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Highly conducting indium tin oxide films grown by ultraviolet-assisted pulsed laser deposition at low temperatures
Thin Solid Films 453 – 454 (2004) 256–261
Highly conducting indium tin oxide films grown by ultraviolet-assisted pulsed laser deposition at low temperatures V. Craciuna,*, D. Craciunb, X. Wangc, T.J. Andersonc, R.K. Singhd a
Major Analytical Instrumentation Center, University of Florida, Gainesville, FL 32611, USA b National Institute for Laser, Plasma and Radiation Physics, Bucharest, Romania c Chemical Engineering, University of Florida, Gainesville, FL, USA d Materials Science and Engineering, University of Florida, Gainesville, FL, USA
Abstract Indium tin oxide films grown by conventional and ultraviolet-assisted pulsed laser deposition technique (PLD and UVPLD) on Si and Corning glass were analyzed by grazing and symmetric incidence X-ray diffraction. Films deposited at substrate temperatures up to 70 8C were amorphous, while films deposited at temperatures of 120 8C and higher showed good crystallinity, with a (2 2 2) texture. The diffraction peaks were split indicating that the films contained a two-layer structure. X-ray reflectivity studies showed that the film surface roughness was below 0.5 nm and their density approximately 7.20 gycm3. Spectroscopic ellipsometry measurements indicated that the films were highly transparent, while X-ray photoelectron spectroscopy investigations showed homogeneous bulk chemical composition and a very small Sn enrichment in the surface region. The carrier mobility was rather high for films deposited at 40 and 70 8C and then decreased with the increase of the substrate temperature, because the onset of crystallization at such low temperatures produces a defective structure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide; Transparent and conductive oxides; Laser ablation; Ultraviolet
1. Introduction The use of temperature-sensitive substrates in solar cells and display technology requires low deposition temperatures for the processing of transparent conducting oxide films. Sputtering techniques, although enabled the deposition of good quality indium tin oxide (ITO) films even at room temperature w1–4x, resulted in a surface roughness too large for some of these applications w3–5x. Much smoother ITO films that also exhibit good electrical and optical properties were obtained by using the pulsed laser deposition (PLD) technique w5– 7x. We have shown recently that the surface morphology of ITO thin films can be further improved by using an in situ ultraviolet-assisted PLD (UVPLD) technique w8– 10x. New results about the effect of UV irradiation on the structural and electrical properties of ITO films deposited at low temperatures are presented here. *Corresponding author. Tel.: q1-352-392-5714; fax: q1-352-3920390. E-mail address: [email protected] (V. Craciun).
2. Experiment Our PLD system uses an excimer laser (KrF, ls248 nm, fluence ;2 Jycm2, repetition rate 5 Hz) to ablate ITO targets (99.99% purity, 10% by weight SnO2). The oxygen pressure was set at 10 mTorr, the optimum value for our setup w8,9x. Low pressure Hg lamps were fitted into the deposition system to irradiate the substrate during the laser ablation-growth process. The power of the 185-nm radiation reaching the substrate was approximately 200 mWycm2. Films were deposited onto (1 0 0) Si wafers that had their native oxide removed by a dip in diluted HF and on corning glass substrates. The thickness and optical properties of films deposited on Si were investigated by spectroscopic ellipsometry (SE, Woollam Co.) at 708 incidence. The film surface roughness, thickness and density were investigated by X-ray reflectivity (XRR, X’Pert MRD Panalytical). The same instrument was used to analyze the crystalline structure of the films in grazing angle incidence (GIXD) and symmetric u–2u geometry (XRD). XPS investigations were performed with a Perkin Elmer instrument
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.132
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
257
Fig. 1. GIXD spectra of the (2 2 2) peak acquired from ITO films deposited on glass by PLD (A) and UVPLD (B) at the incidence angles: Vs 0.2758 (a), 0.3008 (b), 0.3258 (c), 0.3508 (d), 0.3758 (e), 0.4008 (f), 0.4508 (g), 18 (h). The symmetric u–2u spectra were also included (i).
using Mg Ka radiation. Data were acquired at two takeoff angles: 458 (information from the top 5.0 nm) and 908 (information from the top 7.0 nm). Binding energies were referenced to the position of adventitious C 1s at 284.6 eV. The resistivity, carrier density and Hall mobility of the films were determined by Hall effect measurements in a van der Pauw configuration. 3. Results Films deposited up to a substrate temperature of 70 8C were amorphous, while those deposited at 120 8C were crystalline, more so for those grown by UVPLD. The use of GIXD technique, which is more appropriate for thin films, revealed the presence of several crystalline orientations in the grown films. It was found that the diffraction peaks corresponding to the (2 2 2) planes were, as expected, the most intense ones. These (2 2 2) peaks as well as several other more intense peaks such as (4 4 0) and (2 1 1) were found to be split in two
peaks. The relative intensity of these peaks was noted to change with the increase of the substrate temperature or film thickness. Figs. 1 and 2 display GIXD spectra acquired at several incidence angles from ITO films deposited on glass and Si, respectively. It is clear that the structure consists of two layers. The surface layer peak is located at approximately 29.68–30.08 that corresponds to an interplanar distance d2 2 2;0.297–0.300 nm, which gives a lattice parameter a0s1.023 nm, very close to ITO tabulated value. The layer close to the interface has a larger lattice parameter than the tabulated value, indicating the presence of a compressive stress. It has been reported that the diffraction peak splitting for ITO thin films is caused by the presence of a twolayer structure w11–13x. During the initial growth stage a rather defective structure that is stressed is formed at the interface with the substrate. As growth continues and the films become thicker, new layers with a lower defect density and therefore less stressed are observed.
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V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
Fig. 2. GIXD spectra of the (2 2 2) peak from ITO films deposited on Si by PLD (A) and UVPLD (B) acquired at different incidence angles: Vs0.2758 (a), 0.3008 (b), 0.3258 (c), 0.3508 (d), 0.3758 (e), 0.4008 (f), 0.4508 (g), 18 (h). The symmetric u–2u spectra were also included (i).
The fact that it was possible to observe this peak splitting for ITO film thicknesses below 100 nm indicates that the transition from the first and defective layer towards the higher crystallinity layer started earlier during the growth than that in other deposition techniques. Furthermore, it started even earlier during the UVPLD process than that in the case of conventional PLD. XPS survey-type spectra showed the presence of In, Sn, O and the usual adventitious C. High resolution spectra of In 3d and Sn 3d regions revealed symmetric peaks, an indication of single oxidation states, which were identified as corresponding to In3q and Sn4q w14– 16x. The oxygen peak acquired from the as-received surface exhibited a more complicated shape that required, as shown in Fig. 3, three peaks for a good fitting. After sputtering for 5 min with Arq the O 1s peak was deconvoluted in only two peaks.
There is an ongoing debate whether the peak denoted B in Fig. 3a and Table 1 corresponds to O atoms bonded to Sn or to a substoichiometric In oxide w16x. A very clear assignment of this oxygen peak is complicated by the fact that during laser ablation under high oxygen pressures some oxygen atoms are trapped inside the growing films and appear as a shoulder towards higher binding energies. In addition, the surface of oxide films sometimes contains hydroxide compounds that also exhibit higher BE oxygen than the non-hydrated oxides. However, as one can see in Table 1, which presents the composition of the ITO films at the surface region and after 5 and 10 min of Arq sputtering for two takeoff angles for a depth-dependence indication, one could not find a simple correlation between the area of this peak and that of Sn peak. Therefore, we suggest that this peak is not related to oxygen atoms bonded to Sn atoms.
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
From Table 1 data one can note a small Sn segregation at the surface. Similar Sn segregation phenomena, both at surface and grain boundaries, were previously reported for ITO films. However, it should be mentioned that the Sn segregation observed in this study for PLD and UVPLD grown films is much smaller than those values w17x. The thickness of the films was obtained by fitting acquired XRR spectra with a three-layer structure (interfacial layer, ITO film and surface contamination layer) using the WINGIXA娃 software. The use of this threelayer model allowed a very good fitting of the acquired spectra. The surface roughness was always below 0.5 nm and the density approximately 7.20 gycm3, the tabulated value of ITO w1,18x. SE measurements as those presented in Fig. 4 showed that the refractive index coefficients were quite close to the reference values previously reported for high quality ITO films and that the extinction coefficients of the
259
Table 1 XPS calculated stoichiometry of as-received and Arq sputtered ITO films grown on Si at 180 8C Oxygen-A
Oxygen-B
In
Sn
UVPLD as-re_458 as-re_908 5 min_458 10 min_458 10 min_908
0.42 0.42 0.45 0.47 0.47
0.14 0.15 0.10 0.07 0.07
0.37 0.37 0.40 0.40 0.40
0.07 0.07 0.06 0.06 0.06
PLD as-re_458 as-re_908 5 min_458 10 min_458 10 min_908
0.40 0.43 0.48 0.46 0.48
0.15 0.12 0.05 0.08 0.06
0.40 0.39 0.41 0.41 0.41
0.06 0.06 0.05 0.05 0.05
The takeoff angle is also indicated.
Fig. 3. High resolution XPS spectra of the O 1s region acquired from an ITO film deposited on Si at 180 8C by UVPLD: as-received surface (a) and after 5 min Arq sputtering. The deconvolution of the spectrum in the component peaks is also shown.
260
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
deposited films were below 0.1 in the visible region, a characteristic of very transparent films w8x. Because the thickness values were very important for calculations of the electrical resistivity, mobility and carrier density values from the Hall measurements, we compared in Table 2 the results obtained from XRR modeling with those obtained from SE for films deposited on Si substrates. A similar three-layer model was used for SE simulations. The surface contaminant layer was modeled like a mixture between ITO and voids. As one can note from the values displayed in Table 2, the agreement was quite good. The results of the electrical measurements are shown in Table 3. ITO films deposited at 40 and 70 8C exhibited rather high mobility values. The onset of crystallization, which as shown by GIXD investigations resulted in a rather defective structure, could explain the observed decrease of the mobility values for films deposited at 120 8C. However, this decrease was partially compensated for PLD grown films by an increase
Table 2 Comparison of SE and XRR simulation results for ITO film thickness Ts
Structure
(8C)
Thickness (nm) SE
Composition
XRR SE
180
Interfacial layer 1.0 0.7 ITO layer 55.8 50.4 Surface layer 2.0 1.0
SiO2 ITO ema(ITOq40% void)
UVPLD 180 噛83
Interfacial layer 1.0 0.8 ITO layer 66.7 64.2 Surface layer 1.7 1.0
SiO2 ITO ema(ITOq37% void)
PLD 噛76
in carrier concentrations that still resulted in a rather low resistivity value. 4. Conclusions High quality ITO films have been grown from 40 to 180 8C on glass and Si substrates using an in situ
Fig. 4. Spectral dependence of refractive index and extinction coefficient for ITO films deposited on Si by PLD and UVPLD.
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
261
Table 3 Electrical properties of ITO films deposited on glass Sample number
Substrate temperature (8C)
Thickness XRR (nm)
PLD 噛79 噛78 噛77
40 70 120
60 64 63
UVPLD 噛80 噛82 噛81
40 70 120
64 65 62
Resistivity vdP (10y4, V cm)
Carrier concentration (1020ycm3)
Mobility Hall (cm2 yV s)
4.10 3.75 4.95
5.50 3.46 9.11
27 48 14
3.34 3.36 11.0
7.29 5.63 3.84
26 33 13
UVPLD technique. The films exhibited density values approximately 7.20 gycm3 and very smooth surfaces. X-ray photoelectron spectroscopy showed uniform bulk composition consisting of fully oxidized metallic peaks and minimal Sn segregation at the surface. SE measurements showed refractive index and extinction coefficient values close to reference values measured for high quality transparent films. Films deposited at temperatures lower than 70 8C were amorphous and exhibited high carrier mobility and low resistivity values. The onset of crystallization resulted in the formation of a two-layer structure containing a defective layer that decreased the carrier mobility. References w1x H. Wulff, M. Quaas, H. Steffen, Thin Solid Films 355–356 (1999) 395. w2x J.L. Huang, Y.T. Jah, B.S. Yau, C.Y. Chen, H.H. Lu, Thin Solid Films 370 (2000) 33. w3x Z.W. Yang, S.H. Han, T.L. Yang, L. Ye, D.H. Zhang, H.L. Ma, C.F. Cheng, Thin Solid Films 366 (2000) 4. w4x J.H. Shin, S.H. Shin, J.I. Park, H.H. Kim, J. Appl. Phys. 89 (2001) 5199. w5x H. Kim, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 74 (1999) 3444.
w6x H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451. w7x H. Kim, J.S. Horwitz, G.P. Kushto, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 79 (1999) 284. w8x V. Craciun, C. Chiritescu, F. Kelly, R.K. Singh, J. Optoelectron. Adv. Mater. 4 (2002) 21. w9x V. Craciun, D. Craciun, Z. Chen, J. Hwang, R.K. Singh, Appl. Surf. Sci. 168 (2000) 118. w10x V. Craciun, D. Craciun, X. Wang, T.J. Anderson, R.K. Singh, J. Optoelectron. Adv. Mat., 5 (2003) 401. w11x H.Y. Yeom, N. Popovich, E. Chason, D.C. Paine, Thin Solid Film 411 (2002) 17. w12x C.H. Yi, Y. Shigesato, I. Yasui, S. Takaki, Jap. J. Appl. Phys. 34 (1995) L244. w13x F.O. Adurodija, H. Izumi, T. Ishihara, H. Yoshioka, M. Motoyama, J. Mater. Sci.: Mater. Electron. 12 (2001) 57. w14x S.K. So, W.K. Choi, C.H. Cheng, L.M. Leung, C.F. Kwong, Appl. Phys. A 68 (1999) 447. w15x W. Song, S.K. So, D. Wang, Y. Qiu, L. Cao, Appl. Surf. Sci. 177 (2001) 158. w16x J.S. Kim, P.K.H. Ho, D.S. Thomas, R.H. Friend, F. Cacialli, G.W. Bao, S.F.Y. Li, Chem. Phys. Lett. 315 (1999) 307. w17x Y. Shigesato, Y. Hayashi, T. Haranoh, Appl. Phys. Lett. 61 (1992) 73. w18x H. Ohta, M. Orita, M. Hirano, H. Hosono, J. Appl. Phys. 91 (2002) 3547.
Highly conducting indium tin oxide films grown by ultraviolet-assisted pulsed laser deposition at low temperatures V. Craciuna,*, D. Craciunb, X. Wangc, T.J. Andersonc, R.K. Singhd a
Major Analytical Instrumentation Center, University of Florida, Gainesville, FL 32611, USA b National Institute for Laser, Plasma and Radiation Physics, Bucharest, Romania c Chemical Engineering, University of Florida, Gainesville, FL, USA d Materials Science and Engineering, University of Florida, Gainesville, FL, USA
Abstract Indium tin oxide films grown by conventional and ultraviolet-assisted pulsed laser deposition technique (PLD and UVPLD) on Si and Corning glass were analyzed by grazing and symmetric incidence X-ray diffraction. Films deposited at substrate temperatures up to 70 8C were amorphous, while films deposited at temperatures of 120 8C and higher showed good crystallinity, with a (2 2 2) texture. The diffraction peaks were split indicating that the films contained a two-layer structure. X-ray reflectivity studies showed that the film surface roughness was below 0.5 nm and their density approximately 7.20 gycm3. Spectroscopic ellipsometry measurements indicated that the films were highly transparent, while X-ray photoelectron spectroscopy investigations showed homogeneous bulk chemical composition and a very small Sn enrichment in the surface region. The carrier mobility was rather high for films deposited at 40 and 70 8C and then decreased with the increase of the substrate temperature, because the onset of crystallization at such low temperatures produces a defective structure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide; Transparent and conductive oxides; Laser ablation; Ultraviolet
1. Introduction The use of temperature-sensitive substrates in solar cells and display technology requires low deposition temperatures for the processing of transparent conducting oxide films. Sputtering techniques, although enabled the deposition of good quality indium tin oxide (ITO) films even at room temperature w1–4x, resulted in a surface roughness too large for some of these applications w3–5x. Much smoother ITO films that also exhibit good electrical and optical properties were obtained by using the pulsed laser deposition (PLD) technique w5– 7x. We have shown recently that the surface morphology of ITO thin films can be further improved by using an in situ ultraviolet-assisted PLD (UVPLD) technique w8– 10x. New results about the effect of UV irradiation on the structural and electrical properties of ITO films deposited at low temperatures are presented here. *Corresponding author. Tel.: q1-352-392-5714; fax: q1-352-3920390. E-mail address: [email protected] (V. Craciun).
2. Experiment Our PLD system uses an excimer laser (KrF, ls248 nm, fluence ;2 Jycm2, repetition rate 5 Hz) to ablate ITO targets (99.99% purity, 10% by weight SnO2). The oxygen pressure was set at 10 mTorr, the optimum value for our setup w8,9x. Low pressure Hg lamps were fitted into the deposition system to irradiate the substrate during the laser ablation-growth process. The power of the 185-nm radiation reaching the substrate was approximately 200 mWycm2. Films were deposited onto (1 0 0) Si wafers that had their native oxide removed by a dip in diluted HF and on corning glass substrates. The thickness and optical properties of films deposited on Si were investigated by spectroscopic ellipsometry (SE, Woollam Co.) at 708 incidence. The film surface roughness, thickness and density were investigated by X-ray reflectivity (XRR, X’Pert MRD Panalytical). The same instrument was used to analyze the crystalline structure of the films in grazing angle incidence (GIXD) and symmetric u–2u geometry (XRD). XPS investigations were performed with a Perkin Elmer instrument
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.132
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
257
Fig. 1. GIXD spectra of the (2 2 2) peak acquired from ITO films deposited on glass by PLD (A) and UVPLD (B) at the incidence angles: Vs 0.2758 (a), 0.3008 (b), 0.3258 (c), 0.3508 (d), 0.3758 (e), 0.4008 (f), 0.4508 (g), 18 (h). The symmetric u–2u spectra were also included (i).
using Mg Ka radiation. Data were acquired at two takeoff angles: 458 (information from the top 5.0 nm) and 908 (information from the top 7.0 nm). Binding energies were referenced to the position of adventitious C 1s at 284.6 eV. The resistivity, carrier density and Hall mobility of the films were determined by Hall effect measurements in a van der Pauw configuration. 3. Results Films deposited up to a substrate temperature of 70 8C were amorphous, while those deposited at 120 8C were crystalline, more so for those grown by UVPLD. The use of GIXD technique, which is more appropriate for thin films, revealed the presence of several crystalline orientations in the grown films. It was found that the diffraction peaks corresponding to the (2 2 2) planes were, as expected, the most intense ones. These (2 2 2) peaks as well as several other more intense peaks such as (4 4 0) and (2 1 1) were found to be split in two
peaks. The relative intensity of these peaks was noted to change with the increase of the substrate temperature or film thickness. Figs. 1 and 2 display GIXD spectra acquired at several incidence angles from ITO films deposited on glass and Si, respectively. It is clear that the structure consists of two layers. The surface layer peak is located at approximately 29.68–30.08 that corresponds to an interplanar distance d2 2 2;0.297–0.300 nm, which gives a lattice parameter a0s1.023 nm, very close to ITO tabulated value. The layer close to the interface has a larger lattice parameter than the tabulated value, indicating the presence of a compressive stress. It has been reported that the diffraction peak splitting for ITO thin films is caused by the presence of a twolayer structure w11–13x. During the initial growth stage a rather defective structure that is stressed is formed at the interface with the substrate. As growth continues and the films become thicker, new layers with a lower defect density and therefore less stressed are observed.
258
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
Fig. 2. GIXD spectra of the (2 2 2) peak from ITO films deposited on Si by PLD (A) and UVPLD (B) acquired at different incidence angles: Vs0.2758 (a), 0.3008 (b), 0.3258 (c), 0.3508 (d), 0.3758 (e), 0.4008 (f), 0.4508 (g), 18 (h). The symmetric u–2u spectra were also included (i).
The fact that it was possible to observe this peak splitting for ITO film thicknesses below 100 nm indicates that the transition from the first and defective layer towards the higher crystallinity layer started earlier during the growth than that in other deposition techniques. Furthermore, it started even earlier during the UVPLD process than that in the case of conventional PLD. XPS survey-type spectra showed the presence of In, Sn, O and the usual adventitious C. High resolution spectra of In 3d and Sn 3d regions revealed symmetric peaks, an indication of single oxidation states, which were identified as corresponding to In3q and Sn4q w14– 16x. The oxygen peak acquired from the as-received surface exhibited a more complicated shape that required, as shown in Fig. 3, three peaks for a good fitting. After sputtering for 5 min with Arq the O 1s peak was deconvoluted in only two peaks.
There is an ongoing debate whether the peak denoted B in Fig. 3a and Table 1 corresponds to O atoms bonded to Sn or to a substoichiometric In oxide w16x. A very clear assignment of this oxygen peak is complicated by the fact that during laser ablation under high oxygen pressures some oxygen atoms are trapped inside the growing films and appear as a shoulder towards higher binding energies. In addition, the surface of oxide films sometimes contains hydroxide compounds that also exhibit higher BE oxygen than the non-hydrated oxides. However, as one can see in Table 1, which presents the composition of the ITO films at the surface region and after 5 and 10 min of Arq sputtering for two takeoff angles for a depth-dependence indication, one could not find a simple correlation between the area of this peak and that of Sn peak. Therefore, we suggest that this peak is not related to oxygen atoms bonded to Sn atoms.
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
From Table 1 data one can note a small Sn segregation at the surface. Similar Sn segregation phenomena, both at surface and grain boundaries, were previously reported for ITO films. However, it should be mentioned that the Sn segregation observed in this study for PLD and UVPLD grown films is much smaller than those values w17x. The thickness of the films was obtained by fitting acquired XRR spectra with a three-layer structure (interfacial layer, ITO film and surface contamination layer) using the WINGIXA娃 software. The use of this threelayer model allowed a very good fitting of the acquired spectra. The surface roughness was always below 0.5 nm and the density approximately 7.20 gycm3, the tabulated value of ITO w1,18x. SE measurements as those presented in Fig. 4 showed that the refractive index coefficients were quite close to the reference values previously reported for high quality ITO films and that the extinction coefficients of the
259
Table 1 XPS calculated stoichiometry of as-received and Arq sputtered ITO films grown on Si at 180 8C Oxygen-A
Oxygen-B
In
Sn
UVPLD as-re_458 as-re_908 5 min_458 10 min_458 10 min_908
0.42 0.42 0.45 0.47 0.47
0.14 0.15 0.10 0.07 0.07
0.37 0.37 0.40 0.40 0.40
0.07 0.07 0.06 0.06 0.06
PLD as-re_458 as-re_908 5 min_458 10 min_458 10 min_908
0.40 0.43 0.48 0.46 0.48
0.15 0.12 0.05 0.08 0.06
0.40 0.39 0.41 0.41 0.41
0.06 0.06 0.05 0.05 0.05
The takeoff angle is also indicated.
Fig. 3. High resolution XPS spectra of the O 1s region acquired from an ITO film deposited on Si at 180 8C by UVPLD: as-received surface (a) and after 5 min Arq sputtering. The deconvolution of the spectrum in the component peaks is also shown.
260
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
deposited films were below 0.1 in the visible region, a characteristic of very transparent films w8x. Because the thickness values were very important for calculations of the electrical resistivity, mobility and carrier density values from the Hall measurements, we compared in Table 2 the results obtained from XRR modeling with those obtained from SE for films deposited on Si substrates. A similar three-layer model was used for SE simulations. The surface contaminant layer was modeled like a mixture between ITO and voids. As one can note from the values displayed in Table 2, the agreement was quite good. The results of the electrical measurements are shown in Table 3. ITO films deposited at 40 and 70 8C exhibited rather high mobility values. The onset of crystallization, which as shown by GIXD investigations resulted in a rather defective structure, could explain the observed decrease of the mobility values for films deposited at 120 8C. However, this decrease was partially compensated for PLD grown films by an increase
Table 2 Comparison of SE and XRR simulation results for ITO film thickness Ts
Structure
(8C)
Thickness (nm) SE
Composition
XRR SE
180
Interfacial layer 1.0 0.7 ITO layer 55.8 50.4 Surface layer 2.0 1.0
SiO2 ITO ema(ITOq40% void)
UVPLD 180 噛83
Interfacial layer 1.0 0.8 ITO layer 66.7 64.2 Surface layer 1.7 1.0
SiO2 ITO ema(ITOq37% void)
PLD 噛76
in carrier concentrations that still resulted in a rather low resistivity value. 4. Conclusions High quality ITO films have been grown from 40 to 180 8C on glass and Si substrates using an in situ
Fig. 4. Spectral dependence of refractive index and extinction coefficient for ITO films deposited on Si by PLD and UVPLD.
V. Craciun et al. / Thin Solid Films 453 – 454 (2004) 256–261
261
Table 3 Electrical properties of ITO films deposited on glass Sample number
Substrate temperature (8C)
Thickness XRR (nm)
PLD 噛79 噛78 噛77
40 70 120
60 64 63
UVPLD 噛80 噛82 噛81
40 70 120
64 65 62
Resistivity vdP (10y4, V cm)
Carrier concentration (1020ycm3)
Mobility Hall (cm2 yV s)
4.10 3.75 4.95
5.50 3.46 9.11
27 48 14
3.34 3.36 11.0
7.29 5.63 3.84
26 33 13
UVPLD technique. The films exhibited density values approximately 7.20 gycm3 and very smooth surfaces. X-ray photoelectron spectroscopy showed uniform bulk composition consisting of fully oxidized metallic peaks and minimal Sn segregation at the surface. SE measurements showed refractive index and extinction coefficient values close to reference values measured for high quality transparent films. Films deposited at temperatures lower than 70 8C were amorphous and exhibited high carrier mobility and low resistivity values. The onset of crystallization resulted in the formation of a two-layer structure containing a defective layer that decreased the carrier mobility. References w1x H. Wulff, M. Quaas, H. Steffen, Thin Solid Films 355–356 (1999) 395. w2x J.L. Huang, Y.T. Jah, B.S. Yau, C.Y. Chen, H.H. Lu, Thin Solid Films 370 (2000) 33. w3x Z.W. Yang, S.H. Han, T.L. Yang, L. Ye, D.H. Zhang, H.L. Ma, C.F. Cheng, Thin Solid Films 366 (2000) 4. w4x J.H. Shin, S.H. Shin, J.I. Park, H.H. Kim, J. Appl. Phys. 89 (2001) 5199. w5x H. Kim, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 74 (1999) 3444.
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