LEDs ♦ news digest
processing, and laser production processes, a joint venture has developed a true green laser diode
Sumitomo Electric and Sony have jointly developed what the firms say is the world’s first semiconductor laser diode with an optical output power of over 100mW in the true green region.
At a wavelength of 530 nm, this laser diode (LD) can be mounted on laser projectors and many other display devices. The new device features twice the luminosity of conventional GaN green laser diodes (which are considered to operate at 60mW or less and at 520nm or less), and a colour gamut broadened by 182% based on the NTSC standard (CIE 1976 colour gamut). T
his standard compares the new true green LD with current red and blue laser diodes. This development could lead to a significant improvement in the performance of laser projectors and other display devices, to reproduce vibrant video and images. Red and blue laser diodes have been commercially available among the primary red-green-blue (RGB) colours, and the technology of these LDs is well known. But there has been a far greater need for high output green laser diodes to enhance the overall performance of high laser projectors and display devices which are needed today. Currently, green lasers are generated by converting the wavelength of infrared laser light from a light source using optical materials, but the light source is large and expensive. What’s more, conventional GaN-based green lasers have problems achieving sufficient luminosity as their performance is limited to an output power of several tens of milliwatts at a wavelength of 520nm or less.
To overcome these challenges, Sony and Sumitomo Electric collaborated on the development of a true green semiconductor laser for practical use. The research drew on Sumitomo Electric’s semi-polar GaN substrate, crystal growth, and wafer processing technologies and Sony’s GaN- based laser technology, acquired through Blu-ray development. By introducing new techniques and
improving the entire semiconductor laser production process, the firms were able to develop a true green semiconductor laser with an optical output power of more than 100mW at a wavelength of 530nm. Changes in the process included structural design, crystal growth, wafer processing, and electrode configuration. This true green semiconductor laser diode is claimed to be highly reliable as it has a wall-plug efficiency of over 8%. The development of the true green semiconductor laser diode completes the three basic colours of the RGB laser light sources. Sumitomo Electric and Sony anticipate a wide variety of applications for this technology such as incorporation into advanced laser projectors with high luminosity and broad colour gamut, as well as compact, light, energy-efficient laser projectors.
The device was grown using a semi-polar (20-21} GaN substrate. In this instance, the plane is tilted 75 degrees compared to the conventional GaN crystal c-plane. The semi-polar GaN substrate enables the sustainable production of homogenous indium-based active layers, leading to the successful growth of a high quality light-emitting layer.
In GaN-based green light-emitting devices, luminous efficiency declines in the green region due to the internal field effects resulting from distortion in the crystal structure and the resultant internal polarisation. It is possible to suppress the internal field effects by adopting semi-polar GaN substrates. However, this isn’t enough; it is also necessary to optimise the entire laser production process, and this is exactly what the joint venture did, building on Sony’s knowledge. Sumitomo Electric and Sony managed to reduce the operating current and voltage in the laser through the optimisation of the laser structure, impurity control in crystal, and minimisation of electrode resistance.
July 2012
www.compoundsemiconductor.net 61
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