Angular emission profiles and coherence length measurements of highly efficient, low voltage resonant cavity light emitting diodes operating around 650nm

We present results on Resonant Cavity Light Emitting Diodes (RCLEDs) emitting at 650nm, which have high efficiencies and low voltages. In particular, we report on the angular properties of these devices, and highlight the observation that overall spectral linewidth increases with collection angle. This unusual property of RCLEDs is largely a consequence of employing a microcavity in the design. An additional contributing factor is the relative distribution of gain amongst the cavity modes (i.e. the level of tuning or detuning of the underlying emission, defined with respect to the longitudinal cavity mode). We have used measurement techniques which spectrally resolve angular radiation profiles to determine the (de)tuning directly. Moreover, these profiles demonstrate how the overall spectral linewidth increases with collection angle. To this end, we have developed a semi-empirical method for determining the overall linewidth as a function of emission numerical aperture (NA). A 4nm detuned device has been investigated and linewidths have been found to increase from 3.1nm to 13.6nm over a range of NA approximate to0 to NA=1, an increase by a factor of around 4. Obviously, a variable Linewidth also implies a variable coherence length with NA. Consequently, the coherence length was found to decrease from 30 mum to 9 mum over the same range. Independent coherence length measurements were carried out by direct interferometric measurements, and confirmed the expected trends.