Light emitting diode (LED) excitation of thermographic phosphors for temperature measurements was investigated in the present work. A near-UV and a green LED excited the phosphorescence of sol–gel deposited chromium-doped alumina (Cr3+:Al2O3, ruby) on silicon in different experiments. The influence of the pulse length on signal intensity was measured and is discussed theoretically. From this, measurement strategies are recommended in order to obtain high signal levels. The temperature-dependent phosphorescence between room temperature and 800 K is compared to previously obtained laser excited phosphorescence. The phosphorescence decay time of the ruby films is found to be slightly dependent on the excitation pulse duration but neither on the excitation wavelength nor on the oxygen partial pressure in the atmosphere. In many situations the less expensive LED excitation appears to be a good alternative to the laser excitation methods. In addition, a strategy is proposed for measuring two-dimensional surface temperatures using two gated cameras; the validity was proven for pointwise measurements.
The microscopic behavior of iron in relaxed Si 1Ϫx Ge x alloy is addressed in the present work where various new aspects are highlighted. In p-type materials two types of defects involving iron may coexist under equilibrium; the isolated form, Fe i , and the iron-acceptor pair, Fe i -A s . The latter complex is favored over the former because it is thermodynamically more stable. In each case the iron atom stabilizes at the interstitial tetrahedral site. When boron is the acceptor impurity, both the isolated and the paired forms introduce donorlike levels, distant from each other by 0.28 eV. In the relaxed Si 1Ϫx Ge x bulk alloy, these levels are shown to remain separated by the same amount. However, they shift toward the valence band much faster than the shrinkage of the band gap when the Ge content is increased. The consequence is that the pair-related donor level merges with the valence band at a fairly low alloy composition (xу7%) while the iron donor level is predicted to disappear from the gap for xу25%. We also show that neither the entropy nor the enthalpy of migration of free iron, whose experimental determination requires one to take into account the abovementioned shift, are affected by alloying. Therefore, the fast diffusing character, attributed to iron in silicon, still holds in the alloy. The origin of spectral broadening, related to the chemical disorder, is discussed. Finally, the major technological implication emerging from our new findings is addressed. In particular, we show that both the gettering by segregation, routinely used in silicon, and the field-induced outdiffusion, established in n-type silicon ten years ago, are totally inefficient in the Si 1Ϫx Ge x alloy.
One promising approach for the measurement of surface temperatures is the use of thermographic phosphor coatings. Here the deposition of chromium doped (0–7%) aluminum oxide films (ruby) with respect to this application is investigated. The optical properties of the grown films are presented in a further study. The sol–gel technique is used for film deposition. The method is rather simple and omits an expensive setup, so cheap coatings are easily prepared. Aluminum tri-isopropoxide (ATI) is used as an aluminum source and chromium acetylacetonate (Cr(acac)3) is used for chromium doping. The deposition process is analyzed by thermogravimetric measurements (TG/DTA). The film morphologies are characterized by scanning electron microscopy (SEM), the chemical compositions are analyzed by energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) is used for the phase analysis. It is shown that amorphous aluminum oxide is prepared during sol–gel deposition at 563 K. It becomes well ordered (α-Al2O3:Cr3+) after annealing at 1403 K in an O2-atmosphere. The chromium content is homogeneous and identical to the stoichiometry of the aluminum and chromium sources. By using a multicycle deposition procedure well adhering films of around 3 µm thickness are grown.
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