High-Temperature Performance of Stacked Silicon Nanowires for Thermoelectric Power Generation

Deep reactive-ion etching at cryogenic temperatures (cryo-DRIE) has been used to produce arrays of silicon nanowires (NWs) for thermoelectric (TE) power generation devices. Using cryo-DRIE, we were able to fabricate NWs of large aspect ratios (up to 32) using a photoresist mask. Roughening of the NW sidewalls occurred, which has been recognized as beneficial for low thermal conductivity. Generated NWs, which were 7 μm in length and 220 nm to 270 nm in diameter, were robust enough to be stacked with a bulk silicon chip as a common top contact to the NWs. Mechanical support of the NW array, which can be created by filling the free space between the NWs using silicon oxide or polyimide, was not required. The Seebeck voltage, measured across multiple stacks of up to 16 bulk silicon dies, revealed negligible thermal interface resistance. With stacked silicon NWs, we observed Seebeck voltages that were an order of magnitude higher than those observed for bulk silicon. Degradation of the TE performance of silicon NWs was not observed for temperatures up to 470°C and temperature gradients up to 170 K.     

Molecular beam epitaxy of alternating-strain ZnSe-based multilayer heterostructures for blue-green lasers

High-quality ZnSe-based heterostructures are grown by uninterrupted molecular beam epitaxy using the concept of strain compensation and alternating-strain multilayers. To verify the advantages of this technique, optically pumped ZnSSe/ZnCdSe laser structures containing short-period superlattices or multiple quantum wells have been grown and studied. A room-temperature injection laser diode with a BeZnSe/ZnSe superlattice waveguide is described. Fiz. Tekh. Poluprovodn. 32, 1272–1276 (October 1998)     

Highly Selective SAM–Nanowire Hybrid NO2 Sensor: Insight into Charge Transfer Dynamics and Alignment of Frontier Molecular Orbitals

Organic–inorganic hybrid gas sensors can offer outstanding performance in terms of selectivity and sensitivity towards single gas species. The enormous variety of organic functionalities enables novel flexibility of active sensor surfaces compared to commonly used pure inorganic materials, but goes along with an increase of system complexity that usually hinders a predictable sensor design. In this work, an ultra‐selective NO₂ sensor is realized based on self‐assembled monolayer (SAM)‐modified semiconductor nanowires (NWs). The crucial chemical and electronic parameters for an effective interaction between the sensor and different gas species are identified using density functional theory simulations. The theoretical findings are consistent with the experimentally observed extraordinary selectivity and sensitivity of the amine‐terminated SnO₂ NW towards NO₂. The energetic position of the SAM–gas frontier orbitals with respect to the NW Fermi level is the key to ensure or impede an efficient charge transfer between the NW and the gas. As this condition strongly depends on the gas species and the sensor system, these insights into the charge transfer mechanisms can have a substantial impact on the development of highly selective hybrid gas sensors.     

Measurements of differential scattering cross section using a ring transducer

A procedure for the measurement of intrinsic scattering object properties is presented and used to obtain illustrative results. The procedure is based on the measurement of the scattered acoustic field as a function of scattering angle and frequency. Measurements are normalized using analytically determined expressions for emitter and detector beams resulting from a combination of unfocused linear elements arranged in a circular configuration. The spatial effects of finite emitter pulse length and detector gate length are represented by a convolution formula valid for narrow-band transmitted signals and long receiver gates. The normalization includes correction for target absorption as well as measurement of the directly transmitted acoustic power in the free field and yields the average differential scattering cross section per unit volume. Under the Born approximation, this quantity is directly proportional to the spatial-frequency spectrum of the scattering medium inhomogeneities. Measured results are reported for two phantoms consisting of glass microspheres embedded in a weakly absorbing agar background medium. For the phantoms employed, scattering effects, rather than increased absorption, are shown to account for most of the difference in transmission loss between pure agar and agar with glass spheres. The measured differential scattering cross sections are compared with theoretical cross sections for distributions of glass spheres measured experimentally. The measured values show good relative agreement with theory for varying angle, frequency, and phantom properties. The results are interpreted in terms of wave space resolution and the potential for tissue characterization using similar fixed transducer configurations.     

Gallium nitride heterostructures on 3D structured silicon

We investigated GaN-based heterostructures grown on three-dimensionally patterned Si(111) substrates by metal organic vapour phase epitaxy, with the goal of fabricating well controlled high quality, defect reduced GaN-based nanoLEDs. The high aspect ratios of such pillars minimize the influence of the lattice mismatched substrate and improve the material quality. In contrast to other approaches, we employed deep etched silicon substrates to achieve a controlled pillar growth. For that a special low temperature inductively coupled plasma etching process has been developed. InGaN/GaN multi-quantum-well structures have been incorporated into the pillars. We found a pronounced dependence of the morphology of the GaN structures on the size and pitch of the pillars. Spatially resolved optical properties of the structures are analysed by cathodoluminescence.     

Thermoelectric Properties of High-Doped Silicon from Room Temperature to 900 K

Silicon is investigated as a low-cost, Earth-abundant thermoelectric material for high-temperature applications up to 900 K. For the calculation of module design the Seebeck coefficient and the electrical as well as thermal properties of silicon in the high-temperature range are of great importance. In this study, we evaluate the thermoelectric properties of low-, medium-, and high-doped silicon from room temperature to 900 K. In so doing, the Seebeck coefficient, the electrical and thermal conductivities, as well as the resulting figure of merit ZT of silicon are determined.     

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.     

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