AlN/IDT/AlN/Sapphire SAW Heterostructure for High-Temperature Applications.

Recent studies have evidenced that Pt/AlN/Sapphire surface acoustic wave (SAW) devices are promising for high-temperature high-frequency applications. However, they cannot be used above 700°C in air atmosphere as the Pt interdigital transducers (IDTs) agglomerate and the AlN layer oxidizes in such conditions. In this paper, we explore the possibility to use an AlN protective overlayer to concurrently hinder these phenomena. To do so, AlN/IDT/AlN/Sapphire heterostructures undergo successive annealing steps from 800°C to 1000°C in air atmosphere. The impact of each step on the morphology, microstructure, and phase composition of AlN and Pt films is evaluated using optical microscopy, scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS). Finally, acoustical performance at room temperature of both protected and unprotected SAW devices are compared, as well as the effects of annealing on these performance. These investigations show that the use of an overlayer is one possible solution to strongly hinder the Pt IDTs agglomeration up to 1000°C. Moreover, AlN/IDT/AlN/Sapphire SAW heterostructures show promising performances in terms of stability up to 800°C. At higher temperatures, the oxidation of AlN is more intense and makes it inappropriate to be used as a protective layer.

Investigations of AlN thin film crystalline properties in a wide temperature range by in situ X-ray diffraction measurements: Correlation with AlN/sapphire-based SAW structure performance.

Aluminum nitride on sapphire is a promising substrate for SAW sensors operating at high temperatures and high frequencies. To get a measure of the suitability and temperature stability of such devices, an experimental relationship between the SAW performance and the structural properties of the AlN thin films was investigated in the temperature range between the ambient temperature and 1000°C. The crystalline structure of the AlN films was examined in situ versus temperature by X-ray diffraction. The results reveal that the AlN films remain (002) oriented even at high temperatures. A gradual increase of the tensile stress in the film due to the thermal expansion mismatch with the substrate has been observed. This increase accelerates around 600°C as the AlN film crystalline quality improves. This phenomenon could explain the amelioration in the SAW performance of AlN/sapphire devices observed previously between 600°C and 850°C. At higher temperatures, surface oxidation of the AlN films reduces the SAW performance. The potential of ZnO thin films as protective layers was finally examined.

KCN Chemical Etch for Interface Engineering in Cu2ZnSnSe4 Solar Cells.

The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.

Local modification of the microstructure and electrical properties of multifunctional Au-YSZ nanocomposite thin films by laser interference patterning.

Nanocomposite films consisting of gold nanoparticles embedded in an yttria-stabilized zirconia matrix (Au-YSZ) have been synthesized with different gold loadings by reactive magnetron sputtering followed by ex situ annealing in air or laser interference patterning (LIP) treatment. It is shown that the electrical conductivity of the nanocomposite films can be modified to a large extent by changing the gold loading, by thermal annealing, or by LIP. The structural and microstructural analyses evidenced the segregation of metallic gold in crystalline form for all synthesis conditions and treatments applied. Thermal annealing above 400 °C is observed to trigger the growth of pre-existing nanoparticles in the volume of the films. Moreover, pronounced segregation of gold to the film surface is observed for Au/(Au + Zr + Y) ratios above 0.40, which may prevent the use of thermal annealing to functionalize gold-rich Au-YSZ coatings. In contrast, significant modifications of the microstructure were detected within the interference spot (spot size close to 2 × 2 mm) of LIP treatments only for the regions corresponding to constructive interference. As a consequence, besides its already demonstrated ability to modify the friction behavior of Au-YSZ films, the LIP treatment enables local tailoring of their electrical resistivity. The combination of these characteristics can be of great interest for sliding electrical contacts.