Abnormal positive bias stress instability of In–Ga–Zn–O thin-film transistors with low-temperature Al2O3 gate dielectric

Abstract

Low-temperature atomic layer deposition (ALD) was employed to deposit Al 2 O 3 as a gate dielectric in amorphous In-Ga-Zn-O thin-film transistors fabricated at temperatures below 120 C. The devices exhibited a negligible threshold voltage shift (DV T) during negative bias stress, but a more pronounced DV T under positive bias stress with a characteristic turnaround behavior from a positive DV T to a negative DV T. This abnormal positive bias instability is explained using a two-process model, including both electron trapping and hydrogen release and migration. Electron trapping induces the initial positive DV T , which can be fitted using the stretched exponential function. The breakage of residual AlO-H bonds in low-temperature ALD Al 2 O 3 is triggered by the energetic channel electrons. The hydrogen atoms then diffuse toward the In-Ga-Zn-O channel and induce the negative DV T through electron doping with power-law time dependence. A rapid partial recovery of the negative DV T after stress is also observed during relaxation. V

Effect of Simultaneous Mechanical and Electrical Stress on the Electrical Performance of Flexible In-Ga-Zn-O Thin-Film Transistors

“…However, it increases more pronouncely after PBS in the IGZO TFT under mechanical bending stress than that at the flat state. In previous reports, the negative shift of V TH after a long stress time was mainly attributed to the hydrogen diffusion from a low-temperature ALD Al 2 O 3 gate dielectric into an IGZO layer [28]. As the hydrogen acts as an effective donor in IGZO, it generates the donor states near E C and shifts the V TH of the IGZO TFT in the negative direction.…”

“…V TH shifts in the positive direction during the initial 500 s; but shifts in the negative direction after 500 s of stress. In previous works, the PBS-induced V TH turnaround behavior was already observed in IGZO TFTs with a low-temperature ALD Al 2 O 3 gate dielectric and was mainly ascribed to the effect of electron trapping and hydrogen release and diffusion [28]. The positive shift of V TH at the initial stage of PBS was explained by the electron trapping in traps at the Al 2 O 3 /IGZO interface or bulk Al 2 O 3 , and the negative shift of V TH after a long stress time was mainly attributed to the hydrogen diffusion from a low-temperature ALD Al 2 O 3 gate dielectric into an IGZO layer.…”

“…The positive shift of V TH at the initial stage of PBS was explained by the electron trapping in traps at the Al 2 O 3 /IGZO interface or bulk Al 2 O 3 , and the negative shift of V TH after a long stress time was mainly attributed to the hydrogen diffusion from a low-temperature ALD Al 2 O 3 gate dielectric into an IGZO layer. The hydrogen atom was assumed to be generated from the breakage of residual AlO-H bonds in the ALD Al 2 O 3 by the energetic electrons in the TFT channel during PBS application [28]. When Al 2 O 3 is deposited using ALD at low temperatures, the chemical reaction between AlO-H and TMA is less complete, which causes considerable AlO-H residues in Al 2 O 3 [29].…”

Tailoring IGZO Composition for Enhanced Fully Solution-Based Thin Film Transistors

“…The abnormal shift in V T when applying PBS is associated to the hydrogen release from residual AlO-H bonds in the AlO x gate dielectric and their migration to the IGZO channel. By diffusing the hydrogen atoms in the channel, a negative Δ V T is induced through electron doping power-law time dependence [42]. Initially V T shifts abruptly however after 30 min the devices stabilize with maximum Δ V T = −0.22 V after 1 h of PBS.…”

Threshold-Variation-Tolerant Coupling-Gate α-IGZO Synaptic Transistor for More Reliably Controllable Hardware Neuromorphic System

“…Thus, in the proposed device, vertical gate voltage has the controllability over the hydrogen ions which are weakly bound to AlO and easy to break the bond and move as shown in Fig. 4(a) [22]. This operation provides the modulation of channel conductance which can be inferences as synaptic weight, by which the biological analogy to change in strength of synaptic connectivity schematically shown in Fig.…”