Formation Of Nanodots Research Articles

IrO2 nanodot formation by plasma enhanced atomic layer deposition as a charge storage layer.

IrO2 nanodot (ND) growth by plasma enhanced atomic layer deposition as a charge storage layer has been investigated. As-deposited IrO2 NDs showed a high density of 2.38 x 10(12)/cm2 and a small mean diameter of 3.22 nm, and turned into a spherical shape with a large dot-to-dot separation after annealing. The metal-oxide-semiconductor capacitor with Al2O3 blocking and IrO2 ND charge storing layers showed a memory window of 7.2 V. Compared with the sample without Ir2O3 NDs, the sample with NDs showed superior memory characteristics.

Engineering the strain in graphene layers with Au decoration

Graphene sheets decorated with Au nanodots are synthesized by deposition of Au of three different thicknesses and subsequent annealing at 400 °C. Different thicknesses of Au film for the formation of Au nanodots on graphene are measured using Rutherford backscattering spectrometry and morphology is studied using scanning electron microscopy. Raman spectroscopy indicates 3–6-fold increase in ID/IG ratio depending on the content of Au deposited on graphene. The increase in disorder in Au decorated graphene layers is explained on the basis of interaction of Au atoms with Π bonds of graphene. The splitting and blueshift in G band signifies compressive strain in Au deposited graphene. X-ray diffraction studies using synchrotron radiation source confirm compressive strain in graphene, which increases with increase of Au film thickness.

Control of molecular orientation and morphology in organic bilayer solar cells: Copper phthalocyanine on gold nanodots

Molecular orientation, morphology of donor (D)/acceptor (A) interface and photoabsorptivity in organic bilayer solar cells were controlled using Au nanodots with an ~20nm diameter inserted between the bottom electrode and the organic layer. Copper phthalocyanine (CuPc) molecules deposited onto the Au nanodot-coated electrode were mostly oriented face-on with large surface roughness, which is beneficial for photoabsorption, charge separation and transport. Furthermore, Au nanodots exhibit blue-shifted plasmon bands so that CuPc absorbs light more efficiently than that on thin Au layer. Bilayer C60/CuPc solar cells containing Au nanodots exhibited 1.4 times higher photoelectric conversion efficiency than those without Au nanodots. Factors for the enhanced efficiency are (i) improvement of the optical absorption characteristics by face-on orientation of CuPc and (ii) increase of the D/A heterointerface area. In addition, the shift of the plasmon absorption band of Au by the formation of nanodots makes absorption of the CuPc layer much more efficiently, resulting in better photovoltaic output.

Comparative study of the carbon nanofilm and nanodots grown by plasma-enhanced hot filament chemical vapor deposition

Carbon nanofilm and nanodots were grown by plasma-enhanced hot filament chemical vapor deposition using methane, hydrogen and nitrogen as the reactive gases. The results of field emission scanning electron microscopy, micro-Raman spectroscopy and X-ray photoelectron spectroscopy indicate that the amorphous carbon nanofilm and nanodots are formed without and with nitrogen, respectively. The formation of carbon nanofilm and nanodots is the consequence of different sputtering-etching effects. The photoluminescence (PL) of carbon nanofilm and nanodots was studied in a SPEX 1403 Ramalog system using a 325nm He–Cd laser as an excitation source and the PL spectra show the PL bands centered at about 411 and 513nm for the carbon nanofilm and 405 and 504nm for the carbon nanodots. Simultaneously, the PL results also indicate that the intensity of PL bands of carbon nanofilm is lower than that of carbon nanodots. The generation of different PL bands was interpreted by the transition mechanism. The difference in the intensity of PL bands is related to the size of carbon nanodots. The electron field emission (EFE) characteristics of carbon nanofilm and nanodots were investigated in a high-vacuum system. The results show that Fowler–Nordhelm curves are composed of two or three straight lines and the carbon nanofilm can emit a high current density, which originate from the diversification of carbon nanodots. The difference in the EFE results of carbon nanofilm and nanodots is associated to the size and number of carbon nanodots. These results can enrich our knowledge about carbon-based nanomaterials and are important to fabricate the carbon-based solid nanodevices in the field of optoelectronics.

Formation of ruthenium-dots on counter electrodes for dye sensitized solar cells

A 34 nm-thick ruthenium layer was deposited on a flat glass substrate by atomic layer deposition at low temperatures followed by the formation of self-aligned Ru micro or nano-dots by rapid thermal annealing for 30 seconds at 400°C. The resulting substrates were re-coated with another 34 or 69 nm-thick Ru layers by ALD at 250°C. Finally, the effective area of 0.45 cm2 dye sensitized solar cell with a glass / FTO / TiO2 / dye / electrode / (nano-dots Ru / Ru)/glass structure was fabricated. The microstructure was examined by field emission scanning electron microscopy and atomic force microscopy. The photovoltaic properties, such as the short circuit current density, open circuit voltage, fill factor, and energy conversion efficiency, were characterized using a solar simulator and potentiostat. FE-SEM confirmed that the 34 nm and 69 nm-thick Ru layers re-deposited by the second ALD process showed a 65% and 49% larger surface area, respectively, than the flat glass substrate due to the rearranged nano-dots. The ECE of the final DSSC was 2.62%, which was a 1.45 times larger than that of the 34 nm-Ru flat glass substrate. These results suggest that the efficiency of DSSCs can be improved by increasing the effective surface area of a counter electrode by low temperature ALD.

Synthesis, characterization, and effect of concentration variation on metal oxide nanostructures

Zinc oxide (ZnO) has been successfully synthesized by an anodization method, and it has been fabricated through anodization method with different concentration of zinc nitrate. The element composition, surface inspection, structural, and morphological features of the products are depend on the concentration of zinc nitrate. At lower concentration (0.01M), SEM image shows ZnO nanowires with average width of about 30 and 50 nm. With increase in the concentration of zinc nitrate from 0.01 to 0.05 M, the nanowires change into the nanosheets with average width of about 0.5 and 1.5 μm. For samples (0.1 M) exhibits nanodots, morphology was composed of hundreds of nanosheets with thickness is about 90 nm on average. When the concentration increases to 0.2 M, the nanodots trench became bigger with diameter about 1.2–2.0 μm. When the concentration of zinc nitrate is 0.3 M, the average diameter of nanodots is about 2–2.5 μm. The trench of nanosheets becomes thinner and shorter, but the number of nanosheets increases with diameter 20–50 nm. The formation of nanowires, nanodots, and nanosheets nanostructures is also believed to result from actions on concentration of zinc nitrate as an aquas medium. The EDX result shows the atomic percentage (at.%) of the oxygen increased when the concentration of zinc nitrate increased. The pattern of EDX indicates that the ZnO nanostructures are composed of Zn, O, and Al. They represent Al composition in the sample because the anode using the aluminum rod during experiment.

High Density Formation of Iron Nanodots on SiO<sub>2</sub> Induced by Remote Hydrogen Plasma

We have successfully formed Fe-nanodots with an areal dot density as high as ~2.4×1011 cm-2 on thermally grown SiO2 by exposing a 3-nm-thick Fe layer to a remote plasma of pure H2 without external heating. During remote H2 plasma exposure, the surface temperature rising up to ~500 °C was caused by surface recombination of atomic H, which enhanced surface migration of Fe atoms and promoted self-assembling nanodots. Electrical separation among Fe-nanodots was also verified from the changes in surface potential after charge injection using an AFM/Kelvin probe technique. The magnetic characterization by using magnetic force microscopy suggests that Fe-nanodots act as not only charge storage nodes but also spin-dependent active elements.

Characterization of Ge Films on Si(001) Substrates Grown by Nanocontact Epitaxy

We quantitatively characterized Ge films epitaxially grown on Si(001) substrates by nanocontact epitaxy. The naocontact epitaxy was composed of three stages: elastically-strain-relaxed nanodot (ND) formation, second layer growth for surface flattening, and final layer growth for high crystallinity. X-ray diffraction results showed that the lattice mismatch strain was almost-fully relaxed on the order of ∼0.05% in Ge films as thin as 100 nm, and was independent of the growth condition of films (second and final layers) above NDs. This supports the idea of a mechanism in which strain relaxation is caused by elastically-strain-relaxed ND formation. It was also verified by simple elastic theory calculation and plan-view transmission electron microscopy. Photoluminescence was observed from the thin film at ∼0.8 eV, suggesting high quality with less point defects.

Silicon nanodot formation and self‐ordering under bombardment with heavy Bi3 ions

AbstractSi nanodots of high density and hexagonal short‐range order are observed upon normal‐incidence bombardment of hot, crystalline Si with Bi3+ ions having a kinetic energy of a few tens of keV. The heights of nanodots are comparable to their widths of ∼20 nm. The implanted Bi accumulates in tiny Bi nanocrystals in a thin Si top layer which is amorphous due to implantation damage. Light and heavy ions up to Xe cause smoothing of surfaces, but Bi3+ ions considered here have a much higher mass. Atomistic simulations prove that each Bi3+impact deposits an extremely high energy density resulting in a several nanometer large melt pool, which resolidifies within a few hundreds of picoseconds. Experiments confirm that dot patterns form only if the deposited energy density exceeds the threshold for melting. Comparing monatomic and polyatomic Bi ion irradiation, Bi–Si phase separation and preferential ion erosion are ruled out as driving forces of pattern formation. A model based on capillary forces in the melt pool explains the pattern formation consistently.magnified imageHigh‐density Si nanodots are formed by polyatomic Bi ion irradiation of hot Si surfaces. Each impact causes local transient melt pools smaller than the dots. Hexagonally ordered patterns evolve by self‐organization driven by repeated ion‐induced melting of tiny volumes. Homogeneously distributed Bi nanocrystals are found in the a‐Si film. These nanocrystals are related to particularities of the Si–Bi phase diagram. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent progress in thin film epitaxy across the misfit scale (2011 Acta Gold Medal Paper)

This paper discusses recent progress in thin film epitaxy across the misfit scale through the paradigm of domain matching epitaxy (DME). This epitaxy across the misfit scale is critical for integrating multifunctionality on a chip and creating smart structures for next-generation solid-state devices. There are three sources of strains that are cumulative at the growth temperature, and the relaxation process starts during the growth process. Upon cooling, unrelaxed lattice, thermal and defect strains give rise to net residual strains. In large misfit (ε⩾10%) systems, where lattice misfit strain is predominant, it can be relaxed completely, and then only thermal and defect strains remain upon cooling. In low misfit systems, all three sources contribute to the residual strain upon cooling, as result of incomplete lattice relaxation. The predominant strain relaxation mechanism in thin films is by nucleation of dislocations at the free surface, as the nucleation energy in the bulk is considerably higher. At the free surface, the activation barrier for dislocation nucleation is considerably lower at the steps. Since the step formation energy is lower under a compressive stress compared with tensile stress, it reduces nucleation energy under compressive stress and lowers the critical thickness compared with tensile stresses in thin films. Once the dislocation nucleates, it propagates or glides to the interface to relieve the strain. However, if lattice frictional stress in the film is high, most dislocations may not reach the interface, depending upon the growth temperature and rate. Thus, these two key steps, dislocation nucleation and propagation, play a critical role in the thin film relaxation process. Once the dislocations reach the interface, the atomic structure of the dislocation at the heterointerfaces determines its electronic properties, specifically trapping and recombination characteristics. It is found that the atomic structure of the dislocation is determined by the interplay between strain and chemical free energies. Thus, the dislocations (representing missing or extra planes) play a critical role in the relaxation of thin film heterostructures. This paper focuses on epitaxy across the misfit scale, based upon matching of integral multiples of lattice planes. If the misfit falls between the integral multiples, it is accommodated by the principle of domain variation, where domains alternate to accommodate the misfit. Details of epitaxy from low misfit (∼4%) in Ge/Si) to large misfit (∼22%) in TiN/Si are shown. In III-nitride/sapphire and II-oxide/sapphire systems, this paper deals with polar orientations, where misfit is uniform in the basal plane, and non-polar orientations, where misfit varies over an order of magnitude in the film plane. It is shown that the DME paradigm is key to the integration of thin film heterostructures across the misfit scale and other complex systems such as vanadium oxide and PZT systems on Si(100) substrates for the integration of functionalities on a computer chip. Finally, it is shown that the formation of epitaxial and self-assembled nanodots on Si(100) provides a critical advance, with tremendous implications for information and data storage and related nanomagnetics applications.

Ru nanodot synthesis using CO2 supercritical fluid deposition

CO2-based supercritical fluid deposition (SCFD) was utilized for the synthesis of ruthenium nanostructures on Si substrates and hafnium oxide (HfO2) films. Ru nanodots were formed by the reduction of a Ru precursor under a hydrogen environment. Ruthenium carbonyl was dissolved in supercritical CO2 in a pre-mix vessel, and this mixture was pumped into a reaction vessel which allowed it to undergo rapid expansion. The formation of Ru nanodots was studied as a function of key process parameters such as the weight of precursor used, the reaction temperature and the H2 reactant pressure. Utilizing the Ru nanodots on a HfO2 control oxide, a flash memory device containing nanodots was fabricated and its memory characteristics were demonstrated.

Formation of Si or Ge nanodots in Si3N4 with in-situ donor modulation doping of adjacent barrier material

Conventional doping of small nanodots does not provide majority carriers due to self-purification effects and much increased ionisation energies of dopants presenting point defects. Adjacent barrier layers to Si-rich Si3N4 can be doped by excess Si in-situ with the segregation anneal for Si nanodot formation. Donor doping of AlxGa1−xN (0 ⩽ x ⩽ 1) with Si is an established process. Material properties and process compatibility of AlxGa1−xN render it suitable as barrier for Si nanodot superlattices. Ab-initio calculations showed that Ge also works as a donor in AlN, extending the material range to Ge and SiGe nanodots in Si3N4.

Formation and Characterization of Hybrid Nanodots Floating Gate for Optoelectronic Application

ABSTRACTWe have fabricated a hybrid nanodots floating gate (FG) in which Si quantum dots (QDs) and silicide nanodots (NDs) are stacked with a very thin SiO2 interlayer in order to satisfy both multiple valued capability and charge storage capacity for a sufficient memory window and to open up novel functionality for optoelectronic application. In electron charging and discharging characteristics measured with application of pulsed gate biases to MOS capacitors with a hybrid NDs FG, stepwise changes in the rates for electron injection and emission were revealed with increasing pulse width at room temperature. Also, nMOSFETs with a hybrid NDs FG show unique hysteresis with stepwise changes in the drain current - gate voltage characteristics. The observed characteristics can be interpreted in terms that the electron injection and storage into silicide-NDs proceed through the discrete charged states of Si-QDs. For MOS capacitors with a triple-stacked hybrid NDs FG fabricated by adding another Si-QDs, by subgap light irradiation from the back side of the Si substrate, a distinct infrared optical response in C-V characteristics was detected at room temperature. The result is attributable to the shift of charge centroid in the hybrid NDs FG as a result of transfer of photoexcited electrons from silicide NDs to Si-QDs.

Thermoelectric characteristics of Sb2Te3 thin films formed via surfactant-assisted electrodeposition

In this study, Sb2Te3 films were electrodeposited potentiostatically at room temperature from acidic nitric baths in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). This resulted in improvements in the surface morphologies of the films. The thus-deposited films also exhibited better adherence to their substrates. In addition, the carrier transport properties of the Sb2Te3 films, including their electrical conductivities and Seebeck coefficient values, could be improved by annealing them at 200 °C. This was due to the formation of Te nanodots in the Sb2Te3 matrix. The resulting Sb2Te3 films with Te nanodots 10–20 nm in size and a dispersion density of 11.4 vol% exhibited a high power factor of 716.0 μW m−1 K−2, which was about two times higher than that of Sb2Te3 films that did not contain Te nanodots. The results of the study suggested that the improvements in the thermoelectric characteristics of the films were due to carrier energy filtering at the Te–Sb2Te3 interface.

Self-assembly of polystyrene-block-poly(4-vinylpyridine) block copolymer on molecularly functionalized silicon substrates: fabrication of inorganic nanostructured etchmask for lithographic use

Block copolymers (BCPs) are seen as a possible cost effective complementary technique to traditional lithography currently used in the semiconductor industry. This unconventional approach has received increased attention in recent years as a process capable of facilitating the ever decreasing device size demanded. Control over microdomain orientation and enhancing long range order are key aspects for the utility of BCPs for future lithographic purposes. This paper provides an efficient route for the fabrication of highly ordered nanostructures suitable for such application. We investigate the significant effect of surface treatment regarding the self-assembly process of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) by employing an ethylene glycol layer, producing well defined perpendicular P4VP cylinders with long range order over large surface areas. Nanopores are generated through surface reconstruction using a preferential solvent, which allows for the incorporation of an inorganic moiety. Treatment of this pattern with UV/Ozone leads to formation of well-ordered iron oxide nanodots with a pitch of ∼26 nm. Furthermore, high aspect ratio silicon nanopillars result following pattern transfer (using Ar/O2).

Real time x-ray studies during nanostructure formation on silicon via low energy ion beam irradiation using ultrathin iron films

Real time grazing incidence small angle x-ray scattering and x-ray fluorescence (XRF) are used to elucidate nanodot formation on silicon surfaces during low energy ion beam irradiation of ultrathin iron-coated silicon substrates. Four surface modification stages were identified: (1) surface roughening due to film erosion, (2) surface smoothing and silicon-iron mixing, (3) structure formation, and (4) structure smoothing. The results conclude that 2.5 × 1015 iron atoms in a 50 nm depth triggers surface nanopatterning with a correlated nanodots distance of 25 nm. Moreover, there is a wide window in time where the surface can have correlated nanostructures even after the removal of all the iron atoms from the sample as confirmed by XRF and ex-situ x-ray photoelectron spectroscopy (XPS). In addition, in-situ XPS results indicated silicide formation, which plays a role in the structure formation mechanism.

Material characterization of high Sn-content, compressively-strained GeSn epitaxial films after rapid thermal processing

We report on the characterization of high Sn-content (∼10% Sn) GeSn films grown on (001) Ge/Si substrates using reduced-pressure chemical vapor deposition. Pseudomorphic 30nm GeSn films were grown on relaxed Ge buffers, exhibit a smooth surface, and display strong photoluminescence (PL) with cavity-resonance fringes from the Ge buffer. Additional luminescence studies confirm that the measured PL originates from the GeSn film. A study on the effects of rapid thermal annealing is presented along with evidence of Sn surface segregation and formation of surface nanodots for anneals at 450°C and above. Anneals at 400°C for up to 500s showed little change in PL intensity or material properties, which suggest that a critical temperature exists for post-growth thermal-processing of high Sn-content, compressively-strained GeSn films.

Structural properties of Si/SiO2 nanostructures grown by decomposition of substoichiometric SiOxNy layers for photovoltaic applications

AbstractThe structural properties of crystalline Si nanodots embedded in a SiO2 matrix are investigated with respect to the exploitation of quantum confinement effects (QCE) in Si solar cells. The nanostructures are grown on crystalline Si (c‐Si) wafers by decomposition of substoichiometric SiOxNy layers with various [O]/[Si] ratios. Cross‐sectional high‐resolution transmission electron microscopy investigations reveal the formation of separated single crystalline nanodots with diameters below 5 nm inside the SiOxNy volume and directly on the c‐Si wafer. The density and diameter of the nanodots decreases with increasing [O]/[Si] ratio, leading to inter‐dot distances above 10 nm for [O]/[Si]>1.3. Photoluminescence (PL) spectra are blue‐shifted relative to the Si bulk PL, which is in good agreement with theoretical QCE models. It is found that for observing the PL signal the nanodots must be covered by a SiO2 shell to reduce charge carrier recombination via defects at the nanodot surface. This requires an [O]/[Si] ratio >0.5 for which the inter‐dot distance becomes too large for charge carrier transport between the nanodots. It is concluded that a better control over the nanodot formation at high [O]/[Si] ratios has to be achieved before QCE can be successfully applied in Si solar cell devices.

Boron doping of silicon rich carbides: Electrical properties

Abstract Boron doped multilayers based on silicon carbide/silicon rich carbide, aimed at the formation of silicon nanodots for photovoltaic applications, are studied. X-ray diffraction confirms the formation of crystallized Si and 3C-SiC nanodomains. Fourier Transform Infrared spectroscopy indicates the occurrence of remarkable interdiffusion between adjacent layers. However, the investigated material retains memory of the initial dopant distribution. Electrical measurements suggest the presence of an unintentional dopant impurity in the intrinsic SiC matrix. The overall volume concentration of nanodots is determined by optical simulation and is shown not to contribute to lateral conduction. Remarkable higher room temperature dark conductivity is obtained in the multilayer that includes a boron doped well, rather than boron doped barrier, indicating efficient doping in the former case. Room temperature lateral dark conductivity up to 10−3 S/cm is measured on the multilayer with boron doped barrier and well. The result compares favorably with silicon dioxide and makes SiC encouraging for application in photovoltaic devices.

A New Process to Fabricate Three Dimensional Ordered Nano Dot Array Structures by Nano Plastic Forming and Dewetting

Three dimensional (3D) nano/quantum dot array structures have attracted more and more attention due to their broad applications. A new fabrication method of multilayer ordered nano dot array with low cost and high throughput is developed in this paper. This process is combination of Top-down and Bottom-up approaches: Nano Plastic Forming (NPF) patterning of metal layer coated on the substrate as Top-down approach and self-organization by dewetting as Bottom-up approach. Effects of process conditions on 3D nano-dot array formation are studied experimentally. Regularity and uniformity of first layer nano-dot array is transferred to the second layer nano-dots by optimizing thickness of the spacer layer and Au coating layer. Multilayer ordered nano dot array structures with good alignment are obtained by repeating coating and annealing processes.