Optical Characterization of Structural Quality in the Formation of In2O3

Sep 4, 2016 - We report on optical examination of structural properties of cubic-In2O3 (c-In2O3) degenerate-semiconducting-oxide thin-film nanostructu...
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Optical Characterization of Structural Quality in the Formation of In2O3 Thin-Film Nanostructures Ching-Hsiang Chan,‡,§ Min-Han Lin,† Liang-Chiun Chao,§ Kuei-Yi Lee,§ Li-Chia Tien,∥ and Ching-Hwa Ho*,†,§ †

Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan § Graduate of Electro-Optical Engineering and Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan ∥ Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan ‡

ABSTRACT: We report on optical examination of structural properties of cubicIn2O3 (c-In2O3) degenerate-semiconducting-oxide thin-film nanostructures grown on Si (100) substrates using vapor transport method driven with vapor−liquid− solid mechanism. Two different types of dense (A) and loose (B) c-In2O3 thin-film nanorods have been, respectively, characterized by photoluminescence (PL), timeresolved photoluminescence (TRPL), thermoreflectance (TR), and Hall and resistivity measurements. Strong excitonic emissions detected from the PL results of the A and B samples identify the high-crystalline quality of the arrow-type cIn2O3 nanorods. Theoretical modeling fits on the TRPL results of the A and B samples characterize and identify defect mechanisms of the nanostructured thin films. Near-band-edge and above-band-edge transitions such as excitons, direct band gap, and Burstein−Moss shift (above direct gap) in the degenerate A and B cIn2O3 have also been detected by TR measurement. Field-emission scanning electron microscopy and field-emission transmission electron microscopy have been implemented to facilitate the observed optical measurement results of the c-In2O3. All experimental analyses show that the TR, PL, and TRPL measurements are powerful techniques for optical characterization of structure and optoelectronics properties of the c-In2O3 degenerate nanostructured thin film available for transparent optoelectronics use.

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INTRODUCTION In2O3 is a wide-band-gap semiconductor (>3 eV), which has been widely applied in transparent conducting oxide (TCO) for acting as an optical window in display-related optoelectronics devices.1−3 The frequently used alloy content is indium−tinoxide (ITO, i.e., In2O3:Sn) with doping Sn in the deposited film to increase conductivity for the In2O3 layer. The In2O3 can usually form different crystal structures of cubic In2O3 (cIn2O3),4 hexagonal In2O3 (h-In2O3), and rhombohedral In2O3 (rh-In2O3). The c-In2O3 is generally the main stable phase of body-centered cubic (bcc) structure with lattice constant a = 10.11 Å (space group Ia3). 4 Because of its specific optoelectronic properties, the In2O3 nanomaterials have also received many other technological applications in nanowirebased field-effect transistors,5 ultraviolet (UV) sensor,6 lightemitting diodes,2 and so on. Despite the variety of technical applications of indium oxide, the nanostructural thin-film growth as well as optical inspection of the crystalline quality of c-In2O3 thin-film nanostructures still remain challenge and need more progress. In this study, two different types of c-In2O3 thin-film nanorods on Si (100) substrate (i.e., denoted as samples A and B) have been grown by vapor transport driven with vapor− © XXXX American Chemical Society

liquid−solid (VLS) mechanism. Owing to different growth conditions, sample A reveals condensed density and larger size nanorods, while sample B has loose density and possesses a smaller diameter for the nanorods. Both samples A and B consist of an initially forming continuous film (CF, thickness around hundreds of nanometers on the Si substrate); then, the nanorods are grown on the CFs. We present and emphasize herein the implementation of optical techniques of thermoreflectance (TR), photoluminescence (PL), and time-resolved photoluminescence (TRPL) measurements to identify and clarify the structure and crystalline quality of the c-In2O3 nanostructured thin films of the A and B samples. In particular, the TR spectra determine the energies of band-edge excitons and above-band-edge transitions with Burstein−Moss shift (BMS) effect for the A and B samples accurately. The carrier density of the degenerate TCO In2O3 A and B thin films can thus be estimated from the BMS energies above the conduction band from TR. The carrier density and degenerate semiconductor property of the c-In2O3 thin-film nanorod samples Received: June 26, 2016 Revised: September 1, 2016

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DOI: 10.1021/acs.jpcc.6b06452 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. FESEM image of (a) top view of sample A, (b) top view of sample B, (c) cross-section view of sample A, and (d) cross-section view of sample B for the as-grown In2O3 thin-film nanorods.

for the growth process of the indium oxide is described as 4 In(g) + 3 O2(g) → 2 In2O3(s). PL experiments have been implemented in two different types of spectrometers that own different resolution. The lowresolution PL experiments have been carried out by a QE65000 spectrometer (focal length = 101 mm), and CCD array detections are employed in the PL measurements. Highresolution PL measurements were carried out in a spectral measurement system where a TRIAX 550 (f = 550 mm) imaging spectrometer equipped with a 600 grooves/mm, 1200 grooves/mm, or 2400 grooves/mm grating acted as the optical dispersion unit. The pumping light source is a Q-switched diode-pumped solid-state laser (λ = 266 nm). For temperaturedependent PL measurements, a closed-cycle cryogenic refrigerator equipped with a digital thermometer controller was used for low-temperature measurements with a temperature stability of 2 K or better. The low-resolution PL spectrometer detects a full-range spectrum from 1.25 to 4 eV, while the high-resolution PL spectrometer focuses on measuring the band-edge emissions of the c-In2O3. Timeresolved photoluminescence (TRPL) has been carried out using time-correlated single photon counting (TCSPC) technique,8 which was equipped with a single photon-sensitive PicoQuant photon photomultiplier assembly (PMA 182) module as the photodetector. The excitation source for the TRPL measurement of the c-In2O3 was achieved by a Picoquant LDH-D-C-375 pulsed diode laser (at λ = 375 nm) with a pulse duration of ∼200 ps. The TR experiments of the semiconducting oxide nanostructures were implemented using indirect heating manner with a gold-evaporated quartz plate as the heating element.9,10 The heated pulses are derived from a function generator and the heating current is driven by a constant current source. The

have been verified and checked by Hall and resistivity measurements. In combination with the TR, PL, and TRPL results, the crystalline quality, defect origin, and excitonic emissions of the c-In2O3 thin-film nanorods are thus realized. The arrow-type dense and loose nanorods in the A and B samples essentially show high-crystalline quality to emit strong excitonic emission, whereas the CF part of the c-In2O3 (initially deposited on the Si substrate) should be a degenerate semiconductor with high defect density and carrier concentration of ∼1019 cm−3 to make a BMS effect detected by TR. All experimental analyses demonstrate that TR, PL, and TRPL are effective optical tools for the inspection of high-quality TCO thin-film growth.

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EXPERIMENTAL SECTION The In2O3 thin-film nanostructures were grown by vapor transport driven with VLS mechanism.7 A horizontal tube furnace was utilized for the growth. Molten indium (0.5 g, 99.9999%) was the source material and put into a boat placed at the center of the quartz tube. A thin layer of gold was predeposited on silicon (100) substrates by sputtering. The Au deposited time of sample A is 10 s, and that of sample B is ∼2 s. The Au film on Si will become many nucleation points for catalytic growing the In2O3 nanowires (nanostructures). At first, the Si substrates were placed in a crucible boat and positioned downstream. The quartz tube was then evacuated and the furnace was heated to 900 °C with a rate of 10 °C/min. Highly pure oxygen carried by argon gas was fed into the quartz tube. The growth temperatures for samples A and B were, respectively, kept at 650 and 520 °C with a background pressure of 0.5 Torr. The typical growth time for the samples was 2 h. After the growth, samples were then cooled under the same ambient gas to room temperature. The chemical reaction B

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Figure 2. TEM images for single c-In2O3 nanorod of sample A in (a) and sample B in (b) along [100] direction. The insets show the corresponding SAED pattern in the nanorod’s center. (c,d) HRTEM images of c-In2O3 in samples A and B, respectively. The interplanar spacings of (011)̅ and (200) were, respectively, determined to be 0.72 and 0.51 nm.

the nanostructured In2O3 thin films. For sample A in the VLS growth, the catalytic Au film that assists in the nuclei-point’s nanorod growth is thicker than that of the sample B, leading to the formation of high-density nanocrystals on the film. From sample A (see Figure 1a,c), high density and highly compact In2O3 nanorods with larger diameters have been successfully grown on Si(100) substrate with respect to those of the other sample B (see Figure 1b,d) with smaller size and loose density. Figure 1 also reveals that the In2O3 nanostructured thin films consist of a compact continuous film (∼100 nm), which follows different sizes of In2O3 nanorods deposited on the Si (100) substrate. Sample B owns tiny-size nanorods that dispersed on a continuous In2O3 film. Because the nanostructured thin films were grown by VLS-type deposition with catalytic nucleation growth, the formation of the thin films may consist of the CF part and the nanorod part. It is slightly different from the other deposition method (e.g., Sol−gel growth) containing mainly the CF part of the thin films.12,13 The temperature of the Au nucleation point can be much higher in the environment in VLS growth14 to facilitate the formation of the nanorods (nonowires) on the CFs. As shown in the FESEM images (Figure 1a,b), many of the nanorods demonstrate a tapered tip for each of the individual rods. Four apparent {111} faces (triangular shape) usually consist of the wire tip due to different surface energies affecting the growth rate to the crystal planes with {111} < {100} < {110}.15 The formation of the {111} face verified the cubic crystalline phase of the In2O3 nanorods (i.e., c-In2O3).

samples were closely attached on the heating element by silicone grease. The on−off heating disturbance uniformly modulates the nanostructured thin film periodically. An 150 W xenon-arc lamp filtered by a PTI 0.2 m monochromator provided the monochromatic light. The incident light was focusing onto the sample with a spot size of ∼100 μm2. The reflected and scattering lights from the thin-film nanorods were collected and detected by a Hamamatsu H3177-51 PMT module. The signal was detected and recorded via EG&G model 7265 lock-in amplifier and personal computer. Measurements of resistivity and Hall effect were achieved by a Keithley model 6220 as the current source, model 2182 as the voltmeter. A regulated dc power supply is equipped for providing the current of the electromagnet (max: 10 kGauss). An RMC model 22 closed-cycle cryogenic refrigerator equipped with a 4075 digital thermometer controller together with a turbo pump was used for low-temperature experiments. The electrical resistivity and Hall effect measurements were carried out by using the van der Pauw technique.11 The ohmic contacts on the samples were made by silver paste.

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RESULTS AND DISCUSSION Figure 1a,b shows the top views of field-emission scanningelectron-microscope (FESEM) images of the two as-grown In2O3 nanostructured thin films for samples A and B, respectively. Figure 1c,d also displays the cross-section views for the corresponding thin-film nanorods in Figure 1a,b. It is known that different growth temperatures and catalytic conditions will lead to different crystal size and crystallinity of C

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In2O3 single crystal existing in sample A. It is inferred that the NBE emissions including FX, BECs, and DAP are coming from the highly crystalline part of the tapered nanorods (see Figure 2), whereas the defect emissions may largely come from the initially deposited CFs part of the c-In2O3, as seen in the FESEM images in Figure 1. The CF part does not contain welldefined crystal shape in the cubic In2O3 such as the {111} and {100} planes of the nanorods in the c-In2O3. The CF part may also contribute more surface carrier density from defects (like oxygen vacancy) to render a degenerate semiconducting behavior in the thin films. We would evaluate defect mechanism and degenerate semiconductor property of the c-In2O3 later. To probe luminescence defect lifetime of the c-In2O3 thinfilm nanorods, TRPL experiments of the samples A and B have been implemented. Figure 4a shows the room-temperature TRPL decay curves measured at ∼1.98 eV (VO → VIn; see Figure 3) for the two different c-In2O3 thin-film nanorods samples. The TRPL measurement data of the c-In2O3 can be analyzed by using an exponential decay function with the Levenberg−Marquardt method expressed as17,18

Figure 2a−d displays transmission-electron-microscope (TEM) images from the single c-In2O3 nanorod with a tapered tip on the wire end. The growth direction of the onedimensional (1D) c-In2O3 nanorod was along . The diameters of the nanorods in Figure 2a,b are about 270 and 102 nm, respectively, derived from samples A and B. The tapered tips and tower edges are usually easily forming structures for the c-In2O3 nanorods.16 High-resolution TEM (HRTEM) images in Figure 2c,d were magnified from a center box (c) and (d), respectively. The interplanar spacings of d(011̅) and d(200) were determined to be 0.72 and 0.51 nm, which matched well with the cubic structure of In2O3. The selected-area electron diffraction (SAED) pattern in Figure 2a,b (inset) also sustains the cubic crystalline phase of the c-In2O3 tapered nanorods. The dotted-like SAED pattern also identifies the high-crystalline quality of the nanorods. Near-band-edge (NBE) emission of PL is an index of crystalline quality of semiconductor materials. For characterization of below-band-edge transitions in c-In2O3, we carried out low-resolution PL measurements in a wide energy range from 1.25 to 4 eV at 300 K. Figure 3 shows the room-

y(t ) = y0 + A1 exp[−(t − t0)/τ1] + A 2 exp[− (t − t0)/τ2] (1)

where τ1 and τ2 are the defect lifetime from different defect mechanisms, and A1 and A2 are the related amplitudes. The fitting analysis using eq 1 in Figure 4a reveals that the sample B has only one slope of τ1 ≈ 580 ns, while the sample A has two lifetimes (two slopes) of τ1 ≈ 580 ns and τ2 ≈ 95 ns, respectively. The long lifetime (i.e., τ1) of PL recombination is maybe coming from the CF part of the c-In2O3 thin film (see upper inset of Figures 4a and 1b), while the shorter lifetime term (i.e., τ2) may be caused by the arrow-like nanostructures (nanorods) on the film, as shown in the lower inset of Figure 4a. The shorter PL lifetime for the arrow-like nanorods indicates the lower defect density and better crystallinity in the nanorods with respect to that of the other part of initially deposited continuous film CF of the c-In2O3. That is why the sample B in Figure 4a (i.e., loose density of nanorods) can get only one lifetime τ1 (one slope) from the CF part but the sample A (i.e., dense nanorods) obtains both τ1 and τ2 from the CF and nanorod parts of the c-In2O3. The shorter τ2 in nanorod than that of τ1 in the CF part indicates that lower gap levels (defects) existed in the nanorod to avoid multiple carrier bounce between the gap levels for extending emission lifetime of the final carrier recombination. Figure 4b depicts the dependence of lifetime (τ1) measured at different PL emission energies (centered at 1.98 eV) from 1.65 to 2.75 eV for samples A and B. The obtained τ2 (∼95 ns, shorter lifetime) measured at 1.98 eV from the nanorods of sample A is also indicated with green cycle (with error bar) in Figure 4b for comparison. With the PL emission energy increases from 1.65 to 2.75 eV, the lifetime τ1 (i.e., longer lifetime slope for the CF part) of the cIn2O3 slightly decreases from ∼750 to ∼550 ns. The TRPL results in Figure 4 sustain the inference of better crystallinity for the nanorods but poor crystalline quality for the CF part of the c-In2O3 thin films. To probe clearly the near-band-edge property of c-In2O3 thin-film nanowires, TR measurement was implemented for the samples A and B nanostructured thin films. The TR experiments can keep uniformity of periodically thermal modulation to the CF part and to each of the nanorods for resulting in easily resolved derivative line-shape spectrum of cIn2O3.19 As shown in Figure 5a,b are the TR spectra at 300 and

Figure 3. Low-resolution PL spectra of the A and B c-In2O3 thin-film nanorods samples at 300 K. The inset shows the high-resolution PL spectra of near-band-edge emission features in the A and B samples at 20 K. The corresponding near-band-edge emissions of FX, BECs, and DAP are shown in the inset for comparison.

temperature PL spectra for the two kinds of c-In2O3 samples (A and B). In addition to the NBE emission features around 3.23 eV, one defect-related emission was observed in the PL spectra near 1.98 eV. It may originate from the defect emission of oxygen vacancy (VO) to indium vacancy (VIn) recombination.16 As shown in Figure 3, the NBE emission of sample A is much stronger than that of sample B, indicating that the average crystalline quality of sample A is higher. The inset in Figure 3 shows the NBE emission features at 20 K. They are also similar to that of 300 K with stronger NBE strength in A but weaker NBE intensity in B. The emission features include free exciton (FX = 3.377 eV), bound-excitons complexes (BECs, A = 3.366 eV, B = 3.358 eV, and C = 3.352 eV), and donor−acceptor pair (DAP = 3.314 eV) emissions. The intensity of near-band-edge emission of sample A is much higher than defect-related emission in Figure 3, while the defect-related emission is more dominant in the PL spectrum of sample B. This phenomenon indicates that there is a significant proportion of high-quality cD

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Figure 4. (a) Comparison of TRPL analysis of the defect-related emission at ∼1.98 eV for the A and B c-In2O3 thin-film nanorods samples. (b) Energy dependence of PL recombination lifetime derived from the TRPL measurements of the A and B samples at 300 K. The fastest lifetime of ∼95 ns (τ2) at 1.98 eV is inferred to derive from the dense tapered nanorods (as the lower insets in panels a and b), while the slower lifetime larger than 500 ns (τ1) may come from the CF part as shown in the upper inset in panel a.

film with degenerate property and high carrier density.25 The high electron density may be caused by surface electron accumulation by oxygen vacancies20,21 of the CF part. On the contrary, the higher crystal quality of the nanorod part can also be evident by a lower transconductance (i.e., lower channel carrier density) of the In2O3 nanowires FET (11.6 nA-V−1)22 as well as a complete series of band-edge excitons detected in the In2O3 nanowires.23 All previously experimental results sustain that the nanorod part has higher crystalline quality than that of the CF portion. As shown in Figure 5a, the transition feature E1 (∼3.298 eV) is detected at 300 K for sample A, whereas the bound exciton BX, exciton excited state E2, and acceptor-toFermi level EA‑CF feature are weaker and not clearly detected in Figure 5a. It is owing to thermal alloy-scattering effect in In2O3.16 The EV‑CF feature is the direct band gap by taking into account the BMS effect in c-In2O3. The energy of EV‑CF ≈ 3.434 eV in Figure 5a agrees well with our previous absorption results (not showing here) by taking into account the opticalabsorption coefficient (α) using the relation of (αhν)2 versus hν24 for obtaining direct band gap of In2O3 with BMS. Figure 6 depicts the possible band-edge scheme of the cIn2O3 owning the nanorod portion and the CF part by taking

Figure 5. Experimental TR and PL spectra of the A and B samples for the c-In2O3 nanostructured thin films at (a) 300 and (b) 20 K, respectively. At 300 K, only the sample A (dominated by mainly highquality nanorods) can detect free-exciton transition feature (i.e., E1 in TR and FX in PL). Whereas the lower crystalline quality of the CF part in sample B shows only the EV‑CF transition feature with BMS effect. The PL emission of FX (BX) in panels a and b may come from the highly crystalline part of the nanorods.

20 K, respectively. The low-resolution PL spectra of excitonic emission (FX and BX) at 300 and 20 K are also included for comparison. As seen in Figure 5b, in comparison with the lowtemperature TR spectra of the two samples, the excitonic transitions E1 (i.e., FX in PL) and E2 may correlate with the highly crystalline part of the nanorods, while the transitions EA‑CF and EV‑CF are inferred to come from the poor-quality CF part of the continuous In2O3 films owing to a surface electron accumulation layer and high density oxygen vacancies (VO) that exist in the In2O3. The nanorods in sample B are loose and slim (see Figure 1) so that the E1 transition (occurred for the nanorods) is much weaker than that of sample A displayed in Figure 5b. Furthermore, the TR transition feature EV‑CF in sample B is enhanced significantly in both Figure 5a,b. It lends evidence that the EV‑CF may largely come from the CF part of the In2O3. The CF part of the c-In2O3 may be a degenerate semiconductor with an interband transition higher than band gap (i.e., EV‑CF > Eg) to show the Burstein−Moss shift (BMS) effect. This situation is similar to an as-grown continuous In2O3

Figure 6. Possible band-edge scheme for the c-In2O3 thin-film nanorods by taking into account band-edge excitons, defects, and Burstein−Moss shift. The excitonic transitions of FX, E1, and E2 may come from the high-quality part of the nanorods, while the EV‑CF and EA‑CF transitions are caused by the BMS effect in the CF part. The optical techniques used for detecting the transition features are also indicated. E

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temperature suggest that at least two competition mechanisms are coexisted in the TCO films. The negative TCR below ∼100 K indicates localization and ionization of the electrons,29,30 and positive TCR above transition temperature shows delocalization of the ionized carriers. This delocalization of electrons may lead to metallic-like behavior of resistivity, which can be the general characteristic of a degenerate semiconductor.29

into account band-edge excitons, defects, and BMS effect. It is clear that the FX, E1, and E2 transitions are mainly detected by the nanorods’ contribution and EV‑CF and EA‑CF are the transitions that largely come from the continuous film CF. The EA‑CF is transited from acceptor states to the conductionband Fermi level with BMS. In In2O3, the acceptors may be indium vacancies (VIn) or oxygen vacancy and indium vacancy complexes (VO−VIn), and donors are mainly caused by oxygen vacancy (VO). The optical techniques used for detecting the transition features are included in Figure 6 for comparison. To verify the degenerate semiconductor property (BMS effect), temperature-dependent resistivity and Hall measurements of the c-In2O3 thin-film nanorods from 30 to 300 K were also carried out. Figure 7 reveals the temperature dependence

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CONCLUSIONS Optical determination of structure, light-emission, electrical, and optoelectronics properties of c-In2O3 thin-film nanostructures has been demonstrated herein. Two different kinds of In2O3 thin-film nanorods on Si (100) have been grown by vapor transport driven with VLS mechanism. The FESEM images indicate that different growth temperature and catalytic condition lead to different size and crystallinity of the nanostructured In2O3 thin films. Sample A is formed by compact and larger-diameter nanorods grown on CF, and sample B consists of loose and tiny nanorod distribution on the CF part of the In2O3. From the optical characterization, we clearly observe the band-edge excitonic transitions by TR in the part of In2O3 nanorods, which has better crystallinity. The BMS effect of a degenerate semiconductor TCO above band edge has also been detected by TR measurement for the CF part of the c-In2O3 thin films. The carrier densities of the A and B samples obtained by the BMS energies of the TR spectra and Hall measurement are about 1.02 × 1019 and 3.35 × 1019 cm−3 at 300 K. The defect recombination lifetime of the samples A and B was characterized using TRPL experiments around a defect emission of ∼1.98 eV. The result shows that sample A possesses highly crystalline quality of nanorods and reveals shorter lifetime and lower defect density, whereas the B sample consists of mainly the CF part and displays longer recombination lifetime and high defect density. All experimental studies (e.g., TR, PL, and TRPL) strongly support the observation of the Burstein−Moss shift effect and excitonic transitions in the degenerate semiconductor TCO systems. A good agreement between the analyses of physical, optical, and electrical experiments sustains the effective optical methods that were applied for TCO characterizations.

Figure 7. Temperature-dependent carrier density of samples A and B measured by Hall measurement. The temperature-dependent resistivity values of the two samples are shown in the lower inset for comparison.

of the carrier density of sample A and sample B thin-film nanorods. At first we focus on studying sample B because there is a simply continuous In2O3 film for the planar carrier transport. The average carrier density of c-In2O3 CF in sample B is ∼3.35 × 1019 cm−3. This value is approximately comparable to the experimental value for the c-In2O3 thin film obtained by BMS energy calculation in TR experiment.16 Resistivity measurement of the c-In2O3 thin films was also implemented. The resistivity value of sample B is ∼5.36 × 10−2 Ω-cm at 300 K, and the Hall mobility of sample B is hence determined to be ∼4.8 cm2/V-s. The values of resistivity, electron concentration, and Hall mobility are in reasonable agreement with previous results obtained by undoped-degenerate c-In2O3 thin-film nanostructures.25,26 Moreover, for sample A, the c-In2O3 thinfilm nanostructures consist of dense and wide nanorods together with a CF part. The average carrier density and resistivity of sample A are around 1.02 × 1019 cm−3 and 5.61 × 10−2 Ω-cm at 300 K. The average crystallinity of sample A is better than that of sample B because of the formation of dense and wide nanorods with high crystal quality on the In2O3 film. Temperature-dependent electrical resistivity of c-In2O3 of samples A and B is shown in the inset of Figure 7. A semiconductor-to-metallic state transition may occur at ∼100 K for the two thin-film nanorods samples. The metal−semiconductor transition temperature is correlated with the degree of disorder, and this phenomenon usually displays the degenerate semiconductor property of the TCO films.27−29 The negative temperature coefficient of resistivity (TCR) below transition temperature and positive TCR above transition

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +886 2 27303772. Fax: +886 2 27303733. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology, Taiwan under grant no. MOST 104-2112-M011-002-MY3.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b06452 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b06452 J. Phys. Chem. C XXXX, XXX, XXX−XXX