Spectral Properties of Proton Irradiated Gallium Nitride Blue Diodes

The permanent damage induced by 2 MeV proton irradiation at room temperature is reported for gallium nitride based blue emitting diodes (CREE model C430-DH85). Both optical and electrical device characteristics were measured. The I-V dependence was obtained as a function of temperature. At low voltages, the current is proportional to the exponential of the voltage at a constant temperature and the slope of the I-V curve is independent of temperature for the range 75-350 K, confirming the tunneling mechanism of the carrier injection. The room-temperature curve was studied as a function of 2-MeV proton irradiation in the fluence range 10/sup 11/ to 10/sup 15/ cm/sup -2/. It is hardly affected up to a fluence of 3/spl times/10/sup 12/ cm/sup -2/. Higher fluences do not affect the tunneling mechanism, but proton irradiation affects the saturation value of the current. The integrated electroluminescence versus voltage curves were obtained as a function of fluence, but the results were not amenable to a degradation constant interpretation. To gain insight into the degradation mechanism, the electroluminescence was analyzed spectrally and found to be the sum of the band-to-band transition in blue color at /spl ap/430 nm and a parasitic yellow band. The contribution of each transition was determined. The ratio of the contributions depends on driving current, temperature, and fluence. Treated individually, both the band-to-band and the yellow transition are related to fluence. The 2-MeV proton radiation damage constant is (7/spl plusmn/1)/spl times/10/sup -14/ cm/sup -2/ for the band-to-band and (2.0/spl plusmn/0.4)/spl times/10/sup -14/ cm/sup -2/ for the yellow transitions. The degradation of space charge recombination and diffusion of minority carriers cause the degradation of the electroluminescence. GaN light-emitting diodes (LEDs) are about two orders of magnitude more resistant to 2-MeV proton irradiation than GaAs LEDs.