Nichia Raises The Bar For Blue And Green VCSEL Efficiency
VCSELs with dielectric and GaN-based mirrors break efficiency records
Nichia is claiming to have broken the efficiency records for GaN-based blue and green VCSELs
This triumph, announced at the SPIE Photonics West meeting this March, returns by this Japanese LED and laser diode manufacturer to the forefront of GaN-based VCSEL development. Nichia pioneered this class of laser more than a decade ago, producing the world's first GaN-based VCSEL operating in continuous-wave mode at room-temperature, before appearing to abandon further development for many years.
Speaking on behalf of the company at the Photonics West, Kenichi Terao revealed that the primary target for these VCSELs is the light source in smart glasses. In these retinal displays, red, green and blue sources only need to produce a milliwatt a most - a specification met by the team's latest lasers.
Terao said that one of the merits of using a VCSEL in smart glasses, rather than an edge-emitting laser, is the far lower threshold current. It is around 1 mW, rather than 10 mW or more, due to an emission volume that is lower by an order of magnitude. Thanks to the lower threshold current, power consumption falls, lengthening battery life.
“Another VCSEL feature is peak power saturation, in a lower output power than an edge-emitting laser, due to self-heating,” said Terao. “This ensures eye safety.”
Nichia's first foray into VCSEL development brought much initial success. Milestones from that era included a violet-emitting room-temperature VCSEL operating in CW mode in 2008, and blue and green siblings in 2011 and 2012.
However, the fabrication process for this design, featuring a pair of dielectric mirrors, is incompatible with mass production. To define the dimensions of the cavity, epilayers must be removed from the substrate and polished down to the ideal thickness. Unfortunately, this cannot be accomplished routinely at the nanometre scale, preventing mass production with acceptable yield.
To overcome this issue, Terao and co-workers have turned to a design that combines a dielectric top mirror with a bottom mirror based on an Al0.8In0.2N/GaN distributed Bragg reflector. Although they are not the pioneers of this architecture, they have made significant advances with it, including the first lasing in the green, and record-breaking efficiencies for all forms of GaN-based VCSEL.
The blue VCSELs produced by the team have: a dielectric DBR made from the pairing of SiO2 and Nb2O5; a 4 µm-diameter aperture defined by passivation of the p-GaN surface; and a cavity length equating to just 4.5 lambda, ensuring that this design is short enough for single-mode lasing.
These devices emit at 442.3 nm, have a threshold current and voltage of 0.40 mA and 3.75 V, and produce a peak wall plug efficiency of 13.6 percent. Measurements on seven of them, producing 0.6 mW when running at 25degC, reveal stable CW operation beyond 1,000 hours.
Formed from the epiwafers featuring a 2-inch GaN substrate, yield for these blue VCSELs exceeded 90 percent. This figure considered an exclusion zone of less than 4 mm, included to exclude material very close to the edge of the wafer, where photolithography processes are unstable.
Measurements at driving currents of 1 mA, 3 mA and 5 mA revealed a single peak for the VCSELs, indicating a stable single longitudinal and single transverse mode. Emission wavelength shifted by 0.11 nm /mA, due to changes in refractive index resulting from self-heating. Further investigations of self-heating uncovered a variation of 0.017 nm/degC, which is smaller than that for an edge-emitting laser, according to Terao.
The design of the team's green VCSEL is very similar to its blue cousin. Terao said that the primary difference is a larger aperture - it's been widened from 4 µm to 5 µm - to prevent an increase in drive voltage. The green VCSEL also features a thinner active layer, to supress the piezoelectric field.
Characteristics of the green VCSEL, which emits at 514.9 nm, include a wall plug efficiency of 3.7 percent, a threshold voltage of 5.02 V and a threshold current of 2.8 mA. Optical output power is over 1.5 mW, smashing the previous record of 0.1 mW held by Sony.
K. Terao et al. SPIE Photonics West 11686-1 (2021)