Growth Control and Heterostructures

General growth control An understanding of the fundamentals of nanowire growth is of greatimportance to optimize nanowire positioning (Fig. 3a), growth direction, morphology, and crystal structure (hexagonal/cubic). This will require further knowledge of the conditions under which one-dimensional crystals preferentially form, and how the various growth mechanisms differ from each other. In particular, the role of the commonly used metal collector particle must be better understood. The development of models for particle-free nanowire growth is also critical. Detailed investigation of the physical and chemical processesinvolved (adsorption, diffusion, interactions) is key to this understanding.   Heterostructures
	Figure 3. (a) Position-controlled InP nanowires grown on an InP substrate11.  (b) High-resolution transmission electron microscopy image of a low bandgap InAs nanowire (dark) with two InP tunnel-barrier segments (bright). (Part (a) reprinted with permission from [12]. © 2004 American Chemical Society.)
Nanowire heterostructures are mainly divided into two categories, axial and radial. Axial heterostructures, where the wire material is varied in the growth direction, are produced using normal nanowire growth techniques, including the common particle-assisted growth mechanism. Efficient strain relaxation in thin nanowires allows the combination of non-lattice-matched materials [13] (Fig. 3b). The band gap can thus be tuned locally along the length of a nanowire, giving access to completely new alternatives for device optimization, e.g., allowing formation of tunnel barrier and quantum confined structures. Radial heterostructures are formed under bulk-like conditions around an existing nanowire. These can, for example, be used to cap the nanowire with a higher bandgap material, either for passivation or modulation doping. Furthermore, strain from a lattice mismatch between the core and the shell may be used as an additional means of altering the energy structure within the nanowire.
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