Research Article www.acsami.org
Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2679−2687
High-Mobility and Hysteresis-Free Flexible Oxide Thin-Film Transistors and Circuits by Using Bilayer Sol−Gel Gate Dielectrics Jeong-Wan Jo,†,∥ Kwang-Ho Kim,‡,∥ Jaeyoung Kim,§ Seok Gyu Ban,† Yong-Hoon Kim,*,§ and Sung Kyu Park*,† †
School of Electrical and Electronics Engineering, Chung-Ang University, Seoul 06980, Korea Korea Electronics Technology Institute, Seongnam 13509, Korea § School of Advanced Materials Science and Engineering, and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Korea
ACS Appl. Mater. Interfaces 2018.10:2679-2687. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/27/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: In this paper, we demonstrate high-performance and hysteresis-free solution-processed indium−gallium−zinc oxide (IGZO) thin-film transistors (TFTs) and high-frequency-operating seven-stage ring oscillators using a low-temperature photochemically activated Al2O3/ZrO2 bilayer gate dielectric. It was found that the IGZO TFTs with single-layer gate dielectrics such as Al2O3, ZrO2, or sodium-doped Al2O3 exhibited large hysteresis, low field-effect mobility, or unstable device operation owing to the interfacial/bulk trap states, insufficient band offset, or a substantial number of mobile ions present in the gate dielectric layer, respectively. To resolve these issues and to explain the underlying physical mechanisms, a series of electrical analyses for various single- and bilayer gate dielectrics was carried out. It is shown that compared to single-layer gate dielectrics, the Al2O3/ZrO2 gate dielectric exhibited a high dielectric constant of 8.53, low leakage current density (∼10−9 A cm−2 at 1 MV cm−1), and stable operation at high frequencies. Using the photochemically activated Al2O3/ZrO2 gate dielectric, the seven-stage ring oscillators operating at an oscillation frequency of ∼334 kHz with a propagation delay of 800 °C) is generally required. Overall, using a single-layer oxide gate dielectric, it is rather difficult to obtain the key requirements for gate dielectrics at low process temperatures. Therefore, to meet these requirements simultaneously at a low temperature, a combination of gate dielectrics such as Al2O3·ZrO2·Na-Al2O3 (sodium-doped Al2O3) may be favorable rather than by using a single-layered gate dielectric layer. In this study, we demonstrated a high-performance and hysteresis-free indium−gallium−zinc oxide (IGZO) TFTs by using bilayer-structured solution-processed gate dielectrics which outperform those with single-layer gate dielectrics. More importantly, the bilayer-structured gate dielectrics could be fabricated at a low temperature by using DUV-induced photochemical activation process.5,14,15 Here, we systematically investigated the electrical properties of various single- and bilayer oxide gate dielectrics (Al2O3, ZrO2, Na-Al2O3, Al2O3/ ZrO2, ZrO2/Al2O3, and Na-Al2O3/ZrO2) for high-performance IGZO TFTs. Also, the correlation between the gate dielectric structure and the electrical performance of IGZO TFTs was investigated such as the field-effect mobility and hysteresis behaviors. Furthermore, by using the optimized gate dielectric structure of Al2O3/ZrO2 bilayer, high-performance and highfrequency-operating IGZO TFTs and seven-stage ring oscillators were demonstrated on a flexible polymer substrate.
2. EXPERIMENTAL SECTION 2.1. Preparation of Metal Oxide Precursor Solutions. To prepare the precursor solutions for desired gate dielectrics, metal precursors of aluminum nitrate nonahydrate (Al(NO3)3·9H2O), 2680
DOI: 10.1021/acsami.7b10786 ACS Appl. Mater. Interfaces 2018, 10, 2679−2687
Research Article
ACS Applied Materials & Interfaces
Figure 1. Electrical characterization of various gate dielectric layers. (a) Schematic of the device structures for TFTs with single-layer (ZrO2, thick ZrO2, Al2O3, and Na-Al2O3) and bilayer (Al2O3/ZrO2, ZrO2/Al2O3, and Na-Al2O3/ZrO2) gate dielectrics; (b) leakage current density vs applied electric field; and (c) capacitance vs frequency measured in metal/dielectric/metal configuration using various gate dielectrics. The thicknesses of ZrO2, thick ZrO2, Al2O3, Na-Al2O3, Al2O3/ZrO2, ZrO2/Al2O3, and Na-Al2O3/ZrO2 gate dielectrics were 12, 60, 60, 49, 65, 69, and 54 nm, respectively.
Table 1. Electrical Properties of IGZO TFTs Using Different Gate Dielectrics dielectric Al2O3 Na-Al2O3 Al2O3/ZrO2 ZrO2/Al2O3 Na-Al2O3/ZrO2
dielectric constant (at 100 Hz)
leakage current density (at 1 MV cm−1) [J/cm2]
6.37 7.91 8.53 8.71 9.83
4.51 3.71 2.74 4.68 1.99
× × × × ×
−9
10 10−7 10−9 10−9 10−5
mobility [cm2 V−1 s−1]
hysteresis [V]
3.2 12.6 13.5 4.2 23.6
+1.4 −1.4 ∼0 +2.4 −2.2
Al2O3 films44,45 attributing to a loose metal−oxygen−metal (M−O−M) network or the presence of voids within the film.9 In an aim to increase the dielectric constant of the Al2O3 film, sodium (Na) doping was carried out,10 and with a 5 at. % doping of Na, the dielectric constant could be increased up to 7.91 as shown in Figure 1c and Table 1. However, as a trade-off, the Na-doped Al2O3 (Na002DAl2O3) film showed an increase in the leakage current density (3.71 × 10−7 A cm−2 at 1 MV cm−1) mainly attributed to the increased number of mobile ions (Na+) in the film.10,46 In addition to single-layer gate dielectrics, various types of bilayer-structured gate dielectrics were investigated as shown in Figure 1a. Three types of bilayer structures were chosen to identify the influences of stacking order, material combination, and interfacial property on the electrical properties of IGZO TFTs: these structures include Al2O3/ZrO2, ZrO2/Al2O3, and Na-Al2O3/ZrO2. As shown in Figure 1b, by inserting an additional layer of Al2O3 on top or bottom of the ZrO2 layer (Al2O3/ZrO2 or ZrO2/Al2O3), the gate leakage current was effectively suppressed showing leakage current densities of ∼10−9 A cm−2 at 1 MV cm−1. In addition, as shown in Table 1 and Figure 1c, the Al2O3/ZrO2 and ZrO2/Al2O3 gate dielectrics exhibited relatively high dielectric constants of 8.53 and 8.71, respectively. Furthermore, the Al2O3/ZrO2 and ZrO2/Al2O3 gate dielectrics showed a relatively stable operation up to 1 MHz. In the case of Na-Al2O3/ZrO2 gate dielectric, a relatively high dielectric constant of 9.83 was observed; however, the
3. RESULTS AND DISCUSSION A schematic structure of IGZO TFTs using solution-processed gate dielectrics is shown in Figure 1a. As described above, as gate dielectric layers, various types of single- and bilayer structures were employed. More detailed information on the gate dielectric structure is summarized in Table 1. In Figure 1b, the leakage current density characteristics of various gate dielectrics are shown which were measured in an IZO/gate dielectric/Cr structure. As shown here, the single-layer ZrO2 film (12 nm thick) exhibited relatively high leakage current density, which can be attributed to their small thickness and narrow band gap property (∼5.8 eV).11,41,42 Increasing the ZrO2 thickness to ∼60 nm or using a different metal precursor for the ZrO2 precursor solution (zirconium oxynitrate hydrate) had no significant influence on the leakage current characteristics of ZrO2 gate dielectric as shown in Figures S2 and S3 (Supporting Information). In contrast, single-layer Al2O3 showed much lower leakage current density of 4.51 × 10−9 A cm−2 (at 1 MV cm−1), mainly owing to its wide band gap property (∼8.7 eV) and low defect density.15,40,41,43 Also, the capacitance of the Al2O3 gate dielectric showed reasonably stable frequency-dependent characteristics as shown in Figure 1c. Particularly, when the frequency was increased from 100 Hz to 1 MHz, the areal capacitance decreased slightly from 94 to 89 nF cm−2. In spite of the decent gate dielectric properties of Al2O3, single-layer Al2O3 exhibited a relatively low dielectric constant (∼6.37) compared to those of the vacuum-deposited 2681
DOI: 10.1021/acsami.7b10786 ACS Appl. Mater. Interfaces 2018, 10, 2679−2687
Research Article
ACS Applied Materials & Interfaces
Figure 2. AFM surface scan images and rms roughness of various gate dielectric layers. Images of single-layer (a) Al2O3 film, (b) ZrO2 film, and (c) Na-Al2O3 film. Images of bilayer (d) Al2O3/ZrO2 film, (e) ZrO2/Al2O3 film, and (f) Na-Al2O3/ZrO2 film.
Figure 3. Electrical characteristics of IGZO TFTs using various gate dielectrics. Transfer (upper panels) and output (bottom panels) characteristics of IGZO TFTs using (a) Al2O3 single-layer gate dielectric, (b) Na-Al2O3 single-layer gate dielectric, (c) Al2O3/ZrO2 bilayer gate dielectric, (d) ZrO2/Al2O3 bilayer gate dielectric, and (e) Na-Al2O3/ZrO2 bilayer gate dielectric.
2682
DOI: 10.1021/acsami.7b10786 ACS Appl. Mater. Interfaces 2018, 10, 2679−2687
Research Article
ACS Applied Materials & Interfaces
Figure 4. Illustration of metal-gate dielectric/semiconductor structure depicting mechanism of variations in the electrical properties of IGZO TFTs by using different gate dielectrics. Energy band diagram of the (a) Al2O3 single-layer gate dielectric, (b) Na-Al2O3 single-layer gate dielectric, (c) Al2O3/ZrO2 bilayer gate dielectric, (d) ZrO2/Al2O3 bilayer gate dielectric, and (e) Na-Al2O3/ZrO2 bilayer gate dielectric.
leakage current density (1.99 × 10−5 A cm−2 at 1 MV cm−1) was much higher than those of Al2O3/ZrO2 and ZrO2/Al2O3 gate dielectrics. To investigate the transistor performance variation by using different gate dielectrics, solution-processed IGZO TFTs with single- or bilayer gate dielectrics were fabricated. Initially, the surface roughness of each gate dielectric layer was analyzed since a rough gate dielectric/semiconductor interface can cause substantial carrier scattering in the channel layer and lead to the formation of interfacial trap states.9,40,43 However, as shown in Figure 2, all of the single- and bilayer gate dielectrics exhibited smooth surfaces with root-mean-square roughness values in the range of 0.1−0.2 nm. Therefore, the influence of surface roughness on the transistor performance is likely to be minimal in this experiment. Figure 3 shows the series of transfer and output curves of IGZO TFTs with different gate dielectrics. Using single-layer Al2O3 as a gate dielectric, the TFT showed a relatively poor performance, having a field-effect mobility of 3.2 cm2 V−1 s−1, Ion of 3.9 × 10−5 A (VDS = VGS = 10 V), and a large clockwise hysteresis (Figure 3a). With the Na-Al2O3 gate dielectric, the IGZO TFTs showed a field-effect mobility and Ion of 12.6 cm2 V−1 s−1 and ∼1.7 × 10−4 A, respectively (Figure 3b), which are comparably higher than those with the Al2O3 gate dielectric. However, a large counter-clockwise hysteresis was observed originating from the mobile Na ions in the dielectric. Interestingly, by using the bilayer Al2O3/ZrO2 gate dielectric layer, IGZO TFTs with a high field-effect mobility of 13.5 cm2 V−1 s−1 and a negligible hysteresis could be obtained as shown in Figure 3c. Considering that the solution-processed IGZO TFTs made on an SiO2 gate dielectric typically showed an average field-effect mobility of ∼2 cm2 V−1 s−1;5 these results suggest that the bilayer Al2O3/ZrO2 gate dielectric can be a promising candidate for enhancing the device performance. To further investigate the origin of the enhanced mobility and hysteresis attenuation by using the Al2O3/ZrO2 gate dielectric, the IGZO TFTs with other types of bilayer gate dielectrics were also fabricated (ZrO2/Al2O3 and Na-Al2O3/ZrO2). In the case of ZrO2/Al2O3 gate dielectric (Figure 3d), the field-effect mobility and the hysteresis behaviors were largely different from the IGZO TFTs using the Al2O3/ZrO2 gate dielectric, exhibiting field-effect mobility of 4.2 cm2 V−1 s−1 and large clockwise hysteresis. Moreover, by combining the Na-Al2O3 and ZrO2 films (Na-Al2O3/ZrO2), it was possible to obtain
IGZO TFTs with a high mobility of 23.6 cm2 V−1 s−1. However, large counter-clockwise hysteresis was observed as shown in Figure 3e. Figure S5 also presents the hysteresis characteristics in the C−V curves measured from various gate dielectrics in MIS structures (single- or bilayer gate dielectrics). The measured C−V curves in Figure S5 were almost identical to the trends observed in the I−V characteristics in TFT devices, as shown in Figure 2. To describe the transistor performance variation by using different gate dielectrics, we constructed energy band models for IGZO TFTs as illustrated in Figure 4. For ideal dielectrics or well-made layer-by-layer-deposited dielectrics,47 typically only positive/negative polarization can be at the interface. In those cases, we can consider the interfacial polarization and their relaxation and then analyze the conduction mechanism of the bilayer dielectrics by using Maxwell−Wagner effect48 and Debye polarization theory as the thickness variation of each dielectric film. However, most of the large-scaled or solutiondeposited engineering dielectrics generally include a nonnegligible amount of charged states or species inside the dielectric films, which are often used as a gate dielectric in the TFT process.16,17,49 In this paper, we are trying to understand and analyze the conduction mechanism of the engineering bilayer dielectric system with the observed phenomenon and the experimental results. In the case of Al2O3 gate dielectric, electron trapping at the acceptor-like trap states in Al2O3 bulk or at the IGZO/Al2O3 interface is occurred as shown in Figure 4a, resulting in a clockwise hysteresis behavior and a relatively low field-effect mobility.17,50−52 In the case of Na-Al2O3 gate dielectric (Figure 4b), a substantial number of mobile Na+ ions are present in the film, and under positive gate-bias condition (VG > 0 V), the Na+ mobile ions are likely to drift toward the IGZO/Na-Al2O3 interface inducing excessive electrons in the IGZO channel layer. As a consequence, the TFTs exhibit high field-effect mobility and counter-clockwise hysteresis because of the ionic charge displacement.16,53 In the case of bilayer gate dielectric structure, the drift of mobile ions and trap-assisted tunneling effect can be more complex.54,55 Particularly, in the case of Al2O3/ZrO2 gate dielectric, the hysteresis behavior is attenuated compared to that of a single-layer Al2O3 gate dielectric, even though the semiconductor/gate dielectric interface is not changed. This suggests that the observed hysteresis behavior is not solely caused by the electron trapping 2683
DOI: 10.1021/acsami.7b10786 ACS Appl. Mater. Interfaces 2018, 10, 2679−2687
Research Article
ACS Applied Materials & Interfaces
Figure 5. Flexible IGZO TFTs and circuits using an optimized Al2O3/ZrO2 bilayer gate dielectric. (a) Transfer, (b) output characteristics, and (c) statistical distribution of IGZO TFTs on a PI substrate. (d) VTH shift of unpassivated IGZO TFTs with various gate dielectrics and substrates under positive gate-bias stress (VGS = +5 V). (e) Photographs of a flexible IGZO TFT array and IGZO-based seven-stage ring oscillator fabricated on a PI substrate. (f) Oscillation frequency (red) and per-stage propagation delay (blue) of a seven-stage ring oscillator as a function of supply voltage (VDD). (g) Output waveforms of the seven-stage ring oscillator operating with supply voltages of 15 V and an oscillation frequency of 334 kHz.
also fabricated and compared the IGZO TFTs using bilayer SiO2/ZrO2 (compared to single-layer SiO2 and ZrO2) gate dielectrics to demonstrate the accuracy of proposing mechanism for other systems. As shown in Figure S6d, the decrease of hysteresis and increase of on-current in IGZO TFTs with SiO2/ ZrO2 can be one of the evidence for the proposed mechanism (the IGZO TFTs with Al2O3/ZrO2). Using the Al2O3/ZrO2 gate dielectric and low-temperature photoactivation process, flexible IGZO TFTs were fabricated on PI substrates. Figure 5a,b shows the transfer and output characteristics of flexible IGZO TFTs. The TFTs exhibited negligible hysteresis (Figure 5c) and an average field-effect mobility of 12.9 cm2 V−1 s−1 which is comparable to those fabricated on a glass substrate. In addition, to verify the operational stability of the devices, positive gate-bias stress tests were carried out. As displayed in Figures 5d and S4 (Supporting Information), the flexible IGZO TFTs showed a threshold voltage shift (ΔVTH) of +3.3 V after a stress time of 10 000 s without any passivation layer which is also comparable to those obtained from IGZO TFTs fabricated with atomiclayer-deposited Al2O3 gate dielectrics (on a PAR substrate; ΔVTH = +4.6 V, on a glass substrate; ΔVTH = +1.1 V).5 The ΔVTH values for IGZO TFTs on Al2O3, ZrO2/Al2O3, and Al2O3/ZrO2 dielectrics were 3.8, 3.4, and 3.3 V, respectively, as shown in Figure S1 of the Supporting Information. The slight variation of subthreshold swing (SS) indicates that the state creation is negligible, so the positive VTH shift is caused by electron trapping in an acceptor-like state in the gate dielectric or at the channel/dielectric interface. To obtain the maximum
at the IGZO/Al2O3 interface, but an additional effect may exist which compromises the clockwise hysteresis behavior. As illustrated in Figure 4c, with an increase of the transverse electric field, electrons in the bulk trap states in ZrO2 begin to emit into the Cr gate electrode as to minimize their energy state (process (1) in Figure 4c).17 As the electrons are injected into the Cr gate electrode, positively charged states remain in the ZrO2 layer,17 resulting in a buildup of additional electrons in the channel layer and subsequent trap filling at the IGZO/ Al2O3 interface. The hysteresis attenuation observed in the transfer characteristics shown in Figure 3c supports this phenomenon. In addition, concerning the gate leakage current, although the injection of electrons from ZrO2 to Cr gate electrode might influence the gate leakage current, the influence was minimal as also observed in a previous report,17 and the leakage current remained low enough (
