Surface emitting optical devices with a vertical cavity have been investigated for applications in optical interconnections. To integrate these devices into a two-dimensional array, it is necessary to improve the conversion efficiency from electrical power to optical power. To meet this requirement, the series resistance of the distributed Bragg reflectors that form the vertical cavity must be reduced. This article demonstrates the reduction in the series resistance of the distributed Bragg reflector by introducing quasi-graded superlattices at the heterointerfaces. By using this structure, we obtain a low series resistance distributed Bragg reflector without compromising the high reflectivity. The mechanism of the reduction in the series resistance is studied and it is found that an increase in tunneling current leads to a decrease in the resistance. The dependence of tunneling current on doping concentration of the distributed Bragg reflector and the superlattice structure is also discussed.
Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, x Bi , lower than 0.2 at.%, an excess deep-donor level (activation energy E d Ϸ 0.03-0.04 eV) appears, and Hall mobility is low. In contrast, for x Bi Ͼ 0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 10 17 cm Ϫ3 . Inductive coupled plasma (ICP) emission analysis shows that, for x Bi ϭ 1 at.%, Bi concentration in the epitaxial layer is as high as N Bi ϭ 2.3-2.7 ϫ 10 19 cm Ϫ3 . These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bidoped layers shows a minimum at a Te vapor pressure of 2.2 ϫ 10 Ϫ5 torr for growth temperature 470ºC, which is coincident with that of the undoped PbTe.
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