The IES standard TM-21-11 provides a guideline for lifetime prediction of LED devices. As it uses average normalized lumen maintenance data and performs non-linear regression for lifetime modeling, it cannot capture dynamic and random variation of the degradation process of LED devices. In addition, this method cannot capture the failure distribution, although it is much more relevant in reliability analysis. Furthermore, the TM-21-11 only considers lumen maintenance for lifetime prediction. Color shift, as another important performance characteristic of LED devices, may also render significant degradation during service life, even though the lumen maintenance has not reached the critical threshold. In this study, a modified Wiener process has been employed for the modeling of the degradation of LED devices. By using this method, dynamic and random variations, as well as the non-linear degradation behavior of LED devices, can be easily accounted for. With a mild assumption, the parameter estimation accuracy has been improved by including more information into the likelihood function while neglecting the dependency between the random variables. As a consequence, the mean time to failure (MTTF) has been obtained and shows comparable result with IES TM-21-11 predictions, indicating the feasibility of the proposed method. Finally, the cumulative failure distribution was presented corresponding to different combinations of lumen maintenance and color shift. The results demonstrate that a joint failure distribution of LED devices could be modeled by simply considering their lumen maintenance and color shift as two independent variables.

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The nucleation of superconductivity in a superconducting disk with a Co/Pt magnetic triangle was studied. We demonstrate that when the applied magnetic field is parallel to the magnetization of the triangle, the giant vortex state of vorticity three splits into three individual Φ0-vortices, due to a pronounced influence of the C3 symmetry of the magnetic triangle. As a result of a strong pinning of the three vortices by the triangle, their configuration remains stable in a broad range of applied magnetic fields. For sufficiently high fields, Φ0-vortices merge and the nucleation occurs through the giant vortex state. The theoretical analysis of this novel reentrant behaviour at the phase boundary, obtained within the Ginzburg -Landau formalism, is in excellent agreement with the experimental data. PACS numbers: 74.78.Na., 75.75.+a, 74.25.Dw The nucleation of superconductivity in mesoscopic samples, whose dimensions are comparable to the superconducting coherence length ξ(T ) and the penetration depth λ(T ), is substantially affected by the sample boundary (see Ref.[1] and references therein). It is well established that in circular mesoscopic disks and loops the onset of superconductivity mostly occurs through the giant vortex state (GVS) due to their cylindrical symmetry [2]. On the other hand, in superconducting squares and triangles for certain magnetic fields the GVS easily splits into individual Φ 0 -vortices (Φ 0 is the superconducting flux quantum), with a possible generation of additional antivortices [3]. The transition from the GVS to a set of Φ 0 -vortices is caused by the reduced axial symmetry of squares and triangles and ensures that a vortex pattern conforms to the symmetry imposed by the boundary of the sample.In this paper we investigate the onset of superconductivity in a mesoscopic disk on which a magnetic triangle with out-of-plane magnetization is placed. The nucleation of superconductivity in hybrid superconductor/ferromagnet disks and loops has been studied previously, both experimentally and theoretically, as these are a good model system to gain insight into the interplay between superconductivity and magnetism in the context of magnetic vortex pinning and related phenomena [5,6,7,8]. In this paper we show that the symmetry of the magnetic triangle has a profound effect on the onset of superconductivity in the disk. Due to the competition between the cylindrical symmetry of the superconducting disk and the C 3 symmetry of the stray field, superconductivity nucleates as a GVS for vorticity L = 2, whereas for L = 3 the GVS splits into three Φ 0 -vortices. As a result of a strong pinning of the three individual vortices their configuration remains stable for L = 4 and L = 5. FIG. 1: The calculated spatial profile (a) of the stray field and (b) a magnetic force micrograph of the triangle. The field values were calculated using the saturation magnetization of bulk Co.The sample was prepared by electron beam lithography, using double resist technique and lift-off in two steps. For details we ...

We have studied the nucleation of superconductivity in a mesoscopic Al disk with a Co/Pd magnetic dot placed on the top by measuring the normal/superconducting phase boundary Tc(B). The measurements have revealed a pronounced asymmetry in the phase boundary with respect to the direction of the applied magnetic field, indicating an enhancement of the critical field when an applied magnetic field is oriented parallel to the magnetization of the magnetic dot. The theoretical Tc(B) curve is in a good agreement with the experimental data. PACS numbers: 74.78.Na, 74.25.Dw,

More and more mid-power white-light LED (MP LED) solutions have been used in outdoor illumination due to their good performance, cost attractiveness, and low energy consumption as compared with conventional lighting solutions. Hence, there is a need for MP LED manufacturers to develop more robust MP LEDs aimed at outdoor applications but still offer a significant cost benefit as compared with currently widely used high-power LEDs. This implies that MP LEDs would be operated in an environment with high humidity and high temperature. This may lead to serious degradation with different failure modes compared with an indoor operation. However, the combined effect of temperature and humidity on the MP LED reliability has not been extensively studied in literature. In this paper, MP LEDs were studied by the wet high-temperature operation life (WHTOL) test in order to understand their degradation mechanisms due to the combined effect of temperature and humidity. It is found that encapsulant discoloration (yellowing) is the major degradation mechanism in the WHTOL test, which will induce serious lumen degradation and color shift. Furthermore, it has been found that electrical degradation in terms of forward voltage increase will also affect the lumen maintenance. Finally, statistical analysis shows that lumen degradation mechanisms in the WHTOL test are similar to a failure in the LM-80-08 test, demonstrating that the WHTOL test is an efficient accelerated degradation test method, which dramatically reduces the test duration compared with the LM-80-08 test.

Serious silicone carbonization has been reported in mid-power white light LEDs during highly-accelerated temperature and humidity stress test (HAST), even though the junction temperature of the LED chip is much lower than the critical temperature of silicone carbonization (300 °C). This paper presents a comprehensive investigation, through which, the effects of Joule heating and phosphor self-heating has been eliminated as main failure mechanisms. As a result, the over-absorption of blue lights by silicone bulk, is considered as the root cause for silicone carbonization. This is mainly due to the scattering effect of water particles inside the silicone materials, which can increase the silicone temperature significantly. Furthermore, finite element thermal simulation has also been performed, confirming the validity of failure mechanism.

We have studied temperature and field dependencies of the critical current IC in the Nb-Fe0.1Si0.9-Nb Josephson junction with tunneling barrier formed by paramagnetic insulator. We demonstrate that in these junctions the co-existence of both the 0 and the π states within one tunnel junction takes place which leads to the appearance of a sharp cusp in the temperature dependence IC(T ) similar to the IC(T ) cusp found for the 0 − π transition in metallic π junctions. This cusp is not related to the 0 − π temperature induced transition itself, but is caused by the different temperature dependencies of the opposing 0 and π supercurrents through the barrier. PACS numbers: 74.50.+r, 85.25.Cp As first predicted by Josephson 1 , the supercurrent I S through the tunnel barrier is driven by the phase difference ϕ across the junction applied to the superconducting wave function. In conventional Josephson junctions (JJ) this current is described by the relationRecently, a considerable attention has been devoted to the investigation of the π JJs 2,3,4 . In this case the relation between the supercurrent and the phase difference is I S = I C sin(ϕ + π) = −I C sinϕ 5 . One of the possible realizations of the π junctions is the superconductorferromagnetic metal-superconductor (S-FM-S) tunnel junction, wherein the spatial oscillations of the superconducting order parameter occur in the ferromagnetic metal as a consequence of the exchange splitting of the conduction band 6 . The transition between the 0 and the π states was experimentally observed as the vanishing of the Josephson current. The 0 − π transition can be induced by the varying barrier thickness 2,4 or the temperature 3,4 . As the absolute value of the current is measured, e.g. for proper values of the ferromagnetic barrier thickness, a sharp cusp in the temperature dependence of the critical current I C (T ) is observed as a consequence of the 0 − π transition.It is also predicted that JJs with magnetic impurities within an insulating barrier can produce the π state 5 . Later on, the possibilities to observe the π junctions in JJs with ferromagnetic insulating or semiconducting barrier (S-FI-S) were analyzed theoretically in 7,8 . In such types of JJs the proximity effect in the barrier is much weaker, as compared with the ferromagnetic metal, and can be disregarded. In this case the formation of the π junction is caused by the quasiparticle scattering on a magnetically active interfaces 8 . It can result in the splitting of the Andreev interface bound-state energies into two spin channels 7 . Theoretically, if these channels com-pensate each other the 0 − π transition is observed. Up to now the π state in the JJs with insulating magnetic barrier has not been found experimentally.In this letter we present the experimental evidence of the existence of the π state in the Josephson junction with magnetic impurities in the insulating barrier. We also demonstrate that the co-existence of both the 0 and the π states within one tunnel junction leads to the appearance of a...

A new characterization technique and an improved model for charge injection and transport through ONO gate stacks are used to investigate the program/retention sequence of silicon nitride-based (SONOS/TANOS) nonvolatile memories. The model accounts for drift-diffusion transport in the conduction band of silicon nitride (SiN). A priori assumptions on the spatial distribution of the charge at the beginning of the program/retention operations are not needed. We show that the carrier transport in the SiN layer impacts the spatial distribution of the trapped charge and, consequently, several aspects of program and retention transients. A few model improvements allow us to reconcile the apparent discrepancy between the values of silicon nitride trap energies extracted from program and retention experiments, thus reducing the number of model parameters