Effect of Buffer Quality on the Performance of InAs/AlSb Heterostructure Backward Tunneling Diode

InAs/AlSb Heterostructure Backward Tunneling Diodes (HBTDs) were grown on semi-insulating GaAs (100) substrate using molecular beam epitaxy (MBE). The current-voltage characteristics of the InAs/AlSb HBTDs, both at room temperatures (RT) and cryogenic temperatures, have been studied as a function of the InAs buffer thickness. It has been found that a thicker InAs buffer doesn’t improve the surface roughness but decreases the threading dislocation (TD) density, thus a higher curvature coefficient in the current-voltage characteristics near zero-bias is obtained

Noise properties of high-T-c superconducting flux transformers fabricated using chemical-mechanical polishing

Reproducible high-temperature superconducting multilayer flux transformers were fabricated using chemical mechanical polishing. The measured magnetic field noise of the flip-chip magnetometer based on one such flux transformer with a 9 x 9 mm(2) pickup loop coupled to a bicrystal dc SQUID was 15 fT/Hz(1/2) above 2 kHz. We present an investigation of excess 1/f noise observed at low frequencies and its relationship with the microstructure of the interlayer connections within the flux transformer. The developed high-T-c SQUID magnetometers may be advantageous in ultra-low field magnetic resonance imaging and, with improved low frequency noise, magnetoencephalography applications

The effect of temperature ramp rate on flip-chip joint quality and reliability using anisotropically conductive adhesive on FR-4 substrate

In this work, the effect of temperature ramp rate on flip-chip anisotropically conductive adhesive joint quality and reliability has been studied. The experiments were performed on bumped and unbumped die. They were assembled onto bare ITO-glass and FR-4 substrates. The reason for using the transparent glass substrate is that the particle deformation and settlement can be visualised without destroying the assembled module. The temperature ramp rates studied ranged between 8.1 and 65.7°C/s. The experiments show that the best joint quality is obtained when a slow temperature ramp rate is applied to unbumped dies. A good joint is achieved when many particles have been entrapped on the die pad and when there is a significant degree of particle deformation. A large degree of deformation of particles results in a large contact area for the electrical conduction path. When a high temperature ramp rate is applied, there is a risk that the adhesive is already cured before full compression is reached. This will prevent the particles in the adhesive from contacting the bonding surface. When assembling bumped die, the temperature ramp rate does not seem to have a significant influence on the result. The joint quality evaluation has been performed using Scanning Electron Microscopy (SEM) and Optical Microscopy (OM). Furthermore, temperature cycling between -40 to +125 °C, 1000 cycles, has been performed to characterise the joint reliability under the optimum temperature ramp rate conditions. The electrical resistance has been measured continuously. A theoretical simulation of the influence of the temperature ramp rate on the adhesive joint quality has been performed using the same test module conditions as for the experimental work. The results coincide with the experimental results, particularly in the range of low bonding pressure value

Templated Growth of Covalently Bonded Three-Dimensional Carbon Nanotube Networks Originated from Graphene

A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material

Activated Recrystallization of Non-Sag Tungsten Wire

The objective of the present investigation was to explore the mechanism of activated recrystallization of heavily drawn doped (non-sag) tungsten wire material. The recrystallization can be induced at low temperatures by the presence of some metals, such as palladium, nickel, iron and others, on the wire surface, which is important as it can seriously deteriorate the mechanical properties of non-sag tungsten wires, when the wires serve as reinforcement material in metal matrix composites. This investigation, which included the recrystallization nucleation mechanism of doped tungsten wire, the short-circuit paths for activator diffusion into tungsten wire, the activated recrys- tallization mechanism of non-sag tungsten wire, and the precipitation of activators in recrystallized tungsten wire, was carried out using atom probe field ion microscopy, optical metallography and electron microscopy. <p> TEM observations about the initial stage of recrystallization of 0.18 mm doped tungsten wire showed that the formation, through dislocation rearrangement, of new grains with a relative misorientation could play a critical role in the recrystallization process. The impeding effect of potassium bubbles both on dislocation rearrangement and boundary migration resulted in the abnormal recrystallization behaviour of the heavily drawn doped tungsten wire. In addition, it was found that the non-<110> orientation of recrystallized tungsten grains could originate either from the grains with a non-<110> orientation in the starting material or from the newly nucleated grains. These observations could explain the recrystallization features of tungsten wire which the current theory could not account for. <p> The activated recrystallization was studied both for nickel plated and palladium plated 0.18 mm cold drawn doped tungsten wire. A recrystallized structure formed in both types of wires after annealing at 1100°C for 100 hours, but their structural features were different. The recrystallization behaviour caused by palladium presence was very similar to that of undoped tungsten wire, whereas the behaviour of nickel plated wire was similar to the general recrystallization of doped wire, except for the much lower recrystallization temperature. This indicated that palladium was more effective as an activator than nickel was. <p> It was shown, using APFIM analysis, that grain boundaries and lattice dislocations could act as fast paths for diffusion of activators in tungsten wire during annealing. The presence of the large potassium bubbles and the segregation of activators on potassium bubbles indicated that the effect of activators was to neutralize the impeding influence of potassium bubbles on dislocation movement and boundary migration. <p> Two types of nickel rich precipitates were found. One was a Ni(W) solid solution, which formed on the bubbles at grain boundaries, and another type, with Ni4W structure, was found on the potassium bubbles connected with dislocations. Palladium rich precipitates were present only in triple junctions. Their structure could not be established

Activated Recrystallization of Non-Sag Tungsten Wire

The objective of the present investigation was to explore the mechanism of activated recrystallization of heavily drawn doped (non-sag) tungsten wire material. The recrystallization can be induced at low temperatures by the presence of some metals, such as palladium, nickel, iron and others, on the wire surface, which is important as it can seriously deteriorate the mechanical properties of non-sag tungsten wires, when the wires serve as reinforcement material in metal matrix composites. This investigation, which included the recrystallization nucleation mechanism of doped tungsten wire, the short-circuit paths for activator diffusion into tungsten wire, the activated recrys- tallization mechanism of non-sag tungsten wire, and the precipitation of activators in recrystallized tungsten wire, was carried out using atom probe field ion microscopy, optical metallography and electron microscopy. TEM observations about the initial stage of recrystallization of 0.18 mm doped tungsten wire showed that the formation, through dislocation rearrangement, of new grains with a relative misorientation could play a critical role in the recrystallization process. The impeding effect of potassium bubbles both on dislocation rearrangement and boundary migration resulted in the abnormal recrystallization behaviour of the heavily drawn doped tungsten wire. In addition, it was found that the non- orientation of recrystallized tungsten grains could originate either from the grains with a non- orientation in the starting material or from the newly nucleated grains. These observations could explain the recrystallization features of tungsten wire which the current theory could not account for. The activated recrystallization was studied both for nickel plated and palladium plated 0.18 mm cold drawn doped tungsten wire. A recrystallized structure formed in both types of wires after annealing at 1100\ub0C for 100 hours, but their structural features were different. The recrystallization behaviour caused by palladium presence was very similar to that of undoped tungsten wire, whereas the behaviour of nickel plated wire was similar to the general recrystallization of doped wire, except for the much lower recrystallization temperature. This indicated that palladium was more effective as an activator than nickel was. It was shown, using APFIM analysis, that grain boundaries and lattice dislocations could act as fast paths for diffusion of activators in tungsten wire during annealing. The presence of the large potassium bubbles and the segregation of activators on potassium bubbles indicated that the effect of activators was to neutralize the impeding influence of potassium bubbles on dislocation movement and boundary migration. Two types of nickel rich precipitates were found. One was a Ni(W) solid solution, which formed on the bubbles at grain boundaries, and another type, with Ni4W structure, was found on the potassium bubbles connected with dislocations. Palladium rich precipitates were present only in triple junctions. Their structure could not be established