Olan Lotty

Doping controlled roughness and defined mesoporosity in chemically etched silicon nanowires with tunable conductivity

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 ...

Porous to Nonporous Transition in the Morphology of Metal Assisted Etched Silicon Nanowires

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.

Containing the catalyst: diameter controlled Ge nanowire growth

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.

Fabrication and Characterization of Single-Crystal Metal-Assisted Chemically Etched Rough Si Nanowires for Lithium-Ion Battery Anodes

Higher Education Authority (under the framework of the INSPIRE programme, funded by the Irish Governments Programme for Research in Third Level Institutions, Cycle 4, National Development Plan 2007-2013)

Raman Scattering Spectroscopy of Metal-Assisted Chemically Etched Rough Si Nanowires

Higher Education Authority (under the framework of the INSPIRE programme, funded by the Irish Governments Programme for Research in Third Level Institutions, Cycle 4, National Development Plan 2007-2013)

Silicon and Germanium Junctionless Nanowire Transistors for Sensing and Digital Electronics Applications

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.

Attomolar streptavidin and pH, low power sensor based on 3D vertically stacked SiNW FETs

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 (HfO2) 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 (ΔId/pH) of ~0.70 dec/pH were measured for structures with APTES functionalized SiO2 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.

Diameter-driven crossover in resistive behaviour of heavily doped self-seeded germanium nanowires

Summary 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.