The 65th Japan Society of Applied Physics (JSAP) Spring Meeting 2018
Device reliability of OLED and oxide-TFT are enhanced by various methods

March 17-20, “The 65th Japan Society of Applied Physics (JSAP) Spring Meeting 2018” was convened in Waseda University. In this article, some notice oral lectures are picked up among OLED, organic-TFT, oxide-TFT and process technology of electronics devices.

Thermal stability of evaporated organic film is enhanced by multi-layer

First of all, as regards OLED, research group of Yamagata University reported estimate result of thermal stability of evaporated organic film against the annealing process.

As you know, in vacuum evaporated organic film, molecule orientation and film density are changed by the annealing process; as a result, transition occurred in the film. For this reason, research group estimated transition temperature of various film structures by in-situ ellipsometry analysis.

Classical hole transporting material α-NPD film was evaporated on silicon wafer substrate at 100 nm films thickness, and then, it was annealed at annealing speed of 1℃/min. Transition phenomenon occurred at 111 ℃. The next, α-NPD/TCTA multi-layer was prepared by stacking of TCTA (Tg＝152℃) on α-NPD film. In this 2 layer structure, transition phenomenon occurred at 120 ℃. This is reason why motion latitude of α-NPD molecule in boundary of multi-layer was reduced and timing of transition was delayed by covering of high Tg organic material on α-NPD single film, which has high motion latitude of molecule. And also, transition timing was delayed according as film thickness of TCTA, however, in case of 10 nm and over, suppression of transition was saturated and not changed.

Furthermore, in case of TCTA/NPD/TCTA triple layer structure, transition temperature was 120 ℃, and transition finishing time was 5.1 min. In short, among various film structures, best result was gained in the context of transition. In this triple film structure, film thickness of α-NPD was changed as 20 nm, 100 nm, and 150 nm respectively, however, film thickness dependence was not observed. Anyway, in case of using low Tg organic material, thermal stability of its film is enhanced by stacking another high Tg organic material.

New inorganic material is effective for HIL of OLED

The research group of Tokyo Institute of Technology proposed a new inorganic material was used as HIL (hole injection layer) material of OLED.

It is 2 composition series, which is composed of conventional amorphous oxide semiconductor (AOS) and MoOx. It has big electron affinity same as HOMO level of organic layer. In sputtering deposited In-Mo-O (In:Mo＝7:3) film, bandgap, electron affinity, and hole carrier mobility are 3.1 ev, 5.6 eV, 1 cm²/V･s respectively.

By making use of this material as HIL, a sample OLED (ITO/HIL/α-NPD/Alq3/LiF/Al) was pilot-produced. As a result, electric property of this device was higher compared to that of reference device with MoOx HIL. Furthermore, if film thickness of HIL was thickened same as 95 nm, electric property was almost same as that of thin film device (0.7 nm). In short, film thickness dependence was not observed. For this reason, if relatively thick In-Mo-O is used as HIL, suppression of leak current and yield are expected to be improved.

Low damage FTS was furthermore advanced as metal deposition method

The research group of Tokyo Polytechnic University proposed to use low damage sputtering method with IR annealing, to minimize damage against organic films in metal film deposition process.

In this research, a sample device (ITO (70 nm)/ITO buffer (0.6 nm)/NPB (40 nm)/Alq3 (30 nm)/BCP (30 nm)/LiF (0.6 nm)/Al (40 nm)) was manufactured on the glass substrate. Al electrode film was deposited by the facing target sputtering (FTS) method, which is known as a low damage deposition method. And also, it was deposited with annealing process by making use of an IR heater.

Figure 2 shows emission characteristics of sample device. Emitting voltage of the sample was almost same as that of the conventional device with evaporated Al cathode by optimization of IR irradiation condition. And also, as figure 3, current-emission characteristics was changed by IR irradiation condition, and then, emitting characteristics of sample device was higher than that of the conventional device with evaporated Al cathode.

Characteristics and reliability are enhanced by annealing in non-exposed surface of IGZO

As concerns oxide-TFT, the research group of Nihon University and Sumitomo Metal Mining reported that device characteristics of a-IGZO-TFT was improved by annealing methods after deposition of a-IGZO.

As you know, if a-IGZO film is annealed in as-depo state, film property is changed because of high vapor pressure, and then, Vth shift generated in NBIS (Negative Bias Illumination Stress) environment. Therefore, it's necessary to form a passivation layer on a-IGZO film in the practical device; however, film property may be degraded in the device manufacturing process. For this reason, deposited a-IGZO film was annealed at non-exposed state of surface.

Concretely, silicon wafer with a-IGZO film in reversed state and in covered state by glass substrate were annealed in atmosphere condition at 350 ℃ for 1 hour. By the way, IGZO film was deposited at 150W power, 0.5 Pa pressure, and O2：Ar ratio 100：1. And then, SiO2 passivation film was deposited on a-IGZO film.

As figure 4, carrier mobility and subthreshold swing characteristics of sample device were increased compared to those of reference device (annealing in conventional state). Furthermore, as figure 5, NBIS characteristics was enhanced, as a result, Vth shift was suppressed. Of course, this is reason why composition ratio is not almost changed because of difficulty of volatilization of ZnO.

Electric property of wettable oxide-TFT is enhanced by ELA treatment

On the other hand, the research group of NAIST and Kyushu University reported InZnO-TFT, which was manufactured by wet process.

To show the activation of InZnO by a photo-assisted process, research group fabricated bottom gate top contact InZnO-TFT. The InZnO precursor was spin-coated on a SiO2 gate insulator/Si gate substrate, and baked using a 2-step baking process at 150℃ and 300℃, to form a thin (10 nm) InZnO film. This treatment was repeated 5 times to form a 5-layered InZnO channel (50 nm). 20 nm Pt on 80 nm Mo for source/drain electrodes were deposited by RF magnetron sputtering. Electrical characteristics were then measured before the photo-assisted process (Fig. 6(a)). Instead of a 300℃ furnace annealing for 2 hours, two types of photo-assisted process were performed: ELA (Excimer Laser Annealing) and a combination of ELA and UV/O3 treatment at 290℃.

Fig 6(b) shows the improvement of electrical characteristics from an average carrier mobility of 0.15 cm²/V･s before ELA to an average of 3.3 cm²/V･s after ELA at 80 mJ/cm2. A similar mobility enhancement (1.16 cm2/V･s) was observed with the combination of ELA and UV/O3. Photo-assisted methods can also induce a semiconductor-to-conductor transformation by decreasing the sheet resistance of InZnO (Table 2).

Furthermore, research group fabricated a transparent all solution-processed oxide-TFT by selectively converting InZnO semiconductor regions into conductors to form source, drain, and gate electrodes (Fig 7).

Work function of electrode is controlled by VUV irradiation against SAM

With respect to organic-TFT, the research group of The University of Tokyo proposed carrier injection height of inorganic electrode and organic semiconductor was reduced by SAM (Self Assembled Monolayers).

Sample device is composed of glass/Cr/Au/FSAM. Firstly, Cr film and Au film were evaporated at 1 nm and 30 nm film thickness respectively. The next, the substrate was dipped into FSAM (1H, 1H, 2H, 2H- perfluorodecanethiol：PFDT) ethanol liquid for 18 hours. Finally, surface of SAM was modified by irradiation of VUV (172 nm).

Figure 9 shows water contact angle in sample device. Water contact angle was changed a range of 115 - 29°by surface modification time. Furthermore, work function could be controlled a range of 5.7 eV - 5.0 eV linearly.

Reference
1)Shiomoto, et.al.：Development of New Amorphous Oxide Semiconductors with Deep Conduction Band Minimum and Their OLED Application, The 65th JSAP Spring Meeting, 2018, 11-018 (2018.3)
2)Yamamoto, et.al.：Effects of adjacent organic films on thermal stability of vacuum-deposited OLED films, The 65th JSAP Spring Meeting, 2018, 11-092 (2018.3)
3)Hoshi, et.al.：Sputter-deposition of Al top electrode films of OLED under an infrared irradiation, The 65th JSAP Spring Meeting, 2018, 11-106 (2018.3)
4)Shimizu, et.al.：Influence of annealing conditions on the electrical performance and reliability of amorphous oxide TFTs, The 65th JSAP Spring Meeting, 2018, 16-009 (2018.3)
5)Juan Paolo Bermundo, et.al.：Photo-assisted activation and conductivity enhancement of solution-processed InZnO, The 65th JSAP Spring Meeting, 2018, 16-008 (2018.3)
6)Kayashima, et.al.：Surface Modification of Fluorinated Self-Assembled Monolayer using Vacuum Ultravioles, The 65th JSAP Spring Meeting, 2018, 11-493 (2018.3)