We demonstrate the possibility of extending the spectral range of luminescence due to InAs quantum dots ͑QDs͒ in a GaAs matrix up to 1.7 m. Realization of such a long wavelength emission is related to formation of lateral associations of QDs during InAs deposition at low substrate temperatures ͑ϳ320-400°C͒. © 1999 American Institute of Physics. ͓S0003-6951͑99͒00242-9͔ In recent years self-organized quantum dots ͑QDs͒ 1 have found significant interest. One of the important advantages of QDs is

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... eir potential to shift the emission wavelength beyond the range available for quantum well ͑QW͒ structures on the same substrate. 2-4 Recently 1.3 m GaAs-based InAs QD lasers are shown to have low threshold current densities ͑Ͻ100 A/cm 2 ͒. 2,3 High temperature stability of threshold current, low internal losses, and good differential efficiency are also demonstrated. 3 For applications in long-haul transmitters, cost-effective 1.55 m emitting GaAs-based lasers are desirable. High-power 1.48 m GaAs-based devices could serve as pumps for optical-fiber amplifiers. In this letter we show that a spectral range up to 1.75 m can be covered by InAs QDs in a GaAs matrix. The main approach to reach long wavelength emission is using lateral associates of QDs which were earlier observed in the upper stacks of vertically coupled QDs. 4 Samples are grown by elemental-source molecular-beam epitaxy ͑MBE͒ on GaAs ͑001͒ substrates using a Riber-32 MBE system. The evolution of the surface morphology is studied in situ, employing reflection high energy electron diffraction ͑RHEED͒. The growth procedure is as follows. After oxide desorption, a 0.5-m-thick GaAs buffer is grown at 600°C followed by a 2 nm/2 nm GaAs-AlAs short period superlattice ͑ten periods͒ and a 100 nm GaAs layer. Then the substrate temperature is lowered to 300-480°C and the QDs are deposited. Afterwards the dots are overgrown by 10 nm of GaAs at the same substrate temperature. The temperature is then increased again to 600°C and a 20-nm-thick GaAs layer is grown. This layer is followed by six periods of 2 nm/2 nm GaAs-AlAs short period superlattice and 5-nmthick GaAs cap layer for surface protection. The sample geometry enables optical studies as well as cross-section and plan-view ex situ characterization using transmission elec-tron microscopy ͑TEM͒ and high resolution electron microscopy ͑HREM͒. TEM and HREM studies are performed using a high voltage JEOL JEM1000 ͑1 MV͒ microscope. Photoluminescence ͑PL͒ is excited by an Ar ϩ laser and detected by Ge or InSb photodiodes. The spectra are corrected according to the spectral sensitivity curves. For photoluminescence excitation ͑PLE͒ experiments, samples are mounted into a continuous flow He cryostat at 6 K. PLE spectra are recorded using the light of a halogen lamp dispersed through a monochromator. Structural and optical properties of QDs deposited at temperatures of 460-520°C were studied in our previous work and by other researchers. 5,6 It was shown that deposition of 4 ML of InAs leads to the formation of a dense array of QDs having pyramidal shape with a square base. 6 The principal axes of the pyramid's base are close to the ͗100͘ or ͗010͘ directions. 6 For QDs grown at 520°C the average length of the pyramid base is 18 nm with the dot density about 2ϫ10 10 cm Ϫ2 . A reduction of the substrate temperature to 460°C leads to a decrease of the average length of the dot base to 12 nm and to an increase in the density of QDs to 1ϫ10 11 cm Ϫ2 . These QDs demonstrate bright PL emission in the range of 1.07-1.1 eV at 10 K. We note that the density of QDs deposited at 460°C is so high that lateral interaction effects via the strain fields during the QDs formation become important and govern the QD arrangement. 7 One can expect that a further reduction of substrate temperature during InAs deposition will result in a further increase in the density of QDs making the lateral interaction of QDs more important. Indeed, TEM studies of the samples deposited at 325-350°C demonstrate that the QD density can increase up to 10 12 cm Ϫ2 , while the QD size decreases down to 6-7 nm. However, an increase in the deposited thickness above 2 ML results in a qualitative change in the arrangement of QDs. Figure 1 shows plan-view ͑a͒ and the cross-section ͑b͒ TEM images of a sample with QDs formed by 4 ML of InAs deposited at 325°C. Individual QDs with a size of about 10 nm are seen in this image. Their areal density is about a͒ Also at the