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p-type crystalline silicon heterojunction diodes
Journal of Non-Crystalline Solids 321 (2003) 175–182 www.elsevier.com/locate/jnoncrysol
Effects of fluorine incorporation on the properties of amorphous carbon/p-type crystalline silicon heterojunction diodes L. Valentini, V. Salerni, I. Armentano, J.M. Kenny *, L. Lozzi, S. Santucci Department of Civil and Environmental Engineering, Materials Engineering Center, Universit a di Perugia, Loc Pentima Bass 21, 05100 Terni, Italy Dipartimento di Fisica - Unit a INFM, Universit a dellÕAquila, 67010 Coppito (AQ), Italy Received 14 June 2002; received in revised form 20 January 2003
Abstract Heterojunction diodes fabricated by plasma enhanced chemical vapor deposition of hydrogenated amorphous carbon (a-C:H) and fluorine-doped amorphous carbons (a-C:H:F) on p-type silicon are analyzed in terms of their electronic and photovoltaic properties. Their structural and optical properties were identified by Raman spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and UV–VIS transmittance. The nature of the heterojunction is confirmed by the rectifying current–voltage characteristic of carbonaceous deposits/p-Si junction. The diodes show a behavior dependent on the amount of the fluorine content. The photovoltaic behavior of the junction is investigated as a function of both fluorine incorporation and thermal treatment of the a-C:H:F films after the deposition. Better photovoltaic effects were observed from annealed a-C:H:F heterojunction structures. The optical and structural characterization performed on films after the thermal treatment indicates that this behavior is most likely due to an extended graphitization. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H) and/or diamond-like carbon (DLC), have attracted attention as an en-
*
Corresponding author. Address: Department of Civil and Environmental Engineering, Materials Engineering Center, Universit a di Perugia, Loc Pentima Bass 21, 05100 Terni, Italy. Tel.: +390-744 492 939; fax: +390-744 492 925. E-mail address: [email protected] (J.M. Kenny).
vironmentally benign and economically viable opto-electronic device material above others, such as amorphous silicon (a-Si and/or a-Si:H) due to various advantages [1]. The application of amorphous carbon as a semiconductor in opto-electronic devices such as photovoltaic solar cells has been recently attempted [1–5], though it is still rather limited to date. Moreover, the reported researches are practically limited to heterojunctions of configurations, a-C or a-C:H or DLC on well known single crystal semiconductors such as Si, GaAs, InP. The recent successful attempts to
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L. Valentini et al. / Journal of Non-Crystalline Solids 321 (2003) 175–182
increase the conductivity of carbonaceous materials [6,7] using phosphorus and nitrogen in the solid and gaseous phase, respectively, seem to have improved the prospects for their use with Si in heterojunction bipolar transistors. In fact amorphous carbon and its hydrogenated forms showed more than 10 orders of magnitude differences in room temperature conductivity depending on how they were prepared [8,9]. Such a large variation in electrical behavior can be expected due to the diverse bonding possibilities of carbon atoms. It is generally accepted that, p states of sp2 hybridizated carbon atoms dominate electrical properties in amorphous carbon because the r states of both sp2 and sp3 hybridizated carbons are further away from the Fermi level [10,11]. Electronic structures of amorphous carbon other than sp2 content are expected to vary with clustering properties of sp2 sites [10] which is also affected by thermal treatment after the deposition process. Thus, the proportion of sp2 carbon atoms and their arrangement in smaller or larger clusters determine the optical gap and have a significant influence on the electrical transport as well. The main motivation of this work is to compare the sp2 fraction in undoped and fluorine-doped (a-C:H:F) films with their optical band gap, which has the ability of performing as a part of a heterojunction diode. For this purpose a-C:H and a-C:H:F films were deposited using the radiofrequency glow discharge plasma deposition system in an ambient of pure CH4 and CH4 /CF4 , respectively. Ellipsometry and UV–VIS spectrometry were then applied to investigate the macroscopic optical behavior of the films, studying both the refractive index and the optical gap as a function of fluorine content in the film, before and after thermal treatment. Raman spectroscopy was used to explore the evolution of the sp2 carbon atom bonding during annealing. The modification of the chemical bond states upon fluorine incorporation in the films was studied using X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. After structural characterization, they were examined regarding their ability to perform as parts of different electronic devices. Thus, the DLC films on silicon were subjected to current–voltage (I–V ) measurements after subsequent metalization
in vacuum with aluminium. The a-C:H films were subsequently prepared adding fluorine and annealed in the air: the role of the amount of fluorine incorporation and the effects of thermal treatment were then examined in terms of the device photovoltage response.
2. Experimental details Amorphous fluorocarbon films were deposited on (1 0 0) silicon substrates (resistivity of 1–12 X cm) by the plasma-assisted decomposition of CF4 –CH4 mixtures employing a 13.56 MHz radiofrequency plasma system. Before loading it into the chamber, the substrate (10 mm 10 mm 0.5 mm) was cleaned in an ultrasonic bath of acetone to remove residual organic contaminants, washed in deionised water and nitrogen dried. Prior to deposition, the chamber was evacuated (102 Pa), and then Ar was introduced for sputter cleaning in order to eliminate any impurity on the substrate (conditions: )200 V bias, 10 Pa, 5 min). These pretreatments are typically applied to remove the oxide layers. Subsequently, the chamber was again evacuated to 102 Pa. The film depositions were carried out at room temperature while the deposition pressure was fixed at 53 Pa. The controlled process variable was the dilution of CH4 with CF4 . The ratio of the CF4 gas flow rate to the total precursor gas flow rate, r ¼ [CF4 ]/([CF4 ]+[CH4 ]), was fixed at 0% and 62% while the total gas flow rate was kept constant at 40 sccm. The film deposition was performed with a RF bias voltage fixed at )400 V, resulting in a thickness of carbonaceous thin film of about 100 nm. Afterwards film deposition thermal annealing on a-C:H:F films was carried out in an atmospheric oven for 1 h at 300 °C. The front and back contacts of the diodes were made with Al electrodes by depositing a film of thickness of 40 nm on top of the carbonaceous thin film and on the bottom of the p-Si substrate, using a conventional vacuum evaporation system. Ohmic behavior of the contacts was checked by measuring the current–voltage characteristics. The photovoltaic behavior of the fabricated heterojunctions was investigated under the illumination of a lamp with a wavelength ranging
L. Valentini et al. / Journal of Non-Crystalline Solids 321 (2003) 175–182
from 400 to 800 nm. Information about the chemical bonding and the elemental composition of samples were obtained by XPS (monochromatized Al Ka source, hm ¼ 1486:6 eV, resolution 0.3 eV). Raman scattering spectra were recorded by a notch filter system in backscattering geometry. A 632.8 nm (1.96 eV) He–Ne laser was used as the light source and the power of the laser was adjusted by optical filters. By using a 100 objective lens, the illuminated spot on the sample surface was focused to about 2 lm in diameter. The resolution of Raman spectra was better than 1 cm1 . Typical acquisition time for the spectra was 30 s. Infrared transmission spectroscopy (IR) was performed in the air using a FTIR spectrophotometer in the 2500–3500 cm1 range. Spectra were normalized with respect to film thickness to allow a direct comparison between different samples. The refractive index was measured at a fixed angle of 70°, by an ellipsometer, operating at three different wavelengths: two in the infrared, at 1530 and 1300 nm, and one in the visible at 632.8 nm. Films of 100 nm thickness were measured with a UV–VIS spectrometer in order to evaluate the transparency in the visible light range of 350–900 nm. The absorption coefficients were calculated based on the transmittance measurements and the thickness data. The optical band gap was then determined (Fig. 1) by fitting the absorption data 1=2 to the Tauc relation, ðahmÞ ¼ Bðhm Eg Þ, where a is the absorption coefficient as a function of photon energy, Eg is the optical gap and B is proportional to the joint density of the states. The
900
]
600
(αh ν)
1/2
[cm
-1/2
-1/2
700
eV
800
r = 62% As Deposited
177
I–V characteristics were measured with a precision semiconductor parameter analyzer.
3. Results As reported in Table 1, the deposition rate increases with CF4 partial pressure in the deposition chamber, while relative changes in the normalized film thickness after 1 h of annealing at 300 °C become more appreciable when CF4 is added in the plasma. Since the main precursors of a-C:H:F growth are CFx radicals [12], the higher deposition rate observed in the CF4 rich plasma atmosphere could be attributed to higher CFx radical density. Under annealing, the thickness of the films decreases up to 70% of their as deposited thickness for films grown with CF4 in the plasma atmosphere. Thus, the observed film shrinkage could be explained in terms of desorption of chemical species containing fluorine. XPS has been used as a diagnostic tool to detect the amount of fluorine atoms. C1sÕ XPS spectra of the film deposited with r ¼ 62% before and after annealing are shown in Fig. 2. The XPS C1sÕ results for the fluorine free film generally shows C1sÕ peak position located around 284.5 eV, which can be associated with an aromatic structure (pure graphitic) [13]. Upon fluorine incorporation the peaks were deconvoluted into two components: C– CF at 287 eV and CF at 289 eV [13,14]. Generally, the increase of CF4 concentration enhances the degree of fluorination, as indicated by the features assigned to C–CF and CF groups. Fluorine to carbon ratios, before and after annealing, obtained by the integrated intensities of XPS C1sÕ and F1sÕ features (inset of Fig. 2) and by the intensities of the carbon component (fitting of C1sÕ level) are reported in Table 2. For the as deposited films, a
500 400 300
After Annealing
200 100 0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Table 1 Deposition rate as a function of CF4 ratio and thickness of the films after annealing normalized by their as deposited thickness CF4 /(CF4 +CH4 ) [%]
Deposition rate [nm/min]
Normalized thickness [%]
0 62
41 1 71 1
99 1 71 1
4.5
Energy [eV] Fig. 1. Tauc plots of a-C:H:F and a-C:H:F annealed films.
a) F1s
680
684
688
692
Binding energy [eV]
II)
b)
1. as deposited 2. after thermal annealing
Absorbance [a. u.]
I)
Intensity [counts]
Intensity [counts]
Intensity [counts]
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Intensity [counts]
178
1. as deposited 2. after thermal annealing
1 2 1
2
F1s
2800
2900
3000
1080 -1
Wavenumber [cm ]
680
684
688
692
Binding energy [eV]
280 282 284 286 288 290 292 294 296 298
Binding energy [eV] Fig. 2. Comparison of C1sÕ photoemission spectra from the sample grown with r ¼ 62% recorded before (I) and after (II) annealing at 300 °C.
value of F/C area ratio equal to 14% was obtained when the CF4 was introduced into the plasma chamber. While this value corresponds to the total fluorine content in the film, a lower percentage of fluorine (Table 2) is chemically bonded to the carbon matrix. From Table 2, it is evident that the total fluorine content in the films after annealing mostly corresponds to that chemically bonded to carbon before the thermal treatment. The chemical bonding of carbon atoms to hydrogen and fluorine was also investigated by IR spectroscopy. In Fig. 3 the transmittance spectra of a-C:H:F film taken before and after annealing are presented. The main features of the IR spectra are two broad bands, one in the region between 1000 and 1300 cm1 , corresponding to CFx modes [15], and the second in the 2700–3200 cm1 region
1120
1160 -1
Wavenumber [cm ]
Fig. 3. IR spectra from the sample with r ¼ 62% recorded before and after annealing at 300 °C showing the CHn stretching region (a) and CFx stretching region (b).
due to CHn vibration modes [16]. While the intensity of the CFx band remains almost constant after annealing (Fig. 3(b)), the intensity of the CHn band decreases (Fig. 3(a)). From the spectra comparison in the CFx absorbing region (Fig. 3(b)), it is clear that the amount of bonded fluorine incorporated into the film is not strongly affected by thermal annealing. In order to achieve a structural investigation, Raman spectra recorded from the different samples between 1000 and 2200 cm1 were measured before and after annealing and are reported in Fig. 4, where the common features of amorphous carbon materials can be observed [17–20], i.e., the D (disorder) and G (graphitic) Raman bands. The main G band peak is located at about 1530–1550 cm1 while the D band appears centred at about 1370 cm1 in the form of a shoulder of the main G band. Each spectrum was least square fitted by two Gaussian curves superimposed to a linear background to take information on the evolution of the sp2 carbon bonds of the films. The frequencies of the D and G bands, as well as their intensities and
Table 2 Comparison of fluorine to carbon ratios obtained by the integrated intensities of XPS C1sÕ and F1sÕ signals and by the intensities of the carbon components (fitting of C1sÕ level), from films as deposited and annealed at different feed composition CF4 /(CF4 +CH4 ) [%]
F/C (F tot. before annealing) [%]
F/C (F bounded before annealing) [%]
F/C (F tot. after annealing) [%]
62
14 1
61
51
Intensity [arb.units]
L. Valentini et al. / Journal of Non-Crystalline Solids 321 (2003) 175–182
r = 62%
r = 0%
1000
1500
2000 -1
Wavenumber [cm ]
Intensity [arb.units]
(a)
1000
(b)
r = 62%
r = 0% 1500
2000 -1
Wavenumber [cm ]
Fig. 4. Raman spectra of a-C:H films grown with different r values before (a) and after (b) annealing.
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widths were used as fitting parameters. Results of this fitting procedure are reported in Table 3, in which the ID =IG ratio, the position and width of the G band are reported as a function of the CF4 content in the plasma chamber. Generally, when the CF4 is added to the plasma, the ID =IG ratio also grows, while the G bandwidth tends to decrease. The ID =IG ratio remains constant while the G band shifts to higher wavelengths decreasing in width for the annealed a-C:H:F sample. The results of the optical characterization performed at a wavelength of 632.5 nm (Table 4) indicates that, as the CF4 is introduced, a decrease of the refractive index is observed and the measured values remain practically unaffected by thermal treatment. The calculated optical band gap is also shown in Table 4. Associating the fluorine concentration in Table 2 with the data in Table 4, it is possible to conclude that increasing fluorine content decreases the optical gap. For a fixed plasma composition instead, the optical gap tends to remain almost constant after the thermal treatment. The current–voltage characteristics of the a-C: H:F/p-Si cell, without light irradiation, displayed rectifying characteristics (Fig. 5). Having confirmed the ohmic characteristics of the contacts we attribute this rectifying behavior to the interface of the n:p junction between the semiconducting a-C: H:F films and silicon. For comparison a-C:H/p-Si cell does not exhibit a rectifying behavior. This is
Table 3 Raman data from the as deposited and annealed films as a function of CF4 fraction: G peak position, G peak width, and intensity ratio of the D and G bands (ID =IG ) CF4 /(CH4 +CF4 ) [%]
0 62
Before annealing
After annealing
G band position [cm1 ]
G band width [cm1 ]
ID =IG
G band position [cm1 ]
G band width [cm1 ]
ID =IG
1532 1538
157 143
0.33 0.60
1535 1550
146 132
0.38 0.55
Table 4 Refractive index and optical band gap of a-C:H films grown with different r values before and after annealing at 300 °C CF4 /(CH4 +CF4 ) [%] 0 62
Tauc gap [eV]
Refractive index
As deposited
After annealing
As deposited
After annealing
1.2 0.5
1.1 0.4
2.19 1.81
2.08 1.84
L. Valentini et al. / Journal of Non-Crystalline Solids 321 (2003) 175–182
2.0x10
-3
1.5x10
-3
1.0x10
-3
5.0x10
-4
0.01
a-C:H:F/p-Si
1E-3 2
Current [A/cm ]
2
Current [A/cm ]
180
0.0 -3
1E-4 1E-5 1E-6 1E-7 1E-8
-2
-1
0
1
2
3
1E-9
Voltage [V]
-3
Fig. 5. The current–voltage characteristics of the a-C:H:F/p-Si heterojunction diode in the dark.
a-C:H:F Annealed -2
-1
0
1
2
3
1
2
3
Voltage [V]
(a) 1E-3
4. Discussion The experimental findings suggest that chemical bonding of carbon and fluorine atoms, more than
2
Current [A/cm ]
1E-4 1E-5 1E-6 1E-7 1E-8 1E-9
a-C:H:F as deposited
1E-10 -3
-2
-1
0
Voltage [V] (b) 1E-3 1E-4 2
Current [A/cm ]
likely due to the band offset of a-C:H:F/p-Si being more asymmetric than for a-C:H film, so it is easier to conduct holes across the junction. The photovoltaic behavior of the fabricated heterojunction diodes is depicted in Fig. 6. From this figure, it is clear that the a-C:H heterojunction structures are not sensitive to light and thus no photovoltaic behavior was detected. On the contrary, when a-C:H:F and a-C:H:F annealed heterojunctions structures are exposed to the light, the reverse bias current increases by about a two order magnitude, i.e. the reverse bias current of a-C:H:F heterojunction increases from 1.46 104 to 1.07 103 A cm2 with a reverse bias of 2.5 V. The photovoltaic parameters, such as the open circuit voltage (VOC ), short circuit current (ISC ) and the fill factor (FF) can be extracted from the I–V measurements. The VOC and ISC of the a-C:H:F heterojunction structures are about 0.24 V and 7.5 109 A cm2 , and the VOC and ISC of the annealed aC:H:F hetereojunction structures are about 0.38 V and 1.47 106 A cm2 , respectively. The ISC of the annealed a-C:H:F heterojunction is about three orders higher than the a-C:H:F heterojunction. The FF is 0.26 and 0.22 for a-C:H:F and annealed a-C:H:F heterojunction structures, respectively.
1E-5 1E-6 1E-7 1E-8 a-C:H/p-Si
1E-9 -3
(c)
-2
-1
0
1
2
3
Voltage [V]
Fig. 6. I–V curves for the a-C:H, a-C:H:F and annealed aC:H:F heterojunction diodes in the dark (straight line) and under illumination (dashed line).
the total fluorine concentration, is a decisive factor to improve the photovoltaic behavior of a-C:H:F films. XPS results indicated that an increase of the total fluorine concentration in the as deposited films is associated with an increase in the fraction
L. Valentini et al. / Journal of Non-Crystalline Solids 321 (2003) 175–182
of unbounded fluorine. In addition, under thermal annealing, IR spectra indicated that C–H bonds [21] (binding energy: 3.5 eV) are broken more easily than C–F bonds (binding energy: 5.6 eV), whose content remains unchanged after the thermal treatment. Combined XPS and IR results show that the total fluorine content after annealing is equal to the amount of fluorine bonded to carbon into as deposited samples. The chemical investigations clearly evidence that only hydrogen bonded to carbon and unbounded fluorine desorb from the film at 300 °C. Moreover the absence of CF2 and CF3 features in the XPS spectra indicates that the formation of one dimensional chains are prevented in our films. The CF2 and CF3 groups are considered weaker against thermal annealing than more crosslinked structures which are typical of diamond-like fluorine free films. This structure is preserved by the fluorine incorporation in the CF bonding configuration. Moreover, it cannot be excluded that the observed two order magnitude increase in current in the fluorine-doped heterojunctions is due to a structural rearrangement occurring after fluorine incorporation. This current increase may be due to the conversion of the diamond-like sp3 group to the graphite-like sp2 group, when the sample is doped. On the samples obtained with CF4 added in the plasma chamber, the ID =IG ratio grows while the G bandwidth tends to decrease. Similar results, reported for a-C:H [19,20] films, are attributed to an increase of the sp2 hybridizated carbon domains. According to this interpretation, for the as deposited a-C:H:F films, as the fluorine content increases the sp2 carbon sites begin to condense into clusters of increasing size. In particular, the increasing intensity of the D band clearly indicates that the sp2 sites begin to form aromatic clusters and the related increasing ID =IG ratio can be referred to as a measure of the ordering of the sp2 carbon atoms in the film [22]. The formation of larger clusters of sp2 coordinated carbon atoms under fluorine incorporation decreases the distribution of the barrier for p electrons and the increasing p electron delocalization [10] results in a more pronounced enhancement of conductivity for fluorine-doped samples with respect to a-C:H films. Upon thermal an-
181
nealing, the ID =IG remains almost constant for each sample, indicating that a further clustering of carbon atoms is not thermally induced. The observed shift upward of the G peak position for the annealed samples is higher for a higher fluorine content in the film suggesting that carbon atoms, due to hydrogen loss, can recombine in a lower stable configuration (sp2 ), resulting in a form of ordered graphite by thermal annealing. In these samples, the observed similar photovoltaic behavior with a smaller extent of changes caused by annealing is a consequence of the already higher degree of clustering in unannealed states proven by the Raman spectra. From these considerations, it is clear that p states of sp2 carbon atoms form tail states connecting to the valence band and conduction band and giving rise to the density of localized states in the gap. In this way, the concentration of sp2 carbon atoms and their arrangement in smaller or larger clusters affect the optical gap and electrical conductivity as well. It is already known [23] that the main structural feature of both fluorine free and fluorinated amorphous carbon films is the presence of both sp3 and sp2 carbon. For as deposited films without CF4 in the plasma chamber, the material is highly sp3 bonded with only short sp2 chains dispersed into the sp3 matrix. For these films, the small number of sp2 sites limits the formation of graphitic clusters so that the measured refractive index remains high (Fig. 5). Upon fluorine incorporation, the a-C:H sp3 diamond-like matrix collapses into sp2 increasing carbon domains (Fig. 3), then the refractive index also decreases. For the interpretation of the results obtained for the annealed samples, one should take into account the recent model proposed by Ferrari and Robertson [19]. These authors showed that there is a relationship between the optical properties and the graphite cluster sizes in a-C:H films; i.e. the Raman ID =IG peak ratio is found to vary inversely with the square of the gap. In our case the invariance of the ID =IG values, combined with the unchanged bonded fluorine content upon thermal annealing, may explain the invariance of both the refractive index and optical gap. This could be understood considering the enhancement of the sp2 fraction, which does not lead to a clustering of graphitic nanocrystallites.
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5. Conclusions To summarize, the a-C:H:F and annealed aC:H:F heterojunction diodes were successful fabricated and a photovoltaic effect was observed in the a-C:H:F and in the annealed a-C:H:F heterojunction structures. The analysis on the I–V characteristics under dark and illumined conditions indicates that the annealed a-C:H:F heterojunction structure show more promising photovoltaic behavior than the a-C:H:F one. The photovoltaic effects are explained using the results of the structural investigation, which shows that by raising the fluorine content in the film, the sp2 carbon sites undergo a gradual ordering into graphitic domains of increasing size and/or number, while the cluster size practically remains unaffected by thermal treatment at 300 °C. These results are explained in terms of a desorption of unbounded fluorine and the partial removal of hydrogen bounded to carbon after the thermal annealing in our a-C:H:F films. References [1] T. Soga, T. Jimbo, K.M. Krishna, M. Umeno, Int. J. Modern Phys. B 14 (2000) 206. [2] V.S. Veerasamy, G.A.J. Amaratunga, J.S. Park, W.I. Milne, H.S. MacKenzie, D.R. McKenzie, Appl. Phys. Lett. 64 (1994) 2297. [3] M. Maldei, D.C. Ingram, in: 13th European PVSC, 1995, p. 1258.
[4] N. Konofaos, C.B. Thomas, J. Appl. Phys. 81 (1997) 6238. [5] T. Soga, T. Jimbo, K.M. Krishna, M. Umeno, Carbon 37 (1999) 531. [6] V.S. Veerasamy, G.A.J. Amaratunga, J.S. Park, W.I. Milne, H.S. MacKenzie, D.R. McKenzie, IEEE 42 (1995) 577. [7] L.K. Chean, X. Shi, E. Liu, J.R. Shi, Appl. Phys. Lett. 73 (1998) 2473. [8] B. Meyerson, F.W. Smith, J. Non-Cryst. Solids 35&36 (1980) 435. [9] A. Helmold, P. Hammer, J.U. Thiele, K. Rowher, D. Meissner, Philos. Mag. B 72 (1995) 335. [10] J. Robertson, Adv. Phys. 35 (1986) 317. [11] J. Robertson, E.P. OÕReilly, Phys. Rev. B 35 (1987) 2946. [12] H. Yokomichi, A. Masuda, J. Appl. Phys. 86 (1999) 2468. [13] D.T. Clark, D. Shuttleworth, J. Polym. Sci. Polym. Chem. 18 (1980) 27. [14] R. DÕAgostino, R. Lamendola, P. Favia, A. Giquel, J. Vac. Sci. Technol. A 12 (1994) 308. [15] R.M. Silverstein, G.C. Bassler, T.C. Morill, Spectrometric Identification of Organic Compounds, 3rd Ed., Wiley, New York, 1975. [16] B. Dischler, A. Bubenzer, P. Koidl, Solid State Commun. 48 (1983) 105. [17] R.O. Dillon, J.A. Woollam, V. Katkanant, Phys. Rev. B 29 (1984) 3482. [18] D. Beeman, J. Silverman, R. Lynds, M.R. Anderson, Phys. Rev. B 30 (1984) 870. [19] A.C. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 75414. [20] S.E. Rodil et al., J. Appl. Phys. 89 (2001) 5425. [21] R.S. Butter, D.R. Waterman, A.H. Lettington, R.T. Ramos, E.J. Fordham, Thin Solid Films 311 (1997) 107. [22] M. Chowalla, A.C. Ferrari, J. Robertson, G.A.J. Amaratunga, Appl. Phys. Lett. 76 (2000) 1419. [23] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095.
Effects of fluorine incorporation on the properties of amorphous carbon/p-type crystalline silicon heterojunction diodes L. Valentini, V. Salerni, I. Armentano, J.M. Kenny *, L. Lozzi, S. Santucci Department of Civil and Environmental Engineering, Materials Engineering Center, Universit a di Perugia, Loc Pentima Bass 21, 05100 Terni, Italy Dipartimento di Fisica - Unit a INFM, Universit a dellÕAquila, 67010 Coppito (AQ), Italy Received 14 June 2002; received in revised form 20 January 2003
Abstract Heterojunction diodes fabricated by plasma enhanced chemical vapor deposition of hydrogenated amorphous carbon (a-C:H) and fluorine-doped amorphous carbons (a-C:H:F) on p-type silicon are analyzed in terms of their electronic and photovoltaic properties. Their structural and optical properties were identified by Raman spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and UV–VIS transmittance. The nature of the heterojunction is confirmed by the rectifying current–voltage characteristic of carbonaceous deposits/p-Si junction. The diodes show a behavior dependent on the amount of the fluorine content. The photovoltaic behavior of the junction is investigated as a function of both fluorine incorporation and thermal treatment of the a-C:H:F films after the deposition. Better photovoltaic effects were observed from annealed a-C:H:F heterojunction structures. The optical and structural characterization performed on films after the thermal treatment indicates that this behavior is most likely due to an extended graphitization. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H) and/or diamond-like carbon (DLC), have attracted attention as an en-
*
Corresponding author. Address: Department of Civil and Environmental Engineering, Materials Engineering Center, Universit a di Perugia, Loc Pentima Bass 21, 05100 Terni, Italy. Tel.: +390-744 492 939; fax: +390-744 492 925. E-mail address: [email protected] (J.M. Kenny).
vironmentally benign and economically viable opto-electronic device material above others, such as amorphous silicon (a-Si and/or a-Si:H) due to various advantages [1]. The application of amorphous carbon as a semiconductor in opto-electronic devices such as photovoltaic solar cells has been recently attempted [1–5], though it is still rather limited to date. Moreover, the reported researches are practically limited to heterojunctions of configurations, a-C or a-C:H or DLC on well known single crystal semiconductors such as Si, GaAs, InP. The recent successful attempts to
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00181-9
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increase the conductivity of carbonaceous materials [6,7] using phosphorus and nitrogen in the solid and gaseous phase, respectively, seem to have improved the prospects for their use with Si in heterojunction bipolar transistors. In fact amorphous carbon and its hydrogenated forms showed more than 10 orders of magnitude differences in room temperature conductivity depending on how they were prepared [8,9]. Such a large variation in electrical behavior can be expected due to the diverse bonding possibilities of carbon atoms. It is generally accepted that, p states of sp2 hybridizated carbon atoms dominate electrical properties in amorphous carbon because the r states of both sp2 and sp3 hybridizated carbons are further away from the Fermi level [10,11]. Electronic structures of amorphous carbon other than sp2 content are expected to vary with clustering properties of sp2 sites [10] which is also affected by thermal treatment after the deposition process. Thus, the proportion of sp2 carbon atoms and their arrangement in smaller or larger clusters determine the optical gap and have a significant influence on the electrical transport as well. The main motivation of this work is to compare the sp2 fraction in undoped and fluorine-doped (a-C:H:F) films with their optical band gap, which has the ability of performing as a part of a heterojunction diode. For this purpose a-C:H and a-C:H:F films were deposited using the radiofrequency glow discharge plasma deposition system in an ambient of pure CH4 and CH4 /CF4 , respectively. Ellipsometry and UV–VIS spectrometry were then applied to investigate the macroscopic optical behavior of the films, studying both the refractive index and the optical gap as a function of fluorine content in the film, before and after thermal treatment. Raman spectroscopy was used to explore the evolution of the sp2 carbon atom bonding during annealing. The modification of the chemical bond states upon fluorine incorporation in the films was studied using X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. After structural characterization, they were examined regarding their ability to perform as parts of different electronic devices. Thus, the DLC films on silicon were subjected to current–voltage (I–V ) measurements after subsequent metalization
in vacuum with aluminium. The a-C:H films were subsequently prepared adding fluorine and annealed in the air: the role of the amount of fluorine incorporation and the effects of thermal treatment were then examined in terms of the device photovoltage response.
2. Experimental details Amorphous fluorocarbon films were deposited on (1 0 0) silicon substrates (resistivity of 1–12 X cm) by the plasma-assisted decomposition of CF4 –CH4 mixtures employing a 13.56 MHz radiofrequency plasma system. Before loading it into the chamber, the substrate (10 mm 10 mm 0.5 mm) was cleaned in an ultrasonic bath of acetone to remove residual organic contaminants, washed in deionised water and nitrogen dried. Prior to deposition, the chamber was evacuated (102 Pa), and then Ar was introduced for sputter cleaning in order to eliminate any impurity on the substrate (conditions: )200 V bias, 10 Pa, 5 min). These pretreatments are typically applied to remove the oxide layers. Subsequently, the chamber was again evacuated to 102 Pa. The film depositions were carried out at room temperature while the deposition pressure was fixed at 53 Pa. The controlled process variable was the dilution of CH4 with CF4 . The ratio of the CF4 gas flow rate to the total precursor gas flow rate, r ¼ [CF4 ]/([CF4 ]+[CH4 ]), was fixed at 0% and 62% while the total gas flow rate was kept constant at 40 sccm. The film deposition was performed with a RF bias voltage fixed at )400 V, resulting in a thickness of carbonaceous thin film of about 100 nm. Afterwards film deposition thermal annealing on a-C:H:F films was carried out in an atmospheric oven for 1 h at 300 °C. The front and back contacts of the diodes were made with Al electrodes by depositing a film of thickness of 40 nm on top of the carbonaceous thin film and on the bottom of the p-Si substrate, using a conventional vacuum evaporation system. Ohmic behavior of the contacts was checked by measuring the current–voltage characteristics. The photovoltaic behavior of the fabricated heterojunctions was investigated under the illumination of a lamp with a wavelength ranging
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from 400 to 800 nm. Information about the chemical bonding and the elemental composition of samples were obtained by XPS (monochromatized Al Ka source, hm ¼ 1486:6 eV, resolution 0.3 eV). Raman scattering spectra were recorded by a notch filter system in backscattering geometry. A 632.8 nm (1.96 eV) He–Ne laser was used as the light source and the power of the laser was adjusted by optical filters. By using a 100 objective lens, the illuminated spot on the sample surface was focused to about 2 lm in diameter. The resolution of Raman spectra was better than 1 cm1 . Typical acquisition time for the spectra was 30 s. Infrared transmission spectroscopy (IR) was performed in the air using a FTIR spectrophotometer in the 2500–3500 cm1 range. Spectra were normalized with respect to film thickness to allow a direct comparison between different samples. The refractive index was measured at a fixed angle of 70°, by an ellipsometer, operating at three different wavelengths: two in the infrared, at 1530 and 1300 nm, and one in the visible at 632.8 nm. Films of 100 nm thickness were measured with a UV–VIS spectrometer in order to evaluate the transparency in the visible light range of 350–900 nm. The absorption coefficients were calculated based on the transmittance measurements and the thickness data. The optical band gap was then determined (Fig. 1) by fitting the absorption data 1=2 to the Tauc relation, ðahmÞ ¼ Bðhm Eg Þ, where a is the absorption coefficient as a function of photon energy, Eg is the optical gap and B is proportional to the joint density of the states. The
900
]
600
(αh ν)
1/2
[cm
-1/2
-1/2
700
eV
800
r = 62% As Deposited
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I–V characteristics were measured with a precision semiconductor parameter analyzer.
3. Results As reported in Table 1, the deposition rate increases with CF4 partial pressure in the deposition chamber, while relative changes in the normalized film thickness after 1 h of annealing at 300 °C become more appreciable when CF4 is added in the plasma. Since the main precursors of a-C:H:F growth are CFx radicals [12], the higher deposition rate observed in the CF4 rich plasma atmosphere could be attributed to higher CFx radical density. Under annealing, the thickness of the films decreases up to 70% of their as deposited thickness for films grown with CF4 in the plasma atmosphere. Thus, the observed film shrinkage could be explained in terms of desorption of chemical species containing fluorine. XPS has been used as a diagnostic tool to detect the amount of fluorine atoms. C1sÕ XPS spectra of the film deposited with r ¼ 62% before and after annealing are shown in Fig. 2. The XPS C1sÕ results for the fluorine free film generally shows C1sÕ peak position located around 284.5 eV, which can be associated with an aromatic structure (pure graphitic) [13]. Upon fluorine incorporation the peaks were deconvoluted into two components: C– CF at 287 eV and CF at 289 eV [13,14]. Generally, the increase of CF4 concentration enhances the degree of fluorination, as indicated by the features assigned to C–CF and CF groups. Fluorine to carbon ratios, before and after annealing, obtained by the integrated intensities of XPS C1sÕ and F1sÕ features (inset of Fig. 2) and by the intensities of the carbon component (fitting of C1sÕ level) are reported in Table 2. For the as deposited films, a
500 400 300
After Annealing
200 100 0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Table 1 Deposition rate as a function of CF4 ratio and thickness of the films after annealing normalized by their as deposited thickness CF4 /(CF4 +CH4 ) [%]
Deposition rate [nm/min]
Normalized thickness [%]
0 62
41 1 71 1
99 1 71 1
4.5
Energy [eV] Fig. 1. Tauc plots of a-C:H:F and a-C:H:F annealed films.
a) F1s
680
684
688
692
Binding energy [eV]
II)
b)
1. as deposited 2. after thermal annealing
Absorbance [a. u.]
I)
Intensity [counts]
Intensity [counts]
Intensity [counts]
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Intensity [counts]
178
1. as deposited 2. after thermal annealing
1 2 1
2
F1s
2800
2900
3000
1080 -1
Wavenumber [cm ]
680
684
688
692
Binding energy [eV]
280 282 284 286 288 290 292 294 296 298
Binding energy [eV] Fig. 2. Comparison of C1sÕ photoemission spectra from the sample grown with r ¼ 62% recorded before (I) and after (II) annealing at 300 °C.
value of F/C area ratio equal to 14% was obtained when the CF4 was introduced into the plasma chamber. While this value corresponds to the total fluorine content in the film, a lower percentage of fluorine (Table 2) is chemically bonded to the carbon matrix. From Table 2, it is evident that the total fluorine content in the films after annealing mostly corresponds to that chemically bonded to carbon before the thermal treatment. The chemical bonding of carbon atoms to hydrogen and fluorine was also investigated by IR spectroscopy. In Fig. 3 the transmittance spectra of a-C:H:F film taken before and after annealing are presented. The main features of the IR spectra are two broad bands, one in the region between 1000 and 1300 cm1 , corresponding to CFx modes [15], and the second in the 2700–3200 cm1 region
1120
1160 -1
Wavenumber [cm ]
Fig. 3. IR spectra from the sample with r ¼ 62% recorded before and after annealing at 300 °C showing the CHn stretching region (a) and CFx stretching region (b).
due to CHn vibration modes [16]. While the intensity of the CFx band remains almost constant after annealing (Fig. 3(b)), the intensity of the CHn band decreases (Fig. 3(a)). From the spectra comparison in the CFx absorbing region (Fig. 3(b)), it is clear that the amount of bonded fluorine incorporated into the film is not strongly affected by thermal annealing. In order to achieve a structural investigation, Raman spectra recorded from the different samples between 1000 and 2200 cm1 were measured before and after annealing and are reported in Fig. 4, where the common features of amorphous carbon materials can be observed [17–20], i.e., the D (disorder) and G (graphitic) Raman bands. The main G band peak is located at about 1530–1550 cm1 while the D band appears centred at about 1370 cm1 in the form of a shoulder of the main G band. Each spectrum was least square fitted by two Gaussian curves superimposed to a linear background to take information on the evolution of the sp2 carbon bonds of the films. The frequencies of the D and G bands, as well as their intensities and
Table 2 Comparison of fluorine to carbon ratios obtained by the integrated intensities of XPS C1sÕ and F1sÕ signals and by the intensities of the carbon components (fitting of C1sÕ level), from films as deposited and annealed at different feed composition CF4 /(CF4 +CH4 ) [%]
F/C (F tot. before annealing) [%]
F/C (F bounded before annealing) [%]
F/C (F tot. after annealing) [%]
62
14 1
61
51
Intensity [arb.units]
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r = 62%
r = 0%
1000
1500
2000 -1
Wavenumber [cm ]
Intensity [arb.units]
(a)
1000
(b)
r = 62%
r = 0% 1500
2000 -1
Wavenumber [cm ]
Fig. 4. Raman spectra of a-C:H films grown with different r values before (a) and after (b) annealing.
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widths were used as fitting parameters. Results of this fitting procedure are reported in Table 3, in which the ID =IG ratio, the position and width of the G band are reported as a function of the CF4 content in the plasma chamber. Generally, when the CF4 is added to the plasma, the ID =IG ratio also grows, while the G bandwidth tends to decrease. The ID =IG ratio remains constant while the G band shifts to higher wavelengths decreasing in width for the annealed a-C:H:F sample. The results of the optical characterization performed at a wavelength of 632.5 nm (Table 4) indicates that, as the CF4 is introduced, a decrease of the refractive index is observed and the measured values remain practically unaffected by thermal treatment. The calculated optical band gap is also shown in Table 4. Associating the fluorine concentration in Table 2 with the data in Table 4, it is possible to conclude that increasing fluorine content decreases the optical gap. For a fixed plasma composition instead, the optical gap tends to remain almost constant after the thermal treatment. The current–voltage characteristics of the a-C: H:F/p-Si cell, without light irradiation, displayed rectifying characteristics (Fig. 5). Having confirmed the ohmic characteristics of the contacts we attribute this rectifying behavior to the interface of the n:p junction between the semiconducting a-C: H:F films and silicon. For comparison a-C:H/p-Si cell does not exhibit a rectifying behavior. This is
Table 3 Raman data from the as deposited and annealed films as a function of CF4 fraction: G peak position, G peak width, and intensity ratio of the D and G bands (ID =IG ) CF4 /(CH4 +CF4 ) [%]
0 62
Before annealing
After annealing
G band position [cm1 ]
G band width [cm1 ]
ID =IG
G band position [cm1 ]
G band width [cm1 ]
ID =IG
1532 1538
157 143
0.33 0.60
1535 1550
146 132
0.38 0.55
Table 4 Refractive index and optical band gap of a-C:H films grown with different r values before and after annealing at 300 °C CF4 /(CH4 +CF4 ) [%] 0 62
Tauc gap [eV]
Refractive index
As deposited
After annealing
As deposited
After annealing
1.2 0.5
1.1 0.4
2.19 1.81
2.08 1.84
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2.0x10
-3
1.5x10
-3
1.0x10
-3
5.0x10
-4
0.01
a-C:H:F/p-Si
1E-3 2
Current [A/cm ]
2
Current [A/cm ]
180
0.0 -3
1E-4 1E-5 1E-6 1E-7 1E-8
-2
-1
0
1
2
3
1E-9
Voltage [V]
-3
Fig. 5. The current–voltage characteristics of the a-C:H:F/p-Si heterojunction diode in the dark.
a-C:H:F Annealed -2
-1
0
1
2
3
1
2
3
Voltage [V]
(a) 1E-3
4. Discussion The experimental findings suggest that chemical bonding of carbon and fluorine atoms, more than
2
Current [A/cm ]
1E-4 1E-5 1E-6 1E-7 1E-8 1E-9
a-C:H:F as deposited
1E-10 -3
-2
-1
0
Voltage [V] (b) 1E-3 1E-4 2
Current [A/cm ]
likely due to the band offset of a-C:H:F/p-Si being more asymmetric than for a-C:H film, so it is easier to conduct holes across the junction. The photovoltaic behavior of the fabricated heterojunction diodes is depicted in Fig. 6. From this figure, it is clear that the a-C:H heterojunction structures are not sensitive to light and thus no photovoltaic behavior was detected. On the contrary, when a-C:H:F and a-C:H:F annealed heterojunctions structures are exposed to the light, the reverse bias current increases by about a two order magnitude, i.e. the reverse bias current of a-C:H:F heterojunction increases from 1.46 104 to 1.07 103 A cm2 with a reverse bias of 2.5 V. The photovoltaic parameters, such as the open circuit voltage (VOC ), short circuit current (ISC ) and the fill factor (FF) can be extracted from the I–V measurements. The VOC and ISC of the a-C:H:F heterojunction structures are about 0.24 V and 7.5 109 A cm2 , and the VOC and ISC of the annealed aC:H:F hetereojunction structures are about 0.38 V and 1.47 106 A cm2 , respectively. The ISC of the annealed a-C:H:F heterojunction is about three orders higher than the a-C:H:F heterojunction. The FF is 0.26 and 0.22 for a-C:H:F and annealed a-C:H:F heterojunction structures, respectively.
1E-5 1E-6 1E-7 1E-8 a-C:H/p-Si
1E-9 -3
(c)
-2
-1
0
1
2
3
Voltage [V]
Fig. 6. I–V curves for the a-C:H, a-C:H:F and annealed aC:H:F heterojunction diodes in the dark (straight line) and under illumination (dashed line).
the total fluorine concentration, is a decisive factor to improve the photovoltaic behavior of a-C:H:F films. XPS results indicated that an increase of the total fluorine concentration in the as deposited films is associated with an increase in the fraction
L. Valentini et al. / Journal of Non-Crystalline Solids 321 (2003) 175–182
of unbounded fluorine. In addition, under thermal annealing, IR spectra indicated that C–H bonds [21] (binding energy: 3.5 eV) are broken more easily than C–F bonds (binding energy: 5.6 eV), whose content remains unchanged after the thermal treatment. Combined XPS and IR results show that the total fluorine content after annealing is equal to the amount of fluorine bonded to carbon into as deposited samples. The chemical investigations clearly evidence that only hydrogen bonded to carbon and unbounded fluorine desorb from the film at 300 °C. Moreover the absence of CF2 and CF3 features in the XPS spectra indicates that the formation of one dimensional chains are prevented in our films. The CF2 and CF3 groups are considered weaker against thermal annealing than more crosslinked structures which are typical of diamond-like fluorine free films. This structure is preserved by the fluorine incorporation in the CF bonding configuration. Moreover, it cannot be excluded that the observed two order magnitude increase in current in the fluorine-doped heterojunctions is due to a structural rearrangement occurring after fluorine incorporation. This current increase may be due to the conversion of the diamond-like sp3 group to the graphite-like sp2 group, when the sample is doped. On the samples obtained with CF4 added in the plasma chamber, the ID =IG ratio grows while the G bandwidth tends to decrease. Similar results, reported for a-C:H [19,20] films, are attributed to an increase of the sp2 hybridizated carbon domains. According to this interpretation, for the as deposited a-C:H:F films, as the fluorine content increases the sp2 carbon sites begin to condense into clusters of increasing size. In particular, the increasing intensity of the D band clearly indicates that the sp2 sites begin to form aromatic clusters and the related increasing ID =IG ratio can be referred to as a measure of the ordering of the sp2 carbon atoms in the film [22]. The formation of larger clusters of sp2 coordinated carbon atoms under fluorine incorporation decreases the distribution of the barrier for p electrons and the increasing p electron delocalization [10] results in a more pronounced enhancement of conductivity for fluorine-doped samples with respect to a-C:H films. Upon thermal an-
181
nealing, the ID =IG remains almost constant for each sample, indicating that a further clustering of carbon atoms is not thermally induced. The observed shift upward of the G peak position for the annealed samples is higher for a higher fluorine content in the film suggesting that carbon atoms, due to hydrogen loss, can recombine in a lower stable configuration (sp2 ), resulting in a form of ordered graphite by thermal annealing. In these samples, the observed similar photovoltaic behavior with a smaller extent of changes caused by annealing is a consequence of the already higher degree of clustering in unannealed states proven by the Raman spectra. From these considerations, it is clear that p states of sp2 carbon atoms form tail states connecting to the valence band and conduction band and giving rise to the density of localized states in the gap. In this way, the concentration of sp2 carbon atoms and their arrangement in smaller or larger clusters affect the optical gap and electrical conductivity as well. It is already known [23] that the main structural feature of both fluorine free and fluorinated amorphous carbon films is the presence of both sp3 and sp2 carbon. For as deposited films without CF4 in the plasma chamber, the material is highly sp3 bonded with only short sp2 chains dispersed into the sp3 matrix. For these films, the small number of sp2 sites limits the formation of graphitic clusters so that the measured refractive index remains high (Fig. 5). Upon fluorine incorporation, the a-C:H sp3 diamond-like matrix collapses into sp2 increasing carbon domains (Fig. 3), then the refractive index also decreases. For the interpretation of the results obtained for the annealed samples, one should take into account the recent model proposed by Ferrari and Robertson [19]. These authors showed that there is a relationship between the optical properties and the graphite cluster sizes in a-C:H films; i.e. the Raman ID =IG peak ratio is found to vary inversely with the square of the gap. In our case the invariance of the ID =IG values, combined with the unchanged bonded fluorine content upon thermal annealing, may explain the invariance of both the refractive index and optical gap. This could be understood considering the enhancement of the sp2 fraction, which does not lead to a clustering of graphitic nanocrystallites.
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5. Conclusions To summarize, the a-C:H:F and annealed aC:H:F heterojunction diodes were successful fabricated and a photovoltaic effect was observed in the a-C:H:F and in the annealed a-C:H:F heterojunction structures. The analysis on the I–V characteristics under dark and illumined conditions indicates that the annealed a-C:H:F heterojunction structure show more promising photovoltaic behavior than the a-C:H:F one. The photovoltaic effects are explained using the results of the structural investigation, which shows that by raising the fluorine content in the film, the sp2 carbon sites undergo a gradual ordering into graphitic domains of increasing size and/or number, while the cluster size practically remains unaffected by thermal treatment at 300 °C. These results are explained in terms of a desorption of unbounded fluorine and the partial removal of hydrogen bounded to carbon after the thermal annealing in our a-C:H:F films. References [1] T. Soga, T. Jimbo, K.M. Krishna, M. Umeno, Int. J. Modern Phys. B 14 (2000) 206. [2] V.S. Veerasamy, G.A.J. Amaratunga, J.S. Park, W.I. Milne, H.S. MacKenzie, D.R. McKenzie, Appl. Phys. Lett. 64 (1994) 2297. [3] M. Maldei, D.C. Ingram, in: 13th European PVSC, 1995, p. 1258.
[4] N. Konofaos, C.B. Thomas, J. Appl. Phys. 81 (1997) 6238. [5] T. Soga, T. Jimbo, K.M. Krishna, M. Umeno, Carbon 37 (1999) 531. [6] V.S. Veerasamy, G.A.J. Amaratunga, J.S. Park, W.I. Milne, H.S. MacKenzie, D.R. McKenzie, IEEE 42 (1995) 577. [7] L.K. Chean, X. Shi, E. Liu, J.R. Shi, Appl. Phys. Lett. 73 (1998) 2473. [8] B. Meyerson, F.W. Smith, J. Non-Cryst. Solids 35&36 (1980) 435. [9] A. Helmold, P. Hammer, J.U. Thiele, K. Rowher, D. Meissner, Philos. Mag. B 72 (1995) 335. [10] J. Robertson, Adv. Phys. 35 (1986) 317. [11] J. Robertson, E.P. OÕReilly, Phys. Rev. B 35 (1987) 2946. [12] H. Yokomichi, A. Masuda, J. Appl. Phys. 86 (1999) 2468. [13] D.T. Clark, D. Shuttleworth, J. Polym. Sci. Polym. Chem. 18 (1980) 27. [14] R. DÕAgostino, R. Lamendola, P. Favia, A. Giquel, J. Vac. Sci. Technol. A 12 (1994) 308. [15] R.M. Silverstein, G.C. Bassler, T.C. Morill, Spectrometric Identification of Organic Compounds, 3rd Ed., Wiley, New York, 1975. [16] B. Dischler, A. Bubenzer, P. Koidl, Solid State Commun. 48 (1983) 105. [17] R.O. Dillon, J.A. Woollam, V. Katkanant, Phys. Rev. B 29 (1984) 3482. [18] D. Beeman, J. Silverman, R. Lynds, M.R. Anderson, Phys. Rev. B 30 (1984) 870. [19] A.C. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 75414. [20] S.E. Rodil et al., J. Appl. Phys. 89 (2001) 5425. [21] R.S. Butter, D.R. Waterman, A.H. Lettington, R.T. Ramos, E.J. Fordham, Thin Solid Films 311 (1997) 107. [22] M. Chowalla, A.C. Ferrari, J. Robertson, G.A.J. Amaratunga, Appl. Phys. Lett. 76 (2000) 1419. [23] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095.