Spectral Behavior and Coherence Length of GaN-, and AlGaInP-based Light-Emitting-Diodes

The coherence length of electromagnetic waves created by different light sources is a largely overlooked parameter. Since the coherence length and its spectral distribution are essential for the entire field of interference, ranging from unintended destructive and constructive pattern to intended interference determining the nature of structural colors. We studied the spectral emission behaviour of GaN- and AlGaInP-based Light-Emitting-Diodes (LED) under thermodynamic equilibrium conditions driven by direct current as well as by short current pulses (500 ns) in the temperature range from 4.2 to 390 Kelvin. The coherence length under the different driving conditions was measured via a Fabry-Perot interference setup. We discuss the validity and limitations of the conventional determination method from the emission linewidth and lineshape. Besides the distinct shifts of the emission wavelength accompanied by significant changes of the full width at half maximum, we found quite high values for the coherence length exceeding 0.15 millimeters for the blue emission and 0.4 millimeters for the red emission at room temperature, respectively. Furthermore this contribution will also discuss the nature and interrelationships of coherence length, emission peak wavelength and the spectral distribution (lineshape and linewidth) of the investigated LEDs.