Research Review: UCSB Reveals Green Laser Secrets
Engineers at the University of California, Santa Barbara, (UCSB) have shown that AlGaN barriers hold the key to the growth of a high-quality active region for a green semipolar laser.
The team’s investigation focused on devices grown on the (2021) plane of GaN, a cut that Sumitomo employed last summer to win the race for the first green laser.
While Sumitomo has said very little about the architecture of its active region, UCSB, in partnership with substrate supplier Mitsubishi Chemical, is now revealing some important findings about this light-emitting region on semi-polar GaN .
This partnership’s recent study involved a comparison of the material quality and device performance of green lasers built with three different types of active region.
Variants fabricated with 10 nm-thick barriers made from GaN, Al0.05Ga0.95N and In0.03Ga0.97N, all produced a spontaneous peak emission wavelength between 520 nm and 540 nm.
All three laser designs featured 4.5 nm-thick InGaN wells, a 1.2 mm-long cavity and highreflectivity, distributed Bragg reflectors. These mirrors that were formed by sputtering provide reflectivity at the front and rear facets of 97 percent and 99 percent, respectively.
Hioraki Ohta from UCSB admitted that the use of AlGaN causes inferior optical confinement due to its lower refractive index. “However, material quality, and in turn internal quantum efficiency, was much better."
Fluorescence microscopy exposed the superior material quality stemming from the AlGaN barrier. Structures with GaN and InGaN barriers featured many nonluminescent triangles with sides of 100 μm or more, which are presumed to contain many non-radiative recombination centers. These triangles were absent in the structure with AlGaN barriers.
A laser with the AlGaN barrier produced 516.3 nm emission when driven in pulsed mode with a 0.01 percent duty cycle.
Threshold current density was 30 kA/cm2, and the device delivered 8 mW at a drive current of nearly 1A and a 40 V operating voltage.
Ohta says that the team will now focus on further improvement of the active region through optimization of the structure and the growth process.
Researchers at UCSB have fabricated an 8 mW laser emitting at 516 nm.
Y.-D. Lin et al. Appl. Phys. Express 3 082001 (2010)