Structural and optical properties of epitaxially overgrown third-order gratings for InGaN/GaN-based distributed feedback lasers

Abstract

Laser-diode heterostructures of InGaAlN containing a third-order diffraction grating for distributed optical feedback have been examined with transmission electron microscopy ͑TEM͒ and scanning electron microscopy ͑SEM͒. The grating was defined holographically and etched by chemically assisted ion-beam etching into the upper GaN confinement layer of the laser structure. After the etch step, it was overgrown with an Al 0.08 Ga 0.92 N upper cladding layer. Threading dislocations were present that initiated at the sapphire substrate, but no new dislocations were observed at the grating/Al 0.08 Ga 0.92 N interface. A comparison of TEM and SEM micrographs reveals that there is a compositional gradient in the AlGaN upper cladding layer; however, calculations show that it did not reduce the optical coupling coefficient of the grating.Within the last couple of years, research on semiconductor lasers with emission wavelengths around 400 nm has received a great deal of attention. Recent milestones in the development of nitride-based light emitters have been the demonstration of high-brightness blue/green light emitting diodes and both pulsed and continuous-wave laser operation at room temperature. [1][2][3][4][5] Mainly for the purpose of overcoming difficulties in high-quality mirror fabrication, but also in order to improve mode selection and wavelength stability, distributed feedback ͑DFB͒ blue lasers have been demonstrated recently.6 Etching and regrowth of the grating, which typically completes the device structure, is a major obstacle for the fabrication of DFB lasers in material systems other than the nitrides. The strong chemical bonds between gallium and nitrogen result in a very stable and chemically inert surface, compared to GaAs materials for example, where oxides are readily formed on the surface.In this letter, we present a structural evaluation of a third-order diffraction grating from InGaN/GaN-based DFB lasers. The grating was defined by holography and etched into the GaN upper confinement layer by chemically assisted ion-beam etching ͑CAIBE͒. After etching, it was overgrown with an Al 0.08 Ga 0.92 N upper cladding layer and a GaN contact layer. We compare scanning electron microscopy ͑SEM͒ and transmission electron microscopy ͑TEM͒ cross sections of the grating before and after overgrowth. Based on these observations, we estimate how an effective alteration of the grating profile due to a compositional gradient might affect the grating's coupling coefficient and thus the threshold gain of the laser.The fabrication of these devices relied on growing a 4 m thick n-type GaN:Si layer on C-face sapphire. On top of this layer, we grew a 500 nm thick, n-type Al 0.08 Ga 0.92 N:Si lower cladding layer, a 100 nm thick n-type GaN:Si lower waveguiding layer, an active region with five 3.5 nm thick In 0.1 Ga 0.9 N quantum wells and 9.0 nm thick GaN barriers, and a 180 nm thick p-type GaN:Mg upper waveguiding layer. More details about growth conditions and doping levels can be found in Ref. 5. The thicknesses of the ...

Realization of a complex-coupled InGaN/GaN-based optically pumped multiple-quantum-well distributed-feedback laser

“…However, in combination with other images taken with g parallel to the interface, no structural defects were found at the grating interface. This is similar to our earlier observations of index-coupled DFB lasers, although the grating in that case consisted of an AlGaN layer over a GaN grating, 8 instead of an AlGaN layer over an InGaN grating as shown here. We note, however, that the consequences of a threading dislocation may be more detrimental in the current structure, because we are effectively reducing the amount of active material required for lasing.…”

Narrow-line InGaN/GaN green laser diode with high-order distributed-feedback surface grating

“…Different types of DFB gratings have been achieved including buried, [19][20][21] surface, [22][23][24][25] and ridge sidewall gratings. [26][27][28][29] The surface and sidewall gratings are the most suitable for green LDs since buried gratings require overgrowth, 30) which tends to compromise the material quality, particularly critical for InGaN-based green laser structures. 3) Related to fabrication, low-order gratings (first, second, etc.)…”

Inversion of wurtzite GaN(0001) by exposure to magnesium

“…The IDs initiate in the layer above the regrowth interface and are approximately 30 nm in diameter. Similar regrowth is used for fabricating distributed feedback lasers [18] but the overgrown layer in that case is AlGaN. No IDs are found in those AlGaN layers, suggesting possible differences in the probability of IDB formation between AlGaN and GaN.…”