Propelling The GaN VCSEL To The Green
Introducing quantum dots enables GaN VCSELs to penetrate the "˜green gap' by Baoping Zhang from Xiamen University
What do you believe is most desired in a laser? Is a high modulation speed? Or is it a low-threshold, a high efficiency, or a circular emission profile that tops your wish list?
The good news is that there is a class of laser that can fulfil all of these attributes and more: the VCSEL. Thanks to its all-round capability, sales are flourishing, with devices based on GaAs being deployed in optical mice, sensors and optical networks.
To grow revenues even more, this device must serve even more applications. That's possible by broadening the range of VCSEL emission wavelengths. When GaAs VCSELs emitting in the red and infra-red are joined by GaN-based cousins covering the blue and green, VCSELs can be used in projectors, displays, and solid-state lighting.
Unfortunately, making a GaN-based VCSEL is far more challenging than making a GaAs-based one. That's not to say that there has been no success "“ several groups have reported electrically driven GaN-based VCSELs that have InGaN/GaN quantum well active layers and produce room-temperature, continuous-wave emission in the near ultra-violet and the blue. But in the green, only Nichia of Japan has made any significant progress. Its team of researchers has succeeded in producing pulsed emission from a VCSEL emitting at 503 nm. However, even this is not true green, but cyan.
The green gap
Propelling InGaN-based devices from the blue to the green is not easy. The problem is well known, and has been given the moniker "˜the green gap'.
One of the reasons why it is so difficult to produce efficient emitters between 500 nm and 600 nm is that there is a lattice mismatch between InGaN and GaN. This gives rise to strain during epitaxial growth of InGaN on GaN, and culminates in the creation of a piezoelectric field. This causes band inclination, and pulls apart electrons and holes to different sides of the quantum well, reducing their chances for radiative recombination. Efficient emission is also impaired by defects in the InGaN layer. These imperfections, which are prevalent when the InGaN layer is too thick or the indium content is too high, can capture freely-moving carriers.
Another impediment to producing a high-performance, green-emitting VCSEL is the large effective masses of the carriers in the GaN-based material system. With InGaAs and its related alloys, carrier masses are far lower, leading to a lower transparent carrier density and a lower threshold current.
Due to all the issues detailed above, the threshold current in the GaN VCSEL is relatively high. But it doesn't need to be. Efforts by our team at Xiamen University, China, have shown that green GaN VCSELs can produce a great performance when their active region is made from quantum dots, rather than quantum wells. Note that this idea is not new, and has already led to some very impressive results with GaAs- and InP-based lasers.
In the GaN material system, the introduction of quantum dots transforms the device. Formation of these structures undergoes strain relaxation, leading to the elimination of electric fields that peg back radiative recombination. What's more, the dots tightly freeze the carriers, so they are no longer captured by defects; and their energy is quantized into restricted values, so the issues associated with a large effective mass disappear. And last but by no means least, the dots have a far higher differential gain than the wells do. The upshot of all these factors is the promise of low-threshold lasing.
VCSELs are formed by surrounding an active region with a pair of mirrors, in the form of distributed Bragg reflectors, which are capable of a reflectivity in excess of 99 percent. The production of GaAs-based VCSELs involves the epitaxial growth of the mirrors, which are based on the pairing of GaAs and AlGaAs. This duo is very good, combining lattice matching with a significant difference in refractive index.
Fabricating GaN-based VCSELs is far more challenging. The only option for making a lattice-matched distributed Bragg reflector with the nitrides is to pair GaN with an AlInN alloy with a specific indium content. This combination is not ideal, however, because the mirrors that result have a narrow stop band, due to the small difference in refractive indices. In addition, it is not easy to control the alloy content in the whole of the distributed Bragg reflector. Note that switching to the combination of AlGaN and GaN only makes matters worse, as it leads to strain accumulation, which causes defects.
In our view, it is better to form mirrors from dielectric materials, such as SiO2, Ta2O5 and TiO2. These oxides are well used in optical coatings, and they can be deposited by mature methods, such as electron-beam evaporation and magnetron sputtering. In addition, the stop band of reflectivity of a dielectric distributed Bragg reflector is much broader than that of GaN-based variant, making it much easier to achieve optical alignment.
Incorporating the dielectric distributed Bragg reflectors into the VCSEL requires a thinning of the wafer. For the team at Nichia, which grows the epitaxial layers on a native substrate, GaN must be thinned from a few hundred microns to just a few microns. This is a very complicated task, and it is easy to cause damage to the active region.
We employ an alternative approach, growing epilayers on sapphire, before removing this substrate by laser lift-off. To liberate the GaN epistructure, light from a violet laser impinges on the epiwafer through its backside. This radiation is absorbed at the interface with GaN. Thanks to a very high photon density, GaN is heated beyond its melting point, allowing the epitaxial layers to detach from sapphire.
One of the merits of this approach is that the liberated structure is just a few microns thick. As that's close to the thickness of the cavity, there is no need to undertake a complicated thinning process.