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Laser ablation lithography on thermoelectric semiconductor
Applied Surface Science 252 (2006) 4759–4762 www.elsevier.com/locate/apsusc
Laser ablation lithography on thermoelectric semiconductor O.Yu. Semchuk a,*, V.N. Semioshko a, L.G. Grechko a, M. Willander b, M. Karlsteen b a
Institute of Surface Chemistry NAS of Ukraine, 17 General Naumov Street, 03164 Kyiv, Ukraine b Goteborg University, SE-412 96 Gothenburg, Sweden Received 3 May 2005; accepted 15 July 2005 Available online 7 November 2005
Abstract In this paper, experimental results of the investigation of the periodic structure on thermoelectric semiconductor Cu2Se are presented. Periodic structures were formed on surfaces of semiconductors due to multi-beam interaction of Q-switched Nd:YAG laser, which was operated in the lowest order of Gaussian mode and pulse duration 7 ns. Surface temperature evolution and transient reflectivity are studied during laser treatment. Creation of Cu islands in the maximal intensity of interference pattern was found. # 2005 Elsevier B.V. All rights reserved. Keywords: Laser ablation; Fluence; Laser-induced structures; Surface; Thermoelectric semiconductors; Cooper islands
1. Introduction In recent years, different works using high power laser have shown that laser materials processing has a great potential for industrial applications, especially in microelectronics. The laser technique presents advantages in material processing due to the numerous possibilities of treatment: cleaning [1–3], decontamination [4,5], surface modification [6,7] and nanoparticles deposition [8,9]. Several methods based on the application of laser beams make it possible to generate micrometer and sub-micrometer periodic structures on semiconductors and polymers. Grating-like pattern develops as a result of the irradiation by laser light through a photo-mask [10], phase-mask [11] or caused by multi-beams interference technique. The features of them are influenced by the ablation characteristic of the material. The illuminations of semiconductors by interference pattern of laser beams having fluence close to ablation threshold result in laser-induced periodic surface structure (LIPSS) formation. The characteristic size of the LIPSS is the order of the magnitude of the laser wavelength. In this study, we combined transient reflectivity technique, Auger electron spectroscopy and atomic force microscopy to
realize complex analysis periodic structure generated on surface of thermoelectric semiconductors Cu2Se. 2. Multi-beam interference Two or more overlapping coherent and linear polarized laser beams produce an interference pattern which is defined of geometry depending on the used wavelength and angles between the beams. The two-dimensional pattern created by two beams has an intensity distribution as: 4px u IðxÞ ¼ 2I0 cos sin þ1 ; l 2
(1)
where I0 is the intensity of one laser beam, l the wavelength and u is the angle between the beams. The periodicity of interference pattern can be defined by the wavelength and by the angle between the beams, and is d = l/(2 sin u/2). Three beams produce a two-dimensional pattern. Four planar beams interference realizes a line pattern with thinner and higher peaks and thicker areas of low intensity compared to a two beam pattern. 3. Experimental
* Corresponding author. Tel.: +380 44 442 9697; fax: +380 44 424 3567. E-mail address: [email protected] (O.Yu. Semchuk). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.112
The laser structuring experiments were carried out in vacuum chamber under pressure 104 Pa using Q-switched Nd:YAG laser which operated in the lowest order of Gaussian
4760
O.Y. Semchuk et al. / Applied Surface Science 252 (2006) 4759–4762
Fig. 1. A schematic drawing of experimental set up (M—mirror, BS—beam splitter, S—sample and D—detector).
mode with a pulse duration 7 ns. The fundamental emission 1064 nm is converted to 532 nm and 355 nm by means of a nonlinear crystal. The laser beam had Gaussian intensity distribution with a diameter of 5 mm (1/e2) and was focused to 1.8 mm by lenses. The intensity was controlled by l/2 wave plate polarization rotator and flash lamp. The number of pulses was selected by a mechanical shutter. Beam splitters were used to obtain two or more coherent beams. Mirrors guided the splitted beams on the sample surface. An He:Ne laser beam and detector connected to digital oscilloscope were used to study transient reflectivity under laser treatment of the surface of the sample. A schematic drawing of experimental set up is sketched in Fig. 1. 4. Surface temperature evolution and transient reflectivity To estimate the temporal temperature evolution we have used heat transfer equation with interference pattern as heat source: rðTÞcp ðTÞ
@Tð~ r; tÞ @t
r½kðTÞ rTð~ r; t ¼ aI0 ð~ rÞð1 RÞ expðazÞ rÞ ¼ I0 ð~
IðxÞ 2ps expfðt tp Þ2 =2s 2 g
(2) ;
(3)
tp s ¼ pffiffiffiffiffiffiffiffiffiffiffi ; 2 2 ln 2
(4)
Fig. 2. Surface temperature evolution (1—laser fluence is 0.5 J/cm2 and 2— laser fluence is 0.75 J/cm2).
where r(T) is the density, T the absolute temperature of sample, cp(T) the heat capacity, r the position vector, k(T) the heat conductivity, tp the pulse time, tp the laser pulse duration, R the reflectivity of the surface, a the absorption coefficient and I(x) is the intensity distribution of interference pattern, which is described by Eq. (1). We have simulated the temperature evolution during laser interaction. The laser light is absorbed in the near surface region. The power absorbed by the solid can be expressed by the source term which is time dependent. The absorbed light energy is then converted into heat which diffused into the bulk by thermal conduction. We assumed first that the optical length absorption and the reflectivity of semiconductors are constant. Material characteristics used for temperature simulation are given in Table 1. Results of numerical simulation of the surface temperature evolution during laser interaction with thermoelectric semiconductors are presented in Fig. 2. The recorded time dependence of reflected signal from cw He–Ne laser is shown in Fig. 3 for two different fluence. The maximum reflecting rate is reached after the end of the pulse and at higher fluence is seen another temporal behavior than at lower. Several effects may be responsible for transient reflectivity changes of Cu2Se caused by intense laser radiation. One of them is temperature-dependent real and imaginary part of refractive index [12]; the second, which has been investigated in [13], is reflectivity changes due to melting; last but not least is phase transformation of material from one state to another [14]. At lower fluence meting temperature 1386 K does not reach and transient changes in reflectivity are caused by temperaturedependent real and imaginary parts of refractive index.
Table 1 Material characteristics of Cu2Se used for temperature simulation Wavelength (nm)
Reflectivity (a.u.)
Absorption coefficient (108 m1)
Heat capacity (J kg1 K1)
Heat conductivity (W m1 K1)
Density (kg m3)
Latent heat of fusion (105 J kg1)
1064 633 532 355
0.4 0.41 (liquid: 0.52) 0.42 0.32
3.1 – 10.1 27
510
95
6739
1.2
O.Y. Semchuk et al. / Applied Surface Science 252 (2006) 4759–4762
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Fig. 5. AFM picture of structured Cu2Se sample.
Fig. 3. Time evolution of transient reflectivity during laser pulse (1—laser fluence is 0.75 J/cm2 and 2—laser fluence is 0.5 J/cm2).
At higher fluence the reflectivity abruptly increases when the surface of semiconductors melts. Then, reflectivity is consistent with the value of reflectivity to liquid semiconductors at 633 nm. The reflectivity stays at this given time by the flat portion of reflectivity curve and decreases when the temperature decreases in crystalline semiconductors. Similar behavior of transient reflectivity due to meting of silicon wafer has been obtained recently during laser treatment with single laser beam. Marine et al. [15] were able to detect self-diffraction and forced scattering during and after self-diffraction excitation with 40 ns ruby laser on GaAs film. They got sharp kink on time dependence of scattered signals, which has been attributed to melting. We made steady-state measurements of reflectivity before and after laser treatment. The change between initial reflectivity and final was about 2.5%. It may be caused by phase transformation, or formation of Cu islands on surface of semiconductors Cu2Se. Additional studies performed by means of Auger spectroscopy confirmed the increase of concentration of Cu atoms in the maximum value of intensity of interference
Fig. 4. Concentration dependence of the copper atoms versus number of laser pulses on the surface in maximum value of interference pattern.
pattern. The dependence of concentration of the copper atoms versus number of laser pulses is shown in Fig. 4. Two important effects must be considered for nanosecond time scale of laser ablation processes. The first involves the interaction between the laser beam and expanding plume, while the second is due to long transfer time of the laser energy on to target material. Moreover, the latter induces a thermal ablation mechanism, whereas the former causes a strong ionization and dissociation of the ablated species [16]. Both dissociation of the Cu2Se and strong evaporation of the Se are responsible for the formation of Cooper islands. The surface of the Cu2Se was investigated by atomic force microscopy (TMX 2000) in tapping mode realized in air. We made measurements before and after laser treatment. It was found that untreated surface had average height Dh 20 nm. AFM image of a structured surface copper selenide is shown in Fig. 5. The etch depth of Cu2Se in dependence of laser fluence at laser wavelength of 1064 nm, 532 nm and 355 nm is compared in Fig. 6. The slope of etch depth is different for three wavelengths. For a one-photon absorption model, the fluence dependence of the etch depth of slightly above threshold is expressed by the relation [17]: 1 F log ; (5) a Fn where F and F n designate laser fluence and threshold fluence, respectively. According to Eq. (5), a linear relation between D D¼
Fig. 6. The fluence dependence of the structure depth induced by laser fluency (1—laser wavelength is 1064 nm, 2—laser wavelength is 532 nm and 3—laser wavelength is 355 nm).
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O.Y. Semchuk et al. / Applied Surface Science 252 (2006) 4759–4762
and log FFn is expected. A deviation at high fluence is observed, which may be caused by plume absorption and formation of Cu islands on surface of thermoelectric semiconductors. 5. Conclusion It was shown that a laser interference pattern allows a micro– nano structuring of surfaces of thermoelectric semiconductors. Firstly, the properties of surface periodic structures depend on laser wavelength and ablation characteristic of semiconductors. Secondly, the laser interference structuring can generate creation of Cu islands on a surface of Cu2Se. Furthermore, the heat diffusion is not important on nanosecond time scale for generation of LIPSS with spacing of 5–10 mm. Acknowledgement We acknowledge financial support through a grant (220180-124G) from the Royal Swedish Academy of Sciences. References [1] J.M. Lee, C. Curran, K.C. Watking, Appl. Phys. A 73 (2001) 219.
[2] P. Maravelaki-Kalaitzki, V. Zafiropulus, C. Fotakis, Appl. Surf. Sci. 148 (1999) 92. [3] P. Neves, M. Arronte, R. Vilar, A.M. Botelho, Appl. Phys. A 74 (2002) 191. [4] M. Sentis, P. Delaporte, M. Gaslaud, W. Marine, O. Uteza, Quantum Electron. 30 (2000) 495. [5] P. Delaporte, M. Gaslaud, W. Marine, M. Sentis, Appl. Surf. Sci. 197–198 (2002) 826. [6] A.C. Agguedelo, J.R. Gacedo, J.F. Marco, M.F. Creus, E. GallegoLluesma, J. Desimoni, R.C. Mercader, Appl. Surf. Sci. 148 (1999) 171. [7] P. Shaaf, Prog. Mater. Sci. 47 (2002) 1. [8] A. Pereira, A. Cros, P. Delaporte, W. Marine, M. Sentis, Appl. Surf. Sci. 197–198 (2002) 845. [9] W. Marine, L. Patrone, B. Luk’yanchuk, M. Senits, Appl. Surf. Sci. 154– 155 (2000) 345. [10] K. Sturm, S. Fahler, H.U. Krebs, Appl. Surf. Sci. 154–155 (2000) 462. [11] L. Zbroniec, T. Sasaki, N. Koshizaki, Appl. Surf. Sci. 197–198 (2002) 8. [12] T.A. Wiggins, J.A. Bellay, A.N. Carrieti, Appl. Opt. 17 (1978) 526. [13] M. Hernandez, J. Venturini, D. Zahorski, J. Boulmer, D. Debarre, G. Kerrien, T. Sarnet, C. Lavorin, M.N. Semeria, D. Camel, J.L. Satailler, Appl. Surf. Sci. 208–209 (2003) 345. [14] M.K. Kelly, J. Rogg, C.E. Nebel, M. Stutzman, Sz. Katai, Phys. Status Solidi (a) 166 (1998) 651. [15] W. Marine, J. Marfaing, F. Salvan, J. Phys. Lett. 44 (1983) L271. [16] H. Hirayama, M. Obara, J. Appl. Phys. 90 (2001) 6447. [17] E. Sarantopoulou, Z. Samardzija, S. Kobe, Z. Kolia, A. Cefalas, Appl. Surf. Sci. 208–209 (2003) 311.
Laser ablation lithography on thermoelectric semiconductor O.Yu. Semchuk a,*, V.N. Semioshko a, L.G. Grechko a, M. Willander b, M. Karlsteen b a
Institute of Surface Chemistry NAS of Ukraine, 17 General Naumov Street, 03164 Kyiv, Ukraine b Goteborg University, SE-412 96 Gothenburg, Sweden Received 3 May 2005; accepted 15 July 2005 Available online 7 November 2005
Abstract In this paper, experimental results of the investigation of the periodic structure on thermoelectric semiconductor Cu2Se are presented. Periodic structures were formed on surfaces of semiconductors due to multi-beam interaction of Q-switched Nd:YAG laser, which was operated in the lowest order of Gaussian mode and pulse duration 7 ns. Surface temperature evolution and transient reflectivity are studied during laser treatment. Creation of Cu islands in the maximal intensity of interference pattern was found. # 2005 Elsevier B.V. All rights reserved. Keywords: Laser ablation; Fluence; Laser-induced structures; Surface; Thermoelectric semiconductors; Cooper islands
1. Introduction In recent years, different works using high power laser have shown that laser materials processing has a great potential for industrial applications, especially in microelectronics. The laser technique presents advantages in material processing due to the numerous possibilities of treatment: cleaning [1–3], decontamination [4,5], surface modification [6,7] and nanoparticles deposition [8,9]. Several methods based on the application of laser beams make it possible to generate micrometer and sub-micrometer periodic structures on semiconductors and polymers. Grating-like pattern develops as a result of the irradiation by laser light through a photo-mask [10], phase-mask [11] or caused by multi-beams interference technique. The features of them are influenced by the ablation characteristic of the material. The illuminations of semiconductors by interference pattern of laser beams having fluence close to ablation threshold result in laser-induced periodic surface structure (LIPSS) formation. The characteristic size of the LIPSS is the order of the magnitude of the laser wavelength. In this study, we combined transient reflectivity technique, Auger electron spectroscopy and atomic force microscopy to
realize complex analysis periodic structure generated on surface of thermoelectric semiconductors Cu2Se. 2. Multi-beam interference Two or more overlapping coherent and linear polarized laser beams produce an interference pattern which is defined of geometry depending on the used wavelength and angles between the beams. The two-dimensional pattern created by two beams has an intensity distribution as: 4px u IðxÞ ¼ 2I0 cos sin þ1 ; l 2
(1)
where I0 is the intensity of one laser beam, l the wavelength and u is the angle between the beams. The periodicity of interference pattern can be defined by the wavelength and by the angle between the beams, and is d = l/(2 sin u/2). Three beams produce a two-dimensional pattern. Four planar beams interference realizes a line pattern with thinner and higher peaks and thicker areas of low intensity compared to a two beam pattern. 3. Experimental
* Corresponding author. Tel.: +380 44 442 9697; fax: +380 44 424 3567. E-mail address: [email protected] (O.Yu. Semchuk). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.112
The laser structuring experiments were carried out in vacuum chamber under pressure 104 Pa using Q-switched Nd:YAG laser which operated in the lowest order of Gaussian
4760
O.Y. Semchuk et al. / Applied Surface Science 252 (2006) 4759–4762
Fig. 1. A schematic drawing of experimental set up (M—mirror, BS—beam splitter, S—sample and D—detector).
mode with a pulse duration 7 ns. The fundamental emission 1064 nm is converted to 532 nm and 355 nm by means of a nonlinear crystal. The laser beam had Gaussian intensity distribution with a diameter of 5 mm (1/e2) and was focused to 1.8 mm by lenses. The intensity was controlled by l/2 wave plate polarization rotator and flash lamp. The number of pulses was selected by a mechanical shutter. Beam splitters were used to obtain two or more coherent beams. Mirrors guided the splitted beams on the sample surface. An He:Ne laser beam and detector connected to digital oscilloscope were used to study transient reflectivity under laser treatment of the surface of the sample. A schematic drawing of experimental set up is sketched in Fig. 1. 4. Surface temperature evolution and transient reflectivity To estimate the temporal temperature evolution we have used heat transfer equation with interference pattern as heat source: rðTÞcp ðTÞ
@Tð~ r; tÞ @t
r½kðTÞ rTð~ r; t ¼ aI0 ð~ rÞð1 RÞ expðazÞ rÞ ¼ I0 ð~
IðxÞ 2ps expfðt tp Þ2 =2s 2 g
(2) ;
(3)
tp s ¼ pffiffiffiffiffiffiffiffiffiffiffi ; 2 2 ln 2
(4)
Fig. 2. Surface temperature evolution (1—laser fluence is 0.5 J/cm2 and 2— laser fluence is 0.75 J/cm2).
where r(T) is the density, T the absolute temperature of sample, cp(T) the heat capacity, r the position vector, k(T) the heat conductivity, tp the pulse time, tp the laser pulse duration, R the reflectivity of the surface, a the absorption coefficient and I(x) is the intensity distribution of interference pattern, which is described by Eq. (1). We have simulated the temperature evolution during laser interaction. The laser light is absorbed in the near surface region. The power absorbed by the solid can be expressed by the source term which is time dependent. The absorbed light energy is then converted into heat which diffused into the bulk by thermal conduction. We assumed first that the optical length absorption and the reflectivity of semiconductors are constant. Material characteristics used for temperature simulation are given in Table 1. Results of numerical simulation of the surface temperature evolution during laser interaction with thermoelectric semiconductors are presented in Fig. 2. The recorded time dependence of reflected signal from cw He–Ne laser is shown in Fig. 3 for two different fluence. The maximum reflecting rate is reached after the end of the pulse and at higher fluence is seen another temporal behavior than at lower. Several effects may be responsible for transient reflectivity changes of Cu2Se caused by intense laser radiation. One of them is temperature-dependent real and imaginary part of refractive index [12]; the second, which has been investigated in [13], is reflectivity changes due to melting; last but not least is phase transformation of material from one state to another [14]. At lower fluence meting temperature 1386 K does not reach and transient changes in reflectivity are caused by temperaturedependent real and imaginary parts of refractive index.
Table 1 Material characteristics of Cu2Se used for temperature simulation Wavelength (nm)
Reflectivity (a.u.)
Absorption coefficient (108 m1)
Heat capacity (J kg1 K1)
Heat conductivity (W m1 K1)
Density (kg m3)
Latent heat of fusion (105 J kg1)
1064 633 532 355
0.4 0.41 (liquid: 0.52) 0.42 0.32
3.1 – 10.1 27
510
95
6739
1.2
O.Y. Semchuk et al. / Applied Surface Science 252 (2006) 4759–4762
4761
Fig. 5. AFM picture of structured Cu2Se sample.
Fig. 3. Time evolution of transient reflectivity during laser pulse (1—laser fluence is 0.75 J/cm2 and 2—laser fluence is 0.5 J/cm2).
At higher fluence the reflectivity abruptly increases when the surface of semiconductors melts. Then, reflectivity is consistent with the value of reflectivity to liquid semiconductors at 633 nm. The reflectivity stays at this given time by the flat portion of reflectivity curve and decreases when the temperature decreases in crystalline semiconductors. Similar behavior of transient reflectivity due to meting of silicon wafer has been obtained recently during laser treatment with single laser beam. Marine et al. [15] were able to detect self-diffraction and forced scattering during and after self-diffraction excitation with 40 ns ruby laser on GaAs film. They got sharp kink on time dependence of scattered signals, which has been attributed to melting. We made steady-state measurements of reflectivity before and after laser treatment. The change between initial reflectivity and final was about 2.5%. It may be caused by phase transformation, or formation of Cu islands on surface of semiconductors Cu2Se. Additional studies performed by means of Auger spectroscopy confirmed the increase of concentration of Cu atoms in the maximum value of intensity of interference
Fig. 4. Concentration dependence of the copper atoms versus number of laser pulses on the surface in maximum value of interference pattern.
pattern. The dependence of concentration of the copper atoms versus number of laser pulses is shown in Fig. 4. Two important effects must be considered for nanosecond time scale of laser ablation processes. The first involves the interaction between the laser beam and expanding plume, while the second is due to long transfer time of the laser energy on to target material. Moreover, the latter induces a thermal ablation mechanism, whereas the former causes a strong ionization and dissociation of the ablated species [16]. Both dissociation of the Cu2Se and strong evaporation of the Se are responsible for the formation of Cooper islands. The surface of the Cu2Se was investigated by atomic force microscopy (TMX 2000) in tapping mode realized in air. We made measurements before and after laser treatment. It was found that untreated surface had average height Dh 20 nm. AFM image of a structured surface copper selenide is shown in Fig. 5. The etch depth of Cu2Se in dependence of laser fluence at laser wavelength of 1064 nm, 532 nm and 355 nm is compared in Fig. 6. The slope of etch depth is different for three wavelengths. For a one-photon absorption model, the fluence dependence of the etch depth of slightly above threshold is expressed by the relation [17]: 1 F log ; (5) a Fn where F and F n designate laser fluence and threshold fluence, respectively. According to Eq. (5), a linear relation between D D¼
Fig. 6. The fluence dependence of the structure depth induced by laser fluency (1—laser wavelength is 1064 nm, 2—laser wavelength is 532 nm and 3—laser wavelength is 355 nm).
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O.Y. Semchuk et al. / Applied Surface Science 252 (2006) 4759–4762
and log FFn is expected. A deviation at high fluence is observed, which may be caused by plume absorption and formation of Cu islands on surface of thermoelectric semiconductors. 5. Conclusion It was shown that a laser interference pattern allows a micro– nano structuring of surfaces of thermoelectric semiconductors. Firstly, the properties of surface periodic structures depend on laser wavelength and ablation characteristic of semiconductors. Secondly, the laser interference structuring can generate creation of Cu islands on a surface of Cu2Se. Furthermore, the heat diffusion is not important on nanosecond time scale for generation of LIPSS with spacing of 5–10 mm. Acknowledgement We acknowledge financial support through a grant (220180-124G) from the Royal Swedish Academy of Sciences. References [1] J.M. Lee, C. Curran, K.C. Watking, Appl. Phys. A 73 (2001) 219.
[2] P. Maravelaki-Kalaitzki, V. Zafiropulus, C. Fotakis, Appl. Surf. Sci. 148 (1999) 92. [3] P. Neves, M. Arronte, R. Vilar, A.M. Botelho, Appl. Phys. A 74 (2002) 191. [4] M. Sentis, P. Delaporte, M. Gaslaud, W. Marine, O. Uteza, Quantum Electron. 30 (2000) 495. [5] P. Delaporte, M. Gaslaud, W. Marine, M. Sentis, Appl. Surf. Sci. 197–198 (2002) 826. [6] A.C. Agguedelo, J.R. Gacedo, J.F. Marco, M.F. Creus, E. GallegoLluesma, J. Desimoni, R.C. Mercader, Appl. Surf. Sci. 148 (1999) 171. [7] P. Shaaf, Prog. Mater. Sci. 47 (2002) 1. [8] A. Pereira, A. Cros, P. Delaporte, W. Marine, M. Sentis, Appl. Surf. Sci. 197–198 (2002) 845. [9] W. Marine, L. Patrone, B. Luk’yanchuk, M. Senits, Appl. Surf. Sci. 154– 155 (2000) 345. [10] K. Sturm, S. Fahler, H.U. Krebs, Appl. Surf. Sci. 154–155 (2000) 462. [11] L. Zbroniec, T. Sasaki, N. Koshizaki, Appl. Surf. Sci. 197–198 (2002) 8. [12] T.A. Wiggins, J.A. Bellay, A.N. Carrieti, Appl. Opt. 17 (1978) 526. [13] M. Hernandez, J. Venturini, D. Zahorski, J. Boulmer, D. Debarre, G. Kerrien, T. Sarnet, C. Lavorin, M.N. Semeria, D. Camel, J.L. Satailler, Appl. Surf. Sci. 208–209 (2003) 345. [14] M.K. Kelly, J. Rogg, C.E. Nebel, M. Stutzman, Sz. Katai, Phys. Status Solidi (a) 166 (1998) 651. [15] W. Marine, J. Marfaing, F. Salvan, J. Phys. Lett. 44 (1983) L271. [16] H. Hirayama, M. Obara, J. Appl. Phys. 90 (2001) 6447. [17] E. Sarantopoulou, Z. Samardzija, S. Kobe, Z. Kolia, A. Cefalas, Appl. Surf. Sci. 208–209 (2003) 311.