Current-injection InGaAlN heterostructure laser diodes grown by metalorganic chemical vapor deposition on sapphire substrates are demonstrated with mirrors fabricated by chemically assisted ion beam etching. Due to the independent control of physical and chemical etching, smooth vertical sidewalls with a root-mean-squared roughness of 4–6 nm have been achieved. The diodes lased under pulsed current-injection conditions at wavelengths in the range from 419 to 423 nm. The lowest threshold current density was 25 kA/cm2. Lasing was observed in both gain-guided and ridge-waveguide test diodes, with cavity lengths from 300 to 1000 μm; and output powers of 10–20 mW were achieved. Laser performance is illustrated with light output-current and current–voltage characteristics and with a high-resolution optical spectrum.
We demonstrate an optically pumped InGaN/GaN-based multiquantum well distributed feedback laser in the blue spectral region. The third-order grating providing feedback was defined holographically and dry etched into the upper waveguiding layer by chemically assisted ion-beam etching. When aligning the stripe-shaped pump beam either parallel or perpendicular to the grating grooves, we found a considerably lower pumping threshold, higher slope efficiency, a slightly longer emission wavelength, and a much narrower linewidth for the geometry with the pump beam orthogonal to the grating lines. A nearly constant emission wavelength of 400.85 nm and a linewidth of 0.7 Å were observed under various pump intensities. To the best of our knowledge, this is the narrowest linewidth ever reported for an optically pumped device in this material system.The fabrication of an electrically pumped laser in the blue spectral region has attracted a lot of attention in the past couple of years. Both pulsed and continuous-wave operation of InGaN/GaN-based devices have been demonstrated at room temperature. 1,2 One of the main concerns in nitride lasers is the fabrication of high-quality mirrors. Up to now, most research groups use sapphire substrates for GaN growth; however, the misorientation between the sapphire and the GaN cleavage planes does not readily permit cleaving of the facets. Polishing of the laser mirrors also does not entirely satisfy because it is a very time-consuming process. Although dry-etched mirrors with high reflective coatings seem to work well in this material system, there is still a certain demand to improve the cavity properties of nitride lasers as far as mirror loss and especially mode selection are concerned. One of the possible solutions of this problem is the implementation of distributed feedback ͑DFB͒, which eliminates the need for excellent cavity mirrors. A relatively straight forward way to explore this idea is the fabrication of an optically pumped device. According to this, Hofmann et al. 3 reported an optically pumped DFB laser in the InGaN/ GaN material system. However, their device was a simple double heterostructure, employed a second-order diffraction grating for surface emission, and the grating was generated using e-beam direct writing. In this letter, we demonstrate the fabrication of a nitride-based multiquantum well ͑MQW͒ DFB laser with a holographically defined third-order grating, which results in edge emission. When compared to the Fabry-Perot-type emission observed from the same material, a reduced threshold pump intensity, a higher slope efficiency, and a much narrower linewidth were seen.The fabrication of these devices relied on growing a 4 m thick GaN layer on C-face sapphire. On top of this layer, we grew a 500 nm thick Al 0.08 Ga 0.92 N lower cladding layer, a 100 nm thick GaN lower waveguiding layer, a 40 nm thick active region with five In 0.15 Ga 0.85 N quantum wells ͑QWs͒ and GaN barriers, and a 180 nm thick GaN upper waveguiding layer. The third-order grating with a perio...
We demonstrate room-temperature pulsed currentinjected operation of InGaAlN heterostructure laser diodes with mirrors fabricated by chemically assisted ion beam etching. The multiple-quantum-well devices were grown by organometallic vapor phase epitaxy on c-face sapphire substrates. The emission wavelengths of the gain-guided laser diodes were in the range from 419 to 432 nm. The lowest threshold current density obtained was 20 kA/cm 2 with maximum output powers of 50 mW. Longitudinal Fabry-Perot modes are clearly resolved in the highresolution optical spectrum of the lasers, with a spacing consistent with the cavity length. Cavity length studies on a set of samples indicate that the distributed losses in the structure are on the order of 30-40 cm 01 .
The threshold current density of narrow-stripe gain-guided nitride laser diodes increases very rapidly as the stripe width is made narrow. To examine this behavior, waveguide simulations, incorporating the complex refractive indices associated with optical gain, have been used to analyze the lateral optical modes of gain-guided laser diodes. Threshold current was then determined from the gain–current relationship of our laser material, which was obtained experimentally. These evaluations reveal that gain guiding, coupled with a carrier-induced index depression, offer a reasonable explanation for the rapid increase in threshold when the stripe width becomes less than 5 μm.
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