Using a combination of a seed layer, low-growth rates, and different growth temperatures, we have produced InAs/GaAs quantum dots ͑QD's͒ that emit at very long wavelengths ͑up to 1.39 m at 293 K͒ with an ultranarrow inhomogeneous broadening ͑full width at half maximum of 14 meV at 10 K͒. The results are discussed in terms of strain relaxation and reduced In/Ga intermixing in the second layer. These two phenomena are interrelated and their control is crucial for achieving long wavelength emission. The QD structures also exhibit interlayer electronic coupling effects. Finally, combining this method with the use of InGaAs in the barrier instead of GaAs, emission wavelengths around 1.5 m at 293 K have been achieved.

Atomic force microscopy and photoluminescence spectroscopy ͑PL͒ have been used to study asymmetric bilayer InAs quantum dot ͑QD͒ structures grown by molecular-beam epitaxy on GaAs͑001͒ substrates. The two QD layers were separated by a GaAs spacer layer ͑SL͒ of varying thickness and were grown at different substrate temperatures. Grown independently, these two layers would exhibit a widely different QD number density, and this technique therefore enables us to assess the influence of the strain fields created by the dots in the first layer on the second-layer QD nucleation and characteristics. For very large SLs ͑Ͼ40 nm͒, total strain relief causes the QD nucleation to be controlled exclusively by the substrate temperature, which influences the migration of In adatoms. In this case, the optical and morphological properties of the second QD layer are identical to a structure with a single QD layer grown at the same temperature. In structures with a much smaller SL, strain effects dominate over the effect of temperature in controlling the nucleation of the QDs, thereby fixing the second-layer QD number density to that of the first ͑templating effect͒. There is also evidence that strain relaxation is present in the QDs of the second layer and that this is crucial for extending their emission wavelength. The optimum SL thickness is shown to be 11 nm, for which low-temperature PL emission peaks at 1.26 m, with a full width at half-maximum of only 15 meV. Intermediate SL thicknesses exhibit broad QD size distributions, with strain effects only partly influencing the QD growth in the second layer.

The dependence of the optical properties of InAs/GaAs quantum dot ͑QD͒ bilayers on seed layer growth temperature and second layer InAs coverage is investigated. As the seed layer growth temperature is increased, a low density of large QDs is obtained. This results in a concomitant increase in dot size in the second layer, which extends their emission wavelength, reaching a saturation value of around 1400 nm at room temperature for GaAs-capped bilayers. Capping the second dot layer with InGaAs results in a further extension of the emission wavelength, to 1515 nm at room temperature with a narrow linewidth of 22 meV. Addition of more InAs to high density bilayers does not result in a significant extension of emission wavelength as most additional material migrates to coalesced InAs islands but, in contrast to single layers, a substantial population of regular QDs remains.

Reflection high-energy electron diffraction measurements of the critical thickness crit for quantum dot ͑QD͒ formation have been used to quantify the effects of indium segregation and strain on the growth of bilayer InAs/ GaAs͑001͒ QD structures. These are not straightforward to deconvolute, because of the complex issues that arise during the growth and capping of the QDs. Segregation and out diffusion of In from buried QDs are shown to occur for GaAs thicknesses up to ϳ6 nm at 580°C. The existence of a floating In adlayer on the surface of the GaAs-capping layer as a result of In segregation is apparent at much lower substrate temperatures ͑510°C͒. The relative contribution of both segregation and strain on the reduction of crit during the growth of a second InAs layer is assessed. Compared with segregation, strain from the buried QDs can be measured through significantly larger capping thicknesses ͑ϳ30 nm͒ under these conditions.

We show that it is possible to obtain emission up to 1.5 µm from InAs/GaAs quantum dots using a combination of low growth rates, a seed layer, variable substrate temperature, and InGaAs capping. These strain engineered structures exhibit a remarkably small linewidth (14 meV) consistent with islands that are very uniform in both composition and size.1 Introduction Optoelectronic components for telecommunications applications around 1.3 and 1.55 µm are currently based on InP substrates. Over the last few years it has been shown that InAs/GaAs quantum dots (QDs) are capable of reaching the lower wavelength and although commercial products are beginning to become available, there are still problems that need to be overcome in order to exploit fully the potential of QD emitters. Key issues are the low gain associated with the QD ground state (GS) and the large inhomogeneous broadening which means that only a subset of the ensemble contributes to the gain at the operating wavelength.We have pioneered a method of achieving emission around 1.3 µm based on reducing the growth rate of the QDs which results in relatively large islands with a composition that is close to pure InAs [1, 2]. There is a concomitant reduction in the inhomogeneous linewidth to ~25 meV consistent with the increased island height but the island density is reduced by a factor of three. This exacerbates the problem of low gain, which we have attempted to overcome by growing multilayer samples optimised so that each layer emits at the same wavelength [3]. Other techniques such as atomic layer epitaxy [4], seed layers [5], strain reducing layers [6] and dots in a well (DWELL) [7] have also demonstrated emission around 1.3-1.35 µm but this appears to be the limit for structures which show good room temperature emission. In this paper we demonstrate that the emission can be extended to 1.5 µm by relieving the strain using a seed layer and by growing and capping at a low temperature with InGaAs. The seed layer also dictates the dot density allowing independent control of density and size (composition). The linewidth is reduced to ~14 meV. These results suggest that QDs may become the active layer of choice for telecommunications applications.