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Review
. 2010 Sep 10;11(4):044305.
doi: 10.1088/1468-6996/11/4/044305. eCollection 2010 Aug.

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

Toshio Kamiya  1 , Kenji Nomura  2 , Hideo Hosono  3
Affiliations
Review

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

Toshio Kamiya et al. Sci Technol Adv Mater. .

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.

Keywords: amorphous oxide semiconductor; liquid crystal display; mass production; mobility; organic light-emitting diode display; stability; thin-film transistor.

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Figures

Figure 1
Figure 1
Flexible and transparent TFT using AOS fabricated on flexible PET substrate. Good TFT performance with saturation mobility above 7 cm2 V-1 s-1 is maintained even after a bending test with a curvature radius of 30 mm.
Figure 2
Figure 2
Graphical summary of required carrier mobility for future displays [38].
Figure 3
Figure 3
Development history of prototype displays using AOS TFTs. The panel size (diagonal size) and resolution (note that ‘# of pixels’ does not count RGB pixels separately, and is simply the product of the horizontal resolution and vertical resolution) are plotted for different companies and research groups.
Figure 4
Figure 4
Photographs of some prototype displays using AOS TFTs. Refer to table 2 for the abbreviations of display resolutions.
Figure 5
Figure 5
Facilities targeting mass production of AOS FPDs. (a) Multicathode AC sputtering system, and 8G-size a-IGZO sputtering targets manufactured by (b) ULVAC and (c) Nippon Mining and Metals Co. Ltd.
Figure 6
Figure 6
Typical device structures used for AOS TFTs.
Figure 7
Figure 7
Typical output (a) and transfer (b) electrical characteristics of an a-IGZO TFT. The TFT is formed on a 150-nm-thick a-SiO2/n+-Si wafer with a top-contact structure (the dielectric constant of a-SiO2 is 3.9ε0). The TFT was annealed in air at 400 °C before forming the source/drain electrodes. The device dimensions are W/L=300/50 μm and tc=40 nm.
Figure 8
Figure 8
Analysis of characteristics of the a-IGZO TFT shown in figure 7. (a) Saturation mobility μsat, (b) field-effect mobility μFE, (c) μFE as a function of VGS and (d) S value. Note that only the data at VDS=10 V are valid for evaluating μsat in (a) and that VDS=2 V (the thick curve) should be used for μFE in (c).
Figure 9
Figure 9
Typical output (a) and transfer (b) electrical characteristics of an a-Si:H TFT. The TFT has an inverted staggered structure with a 200-nm-thick a-SiNx:H gate insulator (dielectric constant εi=7.3ε0). The device dimensions are W/L=28/6 μm and tc=200 nm.
Figure 10
Figure 10
Schematic models of subgap DOS in (a) a-Si:H and (b) a-IGZO.
Figure 11
Figure 11
(a) Subgap DOS of a-IGZO obtained by device simulations (TCAD) and the C–V method (C–V). Depletion-type and enhancement-type a-IGZO TFTs were annealed in air at 400 °C. The DOS of a-Si:H is also shown for comparison. (b) Transfer characteristics of unannealed and annealed a-IGZO TFTs, which correspond to ‘C–V, unannealed’ and ‘annealed’ in (a), respectively.
Figure 12
Figure 12
(a) Concentrations of interface (Dit, circles) and bulk defects (Nsg, squares) for unannealed and annealed a-IGZO TFTs before and after constant current stress tests, and (b) concentration ratios of bulk/surface defects, Nsgtc/Dit, as functions of channel thickness tc.
Figure 13
Figure 13
(a) Hall mobilities of InGaO3(ZnO)m as functions of electron density. (b) Illustration to explain the percolation conduction model. c-IGZO1 and c-IGZO5 represent crystalline phases with m = 1 and 5, respectively. HQ and LQ denote high-quality and low-quality, respectively, as defined in [107].
Figure 14
Figure 14
Hard x-ray photoemission spectra (HX-PES) of the (a) valence band region and (b) band-gap region. VBM is obtained by extrapolating the onset signal of the valence band spectrum as shown by the red line.
Figure 15
Figure 15
Simulated transfer curve of a-IGZO TFT. The parameters are taken from [98]. The scatter in the reverse-bias currents is due to their extremely low values.
Figure 16
Figure 16
(a) Typical response to monochromatic light of transfer characteristic of annealed a-IGZO TFT. The photon flux was fixed at ∼1×1014 photons (cm-2 s-1). The blue dashed lines correspond to illumination above the band gap (>3.1 eV) and the black solid lines correspond to subgap illumination. (b) Model to explain NBL instability.
Figure 17
Figure 17
Pseudo-band structures of (a) a-InGaZnO4 and (b) a-Si.
Figure 18
Figure 18
(a, b) Temperature dependences of (a) electron density and (b) conductivity measured using the Hall effect. The symbols show experimental data and the solid lines were calculated using the percolation conduction model. (c) Electronic structure in the conduction band of a-IGZO extracted from the data in (a, b). (d) Examination of the Meyer–Neldel rule. The circles show c-IGZO data and triangles correspond to a-IGZO.
Figure 19
Figure 19
Effects of band filling in a-IGZO [5] and a-IZO [165]; the effective masses m*e were calculated using the straight lines in the figure.
Figure 20
Figure 20
Optical spectra of various a-IGZO. HQ, LQ, as and ann denote high-quality, low-quality, as-deposited and annealed films, respectively (see [107]). (a) Real and (b) imaginary parts of dielectric function. (c) Optical absorption spectrum. [110].
Figure 21
Figure 21
Total and projected DOSs of a-IGZO calculated using a (InGaZnO4)17 cell.
Figure 22
Figure 22
Local coordination structures of some oxygen deficiencies. The small red spheres represent O ions, green spheres are Ga, gray spheres are Zn and pink spheres are In atoms. The red spheres indicated by the arrows are oxygen vacancy sites. ‘Corner-share’, ‘Free space’ and ‘Edge/Face-share’ describe the structures around the oxygen vacancy sites.
Figure 23
Figure 23
Formation energies for various defects in a-IGZO calculated by DFT.
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