Wouter Oepts

DIELECTRIC BREAKDOWN OF FERROMAGNETIC TUNNEL JUNCTIONS

The time-dependent dielectric breakdown of Co/Al2O3/Co(-Fe) magnetic tunnel junctions is investigated. At voltages larger than 1.2 V, almost immediate breakdown of the junction is observed, leading to a decreased (magneto)resistance. The shorts, which are local hot spots, were visualized by making use of a liquid crystal film on top of the junction. The breakdown voltages of a series of nominally identical tunnel junctions measured in a voltage-ramp experiment are shown to increase with increasing ramp speed. The results are analyzed in the framework of several models for the voltage dependent breakdown probability.

Analysis of breakdown in ferromagnetic tunnel junctions

Due to their very thin tunnel barrier layer, magnetic tunnel junctions show dielectric breakdown at voltages of the order of 1 V. At the moment of breakdown, a highly conductive short is formed in the barrier and is visible as a hot spot. The breakdown effect is investigated by means of voltage ramp experiments on a series of nominally identical Co/Al2O3/Co tunnel junctions. The results are described in terms of a voltage dependent breakdown probability, and are further analyzed within the framework of a general model for the breakdown probability in dielectric materials, within which it is assumed that at any time the breakdown probability is independent of the (possibly time-dependent) voltage that has been previously applied. The experimental data can be described by several specific forms of the voltage breakdown probability function. A comparison with the models commonly used for describing thin film SiO2 breakdown is given, as well as suggestions for future experiments.

38.1: Optical Feedback for AMOLED Display Compensation using LTPS and a-Si:H Technologies

New optical feedback pixel circuits for a-Si:H and LTPS technologies will be presented. The circuits will enable correction of threshold voltage drift of the drive TFT and degradation of the OLED. In the a-Si:H case this will be achieved with a standard a-Si:H process and for LTPS an a-Si NIP photodiode is integrated. Operation, technology and measurements will be presented.

Asymmetric bias voltage dependence of the magnetoresistance of Co/Al2O3/Co magnetic tunnel junctions: Variation with the barrier oxidation time

Recently it has been observed that the magnetoresistance (MR) of plasma oxidized exchange biased Co/Al2O3/Co tunnel junctions can have a strongly asymmetric bias voltage (Vbias) dependence. In this article we report on the dependence of this phenomenon on barrier oxidation time tox. For junctions based on 1.5 nm Al, tox was varied from 20 to 120 s. For tox=20 s, for which the MR is approximately 20% at Vbias=0, and for tox⩾90 s symmetric MR(Vbias) curves are found, with the MR decreasing monotonically with |Vbias|. A strong asymmetric bias voltage dependence was observed for intermediate oxidation times, which correspond to essentially full oxidation of the Al layer, but almost no formation of stoichiometric CoO at the bottom electrode. Samples with tox=60 s show even an asymmetric double peak in MR(Vbias). Due to its strength, it has an important consequence for device applications: for a series of junctions with variable tox the maximum signal voltage (at a fixed current) is not necessarily obtained for...

Observation and analysis of breakdown of magnetic tunnel junctions

At voltages larger than 1.2 V Co/Al2O3/Co(–Fe) magnetic tunnel junctions show almost immediate breakdown, leading to a decrease in (magneto) resistance. The position of the shorts formed is visualized by making use of a liquid crystal film on top of the junction. The breakdown voltages of a series of nominally identical junctions measured in a voltage-ramp experiment increased with larger ramp rate. The extrapolated lifetime, at lower voltages, is evaluated.

PC Board Thermal Management of High Power LEDs

Increasing requirements of closely packed high power LEDs pose challenges for PC board thermal management. This paper presents a cost-effective thermal solution using FR4 based PCB technology and filled and capped vias. This robust technology enables superior thermal performance on board level for closely packed power LEDs. No solder joint or board level reliability failures were found during a temperature cycling test (TCT) from -40degC to 125degC after 1000 cycles. Moreover no solder joint and board level failure was found after 4000 cycles for open via FR4 when monitoring changes of board thermal resistance. Trade-offs are given for board thermal resistance versus packing density, various board designs, PCB technology, solder joint reliability and PCB board level reliability including open vias and filled and capped vias.

Improved optical feedback for OLED differential ageing correction

— To compete with LCDs and to meet standard display product specifications, OLED displays must have a high degree of tolerance to differential ageing or “burn-in.” A new optical feedback pixel circuit is presented that enables accurate differential ageing correction, can have low power consumption, and enables a high degree of non-uniformity correction. The circuit can be implemented in LTPS, and a-Si:H TFT technologies and circuits for both cases are shown. The a-Si:H approach is low cost and enables correction of both TFT threshold voltage drift and OLED degradation at the same time. The circuit analysis, operation, and technology will be described and results presented.

Enhanced anisotropy of permalloy layers sputter deposited on V-grooved substrates and tilted surfaces

The magnetic anisotropy of 8–100 nm thick Ni80Fe20 films sputter deposited on nanostructured V-grooved substrates and on tilted surfaces is investigated. Films deposited on a V-grooved substrate (200 nm period) with the sidefaces at an angle of 55° to the substrate plane, show a very large and essentially thickness-independent magnetic anisotropy field, viz. 25±3 kA/m. Planar reference films deposited also at an angle of 55° to their substrate normal show an increase of the magnetic anisotropy as well, but only to 8 kA/m, independent of the film thickness, which is explained as a growth-induced effect. It is argued that the enhanced anisotropy observed in the V-grooved substrates is not the result of shape anisotropy induced by the V-grooves. This leads to the conclusion that the observed enhanced anisotropy must also be a growth-induced effect, enhanced by the specific geometry of the V-grooves.

Magnetothermopower of cobalt/copper multilayers with gradient perpendicular to planes

The magnetothermopower and magnetoresistance of two copper/cobalt multilayers grown on grooved indium phosphide were measured between 100 K and 295 K. One sample was deposited at an angle onto a grooved substrate such that transport properties are measured between layers rather than parallel to them (CPP); the other was deposited perpendicular to the substrate, so that the transport properties are measured at an angle to the planes (CAP). The data show the magnetothermopower to vary inversely with the resistivity, as predicted by the Mott formula if the magnetoresistance is determined by the ratio α of majority to minority densities of states at the Fermi energy EF in the magnetic metal. Using this result, we can collapse the thermopower data at various temperatures to find that α′/α is 0.6 eV−1 for the CPP sample and 0.7 eV−1 for the CAP sample, where α′≡(∂α/∂E)EF.

Perpendicular giant magnetoresistance in magnetic multilayers deposited on grooved substrates

Abstract We have deposited Co/Cu magnetic multilayers at an angle onto grooved InP substrates with 0.2 μm groove period and measured the giant magnetoresistance with the current perpendicular to the layer plane. We have determined as a function of temperature the spin-dependent scattering parameters due to magnetic bulk and interface scattering, using the two channel model for electron transport. We find that the decrease of the magnetoresistance from 4.2 K to room temperature is mainly due to an increase of the Co and Cu bulk resistivities, while the temperature dependence of the interface resistance and of the spin-asymmetry scattering parameters is very small.