Search

In oxide-based RRAMs using reactive electrodes such as Al, the properties of spontaneously formed interfacial layers are critical factors in determining the resistive switching (RS) performance and reliability. This interfacial layer can provide the beneficial function of oxygen reservoir and series resistance, but is very labile and prone to deterioration, causing fatal reliability problems. Moreover, there are technical difficulties in manipulating and improving the functional interfacial layer due to the various interaction dynamics near the interface and the unstable thermodynamic properties of Al. In this work, laser-assisted interface engineering, which allows exquisite manipulation of the labile interfacial layer, is proposed to improve the reliability and performance of Al/ZnO/Al RRAMs. In addition to photothermal and photochemical effects, the proposed laser process enables fine control over out-diffusions of Al atoms in the vicinity of the ZnO/Al interface, forming a robust interfacial layer with a uniform morphology and abundant oxygen Frenkel pairs. This laser-engineered interfacial layer increases the RHRS/RLRS ratio by over 100-fold and reduces RHRS variation with improved oxygen reservoir ability. It also appears to reduce leakage current and power consumption by acting as a stable series resistance. The correlation between structural and stoichiometric properties of the functional interfacial layer and the performance and reliability of the RRAM is explicated. The results suggest that laser-assisted interface engineering can be one of the most promising methods to implement highly reliable, high-performance Al/ZnO/Al RRAMs.

Gel polymer electrolyte (GPE) is recently attracting a significant attention for lithium air batteries (LABs) due to better reliability compared with liquid type electrolyte and higher ionic conductivity compared with solid type electrolyte. In this paper, we report a new combination of polyacrylonitrile (PAN)-based GPE with tetrachloro-1,4-benzoquinone (tCl-pBQ) as the redox mediator for the LAB application. The synthesized PAN/tCl-pBQ GPE with lithium bis (tri-fluoro-methane-sulfonyl)imide (LiTFSI)/tetraethylene glycol dimethyl ether (TEGDME) displays a decrease in the LAB charge voltage from ~ 4.2 to 3.6 V and a ~ 10% increase in energy cycle efficiency compared with PAN GPE without tCl-pBQ. The decrease in charge voltage improves the cyclability from 8 to 98 cycles. Structural analyses reveal that addition of tCl-pBQ accelerates the formation of an amorphous phase in the PAN-based gel polymer electrolyte matrix, improving the ionic conductivity from 7.64 to 12.5 mS cm−1 at room temperature.

Monolayer doping (MLD) has been regarded as the most suitable doping method for future semiconductor devices. MLD based on surface functionalization can be seriously affected by the surface states, including the orientations, crystallinities, and defects. We report for the first time the effect of surface states on boron-MLD (B-MLD) process and discuss the applicability of MLD for a fin structures. Depending on the surface states, the monolayer formation reaction is restricted, which causes more than five-fold differences in the doping level. Therefore, the surface states should be gravely considered before applying MLD and are crucial constraints in the MLD process on non-planar structures that have different surface states depending on its structural position. The B-MLD process on as-cleaned (100) and (110) silicon surfaces provides doping levels of 4.69 × 1020 and 2.48 × 1020 atoms/cm3, respectively. The MLD efficiency on the (110) orientation is degraded by insufficient reaction sites for the hydrosilylation reaction on the monohydride-terminated (110) silicon surface. Additionally, the surface damage interrupts the formation of a dopant-containing monolayer, causing a poor doping level and dose uniformity. Our research provides new insights into the development of wet-chemical doping methods for non-planar devices by studying the effect of surface states on MLD efficiency.

A novel 3D interconnected web-like carbon material with high electrocatalytic activities toward oxygen reduction reaction (ORR) has been developed for the first time via direct carbonization of a composite (Zn-MOF-74@CNFs) comprising Zn-MOF-74s grown on carbon nanofibers (CNFs) web. The hexagonal pillar shaped Zn-MOF-74s with a diameter which ranges from 300 to 600 nm grow along the CNFs web by solvothermal method. After carbonization of Zn-MOF-74@CNFs, effective interconnections promoting electron transfer are successfully formed between carbonized Zn-MOF-74 (C-Zn-MOF-74) and on CNFs as well as C-Zn-MOF-74 themselves. The extraordinary 3D structure thus fabricated significantly improves the electrocatalytic activity toward ORR. The calculated electron transfers number (n) values for carbonized Zn-MOF-74@CNFs (C-Zn-MOF-74@CNFs) are nearly 4 at potentials ranging from 0.4 to 0.6 V (vs. reversible hydrogen electrode), demonstrating that the ORR process occurs dominantly through a direct four-electron pathway. Tafel slope of C-Zn-MOF-74@CNFs at low over-potential are lower than those from C-Zn-MOF-74 and even commercial Pt/C. Durability is also found to exceed that of commercial Pt/C. This study provides a novel 3D interconnected carbon material as a non-metal ORR electrocatalyst and design strategy for a large-area, self-standing and binder-free carbon-based electrochemical electrode.

Photo-thermal processing assisted by laser irradiation is proposed as a novel method to control the phase of nickel silicide with reduction in the diffusion of nickel into the silicon substrate. The third harmonics of Nd3+:Y3Al5O12 laser (wavelength, 355 nm) is used for photo-thermal processing. Optical and thermal simulations are performed to obtain an optimum thickness (30 nm) of the nickel film for photo-thermal processing and to predict the temperature profile of the nickel-silicon interface during laser irradiation. It is confirmed that Ni2Si, NiSi and NiSi2 phase are effectively formed at the laser energy densities of 15, 20–40, and 50 mJ/cm2, respectively. We demonstrate that the phases of nickel silicide determined by Raman spectroscopy and X-ray diffraction analyses are in good agreement with those predicted by the heat transfer simulation. In addition, undesirable diffusion of nickel into silicon substrate is considerably reduced by instantaneous photo-thermal processing using the nano-second laser (pulse duration, 6 ns).

Many efforts are continuously devoted to developing high-efficiency, low-cost, and highly scalable oxygen reduction reaction (ORR) electrocatalysts to replace precious metal catalysts. Herein, we successfully synthesize Co- and defect-rich carbon nanofibers (CNFs) using an efficient heat treatment approach involving the pyrolysis of electrospun fibers at 370 °C under air. The heat treatment process produces Co-decorated CNFs with a high Co mass ratio, enriched pyridinic N, Co-pyridinic Nx clusters, and defect-rich carbon structures. The synergistic effects from composition and structural changes in the designed material increase the number of catalytically active sites for the ORR in an alkaline solution. The prepared Co- and defect-rich CNFs exhibit excellent ORR activities with a high ORR onset potential (0.954 V vs. RHE), a large reduction current density (4.426 mA cm−2 at 0.40 V), and a nearly four-electron pathway. The catalyst also exhibits a better long-term durability than commercial Pt/C catalysts. This study provides a novel hybrid material as an efficient ORR catalyst and important insight into the design strategy for CNF-based hybrid materials as electrochemical electrodes.

In this paper, thermal stability of a one-dimensional metallodielectric photonic crystal (1D MDPhC) structure based on W and SiO2 for thermophotovoltaic systems is reported. The thermal degradation mechanism of the structure, in its operating temperature range, is thoroughly investigated by using energy dispersive spectroscopy (EDS) with transmission electron microscope (TEM) and depth-profiling X-ray photoelectron spectroscopy (XPS). It is found that the structure is entirely destroyed under 1400 K by an inter-diffusion process forming a mixture of W and SiO2 without measurable oxidization of W. But, long-term annealing results in oxidization of W layer even at a lower temperature of 1300 K. During the long-term annealing, oxygen atoms in outside atmosphere are believed to cause oxidation of the upper W layer below the top SiO2 layer. Additionally, delaminated spots are observed over the surface. These thermal behaviors are potential clues to prevent or minimize thermal degradation of the multilayer structure under high temperature operation.

We investigate the effect of the number of laser pulses on the formation of p+/n silicon ultra-shallow junctions during non-melt ultra-violet laser (wavelength, 355 nm) annealing. Through surface peak temperature calculating by COMSOL Multiphysics, the non-melt laser thermal annealing is performed under the energy density of 130 mJ/cm2. We demonstrate that increasing the number of laser pulses without additional pre-annealing is an effective annealing method for achieving good electrical properties and shallow junction depth by analyzing sheet resistance and junction depth profiles. The optimal number of laser pulses is eight for achieving a high degree of activation of dopant without further increase of junction depth. We have also explained the improved electrical characteristics of the samples on the basis of fully recovered crystallinity as revealed by Raman spectroscopy. Thus, it is suggested that controlling the number of laser pulses with moderate energy density is a promising laser annealing method without additional pre-annealing.

Emissivity-tunable metamaterials of layered refractory metal and dielectric have great potentials as a simple thermophotovoltaic (TPV) selective emitter due to its near-omnidirectional, polarization-independent, and broadband selective emissivity. However, it is known that the stability of the layered structure is limited by the oxidation of metals. While there still exists ambiguity concerning the main source of oxygen between adjacent oxide layers and external residual oxygen, most reports focus on the adjacent layers. In this report, thermal stability of a tungsten-based layered metamaterial is investigated under a high-vacuum environment with great care to reduce residual oxygen. The results show unprecedented thermal stability up to 1200 °C for 3 h without any measurable oxidation of metal. This implies that the interlayer diffusion of oxygen from adjacent oxide layers is not exclusively responsible for the oxidation of metal. At such a high temperature, the layered metamaterial theoretically yields a high convertible radiative power density of 3.04 W/cm2 with comparable spectral efficiency of 40.2%. Finally, after performing series of thermal tests under higher thermal loads, we propose a novel high-temperature degradation model for layered metamaterials, the stability of which is ultimately limited by the agglomeration of thin metal layers.

Compared to conventional electrode, a self-standing structure electrode is an effective way to achieve high performance of supercapacitor by maximizing the use of active material. We present a new combination of nickel oxide–carbon composites fabricated by directly carbonizing a nickel metal–organic framework (Ni-MOF)@carbon nanofiber (CNF) for a self-standing electrode of the supercapacitor application. The new scheme utilizes the CNF film as a substrate with high electron transferring capability and as a backbone of a self-standing electrode as well. It is observed that the MOF-derived NiOs with a diameter of ~ 8 nm are uniformly distributed in the carbon matrix and result in the improvement of the electrical conductivity. The self-standing electrode, NiO/C@CNF composite, provides a high rate capability with high specific capacitances of 742.2 and 671.1F g−1 (at 1 and 10 A g−1), respectively. An asymmetric supercapacitor (ASC) constructed from the NiO/C@CNF and the activated carbon exhibits an excellent specific energy density of 58.43 Wh kg−1 at a power density of 1,947 W kg−1. It is also confirmed that the ASC shows a good cycle stability from the long-term cycling test. It is demonstrated that the proposed nickel oxide–carbon composite has a potential as promising self-standing electrode materials for supercapacitors.

Metal-organic framework(MOF)-derived carbon material is a promising material for lithium-air battery cathode, but its pores are small (

The fabrication of metal-organic framework (MOF) films on solid substrates is challenging due to poor wettability and lack of sufficient reactive functional groups on the surface, which prevents the stable attachment and growth of MOF films. For this reason, many studies have been conducted to directly grow MOF films on solid substrates such as metals, metal oxides and ceramics using polymer or functional groups. In this work, silicon (100) substrates were functionalized with carboxylic functional groups using a mixture of acrylic acid and hydrofluoric acid through hydrosilylation process. Mg-MOF-74 films formed on silicon substrates by 24 hours of solvothermal synthesis exhibited high surface areas (965 m2/g) and high CO2 gas capture behavior (186 cm3/g). This approach allowed the direct growth of Mg-MOF-74 films on silicon substrates by solvothermal synthesis. Further, this work provides a concept for the surface treatment to form sufficient reactive functional groups based on carboxylic acid on silicon substrates.

In this study, a facile microwave-hydrothermal method was successfully applied to synthesize boron and nitrogen co-doped graphene (BNG) electrocatalyst for the oxygen reduction reaction (ORR). It consists of an efficient two-step process involving simultaneous doping with different heteroatoms (B and N) and reduction of doped graphene oxide. It was found that the B and N contents of highly reduced BN co-doped graphene (HRBNG) are 3.55 and 4.43 at%, respectively. The HRBNG exhibited clearly enhanced electrocatalytic activity towards the ORR in alkaline electrolytes. The electron transfer number (n) was obtained 3.53 ∼ 3.84 in potential range of 0.465 V–0.225 V, indicating that the HRBNG favors the four-electron pathway for the reduction of oxygen. These results demonstrate that the synthesized HRBNG has potential to replace expensive precious metal catalysts and also provide a new strategy to synthesize heteroatom-doped graphene-based catalyst.

In this study, the poly-vinylidene fluoride (PVDF)/p-benzoquinone (pBQ) gel polymer electrolyte (GPE) is synthesized well with LiTFSI/TEGDME for Li-air battery (LAB) applications. The PVDF/pBQ GPEs are characterized by electrochemical analysis and compared to the PVDF GPE. We confirm that pBQ molecule in PVDF GPE shows redox mediate behavior during charge-discharge process. Employing PVDF/pBQ GPE for LAB leads to an improved cycleability from 2 to over 30. In addition it results in a charge voltage decrease from around 4.2 V to around 3.3 V. The energy cycle efficiency (ECE) is improved about 15%. The pBQ accelerates to the formation of an amorphous phase on synthesized PVDF GPE matrix by acting inhibitor for polymerization. It leads to ionic conductivity improvement of synthesized GPE from 2.83 mScm−1 to 4.98 mScm−1.

Selective and localized process technology is a key enabler for the monolithic three-dimensional (M3D) integration of high-performance oxide-based resistive random access memory (RRAM) which is emerging as a post-flash nonvolatile memory solution. In this work, a photo-thermal laser process in a ZnO/Pt multi-layered structure is proposed as a new strategy to modulate the crystallinity at the ZnO/Pt interface and the oxygen vacancy (VO) distribution in the ZnO layer. Intensive optical and thermal simulation has been utilized for the optimized laser processing of the ZnO/Pt interface. It is verified that the optimized laser process can produce a thin ZnO crystallized layer on the ZnO/Pt interface and a significant concentration gradient of VO within the ZnO layer, without any deformation of the multi-layered structure. The switching characteristics of the laser-processed Pt/ZnO/Pt RRAM reveal that the laser process can increase the RHRS/RLRS ratio by up to tenfold and decrease the VSET by about 25%. The reliability of the RRAMs is also considerably improved. Our research demonstrates that the photo-thermal approach is a very promising method for improving oxide-based RRAM performance while meeting the critical requirements of M3D integration.

In this report, a selective interface laser annealing (SILA) process is applied to the atomic-layer-deposited Al2O3 film/p-type silicon interface using an Nd3+: YAG laser with a wave-length of 532 nm. The absorptivity of the laser wavelength confirms a full absorption of the laser power to the Al2O3/Si interface without any loss through the Al2O3 film, which results in a successful selective annealing of the interface. Compared with conventional forming gas annealing (FGA), the SILA is found to build a thinner SiO2 interface layer and lead to a less reduction of capacitance. It is also revealed that the laser-annealed sample exhibits a surface blister density of 31 counts/mm2 which is much lower than the value of 250 counts/mm2 obtained from FGA. The suppression of the surface blister is attributed to the much shorter processing time of lasing (

In this study, self-standing, binder-free ZIF-9 derived Co3O4/carbon nanofiber composites were synthesized via electrospinning and post thermal treatment for use as the cathode in a non-aqueous Li-air battery. Due to possessing a three-dimensional cross-linked web structure, Co3O4/carbon nanofiber composites are used directly as the cathode for Li-air batteries without the use of any binders or conductive metal foam, thus alleviating undesirable chemical reactions. We confirm that metallic cobalt (Co) in ZIF-9 is successfully oxidized to cobalt oxide (Co3O4) following two thermal treatment steps by analysis of XRD and XPS. The initial discharge capacity of Co3O4/carbon nanofiber composites exceeds 760 mAh g−1, which is a much higher discharge capacity compared to pristine carbon nanofiber (72 mAh g−1). Additionally, Co3O4/carbon nanofiber composite based cells exhibit improved cycling properties and a lower charge overpotential at various current densities. The improved electrochemical properties of the Co3O4/carbon nanofiber composites are attributed to the catalytic activity and stable contact with the homogeneously distributed Co3O4 in the carbon nanofiber structure. This work demonstrates that the synthesized Co3O4/carbon nanofiber composites could possibly be applied for use as next generation electrode materials for energy storage and conversion devices, particularly Li-air batteries.

In this letter, we utilize a simplified hydrothermal method without any addition of catalyst to synthesize one-and three-dimensional structured pure vanadium pentoxide (V 2 O 5 ) particles, V 2 O 5 nano-belt, micro-flower and micro-plane-flower. The synthesis is made possible by the formation of shcherbinaite phase and its cleavaging property along (001) facet by physical force. The V 2 O 5 nano-belt has been derived from the V 2 O 5 precursor in de-ionized (D.I.) water without catalysts by using stirred autoclave system. And V 2 O 5 micro-flower and micro-plane-flower have been synthesized in ethylene glycol solvent under controlled pH condition by HNO 3 or NH 4 OH. The synthesized nano-belts are less than 100 nm in width and less than 50 nm in thickness, respectively. The diameters of the synthesized micro-flower and micro-plane-flower are 5–8 μm. It is demonstrated that the prepared specimens are pure V 2 O 5 composition with shcherbinaite phase.

We develop a light-emitting diode (LED) metal package consisting of a reflector integrated with a heat spreader as one body. The structure functions obtained from the thermal transient measurement reveals that the integrated structure of the metal package leads to improved thermal performance. We make significant efforts to clarify the interaction between the extracted light from the LED chip and the inside surface of the metal package. The radiant flux from the LED chip is calculated from the definition of thermal resistance and is implemented as an input parameter to predict the optical power and ray tracing. The simulated optical output power in the far-field mode is 145.3 mW, which is in good agreement with the value measured by an integrating sphere. It is shown that a large portion of the emitted light (37.12 mW) is lost by the interaction with the package surface before it reaches the far-field receiver. It is demonstrated that the angle of the first reflector cup is one of the critical design parameters in the metal package structure that we investigate. When the first reflector angle is optimized from 80° to 55° (with the second reflector angle of 50°), the optical power increases from its original value of 145.3-170.15 mW with an improved optical extraction efficiency from 75.63% to 88.57%. The ray tracing and illuminance raster charts also confirm that the improved optical performance of the metal package mainly stems from the optimization of the reflector angles.

This paper reports on the thermal analysis and improvement of the light emitting diode (LED) module as a lighting source. The analysis was made by transient thermal measurement and thermal simulation using the Finite Volume Method. Two basic thermal schemes were applied for the decrease of the junction temperature of the LED module. Thermal resistance was analytically defined for the LED module with multi LED packages and was confirmed by the experimental data obtained from the thermal transient method. It was found that the thermal improvement of the LED module led to the enhancement of the light output power and radiant intensity. The thermally designed LED module exhibited about 20% decrease in junction temperature compared with a basic structure before thermal design. The temperature calibrating factor, 0.046 nm/°C, was calculated from the peak wavelengths of the LED modules.

This paper reports on the direct thermal observation of the pentacene – based organic thin-film transistors (OTFTs) under the real operating conditions. Liquid crystal (LC) spreading method was utilized for the thermal investigation of an active layer of the OTFT package. Temperature variation in the OTFT package was recorded for the different input power and significant heat generation was observed in the confined active layer. Detailed thermal performance of the OTFT package was projected using a Computational Fluid Dynamics (CFD) method as well. It was shown that the driving of the OTFT package with the drain voltage of −15 V resulted in the active layer temperature of about 53.2 °C. The result indicates that the device design with effective thermal dissipation is imperative for reliable operation of the OTFT package.

In this work, we have analyzed the thermal properties of a GaN-based LD (Laser Diode) as functions of input powers, cooling conditions, and ambient temperatures. It was found that the thermal resistance has a slight change with input current under the forced cooling condition. In contrast to the forced cooling condition, significant change of thermal resistance was observed under the natural cooling condition. When the ambient temperature was increased from 0 °C to 50 °C, the measured thermal resistance was increased from 20 K/W to 27.5 K/W.

In this paper, we describe an optimization process of thermal design for the light lamp which utilizes the Light Emitting Diode (LED) module as a lighting source. The thermal performance of the LED module was shown to significantly improve by the optimization process of cooling system attached with it. A wind circulating cooling system was designed for the LED arrays with the input power of 60 Watts and another heat pipe cooling system was designed for the input power of 144 Watts. The effects of design parameters such as air temperature and air velocity on the thermal performance of LED lamp were investigated. Thermal performance of the two cooling systems was compared for practical application.

In this letter, a theoretical degradation model of LEDs at dynamic working conditions was proposed. The Arrhenius effective temperature (AET) was used to describe the effect of dynamic working temperature. Utilizing the concept of AET, the lifetime prediction of LEDs at dynamic working conditions can be carried out based on the data base at steady state. It was found that the activation energy has a great effect on the numerical value of AET. The extraction of activation energy from steady state measurement made the degradation model available to predict the lifetime of LEDs at various input. The AET was found to be always higher than the arithmetical average temperature. The lifetime of LEDs was found to increase with the frequency and decrease with the duty cycle for the typical dynamic working conditions–pulse input.

In this paper, we present the thermal, electrical, and optical analyses of light emitting diode (LED) packages with different chip sizes. The LED packages under investigation employed the same configuration of package components, except for the chip sizes. The forward current was found to increase with the chip size at the same forward voltage due to the area increase of current spreading. The luminous flux and optical power were found to increase with the chip size at the same current density. The thermal analysis was made by the transient thermal measurement and thermal simulation using the finite volume method. It was demonstrated that the thermal resistance decreased with the chip size under the same package conditions both by simulation and experiment. The bulk thermal resistance and spreading thermal resistance were combined together to give out a quantitative investigation of the partial thermal resistance variation. Moreover, the spreading thermal resistance was found to have a great effect on the total thermal resistance of LED packages.

In this letter, the thermal evaluation of high-power LED packages at pulse conditions was reported. A theoretical calculation model was proposed based on the analogy between the thermal and electrical RC circuits. The thermal performance of LED packages driven by pulse input was calculated using the RC network extracted from transient thermal measurement. The junction temperature fluctuation band decreases with the frequency at certain duty cycles. The saturated average junction temperature rise linearly increases with the duty cycle at certain frequencies. These predictions were verified by the real-time junction temperature measurement using the peak shift method at pulse conditions. The theoretical model was found to be effective and applicable to the evaluation of the thermal performance of LEDs working at pulse conditions.

In this paper, we present the thermal and mechanical analysis of high-power light-emitting diodes (LEDs) with ceramic packages. Transient thermal measurements and thermomechanical simulations were performed to study the thermal and mechanical characteristics of ceramic packages. Thermal resistances from the junction to the ambient were decreased from 79.6 to 46.7degC/W by replacing the plastic mold with a ceramic mold for LED packages. Thermomechanical stress induced in the heat-block test was simulated using a finite-element method. Higher level of thermomechanical stress in the chip was found for LEDs with ceramic packages, despite less mismatching coefficients of thermal expansion, compared with that with plastic packages. The thermomechanical-stress components in the direction of the thickness were found to be larger than that in other two directions. The results suggest that the thermal performance of LEDs can be improved by using ceramic packages, but the mounting process of the high-power LEDs with ceramic packages is critically important and should be the reason for causing delaminating interface layers in the packages.

We investigated thermal behavior of GaN-based laser diode (LD) packages as a function of cooling systems, die attaching materials, chip loading conditions, and optical performances. The electrical thermal transient technique was employed for the thermal measurement of junction temperature and thermal resistance of LD packages. The results demonstrate that the total thermal resistance of LD packages is controlled mainly by the packaging design rather than the chip structure itself. Significant changes in thermal resistance with input current were observed under a natural cooling system because of the sensitive change in the heat transfer coefficient with the change in temperature. Employment of PbSn as a die attachment was more advantageous over the Ag-paste in the thermal behavior of LD packages. The LD package with epi-down structure resulted in the lower thermal resistance compared to one with epi-up structure. A continuous increase in junction temperature was measured after lasing. It was attributed to an increase in the thermal resistance of LD when it took the optical power into an account. Effective input power was decreased by the lasing and led to high thermal resistance values.

Thermal transient measurements of high power GaN-based light-emitting diodes (LEDs) with multichip designs are presented and discussed in the paper. Once transient cooling curve was obtained, the structure function theory was applied to determine the thermal resistance of packages. The total thermal resistance from junction to ambient considering optical power is 19.87 K/W, 10.78 K/W, 6.77 K/W for the one-chip, two-chip and four-chip packages, respectively. The contribution of each component to the total thermal resistance of the package can be determined from the cumulative structure function and differential structure function. The total thermal resistance of multichip packages is found to decrease with the number of chips due to parallel heat dissipation. However, the effect of the number of chips on thermal resistance of package strongly depends on the ratio of partial thermal resistance of chip and that of slug. Therefore, an important thermal design rule for packaging of high power multichip LEDs has been analogized.

Degrading characteristics of InGaN/GaN-based blue light-emitting diodes (LEDs) in the optical increase stage (stage I) were investigated. The LEDs were under an ageing with dc current of different levels. Different ageing phenomena were observed for both the electrical and the optical characteristics. The tunnelling components of the forward current were found to change with different trends for the samples aged under current stresses of 100 mA and 1000 mA, respectively. The former decreased monotonically, while the latter decreased in the early period, and then kept increasing. Different ageing rates but the same trends were observed for the optical degradation of the samples aged under different stresses. The ageing factors induced by the dopant activation and the change of defects competed with each other in the ageing process and were proposed to be responsible for the ageing characteristics.

In this paper we report on the effects of variables in thermal resistance measurement of high power GaN-based light-emitting diodes (LEDs). The investigated variables include ambient temperature, thermal interface material (TIMs) at different pressure. The combination of transient thermal measurement method and optical measurement was employed for the study. The measured thermal resistance of LED packages was found to increase with the ambient temperature. The temperature dependence of optical efficiency, forward voltage, and thermal properties of packaging materials are thought to be responsible for the increase of thermal resistance with the ambient temperature. The interface effect on the thermal resistance was studied by applying different external pressure on the interface with different TIMs. And the measured thermal resistances were found to reach stabilization at certain pressure level after initial decrease with the external applied pressure.

This paper discusses about thermal performance of high power light emitted diode (HPLED) implemented with sintered metal wick type heat pipe(SWHP). The HPLED(2.5 W) samples were surface mounted device(SMD) package used in our experiments. The experiments were made for SWHP with diameters of 6.0 mm. The length of the SWHP is 150 mm. The working fluid in the heat pipes is pure water. The electrical-thermal transient technique was employed for the junction temperature measurement. It was found that the SWHP leads to decreased of thermal resistance by 35 % compared with a simple copper bar in oil bath (forced cooling condition). Employment of copper cap as a LED attachment was more advantageous over the phosphor bronze. After the increase of input power, the thermal resistance of HPLED package has decreased with the increase of effective thermal conductivity of SWHP.

Ceramic packages with different paths of thermal dissipation were designed by a simulation using the finite-volume method. Thermal resistances of the packages were obtained from the calculation of temperatures of LED (light emitting diode) chips. It was shown that the thermal characteristics of LED packages were strongly affected by the contact area of a thermal via in contact with a LED chip rather than that in contact with a ceramic mould. Ceramic LED packages were fabricated and characterized. A deviation of simulated results from experimental results was found. The difference was attributed to the poor contact roughness between the LED chip and the thermal via.

In this paper we present thermal analysis of three kinds of ceramic package designs for high power LEDs. The analysis was made by transient thermal measurement and thermal simulation using the Finite Volume Method. The three ceramic packages under investigation employ same configuration of GaN-based chip, but they have different size and distribution of thermal vias. Three designs of LED packages resulted in significantly different thermal behaviors. Thermal behaviors, described as thermal resistance, of the three packaging designs were compared and evaluated as functions of bulk thermal resistance, spreading resistance, and surface roughness. The deviation between the simulated results and measured data were attributed to the different surface roughness in the interfaces between the packaging components. It was demonstrated that the junction temperature decreases with the effective contact area ratio in the LED packages.

In this paper we report on the thermal analysis of high-power light-emitting diodes (LEDs) as a function of input power. The transient thermal measurement method using structure function was employed to investigate the thermal behavior of LEDs at different working conditions. It was shown that a 5 W LED package installed with four chips exhibited an increase of thermal resistance from 7.06 K/W to 8.83 K/W with the input current ranging from 50 mA to 700 mA at the ambient temperature of 25 °C. However, the thermal resistance for a 1 W LED package with one chip was decreased from 20.33 K/W to 19.54 K/W with the input current ranging from 50 mA to 350 mA. Based on our improved LED model, the different trends for the two packages are attributed to the difference in the power dissipated by internal series electrical resistances. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports on the thermal behavior of GaN-based laser diode (LD) package as functions of cool-ing systems, die attaching materials, and chip loading conditions. Thermal resistance and junction tem-perature was determined by electrical-thermal transient method. Significant change of thermal resistance with input current was observed under natural cooling condition due to the sensitive change of heat trans-fer coefficient (h ) with temperature. Employment of PbSn as a die attachment was more advantageous over Ag-paste in thermal behavior of LD package. Compare the thermal resistance of LD packages epi-down and epi-up structures. The partial thermal resistance from junction to submount is 4.68 K/W for epi-down structure, and 9.65 K/W to epi-up structure. The results demonstrate that the total thermal resistance of LD package be controlled mainly by the packaging design rather than the chip structure itself. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we present the analysis of delamination in light-emitting diode (LED) packages by transient thermal measurement and thermo-mechanical simulation. The LED samples were subjected to various moisture preconditioning treatments, and subsequently heat block testing. Transient thermal measurements were used to investigate the thermal behavior of the delaminated LEDs. Thermal resistance from chip to lead frame was increased from 41.7 to 70.9 K/W by a preprocessing for 24 h. Blocking of thermal path induced by delamination was regarded as being responsible for the increased thermal resistance. The maximum thermal stress for the die attach obtained from FEA simulation is 91.5 Mpa, and the thermal stress distribution is consistent with the observed micrograph for the delamination in the lead frame. Thermal stress induced in the reflow process caused delamination and moisture absorption expanded the delamination in the LED package investigated.

We present thermal modeling and measurement results of AlGaN-GaN heterojunction field effect transistors fabricated on sapphire and SiC substrates, respectively. The device structures are identical except for the substrate material used to grow the AlGaN-GaN heterostructure. One objective is to study the effect of substrate material on the thermal and electrical performance of the resulting devices. To compute the temperature profiles, in-house PAMICE code developed for a three-dimensional structure was used. To measure the temperatures on the chip surface, nematic liquid crystal thermography was used. This technique is nondestructive and can be performed in realtime during device operation. It has submicrometer spatial resolution and /spl plusmn/1/spl deg/C temperature accuracy. The measured temperatures agree well with the calculated ones. The relationship between the measured temperature and power is almost linear for both types of devices. The junction-to-case thermal resistance of the device fabricated on sapphire substrate is 4.4 times that of the device built on SiC substrate.

This paper reports on the thermal modelling of GaN-based LEDs. The theoretical calculation was made by combining an analytical simulation employing the Unit Temperature Profile Approach (UTPA) and Finite Element Method (FEM). An interfacing process was made by the optimization of the modelling input parameters used in the analytical simulator. The calculated temperatures of the LED chip inside the epoxy package was compared with the experimentally measured data and the optimized heat transfer coefficients were extracted. The extracted parameters were implemented into the numerical thermal calculation of the package surface using FEM. By the effective interfacing process between the two modelling tools, it is demonstrated that the analytical simulator can be utilized for the accurate prediction of the surface temperatures of LED packaging with non-flat surface.

This paper reports on the experimental methods of the determination of junction temperature and thermal resistance in GaN-based LEDs. For the direct temperature measurement and investigation of thermal distribution on the operating LED chip, nematic liquid crystal thermographic technique was employed. Hot spot was observed and its size was increasing with the driving input power. The initial hot spot with an anisotropic–isotropic transition of 29 °C appeared near the cathode region under the drive voltage of 2.95 V and the current of 8.1 mA. The size of the hot spot was increased with input power. Micro thermocouple was embedded into the epoxy package for the investigation of its size effects on thermal behavior. For the specific structure of LED package investigated the thermal resistances were calculated to be 265 °C/W and 215 °C/W for the low epoxy domed package and high epoxy domed package, respectively. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The spin transport in a lateral spin-injection device with an FeCo/Si/FeCo junction has been investigated. Magnetoresistance (MR) signals were found to appear at low magnetic fields in the range 4 – 300 K. This is attributable to the switching of the magnetization of the two ferromagnetic contacts in the device for certain magnetic fields over which the magnetization in one contact is aligned antiparallel to that in the other. Our results suggest that the spin-polarized electrons are injected from the first contact and, after propagating through the bulk Si, are collected by the second contact.

In this paper, a thermal analysis was made on blue and white GaN-based LEDs. The thermal analysis consists of experimental temperature measurements and finite element calculations on the LED chips and surface of epoxy package. The direct on-chip temperature measurement using a nematic liquid crystal resulted in a hot spot with a transition boundary of 43 °C in a range of about 450 to 500 μm under a forward voltage of 3.9 V. The surface temperature of the epoxy package was measured as a function of input power and it exhibits a linear relationship. The finite element method was used for the calculation of temperature distributions for samples and the simulated data showed good agreement with the experimental results. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we report on the analytical thermal modeling and temperature measurement of AlGaN/GaN HFETs fabricated using the sapphire and SiC substrates. The thermal modeling was conducted by the unit temperature profile approach and the thermal measurement was made using the nematic liquid crystal thermogrphic method. The calculated data and the measured results were found out to agree well and exhibited the linear relationship of peak temperature and input power. For the same drive condition at gate voltage, Vgs = 0 V, it was found out that the AlGaN/GaN HFET built on SiC delivers twice more current comparing to its sapphire counterpart. Under the input power of 0.63 W, the device with the sapphire substrate exhibits the peak temperature of 110 °C, but the peak temperature of the device built on the SiC substrate is measured to be about 40 °C.

Several dry etching methods which are commonly used for the fabrication of SiC devices are known to result in ion-induced damage on the etched surface, which is highly undesirable for the high frequency and high power device operation. In this paper we report on photoelectrochemical etching process of semiconducting 6H–SiC wafer. The etching process consists of illumination of UV in HF-based electrolyte, thermal oxidation, and HF dipping. An etching rate of 760 A min−1 was obtained using a dilute HF(1.4 wt.% in H2O) electrolyte with the etching potential of 3.5 V. The etching rate was found out to increase with the bias voltage. Double-step oxidation process was found out to be useful for the decrease of the surface roughness. It was shown that the addition of H2O2 into the HF solution improves the etching rate of the SiC surface. The PEC etching resulted in a highly anisotropic etching characteristics and shown to have a potential for the fabrication of SiC devices.

This is the first report on the fabrication of AlGaN/GaN HFETs which has a recessed gate structure achieved by the photoelectrochemical etching technique. Optimal photoelectrochemical wet etching conditions were stabilized and applied for the device fabrication. The DC and large-signal RF performance of thus fabricated device is presented as well. The ohmic contacts fabricated on the n + -GaN layer exhibited contact resistivity of mid 10 −6 Ω cm 2 and resulted in a linear I – V characteristics during an operation of device. The maximum drain–source current density is approximately 174 mA mm −1 (at V GS =1 V), and the transconductance of approximately 68 mS mm −1 (at V GS =−1.1 V, V DS =6 V). The maximum frequency is measured to be approximately 31 GHz, and an RF power of 84 mW mm −1 at 1.8 GHz for a 1400-μm wide gate device.

This reports on the direct thermal observation of the pentacene-based Organic Thin-Film Transistors (OTFT) under the real operating conditions. An electrical technique which employs a thermal transient method was utilized for the investigation of determination of inner structure of device. Applying different input power, the temperature variation in the channel was determined. Under the driving power of 0.226 W, a thermal resistance of 240 °C/W was measured for the OTFT package. It was demonstrated that the heat generation inside the OTFT package increases with the input power to the OTFT package. Thermal distribution inside the OTFT package was mapped using a CFD (Computational Fluid Dynamics) method for the various input power. The results indicate that the design with effective thermal dissipation is imperative for reliable operation of the OTFT package.