Boron-doped Silicon Nanowires Research Articles

Enhanced thermoelectric figure of merit in highly-doped silicon nanowires via a corrugated surface modulation

Silicon nanowire has attracted considerable attention in thermoelectric devices because the diameter reduction can reduce thermal conductivity with boundary scattering, improving the figure of merit. Nevertheless, there are some challenges in device fabrication due to the relatively low efficiency and the difficulties in mass production. Here, we suggest the silicon nanowire with a corrugated surface to overcome these drawbacks by using a deep reactive-ion etcher. Corrugated silicon nanowires can significantly increase the figure of merit, which is caused by the further reduction of the thermal conductivity originating from phonon backscattering. Therefore, the phosphorus- and boron-doped silicon nanowires with a diameter of 173 nm and 191 nm show very low thermal conductivities of 6.1 and 4.9 W·m–1·K–1, respectively. Notably, these values stand as the lowest values among silicon nanowires previously reported by dry etching at 300 K. As a result, the figure of merit reaches 0.314 for n-type and 0.268 for p-type at 400 K without the diameter scaling of a nanowire.

Binder-free boron-doped Si nanowires toward the enhancement of lithium-ion capacitor

Lithium-ion capacitors (LICs) are next-generation electrochemical storage devices that combine the benefits of both supercapacitors and lithium-ion batteries. Silicon materials have attracted attention for the development of high-performance LICs owing to their high theoretical capacity and low delithiation potential (∼0.5 V versus Li/Li+). However, sluggish ion diffusion has severely restricted the development of LICs. Herein, a binder-free anode of boron-doped silicon nanowires (B-doped SiNWs) on a copper substrate was reported as an anode for LICs. B-doping could significantly improve the conductivity of the SiNW anode, which could enhance electron/ion transfer in LICs. As expected, the B-doped SiNWs//Li half-cell delivered a higher initial discharge capacity of 454 mAh g−1 with excellent cycle stability (capacity retention of 96% after 100 cycles). Furthermore, the near-lithium reaction plateau of Si endows the LICs with a high voltage window (1.5–4.2 V), and the as-fabricated B-doped SiNWs//AC LIC possesses the maximum energy density value of 155.8 Wh kg−1 at a battery-inaccessible power density of 275 W kg−1. This study provides a new strategy for using Si-based composites to develop high-performance LIC.

Determination of the Free Charge Carrier Concentration in Boron-Doped Silicon Nanowires Using Attenuated Total Reflection Infrared Spectroscopy

Attenuated total reflection infrared spectroscopy is used to determine the free charge carrier concentration in arrays of silicon nanowires with characteristic transverse sizes of 50–100 nm and a length of the order of 10 μm formed on lightly doped crystalline p-type silicon by metal-assisted chemical etching and subjected to the additional thermal-diffusion doping of boron at temperatures of 850–1000°C. It is found that the free hole concentration in arrays varies from 5 × 1018 to 3 × 1019 cm–3 depending on the annealing temperature and is maximal at temperatures of 900–950°C. These results can be used to extend the range of potential application of silicon nanowires in photonics, sensorics, and thermoelectric power converters.

Uniform p-type doping of silicon nanowires synthesized via vapor-liquid-solid growth with silicon tetrachloride

Boron-doped silicon nanowires (SiNWs) grown by the vapor-liquid-solid growth mechanism using silicon tetrachloride (SiCl4) as the silicon precursor and trimethylboron (TMB) as the boron source were studied to understand the axial and radial doping uniformity. TMB-doped SiNWs with diameters up to 400 nm and lengths > 7.5 μm were integrated into a global back-gated test structure with multiple electrodes for electrical characterization. From gate modulated measurements, the SiNWs were confirmed to be heavily doped p-type. Multiple four point resistivity measurements across a total length of 7.5 μm were taken on as-grown SiNWs. Resistivity, corrected for surface charge, was determined to be 0.01 +/− 0.002 Ω cm along the entire length of the as-grown boron doped SiNWs. This was also observed in the axial direction for etched SiNWs, with corrected resistivity of 0.01 +/− 0.003 Ω cm, therefore confirming the uniform p-type doping of SiNWs using TMB and SiCl4 as precursors.

Hole Mobility in Low-Doped Silicon Nanowires: An Atomistic Approach

We performed first-principle calculations by the Density Functional Theory in order to study the scattering properties of silicon nanowires with two different crystalline orientations, [100] and [110], and different types of dopants. The nanowire axis orientation is found to have a strong influence on transmission, which can alter the normal behavior of dopants. Boron, a p-type dopant, can act indeed as a strong scatterer in the conduction band of [100]-oriented silicon nanowires. Using Boltzmann transport theory, we calculated the charge mobility of boron-doped silicon nanowires with different diameters. Although the scattering strength is shown to be strongly dependent on dopant locations and wire cross-sectional size, the hole mobilities are rather insensitive. It was found that the doping density and the nanowire width strongly influence the hole mobility. It was also found that in small diameter nanowires scattering by neutral impurities can be quite significant and act as a limiting factor to the mobility. At low doping densities, the hole mobility increases monotonically with the width of the nanowire.

Laser direct synthesis of silicon nanowire field effect transistors

We demonstrate a single-step, laser-based technique to fabricate silicon nanowire field effect transistors. Boron-doped silicon nanowires are synthesized using a laser-direct-write chemical vapor deposition process, which can produce nanowires as small as 60 nm, far below the diffraction limit of the laser wavelength of 395 nm. In addition, the method has the advantages of in situ doping, catalyst-free growth, and precise control of nanowire position, orientation, and length. Silicon nanowires are directly fabricated on an insulating surface and ready for subsequent device fabrication without the need for transfer and alignment, thus greatly simplifying device fabrication processes. Schottky barrier nanowire field effect transistors with a back-gate configuration are fabricated from the laser-direct-written Si nanowires and electrically characterized.

Controlled Pore Formation on Mesoporous Single Crystalline Silicon Nanowires: Threshold and Mechanisms

Silicon nanowires are prepared by the method of the two‐step metal‐assisted wet chemical etching. We analyzed the structure of solid, rough, and porous nanowire surfaces of boron‐doped silicon substrates with resistivities of ρ > 1000 Ωcm, ρ = 14–23 Ωcm, and ρ < 0.01 Ωcm by scanning electron microscopy and nitrogen gas adsorption. Silicon nanowires prepared from highly doped silicon reveal mesopores on their surface. However, we found a limit for pore formation. Pores were only formed by etching below a critical H2O2 concentration ( M). Furthermore, we determined the pore size distribution dependent on the etching parameters and characterized the morphology of the pores on the nanowire surface. The pores are in the regime of small mesopores with a mean diameter of 9–13 nm. Crystal and surface structure of individual mesoporous nanowires were investigated by transmission electron microscopy. The vibrational properties of nanowire ensembles were investigated by Raman spectroscopy. Heavily boron‐doped silicon nanowires are highly porous and the remaining single crystalline silicon nanoscale mesh leads to a redshift and a strong asymmetric line broadening for Raman scattering by optical phonons at 520 cm−1. This redshift, λSi bulk = 520 cm−1 →λSi nanowire = 512 cm−1, hints to a phonon confinement in mesoporous single crystalline silicon nanowires.

Preparation of silicon nanowires by in situ doping and their electrical properties

Individual silicon nanowires (SiNWs) doped by in situ dopant incorporation during growth of the SiNWs was investigated. Electrical charge transport measurements were taken before and after removing the material from the boron-doped SiNW surfaces by wet chemical etching. AFM (atomic force microscopy) images showed the radial cross section of the ∼10nm thick material that was cut off from the SiNW surface. The electrical transport characteristics after each etching cycle revealed the radial core–shell distribution of the activated dopants. The carrier concentration close to the surface of the boron-doped SiNWs was a factor of 6 higher than the value in the core. Using a simple, rapid and reliable technique, the natural distribution of the surface dopants in the SiNWs, which was distinct from many top-down fabricated NWs, explained the enhancement in the charge transport properties of these NWs. This could yield robust properties in ultra-small devices that are often dominated by random dopants fluctuations.

Visible and Infra-red Light Emission in Boron-Doped Wurtzite Silicon Nanowires

Silicon, the mainstay semiconductor in microelectronic circuitry, is considered unsuitable for optoelectronic applications owing to its indirect electronic band gap, which limits its efficiency as a light emitter. Here we show the light emission properties of boron-doped wurtzite silicon nanowires measured by cathodoluminescence spectroscopy at room temperature. A visible emission, peaked above 1.5 eV, and a near infra-red emission at 0.8 eV correlate respectively to the direct transition at the Γ point and to the indirect band-gap of wurtzite silicon. We find additional intense emissions due to boron intra-gap states in the short wavelength infra-red range. We present the evolution of the light emission properties as function of the boron doping concentration and the growth temperature.

Bias Dependence of Sub-Bandgap Light Detection for Core–Shell Silicon Nanowires

We experimentally demonstrate a vertically arrayed silicon nanowire-based device that exhibits voltage dependence of photoresponse to infrared sub-bandgap optical radiation. The device is fabricated using a proximity solid-state phosphorus diffusion method to convert the surface areas of highly boron-doped silicon nanowires into n-type, thus forming a radial core-shell p-n junction structure. Prominent photoresponse from such core-shell Si nanowires is observed under sub-bandgap illumination at 1310 nm. The strong bias dependence of the photoresponse and other device characteristics indicates that the sub-bandgap absorption is attributed to the intrinsic properties of core-shell Si nanowires rather than the surface states. The attractive characteristics are based on three physical mechanisms: the Franz-Keldysh effect, quasi-quantum confinement effect, and the impurity-state assisted photon absorption. The first two effects enhance carrier tunneling probability, rendering a stronger wave function overlap to facilitate sub-bandgap absorption. The last effect relaxes the k-selection rule by involving the localized impurity states, thus removing the limit imposed by the indirect bandgap nature of Si. The presented device uses single-crystal silicon and holds promise of fabricating nanophotonic systems in a fully complementary metal-oxide-semiconductor (CMOS) compatible process. The concept and approach can be applied to silicon and other materials to significantly extend the operable wavelength regime beyond the constraint of energy bandgap.

Porous Nanostructures and Thermoelectric Power Measurement of Electro-Less Etched Black Silicon

We report the morphology evolution of porous silicon nanostructures and thermoelectric characterization of silicon nanowires (SiNWs) of Electro-Less Etched (ELE) black silicon. Along the axial direction of NWs, the nanopore density (porosity) increases gradually for both highly doped n-Si (n+-Si) and highly doped p-Si (p+-Si). The porosity of silicon nanostructures has been demonstrated to be determined mostly by the wafer doping level and etching time. The formation of porous n+-SiNWs and porous n++-Si film can be understood by an enhanced electron tunnelling from the silicon to the electrolyte through a narrowed space charge layer, while the porous p+-SiNW formation could be the result of an increased thermionic emission current over a lower barrier due to a lower band bending. With microfabricated heaters and thermometers, we measured simultaneously electrical resistivity and thermoelectric power (TEP) of boron-doped SiNWs prepared from a Si wafer with resistivity of 0.2–0.4 Ω cm. The electrical conduc...

Radial Distribution of Active Impurities in Individual In situ Boron-Doped Silicon Nanowires: A Raman Scattering Study

The radial distribution of electrically active B in individual in situ B-doped Si nanowires (SiNWs) grown by the vapor liquid solid (VLS) process is studied by combining micro-Raman spectroscopy and wet chemical etching. The etching of the surface layer results in significant changes in the Raman spectra; Fano-type asymmetry, which is a characteristic of heavily doped p-type Si, disappears and the spectra become almost symmetric. The Raman data reveal that the as-grown B-doped SiNWs consist of a heavily B-doped shell and an almost intrinsic core.

Correlating dopant distributions and electrical properties of boron-doped silicon nanowires

Quantitative nonuniform radial doping profiles in vapor liquid solid grown boron-doped silicon nanowires are correlated with axial variations in electrical properties. Boron concentrations measured by atom probe tomography are lower for the core material grown from a gold catalyst than for material deposited on the nanowire surface. Transistors fabricated along a single nanowire exhibit a transition from nonlinear contact-dominated behavior to linear behavior with increasing thickness of the dopant-enriched surface layer. Simple models confirm that the surface is doped to a level that enables the contact resistance to become comparable to the channel resistance, suggesting that unintentional surface doping may play a role in lowering contact resistances in some nanowire devices.

Charge Transport Characteristics in Boron-Doped Silicon Nanowires

We report the charge transport and inferred surface depletion characteristics of silicon nanowires (Si NWs) with diameters of 90-170 nm after boron doping to 8 times 1017 and 4 times 1019 cm-3 by a proximity diffusion doping technique. Four-probe current-voltage measurements were performed to obtain the NW resistivity, and the electrically active dopant concentration and surface oxide charge density were extracted by varying the NW diameter. The Ti/Au to Si NW contact resistance and specific contact resistivity were also obtained, and specific contact resistivities as low as 2 times 10-5 Omega middot cm2 were achieved. The derived parameters for these ex situ boron-doped Si NWs agree reasonably well with the expected characteristics and earlier reported results for in situ boron-doped Si NWs. Interface charge creates a surface depletion region in p-type Si NWs, which decreases the conducting area of the NW. This effect increases the NW resistance and becomes increasingly significant with decreasing dopant concentration and NW diameter. A simple method is presented to estimate the relative influence of surface charge density on electrical transport in NWs for this case.

Surface depletion thickness of p-doped silicon nanowires grown using metal-catalysedchemical vapour deposition

An accurate evaluation of the radial dopant profile in a nanowire is crucial for designingfuture nanoscale devices synthesized using bottom-up techniques. We developed a very slowwet chemical etchant for gradually reducing the diameters of metal-catalysed,boron-doped silicon nanowires with varying diameters and lengths. Particularcare has been taken to perform the experiment at room temperature to preventdopant segregation, which is common in high temperature processes. By ensuringidentical surface conditions subsequent to diameter reduction, the resistance of thenanowires was measured and, as anticipated, was found to increase with decreasingdiameter. As the diameters were shrunk using wet-chemical etching, nanowiresexhibited a non-linear increase of the resistance when the diameter was reduced to∼50 nm. This is an indication of near-complete depletion in the nanowires caused bynanowire surface charges. The dopant concentration of the nanowires was found to be2.1 × 1018 cm−3 and the corresponding surface charge density was around2.6 × 1012 cm−2.

The Influence of B2H6 on the Growth of Silicon Nanowire

The un-doped and boron-doped silicon nanowires (SiNWs) were grown via vapor–liquid–solid (VLS) mechanism by low pressure chemical deposition (LPCVD). The diameters of un-doped and boron-doped SiNWs varied from 18.5 to 75.3 nm and 26.6 to 66.1 nm, respectively. The critical growth temperature of boron-doped SiNWs is 10°C lower than that of un-doped ones and the diameters of the boron-doped SiNWs is always larger than that of the un-doped ones under different growth temperatures. This is because that the introduction of diborane enhanced the dissociation of SiH4 which determines the growth process of SiNW. A growth process of silicon nanowire is proposed to describe the influence of B2H6.

Effect of diborane on the microstructure of boron-doped silicon nanowires

Boron-doped silicon (Si) nanowires, with nominal diameters of 80 nm, were grown via the vapor–liquid–solid (VLS) mechanism using gold (Au) as a catalyst and silane (SiH 4) and diborane (B 2H 6) as precursors. The microstructure of the nanowires was studied by scanning electron microscopy, transmission electron microscopy and electron energy-loss spectroscopy. At lower B 2H 6 partial pressure and thus lower doping levels (⩽1×10 18 cm −3), most of the boron-doped Si nanowires exhibited high crystallinity. At higher B 2H 6 partial pressure (∼2×10 19 cm −3 doping level), the majority of the wires exhibited a core–shell structure with an amorphous Si shell (20–30 nm thick) surrounding a crystalline Si core. Au nanoparticles on the outer surface of the nanowires were also observed in structures grown with high B/Si gas ratios. The structural changes are believed to result from an increase in the rate of Si thin-film deposition on the outer surface of the nanowire at high B 2H 6 partial pressure, which produces the amorphous coating and also causes an instability at the liquid/solid interface resulting in a loss of Au during nanowire growth.

Photoluminescence and Raman Scattering Correlated Study of Boron-Doped Silicon Nanowires

Boron-doped (B-doped) silicon nanowires (SiNWS) have been prepared and characterized by Raman scattering and photoluminescence (PL). B-doped SiNWS were grown by plasma enhanced chemical vapor deposition (PECVD), using diborane (B2H6) as the dopant gas. Raman spectra show a band at 480cm-1,which is attributed to amorphous silicon. Photoluminescence at room temperature exhibits three distinct emission peaks at 1.34ev,1.42ev,1.47ev. Possible reason for these is suggested. PACS: 36.40._c; 81.15.Gh; 81.20._n

Structural and electrical properties of trimethylboron-doped silicon nanowires

Trimethylboron (TMB) was investigated as a p-type dopant source for the vapor–liquid–solid growth of boron-doped silicon nanowires (SiNWs). The boron concentration in the nanowires was measured using secondary ion mass spectrometry and results were compared for boron-doping using TMB and diborane (B2H6) sources. Boron concentrations ranging from 1×1018 to 4×1019cm−3 were obtained by varying the inlet dopant∕SiH4 gas ratio. TEM characterization revealed that the B2H6-doped SiNWs consisted of a crystalline core with a thick amorphous Si coating, while the TMB-doped SiNWs were predominantly single crystal even at high boron concentrations. The difference in structural properties was attributed to the higher thermal stability and reduced reactivity of TMB compared to B2H6. Four-point resistivity and gate-dependent conductance measurements were used to confirm p-type conductivity in the TMB-doped nanowires and to investigate the effect of dopant concentration on nanowire resistivity.

Optical properties of boron-doped Si nanowires

Raman scattering and photoluminescence (PL) of boron-doped silicon nanowires have been investigated. Raman spectra showed a band at 480 cm −1, indicating that the crystallinity of the nanowires was suppressed by boron doping. PL taken from B-doped SiNWS at room temperature exhibited three distinct emission peaks at 1.34, 1.42, and 1.47 eV and the PL intensity was much stronger than that of undoped SiNWS. The increased PL intensity should be very profitable for nano-optoelectronics.