A Study on Electrical Characterization of IZTO Thin-Film Transistors with Transparent Conducting Oxide Electrodes

Abstract

Over the past decade, amorphous and polycrystalline silicon-based TFTs have been widely used in flat-panel displays. However, these TFTs have several critical problems such as light sensitivity, low mobility, and bias induced metastability. Recently, the necessity for flat-panel display with large size, working at high speed and low voltage, became essential. But such operating properties are not easy to achieve by modifying the conventional amorphous silicon material. Oxide semiconductors have received great attention for practical applications in high speed TFTs due to relative high electron mobility. Furthermore, the characteristics of oxide semiconductor TFTs do not degrade on exposure to visible light due to wide band gap. In addition, oxide semiconductors have amorphous phase in general, which shows uniform electric properties at large scale substrate and is strongly advantageous for the display applications. In this dissertation, some of the issues concerning fabrication and characterization of the IZTO TFTs with IZTO channel layer were explored. Also, the characterizations of transparent conducting oxide with oxide semiconductor/metal/oxide semiconductor structure were investigated to replace conventional electrodes used as source/drain, and gate First of all, Effects of Ag film thickness on the optical and the electrical properties in CuAlO2/Ag/CuAlO2 multilayer films grown on glass substrates were investigated. Atomic force microscopy images showed that Ag films with a thickness of a few nanometers had island structures. X-ray diffraction patterns showed that the phase of the CuAlO2 layer was amorphous. The resistivity of the 40 nm-CuAlO2/18 nm-Ag/40 nm-CuAlO2 multilayer films was 2.8 × 10-5 Ω•cm, and the transmittance of the multilayer films with an Ag film thickness of 8 nm was approximately 89.16%. These results indicate that CuAlO2/Ag/CuAlO2 multilayer films grown on glass substrates holds promise for potential applications as transparent conducting electrodes in high-efficiency transparent devices. Secondly, the structural, electrical and optical properties of transparent conducting oxide (TCO) films consisting of IZTO/Ag/IZTO multilayer were investigated in relation to thickness effect of IZTO and Ag films. The IZTO films were grown on glass substrates by using a radio-frequency magnetron co-sputtering of IZO and ZTO at room temperature. X-ray diffraction (XRD) measurements for the IZTO/Ag/IZTO multilayer structures grown on glass substrates showed that the IZTO layers in the multilayer structures were amorphous. The morphology of Ag film surface on IZTO film was observed by using scanning electron microscopy (SEM). The homogeneous morphologies of the Ag films inserted between the IZTO films basically influence the optical transmittance and the resistivity of the IZTO films on glass substrate. These multilayer structures also showed high transmittance in the visible spectral region. The electrical properties of the IZTO/Ag/IZTO multilayer structures grown on glass substrates are enhanced by using Ag thin interlayer and by optimum thickness of the IZTO thin film, compared to the IZTO single layer. Thirdly, the a-IZTO TFTs with dual channel layers on heavily dope p-type silicon substrate were prepared by tilted dual target RF magnetron sputtering system. To determine the device characteristics of the transistor, we measured the transfer characteristics of the fabricated IZTO TFTs with dual channel layers. The IZTO channel layer deposited at a low oxygen partial pressure has a relatively high conductivity. The Vth value shifted in the positive direction as the oxygen partial pressure increased. This is due to the carrier concentration changed. The IZTO channel layer deposited at a higher oxygen partial pressure has lower density of oxygen vacancies. The dual channel IZTO TFTs with top channel of high oxygen partial pressure and bottom channel of low oxygen partial pressure show that the operation characteristics of threshold voltage (Vth), mobility (usat), Ion/off ratio and subthreshold swing (SS) are improved by dual channel layers. Finally, the a-IZTO TFTs on transparent glass substrate were prepared by tilted dual target RF magnetron sputtering system. The transparent IZTO TFTs with IZTO/Ag/IZTO and ITO electrodes as source/drain exhibited high optical transmittance in the visible region even though Ag layer inserted in between IZTO layers. The IZTO TFTs with IZTO/Ag/IZTO as source/drain electrode showed transistor behavior with threshold voltage Vth of 2.7 V, usat of 2.6 cm2/Vs, and Ion/Ioff ratio of 3.1×106. The TFT performance varies with source/drain materials because of the contact resistance between the channel layer and source/drain electrodes. The extracted RSD of IZTO TFT with IZTO/Ag/IZTO electrodes was smaller than IZTO TFT with ITO electrodes. The contact resistance of source/drain electrodes in the IZTO TFTs with IZTO/Ag/IZTO electrodes could be reduced because homojunction is formed between the IZTO channel layer and source/drain electrodes in comparison with TFTs with ITO electrodes. |지난 수십년간 비정질 그리고 다결정 실리콘을 기반으로 하는 박막트랜지스터가 평판디스플레이 분야에서 널리 사용되어 왔다. 그러나 이러한 비정질 또는 다결정 실리콘 박막트랜지스터의 경우 빛의 민감도, 낮은 이동도 그리고 전압인가에 따른 불안정한 동작 등과 같은 치명적인 문제점을 갖고 있다. 최근 디스플레이 동향은 큰 크기와 빠른 동작속도 그리고 낮은 전압에서 구동되는 평판디스플레이를 필요로 하고 있다. 그러나 이러한 특성들은 기존의 비정질 실리콘 물질을 사용하여 얻기는 쉽지 않다. 산화물 반도체는 상대적으로 높은 이동도 때문에 고속 박막트랜지스터의 응용으로 많은 관심을 받고 있다. 더욱이 산화물 반도체 박막트랜지스터의 특성들은 넓은 밴드갭에 의해 가시광에 의한 노출로 감소하지 않는다. 또한 산화물 반도체는 비정질 상태로 큰 크기의 기판위에도 균일한 전기적 특성을 유지할 수 있어 디스플레이 분야에서 강한 장점을 갖는다. 본 연구에서는 IZTO층을 채널로 사용하는 박막트랜지스터의 제조와 특성에 관한 연구를 수행하였다. 또한 소스와 드레인으로 사용하는 기존의 물질을 대체하기 위해 산화물반도체/금속/산화물반도체 구조의 투명전극에 대한 특성에 관한 연구를 수행하였다. 2장에서는 투명전극층으로 CuAlO2/Ag/CuAlO2, IZTO/Ag/IZTO다층 구조의 전기적, 광학적 특성을 서술하였다. Ag 층이 일정한 두께를 가질 때 가시영역에서 높은 투과성과 매우 낮은 비저항값을 갖는 것을 확인하였다. 단일 CuAlO2, IZTO 층과 비교하여 최적화된 두께의 Ag층이 사용될 때 증가된 전기적 특성 결과를 얻을 수 있었다. 3장에서는 두개의 채널층을 갖는 IZTO 박막트랜지스터의 동작특성을 서술하였다. 두개의 채널층은 서로 다른 산소분압을 갖도록 구성하였으며 단일 채널층에 비해 두개의 채널층을 갖는 박막트랜지스터의 동작특성이 향상된 것을 확인할 수 있었다. 4장에서는 완전히 투명한 IZTO 박막트랜지스터를 제작하여 그것에 대한 동작특성을 기술하였다. 소스와 드레인으로는 IZTO/Ag/IZTO를 전극으로하여 박막트랜지스터를 제작하였고 충분히 기존의 ITO를 대체할 수 있는 동작특성들을 얻을 수 있었다.; Over the past decade, amorphous and polycrystalline silicon-based TFTs have been widely used in flat-panel displays. However, these TFTs have several critical problems such as light sensitivity, low mobility, and bias induced metastability. Recently, the necessity for flat-panel display with large size, working at high speed and low voltage, became essential. But such operating properties are not easy to achieve by modifying the conventional amorphous silicon material. Oxide semiconductors have received great attention for practical applications in high speed TFTs due to relative high electron mobility. Furthermore, the characteristics of oxide semiconductor TFTs do not degrade on exposure to visible light due to wide band gap. In addition, oxide semiconductors have amorphous phase in general, which shows uniform electric properties at large scale substrate and is strongly advantageous for the display applications. In this dissertation, some of the issues concerning fabrication and characterization of the IZTO TFTs with IZTO channel layer were explored. Also, the characterizations of transparent conducting oxide with oxide semiconductor/metal/oxide semiconductor structure were investigated to replace conventional electrodes used as source/drain, and gate First of all, Effects of Ag film thickness on the optical and the electrical properties in CuAlO2/Ag/CuAlO2 multilayer films grown on glass substrates were investigated. Atomic force microscopy images showed that Ag films with a thickness of a few nanometers had island structures. X-ray diffraction patterns showed that the phase of the CuAlO2 layer was amorphous. The resistivity of the 40 nm-CuAlO2/18 nm-Ag/40 nm-CuAlO2 multilayer films was 2.8 × 10-5 Ω•cm, and the transmittance of the multilayer films with an Ag film thickness of 8 nm was approximately 89.16%. These results indicate that CuAlO2/Ag/CuAlO2 multilayer films grown on glass substrates holds promise for potential applications as transparent conducting electrodes in high-efficiency transparent devices. Secondly, the structural, electrical and optical properties of transparent conducting oxide (TCO) films consisting of IZTO/Ag/IZTO multilayer were investigated in relation to thickness effect of IZTO and Ag films. The IZTO films were grown on glass substrates by using a radio-frequency magnetron co-sputtering of IZO and ZTO at room temperature. X-ray diffraction (XRD) measurements for the IZTO/Ag/IZTO multilayer structures grown on glass substrates showed that the IZTO layers in the multilayer structures were amorphous. The morphology of Ag film surface on IZTO film was observed by using scanning electron microscopy (SEM). The homogeneous morphologies of the Ag films inserted between the IZTO films basically influence the optical transmittance and the resistivity of the IZTO films on glass substrate. These multilayer structures also showed high transmittance in the visible spectral region. The electrical properties of the IZTO/Ag/IZTO multilayer structures grown on glass substrates are enhanced by using Ag thin interlayer and by optimum thickness of the IZTO thin film, compared to the IZTO single layer. Thirdly, the a-IZTO TFTs with dual channel layers on heavily dope p-type silicon substrate were prepared by tilted dual target RF magnetron sputtering system. To determine the device characteristics of the transistor, we measured the transfer characteristics of the fabricated IZTO TFTs with dual channel layers. The IZTO channel layer deposited at a low oxygen partial pressure has a relatively high conductivity. The Vth value shifted in the positive direction as the oxygen partial pressure increased. This is due to the carrier concentration changed. The IZTO channel layer deposited at a higher oxygen partial pressure has lower density of oxygen vacancies. The dual channel IZTO TFTs with top channel of high oxygen partial pressure and bottom channel of low oxygen partial pressure show that the operation characteristics of threshold voltage (Vth), mobility (usat), Ion/off ratio and subthreshold swing (SS) are improved by dual channel layers. Finally, the a-IZTO TFTs on transparent glass substrate were prepared by tilted dual target RF magnetron sputtering system. The transparent IZTO TFTs with IZTO/Ag/IZTO and ITO electrodes as source/drain exhibited high optical transmittance in the visible region even though Ag layer inserted in between IZTO layers. The IZTO TFTs with IZTO/Ag/IZTO as source/drain electrode showed transistor behavior with threshold voltage Vth of 2.7 V, usat of 2.6 cm2/Vs, and Ion/Ioff ratio of 3.1×106. The TFT performance varies with source/drain materials because of the contact resistance between the channel layer and source/drain electrodes. The extracted RSD of IZTO TFT with IZTO/Ag/IZTO electrodes was smaller than IZTO TFT with ITO electrodes. The contact resistance of source/drain electrodes in the IZTO TFTs with IZTO/Ag/IZTO electrodes could be reduced because homojunction is formed between the IZTO channel layer and source/drain electrodes in comparison with TFTs with ITO electrodes.