technology green lasers
Part of the reason behind the high current densities required for lasing in long-wavelength nitride lasers is that the wurtzite III-N crystal has a polar nature. The polarization fields that result from this particular structure reduce electron-hole overlap in the well, a phenomena known as the quantum confined Stark effect (QCSE). To overcome optical losses, current densities in blue and green laser diodes need to hit 2 kA cm-2
and over 4 kA cm-2
, respectively. These values
are an order of magnitude higher than those for lasers based on InGaAs quantum wells.
The low optical gain found in green lasers can be partially addressed with semi-polar substrates: Differential optical gain doubles when a green InGaN quantum well is grown on a semi-polar orientation, rather than the conventional polar one. This benefit of semi-polar orientation is due to a reduction in the QCSE and anisotropic optical gain by the breaking of the 90-degree rotational symmetry of the wurtzite crystal semi-polar plane – a property not associated with conventional III-V compounds, which have a cubic symmetry.
Working with semi-polar lasers is relatively new, and this type of epitaxial structure is still immature. Limits of this approach are still being explored, including the strain management techniques already outlined above. One downside of working with semi-polar lasers is that optical gain is, in most cases, only high in one direction – and this direction is not always favourable for cleaved facet formation. This state of affairs also has its origins in the breaking of plane rotational symmetry.
Low gain is only partly to blame for high-threshold currents, which also result from high optical loss. In the red and infrared spectral ranges served by III-V lasers, intersubband absorption and free carrier absorption are the primary causes of optical loss, and they can be trimmed to less than 1 cm-1
for 980 nm lasing. With III-N
lasers operating in blue-green, however, losses are ten times higher, due to acceptor-bound absorption.
One undesirable trait associated with III-Ns is the high activation energy of acceptors. Very high acceptor concentrations are needed to realise a desired hole conductivity. With GaN and its related alloys, the only practical acceptor is magnesium, which has an activation energy in excess of 160 meV – at least four times that associated with a range of elements for doping more traditional III-Vs. This means that when using magnesium, if the acceptor concentration is about 1x1019
cm-3 , less than 2 percent of holes contribute to
conductivity. So very high levels of magnesium are needed to form layers that have reasonable p- conductivity, a necessity for making devices with a low operating voltage. However, a penalty must be paid – high optical absorption.
Figure
4.Deep charge carrier confinement inside the QW leaves limited opportunity for carrier distribution among several QWs and is the root cause of injection asymmetry needed to obtain p-conductivity
.Heavy magnesium doping is .Asymmetry of carrier injection in the
active region causes electron leakage to a heavily p-doped region where carriers can recombine non-radiatively if this process is not blocked
Lasers designers can use several tricks to trim the total optical loss of their devices. They can select a relatively high reflectivity for the front mirror, which also leads to a reduced slope efficiency of output light. In addition, they can keep the p-doped region away from the optical mode, while still ensuring that there is sufficient hole injection into the quantum well and the operating voltage of the laser is reasonable. One neat way to do this is to use an asymmetrical waveguide refractive index profile, which shifts the optical mode towards the n-side. Although this slightly reduces optical overlap with the active region, it substantially cuts overlap with the p-layers.
Injecting electrons and holes The third major challenge facing developers of green III-N lasers is the injection of carriers into quantum wells that are deep and have a band structure that is severely distorted by the QCSE.
With infrared laser diodes, good carrier transport results from the use of a graded-index, separate-confinement heterostructure. In this class of device, the bandgap increases gradually in both directions away from the quantum well region, and carriers can move in and out of each quantum well, leading to similar populations in every well. However, electrons and holes are still confined within the multi-quantum well region, because this is surrounded by a bandgap gradient that creates electric fields preventing carrier out-diffusion. What’s more, this approach also leads to optical confinement, because the material furthest from the wells has the widest bandgap and the lowest refractive index.
June 2012
www.compoundsemiconductor.net 17
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