Behaviour of 1.2 kV SiC JBS diodes under repetitive high power stress

A methodology and a dedicated test-bench to evaluate the behaviour of SiC JBS diodes under repetitive high power stresses and surge currents have been developed and illustrated with JBS and Schottky 1.2 kV SiC diodes. The concept of dissipated energy versus the number of cycles is introduced to characterize the degradation evolution of the tested device. The sweeping current pulse technique offers the possibility to evaluate the effect of the self-heating on the I (V). This is especially relevant for JBS SiC diodes whose series resistance highly depends on the junction temperature. JBS diodes show a ×2.66 (×4.16) higher surge current capability at 25 °C (225 °C) than the pure Schottky diodes. The fabricated SiC JBS and Schottky diodes have been submitted to more than 300,000 power cycles at a dissipated energy of 0.7 J, showing no relevant degradation.     

Unipolar nonvolatile memory devices with composites of poly(9-vinylcarbazole) and titanium dioxide nanoparticles

Organic-based devices with an 8 × 8 array structure using titanium dioxide nanoparticles (TiO₂ NPs) embedded in poly(9-vinylcarbazole) (PVK) film exhibited bistable resistance states and a unipolar nonvolatile memory effect. TiO₂ NPs were a key factor for realizing the bistability and the concentration of TiO₂ NPs influenced ON/OFF ratio. From electrical measurements, switching mechanism of PVK:TiO₂ NPs devices was closely associated with filamentary conduction model and it was found that the OFF state was dominated by thermally activated transport while the ON state followed tunneling transport. PVK:TiO₂ NPs memory devices in 8 × 8 array structure showed a uniform cell-to-cell switching, stable switching endurance, and a high retention time longer than 10⁴ s.     

Design,development and reliability testing of a low power bridge-type micromachined hotplate

In this paper, the development and reliability of a platinum-based microheater with low power consumption are demonstrated. The microheater is fabricated on a thin SiO₂ bridge-type suspended membrane supported by four arms. The structure consists of a 0.6 μm-thick SiO₂ membrane of size 50 μm × 50 μm over which a platinum resistor is laid out. The simulation of the structure was carried out using MEMS-CAD Tool COVENTORWARE. The platinum resistor of 31.0 Ω is fabricated on SiO₂ membrane using lift-off technique. The bulk micromachining technique is used to create the suspended SiO₂ membrane. The temperature coefficient of resistance (TCR) of platinum used for temperature estimation of the hotplate is measured and found to be 2.2 × 10⁻³/°C. The test results indicate that the microhotplate consumes only 11.8 mW when heated up to 400 °C. For reliability testing, the hotplate is continuously operated at higher temperatures. It was found that at 404 °C, 508 °C and 595 °C, the microhotplate continuously operated up to 16.5 h, 4.3 h and 4 min respectively without degrading its performance. It can sustain at least 53 cycles pulse-mode of operation at 540 °C with ultra-low resistance and temperature drifts. The structure has maximum current capability of 19.06 mA and it can also sustain the ultrasonic vibration at least for 30 min without any damage.     

NEMS switch with 30 nm-thick beam and 20 nm-thick air-gap for high density non-volatile memory applications

We developed two types of titanium nitride (TiN) based nanoelectromechanical systems (NEMS) switches with the smallest dimensions ever made by typical “top-down” complementary metal–oxide–semiconductor (CMOS) fabrication technology. NEMS cantilever switch (NCLS) and NEMS clamp switch (NCS) with 30 nm-thick TiN beam and 20 nm-thick air-gap were successfully fabricated and electrically characterized. The fabricated NCLS showed ideal on/off current characteristics with an essentially zero off current, a sub-threshold slope of less than 3 mV/decade, and an on/off current ratio over 10⁵ in air ambient. Also, the NCLS exhibited an endurance of over several hundred of switching cycles under dc and ac bias conditions in air ambient. Suspended beam memory (SBM) cell array structure was suggested for high density non-volatile memory applications.     

Study on the thermal stability of Ga-doped ZnO thin film: A transparent conductive layer for dye-sensitized TiO2 nanoparticles based solar cells

Generally, optoelectronic devices are fabricated at a high temperature. So the stability of properties for transparent conductive oxide (TCO) films at such a high temperature must be excellent. In the paper, we investigated the thermal stability of Ga-doped ZnO (GZO) transparent conductive films which were heated in air at a high temperature up to 500 °C for 30 min. After heating in air at 500 °C for 30 min, the lowest sheet resistance value for the GZO film grown at 300 °C increased from 5.5 Ω/sq to 8.3 Ω/sq, which is lower than 10 Ω/sq. The average transmittance in the visible light of all the GZO films is over 90%, and the highest transmittance is as high as 96%, which is not influenced by heating. However, the transmittance in the near-infrared (NIR) region for the GZO film grown at 350 °C increases significantly after heating. And the grain size of the GZO film grown at 350 °C after annealing at 500 °C for 30 min is the biggest. Then dye-sensitized TiO2 NPs based solar cells were fabricated on the GZO film grown at 350 °C (which exhibits the highest transmittance in NIR region after heating at 500 °C for 30 min) and 300 °C (which exhibits the lowest sheet resistance after heating at 500 °C for 30 min). The dye-sensitized solar cell (DSSC) fabricated on the GZO film grown at 350 °C exhibits superior conversion efficiency. Therefore, transparent conductive glass applying in DSSCs must have a low sheet resistance, a high transmittance in the ultraviolet–visible–infrared region and an excellent surface microstructure.     

Multilevel characteristics and operating mechanisms of nonvolatile memory devices based on a floating gate of graphene oxide sheets sandwiched between two polystyrene layers

Nonvolatile organic memory devices were fabricated utilizing a graphene oxide (GO) layer embedded between two polystyrene (PS) layers. Scanning electron microscope images of GO sheets sandwiched between two PS layers showed that the GO sheets were clearly embedded in the PS layers. Capacitance–voltage (C–V) curves of the Al/PS/GO/PS/n-type Si devices clearly showed hysteresis behaviors with multilevel characteristics. The window margin of the nonvolatile memory devices increased from 1 to 7 V with increasing applied sweep voltages from 6 to 32 V. The cycling retention of the ON/OFF switching for the devices was measured by applying voltages between +15 and −15 V. While the capacitance of the memory devices at an ON state have retained as 230 pF up to 10⁴ cycles, that at an OFF state maintained as 16 pF during three times of repeated measurements. The extrapolation of the retention data for the devices maintained up to 10⁶ cycles. The operating mechanisms of the nonvolatile organic memory devices with a floating gate were described by the C–V results and the energy band diagrams.     

Photocatalytic degradation of methyl orange and gas-sensing performance of nanosized ZnO

ZnO nanoparticles were synthesized by calcining composites of zinc nitrate and poly(vinyl pyrrolidone) (PVP, molecular weight 30 000) at a mass ratio of 1:2 at 500 °C for 2 h. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used to characterize the as-synthesized ZnO nanoparticles. The particles ranged in size from 30 to 50 nm. Infrared spectra of PVP and the PVP+Zn(NO₃)₂·6H₂O composite revealed coordination between the carbonyl (C=O) of PVP and Zn²⁺ of zinc nitrate, which led to a uniform nanoparticle morphology. The gas-sensing properties and photocatalytic performance of the final product were systematically investigated. The results show that the ZnO nanoparticles exhibit both a high response for ethanol detection and excellent photocatalytic activity for degradation of methyl orange under UV irradiation for 30 min.     

Preparation and characterization of conductive polypyrrole/kaolinite composites

Conductive polypyrrole (PPy)/kaolinite clay composites were prepared by in situ chemical polymerization of pyrrole in the presence of kaolinite using FeCl₃ as oxidant. The PPy content and conductivity of the composites reached 32.8% and 8.3×10^?2 S/cm at HCl concentrations of 1.5 M and 0.5 M, respectively. The microhardness of the composites containing different amounts of PPy was higher than that of the PPy and kaolinite components. The highest microhardness observed was 30.17 kg/mm² for the composite containing 9.6% PPy. The electrical resistance of the composites was monitored during heating–cooling cycles over the range 5–120 °C. The change in resistance with temperature was more repeatable for the composite than for PPy. The composites were characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The humidity-sensing properties were also examined.     

A novel 1,1′-bis[4-(5,6-dimethyl-1H-benzimidazole-1-yl)butyl]-4,4′-bipyridinium dibromide (viologen) for a high contrast electrochromic device

A new electrochromic viologen, 1,1′-bis-[4-(5,6-dimethyl-1H-benzimidazole-1-yl)-butyl]-4,4′-bipyridinium dibromide (IBV) was synthesized by di-quaternization of 4,4′-bipyridyl using 1-(4-bromobutyl)-5,6-dimethyl-1H-benzimidazole. X-ray photoelectron spectroscopy confirmed the formation of the IBV (viologen) salt as distinct signals due to quaternary nitrogen and neutral nitrogen, and ionic-bonded bromide were identified. An electrochromic device encompassing a dicyanamide ionic liquid based gel polymeric electrolyte with high ionic conductivity, a thermal decomposition temperature above 200 °C, and a stable voltage window of ～4 V with the IBV viologen dissolved therein, was constructed. IBV is a cathodically coloring organic electrochrome and the device underwent reversible transitions between transparent and deep blue hues; the color change was accompanied by an excellent optical contrast (30.5% at 605 nm), a remarkably high coloration efficiency of 725 cm² C^?1 at 605 nm and switching times of 2–3 s. Electrochemical impedance spectroscopy revealed an unusually low charge transfer resistance at the IBV salt/gel interface, which promotes charge propagation and is responsible for the intense coloration of the reduced radical cation state. The device was subjected to repetitive switching between the colored and bleached states and was found to incur almost no loss in redox activity, up to 1000 cycles, thus ratifying its suitability for electrochromic window/display applications.     

Electrical switching and conduction mechanisms of nonvolatile write-once-read-many-times memory devices with ZnO nanoparticles embedded in polyvinylpyrrolidone

We reported on the influence of zinc oxide nanoparticles (ZnO NPs) on the electrical bistable behavior of nonvolatile write-once-read-many-times (WORM) memory devices based on an indium-tin oxide/polyvinylpyrrolidone (PVP):ZnO NPs/aluminum (ITO/PVP:ZnO/Al) structure. The maximum ON/OFF current ratio of the nonvolatile WORM memory devices was approximately 3 × 10³ and the devices remained in the ON state even after the applied voltage was turned off. In addition, reliability studies for response time and once write/continuous read operations of the optimal ZnO NPs concentration are presented. The response times of both rise-time and fall-time were about 3 and 6 μs respectively. The conduction mechanisms of all voltage regions of the device were analyzed by theoretical models and electron trapping in the ZnO NPs of the electron tunneling among a PVP matrix was discussed.     

Photocurable propyl-cinnamate-functionalized polyhedral oligomeric silsesquioxane as a gate dielectric for organic thin film transistors

A polyhedral oligomeric silsesquioxane (POSS)-based insulating material with photocurable propyl-cinnamate groups (POSS-CYNNAM) was designed and synthesized through simple single step reaction for use as a gate dielectric in organic thin-film transistors (OTFT). POSS-CYNNAM was soluble in common organic solvents and formed a smooth thin film after spin-casting. A thin film of POSS-CYNNAM was cross-linked and completely solidified under UV irradiation without the use of additives such as photoacid generators or photoradical initiators. ITO/insulator/Au devices were fabricated and characterized to measure the dielectric properties of POSS-CYNNAM thin films, such as leakage current and capacitance. A pentacene-based OTFT using the synthesized insulator as the gate dielectric layer was fabricated on the transparent indium tin oxide (ITO) electrode, and its performance was compared to OTFTs using thermally cross-linked poly(vinyl phenol) (PVP) as the insulator. The fabricated POSS-CYNNAM OTFT showed a comparable performance to devices based on the PVP insulator with 0.1 cm²/Vs of the field effect mobility and 4.2 × 10⁵ of an on/off ratio.     

Bimodal and monomodal diamond particle effect on the thermal properties of diamond-particle-dispersed Al–matrix composite fabricated by SPS

Diamond-particle-dispersed aluminum (Al) matrix composites consisting of monomodal and bimodal diamond particles were fabricated in spark plasma sintering process, where the mixture of diamond, pure Al and Al–5 mass% Si alloy powders were consolidated in liquid and solid co-existent state. Microstructures and thermal properties of the composites fabricated in such a way were investigated and the monomodal and bimodal diamond particle effect was evaluated on the thermal properties of the composites. The composites can be well consolidated in a temperature range between 773 K and 878 K and scanning electron microscopy detects no reaction product at the interface between the diamond particle and the Al matrix. Relative packing density of the composite containing monomodal diamond particles decreased from 99.1% to 87.4% with increasing volume fraction of diamond between 50% and 60%, whereas that of the composite containing bimodal diamond particles was higher than 99% in a volume fraction of diamond up to 65%. The thermal conductivity of the composite containing bimodal diamond particles was higher than that of the composite containing monomodal diamond particles in a volume fraction of diamond higher than 60%. The coefficients of thermal expansion (CTEs) of the diamond-particle-dispersed Al–matrix composites fall in the upper line of Kerner model, indicating good bonding between the diamond particle and the Al matrix in the composite. The thermal conductivity of the composite containing 70 vol.% bimodal diamond particles was 578 W/m K and its CTE was 6.72 × 10⁻⁶ at R.T.     

Twistable nonvolatile organic resistive memory devices

We fabricated an 8 × 8 cross-bar array-type organic nonvolatile memory devices on twistable poly(ethylene terephthalate) (PET) substrate. A composite of polyimide (PI) and 6-phenyl-C61 butyric acid methyl ester (PCBM) was used as the active material for the memory devices. The organic memory devices showed a high ON/OFF current ratio, reproducibility with good endurance cycle, and stability with long retention time over 5 × 10⁴ s on the flat substrate. The device performance remained well under the twisted condition with a twist angle up to ～30°. The twistable organic memory device has a potential to be utilized in more complex flexible organic device configurations.     

Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Electrical and thermal stability of Ag ohmic contacts for GaN-based flip-chip light-emitting diodes by using an AgAl alloy capping layer

We have investigated Ag(200 nm)/AgAl(100 nm) ohmic contacts to p-type GaN for near-UV (405 nm) flip-chip light-emitting diodes (LEDs). It is shown that the use of an AgAl alloy capping layer (with 8 at% Al) results in better electrical and optical properties as compared to single Ag contacts when annealed at 430 °C. For example, Ag/AgAl (8 at% Al) contacts give specific contact resistance of 4.6×10^–4 Ω cm² and reflectance of 90% at a wavelength of 405 nm. However, use of an AgAl (with 50 at% Al) layer is not effective. LEDs fabricated with the Ag/AgAl (8 at% Al) reflectors produce higher light output as compared with the ones with single Ag reflectors. Ohmic mechanisms of the Ag/AgAl (8 at% Al) contacts are described and discussed.     

Analysis of nanometer-scale InGaAs/InAs/InGaAs composite channel MOSFETs using high-K dielectrics for high speed applications

The outstanding electron transport properties of InGaAs and InAs semiconductor materials, makes them attractive candidates for future nano-scale CMOS. In this paper, the ON state and OFF state performance of 30 nm gate length InGaAs/InAs/InGaAs buried composite channel MOSFETs using various high-K dielectric materials is analyzed using Synopsys TCAD tool. The device features a composite channel to enhance the mobility, an InP spacer layer to minimize the defect density and a heavily doped multilayer cap. The simulation results show that MOSFETs with Al₂O₃/ZrO₂ bilayer gate oxide exhibits higher gm/I_D ratio and lower sub threshold swing than with the other dielectric materials. The measured values of threshold voltage (V_T), on resistance (R_ON) and DIBL for Lg = 30 nm In_0.53Ga_0.47As/InAs/In_0.53Ga_0.47As composite channel MOSFET having Al₂O₃/ZrO₂ (EOT = 1.2 nm) bilayer dielectric as gate oxide are 0.17 V, 290 Ω-µm, and 65 mV/V respectively. The device displays a transconductance of 2 mS/µm.     

Complementary-like inverters based on an ambipolar solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative

We report on high-mobility top-gate organic field-effect transistors (OFETs) and complementary-like inverters fabricated with a solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative as the channel semiconductor and a CYTOP/Al₂O₃ bilayer as the gate dielectric. The OFETs showed ambipolar behavior with average electron and hole mobility values of 1.2 and 0.01 cm² V^?1 s^?1, respectively. Complementary-like inverters fabricated with two ambipolar OFETs showed hysteresis-free voltage transfer characteristics with negligible variations of switching threshold voltages and yielded very high DC gain values of more than 90 V/V (up to 122 V/V) at a supply voltage of 25 V.     

The effect of heat treatments on the properties of Ti/Pt heating elements for gas sensor applications

Ti/Pt as heating element for gas sensor applications was fabricated on silicon (Si) wafer substrate. The fabricated device was subjected to heat treatment at different prescribed time periods for thermal stability. The energy dispersion spectroscopy (EDS) results of the device indicated that there were no Ti traces on the Pt surface after heat treatment at 450 °C for 3 and 4 h in an argon (Ar) atmosphere. A maximum temperature coefficient of resistance (TCR) with a value of 2.88×10^?3 K^?1 was obtained for the device with 3 h heat treatment.     

Direct photo-patternable,low-temperature processable polyimide gate insulator for pentacene thin-film transistors

We have developed photo-sensitive, low-temperature processable, soluble polyimide (PSPI) gate insulator with excellent resistance to the photo-patterning process. The PSPI was synthesized through one-step condensation polymerization of monomers 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and 3,5-diaminobenzyl cinnamate (DABC). PSPI thin film, fabricated at 160 °C, has a dielectric constant of 3.3 at 10 kHz, and leakage current density of <1.7 × 10^?10 A/cm², while biased from 0 to 100 V. PSPI could be easily patterned by selective UV-light exposure and dipping into γ-butyrolactone. To investigate the potential of the polyimide as the photo-patternable gate insulator, we fabricated pentacene OTFTs and confirmed the PSPI’s resistance to the photo-patterning process. The photo-patternable polyimide shows promise as gate dielectrics for OTFTs.     

Analysis of quantum conductance,read disturb and switching statistics in HfO2 RRAM using conductive AFM

Most studies on resistance switching have been carried out at the device level with the standard electrical characterization setup, which allows for effective automated reliability test and extensive characterization of the lifetime of an RRAM device. However, it is equally important to be able to probe the switching phenomenon at the nanoscale so as to improve insight on the bias-dependent kinetic behavior of the filament during multiple reversible breakdown and recovery cycles. This study aims to do just that by probing HfO₂ blanket films (~ 4 nm) with a W bottom electrode using an ultra-sharp Pt-wire conductive AFM (CAFM) tip with an areal resolution of ~10–20 nm at ambient conditions. The use of the CAFM allows for a more reliable assessment of single filament evolution behavior as possible multiple filamentation events (common at the device level) are rare for such small probing areas. The role of oxygen vacancy induced filaments is studied here by using low compliance setting and moderate voltage levels, ensuring operation in the sub-quantum conductance regime. Our results show good repeatable switching trends and also provide insight on the quantum conductance phenomenon in oxygen vacancy based filaments. The read disturb trends in switching are investigated for the high resistance state (HRS) and the impact of tip-induced mechanical stresses on forming lifetime is also presented, which could serve as a motivator for further studies on non-volatile memory (NVM) reliability for flexible electronics devices and system on chip (SoC) applications.