Kyocera in Japan has developed a new thin-film process technology for making silicon substrates for gallium nitride (GaN) microLEDs and short-cavity lasers. These can be used for low cost, low power lasers as small as 100 microns for AR smart glasses and transparent displays in cars.
Reducing the size, cost and power consumption of laser displays is key to the volume adoption of smart glasses, which Kyocera and other companies have identified as a key trend to replace smartphones in the future.
Researchers at the Research Institute for Advanced Materials and Devices in Kyoto grew a GaN layer on Si substrate which is available in high volumes at a low cost. The GaN layer is then masked with a non-growing material that features an opening in the centre.
After this, when a GaN layer is formed on the Si substrate, GaN nuclei grow over the opening in the mask. The GaN layer, which is a growing nucleus, has many defects at the initial stage of growth but, by forming the GaN layer laterally, high-quality GaN layers with low defect density can be created, and devices can be fabricated successfully from this low-defect region of the GaN layer.
GaN-based light-source devices, both microLED and lasers, have typically been built on sapphire and GaN substrates. Conventional processes involve forming a thin GaN device layer directly onto the sapphire substrate by heating it to over 1,000 °C in a controlled gas atmosphere. This layer is then peeled from the substrate to create a GaN-based micro-light-source device. There is rising demand for smaller devices, but there are several challenges says Kyocera.
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In case of micro-LEDs, current processes require difficult steps to divide the device layer into individual light sources on the substrate; and then, to separate the device layer from the substrate. As devices become smaller, the technical challenge of this peeling process can result in low yield
Fabrication of micro-light sources is also problematic because device layers must be deposited onto sapphire, silicon, or other materials with crystal structures that differ from that of the device layer. This creates high defect density and inherent quality control challenges.
The new process enables easier peeling of the GaN device layer as masking the GaN layer with a material that does not grow suppresses bonding between the Si substrate and the GaN layer.
While conventional semiconductor lasers for AR have been miniaturized to as small as 300 microns in length, Kyocera is the first in the world to achieve a size of just 100 microns. This novel cleaving method results in a size reduction of about 67% and helps minimize power consumption to reduce the size and weight of the battery.
The reliable separation of the GaN device layer from the relatively inexpensive Si substrate also reduces the manufacturing costs for high volume applications.
Kyocera will offer a wide range of platform, substrate, and process technologies for low-cost micro-light sources to market in the near future.
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