N. V. Sibirev

New Mode of Vapor−Liquid−Solid Nanowire Growth

We report on the new mode of the vapor-liquid-solid nanowire growth with a droplet wetting the sidewalls and surrounding the nanowire rather than resting on its top. It is shown theoretically that such an unusual configuration happens when the growth is catalyzed by a lower surface energy metal. A model of a nonspherical elongated droplet shape in the wetting case is developed. Theoretical predictions are compared to the experimental data on the Ga-catalyzed growth of GaAs nanowires by molecular beam epitaxy. In particular, it is demonstrated that the experimentally observed droplet shape is indeed nonspherical. The new VLS mode has a major impact on the crystal structure of GaAs nanowires, helping to avoid the uncontrolled zinc blende-wurtzite polytylism under optimized growth conditions. Since the triple phase line nucleation is suppressed on surface energetic grounds, all nanowires acquire pure zinc blende phase along the entire length, as demonstrated by the structural studies of our GaAs nanowires.

The role of surface diffusion of adatoms in the formation of nanowire crystals

A model of the formation of nanowire crystals on surfaces activated by droplets of the catalyst of growth is developed. In the model, the diffusion of adatoms from the surface of the substrate to the lateral surface of the crystals is taken into account. The exact solution of the diffusion problem for the flow of adatoms from the surface to the nanowire crystals is obtained, and the particular cases of the solution for the short and long diffusion lengths of adatoms, λs, are analyzed. A general expression for the length of the nanowire crystals, L, in relation to their radius R and to the conditions of growth is derived. The expression is applicable to a large variety of technologies of growth. The theoretical results are compared with the experimental dependences L(R) in the range of R = 20–250 nm for GaAs nanowire crystals grown by molecular-beam epitaxy on the GaAs (111) V Ga surface activated by Au. It is shown that, in some range of the parameters, the dependence L(R) follows the function 1/R2ln(λs/R), which is radically different from the classical diffusion dependence 1/R.

The Diffusion Mechanism in the Formation of GaAs and AlGaAs Nanowhiskers during the Process of Molecular-Beam Epitaxy

The formation of GaAs and AlGaAs nanowhiskers using molecular-beam epitaxy on GaAs (111)B surfaces activated with Au is theoretically and experimentally studied. It is experimentally shown that nanowhiskers whose length exceeds the effective thickness of the deposited GaAs by an order of magnitude can be grown. It is found that the experimental dependences of the nanowhisker length L on its diameter D can differ radically from those observed in the case of a vapor-liquid-solid growth mechanism. The L(D) dependences obtained in this study are decreasing functions of D. The above effects are related to the existence of the diffusion transport of atoms from the surface towards the tips of the whiskers, which leads to a considerable increase in the growth rate of thin whiskers. A theoretical model of the formation of nanowhiskers in the process of molecular-beam epitaxy is developed. The model provides a unified description of the vapor-liquid-solid and diffusion growth mechanisms and qualitatively explains the experimental results obtained.

Influence of shadow effect on the growth and shape of InAs nanowires

The influence of shadow effect originating from the neighboring nanowires on the nanowire growth is theoretically investigated. The nanowire axial and radial growth rates and the nanowire shape are shown to be strongly dependent on the nanowire surface density and the direction of incident flux. Theoretical predictions are compared with the experimental shapes of InAs nanowires grown by the Au-catalyzed molecular beam epitaxy. In particular, the barrel-like shape observed in dense arrays of InAs nanowires is well described by the model. Very importantly, we show that the shadow effect helps to avoid otherwise enabled radial growth and to preserve the cylindrical nanowire shape.

Composition-dependent interfacial abruptness in Au-catalyzed Si(1-x)Ge(x)/Si/Si(1-x)Ge(x) nanowire heterostructures.

As MOSFETs are scaled down, power dissipation remains the most challenging bottleneck for nanoelectronic devices. To circumvent this challenge, alternative devices such as tunnel field effect transistors are potential candidates, where the carriers are injected by a much less energetically costly quantum band to band tunneling mechanism. In this context, axial nanowire heterointerfaces with well-controlled interfacial abruptness offer an ideal structure. We demonstrate here the effect of tuning the Ge concentration in a Si1-xGex part of the nanowire on the Si/Si1-xGex and Si1-xGex/Si interfacial abruptness in axial Si-Si1-xGex nanowire heterostructures grown by the Au-catalyzed vapor-liquid-solid method. The two heterointerfaces are always asymmetric irrespective of the Ge concentration or nanowire diameter. For a fixed diameter, the value of interface abruptness decreases with increasing the Ge content for the Si/Si1-xGex interface but shows no strong Ge dependence at the Si1-xGex/Si interface where it features a linear correlation with the nanowire diameter. To rationalize these findings, a kinetic model for the layer-by-layer growth of nanowire heterostructures from a ternary Au-Ge-Si alloy is established that predicts a discrepancy in Ge concentration in the layer and the catalyst droplet. The Ge concentration in each layer is predicted to be dependent on the composition of the preceding layer. The most abrupt heterointerface (∼5 nm) is achieved by growing Si1-xGex with x = 0.85 on Si in a 25 nm diameter nanowire.

Kinetic model of the growth of nanodimensional whiskers by the vapor-liquid-crystal mechanism

A new kinetic model describing the growth of nanodimensional whiskers according to the vapor-liquid-crystal (VLC) mechanism is proposed. A self-consistent equation determining the whisker growth rate as a function of the liquid drop radius and the growth conditions is obtained. A manifold increase in the growth rate on the drop containing an activating substance as compared to the case of growth on the nonactivated crystal surface is explained. The proposed approach generalizes the phenomenological Givargizov-Chernov model and allows the functional form and the kinetic coefficients in the dependence of the growth rate on the control and energy parameters to be determined. The results of numerical calculations of the whisker growth rate as a function of the drop radius for various growth conditions are presented and compared to experimental data on the growth of nanodimensional GaAs whiskers on gold-activated GaAs(111)B surface.

Elastic energy relaxation and critical thickness for plastic deformation in the core-shell InGaAs/GaAs nanopillars

We report on the core-shell InGaAs/GaAs nanopillars grown by metal organic chemical vapor deposition on silicon substrates. The core diameter typically amounts to 600 nm, the shell thickness is around 160 nm, and the lattice mismatch amounts to 2% for the 20% In content used in our growth experiments for wurtzite crystal structure. Surprisingly, the transmission electron microscopy studies reveal an excellent crystal quality in the entire pillar with no noticeable defects even though the critical thickness for dislocation formation in GaAs shell is only 10 nm in the thin film case. To explain the observed effect, we develop a theoretical model that is capable of describing a huge increase of the critical thickness for plastic deformation owing to the core-shell geometry.

Hexagonal structures in GaAs nanowhiskers

We have studied the crystal structure of GaAs nanowhiskers grown by molecular beam epitaxy (MBE) on gold-activated GaAs(111)B substrates. The results of reflection high-energy electron diffraction and transmission electron microscopy showed that the MBE-grown GaAs nanowhiskers can form a crystal structure of sphalerite, wurtzite, or an intermediate phase close to 4H polytype, depending on the deposition conditions and the size of catalyst droplets. The results are interpreted within the framework of a thermodynamic model.

Threshold Behavior of the Formation of Nanometer Islands in a Ge/Si(100) System in the Presence of Sb

Atomic-force microscopy is used to study the behavior of an array of Ge islands formed by molecular-beam epitaxy on an Si (100) surface in the presence of an antimony flux incident on the surface. It is shown that, as the Sb flux increases to a certain critical level, the surface density of the islands increases; however, if this critical level is exceeded, nucleation of the islands is suppressed and mesoscopic small-height clusters are observed on the surface. This effect is explained qualitatively in the context of a kinetic model of the islands’ formation in heteroepitaxial systems mismatched with respect to their lattice parameters.

The initial stage of growth of crystalline nanowhiskers

The initial stage of formation of semiconductor nanowhiskers by the vapor-liquid-solid mechanism is studied theoretically and experimentally. It is shown that small-sized droplets either are overgrown or emerge together with the surface of the two-dimensional epitaxial layer. The geometric shape of the nanowhiskers at the initial stage of growth is calculated. Experimental data on the formation of GaAs nanowhiskers by molecular beam epitaxy on the GaAs(111)As substrates activated with Au are reported.