TECHNOLOGY TELECOMS


recombination, which dominates conventional telecom lasers and SOAs, wasting about 80 percent of the input electrical power. Thanks to this energy saving, our devices will generate far less heat, and will not require power-hungry thermoelectric coolers for temperature control.


Addressing Auger The energy sapping, Auger loss mechanism that severely degrades the efficiency in today’s InP- based devices stems from the recombination of an electron in the conduction band and a heavy-hole in the valence band (see Figure 1). Instead of interacting to emit a photon, this pair of oppositely charged carriers excites a hole from near the valence band maximum into the spin-split-off band. The hole then relaxes, releasing energy in the form of heat.


Figure 1: (Left) Auger recombination is the dominant loss mechanism in conventional InP-based lasers. Here a conduction electron (1) recombines with a valence hole (2), with the released energy exciting a valence hole (3) to the spin-split-off band (4) instead of creating a photon. (Right) This Auger loss mechanism can be suppressed when the spin-orbit-splitting energy exceeds the band gap. When this occurs, the Auger recombination process is forbidden, due to conservation of energy, because the energy required to excite a hole to the spin-split-off band exceeds the energy produced by electron-hole recombination


Over the years, incremental approaches have been pursued to reduce these Auger-related inefficiencies, but they have failed to address its fundamental cause: It originates from the electronic band structure associated with the constituent materials in the device’s active region. Manipulating the band structure is the only way to tackle this issue head-on – and that is what we are doing by turning to the unique properties of bismuth-containing alloys, which enable the design of Auger-free lasers.


One attractive attribute of bismide alloys is the behaviour of their energy gap: It decreases very rapidly with bismuth composition, allowing growth of telecom lasers on a GaAs substrate. But even more important than that – and the key idea behind the suppression of the dominant Auger loss process – is that GaBi, in contrast to conventional III-V materials emitting in the near-infrared, is predicted to have a very large spin-orbit-splitting energy (see Figures 2 and 3). Its value, which is the difference between the valence band maximum and the lower lying spin-split-off valence band, is of the order of 2.2 eV – large and controllable


Kerstin Volz (left) and Peter Ludewig (right) standing next to the MOCVD tool at Philipps University Marburg, Germany.


54 www.compoundsemiconductor.net August / September 2013


Figure 2: Spin-orbit splitting energy for various III-V materials. The very large value for the spin-orbit-splitting energy ΔSO


for GaBi holds the key to suppressing Auger-related losses in GaBiAs/GaAs-based lasers


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