Research Review: Corning Simplifies Semi-polar Green Lasers
REMOVING the electron-blocking layer (EBL) has little impact on the performance of green semi-polar lasers, according to experimental efforts by researchers at Corning. This US team has shown that even when the EBL is omitted, semi-polar lasers can still have good injection efficiencies and deliver in a relatively similar perforance over a wide temperature range. These beneficial characteristics strengthen the case that the semi-polar green laser is the best candidate for providing a singlechip green source for various ‘RGB’ displays, including pico-projectors. Lead-author Dmitry Sizov argues that the team’s findings should not raise any eyebrows, given that EBLs are rarely used in semiconductor systems that are free from polarization fields. “Semi-polar structures with reduced polarization fields are closer to those systems, which may be part of the reason why the EBL is not critical," says Sizov. He and his co-workers also believe that the carrier injection mechanism in semipolar laser-diodes differs from those in their conventional cousins. Removing the EBL from semi-polar lasers could make the semi-polar lasers more attractive, because it enables the devices to combine superior injection architectures with lower operating voltages. Corning’s engineers exposed the lack of importance of the EBL by comparing the performance of 11 semi-polar devices with ridge widths of 1.0 µm, 1.5 µm and 2.0 µm and emission wavelengths ranging from 508 nm to 522 nm. Four of these MOCVD-grown lasers had an Al0.28Ga0.78N EBL positioned 10 nm above the top-most QW; the other seven were EBL-free devices. Driven in pulsed mode, all of these lasers show no significant change in slope efficiency with temperature, indicating that variations in design had little impact on injection efficiency. Temperature insensitivity, which was determined by values for the ‘characteristic temperature of lasing threshold’, T0, did not depend on the inclusion or absence of the EBL. However, the value of T0 for all these lasers, which ranged from 161 K to 246K, is significantly higher than the typical value for conventional green laser diodes.The team attribute’s the superior temperature insensitivity of the semi-polar laser to intrinsic quantum well properties, such as a higher recombination rate. The team’s most impressive EBL-free laser emits at 519.4 nm and delivers a CW output of 60 mW at 10 oC, falling to 35 mW at 60 oC. Sivoz and his co-workers plan to continue working on the development of novel approaches for creating efficient laser devices, including GaN-based lasers. D. Sivoz et al. Appl. Phys. Express 4 102103 (2011).