Characterization of AlGaN/GaN structures on various substrates grown by radio frequency-plasma assisted molecular beam epitaxy

The structural properties and surface morphology of AlGaN/GaN structures grown on LiGaO₂ (LGO), sapphire, and hydride vapor phase epitaxy (HVPE)-grown GaN templates are compared. AlGaN grown on LGO substrates shows the narrowest x-ray full width at half maximum (FWHM) for both symmetric 〈00.4〉 and asymmetric 〈10.5〉 reflections. Atomic force microscopy (AFM) analysis on AlGaN surfaces on LGO substrates also show the smoothest morphology as determined by grain size and rms roughness. The small lattice mismatch of LGO to nitrides and easily achievable Ga-polarity of the grown films are the primary reasons for the smoother surface of AlGaN/GaN structure on this alternative substrate. Optimizations of growth conditions and substrate preparation results in step flow growth for an AlGaN/GaN structure with 300 Å thick Al_0.25Ga_0.75N on 2.4 μm thick GaN. A high III/V flux ratio during growth and recently improved polishing of LGO substrates aids in promoting two dimensional step flow growth. The GaN nucleation layer directly on the LGO substrate showed no evidence of mixed phase cubic and hexagonal structure that is typically observed in the nucleation buffer on sapphire substrates. Cross-sectional high-resolution transmission electron microscopy (HRTEM) was performed on an AlGaN/GaN heterostructure grown on LGO. The atomic arrangement at the AlGaN/GaN interface was sharp and regular, with locally observed monolayer and bilayer steps.     

The growth of GaN on lithium gallate (LiGaO2) substrates for material integration

Compliant substrates offer a new approach for strain management in semiconductors. Various implementations and processes for achieving substrate compliancy have been proposed and demonstrated. These include the use of twist-, glass-, and metal-bonds. A recent focus in our work has been on the growth of GaN on a novel and easily removable substrate—lithium gallate—for the ultimate regrowth on a bonded GaN template. The bonding technology is important to reduce thermal stresses during the regrowth step. Herein, we focus on the understanding of the growth of GaN on lithium gallate.     

Design of material removal rate to reduce machining time of dental crown

Journal of Mechanical Science and Technology - Freeform surface grinding is one of the commonly used dental medical manufacturing processes. In dentistry machining, zirconia is one of main...     

The heterogenous integration of GaN thin-filmmetal-semiconductor-metal photodetectors onto silicon

The heterogeneous integration of GaN thin-film metal-semiconductor-metal (MSM) photodetectors onto a host substrate of SiO₂-Si is reported. Thin-film GaN photodetectors were separated from the lithium gallate (LiGaO₂) growth substrate using selective etching, and contact bonded onto a SiO₂-Si host substrate. The thin-film MSMs exhibited a dark current of 13.36 pA and an UV photoresponse at 308 nm of 0.11 A/W at a reverse bias voltage of 20 V. This first demonstration of GaN thin-film device integration onto SiO₂-Si using a low-temperature integration process, combined with the advances in GaN material quality on LiGaO₂ substrates, enables the integration of GaN devices with Si circuitry for heterogeneously integrated systems     

Enhanced write performance of a 64-mb phase-change random access memory

The write performance of the 1.8-V 64-Mb phase-change random access memory (PRAM) has been improved, which was developed based on 0.12-/spl mu/m CMOS technology. For the improvement of RESET and SET distributions, a cell current regulator scheme and multiple step-down pulse generator were employed, respectively. The read access time and SET write time are 68 ns and 180 ns, respectively.     

Active control of low-frequency noise in bubbly water-filled pipes

This paper describes an active silencer system for low-frequency noise in water-filled pipes, with implementation in marine vessels and structures in mind. Active noise control in underwater environment has its unique challenges arising from bubbles. In this light the effect of bubbles on active noise control in water-filled pipes is explored, and a design guideline for a robust active silencer is proposed. The silencer consists of an underwater sound source, an error hydrophone, and an electronic controller programmed with an adaptive filter. The adaptive filter is updated using the filtered-x least mean square algorithm. Estimation of the incoming noise is performed using an adaptive notch filter. The performance of the active silencer is tested for pure tone noises below 500 Hz in a water-filled pipe connected to a reservoir. Notable reduction of noise is achieved with varying degrees of success at different bubble fractions.

     

Capillary Performance of Micropillar Arrays in Different Arrangements

In this study, permeability and capillary pressure of copper micropillar structures (height: 50 µm, diameter: 50 µm) in different arrangements (hexagonal, rectangular, and square) and different porosities (0.45/0.5, 0.6, 0.7, 0.8) are compared experimentally and numerically. The micropillar structures are fabricated on copper clad printed circuit board with electroplating, and the samples are coated with silica nanoparticles to enhance wettability. A forced liquid flow test is used to measure permeability of the samples, and capillary rate-of-rise measurement technique is used to determine the capillary pressure of the wicks. In the permeability model, the effect of meniscus curvature is considered, and the results are compared with other permeability models. Capillary pressure is predicted by using surface energy minimization tool, Surface Evolver. The test results show that the micropost array in rectangular arrangement have the highest permeability, and similar capillary pressure compared to other pillar arrangements with the same porosity, and thus show the highest capillary performance parameter. The effect of gravity on the sample characterization with capillary rate-of-rise test is also studied to investigate the feasibility of applying Washburn’s equation to test data.     

A 90 nm 1.8 V 512 Mb Diode-Switch PRAM With 266 MB/s Read Throughput

A 512 Mb diode-switch PRAM has been developed in a 90 nm CMOS technology. The vertical diode-switch using the SEG technology has achieved minimum cell size and disturbance-free core operation. A core configuration, read/write circuit techniques, and a charge-pump system for the diode-switch PRAM are proposed. The 512 Mb PRAM has achieved read throughput of 266 MB/s through the proposed schemes. The write throughput was 0.54 MB/s in internal x2 write mode, and increased to 4.64 MB/s with x16 accelerated write mode at 1.8 V supply.     

Sound visualization of smartphone-radiated noise using nearfield acoustic holography

A smartphone contains electronic components that can inadvertently act as sources of unwanted vibration and noise. Also known as the “buzz,” the noise is produced primarily by multilayer ceramic capacitors, the piezoelectric pulsation of which drives the circuit board into vibration, hence sound radiation. Given the close proximity of the smartphone to the ear, the audible noise, albeit low in amplitude, can be a nuisance and degrade the call quality. Thus accurate measurement and analysis of the noise are needed as the first step toward resolving the problem. The popular farfield-based techniques, however, are not applicable to smartphone-radiated noise because of its low sound pressure level (∼20 dB). This paper presents an alternative method based on nearfield acoustic holography (NAH), here tailored for a small mobile device radiating low-intensity noise. The NAH method is shown to be capable of visualizing sound pressure and intensity anywhere near the smartphone as well as the vibration of the circuit board, which could lead to an effective design strategy for a quieter smartphone.