Capacitance-voltage measurements – revealing a promising approach to device optimization of UV LEDs
Fig 1: Typical image of a flip-chip mounted UV LED on an AlN submount
Fig. 2: Change in measured device capacitance due to a 200 h constant current stress as a function of heat sink temperature during stress
Fig. 3: Capacitance-voltage profiles of two p-contacts measured in series before (black) and after (red) stress of the LED-structure underneath one of these contacts
AlGaN-based light emitting diodes (LEDs) with ultraviolet (UV) emission at wavelengths between 250 nm and 320 nm are promising devices for a variety of applications such as water purification, gas sensing, and UV curing. However, the lifetime of these UV LEDs is still limiting their applicability.
Within the Joint Lab GaN Optoelectronics of FBH and the Institute of Solid State Physics at the TU Berlin, UV-B and UV-C LEDs with emission wavelengths of 310 nm and 265 nm have been developed. To enhance the lifetime, profound knowledge about physical mechanisms behind the stress-induced degradation is needed. Therefore, the UV-B and UV-C LEDs were exposed to an accelerated aging for 200 hours at a constant current density of 140 A/cm2 and different temperatures. It turned out that the reduction of the optical power is more pronounced and strongly temperature-dependent for UV-C LEDs compared to UV-B LEDs. To better understand this discrepancy, the LEDs have been analyzed before and after stress by means of capacitance-voltage (CV) measurements.
This method allows access to the widths of space charge regions (SCR) within the LEDs. Comparing the capacitances before and after stress reveals an impact of the temperature during the stress (Fig. 2). While the capacitance gets smaller for the investigated UV-C LEDs, what can be attributed to an enlargement of the SCR, the capacitance of the UV-B LEDs increased for stress temperatures ≤ 60°C. This opposite behavior indicates that at least two different degradation mechanisms are prominent in these devices.
Further investigations of the UV-B LEDs showed that the capacitance increases mostly at the beginning of stress. This leads to the question where exactly the changed SCR is located within the heterostructure. Therefore, we developed a method to measure solely the p-side of the stressed device. This allows to separate effects of the p-side from those of the pn-junction and the n-side of the heterostructure. Our investigations show (Fig. 3) that the initial increase in capacitance can be attributed to changes in the p-side of the LED. The stress is assumed to activate magnesium acceptors in the semiconductor. The described method offers new possibilities for degradation studies in UV LEDs. Especially the possibility to locate the changes in the LED structure should enable a more specific approach to device optimization.
This work was partially supported by the German Federal Ministry of Education and Research (BMBF) through the consortia project "Advanced UV for Life" under contracts 03ZZ0105A and 03ZZ0105B. Further support was given by the Federal Ministry for Economic Affairs and Energy (BMWi) through the project "UV-Berlin" under contract 03EFCBE067 as well as by the Deutsche Forschungsgemeinschaft within the Collaborative Research Center "Semiconductor NanoPhotonics" (CRC 787).
Publications:
J. Glaab, N. Lobo Ploch, J. Rass, T. Kolbe, T. Wernicke, F. Mehnke, C. Kuhn, J. Enslin, C. Stoelmacker, V. Kueller, A. Knauer, S. Einfeldt, M. Weyers, M. Kneissl, Influence of the LED heterostructure on the degradation behavior of (InAlGa)N-based UV-B LEDs, Proc. of SPIE 9748, 97481O-1 (2016).
D. Monti, M. Meneghini, C. De Santi, G. Meneghesso, E. Zanoni, J Glaab, J. Rass, S. Einfeldt, F. Mehnke, J. Enslin, T. Wernicke, M. Kneissl, Defect-Related Degradation of AlGaN-Based UV-B LEDs, IEEE Transactions on Electron Devices, Volume 64 (1), 200-205 (2017)