technology green lasers
Figure 1). Substituting another group V element for nitrogen is not easy, either, so there are only three ternary alloys available. Two are AlGaN and InGaN, which cannot be lattice matched, and the third is AlInN, which is rarely used due to the challenges associated with the growth of uniform material.
There is also a family of quaternary alloys that allow some degree of bandgap tuning for a given lattice constant: AlInGaN. But this class of material is rarely used, due to strong fluctuations in composition and a tendency for morphology roughening resulting from a large difference in the preferred growth temperatures of AlGaN and InGaN.
Consequently, designers of deep blue and green nitride lasers tend to select AlGaN for the cladding layers and InGaN for the waveguides. To prevent excessive strain, concentrations of aluminium and indium in AlGaN and InGaN are kept within a few percent of each other. However, this leads to a relatively low refractive index contrast between the waveguide core and claddings. Making matters worse, at longer wavelengths, the differences in refractive index between given alloy compositions diminish.
One choice facing every nitride laser designer is that of substrate orientation. Traditionally, nitride lasers have been formed on the c-plane, an orientation that allows for significant strain without defect formation, thanks to a relatively large force needed for glide along available glide planes.
Relaxation in nitrides due to dislocation glide occurs at far lower levels of strain when lasers are formed on semi-polar planes – planes in a wurtzite crystal tilted by an angle ß (0<ß<90 and 90<ß<180) with respect to the c-plane. There are many semi-polar planes to work with, and it is possible to find orientations that allow sufficient indium incorporation to realise green emission. More importantly, semi-polar planes enable the formation of epitaxial structures with higher optical gain. This is a key advantage for making green lasers, where optical gain is limited (this is discussed in detail later on).
When nitride layers are deposited on semi-polar substrates, strain leads to misfit dislocation arrays at AlGaN/InGaN interfaces between layers over 100 nm- thick that have aluminium and indium contents of just a few percent. If the impact of these misfit dislocations is similar to that in other III-V devices, they will jeopardize reliability.
Our team of researchers at Corning has found that misfit dislocations also act as strong non-radiative recombination channels, and in addition they degrade surface morphology, just like they do in other III-V structures. Fortunately, there are special tricks for sidestepping these relaxation effects and ultimately achieving sufficient optical confinement in semi-polar green lasers with good quantum efficiency. Thanks to these tricks, it is possible to realise high optical gain in green lasers.
Figure 1.Differences in crystal structure makes the alloying of wurtzite III/N with traditional III/V compounds challenging
Keeping misfit dislocations away from the active region is one way to manage strain. Do this, and strain relaxation is actually beneficial. The key is to grow an InGaN waveguide core over the cladding in a way that leads to dislocations forming at this interface. This increases the lattice constant of the InGaN layer, and it enables the subsequent growth of a green InGaN quantum well with much lower stress, thereby preventing relaxation of this layer.
This must be followed up with the growth of a second InGaN waveguiding layer on top of the well that has the same lattice constant as the bottom waveguide. Doing this enables the coherent growth of a multi- quantum well region sandwiched between InGaN waveguiding layers.
Figure 2. Green laser diodes are composed of lattice- mismatched heterostructures because most of the conventional III/N alloys are lattice mismatched with GaN substrates
June 2012
www.compoundsemiconductor.net 15
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105