Stoichiometric SiO2 Research Articles

Effect of О<sub>2</sub><sup>+</sup> Ion Implantation on the Elemental and Chemical Composition of the Si(111) Surface

Using the methods of secondary ion mass spectrometry, elastic peak electron spectroscopy and Auger electron spectroscopy, the elemental and chemical composition of the surface, concentration profiles of the distribution of atoms over the depth of silicon implanted with O2+ ions with energy E0 = 1 keV at a dose of D = 6 × 1016 cm–2 were studied. It was found that oxides and suboxides of Si (SiO2, Si2O and SiO0.5) were formed in the ion-doped layer, and it also contained unbound O and Si atoms. Post-implantation annealing at 850–900 K led to the formation of a stoichiometric SiO2 layer ~25–30 Å thick.

Uniform broad-area deposition and patterning of SiO2 nanofilms by 172 nm photochemical conversion of liquid tetraethoxysilane layers at 300 K

Oxide films of the quality required for the fabrication of electronic and photonic devices are typically deposited at elevated temperatures and thermal equilibrium, thereby adversely impacting thermal budgets. We report the deposition and patterning of silicon dioxide (SiO2) films of high electrical and optical quality on Si(100) or polymer substrates in a N2 atmosphere and at 300 K by the photochemical conversion of thin liquid tetraethoxysilane (TEOS) layers with narrowband vacuum ultraviolet radiation [vacuum ultraviolet (VUV), 172 nm] provided by efficient microplasma lamps. Irradiating liquid TEOS layers, produced by spin-coating the precursor onto a substrate, with a VUV intensity of 13 mW cm−2, yields 40 nm-thick SiO2 films having a dielectric breakdown strength (Eb) of 5 MV cm−1, for which no precedent exists in the deposition of oxide films at 300 K. If room temperature-deposited films are post-annealed at 200 °C, Eb rises to 7.5 MV cm−1, which is <12% below the measured value (8.5 MV cm−1) for 40 nm SiO2 films grown by thermal oxidation. The deposition of 1 µm thick, stoichiometric SiO2 films requires only 20 min of VUV illumination at low optical fluences, and films with thicknesses of ∼35–60 nm exhibit a refractive index of 1.45 (633 nm). X-ray photoelectron spectrometry and Rutherford backscattering analysis indicate that, despite the deposition temperature, hydrocarbon impurity levels are near or below the detection limit. The capability for depositing 960 nm-thick SiO2 films uniformly (to within 0.6%) by liquid → solid photochemical conversion over a 5 cm diameter Si substrate and patterning films onto flexible polymer substrates has also been demonstrated.

Hybrid silica and thin-film lithium niobate waveguides and Y-junctions

Silica (SiO2) strip-loaded waveguides and Y-junctions on lithium niobate (LN) thin film were designed, fabricated and experimentally investigated. We first studied the optical confinement factor distribution in strip-loaded waveguides. The Simulations showed that most of the power (>82%) was confined in the LN layer, and the dimensions of the SiO2 loading strip had a relatively small influence on the optical confinement factor distribution. The SiO2 film was deposited with the magnetron sputtering method. The deposited SiO2 layer approached stoichiometric SiO2, and a smooth surface was obtained. A group of waveguides with widths of 2–5 μm were fabricated using lift-off technology. The 2 μm-wide waveguide displayed low propagation losses of 0.2 dB/cm for the quasi-TE (q-TE) mode and 0.8 dB/cm for the quasi-TM (q-TM) mode at the wavelength of 1550 nm. Y-junctions with small sizes were designed and fabricated based on the low-loss waveguides. The transmission could reach 70–80%, and the splitting ratio maintained at a stable level near 1:1 for the Y-junctions S1 and S4. The low-loss strip-loaded waveguides and Y-junctions offer the possibility of combining SiO2 and LN materials for integrated optics. Additional high-performance photonic devices and circuits using this method are expected in the future.

Investigating wet chemical oxidation methods to form SiO2 interlayers for self-aligned Pt-HfO2-Si gate stacks

Self-aligned metal-oxide-semiconductor (MOS) capacitors are studied with several low-temperature, wet chemical silicon dioxide (SiO2) interlayers to understand their impact on electrical performance. Self-aligned MOS capacitors are fabricated with a bottom-up patterning technique that uses a poly(methyl methacrylate) brush and dopant-selective KOH etch combined with area-selective atomic layer deposition of hafnium dioxide (HfO2) and Pt. The wet chemical pretreatments used to form the SiO2 interlayer include hydrofluoric acid (HF) etch, 80 °C H2O, and SC-2. Capacitance-voltage measurements of these area-selective capacitors exhibit a HfO2 dielectric constant of ∼19, irrespective of pretreatment. After a forming gas anneal, the average interface state density decreased between 1.8 and 7.5 times. The minimum observed Dit is 1 × 1011 eV−1 cm−2 for the HF-last treatment. X-ray photoelectron spectroscopy shows an increase in stoichiometric SiO2 in the interfacial layer after the anneal. Additional carbon is also observed; however, comparison with capacitors fabricated in a nonselective process reveals minimal impact on performance.

In-situ PECVD-based stoichiometric SiO2 layer for semiconductor devices

We propose an in-situ stoichiometric SiO2 layer deposition using plasma-enhanced chemical vapor deposition (PECVD) to replace typical outside SiO2 deposition for semiconductor devices, such as thermal oxidation and/or wet chemical oxidation. The SiO2 films deposited in a silane (SiH4), argon (Ar), and carbon dioxide (CO2) precursor gas mixture are compared to those deposited in a standard SiH4+CO2 plasma environment. Ar gases are introduced into the plasma environment of silane (SiH4) and carbon dioxide (CO2) to improve the optoelectronic properties and stoichiometry of SiO2. The addition of the Ar gases promotes the dissociation of SiH4 and CO2, resulting in an increase in the deposition rate and a decrease in the SiO2 refractive index. The addition reduces the dielectric constant of SiO2 by analyzing the metal-oxide-semiconductor structure. The resultant SiO2 shows excellent surface passivation of crystalline silicon wafers. The mechanism of SiO2 formation in PECVD with mixture gases is thoroughly addressed. The in-situ PECVD-based stoichiometric SiO2 layer is expected to reduce the complexity of the semiconductor device procedure.

Bimolecular Reaction Mechanism in the Amido Complex-Based Atomic Layer Deposition of HfO2.

The surface chemistry of the initial growth during the first or first few precursor cycles in atomic layer deposition is decisive for how the growth proceeds later on and thus for the quality of the thin films grown. Yet, although general schemes of the surface chemistry of atomic layer deposition have been developed for many processes and precursors, in many cases, knowledge of this surface chemistry remains far from complete. For the particular case of HfO2 atomic layer deposition on a SiO2 surface from an alkylamido-hafnium precursor and water, we address this lack by carrying out an operando atomic layer deposition experiment during the first cycle of atomic layer deposition. Ambient-pressure X-ray photoelectron spectroscopy and density functional theory together show that the decomposition of the metal precursor on the stoichiometric SiO2 surface in the first half-cycle of atomic layer deposition proceeds via a bimolecular reaction mechanism. The reaction leads to the formation of Hf-bonded methyl methylene imine and free dimethylamine. In addition, ligand exchange takes place involving the surface hydroxyls adsorbed at defect sites of the SiO2 surface.

CO2 capture by Li2CaSiO4 and enhancement with alkali carbonates.

Alkali and alkali earth oxides show good CO2 capture performance for carbonation, while their regeneration occurs at high temperatures, leading to a high energy penalty. When alkali oxides and alkali earth oxides combine with SiO2 to form oxysalts, the regeneration temperatures can be reduced, and the CO2 adsorption capacity is maintained. In this study, the reaction between CO2 and Li2CaSiO4, composed of stoichiometric CaO, Li2O, and SiO2, was evaluated thermodynamically by DFT. The synthesized Li2CaSiO4 with and without alkali carbonates was used as CO2 sorbents, and their CO2 adsorption performances were examined using thermal analyses. The phase and morphology of Li2CaSiO4 before and after CO2 adsorption were characterized by XRD and SEM. According to the thermodynamic evaluation and the XRD results, Li2CaSiO4 could adsorb CO2 and form CaCO3 and Li2SiO3. The thermal analyses showed that the regeneration of Li2CaSiO4 started from 575 °C, at which it was difficult to realize the CO2 diffusion through the solid CaCO3 product layer. The mixed alkali carbonates can improve the kinetics and facilitate the CO2 adsorption of Li2CaSiO4. Alkali carbonates were effective in reducing the activation energy of the reaction and CO2 diffusion at low temperatures and improving the cyclic stability because of the dispersing carbonation products.

Synchrotron-Excited Luminescence and Converting of Defects and Quantum Dots in Modified Silica Films

Optically active defects in modified silicon oxide films on a silicon substrate have been studied by low-temperature photoluminescence (PL) spectroscopy using excitation by synchrotron radiation in the vacuum ultraviolet region. Films of dry thermal silicon oxide obtained by treatment of stoichiometric SiO2 in hydrogen plasma, films of wet thermal silicon oxide, and films with low dielectric constant (so called "low-k" dielectrics) have been studied. Investigations of various type SiO2 films detect the PL centers, which can be conditionally divided into two groups. The first group includes intrinsic point defects, the different variants of oxygen-deficient centers (ODCs). The second group contains centers like the spatially confined excitons in silicon quantum dots (SiQDs). It is shown that SiQDs differ in spectral characteristics, are formed in different ways, due to the transformation and clustering of point defects such as E'-centers, ODC(I) and ODC(II). In this case, the size and spectral properties of quantum dots depend on the mechanism of their formation. Schemes for the conversion of these defects are proposed.It was found that for wet films and nonstoichiometric SiOx films treated in hydrogen plasma, "metastable" centers ODC(I) and ODC(II) arise only under the action of synchrotron radiation in the region of interband or exciton absorption and then they decay because of radiative relaxation. The absence of stable ODC centers in these films is due to their clustering and the formation of SiQDs. In "low-k" mesoporous films, on the contrary, silicon quantum dots SiQDs are not formed, but ODC(I) and ODC(II) centers are clearly manifested. A scheme of electronic transitions upon indirect excitation of silicon quantum dots through exciton states of the SiO2 matrix is considered. The presented results demonstrate the possibility of controlling the optical properties of silicon thin-film structures by creating in them both oxygen-deficient point defects and quantum dots of various sizes.

Study of the Effect of Nitric Acid in Electrochemically Synthesized Silicon Nanocrystals: Tunability of Bright and Uniform Photoluminescence

In this work, we show a correlation between the composition and the microstructural and optical properties of bright and uniform luminescent porous silicon (PSi) films. PSi films were synthesized by electrochemical etching using nitric acid in an electrolyte solution. PSi samples synthesized with nitric acid emit stronger (up to six-fold greater) photoluminescence (PL) as compared to those obtained without it. The PL peak is shifted from 630 to 570 nm by changing the concentration ratio of the HF:HNO3:(EtOH-H2O) electrolyte solution, but also shifts with the excitation energy, indicating quantum confinement effects in the silicon nanocrystals (Si-NCs). X-ray photoelectron spectroscopy analysis shows a uniform silicon content in the PSi samples that emit the strongest PL. High-resolution transmission electron microscopy reveals that the Si-NCs in these PSi samples are about ~2.9 ± 0.76 nm in size and are embedded in a dense and stoichiometric SiO2 matrix, as indicated by the Fourier transform infrared analysis. On the other hand, the PSi films that show PL of low intensity present an abrupt change in the silicon content depth and the formation of non-bridging oxygen hole center defects.

Electrical properties of low-temperature SiO2 thin films prepared by plasma-enhanced atomic layer deposition with different plasma times

Silicon dioxide (SiO2) thin films were prepared by plasma-enhanced atomic layer deposition (PEALD) at a low temperature of 150 °C using di-isopropylaminosilane and oxygen with different plasma times. While SiO2 films deposited with a short plasma time of 0.5 s exhibited high leakage current, SiO2 films deposited with a plasma time of 7 s at 150 °C showed excellent dielectric properties, including a low current density of 4.8 × 10−9 A/cm2 at 1 MV/cm and a high breakdown field of 10.5 MV/cm, comparable to those of PEALD-SiO2 films deposited at 350 °C. As the plasma time increased from 0.5 to 7 s, the dielectric constant of SiO2 films decreased from 7.5 to 4.0, which was close to the value of stoichiometric SiO2. Appropriate conduction mechanisms of these SiO2 films with differing electrical characteristics by plasma time were examined. Analyses by x-ray photoelectron spectroscopy and secondary ion mass spectrometry revealed that the quality of SiO2 films largely depended on the amount of defects such as hydroxyl and hydrogen-related species generated by low-temperature deposition.

High-temperature CO2 treatment for improving electrical characteristics of 4H-SiC(0001) metal-oxide-semiconductor devices

High-temperature CO2 treatments for 4H-SiC(0001) surfaces and SiO2/SiC structures were investigated. A stoichiometric SiO2 insulating layer was found to be grown on SiC by thermal oxidation in atmospheric CO2 ambient with an activation energy of 7.5 eV. Post-oxidation annealing (POA) in CO2 at a temperature of 1200 °C or more was effective in reducing interface fixed charges, oxide traps, and defect precursors, which are intrinsically involved in SiO2/SiC system grown with O2 oxidant. Furthermore, modification of the SiO2/SiC interface promoted by oxidizing SiC with CO2 at 1400 °C or more reduced the amount of interface states, thus the SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) treated with CO2 at 1400 °C exhibited improved channel mobility with a stable threshold voltage.

Sub-2 nm boron doping in silicon using novel ultra-thin SiO2 film produced by sol–gel dip coating as a capping layer

Advancements in semiconductor electronics and sensor technologies demand a more robust doping approach and process to fabricate ultra-shallow doping with abrupt depth, particularly on complex structured surfaces. Molecular monolayer doping (MLD) is a potential approach, but is limited by the difficulty in depositing a high-quality SiO2 capping film without damaging the dopant molecular layer. In this work, a relative humidity condition during deposition of the film in the sol–gel dip-coating process is found to produce continuous, dense and stoichiometric ultra-thin SiO2 films of quality equivalent to that produced by complicated thermal oxidation/etching and sophisticated chemical vapor deposition and atomic layer deposition methods. Applying these ultra-thin SiO2 films as capping layers in the molecular MLD method, B doping in Si with sub-2 nm effective depth and sub-5 nm abrupt depth is achieved. Remarkably, around 82%–86% of doped B atoms in Si are found to be electrically active as estimated from sheet resistance measurements. The established sol–gel dip coating conditions to deposit ultra-thin SiO2 films are generic and can be extended to produce high-quality ultra-thin films of other metal oxide materials required for advanced technological applications.

Passivation of n- and p-Type Silicon Surfaces With Spray-Coated Sol-Gel Silicon Oxide Thin Film

Thermally grown SiO2 is widely used for silicon surface passivation in solar cells and other applications due to excellent interfacial properties. SiO2 films with thickness in the range of 17 nm deposited by industrially viable spray-coating technique on both n- and p-type silicon are reported. Low values of interface state density of ${1.4}\times {10}^{{10}}$ cm $^{\boldsymbol -{2}}$ eV $^{\boldsymbol -{1}}$ on n-type and ${2.0}\times {10}^{{10}}$ cm $^{\boldsymbol -{2}}$ eV $^{\boldsymbol -{1}}$ on p-type were achieved. Fixed oxide charges in the range of 7.1– ${9.4}\times {10} ^{{11}}$ cm $^{\boldsymbol -{2}}$ and 3.9– ${6.5}\times {10} ^{{11}}$ cm $^{\boldsymbol -{2}}$ on n- and p-type, respectively, are obtained. Excellent passivation results in the effective surface recombination velocity of 0.97 cms $^{\boldsymbol -{1}}$ and 8.07 cms $^{\boldsymbol -{1}}$ at minority carrier concentration of $10^{{15}}$ cm $^{\boldsymbol -{3}}$ on n- and p-type Czochralski silicon, respectively, without the use of capping layer. SiO2 film exhibits dielectric breakdown field strength of 4.3 MVcm $^{\boldsymbol -{1}}$ and 5.6 MVcm $^{\boldsymbol -{1}}$ and leakage current density of ${2.2}\times 10 ^{\boldsymbol -{8}}$ Acm $^{\boldsymbol -{2}}$ and ${1.1}\times {10} ^{\boldsymbol -{8}}$ Acm $^{\boldsymbol -{2}}$ at 1 MVcm $^{\boldsymbol -{1}}$ on n- and p-type silicon, respectively. These values are superior to sol-gel-based SiO2 films reported previously and are comparable to or better than those reported for other methods for the growth or deposition of SiO2 on silicon. Amorphous nature of the film is validated by high-resolution transmission electron microscopy (TEM). The X-ray photoelectron spectroscopy (XPS) analysis shows nearly stoichiometric SiO x film.

Energy transfer channel between silicon nanocrystals and an optical center emitting above their bandgap

This paper reports on the observation of an electronic energy transfer channel between silicon nanocrystals and the SiO2 matrix they are embedded in. This energy transfer manifests itself by photoluminescence at 650 nm, i.e. above the indirect band gap, in superlattices formed by alternating layers of P- or B-doped as well as undoped silicon nanocrystals (NCs) and stoichiometric SiO2 or SiOxNy. This band is only observable under strong (above ~13 mJ/cm2) UV femtosecond excitation. A detailed investigation of the excitation intensity dependence together with luminescence dynamics enabled us to ascribe this band to a non-bridging oxygen hole center located in the SiO2 (or SiOxNy) matrix, to study the influence of NC treatment on its photoluminescence and to determine the energy transfer mechanism.

Remarkable improvements in the performance and stability of Si photoanodes adopting nanocrystalline NiOx electrocatalyst and stoichiometric SiO2 protection

Silicon photoanodes have been studied intensively for efficient photoelectrochemical (PEC)-mediated catalysis of the oxygen evolution reaction, but their performance and stability still need to be improved before they can be applied practically. Here, we demonstrate that the porous, nanocrystalline NiOx electrocatalyst integrated with a thermally-grown SiO2 protection layer results in unexpectedly high PEC performances as well as long-term stability in Si photoanodes. The porous structure of NiOx and its large active area in contact to the electrolyte improves electrocatalytic activity and greatly reduces the kinetic overpotential required for charge transfer. In addition, the redox-active surface contributed by nanocrystalline nature of NiOx results in the accumulation of charges during the PEC reaction, predominantly affecting the interface energetics of NiOx/n-Si junctions. This feature enables a high open circuit potential of ~0.6 V even without p-n junctions. Stoichiometric SiO2 affords strong electrochemical corrosion resistance during the PEC water oxidation in 1 M KOH electrolyte, in contrast to the wet chemical-grown, nonstoichiometric SiOx. The porous and nanocrystalline NiOx and stoichiometric SiO2 incorporated in the n-Si photoanode ensures continuous operation for 100 h at high performance. These results can help inform a new way to design a high performance Si photoanodes with long-term stability.

Formation and Thermal Behaviors of Ternary Silicon Oxycarbides derived from Silsesquioxane Derivatives.

Silsesquioxane (SQ) derivatives possessing intramolecular H2C = CH- groups and Si-H groups were designed as precursors for ternary silicon oxycarbide (SiOC). By using R-Si(OMe)3, H-Si(OEt)3 and (H-Si(Me)2)2O as starting compounds, SQ derivatives of VH-SQ (R = vinyl) and St-H-SQ (R = stylyl) were successfully synthesized through the conventional sol-gel route. Simultaneous thermogravimetric and mass spectroscopic analyses up to 1000 °C revealed that in situ cross-linking via hydrosilylation and demethanation of VH-SQ suppressed the evolution of gaseous hydrocarbon species to afford amorphous SiOC having a composition close to the desired stoichiometric SiO2(1−x)Cx (x = ca. 0.3) with a high yield. The effect of carbon content on the phase separation and crystallization of the SQ-derived amorphous SiOC was studied by several spectroscopic analyses and TEM observation. The results were discussed aiming to develop a novel polymer-derived ceramics (PDCs) route for in situ formation of binary β-SiC-amorphous SiO2 nanocomposites with enhanced thermal and mechanical stability.

Optical grade SiO2 films prepared by HWCVD

Using a large-scale HWCVD in-line deposition system with maximum deposition areas of 500×600mm2 SiOx films were deposited from gas mixtures of SiH4 and O2 at pressures between 1.5 and 3.5Pa with substrate temperatures of 140 up to 360°C. Activation of the gas atmosphere was achieved via tungsten wires at temperatures of 1900 up to 2100°C. Keeping the silane gas flow at constant levels between 50 and 150sccm the ratio of oxygen gas flow to silane was varied from 0 to ~170% in order to vary the stoichiometry of the deposited films. Optical transmission was measured on Schott B270 and quartz glass substrates. Starting at oxygen ratios O2/SiH4<100% an increase of oxygen gas flow at first leads to an increase of transparency. After passing through transparency maxima at O2/SiH4 ratios of about 110–140%, further increase of oxygen gas flow leads to a decrease of transparency due to incorporation of partly vaporized tungsten wires. Process conditions for the optima of transparency and other film properties were investigated in detail as a function of the deposition parameters by using design of experiments. Due to a lack of ion-bombardment by HWCVD process film damage with high stress levels could be prevented. After optimization of the process parameters, highly transparent stoichiometric SiO2 films with residual compressive stresses of about 150MPa with high deposition rates (a>2nm/s) were deposited. With this development, antireflective coatings based on Si3N4-SiO2 layers on 10×10cm2 were both deposited by in-line HWCVD processes to demonstrate the potential for optical applications for the first time.

Single-step generation of metal-plasma polymer multicore@shell nanoparticles from the gas phase

Nanoparticles composed of multiple silver cores and a plasma polymer shell (multicore@shell) were prepared in a single step with a gas aggregation cluster source operating with Ar/hexamethyldisiloxane mixtures and optionally oxygen. The size distribution of the metal inclusions as well as the chemical composition and the thickness of the shells were found to be controlled by the composition of the working gas mixture. Shell matrices ranging from organosilicon plasma polymer to nearly stoichiometric SiO2 were obtained. The method allows facile fabrication of multicore@shell nanoparticles with tailored functional properties, as demonstrated here with the optical response.

Luminescent Properties of Silicon Nanocrystals:Spin on Glass Hybrid Materials

The photoluminescence characteristics of films consisting of Si nanocrystals either coated with or embedded into Spin on Glass (SOG) were studied. Si nanocrystals showing red or blue luminescence when suspended in alcohol solution were obtained from porous silicon films. These were then either deposited in Si substrates and coated with SOG, or mixed in an SOG solution that was later spun on Si substrates. Both types of films were thermally annealed at 1100 °C for three hours in N2 atmosphere. Transmission electron microscopy measurements showed a mean diameter of 2.5 nm for the Si nanocrystals, as well as the presence of polycrystalline Si nanoagglomerates. These results were confirmed by X-ray diffraction studies, which revealed the (111), (220) and (311) Bragg peaks in Si nanocrystals. Fourier transform infrared spectroscopy studies showed that the coated films present higher chemical reactivity, promoting the formation of non-stoichiometric SiO2, while the embedded films behave as a stoichiometric SiO2 after the thermal annealing. The PL (photoluminescence) characterization showed that both embedded and coated films present emission dominated by the Quantum Confinement Effect before undergoing any thermal treatment. After annealing, the spectra were found to be modified only in the case of the coated films, due to the formation of defects in the nanocrystals/SiO2 interface.

(Invited) Intense Red Light Emitting Mesoporous Silicon Nanowires for Luminescent Devices

Recent advancements in the growth and formation of semiconducting nanostructures, particularly group 14 metalloid semiconductors such as silicon and germanium have shown how quantum effects can be engineered and controlled (1,2) to modify thermal and optical properties (3). In particular, the nanostructuring of silicon materials mediates phonon scattering and confinement, including reduction of thermal conductivity (4,5) and Si-based nanowire heterostructures have been employed as solar cell materials and nanoelectronic power sources. Crystal sizes below the Bohr radius also results in light emission, and effective bandgap modification can influence absorbed thermopower in nanoscale silicon, and for photovoltaics. We report intense red luminescence from mesoporous n+-Si(100) nanowires (NWs) and nanocrystal-decorated p-Si NWs fabricated using electroless metal assisted chemical (MAC) etching (6-9). n+-Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure. p-type Si(100) NWs exhibit solid core structure, with a surface roughness that contains surface-bound nanocrystals. Both mesoporous n+-Si NWs and rough, solid p-Si NWs exhibit red luminescence at ~1.7 and ~1.8 eV, respectively. Time-resolved photoluminescence (PL) measurements indicated long (tens of µs) radiative recombination lifetimes. The red luminescence is visible with the naked eye and the red light is most intense from mesoporous n+-Si NWs, which exhibit a red-shift in the emission maximum to 1.76 eV at 100 K. The red PL from monolithic arrays of p-type NWs with nanocrystal-decorated rough surfaces is comparatively weak, but originates from the surface bound nanocrystals. Significant PL intensity increase is found during excitation for mesoporous NWs. X-ray photoelectron spectroscopy identifies a stoichiometric SiO2 on the rough p-Si NWs with a SiOx species at the NW surface. No distinct oxide is found on the mesoporous NWs. The analysis confirms that long life-time PL emission arises from quantum confinement from internal nanoscale crystallites, and oxidized surface-bound crystallites, on n+- and p-Si NWs respectively. References (1) G. S. Armatas and M. G. Kanatzidis, Science, 313, 817 (2006). (2) B. Kiraly, S. Yang and T. J. Huang, Nanotechnology, 24, 245704 (2013) (3) L. T. Canham, Appl. Phys. Lett., 57, 1046 (1990). (4) A. I. Hochbaum, R. Chen, R. D. Delgado, W. Liang, E. C. Garnett, M. Najarian, A. Majumdar and P. Yang, Nature, 451, 163 (2008). (5) A. I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W. A. Goddard Iii and J. R. Heath, Nature, 451, 168 (2008). (6) C. O'Dwyer, W. McSweeney and G. Collins, ECS J. Solid State Sci. Technol., 5, R3059 (2016). (7) W. McSweeney, H. Geaney and C. O'Dwyer, Nano Res., 8, 1395 (2015). (8) W. McSweeney, C. Glynn, H. Geaney, G. Collins, J. D. Holmes and C. O'Dwyer, Semicon. Sci. Technol., 31, 014003 (2015). (9) E. G. Chadwick, V. Mogili, C. O’Dwyer, J. Moore, J. Fletcher, F. Laffir, G. Armstrong and D. A. Tanner, RSC Adv., 3, 19393 (2013).