Interpreting Mössbauer spectra reflecting an infinite number of sites: an application to Fe1-xSi synthesized by pulsed laser annealing

We present a study on the interpretation of conversion electron Mossbauer spectra reflecting an infinite number of sites, in casu Mossbauer spectroscopy on Fe1-xSi layers on Si, synthesized by pulsed laser annealing. These spectra display a broad double-peaked resonance, reflecting the numerous different environments of the Fe-57 probe due to a distribution of vacancies on the Fe sublattice. The spectra can be fitted in many different ways; hence finding a reliable physical interpretation is not straightforward. Therefore ab initio calculations have been performed in order to obtain a priori information about the hyperfine interaction parameter distributions. For this material, the electric-field gradient on the Fe-57 atoms turns out to depend on details in the configuration of neighbors as far as the sixth neighbor shell. The isomer shift appears to be determined by the number of Fe atoms in the first and second Fe neighbor shells only. This leads to the construction of an ab initio based model predicting the mean isomer shift and its distribution for a given Fe1-xSi layer with a known Fe concentration profile. By applying this model new information from the experimental data can be extracted: we show that after applying one or two laser pulses, the Fe atoms are not completely randomized at an atomic scale. The relation of this model to other approaches of analyzing Mossbauer spectra with a distribution of sites is discussed, as well as the difference between the present results on [CsCl]Fe1-xSi and earlier interpretations in the literature. This work reveals how a combination of Mossbauer experiments and ab initio calculations leads to a more reliable interpretation of Mossbauer spectra reflecting an infinite number of sites

Formation and microstructure of cubic metastable iron silicides synthesized during pulsed laser annealing

The phase formation and crystallization processes of metastable [CsCl]Fe1-xSi phases were investigated by irradiating epsilon-FeSi/Si(111) thin films with a pulsed excimer laser in the energy density range 300-900 mJ/cm(2). The samples were analysed by Rutherford backscattering and channeling spectrometry (RBS/C), cross-sectional transmission electron microscopy (TEM) and conversion electron Mossbauer spectroscopy (CEMS). Laser irradiation results in mixing of the FeSi with the Si substrate, with the final concentration depending on the laser energy density. Due to the extremely rapid quench of the melt, a non-uniform Fe concentration is obtained. Analysis by cross-sectional transmission electron microscopy confirmed that this phase, which exhibits epitaxial ordering, corresponds to the metastable [CsCl]Fe1-xSi phase, which converts into the semiconducting beta-FeSi2 upon annealing at 600degreesC. CEMS indicates that no stable Fe- silicide phase nor a combination of stable phases have been formed. The CEM spectra consist of a distribution of quadrupole doublets and isomer shifts, in agreement with a [CsCl]Fe1-xSi phase that exhibits a (i) composition gradient and (ii) a random number of Fe vacancies in the neighbouring shells. These distributions make the CEM spectra hard to interpret. Full-Potential Linearized Augmented Plane Wave (FLAPW) calculations were performed to gain more insight in the hyperfine interaction parameters of the metastable [CsCl]Fe1-xSi phase and their dependence on a concentration variation. These calculations confirm the decreasing trend of the isomer shift with increasing number of laser pulses

A Ratiometric Readout Circuit for Thermal-Conductivity-Based Resistive CO₂ Sensors

This paper reports a readout circuit for a resistive CO2 sensor, which operates by measuring the CO2-dependent thermal conductivity of air. A suspended hot-wire transducer, which acts both as a resistive heater and temperature sensor, exhibits a CO2-dependent heat loss to the surrounding air, allowing CO2 concentration to be derived from its temperature rise and power dissipation. The circuit employs a dual-mode incremental delta-sigma ADC to digitize these parameters relative to those of an identical, but isolated, reference transducer. This ratiometric approach results in a measurement that does not require precision voltage or power references. The readout circuit uses dynamically-swapped transducer pairs to cancel their baseline-resistance, so as to relax the required dynamic range of the ADC. In addition, dynamic element matching (DEM) is used to bias the transducer pairs at an accurate current ratio, making the measurement insensitive to the precise value of the bias current. The readout circuit has been implemented in a standard 0.16 μm CMOS technology. With commercial resistive micro-heaters, a CO2 sensing resolution of about 200 ppm (1σ) was achieved in a measurement time of 30 s. Similar results were obtained with CMOS-compatible tungsten-wire transducers, paving the way for fully-integrated CO2 sensors for air-quality monitoring.Accepted Author ManuscriptElectronic InstrumentationMicroelectronic