- Email: [email protected]
Effect of film thickness on the optical constants and optical absorption properties of NiOx thin films
Physica B 405 (2010) 3875–3878
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effect of film thickness on the optical constants and optical absorption properties of NiOx thin films Ying Zhou a,b,n, Yongyou Geng a, Donghong Gu a, Weibing Gu a,b, Zhi Jiang a,b a b
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China. Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e in fo
abstract
Article history: Received 2 January 2007 Received in revised form 6 February 2007 Accepted 17 February 2007
NiOx (x 4 1) thin films were deposited by reactive DC-magnetron sputtering from a nickel metal target in Ar +O2 with the relative O2 content 5%. The reflectivity and transmission of the films were measured from 350 to 700 nm. The films coated to thicknesses of 17, 27 and 34 nm had the same absorption mechanism which is a direct allowed transition. The refractive index n and extinction coefficient k of the films were calculated from the measured reflectivity and transmission of these films, and the relationship of the optical band gap with variation in film thickness was studied as well. All results showed that the optical properties of NiOx thin films depended on the film thickness. & 2010 Elsevier B.V. All rights reserved.
Keywords: NiOx thin films Optical constants Optical band gap Film thickness Annealing
Introduction NiO films have a wide range of applications due to their excellent chemical stability as well as optical, electrical and magnetic properties [1].The have been used as p-type transparent conducting films [2], an antiferromagnetic materials [3], in electrochromic devices [4], and as a functional sensor layer for chemical sensors [5]. Using NiO films as the recording materials of optical disc is one of their new applications. Recently, NiOx films as a new write-once recording medium of blue laser recording have been proposed [6], and the carrier to noise ratio (CNR) was more than 47 dB and jitter was less than 13%. The optical constants (refractive index n and extinction coefficient k) and the film thickness are important parameters that affect the performance of an optical film in an optical system [7] and the optical simulation results are strongly dependent on the optical constants of the films [8]. Up to now, although a number of reports have discussed the properties of sputtered NiOx films [9–11], little research has been done to investigate the properties of low thickness NiOx films. In blue laser recording, the thickness of the recording layer was only about 30 nm; therefore many studies must be focused on the optical properties of thin films. In the present work, the influence of the thickness variation on the
n
Corresponding author at: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Number 390, Qing he Lu (Road), Jia ding, Shanghai 201800, PR China. Tel.:+ 86 216 9918592; fax: + 86 216 9918800. E-mail address: [email protected] (Y. Zhou). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.02.100
optical properties of NiOx films deposited DC-magnetron sputtering is considered.
by
reactive
Experimental Single-layer NiOx films were deposited on K9 glass or single-crystal silicon substrates of 30 mm in diameter by reactive DC-magnetron sputtering using a 99.99% pure nickel target in a mixture of argon–oxygen environment. The flow rates of argon and oxygen were individually controlled using a flow meter and O2/Ar ratio was 5%. The base pressure in the deposition chamber was typically 7.0 10 4 Pa. The sputtering pressure was 0.6 Pa and the sputtering power was 250 W. The sputtering time was changed in order to obtain different film thicknesses. After deposition, some samples were annealed at 400 1C for 30 min in air. The thickness of films was examined by an Alpha-step 500 surface profilometer (Tencor, America). Energy dispersive spectroscopy (EDS) measurements were carried out on a JEOL JEM-2011 energy dispersive spectrometer to identify the elements in the films. The surface morphology of the films was investigated by atom force microscopy (AFM, Seiko SPA-500). The reflectance and transmittance of the films were measured by a Perkin-Elmer Lambda (900UV/VIS/NIR) spectrophotometer in the wavelength range of 350–700 nm. Calculate method A simple method for computation of the optical constants, using the optical transmission and reflectivity spectrum at normal
3876
Y. Zhou et al. / Physica B 405 (2010) 3875–3878
incidence, is employed here. When the extinction coefficient of the substrate is kg ¼0 and the refractive index of the atmosphere is n0 ¼1, the Hadley equation is given by [12,13] Rc ¼
M1 cosh r þ M2 sinh bM3 cos b þ M4 sin b G1 cosh r þ G2 sinh bG3 cos b þ G4 sin b
ð1Þ
Tc ¼
8ng ðn2 þ k2 Þ G1 cosh r þ G2 sinh bG3 cos b þ G4 sin b
ð2Þ
Here
r ¼ 4pkd=l, b ¼ 4pnd=l M1 ¼ ðn2 þ k2 þ 1Þðn2 þk2 þn2g Þ4n2 ng M2 ¼ 2n½ng ðn2 þ k2 þ 1Þðn2 þ k2 þn2g Þ M3 ¼ ðn2 þ k2 1Þðn2 þk2 n2g Þ þ 4k2 ng M4 ¼ 2k½ng ðn2 þk2 1Þðn2 þ k2 n2g Þ G1 ¼ ðn2 þk2 þ 1Þðn2 þ k2 þ n2g Þ þ 4n2 ng G2 ¼ 2n½ng ðn2 þ k2 þ1Þ þðn2 þk2 þn2g Þ G3 ¼ ðn2 þk2 1Þðn2 þ k2 n2g Þ4k2 ng G4 ¼ 2k½ng ðn2 þ k2 1Þ þ ðn2 þ k2 n2g Þ where d is the thicknesses of the films, ng is the refractive index of the substrate and l is the wavelength. However, Rc and Tc are affected by the underside surface of the substrate. Therefore, the total transmission and reflectivity of the films are given by [14] Rðn,k,d, lÞ ¼ Rc þ Tðn,k,d, lÞ ¼
Tc2 R0 1R0 R1
Tc T0 1R0 R1
ð3Þ
ð4Þ
where R0 is the reflectivity of the underside surface of the substrate and R0 ¼ ½ð1ng Þ=ð1 þ ng Þ2 , T0 is the transmission of the underside surface of the substrate and T0 ¼ 1R0 , R1 can be calculated through the equation R1 ¼
M1 cosh rM2 sinh bM3 cos bM4 sin b G1 cosh r þ G2 sinh bG3 cosb þG4 sin b
ð5Þ
R and T can be measured by spectrophotometer, so n and k of the films are obtained through the nonlinear combination equation composed of Eqs. (3) and (4). The optical absorption coefficient a of the investigated thin films as a function of transmission T and reflectivity R was given by [15] 8 8" 91=2 9 #2 > = = > 1 <ð1RÞ2 < ð1RÞ2 2 ð6Þ þ a ¼ ln þR > > : ; d : 2T 2T ; The optical band gap Eg of the films can be determined from the dependence of absorption coefficient on the photon energy. The direct or indirect nature of optical transition between parabolic bands can be studied using the relation [16,17]
ahv ¼ BðhvEg Þg
ð7Þ
where B is the quality factor depending on the transition probability, hv is the photoenergy, and the power term g is an index that characterizes the optical absorption process. g can take values of 2, 3, 1/2 or 3/2 depending on the nature of electronic transitions.
Results and discussion The elemental analysis with EDS as shown in Fig. 1 demonstrates that the as-deposited films contain Ni, O and Si. Si comes from the single-crystal silicon substrates due to the low
Fig. 1. Elemental analysis of as-deposited films by EDS.
thickness of the films and the results show that the as-deposited films are oxygen-excess NiOx films (x41). Our previous studies showed that the as-deposited NiOx films contained both Ni2 + and Ni3 + through X-ray photoelectron spectroscopy (XPS) [18]. The Ni3 + ion of Ni2O3 were produced as color centers in NiOx thin films. This may be related to many Ni2 + vacancies existed in NiOx films, and to keep the charge near the Ni2 + vacancies neutral, some of the Ni2 + were oxidized to Ni3 + [19]. It was also reported in literature that oxygen-excess NiOx films were thermally unstable. Depending on the NiOx composition and the annealing atmosphere, they decomposed into thermally more stable, oxygen-poorer NiO and/or Ni at or above 300 1C [20]. Chang et al. [6] proposed that the recording mechanism of NiOx films was due to NiOx-NiO+ O2 released O2, which pushed the soft substrate while the recording layer was irradiated. Fig. 2 shows the AFM images of as-deposited and annealed films. It was found that the surface morphology of the as-deposited films was smooth and compact, and the films annealed at 400 1C became very rough due to the bubble formation coming from the decomposition of NiOx. Fig. 3 shows the reflectivity spectra of the films recorded at different film thicknesses. It was observed that the reflectivity of both the as-deposited and annealed films at different film thicknesses increased with the increase in wavelength. In addition, the reflectivity of these films increased with increase in the film thickness. It was accepted that the growth of films by sputtering method was mainly composed by island, network and continuous stages. The crystallite sizes of NiO films increasing with increase in film thickness have been observed [10]. Therefore, the improvement of NiO proportion with the film thickness may result in the increasing reflectivity [21]. After annealing, the reflectivity of the films at different film thicknesses decreased obviously due to NiOx-NiO+ O2 released O2, leading to a high optical contrast between as-deposited and annealed films, which was enough for optical recording. Transmission spectra of NiOx films at different film thicknesses are shown in Fig. 4. The transmission of the films decreased as the film thickness increases. In addition, the transmission of the films deposited at different film thicknesses increased after annealing, which was related to the color center of Ni3 + becoming the bleached state of Ni2 + . When film thickness exceeded a certain critical value, the optical constants did not change significantly with film thickness.
Y. Zhou et al. / Physica B 405 (2010) 3875–3878
3877
Fig. 2. AFM images of NiOx films: (a) as-deposited and (b) annealed at 400 1C.
Fig. 3. Reflectance spectra of NiOx films at different film thicknesses.
In this case, the optical constants of the recording materials were assumed to be constant regardless of film thickness in conventional optical design and thermal simulation of multilayer structure optical discs [22]. However, this assumption was not valid in blue laser recording because the thickness of recording materials became very low. The refractive index n and extinction coefficient k of the films dependence of the wavelength are shown in Figs. 5 and 6. It was observed that both refractive index n and extinction coefficient k of the films at different thicknesses increased with the increase in wavelength. It could be also seen that the refractive index n and extinction coefficient k of the films changed with films thickness. After annealing, the refractive index n of the films at different thicknesses changed slightly, meanwhile, the extinction coefficient k of the films decreased from 1.6–2.7 to 0.2–1.3 obviously. Therefore, the high optical contrast between as-deposited and annealed films can be related to the changes of the extinction coefficient k of the films after annealing. The direct or indirect nature of optical transition between parabolic bands can be studied using Eq. (7). In the present study, the variation of absorption coefficient with photoenergy followed the above relation for g ¼1/2, indicating that the transition should correspond to direct allowed transition. As shown in Fig. 7, a linear relation between (ahv)2 and hv was found for NiOx thin films of three different thicknesses. The optical band gap Eg could be obtained by extrapolating the linear portion to ahv¼0 from
Fig. 4. Transmission spectra of NiOx films at different film thicknesses.
Fig. 5. The variation of refractive index with wavelength for films at different film thicknesses.
Fig. 7. The values of Eg for the films coated with thicknesses of 17, 27 and 34 nm were 2.37, 2.50 and 2.57 eV, respectively. It was well know that unsaturated bonds lead to the formation of some
3878
Y. Zhou et al. / Physica B 405 (2010) 3875–3878
films. Varkey and Fort reported Eg ¼3.25 eV for NiO films [24]. The optical band gaps of the NiOx thin films calculated in the present study were smaller than the previous studies. The difference in the optical band gap could come from low thickness and vacancies existed in the films.
Conclusion
Fig. 6. The variation of extinction coefficient with wavelength for films at different film thicknesses.
NiOx (x41) thin films have been deposited by reactive DCmagnetron sputtering from a nickel metal target in Ar+ O2 with the relative O2 content 5%. The reflectivity of these films increased and the transmission decreased with increase in the film thickness. After annealing, the reflectivity of the films at different film thicknesses decreased obviously due to NiOx-NiO+ O2 released O2, leading to a high optical contrast between asdeposited and annealed films. The high optical contrast can be related to the changes of the extinction coefficient k of the films after annealing. Both the refractive index n and extinction coefficient k of the films changed with films thickness. The absorption mechanism of NiOx thin films was due to direct allowed transition. The optical band gaps of the NiOx films were 2.37, 2.50 and 2.57 eV corresponding to the films thicknesses of 17, 27 and 34 nm, respectively.
Acknowledgments The present work is supported by the National High Technology Development Program (2004AA31G230) and National Natural Science Foundation (50672108, 60644002) of China, which are greatly appreciated. References
Fig. 7. Absorption coefficient plotted as (ahv)2 versus the photoenergy for films at different film thicknesses.
defects in the small thickness films and increase the out-of-order structure of films. In thicker films, while keeping the same conditions, greater time deposition builds up more homogeneous network, minimizing the number of defects and the localized states, thus increasing the optical band gap [23]. Therefore, the optical band gap of NiOx thin films increased from 2.37 to 2.57 eV with increasing the film thickness from 17 to 34 nm. Several other researchers have studied the absorption of photoenergy for NiO
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Y.M. Lu, W.S. Hwang, J.S. Yang, Surf. Coat. Technol. 155 (2002) 231. H. Sato, T. Minami, S. Takata, et al., Thin Solid Films 236 (1993) 27. E. Fujii, A. Tomozawa, H. Torii, et al., Jpn. J. Appl. Phys. 35 (1996) L328. M. Kitao, K. Izawa, K. Urabe, et al., Jpn. J. Appl. Phys. 33 (1994) 6656. M. Ferroni, D. Boscarion, E. Comini, et al., Sensors Actuators B 58 (1999) 289. H.L. Chang, T.R. Jeng, J.P. Chen, et al., Jpn. J. Appl. Phys. 44 (2005) 6109. A.M. El-Naggar, Opt. Laser Technol. 33 (2001) 237. X.S. Miao, T.C. Chong, Y.M. Huang, et al., Jpn. J. Appl. Phys. 38 (1999) 1638. I. Hotovy, J. Huran, J. Janlk, et al., Vacuum 51 (1998) 157. H.L. Chen, Y.M. Lu, W.S. Hwang, Thin Solid Films 498 (2006) 266. J.L. Yang, Y.S. Lai, J.S. Chen, Thin Solid Films 488 (2005) 242. D.E. Gray, in: Handbook Of Optical Constants Of Solids, Mcgraw-Hill, New York, 1963. X. Sun, R. Hong, H. Qi, et al., Acta Phys. Sin. 55 (2006) 4923 In Chinese. L. Harris, A.L. Loeb, J. Opt. Soc. Am. 45 (1954) 179. J. Herrero, C. Guillen, J. Appl. Phys. 69 (1991) 429. A. El-Korashy, H. El-Zahed, M. Radwan, Physica B 334 (2003) 75. V. Pandey, S.K. Tripathi, A. Kumar, Physica B 388 (2007) 200. Y. Zhou, Y. Geng, D. Gu, Mater. Lett. 61 (2007) 2482. S.R. Jiang, P.X. Yan, B.X. Feng, et al., Mater. Chem. Phys. 77 (2002) 384. W. Bruckner, R. Kaltofen, J. Thomas, et al., J. Appl. Phys. 94 (2003) 4853. H. Du, J.Q. Xiao, Y.S. Zou, et al., Opt. Mater. 28 (2006) 944. J.Y. Li, L.S. Hou, H. Rao, et al., Proc. SPIE 4085 (2001) 129. M. Fang, Q.H. Li, F.X. Gan, Chin. Opt. Lett. 2 (2004) 177. A.J. Varkey, A.F. Fort, Thin Solid Films 235 (1993) 47.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effect of film thickness on the optical constants and optical absorption properties of NiOx thin films Ying Zhou a,b,n, Yongyou Geng a, Donghong Gu a, Weibing Gu a,b, Zhi Jiang a,b a b
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China. Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e in fo
abstract
Article history: Received 2 January 2007 Received in revised form 6 February 2007 Accepted 17 February 2007
NiOx (x 4 1) thin films were deposited by reactive DC-magnetron sputtering from a nickel metal target in Ar +O2 with the relative O2 content 5%. The reflectivity and transmission of the films were measured from 350 to 700 nm. The films coated to thicknesses of 17, 27 and 34 nm had the same absorption mechanism which is a direct allowed transition. The refractive index n and extinction coefficient k of the films were calculated from the measured reflectivity and transmission of these films, and the relationship of the optical band gap with variation in film thickness was studied as well. All results showed that the optical properties of NiOx thin films depended on the film thickness. & 2010 Elsevier B.V. All rights reserved.
Keywords: NiOx thin films Optical constants Optical band gap Film thickness Annealing
Introduction NiO films have a wide range of applications due to their excellent chemical stability as well as optical, electrical and magnetic properties [1].The have been used as p-type transparent conducting films [2], an antiferromagnetic materials [3], in electrochromic devices [4], and as a functional sensor layer for chemical sensors [5]. Using NiO films as the recording materials of optical disc is one of their new applications. Recently, NiOx films as a new write-once recording medium of blue laser recording have been proposed [6], and the carrier to noise ratio (CNR) was more than 47 dB and jitter was less than 13%. The optical constants (refractive index n and extinction coefficient k) and the film thickness are important parameters that affect the performance of an optical film in an optical system [7] and the optical simulation results are strongly dependent on the optical constants of the films [8]. Up to now, although a number of reports have discussed the properties of sputtered NiOx films [9–11], little research has been done to investigate the properties of low thickness NiOx films. In blue laser recording, the thickness of the recording layer was only about 30 nm; therefore many studies must be focused on the optical properties of thin films. In the present work, the influence of the thickness variation on the
n
Corresponding author at: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Number 390, Qing he Lu (Road), Jia ding, Shanghai 201800, PR China. Tel.:+ 86 216 9918592; fax: + 86 216 9918800. E-mail address: [email protected] (Y. Zhou). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.02.100
optical properties of NiOx films deposited DC-magnetron sputtering is considered.
by
reactive
Experimental Single-layer NiOx films were deposited on K9 glass or single-crystal silicon substrates of 30 mm in diameter by reactive DC-magnetron sputtering using a 99.99% pure nickel target in a mixture of argon–oxygen environment. The flow rates of argon and oxygen were individually controlled using a flow meter and O2/Ar ratio was 5%. The base pressure in the deposition chamber was typically 7.0 10 4 Pa. The sputtering pressure was 0.6 Pa and the sputtering power was 250 W. The sputtering time was changed in order to obtain different film thicknesses. After deposition, some samples were annealed at 400 1C for 30 min in air. The thickness of films was examined by an Alpha-step 500 surface profilometer (Tencor, America). Energy dispersive spectroscopy (EDS) measurements were carried out on a JEOL JEM-2011 energy dispersive spectrometer to identify the elements in the films. The surface morphology of the films was investigated by atom force microscopy (AFM, Seiko SPA-500). The reflectance and transmittance of the films were measured by a Perkin-Elmer Lambda (900UV/VIS/NIR) spectrophotometer in the wavelength range of 350–700 nm. Calculate method A simple method for computation of the optical constants, using the optical transmission and reflectivity spectrum at normal
3876
Y. Zhou et al. / Physica B 405 (2010) 3875–3878
incidence, is employed here. When the extinction coefficient of the substrate is kg ¼0 and the refractive index of the atmosphere is n0 ¼1, the Hadley equation is given by [12,13] Rc ¼
M1 cosh r þ M2 sinh bM3 cos b þ M4 sin b G1 cosh r þ G2 sinh bG3 cos b þ G4 sin b
ð1Þ
Tc ¼
8ng ðn2 þ k2 Þ G1 cosh r þ G2 sinh bG3 cos b þ G4 sin b
ð2Þ
Here
r ¼ 4pkd=l, b ¼ 4pnd=l M1 ¼ ðn2 þ k2 þ 1Þðn2 þk2 þn2g Þ4n2 ng M2 ¼ 2n½ng ðn2 þ k2 þ 1Þðn2 þ k2 þn2g Þ M3 ¼ ðn2 þ k2 1Þðn2 þk2 n2g Þ þ 4k2 ng M4 ¼ 2k½ng ðn2 þk2 1Þðn2 þ k2 n2g Þ G1 ¼ ðn2 þk2 þ 1Þðn2 þ k2 þ n2g Þ þ 4n2 ng G2 ¼ 2n½ng ðn2 þ k2 þ1Þ þðn2 þk2 þn2g Þ G3 ¼ ðn2 þk2 1Þðn2 þ k2 n2g Þ4k2 ng G4 ¼ 2k½ng ðn2 þ k2 1Þ þ ðn2 þ k2 n2g Þ where d is the thicknesses of the films, ng is the refractive index of the substrate and l is the wavelength. However, Rc and Tc are affected by the underside surface of the substrate. Therefore, the total transmission and reflectivity of the films are given by [14] Rðn,k,d, lÞ ¼ Rc þ Tðn,k,d, lÞ ¼
Tc2 R0 1R0 R1
Tc T0 1R0 R1
ð3Þ
ð4Þ
where R0 is the reflectivity of the underside surface of the substrate and R0 ¼ ½ð1ng Þ=ð1 þ ng Þ2 , T0 is the transmission of the underside surface of the substrate and T0 ¼ 1R0 , R1 can be calculated through the equation R1 ¼
M1 cosh rM2 sinh bM3 cos bM4 sin b G1 cosh r þ G2 sinh bG3 cosb þG4 sin b
ð5Þ
R and T can be measured by spectrophotometer, so n and k of the films are obtained through the nonlinear combination equation composed of Eqs. (3) and (4). The optical absorption coefficient a of the investigated thin films as a function of transmission T and reflectivity R was given by [15] 8 8" 91=2 9 #2 > = = > 1 <ð1RÞ2 < ð1RÞ2 2 ð6Þ þ a ¼ ln þR > > : ; d : 2T 2T ; The optical band gap Eg of the films can be determined from the dependence of absorption coefficient on the photon energy. The direct or indirect nature of optical transition between parabolic bands can be studied using the relation [16,17]
ahv ¼ BðhvEg Þg
ð7Þ
where B is the quality factor depending on the transition probability, hv is the photoenergy, and the power term g is an index that characterizes the optical absorption process. g can take values of 2, 3, 1/2 or 3/2 depending on the nature of electronic transitions.
Results and discussion The elemental analysis with EDS as shown in Fig. 1 demonstrates that the as-deposited films contain Ni, O and Si. Si comes from the single-crystal silicon substrates due to the low
Fig. 1. Elemental analysis of as-deposited films by EDS.
thickness of the films and the results show that the as-deposited films are oxygen-excess NiOx films (x41). Our previous studies showed that the as-deposited NiOx films contained both Ni2 + and Ni3 + through X-ray photoelectron spectroscopy (XPS) [18]. The Ni3 + ion of Ni2O3 were produced as color centers in NiOx thin films. This may be related to many Ni2 + vacancies existed in NiOx films, and to keep the charge near the Ni2 + vacancies neutral, some of the Ni2 + were oxidized to Ni3 + [19]. It was also reported in literature that oxygen-excess NiOx films were thermally unstable. Depending on the NiOx composition and the annealing atmosphere, they decomposed into thermally more stable, oxygen-poorer NiO and/or Ni at or above 300 1C [20]. Chang et al. [6] proposed that the recording mechanism of NiOx films was due to NiOx-NiO+ O2 released O2, which pushed the soft substrate while the recording layer was irradiated. Fig. 2 shows the AFM images of as-deposited and annealed films. It was found that the surface morphology of the as-deposited films was smooth and compact, and the films annealed at 400 1C became very rough due to the bubble formation coming from the decomposition of NiOx. Fig. 3 shows the reflectivity spectra of the films recorded at different film thicknesses. It was observed that the reflectivity of both the as-deposited and annealed films at different film thicknesses increased with the increase in wavelength. In addition, the reflectivity of these films increased with increase in the film thickness. It was accepted that the growth of films by sputtering method was mainly composed by island, network and continuous stages. The crystallite sizes of NiO films increasing with increase in film thickness have been observed [10]. Therefore, the improvement of NiO proportion with the film thickness may result in the increasing reflectivity [21]. After annealing, the reflectivity of the films at different film thicknesses decreased obviously due to NiOx-NiO+ O2 released O2, leading to a high optical contrast between as-deposited and annealed films, which was enough for optical recording. Transmission spectra of NiOx films at different film thicknesses are shown in Fig. 4. The transmission of the films decreased as the film thickness increases. In addition, the transmission of the films deposited at different film thicknesses increased after annealing, which was related to the color center of Ni3 + becoming the bleached state of Ni2 + . When film thickness exceeded a certain critical value, the optical constants did not change significantly with film thickness.
Y. Zhou et al. / Physica B 405 (2010) 3875–3878
3877
Fig. 2. AFM images of NiOx films: (a) as-deposited and (b) annealed at 400 1C.
Fig. 3. Reflectance spectra of NiOx films at different film thicknesses.
In this case, the optical constants of the recording materials were assumed to be constant regardless of film thickness in conventional optical design and thermal simulation of multilayer structure optical discs [22]. However, this assumption was not valid in blue laser recording because the thickness of recording materials became very low. The refractive index n and extinction coefficient k of the films dependence of the wavelength are shown in Figs. 5 and 6. It was observed that both refractive index n and extinction coefficient k of the films at different thicknesses increased with the increase in wavelength. It could be also seen that the refractive index n and extinction coefficient k of the films changed with films thickness. After annealing, the refractive index n of the films at different thicknesses changed slightly, meanwhile, the extinction coefficient k of the films decreased from 1.6–2.7 to 0.2–1.3 obviously. Therefore, the high optical contrast between as-deposited and annealed films can be related to the changes of the extinction coefficient k of the films after annealing. The direct or indirect nature of optical transition between parabolic bands can be studied using Eq. (7). In the present study, the variation of absorption coefficient with photoenergy followed the above relation for g ¼1/2, indicating that the transition should correspond to direct allowed transition. As shown in Fig. 7, a linear relation between (ahv)2 and hv was found for NiOx thin films of three different thicknesses. The optical band gap Eg could be obtained by extrapolating the linear portion to ahv¼0 from
Fig. 4. Transmission spectra of NiOx films at different film thicknesses.
Fig. 5. The variation of refractive index with wavelength for films at different film thicknesses.
Fig. 7. The values of Eg for the films coated with thicknesses of 17, 27 and 34 nm were 2.37, 2.50 and 2.57 eV, respectively. It was well know that unsaturated bonds lead to the formation of some
3878
Y. Zhou et al. / Physica B 405 (2010) 3875–3878
films. Varkey and Fort reported Eg ¼3.25 eV for NiO films [24]. The optical band gaps of the NiOx thin films calculated in the present study were smaller than the previous studies. The difference in the optical band gap could come from low thickness and vacancies existed in the films.
Conclusion
Fig. 6. The variation of extinction coefficient with wavelength for films at different film thicknesses.
NiOx (x41) thin films have been deposited by reactive DCmagnetron sputtering from a nickel metal target in Ar+ O2 with the relative O2 content 5%. The reflectivity of these films increased and the transmission decreased with increase in the film thickness. After annealing, the reflectivity of the films at different film thicknesses decreased obviously due to NiOx-NiO+ O2 released O2, leading to a high optical contrast between asdeposited and annealed films. The high optical contrast can be related to the changes of the extinction coefficient k of the films after annealing. Both the refractive index n and extinction coefficient k of the films changed with films thickness. The absorption mechanism of NiOx thin films was due to direct allowed transition. The optical band gaps of the NiOx films were 2.37, 2.50 and 2.57 eV corresponding to the films thicknesses of 17, 27 and 34 nm, respectively.
Acknowledgments The present work is supported by the National High Technology Development Program (2004AA31G230) and National Natural Science Foundation (50672108, 60644002) of China, which are greatly appreciated. References
Fig. 7. Absorption coefficient plotted as (ahv)2 versus the photoenergy for films at different film thicknesses.
defects in the small thickness films and increase the out-of-order structure of films. In thicker films, while keeping the same conditions, greater time deposition builds up more homogeneous network, minimizing the number of defects and the localized states, thus increasing the optical band gap [23]. Therefore, the optical band gap of NiOx thin films increased from 2.37 to 2.57 eV with increasing the film thickness from 17 to 34 nm. Several other researchers have studied the absorption of photoenergy for NiO
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Y.M. Lu, W.S. Hwang, J.S. Yang, Surf. Coat. Technol. 155 (2002) 231. H. Sato, T. Minami, S. Takata, et al., Thin Solid Films 236 (1993) 27. E. Fujii, A. Tomozawa, H. Torii, et al., Jpn. J. Appl. Phys. 35 (1996) L328. M. Kitao, K. Izawa, K. Urabe, et al., Jpn. J. Appl. Phys. 33 (1994) 6656. M. Ferroni, D. Boscarion, E. Comini, et al., Sensors Actuators B 58 (1999) 289. H.L. Chang, T.R. Jeng, J.P. Chen, et al., Jpn. J. Appl. Phys. 44 (2005) 6109. A.M. El-Naggar, Opt. Laser Technol. 33 (2001) 237. X.S. Miao, T.C. Chong, Y.M. Huang, et al., Jpn. J. Appl. Phys. 38 (1999) 1638. I. Hotovy, J. Huran, J. Janlk, et al., Vacuum 51 (1998) 157. H.L. Chen, Y.M. Lu, W.S. Hwang, Thin Solid Films 498 (2006) 266. J.L. Yang, Y.S. Lai, J.S. Chen, Thin Solid Films 488 (2005) 242. D.E. Gray, in: Handbook Of Optical Constants Of Solids, Mcgraw-Hill, New York, 1963. X. Sun, R. Hong, H. Qi, et al., Acta Phys. Sin. 55 (2006) 4923 In Chinese. L. Harris, A.L. Loeb, J. Opt. Soc. Am. 45 (1954) 179. J. Herrero, C. Guillen, J. Appl. Phys. 69 (1991) 429. A. El-Korashy, H. El-Zahed, M. Radwan, Physica B 334 (2003) 75. V. Pandey, S.K. Tripathi, A. Kumar, Physica B 388 (2007) 200. Y. Zhou, Y. Geng, D. Gu, Mater. Lett. 61 (2007) 2482. S.R. Jiang, P.X. Yan, B.X. Feng, et al., Mater. Chem. Phys. 77 (2002) 384. W. Bruckner, R. Kaltofen, J. Thomas, et al., J. Appl. Phys. 94 (2003) 4853. H. Du, J.Q. Xiao, Y.S. Zou, et al., Opt. Mater. 28 (2006) 944. J.Y. Li, L.S. Hou, H. Rao, et al., Proc. SPIE 4085 (2001) 129. M. Fang, Q.H. Li, F.X. Gan, Chin. Opt. Lett. 2 (2004) 177. A.J. Varkey, A.F. Fort, Thin Solid Films 235 (1993) 47.