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We report the demonstration of novel GaAs/AlGaAs integrated optical 1-to-N way beam splitters which use symmetric mode mixing in center-fed multimode planar waveguides. Each device has one single-mode input guide, a carefully chosen length of parallel sided multimode guide, and N equally spaced single-mode output guides. The mixing of symmetric modes shares the input light equally between the output guides by a symmetric form of the self-imaging process. We demonstrate experimentally that this type of beam splitter can be used to divide power equally, with high accuracy and low loss, between the N output guides, for values of N between 2 and 20.

Details of a method for the characterization of deep levels with large capture cross sections for minority carriers are presented. This technique has been used to investigate centers in gallium phosphide. Two defects at EV+0.75 eV and EV+0.95 eV are described in detail. Evidence is presented that shows that the shallower of these defects can control the minority-carrier lifetime in n-type gallium phosphide and in fact is the dominant recombination center in most epitaxial layers of this material. The technique uses capacitance as a measure of the charge state of the deep levels in the depletion region of a Schottky barrier. This charge state is perturbed by the capture and subsequent thermal emission of minority carriers. The carriers are generated by irradiation of the semiconductor with low-intensity light at a wavelength near the absorption edge. Minority carriers generated in the neutral material within about a diffusion length of the barrier region are extracted by the depletion field. Majority carriers are excluded by the field and consequently the current through the barrier is due predominantly to minority carriers. These are captured by the defects, the fastest capture being into the levels with the largest capture cross sections. As a result, the technique can in many cases be used selectively to detect the most important recombination centers in a semiconductor and to determine their capture cross sections, concentrations, and energy depths.

The present understanding of the operation of green-emitting GaP LED'S is reviewed. All existing visible LED devices which are made in III-V compound semiconductors are inefficient, and it is known that this is due to the low visible luminescence efficiency of the epitaxial material incorporated in the devices. In green-luminescent GaP this inefficiency is becoming understood, and quantitative analyses of the important mechanisms are described. The radiative processes include free-exciton and boundexciton recombination which are important in p-type and n-type material with and without nitrogen doping, but in all materials the recombination is dominated by nonradiative processes which have proved to be elusive and difficult to eliminate. Particular emphasis is therefore placed on recent advances in the quantitative classification of these dominant non-radiative processes in n-type material. These are : (1) recombination at deep defect levels positioned 0.75 eV from the valence band, (2) diffusion-limited recombination at dislocations, and (3) surface and interface recombination. D R WightTable 1. LED performance comparisons GaAszP1-s GaP Ideal Red Yellow Red Yellow Green Green Peak h (nm) 650 590 695 575 565 556 Visual efficiency (lm W-1) 50 450 15 500 600 620