Comprehensive study on unipolar RRAM charge conduction and stochastic features, a simulation approach

An in-depth analysis of resistive switching (RS) in unipolar devices is performed by means of a new simulator based on resistive circuit breakers of different features. The forming, set and reset processes are described in terms of the stochastic formation and rupture of conductive filaments of several branches in the dielectric. Both, the electric field and temperature dependencies are incorporated in the simulation. The simulation tool was tuned with experimental data of devices fabricated making use of the Ti/HfO2/Si stack. The variability and the stochastic behavior are characterized and reproduced correctly by simulation to understand the physics behind RS. Reset curves with several current steps are explained considering the rupture of different branches of the conductive filament. The simulation approach allows to connect in a natural manner to compact modeling solutions for the devices under study.

Static and Dynamic Characterization of a 3.3 Kv, 45 A 4H-Sic MOSFET

Wide bandgap materials such as Silicon Carbide (SiC) has enabled the use of medium voltage unipolar devices like Metal-Oxide Field Effect Transistors (MOSFETs) and Junction Field Effect Transistors (JFETs), which can switch at much higher frequencies as compared to their silicon counterparts. It is therefore imperative to evaluate the performance of these medium voltage devices. In this paper, the static characterization and the switching performance of the new single die 3.3 kV, 45 A 4H-SiC MOSFET developed by Cree Inc are presented. The switching performance is measured through the conventional Double Pulse Test. Testing is done at a dc-link voltage of 1.5 kV for different values of current, and gate resistances.

Zirconium Tetrakis(8-hydroxyquinolinolate) and Lithium Schiff-Base Cluster Complex for Efficient Charge Injection and Transfer in Green PHOLED processed by OVPD

ABSTRACTTypical electron transport (2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)) and injection (Cs2CO3) materials are successfully replaced by zirconium tetrakis(8-hydroxyquinolinolate) (Zrq4) and lithium 2-((o-tolylimino)methyl)-phenolate (EI-111) in simplified OLED (organic light-emitting diodes) processed by organic vapor phase deposition (OVPD). The performance of combining Zrq4 and EI-111 is analyzed in unipolar devices and compared to devices with configurations of Zrq4/Cs2CO3, TPBi/EI-111 and TPBi/Cs2CO3. Current density-voltage (J-V) measurements are performed and correlated to different carrier injection and transport properties. The investigated material combinations are implemented in the simplified OLED structures and compared to each other. To account for the high HOMO level of Zrq4, 5 nm of TPBi are added to confine holes and excitons in the emissive layer (EML) and to improve device performance. After tailoring the organic stack for OLED with Zrq4, a remarkable boost in device efficiency is observed. The luminous efficacy increased from 3.0 to 21.9 lm/W and the EQE from 2.1 to 11.0 % for a device with Zrq4/EI-111. Furthermore, OLED having Zrq4/Cs2CO3 show an even greater enhancement to 26.3 lm/W and 11.7 %.

An Investigation on Resistive Switching Characteristics Induced by HfOx and Electrode Interfaces

Pt/HfOx/Pt resistive switching devices with symmetric electrodes were fabricated. Bipolar resistive switching (RS) behaviors and unipolar behaviors were then observed under a positive/negative bias applied to the top electrode (TE). A comparison and analysis of bipolar/unipolar RS behaviors under different voltage polarities was then performed.The results demonstrated that bipolar RS was achieved via a drift of anion (O2-) under the electric field resulting in the rupture and recovery of filaments at the interface. When the filaments dissolved and formed at the interface near BE, the performance of the bipolar RS devices was better. However, for unipolar RS devices, when filaments dissolved and formed at the interface near TE, the performance was even better. These results indicated that a drift of O2-caused by electric field and a diffusion of O2-induced by Joule heat were the main reasons for unipolar RS. The different characteristics of the bipolar and unipolar devices can be attributed to the existence of a different number of defects at the active interface of the devices. This was where the rupture and recovery of filaments occurred. The results also indicate that the active interface is more important than other interfaces for RRAM performance.

THE EFFECT OF LIGHT-INDUCED METASTABLE DEFECTS ON THE MAGNETO-CONDUCTANCE OF POLY(PHENYLENE-VINYLENE) DIODES

We studied the changes in the magneto-conductance (MC) response in bipolar and unipolar organic diodes based on the π-conjugated polymer poly[2-methoxy-5-(2-ethylhexyl-oxy)-1,4-phenylene-vinylene] (MEH-PPV), upon prolonged illumination of light in the Visible–UV range. We found that the MC response in the hole-unipolar device gradually changes sign from negative to positive upon illumination, caused by the photogeneration of metastable donor-like defects in the MEH-PPV layer. This conclusion is supported by photoinduced absorption spectroscopy of pristine MEH-PPV film. In contrast, we found very little change in the MC response upon illumination of electron-unipolar devices; however, the MC response substantially increases in bipolar devices. We interpret the sign change in the MC response of hole-unipolar devices as due to illumination induced change in the spin-pairs that participate in the MC, from spin pairs of same charge to spin pairs of opposite charges.

Development of SiC Super-Junction (SJ) Devices by Multi-Epitaxial Growth

Super-junction (SJ) devices have been developed to improve the trade-off relationship between the blocking voltage (VBD) and specific on-resistance in unipolar power devices. This SJ structure effect is expected in SiC unipolar devices. Multi-epitaxial growth is a known fabrication method for SJ structures where epitaxial growth and ion implantation are repeated alternately until a certain drift-layer thickness is achieved. In this study, we fabricated two types of test elemental groups with an SJ structure to evaluate the breakdown voltage (VBD) and specific resistivity of the drift layer (Rdrift). Experimental results show that VBDexceeded the theoretical limit of the 4H-SiC by 300V, and Rdriftagreed well with the estimated value from the device simulation. The beneficial effects of the SJ structure in the SiC material on VBDand Rdriftwere confirmed for the first time.