The antimonide based vertical external cavity surface emitting lasers (VECSELs) operating in the 1.8 to 2.8 Tm wavelength range are typically based on InGaAsSb/AlGaAsSb quantum wells on AlAsSb/GaSb distributed Bragg reflectors (DBRs) grown lattice-matched on GaSb substrates. The ability to grow such antimonide VECSEL structures on GaAs substrates can take advantage of the superior AlAs based etch-stop layers and mature DBR technology based on GaAs substrates. The growth of such III-Sb VECSELs on GaAs substrates is non-trivial due to the 7.78% lattice mismatch between the antimonide based active region and the GaAs/AlGaAs DBR. The challenge is therefore to reduce the threading dislocation density in the active region without a very thick metamorphic buffer and this is achieved by inducing 90 ° interfacial mist dislocation arrays between the GaSb and GaAs layers. In this presentation we make use of cross section transmission electron microscopy to analyze a variety of approaches to designing and growing III-Sb VECSELs on GaAs substrates to achieve a low threading dislocation density. We shall demonstrate the failure mechanisms in such growths and we analyze the extent to which the threading dislocations are able to permeate a thick active region. Finally, we present growth strategies and supporting results showing low-defect density III-Sb VECSEL active regions on GaAs.
Lasers with emission wavelength around 2 _m have been traditionally based on InGaSb quantum wells grown on
GaSb. An alternative is to use self assembled InAs Quantum Dashes grown on InP by the Stranski-Krastanov
growth mode. More speci_cally, InAs quantum dashes embedded in strained GaInAs quantum wells, grown in
InAlGaAs waveguides lattice matched to InP substrates have been successfully used as active medium in edge
emitting lasers with wavelengths in the range from 1.45 _m to 2.1 _m. Advantages of this material system compared
to the GaSb based system include easier lattice matching; i.e. only one group V element is involved. Many
optoelectronic properties of the InAs/InP quantum dash material system are similar to those of InAs quantum
dots grown on GaAs substrates. The latter material system has been very successfully used for VECSELs in the
wavelength region around 1 _m, leading to the highest power VECSEL at this wavelength, mode locking, wide
range tunability as well as intra cavity SHG to generate red light. A challenge in the material system based on
InP substrates is to fabricate a DBR. A lattice-matched DBR can consist of GaAsSb/AlAsSb. Alternatively one
can grow a metamorphic DBR based on either GaAs/AlAs or GaSb/AlSb. The latter requires the DBR to be
grown after the active region. The resultant VECSEL is then a bottom emitter, where the substrate has to be
removed. This can be achieved by introducing an etch stop layer between substrate and active region. Lastly,
the DBR can be grown separately and subsequently wafer bonded to the active region. This paper will discuss
details of these technologies and present results.
We investigate experimentally and theoretically the influence of non-radiative carrier losses on the performance of
VECSELs under pulsed and CW pumping conditions. These losses are detrimental to the VECSEL performance
not only because they reduce the pump-power to output-power conversion efficiency and lead to increased
thresholds, but also because they are strong sources of heat. This heating reduces the achievable output power
and eventually leads to shut-off due to thermal roll-over. We investigate the two main sources of non-radiative
losses, defect recombination and Auger losses in InGaAs-based VECSELs for the 1010nm-1040nm range as well
as for InGaSb-based devices for operation around 2μm. While defect related losses are found to be rather
insignificant in InGaAs-based devices, they can be severe enough to prevent CW operation for the InGaSb-based
structures. Auger losses are shown to be very significant for both wavelengths regimes and it is discussed how
structural modifications can suppress them. For pulsed operation record output powers are demonstrated and
the influence of the pulse duration and shape is studied.
We demonstrate a novel epitaxial process for the growth of low-dislocation density GaSb on GaAs. The
growth mode involves the formation of large arrays of periodic 90° misfit dislocations at the interface
between the two binary alloys which results in a completely strain relieved III-Sb epi-layer without the
need for thick buffer layers. This epitaxial process is used for the growth of antimonide active regions
directly on GaAs/AlGaAs distributed Bragg Reflectors (DBRs) resulting in 2 μm VECSELs on GaAs
substrates.
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