Design of InZnSnO Semiconductor Alloys Synthesized by Supercycle

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Functional Inorganic Materials and Devices

Design of InZnSnO Semiconductor Alloys Synthesized by Supercycle Atomic Layer Deposition and Their Rollable Applications Jiazhen Sheng, TaeHyun Hong, DongHee Kang, Yeonjin Yi, Jun Hyung Lim, and Jin-Seong Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02999 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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ACS Applied Materials & Interfaces

Design of InZnSnO Semiconductor Alloys Synthesized by Supercycle Atomic Layer Deposition and Their Rollable Applications Jiazhen Sheng1, TaeHyun Hong1, DongHee Kang2, Yeonjin Yi2, Jun Hyung Lim*,3, and Jin-Seong Park*,1 1Division

of Materials Science and Engineering, Hanyang University, 222, Wangsimni-ro,

Seongdong-gu, Seoul, 04763, Republic of Korea 2Department

of Physics and VdW Materials Research Center, Yonsei University, Seoul, 03722,

Republic of Korea 3R&D

Center, Samsung Display, Yongin 17113, Republic of Korea

ABSTRACT

Amorphous InGaZnO semiconductors have been rapidly developed as active chargetransport materials in thin film transistors (TFTs) because of their cost-effectiveness, flexibility, and homogeneous characteristics for large-area applications. Recently, InZnSnO (IZTO) with superior mobility (higher than 20 cm2 V−1 s−1) has been suggested as a promising oxide semiconductor material for high-resolution, large-area displays. However, the electrical and

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physical characteristics of IZTO have not been fully characterized. In this study thin IZTO films were grown using a novel atomic layer deposition (ALD) super-cycle process consisting of alternating subcycles of single oxide deposition. By varying the number of deposition subcycles, it was determined that the insertion of a Sn–O cycle improved the mobility and reliability of IZTObased TFTs. Specifically, the IZTO TFT obtained using one In–O cycle, one Zn–O cycle, and one Sn–O exhibited the best performance (saturation mobility: 27.8 cm2 V−1 s−1, and threshold voltage shift: 1.8 V after applying a positive bias temperature stress conditions). Next, the production of rollable and flexible devices was demonstrated by fabricating ALD-processed IZTO TFTs on polymer substrates. The electrical characteristics of these TFTs were retained without drastic degradation for 240,000 bending cycles. These results indicate the supercycle ALD technique is effective for synthesizing multicomponent oxide TFTs for electronic applications requiring high mobility and mechanical flexibility.

Keywords: n-type oxide semiconductor, amorphous semiconductor, thin film transistor, band structure, flexible


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INTRODUCTION Amorphous oxide semiconductors (AOS) have attracted increasing attention as active chargetransport materials in numerous electronic devices because they are cost-effective, scalable, flexible, and meet the large-area application requirements of displays and sensors.1–4 In particular, amorphous InGaZnO (IGZO) has been widely used as a channel material in thin film transistors (TFTs). However, the limited mobility of this material (approximately 10 cm2 V−1 s−1) is insufficient for high-response-speed and high-resolution displays.5,6 To improve the electrical properties of IGZO TFT, combinatorial IGZO compositions have been firstly studied to achieve better field-effect mobility (>10 cm2/V.sec) and to apply for backchannel-etch (BCE) structure and wet-etch selectivity (IGZO active layer vs. metal interconnect layer).7-8 After all, many researchers have focused on designing new active materials based on a combination of metal cations.9 The concept for these alloy designs is to select cations with large ionic radii and spherically symmetric 4s, 5s, or 6s electron orbitals to increase the degree of wavefunction overlap as well as electron delocalization, and finally enhance electron mobility.10 As an example, in Al-doped zinc tin oxide (ZnSnOx), the larger ns0 (n ≥ 4) orbitals of the heavy-metal cations, Sn4+ and Zn2+, increase the degree of overlap and conduction band dispersion. Al enhances stability11-14 because its binding energy with oxygen (511.0 eV) is higher than Zn-O (250eV)15. Various similar alloy designs (Sn–ZnO,16 Hf–ZnSnO,17 In–ZnO,18 Mg–InZnO,19 Hf–InZnO,20 AlSn-InZnO,21 etc.) have been reported. In the case of Hf or Mg doping in other alloy systems, these dopants always act as a carrier suppressor, which enhances reliability, but also degrades the transfer performance of the device.17,19,20 On the other hand, Sn doping16,21 seems to have the potential to preserve device transfer performance while simultaneously enhancing device reliability. Incorporating Sn as a


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dopant in InZnO alloy systems is recognized as an effective and simple method to improve the reliability and mobility of devices because the increased number of carrier electrons and higher electronegativity of Sn can simultaneously provide more carriers and suppress the formation of oxygen deficiencies.22,23 Accordingly, IZTO has potential value because of its relatively high field effect mobility (≥20 cm2 V−1 s−1) and superior reliability.24,25 However, its physical and chemical properties have not been systematically studied to clarify the relationship between device performance and electronic structure. Since the early studies on oxide semiconductors, sputtering has been the most widely used technique for oxide semiconductor deposition because it easily allows the introduction of various additional coating materials via combinatorial sputtering. However, when using a single composition target with this process, it is difficult to accurately control the metal–cation ratios of the as-grown films. To overcome this weakness, various deposition processes, such as chemical vapor deposition (CVD), sol-gel methods, and ALD, have been introduced. Among them, ALD allows accurate control of the film composition as well as thickness, and it can also grow large area, extra thin films with excellent uniformity, compared to the conventional sputtering technique.26,27 In addition, ALD’s wide processing temperature range is advantageous for fabricating devices on polymer substrates.18

It is generally well known that the ALD technique

is suitable for synthesizing binary or ternary oxide systems such as zinc tin oxide,28 indium zinc oxide (IZO),18 and indium gallium oxide29 because it provides accurate control of the chemical composition. In this study, we explored the applicability of novel ALD-processed IZTO TFTs and evaluated the effect of the metal cation ratio on the chemical and electronic structure of films, and the electrical properties of the TFTs, by varying the number of Sn–O deposition subcycles.


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Additionally, to examine the potential of producing flexible devices, ALD-processed IZTO TFTs with high mobility (~25 cm2 V−1 s−1) were fabricated on a polyimide (PI) substrate, and relevant rolling tests were performed.

Experimental ALD Deposition of IZTO Films: IZO and IZTO thin films were deposited using ALD (NCDD100) at 180 °C, using [1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (InCA-1) as the indium precursor, DEZ (Diethylzinc) as the zinc precursor, tetrakis-dimethyl-amine-tin (TDMASn) as the tin precursor, and hydrogen peroxide (30% H2O2 in H2O; Sigma-Aldrich) as the reactant on a Si(100) substrate with a native oxide layer after acetone washing. The indium precursor was maintained at 40 °C to obtain a sufficient pressure and dose amount, as previously reported.28 InOx and ZnO were deposited one cycle at a time in an alternating sequence to obtain the IZO thin film, whereas [TDMASn – H2O2] cycles (1 or 2 cycles) were performed immediately following the [InCA-1 – H2O2 – DEZ – H2O2] sub-cycle to grow the IZTO(111) and IZTO(112) thin films. Chemical and Physical Characterization of IZTO Films: Auger electron spectroscopy (AES) (PHI 700Xi) and spectroscopic ellipsometry (SE) (Elli-SE(UV)-FM8) were conducted to analyze the thickness and chemical composition of the thin ﬁlms. High resolution transmission electron microscopy (HRTEM) (JEM2100F, JEOL) was also used to analyze the microstructure of the films. To obtain the elemental maps, we used an energy dispersive spectrometer (EDS) attached to the TEM while operating in the STEM mode (the nominal electron beam size was 1 nm). Individual IZO(11), IZTO(111), and IZTO(112) samples deposited on glass substrates were used to analyze the samples’ optical properties using UV-vis spectrometry. The optical transmittance


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of each sample was investigated over a wavelength range of 400–900 nm, after subtracting the background originating from the glass substrate. X-ray photoelectron spectroscopy (XPS) (VersaProbe II) was performed using a monochromatic Al Kα (1486.6 eV) source with a pass energy of 20 eV. Ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) measurements were conducted using a SPECS PHOIBOS 150 hemispherical analyzer and a PSP photon detector. For UPS measurements, an ultraviolet discharge lamp was used as the excitation source (He І, 21.22 eV) and a sample bias of −10 V was applied to obtain the secondary electron cutoff (SEC) in the normal emission geometry. For IPES measurements, a low-energy electron gun with a BaO cathode and a band pass filter of 9.5 eV were used in the isochromatic mode. The IPES measurements were carefully performed while monitoring spectral changes to avoid any significant changes originating from sample damage by the electron gun. No spectral changes were observed during 10 scans. Both spectrometers were calibrated with respect to the Fermi step of a cleaned Au reference, and the spectral broadness of the UPS and IPES measurements was 0.1 and 0.5 eV, respectively. The surface morphologies of the ﬁlms were observed by atomic force microscopy (AFM) (XE-100). Fabrication of ALD-Processed TFTs and Analysis of Electrical Properties: IZTO TFTs with a top contact bottom gate structure were fabricated on p++ Si substrates with thermally oxidized SiO2 (100 nm). The 20nm IZO(11), IZTO(111), and IZTO(112) channels were grown at 180 °C and patterned using a combination of wet-etching (MA-SO2, Dongwoo Fine-Chem) and photolithography processes. Sputtering process deposited indium tin oxide (ITO, 100 nm) source and drain electrodes were patterned by a lift-off method. The channel width (W) and length (L) dimensions were 40 and 20 μm, respectively. The devices were post annealed at 350 °C in the air ambient for 1 h. The electrical properties of the devices were measured using an HP 4155A


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parameter analyzer at room temperature. The stability property of the devices was measured under PBTS conditions by applying a VGS of 20 V at 60 °C during 3600s. Flexible bottom-gate, top-contact IZTO (111) and (112) TFTs were fabricated on PI substrates. The thickness of the ITO gate layer was 100nm, which was deposited using the sputtering method at room temperature. The thickness of the AlOx gate-insulator layer was 100nm, and was deposited by ALD at 200 °C using trimethylaluminum (TMA) and H2O. Both layers were patterned by photolithography and a wet-etching process using nitric acid. The rolling test was conducted via a commercialized bending machine (Yuasa System) with a rolling radius of 1.5 mm and a number of cycles.

RESULTS AND DISCUSSION 1. Deposition of Sn-Doped IZO Thin Films and Relative Electronic Structures In this study, we synthesized multicomponent oxide films using supercycle ALD, which consists of alternating subcycles of single metal oxide depositions. The supercycle deposition conditions were optimized by evaluating the film growth rate during each single metal oxide cycle (data not shown). To analyze the microstructures of the as-deposited films, HRTEM was conducted with energy-dispersive EDS mapping, as shown in Figure 1. Films with uniform thicknesses of 40 nm were grown by the ALD process using sequential alternating deposition at a chamber temperature of 180 °C. We designed alloys with various chemical compositions, namely, thin films of InZnO (one In–O cycle and one Zn–O cycle), InZnSnO (one In–O cycle, one Zn–O cycle, and one Sn–O cycle), and InZnSnO (one In–O cycle, one Zn–O cycle, and two Sn–O cycles) (hereafter, denoted as IZO(11), IZTO(111), and IZTO(112), respectively). Long-range ordering of the lattice was not


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observed in the cross-sectional TEM images of any of the films, which was consistent with the selected area electron diffraction patterns (insets of Figure 1a–c). Figure 1d shows a representative image of the as-deposited IZTO(111) film with homogeneous elemental distributions of In, Zn, Sn, and O, as revealed by EDS mapping. In our previous reports, InOx (cubic)31 and ZnO(hexagonal),30 which were produced by ALD using the same precursors, crystallized at the growth temperature (

Design of InZnSnO Semiconductor Alloys Synthesized by Supercycle

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