For pure Ge samples, the influence of the substrate

temperature was investigated at 1.2 and 5.3 kPa.

Higher deposition rates were obtained when the Ts or

the total pressure were risen, keeping constant the

other deposition parameters, with two distinct

temperature regimes before and after �300 8C(Fig. 1). This can be explained by the mentioned

thermal decomposition of GeH4 at temperatures

>280 8C [23], involving a pyrolitic contribution to

the deposition process that led not only to higher

growth rates but also to a change in the film structural

properties, as confirmed by AFM, Raman and XRD.

AFM analysis revealed that rougher pure Ge

coatings were obtained at higher Ts or total pressure.

This could be related to the higher deposition rates,

obtained when both deposition parameters (Ts and P)

increase, or/and to a change in the film structure. The

roughness varied from 1.9 to 26.3 nm for 1.2 kPa and

from 3.4 to 75.4 nm for 5.3 kPa. Thus, the sample

deposited at 400 8C and 5.3 kPa was the roughest one,

with rms = 75.4 nm (Fig. 3a).

Raman spectroscopy analysis (Fig. 3b) showed that

at Ts = 250 8C for both 1.2 kPa and 5.3 kPa, films had

the typical Raman shift around 270 cm�1 for Ge–Ge

bonds in an amorphous structure. However, samples

deposited at higher Ts present a single peak or a broad

band at 300 cm�1, attributed to Ge–Ge vibration mode

for crystalline germanium [24].

X-ray diffraction measurements of pure Ge

samples evidenced the crystallisation of some films.

The background corrected XRD patterns demonstrate

the polycrystallinity of the films deposited at 320 and

400 8C, since (1 1 1), (2 2 0) and (3 1 1) reflections

E. Lopez et al. / Applied Surface Science 248 (2005) 108–112112

Fig. 4. X-ray diffraction spectrum of a Ge film deposited on

Corning glass (7059) at 5.3 kPa and 400 8C.

were observed at 2u values of 27.38, 45.38 and 53.78,respectively (Fig. 4). While samples deposited at

1.2 kPa were randomly oriented, those deposited at

5.3 kPa showed a preferential (2 2 0) orientation with

larger grains at 320 8C (40 nm) than at 400 8C(33 nm). The absence of any diffraction peak for

the rest of the layers deposited at lower temperatures

suggested an amorphous structure.

4. Conclusion

The results concerning the growth of Ge containing

films by ArF-LCVD at 1.2 kPa show that as higher

substrate temperatures were used higher deposition

rates and lower hydrogenation were observed. Surface

roughness and type of bonding are also affected by Ts

but in a different way in SiGe and SiGeC films. While

pure Ge samples were polycrystalline when

Ts � 320 8C, with rougher surfaces and larger grains

at P = 5.3 kPa, no evidence of crystallinity was found

for SiGe or SiGeC alloys.

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 U. Kosch,

C. Serra, F. Fabbri and L. Fornarini for their

extensive technical help and for fruitful discussions.

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