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Using the segmented contact method we separate and numerically evaluate the components making up the threshold current density dependence of quantum dot ridge waveguide lasers. An increasing internal optical mode loss and an increasing lateral out-diffusion current are the significant processes in ridges of widths between 4 and 10 μm, with no significant contribution from a deteriorating gain-mode overlap. By fitting a diffusion length model to the lateral out-diffusion process, we extract the ambipolar diffusion length, Ld, as a function of intrinsic carrier injection-level which covers carrier densities appropriate for functioning light-emitting diode and laser devices. The measured dependence fits a diffusion mechanism involving the thermal redistribution of carriers via the wetting-layer and most significantly leads to two regimes where Ld can be reduced in self-assembled quantum-dot systems. Only one of these is shown to be beneficial to the overall efficiency of the device, while the other is at the expense of undesired high-order nonradiative recombination processes at high injection-levels. Covering a peak modal gain range of approximately 5 to 11 cm−1 over injection-levels of 65 to 122 meV at 350 K, this dependence caused Ld to change from 0.75 to 1.50 μm, with the maximum occurring at 84 meV where the peak modal gain is 6 cm−1. Decreasing the temperature to 300 K reduced Ld to <0.75 μm over approximately the same injection-level range.

A strong variation in the lateral ambipolar diffusion length (L d ) with injectionlevel in quantum-dot lasers results in two regimes of low L d . Only one of these should be used for efficient laser operation.As the size of semiconductor lasers and integrated structures progressively reduces or where etching through the active region (for e.g. micro-pillar cavities or the integration of deep-etched photonic features) is necessary, 'lateral' carrier diffusion in the plane of the active region makes an increasingly important contribution to the overall performance and efficiency. Self-assembled quantum-dots (QDs) are anticipated to inhibit lateral diffusion by virtue of their zero-dimensionality 1 . In the literature, studies have attempted to compare the lateral ambipolar diffusion length (L d ) in QDs and shown it to be smaller than QWs 2,3,4 . However the values for L d range considerably (0.1 -5 µm) and there still remains some ambiguity in the mechanism of carrier transport in self-assembled quantum-dot material. To resolve this ambiguity, here we perform an investigation of the lateral ambipolar diffusion length at carrier densities appropriate for functioning LED and laser devices and demonstrate a strong variation in L d as a function of both the carrier injection-level and temperature. The observed dependence is consistent with a diffusion mechanism involving the thermal redistribution of carriers via the wetting-layer and most significantly results in two distinct regimes where L d can be reduced, with only one of these being beneficial to overall device efficiency.The approach we use to measure L d is in line with those which fit the threshold current density dependence on ridge laser width with a standard ambipolar diffusion length model. However we use the segmented contact method 5 to separate and numerically evaluate the components making up threshold current density and their dependence on ridge width i.e. that of an increasing internal optical loss, deteriorating gain mode overlap, and increasing fraction of lateral carrier out-diffusion loss. This allows us to unambiguously apply a diffusion length model to the out-diffusion process alone, and moreover perform the analysis as a function of injection-level rather than being restricted to a single operating point such as threshold.Measurements of the internal optical loss are made from the net modal gain and loss spectra. At low energy, where the optical gain and absorption tend to zero, both spectra tend to the internal optical loss, α i , as illustrated in Fig. 1. In ridges between 10 and 4 µm in width we find an increase in α i by a factor of 2.3 to be one of the significant mechanisms in the threshold dependence with ridge width. To take account of this component and evaluate the other mechanisms, we analyze the characteristics as a function of the quasi-Fermi level (ΔE f ) separation normalized to half of peak ground state absorption (E trans ). This was introduced in [6] as a means to express the intrinsic carrier injection-level (or relative ...

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