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The dependence of the resistivity with changing diameter of heavily-doped self-seeded germanium nanowires was studied for the diameter range 40 to 11 nm. The experimental data reveal an initial strong reduction of the resistivity with diameter decrease. At about 20 nm a region of slowly varying resistivity emerges with a peak feature around 14 nm. For diameters above 20 nm, nanowires were found to be describable by classical means. For smaller diameters a quantum-based approach was required where we employed the 1D Kubo–Greenwood framework and also revealed the dominant charge carriers to be heavy holes. For both regimes the theoretical results and experimental data agree qualitatively well assuming a spatial spreading of the free holes towards the nanowire centre upon diameter reduction.

The Li+ insertion and extraction characteristics at n-type and p-type Si(100) electrodes with different carrier density and doping type are investigated by cyclic voltammetry and constant current measurements. The insertion and extraction potentials are demonstrated to vary with cycling and the occurrence of an activation effect is shown in n-type electrodes where the charge capacity and voltammetric currents are found to be much higher than p-type electrodes. A rate-dependent redox process influenced by the surface region electronic density, which influences the magnitude of cyclic voltammetry current is found at Si(100) surface regions during Li insertion and extraction. At p-type Si(100) surface regions, a thin, uniform film forms at lower currents, while also showing a consistently high (>70%) Coulombic efficiency for Li extraction. The p-type Si(100) surface region does not undergo crack formation after deintercalation and the amorphization was demonstrated using transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) and Raman scattering demonstrate that highly doped n-type Si(100) retains Li as a silicide and converts to an amorphous phase as a two-step phase conversion process. The findings show the succinct dependence of Li insertion and extraction processes for uniformly doped Si(100) single crystals and how the doping type and its effect on the semiconductor-solution interface dominate Li insertion and extraction, composition, crystallinity changes and charge capacity.

From Springer description: "We present the design considerations of an autonomous wireless sensor and discuss the fabrication and testing of the various components including the energy harvester, the active sensing devices and the power management and sensor interface circuits. A common materials platform, namely, nanowires, enables us to fabricate state-of-the-art components at reduced volume and show chemical sensing within the available energy budget. We demonstrate a photovoltaic mini-module made of silicon nanowire solar cells, each of 0.5 mm2 area, which delivers a power of 260 μW and an open circuit voltage of 2 V at one sun illumination. Using nanowire platforms two sensing applications are presented. Combining functionalised suspended Si nanowires with a novel microfluidic fluid delivery system, fully integrated microfluidic–sensor devices are examined as sensors for streptavidin and pH, whereas, using a microchip modified with Pd nanowires provides a power efficient and fast early hydrogen gas detection method. Finally, an ultra-low power, efficient solar energy harvesting and sensing microsystem augmented with a 6 mAh rechargeable battery allows for less than 20 μW power consumption and 425 h sensor operation even without energy harvesting."

Porous semiconductors structured top-down by electrochemical means, and from bottom-up growth of arrays and arrangements of nanoscale structures, are shown to be amenable to a range of useful thermal, optical, electrical and electrochemical properties. This paper summarises recent investigations of the electrochemical, electrical, optical, thermal and structural properties of porous semiconductors such as Si, In2O3, SnO2 and ITO, and dispersions, arrays and arrangements of nanoscale structures of each of these materials. We summarize the property-inspired application of such structurally engineered arrangements and morphologies of these materials for antireflection coatings, broadband absorbers, transparent contacts to LEDs that improve transmission, electrical contact and external quantum efficiency. Additionally the possibility of thermoelectric performance through structure-mediated variation in thermal resistance and phonon scattering without a p-n junction is shown through phonon engineering in roughened nanowires. Lastly, we show that bulk crystals and nanowires of p- and n-type doped Si are promising for use as anodes in Li-ion batteries.

Silicon nanowires were fabricated by a metal assisted chemical (MAC) etching process and routes toward ohmic contacting of substrates for Li-ion battery anode application were developed. Si nanowire layers are comprised of wires that are single crystal with rough outer surfaces. The nanowires are epitaxial with the underlying Si(100) substrate, maintain equivalent doping density and crystal orientation, and are coated with a stoichiometric SiO2. Electrical backside contacting using an In-Ga eutectic allows low-resistance ohmic contacts to low-doped nanowire electrodes for electrochemical testing.

We have shown that SiNWs formed on (100) Si substrates by metal-assisted chemical etching can exhibit different Raman scattering processes that are dependent on the orientation of examined NWs to the incident excitation light. The SiNWs retain the single crystal orientation and doping from the original bulk substrate and form as rough and mesoporous NWs with a SiOx shell surrounding the NW. The Raman scattering spectra of vertical and horizontally lying SiNWs showed quantum confined phonon scattering processes from narrow and roughened NWs, whose spectral resolution was increased by orienting NW horizontal to the beam to maximize probe cross-section. SiOx contributions were not evident and specific substrate Raman modes were suppressed for horizontal NWs. Beam induced heating to 425 K showed pronounced red-shifting and asymmetry of the TO, 2TO and 2TA phonon modes consistent with phonon quantum confinement effects not observable when the NW were parallel to incident excitation.

3D vertically stacked silicon nanowire (SiNW) field effect transistors (FET) with high density arrays (up to 7×20) of fully depleted and ultra-thin (15–30 nm) suspended channels were fabricated by a top-down CMOS compatible process on silicon on insulator (SOI). The channels can be wrapped by conformal high-κ gate dielectrics (HfO 2 ) and their conductivity can be excellently controlled either by a reference electrode or by three local gates; a backgate (BG) and two symmetrical Pt side-gates (SG) offering unique sensitivity tuning. Such 3D structure has been (3-Aminopropyl)-triethoxysilane (APTES) functionalized and biotynilated for pH and streptavidin (protein) sensing, respectively. An ultra-low concentration of 17 aM of streptavidin was measured, the lowest ever reported in literature. Extremely high quasi-exponential drain current responses (ΔI d /pH) of ∼0.70 dec/pH were measured for structures with APTES functionalized SiO 2 gate dielectrics when operated in the subthreshold regime. Also, high drain current responses > 20 µA/pH and high sensitivities (S ∼ 95%) were measured for structures with a native oxide gate dielectrics when operated in the strong-inversion regime.

Top-down fabrication of sub-50nm silicon nanowire sensors with various geometries.Excellent performance as backgated junctionless nanowire transistors (JNTs).Very good sensing capabilities.Among the smallest top-down fabricated nanowire sensing devices reported to date. This article reports the fabrication of sub-50nm field effect transistor (FET)-type silicon (Si) nanowire (Si NW) chemical and biological sensing devices with a junctionless architecture, as well as on the initial characterisation of their electrical and sensing performance.The devices were fabricated using a fully complementary metal-oxide-semiconductor (CMOS)-compatible top-down process on silicon-on-insulator (SOI) wafers. The fabrication process was mainly based on high-resolution electron beam lithography (EBL) and reactive ion etching (RIE) but also included photolithography (mix-and-match lithography), thin film deposition by electron beam evaporation, lift-off, thermal annealing and wet etching.The sensing performance of a matrix of nanowire devices, i.e. containing 1, 3 and 20 NWs with lengths of 0.5, 1 and 10µm was examined. Each element of the matrix also contained five devices with different NW widths: 10, 20, 30, and 50nm and 5µm (a Si belt reference device). Electrical characterisation of the devices showed excellent performance as backgated junctionless nanowire transistors (JNTs): high on-currents in the range of 1-10µA and high ratios between the on-state and off-state currents (Ion/Ioff) of 6-7 orders of magnitude. In addition, the results of ionic strength sensing experiments demonstrate the very good sensing capabilities of these devices. To the best of our knowledge, these nanowire sensors are among the smallest top-down fabricated Si NW devices reported to date.

In this chapter, we introduce two specific types of junctionless nanowire transistors (JNTs): (i) silicon-on-insulator (SOI) back-gated JNTs for sensing applications and (ii) germanium-on-insulator (GeOI) top-gated JNTs for digital logic applications. We discuss in detail the suitability of junctionless architecture for these particular applications and present results on device fabrication and characterisation. Back-gated JNTs of 45 different channel geometries (different numbers, lengths, and widths of channel nanowires) have been designed and fabricated with very high precision (down to 10 nm widths of the nanowires) on SOI wafers using a fully CMOS-compatible fabrication process. Electrical characterisation of the fabricated devices has demonstrated their excellent performance as back-gated JNTs. Furthermore, data from pH and streptavidin sensing experiments have proven their good sensing properties. These JNTs are among the smallest top-down fabricated nanowire sensing devices reported to date. Top-gated JNTs with Ge nanowire channels of widths down to 20 nm have been fabricated by a simple CMOS-compatible process on GeOI wafers with a highly p-doped (~1×1019 cm−3) top germanium layer. The fabricated devices have demonstrated decent output and transfer characteristics with relatively high I on /I off current ratios of up to 2.0 × 105 and steep subthreshold slopes of 189 mV/dec. To the best of our knowledge, these are the first reported Ge JNTs.

A 3D, vertically stacked silicon nanowire (SiNW) field effect transistor (FET) featuring a high density array (7×20) of fully depleted channels has been successfully fabricated by a CMOS compatible process on silicon on insulator (SOI) and functionalized for streptavidin detection for the first time. The channels are surrounded by conformal high-κ gate dielectrics (HfO2), and their conductivity can be uniquely tuned by three gates; a backgate (BG) and two symmetrical Pt side gates (SG) through a liquid, offering unique sensitivity tuning with high gate coupling (SS=75 mV/dec, α'=SS60mV/dec/SSmeasured=0.8, with highest sensitivity=93-99%, obtained for I=1 0pA-10nA, in weak inversion) ever published.

peer-reviewed By using Si(100) with different dopant type (n(++)-type (As) or p-type (B)), we show how metal-assisted chemically etched (MACE) nanowires (NWs) can form with rough outer surfaces around a solid NW core for p-type NWs, and a unique, defined mesoporous structure for highly doped n-type NWs. We used high resolution electron microscopy techniques to define the characteristic roughening and mesoporous structure within the NWs and how such structures can form due to a judicious choice of carrier concentration and dopant type. The n-type NWs have a mesoporosity that is defined by equidistant pores in all directions, and the inter-pore distance is correlated to the effective depletion region width at the reduction potential of the catalyst at the silicon surface in a HF electrolyte. Clumping in n-type MACE Si NWs is also shown to be characteristic of mesoporous NWs when etched as high density NW layers, due to low rigidity (high porosity). Electrical transport investigations show that the etched nanowires exhibit tunable conductance changes, where the largest resistance increase is found for highly mesoporous n-type Si NWs, in spite of their very high electronic carrier concentration. This understanding can be adapted to any low-dimensional semiconducting system capable of selective etching through electroless, and possibly electrochemical, means. The process points to a method of multiscale nanostructuring NWs, from surface roughening of NWs with controllable lengths to defined mesoporosity formation, and may be applicable to applications where high surface area, electrical connectivity, tunable surface structure, and internal porosity are required. (C) 2013 AIP Publishing LLC. PUBLISHED peer-reviewed

We report the controlled self-seeded growth of highly crystalline Ge nanowires, in the absence of conventional metal seed catalysts, using a variety of oligosilylgermane precursors and mixtures of germane and silane compounds (Ge:Si ratios between 1:4 and 1:1). The nanowires produced were encased in an amorphous shell of material derived from the precursors, which acted to isolate the Ge seed particles from which the nanowires were nucleated. The mode diameter and size distribution of the nanowires were found to increase as the growth temperature and Ge content in the precursors increased. Specifically, a model was developed to describe the main stages of self-seeded Ge nanowire growth (nucleation, coalescence, and Ostwald ripening) from the oligosilylgermane precursors and, in conjunction with TEM analysis, a mechanism of growth was proposed.

A single step metal assisted etching (MAE) process, utilizing metal ion-containing HF solutions in the absence of an external oxidant, has been developed to generate heterostructured Si nanowires with controllable porous (isotropically etched) and non-porous (anisotropically etched) segments. Detailed characterisation of both the porous and non-porous sections of the Si nanowires was provided by transmission electron microscopy studies, enabling the mechanism of nanowire roughening to be ascertained. The versatility of the MAE method for producing heterostructured Si nanowires with varied and controllable textures is discussed in detail.

Silicon nanostructures and methods to grow them and influence their structure and morphology has recently been demonstrated to offering a useful strategy to control thermal conductivity and light-matter interactions. The transport and scattering of phonons in Si has demonstrated effective thermoelectric performance largely due to effects caused by surface roughening and nanoscale irregularities in the crystal structure that contribute to diffuse boundary scattering mechanisms. When confinement effects are introduced in crystalline materials the electron and phonon transport can be significantly due to three discrete effects: increased boundary scattering, changes in phonon dispersion, and quantization of phonon transport. Using both room-temperature and in-situ thermal Raman scattering spectroscopy the transport of phonons within Si and its nanostructures can be probed quickly and with little sample preparation. Both ambient and high temperature characteristics of the Si are examined and phonon transport changes between the b-Si and the SiNWs can be linked to scattering effects due to the different morphologies possible using metal-assisted chemical etching to form NWs with tunable porosity and surface roughness. This work demonstrates that by increasing the number of available scattering sites in nanostructured Si within the structure (not just boundary scattering at rough surfaces), the role of four phonon processes on the phonon transport within Si can be optimised. This study is important for providing information on the phonon transport within Si and its nanostructures allowing morphologies that affect phonon transport to be exploited for engineering the thermoelectric properties of future Si devices. References J. Lim, K. Hippalgaonkar, S. C. Andrews, A. Majumdar, P. Yang, Nano Letters, 12, 2475 (2012). J. Tang, H.-T. Wang, D. H. Lee, M. Fardy, Z. Huo, T. P. Russell, P. Yang, Nano Letters, 10, 4279 (2010). A. I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E. C. Garnett, M. Najarian, A. Majumdar, P. Yang, Nature, 451, 163 (2008). A. I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J. K. Yu, W. A. Goddard, J. R. Heath, Nature, 451, 168 (2008). K. Q. Peng, J. J. Hu, Y. J. Yan, Y. Wu, H. Fang, Y. Xu, S. T. Lee, J. Zhu, Advanced Functional Materials, 16, 387 (2005).

Sub-20 nm diameter Ge nanowires with narrow size distributions were grown from Ag nanoparticle seeds in a supercritical fluid (SCF) growth process. The mean Ge nanowire diameter and size distribution was shown to be dependent upon Ag nanoparticle coalescence, using both spin-coating and a block copolymer (BCP) templating method for particle deposition. The introduction of a metal assisted etching (MAE) processing step in order to "sink" the Ag seeds into the growth substrate, prior to nanowire growth, was shown to dramatically decrease the mean nanowire diameter from 27.7 to 14.4 nm and to narrow the diameter distributions from 22.2 to 6.8 nm. Hence, our BCP-MAE approach is a viable route for controlling the diameters of semiconductor nanowires whilst also ensuring a narrow size distribution. The MAE step in the process was found to have no detrimental effect on the length or crystalline quality of the Ge nanowires synthesised.

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Silicon nanowire (Si NW) sensors have attracted great attention due to their ability to provide fast, low-cost, label-free, real-time detection of chemical and biological species. Usually configured as field effect transistors (FETs), they have already demonstrated remarkable sensitivity with high selectivity (through appropriate functionalisation) towards a large number of analytes in both liquid and gas phases. Despite these excellent results, Si NW FET sensors have not yet been successfully employed to detect single molecules of either a chemical or biological target species. Here we show that sensors based on silicon junctionless nanowire transistors (JNTs), the simplest possible transistors, are capable of detecting the protein streptavidin at a concentration as low as 580 zM closely approaching the single molecule level. This ultrahigh detection sensitivity is due to the intrinsic advantages of junctionless devices over conventional FETs. Apart from their superior functionality, JNTs are much easier to fabricate by standard microelectronic processes than transistors containing p–n junctions. The ability of JNT sensors to detect ultra-low concentrations (in the zeptomolar range) of target species, and their potential for low-cost mass production, will permit their deployment in numerous environments, including life sciences, biotechnology, medicine, pharmacology, product safety, environmental monitoring and security.