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Optical and structural properties of amorphous carbon thin films deposited by microwave surface-wave plasma CVD
Diamond & Related Materials 15 (2006) 188 – 192 www.elsevier.com/locate/diamond
Optical and structural properties of amorphous carbon thin films deposited by microwave surface-wave plasma CVD Sudip Adhikari a,*, Sunil Adhikary b, Ashraf M.M. Omer a, Mohamad Rusop c, Hideo Uchida b, Tetsuo Soga c, Masayoshi Umeno b a Department of Electrical and Electronic Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-850, Japan Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan Department of Environmental Technology and Urban planning, Nagoya Institute of Technology, Gokiso-cho, show-ku, Nagoya 466-8555, Japan b
c
Available online 20 October 2005
Abstract Nitrogen doped amorphous carbon (a-C : N) thin films were deposited on silicon and quartz substrates by microwave surface-wave plasma chemical vapor deposition technique at low temperature (< 100 -C). We used argon (Ar), camphor dissolved in alcohol and nitrogen (N) as carrier, source and dopant gases, respectively. Optical band gap (E g) decreased from 4.1 to 2.4 eV when the N gas concentration increased from 0 to 4.5%. The films were annealed at different temperatures ranging from 150 to 450 -C in Ar gas environment to investigate the optical and electrical properties of the films before and after annealing. Both E g and electrical resistivity (q) decreased dramatically to 0.95 eV and 5.7 104 (V-cm) at 450 -C annealing. The structural modifications of the films leading to more graphite as a function of the annealing temperature was confirmed by the characterization of Raman spectra. D 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon; Nitrogen doping; Microwave surface-wave plasma CVD; Annealing; Optical band gap; Electrical resistivity
1. Introduction Silicon (Si)-based solar cells fabricated to date are very expensive to use on a commercial basis [1,2]. The cost reduction of solar cells and establishment of environmentally friendly production process are very important for further spread of photovoltaic technology. Carbon is highly stable, cheap and non-toxic material which can be obtained from precursors those are sufficiently available in nature [3,4]. Furthermore, amorphous carbon (a-C) has been an attractive material for the fabrication of photovoltaic solar cells because of its outstanding properties such as chemical inertness, high hardness, high thermal conductivity, infrared optical transparency and optical band gap varying over a wide range from about 5.5 eV for insulating diamond to 0.0 eV for metallic graphite [5]. Unlike amorphous silicon (a-Si) where only the stable sp3 configuration is possible, a-C consists of a mixture of sp2 and sp3 configura-
* Corresponding author. Tel.: +81 568 51 9244; fax: +81 568 51 1478. E-mail address: [email protected] (S. Adhikari). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.08.069
tions and hence possible to vary the optical band gap (E g) by simply varying the relative proportion of the sp2/sp3 hybridization [6]. Also, like other amorphous semiconducting materials, it can be doped and made n- or p-type [7]. The properties of a-C thin films depend strongly on the precursor material, method of deposition and thermal annealing. Hydrogen content in a-C thin films modifies the properties of the films by increasing the percentage of sp3 configuration, causing an increase in the E g [8]. Doping of a-C with n-type dopants such as phosphorus (P) and nitrogen (N) has been attempted by several researchers [5,9]. N being a gaseous phase has the advantage of better control of dopant concentration over P in physical deposition system. Successful control of N doping in a-C helps to realize the photovoltaic application. Although N doped a-C (a-C : N) films have been deposited by various methods [10,11], properties of aC : N films deposited by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD), an improved newly developed thin film deposition method [12] are not yet clearly understood. In this paper, we report the optical, electrical and structural properties of a-C : N thin films deposited on silicon (Si) and
S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
Slot Antenna
Fan
Quartz Plate
2.45GHz Microwave
Short Plunger
Waveguide Surface wave Plasma
Substrate Stage
Fig. 1. Schematic diagram of MW SWP-CVD.
quartz substrates by MW SWP-CVD, before and after thermal annealing of the films. Our experimental purpose is to control properties of the films for suitable solar cell application.
189
The maximum MW power of the system is 2500 W, whereas stage temperature can be controlled up to 650 -C (T5 -C). In this deposition system MW power, gas flow rate and deposition duration can be controlled by touch-screen computer system. For film deposition, we used Ar (280 ml/min) as carrier gas, camphor (C10H16O) dissolved with ethyl alcohol (C2H5OH) (10 ml/min) composition as source gas and N (0 to 6.4% in the gas composition) as a doping gas. The CVD chamber was evacuated at 3.5 10 4 Pa and total gas pressure was held fixed at 60 Pa during film deposition. The substrates were cleaned beforehand by acetone and methanol in ultrasonic bath and only for Si substrates were etched with diluted hydrofluoric acid (HF : H2O) (1 : 10) in order to remove the native oxide layer from the surface. The lunched MW power was typically 500 W. One set of the as-deposited a-C : N films was annealed in a quartz tube at different temperatures (150 to 450 -C) for 20 min in Ar gas environment. The as-deposited and the annealedfilms were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), UV/VIS/NIR spectroscopy, Nanopics 2100/NPX200 surface profiler, 4-point probe resistance measurement and Raman spectroscopy. 3. Results and discussion
2. Experimental setup a-C : N thin films were deposited on Si and quartz substrates by MW SWP-CVD at low temperature (< 100 -C). Fig. 1 shows schematic diagram of the MW SWP-CVD system. This method is useful to avoid plasma induced damages of the substrates surfaces and has a relatively large stage diameter (20 cm) that enables to deposit a relatively large area thin film in uniform condition. The SWP was produced in a 30 cm diameter cylindrical vacuum chamber by introducing 2.45 GHz microwave through a quartz window via slot antennae.
Fig. 2 shows AFM image of three-dimensional surface structure of the a-C : N film deposited on Si substrate as an example. The root mean square (RMS) roughness of the film was found to be 0.50 nm. The a-C : N thin films deposited by MW SWP-CVD are very smooth compared to a-C : N films deposited by other methods [13,14]. The analysis of XPS is one of the most useful techniques for characterization of the chemical bonding structure and to acquire useful information on the chemical environment. Fig. 3 shows the information of chemical composition in the as-
Fig. 2. AFM image (scanned area: 3 3 Am2) of three-dimensional surface morphology of the a-C : N (N / Alcohol + Camphor flow ratio: 5 : 10) film deposited on silicon substrates. The RMS roughness of the film was found to be 0.50 nm.
190
S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
4.4
4
Optical band gap (eV)
Intensity (a.u.)
C 1s (281.7 eV)
N 1s (396 eV)
O 1s (529.7 eV)
3.6
3.2
2.8
2.4
2 100
200
300
400
500
600
0
700
deposited film as an example; here the peak position of carbon (C) 1s, nitrogen (N) 1s and oxygen (O) 1s are at 281.7, 396 and 529.7 eV, respectively. The N peak found in the XPS spectrum indicates that N has been incorporated into the a-C : N film [15]. To study the optical characteristics of both as-deposited and the annealed films, we carried out the reflectance and transmittance measurements by UV/VIS/NIR spectrophotometer in the range of 200– 2000 nm. The absorption coefficient (a) was calculated by the spectral reflectance and transmittance, and film thickness data. Thickness of the films were measured by Nanopics 2100/NPX200 surface profiler. E g was obtained by Tauc plot [16]. The Tauc E g was obtained from the extrapolation of the linear part of the curve at a = 0 by using the Tauc equation, ðahvÞ1=2 ¼ B Eg hv ð1Þ where B is the density of the localized state constant. Fig. 4 shows E g rapidly decreased (from 4.1 eV at N = 0%) in the beginning, with increasing N concentration during film deposition. The minimum E g is found 2.4 eV when N concentration is 4.5 %. The similar behavior has also been reported by Chen and Robertson [9] and Hayashi et al. [17], and the trend of decreasing E g with increasing N concentration was interpreted as an increase in the disorder as determined by the measurement of the Urbach tail width. In order to examine the annealing effects in our films, the films were annealed at various temperatures. Fig. 5 shows that film thickness decreased gradually until 300 -C, beyond that it decreased significantly with increasing annealing temperature. Fig. 6 (a) shows a plotted as a function of photon energy (eV), for as-deposited and the annealed a-C : N thin films. The a increased gradually with increasing annealing temperature until 350 -C, and then jumped abruptly towards higher a values. Our result is qualitatively similar with that of Cheah et
2
3
4
5
6
7
Fig. 4. Optical band gap as a function of N concentration.
al. [18] and Cody [19]. Fig. 6 (b) shows a plot of E g versus annealing temperature. The E g decreased (2.4 to 2.3 eV) slightly in the beginning until 150 -C annealing, and beyond that it decreased monotonically to 0.95 eV at 450 -C. Qualitatively similar effects of annealing on optical properties of carbon films have been found by other researchers [20 – 22], and the decrease of E g with annealing temperature was attributed to graphitization of the films (i.e. increasing sp2 bonds). The Fig. 7 shows that the electrical resistivity (U) of the a-C : N films decreased with increasing annealing temperature. The q decreased gradually from 7.5 105 to 5.8 105 (V-cm) with increasing annealing temperature up to 250 -C. Beyond that it decreased rapidly to 5.7 104 (V-cm) at 450 -C annealing. The change of the optical and electrical properties of a-C : N film with increasing annealing temperature probably due to the modification of C –N bonding (decreasing sp3 bonds) configuration leading to more graphitization. This 350
300
Films thickness (nm)
Fig. 3. Core level X-ray photoelectron spectra of carbon (C 1s), nitrogen (N 1s) and oxygen (O 1s) for a-C : N thin film (N / Alcohol + Camphor flow ratio: 5 : 10).
1
Nitrogen concentration (%)
Binding energy (eV)
250
200
150
100
50 0
100
200
300
400
Annealing temperature (°C) Fig. 5. Films thickness as a function of annealing temperature.
S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
(a)
191
(b)
4E+5
2.5
b 150 °C c 200 °C
3E+5
Optical band gap (eV)
Optical absorption coefficient (cm-1)
a as-deposited
d 250 °C e 300 °C f 350 °C
2E+5
g 400 °C h 450 °C g
1E+5
h
2
1.5
1
f e dc b a
0.5
0E+0 1
2
3
4
5
0
100
200
300
400
Annealing temperature (°C)
Photon energy (eV)
Fig. 6. (a) Optical absorption coefficient (a) versus photon energy; (b) Optical band gap as a function of annealing temperature, of a-C : N thin films.
phenomenon has been supported by Raman spectroscopy measurements as described below. Fig. 8 shows Raman spectra of the as-deposited and the annealed a-C : N films. The figure clearly shows that the broad band of the as-deposited film gradually splitted into two peaks (commonly known as D and G peaks) with increasing annealing temperature. Moreover, the slight up shift of G peak towards higher wave number and peaks becoming narrow with higher annealing temperature, indicates that the films progressively changed to graphite in nature and crystalline have a very small grain size with increasing annealing temperature [23,24]. The shift of G line from about 1560 to 1586 cm 1 (Fig. 9) indicates the formation of disorder nano-crystalline graphitic particles upon thermal annealing. Also, Fig. 9 shows the shift of D peak as
a function of annealing temperature. The shifting trend of D peak towards higher wave number is similar to that of the G peak, which indicates amorphous nature of the carbon films [25]. 4. Conclusions The effects of N doping and thermal annealing on optical, electrical and structural properties of a-C : N thin films deposited by MW SWP-CVD are investigated. The N doping into the films during deposition resulted the decrease of E g of the films from 4.1 to 2.4 eV. The XPS measurement showed the successful
D peak
G peak
80
450 °C
Raman Intensity (a.u.)
Electrical resistivity (×10 4Ω-cm )
70 60 50 40 30
400 °C 350 °C 300 °C 250 °C 200 °C
20
150 °C 10
as-deposited 0 0
100
200
300
400
Annealing temperature (°C) Fig. 7. Electrical resistivity of the a-C : N films as a function of annealing temperature.
1000
1200
1400
1600
1800
Raman shift (cm-1) Fig. 8. Raman spectra of the as-deposited and the annealed a-C : N films.
1600
1400
1580
1380
1560
1360
1540
1340
1520
1320
Position of D band (cm-1)
S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
Position of G band (cm-1)
192
1300
1500 0
100
200
300
400
Annealing temperature (°C) Fig. 9. Raman G (left axis) and D (right axis) band position of the as-deposited and the annealed a-C : N films.
doping of N in the a-C films. The E g of a-C : N films can be tuned from 2.4 to 0.95 eV by thermal annealing of the films. The q decreased from 7.5 105 to 5.7 104 (V-cm) with increasing thermal annealing up to 450 -C. The result shows that annealing at 300 -C is the most appropriate annealing temperature for getting suitable E g (¨ 1.5 eV) of a-C : N thin film for solar cell application. The structural modification of the films as a function of the annealing temperature was due to graphitization, which was confirmed by the characterization of Raman spectra. Our results show that it is possible to control E g of a-C thin films partially by N doping during film deposition and largely by post growth annealing of the films. These results can be important references to optimize the properties of the MW SWP-CVD deposited films applicable for photovoltaic solar cell. Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the
Ministry of Economy, Tread and Industry (METI), Government of Japan. We are also grateful to the Ministry of Education, Culture, Sports, Science and Technology (MONBUKAGAKUSHO) of the Japan Government for providing a scholarship to Sudip Adhikari for his Ph.D. research. References [1] Z.Q. Ma, B.X. Liu, Sol. Energy Mater. Sol. Cells 69 (2001) 339. [2] Z.B. Zhou, R.Q. Cui, Q.J. Pang, G.M. Hadi, Z.M. Ding, W.Y. Li, Sol. Energy Mater. Sol. Cells 70 (2002) 487. [3] K.M. Krishna, T. Soga, K. Mukhopadhyay, M. Sharon, M. Umeno, Sol. Mater. Sol. Cells 48 (1997) 25. [4] M. Kumar, Y. Ando, Diamond Relat. Mater. 12 (2003) 1845. [5] M. Rusop, S.M. Mominuzzaman, T. Soga, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 42 (2003) 2339. [6] X. Wang, H.R. Harris, K. Bouldin, et al., J. Appl. Phys. 87 (2000) 621. [7] J. Robertson, E.P. O’Reilly, Phys. Rev., B 35 (1987) 2946. [8] B. Dischler, A. Bubenzer, P. Koidl, Solid State Commun. 48 (1983) 105. [9] C.W. Chen, J. Roberson, Carbon 37 (1999) 839. [10] G. Lazer, I. Lazer, J. Non-Cryst. Solids 331 (2003) 70. [11] S. Bhattacharyya, M. Hietschold, F. Richter, Diamond Relat. Mater. 9 (2002) 544. [12] M. Nagatsu, T. Sano, N. Takada, N. Toyoda, M. Tange, H. Sugai, Diamond Relat. Mater. 11 (2002) 976. [13] T. Sharda, T. Soga, T. Jimbo, M. Umeno, Diamond Relat. Mater. 9 (2000) 1331. [14] M. Rusop, T. Kinugawa, T. Soga, T. Jimbo, Diamond Relat. Mater. 9 (2002) 544. [15] J.H. Kim, Y.H. Kim, D.J. Choi, H.K. Baik, Thin Solid Films 289 (1996) 79. [16] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627. [17] Y. Hayashi, G. Yu, M.M. Rahaman, J. Appl. Phys. 89 (2001) 7924. [18] L.K. Cheah, X. Shi, J.R. Shi, E.J. Liu, S.R.P. Sliva, J. Non-Cryst. Solids 242 (1998) 331. [19] G.D. Cody, in: J.I. Pankove (Ed.), Semiconductors and Semimetals Part B, vol. 21, Academic Press, Orland, 1984, p. 11. [20] M.A. Tamor, W.C. Vasell, J. Appl. Phys. 76 (1994) 3823. [21] A.M.M. Omer, S. Adhikari, S. Adhikary, H. Uchida, M. Umeno, Diamond Relat. Mater. 13 (2004) 2136. [22] S.M. Mominuzzaman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) 658. [23] B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 (1983) 636. [24] S.M. Mominuzzaman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, Carbon 38 (2000) 127. [25] E.H. Lee, J.D.M. Hembree, G.R. Rao, L.K. Mansur, Phys. Rev., B 48 (1993) 15540.
Optical and structural properties of amorphous carbon thin films deposited by microwave surface-wave plasma CVD Sudip Adhikari a,*, Sunil Adhikary b, Ashraf M.M. Omer a, Mohamad Rusop c, Hideo Uchida b, Tetsuo Soga c, Masayoshi Umeno b a Department of Electrical and Electronic Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-850, Japan Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan Department of Environmental Technology and Urban planning, Nagoya Institute of Technology, Gokiso-cho, show-ku, Nagoya 466-8555, Japan b
c
Available online 20 October 2005
Abstract Nitrogen doped amorphous carbon (a-C : N) thin films were deposited on silicon and quartz substrates by microwave surface-wave plasma chemical vapor deposition technique at low temperature (< 100 -C). We used argon (Ar), camphor dissolved in alcohol and nitrogen (N) as carrier, source and dopant gases, respectively. Optical band gap (E g) decreased from 4.1 to 2.4 eV when the N gas concentration increased from 0 to 4.5%. The films were annealed at different temperatures ranging from 150 to 450 -C in Ar gas environment to investigate the optical and electrical properties of the films before and after annealing. Both E g and electrical resistivity (q) decreased dramatically to 0.95 eV and 5.7 104 (V-cm) at 450 -C annealing. The structural modifications of the films leading to more graphite as a function of the annealing temperature was confirmed by the characterization of Raman spectra. D 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon; Nitrogen doping; Microwave surface-wave plasma CVD; Annealing; Optical band gap; Electrical resistivity
1. Introduction Silicon (Si)-based solar cells fabricated to date are very expensive to use on a commercial basis [1,2]. The cost reduction of solar cells and establishment of environmentally friendly production process are very important for further spread of photovoltaic technology. Carbon is highly stable, cheap and non-toxic material which can be obtained from precursors those are sufficiently available in nature [3,4]. Furthermore, amorphous carbon (a-C) has been an attractive material for the fabrication of photovoltaic solar cells because of its outstanding properties such as chemical inertness, high hardness, high thermal conductivity, infrared optical transparency and optical band gap varying over a wide range from about 5.5 eV for insulating diamond to 0.0 eV for metallic graphite [5]. Unlike amorphous silicon (a-Si) where only the stable sp3 configuration is possible, a-C consists of a mixture of sp2 and sp3 configura-
* Corresponding author. Tel.: +81 568 51 9244; fax: +81 568 51 1478. E-mail address: [email protected] (S. Adhikari). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.08.069
tions and hence possible to vary the optical band gap (E g) by simply varying the relative proportion of the sp2/sp3 hybridization [6]. Also, like other amorphous semiconducting materials, it can be doped and made n- or p-type [7]. The properties of a-C thin films depend strongly on the precursor material, method of deposition and thermal annealing. Hydrogen content in a-C thin films modifies the properties of the films by increasing the percentage of sp3 configuration, causing an increase in the E g [8]. Doping of a-C with n-type dopants such as phosphorus (P) and nitrogen (N) has been attempted by several researchers [5,9]. N being a gaseous phase has the advantage of better control of dopant concentration over P in physical deposition system. Successful control of N doping in a-C helps to realize the photovoltaic application. Although N doped a-C (a-C : N) films have been deposited by various methods [10,11], properties of aC : N films deposited by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD), an improved newly developed thin film deposition method [12] are not yet clearly understood. In this paper, we report the optical, electrical and structural properties of a-C : N thin films deposited on silicon (Si) and
S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
Slot Antenna
Fan
Quartz Plate
2.45GHz Microwave
Short Plunger
Waveguide Surface wave Plasma
Substrate Stage
Fig. 1. Schematic diagram of MW SWP-CVD.
quartz substrates by MW SWP-CVD, before and after thermal annealing of the films. Our experimental purpose is to control properties of the films for suitable solar cell application.
189
The maximum MW power of the system is 2500 W, whereas stage temperature can be controlled up to 650 -C (T5 -C). In this deposition system MW power, gas flow rate and deposition duration can be controlled by touch-screen computer system. For film deposition, we used Ar (280 ml/min) as carrier gas, camphor (C10H16O) dissolved with ethyl alcohol (C2H5OH) (10 ml/min) composition as source gas and N (0 to 6.4% in the gas composition) as a doping gas. The CVD chamber was evacuated at 3.5 10 4 Pa and total gas pressure was held fixed at 60 Pa during film deposition. The substrates were cleaned beforehand by acetone and methanol in ultrasonic bath and only for Si substrates were etched with diluted hydrofluoric acid (HF : H2O) (1 : 10) in order to remove the native oxide layer from the surface. The lunched MW power was typically 500 W. One set of the as-deposited a-C : N films was annealed in a quartz tube at different temperatures (150 to 450 -C) for 20 min in Ar gas environment. The as-deposited and the annealedfilms were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), UV/VIS/NIR spectroscopy, Nanopics 2100/NPX200 surface profiler, 4-point probe resistance measurement and Raman spectroscopy. 3. Results and discussion
2. Experimental setup a-C : N thin films were deposited on Si and quartz substrates by MW SWP-CVD at low temperature (< 100 -C). Fig. 1 shows schematic diagram of the MW SWP-CVD system. This method is useful to avoid plasma induced damages of the substrates surfaces and has a relatively large stage diameter (20 cm) that enables to deposit a relatively large area thin film in uniform condition. The SWP was produced in a 30 cm diameter cylindrical vacuum chamber by introducing 2.45 GHz microwave through a quartz window via slot antennae.
Fig. 2 shows AFM image of three-dimensional surface structure of the a-C : N film deposited on Si substrate as an example. The root mean square (RMS) roughness of the film was found to be 0.50 nm. The a-C : N thin films deposited by MW SWP-CVD are very smooth compared to a-C : N films deposited by other methods [13,14]. The analysis of XPS is one of the most useful techniques for characterization of the chemical bonding structure and to acquire useful information on the chemical environment. Fig. 3 shows the information of chemical composition in the as-
Fig. 2. AFM image (scanned area: 3 3 Am2) of three-dimensional surface morphology of the a-C : N (N / Alcohol + Camphor flow ratio: 5 : 10) film deposited on silicon substrates. The RMS roughness of the film was found to be 0.50 nm.
190
S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
4.4
4
Optical band gap (eV)
Intensity (a.u.)
C 1s (281.7 eV)
N 1s (396 eV)
O 1s (529.7 eV)
3.6
3.2
2.8
2.4
2 100
200
300
400
500
600
0
700
deposited film as an example; here the peak position of carbon (C) 1s, nitrogen (N) 1s and oxygen (O) 1s are at 281.7, 396 and 529.7 eV, respectively. The N peak found in the XPS spectrum indicates that N has been incorporated into the a-C : N film [15]. To study the optical characteristics of both as-deposited and the annealed films, we carried out the reflectance and transmittance measurements by UV/VIS/NIR spectrophotometer in the range of 200– 2000 nm. The absorption coefficient (a) was calculated by the spectral reflectance and transmittance, and film thickness data. Thickness of the films were measured by Nanopics 2100/NPX200 surface profiler. E g was obtained by Tauc plot [16]. The Tauc E g was obtained from the extrapolation of the linear part of the curve at a = 0 by using the Tauc equation, ðahvÞ1=2 ¼ B Eg hv ð1Þ where B is the density of the localized state constant. Fig. 4 shows E g rapidly decreased (from 4.1 eV at N = 0%) in the beginning, with increasing N concentration during film deposition. The minimum E g is found 2.4 eV when N concentration is 4.5 %. The similar behavior has also been reported by Chen and Robertson [9] and Hayashi et al. [17], and the trend of decreasing E g with increasing N concentration was interpreted as an increase in the disorder as determined by the measurement of the Urbach tail width. In order to examine the annealing effects in our films, the films were annealed at various temperatures. Fig. 5 shows that film thickness decreased gradually until 300 -C, beyond that it decreased significantly with increasing annealing temperature. Fig. 6 (a) shows a plotted as a function of photon energy (eV), for as-deposited and the annealed a-C : N thin films. The a increased gradually with increasing annealing temperature until 350 -C, and then jumped abruptly towards higher a values. Our result is qualitatively similar with that of Cheah et
2
3
4
5
6
7
Fig. 4. Optical band gap as a function of N concentration.
al. [18] and Cody [19]. Fig. 6 (b) shows a plot of E g versus annealing temperature. The E g decreased (2.4 to 2.3 eV) slightly in the beginning until 150 -C annealing, and beyond that it decreased monotonically to 0.95 eV at 450 -C. Qualitatively similar effects of annealing on optical properties of carbon films have been found by other researchers [20 – 22], and the decrease of E g with annealing temperature was attributed to graphitization of the films (i.e. increasing sp2 bonds). The Fig. 7 shows that the electrical resistivity (U) of the a-C : N films decreased with increasing annealing temperature. The q decreased gradually from 7.5 105 to 5.8 105 (V-cm) with increasing annealing temperature up to 250 -C. Beyond that it decreased rapidly to 5.7 104 (V-cm) at 450 -C annealing. The change of the optical and electrical properties of a-C : N film with increasing annealing temperature probably due to the modification of C –N bonding (decreasing sp3 bonds) configuration leading to more graphitization. This 350
300
Films thickness (nm)
Fig. 3. Core level X-ray photoelectron spectra of carbon (C 1s), nitrogen (N 1s) and oxygen (O 1s) for a-C : N thin film (N / Alcohol + Camphor flow ratio: 5 : 10).
1
Nitrogen concentration (%)
Binding energy (eV)
250
200
150
100
50 0
100
200
300
400
Annealing temperature (°C) Fig. 5. Films thickness as a function of annealing temperature.
S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
(a)
191
(b)
4E+5
2.5
b 150 °C c 200 °C
3E+5
Optical band gap (eV)
Optical absorption coefficient (cm-1)
a as-deposited
d 250 °C e 300 °C f 350 °C
2E+5
g 400 °C h 450 °C g
1E+5
h
2
1.5
1
f e dc b a
0.5
0E+0 1
2
3
4
5
0
100
200
300
400
Annealing temperature (°C)
Photon energy (eV)
Fig. 6. (a) Optical absorption coefficient (a) versus photon energy; (b) Optical band gap as a function of annealing temperature, of a-C : N thin films.
phenomenon has been supported by Raman spectroscopy measurements as described below. Fig. 8 shows Raman spectra of the as-deposited and the annealed a-C : N films. The figure clearly shows that the broad band of the as-deposited film gradually splitted into two peaks (commonly known as D and G peaks) with increasing annealing temperature. Moreover, the slight up shift of G peak towards higher wave number and peaks becoming narrow with higher annealing temperature, indicates that the films progressively changed to graphite in nature and crystalline have a very small grain size with increasing annealing temperature [23,24]. The shift of G line from about 1560 to 1586 cm 1 (Fig. 9) indicates the formation of disorder nano-crystalline graphitic particles upon thermal annealing. Also, Fig. 9 shows the shift of D peak as
a function of annealing temperature. The shifting trend of D peak towards higher wave number is similar to that of the G peak, which indicates amorphous nature of the carbon films [25]. 4. Conclusions The effects of N doping and thermal annealing on optical, electrical and structural properties of a-C : N thin films deposited by MW SWP-CVD are investigated. The N doping into the films during deposition resulted the decrease of E g of the films from 4.1 to 2.4 eV. The XPS measurement showed the successful
D peak
G peak
80
450 °C
Raman Intensity (a.u.)
Electrical resistivity (×10 4Ω-cm )
70 60 50 40 30
400 °C 350 °C 300 °C 250 °C 200 °C
20
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S. Adhikari et al. / Diamond & Related Materials 15 (2006) 188 – 192
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doping of N in the a-C films. The E g of a-C : N films can be tuned from 2.4 to 0.95 eV by thermal annealing of the films. The q decreased from 7.5 105 to 5.7 104 (V-cm) with increasing thermal annealing up to 450 -C. The result shows that annealing at 300 -C is the most appropriate annealing temperature for getting suitable E g (¨ 1.5 eV) of a-C : N thin film for solar cell application. The structural modification of the films as a function of the annealing temperature was due to graphitization, which was confirmed by the characterization of Raman spectra. Our results show that it is possible to control E g of a-C thin films partially by N doping during film deposition and largely by post growth annealing of the films. These results can be important references to optimize the properties of the MW SWP-CVD deposited films applicable for photovoltaic solar cell. Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the
Ministry of Economy, Tread and Industry (METI), Government of Japan. We are also grateful to the Ministry of Education, Culture, Sports, Science and Technology (MONBUKAGAKUSHO) of the Japan Government for providing a scholarship to Sudip Adhikari for his Ph.D. research. References [1] Z.Q. Ma, B.X. Liu, Sol. Energy Mater. Sol. Cells 69 (2001) 339. [2] Z.B. Zhou, R.Q. Cui, Q.J. Pang, G.M. Hadi, Z.M. Ding, W.Y. Li, Sol. Energy Mater. Sol. Cells 70 (2002) 487. [3] K.M. Krishna, T. Soga, K. Mukhopadhyay, M. Sharon, M. Umeno, Sol. Mater. Sol. Cells 48 (1997) 25. [4] M. Kumar, Y. Ando, Diamond Relat. Mater. 12 (2003) 1845. [5] M. Rusop, S.M. Mominuzzaman, T. Soga, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 42 (2003) 2339. [6] X. Wang, H.R. Harris, K. Bouldin, et al., J. Appl. Phys. 87 (2000) 621. [7] J. Robertson, E.P. O’Reilly, Phys. Rev., B 35 (1987) 2946. [8] B. Dischler, A. Bubenzer, P. Koidl, Solid State Commun. 48 (1983) 105. [9] C.W. Chen, J. Roberson, Carbon 37 (1999) 839. [10] G. Lazer, I. Lazer, J. Non-Cryst. Solids 331 (2003) 70. [11] S. Bhattacharyya, M. Hietschold, F. Richter, Diamond Relat. Mater. 9 (2002) 544. [12] M. Nagatsu, T. Sano, N. Takada, N. Toyoda, M. Tange, H. Sugai, Diamond Relat. Mater. 11 (2002) 976. [13] T. Sharda, T. Soga, T. Jimbo, M. Umeno, Diamond Relat. Mater. 9 (2000) 1331. [14] M. Rusop, T. Kinugawa, T. Soga, T. Jimbo, Diamond Relat. Mater. 9 (2002) 544. [15] J.H. Kim, Y.H. Kim, D.J. Choi, H.K. Baik, Thin Solid Films 289 (1996) 79. [16] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627. [17] Y. Hayashi, G. Yu, M.M. Rahaman, J. Appl. Phys. 89 (2001) 7924. [18] L.K. Cheah, X. Shi, J.R. Shi, E.J. Liu, S.R.P. Sliva, J. Non-Cryst. Solids 242 (1998) 331. [19] G.D. Cody, in: J.I. Pankove (Ed.), Semiconductors and Semimetals Part B, vol. 21, Academic Press, Orland, 1984, p. 11. [20] M.A. Tamor, W.C. Vasell, J. Appl. Phys. 76 (1994) 3823. [21] A.M.M. Omer, S. Adhikari, S. Adhikary, H. Uchida, M. Umeno, Diamond Relat. Mater. 13 (2004) 2136. [22] S.M. Mominuzzaman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) 658. [23] B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 (1983) 636. [24] S.M. Mominuzzaman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, Carbon 38 (2000) 127. [25] E.H. Lee, J.D.M. Hembree, G.R. Rao, L.K. Mansur, Phys. Rev., B 48 (1993) 15540.