technology LEDs
Figure 1(a). Schematic cross-sectional view of vertical deep UV lamp. To the right is an image of emission from the back n-contact side. The total emission area is 850 µm x 850 µm. Fig. 1(b). A packaged MicroLED-array- based deep UV lamp with 4x4 pixels. Each pixel has a grid of 10x10 micro-pixels each having a diameter of 20 µm. Total emission area is 700 µm x 700 µm. To the right is an expanded emission image of each pixel
around 250 nm, which is an extension of our efforts at South Carolina that resulted in sub-milliwatt power 250 nm LEDs. Recently the Riken research group has succeeded in fabricating 250 nm devices that produce 1 mW at a 20 mA pump current.
Device
processing facilities used by Nitek Inc. personnel
Another direction that deep UV LED research is taking is the development of low-defect AlGaN templates for subsequent deposition of deep-ultraviolet LED layers. The objective of this research is to take device efficiency beyond the 1 percent value that was realized by our efforts at South Carolina several years ago. Efforts in this direction are taking place at Nitek, a spin-off from the University of Carolina that we are all involved with (see Figure 1).
We recently presented some encouraging results at International Workshop on Nitride Semiconductors conference that was held in Montreux, Switzerland, in the Fall of 2008. At this gathering we reported the use of pulsed lateral overgrowth to fabricate low-defect AlGaN templates with a thickness of well above 10 microns. These templates are not just beneficial to the emission efficiency - they also significantly improve thermal management, leading to an increase in device lifetime by approximately 50%.
The third goal being targeted by deep ultraviolet LED researchers is the development of large-area lamps that realize higher output powers, through a hike in drive currents to 200 mA or more. Thanks to the larger emission area of these lamps, it is possible to employ pump current densities – and levels of device heating – that are similar to those for small-area, discrete devices operating at 20 mA.
Through Nitek, we are pursuing two different approaches to making monolithic, large-area, deep ultraviolet lamps for room-temperature operation (see Figure 1). The distinction between these two approaches is a difference in current conduction geometry and the configuration of the p- electrodes.
Our first device architecture is a single, large-area pixel with vertical-current conduction geometry (Fig. 1(a)). This form of conduction is realized by removing the sapphire substrates, and then creating n-electrodes on the backside of the bottom, nitrogen-face n-AlGaN layers. In the second scheme, we have turned to a lateral- conduction geometry, and employed several micropixel electrodes to define the emission area (see Figure 1(b)). This device geometry does not require the removal of the sapphire substrate.
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www.compoundsemiconductor.net June 2010
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