Optimisation of cavity lengths and facet reflectivity yields powerful, efficient semi-polar lasers

Sony and Sumitomo Electric Industries have produced a portfolio of green lasers that are setting new benchmarks for output power and spectral coverage. Semi-polar devices produced by these companies, which started working together in 2010, deliver output powers of up to 168 mW, emit at wavelengths of up to 536.6 nm, and have a lifetime of more than 5000 hours when operating continuously at an output of 50 mW.

Thanks to this great set of characteristics, these devices are very attractive candidates for the green source in pico-projectors. According to the team, requirements for laser projection include an output power in excess of 50 mW, a wall-plug efficiency (WPE) of more than 4.5 percent, and a lasing wavelength of at least 515 nm – and preferably 525 nm or more. According to Shimpei Takagi from Sumitomo, wavelengths above 525 nm are needed to realise a good colour gamut.

The best GaN plane for making a green laser is a hot topic within the nitride community. The likes of Nichi and Osram have developed green lasers on the conventional c-plane of GaN, while others have turned to semi-polar planes.

Researchers from Sony and Sumitomo claim that the semi-polar plane has several benefits over its more traditional cousin: It is easier to grow green-emitting InGaN quantum wells with a high degree of homogeneity; piezoelectric fields are weaker, leading to a higher radiative recombination efficiency; and for emission wavelengths beyond 520 nm, the threshold current density is far lower. The nitride community has used several semi-polar planes to build green lasers and the Japanese team believes that the {2021} orientation produces the best devices.

The team fabricated a series of ridge-waveguide lasers on {2021}GaN substrates with a threading dislocation density of less than 1 x 106cm-2. These semi-polar devices featured InAlGaN cladding layers, InGaN waveguides and an InGaN multiple quantum well with a well thickness of 2-3 nm. Photolithography and dry etching formed ridge waveguide structures with 1.5-2 µm widths, and cleaving created 500 µm cavities with facets that were subsequently coated with dielectric films.

Very high output powers at emission wavelengths above 520 nm resulted from optimisation of the cavity length and mirror reflectivity (see table for details). “We optimized cavity length by both calculation and experiments,” explains Takagi. “Cavity length and proper selection of reflectivity have a major impact on device performance.” Improvements in both these areas enabled a four-fold increase in slope efficiency compared to devices reported by Sumitomo in 2010.

The WPE of the team’s lasers is very high, but decreases at longer wavelengths. Lasers emitting in the 525-530 nm range have a WPE of more than 8 percent, falling to just above 5 percent for the 536.6 nm chip, which is claimed to produce the longest emission wavelength for any green laser. This series of devices has a significantly higher WPE than those reported in 2010, thanks in part to a superior doping profile that reduced the threshold voltage from 6.4 V to 4.7 V.

Impressive performance at high temperatures is possible with these semi-polar lasers. At 80 °C, devices can deliver 90 mW, indicating that they are suitable for deployment in portable devices that are only able to house heat sinks with limited capability.

One issue being debated by semi-polar laser developers from various institutions is the extent of the benefit of the electron-blocking layer in the device. Recently, Dmitry Sizov and co-workers from Corning argued that this layer plays a minor role in device operation. In contrast, the Japanese team have found that the electron-blocking layer supressed carrier leakage and helps the laser to produce a high characteristic temperature (T0). The researchers have also performed reliability measurements on lasers emitting at 527.5 - 530.8 nm.  They defined the lifetime as the estimated time for a 30 percent increase in operating current. “For GaN-based lasers for Blu-ray Discs, this is a standard definition,” points out Katsunori Yanashima, a member of the Advanced Materials Laboratory at Sony.
 

 
The Sumitomo-Sony partnership has set a new benchmark for power output of long-wavelength nitride lasers

 

Semi-polar green lasers combine wall-plug efficiencies of more than 5 percent with output power of up to 168 mW and long lifetimes

Operating at 50 mW and 55 °C, these lasers had a lifetime of 5000 hours, and when the output increased to 70 mW, lifetime shortened to 2000 hours. The team claims that these values for lifetime are longer than those reported for c-plane lasers, thanks to weaker polarization fields, better material homogeneity in the quantum wells and a low resistivity of the p-contact. Improvements to the latter characteristic stemmed from optimisation of the magnesium dopant concentration and its activation treatment. 

The collaboration is now trying to increase the output power and efficiency of its semi-polar lasers.

S. Takagi et al. Appl. Phys. Express 5 082102 (2012)

K. Yananshima et al. Appl. Phys. Express 5 082103 (2012)