The interrelation between the antidiffusion properties of titanium diboride films and their nanocrystalline structure is investigated. We made a valid assumption that the main reason for degradation of contacts with TiB2-based diffusion layers is diffusion through the TiB2 film through dislocations (formed due to stresses that appear in the course of ohmic contact formation) rather than chemical interaction. In that case, increase of mechanical strength of the TiB2 film at reduction of grain size will affect its diffusion strength more strongly than growth of diffusion penetrability owing to increase of grain boundary role. Our investigations showed that, to ensure maximal mechanical strength and heat stability, the size of nanocrystallites in films forming diffusion barriers has to lie within 3—15 nm. The TiB2 films with optimal nanocrystallite parameters can be obtained using magnetron sputtering with discharge current of 0,4 A and oxygen content in a target up to 8 at.%. Application of TiB2-based nanocrystal films as antidiffusion layers in contacts to wide-gap semiconductors makes it possible to raise heat stability of devices based on such semiconductors.
The emission uniformity of LED chips in the entire range of brightness and colors is the problem in LED displays manufacture process. It was approved that at lowering brightness gradations appearing the radiation nonuniformity between LED chips, and the higher disorders will be seen on the lesser emission levels. The RGB LED chips, observed by us, were based on AlGaInP (red), In0.3Ga0.7N/GaN (green), and In0.2Ga0.8N/GaN (blue) and had nominal working current 20 mA. Analysis of the current-voltage characteristics and capacitance-voltage characteristics showed the presence of inhomogeneous semiconductor junctions and ohmic contacts in blue LEDs structures that are the source of possible irregularities in the final emission of LED displays. The variation of voltages (at current 10 mA) was 2.81—2.98 V for blue structures, and 1.9—2.0 V for red ones. Some of the blue structures had additional parasite current-flow mechanism at low current. Rise time and fall time of electroluminescence pulse of blue structures were measured. The shortest LED pulse time in LED displays is limited by duration of 6—8 ns. The quality of LED displays may be improved by implementing the preliminary control of LED chips in a narrower range of voltage variation at operating currents, or more narrow interval of I—V variation. This additional LEDs sorting will of course reduce the percentage of the total amount of light-emitting diodes, but may improve the image (video, photo) quality at translations by using a lower brightness gradations.
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