Aerosol Metal Nanoparticles and their Role in Particle-Assisted Growth of III–V Nanowires
Abstract: Semiconductor nanowires have properties that make them potentially useful for applications in future electronic, photovoltaic, and optoelectronic devices. A powerful nanowire fabrication technique is the use of a nanoparticle as a preferential nucleation site, from which a nanowire grows. There has been significant progress in nanowire growth assisted by Au nanoparticles over the past 20 years. However, the high cost of gold and its incompatibility with silicon are obstacles to the industrial production of III–V nanowires. Therefore, significant efforts have been devoted to developing alternatives to Au-seeded nanowire growth. Although numerous metals and alloys are available, the development of alternatives to Au has been slow. The metal nanoparticles must have a high material quality and narrow size distribution, and their concentration must be controlled to enable precise experiments, and, in the future, high yields of identical devices.In the work described in this thesis, a spark discharge generator was used to synthesize and characterize a wide range of metal nanoparticles: Ag, Au, Bi, Co, Pb, Pd, Pt, Rh, and Sn, as well as three alloys: Ag25Au75, AgAu, and Ag75Au25. The nanoparticles were formed as an aerosol in an inert carrier gas of N2, and H2 was added to prevent the oxidation of base metals such as Bi, Co, and Sn. The nanoparticles were then either deposited on III–V substrates for nanowire growth using metal organic chemical vapor deposition, or used directly for gas- phase nanowire growth in an Aerotaxy reactor. The nanoparticle melting temperature is governed by its composition, primarily controlled by the choice of initial metal and the concentration of the group III material during nanowire growth. The nanoparticle composition was investigated after growth using transmission electron microscopy. For Pd-seeded GaAs or InAs a high group III content was found to be correlated with straight vertical nanowires, in contrast to kinked curly nanowires. Nearly identical InAs nanowires could be grown from In-rich Au and Pd nanoparticles, and the transition to curly nanowires could be triggered by reducing the In concentration. Initiating growth in a stable In-rich regime might provide a means of using other seed metals, which could reduce the cost, may be compatible with Si, and potentially add new benefits of the added seed metal. For example, the growth of GaAs nanowires by Aerotaxy from Au and AgAu nanoparticles is simple, but when pure Ag nanoparticles were used the nanowires developed kinks at an early stage of growth, possibly due to the higher melting point.
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