Reduction Of Threshold Current Density Research Articles

In1−xGaxP1−zAsz double-heterojunction-laser operation (77 °K, yellow) in an external grating cavity

In1−xGaxP1−zAsz double-heterojunction lasers fabricated by liquid-phase epitaxy are operated at λ ≲6000 Å (77 °K) in an external grating cavity to provide wavelength-selective optical feedback. Diodes with and without antireflecting coatings and with heterobarrier heights of ∼20, ∼65, and ∼90 meV are examined. With external grating feedback, laser operation occurs at reduced threshold current densities and narrower linewidths and, depending upon heterobarrier height, can be tuned over an appreciable wavelength range. Within certain tuning limits on either side of line center, the carrier recombination is drawn to the grating wavelength and, as in earlier work, laser operation on a homogeneously broadened line is observed. Beyond these limits, particularly for excitation just above the threshold of stimulated emission, laser operation on an inhomogeneously broadened line can be distinguished. These observations showing that homogeneous and inhomogeneous line broadening occur in a semiconductor laser are accounted for by a simple model of the recombination kinetics.

Effect of gas-phase stoichiometry on the minority-carrier diffusion length in vapor-grown GaAs

The electron diffusion length in Zn-doped (p ∼1×1019 cm−3) vapor-grown GaAs has been found to depend strongly on the gas-phase stoichiometry used during epitaxial deposition. Specifically, near-stoichiometric or Ga-rich flow conditions provide diffusion lengths as large as 4.8 μm; this is about a factor of two larger than that of VPE GaAs prepared under commonly used As-rich flow conditions, and is comparable to that of LPE GaAs. Furthermore, the modified gas-flow conditions provide reduced threshold current densities for a room-temperature double-heterojunction laser of GaAs/In0.5Ga0.5P.

GaAs-AlGaAs double heterostructure lasers with taper-coupled passive waveguides

Taper-coupled laser structures suitable for integrated optics applications were grown by a liquid phase epitaxy process. Reduced threshold current densities, j0=6.3 kA/cm2 μm, and external differential quantum efficiencies ηd as high as 28% were measured with nominally undoped n-type active layers. Control units yielded j0=5.3 kA/cm2 μm with ηd as high as 37%. An internal quantum efficiency of 78% and a taper coupling efficiency ≳50% are deduced from the data.

Control of Optical Losses in p-n Junction Lasers by Use of a Heterojunction: Theory and Experiment

The optical losses of homojunction and heterojunction ``close-confinement'' GaAs injection lasers fabricated by liquid phase epitaxy have been described in terms of a simple waveguide model. By introducing an (AlGa) As–GaAs p+-p heterojunction (ΔEg∼0.1 eV) within ∼2 μ of the p-n junction, the room-temperature absorption loss is substantially reduced and the laser efficiency increased. This reduced absorption is due to combination of improved optical waveguiding and reduced absorption in the p+ material adjoining the active region. In addition, the laser gain coefficient can be increased in some cases by the addition of a heterojunction with the degree of improvement depending on the electron diffusion length, width, and doping level of the active region. The increased gain coefficient can contribute substantially to the reduced threshold current density of ``close-confinement'' lasers at room temperature. However, the relative contributions of reduced laser loss and increased gain coefficient to the improved performance depends critically on the details of the laser construction, and no generally valid conclusions can be made applicable to all heterojunction lasers. Values of the exterior differential quantum efficiency of 43% are obtained at 300°K, which means that much of the light internally generated is emitted. Because of the reduced internal loss, the spontaneous exterior efficiency is also greatly increased (a factor of 2–3). State-of-the art values of the threshold current density for the optimum structures are 10 000 A/cm2 for a cavity length of 20 mils, with values of 8000 A/cm2 in exceptional units. A power conversion efficiency at 300°K of 10% has been measured, which agrees with the theoretically predicted value. It is noteworthy that the diode series resistance is comparable in the solution-grown lasers with and without the heterojunction. The laser behavior has been studied as a function of varying acceptor concentration, donor concentration and bandgap energy discontinuity at the p+-p interface. Semiquantitative agreement for the laser loss between theory and experiment at 300° and 77°K is obtained for an active region width of ∼2 μ, using reasonable values of the index-of-refraction differences between the various laser regions and free-carrier absorption.

Annealing Effects of a Diffused GaAs Junction Laser

The annealing effect, namely the reduction of the threshold current densities in diffused type GaAs junction lasers at room temperature, was investigated. The optimum annealing temperature changes with the concentration of impurity in the substrate. It was found that annealing 2 h at 900°C and 2 h at 950°C is the best for 1.1×1018 and 9.3×1018 cm-3 Te doped substrates, respectively. In a lightly doped diode the improvement of internal quantum efficiency is decisive for the reduction of threshold current density. In a highly doped diode the reduction of diffraction loss should also be considered. Further band tailing does not occur in the lightly doped diode by annealing.