Hot electron injection laser : variable carrier heating for high-speed, low-chirp direct modulation

The novel Hot Electron Injection Laser (HEL), a three-terminal vertically integrated transistorlaser structure, is designed to investigate and possibly utilize the effects of carrier heating on the optical gain and wavelength chirp. Simulations show the potential of carrier heating assisted gain switching to directly modulate the optical field intensity at frequencies up to 100Ghz while minimizing the parasitic wavelength chirp. The HEL is designed to demonstrate these results through independent but complementary control over both the concentration and the energy of the electrons injected into the active layer. It utilizes a strong electric field to accelerate the electrons and distributes their energy inside the active layer. There the energy is used to modulate the material gain and to control the wavelength chirp. The electrons are heated and cooled by increasing or decreasing the energy of the injected carriers. Both the effectiveness of the launcher to increase the temperature of the electrons inside the active layer as well as the effect of higher electron temperatures on the material gain are investigated here. The Hot Electron Injection Laser derives its properties from the vertical integration of a diode laser with a heterojunction bipolar transistor. Joining the layer stacks of these devices puts extra emphasis on the epitaxial design to ensure proper transistor and laser behavior as well as the required electron heating. The epitaxial design rules are deduced and explained. Fabricating the Hot Electron Injection Laser involves the actual epitaxial growth of the designed layer stack and the subsequent characterization of these layers. It also involves transferring the patterns of the mask design onto the grown wafer. The three-terminal Hot Electron Injection Laser differs strongly from any conventional two-terminal diode laser in that it puts stronger requirements on the epitaxial layers and that it requires the additional base contact to control the base potential and thus the electric field across the launcher. And in spite of its narrow elongated design, the processing should still result in homogeneous carrier injection and a constant base potential along the cavity of the laser. The correct vertical integration of the transistor and the laser has proven to be the most challenging part of this thesis. The basic transistor current-voltage curves were measured first. The measurements continued by obtaining the optical properties like optical power versus current curves, threshold current densities and the optical spectrum. Finally these result were used to estimate the carrier heating efficiency. The measurement results indicate a certain level of heating voltage induced gain switching to be present. The possible effects on which that gain switching could be based are discussed and estimates for their relative contributions are given. The heating voltage induced carrier heating is within the range of carrier heating predicted by simulations based on the Monte Carlo method. Compensating for other possible electric field induced gain switching, such as the Electro-Optic or Pockels effect and the Franz-Keldysh effect, the remaining carrier heating induced gain switching is smaller than expected. Various improvements to the current implementation are discussed to increase the carrier heating induced gain switching.