Identifying Crystallization- and Incorporation-Limited Regimes during Vapor–Liquid–Solid Growth of Si Nanowires

The vapor–liquid–solid (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 (365–480 °C), diameters (30–200 nm), and pressures (0.1–1.6 Torr SiH₄). We develop a kinetic model of VLS growth that includes (1) Si incorporation into the liquid Au–Si catalyst, (2) Si evaporation from the catalyst surface, and (3) Si crystallization at the catalyst–NW 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

Encoding Abrupt and Uniform Dopant Profiles in Vapor–Liquid–Solid Nanowires by Suppressing the Reservoir Effect of the Liquid Catalyst

Semiconductor nanowires (NWs) are often synthesized by the vapor–liquid–solid (VLS) mechanism, a process in which a liquid dropletsupplied with precursors in the vapor phase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 “reservoir effect” 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 (≤250 nm/min) and low reactor pressures (≤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

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

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 vapor–liquid–solid 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 vapor–solid–solid 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

Encoding Highly Nonequilibrium Boron Concentrations and Abrupt Morphology in p‑Type/n-Type Silicon Nanowire Superlattices

Although silicon (Si) nanowires (NWs) grown by a vapor–liquid–solid (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 liquid–solid 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 × 10¹⁹ cm^–3, 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 ∼10 nm. The results presented herein enable the growth of complex, degenerately doped p–n junction nanostructures that can be explored for a variety of advanced applications, such as Esaki diodes, multijunction solar cells, and tunneling field-effect transistors

Synthetically Encoding 10 nm Morphology in Silicon Nanowires

Si nanowires (NWs) have been widely explored as a platform for photonic and electronic technologies. Here, we report a bottom-up method to break the conventional “wire” symmetry and synthetically encode a high-resolution array of arbitrary shapes, including nanorods, sinusoids, bowties, tapers, nanogaps, and gratings, along the NW growth axis. Rapid modulation of phosphorus doping combined with selective wet-chemical etching enabled morphological features as small as 10 nm to be patterned over wires more than 50 μm in length. This capability fundamentally expands the set of technologies that can be realized with Si NWs, and as proof-of-concept, we demonstrate two distinct applications. First, nanogap-encoded NWs were used as templates for Noble metals, yielding plasmonic structures with tunable resonances for surface-enhanced Raman imaging. Second, core/shell Si/SiO₂ nanorods were integrated into electronic devices that exhibit resistive switching, enabling nonvolatile memory storage. Moving beyond these initial examples, we envision this method will become a generic route to encode new functionality in semiconductor NWs

Synthetically encoded ultrashort-channel nanowire transistors for fast, pointlike cellular signal detection.

Nanostructures, which have sizes comparable to biological functional units involved in cellular communication, offer the potential for enhanced sensitivity and spatial resolution compared to planar metal and semiconductor structures. Silicon nanowire (SiNW) field-effect transistors (FETs) have been used as a platform for biomolecular sensors, which maintain excellent signal-to-noise ratios while operating on lengths scales that enable efficient extra- and intracellular integration with living cells. Although the NWs are tens of nanometers in diameter, the active region of the NW FET devices typically spans micrometers, limiting both the length and time scales of detection achievable with these nanodevices. Here, we report a new synthetic method that combines gold-nanocluster-catalyzed vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) NW growth modes to produce synthetically encoded NW devices with ultrasharp (nm) n-type highly doped (n(++)) to lightly doped (n) transitions along the NW growth direction, where n(++) regions serve as source/drain (S/D) electrodes and the n-region functions as an active FET channel. Using this method, we synthesized short-channel n(++)/n/n(++) SiNW FET devices with independently controllable diameters and channel lengths. SiNW devices with channel lengths of 50, 80, and 150 nm interfaced with spontaneously beating cardiomyocytes exhibited well-defined extracellular field potential signals with signal-to-noise values of ca. 4 independent of device size. Significantly, these "pointlike" devices yield peak widths of ∼500 μs, which is comparable to the reported time constant for individual sodium ion channels. Multiple FET devices with device separations smaller than 2 μm were also encoded on single SiNWs, thus enabling multiplexed recording from single cells and cell networks with device-to-device time resolution on the order of a few microseconds. These short-channel SiNW FET devices provide a new opportunity to create nanoscale biomolecular sensors that operate on the length and time scales previously inaccessible by other techniques but necessary to investigate fundamental, subcellular biological processes.</p

Reversible Strain-Induced Electron–Hole Recombination in Silicon Nanowires Observed with Femtosecond Pump–Probe Microscopy

Strain-induced changes to the electronic structure of nanoscale materials provide a promising avenue for expanding the optoelectronic functionality of semiconductor nanostructures in device applications. Here we use pump–probe microscopy with femtosecond temporal resolution and submicron spatial resolution to characterize charge–carrier recombination and transport dynamics in silicon nanowires (NWs) locally strained by bending deformation. The electron–hole recombination rate increases with strain for values above a threshold of ∼1% and, in highly strained (∼5%) regions of the NW, increases 6-fold. The changes in recombination rate are independent of NW diameter and reversible upon reduction of the applied strain, indicating the effect originates from alterations to the NW bulk electronic structure rather than introduction of defects. The results highlight the strong relationship between strain, electronic structure, and charge–carrier dynamics in low-dimensional semiconductor systems, and we anticipate the results will assist the development of strain-enabled optoelectronic devices with indirect-bandgap materials such as silicon

Passivation of Nickel Vacancy Defects in Nickel Oxide Solar Cells by Targeted Atomic Deposition of Boron

Localized trap states, which are deleterious to the performance of many solar-energy materials, often originate from the under-coordinated bonding associated with defects. Recently, the concept of targeted atomic deposition (TAD) was introduced as a process that permits the passivation of trap states using a vapor-phase precursor that selectively reacts with only the surface defect sites. Here, we demonstrate the passivation of nickel oxide (NiO) with the TAD process using diborane gas for selective, low-temperature deposition of boron (B) under continuous flow in a chemical vapor deposition (CVD) system. NiO is a ubiquitous cathode material used in dye-sensitized solar cells (DSSCs), organic photovoltaic devices, and organo-lead halide perovskite solar cells. The deposition of B at 100 °C is shown to follow first-order kinetics, exhibiting saturation at a B to Ni atomic ratio of ∼10%. Electrochemical measurements, combined with first-principles calculations, indicate that B passivates Ni vacancy defects by partially saturating the bonding of the oxygen atoms adjacent to the vacancy. p-Type DSSCs were fabricated using TAD-treated NiO and show a modest improvement in photovoltaic performance metrics. The results highlight the potential ubiquity of TAD passivation with a range of atomic precursors and vapor-phase processes

Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy

We report the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a nondestructive method to map free-carriers in axially modulation-doped silicon nanowires (SiNWs) with nanoscale spatial resolution. Using this technique, we can detect local changes in the electrically active doping concentration based on the infrared free-carrier response in SiNWs grown using the vapor–liquid–solid (VLS) method. We demonstrate that IR s-SNOM is sensitive to both p-type and n-type free-carriers for carrier densities above ∼1 × 10¹⁹ cm^–3. We also resolve subtle changes in local conductivity properties, which can be correlated with growth conditions and surface effects. The use of s-SNOM is especially valuable in low mobility materials such as boron-doped p-type SiNWs, where optimization of growth has been difficult to achieve due to the lack of information on dopant distribution and junction properties. s-SNOM can be widely employed for the nondestructive characterization of nanostructured material synthesis and local electronic properties without the need for contacts or inert atmosphere