LEDs technology
Both of these architectures have nearly identical emission area, and their fabrication required overcoming challenges related to materials growth, device processing and packaging. These included improving epitaxial growth uniformity and developing new device processing and packaging procedures as dictated by the large area and the vertical conduction geometry.
Our second architecture - deep ultraviolet lamps with lateral conduction - involves a 4x4 pixel geometry. However, each pixel itself comprises of 20 µm diameter micropixels. The total emission area is about 700 µm x 700 µm. These lamp chips are flip-chip mounted onto quasi-metallic carriers to improve heat sinking, and the carrier/chip assembly is bonded to a TO3-type, gold- plated metallic header. In this configuration the 280 nm lamp can realize a room-temperature output of 52 mW at a cw-pump current of 750 mA (see Figure 2). The emission spectra are very clean with a peak-to-valley ratio at 280 nm well in excess of 500.
We believe that even higher output powers are possible. An increase in output should be possible by turning to a packaging scheme that will also collect light traveling in the lateral direction due to waveguiding in the AlGaN layers. Initial measurements indicate that the room temperature lifetime for these 280 nm lamps at continuous wave operation is around 1000 hours, and this should increase with better thermal management.
Fabrication of the vertical conduction, deep ultraviolet lamps involves two new processing steps: removal of the sapphire substrate; and formation of n-ohmic contacts on the backside of the bottom n-AlGaN layer. We have pioneered the laser lift-off of sapphire from AlGaN-based
Initial measurements indicate that the room
temperature lifetime for these 280 nm lamps at continuous wave operation is around 1000 hours, and this should increase with better thermal management
deep ultraviolet LED structures that utilize an AlN buffer layer. In addition, we have developed a new processing scheme for vertical-conduction, thin-film, deep ultraviolet lamps.
To make a vertical conduction structure - rather than one based on lateral conduction - requires a reversing of the order of fabricating the two different contacts. The p- contacts must be formed before the n-contacts in a vertical conduction LED, and this complicates device fabrication, because the p-contact degrades due to the higher temperatures needed for the fabrication of the n- contact.
However, we have found a remedy to this problem that has led to significant improvements in deep-ultraviolet LED electrical characteristics, along with a record DC power of 6.2 mW for the 280 nm lamp driven at 260 mA (see Figure 3).
Efforts are now being directed at improving electrical characteristics and the output powers of our lamps. New packaging schemes are also being developed to improve thermal management, minimize device heating and ultimately increase device lifetimes.
Figure 2. DC current-voltage (I-V) and current-power (I-L) characteristics of a 280 nm lateral UVC lamp operating at room temperature
Figure 3. DC current-voltage (I-V) and current-power (I-L) characteristics of a 280 nm vertical UVC lamp operating at room temperature
June 2010
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