Carbon incorporation in silicon–carbon films grown at different substrate temperatures

Carbon incorporation in silicon–carbon films grown at different substrate temperatures

Thin Solid Films 515 (2007) 7634 – 7638 www.elsevier.com/locate/tsf Carbon incorporation in silicon–carbon films grown at different substrate tempera...

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Thin Solid Films 515 (2007) 7634 – 7638 www.elsevier.com/locate/tsf

Carbon incorporation in silicon–carbon films grown at different substrate temperatures U. Coscia a,b,⁎, G. Ambrosone c,b , P. Maddalena c,b , A. Setaro c,b , A.R. Phani a,d,e , M. Passacantando a,d,e b

e

a CNISM, Italy Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Napoli, Italy c CNR-INFM Coherentia, Italy d CASTI, Italy Dipartimento di Fisica, Università dell'Aquila, I-67010 Coppito (L'Aquila), Italy

Available online 16 January 2007

Abstract Hydrogenated microcrystalline silicon–carbon thin films have been deposited by plasma enhanced chemical vapour deposition technique at the substrate temperatures of 250 °C and 400 °C varying the radio frequency (RF) power in the 10–100 W range. The effects of substrate temperature and RF power on the structural, compositional, optical, and electrical properties have been investigated. The increase of substrate temperature or RF power leads to a decrease of crystallinity degree and an enhancement of carbon content. Optical absorption in the UV-visible region and electrical conductivity are affected in a different way by the RF power and substrate temperature variations. Silicon grain nucleation of films deposited at the temperature of 250 °C on commercial doped tin oxide substrate has been explored, for different RF power, by means of X-ray diffraction measurements. © 2006 Elsevier B.V. All rights reserved. Keywords: Microcrystalline silicon–carbon; RF-PECVD; Crystallinity; Absorption coefficient

1. Introduction Hydrogenated microcrystalline silicon–carbon (μc-Si1 − xCx: H) is an inhomogeneous material composed of Si and/or SiC crystallites of few nanometers embedded in an amorphous silicon–carbon matrix [1–3]. μc-Si1 − xCx:H shows higher conductivity, mobility and doping efficiency than the amorphous counterpart due to the presence of crystallites. On the other hand, carbon incorporation in silicon–carbon alloys enlarges the energy gap compared to that of microcrystalline silicon [4,5]. These properties justify the great interest devoted to μc-Si1 − xCx:H for applications such as solar cells, thin film transistors and electroluminescent devices [6–8]. In previous papers [9,10] it has been reported that, starting from silane–methane gas mixtures which are highly diluted in

hydrogen in a plasma enhanced chemical vapour deposition (PECVD) system, it is possible to tune the carbon content of the material by varying RF power at substrate temperature, TS, of 400 °C. Furthermore, it has been investigated [11,12] that the increase of carbon content in these films leads to a decrease of fraction and/or size of silicon crystallites. For device applications the deposition temperature has to be lower than 400 °C to avoid the damage of the substrate and/or interfaces [13]. Since RF power plays an important role on μc-Si1 − xCx:H growth it is useful to study, at low TS, the effect of RF power on carbon incorporation and degree of crystallinity. In this paper, data on the physical properties of samples deposited at TS of 250 °C as a function of RF power are presented and compared with ones obtained by μc-Si1 − xCx:H grown at 400 °C [9,10]. 2. Experimental procedure

⁎ Corresponding author. Dipartimento di Scienze Fisiche, Complesso Universitario MSA, Via Cintia, 80126 Napoli, Italy. Tel.: +39 081 676102; fax: +39 081 676346. E-mail address: [email protected] (U. Coscia). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.123

Hydrogenated silicon–carbon films have been deposited in a high vacuum PECVD system capacitively coupled to a RF generator of 13.56 MHz at the substrate temperature of 250 °C.

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Films have been grown by using, as a source for Si and C, silane (SiH4) and methane (CH4) gas mixtures highly diluted in hydrogen (H2). Films have been deposited onto different substrates namely Corning 7059 glass and commercial SnO2:F coated glass (Asahi type U). In all the depositions the pressure, the total gas flow rate, hydrogen dilution [H2] / ([SiH4] + [CH4]), and the methane fraction [CH4] / ([SiH4] + [CH4]), have been fixed at values of 226 Pa, 300 sccm, 250 and 0.50, respectively, while the RF power (w) has been varied from 10 to 100 W (RF power density range 0.065–0.65 W/cm2). The deposition conditions are close to ones used by some authors [14] to grow polymorphous silicon–carbon alloys (formed by a-Si1 − xCx:H matrix containing silicon nanocrystals) by plasma phase crystallization. For deposited films structural, compositional, optical and electrical characterizations have been performed. The structural properties have been investigated by X-ray Diffraction (XRD) measurements, in grazing incidence geometry (incident angle of 0.7°) using a Bruker D-5000 diffractometer, equipped with a Cu target and a solid state detector for scattered X-rays. A Göbel mirror has been used to collimate the incident X-ray beam and to select the Cu Kα emission line (Kα1 = 1.5406 Å, Kα2 = 1.5444 Å). The elemental contents of carbon and silicon are determined by combining the results of ion beam techniques such as 2.2 alpha-particle Rutherford Backscattering with 1.0 to 2.0 MeV proton non-Rutherford Backscattering along the entire film depth. The optical transmission spectra in the 200– 2500 nm range have been collected using a dual beam Perkin Elmer Lambda 900 spectrophotometer. From these spectra the absorption coefficient in UV-visible region and the thickness of films have been determined. The film thickness lays between 0.26 and 1.28 μm. Dark conductivity, σd, and photoconductivity, σph, under white light of 100 mW/cm2 (AM1 conditions), have been measured at room temperature in coplanar configuration using a Keithley 617 electrometer unit. 3. Results and discussion XRD spectra of μc-Si1 − xCx:H films deposited at TS = 250 °C in the RF power range of 10 to 100 W are shown in Fig. 1. The broad peak around 2θ = 27° is due to the glass substrate. Apart from the 100% intensity peak of Si (111), other diffraction planes (220) and (311) of c-Si have also been detected as a function of RF power. These diffraction planes have been identified at 2θ = 28.4°, 47.3° and 56.1°, respectively. In Fig. 1, the diffraction peak of (111) plane of film deposited at w = 35 W is scarcely detectable and the spectra of the ones grown at w ≥ 50 W are typical of amorphous films. The measured reflections belong to cubic phase of Si (JCPDS card #: 27– 1402). It is note worthy to mention that no diffraction planes associated with c-SiC or crystalline forms of carbon (sp3 or sp2) have been observed. In the inset of the Fig. 1, the normalized intensity of peak for the Si (111) diffraction plane widens with RF power from 10 W to 35 W indicating the decrease in the grain size. The average grain size (δ), calculated from the well known Scherrer's formula, has been plotted in Fig. 2(a) as function of

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Fig. 1. XRD spectra of silicon–carbon films deposited on Corning glass at TS of 250 °C and different RF powers. (Inset shows the normalized intensity peak for the diffraction plane (111) of some XRD spectra.)

RF power. For comparison δ values for the films deposited at 400 °C have also been depicted. The crystalline volume fraction ( f ) were calculated by Raman spectra reported elsewhere [9,15] and plotted against w in the Fig. 2(b). f of both sets deposited at different TS decreases as a function of RF power and a transition from microcrystalline to amorphous phase occurs. Since in the whole RF power range δ and f are higher for thin films deposited at T S = 250 °C, it can be inferred that the crystallization process is favoured at lower TS. It is known that in μc-Si:H growth the increase of RF power density leads to a deterioration of crystallinity because of the augment of ionic bombardment of the film surface [16], while the increase of TS, in the 250–400 °C range, improves the crystallinity as a consequence of an enhanced surface mobility of precursors, which can diffuse and form more ordered structures [16]. In order to explain the behaviour of the crystallinity as a function of w and TS, for μc-Si1 − xCx:H, besides the influence of ionic bombardment and mobility of precursors, also the effect of carbon content has been considered. The carbon content (x, x = C / (C + Si)) versus w has been plotted in the Fig. 3 for both TS. It is evident that with increasing either RF power or TS, x increases. Since the crystalline phase is composed of Si crystallites, the carbon is incorporated only in the amorphous phase of the deposited films. The increase of RF power increases the number of dissociated CH4 molecules leading to an augment of carbon incorporation in films [17,18]. The introduction of C atoms into Si network [11,12] causes distorted bond lengths and angles, which prevent the formation of Si crystallites, worsening the effect of ion bombardment of film surface on crystallization process. TS effect on carbon content can be explained [19], on the basis of assumption that the increase of TS induces an enhancement of the mobility of hydrocarbon radicals and a more reactive surface, which promotes the adhesion of the precursors to the growth sites. This mechanism enhances the carbon content in the film [19] leading to more disorder in the network that hinders the formation of Si crystallites, contrary to what happened in the case of μc-Si:H deposited by SiH4 + H2 gas mixtures.

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Fig. 2. a) Grain size (δ) and b) crystalline volume fraction ( f ) vs. RF power for films deposited at two different substrate temperatures (250 °C and 400 °C).

Fig. 4. Absorption coefficient (α) of silicon–carbon films deposited at TS of 250 °C and 400 °C for different RF powers w: a) 10 W; b) 35 W; and c) 100 W.

As regards the structural properties of the amorphous phase, infrared spectra of the films, reported elsewhere [15], show that no C-C vibrational modes are detectable while Si–H and Si–C bond concentrations increase. In these alloys carbon is mainly bonded to silicon and both carbon and hydrogen content may affect optical and transport properties. In the following only the effect of carbon content on the film properties will be discussed.

Optical absorption spectra in the UV-visible region, of silicon–carbon films at low and high TS is shown in the Fig. 4 (a)–(c). With increasing RF power, the absorption spectra at TS = 400 °C shift toward higher energies as shown in the curves 1, 3, and 5, respectively. For TS = 250 °C, up to w = 35 W the spectra enhance toward low energy, in a wide wavelength range, then, turn back for w N 35 W as shown in curves 2, 4, and 6, respectively. Furthermore, from Fig. 4(a)–(c), the TS effects on

Fig. 3. Carbon content x vs. RF power in the films deposited at two different substrate temperatures (250 °C and 400 °C).

Fig. 5. Dark conductivity (σd) and photoconductivity (σph) as a function of the RF power for silicon–carbon films deposited at TS of 250 °C and 400 °C.

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Fig. 6. XRD spectra of silicon–carbon films deposited on SnO2:F substrate at TS = 250 °C for different RF powers. (⁎) indicates the SnO2:F substrate reflections.

the optical properties of the films deposited at different RF powers can be observed. Indeed, the increase of TS from 250 °C to 400 °C, at w = 10 W causes a shift of the spectrum towards low energy (Fig. 4(a)), at w = 35 W there are no significant differences between the spectra (Fig. 4(b)), while at w = 100 W the augment of TS shifts the absorption spectrum toward high energy. All these trends can be interpreted pointing out that with increasing RF power or TS, the enhancement of the absorption coefficient in the UV-visible region, due to the decrease of crystallinity fraction, is hindered by the increase of carbon content, which promotes the lowering of the absorption spectra. This argument can be used to describe the experimental data shown in Fig. 4(a)–(c). As regard TS effects, at w = 10 W the absorption is mainly governed by the crystalline phase, because both films have high degree of crystallinity and low carbon content (see Figs. 2 and 3). Thus, the film deposited at lower TS, having higher degree of crystallinity, is more transparent (Fig. 4(a)). At w = 35 W, increasing TS the enhancement in the absorption was expected, due to the lowering in degree of crystallinity, but it is balanced by the effect of the increase in carbon content. At w N 35 W the absorption is mainly governed by the amorphous silicon–carbon phase (see Figs. 2 and 3), than films deposited at TS = 400 °C, having higher carbon content, are more transparent (see Fig. 4(c)). In order to investigate the electrical properties of the deposited films, dark conductivity (σd) and photoconductivity (σph) have been plotted as a function of RF power as shown in the Fig. 5. σd decreases several orders of magnitude with increasing w for both sets of samples. This result could be attributed to a combined effect of the decrease of crystallinity and the increase of carbon content. TS has less effect than RF power on σd variation. Indeed at higher TS, σd is slightly lower for w ≤ 35 W and it is lower than approximately one order of magnitude for w N 35 W, because the amorphous phase becomes predominant. Photoconductivity is affected by RF power as σd, but it is not significantly influenced by TS in all the investigated w range except for the film deposited at w = 15 W.

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In device applications, it is necessary to deposit films on glass substrate coated by a transparent conducting oxide where a low substrate temperature is required. In order to explore silicon grain nucleation on commercial doped tin oxide (TO), a set of silicon–carbon thin films were deposited on SnO2:F (Asahi type U) substrates at the temperature of 250 °C with different RF power from 10 W to 100 W (see par. 2). Fig. 6 presents XRD spectra of some silicon–carbon films deposited on SnO2:F. The diffraction peaks from the reflection planes of Si and reflections of SnO2 substrate in the 2θ range from 20° to 70° have been recorded. At w ≤ 35 W the diffraction peaks of Si such as (111), (220) and (311) decrease as a function of RF power. At w = 50 W only a minute structure at 2θ = 28.4° could be detected revealing the presence of Si crystallites in the (111) direction. On the other hand, films deposited at w N 50 W exhibit amorphous behaviour. By comparing these results with those obtained from the films deposited on Corning glass (Fig. 1), it can be deduced that SnO2 substrate favours the growth process of Si crystallites over a wider RF power range. Moreover, it can be noticed that the diffraction peaks of SnO2 strongly decrease with increasing w denoting a degradation of the TO layer. This behaviour can be attributed, as in the case of μc-Si:H growth [20,21], to a strong chemical reduction of the top surface of TO exposed to high power plasma highly diluted in H2. The degradation of TO film affects TO/μc-Si1 − xCx:H interface limiting the device efficiency. More work has to be carried out in order to individuate suitable deposition conditions for a better interface fabrication. 4. Conclusions Structural, optical and electrical properties of two sets of silicon–carbon samples deposited at TS of 250 and 400 °C have been investigated. It has been observed that with increasing RF power or substrate temperature the degree of crystallinity decreases and carbon content increases in the films. The behaviour of the optical absorption in the UV-visible region vs. RF power and TS have been described in terms of the enhancement of absorption coefficient, due to the decrease of crystallinity and the lowering of the absorption spectra, caused by the increase of carbon content. Electrical properties are strongly affected by the RF power, while they are not significantly influenced by TS. It has been observed that SnO2:F substrate, compared to Corning glass, favours the growth of Si crystallites over a wider range, however a degradation of tin oxide layer takes place at higher RF power. References [1] [2] [3] [4]

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