This study investigates the impact of Mg doping concentration in GaN thin films on their physical and mechanical properties, specifically on their conductivity transition, luminescence, hardness, Young's modulus, and creep behavior. Mg-doped GaN layers were grown on sapphire substrates using the MOCVD technique, with trime-thylgallium (TMG) and bis(cyclopentadienyl) magnesium (Cp2Mg) as precursors for Ga and Mg, respectively. The TMG flow rate was set at 20 & mu;mol/min, while the Mg flow rate was adjusted by tuning the temperature of the thermostatic bath of Cp2Mg. Three different Cp2Mg flow rates: 2, 7 and 9 & mu;mol/min were utilized to explore their effects on the physical and mechanical properties of the grown epilayers. The results demonstrate that an in-crease in the Mg concentration leads to a conductivity transition from n-type to p-type, with the highest hole concentration (4.5 & PLUSMN; 0.2) x 1017 cm-3 and blue luminescence attained with Mg concentration equal to 2.1 x 1019 atoms/cm3. Moreover, the hardness, Young's modulus and the intensity of compressive stress increase with the Cp2Mg flow rate due to the evolution of pre-existing dislocation density and point defects incorporation. Additionally, this study evaluates the creep behavior of the Mg-doped GaN thin films. It shows that both the creep stress exponent and the maximal creep depth decrease with the increase of the Cp2Mg flow rate. This is attributed to dislocation glides and climbs governing the creep mechanism. Accordingly, this investigation highlights the importance of understanding the relationship between physical properties and mechanical char-acteristics of Mg-doped GaN thin films, which have potential implications for various technological applications.

We report on the simulation and fabrication of 2.5D micro-thermoelectric generators (mu TEGs) with a thermopile topology periodically folded and distributed on a multi-membrane template, capable of harvesting lost heat directly into useful electrical energy. The originality of the folded thermopile is multiple: i/ it uses low-cost and eco-friendly alloy-based thermoelectrics (TEs) in the form of a series of Ni90Cr10/Cu55Ni45 thermocouples (TCs), ii/ in each TC both TE layers are stacked on top of each other allowing a better integration, iii/ the TCs are electrically associated in series and in parallel, reducing drastically the electrical resistance, iv/ the choice of membrane number permits to tune the module thermal resistance. All this results in an improvement of the conversion efficiency of the mu TEG compared to our former all-Silicon planar modules. A finite element simulation allows defining the temperature distribution profiles in the module as a function of its dimensions. Several 2-and 3-membranes based mu TEGs were fabricated using CMOS-compatible Silicon technology and characterized. In the best mu TEG, the harvesting of 1 Watt of heat results in an output power density of 108.3 mu W/cm2. This corresponds to an efficiency factor of 6.82 10-3 mu W.cm-2.K-2 which is better than state-of-the-art metal-based modules.

In this work, we present our novel planar micro-thermoelectric generators (µTEGs) integrating an original “folded” thermopile topology, periodically distributed on suspended membranes. The thermopile integrates eco-friendly and low-cost materials as Constantan (Cu55Ni45) and Chromel (Ni90Cr10) that build up thermocouples associated both in series and in parallel. This dual association allows to drastically reduce the thermopile electrical resistance (down to a few tens to a few hundred ohms) and so to electrically better adapt them. To optimize the structural dimensions of the µTEG, numerical simulations have been performed by 3D-finite element modeling using COMSOL Multiphysics© software. Several µTEG modules integrating this folded thermopile have been manufactured using Silicon micro-technologies that differ from our former expertise. In 3-membrane-based modules the harvested output power can reach 108 µW/cm2 (for 1 W heat injected into the µTEG) with an output voltage up to few hundred of millivolts, which is enough to supply micro devices for IoT applications.

A set of N-polar InN epilayers has been grown at different temperatures by plasma-assisted molecular beam epitaxy (PA-MBE) on GaN/AlN/Al2O3(0001) templates. The purpose is to understand how the variation of crucial factor, such as the temperature, impacts the growth process and the resulting samples' properties. The characterization of these InN samples using atomic force microscopy and scanning electron microscopy showed different island distributions and shapes by varying the growth temperature. High resolution-X-ray diffraction (HRXRD) enabled to identify a single crystalline phase (hexagonal wurtzite), whatever the growth temperature. Actually, the increase of growth temperature up to 560 degrees C has improved the crystalline quality; whereas for high temperature, the crystalline quality degrades. The dislocation density of the epilayer grown at this optimum temperature (around 560 degrees C) is about 1.9 x 10(10) cm(-2), which is determined using HRXRD spectra analysis. High compressive residual stress value of 0.54 GPa was derived using Raman spectroscopy. Room temperature photoluminescence (PL) displays a band gap energy around 0.69 eV. Besides, Burstein-Moss effect, the PL band gap energy measured at 10 K is dictated by the biaxial compressive residual stresses. Nanoindentation tests were carried out on InN epilayers. Only, the sample grown at 560 degrees C exhibited a pop-in event, for which the measured hardness and Young's modulus are of 4.5 +/- 0.5 GPa and of 171 +/- 8 GPa, respectively. Accordingly, the growth temperature of InN epilayers influences the resulting physical and mechanical performances, thus a good compromise between physical and mechanical features permits to manufacture efficient devices. (C) 2021 Elsevier B.V. All rights reserved.

In this paper, we address the optimization of the electrical and thermoelectrical properties of nanostructured polysilicon material for integration in thermoelectric (TE) devices. In particular, we describe an original method to build a superficial and nanostructured porous polysilicon (POpSi) thin film on a Silicon substrate. This latter is electrically isolated from the conductive Silicon substrate by an SiO2 interlayer for Van der Pauw electric and home-made thermoelectric measurements. From the characterization data, we demonstrate that the nanostructuration brought by porosification of polysilicon layers (polySi) breaks their thermal conductivity by a factor of about 30 and has no detrimental impact on their Seebeck coefficient: a POpSi with up to 62% porosity has an experimental Seebeck value of 260 mu V/K when the standard polysilicon, it is built from, is n type with a carrier density of 3.4 x 10(19)/cm(3). On the other hand, the benefit of the thermal conductivity reduction clearly over-compensates the degradation of the experimental electrical conductivity measured in the POpSi layers: in terms of the figure of merit ZT, a 25-fold increase in a POpSi layer with 44% porosity is seen compared to the standard polySi layer (it increases from 0.004 to 0.1). Such a novel material is a viable candidate for planar thermoelectric devices using Silicon technologies. In this effort, we discuss the integration of a POpSi thin film with optimum porosity into a planar TE microgenerator (mu TEG) with suspended membranes supported by a simulated conversion efficiency increased by 28% compared to a mu TEG integrating the standard polySi layer counterpart.

The chemical co-precipitation process was used for the synthesis of Fe-doped ZnO nanopowders (ZnO:Fe) with varying doping concentrations from 0% to 10 %. Doping with iron caused significant alterations in the physical properties of ZnO nanopowders. The XRD analysis revealed that Fe introduction forms a secondary phase of spinel ZnFe2O4 for doping concentration higher than 1%. The average crystallite size was proven to decrease with the increase in Fe concentration. SEM images showed the formation of spherical nanoparticles, where the average grain size decreased with doping. The Fe incorporation into the ZnO host lattice was also confirmed by FTIR and Raman spectroscopy. The optical properties were strongly influenced by Fe doping. The dielectric parameters were subsequently investigated as a function of frequency and composition. The observed dielectric dispersion was interpreted on the basis of Maxwell-Wagner interfacial model and Koops phenomenological theory. Overall, Fe doping improved the electrical conductivity of ZnO nanopowders.

Dans cet article, nous présentons des microgénérateurs thermoélectriques (µTEG) planaires réalisés en technologie Silicium compatible CMOS, et destinés à la récupération de toute forme d'énergie thermique.Ceux-ci utilisent des matériaux faible coût, abondants et respectueux de l'environnement.La captation de la chaleur est effectuée à l'aide d'un concentrateur, en Silicium.La configuration « semi 3D » développée utilise des membranes suspendues et nécessite, pour éviter de les casser, la mise en oeuvre de structures de maintien, appelées bossages.L'objectif de ce travail est d'évaluer l'amélioration des performances de conversion des µTEG résultant de la réduction des pertes thermiques latérales suite à l'anodisation sélective de ces bossages.La simulation thermique de µTEGs intégrant de tels bossages en Si poreux montre que les puissances générées pourront être augmentées jusqu'à 65%.ABSTRACT.In this paper, we present a planar thermoelectric microgenerator (µTEG) based on CMOS-compatible silicon technology to harvest all forms of thermal energy.These have made by using low cost, abundant and ecofriendly materials.The heat harvesting has performed with a silicon concentrator.The developed semi-3D configuration needs safety support called 'boss' to avoid the breaking up of the membranes.The objective of this work is to evaluate the improvement of these μTEGs performance resulting from the reduction of lateral heat losses by using selective anodizing of these bosses.The thermal modeling of µTEGs embedding porous silicon bosses shows an increase in output power up to 65%.

A technology of planar micro thermoelectric generators (mu TEGs) with double thermal deflections is presented in this letter. We focus on the development of an original technic of bonding pillars etched on a first silicon wafer onto the fragile membranes of a second silicon wafer. The monolithic devices realized are able to convert waste heat into electrical power. The maximum output power for a five-membranesbased mu TEG is 12.3 mu W/cm(2), for an input power of 2 W/cm(2). This kind of mu TEG has a high thermal resistance. It is deduced to be 44.3 K/W, which allows a heat adaptation to any environment with a high thermal resistance.

In this paper, Si-doped GaN template layer and nitride-based multiple quantum well (MQW) light-emitting diodes (LEDs) with conventional GaN buffer layer and GaN buffer layer using SiN treatment were elaborated by metalorganic vapor phase epitaxy (MOVPE). On both kinds of structures, five In x Ga1−x N/GaN quantum wells were deposited simultaneously in identical growth conditions. GaN template layer defects that influence on the growth of In x Ga1−x N/GaN MQW LEDs were systematically studied by means of scanning electron microscopy, high-resolution x-ray diffraction, temperature-dependant photoluminescence measurement, and electroluminescence. It is shown that optical properties of In x Ga1−x N/GaN MQWs depend on the defect density of elaborated templates. Thereafter, we report an enhancement of the emission of blue MQW LEDs using SiN treatment, compared to the MQW LED emissions deposited on a conventional GaN buffer layer.

In this paper, we focus on the dispersion performances related to the manufacturing process of heat flux sensors realized in CMOS (Complementary metal oxide semi-conductor) compatible 3-in technology. In particular, we have studied the performance dispersion of our sensors and linked these to the physical characteristics of dispersion of the materials used. This information is mandatory to ensure low-cost manufacturing and especially to reduce production rejects during the fabrication process. The results obtained show that the measured sensitivity of the sensors is in the range 3.15 to 6.56 μV/(W/m2), associated with measured resistances ranging from 485 to 675 kΩ. The dispersions correspond to a Gaussian-type distribution with more than 90% determined around average sensitivity S e ¯ = 4.5 µV/(W/m2) and electrical resistance R ¯ = 573.5 kΩ within the interval between the average and, more or less, twice the relative standard deviation.

This paper presents the modelling, design, fabrication and characterization of a planar micro thermoelectric generator (mu TEG) which is able to convert waste heat into a few microwatts of electrical power. In order to get a better performance under a large variety of heat sources even if their thermal resistance is high, a planar mu TEG with a large thermal resistance was designed and fabricated. It is built of two periodically etched silicon substrates that are respectively used as heat concentrator and heat evacuator, the whole embedding a multilayer membrane which includes a polysilicon-based thermopile with large thermoelement leg length. The thick air cavities etched in the substrates are effective in preventing the direct heat loss from concentrator to evacuator. 3D thermal simulations are carried out to improve the performance of the mu TEG. A new definition of the "efficiency-factor" which involves the thermal input power instead of the temperature difference across the chip is suggested to evaluate the efficiency of this kind of mu TEGs. The advantage of this new efficiency factor is that it takes the thermal resistance of the mu TEG into consideration. With a thermal resistance of 78 K/W, the experimental results show that the mu TEG can work under high temperature difference (up to 267 K). With an optimized structure, i.e. 5 membranes and annealed polySi as TE main material, the maximum output power of our mu TEG is 138 mu W/cm(2) when the input power is 4 W/cm(2) and its corresponding new efficiency factor is 865 mu m(2)/W. (C) 2014 Elsevier B.V. All rights reserved.

A low-cost and non-destructive measurement technique based on the combination of a temperature sensor and a heater integrated in a very sharp tip is proposed for the determination of thermal conductivity of planar materials. The thermal sensor is fabricated by means of microtechnology technique. Associated to the system, an analytical thermal model is developed to express the measured Seebeck voltage as a function of the material thermal resistance. A numerical analysis based on COMSOL Multiphysics. is then developed to extract the thermal conductivity from the thermal resistance. To validate the approach proposed, experiments on planar dielectric materials and metals are conducted. Precision around 0.1 W m(-1) K-1 for thermal conductivities lower than 2 W m(-1) K-1 is obtained.

This paper presents an original integration of polycrystalline SiGe-based quantum dots superlattices (QDSL) into Thermoelectric (TE) planar infrared microsensors (μSIR) fabricated using a CMOS technology. The nanostructuration in QDSL results into a considerably reduced thermal conductivity by a factor up to 10 compared to the one of standard polysilicon layers that are usually used for IR sensor applications. A presentation of several TE layers, QDSL and polysilicon, is given before to describe the fabrication of the thermopile-based sensors. The theoretical values of the sensitivity to irradiance of μSIR can be predicted thanks to an analytical model. These findings are used to interpret the experimental measurements versus the nature of the TE layer exploited in the devices. The use of nanostructured QDSL as the main material in μSIR thermopile has brought a sensitivity improvement of about 28% consistent with theoretical predictions. The impact of QDSL low thermal conductivity is damped by the contribution of the thermal conductivity of all the other sub-layers that build up the device.

To investigate the effect of a seed layer on the growth and properties of ZnO nanorods using hydrothermal technique, various thickness of sputter deposited ZnO thin films were used. The changes in crystallinity, orientation, and optical properties of the nanorods synthesized on these ZnO thin films were examined. These properties were studied simultaneously in two series of samples, wherein in one series the nanorods were unannealed while in the other series they were annealed. Structural characterization revealed that both categories of nanorods were highly crystalline, with a hexagonal phase, and grew along the [0001] direction. The density of the nanorods per unit area increased as the thickness of the seed layer decreased. We also found that the defect related emission in photoluminescence spectra was quite low in both the annealed and non-annealed samples series. Typically, the decay curves obtained from these ZnO nanorods show a combination of two exponential decays. The nonradiative fast decay component was affected by the thickness of the seed layer and its values were higher than those of previously reported ZnO nanostructures grown by the hydrothermal technique. This comprehensive study shows that as grown nanorods lead directly to a high crystalline quality.

This work proposes a new method to improve the efficiency of the heat flow path of a micro thermoelectric generator (mu TEG). A silicon heat concentrator is placed on a planar mu TEG after alignment to guide the heat flow from the heat source to the hot junctions. The results show that the heat concentrator can efficiently isolate the heat source from the cold junctions and guide the heat flux to pass through the planar thermocouples in order to generate a temperature gradient. When a 4W/cm(2) power is injected to a fabricated device, it builds an output voltage of 29V/cm(2) which can supply an output power of 41 mu W/cm(2) to a matched load resistance.

The microstructure of nonpolar heteroepitaxial wurtzite films (GaN and ZnO-based) is dominated by the presence of planar basal stacking faults (BSFs). In this paper, transmission electron microscopy studies of both GaN and ZnO nonpolar films oriented either (11–20) or (1–100) and grown on sapphire substrates, permit to propose and evaluate different mechanisms of BSFs formation. The main mechanism of formation of BSFs results from a Volmer Weber growth mode. The first stage of the growth is a 3D nucleation. The 3D nuclei are relaxed at least along one in-plane orientation. BSFs are created in the coalescence boundaries in order to compensate translations between neighbouring islands. BSFs are well adapted to compensate in-plane translations in the case of nonpolar films. In fact, their plane is perpendicular to the substrate surface and this orientation is similar to the orientation of the coalescence boundaries. Moreover, their displacement vector has a component parallel to the translation between islands and their formation energy is low. On the other hand, in the case of the polar growth, BSFs are not adapted to compensate in-plane translation as their plane is perpendicular to the coalescence boundaries.

Ti(40 nm)/Au(120 nm) contacts have been prepared on n-GaN/sapphire. The contacts were annealed from 400 degrees C to 900 degrees C for 10 min in nitrogen, respectively. The interaction between the contact metallization and the GaN epitaxial layer (on sapphire) was investigated by electron microscopy and X-ray diffraction. It was found that for an annealing at a temperature equal or higher than 700 degrees C, the gold diffused through Ti layer and Au-rich grains were formed under this Ti layer. Between the Au-rich grains and the n-type GaN layer a new Ti-rich thin layer was identified by XTEM. High resolution images and X-ray data showed that it's TiN(0.26) that was formed at the metal/semiconductor interface. FESEM investigations showed that Ti/Au contacts annealed in nitrogen still have a very smooth surface morphology. (C) 2011 Elsevier B.V. All rights reserved.

In this article, infrared thermoelectric microsensors manufactured by CMOS technology with an original design are presented. Packaging is not required because absorbing and reflecting superficial zones, laid on a single membrane, are symmetrically structured with regard to the thermal disturbances of the surrounding gas. An analytical model was developed to determine the geometrical parameters of these microsensors in the aim to optimise the sensitivity to the radiation and the immunity to the ambient air temperature variations. Some sensors with membranes areas varying from 3mm×3mm to 10mm×10mm and 3×3 and 5×2 unpackaged arrays of microsensors were fabricated. A prototype of imager equipped with a polyethylene Fresnel lens was realised. Such devices open the way to a wide range of low resolution–low cost imager applications.

In two dimensional (2D) systems, the electron mobility (μ) depends on the electron concentration (n) showing a bell-like μ(n) dependence, observed in many semiconductor structures. The mechanism responsible for this effect is an interplay of scattering by the interface roughness and screening by the 2D electron gas. Investigation of μ(n) dependence gives important information about basic processes responsible for electrical conductivity. Such studies are usually carried out on large area devices (Hall bars). It is important to verify theoretical prediction concerning electrical transport in the case of modern nanometer field effect transistors (FETs) which enable generation of extremely high electric fields. Experimental determination of the mobility under the gate of a nanometer FET is a difficult task since the traditional methods fail in this case. Recently, a DC magnetoresistance (MR) method was applied to determine μ in nanometer Si MOSFETs and enabled to show an impact of the ballistic motion on μ at 300 K [1, 2]. However, the DC MR method is biased by the contact and access resistance which must be carefully taken into account. Here we present an AC MR method based on measurements of FET magnetoresistance with a simultaneous modulation of the gate potential. We show that in the AC MR method one determines μ and n directly under the gate of a FET, even with a gate of a nanometer length; μ is determined from a classical MR at low magnetic fields, and n from Shubnikov de Haas oscillations at quantizing high fields. We apply the AC MR method to different types of transistors: micrometer AlGaN/GaN HEMTs at 4 K, micrometer and sub micrometer (~ 70 nm) Si MOSFETs at 300 K. We discuss obtained dependencies of the electron scattering times on n, and possibilities of application of AC MR method for investigation of μ on a nanometer scale.