Communication LEDs emitting at 1300 nm, when used with graded-index silica fibers, are suited for high-speed data transmission. A communication LED structure emitting at 1300 nm is shown in Fig. 23.2 (Saul et al., 1985). The light is emitted through the InP substrate, which is transparent at 1300 nm. Accordingly, the device is mounted epi-side down in the LED package.

The device has a GaInPAs active region lattice matched to the InP substrate. No current - spreading layer is used so that the light-emitting region is located directly above the contact. At the light-exit point, an optical lens collimates the light to improve the LED-to-fiber coupling efficiency. The lens is etched into the InP substrate by a photo-electrochemical process (Ostermayer et al., 1983).

The emission spectra measured from the surface and from the edge of the device are shown in Fig. 23.3. The two emission spectra are markedly different. The spectrum emitted towards the edge has a smaller spectral width due to self-absorption. During self-absorption, predominantly photons of the high-energy part of the spectrum are reabsorbed.

Fig. 23.3. Emission spectra along the edge and surface of a GalnPAs/lnP communication LED emitting at 1300 nm. The spectrum emitted along the edge of the LED is narrower due to self-absorption.

A scanning electron micrograph of a GalnPAs/InP LED wafer is shown in Fig. 23.4. The surface of contacts displays roughness due to the annealing process that follows contact deposition. To reduce Fresnel reflection losses at the semiconductor-air boundary, the lens is anti-reflection coated.

Note that the wavelength 1500 nm is of interest for long-distance silica fiber communication. Long-distance communication fibers must have a small core diameter to be single mode and