technology green lasers
prevent this from degrading device behaviour, an electron-blocking layer is inserted between the wells and the p-type region. This thin layer stems the flow of electrons to the p-side, but partially blocks hole injection into the quantum wells and fails to address the problem of non-uniform carrier distribution within the active region. When we tried to build c-plane green lasers with several quantum wells, only the one or two closest to the p-side provided optical gain, with the rest absorbing light, because they are under-pumped, unless extremely high current is applied.
Semi-polar lasers don’t suffer the same fate. Hole injection into several quantum wells is much easier, due to a reduction in the strength of the built-in polarization fields. These weaker fields lower barrier spikes, making carrier fly-over more uniform and ultimately leading to a more even distribution of electrons and holes within the wells.
Unfortunately, up until now it has proved impossible to transfer this elegant design to green nitride lasers. In our view, this is because of peculiarities associated with the bandstructures of GaN and InGaN used in the barriers and wells. These materials have bandgaps of 3.49 eV and 2.34 eV, so the total band offset exceeds 1 eV. It is hard to determine the precise bandstructure, due to significant variations in the values for the offset ratio of the conduction band to the valence band. However, these differences are not important, because there is no question that the barrier height (given in energy ) for each charge carrier exceeds the value of kB
T by a factor of at least ten.
To replicate the graded-index structures found in conventional III-V lasers, engineers must fabricate InGaN barriers with a significantly narrower bandgap than GaN. But this would increase the total amount of indium in the active region, adding compressive strain to wells that are already suffering from this. So the barriers must contain very little or no indium. This means that the rate for carrier thermal escape from a quantum well ground state, which rapidly diminishes with increases in barrier height, is far slower than carrier recombination. Consequently, carriers are trapped in one well until they recombine, and they only travel across the wells by ballistic fly-over.
The upshot of all of this is poor carrier distributions, particularly in c-plane lasers. Here, the piezoelectric effects create potential spikes in the barriers, which lead to a drag on ballistic carrier motion. Holes fare worse than electrons, due to a higher average effective mass and lower mobility, and this results in the accumulation of carriers in the quantum wells near the p-side.
In c-plane green laser diodes and LEDs the well closest to the p-side tends to be the one most populated with carrier. This is part of the reason why the p-n junction is effectively shifted toward the p-side, promoting electron leakage to the p-doped region. Any electrons that get this far undergo parasitic nonradiative recombination. To
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We have found that this superior band structure in semi- polar lasers also allows designers to dispense with the electron-blocking layer, which is not used in III-V infrared lasers. Thanks to a deeper penetration of holes in the multiple quantum well region, fewer electrons are attracted to the p-layers, and there is much less shifting of the position of the p-n junction.
There is a silver lining associated with deep carrier confinement in the quantum wells: Reduced sensitivity to changes in temperature. The measure of this sensitivity in laser diodes is the threshold current characteristic temperature T0
. In arsenide- or phosphide-
based lasers, carrier confinement is typically 0.2 eV or less, and a significant proportion of carriers leak away from the active region at normal operating temperatures. With green nitride lasers, the far stronger carrier confinement prevents any electron escape from quantum wells with temperatures below 400K.
Switching from non-polar to semi-polar green lasers leads to an even higher value of T0
. We believe that this
stems from a combination of the deep carrier confinement found in all forms of nitride laser, plus a particularly large separation of quantum well quantization states. This leads to a lower thermal population of the higher energy bands.
It is clear that III-N materials hold great promise for covering a wide spectral range. When it comes to green III-N lasers, the challenges are quite different to those facing the designers of III-V red and infrared lasers, so a different approach is needed. With III-N green lasers, strain comes into play, there is low optical gain and high optical loss to deal with, and deep carrier confinement combats uniform carrier distributions in the multiple quantum well – but it has a good side too, in the form of diminished device temperature sensitivity. Understanding all of this, and how to navigate a path through these obstacles, holds the key to improving the performance of green lasers.
© 2012 Angel Business Communications. Permission required.
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