Present status of amorphous In–Ga–Zn–O thin-film transistors

Abstract

The present status and recent research results on amorphous oxide semiconductors (AOSs) and their thin-film transistors (TFTs) are reviewed. AOSs represented by amorphous In-Ga-Zn-O (a-IGZO) are expected to be the channel material of TFTs in next-generation flat-panel displays because a-IGZO TFTs satisfy almost all the requirements for organic light-emitting-diode displays, large and fast liquid crystal and three-dimensional (3D) displays, which cannot be satisfied using conventional silicon and organic TFTs. The major insights of this review are summarized as follows. (i) Most device issues, such as uniformity, long-term stability against bias stress and TFT performance, are solved for a-IGZO TFTs. (ii) A sixth-generation (6G) process is demonstrated for 32″ and 37″ displays. (iii) An 8G sputtering apparatus and a sputtering target have been developed. (iv) The important effect of deep subgap states on illumination instability is revealed. (v) Illumination instability under negative bias has been intensively studied, and some mechanisms are proposed. (vi) Degradation mechanisms are classified into back-channel effects, the creation of traps at an interface and in the gate insulator, and the creation of donor states in annealed a-IGZO TFTs by the Joule heating; the creation of bulk defects should also be considered in the case of unannealed a-IGZO TFTs. (vii) Dense passivation layers improve the stability and photoresponse and are necessary for practical applications. (viii) Sufficient knowledge of electronic structures and electron transport in a-IGZO has been accumulated to construct device simulation models.

Structural-relaxation-driven electron doping of amorphous oxide semiconductors by increasing the concentration of oxygen vacancies in shallow-donor states

“…[1][2][3][4][5] A notable characteristic of amorphous oxides, relative to their crystalline counterparts, is that these materials contain structural disorder-related defects (for example, free volume) in addition to non-stoichiometric defects (for example, oxygen vacancies), which significantly affect the corresponding electrical properties. 3,[6][7][8] Moreover, the degree of structural disorder in amorphous metal oxides is always likely to decrease to form more stable structures because internal atomic rearrangement occurs even below the glass transition temperature (T g ). This process is known as structural relaxation (SR) [9][10][11] and results in continuous changes in the electrical properties.…”

“…12,13 These results indicate that oxygen vacancies (V O s), which are inherent chemical defects, act as the dominant electron donors. Interestingly, the V O s can act as deep donors 8 (that is, limited donation of free electrons), electron traps 3,6 and shallow donors, whose electronic states are determined by the local atomic environment. Certain researchers have suggested that all V O s in crystalline IGZO are deep donors; however, certain V O s act as shallow donors in a-IGZO when the structural disorder causes weak interactions between the metal ions and V O s (that is, when the bonding distance between a metal ion and a V O increases, called outward relaxation).…”

Undercoordinated indium as an intrinsic electron-trap center in amorphous InGaZnO4

“…Indium (In)-based amorphous oxide semiconductors are considered as a promising material for next-generation thin-film electronics and optoelectronics because they have high electron mobility, transparency, flexibility and uniformity. [29][30][31][32][33] However, the success of these applications has been limited by the lack of stability in their electrical properties owing to charge trapping.Investigation of the charge-trapping defects on the atomic scale is an essential prerequisite to overcome the instability issue of the indiumbased amorphous oxide semiconductors. An oxygen-vacancy (V O ) defect has been suggested as a metastable hole-trap center.…”

Overview of electroceramic materials for oxide semiconductor thin film transistors

“…), high frame rate (>240 Hz), and three-dimensional (3D) displays. In order to realize such highend products such as 3D televisions that do not require specialized glasses, using either active matrix liquid crystal displays (AMLCDs) or active-matrix, organic light-emittingdiode (AMOLED) panels, high-performance thin film transistor (TFT) devices acting as switching or driving elements are [12,150]. For example, for large AMOLED TVs, the mobility requirement is generally predicted to be over 30 cm 2 /V · s (depending on display resolution and pixel-circuit designs) because OLED pixels need high current in order to emit light through current injection.…”

“…They have since been taking the place of a-Si and LTPS, given several key qualities, including high electrical mobility, amorphicity, and high optical transparency. Moreover, AOSs have a lower density-of-states (DOS), including tail and deep level states, compared to that of a-Si, resulting in superior electrical stability [12].…”