Surface-emitting Burrus-type communication LEDs emitting at 870 nm
One of the first LED structures suitable for optical fiber communication was developed by Charles Burrus of AT&T Bell Laboratories (Burrus and Miller, 1971; Saul et al., 1985). The Burrus-type LED is shown in Fig. 23.1 (a) and consists of a double heterostructure with a GaAs active region grown lattice-matched on a GaAs substrate. The original structure proposed and demonstrated by Burrus was just a homojunction. However, such homojunction LEDs are no longer in use due to unwanted light reabsorption in the layers adjoining the active region.
The Burrus-type structure has several features making it suitable for communication applications. Firstly, the light is generated in an active region of small lateral extent. The lateral size of the active region is determined by the p-type ohmic contact size of the LED. If the p-type confinement layer is sufficiently thin, no current spreading occurs, and the lateral extent of the light-emitting region is the same as the contact size. By design, the lateral extent of the active region is smaller than the core diameter of the optical fiber to maximize coupling efficiency. Secondly, the opaque GaAs substrate is partially removed, as shown in Fig. 23.1, to reduce absorption of light in the substrate. The substrate can be thinned by chemically assisted mechanical polishing to about 150 thickness followed by a wet chemical etch. If the GaAs membrane created by etching is too thin, it tends to break easily during fiber coupling. On the other hand, a thick membrane reduces radiative efficiency due to absorption.
Figure 23.1 (b) shows a typical coupling arrangement of the Burrus-type LED to an optical fiber. Epoxy is used to permanently attach the fiber to the LED. Note that the p-type contact also serves as a sink for the heat generated in the active region. The heat sink is particularly efficient if the p-type contact includes a thick layer of electroplated gold.