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Quinternary barrier propels GaSb lasers to longer wavelengths


Improving hole confinement in the active region stretches GaSb-based laser emission to 3.7 microns


THE record for long wavelength emission for room-temperature GaSb-based type I lasers has been broken by a pair of researchers from the Technical University of Munich.


Extending the emission of this form of GaSb-based laser from its previous record of 3.44 µm to the new benchmark of 3.73 µm makes this device a more promising candidate for various applications. That’s because the spectral range around 3.6 µm is well suited to gas sensing, free-space


communications and spectroscopy, thanks to its location within the water-absorption-free atmospheric window spanning 3.4-5.0 µm.


Emission spectra of 3.6 – 3.7 µm Fabry-Perot lasers at 15 °C under pulsed operation


“It enables one to monitor greenhouse gases in the atmosphere without water interference,” explains corresponding author Kristijonas Vizbaras, who points out that methane and ozone can be monitored using 3.6 µm lasers. “Moreover, wavelengths around 3.6 µm can access the first absorption lines of sulphur dioxide, which is a very important pollutant gas and is highly interesting for various combustion control processes.”


According to Vizbaras, the key to extending the emission of these GaSb- based lasers beyond 3.5 µm is the improvement of the epitaxial growth technology for the quinternary AlGaInAsSb barrier, so that this enables good hole confinement in the quantum well. Optimising this layer requires tremendous effort because characterisation of the quinternary is complex and time- consuming.


Better hole confinement is not the only benefit stemming from the team’s quinternary barrier – it also helps to increase the laser’s emission wavelength. During the growth of the quantum well these layers are subjected to an in-situ anneal that increases their bandgap, leading to a reduction in the laser’s


emission wavelength. Vizbaras and his colleague Markus-Christian Amann have found that this unwanted blue shift is smaller in lasers incorporating their AlGaInAsSb barrier.


The spectral range around 3.6 µm can also be accessed with quantum cascade lasers. However, Vizbaras argues that this type of laser is inferior to ones based on GaSb in several respects: Its operating voltage and current are higher, making it far less suited to portable applications requiring batteries; room-temperature operation is far more challenging; and device growth is more complex and time consuming.


Vizbaras and Amann fabricated laser diode epistructures on GaSb substrates by MBE using a growth temperature of 550 °C. Epiwafers featuring five compressively strained, 10 nm-thick Ga0.3


In0.7 As0.48 Sb0.52


quantum wells were processed into ridge waveguide lasers with cavity lengths of 1-3 mm and 30 µm ridges. These devices with uncleaved facets were mounted epi- side up on copper heatsinks.


Driven with 500 ns pulses at a 8.3 kHz repetition rate, 2 mm-long, 3.58 µm lasers


50 www.compoundsemiconductor.net October 2011


produced an average output power of 29 mW per facet. Lasing was possible up to a heatsink temperature of 27 °C, limited by a loss-mechanism of unknown origin kicking in at 17 °C.


After reporting these results in a very recent edition of Electronics Letters, the researchers have continued to extend the wavelength of their GaSb-based lasers. “We have just demonstrated room- temperature emission at 3.73 µm with 20 percent lower threshold current densities than for the published 3.6 µm lasers,” says Vizbaras. “The most recent results are being prepared for publication at the moment.”


Further improvements in the performance of these lasers are possible, according to Vizbaras: “The threshold current densities are reasonably small for a proto-type laser, and further improvement should definitely lead to continuous-wave (CW) operation.”


His foundation for making this claim is the progress of 3.3 µm lasers, which had pulsed threshold current densities of 5 kA/cm2


when first fabricated in 2005 and now operate in CW mode at room temperature. “Similar progress is expected for 3.6 µm lasers.”


Vizbaras points out that a hike in the differential gain of their laser should result from increasing the strain in the active region, and lower operating voltages could be realised through contact optimisation.


“Additionally, more elaborate mounting and heat-sinking should boost performance much further and eventually enable CW, room-temperature operation.”


The team will now try to improve the performance of their lasers and extend their emission to 4 µm and beyond.


K. Vizbaras et al. Electron. Lett. 47 980 (2011)


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