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Vacuum 82 (2008) 1525–1528

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Vacuum

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A growth rate, structure and surface morphology study of Si1-x-yGexCy filmsdeposited by ArF-LCVD in tilted geometry

E. Lopez a,*, S. Chiussi a, U. Kosch a, P. Gonzalez a, J. Serra a, C. Serra b, B. Leon a

a Dpto. Fısica Aplicada, Universidade de Vigo, Campus Universitario, Lagoas-Marcosende, 36310 Vigo, Spainb CACTI, Universidade de Vigo, Campus Universitario, Lagoas-Marcosende, 36310 Vigo, Spain

Keywords:ArF-LCVDGrowth rateRoughnessCrystallinitySiGeCTilted geometry

* Corresponding author. Tel.: þ34986812216; fax: þE-mail address: fani@uvigo.es (E. Lopez).

0042-207X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.vacuum.2008.03.018

a b s t r a c t

Ge, SiGe, SiC and SiGeC films were grown by ArF-Excimer laser induced chemical vapor deposition. Theresults demonstrate that in ArF-LCVD a fine and effective control of both the deposition rate, filmproperties and surface morphology is possible without altering the gas composition and pressure, bychanging exclusively the distance between laser beam and substrate surface or by the fact of irradiatingor not the films. Different distances have been achieved by tilting the sample 30� with respect to thebeam, a geometry simultaneously producing both, an irradiated and a no irradiated zone in the samefilm.The extensive characterisation of these films was carried out through different techniques in order to seethe influence of the irradiation geometry on composition, microstructure and roughness. The evaluationof the deposition rate and the XPS results revealed different growth rate behaviour along the filmwithout considerable variations in composition. AFM proved the small roughness of the films and itsstrong dependence on the laser beam to substrate distance. Raman spectroscopy and XRD were used todetermine the main structural properties. Additional information about surface morphology was alsoobtained through SEM.For pure Ge, some more studies have been performed due to the tendency of these films to show sig-nificant changes, especially in growth rate and roughness, both when the laser irradiated the samplesand when temperature, pressure and laser power were varied.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

After an extensive research on pure Si, Ge and C films for mi-croelectronic and optoelectronic devices, it has been demonstratedthat alloying Ge with Si lowers the optical band gap, thus improvingthe photoresponse in the longer wavelength region. This impliesthe development of silicon-based devices such as solar cells, pho-todetectors, thin film transistors (TFTs) and flat panel displays [1].On the other hand, alloying Si with C provides a widening of the Siband gap, increasing its applications in low wavelength absorbingdevices and window materials. The most recent goal was the ad-dition of small amounts of carbon into the SiGe system. Forming theternary alloy implies strain compensation [2] and, at the same time,band gap engineering [3] for advanced microelectronic devices.However, obtaining defect-poor high quality layers with well-de-fined properties is strongly influenced by composition, dislocationgeneration and surface morphology. Film properties are usuallydictated by the growth process. Consequently, the research on new

34986812135.

All rights reserved.

and flexible processing techniques that assure a fine control of thedeposition process is still of great importance.

Laser induced chemical vapor deposition (LCVD) has proved tobe a suitable technique for growing group IV semiconductors[4–12]. This method not only allows to tailor film properties bycontrolling the precursor gas flows and substrate temperature, butalso by the use of different irradiation geometries and laserfluences. Depending if the direction of the laser beam has a parallelor perpendicular orientation with respect to the substrate, theresulting material properties can be considerably different. Acombination of both geometries for studying the influence of thedistance to the laser on film properties is possible through a tiltedconfiguration in which the laser irradiates part of the substratedepending on the chosen angle between the laser beam and thesubstrate [13–15].

Other advantages of LCVD are that this technique allows de-positing in small selected regions as well as on large areas throughthe use of masks or lens and that it offers the possibility of singlechamber processing. Especially this last point makes the techniqueversatile and compatible with other conventional and also laserassisted processes, such as ELC (Excimer Laser Crystallisation) forobtaining nano- or micro- or polycrystalline coatings and PLIE

E. Lopez et al. / Vacuum 82 (2008) 1525–15281526

(Pulsed Laser Induced Epitaxy) for producing heteroepitaxiallayers [16,17].

The aim of this work is to demonstrate the effectiveness of ArF-LCVD in controlling deposition rate, structure and surface mor-phology by changing exclusively the distance between the loweredge of the laser beam and the substrate surface, called off-surfacedistance, or by the fact of irradiating the film or not.

2. Experimental

Different adherent Ge containing films were grown on Corning(7059) by ArF-LCVD in tilted configuration at 30� for simulatingdifferent off-surface distances. Attending to the dimensions of thelaser beam, it has to be noted that, at this angle, part of the sub-strate was irradiated by the laser (Fig. 1). This laser-irradiated zone,which is above the low edge of the laser beam, is identified withoff-surface distances negative values from �3.5 to 0 mm. The non-irradiated region comprises off-surface distances values from 0 to5.5 mm.

The experimental set-up consisted of a hybrid self-made stain-less steel HV/UHV chamber (base pressure of 0.03 mPa) connectedto a gas supply handing system that have been described in a pre-vious paper [4]. The substrate holder can be positioned at any anglefrom below to above the laser beam through a rotatable manipu-lator situated outside the vacuum chamber. During the LCVD pro-cess, the substrate temperature and the total pressure were keptconstant at 250 �C and 1.2 kPa, respectively.

The SiGe, SiC and SiGeC films were grown, depending on thedesired film, using a mixture of 1 sccm of disilane (Si2H6), 0 or0.5 sccm of germane (GeH4) and 0 or 4 sccm of ethylene (C2H4)diluted in He. These precursor gases enter the chamber through anadjustable nozzle in the near vicinity of the substrate and werephotolitically decomposed by 193 nm ArF-Excimer laser (LambdaPhysik LPX 220i) radiation with a constant power density of 0.7W/cm2 in the case of SiGe, SiC and SiGeC films. Depositing pure Gefilms at these substrate temperature and pressure required the useof higher germane flow (2 sccm) and laser power density (4.5W/cm2) for obtaining growth rates comparable with the alloys.

Taking into account the significant differences that pure Ge filmsshowed in contrast with other SiGe, SiC or SiGeC samples, someadditional studies have been performed varying the pressure

Fig. 1. Scheme of (a) the up-view of the sample holder of the UHV system for LCVD (b) thconfiguration (laser beam–substrate distance¼ 2.7 mm).

(5.3 kPa), the substrate temperature (400 �C) or the laser powerdensity (2.7 W/cm2).

For studying the influence of the different beam to substratedistance on the film properties, various aligned points situated inthe middle of the sample have been analysed in detail. The thick-ness of the coatings was characterised by profilometry (Dektak3ST-Veeco) and the composition of the films was determined by X-RayPhotoelectron Spectroscopy (XPS; Escalab 250iXL-VG Scientific)using monochromatic AlKa radiation at 1486.92 eV. The surfacemorphology of the samples was studied by Atomic Force Micros-copy (AFM, Discoverer-Topometrix) in contact mode and ScanningElectron Microscopy (SEM, XL30-PHILIPS). Surface roughness wasevaluated by the root mean square (RMS) of the AFM profile.

X-ray diffraction was performed on a conventional q–2q dif-fractometer (Rigaku, Geigerflex) in reflection geometry and thediffraction patterns were collected in q–2q coupled mode using CuKa radiation (l¼ 0.154 nm).

3. Results and discussion

All films showed two different behaviours of the deposition rate,depending on the existence of substrate laser irradiation (Fig. 2). Inthe not irradiated zone, the laser causes the decomposition of theprecursor gases near the substrate surface avoiding additionalsubstrate heating and the possible modification of the growing filmand the underlying substrate through laser radiation. In the irra-diated zone, the laser heats the substrate directly and the gasesnear the substrate surface are additionally heated by diffusion andconvection phenomena.

For SiGe, SiC and SiGeC samples produced at 250 �C and 1.2 kPa,the deposition rate reaches its highest value in the domain wherethe laser directly irradiates the substrate and the just depositedfilm. It keeps almost constant, around 3.5 nm/min for SiGe, 3.3 nm/min for SiC and 3.7 nm/min for SiGeC films, till the proximity of anoff-surface distance of zero. These high values found in the laser-irradiated zone are supposed to be influenced by a pyrolytic con-tribution added to the photolytic phenomenon that provokes thedeposition of the precursor gases.

However, a decreasing of the deposition rate with the off-surface distance is observed in the non-irradiated zone that can beexplained by gas phase chemical reaction of the growth precursors

e tilted configuration with laser partially irradiating the substrate and (c) the parallel

-4 -3 -2 -1 0 1 2 3 4 5 6

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

De

po

sitio

n R

ate

(n

m/m

in

)

Off-surface distance (mm)

Laser irradiated zone

Fig. 2. Deposition rate at different off-surface distances for Ge ( ), SiC (-C-), SiGe(---) and SiGeC (-:-) films obtained on 30� tilted glass substrates at 250 �C. Thepatterned area represents the laser-irradiated zone.

20 30 40 50 60 70 80

(311)

(220)

a

b

(111)

CP

S

2 theta (degrees)

Fig. 4. (a) Scanning electron micrograph, (b) X-ray diffraction spectrum of a non-irradiated zone of a pure Ge film deposited on Corning glass (7059) at 400 �C, 5.3 kPaand 4.5 mJ/cm2.

E. Lopez et al. / Vacuum 82 (2008) 1525–1528 1527

as they diffuse to the substrate. When the off-surface distance in-creased from 0 to 5.5 mm, the growth rate was reduced from 3.1 to0.7 nm/min for SiGe, from 3.1 to 0.9 nm/min for SiC and from 3.7 to1.4 nm/min for SiGeC. The similar values obtained for the Si con-taining samples at these deposition conditions shows that the ad-dition of C2H4 or/and GeH4 to the Si2H6 does not have a stronginfluence on the behaviour of the deposition rate observed inprevious experiments, realized in parallel configuration [4].

AFM analysis showed that SiGe, SiC and SiGeC films presentedlow surface roughness, particularly in the laser-irradiated domainof both sample types showing surfaces with constant RMS values ofz2 nm. Anyway, an enhancement of the roughness with increasingoff-surface distance was observed in the not irradiated zone,especially for Ge containing films. The RMS values at a beam tosubstrate distance of 2.7 mm were of 8.3 nm for SiGe, 2.6 nm for SiCand 6.5 nm for SiGeC. A reasonable explanation of this behaviourmight be that the longer path of the photo dissociated molecules,needed for reaching the substrate surface, favours the formation ofbigger sized particles. However, a typical structural feature, the

-5 -4 -3 -2 -1 0 1 2 3 4 5 60,0

0,5

1,0

1,5

2,0

25,0

27,5

30,0

32,5

35,0

37,5

40,0

Dep

ositio

n R

ate (n

m/m

in

)

Off-surface distance (mm)

Laser irradiated zone

Fig. 3. Deposition rate for pure Ge films obtained on 30� tilted glass substrates atdifferent off-surface distances with different deposition parameters: 400 �C, 5.3 kPaand 4.5 W/cm2 (-C-), 250 �C, 1.2 kPa and 4.5 W/cm2 (---) and 250 �C, 5.3 kPa and2.7 W/cm2 (-:-). The patterned area represents the laser-irradiated zone.

cauliflower structure that is reported to depend on the beam tosubstrate distance [18], was not observed by SEM.

The evaluation of the XPS composition analysis made for dif-ferent beam to substrate distances revealed that the compositionwas practically unaffected by increasing off-surface distance as itwas predicted in a previous paper [15]. Only insignificant variationswere found between the two distinguished domains showing thatcomposition is less affected than growth rate. An average compo-sition of the alloys of Si0.88Ge0.12, Si0.83C0.17 and Si0.85Ge0.11C0.04 hastherefore been calculated for the analysed binary and ternaryalloys.

For pure Ge coatings, some differences with respect to the Gecontaining alloys were found. The absence of a silicon source in theprecursor gases mixture leads to a considerable reduction of thedeposition rate in contrast to SiGe or SiGeC coatings (Fig. 2). It in-creases in the laser-irradiated zone, reaches a maximum that,depending on the substrate temperature, is situated in the prox-imity of the low edge of the laser or in the non-irradiated zone, andfinally decreases again.

Table 1Some RMS values obtained for pure Ge films grown on Corning glass at differentparameters conditions in tilted geometry (30�)

Off-surface distance RMS (nm) values for pure Ge films

250 �C, 1.2 kPa 250 �C, 5.3 kPa 400 �C, 5.3 kPa

�1.5 mm 16.3 2.2 31.40.63 mm 5.7 4.2 15.62.7 mm 4.6 3.1 14.6

E. Lopez et al. / Vacuum 82 (2008) 1525–15281528

The comparison among various pure Ge films deposited at dif-ferent deposition parameters (Fig. 3) revealed that the depositionrate was significantly affected by an increase of the total pressurefrom 1.2 to 5.3 kPa, but also that these changes can be compensatedby lowering the laser power from 13 to 8 W, respectively. It is alsoevident that an enhancement of the deposition rate values is ob-served when the substrate temperature is risen to 400 �C. Thepyrolitic contribution caused by thermal decomposition of GeH4 attemperatures >280 �C [19] led not only to higher growth rates, butalso to a considerable change in the film structure, as corroboratedby Raman spectroscopy analysis. Raman spectrum presents a singlepeak at 300 cm�1 attributed to Ge–Ge vibration mode for crystal-line germanium [20], both in the laser-irradiated and non-irradi-ated zone of the film deposited at 400 �C, which was not detectedfor the rest of the samples. In addition, SEM imaging (Fig. 4a) of thissample showed rougher surfaces evidencing sub-micro-crystalsthat have not been observed for the extremely smooth Ge filmsdeposited at lower substrate temperatures as studied in detailthrough AFM (Table 1).

No evidences of crystallinity in the Si containing samples couldbe found by X-ray diffraction measurements, the diffraction pat-terns of pure Ge samples evidenced the crystallisation of the filmsdeposited at 400 �C, 5.3 kPa. The XRD spectra demonstrate thepolycrystallinity of the film in both zones as (111), (220) and (311)reflections were observed at 2q values of 27.3, 45.3 and 53.7�, re-spectively (Fig. 4b).

The different behaviour of the deposition rate in the laser-irra-diated zone, which has only been observed for pure Ge samples,might be explained by possible ablation phenomena. It is veryprobable that the laser radiation reaches the low ablation thresholdof Ge that is estimated to be around 60 mJ/cm2 [21]. Consideringthe inhomogeneity of a conventional Excimer laser beam withouthomogeniser, it is obvious that the ablation increases when thegrowing film is irradiated with the inner, more intense, region ofthe beam corresponding to an off-surface distance of approxi-mately �4 mm. However, the process in this irradiated region isvery complex due to the fact that the impinging radiation alsoheated up the growing film, thus promoting an additional pyrolyticdecomposition of the precursor gas that should compensate theablation effect. This complex point requires a more detailed studythat will be performed in the near future.

4. Conclusion

The growths of various adherent Si1-x-yGexCy films have beenperformed by ArF-LCVD in tilted configuration. Introducing a tilting

angle of 30� between the laser beam and the substrate allowed toobtain an irradiated and a non-irradiated zone in the same sampleand, at the same time, simulate different distances, laser beam–substrate. We demonstrated that, while stoichiometry remainedconstant along the film, other film properties such as growth rate,surface roughness, surface morphology and structure varied ina different way depending on the composition of the coating andseveral deposition parameters such as substrate temperature, totalpressure or laser power.

Acknowledgements

This work has been partially supported by EU as well as bySpanish contracts and grants: HA1999-0106, XUGA32107BB92-DOG211, MAT2000-1050, MAT2003-04908, UV62903I5F4,PGIDT01PX130301PN and PR405A2001/35-0. The authors wish tothank J.B.Rodrıguez (Univ. Vigo) for his extensive technical help andfor fruitful discussions.

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