Formation mechanisms of low-dimensional semiconductor nanostructures grown by OMCVD on nonplanar substrates

Semiconductor quantum wires (QWRs) are promising structures for optoelectronics applications, since they can provide quantum confinement for charge carriers in two dimensions. The advantage that they offer over conventional quantum wells (QWs) is due to the sharper density of states characteristic of these structures, yielding narrower spectral lines and higher optical gain. However, to exhibit clear confinement characteristics, QWRs must meet stringent requirements in terms of size, uniformity and interfacial quality. Different methods have been explored for QWR fabrication. Techniques based on etching and regrowth suffer from defect incorporation into the lateral interfaces, since they are not formed in situ, and are limited in size by the lithographic features. On the other hand, growth of fractions of monolayers on vicinal substrates, although overcoming the above limitations, gives rise to size nonuniformities and graded interfaces. In this project, (In)GaAs/AlxGa1-xAs QWRs are obtained by organometallic chemical vapor deposition (OMCVD) growth of quantum wells on patterned, V-grooved substrates. In this way, the lithographically defined pattern serves as a seed for QWR formation. The self-ordering properties of OMCVD on nonplanar surfaces ensure the creation of a self-limiting profile at the bottom of the grooves, on which the wires are grown. This method overcomes the size limitations imposed by lithography, allows the in situ formation of interfaces and, thanks to the self-ordering mechanism, yields structures with high uniformity, whose characteristics are determined solely by the growth conditions. Although nonplanar growth has been employed for more than ten years for QWR fabrication, the understanding of the self-ordering mechanisms originating the profiles at the bottom of the grooves has been until now only phenomenological. The attainment of self-limiting profiles takes place via transients of the growth rates at the bottom of the groove. Current models of nonplanar growth can predict the formation, evolution or disappearance of facets at the 100nm-μm size. However, they cannot describe the transient behaviors at the nm scale that lead to self-limiting growth. This thesis project has been aimed at elucidating the physical mechanisms of this self-organized growth. A fundamental part of the project has been the creation of a wide experimental database to understand the dependence of the self-limiting profiles on the materials and growth conditions. The profiles at the bottom of the groove exhibit a faceted structure, consisting of a central (100) plane, surrounded by two {311}A ones. Cross-sectional transmission electron microscopy (TEM) shows that the bottom facets become wider as the growth temperature increases and as the Al mole fraction x of AlxGa1-xAs layers decreases. It appears therefore that surface diffusion is a key element in determining self-limiting growth. TEM cross sections show also that the establishment of self-lim