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Influence of layer thickness on the photoluminescence and Raman scattering of a-C:H prepared from benzene
Diamond & Related Materials 15 (2006) 967 – 971 www.elsevier.com/locate/diamond
Influence of layer thickness on the photoluminescence and Raman scattering of a-C:H prepared from benzene S. Tóth ⁎, M. Veres, M. Füle, M. Koós Research Institute for Solid State Physics and Optics, Pf. 49, Budapest 1525, Hungary Available online 7 February 2006
Abstract How the layer thickness influences the atomic scale bonding structure and the light emission of hydrogenated amorphous carbon (a-C:H) was addressed in our investigations. For this purpose a-C:H films of different thicknesses (60–2249 nm) were prepared from benzene by plasma enhanced chemical vapour deposition method. Photoluminescence (PL) investigations performed in a broad emission and excitation energy region exhibit a considerable change of luminescence with layer thickness. The photon energy of maximal efficiency shifts into the lower energy region. The optical band gap decreases also as the layer thickness increases. Raman spectra were measured by 785 nm excitation. A considerable broadening of the G band region parallel with its shift to smaller wavenumbers with increasing layer thickness was generally observed. Luminescence properties will be discussed in the framework of structural changes monitored by the Raman scattering investigations. © 2005 Elsevier B.V. All rights reserved. Keywords: Diamond like carbon; Plasma CVD; Optical properties; Bonding configurations
1. Introduction Hydrogenated amorphous carbon (a-C:H) layers has great interest because of its favorable mechanical, electrical and optical properties. Nowadays these films have widespread applications like protective, biomedical and optical coatings. Polymer like amorphous carbon seems to be a promising material as an active layer in electroluminescent displays, light emitting diodes and sensors as well. These applications however require items with different layer thicknesses from hundred nanometers up to micrometer. The motivation of this work is to investigate whether the layer thickness influences the bonding structure and the macroscopic properties of a-C:H thin films or not. Appropriate analyzing techniques which are eligible to investigate the electronic structure and bonding properties is the photoluminescence and Raman scattering. Photoluminescence is very sensitive to explore the electronic structure and its changes, especially of those which are related to the luminescence centers; while Raman spectroscopy is a ⁎ Corresponding author. Tel.: +36 1 392 22 22/3619; fax: +36 1 392 22 15. E-mail address: [email protected] (S. Tóth). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.12.018
useful technique to monitor the bonding structure on the atomic scale. Modification of the electronic structure near band edges results in change of optical band gap, therefore spectral ellipsometric study of a-C:H layers was also performed. 2. Experimental details The a-C:H films were deposited in a capacitively coupled radio frequency (RF) plasma enhanced (PE) CVD system from benzene onto crystalline silicon wafer. Two series of films with different thicknesses were analyzed in detail. These films were prepared at plasma pressure of 18.5 Pa and self-bias of − 30 and − 50 V. By increasing deposition time layers with different thicknesses (60–2300 nm) were prepared; meanwhile the pressure and the self-bias were held at the same value. Steady state photoluminescence was measured at room temperature in reflection geometry with Hitachi model F4500 fluorescence spectrophotometer. Spectral distribution of luminescence was analyzed in the 1.77–4.59 eV emission photon energy region excited by 4.96 eV. The PL excitation spectra (PLE) were measured at peak position of emission
S. Tóth et al. / Diamond & Related Materials 15 (2006) 967–971
band by using excitation photon energy of 2.06–6.19 eV. Correction of measured spectra was made for the spectral response of the measuring system and for the excitation intensity as well. Spectral ellipsometric data obtained by a Woollam M2000F rotating compensator spectroscopic ellipsometer were analyzed in the 1–5 eV photon energy region on the ground of direct n, k fitting method. No detailed results will be presented here, just the optical band gap dependence on the layer thickness. Raman scattering measurements were performed on Renishaw 1000 B micro-Raman spectrometer. Excitation source was a diode laser with wavelength of 785 nm. We present spectra recorded in the wavenumber range of 1100–1750 cm− 1.
(a) 2249 nm
Luminescence intensity [arb.units]
968
1946 nm 1548 nm 1025 nm 940 nm 462 nm 303 nm 183 nm
3. Results
60 nm
(b) Luminescence intensity [arb.units]
In Fig. 1, PL spectra of both series (plasma pressure: 18.5 Pa, self-bias: −30 V (a) and − 50 V (b)) of a-C:H thin films having different layer thicknesses are presented. Layer thicknesses given in Fig. 1 were determined by fitting the results of spectral ellipsometric measurements. All spectra were measured by 4.96 eV excitation and at room temperature. This excitation photon energy was chosen to excite the whole emission band even in the ultraviolet region to get a reliable comparison of the spectral distribution of PL measured on a-C:H layers of different thicknesses and also to avoid the influence of the excitation energy for the shape of the emission band [1,2]. PL spectra shown in Fig. 1 exhibit a structure of different origin. A nearly regular modulation of PL intensity measured on thick layers (N462 nm for series “a” and N638 nm for series “b”) is related to interference effect while composite character of PL spectra is clearly seen on thin layers. The composing bands characterized by 3.61 eV, 3.26 eV, 2.67 eV, 2.45 eV and 2.26 eV reported before [3] are well observable on these (b 303 nm for series “a” and b512 nm for series “b”) layers. It should be mentioned however that a very weak band at 3.67 eV appears even in PL spectrum of most thick samples. The following main tendencies can be established in the change of PL spectral distribution with increasing layer thicknesses for a-C:H samples with relatively low self-bias (b|− 100 V|): (i) the composing bands are gradually transformed into two dominant bands as the thickness increases from 60 nm up to 303 nm for series “a” and from 86 nm to 512 nm for series “b”; (ii) over the 462 nm and 638 nm layer thickness for series “a” and “b”, respectively, one band with a broad distribution characterizes the PL of samples; (iii) the light emission with the increase of layer thickness shifts into lower photon energy region. Similar tendency was observed before [4], where the PL peak energy shifted into the lower energy region with increasing RF power. PL spectra measured on these two series of films are presented to show that this tendency is typical for samples with lower self-bias. Measurements on samples with higher selfbiases were also performed, but it will be published later. In the following part the results obtained from samples with selfbias of − 30 V will be presented. For the quantitative characterization of these changes we have determined the
106 nm
1151 nm 1001 nm 638 nm
512 nm 239 nm 86 nm 2,0
2,5
3,0
3,5
4,0
Luminescence energy [eV] Fig. 1. Change of the photoluminescence spectra excited by 4.96 eV photon energy of two series of a-C:H samples with increasing thickness for samples deposited at 18.5 Pa pressure and (a) − 30 V and (b) − 50 V self-bias.
center of gravity by fitting the PL spectral distribution. The values obtained are depicted in Fig. 2. The center of gravity of the emitted light is lowered by 0.22 eV as the layer thickness increases from 60 nm up to 2249 nm. Dependence of the optical band gap (E04) on the layer thickness is also shown in Fig. 2. E04 values were determined by the direct n, k fitting of the results of spectral ellipsometric measurements. The similarity of the band gap value to the PL center of gravity decreases when the layer thickness increases however its change (∼ 0.5 eV) is two times larger compared to the shift of PL. Hence there is no direct correlation between the change of the band gap value and of the center of emitted light as the layer thickness increases.
S. Tóth et al. / Diamond & Related Materials 15 (2006) 967–971
3,9
2,8
3,8 3,7 3,6 2,6 3,5 3,4
2,5
3,3 2,4
0
500
1000
1500
2000
3,2 2500
Layer thickness [nm] Fig. 2. Thickness dependence of the optical band gap and the center of gravity of the PL emission on samples with − 30 V self-bias.
Luminescence intensity [arb. units]
Whereas the luminescence spectrum represents the spectral distribution of the emitted light, the excitation spectrum, PLE (dependence of the luminescence intensity on the excitation photon energy) gives optical transitions efficient in the excitation of luminescence. In Fig. 3 we show the PLE spectra measured at 2.61 eV emitted photon energy for thin (183 nm) and at 2.44 eV emitted photon energy for thick (1946 nm) a-C:H layers. PLE spectra consist of a weak band practically in the same photon energy region (4–5 eV) for each of samples shown and a broad band with much higher intensity peaked at different photon energies for the thin and thick samples. Apart from the interference modulation of the PLE curve in the case of the thick sample, the photon energy of maximal efficiency shifts by ∼0.25 eV into the lower energy region compared to the thin layer. The relation of the luminescence intensity is not real because these spectra are not corrected for the absorption. Structural modifications, especially which are related to the photoluminescence and the optical transitions at band edges can be well monitored by Raman scattering measure-
2,0
Layer thickness: 1946 nm Emission energy : 2,44 eV
Layer thickness: 183 nm Emission energy: 2,61 eV
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
Excitation energy [eV] Fig. 3. Photoluminescence excitation spectra measured at 2.61 eV emitted photon energy for thin film (183 nm) and at 2.44 eV for 1946 nm thick samples (self-bias: −30 V).
2249 nm 1946 nm Raman intensity [arb.units]
2,7
Band gap [eV]
PL center of gravity [eV]
969
1548 nm 1367 nm 1025 nm 462 nm 303 nm 183 nm 106 nm
60 nm
1100
1200
1300
1400
1500
1600
1700
Raman shift [1/cm] Fig. 4. Raman spectra probed at 785 nm measured on a-C:H samples (self-bias: −30 V) with different thicknesses.
ments. Fig. 4 shows Raman spectra excited by 785 nm for the films of different layer thickness. The spectra are characteristic for amorphous carbon because of the presence of the G and D bands, which appear at 1550–1600 cm− 1 and 1350 cm− 1 respectively. The G band belongs to the stretching mode of the rings and chains, which contains sp2 hybridized carbon atoms, while the D band corresponds to the breathing mode of these rings [8–10]. In the wavenumber region of the widely distributed D band some additional small peaks could also be observed. These peaks found at 1290, 1380 and 1450 cm− 1 are assigned to the C–H deformational vibrations of sp3-CH2 and sp3-CH3 groups. The Raman scattering spectra also contains several sharp peaks which are related to the vibrations of substituted benzene rings, these peaks are the following: 1160, 1190, 1290 and 1450 cm− 1. The most intense peak at 1607 cm− 1 corresponds to the CfC stretching vibration mode of benzene. The development of Raman scattering spectrum with the increase of layer thickness exhibits some characteristic features. The scattering intensity in the G band region increases considerably compared to that of the D band region as the layer thickness increases from 60 nm to 2249 nm. The broad featureless G band near 1550 cm− 1 modifies over 183 nm layer thickness by appearance of a narrow intensive band at 1607 cm− 1 as well as of weak peaks at 1160, 1190 and 1290 cm− 1 due to substituted benzene ring vibrations. The G band broadening and the increase of scattering intensity in the 1500–1650 cm− 1 region is clearly seen.
970
S. Tóth et al. / Diamond & Related Materials 15 (2006) 967–971
4. Discussion Investigation of photoluminescence, optical band gap and the structural properties on atomic bonding scale of PE CVD aC:H films with increasing layer thickness provides experimental evidence for a considerable modification of structural and electronic properties of this material. For a straightforward explanation of the change in energetic position of the PL and PLE spectra at first we take into account the luminescence mechanism characteristic for a-C:H layers. Following photogeneration the electron-hole pairs localized spatially at luminescence center will recombine radiatively and will emit luminescence [5–7]. The sp2 hybridized carbon sites are supposed to behave as luminescence centers. These carbon atoms form π bonds and give rise to electronic density of states near band edges. Because of the energy gain the sp2 hybridized carbons favor clustering into rings and chain structure as well. Extent of π electron localization and hence π–π* splitting is closely related to the size of structural units, containing sp2 carbon sites. Photon energy of optical transitions being the absorption or radiative recombination through these sp 2 hybridized carbon sites depends on the localization extent of π electronic states. Therefore it is quite reasonable to suppose that the ∼ 0.22 eV decrease of PL center of gravity and similarly ∼ 0.25 eV shift of the PLE peak position are the consequence of an increase of localization extent of π electronic states due to structural modification as the layer thickness increases. The very close value of the change in the energetic position of PL and PLE indicates that the excitation and radiative recombination takes place in the same cluster as it is expected from the luminescence model [5–7]. Optical absorption near band edges determines the optical gap. To these transitions contribute π electronic states of each cluster which contains sp2 hybridized carbon sites. The ∼ 0.51 eV decrease of the optical gap as layer thickness increases is more than two times larger compared to the photon energy change of PLE peak position over the same increase of layer thickness. This indicates clearly that a given part of optical transitions at these photon energies contributes only to excitation of photoluminescence. It can also be concluded that optical transitions involving π electronic states of clusters characterized by a larger localization extent of π electrons are ineffective in the excitation of photoluminescence. This explains the difference in the decrease of the PLE peak energy and the optical band gap when the layer thickness increases. Raman scattering experiments exhibit some dominant tendency in the evolution of structure with increasing layer thickness. One of this is the appearance of substituted benzene rings in layers having thickness N106 nm. The contribution of this structure to the scattering in the G band region is dominant up to 1025 nm thickness. Because light emission at large photon energies (N3 eV) gradually decreases as the scattering of benzene groups is developing, they are not in correlation. Therefore this high energy region of PL can be attributed rather to a chain-like structural ordering of sp2 sites characterized with small localization extent. If it is so the PL spectrum should shift
into the lower photon energy region as the delocalization length of photoexcited carriers increases. The decrease of ratio between the scattering intensity of the D and G band when the layers become thicker indicates the formation of chain-like structure containing sp2 hybridized carbons. The enhancement of scattering in the smaller wavenumber region of the G band for layers of N 1025 nm suggests an increase of chain length, which explains the observed changes in optical absorption and radiative recombinations as well. The structural modification with increasing layer thickness monitored by Raman scattering can tentatively be explained in the following way. The substrate material (crystalline silicon) prefers the formation of three dimensional structures with sp3 hybridized carbon sites; therefore the clustering of sp2 carbon atoms is hard. As the layer thickness increases a loose structure is forming because of large hydrogen content and low ion energy used in our deposition process. Due to the low self-bias the benzene rings from the background gas can be incorporated into the layer structure, though this process starts in films of larger thickness because of planar arrangement favored by benzene rings. The breaking of the ring structure in the background gas results in chains with conjugated double bonds of sp2 sites, which are also incorporated into the structure. With the increase of layer thickness the formation of long chains by bonding to each other of small chain segments arriving from the plasma takes place hence scattering from these structural fragments will dominate the G band region of Raman spectra measured on thick layers. 5. Conclusions Photoluminescence and Raman spectroscopic study of PE CVD a-C:H thin layers with relatively low self-bias (b |− 100 V|) with increasing thickness were performed to investigate the electronic and bonding structure and to explore how the layer thickness influences these properties. The shift of PL and PLE spectra into the lower photon energy range as well as the decrease of optical band gap was found with increasing thickness, which indicates the widening of the band tail states. Growing intensity in the G band region of Raman spectra and the shift of this band into the smaller wavenumber region indicates the development of long chains containing sp2 carbon atoms as the layer thickness increases. Besides the growing conjugation length in chain structure the ordered benzene rings start to evolve in the structure also. Acknowledgement This work was supported by the National Research and Development Project NKFP 3A/071/2004. References [1] Rusli, J. Robertson, G.A.J. Amaratunga, J. Appl. Phys. 80 (1996) 2998. [2] I. Pócsik, M. Koós, Diamond Relat. Mater. 10 (2001) 161. [3] M. Koós, M. Füle, M. Veres, S. Tóth, I. Pócsik, J. Non-Cryst. Solids 299– 302 (2002) 852. [4] Rusli, G.A.J. Amaratunga, S.R.P. Silva, Opt. Mater. 6 (1996) 93.
S. Tóth et al. / Diamond & Related Materials 15 (2006) 967–971 [5] T. Heitz, C. Godet, J.E. Bourée, B. Drévillon, Phys. Rev., B 60 (1999) 6046. [6] M. Koós, I. Pócsik, J. Erostyák, A. Buzádi, J. Non-Cryst. Solids 227–230 (1998) 579. [7] S.R.P. Silva, J. Robertson, Rusli, G.A.J. Amaratunga, J. Schwan, Philos. Mag., B 74 (1996) 369. [8] M. Koós, M. Veres, S. Tóth, M. Füle, Raman Spectroscopy of CVD Carbon thin films excited by near-infrared light, in: G. Messina, S. Santangelo
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(Eds.), Carbon: the future material for advanced technology applications, Springer' series topics in Applied Physics, 2005, pp. 415–437. [9] A.C. Ferrari, J. Robertson, Phys. Rev., B 64 (2001) 075414. [10] M. Veres, M. Füle, S. Tóth, M. Koós, I. Pócsik, Diamond Relat. Mater. 13 (2004) 1412.
Influence of layer thickness on the photoluminescence and Raman scattering of a-C:H prepared from benzene S. Tóth ⁎, M. Veres, M. Füle, M. Koós Research Institute for Solid State Physics and Optics, Pf. 49, Budapest 1525, Hungary Available online 7 February 2006
Abstract How the layer thickness influences the atomic scale bonding structure and the light emission of hydrogenated amorphous carbon (a-C:H) was addressed in our investigations. For this purpose a-C:H films of different thicknesses (60–2249 nm) were prepared from benzene by plasma enhanced chemical vapour deposition method. Photoluminescence (PL) investigations performed in a broad emission and excitation energy region exhibit a considerable change of luminescence with layer thickness. The photon energy of maximal efficiency shifts into the lower energy region. The optical band gap decreases also as the layer thickness increases. Raman spectra were measured by 785 nm excitation. A considerable broadening of the G band region parallel with its shift to smaller wavenumbers with increasing layer thickness was generally observed. Luminescence properties will be discussed in the framework of structural changes monitored by the Raman scattering investigations. © 2005 Elsevier B.V. All rights reserved. Keywords: Diamond like carbon; Plasma CVD; Optical properties; Bonding configurations
1. Introduction Hydrogenated amorphous carbon (a-C:H) layers has great interest because of its favorable mechanical, electrical and optical properties. Nowadays these films have widespread applications like protective, biomedical and optical coatings. Polymer like amorphous carbon seems to be a promising material as an active layer in electroluminescent displays, light emitting diodes and sensors as well. These applications however require items with different layer thicknesses from hundred nanometers up to micrometer. The motivation of this work is to investigate whether the layer thickness influences the bonding structure and the macroscopic properties of a-C:H thin films or not. Appropriate analyzing techniques which are eligible to investigate the electronic structure and bonding properties is the photoluminescence and Raman scattering. Photoluminescence is very sensitive to explore the electronic structure and its changes, especially of those which are related to the luminescence centers; while Raman spectroscopy is a ⁎ Corresponding author. Tel.: +36 1 392 22 22/3619; fax: +36 1 392 22 15. E-mail address: [email protected] (S. Tóth). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.12.018
useful technique to monitor the bonding structure on the atomic scale. Modification of the electronic structure near band edges results in change of optical band gap, therefore spectral ellipsometric study of a-C:H layers was also performed. 2. Experimental details The a-C:H films were deposited in a capacitively coupled radio frequency (RF) plasma enhanced (PE) CVD system from benzene onto crystalline silicon wafer. Two series of films with different thicknesses were analyzed in detail. These films were prepared at plasma pressure of 18.5 Pa and self-bias of − 30 and − 50 V. By increasing deposition time layers with different thicknesses (60–2300 nm) were prepared; meanwhile the pressure and the self-bias were held at the same value. Steady state photoluminescence was measured at room temperature in reflection geometry with Hitachi model F4500 fluorescence spectrophotometer. Spectral distribution of luminescence was analyzed in the 1.77–4.59 eV emission photon energy region excited by 4.96 eV. The PL excitation spectra (PLE) were measured at peak position of emission
S. Tóth et al. / Diamond & Related Materials 15 (2006) 967–971
band by using excitation photon energy of 2.06–6.19 eV. Correction of measured spectra was made for the spectral response of the measuring system and for the excitation intensity as well. Spectral ellipsometric data obtained by a Woollam M2000F rotating compensator spectroscopic ellipsometer were analyzed in the 1–5 eV photon energy region on the ground of direct n, k fitting method. No detailed results will be presented here, just the optical band gap dependence on the layer thickness. Raman scattering measurements were performed on Renishaw 1000 B micro-Raman spectrometer. Excitation source was a diode laser with wavelength of 785 nm. We present spectra recorded in the wavenumber range of 1100–1750 cm− 1.
(a) 2249 nm
Luminescence intensity [arb.units]
968
1946 nm 1548 nm 1025 nm 940 nm 462 nm 303 nm 183 nm
3. Results
60 nm
(b) Luminescence intensity [arb.units]
In Fig. 1, PL spectra of both series (plasma pressure: 18.5 Pa, self-bias: −30 V (a) and − 50 V (b)) of a-C:H thin films having different layer thicknesses are presented. Layer thicknesses given in Fig. 1 were determined by fitting the results of spectral ellipsometric measurements. All spectra were measured by 4.96 eV excitation and at room temperature. This excitation photon energy was chosen to excite the whole emission band even in the ultraviolet region to get a reliable comparison of the spectral distribution of PL measured on a-C:H layers of different thicknesses and also to avoid the influence of the excitation energy for the shape of the emission band [1,2]. PL spectra shown in Fig. 1 exhibit a structure of different origin. A nearly regular modulation of PL intensity measured on thick layers (N462 nm for series “a” and N638 nm for series “b”) is related to interference effect while composite character of PL spectra is clearly seen on thin layers. The composing bands characterized by 3.61 eV, 3.26 eV, 2.67 eV, 2.45 eV and 2.26 eV reported before [3] are well observable on these (b 303 nm for series “a” and b512 nm for series “b”) layers. It should be mentioned however that a very weak band at 3.67 eV appears even in PL spectrum of most thick samples. The following main tendencies can be established in the change of PL spectral distribution with increasing layer thicknesses for a-C:H samples with relatively low self-bias (b|− 100 V|): (i) the composing bands are gradually transformed into two dominant bands as the thickness increases from 60 nm up to 303 nm for series “a” and from 86 nm to 512 nm for series “b”; (ii) over the 462 nm and 638 nm layer thickness for series “a” and “b”, respectively, one band with a broad distribution characterizes the PL of samples; (iii) the light emission with the increase of layer thickness shifts into lower photon energy region. Similar tendency was observed before [4], where the PL peak energy shifted into the lower energy region with increasing RF power. PL spectra measured on these two series of films are presented to show that this tendency is typical for samples with lower self-bias. Measurements on samples with higher selfbiases were also performed, but it will be published later. In the following part the results obtained from samples with selfbias of − 30 V will be presented. For the quantitative characterization of these changes we have determined the
106 nm
1151 nm 1001 nm 638 nm
512 nm 239 nm 86 nm 2,0
2,5
3,0
3,5
4,0
Luminescence energy [eV] Fig. 1. Change of the photoluminescence spectra excited by 4.96 eV photon energy of two series of a-C:H samples with increasing thickness for samples deposited at 18.5 Pa pressure and (a) − 30 V and (b) − 50 V self-bias.
center of gravity by fitting the PL spectral distribution. The values obtained are depicted in Fig. 2. The center of gravity of the emitted light is lowered by 0.22 eV as the layer thickness increases from 60 nm up to 2249 nm. Dependence of the optical band gap (E04) on the layer thickness is also shown in Fig. 2. E04 values were determined by the direct n, k fitting of the results of spectral ellipsometric measurements. The similarity of the band gap value to the PL center of gravity decreases when the layer thickness increases however its change (∼ 0.5 eV) is two times larger compared to the shift of PL. Hence there is no direct correlation between the change of the band gap value and of the center of emitted light as the layer thickness increases.
S. Tóth et al. / Diamond & Related Materials 15 (2006) 967–971
3,9
2,8
3,8 3,7 3,6 2,6 3,5 3,4
2,5
3,3 2,4
0
500
1000
1500
2000
3,2 2500
Layer thickness [nm] Fig. 2. Thickness dependence of the optical band gap and the center of gravity of the PL emission on samples with − 30 V self-bias.
Luminescence intensity [arb. units]
Whereas the luminescence spectrum represents the spectral distribution of the emitted light, the excitation spectrum, PLE (dependence of the luminescence intensity on the excitation photon energy) gives optical transitions efficient in the excitation of luminescence. In Fig. 3 we show the PLE spectra measured at 2.61 eV emitted photon energy for thin (183 nm) and at 2.44 eV emitted photon energy for thick (1946 nm) a-C:H layers. PLE spectra consist of a weak band practically in the same photon energy region (4–5 eV) for each of samples shown and a broad band with much higher intensity peaked at different photon energies for the thin and thick samples. Apart from the interference modulation of the PLE curve in the case of the thick sample, the photon energy of maximal efficiency shifts by ∼0.25 eV into the lower energy region compared to the thin layer. The relation of the luminescence intensity is not real because these spectra are not corrected for the absorption. Structural modifications, especially which are related to the photoluminescence and the optical transitions at band edges can be well monitored by Raman scattering measure-
2,0
Layer thickness: 1946 nm Emission energy : 2,44 eV
Layer thickness: 183 nm Emission energy: 2,61 eV
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
Excitation energy [eV] Fig. 3. Photoluminescence excitation spectra measured at 2.61 eV emitted photon energy for thin film (183 nm) and at 2.44 eV for 1946 nm thick samples (self-bias: −30 V).
2249 nm 1946 nm Raman intensity [arb.units]
2,7
Band gap [eV]
PL center of gravity [eV]
969
1548 nm 1367 nm 1025 nm 462 nm 303 nm 183 nm 106 nm
60 nm
1100
1200
1300
1400
1500
1600
1700
Raman shift [1/cm] Fig. 4. Raman spectra probed at 785 nm measured on a-C:H samples (self-bias: −30 V) with different thicknesses.
ments. Fig. 4 shows Raman spectra excited by 785 nm for the films of different layer thickness. The spectra are characteristic for amorphous carbon because of the presence of the G and D bands, which appear at 1550–1600 cm− 1 and 1350 cm− 1 respectively. The G band belongs to the stretching mode of the rings and chains, which contains sp2 hybridized carbon atoms, while the D band corresponds to the breathing mode of these rings [8–10]. In the wavenumber region of the widely distributed D band some additional small peaks could also be observed. These peaks found at 1290, 1380 and 1450 cm− 1 are assigned to the C–H deformational vibrations of sp3-CH2 and sp3-CH3 groups. The Raman scattering spectra also contains several sharp peaks which are related to the vibrations of substituted benzene rings, these peaks are the following: 1160, 1190, 1290 and 1450 cm− 1. The most intense peak at 1607 cm− 1 corresponds to the CfC stretching vibration mode of benzene. The development of Raman scattering spectrum with the increase of layer thickness exhibits some characteristic features. The scattering intensity in the G band region increases considerably compared to that of the D band region as the layer thickness increases from 60 nm to 2249 nm. The broad featureless G band near 1550 cm− 1 modifies over 183 nm layer thickness by appearance of a narrow intensive band at 1607 cm− 1 as well as of weak peaks at 1160, 1190 and 1290 cm− 1 due to substituted benzene ring vibrations. The G band broadening and the increase of scattering intensity in the 1500–1650 cm− 1 region is clearly seen.
970
S. Tóth et al. / Diamond & Related Materials 15 (2006) 967–971
4. Discussion Investigation of photoluminescence, optical band gap and the structural properties on atomic bonding scale of PE CVD aC:H films with increasing layer thickness provides experimental evidence for a considerable modification of structural and electronic properties of this material. For a straightforward explanation of the change in energetic position of the PL and PLE spectra at first we take into account the luminescence mechanism characteristic for a-C:H layers. Following photogeneration the electron-hole pairs localized spatially at luminescence center will recombine radiatively and will emit luminescence [5–7]. The sp2 hybridized carbon sites are supposed to behave as luminescence centers. These carbon atoms form π bonds and give rise to electronic density of states near band edges. Because of the energy gain the sp2 hybridized carbons favor clustering into rings and chain structure as well. Extent of π electron localization and hence π–π* splitting is closely related to the size of structural units, containing sp2 carbon sites. Photon energy of optical transitions being the absorption or radiative recombination through these sp 2 hybridized carbon sites depends on the localization extent of π electronic states. Therefore it is quite reasonable to suppose that the ∼ 0.22 eV decrease of PL center of gravity and similarly ∼ 0.25 eV shift of the PLE peak position are the consequence of an increase of localization extent of π electronic states due to structural modification as the layer thickness increases. The very close value of the change in the energetic position of PL and PLE indicates that the excitation and radiative recombination takes place in the same cluster as it is expected from the luminescence model [5–7]. Optical absorption near band edges determines the optical gap. To these transitions contribute π electronic states of each cluster which contains sp2 hybridized carbon sites. The ∼ 0.51 eV decrease of the optical gap as layer thickness increases is more than two times larger compared to the photon energy change of PLE peak position over the same increase of layer thickness. This indicates clearly that a given part of optical transitions at these photon energies contributes only to excitation of photoluminescence. It can also be concluded that optical transitions involving π electronic states of clusters characterized by a larger localization extent of π electrons are ineffective in the excitation of photoluminescence. This explains the difference in the decrease of the PLE peak energy and the optical band gap when the layer thickness increases. Raman scattering experiments exhibit some dominant tendency in the evolution of structure with increasing layer thickness. One of this is the appearance of substituted benzene rings in layers having thickness N106 nm. The contribution of this structure to the scattering in the G band region is dominant up to 1025 nm thickness. Because light emission at large photon energies (N3 eV) gradually decreases as the scattering of benzene groups is developing, they are not in correlation. Therefore this high energy region of PL can be attributed rather to a chain-like structural ordering of sp2 sites characterized with small localization extent. If it is so the PL spectrum should shift
into the lower photon energy region as the delocalization length of photoexcited carriers increases. The decrease of ratio between the scattering intensity of the D and G band when the layers become thicker indicates the formation of chain-like structure containing sp2 hybridized carbons. The enhancement of scattering in the smaller wavenumber region of the G band for layers of N 1025 nm suggests an increase of chain length, which explains the observed changes in optical absorption and radiative recombinations as well. The structural modification with increasing layer thickness monitored by Raman scattering can tentatively be explained in the following way. The substrate material (crystalline silicon) prefers the formation of three dimensional structures with sp3 hybridized carbon sites; therefore the clustering of sp2 carbon atoms is hard. As the layer thickness increases a loose structure is forming because of large hydrogen content and low ion energy used in our deposition process. Due to the low self-bias the benzene rings from the background gas can be incorporated into the layer structure, though this process starts in films of larger thickness because of planar arrangement favored by benzene rings. The breaking of the ring structure in the background gas results in chains with conjugated double bonds of sp2 sites, which are also incorporated into the structure. With the increase of layer thickness the formation of long chains by bonding to each other of small chain segments arriving from the plasma takes place hence scattering from these structural fragments will dominate the G band region of Raman spectra measured on thick layers. 5. Conclusions Photoluminescence and Raman spectroscopic study of PE CVD a-C:H thin layers with relatively low self-bias (b |− 100 V|) with increasing thickness were performed to investigate the electronic and bonding structure and to explore how the layer thickness influences these properties. The shift of PL and PLE spectra into the lower photon energy range as well as the decrease of optical band gap was found with increasing thickness, which indicates the widening of the band tail states. Growing intensity in the G band region of Raman spectra and the shift of this band into the smaller wavenumber region indicates the development of long chains containing sp2 carbon atoms as the layer thickness increases. Besides the growing conjugation length in chain structure the ordered benzene rings start to evolve in the structure also. Acknowledgement This work was supported by the National Research and Development Project NKFP 3A/071/2004. References [1] Rusli, J. Robertson, G.A.J. Amaratunga, J. Appl. Phys. 80 (1996) 2998. [2] I. Pócsik, M. Koós, Diamond Relat. Mater. 10 (2001) 161. [3] M. Koós, M. Füle, M. Veres, S. Tóth, I. Pócsik, J. Non-Cryst. Solids 299– 302 (2002) 852. [4] Rusli, G.A.J. Amaratunga, S.R.P. Silva, Opt. Mater. 6 (1996) 93.
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