High-Performance Thin-Film Transistors of Quaternary IndiumâZinc

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Functional Inorganic Materials and Devices

High-Performance Thin Film Transistors of Quaternary IndiumZinc-Tin Oxide Films Grown by Atomic Layer Deposition In-Hwan Baek, Jung Joon Pyeon, Seong Ho Han, Ga-Yeon Lee, Byung Joon Choi, Jeong Hwan Han, Taek-Mo Chung, Cheol Seong Hwang, and Seong Keun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03331 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Materials & Interfaces

High-Performance Thin Film Transistors of Quaternary Indium-Zinc-Tin Oxide Films Grown by Atomic Layer Deposition In-Hwan Baek,†,‡ Jung Joon Pyeon,†,§ Seong Ho Han,⊥ Ga-Yeon Lee,⊥ Byung Joon Choi,∥ Jeong Hwan Han,∥ Taek-Mo Chung,⊥ Cheol Seong Hwang,‡ and Seong Keun Kim†,*

†Center

for Electronic Materials, Korea Institute of Science and Technology, Seoul,

02792, South Korea

‡Department

of Materials Science and Engineering and Inter-University Semiconductor

Research Center, Seoul National University, Seoul, 08826, South Korea

§KU-KIST

Graduate School of Converging Science and Technology, Korea University,

Seoul, 02841, South Korea

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

⊥Division

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of Advanced Materials, Korea Research Institute of Chemical Technology,

Daejeon, 34114, South Korea

∥Department

of Materials Science and Engineering, Seoul National University of

Science and Technology, Seoul, 01811, South Korea

KEYWORDS Atomic Layer Deposition, Amorphous Oxide, In-Zn-Sn-O, Thin Film Transistor, Vertical Transistor

ABSTRACT A new deposition technique is required to grow the active oxide semiconductor layer for emerging oxide electronics beyond the conventional sputtering technique. Atomic layer deposition (ALD) has the benefits of versatile composition control, low defect density in the films, and conformal growth over a complex structure, which can hardly be obtained with sputtering. This study demonstrates the feasibility of growing amorphous In-Zn-Sn-


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O (a-IZTO) through ALD for oxide thin film transistor (TFT) applications. In the ALD of the a-IZTO film, the growth behavior indicates that there exists a growth correlation between the precursor molecules and the film surface where the ALD reaction occurs. This provides a detailed understanding of the ALD process that is required for precise composition control. The a-IZTO with In:Zn:Sn = 10:70:20 was chosen for highperformance TFTs among other compositions regarding the field-effect mobility (μFE), turn-on voltage (Von), and subthreshold swing voltage (SS). The optimized TFT device with the a-IZTO film thickness of 8 nm revealed a high performance with a μFE of 22 cm2V-1s-1, Von of 0.8 V, and SS of 0.15 Vdec-1, after the annealing at 400 °C for 30 min. Furthermore, an emerging device such as a vertical channel TFT was demonstrated. Thus, the a-IZTO ALD process could offer promising opportunities for a variety of emerging oxide electronics beyond planar TFTs.

INTRODUCTION


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There is increasing industrial demand for high-mobility semiconductors that can be fabricated under the industry-compatible conditions regarding the film quality and the processing temperature. Amorphous oxide semiconductors are one of the materials that meet the strict requirements of industrial applications. In this regard, amorphous In-GaZn-O (a-IGZO) has led the field due to its relatively high mobility, excellent uniformity over large areas, and a low processing temperature. Hence, devices using a-IGZO films as the active channel layer are already under the mass production for the fabrication of backplane thin film transistors (TFTs) in flat-panel displays, including active-matrix organic light-emitting displays.1-7 Nonetheless, for the next-generation displays requiring higher resolutions and frame rates, a-IGZO may suffer from the insufficient carrier mobility. Thus, other potential candidates, such as ZnON8-9 and bi-layer oxide structures, have been studied.10-12 The sputtering technique has been conventionally used for the growth of a-IGZO for display applications.13 This technique allows low-temperature processing and excellent uniformity over large areas. Despite the advantages, the sputtering technique has several problems. In the sputtering of multicomponent materials, such as a-IGZO, the


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target inevitably suffers from a change in the composition due to aging because each element of these multicomponent materials has a different sputtering yield.14 The highenergy bombardment of ions during the sputtering process produces defects in the grown oxide films, resulting in the degradation of their TFT performance. Furthermore, engineering the cation composition by a gradual change in the channel composition or a bi-layer structured channel has resulted in a high performance of the TFTs,10-12 but these approaches cannot be implemented by the conventional sputtering process that involves a single target. Excellent conformality of the film growth technique is a critical factor for implementing the vertical channel TFTs (VTFTs) in ultrahigh-resolution displays.15-16 The conventional sputtering method is undesirable for the fabrication of VTFTs owing to its feature of directional growth. Atomic layer deposition (ALD) is a thin film growth technique that is based on the surface reaction between precursors that are injected sequentially. It provides excellent reproducibility, precise thickness control, versatile compositional engineering, highquality interfaces, and perfect conformality over complex structures. Therefore, ALD is expected to resolve the problems associated with the sputtering of multicomponent


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oxide semiconductors. Indeed, promising results have been recently demonstrated for TFTs utilizing ALD-grown binary and ternary oxides; for example, field-effect mobilities (μFE) of 20 cm2V-1s-1 for ZnO,17 30 cm2V-1s-1 for In2O3,18 13 cm2V-1s-1 for ZnSnOx,19 and 9.5 cm2V-1s-1 for InGaOx20 have been obtained. The ALD mechanism in the case of multicomponent materials, however, usually deviates from the simple sum of the growths of the constituent materials.21 Hence, it is important to understand the ALD behavior of multicomponent materials for the precise control of the composition of the films. Also, the properties of oxide semiconductors are influenced by the ALD conditions such as the types of precursors and the processing temperature even for the same cation composition. A systematic study on the properties and device characteristics of oxide semiconductors is therefore needed. In this work, ALD-based growth and its application in TFTs are demonstrated based on the quaternary channel oxide a-IZTO, which is one of the promising oxide semiconductors.22-31 The correlations between the growths of the constituent elements were firstly examined for the ALD of a-IZTO, and the TFT properties of the a-IZTO films were subsequently studied over a wide composition range, which was further optimized


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by changing the post-annealing condition and the film thickness. The findings reveal that the a-IZTO films grown by ALD could be a highly feasible channel material for oxide TFTs. Also, an emerging electronic device such as a VTFT was demonstrated based on the ALD of a-IZTO. The implementation of the emerging devices demonstrates that the ALD of a-IZTO could provide an opportunity for a variety of emerging oxide electronics beyond planar oxide TFTs in display applications.

RESULTS AND DISCUSSION The first step in the ALD of multicomponent materials is to determine the temperature window where the ALD windows of all the components overlap because the metalorganic precursors for each cation have different thermal stabilities, depending on the ligand structure. Diethylzinc (DEZ), used as a Zn precursor, is known to have a wide ALD

window,

up

to

300

°C32,

whereas

dimethyl(N-ethoxy-2,2-

dimethylpropanamido)indium (Me2In(EDPA)) and dimethylamino-2-methyl-2-propoxytin(II) (Sn(dmamp)2), used as the In and Sn precursors, respectively, have relatively narrow ALD temperature windows. The authors previously reported that the ALD


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reaction of Me2In(EDPA) and O2 plasma had an ALD temperature window of 90–250 °C.18 Regarding the thermal stability of Sn(dmamp)2, the authors observed a sudden increase in the growth per cycle (GPC) in the ALD of SnO2 involving Sn(dmamp)2 and O3 at 230 °C33 and Sn(dmamp)2 and O2 plasma at 220 °C,34 suggesting that this Sn precursor decomposes thermally at temperatures higher than those values. Generally, growth at higher temperatures within the ALD window improves the properties of the thin films, such as the film density.35 In this work, therefore, the ALD was performed at the fixed temperature of 210 °C considering the ALD temperature windows of all the metalorganic precursors. The results on the phases and optical bandgaps of the component oxides indicate that single-phase thin films of ZnO, In2O3, and SnO2 were well grown by the ALD process with O2 plasma at 210 °C (Fig. S1). The self-limiting characteristic of ALD results in a constant GPC, which allows for precise control of the thickness at the atomic scale. During the ALD of multicomponent materials, however, the GPC frequently varies depending on the sub-cycle ratio, because the reaction surface changes with the type of the injected precursor. The GPC obtained for the ALD of multicomponent materials usually deviates from the simple sum


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of the GPCs observed in the ALD of the constituent materials. Hence, the growth behaviors of ternary oxides such as In-Sn-O, In-Zn-O, and Zn-Sn-O were examined to understand the interplay between two different constituent oxides in ALD. Figures 1 (a)– (c) show the variations in ‘number of the atoms gained per cycle’ during the ALD of InSn-O, In-Zn-O, and Zn-Sn-O films as functions of the Nx/[Nx+Ny] sub-cycle ratio, respectively. Ni (i = ZnO, SnO2, or In2O3) indicates the number of ALD sub-cycles of each oxide. The atomic gain was obtained from the metal cations present in the films, measured by wavelength dispersive X-ray fluorescence (WDXRF). For the growth of InSn-O, as shown in Fig. 1 (a), the number of atoms gained per cycle of Sn and In are nearly invariant, irrespective of the sub-cycle ratio. Hence, the actual composition of the In-Sn-O films follows the rule of mixtures (Fig. 1 (d)). In contrast, a significant change in the number of atoms gained per cycle is observed for the other ternary oxides such as In-Zn-O and Zn-Sn-O. The number of Zn atoms gained per cycle in both the materials considerably increases with decreasing NZnO/[NZnO+NSnO2

or In2O3]

sub-cycle ratio and

varies from 6.7 × 1014 cycle-1cm-2 at NZnO/[NZnO+NSnO2 or In2O3] = 1 to 1.2 × 1015 cycle-1cm2

at NZnO/[NZnO+NSnO2 or In2O3] = 0.09. The number of Sn and In atoms gained per cycle


9


slightly decreases with increasing NZnO/[NZnO+NSnO2

or In2O3]

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sub-cycle ratio. This

suggests that the adsorption of the DEZ precursor is drastically enhanced on the In-O and Sn-O surfaces, while the Sn(dmamp)2 and Me2In(EDPA) precursors are slightly less adsorbed on the Zn-O surface. To further understand the growth enhancement of ZnO on each In-O and Sn-O surfaces, we examine the initial growth of ZnO on ALD-grown In2O3 and SnO2. (Fig. S2) The graphs of the number of Zn atoms gain vs. number of ZnO cycles show a positive y-intercept of approximately 1.5 × 1015 cm-2. This means that the deposited amount of Zn atoms in the first cycle on the In2O3 and SnO2 is much larger than that for a cycle in the steady-state growth, which is well in agreement with the growth enhancement in Figs. 1 (b) and (c). A change in the GPC of the constituent elements on the heterosurface

has

been

reported

for

the

ALD

of

multicomponent

materials

and

nanolaminates.36-37 In particular, Bent’s group reported the correlating growth characteristic of the ALD of Zn-Sn-O films from DEZ, tetrakis(dimethylamido)tin, and H2O.38-39 In contrast to this work on the ALD of Zn-Sn-O, they observed a decrease in the GPC of ZnO ALD immediately after the SnO2 ALD cycle. They claimed that the


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adsorption of DEZ was somewhat restricted on the OH-terminated SnOx surface because of the reduction in the reaction site density.38-39 The contradictory behavior between these reports and our work provides a clue to understand the correlating growth mechanism in this work. The valence state of Sn ions in the SnOx is +4, which is identical to that in our work. (Fig. S3) This indicates that the contradictory behavior does not result from the difference in the valence state of the Sn ions in the reaction surface. The only thing that is different between our work and their work is the type of the functional group on the SnOx reaction surface formed after the injection of the oxygen source; hydroxyl groups are formed on the SnOx reaction surface when H2O is used as the oxygen source in the ALD while the functional group formed on the reaction surface after the injection of O2 plasma has not been well understood yet in the plasmaenhanced ALD. Although the mechanistic details of the growth enhancement are not available, it is probably that the functional group on the SnOx formed after the injection of O2 plasma might increase the density of the reaction site to anchor the DEZ in contrast to the OH-terminated SnOx formed after the injection of H2O.


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The correlating growth characteristics of the ALD of In-Zn-O and Zn-Sn-O films result in the actual compositions of the films deviating from those corresponding to the rule of mixtures. As shown in Figs. 1 (e) and (f), the actual compositions of both In-Zn-O and Zn-Sn-O films shift to a high Zn concentration (or a low NZnO/[NZnO+NSnO2 or In2O3] ratio along the x-axis) from the composition calculated based on the rule of mixtures because of large increases in the number of Zn atoms gained per cycle on the In-O and Sn-O reaction surfaces. Based on the correlation growth characteristics of the ternary oxides, the ALD of films of IZTO, a quaternary oxide, was performed. Figure 2 shows the variations in the number of atoms gained per cycle of In, Zn, and Sn ions in the films as functions of NZnO/[NIn2O3+NZnO+NSnO2] sub-cycle ratio. The number of Zn atoms gained per cycle tends to increase with decreasing NZnO/[NIn2O3+NZnO+NSnO2] sub-cycle ratio, which is consistent with the variations observed in the number of Zn atoms gained per cycle of the In-Zn-O and Zn-Sn-O films in Figs. 1 (b) and (c). Therefore, the target composition of the IZTO quaternary oxide can be modulated based on the data accumulated. Although the data corresponding to the number of Zn atoms gained per cycle (in Fig. 2)


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are rather scattered, this scattering is attributed to the difference in the NIn2O3:NSnO2 ratio at the same NZnO/[NIn2O3+NZnO+NSnO2] sub-cycle ratio. The difference also influences the number of Zn atoms gained per cycle. The composition of the IZTO quaternary oxide is a key factor determining the electrical properties of the grown film such as mobility and carrier concentration, and consequently, governs the device properties of the TFTs that utilize IZTO as the channel material. For the selection of the appropriate composition, bottom-gate staggered TFTs, illustrated in Fig. 3 (a), were fabricated with the IZTO films over a wide range of compositions. The device characteristics are many, and include μFE, on/off current ratio, subthreshold swing (SS), threshold voltage (Vth), and turn-on voltage (Von). Here, two representative TFT characteristics, μFE, and Von, are chosen as the screening criteria to identify appropriate compositions of the IZTO films because these parameters are sensitively dependent on the composition. A high μFE is essential to achieve a high on-current. A positive Von is required for the TFTs operating in the enhancement mode, which is desirable for simple circuit design and for minimizing power dissipation.40 Also, the positive Von implies the possibility of low off-currents in TFTs.


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Figures 3 (b) and (c) show the triangle plots of the (b) Von and (c) μFE of the TFTs with the ALD-grown IZTO films, respectively. All the thin films were annealed at 400 °C in air for 30 min. To compare the performances, the TFT characteristics of sputtered IZTO films reported in the literature are also included in Figs. 3 (d) and (e)23-28, where thesputtered IZTO films were annealed at 300–400 °C. The detailed conditions of the devices reported in the references are described in Table S1. In Fig. 3 (b), positive Von values are generally observed at Zn at.% > 60%. A similar tendency is observed in the case of the sputtered IZTO films. (Fig. 3 (d)) It is reported that the incorporation of Zn ions in IZTO thin films suppresses the formation of oxygen vacancies that generate free carriers.22 This supports the positive shift in the Von with the increasing Zn content, shown in Fig. 3 (b). Therefore, the μFE of the TFTs was mostly examined in the Zn-rich region. In Fig. 3 (c), high μFE values of ~22 cm2V-1s-1 are observed in the region where the compositions of In and Sn are similar. The μFE value decreases to below 15 cm2V-1s-1 in the region with low SnO content. In contrast, the triangle plot of the μFE taken from the sputtered films does not show a clear compositional dependence, although they generally reveal


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higher μFE (Fig. 3 (e)). This might be attributed to the differences in the experimental conditions such as the annealing atmosphere, film thickness, and deposition conditions. To further optimize the composition of IZTO, the transport properties of the TFTs with the as-grown IZTO films were examined in the composition region where they were screened based on the results of Fig. 3. Two different series of compositions— the variation in the Zn content at the fixed In:Sn atomic ratio of 42 at.%:58 at.% and the variation in the In:Sn atomic ratio at the fixed Zn content of 70 at.%. — were exploited (Fig. 4 (a)). The variation in the composition was also verified by X-ray photoelectron spectroscopy (XPS) (Fig. S3). The films examined in both series are very smooth irrespective of the composition (Fig. S4), indicating that the influences of the surface morphology on the electrical performance can be excluded. Figure 4 (b) shows the transfer curves of the IZTO TFTs at the different Zn concentrations of 47, 61, and 71 at.%. The Von shifts to positive values with increasing concentration of Zn in the films and the TFT displays a positive Von at the Zn content of 71 at.%. The Von is related to the carrier density of the films, of which origin is related to the formation of oxygen vacancies.2 The electronegativity difference between Zn and O is much larger than


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those of In-O and Sn-O. In the IZTO films with low Zn contents, therefore, the formation of a considerable number of oxygen vacancies is expected, resulting in a negative shift in the Von. The μFE slightly decreases with increasing Zn content. Figure 4 (c) shows the transfer curves of the IZTO TFTs at the Zn content of approximately 70 at.%, for which the In:Sn atomic ratio was varied. The Von values are commonly ~0 V because of the high Zn content. The increase in the In content slightly shifts the Von into the positive direction. A high μFE of ~15 cm2V-1s-1 is obtained for the IZTO TFT at the composition of In:Zn:Sn = 10:70:20. A further increase in the In content reduces the μFE of the IZTO TFTs, which is inconsistent with the previous report where the higher In is reported to enhance the μFE of IGZO TFTs.41 The electrical properties are also affected by the processing method and conditions and, therefore, the specific results obtained from this study do not necessarily correspond to the general trend. The composition of In:Zn:Sn = 10:70:20 was used for further studies. The output characteristics of all the TFTs, shown in Fig. 4, reveal a clear pinch-off in the positive drain voltage region (Fig. S5). In addition, the uniformity in the device performance of the TFTs is verified from the measurements of seven TFTs fabricated on the same substrate. (Fig. S6)


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Based on the composition screened, a post-annealing process was carried out to improve the electrical properties. Figures 5 (a)–(c) show the transfer curves of the TFTs with (a) the as-grown IZTO layer and with the films annealed at (b) 400 °C and (c) 500 °C in the air atmosphere. The device performance parameters derived from the transfer curves are summarized in Table 1. The μFE increases from 15 cm2V-1s-1 to 22 cm2V-1s-1 upon annealing at 400 °C and to 32 cm2V-1s-1 upon annealing at 500 °C. All the devices show a small clockwise hysteresis of approximately 0.6 V irrespective of the annealing temperature. None of the films were crystallized even after annealing up to 500 °C (Fig. 6 (a)). The amorphous state of the annealed film is also verified from the transmission electron microscopy (TEM) analysis. (Fig. 6 (b)) The IZTO film has a sharp interface between the IZTO and SiO2, and the diffusion of Si into the IZTO layer is not observed even after annealing at 400 °C. (Fig. S7) The root-mean-square roughness of all the films is quite low (108 is observed for the TFTs owing to the extremely low off-currents that result from the low carrier densities. The SS value decreases from 0.24 Vdec-1 for the as-grown film to 0.15 Vdec-1 for the films annealed at 400 °C and 500 °C. This is attributed to a reduction in the trap density of the films upon annealing at the high temperatures. The output characteristics of all the TFTs are typical of nchannel TFTs (Fig. S8). The drain currents are increased after the annealing at the higher temperature, although the Von shifts positively as the annealing temperature increases. This is attributed to the large enhancement in the μFE due to the annealing. The film thickness is another crucial factor in determining the electrical performance of the devices. Figure 7 (a) shows the transfer curves of the IZTO (In:Zn:Sn = 10:70:20) TFTs having different thicknesses of 5, 8, and 16 nm. All the films were annealed at 400 °C in air for 30 min. The electrical performance parameters derived from the transfer curve are summarized in Fig. 7 (b). The μFE saturates to approximately 22 cm2V-1s-1


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above 8 nm, whereas it is reduced to ~10 cm2V-1s-1 at the small thickness of 5 nm. It is known that there is a critical channel thickness to saturate the μFE.44 While the μFE is saturated above the critical thickness, the μFE decreases below the critical thickness. This is related to the thickness of the full accumulation layer formed under application of the gate bias. The Von monotonically decreases with increasing film thickness. It has been reported that the Von value is negatively shifted upon increasing the thickness of ntype oxide semiconductors.45-46 This is because the number of total electronic carriers in the oxide layer is proportional to the film thickness, and thus, a higher voltage is required to fully deplete the carriers in the entire thicker layer of the oxide semiconductor. In contrast to the thickness dependence of the μFE and Von, the SS value is almost constant at ~0.15 Vdec-1, regardless of the channel thickness. The SS value is expressed by the following equation:47 (2)


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where k, T, q, Cox, εs, Nbulk, and Nit are the Boltzmann constant, absolute temperature, electronic charge, capacitance of the gate oxide, dielectric constant of IZTO, bulk trap density of the IZTO layer, and trap density at the oxide/SiO2 interface, respectively. It should be noted that the bulk trap density is thickness-dependent, whereas the interface trap density is independent of the oxide thickness. The thickness-independence of the SS value in Fig. 7 implies that Nit is dominant over Nbulk in IZTO TFTs. Assuming the negligible Nbulk, the Nit calculated using eq. (2) is approximately 3.3 × 1011 eV-1cm-2, which is quite low compared to that reported for other oxide semiconductors.48 Such a low Nit value indicates the high quality of the interface between IZTO and SiO2. ALD has the unique characteristic of conformal growth over a complex-shaped structure, which facilitates the demonstration of emerging devices such as threedimensional TFTs beyond the planar oxide TFTs. We examined the conformality of the ALD IZTO film prior to the demonstration of VTFTs. Figure 8 (a) shows the crosssectional TEM image of the ALD IZTO layer grown on a step structure with a depth of 1.9 μm. The step structure was fabricated by using the reactive-ion etching process of a Si substrate, and a 120-nm-thick Al2O3 film was subsequently grown by ALD. Figures 8


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(b)-(d) show the magnified TEM images of the area indicated in Fig. 8 (a). The physical thickness of the IZTO film is observed to be approximately 12 nm irrespective of the position. In particular, the film thickness is uniform even on the undulating surface on the sidewall which is a feature of the deep reactive-ion etching. (Fig. 8 (c)) In addition to the thickness conformality of the film, the composition conformality is required for the implementation of the VTFT. Figure 8 (e) shows the variation in the cation composition of the ALD IZTO film at various positions indicated in Fig. 8 (a). Despite the multicomponent characteristic of the IZTO, the composition ratio is very uniform at all the positions. This conformal growth suggests that the ALD of the IZTO films shows promise for the applications of emerging devices such as the VTFT. A VTFT was demonstrated by using a 10 nm thick ALD IZTO layer, as shown in Fig. 9 (a). A step structure with a depth of 900 nm was fabricated by using the reactive-ion etching process of the p++-Si substrate (gate), and a 75-nm-thick Al2O3 film was subsequently grown by ALD with trimethylaluminum and H2O at 240 °C. The Al2O3 film was served as the gate dielectric layer. The scanning electron microscope (SEM) image (Fig. 9 (b)) shows the step structure with patterned IZTO/Al2O3 layers. Owing to the


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nature of the deep reactive-ion etching, an undulating surface on the sidewall and a rough surface on the bottom are observed. Nevertheless, the VTFT fabricated on the rough surface showed a high performance demonstrating the high quality of the IZTO film due to its conformal and uniform coverage. Figures 9 (c) and (d) show the transfer curves and the output characteristics of the VTFT, respectively, where the effective channel length and width were 4.9 and 26 μm, respectively. A high on/off current ratio of ~108 is obtained for the VTFT, and reasonably high device performance of μFE of 10 cm2V-1s-1 and SS of 0.24 Vdec-1 are obtained. This demonstrates that the ALD process has a great potential for implementing the VFT in three-dimensional emerging devices. Compared to the planar TFT shown in Fig. 5 (b), however, the μFE and SS values of the VTFT are slightly degraded. These values are dependent on the interface roughness, and the rough surface observed in Fig. 9 (b) might be the cause for the slight degradation observed in the performance.

CONCLUSION


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This study investigated the possibility of using IZTO films grown by ALD as an oxide semiconductor in high-performance TFTs. The ALD of the quaternary oxide IZTO exhibited the correlating growth characteristics of Zn-O with other oxides such as In-O and Sn-O. The adsorption of the Zn precursor was drastically enhanced on the In-O and Sn-O surfaces, while the adsorptions of the In and Sn precursors on Zn-rich surface were only slightly reduced. Based on the ease of controlling the composition of the IZTO films through the ALD process, the electrical properties of the IZTO TFTs were measured over a wide composition range to determine the optimal composition for better performance. Through systematic studies on the influences of the post-annealing step and the film thickness, high-performance TFTs with IZTO grown by ALD were realized showing a high μFE of 22 cm2V-1s-1, positive Von of 0.8 V, SS of 0.15 Vdec-1, and high on/off current ratio of >108. Furthermore, the emerging electronic device such as the VTFT was demonstrated. The excellent conformality of the ALD process allowed the facile fabrication of the VTFT, which cannot be realized by sputtering. The VTFT exhibited a reasonable device performance, with a μFE of 10 cm2V-1s-1, SS value of 0.24


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Vdec-1 and high on/off current ratio of ~108. These results demonstrate that the ALD of IZTO films offers a high potential for application in emerging oxide electronics.

EXPERIMENTAL PROCEDURES

Film growth: The quaternary oxide IZTO and the component materials were deposited via the plasma-enhanced ALD technique. Sn(dmamp)2, Me2In(EDPA), and DEZ were used as the Sn, In, and Zn precursors, respectively. An O2 plasma of power 200 W was generated by using O2 gas (the O source) at the flow rate of 200 sccm. The film was grown at 210 °C. The base pressure was 30 mTorr, and the working pressure is approximately 600 mTorr. The substrates were ultrasonically immersed in acetone, ethyl alcohol, and finally distilled water for 10 minutes each immediately prior to loading the substrate in the chamber. A sub-cycle for each constituent oxide was composed of metal precursor injection, purging, oxygen source injection, and purging steps. The composition of the films was tuned by controlling the ratio of the number of sub-cycles for each constituent oxide. For example, a super-cycle for the growth of the


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In10Zn70Sn20O thin film is composed of one ZnO sub-cycle, one In2O3 sub-cycle, and two SnO2 sub-cycles, and the ALD recipe is shown in Fig. S9.

Characterization: The atomic composition and the layer density of the In, Zn, and Sn cations in the films were obtained by WDXRF (ZSX Primus II, Rigaku). The thickness of the thin films was determined by spectroscopic ellipsometry (SE MG-1000, Nanoview), and was cross-checked by X-ray reflectivity. The crystallinity of the oxide films was analyzed by grazing incidence X-ray diffraction (ATX-G, Rigaku). The morphology of the thin films was observed by atomic force microscopy (AFM, FP-3D-SA, Asylum Research). The chemical states of the films were evaluated by XPS (PHI 5000 Versaprobe, ULVAC) using a standard X-ray source of Al Kα (1486.6 eV). The background pressure was 2.0 × 10-7 Pa for the XPS experiment. A semiconductor parameter analyzer (4155B, Keysight technology) was utilized to measure the electrical transport properties of the devices.

Device fabrication and measurements: A bottom-gate TFT structure was used in this work. Heavily doped p-type Si (

High-Performance Thin-Film Transistors of Quaternary IndiumâZinc

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