ACES Int'l
Secrets & Tips
for Fiber Optics (Page 3)
Allrightee then...let’s go into the next room and see what’s going on in there. Stick close and don’t touch anything!
This room is where Etched-Well Surface LEDs are made. For data rates used in telecommunications (100MHz), the planar LED becomes impractical. These higher data rates usually call for fibers with cores on the order of 50-62 m m. If a planar LED is used, the broad emission pattern of several hundred micro-meters will only allow a few percent of the power to be launched into the small diameter fiber core. We could reduce the emission area of the planar LED, but that could lead to reliability problems. The increase in current density will cause a large temperature rise in the vicinity of the junction, and the thermal path from the junction to the die-attached header (through the confining layer and substrate) is not adequate to draw the heat away from the junction. Continuous operation at higher temperatures would soon increase the non-radiative sites in the LED and the efficiency would drop rapidly. If we mounted the LED chip upside down, the hot spot would be closer to the die-attached surface, but the light would have to pass through the thick substrate. The photon absorption in the substrate would reduce the output power significantly. A solution to this problem was developed by Burris and Dawson, of Bell Labs. The "etched well" or "Burrus" diode was created and is shown in Figure 4.
For the Burrus diode, the thick n-type substrate is the starting point in the wafer. Successive layers of aluminum-gallium-arsenide are grown epitaxially on the substrate. The layer functions (confinement, active, window) are essentially the same as in the planar structure. Watch closely and you’ll see that after the final p-type layer (contact) is grown, it is covered with a layer of SiO2. Small openings are then cut in the SiO2 to define the active emitting area. Metal is then evaporated over the wafer and contacts the p-layer through the small openings. The final processing consists of etching through the substrate. The etched wells are aligned over the active areas defined by the SiO2 openings on the underside of the wafer and remove the heavily-photon-absorptive substrate down to the window layer. As an indication of the delicacy of this operation, it requires double-sided alignment on a wafer about 0.1 mm thick with a final thickness in the opening of about 0.025 mm!
The radiation pattern from the Burrus diode is still
lambertian, but it has a remarkably small emitting area and enables coupling
into very small fibers down to 50 m m. The close proximity of the hot spot
(0.005 mm) to the heatsink at the die attachment makes it a reliable structure. Figure
5 shows some fiber couplings possible with the Burrus Diode.