Advancements in thin-film transistor (TFT) technology have extended to electronics that can withstand extreme bending or even folding. Although the use of ultrathin plastic substrates has achieved considerable advancement towards this end, free-standing ultrathin plastics inevitably suffer from mechanical instability and are very difficult to handle during TFT fabrication. Here, in addition to the use of a 1.5 μm-thick polyimide (PI) substrate, a 1.5 μm-thick PI film is also deposited on top of the TFT devices to ensure that the devices are located at the neutral plane of the two PI films for high folding stability. For mechanical support during TFT fabrication up to the deposition of the top PI film, the PI substrate is spin coated on top of a carrier glass that is coated with a mixture of carbon nanotubes (CNTs) and graphene oxide (GO). The mixture of CNT and GO facilitates mechanical detachment of the neutral plane (NP) TFTs from the carrier glass before they are transferred to a polydimethylsiloxane (PDMS) substrate as islands. Being located in the neutral bending plane, the NP TFT can be transferred to the PDMS without performance degradation and exhibit excellent mechanical stability after stretching the PDMS substrate up to a 25% elastic elongation.

The analysis of current-voltage (I-V) and capacitance-voltage (C-V) characteristics for amorphous indium gallium zinc oxide Thin film transistors as a function of active layer thickness shows that negative bias under illumination stress (NBIS) is quantitatively explained by creation of a bulk double donor, with a shallow singly ionized state ε(0/+) > EC-0.073 eV and a deep doubly ionized state ε(++/+) < EC-0.3 eV. The gap density of states, extracted from the capacitance-voltage curves, shows a broad peak between EC–E = 0.3 eV and 1.0 eV, which increases in height with NBIS stress time and corresponds to the broadened transition energy between singly and doubly ionized states. We propose that the center responsible is an oxygen vacancy and that the presence of a stable singly ionized state, necessary to explain our experimental results, could be due to the defect environment provided by the amorphous network.

We studied the environmental stability of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with single-layer (SiO 2 ) and bilayer (SiO 2 /SiN x ) passivation under high-humidity (80%) storage. During the 30 days of investigation, all single-layer passivated TFTs showed negative turn-ON voltage shifts ( V ON ), the size of which increased with storing time. The negative V ON is attributed to donor generation inside the active a-IGZO caused by the diffusion of ambient hydrogen/water molecules passing through the SiO 2 passivation layer. The X-ray photoelectron spectroscopy depth profile for the SiO 2 passivated structures confirms that the concentration of oxygen vacancies, which is initially larger at the a-IGZO/SiO 2 interface, compared with the bulk a-IGZO, decreases after 30 days of storage under high humidity. This can be explained as the passivation of oxygen vacancies by diffused hydrogen. On the other hand, all bilayer passivated TFTs showed good air stability at room temperature and high humidity (80%).Index Terms-Amorphous-indium-gallium zinc oxide (a-IGZO), oxide thin-film transistors (TFT) reliability, SiO 2 and SiN x passivation layer, TFT.

A 2 in. active‐matrix light‐emitting diode (AMLED) display by integration of the micro‐LED onto the oxide thin‐film transistor (TFT) backplane using flip chip bonding is reported. A blue‐emitting micro‐LED (µ‐LED) with a size of 90 × 50 µm2 is fabricated on the GaN epi grown on a sapphire substrate. The amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) TFT on glass exhibiting the mobility of 18.4 cm2 V−1 s−1, turn‐on voltage (V ON) of 0.2 V, and subthreshold swing 0.25 V dec−1, is used for LED backplane. A two TFT and one capacitance pixel structure is utilized for driving 128 × 384 AMLED with 120 Hz frame rate. The laser lift‐off process with flip‐chip bond allows the transfer of the µ‐LED chips with 49 152 pixels onto the TFT backplane, demonstrating a 2 in. AMLED display with a good gray scale image. The current efficiency of µ‐LED is found to be 12.9 Cd A−1 at the luminance of 630 Cd m−2. Therefore, a‐IGZO TFT backplane can be used for µ‐LED displays.

We have studied the effect of long time post-fabrication annealing on negative bias illumination stress (NBIS) of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film-transistors. Annealing for 100 h at 250 °C increased the field effect mobility from 14.7 cm2/V s to 17.9 cm2/V s and reduced the NBIS instability remarkably. Using X-ray photoelectron spectroscopy, the oxygen vacancy and OH were found to exist at the interfaces of a-IGZO with top and bottom SiO2. Long time annealing helps to decrease the vacancy concentration and increase the metal-oxygen bonds at the interfaces; this leads to increase in the free carrier concentrations in a-IGZO and field-effect mobility. X-ray reflectivity measurement indicated the increment of a-IGZO film density of 5.63 g cm−3 to 5.83 g cm−3 (3.4% increase) by 100 h annealing at 250 °C. The increase in film density reveals the decrease of O vacancy concentration and reduction of weak metal-oxygen bonds in a-IGZO, which substantially helps to improve the NBIS stability.

The evolution with time of interface trap density and bulk density of states in amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs), for negative-bias-under-illumination-stress (NBIS), is traced. Based on the combined analysis of TFT current-voltage and capacitance-voltage characteristics, position of Fermi energy, flat band voltage, interface trap density, and gap state density per unit energy are investigated as function of NBIS time and applied gate voltage. These key parameters help to identify the degradation phenomena responsible for the negative threshold voltage shift caused by NBIS. In particular, the interface trap density becomes more positive; from 0.03 × 1011/cm2 to 0.65 × 1011/cm2, while the gap trap density per unit energy also increases after NBIS, supporting defect creation in the bulk and build-up of positive charge at the gate insulator/active-layer interface as the mechanism responsible for NBIS instability.

Radiating amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with deep ultraviolet light (λ = 175 nm) is found to induce rigid negative threshold-voltage shift, as well as a subthreshold hump and an increase in subthreshold-voltage slope. These changes are attributed to the photo creation and ionization of oxygen vacancy states (VO), which are confined mainly to the top surface of the a-IGZO film (backchannel). Photoionization of these states generates free electrons and the transition from the neutral to the ionized VO is accompanied by lattice relaxation, which raises the energy of the ionized VO. This and the possibility of atomic exchange with weakly bonded hydrogen leads to metastability of the ionized VO, consistent with the rigid threshold-voltage shift and increase in subthreshold-voltage slope. The hump is thus a manifestation of the highly conductive backchannel and its formation can be suppressed by reduction of the a-IGZO film thickness or application of a back bias after radiation. These results support photo creation and ionization of VO as the main cause of light instability in a-IGZO TFTs and provide some insights on how to minimize the effect.

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