Characterization of InGaN/GaN-Based Multi-Quantum Well Distributed Feedback Lasers
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comparable threshold gain, single mode behavior with a higher side mode suppression ratio, and a modehop-free tuning over a temperature range of more than 30 K. FABRICATION The fabrication of these devices relied on growing a 4 lim thick n-type GaN:Si layer on Cface sapphire. On top of this layer, we grew a 500 nm thick, n-doped Alo0 osGao.9,N:Si lower cladding layer, a 100 nm thick n-doped GaN:Si lower waveguiding layer, a 30 nm thick un-doped active region with five 3 nm thick In0.15Gao. 85N quantum wells and 7 nm thick GaN barriers, and a 180 nm thick p-doped GaN:Mg upper waveguiding layer. For the electrically injected, indexcoupled device, the 3rd order grating with a period of 240 nm was defined by a holographic exposure and dry-etched into the upper waveguiding layer by chemically-assisted ion beam etching. A numerical calculation of the coupling coefficient showed that the tooth shape of this 100 nm deep grating was a critical parameter. For our rectangular tooth geometry with rounded tops, the coupling coefficient was relatively weak, on the order of 5 - 10 cm 1 . Together with a cavity length of 1000 gim, this coupling strength corresponds to an effective reflectance of 15 30 %. After grating fabrication, we performed optical pumping experiments in order to confirm the matching between the grating resonance wavelength and the gain peak. Then, we proceeded with an epitaxial re-growth to complete the device structure. This re-growth consisted of a 300 nm thick p-doped Al1 o8Gao. 92N:Mg upper cladding layer and a 100 nm thick p-type GaN:Mg contact layer. Device processing for the electrically pumped devices involved the definition mesas to enable lateral n-type contacting of the devices, and evaporation of standard n- and p-metal Ti/Au layers. The p-metal contact stripes were 4, 10 or 20 gtm wide, and thus defined the width of the gain region. A SiON layer was used to electrically isolate the non-contacted areas and the sidewalls of the mesas, thereby restricting 30 gain to within a narrow stripe in the lateral DFB laser direction. Following the SiON window etch, 10 X1 2 we evaporated another Ti/Au layer in order E
20
to provide the p-metal contact pads for the
To= 100 K
R 15
probe. Finally, the mirrors were etched,
13oC 23 T
S10
again by chemically-assisted ion beam etching. In order to ensure that these lasers would not accidentally oscillate as FabryPerot lasers, we dry-etched only one of the facets exactly vertical while leaving the
33 'C
o0
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5'""
0 0
•other 500
1000
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one at an angle of approximately 20 2500
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Injecton current [mA] / GaN Fig. 1 - L-I-characteristicsof an IGN DFB laser with an active area of 10 x 1DB laser 2 itha attour ivearent ofha t0 1000 1m 2 at four different heatsink
temperatures.
to the vertical.
The fabrication of optically pumped complex-coupled DFB lasers relied on interrupting growth after the QW active region, and etching the grating with a depth
of 45 nm through the QWs. Re-growth of 100 nm GaN and a 50 nm thick A10.08Gao. 92N:Mg laye
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