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b c of periods and repeat spacing of the compositionally modulated nanowire superlattices can be readily varied during growth. High-resolution TEM images show that GaAs/GaP junction regions exhibit a crystalline nanowire core without obvious defects and can be abrupt. Photoluminescence, electrical transport, and electroluminescence measurements of single nanowire superlattices suggest potential applications ranging from nano-barcodes to polarized nanoscale LEDs [14].

Bjork et al. reported the growth of InAs/InP nanowire superlattices in an ultrahigh vacuum chamber designed for chemical beam epitaxy (CBE) [12, 148]. The rapid switching between InAs and InP is obtained via a sequence where growth is interrupted as the indium source (TMIn) is switched off, followed by a change of As or P source. Finally, the supersaturation conditions, as a prerequisite for reinitiation of growth, are reestablished as the indium source is again injected into the growth chamber. Significantly, their lattice-resolved TEM images show that the interfaces are atomically abrupt (Fig. 10). The periods of the modulated nanowire structures range from 100 to just several nanometers. Electrical measurements of thermionic emission across an 80-nm-wide INP heterobarrier, positioned inside InAs whiskers 40 nm in diameter, yield a barrier height of 0.6 eV.

Wu et al. reported a hybrid pulsed laser ablation/chemical vapor deposition process for the synthesis of Si/SiGex nanowire superlattices [13]. Ge vapor is generated in pulsed ablation of a pure Ge target. A gas mixture of H2 and SiCl4 is continuously introduced into the reaction tube. Si precursors continuously deposit onto Au to induce the Si nanowire growth. During this growth process, if the laser is turned on, Ge vapor is generated and will react with Si precursors to grow SiGex. By periodically turning the laser on and off, Si/SiGex nanowire superlattices form. Solanki et al. focused on the growth of ZnSe/CdSe nanowire superlattices using an atomic layer deposition (ALD) technique [149]. ALD is a monolayer stepwise growth process that proceeds by exposing the substrate surface alternately to each precursor followed by an inert gas (N2) pulse to remove the excess species and by-products of the reaction [151]. The precursors for ZnSe and CdSe are elemental Zn, Se, and Cd.

Apart from the important contributions to materials science and nanostructure formation, these results could greatly increase the versatility and power of these building blocks in nanoscale electronic and photonic applications [150]. Potential applications include nano-barcodes, quantum cryptography and computing, "engineered" one-dimensional electron waveguides [14], heterostructures integrated in scanning probe- or field-emission, 1D-0D-1D resonant tunneling devices, one-dimensional superlattice arrays of intercoupled quantum dots [12, 148], nanoscale circuits, thermoelectrics [13], and light-emitting devices [13, 14, 149].

Figure 10. TEM image of an InAs whisker, 40 nm in diameter, containing four sequential InP barriers. The strain fields around each interface can be seen as elliptical, darker areas, having an extension of about 20 nm. In the thickest barrier the crystal is therefore completely relaxed. The magnified views prove that abruptness of the interfaces on the atomic scale is obtained. Reprinted with permission from [12], M. T. Bjork et al., Appl. Phys. Lett. 80, 1058 (2002). © 2002, American Institute of Physics.

Figure 10. TEM image of an InAs whisker, 40 nm in diameter, containing four sequential InP barriers. The strain fields around each interface can be seen as elliptical, darker areas, having an extension of about 20 nm. In the thickest barrier the crystal is therefore completely relaxed. The magnified views prove that abruptness of the interfaces on the atomic scale is obtained. Reprinted with permission from [12], M. T. Bjork et al., Appl. Phys. Lett. 80, 1058 (2002). © 2002, American Institute of Physics.

Semiconductor-metal, semiconductor-semiconductor junctions are important building blocks which can be used for rectifiers, light-emitting diodes, and photodetectors. Recently, one-dimensional nanojunctions with different materials have been fabricated.

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