Coexistence of vapor-liquid-solid and vapor-solid-solid growth modes in Pd-assisted InAs nanowires

Vapor-solid-solid growth dynamics in GaAs nanowires

Semiconductor nanowires are promising material systems for coming-of-age nanotechnology. The usage of the vapor-solid-solid (VSS) route, where the catalyst used for promoting axial growth of nanowires is a solid, offers certain advantages compared to the common vapor-liquid-solid (VLS) route (using a liquid catalyst). The VSS growth of group-IV elemental nanowires has been investigated by other groups in situ during growth in a transmission electron microscope (TEM). Though it is known that compound nanowire growth has different dynamics compared to elemental semiconductors, the layer growth dynamics of VSS growth of compound nanowires have not been studied yet. Here we investigate for the first time controlled VSS growth of compound nanowires by in situ microscopy, using Au-seeded GaAs as a model system. The ledge-flow growth kinetics and dynamics at the wire-catalyst interface are studied and compared for liquid and solid catalysts under similar growth conditions. Here the temperature and thermal history of the system are manipulated to control the catalyst phase. In the first experiment discussed here we reduce the growth temperature in steps to solidify the initially liquid catalyst, and compare the dynamics between VLS and VSS growth observed at slightly different temperatures. In the second experiment we exploit thermal hysteresis of the system to obtain both VLS and VSS at the same temperature. The VSS growth rate is comparable or slightly slower than the VLS growth rate. Unlike in the VLS case, during VSS growth we frequently observe that a new layer starts before the previous layer is completely grown, i.e., 'multilayer growth'. Understanding the VSS growth mode enables better control of nanowire properties by widening the range of usable nanowire growth parameters.

Nanowire morphology and particle phase control by tuning the In concentration of the foreign metal nanoparticle

Controllable particle assisted growth (PAG) of III-V nanowires is today almost exclusively done with Au, Ga or In nanoparticles, whereas other metals often yield nanowires with uncontrolled growth directions. To improve the control of the initial growth direction in PAG, independent of choice of metal, we propose to initiate nanowire growth from a group-III-rich foreign metal particle. For III-V nanowire growth, the group III concentration of the particle can be made to increase or decrease with the relative supply of group III and group V material, which can be used to promote the liquid phase that is necessary for vapor-liquid-solid growth. In this paper, 30 nm Pd nanoparticles are used to develop growth conditions for In-rich PAG of InAs nanowires. The particle size evolution for different growth times and V/III ratios is correlated with changes in nanowire density and morphology. In addition, we demonstrate In-rich Co, Pd, Pt and Rh nanoparticles and optimized In-rich PAG from Au and Pd seeds. The Au and Pd seeded nanowires are remarkably similar and by tuning the particle composition we trigger a morphological change. The vertical nanowire morphology is associated with In-rich nanoparticles that contain a liquid phase. The curly nanowire morphology, with random growth directions have an In concentration less than or equal to that of the most In rich compound of the seed metal-In system.

Polarity and growth directions in Sn-seeded GaSb nanowires

We here investigate the growth mechanism of Sn-seeded GaSb nanowires and demonstrate how the seed particle and its dynamics at the growth interface of the nanowire determine the polarity, as well as the formation of structural defects. We use aberration-corrected scanning transmission electron microscopy imaging methodologies to study the interrelationship between the structural properties, i.e. polarity, growth mechanism, and formation of inclined twin boundaries in pairs. Moreover, the optical properties of the Sn-seeded GaSb nanowires are examined. Their photoluminescence response is compared with one of their Au-seeded counterparts, suggesting the incorporation of Sn atoms from the seed particles into the nanowires.

Demonstration of Sn-seeded GaSb homo- and GaAs-GaSb heterostructural nanowires

The particle-assisted epitaxial growth of antimonide-based nanowires has mainly been realized using gold as the seed material. However, the Au-seeded epitaxial growth of antimonide-based nanowires such as GaSb nanowires presents several challenges such as for example direct nucleation issues and crystal structure tuning. Therefore, it is of great importance to understand the role of seed material choice and properties in the growth behavior of antimonide-based nanowires to obtain a deeper understanding and a better control on their formation processes. In this report, we have investigated the epitaxial growth of GaSb and GaAs-GaSb nanowires using in situ-formed tin seeds by means of metalorganic vapor phase epitaxy technique. This comprehensive report covers the growth of in situ-formed tin seeds and Sn-seeded GaSb nanowires on both GaAs and GaSb (111)B substrates, as well as GaAs-GaSb nanowires on GaAs (111)B substrates. The growth behavior and structural properties of the obtained GaSb nanowires are further investigated and compared with the Au-seeded counterparts. The results provided by this study demonstrate that Sn is a promising seed material for the growth of GaSb nanowires.

Tuning the growth mode of nanowires via the interaction among seeds, substrates and beam fluxes

The growth mechanism of semiconductor nanowires is still an argument of high interest, and it is becoming clearer, investigation after investigation, that simple pictures fail to describe the complex behaviors observed under different growth conditions. We report here on the growth of semiconductor nanowires, maintaining control over the chemical composition and the physical state of the metallic seeds, and tuning the growth mechanism by varying the growth conditions. We focused on Au-assisted ZnSe nanowires grown by molecular beam epitaxy on GaAs(111)B substrates. We show that at sufficiently high temperatures, the Au seed is strongly affected by the interaction with the substrate and that nanowire growth can occur through two different mechanisms, which have a strong impact on the nanowire's morphology and crystal quality. In particular, ZnSe NWs may exhibit either a uniformly oriented, straight morphology when the nanoparticle seed is liquid, or a kinked, worm-like shape when the nanoparticle seed is switched to a solid phase. This switch, which tunes the nanowire growth mechanism, is achieved by controlling the Zn-to-Se beam pressure ratio at the Au-seed surface. Our results allow a deeper understanding of particle-assisted nanowire growth, and an accurate control of nanowire morphology via the control of the growth mechanism.

Metal-seeded growth of III-V semiconductor nanowires: towards gold-free synthesis

Semiconductor nanowires composed of III-V materials have enormous potential to add new functionality to electronics and optical applications. However, integration of these promising structures into applications is severely limited by the current near-universal reliance on gold nanoparticles as seeds for nanowire fabrication. Although highly controlled fabrication is achieved, this metal is entirely incompatible with the Si-based electronics industry. In this Feature we review the progress towards developing gold-free bottom-up synthesis techniques for III-V semiconductor nanowires. Three main categories of nanowire synthesis are discussed: selective-area epitaxy, self-seeding and foreign metal seeding, with main focus on the metal-seeded techniques. For comparison, we also review the development of foreign metal seeded synthesis of silicon and germanium nanowires. Finally, directions for future development and anticipated important trends are discussed. We anticipate significant development in the use of foreign metal seeding in particular. In addition, we speculate that multiple different techniques must be developed in order to replace gold and to provide a variety of nanowire structures and properties suited to a diverse range of applications.

Defect-free <110> zinc-blende structured InAs nanowires catalyzed by palladium

We report the epitaxial growth of defect-free zinc-blende structured InAs nanowires on GaAs{111}(B) substrates using palladium catalysts in a metal-organic chemical vapor deposition reactor. Through detailed morphological, structural, and chemical characterizations using electron microscopy, it is found that these defect-free InAs nanowires grew along the <1[combining overline]1[combining overline]0> directions with four low-energy {111} faceted side walls and {1[combining overline]1[combining overline]3[combining overline]} nanowire/catalyst interfaces. It is anticipated that these defect-free <1[combining overline]1[combining overline]0> nanowires benefit from the fact that the nanowire/catalyst interfaces does not contain the {111} planes, and the nanowire growth direction is not along the <111> directions. This study provides an effective approach to control the crystal structure and quality of epitaxial III-V nanowires.