Negative Bias Illumination Stress Research Articles

Non-Traditional Response to Intensive NBS and NBIS Treatments on IGZO TFTs

This work provides an interpretation of defect and trap state behavior following the application of intensive negative-bias stress (NBS) and negative-bias illumination stress (NBIS) on IGZO TFTs. Staggered bottom-gate TFTs were fabricated with a 50 nm IGZO film sandwiched between a 50 nm gate oxide and a 50 nm passivation oxide layer. The working metal was molybdenum for the gate electrode and source/drain contacts. A passivation anneal was done and an alumina capping layer was deposited via ALD to promote electrical stability; full process details are provided in a previous report. Devices were tested with a Keysight B1500 parameter analyzer to establish initial device characteristics, with +20 V PBS and -10 V NBS stress applications for 20 ks having a negligible response. Intensive stress testing was then done for 10 ks at either high bias (-20 V NBS) or with the addition of illumination (-10 V NBIS at 410 nm). The responses to intensive NBS and NBIS applications were both non-traditional and distinctive.

Sputtered AlOx interlayer for improving performance of a-InGaZnO TFTs: A study on hydrogen diffusion mitigation and electron modulation

In this study, we developed an a-InGaZnO (a-IGZO) thin film transistor (TFT) structure that inserts a sputtered AlOx intermediate layer (IL) within the active layer. The IL not only effectively blocks hydrogen (H) diffusion from the gate insulation (GI) layer to the upper region of a-IGZO but also modifies the energy band structure of the bottom channel region and creates a locally low electron concentration that counteracts the excess electron donated by diffused H. Compared to conventional TFTs, the TFT with the IL exhibits impressive electrical characteristics, including a high saturation mobility (μsat) of 14.5 cm2 V−1 s−1, an on/off current ratio (Ion/Ioff) of 6.2 × 108, and a low subthreshold swing (SS) of 0.16 V/dec. Furthermore, this structure exhibits remarkable stability under negative bias stress and negative bias illumination stress, with ΔVth values of 1.1 and 1.5 V, respectively. The integration of the IL provides a promising approach for enhancing the performance of a-IGZO TFTs, paving the way for next-generation display technologies.

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.

Dependence of a Hydrogen Buffer Layer on the Properties of Top-Gate IGZO TFT.

In this paper, the effect of a buffer layer created using different hydrogen-containing ratios of reactive gas on the electrical properties of a top-gate In-Ga-Zn-O thin-film transistor was thoroughly investigated. The interface roughness between the buffer layer and active layer was characterized using atomic force microscopy and X-ray reflection. The results obtained using Fourier transform infrared spectroscopy show that the hydrogen content of the buffer layer increases with the increase in the hydrogen content of the reaction gas. With the increase in the hydrogen-containing materials in the reactive gas, field effect mobility and negative bias illumination stress stability improve nearly twofold. The reasons for these results are explained using technical computer-aided design simulations.

Effects of Laser Treatment of Terbium-Doped Indium Oxide Thin Films and Transistors.

In this study, a KrF excimer laser with a high-absorption coefficient in metal oxide films and a wavelength of 248 nm was selected for the post-processing of a film and metal oxide thin film transistor (MOTFT). Due to the poor negative bias illumination stress (NBIS) stability of indium gallium zinc oxide thin film transistor (IGZO-TFT) devices, terbium-doped Tb:In2O3 material was selected as the target of this study. The XPS test revealed the presence of both Tb3+ and Tb4+ ions in the Tb:In2O3 film. It was hypothesized that the peak of the laser thermal effect was reduced and the action time was prolonged by the f-f jump of Tb3+ ions and the C-T jump of Tb4+ ions during the laser treatment. Studies related to the treatment of Tb:In2O3 films with different laser energy densities have been carried out. It is shown that as the laser energy density increases, the film density increases, the thickness decreases, the carrier concentration increases, and the optical band gap widens. Terbium has a low electronegativity (1.1 eV) and a high Tb-O dissociation energy (707 kJ/mol), which brings about a large lattice distortion. The Tb:In2O3 films did not show significant crystallization even under laser energy density treatment of up to 250 mJ/cm2. Compared with pure In2O3-TFT, the doping of Tb ions effectively reduces the off-state current (1.16 × 10-11 A vs. 1.66 × 10-12 A), improves the switching current ratio (1.63 × 106 vs. 1.34 × 107) and improves the NBIS stability (ΔVON = -10.4 V vs. 6.4 V) and positive bias illumination stress (PBIS) stability (ΔVON = 8 V vs. 1.6 V).

Facile routes to enhance doping efficiency using nanocomposite structures for high-mobility and stable PEALD-ITGO TFTs

In–Sn–Ga–O (ITGO) thin-film transistors (TFTs) fabricated by atomic layer deposition (ALD) are promising candidates for widespread semiconductor applications because of their high mobility and stability. To further improve the device characteristics, the doping efficiency of each cation must be increased. Here, we propose a facile method to enhance the device characteristics using plasma-enhanced ALD-nanocomposite (NC) structures. The deposition of SnO2 materials within the In2O3 layer of the ITGO film with the NC-ITO structure accelerates the reduction of In2O3 (In0: 35.3 % →46.1 %), thereby stabilizing SnO2 (Sn4+: 71.0 % →89.1 %). In addition, this process significantly decreases the fraction of oxygen-related defects (Odefect: 24.0 % →17.6 %) because Sn–O has a substantially higher bond dissociation energy than In–O. Consequently, the ITGO TFT with the NC-ITO active layer exhibits considerable improvements in the electrical parameters, such as an increase in the field-effect mobility from 41.9 to 54.5 cm2/V s and an enhancement in the subthreshold swing from 69.8 to 64.8 mV/decade. In addition, the device shows excellent results for negative bias illumination stress and positive bias temperature stress, with the differences in the threshold voltage shift being −0.22 and + 0.06 V, respectively.

Double-gate structure enabling remote Coulomb scattering-free transport in atomic-layer-deposited IGO thin-film transistors with HfO2 gate dielectric through insertion of SiO2 interlayer

In this paper, high-performance indium gallium oxide (IGO) thin-film transistor (TFT) with a double-gate (DG) structure was developed using an atomic layer deposition route. The device consisting of 10-nm-thick IGO channel and 2/48-nm-thick SiO2/HfO2 dielectric was designed to be suitable for a display backplane in augmented and virtual reality applications. The fabricated DG TFTs exhibit outstanding device performances with field-effect mobility (μFE) of 65.1 ± 2.3 cm2V−1 s−1, subthreshold swing of 65 ± 1 mVdec−1, and threshold voltage (VTH) of 0.42 ± 0.05 V. Both the (μFE) and SS are considerably improved by more than two-fold in the DG IGO TFTs compared to single-gate (SG) IGO TFTs. Important finding was that the DG mode of IGO TFTs exhibits the nearly temperature independent μFE variations in contrast to the SG mode which suffers from the severe remote Coulomb scattering. The rationale for this disparity is discussed in detail based on the potential distribution along the vertical direction using technology computer-aided design simulation. Furthermore, the DG IGO TFTs exhibit a greatly improved reliability with negligible VTH shift of − 0.22 V under a harsh negative bias thermal and illumination stress condition with an electric field of − 2 MVcm−1 and blue light illumination at 80 °C for 3600 s. It could be attributed to the increased electrostatic potential that results in fast re-trapping of the electrons generated by the light-induced ionization of deep level oxygen vacancy defects.

Light Effect on Amorphous Tin Oxide Thin‐Film Transistors

Amorphous tin oxide (a‐SnOx) is a potential transparent oxide semiconductor candidate for future large‐area electronic applications. The thin‐film transistor (TFT) mobilities reach ≈100 cm2 Vs−1, a mobility higher than other multiple cation‐based oxide semiconductor TFTs. Few optical properties have been reported so far and therefore both the effect of visible light and negative bias illumination stress (NBIS) on a‐SnOx TFT performances, known to dramatically impact oxide semiconductor‐based TFTs, have been investigated. The variation of density of states (DOS) due to NBIS by device simulation is analyzed, and a fourfold increase of the donor‐like states and a decrease in the band edge DOS from 2.3 to 2.0 × 1019 cm−3 eV−1 are showed. The evaluation of the effect of neutral, singly, and doubly ionized oxygen vacancies by density functional theory using 95 atoms reveals not only states in the gap of SnO2, but also variations in the electron density, and modifications in the crystal parameters compared to a structure without an oxygen vacancy. Material and device simulation analysis reveal that the oxygen vacancies have a dramatical impact on the DOS in the gap of SnO2 and can explain the NBIS phenomenon observed in a‐SnOx TFT.

Increased Threshold Voltage of Amorphous InGaZnO Thin-Film Transistors After Negative Bias Illumination Stress

Increased Threshold Voltage of Amorphous InGaZnO Thin-Film Transistors After Negative Bias Illumination Stress

Indium-zinc-tin-oxide thin-film-transistor reliability enhancement using fluoridation with CF4 reactive sputtering

We propose a new fluorine doping scheme to achieve a notable improvement in IZTO thin-film-transistor reliability using the fluorine-doped IZTO channel with the stacked IZTO:F/IZTO layers. The fluorine in IZTO:F film was in-situ doped through IZTO deposition using RF sputtering with CF4 as the reactive gas. Furthermore, the electrical characteristics of the IZTO TFTs with varied IZTO:F/IZTO layer thickness ratios were investigated to optimize the IZTO:F thickness ratio in the channel. The TFTs with the optimal IZTO:F/IZTO layer thickness demonstrated significantly enhanced reliability under various stress conditions because the doped fluorine can occupy oxygen vacancies and provide stronger bonding with the metal ions in the active layer. Specifically, the TFT threshold voltage shift (ΔVth) with and without IZTO:F layers were 2.72 V and 5.77 V under positive bias stress (PBS), respectively, which is 2.12 times reduction. The ΔVth was also decreased from −2.23 V to −0.30 V under negative bias stress (NBS), which shows an improvement of 7.43 times. In addition, these devices also have enhanced reliability under negative bias illumination stress (NBIS). The fluorinated IZTO TFTs with the IZTO:F/IZTO layer exhibits enhanced reliability as a result of the defects passivation in the doped fluorine but also have a low manufacturing cost due to in-situ fluorine doping with no additional doping step. It is believed that fluorinated IZTO TFT with the enhanced device reliability will be a good candidate to support the requirements for the next-generation display backplanes.

Compositional tailoring of indium-free GZO layers via plasma-enhanced atomic layer deposition for highly stable IGZO/GZO TFT

Oxide semiconductor-based thin-film transistors (TFTs) are promising candidates for display backplanes and memory device applications. To achieve high device performance and sustain the electrical properties under prolonged operation, it is important to overcome the mobility–stability trade-off in oxide TFTs. Here, we propose a bilayer-structured semiconductor stack formed by plasma-enhanced atomic layer deposition (PEALD), where an indium-free gallium zinc oxide (GZO) film is grown on top of indium gallium zinc oxide (IGZO), to be implemented in top gate devices. Applying a GZO layer (with optimum Ga to Zn ratio) between the IGZO and the gate insulator (GI) resulted in two major effects: interface engineering and band alignment modulation. While maintaining a sufficiently large field effect mobility (31.9 cm2 V−1 s−1), considerable improvements in device reliability were observed concerning positive-bias thermal stress (PBTS) and negative-bias illumination stress (NBIS) compared to TFTs incorporating IGZO single layers. This suggests that proper composition control through PEALD in the bilayer oxide semiconductor TFT may help achieve an appropriate balance between mobility and reliability.

High transparent and stability indium praseodymium oxide thin-film transistors with tungsten doping by solution method

Indium-oxide-based thin-film transistors are widely investigated and exhibit higher carrier mobility than amorphous silicon. Lanthanide-doped Indium oxide TFTs show relatively high negative bias illumination stress (NBIS) stability but at the cost of mobility. To improve the NBIS stability by the solution method, we doped Tungsten (W) into Praseodymium-doped Indium oxide (InPrO) and fabricated InPrWO TFT. We proposed that W doping could densify the films and suppress oxygen vacancy. With the increase of W doping concentration from 0 to 3 mol%, the contact angle of precursor solutions first decreased and then increased, reaching minimums of ∼13° at 1 mol%. When the W doping concentration increased from 0 to 1 mol%, the density of films increased from 6.155 to 6.479 g/cm3. While doping concentration increased from 1 to 3 mol%, the density of films decreased. Transmittance of InPrWO films was higher than 90 % from 300 to 800 nm. The localized defects of InPrWO films were investigated by μ-PCD. With 1 mol% W doping, deep-level and shallow-level defects were relatively the least. XPS result revealed that W was successfully doped into the films. Besides, the oxygen vacancy ratio first decreased and then increased when the W doping concentration increased. InPrWO film with 1 mol% W doping had the least oxygen vacancy ratio, resulting in the highest stability. The TFT fabricated with optimal 1 mol% W doped InPrWO film showed excellent electrical characteristics (μsat = 5.4 cm2/Vs, Vth = -0.3 V, Ion/Ioff = 8.0 × 105, and SS = 0.30 V/dec). With the W doping concentration increased, the NBIS stability first increased and then decreased, having minimum shift ΔVth = -2.1 V at 1 mol%. We improved the quality of films and NBIS stability of TFT by doping W into InPrO and appropriate post-processing, satisfying the development trend of high transparency and high stability TFTs for future displays.

Improved Performance and Bias Stability of Indium‐Tin‐Zinc‐Oxide Thin‐Film Transistors Enabled by an Oxygen‐Compensated Capping Layer

Herein, the effects of oxygen‐compensated capping layer (CCL) on the electrical performance and stability of indium‐tin‐zinc‐oxide (ITZO) thin‐film transistors (TFTs) are investigated. Two different channel structures, namely, single and dual channels, are tested for the ITZO TFTs. The dual‐channel layer is created by depositing an oxygen CCL on the oxygen‐uncompensated channel layer (UCL), while the single‐channel layer consists only of the oxygen UCL. It is found that the oxygen CCL is critical for enhancing the electrical properties of dual‐channel ITZO TFT and its stability under different stress modes such as dynamic stress, positive bias temperature stress, and negative bias illumination stress. The dual‐channel ITZO TFT exhibits a saturation field‐effect mobility of 16.69 cm2 V−1s−1, a threshold voltage of 6.80 V, and a subthreshold swing of 0.22 V dec−1. Furthermore, it is revealed that the higher metal‐oxide concentration and fewer defects in the dual channel lead to enhanced electrical performance and stability of the device. This work demonstrates the potential of utilizing the oxygen CCL for the highly reliable operation of oxide semiconductor TFTs.

Investigation of Donor-like State Distributions in Solution-Processed IZO Thin-Film Transistor through Photocurrent Analysis

The density of donor-like state distributions in solution-processed indium–zinc-oxide (IZO) thin-film transistors (TFTs) is thoroughly analyzed using photon energy irradiation. This study focuses on quantitatively calculating the distribution of density of states (DOS) in IZO semiconductors, with a specific emphasis on their variation with indium concentration. Two calculation methods, namely photoexcited charge collection spectroscopy (PECCS) and photocurrent-induced DOS spectroscopy (PIDS), are employed to estimate the density of the donor-like states. This dual approach not only ensures the accuracy of the findings but also provides a comprehensive perspective on the properties of semiconductors. The results reveal a consistent characteristic: the Recombination–Generation (R-G) center energy ET, a key aspect of the donor-like state, is acquired at approximately 3.26 eV, irrespective of the In concentration. This finding suggests that weak bonds and oxygen vacancies within the Zn-O bonding structure of IZO semiconductors act as the primary source of R-G centers, contributing to the donor-like state distribution. By highlighting this fundamental aspect of IZO semiconductors, this study enhances our understanding of their charge-transport mechanisms. Moreover, it offers valuable insight for addressing stability issues such as negative bias illumination stress, potentially leading to the improved performance and reliability of solution-processed IZO TFTs. The study contributes to the advancement of displays and technologies by presenting further innovations and applications for evaluating the fundamentals of semiconductors.

Improving the electrical stability of a‐IGZO TFT through gate surround structures

AbstractThis paper delves into a structural modification of dual‐gate oxide thin film transistor (TFT). Diverging from the conventional dual‐gate TFT structure, the authors’ approach connects the bottom and top gate electrodes, effectively enveloping all four sides of the a‐IGZO channel. This shielding configuration ensures stable operation, even under light illumination. Moreover, capitalizing on the stability under light conditions, the authors observed a remarkable threefold improvement in the mobility of the fabricated TFT with this proposed structure, resulting in a substantial 156% increase in current. Furthermore, in the negative bias illumination stress test, the proposed TFT exhibited minimal fluctuations when compared to its single‐gate counterpart, further underscoring its exceptional and robust performance.

Enhancement in Performance and Reliability of Transparent IGZO Thin-Film Transistors by ITO/Ti Stacked Source/Drain Contacts

Enhancement in performance and reliability of transparent IGZO thin-film transistor (TFT) was achieved by adopting a laminated structure with a thin ITO layer and an ultrathin Ti layer as the source/drain (S/D) contacts. Compared with the transparent TFT with pure ITO S/D contacts, the transparent TFT with the ITO/Ti S/D contacts showed three times of enhancement in field-effect mobility from 4.75 to 12.10 cm2/Vs, two times of enhancement in on/off current ratio from 7.0 × 107 to 1.54 × 108, three times of reduction in contact resistance from 15.74 to 4.64 kΩ, and a decrease in threshold voltage from 3.11 V to 2.80 V. The TFT with the ITO/Ti S/D contacts also maintained an extremely low leakage current at zero gate bias (for the device with channel width/channel length of 40 μm/5 μm, the leakage current was ∼1 × 10−13 A). In addition, the TFT with the ITO/Ti S/D contacts showed a hump-free transfer curve and a smaller shift in threshold voltage under negative bias illumination stress. The enhancement in performance and reliability makes the transparent TFT with the ITO/Ti S/D contacts very promising in transparent display applications.

Electrical instabilities of a-IGZO TFTs under different conditions of bias and illumination stress

Amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) have been widely applied in display technologies. An important factor of these devices is the electrical stability to avoid misinterpretations of current or voltage values under different conditions such as: bias stress, recovery time, and illumination. This work presents the effects of ΔVT under positive bias stress, negative bias stress, positive bias illumination stress, negative bias illumination stress (PBS, NBS, PBIS, NBIS), stress recovery, cyclic bias stress, hysteresis, fixed bias, and aging effects. It was found that a full recovery was achieved only under illumination because of the de-trapping effect (PBS) and ionization of oxygen vacancies (NBS). Some tests showed a low ΔVT over time, however, some considerations must be taken to carry out this purpose. Finally, fixed polarization under illumination was shown to find a stable operating region over time showing negligible variations upon aging.

Mobility Enhancement of BCE-Type Amorphous InGaZnO TFTs Using Triple-Layer Channels

The back-channel-etch-type amorphous InGaZnO (a-IGZO) triple-layer thin-film transistor (TL-TFT) consists of a top barrier and a bottom barrier deposited with oxygen flow (OF) and an a-IGZO main channel deposited without OF in between. The TL-TFT has 1.7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> and 1.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> electron mobilities for the bottom gate (BG) operation as compared to the single-layer and double-layer channel TFTs, respectively. The conduction band difference between the barrier layers and the main channel is high enough to confine the carriers in the main channel. The bottom barrier decreases the Coulomb scattering and the surface roughness scattering to increase the mobility, while the top barrier decreases the plasma-induced damage in the channel and the bottom barrier. However, both the hysteresis and subthreshold swing (S.S.) increase if only the bottom barrier is adopted double-layer TFT (DL-TFT) because the shallow states in the IGZO bottom barrier provide extra tunneling paths for the electrons to be trapped inside the BG oxide. With additional top barrier structure, plasma-induced damage in the main channel and the bottom barrier can be mitigated to reduce the degradation of hysteresis and S.S. Moreover, the TL-TFTs demonstrate better reliability under both positive bias stress and negative bias illumination stress than single-layer TFTs (SL-TFTs) and DL-TFTs.

Fabrication and investigation of the electrical performance of nitrogenated bilayer ZnO:N/ZnO thin-film transistors

In this study, a bilayer ZnO:N/ZnO thin-film transistors (TFTs) was designed to improve both the performance and stability of single-layer ZnO TFTs, using the effects of N-treatment on oxygen vacancy (VO) and the interface trap density. Bilayer ZnO:N/ZnO films and related TFTs were fabricated via radiofrequency magnetron sputtering in a mixture of Ar and N2 gas at 150 °C. The electrical performance and bias-stress stability were systematically investigated. The bilayer ZnO:N/ZnO TFT exhibited a linear field-effect mobility of 13.6 cm2/Vs, high current on-to-off ratio (Ion/Ioff) of 1 × 107, small threshold voltage (Vth) of 2.6 V, and low subthreshold swing of 0.32 V/decade compared with single-layer undoped ZnO and ZnO:N TFTs. Moreover, ZnO:N/ZnO TFT exhibited small threshold voltage shifts (∆Vth) of 0.8 and –0.9 V according to positive bias illumination stress and negative bias illumination stress measurements, respectively. X-ray photoelectron spectroscopy analysis showed that the N-treatment passivated the VO defects, reduced the density of interface traps, and controlled the carrier concentration. Thus, the performance and stability of the ZnO:N/ZnO TFTs were improved. Therefore, ZnO:N/ZnO TFTs have good application prospects in transparent thin-film electronics and flat-panel displays.

Channel migration of dual channel a-InGaZnO TFTs under negative bias illumination stress

In this work, the time-dependent behaviors of dual-channel amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) under negative bias illumination stress (NBIS) are systematically discussed. The dual-channel comprised two different IGZO layers fabricated by tuning the oxygen flow during deposition. The presence of heterojunctions enhanced field-effect mobility by 1.5× owing to the confinement of carriers in buried channels because of an energy barrier. However, after long periods of NBIS stress, the degradation of a-IGZO TFTs resulted in the entrapment of photo-generated electron–hole pairs at interface defects. The conduction path migrated to the surface channel. Results from extracting the hysteresis window and utilizing capacitance–voltage measurements have indicated a channel migration phenomenon due to the entrapment of electrons and holes at the surface and buried channel interfaces.