research review New plane promises to aid green lasers
Researchers at the University of California, Santa Barbara (UCSB) and Mitsubishi Chemical claim to have uncovered a superior semi-polar plane for making green lasers.
Last summer Sumitomo won the race for the green laser diode with a device built on the (2021) plane. But corresponding author Po Shan Hsu from UCSB told Compound Semiconductor that even better results should be possible by switching to growth on the (3031) plane.
He says that the (3031) plane offers a much wider laser diode design space.
Theoretical calculations suggest that it is possible to grow In0.06
Ga0.94
60 percent thicker on this plane than the (2021) one.
The opportunity to grow thicker layers enables the growth of waveguide structures with sufficient refractive index contrast that do not use AlGaN, but just InGaN and GaN.
Eliminating AlGaN cladding layers is a big deal. “Growth of thick AlGaN layers are unfavorable because they can increase
N layers that are
device voltage, decrease device lifetime, reduce device yield and reduce reactor stability,” says Hsu.
What’s more, AlGaN layers usually require supperlattices, and this increases epiwafer growth times and ultimately manufacturing costs.
The team used MOCVD growth on free- standing GaN substrates provided by Mitsubishi Chemical to fabricate their blue InGaN/GaN laser diodes on the (3031) plane.
These devices produced a threshold current of 5.6 kA/cm2
to 6.0 kA/cm2 . and a clear lasing peak at 445
nm. Electroluminescence of the laser shifted by just 4 nm when the current density was cranked up from 0.3 kA/cm2
This very small shift in wavelength suggests that the device is capable of being driven at high modulation rates. The laser’s operating voltage was 9.9 V, a relatively high value that is probably due to unoptimized p-contacts and doping profiles.
Shifting the emission of these lasers from the blue to the green is one of the next goals for the team. This requires an increase in the
Growth of lasers on the (3031) plane enables the fabrication of structures free from AlGaN cladding layers. Lasers that do not contain AlGaN cladding layers can have lower operating voltages and last longer.
indium-content of the active region. The team is looking into this, and investigating how indium incorporation on the (3031) plane compares to that on other semi-polar planes. Po Shan Hsu et al. Appl. Phys. Express 3 052702 (2010)
PICs could benefit from square lasers
A team of researchers at the Chinese Academy of Sciences has built the first square microlasers featuring output ports on opposite corners.
These devices that are produced by planar processes could provide a key building block for the fabrication of photonic integrated circuits (PICs).
The shape of microdisk lasers governs the direction of their emission. Square lasers with output ports in the corners were investigated, because this design can realize highly confined modes while still producing directional emission.
Fabrication of the structures begins by using plasma enhanced CVD to deposit an 800 nm-thick SiO2
film onto the top of
epitaxial laser structures grown on InP. Photolithography defines the profile of the
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www.compoundsemiconductor.net June 2010
layer is then deposited, with further photolithography and etching steps defining a window in the cavity. The addition of Ti/Au and Au/Ge/Ni contacts creates n and p- type electrodes.
laser, before plasma and wet etching techniques remove the oxide film. A second SiO2
A 1514 nm laser created by this process delivered 4.8 µW at a 72 mA injection current. “It’s only a concept device, and the threshold current and output power are not good enough for making photonic integrated circuits yet,” explains corresoponding author Yong-Zhen Huang.
He says that the team will now focus on reducing the threshold current and increasing the output power of its microlasers.
“We are now investigating coupled circular
The two-port square microlaser has 20 µm sides and output waveguides on opposite corners. Credit: CAS.
resonators, which can have over 99 percent coupling efficiency, according to finite- difference time-domain simulations,” says Huang.
K-J. Che et al Electron. Lett. 46 585 (2010)
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