technology green lasers
Once this is done, engineers must then deposit a lattice-mismatched, relaxed top cladding layer, which will lead to the formation of another array of misfit dislocations, this time on the lower interface of the upper cladding. With this approach, dislocations are only located in the interfaces between waveguiding and cladding layers, where they are too far away from the active region to contribute to non-radiative recombination losses. It is worth noting that the possibility of instantly forming a misfit dislocation array at the interface and growing a relaxed and low-defect- density layer right over it is quite unique for semipolar III-N heterostructures.
There have also been reports of strain relaxation management in III-V zinc-blende structures, such as near-infrared lasers with In(Ga)As quantum wells, quantum dots grown on GaAs substrates, and multi- junction solar cells. However, in order to avoid defects in active regions, those realizations require the growing of bulk transition layers or superlattices, instead of using single interface, like in semipolar III-Ns.
Alternatively, misfit dislocations can be completely avoided with strain-compensating layers. The trick is to balance compressive strain in the wells and the InGaN waveguiding layers with tensile strain in Al(In)GaN barriers. Apparently, when the indium concentration in InGaN layers of the waveguide core is sufficiently high, strain compensation is possible without degrading optical confinement.
This is thanks to one beneficial characteristic of InGaN: Its refractive index increases superlinearly with indium concentration. In other words, the refractive index increases much faster when the separation between the lasing photon energy and the InGaN material bandgap is smaller. Intuitively, simple insertion of the Al(In)GaN layers inside the InGaN waveguide core would reduce its average refractive index. However, due to strain compensation it in fact enables the use more indium in the waveguide, which overcompensates the refractive index reduction, so it is still higher than for the design without strain compensation.
Cranking up the current
The second major challenge facing the developers of green lasers is to design and build a device capable of operating at the very high current densities required for light amplification. In InGaN quantum wells, carrier density is relatively high, due to the high effective mass of both types of carrier – masses of holes and electrons in InGaN are more than 1.4me compared to values of just 0.51me electrons and holes in GaAs (me
and 0.2me and 0.063me
, respectively, for
is the mass of a free
electron). The higher electron and hole effective masses lead to a higher density of electron and hole states. In turn, the higher density of electron and hole states in the InGaN quantum well leads to a higher transparency carrier density, a pre-requisite for lasing, that is more than twice that required for laser diodes based on InGaAs quantum wells.
This hike in transparency carrier density has unwanted ramifications. Recombination current is a super-linear function of carrier concentration – especially at high current densities where non-radiative Auger recombination dominates – so the current density needed to reach transparency in an InGaN quantum well is several times higher than it is in one made from InGaAs (see Figure 3).
Making matters worse, the increases in indium content that push the emission of InGaN quantum wells towards the green also leads to rapid reductions in differential gain. This is not just the result of a hike in non-radiative recombination – there is also significant inhomogeneous line broadening resulting from InGaN alloy fluctuations when the molar concentration of indium in the ternary is around 30 percent, the fraction required for green emission.
Figure 3.(a) The charge carrier density needed to reach transparency is much higher for InGaN QWs – especially c-plane ones – than III/As QWs.(b) As a result,InGaN QWs need much higher pumping for light
amplification.Despite this,the differential gain is much lower in green InGaN QWs.(This figure drew on data from P 1144 (1991))
.Blood et
al.J.Appl.Phys.70 16
www.compoundsemiconductor.net June 2012
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