review research Simplifying GaN VCSEL fabrication
THE process for making a room- temperature GaN-based VCSEL capable of continuous wave (CW) emission has been simplified, thanks to efforts at National Chiao-Tung University, Taiwan.
Corresponding author Tien-Chang Lu claims that the group is this first to use an AlN/GaN-based bottom DBR in this class of device. “This is the first step toward the all-epi type structure used in mature, GaAs- based, 850 nm VCSELs.”
The breakthrough by Lu and his co-workers could help to spur GaN VCSEL commercialization. These devices have a far lower threshold current than their edge- emitting cousins, and could provide a more efficient blue light source for Blu-ray pick-up heads, portable micro-projectors, and high- resolution laser mice.
Success at National Chiao-Tung University follows Nichia’s report of the fabrication of the first room-temperature, CW VCSEL in 2008. The Taiwanese researchers attribute Nichia’s breakthrough to the introduction of a thinner transparent conducting layer, which cuts internal loss; and the growth of a high-quality active region, thanks to deposition on a GaN substrate.
Nichia’s approach to VCSEL fabrication is complex, and involves laser lift-off and elaborate polishing and bonding processes. A far simpler approach is to grow a nitride- based DBR and an active region on sapphire by MOCVD, before adding a dielectric DBR with an electron gun.
The Taiwanese researchers have pioneered such a process, which begins with the growth of a GaN buffer layer; a 29-pair AlN/GaN DBR; a GaN/InGaN active region featuring ten, 2.5 nm-thick quantum wells; an electron-blocking layer; a 110 nm-thick p- GaN layer; and a 2 nm-thick GaN-cap. AlN/GaN superlattices were inserted into the DBR to prevent cracking.
A 10 nm diameter aperture was defined by deposition and selective removal of SiN. The team deposited a current spreading layer and n- and p-contacts on this, and then added a ten-period Ta2
O5 /SiO2 complete device fabrication.
Lu claims that this is the first GaN VCSEL to feature an AlGaN electron-blocking layer. This resulted in a relatively low threshold current density of 9.7 mA, corresponding to a threshold current density of 12.4 kA/cm2 Turn-on voltage for the 412 nm laser was
. Nichia’s LEDs hit new highs
Nichia has increased the efficacy of its LEDs that are being developed for solid-state lighting.
Results from this Japanese optoelectronic device manufacturer include a LED chip that delivers 1913 lumens at 135 lm/W when driven at 1A. This chip combines four, 450 µm by 450 µm die in series. According to the researchers, this LED has a higher flux than a 20W-class fluorescent lamp and an efficacy 50 percent higher than a tri-phosphor fluorescent lamp.
Nichia’s engineers have also fabricated a 1 mm by 1 mm chip that produces 183 lm/W at 350 mA and 130 lm/W at 1A.
This set of results is broadly comparable to Cree’s recent announcements. Nichia’s record for efficacy at 350 mA is not as high as its US rival, which announced a 208 lm/W LED at 350 mA. However, the dimensions of the more efficient device are unknown. Cree has also unveiled a single chip device, the Xlamp XM LED, which produces 110 lm/W at 2A and is slated for release this Fall.
Nichia’s recent success has stemmed from reducing the operating voltage of its LEDs from 3.1 V to 2.8 V. Improvements in epitaxial wafer quality and the introduction of ‘current-expanding’ electrodes have help to cut operating voltage.
Y. Narukawa et al. J. Phys. D. Appl. Phys. 43 354002 (2010) DBR to
The team’s next goal is to improve laser beam quality. Lu says that this will be accomplished through further reduction of the threshold current, plus the addition of an optical confining structure.
T.-C. Lu et al Appl. Phys. Lett. 97 071114 (2010)
VCSEL growth involves MOCVD deposition of a GaN-based epitaxial stack, followed by ion-assisted electron gun deposition of a Ta2O5/SiO2 DBR
4.3 V, output power peaked at just over 35 µW, and beam divergence was 8o
.
October 2010
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