The evolution of surface morphology in epitaxial InAs and GaAs strained layers grown on InP(001) was investigated by scanning tunneling microscopy. For compressive InAs layers, well-defined nanowires were formed at the onset of the 2D/3D growth mode transition. These wires have base width and height distributions peaked at around 230 Å and 20 Å, respectively. The sharp 2D/3D transition is associated with a large reorganization of matter. In contrast, for tensile GaAs layers the growth mode is characterized by a gradual roughening process.There is increasing interest in the formation of low-dimensional semiconductor structures, such as quantum dots or wires, which, due to quantum confinement effects, exhibit novel physical properties leading to important applications in optoelectronics. One of the promising paths towards the formation of such nanostructures is to exploit spontaneous self-organization phenomena in the early stages of the heteroepitaxial growth of strained semiconductor layers. For a relatively high intrinsic strain, the growth evolves beyond a critical thickness from an initial layer-by-layer growth mode to the formation of coherent 3D islands (Stranski-Krastanov growth mode). This strain-induced self-assembling method has been applied to a wide range of III-V heterostructures, such as In 1−x Ga x As/GaAs [1][2][3][4] or InP/GaAs [5,6]. Quantum dots which exhibit ordering in size, shape and lateral arrangement have been successfully produced by both the metalorganic vapor-phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) growth methods, and their optical properties have been largely characterized [7,8]. Although it was suggested by Tersoff et al.[9] that quantum wires might even be formed in that way, most studies of quantum wire formation have concentrated on the controlled epitaxial growth on patterned or vicinal surfaces [10][11][12]. For optoelectronic applications significant progress must still be achieved to control the size distribution of these 3D nanostructures. Therefore, * Corresponding author a good understanding of the physical processes that govern the strain-induced 3D islanding remains a real challenge.The In 1−x Ga x As/InP system offers a particularly interesting situation because it allows us to study the epitaxial growth of either compressively strained (x < 0.47) or tensilely strained (x > 0.47) layers. In that system the maximum attainable strain corresponds to compressively strained InAs/InP(001) layers (ε = −3.1%) and tensilely strained GaAs/InP(001) layers (ε = 3.7%). We study here the initial stages of growth for these two extreme cases, using scanning tunneling microscopy (STM) to characterize the growing crystal surface. The formation, in the compression case, of well-ordered nanowires at the onset of the 2D/3D growth mode transition will be more particularly described.Epitaxial layers were grown by molecular beam epitaxy in a RIBER 2300 system connected under ultrahigh vacuum to an STM chamber equipped with a beetle-type scanning tunneling microscope [13]. After t...

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