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Effect of laser irradiation on the properties of indium tin oxide films deposited at room temperature by pulsed laser deposition
Vacuum 67 (2002) 209–216
Effect of laser irradiation on the properties of indium tin oxide films deposited at room temperature by pulsed laser deposition F.O. Adurodijaa,*, H. Izumia, T. Ishiharaa, H. Yoshiokaa, M. Motoyamaa, K. Muraib a
Hyogo Prefectural Institute of Industrial Research, 3-1-12, Yukihira-cho, Suma-ku, Kobe 654-0037, Japan b Osaka National Research Institute, Midorigaoka, Ikeda, Osaka 563-8577, Japan Received 12 November 2001; received in revised form 17 December 2001; accepted 17 December 2001
Abstract Indium tin oxide (ITO) thin films 60–100 nm thick have been grown on SiO2 glass by laser irradiation of the substrate during pulsed laser deposition. A laser beam with an energy density of 0.07 J cm2 (size B1 cm2) was directed on to the middle part of the substrates during film growths. Films were deposited from a 95 wt% In2O3–5 wt% SnO2 sintered ceramic target at a room temperature and oxygen pressures ðPO2 Þ ranging from 0.13 to 5.99 Pa. The structural, electrical, and optical properties of the laser-irradiated and the nonirradiated parts of the films were studied as a function of PO2 : Crystalline ITO films with /1 1 1S preferred orientation were observed at all PO2 ; except 0.13 Pa. Under PO2 around 1.33 Pa, minimal resistivities of 1.2 104 and 2.3 104 O cm were obtained on the laser irradiated and the nonirradiated parts of the deposited films. The observed low resistivity for the laser-irradiated part of the films was a consequence of both the high carrier concentration and Hall mobility of the films. High optical transmittance (>85%) to visible light was obtained. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Indium tin oxide; Pulsed laser deposition; Laser irradiation; Laser crystallization; Electrical; Optical; Structural properties
1. Introduction Indium tin oxide (ITO) thin film is an important material for the production of solar cell and flat panel display. The ITO film has unique properties such as low resistivity (B104 O cm), high optical transmittance to visible light, and high nearinfrared reflectance that makes it useful as n-type window layer, particularly for the solar cells [1]. It is well known that these properties are reasonably *Corresponding author. Tel.: +81-78-731-4481; fax: +8178-735-7845. E-mail address: [email protected] (F.O. Adurodija).
obtained at substrate temperatures (Ts ) above 3001C [2]. In the case of applications that demand the deposition of ITO films on heat-sensitive materials like organic thin films, high Ts is not desirable. It is critical to develop low-temperature deposition techniques for the ITO films. Several techniques such as sputtering, electron beam evaporation, and chemical vapor deposition are used for the growth of ITO thin films [1–3]. However, at room temperature (RT), these techniques produce amorphous ITO films with poor electrical and optical properties [4]. The lowresistivity ITO films can be realized by the crystalline phase.
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In order to achieve the low-temperature growth, the amorphous ITO films have been crystallized by thermal annealing at low Ts (120–1651C) and/or irradiation using pulsed laser (pulsed laser annealing) [4,5]. The merits of pulsed laser annealing of ITO films are as follows [5]: (a) ultraviolet (UV) photons from the laser energy beam are essentially absorbed by an ITO film due to its high absorption coefficient of 3 105 cm1 to UV light, (b) pulsed UV laser does not produce continuous rise in temperature, therefore making it feasible to set the optimal conditions of irradiation. The pulsed laser irradiation was also used to enhance the crystallinity of BaTiO3 [6], PbTiO3 [7] and Y–Ba–Cu–O [8–10] thin films. In this paper, we will describe the formation of crystalline ITO films on the glass substrate at RT by laser irradiation of the substrate during pulsed laser deposition (PLD). The structural, electrical, and optical properties of the films were studied as a function of oxygen pressures ðPO2 Þ:
2. Experimental ITO films were deposited on SiO2 fused-quartz glass by the PLD from a sintered ITO target (2-cm
diameter, 95 wt% In2O3–5 wt% SnO2) using a KrF (248 nm) excimer laser system [11]. The laser system was operated at an energy of about 300 mJ and a frequency of 20 Hz. The laser energy beam from the system was divided into two equal parts using a beam splitter. Half of the energy that was focused on the target produced an energy density of 2–3 J cm2 at the target surface. The remainder was directed to the middle part of the substrate surface through a separate quartz glass window using a single convex lens with a focal length of 70 cm, thus creating an energy density of 0.07 J cm2. Fig. 1 shows the experimental setup of the PLD system. The cleaned glass substrates were clamped onto the holder using stainless steel strips, thus producing a well-defined step in the nonirradiated part close to the focused laser beam to enable the film thickness to be properly determined. Films with reasonable uniformity were deposited at RT and PO2 over the range 0.13–5.99 Pa. The PO2 was controlled using a mass flow controller. The above conditions produced deposition rates ranging from 8 to 15 nm min1. After depositions, the middle part of the films that was irradiated with the laser energy pulses that was elliptical in shape with an area of
Fig. 1. An illustration of PLD system.
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about 1 cm2 was observed visually. The thickness of the films varied from 60 to 100 nm measured using a surface profilometer. The thickness of the films was measured from the well-defined edge by the stainless strip on the glass in the nonirradiated part of the film that was close to the laser-irradiated part. Because the energy density of the irradiation beam is far less than the threshold (120 mJ cm2) required to cause re-ablation of the deposited films, no major change in the film thickness was expected [12]. Therefore, the measured thickness on the nonirradiated parts of the films was considered nearly equal to the laser-irradiated parts, since reasonable uniformity of the films was achieved over the substrate areas. The structure and morphology of the films were determined by X( ray diffraction (XRD) using Cu Ka (l ¼ 1:5405 A) radiation and atomic force microscopy (AFM), respectively. The electrical properties were obtained at RT using the van der Pauw measurement equipment. All measurements were performed on the laser-irradiated and the nonirradiated parts of the samples.
3. Results 3.1. Effects of PO2 on the structure of the laser-irradiated films Fig. 2 shows typical XRD patterns of a nonirradiated part of an ITO film deposited at PO2 of 1.33 Pa (a) and the laser-irradiated parts of the films deposited at PO2 over the range 0.13–5.99 Pa (b), respectively. The nonirradiated parts of the ITO films were amorphous, irrespective of the changes in PO2 as indicated by the absence of diffraction peaks [Fig. 2(a)]. In the case of the laser-irradiated parts of the films, the crystalline phase was observed, as shown in Fig. 2(b). The XRD patterns indicated a strong /1 1 1S preferred orientation of ITO, except for the films deposited at low PO2 of 0.13 Pa. The film deposited at PO2 of 0.13 Pa showed splitting of the diffraction peaks.
Fig. 2. (a) XRD spectrum of a nonirradiated part of an ITO film deposited at RT and PO2 of 1.33 Pa, (b) XRD spectra of the laser-irradiated parts of ITO films deposited at PO2 over the range 0.13–5.99 Pa.
3.2. Effects of PO2 on the morphology of the laser-irradiated films Fig. 3 shows typical surface AFM pictures of a nonirradiated part of the film deposited at 1.33 Pa (a), and the laser-irradiated parts of ITO films deposited at PO2 of 1.33 (b), 0.13 (c), and 3.33 Pa (d), respectively. The nonirradiated part of the films exhibited smooth surface feature of amorphous phase. In the laser-irradiated parts, the morphologies varied with PO2 : At a low PO2 of 0.13 Pa, and at high PO2 (>2.7 Pa), the films showed uniform structure containing small crystallites. These films exhibited the poor crystalline structures observed from the XRD analysis [Fig. 2(b)]. However, at PO2 of 1.33 Pa, the ITO films showed a densely packed structure with crystallites. The size of the crystallites was around 200 nm.
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Fig. 3. Fig. 3 shows surface AFM pictures: (a) a nonirradiated part of the film deposited at 1.33 Pa; (b), (c) and (d), the laser-irradiated parts of ITO films deposited at PO2 of 1.33, 0.13, and 3.33 Pa, respectively.
3.3. Effects of PO2 on the electrical properties of the laser-irradiated films Fig. 4 shows the dependence of the resistivity on the PO2 for the laser-irradiated and the nonirra-
diated parts of ITO films. At low PO2 (0.13 Pa), the laser-irradiated part of an ITO film produced a resistivity of 3.1 104 O cm compared to 4.7 103 O cm for the nonirradiated part. With increasing PO2 to 1.33–2.0 Pa, the resistivity of the
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Fig. 4. The dependence of the resistivity (r) on PO2 for the laser-irradiated and the nonirradiated parts of the ITO films.
laser-irradiated and the nonirradiated parts of the films decreased to minimal values of 1.2 104 and 2.3 104 O cm. Further increase in PO2 led to a sharp rise in the resistivity of both parts of the films. Fig. 5 shows the carrier density (n) and the Hall mobility (m) as a function of PO2 for the laserirradiated and the nonirradiated parts of the ITO films. At a low PO2 of 0.13 Pa [Fig. 5(a)], the carrier densities were 7.7 1020 and 4.7 1020 cm3 for the laser-irradiated and the nonirradiated parts of the ITO films, respectively. With increasing PO2 to 2.0 Pa, the carrier density increased to 1.3 1021 and 8.8 1020 cm3 for the laser-irradiated and the nonirradiated parts of the films, respectively. In Fig. 5(b), it is seen that the effect of PO2 on the Hall mobility in both parts of the films followed similar tendencies. Within experimental errors (o5%), at low PO2 (0.13 Pa), the laser-irradiated part of a film showed a Hall mobility of 26.2 cm2 V1 s1. It increased to 40.4–45.2 cm2 V1 s1 as PO2 increases to between 0.67 and 2.7 Pa. Further increase in PO2 to 5.99 Pa resulted in a sharp decrease in the mobility to 2.2 cm2 V1 s1. At PO2 of 0.13 Pa, the nonirradiated parts of the films indicated
Fig. 5. (a) The carrier density (n) and (b) the Hall mobility (m) as a function of PO2 for the laser-irradiated and the nonirradiated parts of the ITO films.
a Hall mobility of 4.5 cm2 V1 s1. The mobility increased rapidly to 30.4 cm2 V1 s1 with the increase of PO2 to 1.33 Pa. For PO2 between 1.33 and 2.7 Pa, no appreciable change in Hall mobility of the nonirradiated part of the films occurred. However, at PO2 of 5.99 Pa, the Hall mobility and carrier density of the nonirradiated part of the film could not be determined due to its high resistivity.
3.4. Effects of PO2 on the optical properties of the laser-irradiated films The optical properties of the films were determined by measuring the transmittance over the wavelength range of 190–2500 nm. Fig. 6 shows the transmittance spectra for the laser-irradiated (a) and the nonirradiated (b) parts of the ITO films deposited at PO2 over the range 0.13–5.99 Pa. The films deposited at low PO2 (=0.13 Pa) showed brownish color with semitransparency (o85%) at 400–800 nm. The visible transmittance of both the
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while the transmittance of the nonirradiated films was slightly below 90%.
4. Discussion 4.1. Effects of PO2 on the structure and morphology of the laser-irradiated films
Fig. 6. Optical transmittance spectra versus the wavelength for (a) the laser-irradiated and (b) the nonirradiated parts of ITO films deposited at PO2 over the range 0.13–5.99 Pa.
laser-irradiated and nonirradiated parts of the ITO films increased with increasing PO2 to 5.99 Pa. At the PO2 region, 1.33–2.0 Pa, transmittance above 90% was obtained for the laser-irradiated films,
In Fig. 2, the ITO films showed highly crystalline structure with a strong /1 1 1S preferred orientation, except for the films deposited at low PO2 of 0.13 Pa where weakening and splitting of diffraction peaks were observed. Peak splitting has been observed in our PLD ITO films deposited on heated substrates at low PO2 [11,13]. This feature was attributed to induced strain due to the highenergy bombardment during growth. Compared to previous results, the present films deposited at PO2 of 0.13 Pa showed weaker peak intensity [13]. Peak splitting has also been reported for sputtered and electron-beam evaporated ITO films [14]. With increasing PO2 (>2.0 Pa) the crystallinity of the films decreased probably due to an increase of adsorbed oxygen atoms on the film surface which would reduce the surface migration of In and/or Sn atoms, leading to a structural disorder [15,16]. The decrease in the crystallinity of the films will be due to a lowering of the energies of the ablated particles and the irradiation beam due to scattering effects. In Fig. 3(a), the nonirradiated part of the films showed a smooth surface feature of amorphous phase as expected. The morphologies of the laserirradiated films varied with the PO2 : The poor morphology observed in the ITO films deposited at low or high PO2 is associated with either insufficient or excessive oxygen atoms present during growth. At PO2 of 1.33 Pa, densely packed structure with crystallites of around 200 nm was evidenced. High-quality ITO films has been produced by PLD method at PO2 of 1.33 Pa [11,17–20]. The distinct change in the structural and morphological properties of the laser-irradiated films is attributed to photochemically induced temperature rise (local heating) during growth [7].
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4.2. Effects of PO2 on the electrical and optical properties of the laser-irradiated films The electrical and the optical properties of the nonirradiated and the laser-irradiated parts of the films were strongly affected by the PO2 : In both cases, a strong influence of PO2 on the resistivity was apparent, particularly at higher PO2 (>2.0 Pa). At low and high PO2 regions, high resistivities were obtained. The laser-irradiated and nonirradiated parts of the films yielded the lowest resistivities of 1.2 104 and 2.3 104 O cm at PO2 of 1.33 Pa, respectively. This observation is consistent with that earlier reported for ITO films deposited by conventional PLD method (i.e., without laser irradiation) [11]. A marked difference between the resistivity, carrier densities and Hall mobilities of the laser-irradiated and the nonirradiated films was apparent, particularly at PO2 o2.0 Pa. At all PO2 regions, the electrical properties of the laser-irradiated parts were better than that of the nonirradiated parts of the ITO films. This improvement may be a consequence of the activation of the Sn atoms into Sn4+ within the host In2O3 crystal lattice during the photochemical-crystallization process. This is expected since Sn atoms (with four valence electrons) substitutes for In atoms (three valence electrons) in the In2O3 crystal lattices. For device applications low-resistivity films (o104 O cm) are preferred. Commercially produced ITO films have resistivity of the order of 2 104 O cm. Therefore, the use of low-resistivity PLD ITO films could yield more efficient devices. High Hall mobility was observed at the optimal PO2 region for the laser-irradiated and the nonirradiated parts of the ITO films. The larger Hall mobility obtained for the laser-irradiated part is associated with an improvement in the crystallinity of the films. The observed relationship between the Hall mobility and PO2 is similar to the case reported for ITO films deposited at RT and 2001C by PLD [20,21]. The poor electrical properties obtained at the low PO2 (0.13 Pa) and the high PO2 (>2.0 Pa) regions was due to a decrease in the crystalline quality, as shown in Figs 2(b) and 3.
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As shown in Fig. 6, the laser irradiation and the change in PO2 during the film growths affected the visible transmittance and the near-infrared reflectance (NIR) of the ITO films. With increasing PO2 ; the NIR of the nonirradiated part of the films decreased accordingly, while that of the laserirradiated part of the films remained reasonably high. In addition, the absorption edge (cut-off wavelength) shifted with changes in PO2 : This shift called the Burstein–Moss shift [22] has been reported in previous studies on PLD ITO films and was also reported by Hamberg and Granqvist in their electron-beam evaporated ITO films [3,11,19]. The Burstein–Moss shift could be related to the structural properties of the films since a shift in the absorption edge produced a corresponding change in the structural properties of the films [Fig. 2(b)]. It is observed that the ITO films with absorption edge at the shorter and longer wavelength regions showed poor crystalline structure. This can be seen from the decrease in the (1 1 1) peak intensity and preferred orientation, and poor morphology at the low and high PO2 region in Figs. 2 and 3. The laser irradiation of the films during PLD has been found to yield higher transmittance (>90%) when compared to the nonirradiated portion of the films.
5. Conclusion Low-resistivity ITO films have been grown on glass by laser irradiation of the substrate during PLD. The properties of the films were very sensitive to change in PO2 : The nonirradiated parts of all the films were amorphous. In contrast, crystalline films with strong /1 1 1S preferred orientation and crystallite sizes of the order of 200 nm were obtained by laser irradiation. The process of film crystallization occurred through a photochemically induced reaction. In this study, ITO films with the best structural, electrical and optical properties were obtained within a narrow PO2 region of 1.33–2.0 Pa. The laser-irradiated part of the films yielded resistivity, carrier concentration and Hall mobility of 1.2 104 O cm, 1.2 1021 cm3 and 40 cm2 V1 s1 compared with 2.3 104 O cm,
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8.8 1020 cm3 and 30 cm2 V1 s1 for the nonirradiated parts of the ITO films. Improved optical properties were also obtained for the laserirradiated part of the ITO films in comparison with the nonirradiated portions.
Acknowledgements This project was sponsored by the New Energy and Industrial Technology Development Organization (NEDO), under the regional consortium projects, R&D of ultrahigh-quality transparent conductive oxide films.
References [1] Chopra KL, Major S, Pandya DK. Thin Solid Films 1983;102:1. [2] Tahar RBH, Ban T, Ohya Y, Takahashi Y. J Appl Phys 1998;83:2631. [3] Hamberg I, Granqvist CG. J Appl Phys 1986;60:R123. [4] Paine DC, Whitson T, Janiac D, Beresford R, Ow-Yang C, Lewis B. J Appl Phys 1999;85:8445. [5] Hosono H, Kurita M, Kawazoe H. Thin Solid Films 1999;351:137. [6] Ito A, Machida A, Obara M. Jpn J Appl Phys 1998;36:L805.
[7] Tabata H, Murata O, Kawai T, Kawai S, Okuyama M. Jpn J Appl Phys 1998;31:L2968. [8] Otsubo S, Minamikawa T, Yonezawa Y, Morimoto A, Shimizu T. Jpn J Appl Phys 1990;29:L73. [9] Morimoto A, Otsubo S, Shimizu T, Minamikawa T, Yonezawa Y. Mater Res Soc Symp Proc 1990;191:31. [10] Estler RC, Nogar NS, Muenchausen RE, Dye RC, Flamme C, Martin JA, Garcia A, Foltyn S. Mater Lett 1990;9:342. [11] Adurodija FO, Izumi H, Ishihara T, Yoshioka H, Matsui H, Motoyama M. Jpn J Appl Phys 1999;38:2710. . enyi T, K!antor Z, laude LD. Appl Surf Sci [12] Szor! 1995;86:219. [13] Adurodija FO, Izumi H, Ishihara T, Yoshioka H, Motoyama M. J Mater Sci: Mater Electron 2001;12:57. [14] Yi CH, Shigesato Y, Yasui I, Takaki S. Jpn J Appl Phys 11995;34:L224. [15] Song PK, Shigesato Y, Yasui I, Ow-Yang CW, Paine DC. Jpn J Appl Phys 1998;37:1870. [16] Thornton JA, Hoffman WD. Thin Solid Films 1989;171:5. [17] Zheng JP, Kwok HS. Thin Solid Films 1993;232:99. [18] Hanus F, Jadin A, Laude LD. Appl Surf Sci 1996; 96–98:807. [19] Kim H, Gilmore CM, Pique A, Horwitz JS, Mattoussi H, Murata H, Kafafi ZH, Chrisey DB. J Appl Phys 1999;86:6451. [20] Wu Y, Maree CHM, Haglund Jr RF, Hamilton JD, Morales Paliza MA, Huang MB, Feldman LC, Weller RA. J Appl Phys 1999;86:991. [21] Adurodija FO, Izumi H, Ishihara T, Yoshioka H, Yamada K, Matsui H, Motoyama M. Thin Solid Films 1999;350:79. [22] Burstein E. Phys Rev 1952;93:632.
Effect of laser irradiation on the properties of indium tin oxide films deposited at room temperature by pulsed laser deposition F.O. Adurodijaa,*, H. Izumia, T. Ishiharaa, H. Yoshiokaa, M. Motoyamaa, K. Muraib a
Hyogo Prefectural Institute of Industrial Research, 3-1-12, Yukihira-cho, Suma-ku, Kobe 654-0037, Japan b Osaka National Research Institute, Midorigaoka, Ikeda, Osaka 563-8577, Japan Received 12 November 2001; received in revised form 17 December 2001; accepted 17 December 2001
Abstract Indium tin oxide (ITO) thin films 60–100 nm thick have been grown on SiO2 glass by laser irradiation of the substrate during pulsed laser deposition. A laser beam with an energy density of 0.07 J cm2 (size B1 cm2) was directed on to the middle part of the substrates during film growths. Films were deposited from a 95 wt% In2O3–5 wt% SnO2 sintered ceramic target at a room temperature and oxygen pressures ðPO2 Þ ranging from 0.13 to 5.99 Pa. The structural, electrical, and optical properties of the laser-irradiated and the nonirradiated parts of the films were studied as a function of PO2 : Crystalline ITO films with /1 1 1S preferred orientation were observed at all PO2 ; except 0.13 Pa. Under PO2 around 1.33 Pa, minimal resistivities of 1.2 104 and 2.3 104 O cm were obtained on the laser irradiated and the nonirradiated parts of the deposited films. The observed low resistivity for the laser-irradiated part of the films was a consequence of both the high carrier concentration and Hall mobility of the films. High optical transmittance (>85%) to visible light was obtained. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Indium tin oxide; Pulsed laser deposition; Laser irradiation; Laser crystallization; Electrical; Optical; Structural properties
1. Introduction Indium tin oxide (ITO) thin film is an important material for the production of solar cell and flat panel display. The ITO film has unique properties such as low resistivity (B104 O cm), high optical transmittance to visible light, and high nearinfrared reflectance that makes it useful as n-type window layer, particularly for the solar cells [1]. It is well known that these properties are reasonably *Corresponding author. Tel.: +81-78-731-4481; fax: +8178-735-7845. E-mail address: [email protected] (F.O. Adurodija).
obtained at substrate temperatures (Ts ) above 3001C [2]. In the case of applications that demand the deposition of ITO films on heat-sensitive materials like organic thin films, high Ts is not desirable. It is critical to develop low-temperature deposition techniques for the ITO films. Several techniques such as sputtering, electron beam evaporation, and chemical vapor deposition are used for the growth of ITO thin films [1–3]. However, at room temperature (RT), these techniques produce amorphous ITO films with poor electrical and optical properties [4]. The lowresistivity ITO films can be realized by the crystalline phase.
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In order to achieve the low-temperature growth, the amorphous ITO films have been crystallized by thermal annealing at low Ts (120–1651C) and/or irradiation using pulsed laser (pulsed laser annealing) [4,5]. The merits of pulsed laser annealing of ITO films are as follows [5]: (a) ultraviolet (UV) photons from the laser energy beam are essentially absorbed by an ITO film due to its high absorption coefficient of 3 105 cm1 to UV light, (b) pulsed UV laser does not produce continuous rise in temperature, therefore making it feasible to set the optimal conditions of irradiation. The pulsed laser irradiation was also used to enhance the crystallinity of BaTiO3 [6], PbTiO3 [7] and Y–Ba–Cu–O [8–10] thin films. In this paper, we will describe the formation of crystalline ITO films on the glass substrate at RT by laser irradiation of the substrate during pulsed laser deposition (PLD). The structural, electrical, and optical properties of the films were studied as a function of oxygen pressures ðPO2 Þ:
2. Experimental ITO films were deposited on SiO2 fused-quartz glass by the PLD from a sintered ITO target (2-cm
diameter, 95 wt% In2O3–5 wt% SnO2) using a KrF (248 nm) excimer laser system [11]. The laser system was operated at an energy of about 300 mJ and a frequency of 20 Hz. The laser energy beam from the system was divided into two equal parts using a beam splitter. Half of the energy that was focused on the target produced an energy density of 2–3 J cm2 at the target surface. The remainder was directed to the middle part of the substrate surface through a separate quartz glass window using a single convex lens with a focal length of 70 cm, thus creating an energy density of 0.07 J cm2. Fig. 1 shows the experimental setup of the PLD system. The cleaned glass substrates were clamped onto the holder using stainless steel strips, thus producing a well-defined step in the nonirradiated part close to the focused laser beam to enable the film thickness to be properly determined. Films with reasonable uniformity were deposited at RT and PO2 over the range 0.13–5.99 Pa. The PO2 was controlled using a mass flow controller. The above conditions produced deposition rates ranging from 8 to 15 nm min1. After depositions, the middle part of the films that was irradiated with the laser energy pulses that was elliptical in shape with an area of
Fig. 1. An illustration of PLD system.
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about 1 cm2 was observed visually. The thickness of the films varied from 60 to 100 nm measured using a surface profilometer. The thickness of the films was measured from the well-defined edge by the stainless strip on the glass in the nonirradiated part of the film that was close to the laser-irradiated part. Because the energy density of the irradiation beam is far less than the threshold (120 mJ cm2) required to cause re-ablation of the deposited films, no major change in the film thickness was expected [12]. Therefore, the measured thickness on the nonirradiated parts of the films was considered nearly equal to the laser-irradiated parts, since reasonable uniformity of the films was achieved over the substrate areas. The structure and morphology of the films were determined by X( ray diffraction (XRD) using Cu Ka (l ¼ 1:5405 A) radiation and atomic force microscopy (AFM), respectively. The electrical properties were obtained at RT using the van der Pauw measurement equipment. All measurements were performed on the laser-irradiated and the nonirradiated parts of the samples.
3. Results 3.1. Effects of PO2 on the structure of the laser-irradiated films Fig. 2 shows typical XRD patterns of a nonirradiated part of an ITO film deposited at PO2 of 1.33 Pa (a) and the laser-irradiated parts of the films deposited at PO2 over the range 0.13–5.99 Pa (b), respectively. The nonirradiated parts of the ITO films were amorphous, irrespective of the changes in PO2 as indicated by the absence of diffraction peaks [Fig. 2(a)]. In the case of the laser-irradiated parts of the films, the crystalline phase was observed, as shown in Fig. 2(b). The XRD patterns indicated a strong /1 1 1S preferred orientation of ITO, except for the films deposited at low PO2 of 0.13 Pa. The film deposited at PO2 of 0.13 Pa showed splitting of the diffraction peaks.
Fig. 2. (a) XRD spectrum of a nonirradiated part of an ITO film deposited at RT and PO2 of 1.33 Pa, (b) XRD spectra of the laser-irradiated parts of ITO films deposited at PO2 over the range 0.13–5.99 Pa.
3.2. Effects of PO2 on the morphology of the laser-irradiated films Fig. 3 shows typical surface AFM pictures of a nonirradiated part of the film deposited at 1.33 Pa (a), and the laser-irradiated parts of ITO films deposited at PO2 of 1.33 (b), 0.13 (c), and 3.33 Pa (d), respectively. The nonirradiated part of the films exhibited smooth surface feature of amorphous phase. In the laser-irradiated parts, the morphologies varied with PO2 : At a low PO2 of 0.13 Pa, and at high PO2 (>2.7 Pa), the films showed uniform structure containing small crystallites. These films exhibited the poor crystalline structures observed from the XRD analysis [Fig. 2(b)]. However, at PO2 of 1.33 Pa, the ITO films showed a densely packed structure with crystallites. The size of the crystallites was around 200 nm.
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Fig. 3. Fig. 3 shows surface AFM pictures: (a) a nonirradiated part of the film deposited at 1.33 Pa; (b), (c) and (d), the laser-irradiated parts of ITO films deposited at PO2 of 1.33, 0.13, and 3.33 Pa, respectively.
3.3. Effects of PO2 on the electrical properties of the laser-irradiated films Fig. 4 shows the dependence of the resistivity on the PO2 for the laser-irradiated and the nonirra-
diated parts of ITO films. At low PO2 (0.13 Pa), the laser-irradiated part of an ITO film produced a resistivity of 3.1 104 O cm compared to 4.7 103 O cm for the nonirradiated part. With increasing PO2 to 1.33–2.0 Pa, the resistivity of the
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Fig. 4. The dependence of the resistivity (r) on PO2 for the laser-irradiated and the nonirradiated parts of the ITO films.
laser-irradiated and the nonirradiated parts of the films decreased to minimal values of 1.2 104 and 2.3 104 O cm. Further increase in PO2 led to a sharp rise in the resistivity of both parts of the films. Fig. 5 shows the carrier density (n) and the Hall mobility (m) as a function of PO2 for the laserirradiated and the nonirradiated parts of the ITO films. At a low PO2 of 0.13 Pa [Fig. 5(a)], the carrier densities were 7.7 1020 and 4.7 1020 cm3 for the laser-irradiated and the nonirradiated parts of the ITO films, respectively. With increasing PO2 to 2.0 Pa, the carrier density increased to 1.3 1021 and 8.8 1020 cm3 for the laser-irradiated and the nonirradiated parts of the films, respectively. In Fig. 5(b), it is seen that the effect of PO2 on the Hall mobility in both parts of the films followed similar tendencies. Within experimental errors (o5%), at low PO2 (0.13 Pa), the laser-irradiated part of a film showed a Hall mobility of 26.2 cm2 V1 s1. It increased to 40.4–45.2 cm2 V1 s1 as PO2 increases to between 0.67 and 2.7 Pa. Further increase in PO2 to 5.99 Pa resulted in a sharp decrease in the mobility to 2.2 cm2 V1 s1. At PO2 of 0.13 Pa, the nonirradiated parts of the films indicated
Fig. 5. (a) The carrier density (n) and (b) the Hall mobility (m) as a function of PO2 for the laser-irradiated and the nonirradiated parts of the ITO films.
a Hall mobility of 4.5 cm2 V1 s1. The mobility increased rapidly to 30.4 cm2 V1 s1 with the increase of PO2 to 1.33 Pa. For PO2 between 1.33 and 2.7 Pa, no appreciable change in Hall mobility of the nonirradiated part of the films occurred. However, at PO2 of 5.99 Pa, the Hall mobility and carrier density of the nonirradiated part of the film could not be determined due to its high resistivity.
3.4. Effects of PO2 on the optical properties of the laser-irradiated films The optical properties of the films were determined by measuring the transmittance over the wavelength range of 190–2500 nm. Fig. 6 shows the transmittance spectra for the laser-irradiated (a) and the nonirradiated (b) parts of the ITO films deposited at PO2 over the range 0.13–5.99 Pa. The films deposited at low PO2 (=0.13 Pa) showed brownish color with semitransparency (o85%) at 400–800 nm. The visible transmittance of both the
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while the transmittance of the nonirradiated films was slightly below 90%.
4. Discussion 4.1. Effects of PO2 on the structure and morphology of the laser-irradiated films
Fig. 6. Optical transmittance spectra versus the wavelength for (a) the laser-irradiated and (b) the nonirradiated parts of ITO films deposited at PO2 over the range 0.13–5.99 Pa.
laser-irradiated and nonirradiated parts of the ITO films increased with increasing PO2 to 5.99 Pa. At the PO2 region, 1.33–2.0 Pa, transmittance above 90% was obtained for the laser-irradiated films,
In Fig. 2, the ITO films showed highly crystalline structure with a strong /1 1 1S preferred orientation, except for the films deposited at low PO2 of 0.13 Pa where weakening and splitting of diffraction peaks were observed. Peak splitting has been observed in our PLD ITO films deposited on heated substrates at low PO2 [11,13]. This feature was attributed to induced strain due to the highenergy bombardment during growth. Compared to previous results, the present films deposited at PO2 of 0.13 Pa showed weaker peak intensity [13]. Peak splitting has also been reported for sputtered and electron-beam evaporated ITO films [14]. With increasing PO2 (>2.0 Pa) the crystallinity of the films decreased probably due to an increase of adsorbed oxygen atoms on the film surface which would reduce the surface migration of In and/or Sn atoms, leading to a structural disorder [15,16]. The decrease in the crystallinity of the films will be due to a lowering of the energies of the ablated particles and the irradiation beam due to scattering effects. In Fig. 3(a), the nonirradiated part of the films showed a smooth surface feature of amorphous phase as expected. The morphologies of the laserirradiated films varied with the PO2 : The poor morphology observed in the ITO films deposited at low or high PO2 is associated with either insufficient or excessive oxygen atoms present during growth. At PO2 of 1.33 Pa, densely packed structure with crystallites of around 200 nm was evidenced. High-quality ITO films has been produced by PLD method at PO2 of 1.33 Pa [11,17–20]. The distinct change in the structural and morphological properties of the laser-irradiated films is attributed to photochemically induced temperature rise (local heating) during growth [7].
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4.2. Effects of PO2 on the electrical and optical properties of the laser-irradiated films The electrical and the optical properties of the nonirradiated and the laser-irradiated parts of the films were strongly affected by the PO2 : In both cases, a strong influence of PO2 on the resistivity was apparent, particularly at higher PO2 (>2.0 Pa). At low and high PO2 regions, high resistivities were obtained. The laser-irradiated and nonirradiated parts of the films yielded the lowest resistivities of 1.2 104 and 2.3 104 O cm at PO2 of 1.33 Pa, respectively. This observation is consistent with that earlier reported for ITO films deposited by conventional PLD method (i.e., without laser irradiation) [11]. A marked difference between the resistivity, carrier densities and Hall mobilities of the laser-irradiated and the nonirradiated films was apparent, particularly at PO2 o2.0 Pa. At all PO2 regions, the electrical properties of the laser-irradiated parts were better than that of the nonirradiated parts of the ITO films. This improvement may be a consequence of the activation of the Sn atoms into Sn4+ within the host In2O3 crystal lattice during the photochemical-crystallization process. This is expected since Sn atoms (with four valence electrons) substitutes for In atoms (three valence electrons) in the In2O3 crystal lattices. For device applications low-resistivity films (o104 O cm) are preferred. Commercially produced ITO films have resistivity of the order of 2 104 O cm. Therefore, the use of low-resistivity PLD ITO films could yield more efficient devices. High Hall mobility was observed at the optimal PO2 region for the laser-irradiated and the nonirradiated parts of the ITO films. The larger Hall mobility obtained for the laser-irradiated part is associated with an improvement in the crystallinity of the films. The observed relationship between the Hall mobility and PO2 is similar to the case reported for ITO films deposited at RT and 2001C by PLD [20,21]. The poor electrical properties obtained at the low PO2 (0.13 Pa) and the high PO2 (>2.0 Pa) regions was due to a decrease in the crystalline quality, as shown in Figs 2(b) and 3.
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As shown in Fig. 6, the laser irradiation and the change in PO2 during the film growths affected the visible transmittance and the near-infrared reflectance (NIR) of the ITO films. With increasing PO2 ; the NIR of the nonirradiated part of the films decreased accordingly, while that of the laserirradiated part of the films remained reasonably high. In addition, the absorption edge (cut-off wavelength) shifted with changes in PO2 : This shift called the Burstein–Moss shift [22] has been reported in previous studies on PLD ITO films and was also reported by Hamberg and Granqvist in their electron-beam evaporated ITO films [3,11,19]. The Burstein–Moss shift could be related to the structural properties of the films since a shift in the absorption edge produced a corresponding change in the structural properties of the films [Fig. 2(b)]. It is observed that the ITO films with absorption edge at the shorter and longer wavelength regions showed poor crystalline structure. This can be seen from the decrease in the (1 1 1) peak intensity and preferred orientation, and poor morphology at the low and high PO2 region in Figs. 2 and 3. The laser irradiation of the films during PLD has been found to yield higher transmittance (>90%) when compared to the nonirradiated portion of the films.
5. Conclusion Low-resistivity ITO films have been grown on glass by laser irradiation of the substrate during PLD. The properties of the films were very sensitive to change in PO2 : The nonirradiated parts of all the films were amorphous. In contrast, crystalline films with strong /1 1 1S preferred orientation and crystallite sizes of the order of 200 nm were obtained by laser irradiation. The process of film crystallization occurred through a photochemically induced reaction. In this study, ITO films with the best structural, electrical and optical properties were obtained within a narrow PO2 region of 1.33–2.0 Pa. The laser-irradiated part of the films yielded resistivity, carrier concentration and Hall mobility of 1.2 104 O cm, 1.2 1021 cm3 and 40 cm2 V1 s1 compared with 2.3 104 O cm,
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8.8 1020 cm3 and 30 cm2 V1 s1 for the nonirradiated parts of the ITO films. Improved optical properties were also obtained for the laserirradiated part of the ITO films in comparison with the nonirradiated portions.
Acknowledgements This project was sponsored by the New Energy and Industrial Technology Development Organization (NEDO), under the regional consortium projects, R&D of ultrahigh-quality transparent conductive oxide films.
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