Electrical and optical properties of Nichia double-heterostructure blue light-emitting diodes, with In 0.06 Ga 0.94 N:Zn, Si active layer, are investigated over a wide temperature range from 10 to 300 K. Current-voltage characteristics have complex character and suggest the involvement of various tunneling mechanisms. At small voltages ͑and currents͒, the peak wavelength of the optical emission shifts with the applied bias across a large spectral range from 539 nm ͑2.3 eV͒ up to 443 nm ͑2.8 eV͒. Light emission takes place even at the lowest temperatures, indicating that a complete carrier freeze-out does not occur.

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We report fabrication and characterization of semiconductor ring lasers with quantum-dot active region. The InAs/ InGaAs/ GaAs "dots-in-a-well" ridge-waveguide ring lasers are monolithically integrated with coupling waveguides and monitoring quantum-dot photodetectors. The lowest threshold current density for semiconductor ring lasers is demonstrated. When enhanced by a forward biased S-section waveguide, stable unidirectional operation with record suppression ratio of counterpropagating waves exceeding 30 dB is achieved.

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