A. M. Mansanares

Sensitivity enhancement in thermoreflectance microscopy of semiconductor devices using suitable probe wavelengths

In this paper we present an experimental and theoretical study of the thermoreflectance response as a function of the probe wavelength for layered microelectronics structures. The investigated sample consists of a polycrystalline silicon conducting track grown on a SiO2-coated Si substrate. Thermoreflectance measurements were carried out in the wavelength range from 450to750nm with the track biased in modulated regime. An oscillating pattern is observed in the spectral region where the upper layer is transparent. Such oscillations are due to the interference resulting from the multiple reflections at the interfaces. Using a thermo-optical model, we show that the optical constants (n and k) of the materials, which are wavelength dependent, as well as their temperature derivatives (dn∕dT and dk∕dT), strongly influence the thermoreflectance signal. The optical thicknesses of the layers, mainly determined by the real part of the refractive indices, define the period of oscillation. On the other hand, the imag...

Magnetocaloric effect in GdGeSi compounds measured by the acoustic detection technique: Influence of composition and sample treatment

In this paper we explore the acoustic detection method applied to the investigation of the magnetocaloric effect in Gd and Gd5(Ge1−xSix)4 compounds, in the temperature range from 230 to 360 K and for magnetic fields up to 20 kOe. Measurements were performed in as-cast materials, both for powder and pellet samples, and in tree samples with compositions around Gd5Ge2Si2 that underwent different thermal treatments. Small differences were observed when comparing powder and pellet samples of Gd and Gd5(Ge1−xSix)4 compounds with 0.500<x≤1.00. For the alloys with composition around Gd5Ge2Si2, which exhibit giant magnetostriction and coexistence of distinct phases, expressive changes were observed when comparing powder and pellet samples. Based on these cases, it is easy to see that the acoustic method can distinguish a second-order phase transition from a first-order magnetic-crystallographic one, and that it presents good sensitivity to detect spurious material phase in small quantities.

Anisotropic magnetocaloric effect in gadolinium thin films: Magnetization measurements and acoustic detection

In this letter, it is demonstrated the ability of the magnetoacoustic technique in detecting the magnetocaloric effect in gadolinium thin films (1.0 μm and 3.0 μm thick), which is not accessible through conventional temperature sensors because of the reduced mass of the samples. The method, which detects the direct effect of the sample temperature variation, proved to be sensitive to the anisotropy of the films, making possible for the investigation of the anisotropic magnetocaloric effect. Magnetization measurements were also carried out, and from these measurements both the adiabatic temperature and the isothermal entropy variations were calculated. The acoustically detected magnetocaloric effect shows very good agreement with these calculations.

Determination of the entropy change using the acoustic detection technique in the investigation of the magnetocaloric effect

In this paper we demonstrate the use of the acoustic detection as an alternative way to determine the entropy variation, ΔST, a parameter normally used to characterize the magnetocaloric effect. The measurements were performed for a Gd sample in the 252–316 K temperature range for magnetic fields from zero up to 50 kOe. The reversible adiabatic curves were built in a T versus H diagram, and specific heat data obtained at zero-magnetic field were employed to assign the entropy values of each curve. Subsequently, the entropy was plotted as a function of temperature for fixed magnetic fields, and therefore the isothermal entropy variation, ΔST, was found as a function of the temperature for several magnetic field steps.

Assessment of the thermal expansion mismatch in lanthanum strontium cobalt ferrite-yttria stabilized zirconia two-layers systems using photoacoustic methodology

In this letter we investigate the thermo-elastic mismatch in lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.2Fe0.8O3−δ) (LSCF) films deposited onto yttria stabilized zirconia (YSZ) substrates, applicable to solid oxide fuel cells. We investigated composite LSCF+YSZ and pure LSCF films deposited onto commercial YSZ substrate. Photoacoustics was used to obtain the effective thermal diffusivity and thermal expansion coefficient of the two-layers samples. Based on a thermal–electrical analogy model, it was possible to get the thermo-elastic properties of the films and to confirm the desired reduction on the thermo-elastic mismatch between film and substrate when comparing composite LSCF+YSZ and pure LSCF films.

Temperature induced stress phase transition in CdTe quantum dots observed by dielectric constant and thermal diffusivity measurements

The authors measured the dielectric constant by capacitance method and the thermal diffusivity by thermal lens technique in the temperature range from 20to300K for CdTe quantum dot doped borosilicate glass samples. Results show a huge difference between the thermal behavior of the pure glass matrix, without quantum dots, and of the doped glass, especially around 90 and 250K. The authors attributed this difference to the phase transition experienced by the CdTe nanocrystals due to the high pressure exerted by the glass matrix over the CdTe quantum dots. The temperature induced stress is caused by the thermal expansion coefficient mismatch between the quantum dot and the glass matrix.

Thermal diffusivity anisotropy in calamitic-nematic lyotropic liquid crystal

The temperature dependence of the thermal diffusivity, α, of a lyotropic liquid crystal is investigated using the thermal lens technique in the range of 12–52 °C. The sample under study presents a calamitic-nematic phase between 15.0 and 47.2 °C. Below and above these transition temperatures it is in the isotropic phase (the phase below 15.0 °C is named reentrant-isotropic, IRE). The sample is aligned in a magnetic field before measurements, with the micelles parallel or perpendicular to the cuvette sidewalls, thus allowing the determination of the thermal diffusivity in both directions. The observed anisotropy in the thermal transport parameter is analyzed based on a model, which considers the geometry of the micelles and the order parameter S as a function of temperature.

A method for thermal diffusivity measurement in fluids

A technique is proposed for thermal diffusivity measurement in fluids. It is based on the Angstrom method, but with excitation of thermal waves by electromagnetic energy absorption and pyroelectric detection. The good agreement between measured thermal diffusivity of air and some test liquids with literature values shows the validity of the method. It is free of some limitations of conventional photopyroelectric technique with length scanning because it is free of moving parts inside the sample and because it avoids problems associated with the non-parallelism between thermal wave generator surface and sensor. It does not require any data normalization procedure or special sample preparation.

Large magnetocaloric effect and refrigerant capacity near room temperature in as-cast Gd5Ge2Si2−xSnx compounds

Large values of isothermal entropy change (ΔST) and refrigerant capacity have been found in Gd5Ge2Si2−xSnx compounds. Values of the order of 20 J kg−1 K−1 for −ΔST were obtained in as-cast samples when submitted to a magnetic field variation of 2 T. First-order-magneto-structural transition is induced by the substitution of silicon by tin and it is shifted to lower temperatures with the tin content. It means that the magnetocaloric effect on this series can be properly tuned to a specific practical thermodynamic cycle, including near room temperature range.

Thermoreflectance microscopy applied to the study of electrostatic discharge degradation in metal-oxide-semiconductor field-effect transistors

We investigated the effect of electrostatic discharge on n-channel metal-oxide-semiconductor field-effect transistors using the thermoreflectance microscopy. The gate terminals of the transistors were submitted to electrostatic pulses on a zap system that respects the human body model. The pulse intensity varied from 40to140V in a cumulative sequence. Electrical characterization showed that the transistor threshold voltage was no longer positive for pulses of 110V and higher. No significant changes in the thermoreflectance maps were observed in these cases. For pulses of 140V a large leakage current appeared, and the thermoreflectance maps revealed strong peaks (localized spot) associated with the induced damage.