Effects of Deposition Temperature on the Device Characteristics of

Jun 27, 2017 - We demonstrated the physical and electrical properties of the In–Ga–Zn–O (IGZO) thin films prepared by atomic-layer deposition (A...
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Effects of Deposition Temperature on the Device Characteristics of Oxide Thin-Film Transistors Using In−Ga−Zn−O Active Channels Prepared by Atomic-Layer Deposition Sung-Min Yoon,*,† Nak-Jin Seong,‡ Kyujeong Choi,‡ Gi-Ho Seo,† and Woong-Chul Shin‡ †

Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin, Gyeonggi-do 17104, Korea ‡ NCD Co. Ltd., Yuseong-gu, Daejeon 34015, Korea S Supporting Information *

ABSTRACT: We demonstrated the physical and electrical properties of the In−Ga−Zn−O (IGZO) thin films prepared by atomic-layer deposition (ALD) method and investigated the effects of the ALD temperature. The film composition (atomic ratio of In:Ga:Zn) and film density were examined to be 1:1:3 and 5.9 g/cm3, respectively, for all the temperature conditions. The optical band gaps decreased from 3.81 to 3.21 eV when the ALD temperature increased from 130 to 170 °C. The amounts of oxygen-related defects such as oxygen vacancies increased with increasing the ALD temperature. It was found from the in situ temperature-dependent electrical conductivity measurements that the electronic natures including the defect structures and conduction mechanism of the IGZO thin films prepared at different temperatures showed marked variations. The carrier mobilities in the saturation regions (μsat’s) for the fabricated thin film transistors (TFTs) using the IGZO channel layers were estimated to be 6.1 to 14.8 cm2 V−1 s−1 with increasing the ALD temperature from 130 to 170 °C. Among the devices, when the ALD temperature was controlled to be 150 °C, the IGZO TFTs showed the best performance, which resulted from the fact that the amounts of oxygen vacancies and interstitial defects could be appropriately modulated at this condition. Consequently, the μsat, subthreshold swing, and on/off ratio for the TFT using the IGZO channel prepared at 150 °C showed 10.4 cm2 V−1 s−1, 90 mV/dec, and 2 × 109, respectively. The threshold voltage shifts of this device could also be effectively reduced to be 0.6 and −3.2 V under the positive-bias and negative-biasillumination stress conditions. These obtained characteristics can be comparable to those for the sputter-deposited IGZO TFTs. KEYWORDS: atomic-layer deposition, oxide semiconductor, thin-film transistor, In−Ga−Zn−O

1. INTRODUCTION Recently, oxide semiconductors have attracted much attention for the backplanes in the flat-panel display (FPD) industries. In particular, amorphous In−Ga−Zn−O (a-IGZO) has been widely researched because of its great advantages such as a high mobility, a superior uniformity, an optical transparency, and a low fabrication temperature.1,2 Radio-frequency (RF) magnetron sputter and solution process has been employed as conventional methods for depositing a-IGZO films.3,4 However, RF magnetron sputter can cause plasma damages to the layers underneath a-IGZO film and have limited process margin for larger-area substrate. Because the solution process has difficulty in controlling and obtaining the reproducibility in film thickness composition, it cannot be applied for mass production yet. On the other hand, the atomic-layer deposition (ALD) process can be a powerful alternative solution to solve these problems. ALD process provides us such benefits as excellent step coverage, precise control of film composition and thickness, and a dense and homogeneous film structure.5,6 Furthermore, the film thickness of the active channel layer is in the range of few tens of nanometers, and hence, the turnaround time of the ALD process is not much of a problem. For © XXXX American Chemical Society

these reasons, oxide thin-film transistors (TFTs) employing an ALD-prepared IGZO channel can be very promising backplane devices for the next-generation FPDs. Actually, some active materials such as Al-doped ZnO and Zn−Sn−O have been reported to be prepared with ALD for TFT applications.7−17 However, there have been few reports on the ALD process for the IGZO with a ternary oxide system. Although Illiberi et al. recently demonstrated the spatial atmospheric ALD techniques for the IGZO TFTs, the obtained device characteristics were far from the conventionally reported sputter-deposited IGZO TFTs.18 In this work, top-gate structured oxide TFTs using aIGZO active channel prepared by ALD process were fabricated and characterized. The effects of ALD temperature on the device characteristics of the IGZO TFTs were extensively investigated. The fabricated TFTs exhibited stable electrical characteristics under positive-bias-stress (PBS) and negativebias-illumination stress (NBIS) conditions. Received: April 1, 2017 Accepted: June 19, 2017

A

DOI: 10.1021/acsami.7b04637 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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istics were analyzed by X-ray photoelectron spectroscopy (XPS) using Al Kα (1486.8 eV) radiation (12.5 kV/20 mA) at a scan step of 0.05 eV. Optical band gaps were analyzed by using a spectroscopic ellipsometer (SE, J. A. Woollam Co.). The electrical conductivities (σc’s) of the prepared films were evaluated by the in situ four-point probe method, which were measured with the temperature change in the range from room temperature to 200 °C.

2. EXPERIMENTAL SECTION The TFTs employing ALD-IGZO channel layers were fabricated to have top-gate bottom-contact structure, as schematically shown in Figure 1a. The source/drain (S/D) electrodes were first patterned on

3. RESULTS AND DISCUSSIONS 3.1. Material Properties of ALD-IGZO Films. Before the device characterizations, such physical properties as film compositions and film densities of the ALD-IGZO films deposited at different temperatures were investigated. Figure 2

Figure 1. (a) Schematic cross-sectional view, (b) microscopic image, and (c) photo image of the ALD-IGZO TFTs fabricated on glass substrate. Figure 2. Variations in atomic ratios of the ALD-IGZO films prepared at different temperatures of 130, 150, and 170 °C. Film compositions were analyzed by XPS measurements.

150-nm-thick indium tin oxide (ITO)-coated glass substrates by a wetetching process. The IGZO channel layers were deposited by using Lucida GD series batch type ALD system (manufactured by NCD Co., Ltd.), in which ALD temperatures were varied to 130, 150, and 170 °C. Newly synthesized indium−gallium single precursor, diethyl zinc (DEZn), and ozone (O3) were employed as In−Ga, Zn, and oxygen sources, respectively. Purge processes for each precursor were carried out by nitrogen. The IGZO layers were prepared with one supercycle composed of several sequential steps of In−Ga−O and Zn−O. In this work, the number of ALD supercycles for the IGZO channel layers were fixed at 120. The film thicknesses were estimated to be 21.4, 24.3, and 27.4 nm at each IGZO film prepared at different ALD temperatures, respectively. A 9-nm-thick Al2O3 was successively deposited at 200 °C right after the deposition of IGZO channel as a channel protection layer with higher film density.19 After the patterning process of active regions, a 100-nm-thick Al2O3 was deposited at 150 °C by ALD as a gate insulator, in which the deposition temperature was lowered to avoid undesirable effects of long-time deposition at high temperature on the electronic natures of the IGZO channel layer. Finally, the ITO was deposited by using RF magnetron sputtering and patterned as gate electrode via a lift-off process. Figure 1b shows the fabricated fully transparent ALD-IGZO TFTs. The postannealing temperature for the fabricated TFTs using the IGZO channels prepared at 130, 150, and 170 °C were optimized at 230, 250, and 230 °C, respectively, for the device evaluations. The TFT characteristics were measured by using a semiconductor parameter analyzer (Keithley- 4200SCS) in a dark box at room temperature. The channel width (W) and length (L) of the evaluated devices were 40 and 20 μm, respectively. To investigate the physical and electrical properties of the IGZO films prepared at different ALD temperatures, IGZO thin films with the same thickness that were employed for the device fabrication were deposited on the Si substrates. Surface morphologies and film densities were examined by means of atomic force microscopy (AFM) and X-ray reflectometry (XRR), respectively. Film compositions and atomic bonding character-

shows the variations in atomic ratios of In:Ga:Zn when the ALD temperatures were varied to 130, 150, and 170 °C, which were analyzed by the XPS measurements. It can be noticeable that the atomic compositions were estimated to be approximately 1:1:3 for all the IGZO films irrespective of the ALD temperature differences. This result suggests that the ALD temperature did not have marked impacts on the film composition during the ALD process for the IGZO thin films. Figure 3a shows the XRR patterns of the IGZO films deposited at 150 °C and compares them before and after the postannealing process at 250 °C. There were no marked changes in total reflection edges even after the postannealing process. This reveals that two IGZO films have an almost the same film density. In XRR analysis, the critical angle for total reflection is proportional to the square root of film density.20,21 Thus, the film thickness, density, and roughness could be determined by a theoretical simulation of the XRR spectra. The film thickness and the density of the IGZO film deposited at 150 °C were estimated to be 24.3 nm and 5.9 g/cm3, respectively. Considering that the film densities of amorphous IGZO thin films prepared by sputtering have been reported to be in the range from 5.0 to 6.2 g/cm3,22−24 the obtained value of film density of the ALD-IGZO was quite comparable to those of the film deposited by a conventional sputtering method. The film thickness values increased from 21.4 to 27.4 nm when the ALD temperature was varied from 130 to 170 °C during the deposition of IGZO film, as summarized in Figure 3b. The increase in the deposition rate was supposed to result B

DOI: 10.1021/acsami.7b04637 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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and 170 °C respectively. It is surprising that the value of the optical band gap of the 130 °C-deposited IGZO film was much larger than those for the 150- and 170 °C-deposited films, even though the temperature difference was as small as 20 °C. A larger band gap induces a lower carrier concentration and lower carrier mobility for the amorphous IGZO semiconductor thin films. For the case of ALD-grown ZnO films, the carrier concentration and electrical conductivity of the ZnO film also showed marked dependencies of the ALD temperature. It was found from previous literature that ZnO films deposited at low ALD temperature contain more OH− than ZnO films deposited at high ALD temperatures.25,26 The ALD reactions of the ZnO film could be incompletely processed at low temperatures. This could induce residual OH− in the ZnO films. The residual O− H bonds may have two influences on ZnO characteristics. First, these bonds act as trap sites because they are located in the interstitial defect sites and could decrease the carrier mobility due to the impurity scattering. Second, the bonds are located in the oxygen vacancy sites and increase the O/Zn ratio. The high O/Zn ratio causes reduction in carrier concentrations of ZnO films. Thus, the carrier concentration and resistivity can sharply change as a function of ALD temperature. This is one of the most feasible scenarios to explain the modulation of electrical conductivity of the ALD-grown ZnO thin films and similar scenarios can be applied for the ALD-grown IGZO thin films. Thus, the differences in optical band gaps might be closely related to the defect-induced electronic natures of the IGZO films prepared at different ALD temperatures, such as amounts and locations of oxygen-related defects. The in situ measurements of temperature-dependent σc’s can provide useful information. To examine the difference in conduction mechanism of the ALD-IGZO films prepared at different deposition temperatures, the variations in σc were measured as a function of temperature from 50 to 200 °C at the forward and reverse sweeps, as shown in Figure 4. The measurement temperature range was appropriately chosen so that the differences in related characteristics could be well distinguished among the samples. The ramping rate was set as 5 °C/min. The initial values of σc for all films were approximately 10−8 S cm−1. From the obtained results, it is useful to separate some specified temperature regions in which different temperature dependencies of the σc were observed for each IGZO film. In region I, designated as the temperature range from 50 to 140 °C, the σc values did not change for all the films. In this temperature region, IGZO films were almost insulating and any carrier did not transport for electronic conduction, owing to insufficient thermal energy. The σc’s of the films started to increase at the beginning of region II with increasing the temperature, which suggests that all the IGZO films showed typical semiconducting behaviors. To further investigate the effect of ALD temperature on the electronic natures of the IGZO films, the activation energies (Ea) were calculated from the Arrhenius equation as follows: σc = σ0 exp(−Ea/kT), where σc and k correspond to the constant factor of conductivity for a given temperature and Boltzmann constant, respectively. The values of Ea calculated for region II for the forward sweep (Ea,for‑II) were approximately 4.60 and 4.59 eV for the 150- and 170 °C-deposited IGZO films, respectively. Alternatively, in region III, denoted as the temperature range higher than 170 °C, the Ea values markedly decreased. The Ea values estimated in region III (Ea,for‑III) for the IGZO films prepared at 130, 150, and 170 °C were approximately 1.00, 1.43, and 1.04 eV, respectively. It was also

Figure 3. (a) XRR profiles of ALD-IGZO film deposited at 150 °C and annealed at 250 °C after the deposition. (b) Variations in film densities and thickness values for the ALD-IGZO films prepared at different temperatures of 130, 150, and 170 °C, which were calculated by XRR profiles.

from the variations in preferred orientation of polycrystalline ZnO layer during the deposition process with increasing the ALD temperatures. Although the growth per cycle might increase by the changes in crystallographic orientation of the ZnO crystallites, the overall film composition and density did not show any marked ALD temperature dependency. Alternatively, all the IGZO films deposited at different temperatures showed almost the same film densities and there were no further improvements after the post heat treatments. It was found from these results that the ALD process can provide significantly dense and homogeneous IGZO thin films without any heat treatment. The surface morphologies of the ALD-IGZO films were measured by AFM in a 5 μm × 5 μm scan area, as shown in Figure S1 of the Supporting Information. The root-mean-square surface roughness (Rq) of the IGZO films showed an increasing trend as the ALD temperatures increased, in which the Rq values were estimated to be approximately 0.30, 0.34, and 0.46 nm for the IGZO films deposited at 130, 150, and 170 °C, respectively. These minute differences in surface morphologies can also be one of the determining parameters for the TFT characteristics. The optical band gaps of the IGZO films prepared at different temperatures were estimated. Figure S2 shows the curve fittings of the absorption coefficient as a function of the photon energy measured by SE analyzer. The optical band gaps of the IGZO thin films appeared to be 3.81, 3.26, and 3.21 eV when the ALD temperatures were controlled to be 130, 150, C

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values between the IGZO films deposited at 150 and 170 °C. Finally, the variations in σc were examined to be irreversible between the forward and reverse sweeps of measurement temperatures. The σc values for all the films did not show any changes when the temperature decreased from 200 to 50 °C. In other words, the ALD-grown IGZO films experienced drastic variations in electronic natures after the thermal treatment at given temperatures. The finally obtained σc values significantly enhanced with increasing the ALD temperature for the IGZO films. Consequently, the ALD temperature could sensitively influence the electrical behaviors, such as carrier concentration and electron mobility, of the ALD-grown IGZO films. To examine the bonding characteristics of oxygen-related species in the IGZO films prepared at various ALD temperatures, the XPS analysis was performed for O 1s spectra.28 Figure 5a−c shows the O 1s spectra and their deconvolution fitting for the ALD-IGZO films prepared at 130, 150, and 170 °C, respectively. To accurately analyze the film characteristics, the deconvolution process of XPS should be carefully performed. The O 1s peaks showed Shirley-type background and background binding energy was set from 528 to 535 eV to generate the accurate standards. The peaks A, B, and C were decomposed to be 529.8, 531.1, and 532.3 eV, respectively. The full width at half-maximum parameter and Lorentzian− Gaussian percentage were set as 1% and 1% for conventional s orbital, respectively. Three deconvoluted peaks correspond to metal oxygen bond (peak A), oxygen vacancy (peak B), and absorbed surface contaminations (peak C) such as −OH, −CO3, and H2O species. Thus, the B and C peaks correspond to the binding energies for the neutral oxygen vacancy sites that trap two free electrons and metal−hydroxyl bonds, respectively.29,30 Most noticeable feature in O 1s peak analysis was that the relative areal ratios of peaks A and B markedly decreased and increased, respectively, with increasing the ALD temperature. This suggests that the IGZO film prepared at higher ALD temperature contain more oxygen-related defects including oxygen vacancies and weak metal−oxygen bonding. These bonding natures and their variations may play important roles in device characteristics and reliabilities of the ALD-IGZO TFTs. Table 1 summarizes the relative areal ratios of each peak to the total area of peaks A, B, and C in the O 1s spectra for the IGZO films deposited at 130, 150, and 170 °C, respectively. 3.2. Device Characterizations of the ALD-IGZO TFTs. Figure 6a shows the drain current−gate voltage (IDS−VGS) characteristics and gate leakage currents (IG) of the fabricated TFTs using the IGZO channels prepared at different ALD temperatures. The measurements were performed in forward and reverse directions of VGS at a drain voltage (VDS) of 10.5 V. As can be seen in a figure, all the TFTs showed excellent transfer characteristics without any hysteretic behaviors in IDS and marked IG components. The on/off current ratio was obtained to be approximately 2.2 × 109. The device parameters such as the field-effect mobility in the saturation region (μsat) and subthreshold swing (SS) for each device were summarized in Figure 6b, in which each value was obtained from more than 20 devices on the same substrate and the device-to-device uniformity was typically examined as shown in Figure S3. It is noteworthy that the μsat increased from 6.1 to 14.8 cm2 V−1 s−1 with increasing the ALD temperature from 130 to 170 °C for IGZO deposition. The obtained highest value of μsat (14.8 cm2 V−1 s−1) was comparable to those of the conventional sputterdeposited-IGZO TFTs and was far superior to that of the previously reported device using ALD-IGZO channel layer.18

Figure 4. Variations in in situ measured electrical conductivity for the ALD-IGZO films prepared at different temperatures of 130, 150, and 170 °C in terms of Arrhenius plot, which were prepared on insulating thermally oxidized Si substrates. The temperature was swept from to 50 to 200 °C in forward and reverse directions. The ramping rate was set as 5 °C/min.

noteworthy that for the 130 °C-deposited film there was no distinct position to separate between the regions II and III. Some interesting and noticeable features can be described from the results mentioned above. First, the starting points of region II were found to be extended to a higher temperature for the IGZO films with increasing the ALD temperature. Higher temperature might be required to initiate the carrier transport for electrical conduction in IGZO films deposited at lower ALD temperature. For the IGZO film prepared at 130 °C, the carrier transport eventually started at region III. The delay of carrier transport at lower temperatures might be caused by residual OH− ions which can suppress the generation of oxygen vacancies, as discussed above. For this situation, desorption of OH− ions demands higher temperatures. Second, almost the same values of Ea,for‑II and Ea,for‑III were estimated for the IGZO films deposited at 150 and 170 °C. However, the IGZO films deposited at 130 °C had an Ea,for‑III corresponding to the first Ea for carrier transport, which was nearly equal to the values of Ea,for‑III corresponding to the second region for the carrier transport of other films. Furthermore, the value of σc for the IGZO film deposited at 130 °C was much lower than those of other IGZO films. Third, in the temperature regions II and III, totally different conduction mechanisms were expected from marked differences between Ea,for‑II and Ea,for‑III. Although it is very difficult to exactly determine the carrier types and conduction mechanisms, the Ea values estimated at specified temperature regions could be determined by a complicated combination of several factors such as thermal excitation from donor levels, atomic rearrangements, and formation energy of oxygen vacancy.27 Actually, for the IGZO films deposited at 170 °C, launching of carrier transport at lower temperature and high Ea,for‑II induced the highest value of σc among three IGZO films, even though there were not marked differences in Ea,for‑III D

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Table 1. Summary of the Relative Area Fraction for Each Peak with Respect to the Total Area of Peaks A, B, and C for the Deconvoluted O 1s Spectrum for the IGZO Films Prepared at ALD Temperatures of 130, 150, and 170 °Ca O 1s A O 1s B O 1s C

binding energy (eV)

130 °C

150 °C

170 °C

529.8 531.1 532.2

48.9 37.1 14.0

35.5 52.0 12.5

23.7 58.6 17.6

a Peaks A, B, and C correspond to metal−oxygen bonding, oxygen vacancies, and contamination on the surface of IGZO films, respectively.

Figure 6. (a) Comparisons of the IDS−VGS transfer characteristics of the fabricated TFTs using the IGZO channel layers deposited at different temperatures of 130, 150, and 170 °C. The VGS was swept in forward and reverse directions and the VDS was set at 10.5 V. (b) Summary of the carrier mobility and subthreshold swing values for each TFT. Figure 5. Variations in the oxygen 1s spectra for the ALD-IGZO films prepared at different temperatures of (a) 130, (b) 150, and (c) 170 °C. Black and green lines correspond to the total O 1s spectrum and background, respectively.

the minimum value of 0.09 V/dec for the TFT using the IGZO prepared at 150 °C. This indicates that the SS values for the ALD-IGZO TFTs were complicatedly determined by the interface defect densities (Nit) but also by the bulk defect sites (Nb) when the ALD temperature was varied during the IGZO deposition. Generally speaking, the SS values in TFTs would be mainly influenced by the interface quality. Furthermore, the good SS values were obtained thanks to the excellent dielectric properties of the Al2O3 gate insulator prepared by ALD

The ALD temperature dependence on the carrier mobility was expected to be closely related to the electronic natures and conduction mechanisms for the IGZO films deposited at different ALD temperatures. On the other hand, the SS showed E

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Figure 7. Variations in the transfer characteristics with the evolution of stress time under PBS condition at a VGS of +20 V for the fabricated TFTs using the IGZO channels prepared at (a) 130, (b) 150, and (c) 170 °C. The stress time and VDS conditions were set at 10 000 s and 10.5 V, respectively. (d) Comparisons in the shifts of VTH for each TFT with time evolution under the PBS conditions.

process.19 However, considering that the fabricated TFTs using the IGZO channel layers deposited at different ALD temperatures had almost the same interfaces with Al2O3 gate insulators via thin PL layer, slight variations in SS value might be caused by the bulk properties of the IGZO channel. Even though it would be quite difficult to perfectly separate the effects of two contributions to each other, we suppose that the lowest value of SS for the TFT using the IGZO deposited at 150 °C could be closely related to the defect structures within the IGZO films, as expected in Figures 4 and 5. The detailed descriptions on the variations in defect structures of the IGZO films prepared at different ALD temperatures will be discussed later to explain the NBIS characteristics of the fabricated devices. It is very important to investigate the device reliabilities of the fabricated devices for practical applications. First, the PBS of the ALD-IGZO TFTs were evaluated at RT, in which the VGS of +20 V was applied to the gate electrode for 104 s and VDS was fixed at +10.5 V, as shown in Figure 7a−c. The shifts of the threshold voltage (ΔVth) were measured as 0.6, 0.6, and 0.8 V for the TFTs using the IGZO channels prepared at 130, 150, and 170 °C, respectively. The positive shifts of the VTH originated from the electron trap at the interfaces between the gate insulator and IGZO channel layers and/or at the bulk traps within the channel. However, the amounts of ΔVth were so small for all the devices, and hence, the ALD-IGZO TFTs were found to be well fabricated to exhibit good PBS characteristics. Next, the NBIS of the devices were also investigated with varying the irradiation wavelengths. Under the negative-bias

stress in dark conditions, the IGZO TFTs generally exhibit stable device characteristics owing to the n-type semiconducting nature of IGZO channel layer.31−33 Contrarily, the VTH values of the IGZO TFTs might experience considerable amounts of negative shifts under the NBIS conditions, which is related to the fact that the donor states can be newly created in shallow levels and provide conduction electrons into the conduction bands.34,35 Sometimes, the generated conduction electrons can also be trapped at the interface between the gate insulator and IGZO channel layers. The NBIS tests were performed under the light illuminations by using the lightemitting diodes with the wavelengths of 635 (red), 530 (green), and 470 nm (blue) corresponding to the energies of 1.95, 2.34, and 2.65 eV. The irradiation intensity was set as 0.1 mW/cm2. Figure 8a−c shows the variations in ΔVth under the NBIS conditions for the TFTs using the IGZO channels prepared at 130, 150, and 170 °C, respectively, in which the VGS of −20 V was applied to the gate terminal for 104 s and VDS was fixed at 10.5 V to obtain saturation IDS. As can be seen in the figures, the values of ΔVth for all devices increased with increasing the irradiation energies during the NBIS tests. When the red wavelength was illuminated at the TFTs using the IGZO channels prepared at 130, 150, and 170 °C, the ΔVth’s were measured to be −0.1, −0.4, and −0.3 V, respectively. Alternatively, the ΔVth’s for each device increased to −1.4, −3.2, and −1.2 V at the irradiation of green wavelength, respectively. It is noteworthy that the ΔVth (3.2 V) for the TFT using the IGZO prepared at 150 °C did not additionally degrade even at the irradiation of blue wavelength. For usual F

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lengths were repeatedly verified for several devices. However, further studies are in progress to strictly confirm the validity of numerical values. This dramatic improvement in NBIS stability for the TFT using the IGZO prepared at 150 °C can be explained by following feasible scenarios: (1) there are two types of defects influencing the NBIS instabilities for the TFTs using the IGZO channel prepared by the ALD process. As mentioned above from the obtained results, the IGZO thin film deposited at 170 °C contains too many conduction carriers generated by vacancy defects. As a result, its σc showed a maximum value [Figure 4] and it includes more oxygen vacancies and weak metal−oxygen bonding sites [Figure 5] among the films deposited at various ALD temperatures. On the other hands, for the IGZO thin film deposited at 130 °C, residual OH− ions caused by insufficient decomposition of precursor source occupy the vacancy sites but introduce as interstitial defects within the matrix. As a result, the σc of the film and μsat of the device showed minimum values, as shown in Figure 4 and Figure 6b, respectively. Furthermore, the temperature-dependent conduction behavior of the 130 °Cdeposited film was totally different from those of the film deposited at 150 and 170 °C. Consequently, the interstitial defects not only degrade the carrier mobility owing to the impurity scattering but also drastically increase the carriers into the conduction band at an illumination of high-energy light source with shorter than critical wavelength. (2) Considering that two types of defects such as oxygen vacancies and O−H interstitials can be supposed to be main origins for the NBIS instabilities of the TFTs using the IGZO channels prepared at 170 and 130 °C, respectively, the ALD temperature of 150 °C can be an optimum condition for appropriately modulating the amounts of oxygen vacancies and for effectively reducing the interstitial defects within the film. This synergic effects for the TFT using the IGZO channel prepared at 150 °C explain its excellent NBIS characteristics.

4. CONCLUSIONS Oxide semiconductor IGZO films were prepared by ALD and their physical properties were extensively investigated. The film composition (In:Ga:Zn) was estimated to be 1:1:3 and did not show any marked dependence of the ALD temperature. Dense and homogeneous IGZO films with a density of approximately 5.9 g/cm3 and smooth surface morphogies of the ALD-IGZO films were obtained. The optical band gaps showed a decreasing trend from 3.81 to 3.21 eV with increasing the ALD temperatures from 130 to 170 °C. These significant differences originated from the ALD temperature conditions had great impacts on the device characteristics of the fabricated IGZO TFTs. The electrical conductivities of the films were evaluated as a function of measurement temperature. The critical temperature to start the carrier transport and the activation energies for electrical conduction at each temperature region showed marked variations when the ALD temperatures were varied from 130 to 170 °C. It was also found that the amounts of oxygen-related defects including oxygen vacancies increased with increasing the ALD temperature. These results suggest that the ALD temperature conditions played important roles in determining the electrical properties, sensitively influenced by defect structures, of the ALD-IGZO films. ALD-temperature-dependent physical and electrical characteristics were well reflected in the device characteristics of the TFTs using the IGZO channels prepared at different ALD temperatures. All the fabricated devices showed excellent PBS

Figure 8. Variations in the ΔVTH values for the fabricated TFTs using the IGZO channels prepared at 130, 150, and 170 °C as a function of stress time during the NBIS tests under (a) red, (b) green, and (c) blue lights, respectively.

cases, the Vth’s of the IGZO TFTs exhibited terrible shifts of larger than −10 V when the higher energy of blue-wavelength light was induced into the devices. Actually, the TFTs using the IGZO channels prepared at 130 and 170 °C experienced the ΔVth’s of −10.7 and −16.7 V, respectively. These evaluations on the NBIS instabilities at irradiations with various waveG

DOI: 10.1021/acsami.7b04637 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces stabilities, in which the ΔVth was estimated to be smaller than 1 V for 104 s. Alternatively, under the NBIS conditions especially at an irradiation of blue wavelength, the instabilities of device behaviors, which were mainly controlled by defect natures of the active channel, were well explained by the effects of the ALD temperature. The TFT using the IGZO channel prepared at 150 °C showed the best NBIS stability. Additional NBIS instabilities in VTH for the TFTs using the IGZO channels prepared at 130 and 170 °C could be explained by the interactions of O−H intersitials and oxygen vacancies, respectively. Consequently, when the IGZO channel layer was prepared at an ALD temperature of 150 °C, the overall device characteristics including the reliabilities could be far enhanced among the fabricated devices. It was clearly suggested that the ALD temperature condition can be one of the most important control parameters to realize high-performance ALD-IGZO TFTs. Furthermore, the obtained device characteristics were sufficiently comparable to those of the conventional sputter-deposited IGZO TFTs. Thus, we can conclude that the ALD process provides new technical strategies for improving both requirements of performance and producitivy for the backplane devices in future FPD industries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04637. Surface morphologies of the ALD-IGZO films; optical band gaps of the ALD-IGZO films; device-to-device uniformity (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sung-Min Yoon: 0000-0001-6535-3411 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Institute for Information and Communications Technology Promotion within the Ministry of Science, ICT, and Future Planning through the Korean Government (The Core Technology Development of Light and Space Adaptable Energy-Saving I/O Platform for Future Advertising Service) under Grant B010116-0133, and in part by the Kyung Hee University through the Samsung Electronics Research and Development Program entitled Flexible Flash Memory Device Technologies for NextGeneration Consumer Electronics.

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DOI: 10.1021/acsami.7b04637 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b04637 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX