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  • Editor: Christian Seassal
  • Vol. 22, Iss. S3 — May. 5, 2014
  • pp: A607–A621
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Annealing of SnO2 thin films by ultra-short laser pulses

D. Scorticati, A. Illiberi, T. Bor, S.W.H. Eijt, H. Schut, G.R.B.E. Römer, D.F. de Lange, and A.J. Huis in t Veld  »View Author Affiliations


Optics Express, Vol. 22, Issue S3, pp. A607-A621 (2014)
http://dx.doi.org/10.1364/OE.22.00A607


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Abstract

Post-deposition annealing by ultra-short laser pulses can modify the optical properties of SnO2 thin films by means of thermal processing. Industrial grade SnO2 films exhibited improved optical properties after picosecond laser irradiation, at the expense of a slightly increased sheet resistance [Proc. SPIE 8826, 88260I (2013)]. The figure of merit ϕ = T10 / Rsh was increased up to 59% after laser processing. In this paper we study and discuss the causes of this improvement at the atomic scale, which explain the observed decrease of conductivity as well as the observed changes in the refractive index n and extinction coefficient k. It was concluded that the absorbed laser energy affected the optoelectronic properties preferentially in the top 100-200 nm region of the films by several mechanisms, including the modification of the stoichiometry, a slight desorption of dopant atoms (F), adsorption of hydrogen atoms from the atmosphere and the introduction of laser-induced defects, which affect the strain of the film.

© 2014 Optical Society of America

1. Introduction

Tin-dioxide (i.e. SnO2), also known as stannic oxide, is a widely applied and studied ceramic material [2

2. M. Batzill and U. Diebold, “The surface and materials science of tin oxide,” Prog. Surf. Sci. 79(2-4), 47–154 (2005). [CrossRef]

]. Un-doped and stoichiometric SnO2 is a semiconductor with a wide bandgap (~3.6 eV). Shallow doping of SnO2 (e.g. with F, Cl or Sb) or small shifts from its perfect stoichiometry (i.e. SnO2-x, with x < 1), which are associated with oxygen vacancies, lead to n-type semi-conductive behavior. Doped SnO2 combines a high electrical conductivity with optical transparency and belongs to a class of materials referred to as Transparent Conductive Oxides (TCOs). These TCOs are important for opto-electronic devices [2

2. M. Batzill and U. Diebold, “The surface and materials science of tin oxide,” Prog. Surf. Sci. 79(2-4), 47–154 (2005). [CrossRef]

, 3

3. R. Gordon, “Criteria for choosing transparent conductors,” MRS Bull. 25(08), 52–57 (2000). [CrossRef]

]. Due to the low fabrication cost, SnO2 thin films find application in different areas, including glass coatings for thermal insulation, oxidation catalysts, gas sensors, flat panel displays, touch panels, flexible electronics, dye-sensitized solar cells (DSSC), thin film silicon and cadmium telluride (CdTe) solar cells, light-emitting diodes (LEDs), etc [3

3. R. Gordon, “Criteria for choosing transparent conductors,” MRS Bull. 25(08), 52–57 (2000). [CrossRef]

]. A strong interest for TCO materials led research into the effects of post deposition annealing by nanosecond (ns) laser pulses on the optical and electrical properties of different TCOs [4

4. S. F. Tseng, W. T. Hsiao, D. Chiang, K. C. Huang, and C. P. Chou, “Mechanical and optoelectric properties of post-annealed fluorine-doped tin oxide films by ultraviolet laser irradiation,” Appl. Surf. Sci. 257(16), 7204–7209 (2011). [CrossRef]

11

11. J. Chae, L. Jang, and K. Jain, “High-resolution, resistless patterning of indium-tinoxide thin films using excimer laser projection annealing process,” Mater. Lett. 64(8), 948–950 (2010). [CrossRef]

]. The use of Rapid Thermal Laser Annealing (RTLA) is an alternative process to conventional furnace annealing. Compared to furnace annealing RTLA reduces thermal damage of sensitive low cost substrates with low melting points [4

4. S. F. Tseng, W. T. Hsiao, D. Chiang, K. C. Huang, and C. P. Chou, “Mechanical and optoelectric properties of post-annealed fluorine-doped tin oxide films by ultraviolet laser irradiation,” Appl. Surf. Sci. 257(16), 7204–7209 (2011). [CrossRef]

]. For a given wavelength of the laser radiation, ultra-short laser pulses -with pulse durations in the order of tens of picoseconds or less- can induce same high surface temperatures of the film, but at a significantly lower energy input per pulse, when compared to nanosecond laser pulses. This can be attributed to the fact that the Heat Affected Zone (HAZ) due to ultra-short laser pulses is significantly smaller than due to ns laser pulses. Hence, ultra-short laser pulsed processing is highly selective in depth of the laser induced transformations. This can be exploited for processing either thin films or selectively process a thin top layer of the substrate under consideration. Moreover, due to the small timescales at which the annealing process occurs during ultra-short laser processing, new and original material structures can be obtained. Despite low heat input during laser annealing, heat accumulation might occur, especially at high pulse rates. However, by careful selection of the laser parameters (i.e. laser fluence, repetition rate, and pulse overlap), heat accumulation can be controlled in order to increase the duration of the heating cycle time, while ensuring that the temperature at the film/substrate interface remains below the damage threshold for the substrate.

In an earlier study [1

1. D. Scorticati, G. R. B. E. Römer, T. Bor, W. Ogieglo, M. Klein Gunnewiek, A. Lenferink, C. Otto, J. Z. P. Skolski, F. Grob, D. F. de Lange, and A. J. Huis in t’ Veld, “Optical and electrical properties of SnO2 thin films after ultra-short pulsed laser annealing,” Proc. SPIE 8826, Laser Material Processing for Solar Energy Devices II, 88260I (2013).

], the effects of UV (λ = 343 nm) picosecond (ps) laser irradiation on the macroscopic optical and electrical properties of thin SnO2 films was demonstrated and discussed. It was shown that the total average optical transmittance (T) in the wavelength range from 400 to 1100 nm increased from 71.4% to 76.1%, while the electrical properties were only slightly deteriorated. That is, the sheet resistance (Rsh) increased from 13.5 to 16.1 Ω/sq. The Figure-of-Merit ϕ = T10 / Rsh [12

12. G. Haacke, “New figure of merit for transparent conductors,” J. Appl. Phys. 47(9), 4086–4089 (1976). [CrossRef]

], which combines the optical and electrical properties in a single performance quantity, increased up to 59% due to the laser treatment. Moreover, it was found that when nanostructures (also known as Laser-Induced Periodic Surface Structures, or LIPSS) form on the surface of the film, due to the laser irradiation, the optical reflectivity of the film reduces and the haze of the film increases. This was attributed, in part, due to the morphology of LIPSS, which was superimposed on the original morphology. Other possible causes, which could smoothen the mismatch of refractive index with air, as well as could explain the decreased optical absorption by the bulk of the material, were not discussed [1

1. D. Scorticati, G. R. B. E. Römer, T. Bor, W. Ogieglo, M. Klein Gunnewiek, A. Lenferink, C. Otto, J. Z. P. Skolski, F. Grob, D. F. de Lange, and A. J. Huis in t’ Veld, “Optical and electrical properties of SnO2 thin films after ultra-short pulsed laser annealing,” Proc. SPIE 8826, Laser Material Processing for Solar Energy Devices II, 88260I (2013).

]. Moreover, it was also pointed out that processing with ultra-short pulses may introduce microscopic modifications in the lattice, which do result in measurable changes in optical and electrical properties.

The present research aims to study the effect of UV (λ = 343 nm) picosecond laser-material interaction on the microscopic structure of SnO2 thin films in order to explain the origin of the macroscopic changes observed in the optical, as well as electrical properties of the film. We discuss the causes at the atomic scale, which can explain the observed decrease of conductivity, such as modification of the density of oxygen vacancies, changes in average grain size, amorphization due to fast quenching, introduction of quenched-in defects during fast thermal processing, as well as stress/strain induced by the defects in the lattice. Also the observed changes in the refractive index n and extinction coefficient k can be understood at the atomic level. That is, the presence of detrimental impurities, formation of different material phases, presence of amorphous structure of the lattice and different carrier densities. The possible causes, which induce a modification of the carrier density, electronic mobility and optical constants n and k, due to the laser treatment, were examined by means of several inspection techniques. The results of these analyses were cross-checked to give an overall exhaustive comprehensive interpretation of the observed effects. The results of this study can be used to further improve the performance of SnO2-based electrodes for solar cells and/or other electronic devices.

2. Experimental

2.1 Laser setup

In the experiments, an Yb:YAG laser source was used, showing a nearly Gaussian power density profile (M2 < 1.3) and with fixed pulse duration of about 6.7 ps. A Third Harmonic Generation (THG) unit was employed to convert the central wavelength λ = 1030 nm of the laser source to 343 nm (UV) and a Galvano-scanner, equipped with an F-theta-lens (focal length 103 mm), was used to scan the focal spot (diameter d = 17 μm) over the surface of the samples. The spot diameter, combined with the scan velocity v and the pulse frequency fp determine the spatial pulse-to-pulse overlap (OL), which is defined here as OL = 1 - v / (dfp). More details on the experimental laser setup can be found in [13

13. D. Scorticati, G. R. B. E. Römer, D. F. de Lange, and A. J. Huis in ’t Veld, “Ultra-short-pulsed laser-machined nanogratings of laser-induced periodic surface structures on thin molybdenum layers,” J. Nanophotonics 6(1), 063528 (2012).

].

2.2 Analysis tools

Several analysis techniques were adopted to study the effects of the laser radiation on the material. A high resolution Scanning Electron Microscope (SEM), as well as an Atomic Force Microscope (AFM) were utilized to investigate the surface morphology of the samples, as well as their cross sections. For the investigation of the crystalline structure of the outermost atomic layers, Transmission Electron Microscope (TEM) was used. X-ray diffraction (XRD) experiments were performed on a system using a mixture of Kα1 and Kα2 of Co radiation with an average wavelength of Kα = 0.17903 nm, to determine the crystallographic structure of the film. To identify changes in stoichiometric composition of the SnO2 thin films and the concentration of impurities, we employed X-ray photoelectron spectroscopy (XPS). Depth profiling of impurities with low concentrations was performed using the Time of Flight Secondary-Ion Mass Spectrometry (TOF-SIMS) technique. The electrical properties of the films, such as electron mobility, carrier density and sheet resistance, were determined by Hall effect measurements and the 4-point probe technique. Last, positron annihilation was adopted to inspect the occurrence of laser-induced defects in the layers after the laser treatment. The Doppler broadening of positron annihilation radiation was measured with a low-energy Variable Energy Positron beam (VEP) using positrons with a kinetic energy in the range of 0-25 keV [14

14. S. W. H. Eijt, R. Kind, S. Singh, H. Schut, W. J. Legerstee, R. W. A. Hendrikx, V. L. Svetchnikov, R. J. Westerwaal, and B. Dam, “Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films,” J. Appl. Phys. 105(4), 043514 (2009). [CrossRef]

]. The depth profiles of the S and W parameters, extracted from the Doppler broadened 511 keV γ-ray photopeak, were analyzed using VEPFIT software [15

15. A. Van Veen, H. Schut, J. de Vries, R. A. Hakvoort, and M. R. Ijpma, “Positron beams for solids and surfaces,” AIP Conf. Proc. 218, 171–196 (1990).

].

2.3 Samples

Fluorine-doped SnO2 samples of 980 nm thickness, deposited by chemical vapor deposition (CVD) on 1 mm thick Borofloat®-glass, showing a sheet resistance of Rsh = 13.5 Ω/sq were used. This combination of a thin SnO2 layer on glass is used for the production of silicon thin film solar cells. Deposition of the samples was performed at TNO in the Netherlands using an industrial process [16

16. A. de Graaf, J. van Deelen, P. Poodt, T. van Mol, K. Spee, F. Grob, and A. Kuypers, “Development of atmospheric pressure CVD processes for high quality transparent conductive oxides,” En. Proc. 2(1), 41–48 (2010). [CrossRef]

].

2.4 Experimental approach

The experimental procedure consisted of three steps. During the first step, laser tracks were created with varying pulse-to-pulse overlap (OL), number of over-scans (OS) and fluence levels (being F0 the peak fluence) by scanning the focal spot over the sample at a fixed pulse repetition frequency of fp = 100 kHz. The second step consisted of finding conditions for damage-free (crack-free and no ablation) treatment of the films. This second step was iterated by choosing finer variation of the processing parameters (OL, OS and fluence) and inspecting the sample afterwards by SEM. The third and final step of the experimental procedure consisted of creating laser treated (3 cm2) areas by varying the pitch (distance) between parallel laser tracks. Following this procedure, three sets of samples were manufactured and subjected to subsequent analysis:

  1. As-deposited SnO2, without laser treatment applied;
  2. Low fluence regime (F0 = 0.17 J/cm2, 1 OS, OL = 98.7%, pitch = 3 µm);
  3. High fluence regime (F0 = 0.26 J/cm2, 1 OS, OL = 98.7%, pitch = 3 µm).

3. Results and discussion

3.1 Surface morphology

For the sample treated at low fluence, where no LIPSSs were found (see Fig. 2(c)), the material remained entirely in the solid state during annealing. In the case of treatment at high fluence, LIPPSs were formed, which only happens when either melting and/or ablation occurs due to laser irradiation. In both cases, the melting temperature must be reached to observe morphological modifications. Therefore, the maximum surface temperature reached during the laser processing of the sample treated at high fluence is expected to have exceeded the melting temperature TM. Several mechanisms can induce melting of the surface in the sample treated at high fluence. First, if the peak fluence F0 of a single pulse is above the fluence FM, required for melting, the surface melts after one single pulse. Experiments based on the so-called D2-method [18

18. J. Bonse, J. M. Wrobel, J. Kruger, and W. Kautek, “Ultrashort-pulse laser ablation of indium phosphide in air,” Appl. Phys., A Mater. Sci. Process. 72(1), 89–94 (2001). [CrossRef]

] were performed to identify the single pulse melting fluence threshold to equal FM = 0.56 J/cm2. As the single peak fluence F0 applied during the experiments (see section 2.4) was well below FM, other mechanisms should be considered to explain the occurrence of surface melting. Two other mechanisms are most likely inducing the melting of the surface. First, inter-pulse heat accumulation and secondly, a reduction of the melting threshold due to laser induced defects when a high overlap (OL) is applied. Usually, a reduction of the thresholds of melting and ablation is observed only upon irradiation with hundreds to thousands of laser pulses. This particular phenomenon is explained by the formation of numerous defects just below the surface of the irradiated material, which in turn reduces the thresholds [19

19. Y. Jee, M. F. Becker, and R. M. Walser, “Laser-induced damage on single-crystal metal surfaces,” J. Opt. Soc. Am. B 5(3), 648–659 (1988). [CrossRef]

]. The latter can also be understood as the chemical storage of heat in the material, by increasing the free Gibbs energy of the lattice, induced by an increasing disorder. However this reduction of the threshold for melting is expected to be a small fraction of FM. At F0 = 0.26 J/cm2, corresponding to the high laser fluence condition, the ratio between the peak fluence and the fluence for melting is small, i.e. F0/FM ≈0.5. For ultra-short laser pulses, thermal dissipation due to heat conduction in the material is limited during the pulse. Hence, the maximum surface lattice temperature T0 of SnO2 at the center of the focal spot (r = 0, z = 0) at F0 = 0.26 J/cm2 will be T0TM/2. The significant difference between TM and T0 cannot be explained by a reduction of the melting temperature in SnO2 caused by the accumulation of defects with the current laser conditions. Therefore, the difference implies that heat accumulation occurs during the treatment and explains why the top surface of the film is melted. That is, heat accumulation allows to reach temperatures above the single-shot maximum temperature, which can explain observed surface modifications of the material after being irradiated with pulses having fluence levels F0 < FM. Moreover, the final structure of a material after the annealing process depends on the local thermal history of the heating-quenching cycle. The occurrence of controlled inter-pulse heat accumulation, which rules the timescale of the annealing cycle, is an important condition to prevent amorphization in material processing via ultra-short laser pulses. On the other hand, it should be mentioned that heat accumulation needs to be controlled by varying the laser parameters (e.g. λ, fp and F0) in order to avoid damage of thermally sensitive substrates.

3.2 Cross sections

Fig. 3 Cross section of SEM images of (a) as-deposited SnO2, (b) SnO2 treated at high laser fluence causing surface melting and the LIPSSs depicted in Figs. 2(e) and (f).
SEM pictures of the cross sections (Fig. 3) show that the grain morphology in the bulk (close to the glass) of the sample treated at high fluence is similar to the morphology of the as-deposited sample. In both samples, small grains are visible near the glass substrate due to the fast nucleation process during deposition. Grain growth along a preferential direction with lower formation energy enables the development of bigger V-shaped grains on top of the small ones. XRD texture measurements performed on the samples indicated the presence of a strong fiber texture with <301> in the direction normal to the plain of the sample. This type of texture has been observed before [20

20. V. Consonni, G. Rey, H. Roussel, and D. Bellet, “Thickness effects on the texture development of fluorine-doped SnO2 thin films: The role of surface and strain energy,” J. Appl. Phys. 111(3), 033523 (2012). [CrossRef]

] and is enhanced in relatively thick (over 500 nm) SnO2:F layers. The fiber texture was not significantly changed due to the laser irradiation. However, the top 100-200 nm of the high fluence laser-treated sample reveals a different contrast and structure than the as-deposited sample.

Fig. 4 (a) and (b): Bright field TEM images of as-deposited SnO2 at two different magnifications. (d) and (e): Bright field TEM images of the sample treated with high laser fluence at two different magnifications. Pictures (c) and (f) show the electron diffraction pattern recorded near the interfaces, respectively corresponding to the as-deposited and high fluence samples.
Arguably, the difference in contrast of the top 100-200 nm in Fig. 3(b) can be attributed either to an increased amount of defects, or to a different chemical composition in the top layer, as a result of a modified lattice structure in the high fluence sample. To investigate the possible presence of an amorphous lattice structure within the modified top part of the high fluence sample, due to fast quenching, the as-deposited and the high fluence samples were glued together and a lamella for TEM inspection was extracted and subsequently mechanically grinded and ion milled. The cross sections of the two samples are shown in Figs. 4(a) and 4(d). Figures 4(b) and 4(e) show a close-up view of the surface layers at the SnO2-glue interface, where lattice fringes indicate crystallinity of the samples all the way up to the surface. The latter is supported by the corresponding electron diffraction pattern recorded near the interfaces shown in Figs. 4(c) and 4(f), showing no amorphous halo. This result was also confirmed by XRD spectra, where no amorphous hump was observed when the same sample was inspected not locally, but over a bigger area (1 × 1 cm) [1

1. D. Scorticati, G. R. B. E. Römer, T. Bor, W. Ogieglo, M. Klein Gunnewiek, A. Lenferink, C. Otto, J. Z. P. Skolski, F. Grob, D. F. de Lange, and A. J. Huis in t’ Veld, “Optical and electrical properties of SnO2 thin films after ultra-short pulsed laser annealing,” Proc. SPIE 8826, Laser Material Processing for Solar Energy Devices II, 88260I (2013).

]. Hence, both measurements indicate the absence of any amorphous structure in the lattice. Therefore, any observed effect of the laser process on the optical and electrical properties of the film cannot be attributed to amorphization.

3.3 Stoichiometry and chemical composition

The optical and electrical properties of an SnO2 film greatly depend on its stoichiometry and chemical composition. Small shifts of stoichiometry, as well as the presence of impurities are associated to interstitial states and band tailing effects. Those can affect the transmittance of light in the bandgap of a semiconductor, as well as the carrier density and the electron mobility of the material. The latter are significant quantities contributing to the sheet resistance (Rsh), being Rsh = 1 / (ne μe qe t), where ne, μe, qe and t respectively are the carrier density, the electron mobility, the electron charge and the thickness of the film. Therefore, to study the effect of laser annealing on the chemical composition and stoichiometry, we carried out XPS and TOF-SIMS analysis of the samples.

Oxygen desorption was inspected by the XPS technique. Tin and oxygen concentrations were measured along the depth of the samples and their signals (respectively the Sn1d5 and O1s peaks) were divided in order to have their ratio O/Sn, see Fig. 5.
Fig. 5 O/Sn ratio from XPS measurements for the as-deposited SnO2 and the sample treated at high fluence.
In perfectly stoichiometric SnO2, this ratio is 2. Irrespective of the fluctuations of the signals over depth, we observed a relative decrease of oxygen concentration in the laser-treated sample at high fluence compared to the concentration on the as-deposited film. The largest shifts in the stoichiometry from SnO2 to SnO2-x, with x as high as 0.1, were found in the sample treated with high fluence, but only in the top 250 nm of the film. We attribute this effect to desorption of surplus interstitial oxygen from the film during laser processing.

3.4 Lattice structure and presence of strain

Fig. 7 Williamson Hall plot of the integral breadth versus the reciprocal lattice spacing for various SnO2 reflections. The intercepts of the extrapolation of the line through the integral breadths of the {110} and {220} determine the size contribution to the broadening for the different samples.
X-ray Diffraction (XRD) was employed to obtain an average value of the SnO2 grain size for the different samples. The grain size can be determined from the width of the measured XRD reflections since the grain size is inversely proportional to the width (the so-called “coherent length”), if no other sources of broadening are present. Often, significant contributions to the total broadening arise from the non-ideal optics of the diffractometer and the wavelength distribution (“instrumental broadening”) and/or from microstructural sources such as dislocations introducing non-homogeneous strain fields (“strain broadening”). The contribution from the instrument can be removed using separately measured reflections of a standard material employing the same instrumental configuration and X-ray wavelength. Here, a standard sample, LaB6, was used for this purpose and an interpolation routine was employed to determine the instrumental line profile for the measured reflections. Subsequently, a deconvolution procedure was adopted to obtain the Fourier coefficients of the only structural broadened profiles [24

24. R. Delhez, Th. H. de Keijser, and E. J. Mittemeijer, “Determination of crystallite size and lattice distortions through X-ray diffraction line profile analysis,” Fresenius Z. Anal. Chem. 312(1), 1–16 (1982). [CrossRef]

]. The widths of these profiles were characterized by the integral breadths determined from the Fourier coefficients. A plot of the integral breadths as a function of 1/d{hkl}, with d{hkl} being the lattice spacing pertaining to the {hkl} reflection considered, is shown in Fig. 7 for all the samples. If grain size broadening is the only source of structural broadening, then it can be shown [25

25. B. E. Warren, X-Ray Diffraction (Addison Wesley, 1969).

] that the integral breadths of all SnO2 reflections are equal. However, Fig. 7 shows that the integral breadth increases as a function of 1/d{hkl} for all samples. Consequently, the samples exhibit lattice defects causing “strain broadening”. For the purpose of this study, we adopted a relatively straightforward approach to separate the “size” and “strain” sources employing multiple orders of the same reflection. Hence, the integral breadth values of the {110} and {220} SnO2 reflections are extrapolated towards the intercept at 1/d{hkl} = 0 [20

20. V. Consonni, G. Rey, H. Roussel, and D. Bellet, “Thickness effects on the texture development of fluorine-doped SnO2 thin films: The role of surface and strain energy,” J. Appl. Phys. 111(3), 033523 (2012). [CrossRef]

]. The so-obtained values represent the net broadening due to the small size of SnO2 crystallites oriented with their <110> normal to the plane of the sample. Here, the integral breadths of the samples are approximately equal to 0.015 nm−1 resulting in an average grain size of 67 nm, when adopting a Scherrer constant equal to one. These results match with the observed grain dimensions obtained from SEM (Fig. 3) and TEM (Fig. 4) analysis, in which epitaxial recrystallization was found.

From the observed increasing trend of the broadening as a function of 1/d{hkl} and the {hkl} dependence of the broadening, the presence of micro-strain sources is also relevant in all samples. The particular dependence of the line broadening on 1/d{hkl} shown in Fig. 7, suggests the presence of distributed defects, probably dislocations, even in the as-deposited sample [26

26. M. Leoni, J. Martinez-Garcia, and P. Scardi, “Dislocation effects in powder diffraction,” J. Appl. Cryst. 40(4), 719–724 (2007). [CrossRef]

31

31. J. E. Dominguez, L. Fu, and X. Q. Pan, “Effects of crystal defects on the electrical properties in epitaxial tin dioxide thin films,” Appl. Phys. Lett. 81(27), 5168–5170 (2002). [CrossRef]

]. The laser treatment causes a further broadening of the reflections only in case of the high fluence sample. Concomitantly to the peak broadening, also a peak shift, not shown here, was observed for this sample indicating the presence of strains at the macro level. Full quantitative analysis of the observed peak shifts and broadening is beyond the scope of this study.

3.5 Formation of defects

Positron annihilation is usually adopted to study the presence of defects, such as vacancies in materials for concentrations down to 1016 cm−3. Signals collected by positron annihilation give information about the position of the trapping sites (lattice defects) for the positrons, thus precisely locating the extension of the affected zone where the changes observed in the SnO2 films have a major impact on the optoelectronic properties. Laser-induced lattice defects (including Sn vacancies, ionized impurities, grain boundaries, or even substitutional atoms) lead to distortion of the lattice and act as traps or scattering centers for free carriers. In turn, these have a detrimental effect on the overall electrical properties, because these defects can induce a reduction of both the carrier density, as well as a reduced electron mobility [30

30. V. Consonni, G. Feuillet, and P. Gergaud, “The flow stress in polycrystalline films: Dimensional constraints and strengthening effects,” Acta Mater. 56(20), 6087–6096 (2008). [CrossRef]

]. In our samples, the measured signal will be a sum of the signals from all different contributions, from intrinsic vacancies to laser-induced defects, which play a primary role on the electrical properties.

The depth-profiles and VEPFIT analysis clearly showed that the properties of the top 130 nm layer of the as-deposited SnO2:F film differ from the properties of the remainder of the film. That is, the thin sub-surface top 130 nm layer shows a low value of the S parameter of 0.4792 ± 0.0003 and high a high value of the W parameter of 0.0700 ± 0.0003. The layer below, towards the glass substrate, shows a relatively higher value of S of 0.4915 ± 0.0003 and a lower value of W of 0.0643 ± 0.0003, when compared to the top 130 nm layer. We attribute this difference in the top and bottom layer of the as-deposited SnO2:F film to the large carbon fractions present in the film in the depth-range up to about 150 nm below the surface (Fig. 6), with the highest concentrations at the surface of up to 44 at% as measured by TOF-SIMS. The concentration of carbon reduces beyond a depth of about 150 nm.

3.6 Effects on the optical properties

Fig. 9 Measured optical transmittance for the three samples, reproduced from [1].
The optical transmittance of the three samples was measured in the wavelength range of 400 to 1100 nm, see Fig. 9. The transmittance varies from an average value of 71.4% in the as-deposited sample to 76.1% in the sample treated at high fluence, see also Table 1. Three main causes can affect the optical transmittance of the films, namely: carbon impurities, stoichiometry and doping concentration.

Carbon impurities, either near the surface or in a TCO film, increase light absorption of the TCO over the complete range of wavelength a solar cell operates (~300-1100 nm), resulting in a lower overall efficiency of the device. In TCO’s, this high absorptivity should be avoided and optical transmittance should be as high as possible. In contrast, a high concentration of carbon at the surface of the film increases the refractive index mismatch at the air-TCO interface, resulting in a higher reflectivity. Taking an average value for the optical absorption coefficient of the carbon-rich outermost layer of about 104 to 105 cm−1 [34

34. N. Laidani, R. Bartali, G. Gottardi, M. Andrele, and P. Cheyssac, “Optical absorption parameters of amorphous carbon films from Forouhi-Bloomer and Tauc-Lorentz models: a comparative study,” J. Phys. Condens. Matter 20(015216), 1–8 (2008).

] in the visible range and considering its distribution with depth, a decrease in absorption of a few percent is predicted in the sample treated at high laser fluence, which is in the same order of the experimentally observed variations in Fig. 9.

The observed decrease in the O/Sn ratio after laser annealing see Fig. 5 might also affect the optical transmittance, since the density of oxygen interstitial states (with O/Sn > 2), which absorb visible radiation, is reduced by the laser annealing.

3.7 Effects on the electrical properties

Hall measurements (see Table 1) show only small differences in the carrier density in the three samples, displaying a maximum value for the as-deposited sample and slightly lower values for the laser-treated samples. The Hall measurements suggest that the decrease of carrier density does not simply follow the increase of the laser fluence. That is, the carrier density of the sample treated at low fluence is lower than the value of the sample treated at high fluence. The latter observation indicates that most likely different causes do affect the carrier density with opposite effects. The total of combined effects depends on the adopted fluence regime during the laser treatment. Since the differences in the measurements are small between the three samples, we can only point out which of the measured properties can possibly play a role in the measured quantities, but not quantify the relative influence.

It is well-known that the carrier density of SnO2 depends on point defects in the lattice associated with oxygen vacancies, which do provide free electrons in the conduction band [2

2. M. Batzill and U. Diebold, “The surface and materials science of tin oxide,” Prog. Surf. Sci. 79(2-4), 47–154 (2005). [CrossRef]

]. The latter could arise from the presence of electrically active impurities acting as donors when they occupy substitutional oxygen sites, as well as from small shifts from the perfect stoichiometry, possibly caused by oxygen desorption [2

2. M. Batzill and U. Diebold, “The surface and materials science of tin oxide,” Prog. Surf. Sci. 79(2-4), 47–154 (2005). [CrossRef]

]. The TOF-SIMS analysis in section 3.3, pointed out a variation in the amount of total ionized impurities, i.e. F and H, while XPS showed shifts in the stoichiometry after the laser treatment. The stoichiometric shift is relatively large, since most of the desorbed atoms are oxygen atoms which are present in excess as interstitials in the lattice above the perfect stoichiometric ratio (O/Sn = 2). Creation of intrinsic shallow donor defects can result in an increase of carrier density up to about 1020 cm−3. Since a fraction of the observed desorbed oxygen can also originate from the lattice, hence increasing the density of oxygen vacancies, this effect is expected to increase the carrier density. Being the observed changes of the carrier density between the three samples in the order of 1019 cm−3, stoichiometric shifts can be considered as one of the possible causes.

Resuming, on the one hand, the carrier density tends to decrease due to a slight desorption of F, but on the other hand, the laser process increases the carrier density by two factors: first, by reducing the stoichiometric ratio of O to Sn due to desorption of oxygen, and secondly, by increasing the amount of adsorbed H (see section 3.3). Presence of laser-induced defects in the lattice can also negatively affect the carrier concentration as well as the electron mobility, and will be discussed at the end of this section.

Table 1 shows a change in electron mobility, due to the laser treatment, measured by the Hall technique. A modified electron mobility in a semiconductor can arise from several causes, such as: scattering by phonons, neutral and ionized impurities, lattice defects and grain boundaries. The total scattering rate is then the sum of the individual rates [36

36. C. Jacoboni, Theory of Electron Transport in Semiconductors, Springer Series in Solid-State Science, Vol. 165 (Springer, 2010).

]. All the mentioned phenomena can be triggered by laser irradiation when a material is quickly heated and subsequently fast cooled by self-quenching. However, for a degenerate semiconductor the situation simplifies, as scattering by phonons and neutral impurities can be neglected.

The contribution of grain-boundary scattering is significant only if the grain size Dg is comparable to the mean electronic free path le, as determined by all scattering mechanisms [37

37. V. I. Kaydanov, T. J. Coutts, and D. L. Young, Studies of Band Structure and Free-Carrier Scattering in Transparent Conducting Oxides Based on Combined Measurements of Electron Transport Phenomena,” NREL/CP-520–29064 (2000).

]. In typical polycrystalline thin TCO films the mean electronic free path estimated in literature, based on the mobility and carrier concentration, is rather small-i.e. le ≈10 nm, while Dg ≈100 nm [38

38. A. Oprea, E. Moretton, N. Barsan, W. J. Becker, J. Wollenstein, and U. Weimar, “Conduction model of SnO2 thin films based on conductance and Hall effect measurements,” J. Appl. Phys. 100, 033716 (2006). [CrossRef]

]. From the obtained XRD data, an estimation can be made about the effect of the laser treatment on the electron mobility of the material. Since the measured mean grain size of 67 nm as derived by XRD analysis is about one order of magnitude larger than le, in all the samples, we conclude that changes in electron mobility of the laser-treated samples are not due to a variation of grain boundary scattering.

4. Conclusions

Areas of 3 cm2 SnO2 films, with a thickness of 980nm, on Borofloat®-glass were irradiated by 6.7 ps, λ = 343nm laser pulses by two sets of laser conditions, referred to as low and high fluence conditions respectively: (i) peak fluence F0 = 0.17 J/cm2, 1 overscan, 98.7% pulse-to-pulse overlap, track-to-track pitch of 3 μm, pulse frequency of fp = 100 kHz, and (ii) F0 = 0.26 J/cm2, 1 OS, OL = 98.7%, pitch = 3 μm, fp = 100 kHz. We observed two main different regimes as results of the laser treatment for the two laser treated samples. While at low fluence the SnO2 layer remained completely in solid state, showing no changes in the surface morphology, the sample treated at high fluence showed that the top layer of 100-200 nm of the SnO2 film was subjected to melting and resolidification. The latter was concluded on the basis of observed Laser-induced Periodic Surface Structures (LIPSSs). Careful analysis revealed a total absence of amorphous material, even in the outermost atomic layers of both laser treated samples. A tuning of the laser parameters (such as F0, fp and OL) was necessary to achieve the optimal inter-pulse heat accumulation, which slows down the resolidification process and avoids amorphization. Simultaneously, the heat accumulation was moderate, which ensured a thermally damage-free film and substrate. The latter is an essential condition to exploit ultra-short pulsed lasers for ultra-shallow thermal treatments even above the melting threshold of the material without damaging the structure of the lattice.

In the two laser treated samples, different properties were identified in the top 100 to 200 nm layer of the SnO2 films, when compared to the remaining layer of the film on glass. This difference is sharper in the sample treated at high fluence, where fast resolidification from the molten state has occurred, than in the sample treated at low fluence. This observation indicates that changes in the opto-electronic properties can be mainly attributed to the top 100 to 200 nm layer of the film and only secondarily to the rest of the layer, which remains almost unaffected by the laser annealing.

Finally, the overall performance of the SnO2 films before and after the laser treatment were compared via the figure of merit ϕ = T10 / Rsh. It was found that the ϕ of already industrially optimized SnO2 films was increased up to 59% due to laser processing.

Acknowledgments

We acknowledge financial support for this research from ADEM, A green Deal in Energy Materials of the Ministry of Economic Affairs of The Netherlands (http://www.adem-innovationlab.nl).

References and links

1.

D. Scorticati, G. R. B. E. Römer, T. Bor, W. Ogieglo, M. Klein Gunnewiek, A. Lenferink, C. Otto, J. Z. P. Skolski, F. Grob, D. F. de Lange, and A. J. Huis in t’ Veld, “Optical and electrical properties of SnO2 thin films after ultra-short pulsed laser annealing,” Proc. SPIE 8826, Laser Material Processing for Solar Energy Devices II, 88260I (2013).

2.

M. Batzill and U. Diebold, “The surface and materials science of tin oxide,” Prog. Surf. Sci. 79(2-4), 47–154 (2005). [CrossRef]

3.

R. Gordon, “Criteria for choosing transparent conductors,” MRS Bull. 25(08), 52–57 (2000). [CrossRef]

4.

S. F. Tseng, W. T. Hsiao, D. Chiang, K. C. Huang, and C. P. Chou, “Mechanical and optoelectric properties of post-annealed fluorine-doped tin oxide films by ultraviolet laser irradiation,” Appl. Surf. Sci. 257(16), 7204–7209 (2011). [CrossRef]

5.

W. Chung, M. O. Thompson, P. Wickboldt, D. Toet, and P. G. Carey, “Room temperature indium tin oxide by XeCl excimer laser annealing for flexible display,” Thin Solid Films 460(1-2), 291–294 (2004). [CrossRef]

6.

J. J. Kim, J. Y. Bak, J. H. Lee, H. S. Kim, N. W. Jang, Y. Yun, and W. J. Lee, “Characteristics of laser-annealed ZnO thin film transistors,” Thin Solid Films 518(11), 3022–3025 (2010). [CrossRef]

7.

G. Legeay, X. Castel, R. Benzerga, and J. Pinel, “Excimer laser beam/ITO interaction: from laser processing to surface reaction,” Phys. Status Solidi 5(10), 3248–3254 (2008). [CrossRef]

8.

C. W. Cheng, C. Y. Lin, W. C. Shen, Y. J. Lee, and J. S. Chen, “Patterning crystalline indium tin oxide by high repetition rate femtosecond laser-induced crystallization,” Thin Solid Films 518(23), 7138–7142 (2010). [CrossRef]

9.

M. F. Chen, K. M. Lin, and Y. S. Ho, “Effects of laser-induced recovery process on conductive property of SnO2:F thin film,” Mater. Sci. Eng. B 176(2), 127–131 (2011). [CrossRef]

10.

B. D. Ahn, W. H. Jeong, H. S. Shin, D. L. Kim, H. J. Kim, J. K. Jeong, S. H. Choi, and M. K. Han, “Effect of excimer laser annealing on the performance of amorphous indium gallium zinc oxide thin-film transistors,” Electrochem. Sol.- St. Lett. 12, H430–H432 (2009).

11.

J. Chae, L. Jang, and K. Jain, “High-resolution, resistless patterning of indium-tinoxide thin films using excimer laser projection annealing process,” Mater. Lett. 64(8), 948–950 (2010). [CrossRef]

12.

G. Haacke, “New figure of merit for transparent conductors,” J. Appl. Phys. 47(9), 4086–4089 (1976). [CrossRef]

13.

D. Scorticati, G. R. B. E. Römer, D. F. de Lange, and A. J. Huis in ’t Veld, “Ultra-short-pulsed laser-machined nanogratings of laser-induced periodic surface structures on thin molybdenum layers,” J. Nanophotonics 6(1), 063528 (2012).

14.

S. W. H. Eijt, R. Kind, S. Singh, H. Schut, W. J. Legerstee, R. W. A. Hendrikx, V. L. Svetchnikov, R. J. Westerwaal, and B. Dam, “Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films,” J. Appl. Phys. 105(4), 043514 (2009). [CrossRef]

15.

A. Van Veen, H. Schut, J. de Vries, R. A. Hakvoort, and M. R. Ijpma, “Positron beams for solids and surfaces,” AIP Conf. Proc. 218, 171–196 (1990).

16.

A. de Graaf, J. van Deelen, P. Poodt, T. van Mol, K. Spee, F. Grob, and A. Kuypers, “Development of atmospheric pressure CVD processes for high quality transparent conductive oxides,” En. Proc. 2(1), 41–48 (2010). [CrossRef]

17.

A. Borowiec and H. K. Haugen, “Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses,” Appl. Phys. Lett. 82(25), 4462–4465 (2003). [CrossRef]

18.

J. Bonse, J. M. Wrobel, J. Kruger, and W. Kautek, “Ultrashort-pulse laser ablation of indium phosphide in air,” Appl. Phys., A Mater. Sci. Process. 72(1), 89–94 (2001). [CrossRef]

19.

Y. Jee, M. F. Becker, and R. M. Walser, “Laser-induced damage on single-crystal metal surfaces,” J. Opt. Soc. Am. B 5(3), 648–659 (1988). [CrossRef]

20.

V. Consonni, G. Rey, H. Roussel, and D. Bellet, “Thickness effects on the texture development of fluorine-doped SnO2 thin films: The role of surface and strain energy,” J. Appl. Phys. 111(3), 033523 (2012). [CrossRef]

21.

B. Zhang, Y. Tian, J. X. Zhang, and W. Cai, “Structural, optical, electrical properties and FTIR studies of fluorine doped SnO2 films deposited by spray pyrolysis,” J. Mater. Sci. 46(6), 1884–1889 (2011). [CrossRef]

22.

W. M. Hlaing Oo, S. Tabatabaei, M. D. McCluskey, J. B. Varley, A. Janotti, and C. G. Van de Walle, “Hydrogen donors in SnO2 studied by infrared spectroscopy and first-principles calculations,” Phys. Rev. B 82(19), 193201 (2010). [CrossRef]

23.

J. R. Vig, “UV/ozone cleaning of surfaces,” J. Vac. Sci. Technol. A 3(3), 1027–1034 (1985). [CrossRef]

24.

R. Delhez, Th. H. de Keijser, and E. J. Mittemeijer, “Determination of crystallite size and lattice distortions through X-ray diffraction line profile analysis,” Fresenius Z. Anal. Chem. 312(1), 1–16 (1982). [CrossRef]

25.

B. E. Warren, X-Ray Diffraction (Addison Wesley, 1969).

26.

M. Leoni, J. Martinez-Garcia, and P. Scardi, “Dislocation effects in powder diffraction,” J. Appl. Cryst. 40(4), 719–724 (2007). [CrossRef]

27.

C. V. Thompson, “Structure evolution during processing of polycrystalline films,” Annu. Rev. Mater. Sci. 30(1), 159–190 (2000). [CrossRef]

28.

R. Carel, C. V. Thompson, and H. J. Frost, “Computer simulation of strain energy effects vs. surface and interface energy effects on grain growth in thin films,” Acta Mater. 44(6), 2479–2494 (1996). [CrossRef]

29.

J. G. Berryman, “Bounds and self-consistent estimates for elastic constants of random polycrystals with hexagonal, trigonal, and tetragonal symmetries,” J. Mech. Phys. Solids 53(10), 2141–2173 (2005). [CrossRef]

30.

V. Consonni, G. Feuillet, and P. Gergaud, “The flow stress in polycrystalline films: Dimensional constraints and strengthening effects,” Acta Mater. 56(20), 6087–6096 (2008). [CrossRef]

31.

J. E. Dominguez, L. Fu, and X. Q. Pan, “Effects of crystal defects on the electrical properties in epitaxial tin dioxide thin films,” Appl. Phys. Lett. 81(27), 5168–5170 (2002). [CrossRef]

32.

W. Mao, B. Xiong, Y. Liu, and C. He, “Correlation between defects and conductivity of Sb-doped tin oxide thin films,” Appl. Phys. Lett. 103(3), 031915 (2013). [CrossRef]

33.

K. Liu, M. Sakurai, and M. Aono, “Controlling Semiconducting and Insulating States of SnO2 Reversibly by Stress and Voltage,” ACS Nano 6(8), 7209–7215 (2012). [CrossRef] [PubMed]

34.

N. Laidani, R. Bartali, G. Gottardi, M. Andrele, and P. Cheyssac, “Optical absorption parameters of amorphous carbon films from Forouhi-Bloomer and Tauc-Lorentz models: a comparative study,” J. Phys. Condens. Matter 20(015216), 1–8 (2008).

35.

A. I. Martinez and D. R. Acosta, “Effect of the fluorine content on the structural and electrical properties of SnO2 and ZnO–SnO2 thin films prepared by spray pyrolysis,” Thin Solid Films 483(1-2), 107–113 (2005). [CrossRef]

36.

C. Jacoboni, Theory of Electron Transport in Semiconductors, Springer Series in Solid-State Science, Vol. 165 (Springer, 2010).

37.

V. I. Kaydanov, T. J. Coutts, and D. L. Young, Studies of Band Structure and Free-Carrier Scattering in Transparent Conducting Oxides Based on Combined Measurements of Electron Transport Phenomena,” NREL/CP-520–29064 (2000).

38.

A. Oprea, E. Moretton, N. Barsan, W. J. Becker, J. Wollenstein, and U. Weimar, “Conduction model of SnO2 thin films based on conductance and Hall effect measurements,” J. Appl. Phys. 100, 033716 (2006). [CrossRef]

39.

H. M. Ng, D. Doppalapudi, T. D. Moustakas, N. G. Weimann, and L. F. Eastman, “The role of dislocation scattering in n-type GaN films,” Appl. Phys. Lett. 73(6), 821–823 (1998). [CrossRef]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(140.7090) Lasers and laser optics : Ultrafast lasers
(350.6050) Other areas of optics : Solar energy
(310.6628) Thin films : Subwavelength structures, nanostructures
(310.7005) Thin films : Transparent conductive coatings

ToC Category:
Materials

History
Original Manuscript: January 8, 2014
Revised Manuscript: February 27, 2014
Manuscript Accepted: February 28, 2014
Published: March 12, 2014

Citation
D. Scorticati, A. Illiberi, T. Bor, S.W.H. Eijt, H. Schut, G.R.B.E. Römer, D.F. de Lange, and A.J. Huis in t Veld, "Annealing of SnO2 thin films by ultra-short laser pulses," Opt. Express 22, A607-A621 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S3-A607


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References

  1. D. Scorticati, G. R. B. E. Römer, T. Bor, W. Ogieglo, M. Klein Gunnewiek, A. Lenferink, C. Otto, J. Z. P. Skolski, F. Grob, D. F. de Lange, and A. J. Huis in t’ Veld, “Optical and electrical properties of SnO2 thin films after ultra-short pulsed laser annealing,” Proc. SPIE 8826, Laser Material Processing for Solar Energy Devices II, 88260I (2013).
  2. M. Batzill and U. Diebold, “The surface and materials science of tin oxide,” Prog. Surf. Sci.79(2-4), 47–154 (2005). [CrossRef]
  3. R. Gordon, “Criteria for choosing transparent conductors,” MRS Bull.25(08), 52–57 (2000). [CrossRef]
  4. S. F. Tseng, W. T. Hsiao, D. Chiang, K. C. Huang, and C. P. Chou, “Mechanical and optoelectric properties of post-annealed fluorine-doped tin oxide films by ultraviolet laser irradiation,” Appl. Surf. Sci.257(16), 7204–7209 (2011). [CrossRef]
  5. W. Chung, M. O. Thompson, P. Wickboldt, D. Toet, and P. G. Carey, “Room temperature indium tin oxide by XeCl excimer laser annealing for flexible display,” Thin Solid Films460(1-2), 291–294 (2004). [CrossRef]
  6. J. J. Kim, J. Y. Bak, J. H. Lee, H. S. Kim, N. W. Jang, Y. Yun, and W. J. Lee, “Characteristics of laser-annealed ZnO thin film transistors,” Thin Solid Films518(11), 3022–3025 (2010). [CrossRef]
  7. G. Legeay, X. Castel, R. Benzerga, and J. Pinel, “Excimer laser beam/ITO interaction: from laser processing to surface reaction,” Phys. Status Solidi5(10), 3248–3254 (2008). [CrossRef]
  8. C. W. Cheng, C. Y. Lin, W. C. Shen, Y. J. Lee, and J. S. Chen, “Patterning crystalline indium tin oxide by high repetition rate femtosecond laser-induced crystallization,” Thin Solid Films518(23), 7138–7142 (2010). [CrossRef]
  9. M. F. Chen, K. M. Lin, and Y. S. Ho, “Effects of laser-induced recovery process on conductive property of SnO2:F thin film,” Mater. Sci. Eng. B176(2), 127–131 (2011). [CrossRef]
  10. B. D. Ahn, W. H. Jeong, H. S. Shin, D. L. Kim, H. J. Kim, J. K. Jeong, S. H. Choi, and M. K. Han, “Effect of excimer laser annealing on the performance of amorphous indium gallium zinc oxide thin-film transistors,” Electrochem. Sol.- St. Lett.12, H430–H432 (2009).
  11. J. Chae, L. Jang, and K. Jain, “High-resolution, resistless patterning of indium-tinoxide thin films using excimer laser projection annealing process,” Mater. Lett.64(8), 948–950 (2010). [CrossRef]
  12. G. Haacke, “New figure of merit for transparent conductors,” J. Appl. Phys.47(9), 4086–4089 (1976). [CrossRef]
  13. D. Scorticati, G. R. B. E. Römer, D. F. de Lange, and A. J. Huis in ’t Veld, “Ultra-short-pulsed laser-machined nanogratings of laser-induced periodic surface structures on thin molybdenum layers,” J. Nanophotonics6(1), 063528 (2012).
  14. S. W. H. Eijt, R. Kind, S. Singh, H. Schut, W. J. Legerstee, R. W. A. Hendrikx, V. L. Svetchnikov, R. J. Westerwaal, and B. Dam, “Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films,” J. Appl. Phys.105(4), 043514 (2009). [CrossRef]
  15. A. Van Veen, H. Schut, J. de Vries, R. A. Hakvoort, and M. R. Ijpma, “Positron beams for solids and surfaces,” AIP Conf. Proc.218, 171–196 (1990).
  16. A. de Graaf, J. van Deelen, P. Poodt, T. van Mol, K. Spee, F. Grob, and A. Kuypers, “Development of atmospheric pressure CVD processes for high quality transparent conductive oxides,” En. Proc.2(1), 41–48 (2010). [CrossRef]
  17. A. Borowiec and H. K. Haugen, “Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses,” Appl. Phys. Lett.82(25), 4462–4465 (2003). [CrossRef]
  18. J. Bonse, J. M. Wrobel, J. Kruger, and W. Kautek, “Ultrashort-pulse laser ablation of indium phosphide in air,” Appl. Phys., A Mater. Sci. Process.72(1), 89–94 (2001). [CrossRef]
  19. Y. Jee, M. F. Becker, and R. M. Walser, “Laser-induced damage on single-crystal metal surfaces,” J. Opt. Soc. Am. B5(3), 648–659 (1988). [CrossRef]
  20. V. Consonni, G. Rey, H. Roussel, and D. Bellet, “Thickness effects on the texture development of fluorine-doped SnO2 thin films: The role of surface and strain energy,” J. Appl. Phys.111(3), 033523 (2012). [CrossRef]
  21. B. Zhang, Y. Tian, J. X. Zhang, and W. Cai, “Structural, optical, electrical properties and FTIR studies of fluorine doped SnO2 films deposited by spray pyrolysis,” J. Mater. Sci.46(6), 1884–1889 (2011). [CrossRef]
  22. W. M. Hlaing Oo, S. Tabatabaei, M. D. McCluskey, J. B. Varley, A. Janotti, and C. G. Van de Walle, “Hydrogen donors in SnO2 studied by infrared spectroscopy and first-principles calculations,” Phys. Rev. B82(19), 193201 (2010). [CrossRef]
  23. J. R. Vig, “UV/ozone cleaning of surfaces,” J. Vac. Sci. Technol. A3(3), 1027–1034 (1985). [CrossRef]
  24. R. Delhez, Th. H. de Keijser, and E. J. Mittemeijer, “Determination of crystallite size and lattice distortions through X-ray diffraction line profile analysis,” Fresenius Z. Anal. Chem.312(1), 1–16 (1982). [CrossRef]
  25. B. E. Warren, X-Ray Diffraction (Addison Wesley, 1969).
  26. M. Leoni, J. Martinez-Garcia, and P. Scardi, “Dislocation effects in powder diffraction,” J. Appl. Cryst.40(4), 719–724 (2007). [CrossRef]
  27. C. V. Thompson, “Structure evolution during processing of polycrystalline films,” Annu. Rev. Mater. Sci.30(1), 159–190 (2000). [CrossRef]
  28. R. Carel, C. V. Thompson, and H. J. Frost, “Computer simulation of strain energy effects vs. surface and interface energy effects on grain growth in thin films,” Acta Mater.44(6), 2479–2494 (1996). [CrossRef]
  29. J. G. Berryman, “Bounds and self-consistent estimates for elastic constants of random polycrystals with hexagonal, trigonal, and tetragonal symmetries,” J. Mech. Phys. Solids53(10), 2141–2173 (2005). [CrossRef]
  30. V. Consonni, G. Feuillet, and P. Gergaud, “The flow stress in polycrystalline films: Dimensional constraints and strengthening effects,” Acta Mater.56(20), 6087–6096 (2008). [CrossRef]
  31. J. E. Dominguez, L. Fu, and X. Q. Pan, “Effects of crystal defects on the electrical properties in epitaxial tin dioxide thin films,” Appl. Phys. Lett.81(27), 5168–5170 (2002). [CrossRef]
  32. W. Mao, B. Xiong, Y. Liu, and C. He, “Correlation between defects and conductivity of Sb-doped tin oxide thin films,” Appl. Phys. Lett.103(3), 031915 (2013). [CrossRef]
  33. K. Liu, M. Sakurai, and M. Aono, “Controlling Semiconducting and Insulating States of SnO2 Reversibly by Stress and Voltage,” ACS Nano6(8), 7209–7215 (2012). [CrossRef] [PubMed]
  34. N. Laidani, R. Bartali, G. Gottardi, M. Andrele, and P. Cheyssac, “Optical absorption parameters of amorphous carbon films from Forouhi-Bloomer and Tauc-Lorentz models: a comparative study,” J. Phys. Condens. Matter20(015216), 1–8 (2008).
  35. A. I. Martinez and D. R. Acosta, “Effect of the fluorine content on the structural and electrical properties of SnO2 and ZnO–SnO2 thin films prepared by spray pyrolysis,” Thin Solid Films483(1-2), 107–113 (2005). [CrossRef]
  36. C. Jacoboni, Theory of Electron Transport in Semiconductors, Springer Series in Solid-State Science, Vol. 165 (Springer, 2010).
  37. V. I. Kaydanov, T. J. Coutts, and D. L. Young, Studies of Band Structure and Free-Carrier Scattering in Transparent Conducting Oxides Based on Combined Measurements of Electron Transport Phenomena,” NREL/CP-520–29064 (2000).
  38. A. Oprea, E. Moretton, N. Barsan, W. J. Becker, J. Wollenstein, and U. Weimar, “Conduction model of SnO2 thin films based on conductance and Hall effect measurements,” J. Appl. Phys.100, 033716 (2006). [CrossRef]
  39. H. M. Ng, D. Doppalapudi, T. D. Moustakas, N. G. Weimann, and L. F. Eastman, “The role of dislocation scattering in n-type GaN films,” Appl. Phys. Lett.73(6), 821–823 (1998). [CrossRef]

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