C. P. Saini

Improved broadband antireflection in Schottky-like junction of conformal Al-doped ZnO layer on chemically textured Si surfaces

Chemically textured Si with improved absorption in the complete range of solar spectrum is investigated by ultraviolet/visible/near-infrared (UV/Vis/NIR) spectroscopy, showing an average specular reflectance of ∼0.4% in the wavelength of 500–3000 nm. The pyramidal structures on such solar-blind Si can reduce the reflectance further below 0.1% in the UV region by conformal growth of granular Al-doped ZnO (AZO) films. X-ray diffraction analyses suggest the growth of polycrystalline AZO on faceted-Si. Moreover, marginal increase in electrical conductivity of AZO is found on textured surfaces, whereas rise in leakage current in Schottky-like Ag/AZO/Si/Ag heterostructure devices is noticed with increasing Si surface area.

Self-organized titanium oxide nano-channels for resistive memory application

Towards developing next generation scalable TiO2-based resistive switching (RS) memory devices, the efficacy of 50 keV Ar+-ion irradiation to achieve self-organized nano-channel based structures at a threshold fluence of 5 × 1016 ions/cm2 at ambient temperature is presented. Although x-ray diffraction results suggest the amorphization of as-grown TiO2 layers, detailed transmission electron microscopy study reveals fluence-dependent evolution of voids and eventual formation of self-organized nano-channels between them. Moreover, gradual increase of TiO/Ti2O3 in the near surface region, as monitored by x-ray photoelectron spectroscopy, establishes the upsurge in oxygen deficient centers. The impact of structural and chemical modification on local RS behavior has also been investigated by current-voltage measurements in conductive atomic force microscopy, while memory application is manifested by fabricating Pt/TiO2/Pt/Ti/SiO2/Si devices. Finally, the underlying mechanism of our experimental results has been...

Defect-engineered optical bandgap in self-assembled TiO2 nanorods on Si pyramids

Transformation of self-assembled crystalline TiO2 nanorods to amorphous layer, and the corresponding impact on optical-bandgap (Eg) on Si pyramids are investigated by irradiating with 50 keV Ar+-ions. Initially, Eg is found to be reduced from 3.23 to 2.94 eV up to a fluence of 1 × 1016 ions/cm2, and discussed in terms of the rise in oxygen vacancies (VO). However, a sudden increase in Eg to 3.38 eV is detected at a fluence of 1 × 1017 ions/cm2 through evolution of voids by over-saturating VO, manifesting the appearance of degenerate states by shifting the Fermi level above the conduction band minimum via Burstein-Moss effect.

Thickness-dependent blue shift in the excitonic peak of conformally grown ZnO:Al on ion-beam fabricated self-organized Si ripples

Al-doped ZnO (AZO) thin films of thicknesses 5,10, 15, 20, and 30 nm were deposited on 500 eV argon ion-beam fabricated nanoscale self-organized rippled-Si substrates at room temperature and are compared with similar films deposited on pristine-Si substrates (without ripples). It is observed that morphology of self-organized AZO films is driven by the underlying substrate morphology. For instance, for pristine-Si substrates, a granular morphology evolves for all AZO films. On the other hand, for rippled-Si substrates, morphologies having chain-like arrangement (anisotropic in nature) are observed up to a thickness of 20 nm, while a granular morphology evolves (isotropic in nature) for 30 nm-thick film. Photoluminescence studies reveal that excitonic peaks corresponding to 5–15 nm-thick AZO films, grown on rippled-Si templates, show a blue shift of 8 nm and 3 nm, respectively, whereas the peak shift is negligible for 20-nm thick film (with respect to their pristine counter parts). The observed blue shifts are substantiated by diffuse reflectance study and attributed to quantum confinement effect, associated with the size of the AZO grains and their spatial arrangements driven by the anisotropic morphology of underlying rippled-Si templates. The present findings will be useful for making tunable AZO-based light-emitting devices.

Self-decorated Au nanoparticles on antireflective Si pyramids with improved hydrophobicity

Post-deposition annealing mediated evolution of self-decorated Au nanoparticles (NPs) on chemically etched Si pyramids is presented. A distinct transformation of Si surfaces from hydrophilic to hydrophobic is initially found after chemical texturing, showing an increase in contact angle (CA) from 58° to 98° (±1°). Further improvement of hydrophobicity with CA up to ∼118° has been established after annealing a 10 nm thick Au-coated Si pyramids at 400 °C that led to the formation of Au NPs on Si facets along with self-ordering at the pyramid edges. Detailed x-ray diffraction studies suggest the evolution of crystalline Au NPs on strained Si facets. Microstructural studies, however, indicate no mixing of Au and Si atoms at the Au/Si interfaces, instead of forming Au nanocrystals at 400 °C. The improved hydrophobicity of Si pyramids, even with Au NPs can be explained in the light of a decrease in solid fractional surface area according to Wenzels model. Moreover, a sharp drop in specular reflectance from Si ...

Probing electron density across Ar+ irradiation-induced self-organized TiO2−x nanochannels for memory application

The variation of electron density in TiO2−x nanochannels, exhibiting resistive switching phenomenon, produced by Ar+ ion-irradiation at the threshold fluence of 5 × 1016 ions/cm2 is demonstrated by X-ray reflectivity (XRR). The transmission electron microscopy reveals the formation of nanochannels, while the energy dispersive X-ray spectroscopy confirms Ti enrichment near the surface due to ion-irradiation, in consistent with the increase in electron density by XRR measurements. Such a variation in Ti concentration indicates the evolution of oxygen vacancies (OVs) along the TiO2−x nanochannels, and thus paves the way to explain the operation and performance of the Pt/TiO2−x/Pt-based memory devices via OV migration.

Probing the impact of energetic argon ions on the structural properties of ZnO:Al/TiO2 heterostructures

The efficacy of 50 keV Ar+-ion irradiation toward the interfacial and stoichiometric engineering of strained Al-doped ZnO (AZO)/TiO2 heterostructure is systematically investigated using a variety of experimental techniques, notably by cross-sectional transmission electron microscopy. Glancing-angle X-ray diffraction evidences the release of in-plane compressive stress from the as-grown AZO/TiO2 bilayer structure at a critical fluence of 1 × 1016 ions/cm2, and we discuss in the light of microcracks and voids formation combined with the dewetting phenomenon. Ion irradiation also leads to an improvement of stoichiometry in both top AZO and underneath amorphous TiO2 layers, as manifested by depth-dependent energy dispersive X-ray spectroscopy owing to the large diffusion of oxygen toward the AZO/TiO2 interfacial region through the AZO defect sites. Such ion beam induced self-healing in stoichiometry of AZO/TiO2 heterostructure has been attributed to a conjunction of sputtering and diffusion phenomena involving the constituent elements (Zn, Ti, and O). Further increase in ion fluence up to 5 × 1016 ions/cm2 causes a complete deterioration of the heterostructure with the formation of a graded layer via intermixing of these elements, followed by the evolution of voids.The efficacy of 50 keV Ar+-ion irradiation toward the interfacial and stoichiometric engineering of strained Al-doped ZnO (AZO)/TiO2 heterostructure is systematically investigated using a variety of experimental techniques, notably by cross-sectional transmission electron microscopy. Glancing-angle X-ray diffraction evidences the release of in-plane compressive stress from the as-grown AZO/TiO2 bilayer structure at a critical fluence of 1 × 1016 ions/cm2, and we discuss in the light of microcracks and voids formation combined with the dewetting phenomenon. Ion irradiation also leads to an improvement of stoichiometry in both top AZO and underneath amorphous TiO2 layers, as manifested by depth-dependent energy dispersive X-ray spectroscopy owing to the large diffusion of oxygen toward the AZO/TiO2 interfacial region through the AZO defect sites. Such ion beam induced self-healing in stoichiometry of AZO/TiO2 heterostructure has been attributed to a conjunction of sputtering and diffusion phenomena involvin...

Oxygen mediated phase transformation in room temperature grown TiO2 thin films with enhanced photocatalytic activity

Room temperature transformation from anatase (A-TiO2) to rutile (R-TiO2) thin films through an intermediate mixed phase on stainless steel driven by a controlled oxygen flow rate (OFR) is investigated. Such OFR dependent phase transition is confirmed by X-ray diffraction and also consistent with X-ray absorption spectroscopy at Ti L and O K-edges, showing a long range ordering in TiO6 octahedral symmetry. X-ray photoelectron spectroscopy reveals a gradual reduction in Ti2O3 and/or TiO intermediate phases with increasing OFR. Finally, an enhanced photocatalytic activity is observed in the mixed phase and discussed in terms of photo-generated charge transport in the type-II staggered band structure between A-TiO2 and R-TiO2 phases.Room temperature transformation from anatase (A-TiO2) to rutile (R-TiO2) thin films through an intermediate mixed phase on stainless steel driven by a controlled oxygen flow rate (OFR) is investigated. Such OFR dependent phase transition is confirmed by X-ray diffraction and also consistent with X-ray absorption spectroscopy at Ti L and O K-edges, showing a long range ordering in TiO6 octahedral symmetry. X-ray photoelectron spectroscopy reveals a gradual reduction in Ti2O3 and/or TiO intermediate phases with increasing OFR. Finally, an enhanced photocatalytic activity is observed in the mixed phase and discussed in terms of photo-generated charge transport in the type-II staggered band structure between A-TiO2 and R-TiO2 phases.