UV LEDs  industry


layers (see Figure 2). Better quality materials also offer additional benefits: a longer carrier non-radiative recombination lifetime, (see Figure2 (a)), higher efficiency and better device reliability. And further improvements in device performance are possible by employing superlattice buffers to reduce strain (see Figure 2(b)), alongside phonon band engineering approaches to negate the negative effects of the polarization fields and realize very high internal quantum efficiency.


Capturing electrons We have also developed new quantum well configurations for deep UV LEDs that incorporate very narrow quantum wells within an wider ‘energy tub’. In the example shown in figure 2(d), the bandstructure is engineered so that the difference in the energy of an electron when it enters the energy tub and when it is at the top of the quantum wells is equal to or greater than the energy of a polar optical phonon in the device material. Electrons emitting this form of phonon cool down much faster on entering the energy tub, and the chances of the carriers remaining there are high, due to the difference in composition on the p-type side of the LED. Now localized in the well, the electrons are more likely to recombine with holes to emit light.


Our efforts at refining band structure, growth methodologies and device processes have culminated in the fabrication of DUV LEDs with CW output powers in the milliwatts range and pulsed powers over 100 mW (see Figure 3). Lifetimes now exceed 5,000 hours for many DUV LED wavelengths for devices run continuously at 20 mA at room temperature without any heat sinking.


Recently, DUV LEDs have also passed space qualification – so far they have demonstrated over 26,000 hours of pulsed operation with no significant power drop or spectral shift. The qualification process was performed at Stanford University and National Security Technologies (NSTec) on our 255 nm UVTOP LEDs, with tests involving extreme radiation hardness, temperature cycling and 14g rms random mechanical vibrations.


For water disinfection applications, CW powers of tens of milliwatts or more are required. To meet this demand, we have developed and launched multi-chip LED lamps and products with powers over 100 mW at 275 nm; each


Figure 2 (a) SEM micrographs of a fully coalesced 20 µm thick AlN sample grown by MELEO (b) Light-induced transient grating (LITG) decay in MOCVD and MEMOCVD-grown AlGaN epilayers for the grating period of 7.7 µm. Carrier lifetimes were estimated by fitting the decay transients with single exponents (lines) (c) Typical deep UV LED design (d) Schematic band diagram of DUV LED for capturing electrons in the light emitting region


lamp containing as many as 100 single chips. In the same manner, DUV LEDs having different emission wavelengths are often combined together in one package to create broadband UV LEDs and multi-wavelength LEDs. Multi-wavelength DUV LEDs have up to 26 individually addressable wavelengths and can be directly coupled to an optical fiber, enabling spectroscopic and fluorometer applications.


Efforts at producing high-power single-chip LEDs are underway. Powers of 100 mW at an emission wavelength of 275 nm have been demonstrated on a 1.5 mm x 1.5 mm chip with an active area of 1 mm2 TO-3 metal can.


packaged in a


Our efforts at refining band structure, growth methodologies and device processes have culminated in the fabrication of DUV LEDs with CW output powers in the milliwatts range and pulsed powers hitting 100 mW


January / February 2011 www.compoundsemiconductor.net 39


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