Negative Bias Illumination Stress Instability Research Articles

Comparative analysis on negative-bias-illumination-stress instabilities between planar- and vertical-channel thin-film transistors using InGaZnO active channels prepared by atomic-layer deposition

It is of the utmost importance to highlight the necessity of characterizing the operational stabilities of vertical-channel thin-film transistors (VTFTs) using In-Ga-Zn-O (IGZO) channel layers as potential back-plane devices for larger-area and higher-resolution display applications. To elucidate the structural variations in channel structures, the device characteristics of the mesa-shaped VTFT were compared with those of the conventional planar-channel TFT (PTFT). The results demonstrated that there were no significant differences in positive-bias stability between the VTFT and the PTFT. Nevertheless, under negative-bias-illumination stress (NBIS), VTFTs and PTFTs exhibited quite distinct instability behavior. The results were suggested to be attributed to the photo-ionization of oxygen vacancy (VO) initiated under illumination stress below a given wavelength. To ascertain the discrepancies in operational behaviors between the two devices, the shift in turn-on voltage (ΔVON) and the change in subthreshold swing (ΔSS) were examined with a lapse of stress time. The ΔVON values were +0.11 V and +0.18 V for the PTFT and VTFT, respectively, under a stress of −2 MV/cm for 3600 s in a dark state. In contrast, under the same conditions with a blue wavelength, they were −0.45 V and −1.81 V, respectively. A larger negative ΔVON for the VTFT was suggested to result from a larger amount of intrinsic defects, such as VO, which originated from the rugged back-channel corresponding to the vertical sidewall of the spacer pattern. The defect-related carrier (DRC) generation may result in a higher degree of VO photo-ionization for the VTFT. As a result, the VTFT showed a positive ΔSS of +120 mV/dec, in contrast to the PTFT, which showed a ΔSS close to zero. This scenario was verified by evaluating the subgap density-of-states within the atomic-layer-deposited IGZO channels for both devices using the thermally-activated electron model. The findings provide valuable insights for the accurate evaluation of the NBIS instability of the IGZO VTFT.

Robustness of Passivated ALD Zinc Tin Oxide TFTs to Aging and Bias Stress

Thin-film electronics fabricated with amorphous oxide semiconductors (AOS) are being studied for beyond-display technologies because of their superior electron transport. To widen the commercial applications, robust devices fabricated using scalable deposition techniques are required. Here, we fabricate bottom-gate, top contact thin-film transistors (TFTs) using zinc tin oxide, an AOS deposited by atomic layer deposition (ALD), and study robustness to aging and bias stress for devices stored in a dark air ambient with and without Al2O3 passivation. All samples exhibit excellent mobility ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu _{\text {FE}}$ </tex-math></inline-formula> ) and subthreshold swing (SS) robustness to aging. Most of the device aging occurs within the first 25 days, after which the performance stabilizes. Changes due to positive bias stress, which are dominated by interactions with ambient molecules, are greatly reduced by passivation. Passivation, however, results in an increase in contact resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{C}$ </tex-math></inline-formula> ), SS, and negative bias illumination stress (NBIS) instability. The increase in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{C}$ </tex-math></inline-formula> is attributed to the diffusion of oxygen vacancies that occurs during the passivation process, while the increase in SS and NBIS instability is attributed to subgap interface defects. The experimental results are compared to Silvaco ATLAS simulations to confirm the proposed physical mechanisms and allow quantification of the associated defects and non-idealities.

(Invited) Approach to Oxide Tfts with High Mobility & Stability

Thin film transistors (TFTs) with the channel of amorphous oxide semiconductors (AOSs) represented by amorphous Indium-Gallium-Zinc Oxide (a-IGZO) are now applied to drive pixels of the state-of the art display based on high mobility, good homogeneity, easy fabrication and optical transparency [1]. Recently, larger mobility TFTs are highly demanded for emerging applications such as ultra-high vision and memories. However, there is a critical obstacle to be overcome, i.e., an empirical trade-off rule in oxide TFTs between mobility and stability. Higher mobility TFTs possess less stability to various types of stress (negative bias-thermal stress instability, NBTS, positive bias-thermal stress instability, PBTS, and negative bias-illumination stress instability, NBIS). The Elucidation of origin for this trade-off rule and new approach are required to realize high mobility and high stability TFTs. We clarify that the origin of this trade-off through comparison in TFTs between IGZO with low mobility and high stability and ITZO with high mobility and low stability. A critical phenomenon was noted during the conventional TFT fabrication; CO-related impurity donates electrons to Indium-Tin-Zinc Oxide (ITZO) with high mobility under the negative bias stress condition but such CO-related impurity effect is negligible in IGZO with lower mobility than ITZO [2]. This finding may be understood by considering that large difference in ECBM determines the transfer of electron generated by adsorbed CO-related species on the back surface of ITZO [3]. Based on this model and this finding, we successfully fabricated the most stable ITZO TFT with a high mobility of 70 cm2/Vs by eliminating CO-related impurity and further passivation; NBTS (-20V, 60 oC, 3600 s, ΔVth: -0.02 V), PBTS (+20 V, 60 oC, 3600 s, ΔVth: 0.12 V) and NBIS (-20V, 15000 lx, 3600s, ΔVth: -1.33 V).[1] Hosono, H., & Kumomi,K., (ed), Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and Memory (Wiley, 2022); Hosono, H. (2018). How we made the IGZO transistor. Nature Electronics, 1(7), 428-428.[2]Shiah, Y. S., Sim, K., Ueda, S., Kim, J., & Hosono, H. (2021). Unintended Carbon-Related Impurity and Negative Bias Instability in High-Mobility Oxide TFTs. IEEE Electron Device Letters, 42(9), 1319-1322.[3] Shiah, Y. S., Sim, K., Shi, Y., Abe, K., Ueda, S., Sasase, M., Kim,J., & Hosono, H. (2021). Mobility–stability trade-off in oxide thin-film transistors. Nature Electronics, 4(11), 800-807.

On the Optimization of Performance and Reliability in a-InGaZnO Thin-Film Transistors by Versatile Light Shielding Design

This work studies the effect of the location of the light shielding (LS) layer on negative bias illumination stress (NBIS) instability in self-aligned top-gate amorphous indium-gallium-zinc oxide thin-film transistors (SA-TG a-InGaZnO TFTs). Although the NBIS instability can be mitigated by introducing a fully covered LS layer, it causes an unwanted parasitic capacitance, as evidenced by capacitance–voltage ( ${C}$ – ${V}$ ) measurements. An alternative solution, in which the device that is partially covered by the LS layer at the source side, is proposed to optimize the tradeoff. This study suggests that an LS layer could be adopted in the SA-TG configuration, as it is versatile in different structural designs, depending on the requirement of targeted applications.

Role of the third metal oxide in In–Ga–Zn–O4 amorphous oxide semiconductors: Alternatives to gallium

We study the role of the third metal oxide in In–Ga–Zn-type oxides (IGZO), Ga2O3, by comparing the calculated electronic properties of various alternatives (Al, Y, Hf, Ti, Si, and W) with Ga. It is found that Ga2O3 causes little disorder in the conduction band minimum (CBM) energy based on In or Zn oxides, and it has a large O vacancy suppression effect, which benefits both a high mobility and a low OFF current of IGZO. However, other alternatives give a pronounced conduction band disorder potential due to their higher CBM energies and thus are not ideal components in amorphous oxide semiconductors. Si and W may reduce the negative bias illumination stress instability by lowering hydrogen-induced states to below the bulk valence band maximum, but Si is not beneficial for mobility. Their role in back-end-of-line transistors is also noted.

Oxygen vacancies and hydrogen in amorphous In-Ga-Zn-O and ZnO

Hydrogen is known to be present as an impurity in amorphous oxide semiconductors at the 0.1% level. The behavior of hydrogen and oxygen vacancies in amorphous In-Ga-Zn-O is studied using density functional supercell calculations. We show that the hydrogens pair up at oxygen vacancies in the amorphous network, where they form metal-H-metal bridge bonds. These bonds are shown to create filled defect gap states lying just above the valence band edge, infrared modes at about 1400 and $1520\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, and they are shown to give a consistent mechanism to explain the negative bias illumination stress instability found in oxide semiconductors such as In-Ga-Zn-O (IGZO).

The Calculation of Negative Bias Illumination Stress-Induced Instability of Amorphous InGaZnO Thin-Film Transistors for Instability-Aware Design

We propose a systematic calculation method for negative bias illumination stress (NBIS)-induced instabilities in amorphous InGaZnO thin-film transistors (TFTs). The proposed method is based on activation energy window (AEW) in subgap energy range, and it can reproduce the NBIS time evolution of ${I}$ – ${V}$ characteristics without long-term stress test. Furthermore, it quantitatively explains the effect of oxygen content on the NBIS instability. The AEW, which is employed for emulating the oxygen vacancy ionization, peroxide formation, and hole trapping, has the order of magnitude of 1016–1017 cm−3 for the bias stress of −20 V and the commercial LED backlight of 300 cd/m2. The proposed method is expected to play such an important role in the instability awareness of amorphous oxide TFTs.

Hydrogen and the Light-Induced Bias Instability Mechanism in Amorphous Oxide Semiconductors

Hydrogen is known to be present as an impurity in amorphous oxide semiconductors at the 0.1% level. Using amorphous ZnO as a simplified model system, we show that the hydrogens pair up at oxygen vacancies in the amorphous network, where they form metal-H-metal bridge bonds. These bonds are shown to create filled defect gap states lying just above the valence band edge and they are shown to give a consistent mechanism to explain the negative bias illumination stress instability found in oxide semiconductors like In-Ga-Zn-O (IGZO).

Negative bias illumination stress instability in amorphous InGaZnO thin film transistors with ITO local conducting buried layer

The investigations on the device instabilities of amorphous InGaZnO thin film transistors (a-IGZO TFTs) with ITO local conducting buried layer (LCBL) under the source/drain region and in the middle of the active channel region have been performed under negative bias and illumination stress. From the increased drain current of a-IGZO with ITO LCBL, one can control the drive current by modulating the length of ITO LCBL without changing the ratio of channel width and length. The reason for the less degradation of a-IGZO TFTs with LCBL under negative bias stress than that of device without LCBL was explained by the fact that ITO LCBL could act to reduce the effective energy barrier and act as a hole damping layer. However, the device degradation of a-IGZO with ITO LCBL under negative bias and illumination stress was more significant than that of one without LCBL due to the electron hole pair generation in ITO layer under illumination.

Hydrogen anion and subgap states in amorphous In–Ga–Zn–O thin films for TFT applications

Hydrogen is an impurity species having an important role in the physical properties of semiconductors. Despite numerous studies, the role of hydrogen in oxide semiconductors remains an unsolved puzzle. This situation arises from insufficient information about the chemical state of the impurity hydrogen. Here, we report direct evidence for anionic hydrogens bonding to metal cations in amorphous In–Ga–Zn–O (a-IGZO) thin films for thin-film transistors (TFT) applications and discuss how the hydrogen impurities affect the electronic structure of a-IGZO. Infrared absorption spectra of self-standing a-IGZO thin films prepared by sputtering reveal the presence of hydrogen anions as a main hydrogen species (concentration is ∼1020 cm−3) along with the hydrogens in the form of the hydroxyl groups (∼1020 cm−3). Density functional theory calculations show that bonds between these hydride ions with metal centers give rise to subgap states above the top of the valence band, implying a crucial role of anionic hydrogen in the negative bias illumination stress instability commonly observed in a-IGZO TFTs.

Negative bias illumination stress instability in amorphous InGaZnO thin film transistors with transparent source and drain

The investigations on the device instabilities in amorphous InGaZnO TFTs with metal (Ni) and transparent (ITO and InGaZnO) source and drain electrodes have been performed under negative bias stress (NBS), negative bias thermal stress (NBTS), negative bias illumination stress (NBIS) and negative bias thermal and illumination stress (NBTIS). From the measured device parameters in dark and under illumination conditions, a-IGZO TFTs with InGaZnO source and drain show an excellent device performances and lower device degradation than the devices with Ni and ITO source and drain under NBS and NBTS. However, amorphous IGZO TFTs with InGaZnO source and drain electrodes show more significant device degradation under NBIS and NBTIS. In order to explain our experimental results, we propose that the center responsible for the device instability is the process-related defects under NBS and NBTS, and the oxygen vacancy under NBIS and NBTIS, respectively.

Effect of top gate bias on photocurrent and negative bias illumination stress instability in dual gate amorphous indium-gallium-zinc oxide thin-film transistor

We have studied the effect of top gate bias (VTG) on the generation of photocurrent and the decay of photocurrent for back channel etched inverted staggered dual gate structure amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film-transistors. Upon 5 min of exposure of 365 nm wavelength and 0.7 mW/cm2 intensity light with negative bottom gate bias, the maximum photocurrent increases from 3.29 to 322 pA with increasing the VTG from −15 to +15 V. By changing VTG from negative to positive, the Fermi level (EF) shifts toward conduction band edge (EC), which substantially controls the conversion of neutral vacancy to charged one (VO → VO+/VO2+ + e−/2e−), peroxide (O22−) formation or conversion of ionized interstitial (Oi2−) to neutral interstitial (Oi), thus electron concentration at conduction band. With increasing the exposure time, more carriers are generated, and thus, maximum photocurrent increases until being saturated. After negative bias illumination stress, the transfer curve shows −2.7 V shift at VTG = −15 V, which gradually decreases to −0.42 V shift at VTG = +15 V. It clearly reveals that the position of electron quasi-Fermi level controls the formation of donor defects (VO+/VO2+/O22−/Oi) and/or hole trapping in the a-IGZO /interfaces.

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.

Investigation on the negative bias illumination stress-induced instability of amorphous indium-tin-zinc-oxide thin film transistors

The quantitative analysis of mechanism on negative bias illumination stress (NBIS)-induced instability of amorphous indium-tin-zinc-oxide thin-film transistor (TFT) was suggested along with the effect of equivalent oxide thickness (EOT) of gate insulator. The analysis was implemented through combining the experimentally extracted density of subgap states and the device simulation. During NBIS, it was observed that the thicker EOT causes increase in both the shift of threshold voltage and the variation of subthreshold swing as well as the hump-like feature in a transfer curve. We found that the EOT-dependence of NBIS instability can be clearly explicated with the donor creation model, in which a larger amount of valence band tail states is transformed into either the ionized oxygen vacancy VO2+ or peroxide O22− with the increase of EOT. It was also found that the VO2+-related extrinsic factor accounts for 80%–92% of the total donor creation taking place in the valence band tail states while the rest is taken by the O22– related intrinsic factor. The ratio of extrinsic factor compared to the total donor creation also increased with the increase of EOT, which could be explained by more prominent oxygen deficiency. The key founding of our work certainly represents that the established model should be considered very effective for analyzing the instability of the post-indium-gallium-zinc-oxide (IGZO) ZnO-based compound semiconductor TFTs with the mobility, which is much higher than those of a-IGZO TFTs.

Examination of the ambient effects on the stability of amorphous indium-gallium-zinc oxide thin film transistors using a laser-glass-sealing technology

The effect of an ambient atmosphere with a positive bias constant current stress (CCS) and a negative bias illumination stress (NBIS) on the stability of amorphous In-Ga-Zn-O thin film transistors (TFTs) is examined by utilizing a glass-hermetic-sealant with a moisture permeability of less than 10−6 g/m2 · day. In the CCS test, the threshold voltage shift (ΔVth) was remarkably suppressed in the glass-sealed TFTs. The unsealed and resin-sealed TFTs exhibited large ΔVth values. During the NBIS tests, the glass-sealed TFTs had almost the same negative ΔVth as the unsealed and resin sealed TFTs. Among the different TFTs, no significant differences were observed in the threshold voltage, the subthreshold swing and the saturation mobility as a function of the photon energy. It is concluded that ambient molecules were the primary origin of the instability of the ΔVth, induced by a CCS, but they were not related to the NBIS instability. The major role of the effective passivation layers in the NBIS test was not to simply block out the ambient effects, but to reduce the extra density of states at/near the surface of the back channel.

Light induced instability mechanism in amorphous InGaZn oxide semiconductors

A model of the negative bias illumination stress instability in InGaZn oxide is presented, based on the photo-excitation of electrons from oxygen interstitials. The O interstitials are present to compensate hydrogen donors. The O interstitials are found to spontaneously form in O-rich conditions for Fermi energies at the conduction band edge, much more easily that in related oxides. The excited electrons give rise to a persistent photoconductivity due to an energy barrier to recombination. The formation energy of the O interstitials varies with their separation from the H donors, which leads to a voltage stress dependence on the compensation.

Improvement in the Photo-Bias Stability of Zinc Tin Oxide Thin-Film Transistors by Introducing a Thermal Oxidized ${\rm TiO}_{2}$ Film as a Hole Carrier Blocking Layer

This paper examined the morphological, structural, and electrical properties of thermal titanium oxide (TiOx) films as a function of the physical thickness. All the thermal TiOx films were assigned to a TiO2 chemical state irrespective of the film thickness. The thinner TiO2 films (≤ 5 nm) showed an amorphous phase, whereas the thicker TiO2 film (≥ 7 nm) had a nanocrystalline structure. This intriguing thickness-dependent crystallization behavior can be explained by the dimensional effect. The mobility of the resulting zinc tin oxide (ZTO) thin-film transistors (TFTs) with a gate-stack of silicon nitride (SiNx) and TiO2/SiNx was monotonously reduced with increasing TiO2 film thickness, which can be attributed to the enhanced Columbic scattering effect of TiO2 films. On the other hand, the negative bias illumination stress instability of the ZTO TFTs can be suppressed significantly to -2.4 V by the adoption of a 5-nm-thick TiO2 film compared with that (-14.4 V) of the ZTO device without a TiO2 film, which is discussed based on the valence band-off structure and the amorphous nature of thermal TiO2 films.

Photo-bias instability of metal oxide thin film transistors for advanced active matrix displays

Abstract

Photobias Instability of High Performance Solution Processed Amorphous Zinc Tin Oxide Transistors

The effects of the annealing temperature on the structural and chemical properties of soluble-processed zinc-tin-oxide (ZTO) films were examined by transmission electron microscopy, atomic force microscopy, high resolution X-ray reflectivity, and X-ray photoelectron spectroscopy. The density and purity of the resulting ZTO channel layer increased with increasing annealing temperature, whereas the oxygen vacancy defect density decreased. As a result, the device performance of soluble ZTO thin film transistors (TFTs) was improved at higher annealing temperature. Although the 300 °C-annealed ZTO TFT showed a marginal field-effect mobility (μFE) and high threshold voltage (Vth) of 0.1 cm(2)/(V s) and 7.3 V, respectively, the 500 °C-annealed device exhibited a reasonably high μFE, low subthreshold gate swing (SS), Vth, and Ion/off of 6.0 cm(2)/(V s), 0.28 V/decade, 0.58 V, and 4.0 × 10(7), respectively. The effects of dark negative bias stress (NBS) and negative bias illumination stress (NBIS) on the degradation of transfer characteristics of ZTO TFTs were also investigated. The instability of Vth values of the ZTO TFTs under NBS and NBIS conditions was suppressed with increasing annealing temperature. To better understand the charge trapping mechanism, the dynamics of Vth shift with NBS and NBIS time for all ZTO TFTs was analyzed on the basis of the stretched exponential relaxation. The negative Vth shift for each transistor was accelerated under NBIS conditions compared to NBS, which resulted in a higher dispersion parameter and smaller relaxation time for NBIS degradation. The relaxation time for NBS and NBIS instability increased with increasing annealing temperature, which is discussed on the basis of the transition mechanism of oxygen vacancy defects.

Composition Dependence of the Negative Bias Light Illumination Instability of Indium Zinc Oxide Transistors

This study examined the effects of the indium fraction in indium zinc oxide (IZO) on the performance and stability of IZO thin film transistors (TFTs). The field-effect mobility and sub-threshold swing were much improved with increasing In fraction; 41.0 cm2/Vs and 0.2 V/decade, respectively, at 85 at% In, compared to 1.1 cm2/Vs and 2.4 V/decade of ZnO TFTs. In contrast, a local minimum negative bias illumination stress instability was observed near 73~77 at% In. This behavior was explained by a poly-crystalline to amorphous phase transition in IZO thin films.