Structural instability of Sn-doped In2O3 thin films during thermal annealing at low temperature

Thin Solid Films 515 (2007) 6686 – 6690 www.elsevier.com/locate/tsf

Structural instability of Sn-doped In2O3 thin films during thermal annealing at low temperature Yanfa Yan ⁎, J. Zhou, X.Z. Wu, H.R. Moutinho, M.M. Al-Jassim National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401, USA Received 29 March 2006; received in revised form 10 November 2006; accepted 23 January 2007 Available online 3 February 2007

Abstract We report on observations of structural stability of Sn-doped In2O3 (ITO) thin films during thermal annealing at low temperature. The ITO thin films were deposited by radio-frequency magnetron sputtering at room temperature. Transmission electron microscopy analysis revealed that the as-deposited ITO thin films are nanocrystalline. After thermal annealing in a He atmosphere at 250 °C for 30 min, recrystallization, coalescence, and agglomeration of grains were observed. We further found that nanovoids formed in the annealed ITO thin films. The majority of the nanovoids are distributed along the locations of the original grain boundaries. These nanovoids divide the agglomerated larger grains into small coherent domains. © 2007 Published by Elsevier B.V. Keywords: In2O3; Thermal annealing; Instability; TEM

1. Introduction Polycrystalline Sn-doped In2O3 (ITO) thin films are the most widely used transparent conductive oxides in applications of optoelectronic devices, flat-panel displays, solar cells, and gas sensors [1–5]. Its low resistivity and high visible transparency have made it very competitive in the family of transparent conductive oxides [6]. It is known that its properties of resistivity and transparency are often influenced by its structural instability under thermal treatment [7–9]. However, the study of structural instability of ITO is very limited [2,5,10,11]. The most systematic study is the recent report on the structural analysis of In2O3 thin films during thermal annealing in the temperature range from 500 °C to 1100 °C [12]. In this study, In2O3 thin films were deposited by spray pyrolysis at temperatures around 500 °C. This study found that the In2O3 thin films were stable during thermal treatment in the temperature range from 25 °C to 500 °C, but were unstable above 500 °C. The reason why the films were stable in the low temperature range may be attributed to the high growth ⁎ Corresponding author. E-mail address: [email protected] (Y. Yan). 0040-6090/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.tsf.2007.01.040

temperature. For many applications, low-temperature deposition is necessary. Therefore, it is very interesting to know if In2O3 thin films deposited at low temperature are still stable under thermal treatment at temperatures below 500 °C. In this paper, we report on observations of structural stability of ITO thin films deposited by radio-frequency (r.f.) magnetron sputtering at room temperature (RT) during thermal annealing at low temperature. Transmission electron microscopy (TEM) analysis revealed that the as-deposited ITO thin films are polycrystalline. After thermal annealing in a He atmosphere at 250 °C for 30 min, recrystallization, coalescence, and agglomeration of grains were observed by TEM and X-ray diffraction (XRD). We further found that nanovoids formed in the annealed ITO thin films. These nanovoids divide the agglomerated larger grains into small coherent domains. The mechanism of the formation of these nanovoids is also discussed. 2. Experimental details The ITO deposition was conducted in a Kurt-Lesker CMS-18 load-locked sputtering system. The deposition was at room temperature and in pure Ar with a thickness of ∼ 150 nm. The r.f.

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3. Results and discussions

Fig. 1. Transmission and absorption measured from as-deposited (circled solid lines) and post-annealed (solid lines) In2O3 thin films.

magnetron sputtering was conducted using a 7.62-cm KurtLesker Torus source fitted with a hot-pressed, fully re-oxidized target (95% In2O3 + 5% SnO2 wt.%, 99.999% purity, Cerac, Inc.). The film thicknesses are about 150 nm. Films for electrical, optical, bulk structural, and atomic force microscopy (AFM) analysis were deposited on cleaned soda–lime glass substrates. Films for TEM analysis were deposited on oxidized-Si wafers for easier preparation of TEM samples. Plan-view TEM samples were prepared by first mechanical polishing the samples to a thickness of ∼ 100 μm and then dimpled down to ∼ 5 μm. Then the samples were subsequently thinned to electron transparency using a 4 kV Ar ion beam at 13° inclination, then cleaned at a lower voltage ∼ 1.5 kV. A liquid N2 cooling stage was used to minimize milling damage. Flow rates of ultra-high-purity Ar and O2 were controlled using mass-flow controllers, and the chamber pressure was controlled with a throttle valve. Postdeposition annealing was conducted in a 7.62-cm quartz-tube furnace in an inert atmosphere, here he was used for better thermal conductivity, at 250 °C for 30 min. Reflection and transmission measurements were performed using UV–Vis spectrometry (Varian Cary 5G). Structural analysis was performed with XRD, AFM, and TEM measurements using Scintag DMS2000, Thermomicroscope Model AutoProbe LS, and FEI Tecnai F20-UT microscope, respectively. The microstructure of the thin films was examined by high-resolution TEM and Z-contrast imaging. Electron energyloss spectroscopy (EELS) was carried out using a Gatan Image Filtering system.

Fig. 2. XRD patterns obtained from as-deposited (lower line) and post-annealed (upper line) In2O3 thin films.

We first look at the effect of thermal annealing on the optical properties. Fig. 1 shows the transmission (blue) and absorption (red) spectra measured from the as-deposited at RT (circled lines) and post-annealed (solid lines) samples. It reveals that transparency (absorption) was increased (decreased) in the short wavelength region, but decreased (increased) in the nearinfrared-wavelength region. As we will see later, these changes on the optical properties can be attributed to the structural changes induced by the thermal annealing. XRD, AFM, and TEM techniques were used to observe structural changes induced by thermal annealing. Fig. 2 shows XRD patterns obtained from the as-deposited (lower line) and post-annealed (upper line) films. The lower XRD pattern does not show any clear diffraction peak, indicating poor crystallinity and very small grain sizes in the as-deposited film. The upper XRD pattern shows many strong diffraction patterns, indicating that the crystallinity and grains should have increased significantly after post-annealing. The XRD pattern also indicates that the grains are randomly oriented. Surprisingly, however, AFM analysis showed that the grain sizes and structural morphology did not change significantly after the thermal annealing. Fig. 3(a) and (b) show AFM images obtained from the as-deposited and post-annealed films,

Fig. 3. AFM images obtained from (a) as-deposited and (b) post-annealed In2O3 thin films.

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respectively. In these images, the higher intensity indicates denser material. These images reveal that the grain boundaries are lower in density than grain interiors in both the as-deposited film and the post-annealed film. The grain sizes and structural morphology are also very similar in both films. These lowmagnification Z-contrast images give similar results as the AFM analysis. However, conventional diffraction contrast imaging and diffraction analysis showed different results. Fig. 5(a) and (b) shows low-magnification bright-field (BF) TEM images obtained from the as-deposited and post-annealed samples, respectively. The insets are the corresponding electron diffraction patterns. The diffraction patterns indicate that the grain sizes are increased significantly after the thermal annealing. According to these patterns, the as-deposited film should be nanocrystalline, whereas the post-annealed film should be polycrystalline. Fig. 5(a) shows blocks surrounded and separated by white dots. These white dots form grainboundary-like morphologies. However, electron diffraction suggests that the grain sizes should be significantly smaller than these blocks, suggesting that these boundaries formed by the white dots are not real grain boundaries. We call them virtual boundaries. TEM images reveal that the virtual boundaries have significantly lower mass density compared to the blocks. The

Fig. 4. Low-magnification Z-contrast images obtained from (a) as-deposited and (b) post-annealed In2O3 thin films.

respectively. It is seen that the grain sizes and structural morphology in the as-deposited film are very similar to those in the post-annealed film, which seems to contradict the XRD results. We know that our XRD results represent the information from the entire film, whereas AFM images represent the information from the surface only. To understand the origin of this seeming contradiction, we examined the films by TEM, which allow us to correlate the grain morphology and crystal structure inside the grains. We applied two different TEM techniques to examine the planview samples. The first technique is the so-called high-angle annular dark-field imaging, also called Z-contrast imaging. The second technique is the conventional diffraction contrast imaging and diffraction. The Z-contrast images were formed by scanning a 1.4-Å probe across a specimen and recording the transmitted high-angle scattering with an annular detector. The image intensity can be described approximately as a convolution between the electron probe and an object function [13]. Thus, the Z-contrast image gives a better view of grain morphology, because it contains very little diffraction contrast. The conventional diffraction contrast imaging and diffraction are more suitable techniques to reveal structural information inside grains. Fig. 4(a) and (b) show low-magnification Z-contrast images obtained from the as-deposited and post-annealed samples,

Fig. 5. BF TEM images obtained (a) as-deposited and (b) post-annealed In2O3 thin films. The insets are the corresponding electron diffraction patterns.

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blocks surrounded by the virtual boundaries are not actual grains, and we call them virtual grains. The virtual grains are composed of many nanocrystallites. The BF TEM image shown in Fig. 5(b) clearly reveals that many virtual grains have agglomerated to form a large real grain, the dark region, after the thermal annealing. Electron diffraction clearly reveals that the large black grain is a single real grain. Thus, the virtual grains have gone through recrystallization during the thermal annealing. However, the virtual boundaries, indicated by the white arrow in Fig. 5(b), remained during the thermal annealing. Because of the lower density, these virtual boundaries are still observed as boundaries in AFM and low-magnification Zcontrast images. This is the reason why both AFM and Zcontrast images showed no significant morphological difference in the as-deposited and post-annealed samples. But in fact, recrystallization and agglomeration occurred during the thermal annealing, as observed by XRD and TEM. The remaining presence of virtual boundaries during thermal annealing is the main reason why different results were concluded from different techniques. The recrystallization and agglomeration can be seen more clearly from high-resolution TEM images. Fig. 6(a) and (b) shows high-resolution TEM images obtained from the asdeposited and post-annealed samples, respectively. Fig. 6(a)

Fig. 7. (a) High-resolution Z-contrast image obtained from the post-annealed sample. (b) Atomic structure of In2O3 projected along the [110] zone axis.

shows that the nanocrystallites are much smaller than the virtual grains seen in Fig. 5(a), confirming that the virtual grains in the as-deposited films are composed of nanocrystallites. After thermal annealing, the nanocrystallites joined together to form perfect large grains, as seen in the high-resolution TEM image [Fig. 6(b)]. However, the original virtual boundaries did not disappear, as indicated by the white arrows. Fig. 6(b) clearly reveals that these boundaries are not real boundaries, but are aligned defects, which are the black dots as indicated by the

Fig. 6. High-resolution TEM images obtained from (a) as-deposited and (b) post-annealed In2O3 thin films.

Fig. 8. O K edge spectra obtained on and off the defects. No change was observed.

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white arrows. The original blocks separated by the virtual boundaries became coherent domains in the large real grains through recrystallization during the thermal annealing. High-resolution Z-contrast imaging analysis revealed that these defects are nanovoids. Fig. 7(a) shows a high-resolution Z-contrast image obtained from the post-annealed sample. This image includes two defects, as indicated by the circles. The electron beam is parallel to the [110] zone axis of In2O3. In this image, the bright spots are In columns. O columns are not observable in this case. The Z-contrast image shows zigzag features, as indicated by the green zigzag lines. Fig. 7(b) shows the atomic structure of In2O3 projected along the [110] zone axis. The black rectangle indicates a unit cell of In2O3. The blue balls are In atoms. The red balls are O atoms. In2O3 has a cubic bixbyite structure with 80 atoms in its unit cell. The In atoms occupy two different positions, the a and b positions, as marked in Fig. 7(b). The distances between column a and column b1 or b3 are shorter than the distance between column a and column b2 or b4. Thus, the columns b4–a–b2 are well resolved in the high-resolution Z-contrast image, but the columns b1–a–b3 are not as well resolved. As a result, the Z-contrast image shows zigzag features, as indicated by the green zigzag lines. The highresolution Z-contrast image shown in Fig. 7(a) does not reveal any structural change, but only intensity decrease in the defect regions, suggesting that the defects are nanovoids. We further carried out EELS on and off the nanovoids to see if they would cause a significant change in the electronic property of the films. Fig. 8 shows O K edges obtained on and off the nanovoids. No significant difference was observed, indicating that the nanovoids do not significantly affect the electronic property of In2O3 films. We now discuss the mechanism of the formation of these nanovoids. Because In2O3 films were deposited at room temperature and in Ar atmosphere, we expect nanocrystalline films with a high concentration of O vacancies. During thermal annealing, O vacancies diffuse into grain boundaries to form nanovoids. In the as-deposited films, O vacancies are distributed in individual sites. These O vacancies create deep levels, which are responsible for the adsorption in the shortwavelength region. The segregation of the individual O vacancies removes the deep-level states induced by O vacancies, leading to enhanced transparency in the short-

wavelength region. The thermal annealing also activates more carriers so that the carrier concentration is increased. This leads to the observed increased absorption in the near-infraredwavelength region. 4. Conclusion We have observed structural instability of Sn-doped In2O3 (ITO) thin films during thermal annealing at low temperature. We found that ITO thin films deposited at room temperature were not stable during thermal annealing in a He atmosphere at 250 °C. After annealing for 30 min, we observed recrystallization, coalescence, and agglomeration of grains. Nanovoids were found to form during the annealing. The majority of the nanovoids were distributed along the locations of the original virtual grain boundaries. These nanovoids divide the agglomerated larger grains into small coherent domains. Acknowledgements This research was performed at NREL and was supported by the U.S. Department of Energy under Contract No. DE-AC3699GO10337. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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