Bulk-Accumulation Oxide-TFT Backplane Technology for Flexible and Rollable AMOLED Displays: Part II
In the second of a two-part series on a new backplane technology for flexible and rollable AMOLED displays, the author describes a system built on a bulk-accumulation (BA) amorphous indium-gallium-zinc-oxide (a-IGZO) TFT.
by Jin Jang
IN the first of our two-part series on a new backplane technology for flexible and rollable AMOLED displays, we reviewed a bulk-accumulation (BA) amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) with 3–5 times the drain current of a comparable conventional single-gate TFT. The advantages of BA TFTs include excellent performance from circuits such as ring oscillators and gate drivers, and also higher robustness under mechanical bending.
The TFT backplane necessary for flexible OLEDs can be realized with low-temperature polycrystalline silicon (LTPS) or oxide semiconductors because of the high performance of these materials. Currently, all AMOLED products manufactured on polyimide substrates use LTPS with excimer-laser annealing.
Another material with promise for use as TFTs on flexible substrates is amorphous oxide. For the last 10 years, a huge number of research groups have been working on amorphous-oxide-semiconductor (AOS) TFTs both on glass and plastic substrates. The first AOS TFT product was introduced in 2003, and since then many LCD and AMOLED-display products with a-IGZO TFT backplanes have been launched.
However, a-IGZO TFTs also have challenges. The low yield, non-uniformity, and bias instability of oxide TFTs limit their wide applications to commercial products. In part one of this article, we explained how our university’s research teams have made significant progress in overcoming some of those limitations by developing a bulk-accumulation TFT, which employs an n-type a-IGZO as its active material, a silicon-dioxide layer as both gate-insulator and passivation layer, and molybdenum as its metal electrodes. In the remainder of this article, part two, we will describe a flexible AMOLED display with integrated gate drivers using BA oxide TFTs that is demonstrated with a carbon-nanotube/ graphene-oxide (CNT/GO) buffer embedded in a plastic substrate.
Flexible AMOLED Display with BA Oxide-TFT Backplanes with CNT/GO Buffer via Non-Laser Detach Technology
Carrier glass is usually used for plastic AMOLED displays where the plastic polyimide (PI) is coated first and then cured before the TFT process is carried out. The schematic view of the plastic TFT backplane is shown in Fig. 1(a), where the CNT/GO buffer layer is coated first before the PI. After the whole AMOLED process, the carrier glass is separated from the AMOLED display by a detach process. The CNT/GO image can be seen in Fig. 1(b), which was taken at the back surface of the PI substrate.
The conventional method for the detach process uses excimer-laser exposure, which breaks the bonds of the PI to the carrier glass. The cost of excimer-laser exposure is high, and a non-laser detach technology offers a lower-cost alternative. We have developed a new technology that uses carbon nanotube (CNT) and graphene oxide (GO), which is coated on the carrier glass first. Then the PI process is carried out.1 The green AMOLED with an integrated gate driver is depicted using the CNT/GO buffer in Fig. 1(c).
Fig. 1: A plastic AMOLED display with gate-driver–in–panel (GIP) is depicted in several views: (a) cross-sectional view, (b) CNT/GO TEM image at the back side of PI substrate, and (c) green AMOLED image on PI substrate with pixel size of 30 um x 30 um and integrated gate driver using BA a-IGZO TFTs. An AMOLED display driven by a 30-µm-pitch integrated gate driver is shown.1,2
A very thin solution-processed CNT/GO backbone is first spin-coated on the glass to decrease adhesion of the PI to the glass; the peel strength of the PI from the glass then decreases, which eases the process of detachment after device fabrication. Given that the CNT/GO remains embedded under the PI after detachment, it minimizes wrinkling and decreases the substrate’s tensile elongation. Device performance is also stable under electrostatic-discharge exposures of up to 10 kV, as electrostatic charge can be released via the conducting CNTs.1
The TFT transfer characteristics are measured with tensile bending down to 2 mm, demonstrating that a BA TFT is robust against strain. This is due to the fast filling of the defects generated in the gap by the strain and by the induced charges in the gap by bulk accumulation. This was confirmed by technology computer-aided design (TCAD) simulation for the transfer characteristics with the same amount of generated defects by strain.
Figure 2(a) left depicts a flexible a-IGZO TFT with an inverted staggered structure fabricated with a BCE process. The samples were first fabricated on polyimide (PI) on glass substrates, then mechanically detached to yield standing-free flexible devices. The performance of the TFTs is initially checked in the flat state, and then while the TFTs are being wound around rods of varying radius (from 10 to 2 mm) to induce tensile strain in the direction parallel to the TFT channel [Fig. 2(a) right]. Figure 2(b) shows the variations in the characteristics of SG (left) and BA (right) TFTs with decreasing bending radii. The SG TFT shows an obvious negative shift of VON, increased off-state leakage currents (IOFF), and on-state currents, while no significant change in performance occurs in any of the bending radii investigated in the case of the BA TFTs. Mechanical strain is reported to result in a change in the value of the Fermi function of the semiconductor, causing an increase in the channel conductivity.4 BA devices are less affected by these small changes in carrier concentration than SG devices, due to the strong gate drive (bulk accumulation/depletion).
Figures 2(c1) through to 2(c3) show that the simulation results using TCAD (solid lines) fit well with the experimental results (symbols).3 A density-of-states model is extracted from the numerical fitting to the electrical performance of the a-IGZO TFTs. An increase in carrier concentration, which is represented herein by the increase in the donor bump, is more representative of the effect of tensile strain. The increase in ionized oxygen vacancies and, as a result, the decrease in deep neutral oxygen vacancies (acceptors) is presented by decreasing NGA. It is interesting to note that although the same density of states (DOS) profile was adopted for single-grain (SG) and BA TFTs, both in the flat and bending states, the simulated transfer characteristics of the SG TFTs shift significantly to the negative VGS direction, whereas those of BA TFTs barely shift. This confirms that BA TFTs are immune to slight changes in the DOS, whereas SG TFTs undergo substantial shift. Given the close matching between simulation and experimental results [Fig. 2 (c3)], it is reasonable to conclude that the high gate drive of BA TFTs indeed makes them immune to slight variations in the density of states that may be caused by application of tensile strain in flexible devices.
Fig. 2: The robustness of the SG and BA a-IGZO TFTs under mechanical strain are demonstrated. In 2(a), left, a schematic cross section of BA TFTs with top-gate offsets of 1 μm on each side is shown. In 2(a), right, an image of the flexible sample being rolled to a cylinder while bent to a radius of 2 mm is shown. 2(b) shows the evolution of the transfer characteristics as a function of bending radius for SG (left) and BA (right) TFTs with channel lengths of 4 μm. 2(c) shows a simulation using TCAD: (c1) a-IGZO density-of-states (DOS) model used in simulation. Simulated transfer curves (solid lines) in flat condition and under a mechanical bending radius of 2 mm for (c2) SG and (c3) BA a-IGZO TFTs. The experimental data (symbols) is also shown.3
A comparison between poly-Si and IGZO TFTs for AMOLED displays is shown in Table 1. The effective mobility of a BA TFT can be 90 cm2/V-sec because the drain current of a BA TFT can be 3–5 times that of an SG TFT. Therefore, a BA-TFT backplane can be used for high-resolution AMOLED displays for mobile applications because the display-driving ability can be similar to that of an LTPS TFT. A flexible 4-in. AMOLED display with an integrated gate driver has been demonstrated.5
The comparisons among poly-Si, IGZO, and BA IZGO TFTs are also shown in Fig. 3. In summary, BA-TFT backplanes have the advantages of manufacturing cost, flexibility, leakage current, and transparency compared with that of LTPS-TFT backplanes for display applications. These capabilities make the technology ideal for supporting future generations of flexible and rollable AMOLED displays.
Fig. 3: The schematic illustrates a comparison among LTPS, oxide, and BA-oxide TFTs for flexible-display applications. The BA-TFT backplane has the advantages of manufacturing cost, flexibility, leakage current, and transparency compared to that of LTPS TFTs.
References
1M. Mativenga, D. Geng, B. Kim, and J. Jang, “Fully transparent and rollable electronics,”ACS Appl. Mater. Interfaces 7, 1578−1585 (2015).
2D. Geng, H. M. Kim, M. Mativenga, Y. F. Chen, and J. Jang, “High Resolution Flexible AMOLED with Integrated Gate-Driver Using Bulk-Accumulation a-IGZO TFTs,” SID Symp. Digest Tech. Papers, 423–426 (2015).
3X. Li, M. M. Billah, M. Mativenga, D. Geng, Y. H. Kim, T. W. Kim, Y. G. Seol, and J. Jang, “Highly robust flexible oxide thin-film transistors by bulk accumulation,” IEEE Electron Device Lett. 36, No. 8, 811–813 (2015).
4N. Münzenrieder et al., “The effects of mechanical bending and illumination on the performance of flexible IGZO TFTs,” IEEE Trans. Electron Devices 50, No. 7, 2041–2048 (2011).
5J. G. Um, D. Geng, M. Mativenga, and J. Jang, “Bulk Accumulation Oxide TFTs for Flexible AMOLED Display with High Yield Integrated Gate Driver,” SID Symp. Digest Tech. Papers 47, No. 1, 872–875 (2016).
Jin Jang is the Director of the Advanced Display Research Center and Department of Information Display at Kyung Hee University in Seoul, South Korea. He can be reached at jjang@khu.ac.kr.