Si Dots Research Articles

Nanosilicon dot arrays with a bit pitch and a track pitch of 25nm formed by electron-beam drawing and reactive ion etching for 1Tbit∕in.2 storage

The formation of very fine Si dots with a bit pitch and a track pitch of less than 25nm using electron-beam (EB) lithography on ZEP520 and calixarene EB resists and CF4 reactive ion etching has been demonstrated. The experimental results indicate that the calixarene resist is very suitable for forming an ultrahigh-packed bit array pattern of Si dots. This result promises to open the way toward 1Tbit∕in.2 storage using patterned media with a dot size of <15nm.

FORMATION OF Si NANOSTRUCTURES USING DRY ETCHING WITH SELF-ORGANIZED METAL OXIDE NANOPILLAR MASKS

ABSTRACT In this study, a novel method is being proposed for the formation of nanostructures, utilizing the electrochemical anodization of Al/metal layers and high density plasma etching. Self-assembled metal oxide pillars formed by anodizing Al/metal layers were employed as an etching mask for nanostructure formation. Once the shape and size of the metal oxide pillars have been determined, the shape and size of the Si dots are dependent on the etching conditions such as etching gas, coil power, and dc-bias. The etch characteristics of Si film and metal oxide pillars were investigated with regard to etch rate, selectivity and profile. Highly ordered Si nanodot arrays were fabricated on a wafer using optimized etching conditions.

Study of Charged States of Si Quantum Dots with Ge Core

Si quantum dots with Ge core were self-assembled on thermally- grown SiO2 from thermal decomposition of alternately pure SiH4 and GeH4 diluted with He. Changes in the surface potential caused by electron charging to and discharging from individual Si dot with and without Ge core were studied by using a Kelvin probe technique in atomic force microscopy (AFM). The potential change attributable to the extraction of one electron from a Si dot with Ge core was observed after tapping an AFM tip biased at +1.2V while for pure Si dot with almost the same size, the tip bias required for causing a similar potential change was as high as +3.0V. Also there was no significant difference in tip bias for electron injection between Si dot with and without Ge core. The result suggests an energy band structure in which Ge core act as a deep potential well for holes but less modulate the conduction band. Correspondingly, stable positively-charged states for Si dots with Ge core were confirmed in comparison with pure Si dots.

Effects of partial self-ordering of Si dots formed by chemical vapor deposition on the threshold voltage window distribution of Si nanocrystal memories

We study the role that the denuded zone around Si nanocrystals obtained by chemical vapor deposition plays on the fluctuations of the dot surface coverage. In fact, the capture mechanism of the silicon adatoms in the proximity of existing dots restricts the number of possible nucleation sites, the final dot size, and the dot position, thus driving the process toward partial self-order. We numerically evaluate the relative dispersion of surface coverage for several gate areas and compare the results to the fully random case. The coverage dispersion is related to the fluctuations from bit to bit of the threshold voltage window (ΔVth) distribution of nanocrystal memories. The evaluations, compared to experimental data on ΔVth extrapolated to small gate areas, provide very favorable projections on the scalability of these memories.

Photoluminescence lifetimes of Si quantum dots

We present a continuum model for the calculation of the electron states in Si dots that accounts for the effects of size, shape, and crystallographic orientation of the dots. This formalism has been used to study the behavior of the photoluminescence (PL) lifetime in Si quantum dots. This is due to the anisotropy of the silicon band structure and the confinement in quantum dots, which result in a cluster of energy levels from the different valleys of Si. Although these levels are very close in energy, they have very different recombination rates. Hence, there are (relatively) fast and slow levels at approximately the same energy. This feature causes a temperature dependence of PL in Si nanostructures, hence it is suggested that dispersion in the magnitude of the PL lifetimes in Si dots is at the origin of the observed stretched exponential behavior of PL lifetime in porous Si. Both zero phonon and phonon-assisted recombinations have been included in the calculations. Zero phonon recombination dominates in small dots (∼2nm) and the lifetime is ∼10μs. In larger dots, of a size of ∼4nm and above, phonon-assisted transitions become dominant and PL lifetimes are of the order of 1–10ms.

Structural and optical properties of Si∕SiO2 superlattices prepared by low pressure chemical vapor deposition

Si ∕ Si O 2 superlattices (SLs) structures were prepared using a low-pressure chemical vapor deposition (LPCVD) method. The structural and optical properties of the SLs materials were characterized using atomic force microscopy (AFM), Fourier transformed infrared (FTIR) absorption, x-ray diffraction, and room-temperature photoluminescence (PL) measurements. The AFM results show that a periodically layered Si∕SiO2 structure was successfully deposited with nanometer-sized Si dots embedded in the Si layers. The FTIR spectra show that the SiO2 near the Si∕SiO2 interface is more ordered than the amorphous SiO2 in the center of the SiO2 layers. The Si∕SiO2 SLs films show a room-temperature PL in the visible-near infrared wavelength region. The PL intensity is significantly enhanced by a high-temperature annealing at 1100°C. The peak position and intensity of the main emission band in the PL spectra strongly depend on the Si layer thickness. A pronounced redshift with increasing Si layer thickness is observed. This emission peak position can be fitted by the theory of quantum confinement effect in Si dots. The emission from recombination through defect and interface states was also observed in the SLs films.

Ge-dot/Si multilayered structures through Ni-induced lateral crystallization

We demonstrate a method for fabricating high-quality Ge-dot/Si multilayered structures. High-density self-assembled Ge dots are grown on amorphous Si layer periodically by low-pressure chemical vapor deposition, and then the amorphous Si are crystallized through Ni-based metal-induced lateral crystallization. Optical microscopy, micro-Raman spectroscopy, and electron microscopy observations reveal that the crystallized Si film has large leaflike grains elongated along the lateral crystallization direction, which shows (110) preference. Furthermore, this preference is found to deliver to different Si layers. The strain shift of Ge dots deduced from Raman spectroscopy reveals a formation of a high-quality interface between the crystallized Si and Ge dot.

The effect of redeposition on the ion flux dependence of Si dot pattern formation during ionsputter erosion

We report an ion flux dependence study of the Si dot pattern formed on Si(100) byAr+ ion sputtering with the ion energy being 1.5 keV, ion dose5 × 1017 ions cm−2, and ion fluxranging from 280 to 1100 µA cm−2. Experimental results show that the lateral dot diameterd and theion flux f basically follow the relationship of , and the surface roughness w decreases with increasing f in an exponential decay manner. Simulations based on a widely accepted continuummodel, namely the noisy Kuramoto–Sivashinsky equation, reproduced the trend ford versusf but failed toexplain that for w versus f. A redeposition consideration was then suggested. It is found that with this correction not only arethe d–f and w–f relationships well explained, but the simulated surface morphology bears closer resemblanceto the experimental one as well. The effect of redeposition becomes important forf> ∼130 µA cm−2 as derived in this work.

Self-aligned TiSi2∕Si heteronanocrystal nonvolatile memory

The titanium silicide/silicon (TiSi2∕Si) heteronanocrystals are fabricated on SiO2 thin films. The metal-oxide-semiconductor structure embedding the TiSi2∕Si heteronanocrystals shows superior performance over the Si dot device. The charge loss rate in the TiSi2∕Si heteronanocrystal device is 7.5 times less than that of the Si dot device. It is also found that the TiSi2∕Si heteronanocrystal device has wider memory window than the Si dot counterpart.

Calculations of the electronic structure of silicon quantum dots: oxidation-induced redshifts in the energy gap

We study oxidation-induced redshifts in the energy gap for spherical Si30, Si42, Si87, Si99, Si167 and Si191 dots (of 1–2 nm in diameter) passivated with hydrogen by self-consistent calculations using the extended Hückel-type nonorthogonal tight-binding method for three different oxygen configurations (double-bonded, backbonded and inserted). While the nature of the lowest unoccupied molecular orbital (LUMO) state does not depend significantly on the dot size, the highest occupied molecular orbital (HOMO) state is associated closely with oxygen in the Si167 or smaller Si dots, and has a much larger Si contribution in the largest Si191 dot, so that the HOMO energy in the Si167 or smaller Si dots depends significantly on the oxygen configuration, while that in the Si191 dot does not. We find that the HOMO–LUMO energy gaps calculated for these Si dots double-bonded to oxygen are all dipole allowed, but gradually decrease from 2.2 eV to about 1.7 eV with increasing dot size, while the inserted oxygen configuration does not cause a significant energy-gap redshift even in the smallest Si dot. Finally, it is found that the energy gaps calculated for the Si dots backbonded to oxygen coincide well with luminescence redshifts observed in porous Si.

Analysis of strain and intermixing in single-layerGe∕Siquantum dots using polarized Raman spectroscopy

The built-in strain and composition of as-grown and Si-capped single layers of Ge/ Si dots grown at various temperatures 460– 800 ° C are studied by a comparative analysis of the Ge-Ge and Si-Ge modes in the polarized Raman spectra of the dots. A pronounced reduction of the strain and Ge content in the dots after deposition of the cap layer at low temperatures is observed, indicating that strain-induced Si diffusion from the cap layer is occurring. For large dots grown at 700– 800 ° C the observations are in agreement with a model of the Ge/ Si dot consisting of a Si-rich boundary region and a Ge-rich core.

Ge‐dots/Si multilayered structures fabricated by Ni‐induced lateral crystallization

AbstractWe report on the fabrication of high‐quality Ge‐dots/Si multilayered structure films by metal‐induced lateral crystallization (MILC). Ge dots are self‐assembled grown on amorphous Si layer using low‐pressure chemical vapor deposition, and then low‐temperature crystallization is carried out by Ni‐based MILC processing. The micro Raman spectroscopy, optical microscopy and electron microscopy observations reveal that the crystallized Si film has large leaf‐like grains elongated along the MILC direction with (110) preference. Moreover, the strain shift of Ge dots reflects the formation of high quality interface between the crystallized Si and Ge dot. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dedicated fabrication of silicon-based ensembles of dot molecules with a specific and unique number of dots

We have performed a two-step liquid phase epitaxy yielding ensembles of SiGe∕Si(001) dot molecules with a specific and unique number of dots. An undersaturation of the initial bismuth solution causes strain-induced pits in the epitaxial Si0.985Ge0.015 layer which are effectively preserved during subsequent Si0.68Ge0.32 dot growth at considerably lower temperatures. Since the latter process happens extremely close to thermodynamic equilibrium, we are able to interrupt it after the formation of ensembles of dimers, trimers or quadruplets, respectively. The crosslike ensemble symmetry is discussed in terms of strain energy distribution as revealed by finite element calculations.

Single-photon-induced random telegraph signal in a two-dimensional multiple-tunnel-junction array

A single-photon detection using a Si-based two-dimensional (2D) multiple-tunnel-junction field-effect transistor (FET) is reported. The single photon is detected as a random telegraph signal (RTS) in the single-hole-tunneling current regime. The frequency of RTS events depends on the light wavelength and intensity, indicating that the RTS occurs due to the single-photon absorption in the Si dots forming the 2D multijunctions. Based on a Monte Carlo simulation using an equivalent circuit representing the 2D multijunction FET, the ``on'' state of RTS appears when the photogenerated charge in the dot sensitively shifts the current level, while the on state returns to the ``off'' state when the charged dot is neutralized. The simulation result also shows that the RTS is triggered not only by the charging of a dot near the current percolation path, but also by the charging of a dot distances away from the path due to the nature of the multijunction system, i.e., the long screening length of the charge-induced potential. These results open up the development of single-photon devices with 2D multijunction systems.

High-density assembly of nanocrystalline silicon quantum dots

This paper reports on a new bottom–up technique of forming silicon nanostructures based on assembly of nanocrystalline (nc) Si dots from the solution. The nc-Si dots with a diameter of 8 ± 1 nm were fabricated by using VHF plasma decomposition of pulsed SiH 4 gas supply and deposited on the substrate randomly. We first studied the method of making the nc-Si dot dispersion solution by immersing the deposited wafer into various kinds of solvent with ultra sonic treatment. We found that methanol works as a suitable solvent for nc-Si dots. We then add a drop of the nc-Si dot solution onto other substrates and evaporated it. During the evaporation the nc-Si dots were assembled in the solution via the lateral capillary meniscus force which works as an attractive force between the dots. Use of SiO 2 substrate with good surface wettability with the solution was found vital to have the maximum meniscus force and to have two-dimensional assembly of the dots. The evaporation speed was carefully controlled via temperature and evaporation pressure to achieve high dot density assembly. In addition, we examined the assembly of the nc-Si dots on the silicon-on-insulator substrates with various kinds of nanoscale patterning and succeeded in making the nc-Si dots cluster bridging between the nanoelectrodes with a gap of as small as 20 nm.

Periodic Alignment of Silicon Dot Fabricated by Linearly Polarized Nd:YAG Pulse Laser

AbstractPeriodically aligned submicron Si dots were fabricated by only irradiating linearly polarized Nd:YAG pulse laser to the amorphous silicon (a-Si) thin film deposited on silicon dioxide (SiO2) film. Interference between the incident beam and the scattered surface wave leads to the spatial periodicity of beam energy density distribution on the surface of the irradiated samples. The a-Si thin film was melted by laser beam, and then, the molten thin Si film was split and condensed due to its surface tensile according to the periodic energy density distribution. The polycrystalline Si (poly-Si) fine lines were formed periodically. After the first irradiation, the sample was rotated by 90o, and the laser beam was irradiated. The periodic energy density distribution was generated on the Si fine lines. Then, the lines were split off and condensed according to the periodic energy density distribution, and the periodically aligned submicron Si dots were fabricated on the SiO2 film.

Characterization of electronic charged states of P-doped Si quantum dots using AFM/Kelvin probe

Phosphorous doping to Si quantum dots was performed by a pulse injection of 1% PH 3 diluted with He during the dot formation on thermally grown SiO 2 from thermal decomposition of pure SiH 4, and electron charging to and discharging from P-doped Si dots were studied to characterize their electronic charged states using a Kelvin probe technique in atomic force microscopy (AFM). The potential change corresponding to the extraction of one electron from each of the P-doped Si dots was observed after applying a tip bias as low as + 0.2 V while for undoped Si dots, with almost the same size as P-doped Si dots, almost the same amount of the potential change was detectable only when the tip bias was increased to ∼ 1 V. It is likely that, for P-doped Si dots, the electron extraction from the conduction band occurs and results in a positively charged state with ionized P donor.

Amplitude-mode electrostatic force microscopy in UHV: Quantification of nanocrystal charge storage

Charge storage in Si and Ge dots $(8--60\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ has been studied via electrostatic force microscopy (EFM) in UHV using a nontraditional amplitude-mode (AM) inversion method to detect cantilever frequency shift and surface potential. Specifics of the AM technique are discussed relating to resonance curve inversion, nonlinear tip-surface coupling, and force vs force gradient ``nulling'' to extract surface potentials. Single dots were charged via contact electrification with the atomic force microscopy (AFM) tip, followed by EFM, to quantitatively determine how the dot charging level depends on injection bias and dot size. Correlating surface potential with dot charge is addressed by calculating the dipole-dipole, dipole-charge, and capacitive forces acting on the AFM tip in a rigorous manner. For this, the tip-substrate capacitive interaction was studied in detail to account for tip geometry and screening effects due to the dot dielectric volume. In every instance, the change in surface potential measured above a charged dot was seen to be carrier-type dependent and vary linearly with injection bias, where the charging efficiency (${e}^{\ensuremath{-}}∕\mathrm{V}$ applied bias) was determined by only the dot-substrate contact area. Charging efficiencies for small dots $(8\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ were $\ensuremath{\sim}10\phantom{\rule{0.2em}{0ex}}{e}^{\ensuremath{-}}∕\mathrm{V}$ and larger dots $(50--60\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ showed $\ensuremath{\sim}200--400\phantom{\rule{0.2em}{0ex}}{e}^{\ensuremath{-}}∕\mathrm{V}$, agreeing well with the predicted single ${e}^{\ensuremath{-}}$ charging behavior of the tip-dot-substrate double tunnel junction (orthodox theory). These experiments demonstrate that AM-EFM can quantitatively measure charge near the level of single electrons---so long as tip shape and screening effects (perturbation of the tip-substrate electric field by the nanostructure being interrogated) are understood and incorporated into models for static charge determination.

Si/SiO2 core shell clusters probed by Raman spectroscopy

Using a pulsed microplasma source, clusters were produced through the ablation of a Si cathode and successive supersonic expansion. The Si cluster beam was deposited onto different substrates and the partial oxidation of the cluster surface avoided the growth of large agglomerates, preserving their nanocrystalline morphology. Micro-Raman spectroscopy was used for an accurate size diagnosis of the deposited nanoparticles. The size of the Si dots ranges between 2 and about 15 nm. The Si dots appear to have a Si oxide shell, as confirmed also by structural and compositional analysis through transmission electron microscopy and atomic force microscopy. Double Raman peaks were attributed to small Si agglomerates having a thin substoichiometric Si-O interface.

Morphology modifications of quantum dots on Si(0 0 1) surface by ion sputtering

The evolution of the surface morphology of ordered, self-organized Ge nanodots samples by sputtering with various Ar+ ion energy and density has been investigated. Under low (300eV) ion energy irradiation the surface smoothing mechanism dominates surface relaxation processes. The result indicates that ion sputtering with low ion energy can be utilized as an effective method for the preparation of ultra-smooth surfaces. In addition, with medium ion energy (650eV) irradiation, the Ge nanodots transform into pure Si dots with the same shape characteristics as the unsputtered nanodots.