General Background of Metal-Oxide-Based Thin-Film Transistors

Introduction

1.1 General Background of Metal-Oxide-Based Thin-Film Transistors

With fast growth of consumer electronic market, the displays with light weight, slim, high resolution, low power consumption and high electrical performance have been substantially implemented to various commercial electronic products. At present, thin-film transistors (TFTs) device technologies used in flat-panel displays, such as liquid crystal display (LCD) and organic light emission displays (OLEDs), are mainly amorphous silicon (α-Si) TFTs and low-temperature poly silicon (LTPS) ones.

However, the field-effect mobility (µFE) of α-Si TFTs is only 0.1~1 cm²·V⁻¹·s⁻¹ and can’t be applied to high speed logic circuitries. Although the LTPS TFTs have a much higher mobility up to 100 cm²·V⁻¹·s⁻¹, the higher fabrication cost as well as the issues associated with the grain boundaries contained in the channel layer limit their application to small-area version.

In recent years, many groups in the world start to explore alternative TFT technologies which can be manufactured at low temperatures and provide intriguing properties such as higher aperture ratio and high mobility. Metal-oxide semiconductor material is one of the possible candidates of channel material to solve the problems mentioned above in the silicon-based TFTs. Table 1.1 compares the properties of silicon-based TFTs with metal-oxide ones. Actually the metal-oxide TFTs have

2

attracted a lot of attentions in the fields of active-matrix liquid crystal display (AMLCD), e-paper, touch-screen technology, and flexible electronics [1-3], because they are transparent and can be fabricated at low temperatures. In 2003, Hoffman, a professor at Oregon State University, had demonstrated the first zinc oxide (ZnO) transparent TFTs (TTFTs) on a glass substrate [4] with excellent on-off ratio of 10⁷ and µFE of around 2.5 cm²·V⁻¹·s⁻¹. The transmittance in the visible light wavelength (400 nm~700 nm) is about 75%. In the next year, Hosono’s group [5] showed an amorphous indium-gallium-zinc oxide (α-IGZO) film deposited on a polyethylene terephthalate substrate at room temperature exhibiting a Hall mobility exceeding 10 cm²·V⁻¹·s⁻¹. Such a value is an order larger than that of hydrogenated α-Si devices and is stable under repetitive bending test of the plastic substrate. Moreover, the high transmittance rate can effectively increase the aperture ratio of the device in a pixel.

ZnO and α-IGZO both have wide band gap (above 3eV) [4, 6], therefore it would not have high photo-induced leakage current as visible lights irradiate the panel.

To develop TTFTs, we need to find suitable materials from the viewpoints of chemical bonding and electrical structure so that the devices can operate with better performance, such as good on-off current ratio (10⁶) and µFE (~10 cm²·V⁻¹·s⁻¹) [7-8].

In 1996, Hosono et al. had proposed a hypothesis about how to choose the channel materials of TTFTs. The hypothesis predicts that the preferred metal-oxide film should be composed of heavy metal cations (HMCs) with an electron configuration (n-1)d¹⁰ns⁰ (n≧4) [1-3]. From Fig. 1.1, it shows that the possible elements which can

use in metal-oxide film. Some elements are poisonous for human body, for example mercury (Hg), arsenic (As) and cadmium (Cd). Indium (In) is rare in the earth crust.

Therefore, how to choose the suitable element from the HMCs is essential to the development of metal oxide TTFTs.

For most wide bandgap semiconductors, the valence band is formed by the 2p

orbits of occupied oxygen. However, the bottom of conduction band is mainly constructed by the orbit of unoccupied metal cations. The unoccupied s orbit of HMCs is isotropic; if the crystal structure allows the HMCs to be close to each other and generate sufficient orbit overlap, the mobile paths of electrons can be formed. Fig.

1.2 shows that the carrier transport difference in crystalline and amorphous semiconductor. In covalent semiconductors such as α-Si the transport paths are composed of strongly directive sp³orbitals, therefore structure randomness would greatly affect the orbital overlap and degrade the carrier mobility. In contrast, amorphous metal-oxide semiconductors (AMOS) are composed of post transition HMCs which provides spherical s orbital. The overlap of HMCs’ orbitals and the oxygen’s 2p orbitals is rather large, and is not significantly affected even in an amorphous state [5]. As a result, AMOS has higher mobility.

ZnO [4, 9-12], α-IGZO [5, 7-8], aluminum-zinc oxide (AZO) [13-15], and indium-zinc oxide (IZO) [8, 16] are the materials that have been widely studied in recent years. ZnO and AZO films are polycrystalline, the film uniformity and controllability of the device characteristics are the issues that need to be concerned.

General requirements for TTFTs include (1) good stability and uniformity, (2) low manufacture temperature, (3) excellent device characteristics, (4) low carrier concentration (10¹⁵ to 10²⁰cm^-3) in the channel for the purposes of suppressing the off-current and control of threshold voltage (Vth) [8]. From literature survey, we choose α-IGZO TFTs as the major subject to study in this work. The reasons are following:

First of all, we could control the carrier concentration of the deposited α-IGZO film. A higher indium (In) content is expected to enhance µFE and promote the on current by a significant increase in the carrier concentration [17]. The gallium

4

than In-O bonding and is effective in suppressing the formation of oxygen vacancy [17-18]. Thus, an appropriate addition of Ga is an effective way to attain lower off current and carrier concentration. The zinc (Zn) contributes to the reduction of shallow tail states [19] below the conduction band and interface states between gate oxide and channel, thus the sub-threshold swing (SS) would be reduced. Secondly, the α-IGZO films have the potential for better TFTs performance and stability than polycrystalline films because the films are free from grain boundaries in the channel.

However, if the Zn composition of an α-IGZO film is too high, it tends to change the film texture from amorphous to polycrystalline which would draw some negative effects on device properties due to the existence of grain boundaries [18]. Finally, the higher µFE for α-IGZO films deposited at or slightly above room temperature (RT) is another major merit.