Delwar Hossain Chowdhury

Light induced instabilities in amorphous indium-gallium-zinc-oxide thin-film transistors

The effect of exposure to ultraviolet radiation on the characteristics of amorphous indium–gallium–zinc–oxide thin-film transistors (TFTs) fabricated by sputtering is investigated. After illumination with 1.5 mW cm−2 of 365 nm radiation, in the absence of any bias stress, a persistent negative shift in the characteristics is observed in the dark. The magnitude of the shift increases with exposure time, saturating after about 10 min. Under these conditions the subthreshold exhibits a rigid shift of around 3.6 V and 7.5 V for TFTs with an active layer thickness of 20 nm and 50 nm, respectively. The shift in the dark increases (decreases) when a negative (positive) bias stress is applied under illumination. The instability behavior caused by exposure to light, in the absence of any bias stress, can be explained on the basis of ionization of neutral oxygen vacancies.

Time-temperature dependence of positive gate bias stress and recovery in amorphous indium-gallium-zinc-oxide thin-film-transistors

We have analyzed the time-temperature dependence of positive bias stress (PBS) and recovery in amorphous indium-gallium-zinc-oxide (a-IGZO) Thin-film-transistors (TFTs) incorporating SiO2 back channel passivation. The data are fitted to stretched exponentials, yielding the time constant τ and stretch parameter β as fitting parameters. As-fabricated samples and samples annealed in vacuum at 250 °C 200 h are compared. The time constant for room temperature stress increases fivefold with the 200 h anneal to the value τ=1.3×106 s. The dependence of τ from stress temperature is well described by an Arrhenius plot, with activation Eτ=0.95 eV. Stress and recovery show very similar activation energies, supporting the defect formation in the bulk or at the gate insulator/a-IGZO interface as the mechanism responsible for PBS.

Light/negative bias stress instabilities in indium gallium zinc oxide thin film transistors explained by creation of a double donor

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 e(0/+) > EC-0.073 eV and a deep doubly ionized state e(++/+) < 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.

Temperature dependence of negative bias under illumination stress and recovery in amorphous indium gallium zinc oxide thin film transistors

We have investigated the temperature dependence of negative bias under illumination stress and recovery. The transfer characteristics exhibits a non-rigid shift towards negative gate voltages. For both stress and recovery, the voltage shift in deep depletion is twice that in accumulation. The results support the mechanism we previously proposed, which is creation and annealing of a double donor, likely to be an oxygen vacancy. The time dependence of stress and recovery can be fitted to stretched exponentials. Both processes are thermally activated with activation energies 1.06 eV and 1.25 eV for stress and recovery, respectively. A potential energy diagram is proposed to explain the results.

Effect of annealing time on bias stress and light-induced instabilities in amorphous indium-gallium-zinc-oxide thin-film transistors

Thermal post annealing was employed for amorphous indium–gallium–zinc-oxide (a-IGZO) thin-film transistors (TFTs) to optimize performance by a low-temperature process, in view of the envisaged role of a-IGZO in plastic electronics. The effect of annealing time at T = 250 and 280 °C on bias stress and light-induced instabilities was investigated. We find that long anneals are effective in reducing bias stress and light-induced instabilities, as well as hysteresis in the transfer characteristics. The time constant τ for positive bias stress and for negative bias stress increases after long anneal, attaining for positive bias stress the value of 1.05 × 106 s, the highest so far reported for these devices. The beneficial effect of long anneals appears to be because of a reorganization of the amorphous network, resulting in a more stable configuration.

Remarkable changes in interface O vacancy and metal-oxide bonds in amorphous indium-gallium-zinc-oxide thin-film transistors by long time annealing at 250 °C

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.

Effect of SiO 2 and SiO 2 /SiN x Passivation on the Stability of Amorphous Indium-Gallium Zinc-Oxide Thin-Film Transistors Under High Humidity

We studied the environmental stability of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with single-layer (SiO2) and bilayer (SiO2/SiNx) passivation under high-humidity (80%) storage. During the 30 days of investigation, all single-layer passivated TFTs showed negative turn-ON voltage shifts (AVON), the size of which increased with storing time. The negative A VON is attributed to donor generation inside the active a-IGZO caused by the diffusion of ambient hydrogen/water molecules passing through the SiO2 passivation layer. The X-ray photoelectron spectroscopy depth profile for the SiO2 passivated structures confirms that the concentration of oxygen vacancies, which is initially larger at the a-IGZO/SiO2 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%).

Low temperature characteristics in amorphous indium-gallium-zinc-oxide thin-film transistors down to 10 K

We report on the characteristics of amorphous indium-gallium-zinc-oxide thin-film transistors (TFTs) in the temperature range of 10–300 K. In the range of 80–300 K, the transfer characteristics are consistent with thermally activated band conduction. Below 80 K, the drain current vs. temperature behavior follows Motts law, exp(−B/T−1/4), with constant B, indicating variable range hopping. The subthreshold swing of the TFT remains unchanged in the band conduction region, but it increases rapidly with decreasing temperature below 80 K. With decreasing temperature, the hopping activation energy decreases and hopping distance increases, and are 16.8 meV and ∼11.6 nm, respectively, at 60 K.

Analysis of Improved Performance Under Negative Bias Illumination Stress of Dual Gate Driving a-IGZO TFT by TCAD Simulation

We report the numerical simulation of the effect of a dual gate (DG) TFT structure operating under dual gate driving on improving negative bias illumination stress (NBIS) of amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs). With respect to the transfer characteristics of a-IGZO TFTs, we show a larger negative threshold voltage shift (ΔVTH) with increasing a-IGZO active layer thickness. This trend is confirmed by TCAD simulation, where the initial transfer curve is plotted under varying a-IGZO thickness keeping a constant density of states. Under varying a-IGZO thickness, TCAD simulation results confirm TFTs under DG driving shows significantly less ΔVTH shift under NBIS compared with that of single gate (SG) driving TFTs. Under 10 K seconds of NBIS, TCAD simulation results show the increase in donor-like states (NGD) by 5.25 × 1017 cm-3 eV-1 and acceptor-like states (NGA) by 7.5 × 1016 cm-3 eV-1.

Improvement in performance of solution-processed indium–zinc–tin oxide thin-film transistors by UV/O3 treatment on zirconium oxide gate insulator

We studied solution-processed amorphous indium–zinc–tin oxide (a-IZTO) thin-film transistors (TFTs) with spin-coated zirconium oxide (ZrOx) as the gate insulator. The ZrOx gate insulator was used without and with UV/O3 treatment. The TFTs with an untreated ZrOx gate dielectric showed a saturation mobility (μsat) of 0.91 ± 0.29 cm2 V−1 s−1, a threshold voltage (Vth) of 0.28 ± 0.36 V, a subthreshold swing (SS) of 199 ± 37.17 mV/dec, and a current ratio (ION/IOFF) of ~107. The TFTs with a UV/O3-treated ZrOx gate insulator exhibited μsat of 2.65 ± 0.43 cm2 V−1 s−1, Vth of 0.44 ± 0.35 V, SS of 133 ± 24.81 mV/dec, and ION/IOFF of ~108. Hysteresis was 0.32 V in the untreated TFTs and was eliminated by UV/O3 treatment. Also, the leakage current decreased significantly when the IZTO TFT was coated onto a UV/O3-treated ZrOx gate insulator.