10 research outputs found
Identifying Crystallization- and Incorporation-Limited Regimes during VaporâLiquidâSolid Growth of Si Nanowires
The vaporâliquidâsolid (VLS) mechanism is widely used for the synthesis of semiconductor nanowires (NWs), yet several aspects of the mechanism are not fully understood. Here, we present comprehensive experimental measurements on the growth rate of Au-catalyzed Si NWs over a range of temperatures (365â480 °C), diameters (30â200 nm), and pressures (0.1â1.6 Torr SiH<sub>4</sub>). We develop a kinetic model of VLS growth that includes (1) Si incorporation into the liquid AuâSi catalyst, (2) Si evaporation from the catalyst surface, and (3) Si crystallization at the catalystâNW interface. This simple model quantitatively explains growth rate data collected over more than 65 distinct synthetic conditions. Surprisingly, upon increasing the temperature and/or pressure, the analysis reveals an abrupt transition from a diameter-independent growth rate that is limited by incorporation to a diameter-dependent growth rate that is limited by crystallization. The identification of two distinct growth regimes provides insight into the synthetic conditions needed for specific NW-based technologies, and our kinetic model provides a straightforward framework for understanding VLS growth with a range of metal catalysts and semiconductor materials
Waveguide Scattering Microscopy for Dark-Field Imaging and Spectroscopy of Photonic Nanostructures
Dark-field microscopy (DFM) is widely
used to optically image and
spectroscopically analyze nanoscale objects. In a typical DFM configuration,
a sample is illuminated at oblique angles and an objective lens collects
light scattered by the sample at a range of lower angles. Here, we
develop waveguide scattering microscopy (WSM) as an alternative technique
to image and analyze photonic nanostructures. WSM uses an incoherent
white-light source coupled to a dielectric slab waveguide to generate
an evanescent field that illuminates objects located within several
hundred nanometers of the waveguide surface. Using standard microscope
slides or coverslips as the waveguide, we demonstrate high-contrast
dark-field imaging of nanophotonic and plasmonic structures such as
Si nanowires, Au nanorods, and Ag nanoholes. Scattering spectra collected
in the WSM configuration show excellent signal-to-noise with minimal
background signal compared to conventional DFM. In addition, the polarization
of the incident field is controlled by the direction of the propagating
wave, providing a straightforward route to excite specific optical
modes in anisotropic nanostructures by selecting the appropriate input
wavevector. Considering the facile integration of WSM with standard
microscopy equipment, we anticipate it will become a versatile tool
for characterizing photonic nanostructures
Encoding Abrupt and Uniform Dopant Profiles in VaporâLiquidâSolid Nanowires by Suppressing the Reservoir Effect of the Liquid Catalyst
Semiconductor nanowires (NWs) are often synthesized by the vaporâliquidâsolid (VLS) mechanism, a process in which a liquid dropletî¸supplied with precursors in the vapor phaseî¸catalyzes the growth of a solid, crystalline NW. By changing the supply of precursors, the NW composition can be altered as it grows to create axial heterostructures, which are applicable to a range of technologies. The abruptness of the heterojunction is mediated by the liquid catalyst, which can act as a reservoir of material and impose a lower limit on the junction width. Here, we demonstrate that this âreservoir effectâ is not a fundamental limitation and can be suppressed by selection of specific VLS reaction conditions. For Au-catalyzed Si NWs doped with P, we evaluate dopant profiles under a variety of synthetic conditions using a combination of elemental imaging with energy-dispersive X-ray spectroscopy and dopant-dependent wet-chemical etching. We observe a diameter-dependent reservoir effect under most conditions. However, at sufficiently slow NW growth rates (â¤250 nm/min) and low reactor pressures (â¤40 Torr), the dopant profiles are diameter independent and radially uniform with abrupt, sub-10 nm axial transitions. A kinetic model of NW doping, including the microscopic processes of (1) P incorporation into the liquid catalyst, (2) P evaporation from the catalyst, and (3) P crystallization in the Si NW, quantitatively explains the results and shows that suppression of the reservoir effect can be achieved when P evaporation is much faster than P crystallization. We expect similar reaction conditions can be developed for other NW systems and will facilitate the development of NW-based technologies that require uniform and abrupt heterostructures
Encoding Highly Nonequilibrium Boron Concentrations and Abrupt Morphology in pâType/n-Type Silicon Nanowire Superlattices
Although
silicon (Si) nanowires (NWs) grown by a vaporâliquidâsolid
(VLS) mechanism have been demonstrated for a range of photonic, electronic,
and solar-energy applications, continued progress with these NW-based
technologies requires increasingly precise compositional and morphological
control of the growth process. However, VLS growth typically encounters
problems such as nonselective deposition on sidewalls, inadvertent
kinking, unintentional or inhomogeneous doping, and catalyst-induced
compositional gradients. Here, we overcome several of these difficulties
and report the synthesis of uniform, linear, and degenerately doped
Si NW superlattices with abrupt transitions between p-type, intrinsic,
and n-type segments. The synthesis of these structures is enabled
by in situ chlorination of the NW surface with hydrochloric acid (HCl)
at temperatures ranging from 500 to 700 °C, yielding uniform
NWs with minimal nonselective growth. Surprisingly, we find the boron
(B) doping level in p-type segments to be at least 1 order of magnitude
above the solid solubility limit, an effect that we attribute to a
high incorporation of B in the liquid catalyst and kinetic trapping
of B during crystallization at the liquidâsolid interface to
yield a highly nonequilibrium concentration. For growth at 510 °C,
four-point-probe measurements yield active doping levels of at least
4.5 Ă 10<sup>19</sup> cm<sup>â3</sup>, which is comparable
to the phosphorus (P) doping level of n-type segments. Because the
B and P dopants are in sufficiently high concentrations for the Si
to be degenerately doped, both segments inhibit the etching of Si
in aqueous potassium hydroxide (KOH) solution. Moreover, we find that
the dopant transitions are abrupt, facilitating nanoscale morphological
control in both B- and P-doped segments through selective KOH etching
of the NW with a spatial resolution of âź10 nm. The results
presented herein enable the growth of complex, degenerately doped
pân junction nanostructures that can be explored for a variety
of advanced applications, such as Esaki diodes, multijunction solar
cells, and tunneling field-effect transistors
Barrierless Switching between a Liquid and Superheated Solid Catalyst during Nanowire Growth
Knowledge
of nucleation and growth mechanisms is essential for
the synthesis of nanomaterials, such as semiconductor nanowires, with
shapes and compositions precisely engineered for technological applications.
Nanowires are conventionally grown by the seemingly well-understood
vaporâliquidâsolid mechanism, which uses a liquid alloy
as the catalyst for growth. However, we show that it is possible to
instantaneously and reversibly switch the phase of the catalyst between
a liquid and superheated solid state under isothermal conditions above
the eutectic temperature. The solid catalyst induces a vaporâsolidâsolid
growth mechanism, which provides atomic-level control of dopant atoms
in the nanowire. The switching effect cannot be predicted from equilibrium
phase diagrams but can be explained by the dominant role of the catalyst
surface in modulating the kinetics and thermodynamics of phase behavior.
The effect should be general to metal-catalyzed nanowire growth and
highlights the unexpected yet technologically relevant nonequilibrium
effects that can emerge in the growth of nanoscale systems
Probing Intrawire, Interwire, and Diameter-Dependent Variations in Silicon Nanowire Surface Trap Density with PumpâProbe Microscopy
Surface
trap density in silicon nanowires (NWs) plays a key role
in the performance of many semiconductor NW-based devices. We use
pumpâprobe microscopy to characterize the surface recombination
dynamics on a point-by-point basis in 301 silicon NWs grown using
the vaporâliquidâsolid (VLS) method. The surface recombination
velocity (<i>S</i>), a metric of the surface quality that
is directly proportional to trap density, is determined by the relationship <i>S</i> = <i>d</i>/4Ď from measurements of the
recombination lifetime (Ď) and NW diameter (<i>d</i>) at distinct spatial locations in individual NWs. We find that <i>S</i> varies by as much as 2 orders of magnitude between NWs
grown at the same time but varies only by a factor of 2 or three within
an individual NW. Although we find that, as expected, smaller-diameter
NWs exhibit shorter Ď, we also find that smaller wires exhibit
higher values of <i>S</i>; this indicates that Ď is
shorter both because of the geometrical effect of smaller <i>d</i> and because of a poorer quality surface. These results
highlight the need to consider interwire heterogeneity as well as
diameter-dependent surface effects when fabricating NW-based devices
Capillarity-Driven Welding of Semiconductor Nanowires for Crystalline and Electrically Ohmic Junctions
Semiconductor
nanowires (NWs) have been demonstrated as a potential platform for
a wide-range of technologies, yet a method to interconnect functionally
encoded NWs has remained a challenge. Here, we report a simple capillarity-driven
and self-limited welding process that forms mechanically robust and
Ohmic inter-NW connections. The process occurs at the point-of-contact
between two NWs at temperatures 400â600 °C below the bulk
melting point of the semiconductor. It can be explained by capillarity-driven
surface diffusion, inducing a localized geometrical rearrangement
that reduces spatial curvature. The resulting weld comprises two fused
NWs separated by a single, Ohmic grain boundary. We expect the welding
mechanism to be generic for all types of NWs and to enable the development
of complex interconnected networks for neuromorphic computation, battery
and solar cell electrodes, and bioelectronic scaffolds
Imaging Charge Separation and Carrier Recombination in Nanowire pâiân Junctions Using Ultrafast Microscopy
Silicon nanowires incorporating p-type/n-type
(p-n) junctions have
been introduced as basic building blocks for future nanoscale electronic
components. Controlling charge flow through these doped nanostructures
is central to their function, yet our understanding of this process
is inferred from measurements that average over entire structures
or integrate over long times. Here, we have used femtosecond pumpâprobe
microscopy to directly image the dynamics of photogenerated charge
carriers in silicon nanowires encoded with p-n junctions along the
growth axis. Initially, motion is dictated by carrierâcarrier
interactions, resulting in diffusive spreading of the neutral electronâhole
cloud. Charge separation occurs at longer times as the carrier distribution
reaches the edges of the depletion region, leading to a persistent
electron population in the n-type region. Time-resolved visualization
of the carrier dynamics yields clear, direct information on fundamental
drift, diffusion, and recombination processes in these systems, providing
a powerful tool for understanding and improving materials for nanotechnology
Ultrafast Carrier Dynamics of Silicon Nanowire Ensembles: The Impact of Geometrical Heterogeneity on Charge Carrier Lifetime
Ultrafast carrier dynamics in silicon
nanowires with average diameters
of 40, 50, 60, and 100 nm were studied with transient absorption spectroscopy.
After 388 nm photoexcitation near the direct band gap of silicon,
broadband spectra from 400 to 800 nm were collected between 200 fs
and 1.3 ns. The transient spectra exhibited both absorptive and bleach
features that evolved on multiple time scales, reflecting contributions
from carrier thermalization and recombination as well as transient
shifts of the ground-state absorption spectrum. The initially formed âhotâ
carriers relaxed to the band edge within the first âź300 fs,
followed by recombination over several hundreds of picoseconds. The
charge carrier lifetime progressively decreased with decreasing diameter,
a result consistent with a surface-mediated recombination process.
Recombination dynamics were quantitatively modeled using the diameter
distribution measured from each sample, and this analysis yielded
a consistent surface recombination velocity of âź2 Ă 10<sup>4</sup> cm/s across all samples. The results indicate that transient
absorption spectroscopy, which interrogates thousands of individual
nanostructures simultaneously, can be an accurate probe of material
parameters in inhomogeneous semiconductor samples when geometrical
differences within the ensemble are properly analyzed
Imaging Charge Separation and Carrier Recombination in Nanowire pâiân Junctions Using Ultrafast Microscopy
Silicon nanowires incorporating p-type/n-type
(p-n) junctions have
been introduced as basic building blocks for future nanoscale electronic
components. Controlling charge flow through these doped nanostructures
is central to their function, yet our understanding of this process
is inferred from measurements that average over entire structures
or integrate over long times. Here, we have used femtosecond pumpâprobe
microscopy to directly image the dynamics of photogenerated charge
carriers in silicon nanowires encoded with p-n junctions along the
growth axis. Initially, motion is dictated by carrierâcarrier
interactions, resulting in diffusive spreading of the neutral electronâhole
cloud. Charge separation occurs at longer times as the carrier distribution
reaches the edges of the depletion region, leading to a persistent
electron population in the n-type region. Time-resolved visualization
of the carrier dynamics yields clear, direct information on fundamental
drift, diffusion, and recombination processes in these systems, providing
a powerful tool for understanding and improving materials for nanotechnology