Negative Bias Stress Stability Research Articles

Enlightenment of Deionized‐Water Bathing IGZO TFTs

AbstractAt present, amorphous indium–gallium–zinc oxide (IGZO) semiconductor has become the most commonly used semiconductor material and is widely used in flat panel displays and various sensors, but its performance is greatly affected by environmental factors, especially water. This study investigates the instability of IGZO thin‐film transistors (TFTs) soaked in deionized water. After 24 h bathing, the field‐effect mobility and threshold voltage change a little, but the subthreshold swing, positive bias stress and negative bias stress stability undergo clear degradation. Through comprehensive examination of the changes in the chemical composition, surface morphology and thickness of the IGZO films, it's found that IGZO experiences selective etching by deionized water, and consequently the film surface becomes rough and rich of In and oxygen vacancies, which can explain the variations on device performance well. In addition to the bathing experiment performed on In2O3, Ga2O3, and ZnO TFTs, a schematic image of the reaction between water and IGZO is depicted, showing the preferential loss of Ga and Zn located next to the oxygen vacancy.

Post-annealing effect of low temperature atomic layer deposited Al2O3 on the top gate IGZO TFT

Electronical properties of top gate amorphous InGaZnO4 thin film transistors (TFTs) could be controlled by post-annealing treatment, which has a great impact on the Al2O3 insulator. To investigate the effect of post-annealing on Al2O3, Al/Al2O3/p-Si MOS capacitoras with Al2O3 films treated under various post-deposition annealing (PDA) temperature were employed to analysis the change of electrical properties, surface morphology, and chemical components by electrical voltage scanning, atomic force microscope (AFM), and x-ray photoelectron spectroscopy (XPS) technologies. After PDA treatment, the top gate TFTs had a mobility about 7 cm2 V−1 s−1 and the minimum subthreshold swing (SS) about 0.11 V/dec, and the threshold voltage (V th) shifted from positive direction to negative direction as the post-annealing temperature increased. Electrical properties of MOS capacitors revealed the existence of positive fixed charges and the variation of trap state density with increasing PDA temperature, and further explained the change of negative bias stress (NBS) stability in TFT. AFM results clarified the increased leakage current, degraded SS, and NBS stability in MOS capacitors and TFTs, respectively. XPS results not only illuminated the origin of fixed charges and the trap density variation with PDA temperatures of Al2O3 films, but also showed the O and H diffusion from Al2O3 into IGZO during post-annealing process, which led to the deviation of V th, the change of current density, and the negative V th shift after positive bias stress in TFTs.

Improved Negative Bias Stress Stability of Sol–Gel-Processed Li-Doped SnO2 Thin-Film Transistors

In this study, sol–gel-processed Li-doped SnO2-based thin-film transistors (TFTs) were fabricated on SiO2/p+ Si substrates. The influence of Li dopant (wt%) on the structural, chemical, optical, and electrical characteristics was investigated. By adding 0.5 wt% Li dopant, the oxygen vacancy formation process was successfully suppressed. Its smaller ionic size and strong bonding strength made it possible for Li to work as an oxygen vacancy suppressor. The fabricated TFTs consisting of 0.5 wt% Li-doped SnO2 semiconductor films delivered the field-effect mobility in a 2.0 cm2/Vs saturation regime and Ion/Ioff value of 1 × 108 and showed enhancement mode operation. The decreased oxygen vacancy inside SnO2 TFTs with 0.5 wt% Li dopant improved the negative bias stability of TFTs.

Observation of Divacancy Formation for ZnON Thin-Film Transistors With Excessive N Content

This study proposes a model of mobility and negative bias stress stability degradation mechanism of zinc oxynitride (ZnON) thin-film transistors (TFTs) with various anion compositions. The subgap density of states (DOS) for ZnON TFTs were extracted using monochromatic photonic capacitance–voltage measurement. The extracted subgap DOS indicated an additional nitrogen-related subgap peak was observed in ZnON TFTs with an excessive nitrogen condition, which might originate from a nitrogen divacancy formation. Furthermore, the shifted threshold voltage components after the bias stress test were quantitatively divided as subgap DOS and interface trapping.

Improved Electrical Performance of Oxide Transistor Utilizing Gallium Doping Both in Channel and Dielectric Layers

Here, we report a new strategy to fabricate high performance thin-film transistors (TFTs) by doping gallium (Ga) both in channel (In2O3) and dielectric (Al2O3) layers. The influence of Ga doping on the microstructure and surface morphology of In2O3 films, as well as the oxygen defects states and electrical properties of Al2O3 films have been systematically investigated. The results show Ga doping frustrates In2O3 crystallization and flattens film surface. As for Al2O3, Ga doping increase the dielectric constant, reflective index and decrease the leakage current density. Furthermore, metal oxide TFTs were developed when using In2O3:Ga as the channel and Al2O3:Ga as the dielectric layers. The devices show improved electrical performance and enhanced positive and negative bias stress stability than undoped devices.

Investigations on the bias temperature stabilities of oxide thin film transistors using In-Ga-Zn-O channels prepared by atomic layer deposition.

Bias temperature stress stabilities of thin-film transistors (TFTs) using In–Ga–Zn–O (IGZO) channels prepared by the atomic layer deposition process were investigated with varying channel thicknesses (10 and 6 nm). Even when the IGZO channel thickness was reduced to 6 nm, the device exhibited good characteristics with a high saturation mobility of 15.1 cm2 V−1 s−1 and low sub-threshold swing of 0.12 V dec−1. Excellent positive and negative bias stress stabilities were also obtained. When positive bias temperature stress (PBTS) stability was tested from 40 to 80 °C for 104 s, the threshold voltages (VTH) of the device using the 6 nm-thick IGZO channel shifted negatively, and the VTH shifts increased from −0.5 to −6.9 V with the increasing temperature. Time-dependent PBTS instabilities could be explained by a stretched-exponential equation, representing a charge-trapping mechanism.

Effects of hydrogen impurities on performances and electrical reliabilities of indium-gallium-zinc oxide thin film transistors

The influences of hydrogen impurities on the performances of indium-gallium-zinc oxide (IGZO) thin film transistors (TFT) are summarized in this article. Firstly, the sources of hydrogen impurities in the IGZO channels of the TFTs are proposed, which could originate from the residual gas in the deposition chamber, the molecules absorbed on the sputtering target surface, the neighbor films that contain abundant hydrogen elements, doping during annealing processes, etc. The hydrogen impurities in the IGZO films can exist in the forms of hydroxyl groups and metal hydride bonds, respectively. The former originates from the reaction between H atoms and the O2- ions. This reaction releases free electrons, leading to a rise of the Fermi level of IGZO, and thus enhancing the mobilities of IGZO TFTs. The latter incurs negative charges on H atoms, and thus changing the distribution of the subgap density of states, hence improving the negative bias (or illumination) stabilities of IGZO TFTs. Subsequently, various methods are also proposed to characterize hydrogen elements in IGZO, such as secondary ion mass spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Finally, the effects of hydrogen impurities on the electrical characteristics of the IGZO TFTs, such as the field effect mobilities, subthreshold swings, threshold voltages, on/off current ratios as well as the positive and negative bias stress stabilities, are discussed. The results indicate that hydrogen element concentration and process temperature are two key factors for the device performances. With the increase of hydrogen element concentration in the IGZO channels, the TFTs exhibit higher electron mobilities, lower subthreshold swings and better reliabilities. However, annealing at too high or low temperatures cannot improve the device performance, and the most effective annealing temperature is 200-300℃. It is anticipated that this review could be helpful to the IGZO TFT researchers in improving the device performances and understanding the underlying mechanism.

Influence of channel layer thickness on the stability of amorphous indium zinc oxide thin film transistors

Amorphous indium zinc oxide (a-IZO) thin film transistors (TFTs) with different channel layer thickness were fabricated on silicon wafers by radio frequency magnetron sputtering method at room temperature. The influence of channel layer thickness on initial electrical characteristics and aging effect of a-IZO-TFTs were investigated. At the same time, the positive / negative bias stress stability of initial and 60-days-aged a-IZO-TFTs were compared. All results indicate that thin channel layer a-IZO-TFTs exhibit good anti-aging effect and bias stability.

Effect of mid-annealing process on the device characteristics of the TFT using Al-doped ZnO active channels prepared by atomic layer deposition

A specified mid-annealing process, which is a thermal treatment in oxygen ambient right after an active layer deposition, was proposed for obtaining a sufficiently wide process window for the atomic layer deposition in order to realize a high performance Al-doped ZnO (AZO) thin-film transistors (TFTs). While the crystalline phases of the AZO thin films were not changed after the mid-annealing process, the electrical conductivities of the films experienced drastic changes owing to the significant reduction of oxygen vacancies during the mid-annealing process. The decrease in the electrical conductivity was more markedly observed for the AZO films prepared at higher deposition temperature. Top-gate-structured TFTs using the mid-annealed AZO active channel layers were fabricated and characterized. Sound on/off switching behaviors of TFTs were obtained at a wider range of deposition temperature. Additionally, the improvements in carrier mobility and negative bias stress stability were successfully confirmed.

Effects of Li doping on the negative bias stress stability of solution-processed ZnO thin film transistors

To investigate the effect of Li dopant on the electrical characteristics under negative bias stress (NBS), we analysed ZnO and Li doped ZnO TFTs. The Li dopant enhanced the field effect mobility and sustained the variation in Von of the ZnO TFTs.

A strategy for performance enhancement of HfInZnO thin film transistors using a double-active-layer structure

We investigate the electrical performance and negative bias stability of thin film transistors (TFTs) using double active layer. Double active layer is consisting of low oxygen content HfInZnO film as bottom channel and high oxygen content HfInZnO film as back channel. The HfInZnO-TFT with double channel layer shows a high mobility of 6.8cm2/Vs, a low threshold voltage of −0.3V, and a threshold voltage shifts of 0.65V under −20V gate bias for 7200s. The results suggest that HfInZnO with low oxygen content as the bottom channel could provide high carrier concentration and good HfInZnO/Al2O3 interface to improve field-effect mobility and negative bias stress stability. HfInZnO with high oxygen content as back channel can control the conductivity to reduce the off current of TFT. Thus, a double-active-layer structure is an effective way to improve the electrical performance of HfInZnO-TFT.

Improvement in Negative Bias Stress Stability of Solution-Processed Amorphous In–Ga–Zn–O Thin-Film Transistors Using Hydrogen Peroxide

We have investigated the effect of hydrogen peroxide (H2O2) on negative bias stress (NBS) stability of solution-processed amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The instability of solution-processed a-IGZO TFTs under NBS is attributed to intrinsic oxygen vacancy defects (Vo) and organic chemical-induced defects, such as pores, pin holes, and organic residues. In this respect, we added H2O2 into an indium-gallium-zinc oxide solution to reduce the defects without any degradation of electrical performance. The field-effect mobility and sub-threshold slope of the a-IGZO TFTs were improved from 0.37 cm(2) V(-1) s(-1) and 0.86 V/dec to 0.97 cm(2) V(-1) s(-1) and 0.58 V/dec, respectively. Furthermore, the threshold voltage shift under NBS was dramatically decreased from -3.73 to -0.18 V. These results suggest that H2O2 effectively reduces Vo through strong oxidation and minimizes organic chemical-induced defects by eliminating the organic chemicals at lower temperatures compared to a conventional solution process.

Role of Self-Assembled Monolayers on Improved Electrical Stability of Amorphous In-Ga-Zn-O Thin-Film Transistors

Self-assembled monolayers (SAMs) have been used to improve both the positive and negative bias-stress stability of amorphous indium gallium zinc oxide (IGZO) bottom gate thin film transistors (TFTs). N-hexylphosphonic acid (HPA) and fluorinated hexylphosphonic acid (FPA) SAMs adsorbed on IGZO back channel surfaces were shown to significantly reduce bias-stress turn-on voltage shifts compared to IGZO back channel surfaces with no SAMs. FPA was found to have a lower surface energy and lower packing density than HPA, as well as lower bias-stress turn-on voltage shifts. The improved stability of IGZO TFTs with SAMs can be primarily attributed to a reduction in molecular adsorption of contaminants on the IGZO back channel surface and minimal trapping states present with phosphonic acid binding to the IGZO surface.

Improved Negative Bias Stress Stability of IZO Thin Film Transistors via Post-Vacuum Annealing of Solution Method

Post-vacuum-annealed amorphous indium zinc oxide (a-IZO) nanostructure thin-film transistors (TFTs) were fabricated by a simple solution-based method. The number of oxygen-related defects in the IZO films was reduced by post-vacuum annealing, as revealed by X-ray photoelectron spectroscopy results. The refractive index studies of as-prepared films indicate the reduced film surface porosity. With the optimal post-vacuum treatment temperature of 250°C, the field-effect mobility and subthreshold slope (S) of TFTs (2.81 cm2/V s and 0.72 V/dec, respectively) improved compared to those of as-prepared IZO TFTs (0.38 cm2/V s and 3.79 V/dec, respectively). The negative bias stress tests of the vacuum-annealed devices was enhanced relatively high electrical stability due to fewer oxygen deficiencies in the IZO films.

Phosphorus Doping Effect in a Zinc Oxide Channel Layer to Improve the Performance of Oxide Thin-Film Transistors

In this study, we fabricated phosphorus-doped zinc oxide-based thin-film transistors (TFTs) using direct current (DC) magnetron sputtering at a relatively low temperature of 100°C. To improve the TFT device performance, including field-effect mobility and bias stress stability, phosphorus dopants were employed to suppress the generation of intrinsic defects in the ZnO-based semiconductor. The positive and negative bias stress stabilities were dramatically improved by introducing the phosphorus dopants, which could prevent turn-on voltage (VON) shift in the TFTs caused by charge trapping within the active channel layer. The study showed that phosphorus doping in ZnO was an effective method to control the electrical properties of the active channel layers and improve the bias stress stability of oxide-based TFTs.