A Study on the Degradation of In-Ga–Zn-O Thin-Film Transistors Under Current Stress by Local Variations in Density of States and Trapped Charge Distribution

Abstract

Thin film transistors using In-Ga-Zn-O (IGZO) semiconductors were evaluated under current stress by applying positive voltages to the gate and drain electrodes. Initially, the transfer characteristics exhibit identical threshold voltages (V T ) when the source and drain electrodes are interchanged during measurement ( forward and reverse V DS sweep). However as stress time increases, larger shifts in V T are observed under forward V DS sweep than under reverse V DS sweep conditions. Sub-gap states analyses based on the photoresponse of capacitance-voltage (C-V) curves suggest that local annihilation of donor-like traps occurs near the drain electrode. Hump-like features are clearly observed in the C-V curves collected between the drain and gate electrodes,while they do not appear in the C-V data obtained between the source and the gate. Based on the above, a local charge trapping model is introduced in order to interpret the device degradation. In this model, the major carrier electrons are trapped more abundantly near the source electrode due to the presence of a Schottky junction between IGZO and the source/drain electrodes.

Analysis of Threshold Voltage Shift for Full VGS/VDS/Oxygen-Content Span under Positive Bias Stress in Bottom-Gate Amorphous InGaZnO Thin-Film Transistors

“…Donor-like states originate from the impact ionization of oxygen vacancies in the IGZO layer (Vo + e − → Vo 2+ + 3e − ). These positively charged states result in a negative V T shift [ 13 , 24 , 25 , 26 , 27 ]. This mechanism is dominant near the drain side because the lateral electric field is higher than the source side.…”

Electrical Performance and Reliability Improvement of Amorphous-Indium-Gallium-Zinc-Oxide Thin-Film Transistors with HfO2 Gate Dielectrics by CF4 Plasma Treatment

“…When the gate bias is applied to the TFT, the HfO 2 gate insulator will provide a strong electric field near the IGZO/HfO 2 interface to bend the Fermi level in the deep of the band gap. The energy level of the neutral oxygen vacancies (V O ), which is higher than the Fermi level, will be ionized (V O ++ ) and two electrons will be contributed to the IGZO film because the highest electron energy level in the IGZO band gap cannot reach the energy level of V O [ 10 , 18 ]. Therefore, the extra free electrons generated during the forward sweep of the gate bias will cause the negative V th shift in the reverse sweep of the gate bias.…”

“…For the a-IGZO TFT without treatment, the device suffers from a more serious negative V th shift and a S.S. degradation, as shown in Figure 6 a,b, which are mainly due to the moisture from the ambient atmosphere absorbed onto the IGZO surface. The absorbed moisture will bond to the metal element in IGZO to form metal-hydroxide (M-OH) bonds, which act as donor-like states to provide extra electrons, but also increase the density of state within the IGZO film [ 18 ]. Thus, the absorbed moisture will cause a negative V th shift and an S.S. degradation.…”

“…However, the electrical stability of the a-IGZO TFT with a high-k gate dielectric layer is one of the critical issues, especially under the positive-gate bias stress (PGBS) [ 10 ]. The threshold voltage (V th ) is abnormally shifted in the negative direction, which is opposite to the a-IGZO TFT with a SiO 2 gate insulator [ 18 , 19 ]. This is due to the electron density generated by the density of states (DOS) near the dielectric/channel layer interface or from absorbed moisture from the air.…”

P‐30: Thermally Activated and Field‐Enhanced Diffusion of Dopants in a‐InGaZnO TFTs Under Circuit Operations and Correlation to Device Stabilities

“…Long term period ~10 1 sec. static stress conditions and local VT variation inside the channel [5]. We found the physical mechanism of thermally activated and field-enhanced diffusion of dopants, which is understood as a change in RSD in terms of device degradation.…”