Introduction and history

The resonant-cavity light-emitting diode (RCLED) is a light-emitting diode that has a lightemitting region inside an optical cavity. The optical cavity has a thickness of typically one-half or one times the wavelength of the light emitted by the LED, i.e. a fraction of a micrometer for devices emitting in the visible or in the infrared. The resonance wavelength of the cavity coincides or is in resonance with the emission wavelength of the light-emitting active region of the LED. Thus the cavity is a resonant cavity. The spontaneous emission properties from a lightemitting region located inside the resonant cavity are enhanced by the resocant-cavity effect. The RCLED is the first practical device making use of spontaneous emission enhancement occurring in microcavities.

The placement of an active region inside a resonant cavity results in multiple improvements of the device characteristics. Firstly, the light intensity emitted from the RCLED along the axis of the cavity, i.e. normal to the semiconductor surface, is higher compared with conventional LEDs. The enhancement factor is typically a factor of 2–10. Secondly, the emission spectrum of the RCLED has a higher spectral purity compared with conventional LEDs. In conventional LEDs, the spectral emission linewidth is determined by the thermal energy kT. However, in RCLEDs, the emission linewidth is determined by the quality factor (Q factor) of the optical cavity.