Enhanced Performance in Al-Doped ZnO Based Transparent Flexible

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Enhanced Performance in Al-Doped ZnO Based Transparent Flexible Transparent Thin-Film Transistors Due to Oxygen Vacancy in ZnO Film with Zn−Al−O Interfaces Fabricated by Atomic Layer Deposition Yang Li,† Rui Yao,† Huanhuan Wang,† Xiaoming Wu,† Jinzhu Wu,† Xiaohong Wu,*,† and Wei Qin*,‡ †

School of Chemistry and Chemical Engineering and ‡School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China S Supporting Information *

ABSTRACT: Highly conductive and optical transparent Aldoped ZnO (AZO) thin film composed of ZnO with a Zn− Al−O interface was fabricated by thermal atomic layer deposition (ALD) method. The as-prepared AZO thin film exhibits excellent electrical and optical properties with high stability and compatibility with temperature-sensitive flexible photoelectronic devices; film resistivity is as low as 5.7 × 10−4 Ω·cm, the carrier concentration is high up to 2.2 × 1021 cm−3. optical transparency is greater than 80% in a visible range, and the growth temperature is below 150 °C on the PEN substrate. Compared with the conventional AZO film containing by a ZnO−Al2O3 interface, we propose that the underlying mechanism of the enhanced electrical conductivity for the current AZO thin film is attributed to the oxygen vacancies deficiency derived from the free competitive growth mode of Zn−O and Al−O bonds in the Zn−Al−O interface. The flexible transparent transistor based on this AZO electrode exhibits a favorable threshold voltage and Ion/Ioff ratio, showing promising for use in high-resolution, fully transparent, and flexible display applications. KEYWORDS: atomic layer deposition, AZO, oxygen vacancy, flexible, TFT

1. INTRODUCTION The ever-increasing demand for high-performance transparent conducting oxide (TCO) has been fuelled by the diverse optoelectronic applications, such as flat-panel displays, smart windows, organic light-emitting diodes (OLED), transparent thin-film transistors (TFTs), and solid-state lighting industries.1−4 To achieve the practical and wide use in industrial device, TCO should satisfy the competing demands of ideal combination of high electrical conductivity, large optical transparency in the visible spectral range, and enduring environmental stability.5,6 Although indium tin oxide (ITO) exhibits satisfactory electrical and optical properties for transparent conducting applications, the seeking for the costeffective alternative to the conventional ITO was still continuously developed due to the toxicity and increasing price of indium in ITO.7 Intrinsic and aluminum (Al)-doped zinc oxide (AZO) thin films, as an alternative to ITO, have an increasing number of prominent applications in electronic and photonic devices because of their excellent properties and abundance, cheapness, and lack of toxicity.8,9 To strike the best balance of large optical transparency and high electrical conductivity, the AZO thin film has been required to minimize the optical absorption and reflection while © 2017 American Chemical Society

preserving a high carrier concentration and restraining carrier scattering.10 To date, the industrial level of AZO films with a stable resistivity of order 10−4 Ω·cm and transmittance >80% can be deposited by sputtering techniques in a micrometer scale.11 However, sputtering deposition was not sufficient for precise thickness control and 3D structure coating.12 Therefore, it is urgent to seek an outstanding synthetic strategy for the transparent oxide conductive AZO film with higher electrical performance, lower growth temperature (under 150 °C), thinner thickness (less than 100 nm), and better compatibility with various substrates such as the glass and the flexible plastics. Atomic layer deposition (ALD) technology has the advantages of pinhole free, low-temperature growth, highly conformality, and accurate controllability of thickness and composition due to the self-limiting and self-saturated surface chemical reaction, which makes ALD suitable for advanced nanodevice applications.13,14 The conventional design strategy for ALD of AZO transparent conductors is to insert Al2O3 dopant layer in the Received: February 22, 2017 Accepted: March 17, 2017 Published: March 17, 2017 11711

DOI: 10.1021/acsami.7b02609 ACS Appl. Mater. Interfaces 2017, 9, 11711−11720

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ACS Applied Materials & Interfaces Scheme 1. Schematic Description of Growth Process of ZnO Thin Film with Zn−Al−O Interface by ALD

2. EXPERIMENTAL SECTION

adjusted to obtain the films with a thickness of approximately 90 nm under base pressure at 0.15 Torr and the growth temperatures in a range of 125−200 °C. The conventional AZO film, the ZnO film with an Al2O3-doping layer, were also prepared by using ALD technique with the identical process parameters of the pressure (0.15 Torr) and the growth temperatures (150 °C). The layered ratio of Al2O3 to ZnO was set as 1:24, and the thickness of the AZO film was adjusted around 100 nm. To determine the rate of growth under varied temperature conditions, an ellipsometer (J. A. Woollam Co., Inc. Alpha-SE) was employed to measure the thickness of AZO films. The crystallinity and composition of the AZO films were investigated by X-ray diffraction (XRD, Rigaku D-Max diffraction) and XPS (TSC K-Alpha, AlK). Each XPS core level spectrum was fitted with a Shirley-type background and deconvoluted into various components. The morphology, energy dispersive X-ray (EDX) analysis, and Zn, Al, O elements mapping images obtained with a transmission electron microscope (TEM, FEI, Tecnai G2S-Twin) and scanning electron microscope (SEM, E-SEM, Quanta 250, FEG). The topography and morphological evaluation of the films were performed with an atomic force microscope (AFM, Asylum Research, MFP-3D-SA). The optical transparency and energy gap were recorded with UV−vis spectrum (Optizen-3220 UV) in the wavelength range of 200−1100 nm. To obtain the oxygen vacancy defects information, PL spectra were inspected using an Accent RPM2000 PL mapping system using 325 nm emission from a He−Cd laser and detected through a 345 nm high-pass filter. Raman spectra were obtained by using a Renishaw inVia Qontor Raman scattering microscope with a laser wavelength of 532 nm. The TEM−EELS system was equipped with probe-corrected high-resolution transmission electron microscope (HRTEM, FEI, Philips Tecnai F20). All of the EELS spectra were calibrated using the zero loss peak position. The resistivity and the carrier mobility of AZO films were measured using van der Pauw geometry with Hall effect testing instrument (SWIN HALL8000, Taiwan) with a magnetic field 0.68 T at room temperature. The thin-film transistor characteristics, including the output and transfer curves of the ALD fabricated device, were evaluated using a semiconductor parameter analyzer (Keithley 4200SCS) in a dark box at room temperature.

ZnO films with Zn−Al−O interface are atomic layer deposited with diethylzinc (DEZ), trimethylaluminum (TMA), and deionized water vapor as zinc, aluminum, and oxygen precursors, respectively. The films were grown in TALD-150 ALD reactor (Kemicro Jiaxing) on glass substrates to evaluate the optical and electrical performance, and the surface morphologies and crystallographic structure of AZO films on silicon substrates were investigated. M cycles of DEZ/H2O followed by one cycle of DEZ/TMA/H2O were repeated for the Zn− Al−O interface incorporation into the ZnO film, where m varied between 9, 19, 24, 29, 34 and 39, and the total super cycles was

3. RESULTS AND DISCUSSION 3.1. Fabrication and Characterization of ZnO Film with a Zn−Al−O Interface. Scheme 1 shows the schematic diagram of the layer structure of ZnO thin film with Zn−Al−O interface deposited by ALD method. To optimize the conductivity, Al content in the AZO film was varied by introducing one DEZ/TMA/H2O cycle in between different pulses of DEZ/H2O. The various Al concentrations (atmos-

growth of i-ZnO matrix to balance the trade-offs between the free carrier concentration (n), the effective carrier mass (m*), and the carrier scattering time (τ) to increase the electrical conductivity σ = e2τ(n/m*).15 According to literature, the resistivity of ZnO/Al2O3 films that meets the requirements of low temperature (under 150 °C) and thin thickness (under 100 nm) was around 10−3 Ω·cm, which was attributed to the electron scattering caused by the amorphous Al2O3 interface and the Al−O−Al clusters and subsequently lead to the degradation of the electrical resistivity.16−18 As an alternative doping concept, the Al-doping introduced by Zn−Al−O pattern rather than Al2O3 interface layer was put forward to improve the conductivity; however, a profound understanding of the origin of the enhanced integrated performance was still challenging.19−21 Herein, we present the structural design, chemical characterization, and electrical properties of a series of ZnO thin films with Zn−Al−O interfaces. We demonstrate their efficacious properties as a transparent electronic conductor for the flexible transistor device. Detailed comparison of the oxygen vacancy defects between ZnO with Zn−Al−O and Al2O3 interfaces based on electron energy-loss spectroscopy (EELS) and X-ray photoelectron spectra (XPS), Raman, and photoluminescence (PL) spectra provides evidence that Zn−Al−O interface can greatly increase the density of oxygen vacancy and enhance electrical properties in AZO thin films. This mechanistic information provides insight into strategies for producing highperformance TCOs for next-generation optoelectronic applications. Beyond TCO applications, this work provides guidance for tailoring the electrical properties of doped semiconductors in general, which can be used for the efficient study of a broad range of thin films prepared via ALD techniques.

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ACS Applied Materials & Interfaces pheric percent, at. %) of films refer to AZO 1:9, 1:19, 1:24, 1:29, 1:34, and 1:39 and with a thickness of approximately 90 nm according to the requirement of the TCO thin films in the practical application. To clarify the distinction between the ZnO thin film with Zn−Al−O interface prepared under the current doping concept and the conventional AZO film, the ZnO film with Al2O3 doping layer were also prepared in this work. The layered ratio of Al2O3 to ZnO was chosen as 1:24 due to the lowest resistivity of the conventional AZO films was obtained for grown with Zn/Al ≈ 20.22 The thickness of the AZO film was adjusted as same as the ZnO/Zn−Al−O (∼100 nm). Figure 1a shows the ALD growth rate per cycle (GPC) as a function of temperature (100−200 °C). The ALD process

Figure 2. Comparison of (a) glancing angle X-ray diffraction patterns and (b) (100) peak position of various ZnO films with Zn−Al−O interface. The insets of panel b show the AFM images of the AZO films.

exhibits almost equivalent intensity for (100) and (002) planes. However, as the increasing of Al doping concentration, the intensity of the (002) plane is gradual decreased, while the (100) plane became a preferred growth plane owing to the suppression effect of the Al precursor on the (002) plane of ZnO.25 Additionally, the (100) peak position was slightly shifted to the higher diffraction angle with increasing Al concentration as shown in Figure 2b, which is due to the introduction of the Al dopants (∼0.54 Å) into the Zn2+ sites (∼0.74 Å). The interplanar spacing of the (100) plane of the AZO films was presented in Figure S1 according to the Bragg equation and Scherrer’s equation. The comparing XRD spectra between the ZnO films with Zn−Al−O and ZnO−Al2O3 interfaces are shown in Figure S2a. It is found that good crystallinity was obtained for both AZO films; however, compared with the conventional AZO film containing by ZnO and Al2O3 interface, the ZnO films with Zn−Al−O interfaces showed the preferred (100) growth plane, the higher XRD (100) area under the peak (Figure S2b), the lowest surface roughness, and the smaller grain size as listed in Table S1. The above features of the ZnO films with Zn−Al−O interfaces all revealed the lower resistivity of AZO films.20,22 The surface topographic images of the AZO films as a function of the Al-to-Zn ratio were taken by using a AFM with a 2 μm scan size, as shown in the insets of Figure 2b. The rootmean-square surface roughness was 4.5 nm for the pure ZnO; with increasing Al doping, it was first decreased until 1.88 nm for an AZO 1:24 film and was then slightly increased to 3.25 nm for an AZO 1:9 film. The decreased roughness with Al doping is due to the fact that substitution of Zn2+ ions by Al3+ ions alters ZnO crystal growth, and the slight increase in the roughness at a 1:9 Al-to-Zn ratio can be attributed to the etching effect of TMA precursor on the ZnO surface, which is consistent with the report by Elam et al.26 To lucubrate the surface morphologies, crystallinity behaviors, and elemental distribution of the AZO films, the representative AZO 1:24 film was detected using SEM, TEM,

Figure 1. Dependence of the growth rates of the AZO films deposited by ALD on (a) temperature and (b) Al-to-Zn ratio.

window was found at 150−175 °C, and the GPC remained almost constant at 1.7 Å/cycle, which indicates self-limiting film growth of AZO films in this temperature range. Either a toolow or a too-high temperature can break the self-limited and self-saturated reaction; therefore, the optimal growth temperature of 150 °C was chosen for subsequent growth according to the best performance of AZO films. As presented in Figure 1b, the thicknesses of AZO films were measured with an ellipsometer to evaluate the influences of Al doping concentration on the film growth rate. The growth rate of AZO was slightly lower than that of the intrinsic ZnO, which is attributed to two reasons: the growth rate of pure Al2O3 was lower than that of the pure ZnO, and the etching of surface Zn atoms via ligand exchange during the Al−O cycles also leads to the lower growth rate of the AZO films.23,24 The glancing-angle X-ray diffraction (GAXRD) spectra of the intrinsic ZnO and different AZO films are shown in Figure 2a. All XRD profiles with sharp diffraction peaks corresponding to hexagonal wurtzite crystal structure (JCPDS, no. 36-1451) clearly demonstrate that the ZnO and AZO films deposited by ALD method possess good crystallinity. The intrinsic ZnO film 11713


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Figure 3. (a) Bright-field and (b) high-resolution TEM images; (c) the selected area diffraction pattern (SADP) (inset shows a schematic diagram of AZO thin films) and (d) EDS spectra with corresponding elemental mapping images of (e) Zn, (f) O, and (g) Al of the deposited AZO 1:24 film.

Figure 4. (a) X-ray photoelectron spectra survey scans of representative AZO 1:24 film and high-resolution spectra of (b) Zn 2p, (c) Al 2p, and (d) O 1s for AZO films with various Al-to-Zn ratio, respectively.

the (100), (002), (101) and other planes of the AZO film, which are in good agreement with the XRD results in Figure 2a. The inset in Figure 3c shows a schematic diagram of the molecular structure of the ZnO thin film with Zn−Al−O interface. The TEM images reveal that no obvious amorphous Al2O3 phase presents in the as-prepared AZO film in

and EDX elemental mapping analysis, as shown in Figure 3. The structural features of the polycrystalline phase of an ALDprepared AZO 1:24 film can be clearly observed in a bright-field TEM image in Figure 3a. The lattice fringes presented in HRTEM image (Figure 3b) and the diffraction rings in selected area diffraction pattern (SADP) (Figure 3c) are associated with 11714


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in the AZO film can be effectively adjusted by the Al-to-Zn growth ratios.32,33 3.2. Optical and Electrical Properties of AZO Films and TFT Performances. The transparency is an important parameter for the applications of TCO material; therefore, the optical transmission was measured by a UV−vis spectrophotometer for all AZO films with various Al concentrations as shown in Figure 5a. In most of the visible light range of 400−

comparison to the AZO films obtained by the other conventional ALD growth method.10,27 The EDX of the AZO 1:24 film in Figure 3d suggests the presence of Zn, O, and Al elements, and the estimated doping percentage of Al was 3.41%. Figure 3e−g corresponds to TEM elemental mapping images of (e) Zn, (f) O, and (g) Al, further confirming the presence and the homogeneous distribution of the elemental Zn, O and Al in the thin film. The elemental homogeneous distribution was also observed in SEM elemental mapping images in Figure S3. The Figure S3a shows the corresponding SEM image of the film deposited on silicon from the top view. It can be seen that the film consists of long wedge-like grains, which are regarded as an aaxis growth (c-axis growth parallel to the substrate). The comparing TEM images for ZnO/Zn−Al−O and ZnO/Al2O3 films were shown in Figure S4. The yellow circles are marked for grain size, and the red arrows indicate the amorphous phases. The electron microscopic observations for microstructural comparison indicate that the continuous crystalline structure with reductive amorphous phase was observed in this novel design of growth method (Figure S4b), which may decrease the carrier scattering and increase the carrier mobility therefore enhance the electrical conductivity of AZO film. To investigate the chemical bonding states and atomic concentrations of Al doping, X-ray photoelectron spectroscopy (XPS) was performed to evaluate the AZO films with various Al-to-Zn ratios. The survey scan of the AZO films and the highresolution spectra of Zn 2p, Al 2p, and O 1s are shown in Figure 4. The calculated atomic concentrations of Al for different AZO films are 1.20%, 1.79%, 3.03%, 3.41%, 3.87%, and 5.19% (Figure S5). In the survey XPS spectrum of the representative AZO 1:24 film shown in Figure 4a, the indexed peaks are corresponding to the elements of Zn, Al, O, and C, where the binding energies are calibrated by taking the carbon C 1s peak (284.6 eV). From the typical Zn 2p spectra shown in Figure 4b, two peaks are assigned to the Zn 2p3/2 state centered at 1021.4 eV and the Zn 2p1/2 state centered at 1044.5 eV, respectively.28 The appearance of Al 2p peaks centered at 74.0 ± 0.1 eV in AZO films indicates that Al doping successfully takes place into the original ZnO lattice through substitution of Zn2+ sites (in Figure 4c).29 In addition, the typical peak positioned at 74.7 eV associated with the Al−O bond of the stoichiometric Al2O3 was not found for all films,30 which indicates no generation of the Al2O3 cluster during the ALD growth process, as observed in HRTEM images in Figure 3a. The comparison of XPS Al 2p peaks between the ZnO films with Zn−Al−O and ZnO−Al2O3 interfaces are shown in Figure S6. Obviously, the one at 74.7 eV associated with the stoichiometric Al2O3 indicates the generation of amorphous Al2O3 phase during the conventional ALD growth process. The Gaussian resolved results for O 1s spectra in Figure 4d show two components of oxygen with different chemical states. The component with lower binding energy centered at 529.8 eV is attributed to O2− ions in the wurtzite structure of hexagonal Zn2+ ion array. Meanwhile, the component with high binding energy located at 531.5 eV is associated with O2− in the oxygen deficient regions within the matrix of ZnO.31 As shown in Figure S7, the relative area ratio of the peak at 531.5 eV to the one at 529.8 eV in the AZO 1:24 film is about 2.99, which is higher than that in other AZO films. In general, the XPS results suggest that the oxygen vacancies and Al doping concentration

Figure 5. (a) Optical transmittance and (b) the band gap energy for AZO films with various Al-to-Zn ratios.

800 nm, all the AZO films present transparency of around 80%. This suggests that the AZO film is feasible as a transparent electrode material for applications in solar cells and other optoelectronic devices. With increasing Al content from 1.20 to 5.19 at. % by the adjustment of the Al-to-Zn ratio, the absorption edge of the corresponding AZO films shifts toward a shorter wavelength, which can be explained by the Burstein−Moss (BM) shift. For all AZO films, a plot of (αhv)2 against the photon energy hv was drawn and the sharp absorption edge could be accurately determined by the linear fit according to the Tauc equation in Figure 5b. It is found that the band gap of the AZO film changed from 3.26 to 3.57 eV for Al doping varying from 0 to 5.19 at. %, respectively. The intrinsic band gap (Eg) of the pure ZnO is 3.27 eV, and that of Al2O3 is 8.7 eV. Thus, the increased band gap is to be expected with increasing concentration of Al. In another way, the increased Eg is due to the increased electron concentration in the AZO films, which is supplied from donor sites associated with oxygen vacancies or excess metal ions. To evaluate the electrical properties of AZO films with different Al percentage, the resistivity, carrier concentration, and mobility were measured at room temperature as shown in Figure 6. Note that all of the films, both AZO and pure ZnO, were n-type semiconductors. In the case of the pure ZnO film, the resistivity, carrier concentration and the mobility were 9.6 × 10−3 Ω·cm, 1.8 × 1020 cm−3 and 10.1 cm2/V·s, respectively. As the Al doping increased, the carrier concentration of AZO films at first was increased rapidly until saturated to 2.2 × 1021 cm−3 11715


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growth temperatures of 230 °C (8.5 × 10−5 Ω·cm).35 Even compared with the conventional atomic layer deposited AZO films, the current ZnO films with Zn−Al−O interface exhibit outstanding conductivity (10−4 Ω·cm) over the films obtained by modulating a cycle ratio of ZnO and Al2O3 (10−3 Ω·cm).27 The higher resistivity for the conventional AZO film is attribute to the inhomogeneous distribution of Al dopant in the ZnO matrix as well as the increased carrier scattering and decreased carrier concentration due to the clustered Al2O3 nanolaminate structures located at the interface between the ZnO layers, which has been proven by other researchers.10,11,36 To our knowledge, the fundamental mechanisms of the superior electrical performance of the ZnO film with a Zn−Al−O interface are not broadly explored yet, and these are essential for understanding the extrinsic doping mechanisms of these films used in many applications such as semiconductors and flexible transparent TFT devices. To investigate the electrical properties and the device stability of the AZO films as electrodes of the flexible transparent thin-film transistor, we fabricated two typical ZnO-based TFT structure using this AZO and traditional AZO thin film, respectively. The schematic structure of the staggered bottom-gate TFT used in this study is shown in Figure 7a. The TFT structure were deposited entirely by ALD method on PEN flexible transparent substrate at 150 °C: AZO film for the gate, source, and drain (GSD) electrodes; Al2O3 film for the gate dielectric; and ZnO film for the channel layer. UV−vis transmittance spectra of the fabricated TFT device is shown in Figure 7b, showing over 80% of transparency in the visible light range. The mechanical flexibility and high transparency of the TFTs are determinant parameters for the practical applications, which can be assessed from the optical image as shown in the inset in Figure 7b. Figure 7c shows the typical output characteristics for ZnO film with Zn−Al−O interface based TFT devices that exhibit enhanced-mode nchannel characteristics. Corresponding transfer characteristic

Figure 6. Comparison of resistivity, carrier concentration, and carrier mobility of ALD-deposited AZO thin films with various Al-to-Zn ratios.

for AZO 1:24 film and then was abruptly reduced. The mobility of the AZO films continuously decreased from 10.1 to 5.9 cm2/ V·s as Al doping increased. Furthermore, the lowest resistivity of 5.7 × 10−4 Ω·cm was obtained for the AZO films grown with 3.41 at. % Al/Zn = 1:24. The resistivity of AZO film deposited by this ALD method decrease continually with the increasing of the thickness due to the homogeneity, continuity, and also the sensitivity of the Hall measurement method for film material and tend to be stable around 5.7 × 10−4 Ω·cm from 60 nm; the resistivity dependent on thickness was shown in Figure S8. This resistivity value is lower than the most reported AZO films prepared by other deposition processes, such as sol−gel, chemical vapor deposition (CVD), pulsed laser deposition (PLD), and frequency magnetron sputtering.11 In particular, note that the current AZO films are only 100 nm thick and under the growth temperatures of 150 °C in comparison to sputtering AZO films with 700−900 nm thicknesses (2.4 × 10−4 Ω·cm)34 and pulsed laser deposited AZO films under the

Figure 7. (a) Schematic of the TFT structure used in this work and (b) UV−vis transmittance spectra of AZO/ZnO/Al2O3/AZO films deposited on PEN substrate. The inset shows the image of fully transparent flexible transistor. (c) Output and (d) transfer characteristic of the TFT fabricated on PEN substrate. 11716


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Figure 8. Comparison of (a) the electrical performances, (b) PL spectra, (c) XPS O 1s peaks, and (d) TEM-EELS spectra of ZnO/Al2O3 and ZnO/ Zn−Al−O layered structure.

522 nm are observed for two types of AZO films. Typically, the UV emission corresponding to the near band edge (NBE) transition originates from the exciton recombination of ZnO. The green luminescence (522 nm) likely results from the oxygen vacancies.39 Compared with the ZnO/Al2O3 film, the emission associated with the oxygen vacancy in the ZnO/Zn− Al−O film was obviously enhanced, which indicates a relatively higher oxygen vacancy content in the ZnO/Zn−Al−O film grown by ALD method. PL spectra suggest that the Zn−Al−O interface facilitate to introduce more oxygen vacancy defects and increase the carrier concentration in the AZO films, resulting in lowering of the resistivity. To clarify the discrepancy of oxygen vacancy defects between Zn−Al−O and Al2O3 interface layers, XPS and Raman shift spectra of two types of AZO films are recorded. It is found that the peak associated with O2− in the oxygen deficient of ZnO/ Zn−Al−O film is obviously greater than that of ZnO/Al2O3 film, as shown in XPS results in Figure 8c. According to the Raman spectra in Figure S10, the most-evident peak located around 436 cm−1 is attributed to the E2 high nonpolar mode of the wurtzite structure of ZnO. To the best signal of the Raman spectrum, the AZO film samples were deposited on quartz substrate; the peak around 471 cm−1 is the typical A1 mode for quartz substrate.40 The weak peak located at 574 cm−1 noted as A1 (LO) phonon mode for AZO is associated with the defects of the oxygen vacancies. Moreover, the A1 (LO) peak intensity of the ZnO/Al2O3 film was lower than that of the ZnO/Zn− Al−O film, indicating more oxygen vacancies in the ZnO/Zn− Al−O film.41−43 To understand the origin of the oxygen vacancy defects in the Zn−Al−O and Al2O3 interface, EELS was adopted on the basis of its sensitivity to the electronic structural change owing to vacancies, bonding state, and so on.44 In Figure 8d, the oxygen K edges of two kinds of the AZO films exhibit significant difference, which indicates that the ZnO/Al2O3 film with a strong first peak in the oxygen K edge is attributed to the

curve measured at a constant drain voltage (VD) of 15 V is shown in Figure 7d. The transfer characteristic curve of the traditional AZO-based TFT device is shown in Figure S9. Compared with the traditional AZO-based TFT, the device performances are significantly enhanced. The traditional AZObased TFT shows the Ion/Ioff ratio of ∼104, a threshold voltage (Vth) of ∼2.7 V with subthreshold swing values of ∼9 V dec−1. Correspondingly, ZnO film with Zn−Al−O interface based TFT devices exhibit low threshold voltage (Vth) of ∼1.5 V and low subthreshold swing values of ∼1.4 V dec−1. The (μSat) was calculated in the saturation regime of the TFT operation. The ZnO/Zn−Al−O-based TFT shows a saturation mobility μSat of ∼2 cm2 V−1 s−1 with an Ion/Ioff ratio of >107. The device performance suggests that our new concept of AZO transparent electrode is competent in the applications of the flexible transparent electro-optical devices.37,38 3.3. Origin of the Oxygen Vacancy Defects and the Mechanism of Electrical Transport. To better understand the commendable electron conduction behavior and device performance of the novel AZO films prepared under the current doping concept, comparative experiments between the ZnO film with Zn−Al−O interface and the conventional ZnO with Al2O3 doping layer AZO film were performed as shown in Figure 8a. According to the hall measurements, the resistivity, carrier concentration, and the carrier mobility of the ZnO/Al2O3 film were 1.2 × 10−3 Ω·cm, 1.1 × 1021 cm−3 and 4.4 cm2/V·s, respectively, which suggest that its electrical performances is remarkably worse than that of the ZnO/Zn−Al−O film. The possible reason is derived from the intrinsic discrepancy between ZnO/Al2O3 and ZnO/Zn−Al−O film. PL emission spectra and Raman spectra were recorded. The PL emission spectrum at an excitation wavelength of 325 nm of the AZO films composed of ZnO/Al2O3 or ZnO/Zn−Al−O are shown in Figure 8b. It can be seen that a strong UV emission centered at 381 nm and a broad visible emission at 11717


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ZnO/Zn−Al−O film and the traditional ZnO/Al2O3 film are performed and reveal that the superior properties could be related to the oxygen deficiency located in Zn−Al−O interface by offering a high electron concentration and reduced layered boundary scattering. The novel film growth concept, excellent electrical performance, low-temperature growth process, high transparency, and chemical stability of our superior TCO film, coupled with the conformity and uniformity of the ALD process, indicate that flexible transparent devices with increasing complexity can be fabricated using our ALD process in the near future.

stable Zn−O bonding state.45 Correspondingly, the EELS data of the ZnO/Zn−Al−O film exhibit the low-intensity ratio (Ifirst/Isecond) of the first and second peaks. According to the relationship between the oxygen K edges and the electronic structural changes, such variations of the oxygen K edge between two kinds of AZO films demonstrate that the intensity ratio (Ifirst/Isecond) is drastically decreased with the existence of oxygen vacancies. Therefore, the decreased Ifirst/Isecond in EELS data for the ZnO/Zn−Al−O film (Figure 8d) seems to be mainly caused by the relatively high density of the oxygen vacancies.46,47 The underlying reason for different oxygen vacancies deficient under two film growth concepts is the free competitive growth mode of Zn−O and Al−O bonding in ALD growth in which the bonding energy of Al−O (511 kJ/mol) is larger than that of Zn−O (159 kJ/mol).48 As a result, the Al is capable of randomly replacing the Zn site to initiate the free competitive growth between the Al−O and Zn−O bond on the surfaces and hence introduces the abundant oxygen vacancies that act as donors and induce superior conductivity by generating carriers in the ALD-AZO films. The oxygen deficiency enhances the carrier concentration by a factor of two because the oxygen vacancy can provide carrier donation sites. Based on the above discussion, we propose a layered structural and electrical transport model for the AZO films, as shown in Figure 9. For the ZnO/Al2O3 layer-by-layer structure,

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02609. Figures showing interplanar spacing of the (100) plane of the AZO films, a comparison of XRD spectra and TEM images, SEM images and corresponding elemental mapping images, concentrations of Al element in atomic layer deposited AZO films, a comparison of XPS Al 2p peaks, the area ratio of the peak for O2−, resistivity dependent on the thickness of ZnO/Zn−Al−O films, output and transfer characteristic curves, and Raman shift spectra. A table comparing film characteristics from XRD, ADM, and ellipsometer technology. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: 86-0451-86402522. ORCID

Xiaohong Wu: 0000-0003-3174-3344 Notes

The authors declare no competing financial interest.

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Figure 9. Schematic of plausible mechanism of electrical transport model for (a) ZnO/Al2O3 and (b) ZnO/Zn−Al−O layered structures.

ACKNOWLEDGMENTS The financial supports of the National Natural Science Foundation of China (grant nos. 51671074 and 51572060), the Excellent Youth Foundation of Heilongjiang Scientific Committee (grant no. JC2015010), and the Doctoral Foundation of Heilongjiang province (grant no. LBHZ14103) are gratefully acknowledged.

the Al2O3 layer can function as boundary for the ZnO layers by generating carrier scattering (Figure 9a). In contrast, the AZO film with ZnO/Zn−Al−O shows increased oxygen deficient bonding in the doper layers. Figure 9b clearly displays these features, including the oxygen deficiency as a low-scattering path between ZnO layers. The act of the low-scattering path has also been observed for other oxide by many researchers when oxygen vacancies were located near the grain boundary.45,49 As a result, the carrier conduction was enhanced by the reduction of layered boundary scattering. The proposed processes can be the possible model for the explanation of the excellent conductivity of the ALD AZO thin film.

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REFERENCES

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4. CONCLUSIONS In summary, we have developed a unique growth concept of ALD ZnO film with an Zn−Al−O interface for transparent AZO films with superior conductivity. The AZO film resistivity and the carrier concentration are 5.7 × 10−4 Ω·cm and 2 × 1021 cm−3 with low temperature (∼150 °C). The fully transparent flexible transistor fabricated on this AZO electrode exhibits a threshold voltage (Vth) of ∼1.5 V, an Ion/Ioff ratio of >107, and a μSat of ∼2 cm2 V−1 s−1. In addition, over 80% transmittance in the wavelength range of 400−700 nm was observed for this TFT device. A series of comparative investigations between the 11718


Research Article

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Enhanced Performance in Al-Doped ZnO Based Transparent Flexible

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