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Intermetallic TiAl-alloys can replace the heavier Ni-based superalloys in several high temperature applications with regards to their mechanical properties, however they can not be used at temperatures above 800°C in oxidizing environments for longer times because of insufficient oxidation resistance. Despite an Al-content of about 45 at.% in technical alloys, no protective alumina layer is formed because the thermodynamic stabilities of titanium oxide and aluminum oxide are of the same order of magnitude. Therefore a mixed TiO2/Al2O3-scale is formed which is fast growing so that the metal consumption rate is quite high. On the other hand the formation of a slow growing alumina layer is promoted by a fluorine treatment. This so called fluorine effect leads to the preferential intermediate formation of gaseous aluminum fluorides at elevated temperatures if the fluorine content at the surface stays within a defined concentration range. These fluorides are converted into solid Al2O3 due to the high oxygen partial pressure of the high temperature service environment forming a protective pure Al2O3 surface scale. In this paper results of high temperature oxidations tests of several technical TiAl-alloys will be presented. Different F-treatments e.g. dipping or spaying which are easy to apply have been used and their results will be compared. The mass change data of the F-treated specimens are always lower than those of the untreated ones. Post experimental investigations such as light microscopy, scanning electron microscopy and energy dispersive X-ray analysis reveal the formation of a thin alumina layer on the F-treated samples after optimization of the process while a thick mixed scale is found on the untreated samples. The results will be discussed in view of an optimized procedure and the future use of TiAl-components in high temperature environments.
The use of flash lamp annealing for processing semiconductor materials is outlined. Specific applications include ultra-shallow junction formation and heteroepitaxial growth of improved quality thin films of cubic silicon carbide. It is demonstrated that flash lamp annealing holds great promise as a technique for fabricating novel devices.
Photoluminescence (PL) and electroluminescence (EL) from Si+- and Ge+-implanted SiO2 layers thermally-grown on a Si substrate were studied. The PL spectra were recorded after annealing at temperatures in the range of 400° to 1200°C. Single-peak PL at 460 nm and doublepeak PL at 420 and 385 nm due to Si+-implanted and Ge+-implanted oxide layers, respectively, reached a maximum following heat treatment at 500°C for 30 min. The EL spectra from the Gerich oxides after annealing at 1000°C correlated very well with the PL signal, and showed a linear dependence on the injection current. The EL emission was strong enough to be readily seen with the naked eye, and the EL efficiency was estimated to be about 5×10−4. High-resolution transmission electron microscopy (HRTEM) analysis showed that the formation of nanocrystals in the implanted oxide layers occurred at annealing temperatures in excess of 800°C. The observed light emission is attributable to the presence of-≡Si-Si≡, ≡Si-Ge≡ and ≡Ge-Ge≡ centres in SiO2.
High-energy, self-ion implantation has been used to form deep gettering layers in Si. Subsequently samples have been contaminated with Cu and subjected to heat treatment. The residual defects act as gettering centres for Cu. The decoration of defects byCu making them detectable by secondary ion mass spectromety analysis. Metastable defect complexes have been detected which, because of their small size, are not directly detectable by other analytical techniques such as transmission electron microscopy and MeV-particle channeling. These defects are probably of interstitial type and have been found mainly midway between the sample and the projected ion range, i.e. around Rp/2. The gettering ability of these small defect complexes may largely exceed that of the post-anneal damage at the projected i.e range, Rp. The results obtained demonstrate that by means of metal gettering the formation, growth and dissolution of very small defect complexes in ion-implanted Si can be studied.
The formation of cavity microstructures in silicon following helium implantation (10 or 40 keV; 1×1015, l×1016 and 5×1016 cm−2) and annealing (800 °C) is investigated by means of Transmission Electron Microscopy (TEM), Rutherford Backscattering Spectrometry and Channeling (RBS/C), and Elastic Recoil Detection (ERD). The processes of cavity nucleation and growth are found to depend critically on the implanted He concentration. For a maximum peak He concentration of about 5×1020 cm−3 the resulting microstructure appears to contain large overpressurized bubbles whose formation cannot be accounted by the conventional gas-release model and bubble-coarsening mechanisms predicting empty cavities. The trapping of Fe and Cu at such cavity regions is studied by Secondary Ion Mass Spectrometry (SIMS).
Metal-Induced-Crystallization (MIC) by the contact of amorphous semiconductors with metals is one of the degradation factors in solar cells. This study has been made on the barrier properties of a ZnO layer between undoped a-SiGe:H and Al metallization films in the structure (001)Si/SiO2/a-SiGe:H/ZnO/Al. Plasma assisted CVD deposition was used to produce a-Si1.xGex:H (x=0 to 1) undoped films over thermally oxidized Si-wafers. There were covered with 500Å and 1000Å thick transparent conductive layers of ZnO. Al and then 1000Å thick films of Al. A set of Al-implanted a-Si, a-Ge, and a-Sio.5Geo.5 films on Si/SiO2 substrates was also prepared to study MIC in an amorphous system with dispersed Al. The structures were annealed in vacuum in the temperature range of 200°C to 400°C for lh. X-ray diffraction studies demonstrated the a-SiGe:H stability against crystallization under ZnO protection up to 400°C. Secondary Ion Mass Spectroscopy didn't reveal any noticeable redistribution of Al inside Al-implanted a-Si:H and a-Si0.5Ge0.5:H samples after annealing at 400°C for lh, but strong Al diffusion was seen in the a-Ge:H layer. Nevertheless, no MIC was observed in any of the Al-implanted a-materials.
Strong blue, red and near-infrared photoluminescence has been observed from Si+-implanted and pulse-annealed SiO2 layers. Raman scattering and high-resolution electron microscopy analyses have revealed a correlation between the structure of the Si inclusions in the SiO2 matrix and the photoluminescence. Structural transformations in the Si-rich SiO2 layers during pulse and furnace annealing have been discussed in terms of the changes in the light emission observed experimentally. Small Si clusters, non-crystalline inclusions and nanocrystals are believed to be the light sources. The blue, red and near-infrared photoluminescence is associated with small complexes of excess Si atoms, non-crystalline Si nanoinclusions and quantum-confined Si nanocrystals, respectively.
Solid solutions of SiC and III-V compound semiconductors are recognized as promising materials for novel semiconductor applications. This paper reports on experiments which explore the possibility of synthesizing thin buried layers of (SiC)l-x(AIN)x having composition of about x = 0.2 by co-implanting N+ and Al+ ions into 6H-SiC wafers maintained at temperatures in the range 200 - 800°C. Structural and compositional evaluation of as-implanted samples was carried out using a combination of Rutherford backscattering/channelling spectrometry and infrared reflectance spectroscopy. It is shown that the structures are highly sensitive to the substrate temperature. The use of sufficiently high temperatures (400 - 800°C) enables the crystallinity of the host material as well as relatively low damage levels to be maintained during implantation. The formation of AI-N bonds within the implanted layers is also confirmed over the temperature range studied.
Single crystal (100) silicon substrates were implanted at 300 keV with substoichiometric oxygen doses ranging from 1 × 1016 to 1 × 1017 cm-2. Samples were annealed for 2 hours over the temperature range from 1100°C to 1250°C and were subsequently analysed by both cross sectional transmission electron microscopy (XTEM) and scanning electron microscopy (SEM). The nucleation and growth of oxide precipitates within the implanted layer was followed during annealing. The emphasis was placed upon studying the process of Ostwald ripening which is known to play an important role in the formation of the incipient buried layer. Besides, a clear trend of the SiO2 precipitates to arrange in well defined regions was revealed and this was attributed, as distinct from the earlier claims, to an inherent process of self organisation.
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