Reliability Improvement in Solution-Processed ZrO2 Dielectrics Due to Addition of H2O2.

In this study, we investigated the effects of hydrogen peroxide (H2O2) on solution-processed zirconium oxide (ZrO2) dielectric materials. The addition of H2O2 into ZrO2 dielectric showed a reduction in hysteresis capacitance-voltage characteristics (from 393 mV to 96 mV). This resulted in a reduction in border trap density (Nbt) of the ZrO2 film (ZrO2: 2.24 × 1011 cm-2, ZrO2 + H2O2: 3.96 × 1010 cm-2). In addition, use of H2O2 in the ZrO2 dielectric improved the interface quality. Specifically, the reduced number of trap sites improved the reliability of the device under a negative bias stress (NBS). The 350 °C annealed ZrO2 dielectric with H2O2 showed excellent leakage current properties (6.7 × 10-9 A/cm2 at gate voltage of -10 V). Based on these results, we fabricated IGZO/ZrO2 + H2O2 TFTs, which showed a high saturation mobility of 6.10 cm2/V · s and excellent switching properties. This study suggests that incorporation of H2O2 into ZrO2 effectively reduced oxygen vacancies through strong oxidation and minimized residual organics that cause impurities or structural defects, such as pores or pin holes, compared to a virgin ZrO2 film.

Incorporation of Si and Zr into Pure HfO2 and Its Effects on Dielectric Integrity.

Hafnium-silicate (HfSiO4, (HfO2)x(SiO2)1-x) and hafnium-zirconate (HfZrO4, (HfO2)x(ZrO2)1-x) films were employed as a gate dielectric to enhance the electrical properties of pure HfO2. (HfO2)x(SiO2)1-x and (HfO2)x(ZrO2)1-x films were formed onto p-Si substrates with varying degrees of Hf content x (x = 1, 0.9, 0.7, and 0.5) via solution processing. With regard to (HfO2)x(SiO2)1-x, the leakage current decreased from 1.94 × 10-8 to 4.29 × 10-9 A/cm2 at a gate voltage of VG = -1 V when the HfO2 content was reduced. These resulted from the reduction of leakage paths through the interface between HfSiO4 and Si substrate. Additionally, (HfO2)x(ZrO2)1-x exhibited the lowest interfacial trap density of 3.4 × 1011 cm-2 eV-1 for x = 0.5 due to a reduction in root mean square (RMS) roughness of the film from 6.0 to 4.2 nm. From the results, it was found that (HfO2)0.5(SiO2)0.5 demonstrated excellent oxide integrity in contact with Si substrates, whereas (HfO2)0.5(ZrO2)0.5 demonstrated an enhanced film morphology and maintained a high dielectric constant value. Finally, the HfZrO4/HfSiO4/Si structure revealed a gate oxide with enhanced integrity compared to pure HfO2-based devices.

Investigation of the Charge Balance in Green Phosphorescent Organic Light-Emitting Diodes by Controlling the Mixed Host Emission Layer.

In this paper, we investigated the use of a mixed host emission layer (MH-EML) in green phosphorescent organic light-emitting diodes (OLEDs). The hole transport type (p-type) material (4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP)) and electron transport type (N-type) material (2,2',2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)) were mixed with different ratios. The electrons were easily injected through the lowest unoccupied molecular orbital (LUMO) of TPBi in the mixed host system. Also, holes were confined in the EML because of the deep highest occupied molecular orbital (HOMO) level of TPBi (6.7 eV). These results indicate that excitons were formed effectively and the recombination zone became wider under a high electric field in MH-EML devices. For these reasons, the lifetime of the MH-OLED device was 1.36 times higher than that of a single host emission layer (SH-EML) device and showed a reduction in Joule heating. Finally, the external quantum efficiency (EQE) roll-off ratio from 1 mA/cm2 to 100 mA/cm2 in the optimized device (30.46%) was 18.12%p lower than that of the SH-EML (48.58%).

Optimization of the Solution-Based Indium-Zinc Oxide/Zinc-Tin Oxide Channel Layer for Thin-Film Transistors.

Double stacked indium-zinc oxide (IZO)/zinc-tin oxide (ZTO) active layers were employed in amorphous-oxide-semiconductor thin-film transistors (AOS TFTs). Channel layers of the TFTs were optimized by varying the molarity of ZTO back channel layers (0.05, 0.1, 0.2, 0.3 M) and the electrical properties of IZO/ZTO double stacked TFTs were compared to single IZO and ZTO TFTs with varying the molarity and molar ratio. On the basis of the results, IZO/ZTO (0.1 M) TFTs showed the excellent electrical properties of saturation mobility (13.6 cm2/V·s), on-off ratio (7×106), and subthreshold swing (0.223 V/decade) compared to ZTO (0.1 M) of 0.73 cm2/V · s, 1 × 107, 0.416 V/decade and IZO (0.04 M) of 0.10 cm2/V · s, 5 × 106, 0.60 V/decade, respectively. This may be attributed to diffusing Sn into front layer during annealing process. In addition, with varying molarity of ZTO back channel layer, from 0.1 M to 0.3 M ZTO back channel TFTs, electrical properties and positive bias stability deteriorated with increasing molarity of back channel layer because of increasing total trap states. On the other hand, 0.05 M ZTO back channel TFT had inferior electrical properties than that of 0.1 M ZTO back channel TFT. It was related to back channel effect because of having thin thickness of channel layer. Among these devices, 0.1 M ZTO back channel TFT had a lowest total trap density, outstanding electrical properties and stability. Therefore, we recommended IZO/ZTO (0.1 M) TFT as a promising channel structure for advanced display applications.

Nonstoichiometric Solution-Processed BaTiO3 Film for Gate Insulator Applications.

Solution processed barium titanate (BTO) was used to fabricate an Al/BaTiO3/p-Si metal-insulator-semiconductor (MIS) structure, which was used as a gate insulator. Changes in the electrical characteristics of the film were investigated as a function of the film thickness and post deposition annealing conditions. Our results showed that a thickness of 5 layers and an annealing temperature of 650 °C produced the highest electrical performance. BaxTi1-xO3 was altered at x = 0.10, 0.30, 0.50, 0.70, 0.90, and 1.0 to investigate changes in the electrical properties as a function of composition. The highest dielectric constant of 87 was obtained for x = 0.10, while the leakage current density was suppressed as Ba content increased. The lowest leakage current density was 1.34×10-10 A/cm2, which was observed at x = 0.90. The leakage current was related to the resistivity of the film, the interface states, and grain densification. Space charge limited current (SCLC) was the dominant leakage mechanism in BTO films based on leakage current analysis. Although a Ba content of x = 0.90 had the highest trap density, the traps were mainly composed of Ti-vacancies, which acted as strong electron traps and affected the film resistivity. A secondary phase, Ba2TiO4, which was observed in cases of excess Ba, acted as a grain refiner and provided faster densification of the film during the thermal process. The absence of a secondary phase in BaO (x = 1.0) led to the formation of many interface states and degradation in the electrical properties. Overall, the insulator properties of BTO were improved when the composition ratio was x = 0.90.

Electrical Characterization of Charge Polarity in AlF3 Anti-Reflection Layers for Complementary Metal Oxide Semiconductor Image Sensors.

In this study, the charge polarity of aluminum fluoride (AlF3) as a function of varying thickness (tAlF3 = 20, 35, 50, 65, and 80 nm) was discussed. AlF3 films were deposited onto p-Si wafers via electron beam sputtering. Thickness dependent charge polarity and reliability issues under bias-temperature stress conditions were identified using a capacitance-voltage (C-V) characterization method. AlF3 was found to possess negative fixed charges, leading to a C-V curve shift toward the positive gate bias direction as tAlF3 was increased up to 50 nm. On the contrary, the C-V characteristics were dominantly affected by the positive charges of mobile ions and/or fluorine vacancies when tAlF3 was increased to more than 50 nm. Additionally, negative bias temperature stress (1 MV/cm, 473 K for 10 mins) increased insulator trapped charges and decreased interface traps in 20 nm thick AlF3 films. These results could be attributed to positively charged fluorine vacancies introduced by broken Al-F bonds within AlF3 films and the passivation of Si dangling bonds due to broken fluorine ions at the interface, respectively. It was believed that 20 nm thick AlF3 films sufficiently attracted holes from the Si substrate, forming a hole accumulation layer on the surface due to total charge polarity of the AlF3 dielectric being entirely governed by negative fixed charges as the thickness of AlF3 decreased. Based on these results, AlF3 films are proposed for use as an anti-reflection layer to replace HfO2 in CMOS image sensors.