On the thermal properties of Ag2Te and Ag2Se in the region of the phase transition

A setup is developed for measuring the thermal properties of Ag₂Te and Ag₂Se near the main phase transition and in the region of it in high vacuum. The samples are of stoichiometric composition with an excess of Ag, Te, and Se. Differential thermal analysis is performed and the temperature differences across and along the samples are measured by the pulsed light method. Using data on the thermal diffusivity a(T), the temperature dependence of the thermal conductivity K = apc is determined, where p is the density and c is the specific heat. The measurement technique used allows us to establish that the main phase transitions are accompanied by intense heat release. New phase transitions before and after the main structural phase transition with heat absorption are found. According to the DTA data, in the Ag₂Te sample with an excess of Ag (0.25 at.%, cn = 1.2 × 10¹⁹ cm^?3) the phase transition at 365 K is also accompanied by heat absorption. Thus, it is established that all of the phase transitions in Ag₂Te and Ag₂Se are first-order.     

Effect of thermal annealing on optical and photoelectric properties of microcrystalline hydrogenated silicon films

The effect of thermal annealing in the temperature range T _a=300–600°C of films of microcrystal-line hydrogenated silicon (μc-Si:H) lightly doped with boron on the spectral dependences of the absorption coefficient (α) at photon energies hν=0.8–2.0 eV, dark conductivity (σ_d), and photoconductivity (Δσ_ph) was studied at room temperature. With increasing annealing temperature, a nonmonotonic variation of α (at hν<1.2 eV), σ_d, and Δσ_ph was observed. The data obtained are attributed to a change in the concentration of electrically active impurities and formation of defects, caused by hydrogen effusion and bond restructuring at high annealing temperatures.     

Thermal measurements and analysis of AlGaInP/GaInP MQW red LEDs with different chip sizes and substrate thicknesses

Thermal properties of AlGaInP/GaInP MQW red LEDs are investigated by thermal measurements and analysis for different chip sizes and substrate thicknesses. To extract the thermal resistance (R_th), junction temperature (T_j) is experimentally determined by both forward voltage and electroluminescence (EL) emission peak shift methods. For theoretical thermal analysis, thermal parameters are calculated in simulation using measured heat source densities. The T_j value increases with increasing the injection current, and it decreases as the chip size becomes larger. The use of a thin substrate improves the heat removal capability. At 450 mA, the T_j values of 315 K and 342 K are measured for 500 × 500 μm² LEDs with 110 μm and 350 μm thick substrates, respectively. For 500 × 500 μm² LEDs with 110 μm thick substrate, the R_th values of 13.99 K/W and 14.89 K/W are obtained experimentally by the forward voltage and EL emission peak shift methods, respectively. The theoretically calculated value is 13.44 K/W, indicating a good agreement with the experimental results.     

Temperature effect on the physical properties of CuIn11S17 thin films for photovoltaic applications

CuIn₁₁S₁₇ compound was synthesized by horizontal Bridgman method using high-purity copper, indium and sulfur elements. CuIn₁₁S₁₇ thin films were prepared by high vacuum evaporation on glass substrates. The glass substrates were heated at 30, 100 and 200 °C. The structural properties of the powder and the films were investigated using X-ray diffraction (XRD). XRD analysis of thin films revealed that the sample deposited at a room temperature was amorphous in nature while those deposited on heated substrates were polycrystalline with a preferred orientation along the (311) plane of the spinel phase. Ultraviolet–visible (UV–vis) spectroscopy was used to study the optical properties of thin films. The results showed that CuIn₁₁S₁₇ thin films have high absorption coefficient α in the visible range (10⁵–10⁶ cm⁻¹). The band gap E_g of the films decrease from 2.30 to 1.98 eV with increasing the substrate temperature (T_s) from 30 to 200 °C. We exploited the models of Swanepoel, Wemple–DiDomenico and Spitzer–Fan for the analysis of the dispersion of the refractive index n and the determination of the optical constants of the films. Hot probe method showed that CuIn₁₁S₁₇ films deposited at T_s=30 °C and T_s=100 °C are p-type conductivity whereas the sample deposited at T_s=200 °C is highly compensated.     

A practical investigation on nickel plated copper heat spreader with different catalytic activation processes for flip-chip ball grid array packages

This study investigates the effects of two different catalytic activation techniques on the thermal performance of the flip-chip heat spreaders. The two activation techniques studied are thin nickel-copper strike and galvanic initiation. Thermal diffusivity and surface roughness of these heat spreaders were studied using the Nano-flash Apparatus and Infinite Focus Microscopy. High temperature storage tests were carried out to investigate the extent of intermetallic diffusion between the nickel and copper layers. The results show that heat spreaders with thin nickel-copper strike catalytic activation technique have a lower thermal diffusivity due to the low thermal conductivity of nickel-copper layer. Moreover, the nickel-copper layers grew thicker from around 0.2 μm at initial time to around 0.55 μm after high temperature storage duration of 168 h. On the other hand, heat spreaders processed using the galvanic initiation technique did not form any nickel-copper intermetallic diffusion layer. As a conclusion, the galvanic initiation technique can potentially provide better thermal performance for heat spreaders used in semiconductor packages.     

Structural and magnetic studies of Ti4+substituted M-type BaFe12O19 hexa-nanoferrites

M-type hexagonal BaTi_xFe_12−(4/3)xO₁₉ nanoparticles, 0≤x≤1, were prepared by the chemical co-precipitation method. XRD, TEM, FT-IR, TGA, DTA and VSM techniques were used to characterize the samples. This study proved the formation of single-phase M-type hexagonal nanoferrites. The average particle and crystallite (R) sizes, lattice parameter c, experimental and theoretical densities, strain (ε), saturation magnetization (M_s), magnetic moment showed decrease against x, whereas the lattice constant a, porosity (P) and specific surface area showed increase. The force constants and trend of band positions and Debye temperature were x dependent, whereas the coercivity, remanent magnetization, squareness and anisotropy constant did not. The net weight loss of the samples led in the range of 0.00418–0.01834% weight where the maximum weight loss occurred before heating at 500 °C. P and R showed strong effect on M_s and ε.     

Sputtered p-Type Sb2Te3/(Bi,Sb)2Te3 Soft Superlattices Created by Nanoalloying

In this work, p-type nanoscale ??soft superlattices?? consisting of multilayer stacks of 25?nm Sb₂Te₃ on 25?nm (Bi_0.2Sb_0.8)₂Te₃ were fabricated by nanoalloying. With this technique, nanoscale layers of the elements Bi, Sb, and Te are deposited by sputtering onto a Si/SiO₂ substrate and subsequently annealed to induce interdiffusion and a solid-state reaction to form the final superlattices. Different combinations of annealing temperatures were used in the annealing process. The in-plane electronic properties (Seebeck coefficient, electrical conductivity, charge carrier concentration, and carrier mobility) of these soft superlattices were examined. The cross-plane thermal conductivity was determined using time-domain thermal reflectance (TDTR). Secondary-ion mass spectrometry (SIMS) depth profiles reveal that the nanostructured thin films exhibit high stability against thermal interdiffusion during the annealing process. X-ray patterns of the samples display very strong texture with preferred c-orientation of the crystallites after the heat treatment. Scanning electron microscopy (SEM) cross-section images of the films show distinctly polycrystalline structure with increasing grain size for higher annealing temperatures, as confirmed by x-ray diffraction (XRD) analysis. Very high power factors exceeding 40???W/cm?K², similar to values for bulk single crystals with comparable compositions, are observed for the soft superlattices. The nanostructure appears to be stable up to 300°C. For a sample annealed at 150°C, a thermal conductivity as low as 0.45?W/mK was determined. Based on different assumptions concerning the degree of anisotropy of the transport properties, a cross-plane figure of merit ZT of 0.6 to 1.9 can be estimated for the thin films annealed at 300°C.     

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (T_s) using glass slides as substrates. At T_s≤673 K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3 μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For T_s=873 K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4 μm. The films obtained at T_s>473 K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Т_s=873 K (0.5×10⁻³) is considerably smaller than for the films deposited at Т_s=773 K (4.0×10⁻³). Increase of the value of T_s improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for T_s=873 K and can be suitable for various applications.     

Characterising Arrhenius moisture diffusivity constants using non-isothermal sorption

The diffusivity of moisture in a substance at an elevated temperature can be characterised using the technique of thermogravimetric desorption. However, the loss of moisture during the initial temperature ramp before stabilising at the test temperature could impair very significantly the accuracy of the evaluated moisture diffusivity. This can be overcome by discarding the data associated with the initial period of non-isothermal sorption followed by translating the residual moisture sorption data by a constant duration, Δt_shift, to the reference time, t_ref, corresponding to the reference temperature, T_ref; regressing between the experimental sorption data, (t_ref, ΔM_t), and the theoretical isothermal sorption data, (t_ref, ΔM_t,eq(D_ref, Δt_shift)), yields the diffusivity, D_ref, at T_ref. Extending the technique of time-translation to the non-isothermal sorption data by time-shifting the non-isothermal sorption data by a dynamic duration, Δt_shift (T), which is a function of the Arrhenius constants of diffusivity, D_o and E_d, to becoming isothermal sorption data at a reference temperature, the two Arrhenius constants of diffusivity can be extracted by regressing (t_ref, ΔM_t) with (t_ref, ΔM_t,eq(D_o, E_d)).     

Effect of Ca Doping on the Thermoelectric Performance of Yb<Subscript>14</Subscript>MnSb<Subscript>11</Subscript>

Complex Zintl phases possess low thermal conductivity and can be easily doped to modify the transport properties. Therefore, these phases have the potential to be good thermoelectric materials by simply controlling carrier concentration. Yb₁₄MnSb₁₁ is a Zintl phase that has shown promise as a p-type thermoelectric material for high-temperature power generation. A Sn-flux synthetic route was used to make the new phase, Yb₁₃CaMnSb₁₁. The high-temperature thermoelectric properties were measured on polycrystalline hot-pressed pellets and compared with Yb₁₄MnSb₁₁. Substitution of the lighter isovalent Ca for Yb should reduce the lattice thermal conductivity by mass disorder scattering, and a noticeable reduction is seen in thermal diffusivity measurements at high temperature. There may also be a carrier concentration effect by employing the more electropositive Ca.     

Correlation between microstructure and drastically reduced lattice thermal conductivity in bismuth telluride/bismuth nanocomposites for high thermoelectric figure of merit

The concept of nanocomposite/nanostructuring in thermoelectric materials has been proven to be an effective paradigm for optimizing the high thermoelectric performance primarily by reducing the thermal conductivity. In this work, we have studied the microstructure details of nanocomposites derived by incorporating a semi-metallic Bi nanoparticle phase in Bi₂Te₃ matrix and its correlation mainly with the reduction in the lattice thermal conductivity. Incorporating Bi inclusion in Bi₂Te₃ bulk thermoelectric material results in a substantial increase in the power factor and simultaneous reduction in the thermal conductivity. The main focus of this work is the correlation of the microstructure of the composite with the reduction in thermal conductivity. Thermal conductivity of the matrix and nanocomposites was derived from the thermal diffusivity measurements performed from room temperature to 150 °C. Interestingly, significant reduction in total thermal conductivity of the nanocomposite was achieved as compared to that of the matrix. A detailed analysis of high-resolution transmission electron microscope images reveals that this reduction in the thermal conductivity can be ascribed to the enhanced phonon scattering by distinct microstructure features such as interfaces, grain boundaries, edge dislocations with dipoles, and strain field domains.     

Photoacoustic study of optical and thermal properties of alloyed CdTexS1−x nanocrystals

Photoacoustic (PA) technique has been applied to study the optical and thermal properties of the alloyed CdTe_xS_1−x (nanocrystals (NCs) with different (Te/(S+Te)) molar ratio (x=0, 0.2, 0.4, 0.6, 0.8 and 1). Increasing x value causes clearly observed red shift of the corresponding exciton peak in PA spectra. The same spectra were compared to those obtained by a regular UV–Vis. absorption. The effective mass approximation (EMA) model was applied to determine the size of the NCs. The calculated sizes of the alloyed NCs are in a good agreement with the directly measured values obtained using high resolution transmission electron microscopy (HRTEM). The values of thermal diffusivity and thermal conductivity obtained using PA technique show at least an order of magnitude larger than that of the bulk values.     

Thermal parameters identification of micrometric layers of microelectronic devices by thermoreflectance microscopy

The objective of this paper is the determination of the thermal properties of micrometric layers of electronic devices using a thermoreflectance probe. Unlike classical thermoreflectance methods, the main point of the method presented in this paper is to be able to quantify the heating energy (by Joule effect) and the effective temperature response (by calibration). It is then possible to estimate the thermal conductivity (in W m⁻¹ K⁻¹) instead of the thermal diffusivity (in m² s⁻¹). A semi-analytical thermal 3D-periodic model then enables to identify a few thermal properties of the layers of the device, and in particular the thermal conductivity of the passivation layer. This methodology has been applied to the study of an industrial device containing interconnect test structures made of copper lines on a silicon wafer with a few micrometers BCB (BenzoCycloButene) polymer passivation layer. The BCB thermal conductivity and the metal heat capacity are obtained using this method.     

Crossed Andreev reflection in superconducting junctions

In this work crossed Andreev reflection (CAR) and elastic cotunneling (EC) are studied for junctions (N₁ISIN₂), where N₁ and N₂ are normal metals, S is a high T_c superconductor and I is an insulator. This study is carried out based on the analytical solutions of Bogoliubov de Gennes equations for anisotropic superconductors. The influence of different pair potential symmetries on the CAR and crossed conductance is analyzed. We show that CAR and EC are higher in d_x²-y² symmetry than in s symmetry. In the case of normal electrodes without magnetization, EC is the predominantly process for d_x²-y² symmetry, while in s symmetry, both processes decay with the same amplitude.     

Effect of fluctuations on electron and phonon processes and thermodynamic parameters of Ag2Te and Ag2Se in the region of phase transition

Temperature dependences of electrical conductivity σ, thermoelectric power α, results of differential thermal analysis ΔT _y , thermal conductivity χ, temperature conductivity κ, and heat capacity C _p were studied in Ag₂Te and Ag₂Se semiconductors in the region of the phase transition. Two extrema are observed in the temperature dependence χ(T): a maximum in the region of the α′ → β′ transition and a minimum in the region of the β′ → β transition; these extrema are caused by the similar dependence C _p (T). It is shown that the α → α′ and β′ → β transitions are displacement transitions, while the α′ → β′ transition is of reconstruction type. It is established that the disorder parameter η in silver chalcogenides is highly smeared in the region of the phase transition; therefore, disordering of phases at the point of the phase transition is incomplete: 73, 62, and 48% in Ag₂Te, Ag₂Se, and Ag₂S, respectively. The minimum volumes V _ph for new phases are calculated; it is shown that the value of V _ph in displacement transitions is larger than in the reconstruction-type transitions.     

Reliability study of power RF LDMOS device under thermal stress

This paper presents the results of comparative reliability study of two accelerated ageing tests for thermal stress applied to power RF LDMOS: Thermal Shock Tests (TST, air-air test) and Thermal Cycling Tests (TCT, air-air test) under various conditions (with and without DC bias, TST cold and hot, different extremes temperatures ΔT). The investigation findings of electrical parameter degradations after various ageing tests are discussed. On-state resistance (R_{ds_on}) is reduced by 12% and feedback capacitance (C_rss) by 24%. This means that the tracking of these parameters enables to consider the hot carrier injection as dominant degradation phenomenon. To reach a better understanding of the physical mechanisms of parameter's shift after thermal stress, a numerical device model (2D, Silvaco-Atlas) was used to confirm degradation phenomena.     

Thermoelectric Properties of CdTe1−x Cl x Material Prepared by Spark Plasma Sintering Method

CdTe compound is a prospective thermoelectric material due to its high Seebeck coefficient and low thermal conductivity. In the present study, we optimized its carrier concentration by substituting Cl on the Te site in order to improve the electrical conductivity and decrease the lattice thermal conductivity. The polycrystalline CdTe_1?x Cl _x (x = 0.005, 0.01, 0.03, 0.05) samples were fabricated by solid state reaction followed with spark plasma sintering, and the relative densities of the sintered samples were higher than 98%. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ). and thermal conductivity (κ), were measured in the temperature range of 300–700 K. The increase of Cl content (x) caused an increase of σ, and the maximum ZT value of 0.2 was obtained at about 630 K for the CdTe_0.97Cl_0.03 sample.     

Transport Properties of Bulk Thermoelectrics: An International Round-Robin Study, Part II: Thermal Diffusivity, Specific Heat, and Thermal Conductivity

For bulk thermoelectrics, improvement of the figure of merit ZT to above 2 from the current values of 1.0 to 1.5 would enhance their competitiveness with alternative technologies. In recent years, the most significant improvements in ZT have mainly been due to successful reduction of thermal conductivity. However, thermal conductivity is difficult to measure directly at high temperatures. Combined measurements of thermal diffusivity, specific heat, and mass density are a widely used alternative to direct measurement of thermal conductivity. In this work, thermal conductivity is shown to be the factor in the calculation of ZT with the greatest measurement uncertainty. The International Energy Agency (IEA) group, under the implementing agreement for Advanced Materials for Transportation (AMT), has conducted two international round-robins since 2009. This paper, part II of our report on the international round-robin testing of transport properties of bulk bismuth telluride, focuses on thermal diffusivity, specific heat, and thermal conductivity measurements.     

Proposal of a new structural thermal vacuum sensor with diode-thermistors combined with a micro-air-bridge heater

New structural Pirani gauge-type thermal vacuum sensor with a microheater and two pn junction diodes, Th-A and Th-B, as a high-sensitive temperature sensor working like a thermistor formed on a micro-air-bridge (MAB) is proposed. The MAB is separated into two regions of A and B. The Th-A and the Th-B can measure temperatures of the region A and the region B connected to the region A with thermal resistance, respectively. The microheater is formed in the region A and can maintain its temperature by feedback control. The diode-thermistor, Th-C, formed on the SOI substrate is provided to measure the ambient temperature T_c. Principle of this Pirani gauge-type thermal vacuum sensor is based on the measurement of the pressure-dependent thermal conductivity of gaseous media due to the heat exchange between the heated MAB (suspended film) and surrounding gas in vacuum. This has more than two orders of magnitude measurable pressure range (2×10⁻³-1×10⁵ Pa) compared with traditional Pirani vacuum sensor, and has very fast response and low power consumption.     

Effects of Frequency on AC Conductivity and Magnetoresistance in Doped La0.65Ca0.35MnO3 Manganites

The effects of frequency on the alternating-current (AC) conductivity of polycrystalline La_0.65Ca_0.35MnO₃ compounds prepared by solid-state reaction with various heat treatments have been studied. The AC conductivity and magnetoresistance (MR) are affected by the heat treatment temperature. The MR is enhanced at and below the transition temperature (T _C). The AC transport behavior of this system shows pronounced negative MR even at small applied magnetic fields for temperatures well below T _C. It is observed that such effects originate from spin-dependent scattering at grain boundaries as a consequence of spin disorder at the interfaces of the boundaries and spin scattering within the grains. The real part of the AC conductivity σ′ exhibits a pronounced frequency dependence, measured in the frequency range f < 100 kHz. The minimum occurs at a frequency that is temperature dependent. The MR below T _C and the frequency dependence of the AC electrical conductivity σ′(f) are interpreted as being consequences of cluster and grain boundary effects.