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Photothermal deflection spectroscopy and transmission measurements of a-C:H films
Journal of Non-Crystalline Solids 254 (1999) 151±155
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Photothermal de¯ection spectroscopy and transmission measurements of a-C:H ®lms T.Y. Leung a, W.F. Man a, S.K. So a, P.K. Lim a,*, W.C. Chan b, F. Gaspari b, S. Zukotynski b a
Department of Physics and Centre for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Hong Kong, People's Republic of China b Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ont., Canada M5S 1A4
Abstract Absorption coecient of a-C:H ®lm prepared by saddle-®eld glow-discharge chemical vapor deposition was measured by means of photothermal de¯ection spectroscopy and transmission measurements. The absorption spectra of these ®lms are divided into a larger absorption region and a smaller absorption region. The larger absorption region can be interpreted in terms of the Tauc model and an optical gap is obtained for the a-C:H ®lm. The smaller absorption region can be analyzed in terms of the Urbach tail and yields information about the band tails of the a-C:H ®lm. The eect of doping on the absorption tails and the optical gap is also reported. We found that both the optical gap and the breadth of the band tail decreased with the concentration of the impurities, phosphorous or hydrogen. Also there is a correlation between the band tail spread and the optical gap. The breadth of the band tail increases with the optical gap. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 71.25.Mg; 72.80.Ng; 78.20.Dj
1. Introduction The study of amorphous diamond-like carbon (DLC) has been an active area of condensed matter physics in recent years [1±3]. Several techniques have been developed for the deposition of DLC ®lms. The plasma assisted techniques dier mainly in the form of excitation, direct current (dc) or radio frequency (rf). The positive attributes of rf and dc diode discharges are combined in the dc saddle-®eld glow-discharge deposition technique.
* Corresponding author. Tel.: +852 2339 7037; fax: +852 2304 6558; e-mail: [email protected]
The details of this technique can be found elsewhere [4]. Films produced by this technique show interesting properties such as resistancy to scratch and transparency in the visible range. In the last few years, a systematic study on how the structure of the ®lm depends on deposition parameters and the eect of doping has been carried out [5±7]. Many techniques have been employed to study the properties of these ®lms. X-ray photoelectron spectroscopy and X-ray stimulated Auger electron spectroscopy were used to study the ®lm composition, chemical states of atoms inside the ®lm and the sp3 /sp2 ratio [5]. Secondary ion mass spectroscopy was used to measure the concentration of each element present in the ®lm [6]. Raman
0022-3093/99/$ ± see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 4 1 - X
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T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
spectroscopy was also used to investigate the structure of the ®lm [5]. Fourier transform infrared spectroscopy was used to study impurities [7]. Photoluminescence [7], optical absorption and electrical conductivity were also measured [6]. Energy band structure information such as the band gap and the band tail are important for a better understanding of the material. Such information can be provided by optical absorption measurements. The traditional means of determining the optical absorption spectrum, through measurements of the optical transmission and re¯ection, are often frustrated by the transparency of DLC ®lms. Therefore, in this work, we employed two techniques, namely photothermal de¯ection spectroscopy (PDS) and optical transmission, to study the optical absorption spectrum in a wide range of light, from near infrared (IR) to ultraviolet (UV), for undoped and phosphorus-doped hydrogenated diamond-like amorphous carbon ®lms. In this article, we report some preliminary results on how the energy gap and band tail of aC:H deposited using saddle-®eld glow-discharge decomposition of methane varies with deposition parameters and how they are aected by the introduction of impurities.
the sample and perpendicular to the pump beam, was used as the probe beam. The de¯ection signal was then detected by a quadrant cell and a digital lock-in ampli®er. The absorption coecient of each ®lm was proportional to the de¯ection signal. The details of the setup and the relation between the absorption coecient and the de¯ection signal can be found from Ref. [8]. Since PDS only measures a relative absorption coecient, calibration is needed to obtain the absolute a. The conventional optical transmission measurement can measure the absolute a in the large absorption region. Therefore, by matching the PDS signal to the optical transmission signal in a region where they both overlap, the PDS signal can be calibrated. Optical transmission of a-C:H ®lms was measured by using a spectrophotometer (Olis Cary14). A 70 W Xe arc lamp (LPS-220 pti) was used as
2. Experimental Both undoped and P-doped a-C:H ®lms were grown on silica plates by using dc saddle-®eld glow discharge deposition. The undoped ®lm was grown using pure methane, while P-doped ®lms were grown using methane diluted with dopant gases (phosphine) in mole fractions ranging from 1 ´ 10ÿ5 to 0.05. The details of sample preparation have been described elsewhere [6]. The optical absorption of a-C:H ®lms was done by PDS. In the PDS experiment, a 1 kW Xe arc lamp was used as the light source and the incident pump beam with wavelength selected by a 1/4 m grating monochromator which was controlled by a stepper motor was used. The pump beam was then modulated at 13 Hz by a mechanical chopper. The sample was placed in a sample holder which was ®lled with a transparent ¯uid, CCl4 . A He±Ne laser, whose light beam was parallel to the surface of
Fig. 1. The absorption coecient of a typical a-C:H ®lm obtained by PDS (circles) and optical transmission measurement (solid dots) after multiple re¯ection taken into consideration. The line is drawn as a guide for the eye.
T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
the light source and all transmission signals were normalized to the power of the light source.
153
4. Discussion
Fig. 1 shows a typical absorption spectrum of a phosphorus-doped a-C:H ®lm measured by PDS and transmission. The circles represent absorption data from PDS and the solid dots represent absorption data from transmission measurement. Multiple re¯ection has been taken into account when calculating the absorption coecient from the transmission data [9]. It can be seen that in the middle energy range, i.e., 2±4 eV, the two sets of data agree with each other within errors of measurement. We did not use PDS to measure absorption at energies >4 eV because the UV intensity in our PDS system decreased.
The absorption spectrum can be divided into two regions, namely, a larger absorption region and a smaller absorption region. The absorption of a-C:H ®lms in the larger absorption region was associated with the transitions from the extended states in the conduction band and valence band [10]. Fig. 2 shows a plot of (aE)1=2 against hm for a typical a-C:H ®lm. For hm > 3:5 eV, the absorption curve can be ®tted to a linear function. This result showed that the band edge of a-C:H ®lms followed the Tauc model. By extending the function to the x-axis, the optical band gap was obtained. In the smaller absorption region, the absorption coecient decreased towards the smaller photon energy region. We attribute the absorption in this energy range to the transition between band tail states in the band gap, which
Fig. 2. Using TaucÕs rule to determine the optical band gap of a-C:H ®lm. PDS signal is represented by circles and optical transmission measurement by solid dots. The curve is a ®t of the Tauc equation to the data.
Fig. 3. Determination of the width of the band tail using UrbachÕs rule. PDS signal (circles) and optical transmission measurement (solid dots). The curve is a ®t of the Urbach equation to the data.
3. Results
154
T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
were induced by the amorphous structure of the aC:H ®lm and thus subgap absorption was produced [10]. In Fig. 3, we plot ln a against hm. For hm < 3 eV, an exponential function ®t the data. By using the Urbach edge equation [11] in this region, the width of band tail of the a-C:H ®lm was determined. The optical band gaps (Eg ) for the phosphorus doped a-C:H ®lms are shown in Fig. 4(a). We found that Eg decreased with increasing concentration of phosphorus. These results were in agreement with that obtained by other groups [12,13]. We assume that the decreasing band gap was due to the impurity states in the band gap. When the impurity concentration increased, the
Fig. 4. Plots of (a) energy band gap and (b) width of the band tail against phosphorus concentration. The lines are drawn as a guide for the eye.
Fig. 5. The correlation between energy band gap and the width of the band tail for undoped and phosphorus-doped ®lms. The data are ®tted to the function Eg A + BE0 . The correlation coecient is R 0.983.
Fig. 6. Plots of (a) energy band gap and (b) width of the band tail against hydrogen content for the phosphorus-doped ®lms. The data are ®tted to two functions, Eg AEg BEg H (conc.) and E0 AE0 BE0 H (conc.). The correlation coecients are REg 0:994 and RE0 0:997.
T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
impurity state becomes an impurity band and this eectively reduced the size of the band gap [10]. The width of the band tail for phosphorus doped samples is plotted in Fig. 4(b). Similar to Eg , the width of the band tail (E0 ) decreased with phosphorus concentration. In Fig. 5, Eg is plotted against E0 for both undoped and phosphorus doped samples. We observe that Eg increased with E0 . In Fig. 6, Eg and E0 are plotted against hydrogen concentration. We found that both Eg and E0 decreased with increasing hydrogen concentration. These results are in agreement with Compagnini et al. [14]. They suggested that the hydrogen content in a-C:H ®lms plays a role in controlling the size of the band gap and the width of the band tail. 5. Conclusions The optical absorption could be ®tted to the Tauc and Urbach equations. The Tauc gap and the Urbach tail decreased with increasing concentrations of P or H. Acknowledgements The authors wish to acknowledge the ®nancial support of Hong Kong Baptist University and the National Sciences and Engineering Research Council of Canada.
155
References [1] J. Robertson, J. Non-Cryst. Solids 137&138 (1991) 825. [2] J. Robertson, J. Non-Cryst. Solids 164±166 (1993) 1115. [3] M.A. Tamor, in: A. Feldman, Y. Tzeng, W. A. Yarbrough, M. Yoshikawa, M. Murakawa (Eds.), Application of Diamond Films and Related Material: Third International Conference, NIST Special Publication 885, Gaithersburg, MD, 1995, p. 691. [4] R.V. Kruzelecky, S. Zukotynski, Mater. Sci. Forum 140± 142 (1993) 89. [5] P.K. Lim, F. Gaspari, S. Zukotynski, J. Appl. Phys. 78 (1995) 5307. [6] W.C.W. Chan, F. Gaspari, T. Allen, P.K. Lim, E. Moreno, E. Sagnes, D. Manage, J. Szurmak, S. Zukotynski, J. Vac. Sci. Technol. A 16 (1998) 889. [7] F. Gaspari, R.V. Kruzelecky, P.K. Lim, L.S. Sidhu, S. Zukotynski, J. Appl. Phys. 79 (1996) 2684. [8] S.K. So, M.H. Chan, L.M. Leung, Appl. Phys. A 61 (1995) 159. [9] T.Y. Leung, MPhil thesis, Hong Kong Baptist University, 1998. [10] N.F. Mott, E.A. Davis, Electronic Processes in NonCrystalline Materials, 2nd ed., Clarendon, Oxford, 1979. [11] F. Urbach, Phys. Rev. 92 (1953) 1324. [12] J.U. Thiele, B. Rubarth, P. Hammer, A. Helmbold, B. Kessler, K. Rohwer, D. Meissner, Diamond Relat. Mater. 3 (1994) 1103. [13] T. Nishimori, A.K. Nakano, H. Sakamoto, Y. Takakuwa, S. Kono, Appl. Phys. Lett. 71 (1997) 945. [14] G. Compagnini, U. Zammit, K.N. Madhusoodanan, G. Foti, Phys. Rev. B 51 (1995) 11168.
www.elsevier.com/locate/jnoncrysol
Photothermal de¯ection spectroscopy and transmission measurements of a-C:H ®lms T.Y. Leung a, W.F. Man a, S.K. So a, P.K. Lim a,*, W.C. Chan b, F. Gaspari b, S. Zukotynski b a
Department of Physics and Centre for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Hong Kong, People's Republic of China b Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ont., Canada M5S 1A4
Abstract Absorption coecient of a-C:H ®lm prepared by saddle-®eld glow-discharge chemical vapor deposition was measured by means of photothermal de¯ection spectroscopy and transmission measurements. The absorption spectra of these ®lms are divided into a larger absorption region and a smaller absorption region. The larger absorption region can be interpreted in terms of the Tauc model and an optical gap is obtained for the a-C:H ®lm. The smaller absorption region can be analyzed in terms of the Urbach tail and yields information about the band tails of the a-C:H ®lm. The eect of doping on the absorption tails and the optical gap is also reported. We found that both the optical gap and the breadth of the band tail decreased with the concentration of the impurities, phosphorous or hydrogen. Also there is a correlation between the band tail spread and the optical gap. The breadth of the band tail increases with the optical gap. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 71.25.Mg; 72.80.Ng; 78.20.Dj
1. Introduction The study of amorphous diamond-like carbon (DLC) has been an active area of condensed matter physics in recent years [1±3]. Several techniques have been developed for the deposition of DLC ®lms. The plasma assisted techniques dier mainly in the form of excitation, direct current (dc) or radio frequency (rf). The positive attributes of rf and dc diode discharges are combined in the dc saddle-®eld glow-discharge deposition technique.
* Corresponding author. Tel.: +852 2339 7037; fax: +852 2304 6558; e-mail: [email protected]
The details of this technique can be found elsewhere [4]. Films produced by this technique show interesting properties such as resistancy to scratch and transparency in the visible range. In the last few years, a systematic study on how the structure of the ®lm depends on deposition parameters and the eect of doping has been carried out [5±7]. Many techniques have been employed to study the properties of these ®lms. X-ray photoelectron spectroscopy and X-ray stimulated Auger electron spectroscopy were used to study the ®lm composition, chemical states of atoms inside the ®lm and the sp3 /sp2 ratio [5]. Secondary ion mass spectroscopy was used to measure the concentration of each element present in the ®lm [6]. Raman
0022-3093/99/$ ± see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 4 1 - X
152
T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
spectroscopy was also used to investigate the structure of the ®lm [5]. Fourier transform infrared spectroscopy was used to study impurities [7]. Photoluminescence [7], optical absorption and electrical conductivity were also measured [6]. Energy band structure information such as the band gap and the band tail are important for a better understanding of the material. Such information can be provided by optical absorption measurements. The traditional means of determining the optical absorption spectrum, through measurements of the optical transmission and re¯ection, are often frustrated by the transparency of DLC ®lms. Therefore, in this work, we employed two techniques, namely photothermal de¯ection spectroscopy (PDS) and optical transmission, to study the optical absorption spectrum in a wide range of light, from near infrared (IR) to ultraviolet (UV), for undoped and phosphorus-doped hydrogenated diamond-like amorphous carbon ®lms. In this article, we report some preliminary results on how the energy gap and band tail of aC:H deposited using saddle-®eld glow-discharge decomposition of methane varies with deposition parameters and how they are aected by the introduction of impurities.
the sample and perpendicular to the pump beam, was used as the probe beam. The de¯ection signal was then detected by a quadrant cell and a digital lock-in ampli®er. The absorption coecient of each ®lm was proportional to the de¯ection signal. The details of the setup and the relation between the absorption coecient and the de¯ection signal can be found from Ref. [8]. Since PDS only measures a relative absorption coecient, calibration is needed to obtain the absolute a. The conventional optical transmission measurement can measure the absolute a in the large absorption region. Therefore, by matching the PDS signal to the optical transmission signal in a region where they both overlap, the PDS signal can be calibrated. Optical transmission of a-C:H ®lms was measured by using a spectrophotometer (Olis Cary14). A 70 W Xe arc lamp (LPS-220 pti) was used as
2. Experimental Both undoped and P-doped a-C:H ®lms were grown on silica plates by using dc saddle-®eld glow discharge deposition. The undoped ®lm was grown using pure methane, while P-doped ®lms were grown using methane diluted with dopant gases (phosphine) in mole fractions ranging from 1 ´ 10ÿ5 to 0.05. The details of sample preparation have been described elsewhere [6]. The optical absorption of a-C:H ®lms was done by PDS. In the PDS experiment, a 1 kW Xe arc lamp was used as the light source and the incident pump beam with wavelength selected by a 1/4 m grating monochromator which was controlled by a stepper motor was used. The pump beam was then modulated at 13 Hz by a mechanical chopper. The sample was placed in a sample holder which was ®lled with a transparent ¯uid, CCl4 . A He±Ne laser, whose light beam was parallel to the surface of
Fig. 1. The absorption coecient of a typical a-C:H ®lm obtained by PDS (circles) and optical transmission measurement (solid dots) after multiple re¯ection taken into consideration. The line is drawn as a guide for the eye.
T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
the light source and all transmission signals were normalized to the power of the light source.
153
4. Discussion
Fig. 1 shows a typical absorption spectrum of a phosphorus-doped a-C:H ®lm measured by PDS and transmission. The circles represent absorption data from PDS and the solid dots represent absorption data from transmission measurement. Multiple re¯ection has been taken into account when calculating the absorption coecient from the transmission data [9]. It can be seen that in the middle energy range, i.e., 2±4 eV, the two sets of data agree with each other within errors of measurement. We did not use PDS to measure absorption at energies >4 eV because the UV intensity in our PDS system decreased.
The absorption spectrum can be divided into two regions, namely, a larger absorption region and a smaller absorption region. The absorption of a-C:H ®lms in the larger absorption region was associated with the transitions from the extended states in the conduction band and valence band [10]. Fig. 2 shows a plot of (aE)1=2 against hm for a typical a-C:H ®lm. For hm > 3:5 eV, the absorption curve can be ®tted to a linear function. This result showed that the band edge of a-C:H ®lms followed the Tauc model. By extending the function to the x-axis, the optical band gap was obtained. In the smaller absorption region, the absorption coecient decreased towards the smaller photon energy region. We attribute the absorption in this energy range to the transition between band tail states in the band gap, which
Fig. 2. Using TaucÕs rule to determine the optical band gap of a-C:H ®lm. PDS signal is represented by circles and optical transmission measurement by solid dots. The curve is a ®t of the Tauc equation to the data.
Fig. 3. Determination of the width of the band tail using UrbachÕs rule. PDS signal (circles) and optical transmission measurement (solid dots). The curve is a ®t of the Urbach equation to the data.
3. Results
154
T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
were induced by the amorphous structure of the aC:H ®lm and thus subgap absorption was produced [10]. In Fig. 3, we plot ln a against hm. For hm < 3 eV, an exponential function ®t the data. By using the Urbach edge equation [11] in this region, the width of band tail of the a-C:H ®lm was determined. The optical band gaps (Eg ) for the phosphorus doped a-C:H ®lms are shown in Fig. 4(a). We found that Eg decreased with increasing concentration of phosphorus. These results were in agreement with that obtained by other groups [12,13]. We assume that the decreasing band gap was due to the impurity states in the band gap. When the impurity concentration increased, the
Fig. 4. Plots of (a) energy band gap and (b) width of the band tail against phosphorus concentration. The lines are drawn as a guide for the eye.
Fig. 5. The correlation between energy band gap and the width of the band tail for undoped and phosphorus-doped ®lms. The data are ®tted to the function Eg A + BE0 . The correlation coecient is R 0.983.
Fig. 6. Plots of (a) energy band gap and (b) width of the band tail against hydrogen content for the phosphorus-doped ®lms. The data are ®tted to two functions, Eg AEg BEg H (conc.) and E0 AE0 BE0 H (conc.). The correlation coecients are REg 0:994 and RE0 0:997.
T.Y. Leung et al. / Journal of Non-Crystalline Solids 254 (1999) 151±155
impurity state becomes an impurity band and this eectively reduced the size of the band gap [10]. The width of the band tail for phosphorus doped samples is plotted in Fig. 4(b). Similar to Eg , the width of the band tail (E0 ) decreased with phosphorus concentration. In Fig. 5, Eg is plotted against E0 for both undoped and phosphorus doped samples. We observe that Eg increased with E0 . In Fig. 6, Eg and E0 are plotted against hydrogen concentration. We found that both Eg and E0 decreased with increasing hydrogen concentration. These results are in agreement with Compagnini et al. [14]. They suggested that the hydrogen content in a-C:H ®lms plays a role in controlling the size of the band gap and the width of the band tail. 5. Conclusions The optical absorption could be ®tted to the Tauc and Urbach equations. The Tauc gap and the Urbach tail decreased with increasing concentrations of P or H. Acknowledgements The authors wish to acknowledge the ®nancial support of Hong Kong Baptist University and the National Sciences and Engineering Research Council of Canada.
155
References [1] J. Robertson, J. Non-Cryst. Solids 137&138 (1991) 825. [2] J. Robertson, J. Non-Cryst. Solids 164±166 (1993) 1115. [3] M.A. Tamor, in: A. Feldman, Y. Tzeng, W. A. Yarbrough, M. Yoshikawa, M. Murakawa (Eds.), Application of Diamond Films and Related Material: Third International Conference, NIST Special Publication 885, Gaithersburg, MD, 1995, p. 691. [4] R.V. Kruzelecky, S. Zukotynski, Mater. Sci. Forum 140± 142 (1993) 89. [5] P.K. Lim, F. Gaspari, S. Zukotynski, J. Appl. Phys. 78 (1995) 5307. [6] W.C.W. Chan, F. Gaspari, T. Allen, P.K. Lim, E. Moreno, E. Sagnes, D. Manage, J. Szurmak, S. Zukotynski, J. Vac. Sci. Technol. A 16 (1998) 889. [7] F. Gaspari, R.V. Kruzelecky, P.K. Lim, L.S. Sidhu, S. Zukotynski, J. Appl. Phys. 79 (1996) 2684. [8] S.K. So, M.H. Chan, L.M. Leung, Appl. Phys. A 61 (1995) 159. [9] T.Y. Leung, MPhil thesis, Hong Kong Baptist University, 1998. [10] N.F. Mott, E.A. Davis, Electronic Processes in NonCrystalline Materials, 2nd ed., Clarendon, Oxford, 1979. [11] F. Urbach, Phys. Rev. 92 (1953) 1324. [12] J.U. Thiele, B. Rubarth, P. Hammer, A. Helmbold, B. Kessler, K. Rohwer, D. Meissner, Diamond Relat. Mater. 3 (1994) 1103. [13] T. Nishimori, A.K. Nakano, H. Sakamoto, Y. Takakuwa, S. Kono, Appl. Phys. Lett. 71 (1997) 945. [14] G. Compagnini, U. Zammit, K.N. Madhusoodanan, G. Foti, Phys. Rev. B 51 (1995) 11168.