The demonstration of Bose-Einstein condensation in atomic gases at micro-Kelvin temperatures is a striking landmark 1 while its evidence for semiconductor excitons 2-5 still is a longawaited milestone. This situation was not foreseen because excitons are light-mass boson-like particles with a condensation expected to occur around a few Kelvins 6, 7 . An explanation can be found in the underlying fermionic nature of excitons which rules their condensation 8 . Precisely, it was recently predicted that, at accessible experimental conditions, the exciton condensate shall be "gray" with a dominant dark part coherently coupled to a weak bright component through fermion exchanges 9 . This counter-intuitive quantum condensation, since

We have studied the influence of InP buffer-layer morphology in the formation of InAs nanostructures grown on InP͑001͒ substrates by solid-source molecular-beam epitaxy. Our results demonstrate that when InP buffer layers are grown by atomic-layer molecular-beam epitaxy, InAs quantum dot-like structures are formed, whereas InP buffer layers grown by MBE produce quantum-wire-like structures. The optical properties of these corrugated structures make them potential candidates for their use in light-emitting devices at 1.55 m. © 2000 American Institute of Physics. ͓S0003-6951͑00͒00309-0͔InAs nanostructures grown on InP͑001͒ are promising candidates for light emitting devices in the wavelength range 1.3-1.55 m. 1-3 A widely investigated technological approach is to use self-organized nanostructures that appear spontaneously as an efficient way to relax elastic strain in lattice-mismatched heteroepitaxy. In order to obtain a welldefined emission wavelength, nanostructures should have a good uniformity in size and shape. Ordering in the growth plane and a precise control of the nanostructure morphology is a key issue for self-organized systems. Due to a 3.2% lattice mismatch of InAs on InP͑001͒, elastic strain relaxation takes place above a certain critical thickness via a change of morphology from two-dimensional ͑2D͒ to threedimensional ͑3D͒. Much of the research developed on selforganized growth of InAs on InP substrates is devoted to quantum-dot ͑QD͒ formation. 1-4 However, very recent results 5 have shown that the chemical composition of the buffer layer ͑InP, InGaAs, or InAlAs͒ is determinant in configuring the final shape of the InAs self-organized nanostructures.In this letter, we demonstrate that the growth conditions of the InP buffer layer also controls the surface rearrangement of the strained InAs layer grown on top. Therefore, it is possible to obtain either QD or quantum-wire ͑QWr͒ structures for identical InAs coverage and growth conditions.There are two sets of samples grown for this work. In the first one, InAs was deposited on a 200-nm-thick InP buffer layer grown either by molecular-beam epitaxy ͑MBE͒ or by atomic-layer molecular-beam epitaxy ͑ALMBE͒ ͑Ref. 6͒ using solid sources. MBE buffer layers were grown in the 2ϫ4 surface reconstruction at T s ϭ460°C and at a beamequivalent pressure ͑BEP͒ (P 2 )ϭ5ϫ10 Ϫ6 Torr. For the ALMBE InP buffer layers, a substrate temperature T s ϭ400°C is used. The P 2 pulsed flux reaching the surface sample is controlled by means of the reflectivity difference ͑RD͒ technique in order to optimize surface stoichiometry for growing InP planar surfaces. 6 The InAs layers, 2.5 ͑ML͒ thick, were deposited at a BEP (As 4 )ϭ1.5-2ϫ10 Ϫ6 Torr, a growth rate of 0.5 ML/s, and T s ϭ400°C, which is chosen to minimize the P/As exchange. After InAs growth, an annealing at 480°C under arsenic pressure during 10-20 s was performed. We observe, in agreement with other authors, 3,4 that InAs growth takes place in a 2D mode. The 2D-3D transition occurs during the annealing process at T s ϭ48...

InAs self-assembled quantum wire structures have been grown on InP substrates and studied by means of photoluminescence and polarized-light absorption measurements. According to our calculations, the observed optical transitions in each sample are consistent with wires of different heights, namely from 6 to 13 monolayers. The nonradiative mechanism limiting the emission intensity at room temperature is related to thermal escape of carriers out of the wires.

We report in situ and in real time quantitative measurements of stress along [110] and [110] directions during the formation of InAs/InP(001) quantum wires (QWr) and consequent stress relaxation. Results show a strong stress anisotropy due to the distortion of As-In bonds along [110] and As-As dimerization along [110]. This anisotropy is claimed to be the origin of QWr formation instead of self-assembled quantum dots. Anisotropic stress relaxation associated to QWr formation is shown to be characteristic of heteroepitaxial systems involving different group V elements grown by MBE under group V stabilized surface (2x4 reconstruction).

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