As an alternative to 3D structures realized by lithography (‘top-down’), 1D nanostructures that are grown ‘bottom-up’ are proposed to be evaluated for application in future technologies. During the last ten years, significant progress has been made in the realization of appropriate 1D nanostructures such as carbon nanotubes, and semiconductor nanowires and nanofibers. However, for their use in various applications, they must possess certain material properties, such as mechanical and structural properties, a desired purity level, or, especially for electronic applications a desired band gap. By selecting the appropriate growth method and the relevant process conditions of the method, it is possible to control the properties of the 1D nanostructure as well as the physical dimensions.

Vapor-liquid-solid (VLS) growth of 1D nanostructures The VLS mechanism is based on the catalytic growth of 1D nanostructures from metal nanoparticles which are heated beyond the eutectic temperature of the metal-semiconductor alloy. The liquid alloy cluster serves as a preferential site for adsorption of reactant from the vapor phase and - when supersaturated - as the nucleation site for crystallization. The supersaturation of the eutectic melt, which is established by catalytic absorption of the gaseous reactants, acts as the driving force for growth in a highly anisotropic manner. The inherent 1D self-organized structures combine high crystalline quality with good controllability of both length and diameter, which may vary over a wide range according to the template used.

As a partner of an ongoing national and transnational project on vertical 1D nanostructures we established three nanowire growth systems:

  • A low pressure CVD (LPCVD) system for the synthesis of Si and Ge nanowires: Currently four gas lines are available for an inert gas, H2 and 2%SiH4 and 2%GeH4 in He. The system is pumped by a turbo molecular pump and an oil free roughing pump. Pressure and gas flow controlled by mass flow controller could be varied over a wide range and the maximum growth temperature is 1000°C.
  • An atmospheric pressure CVD (APCVD) furnace for the synthesis of wide band gap (WBG) materials such as Ga2O3 or ZnO.
  • For the hierarchical heterostructures comprising GaAs, InAs, GaAlAs, an MBE system (Mod Gd. II) and is available at our institute.

Epitaxial and orientation controlled growth of single crystal silicon nanowires has been achieved on Si (111), Si (100) and Si (110) with by gold-gallium nanoparticle catalyzed chemical vapor deposition with a SiH4 precursor. The diameter and length distribution are narrowly dispersed at any growth stages and growth temperatures.
Cross section view of epitaxially grown nanowires on Si (110) substrate.
Tilted view (75°) of highly oriented nanowires perpenticular to the Si (111) substrate surface.
HRTEM demonstrates that these high quality NWs grow mainly along the (111) direction with a large proportion of them even perpendicular to the substrate.
Closer view on the interface between catalyst and nanowire.
By the same method we achieved also epitaxial growth of Ge nanowires on Si and Ge substrates. Size and length of the Ge-NWs grown on Si (111) substrates are very homogeneous and show no kinks. The Ge nanowires grow in the (111) direction and all orientations of the NWs along the four ?111? directions could be observed with a large proportion of the NWs growing perpendicular to the substrate. The closer view on a single NW shows that the NWs are highly tapered for a growth temperature of 340°C. By making a particular choice of growth conditions, it is possible to realize either rod-like or tapered nanowires.
Very recently wide band gap semiconducting Ga2O3 nanowires, nanorods, and nanosheets have been fabricated by an oxygen assisted carbothermal reduction process using Ga2O3 powders mixed with graphite as starting materials at a temperature of 1000°C. SEM, TEM images and SAED show that the nanostructures are pure, thin, uniform, and single-crystalline.

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