technology green lasers
The challenges for going green
Green GaN lasers are very different from their red and infrared III-V cousins: They are strained, plagued by strong internal electric fields and have massive band offsets. But if you can understand these traits and use some of them to your advantage, it is possible to design devices for plugging the green gap, say Dmitry Sizov,
Rajaram Bhat and Chung-En Zah from Corning.
F
ans of high definition movies and Sony Playstations are grateful to the inventors and producers of the GaN laser. However, what they are probably unaware of is that these scientists and engineers that refined and developed this violet-emitting laser, which can read the ones and zeros off of optical discs, had to deal with a multitude of tough challenges that do not exist in the traditional III-Vs: Limited availability of native substrates; higher growth temperatures and low chemical reactivity at room temperature; polarization effects resulting from a wurtzite crystal structure; heavy effective mass; high acceptor ionization energy; and limited availability of strain-free heterostructures.
More recently, these issues have hampered efforts within the nitride community to extend laser emission to longer wavelengths, a spectral region known as the green gap. Green lasers are wanted for colour projection systems that combine the output of red,
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www.compoundsemiconductor.net June 2012
green and blue sources. In addition, these single-chip lasers could win sales in a variety of defence, biomedical, industrial and instrumentation applications.
Chip designers trying to select an architecture for a green laser come up against three major impediments: Strain, because it is not possible to make the device with a set of lattice-matched materials; the need to operate at the high current densities to reach optical amplification, a pre-requisite for lasing; and injecting enough carriers uniformly into an active region plagued with an awkward band structure. The remainder of this article will detail each of these three challenges, before offering some insights into ways to overcome them.
Coping with strain
Lattice matching with nitride materials is very tricky. Differences in crystal structure prevent pairing III-Ns with III-Vs – the wurtzite structure of the former is significantly different from the zinc blende structure of the latter (see
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