Comparison of techniques for in vivo attenuation measurements

The attenuation of an ultrasound pulse within tissue can be estimated from either the amplitude decay or the frequency downshift of returning echoes. The authors compare the results of both analyses as applied to ultrasound B scan echoes from the livers of 49 individuals. The amplitude decay of the backscattered signal Fourier components with depth was used to calculate attenuation coefficients. In addition, the frequency downshift of the same backscattered signals was estimated using both zero-crossing and spectral centroid methods. The results show that the frequency-domain estimators yield consistently higher attenuation coefficients, with higher variability compared to the amplitude decay method. Explanations for the apparent bias and variability of the frequency-shift estimators include both the assumptions regarding tissue and signal which may not be met in practice and the effects of low-frequency electronic noise on spectral estimates     

Nanowire silicon as a material for thermoelectric energy conversion

In order to use silicon as an efficient thermoelectric (TE) material for TE energy conversion, it is necessary to reduce its relatively high thermal conductivity, while maintaining the high power factor. This can be done by structuring silicon into 1-D structures or nanowires (NWs). Due to nanostructuring phonon scattering with the surface of such wires is drastically increased. This can result in a reduction of the thermal conductivity at a factor of 100 with respect to bulk silicon. Fabrication of vertical silicon NWs using ICP-cryogenic dry etching (Inductive Coupled Plasma) is described as a concept for CMOS-compatible integration of NW arrays instead of single NWs into a TE converter device. The uniform height of the NWs allows to connect simultaneously all NWs of an array. The realized NWs have diameters down to 180?nm and their height was selected between 1 and 10?μ. Measurement of electrical and thermal resistance of single silicon NWs with different diameters will be presented which was done in a scanning electron microscope (SEM) equipped with nanomanipulators. Furthermore, measurements will be shown of the thermal conductivity and the Seebeck coefficient of NW arrays.     

Fabrication, packaging, and characterization of p-SOI Wheatstone bridges for harsh environments

Strain gauges in p-type silicon-on-insulator (SOI) were investigated for data logging during deep drilling. Different metallization systems (Ti/Pt/Au, Ti/Pd/Au, and Ti/TiN/Au) were tested for high temperatures by measuring the specific resistance over time. Pressure-assisted Pick-and-Place silver sintering was applied to attach the fabricated p-SOI Wheatstone bridges on substrate carriers. The influence of the joining technique on the offset voltage was investigated as well as the temperature behavior during temperature cycling (50?cycles from 30°C up to 250°C). Furthermore, the temperature dependence of noise was analyzed.     

Shallow and deep dry etching of silicon using ICP cryogenic reactive ion etching process

We achieved to etch nanostructures as well as structures with high aspect ratios in silicon using an inductively coupled plasma cryogenic deep reactive ion etching process. We etched cantilevers, submicron diameter pillars, membranes and deep structures in silicon with etch rates between 13 nm/min and 4 μm/min. These structures find applications as templates for metal organic vapour phase epitaxial growth of GaN-based nanostructures for optoelectronic devices or they are the basic constituents of a nanoparticle balance in the subnanogram range and of a thermoelectric generator.     

Measurements of thermoelectric properties of silicon pillars

Nanostructured silicon as a material for thermoelectrics provides several advantages over conventional materials, such as bismuth telluride or lead telluride. The technological processing of silicon is well advanced due to the rapid development of microelectronics in recent decades. Silicon is largely available and environmentally friendly. The operating temperature of silicon thermoelectric generators is higher (>250 °C) compared to bismuth telluride. So far silicon is rarely used as a thermoelectric material because of its high thermal conductivity. The figure of merit Z, which is the commonly used measure of the thermoelectric properties of materials, is thereby too small for device applications. In order to introduce silicon as an efficient thermoelectric material thermal conductivity has to be drastically reduced. Nanostructuring into wires, i.e. restriction of the device geometry in both dimensions perpendicular to the heat propagation path indicates a route towards lower thermal conductivity. In this study we investigated silicon pillars produced in a wafer-scale top-down process by ICP (Inductive Coupled Plasma) cryogenic dry etching followed by thermal oxidation and oxide stripping. The pillars have diameters from 2 μm down to 170 nm. Their heights vary from 26.7 μm to 32.1 μm. We measured the reduction of thermal conductivity due to decreasing of pillar diameter. 3ω measurements were performed using a Wollaston probe with pillars of diameters down to 700 nm showing a reduction of thermal conductivity of 27% compared to bulk silicon.     

Propagation and backpropagation for ultrasonic wavefront design

Wave backpropagation is a concept that can be used to calculate the excitation signals for an array with programmable transmit waveforms to produce a specified field that has no significant evanescent wave components. This concept can also be used to find the field at a distance away from an aperture based on measurements made in the aperture. For a uniform medium, three methods exist for the calculation of wave propagation and backpropagation: the diffraction integral method, the angular spectrum method, and the shift-and-add method. The boundary conditions that are usually implicitly assumed by these methods are analyzed, and the relationship between these methods are explored. The application of the angular spectrum method to other kinds of boundary conditions is discussed, as is the relationship between wave backpropagation, phase conjugation, and the time-reversal mirror. Wave backpropagation is used, as an example, to calculate the excitation signals for a ring transducer to produce a specified pulsatile plane wave with a limited spatial extent.     

Renormalization of the band gap in highly photoexcited type-II ZnSe/BeTe structures

For the type-II ZnSe/BeTe heterostructures, a large (～0.1 eV) red shift of the edge of interband recombination in the ZnSe layers is observed at high densities of spatially separated photoexcited electrons and holes (～10¹³ cm^?2). The observed magnitude of renormalization of the band gap exceeds the magnitudes predicted by the multiparticle theory for dense type-I electron-hole systems at the same concentrations of two-dimensional charge carriers. Numerical calculations show that macroscopic electric fields induced by separated charges have a profound effect on the energy of direct transitions in type-II structures, resulting in an additional decrease in the energy of the transitions. In wide structures, where the ZnSe layer thickness is ? 15 nm, the renormalization effect is less pronounced. This is attributed to incomplete spatial separation of photoexcited charge carriers in the case of profound band bending and, thus, to the less-pronounced effect of electric fields.     

Time-shift estimation and focusing through distributed aberrationusing multirow arrays

The effects of element height on time-shift estimation and transmit focus compensation are demonstrated experimentally. Multirow ultrasonic transducer arrays were emulated by combining adjacent elements of a 3.0-MHz, 0.6-mm pitch, two-dimensional array to define larger virtual elements. Pulse-echo data were acquired through tissue-mimicking distributed aberrators, and time-shift maps estimated from those data were used for transmit focus compensation. Compensated beams formed by arrays with fine row pitches were similar, but focus restoration was significantly less effective for "1.75-D" arrays with a coarse row pitch. For example, when focus compensation was derived from strongly aberrated random scattering data [70-ns nominal rms arrival time fluctuation with 7 mm FWHM (full-width at half-maximum) correlation length], the mean -20 dB lateral beamwidths were 5.2 mm for f/2.0 arrays with 0.6- and 1.8-mm row pitches and 9.5 mm for an f /2.0 array with 5.4-mm pitch. Time-shift maps estimated from random scattering data acquired with 5.4-mm pitch arrays included large discontinuities caused by low correlation of signals received on vertically and diagonally adjacent emulated elements. The results indicate that multirow arrays designed for use with aberration correction should have element dimensions much less than 75% of the correlation length of the aberration and perhaps as small as 25 to 30% of the correlation length     

Improved degradation stability of blue-green II-VI light-emitting diodes with excluded nitrogen-doped ZnSe-based layers

Degradation characteristics of p-i-n BeZnSe/Zn(Be)CdSe light-emitting diodes were investigated. Undoped short-period superlattices, which provide efficient hole transport from the p ⁺-BeTe:N near-contact region (hole injector) into the active region, were used instead of the p-doped BeZnSe:N emitter. It is demonstrated that this makes it possible to considerably lengthen the operating life of the light-emitting diodes at highest direct current densities (～4.5 kA/cm²) at room temperature.