Lamps Boost Output In The Deep Ultraviolet
Ultraviolet light in the 250-300 nm range can serve many applications, including air and water purification, polymer curing and bio-medicine. Each of these markets is fairly large, and the water purification market alone is estimated to be worth over $5 billion.
Today the main source of deep ultraviolet light for all these applications is the mercury lamp. It is not ideal however, for many reasons: it is large; bulky; requires high operating voltages; cannot be driven in pulsed mode; and its emission wavelength is fixed. On top of this, there are contamination issues arising from the use of mercury, which pose severe challenges when contemplating medical or bio-medical applications.
One attractive alternative to this lamp is the AlInGaNbased deep-ultraviolet LEDs, which can be formed on sapphire substrates that are transparent at these short wavelengths. This type of device has been pioneered by the academic group at South Carolina headed by one of us, Asif Khan, with development dating back to 2002. The key to producing the first successful devices was the combination of pulsed epitaxy and superlattice buffer layers. This mitigated strain in the epistructure that invariably leads to layer cracking when thick, high aluminum-composition AlGaN layers are directly deposited over sapphire. These thick, aluminum-rich layers are essential, because they can address current crowding in the LED that leads to localized heating, and ultimately substantial shortening of device lifetime.
Taking this approach enabled the fabrication of 280 nm LEDs that delivered 1 mW at a DC pump current of 20 mA, and showed a reduction in power output after 1000 hours of operation of only 50 percent. These results were reported in 2006, and since then they have been duplicated by several groups, including Sensor Electronics Technology and Riken. However, to date there has been little progress in terms of higher powers and longer lifetimes.
More recently, the deep UV LED research community has directed efforts in three distinct directions. One of these is the fabrication of higher power, discrete LEDs emitting 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.
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.
Device processing facilities used by Nitek Inc. personnel
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 pelectrodes.
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-arraybased 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
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 lateralconduction 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.
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, goldplated 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.
Figure 2. DC current-voltage (I-V) and current-power (I-L) characteristics of a 280 nm lateral UVC lamp operating at room temperature
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 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 ncontact.
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).
Figure 3. DC current-voltage (I-V) and current-power (I-L) characteristics of a 280 nm vertical UVC lamp operating at room temperature
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.