Project

Discipline

quantum heterostrucures; growth direction; growth mechanisms; molecular beam epitaxy; nanowires

Lead
Croissance de nanofils en semiconducteurs III-V dans la direction (100)
Lay summary
Les nanofils sont des cristaux en forme de filament avec un diamètre entre quelques et quelques centaines de nanomètre. Grâce à leur forme particulière et taille, ils sont particulièrement interessants pour beaucoup d'applications en électronique et optoelectronique. Dans ce projet nous addressons la question que les nanofils sont normalement orientés dans la direction cristallographique (111)B et que les substrats de l'industrie electronique sont orientés dans la direction (100). Nous allons investiguer comment modifier la croissance de ces nanofils pour les adapter aux substrats de l'industrie. Finalement, la croissance en diection (100) aura le deuxième effet positiv de bloquer la formation de défauts d'empliement, et donc la qualité des matériaux sera encore meilleure.

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Last update: 02.10.2016

Active participation

Title	Year

Semiconductor nanowires are filamentary crystals with a tailored diameter in the range from a few to about 100 nm. Thanks to their special morphology and reduced diameter, they hold great promise as building blocks for next generation applications in the electronic, optoelectronic, energy harvesting and storage domain. Research in semiconductor nanowires has made an extraordinary progress in the last decade. Many milestones have been achieved, and industry is progressively interested in these nanostructures. Semiconductor industry, including electronics and solar cell industry, builds devices mostly on (100) oriented substrates. However, nanowires grow mostly in the (111) direction. Recipes for nanowire growth in the (100) direction would significantly ease the integration of nanowires into existing technology platforms. They would for example enable the integration of III-V semiconductors on the CMOS platform. An additional advantage of growing nanowires in the (100) direction is that a growth front in this direction locks the formation of stacking defects (details in the proposal). As a consequence, defect-free single crystalline nanowires can be obtained. Today, III-V (100) oriented nanowires have mostly been obtained by the use of Au-assisted vapor-liquid-solid (VLS) method. Au preferentially gathers vapor precursors that decompose underneath in the form of a solid nanowire. Growth along the (100) direction has been achieved by the Samuelson and Bakkers groups by careful modification of the contact angle of the Au droplet with the substrate/nanowire. This has been obtained by adjusting the surface energies of both the substrate prior to growth and the Au droplet (for example by alloying it with lower energy elements or surfactants). Still, the use of Au in the fabrication of electronic or photonic semiconductor devices is incompatible with the current semiconductor industry. Nanowires obtained in a Au-free manner would offer a much larger technological impact. In our group, we grow III-V nanowires by the so-called self-assisted method where one intentionally avoids the use of Au. For example, for VLS growth of GaAs nanowires along (111) direction, Au droplets have successfully been replaced by Ga droplets. In this project, we propose to go beyond these earlier studies and engineer the self-assisted growth of III-As nanowires in the (100) direction to obtain a high yield of defect-free nanowires. Defect-free nanowires are key for e.g. high-efficiency solar cells and nanowire-based quantum information devices where electron scattering at crystal defects needs to be avoided. To gain technological relevance, large amounts of defect-free nanowires, as proposed here, will be required. In our research plan, we will explore the two materials GaAs and InAs that provide us with different band gaps, effective electron masses, Schottky Barrier heights and spin-related properties (Landé factor, spin-orbit coupling strength). In bulk form, these materials are known for instance for their excellent optoelectronic and high-frequency characteristics, respectively. We intend to answer to following fundamental questions:?What are the underlying mechanisms determining the growth direction of III-As nanowires??What is the effect of the initial contact angle of the Ga droplet in the growth direction of GaAs nanowires on (100) substrates??Is it possible to influence the growth direction of InAs nanowires by using an In droplet at the initial stage of growth??Do the initial conditions for nucleating a (100)-oriented nanowire change for steady-state growth??Does the contact angle of the VLS droplet have an impact on the shape of the liquid-solid interface? If yes, how does this affect the preferred growth direction? ?What growth directions are physically limited due to surface energy considerations??Is it possible to grow (100)-oriented III-V nanowires in an ordered fashion by pre-patterning a substrate and in a self-assisted fashion??Do (100) oriented III-As nanowires provide new possibilities for quantum heterostructures?