The vapour-liquid-solid (VLS) mechanism, and analogues thereof, is the most commonly used route to semiconductor nanowire production. The VLS mechanism relies on a vapour phase precursor of the nanowire material, which impinges on a liquid phase seed particle, from which unidirectional nanowire growth proceeds. The choice of an appropriate seed material has the beneﬁt of allowing control over the diameter of the nanowires produced, while the seed materials can also signiﬁcantly aﬀect the crystalline quality of the nanowire. Analogues of the VLS mechanism include supercritical ﬂuid-liquid-solid (SFLS), supercritical ﬂuid-solid-solid (SFSS), solution-liquid-solid (SLS), vapour-solid-solid (VSS), and oxide assisted growth (OAG) mechanisms. Common to all of these analogues is the existence of a collector or seed particle, which acts as a sink for the nanowire material, and from which unidirectional growth proceeds. Conventionally, the seed particle is a metal with which the nanowire material or component thereof forms an alloy.
Graphical representation of VLS andVSS growth of nanowires.
In MCAG, our focus is to manipulate the growth, morphology and defect formation in nanowires, mainly Group 14 nanowires (Si and Ge). Our goal is to design new functional one-dimensional nanomaterials to address grand challenges in the area of future nanowire devices. Mainly supercritical fluid based bottom-up growth or liquid-injection chemical vapour deposition techniques are employed to achieve controlled growth of nanowires.
At MCAG we try to achieve superior nanowire growth kinetics through the manipulation of inherent thermodynamic and kinetic constraints in eutectic VLS-like nanowire growth. Different type of catalysts (Type A, B and C) could be combined for nanowire growth and defect engineering or to inject dopants through the seed. The figure below shows germanium nanowires grown from AuAg binary alloy catalyst, highlighting faster growth kinetics with Ag inclusion in the Au-Ge eutectic system. Ge equilibrium concentration and supersaturation was controlled through Ag inclusion in Au-Ge binary system.
Germanium nanowires growth from AuAg binary alloy catalysts
We also use in-situ transmission electron microscopy (TEM) to estimate and predict nanowire growth, diameter and defect formation in VLS paradigms, through catalyst shape deformation and the shape evolution of different interfaces. The figure below shows an example of different catalyst-nanowire interface shapes observed at growth temperature (450 ºC) in a Gatan 652 TEM heating stage for different sized and differently seeded nanowires.
In-situ TEM imaging of Ge nanowires
Another goal within MCAG is to control defect and polytype generation in group 14 nanowires. Here we use an epitaxial defect transfer process from catalyst seed to nanowire in sub-eutectic solid-seeded nanowire growth. A few examples of this defect transfer and formation of axial twinning in germanium nanowires are shown below. Our goal is to manipulate strain and electron mobility through twin boundary generation and to form crystal phase quantum dots in group IV nanowire platform.
Apart from these research on Ge and Si we are also exploring range of exciting materials such as phase change materials (GeTe, GeSbTe), direct band group IV materials etc. through bottom-up growth techniques.
Biswas, S.; O’Regan, C.; Morris, M. A.; Holmes, J. D. ‘In-situ observations of nanoscale Effects in germanium nanowire growth with ternary eutectic alloys’ Small 2015, 11 (1), 103-111.
O’Regan, C.; Biswas, S.; Petkov, N.; Holmes, J. D. ‘Recent advances in the growth of germanium nanowires: synthesis, growth dynamics and morphology control’ J. Mater. Chem. C 2014, 2 (1), 14-33.
O’Regan, C.; Biswas, S.; Barth, S.; Morris, M. A.; Petkov, N.; Holmes, J. D. ‘Size-controlled growth of germanium nanowires from ternary eutectic alloy catalysts’ J. Mater. Chem. C 2014, 2 (123), 4597-4605.
Biswas, S.; O’Regan, C.; Petkov, N.; Morris, M. A.; Holmes, J. D. ‘Manipulating the growth kinetics of vapor-liquid-solid propagated Ge nanowires’ Nano Lett. 2013, 13, 4044-4052.
Biswas, S.; Singha, A.; Morris, M. A.; Holmes, J. D. ‘Inherent control of growth, morphology and defect formation in germanium nanowires’ Nano Lett. 2012, 12, 5654-5663.
Barth, S.; Boland, J. J.; Holmes, J. D. ‘Defect transfer from nanoparticles to nanowires’ Nano Lett. 2011, 11, 1550-1555.