UV-B LEDs of high robustness and reliability
Fig. 1: Degradation of optical power over time of 305 nm UV-B LEDs operated at 30 mA and 90 mA.
Fig. 2: Degradation of optical power over time of 305 nm UV-B LEDs at alternating temperature.
The optical output power of (AlGa)N-based LEDs emitting in the UV-B spectral region has been considerably improved over the last years. However, the long-term stability of the electro-optical characteristics of these LEDs is still an issue. The lifetime of currently available UV-B LEDs is limited to some 1,000 hours or less, which impedes their industrial application. Recently, extended stress tests of UV-B LEDs have been started at FBH to study the relevant degradation mechanisms in these devices. The UV-B LEDs with an emission wavelength around 305 nm have been developed in a joint effort between TU Berlin and the FBH. The devices were operated for up to 1,000 hours under varying stress conditions. Fig. 1 shows the evolution of the optical power over time of LED samples which were flip-chip mounted on passively cooled AlN ceramics. The optical power increases within the first hours of operation, strongly decreases within the following hours, and finally drops gradually and slowly for even longer times. The so called "burn-in" phase during the first hours and the subsequent degradation phase have been studied separately.
Fig. 2 shows a UV-B LED driven at 20 mA and a temperature alternating between 122°C and 185°C in the active region. It was found that the interplay of current and temperature determines whether the burn-in is marked by an increase or a decrease of the optical power. Moreover, both the reverse-bias leakage current and the forward-bias leakage current below the band-gap were observed to increase with time, whereas the emission spectra did not change significantly. It is assumed that these effects are related to the generation and/or movement of point defects rather than extended defects in the epitaxial heterostructure. To estimate the total operation lifetime of the LEDs the evolution of the optical power over time was extrapolated to the point where it should have decreased to 50% (L50 level) of the value after the burn-in phase. A L50 lifetime of > 4,000 hours and > 10,000 hours for current densities of 225 A/cm2 and 75 A/cm2, respectively, can be estimated. These numbers are encouraging as they demonstrate the robustness and high reliability of our UV-B LEDs.
Publications:
N. Lobo-Ploch, H. Rodriguez, C. Stölmacker, M. Hoppe, M. Lapeyrade, J. Stellmach, F. Mehnke, T. Wernicke, A. Knauer, V. Kueller, M. Weyers, S. Einfeldt, M. Kneissl, "Effective Thermal Management in Ultraviolet Light Emitting Diodes with Micro-LED Arrays", IEEE Trans. Electron Devices 60, 782 (2013).
T. Kolbe, F. Mehnke, M. Guttmann, C. Kuhn, J. Rass, T. Wernicke, M. Kneissl, "Improved injection efficiency in 290 nm light emitting diodes with Al(Ga)N electron blocking heterostructure", Appl. Phys. Lett. 103, 031109 (2013).
FBH research: 10.02.2014