Temperature-insensitive luminescence linewidth from highly-uniform strain-reduced InAs quantum dots

Because quantum dots (QDs) grown by self-assembling techniques show high emission efficiency, they are now being incorporated into optical devices such as laser diodes). However, obvious characteristic improvements still have not been made mainly because of a wide inhomogeneous broadening which originates from the dot size distribution. When the dot size uniformity is improved so that the luminescence linewidth becomes narrower than the width of ordinary quantum wells (QWs), a reduced threshold current and a high characteristic temperature will be expected in QD lasers. Recently, highly-uniform InAs QDs have been fabricated by using a strained coverage layer (InGaAs) instead of the usual GaAs-based layer. The strain of InAs is reduced due to expansion toward the surface resulting in long-wavelength emission from structures on GaAs substrates. A narrow linewidth of 21 meV is observed in InAs QDs covered with 7 nm-thick strained In/sub 0.2/Ga/sub 0.8/As, presumably because of suppression of In segregation and interface diffusion. Typical photoluminescence (PL) spectra are reported. The linewidth is reduced by almost half in the GaAs-covered QD structure and is almost comparable to the best results observed in ordinary QW structures. Thus, using these strain-reduced QDs will improve device characteristics by reducing the state density of carriers.