Linkai Wang

Efficient single-active-layer organic light-emitting diodes with fluoropolymer buffer layers

In the present Letter, efficient organic light-emitting diodes (OLEDs) with tris(8-hydroxyquinolino) aluminum (Alq3) as a single-active layer have been prepared by using a series of fluoropolymer buffer layers. The OLEDs with a 10-nm-thick poly(tetrafluroethylene-perfluoroalkylvinylethers) (PFA) buffer layer had a current efficiency of 4.46cd∕A at a current density of 2000A∕m2, whereas conventional double-layer OLEDs with N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) and Alq3 showed a current efficiency of only 3.81cd∕A at the same condition in our experiment. The effect of the insulating fluoropolymer buffer layers could be interpreted to enhance hole injection and improve carrier balance.

Effects of anode materials on resistive characteristics of NiO thin films

This letter shows that the NiO-based structure with different anodes has different resistive switching properties. A conical conductive filament (CF) model is proposed for oxygen vacancies distributed in NiO films. Modeling analysis reveals much larger dissolution velocity of CF near anodes than near cathodes during the reset process. Different interfaces shown in Auger electron spectroscopy can be bound with the model to reveal that CF is dissolved in the structure with Pt or Au as anodes, while CF remains constant if the anode material is Ti or Al, which can explain whether switching properties occur in the specific NiO-based structures.

Study of Iridium Bottom Electrode in Ferroelectric Random Access Memory Application

Iridium (Ir) is comprehensively investigated for application in Ferroelectric random access memory. Ir thin films are treated at different temperature and examined to investigate the effects of thermal processes which are unavoidable in fabrication of ferroelectric films. Stress in as deposited Ir films is found to be relieved by 400°C baking in the air. IrO2 starts to form when the temperature reaches 600°C. Then IrO2/Ir electrode formed by 650°C rapid thermal process (RTP) and Ir are used as bottom electrodes in sol-gel lead zirconium titanate (PZT) based capacitors. Effects of bottom electrodes on PZT films are investigated and electrical properties of sol-gel PZT based capacitors including polarization, fatigue and leakage are studied. Capacitors using IrO2/Ir bottom electrodes are found to have larger remnant polarization and better fatigue property. Mechanism for fatigue is studied based on oxygen vacancies according to the effects of IrO2. Leakage performance is investigated from the viewpoint of electrode work functions, which are proposed to be principal factor affecting leakage performance. Besides, surface conditions of the electrodes are examined. Roughness increase and hillock formed at the surface of Ir during thermal process may be another reason to affect leakage current.

Key Technologies for FeRAM Backend Module

Ferroelectric random access memory (FeRAM) is believed to be the most promising candidate for the next generation non-volatile memory due to its fast access time and low power consumption. Fabrication technologies of FeRAM can be divided into two parts: CMOS technologies for circuits which are standard and can be shared with traditional IC process line, and process relating to ferroelectric which is separated with CMOS process and defined as backend module. This paper described technologies for integrating ferroelectric capacitors into standard CMOS, mainly about modeling of ferroelectric capacitors and backend fabrication technologies. Hysteresis loop of the ferroelectric capacitor is the basis for FeRAM to store data. Models to describe this characteristic are the key for the design of FeRAM. A transient behavioral ferroelectric capacitor model based on C-V relation for circuit simulation is developed. The arc tangent function is used to describe the hysteresis loop. “Negative capacitance” phenomenon at reversing points of applied voltage is analyzed and introduced to the model to describe transient behaviors of the capacitor. Compact equivalent circuits are introduced to integrate this model into HSPICE for circuit simulation. Ferroelectric materials fabrication, electrodes integration and etching are the main technologies of FeRAM fabrication process. An metal organic chemical vapor deposition (MOCVD) process is developed to fabricate high quality Pb(Zr 1-x Ti x )O 3 (PZT) films. Pt is known to cause the fatigue problems when used as electrodes with PZT. Ir is used as electrodes to improve the fatigue property of PZT based capacitors, and mechanism of the fatigue is analyzed. Hard mask is used to reduce the size of the capacitors and damage caused in etching process. In our process, Al 2 O 3 is developed as hard mask, which simplifies the FeRAM backend integration process.

Bipolar switching analysis and negative resistance phenomenon in TiO x -based devices

This work addresses the negative resistance phenomenon in TiOx-based resistive-switching cells applied in RRAM. The switching behavior of the Ag (or Pt, top)/TiOx/Pt (bottom) structure was also carefully analyzed. The negative resistance phenomenon and switching behavior can be modeled to space-charge-limited conduction (SCLC) mechanism with asymmetric electron trapping centers. A two-variable model based on SCLC is also proposed.