Lead-free silver nanoparticle pastes have been tested as a replacement for high temperature lead-rich solders used in electronic manufacturing. The pastes contain a small amount of solvent, and primarily consist of submicronsilver powder and passivated silver nanoparticles. The nanoparticles were synthesized from Ag 2 CO 3 and a long-chain alcohol by a method that produced a passivating layer consisting almost exclusively of the carboxylate of the reactant alcohol. The pastes were used to connect a silicon diode chip to copper bases without applied pressure when sintered at 350°C under nitrogen. Diode packages made with sintered silver interconnects had electrical and thermal properties equal to those with lead-soldered interconnects, even after 3000 thermal cycles between À55°C and +150°C. The mechanical strength was half that of lead-rich solder joints, but still strong enough for practical use.
Recently, there has been much interest in the contribution of exciton transitions to the absorption coefficient, especially in 11-VI semiconductors (1). Theories on the line-shape in exciton transitions have been developed by Toyozawa (2, 3). In the case of an e'xciton band where k = 0 is the bottom of the band and where there are no levels in other bands which have the same energy, he pointed out that multi-phonon processes were very important and the line-width is proportional to the square of the absolute temperature (3). In this letter we report that the lowest exciton band of CuCl shows the T-dependence mentioned above. 2Exciton spectra have been obtained from precise transmittance measurements on thin films, and also from Kramers-Kronig analysis of reflection measurements on single crystals. CuCl films which had zincblende structure were prepared by evaporating zone-refined polycrystals onto a fused quartz substrate heated at 150 C.In view of the extremely high absorption coefficient at low temperatures and its large temperature dependence, we prepared several films having different thickness between 250 to 1800 1. The thickness was determined by an interference spectrum obtained from reflection measurements (4). Single crystals were grown by the Bridgeman method as shown in the previous report (5). Spectral measurements were carried out from 360 stabilized metal Dewar, which allowed to obtain a whole spectrogram within temperature variations of 1 o r 2 K. The spectral band width of analysing instruments, consisting of a double monochromator, a photomultiplier, and an amplifier-recorder, was less than 1 1. Absorption spectra were corrected for reflectance and interference effects.Typical absorption and reflection spectra are given in Fig. 1 and 2. The exciton bands are well separated from the band-to-band absorption edge, and this makes it easy to analyze the shape and width of the exciton band. Of the two exciton peaks,
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