Arrays Of Si Nanocrystals Research Articles

Rearrangement of the optical properties of a-SiOx: H films after crystallization of silicon nanoclusters

The results of investigations on crystallization of silicon nanoclusters in a-SiOx matrix have shown that, even at the very fast annealing using pulse photonic annealing (PPA) formation of rather large silicon crystallites (≥ 100 nm) occurs, as well as the arrays of Si nanocrystals with average sizes of ~10 nm and small sizes of ~1–2 nm happens. It is shown that with an increase in the concentration of nanoclusters in the initial film from 15 to 53%, the contribution of large crystals (larger than 100 nm) increases from 15% (at 15% ncl-Si) to 65% (at 53% ncl-Si), which says on an increase in the probability of coalescence of crystallites into substantially larger ones with an increase of ncl-Si content in the initial film, despite the high annealing speed.

Local field corrections to the spontaneous emission in arrays of Si nanocrystals

We present a theory of the local field corrections to the spontaneous emission rate for the array of silicon nanocrystals in silicon dioxide. An analytical result for the Purcell factor is obtained. We demonstrate that the local-field corrections are sensitive to the volume fill factor of the nanocrystals in the sample and are suppressed for large values of the fill factor. The local-field corrections and the photonic density of states are shown to be described by two different effective permittivities: the harmonic mean between the nanocrystal and the matrix permittivities and the Maxwell–Garnett permittivity.

Monte Carlo study of the Electron Transport Properties of an Array of Si Nanocrystals

We have studiedthe conductivity of an array of Si nanocrystals separated by energy barriers using the Monte Carlo technique. The energy band diagram was calculated by self-consistent solution of the Poisson equation. It is shown that the simulation data can be interpreted by an equivalent network of series resistors. A resistance can be attributed to the energy barrier that increases with increasing barrier width. The barrier resistance has been calculated as a function of the barrier width. Our resultscan be useful in studying the thermoelectric properties of composite structures in the presence of energy barriers. The assumption of an in-series resistor network commonly used to study the thermoelectric properties of nanocomposites has been confirmed by our simulations on 1d nanocomposites.

Thermal evolution of the morphology, structure, and optical properties of multilayer nanoperiodic systems produced by the vacuum evaporation of SiO and SiO2

The alternate vacuum evaporation of SiO and SiO2 from separate sources is used to produce amorphous a-SiO x /SiO2 multilayer nanoperiodic structures with periods of 5–10 nm and a number of layers of up to 64. The effect of annealing at temperatures T a = 500–1100°C on the structural and optical properties of the nanostructures is studied. The results of transmission electron microscopy of the samples annealed at 1100°C indicate the annealing-induced formation of vertically ordered quasiperiodic arrays of Si nanocrystals, whose dimensions are comparable to the a-SiO x -layer thickness in the initial nanostructures. The nanostructures annealed at 1100°C exhibit size-dependent photoluminescence in the wavelength range 750–830 nm corresponding to Si nanocrystals. The data on infrared absorption and Raman scattering show that the thermal evolution of structural and phase state of the SiO x layers with increasing annealing temperature proceeds through the formation of amorphous Si nanoinclusions with the subsequent formation and growth of Si nanocrystals.

Functional nanocrystal-based memories with extraction of nanocrystals properties by charge pumping technique

2D arrays of Si nanocrystals embedded in the gate oxide (⩽10nm) of NVRAM-like transistors are synthesized by ultra-low-energy ion implantation (ULE-II). As we have shown previously, characteristics of trapping centers responsible for the memory performance such as type, depth location into the oxide, effective size and density could be obtained by using the two-level charge pumping (CP) technique performed versus frequency. It has also been demonstrated clearly that these trapping centers are the Si-NCs. However, in order to enhance the memory performances (i.e. charge retention and endurance to cycling), the synthesis of the Si-NCs must be carefully tuned, especially by adjusting the annealing conditions. In this paper, we demonstrate both that the Si-NCs are still responsible for the charge trapping while transistors are fabricated with strong NCs annealing conditions and that some memory performances recommended by the ITRS, more specifically the endurance criteria, are satisfied, with more than 106 erase/write cycles without degradation of the memory window. Therefore, our synthesis technique could provide high-performances non-volatile memories, ensuring the charge trapping in NCs.

Analysis of the lateral resolution of electrostatic force gradient microscopy

Signal measured by electrostatic force gradient microscopy (EFGM) is the z-gradient of the electrostatic force acting between the probe of an atomic force microscope (AFM) and the surface under study. A model is presented for calculating the z-gradient of the electrostatic interaction of the AFM probe with local charges in a dielectric layer at the surface. In the EFGM mode, the interaction of charges with only the probe tip apex should be taken into account. In this approach, a simplified expression can be derived for calculating the z-gradient of the electrostatic force. The model makes it possible to estimate the lateral resolution limit for EFGM imaging of individual charges and to simulate experimental EFGM images as a function of the tip-surface distance and the tip radius. The adequacy of the model was confirmed by quantitative simulation of the experimental EFGM images of local charges injected from the AFM probe into a planar array of Si nanocrystals in a thin SiO2 layer on a Si substrate.

Lateral electrical transport and photocurrent in single and multilayers of two-dimensional arrays of Si nanocrystals

We have investigated electrical transport and photocurrent in single and multilayers of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. The films were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. We found that at high voltages, electrical transport is governed by space charge limited currents due to the presence of traps. At low voltages, electrical transport is ohmic. Carrier mobility, carrier concentration, and trap density in the films were extracted from the electrical measurements. Combining photocurrent and absorption measurements for the films with different SiNC sizes, we found a remarkable similarity in the photon energy dependence of the photocurrent and of the absorbed light from the SiNCs, confirming a proportionality relation between the two quantities. Also, from the combined study of electrical transport and photocurrent, minority carrier lifetimes were extracted.

Structural and optical characterization of two-dimensional arrays of Si nanocrystals embedded in SiO2 for photovoltaic applications

We report on the structural and optical characterization of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. Single and multiple SiNC layers were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. The single SiNC layers consisted of one SiNC layer embedded in two silicon dioxide (SiO2) layers, whereas the multi-layered structure consisted of five SiNC layers of equal thickness separated by SiO2 layers. SiNC layers with thicknesses ranging from 2 to 25 nm were investigated. A thorough structural characterization of the films was carried out by combining grazing incidence x-ray diffraction, x-ray reflectivity, and transmission electron microscopy (TEM). Both XRD and TEM measurements revealed that the SiNC layers were polycrystalline in nature and composed of SiNCs, separated by grain boundaries, with their vertical size equal to the SiNC layer and their lateral size characterized by a narrow size distribution. The high resolution TEM (HRTEM) images showed that oxidation of the SiNC layers proceeded by consumption of Si from their top surface, without any detectable oxidation at the grain boundaries. Only in the case of the thinnest investigated SiNC layer (2 nm), the SiNCs were well separated by SiO2 tunnel barriers. From transmission and reflection optical measurements, energy band gaps of the SiNCs were estimated. These results were correlated with the sizes of the SiNCs obtained by HRTEM. A shift of the estimated band gaps with decreasing SiNC size was observed. This was consistent with quantum size effects in the SiNCs. The film containing the smallest SiNCs (2 nm in the growth direction), besides a significant shift of the absorption edge to higher energies, showed light emission at room temperature which is due to radiative recombination of photo-generated carriers in localized SiNCs separated by SiO2 tunnel barriers.

Generating ordered Si nanocrystals via atomic force microscopy

We describe two successful routes for generating ordered arrays of Si nanocrystals by using atomic force microscopy (AFM) and amorphous silicon thin films (200–400nm) on Ti/Ni coated glass substrates. First, we show that field-enhanced metal-induced solid phase crystallization at room temperature can be miniaturized to achieve highly spatially localized (below 100nm) current-induced crystallization of the amorphous silicon films using a sharp tip in AFM. In the second route, resistive nano-pits are formed at controlled positions in the amorphous silicon thin films by adjusting (lowering and/or stabilizing) the exposure currents in the AFM process. Such templated substrates are further used to induce localized growth of Si nanocrystals in plasma-enhanced chemical vapor deposition process. In both cases the crystalline phase is identified in situ as features of enhanced current in current-sensing AFM maps.

Lateral electronic transport in 2D arrays of oxidized Si nanocrystals on quartz: Coulomb blockade effect and role of hydrogen passivation

We report on the lateral transport in a single two-dimensional (2D) array of Si nanocrystals of different sizes grown by low pressure chemical vapor deposition (LPCVD) of silicon on a quartz substrate and subsequent oxidation at high temperature. The initial nanocrystal size in the z-direction was 5 nm, while it was reduced to ∼3 nm after oxidation. The nanocrystals in the x-y plane were connected by grain boundaries and/or by very thin silicon oxide barriers, while a thin oxide layer was formed on their surface. The electrical measurements showed that current in the film is mainly governed by thermionic emission over the barriers (grain boundaries or dielectric barriers) at high temperatures and by tunneling at lower temperatures. Charge traps at the interfaces of the silicon nanocrystals with the oxide and at the grain boundaries cause considerable hysteresis in the current-voltage characteristics. Hydrogen passivation of the charge traps reduces considerably the hysteresis effect and the activation energy of the thermionic emission, while revealing a clear Coulomb gap.

Optical characterization using ellipsometry of Si nanocrystal thin layers embedded in silicon oxide

ABSTRACTWe have developed optical models for the characterization of grain size in nanocrystal thin films embedded in SiO2 and fabricated using low pressure chemical vapor deposition of Si from silane on a quartz substrate, followed by thermal oxidation. The as-grown nanocrystals thin film on quartz was composed of a two-dimensional array of Si nanocrystals (Si-NC) showing columnar structure in the z-direction and touching each other in the x-y plane. The nanocrystal size in the z-direction was equal to the Si nanocrystal film thickness, changing by the deposition time, while their x-y size was almost equal in all the samples, with small size dispersion. After high temperature thermal oxidation, a thin silicon oxide film was formed on top of the nanocrystals layer. The aim of this work was to measure the grain size and the nanocrystallinity of the Si nanocrystal thin films, a quantity related to the change of the dielectric function. We used a definition for the nanorcystallinity that is related to the effective medium analysis (EMA) of the material. The optical technique used for the investigations was spectroscopic ellipsometry. To measure the above sample properties the thickness and composition of several layers on a quartz substrate had to be determined by proper modeling of this complex system. We found that the nanocrystallinity (defined as the ratio of nc-Si/(c-Si+nc-Si) decreases systematically with increasing the Si-NC layer thickness. Using this approach we are sensitive to the lifetime broadening of electrons caused by the scattering on the grain boundaries, and not to the shift of the direct interband transition energies due to quantum confinement.

Ordered arrays of Si nanocrystals in SiO2: Structural, optical, and electronic properties

It is found that irradiation of SiO2 layers containing Si nanocrystals with high-energy heavy ions induces profound structural modifications—specifically, the formation of vertically ordered arrays of nanocrystals along the ion tracks. This effect results in substantial changes in the electrical properties (the conductivity and capacitance—voltage characteristics) and optical (photoluminescence) properties of the layers containing nanocrystals.

Ultra-Low Energy Ion Implantation of Si into HfO2 and HfSiO-based Structures for Non Volatile Memory Applications

AbstractThe fabrication of Si nanocrystals (NCs) in multilayer structures based on HfO2 and alloys for memory applications is carried out using an innovative method, the ultra-low energy (1 keV) ion implantation followed by a post-implantation annealing. Si+ ions are implanted into SiO2 thin layers deposited on top of thin HfO2-based layers. After annealing at high temperature (1050°C), the implantation leads to the formation of a two dimensional array of Si NCs at a distance from the surface larger than expected, due to an anomalous oxidation of the implanted Si. Nevertheless, the best memory windows are obtained at lower thermal budget, when no nanocrystals are present in the layer. This suggests that electrical measurements should always be correlated to structural characterization in order to understand where charge storage occurs.

Si nanocrystal synthesis in HfO2/SiO/HfO2 multilayer structures

The synthesis of two-dimensional arrays of Si nanocrystals in anHfO2 matrix has been achievedby deposition of HfO2/SiO/HfO2 multilayer structures followed by high temperature (1100 °C)thermal treatment in nitrogen atmosphere. Silicon out-diffusion from the SiO layer through theHfO2 films has been shown to be the limiting factor in the formation of the Si nanocrystals.Suitable strategies have been identified in order to overcome this limitation. Si nanocrystalformation has been achieved by properly adjusting the thickness of the SiO layer.

Self-assembled hexagonal ordering of Si nanocrystals embedded inSiO2 nanodots

Highly dense hexagonally ordered two-dimensional arrays of Si nanocrystals embedded inSiO2 nanodots were fabricated on a silicon substrate by using a self-assembled porous anodic aluminathin film as a masking layer through which electrochemical oxidation of the Si substrateand ultralow energy Si implantation took place. After removal of the alumina film and hightemperature annealing of the samples, hexagonally ordered Si nanocrystals embedded withinSiO2 nanodots were obtained, having sizes in the few tens of nanometer range. The fabricatedordered structures show significant potential for applications either in basic physicsexperiments or as building blocks for nanoelectronic and nanophotonic devices.

Oxidation of Si nanocrystals fabricated by ultralow-energy ion implantation in thin SiO2 layers

The effect of thermal treatments in nitrogen-diluted oxygen on the structural characteristics of two-dimensional arrays of Si nanocrystals (NCs) fabricated by ultralow-energy ion implantation (1 keV) in thin silicon dioxide layers is reported. The NC characteristics (size, density, and coverage) have been measured by spatially resolved electron-energy-loss spectroscopy by using the spectrum-imaging mode of a scanning transmission electron microscope. Their evolution has been studied as a function of thermal treatment duration at a temperature (900 °C) below the SiO2 viscoelastic point. An extended spherical Deal-Grove [J. Appl. Phys. 36, 3770 (1965)] model for self-limiting oxidation of embedded silicon NCs has been carried out. It proposes that the stress effects, due to oxide deformation, slow down the NC oxidation rate and lead to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results. Soft oxidation appears to be a powerful way for manipulating the NC size distribution and surface density.

Manipulation of two-dimensional arrays of Si nanocrystals embedded in thin SiO2 layers by low energy ion implantation

In silicon nanocrystal based metal–oxide–semiconductor memory structures, tuning of the electron tunneling distance between the Si substrate and Si nanocrystals located in the gate oxide is a crucial requirement for the pinpointing of optimal device architectures. In this work it is demonstrated that this tuning of the “injection distance” can be achieved by varying the Si+ ion energy or the oxide thickness during the fabrication of Si nanocrystals by ultralow-energy silicon implantation. Using an accurate cross-section transmission electron microscopy (XTEM) method, it is demonstrated that two-dimensional arrays of Si nanocrystals cannot be positioned closer than 5 nm to the channel by increasing the implantation energy. It is shown that injection distances down to much smaller values (2 nm) can be achieved only by decreasing the nominal thickness of the gate oxide. Depth profiles of excess silicon measured by time-of-flight secondary ion mass spectroscopy and Si nanocrystal locations determined by XTEM are compared with Monte-Carlo simulations of the implanted Si profiles taking into account dynamic target changes due to ion implantation, ion erosion, and ion beam mixing. This combination of experimental and theoretical studies gives a safe explanation regarding the unique technological route of obtaining Si nanocrystals at distances smaller than 5 nm from the channel: the formation of nanocrystals requires that the interface mixing due to collisional damage does not overlap with the range profile to the extent that there is no more a local maximum of Si excess buried in the SiO2 layer.

Pattern Formation on Silicon-on-Insulator

ABSTRACTThe strain driven self-assembly of faceted Ge nanocrystals during epitaxy on Si(001) to form quantum dots (QDs) is by now well known. We have also recently provided an understanding of the thermodynamic driving force for directed assembly of QDs on bulk Si (extendable to other QD systems) based on local chemical potential and curvature of the surface. Silicon-on-insulator (SOI) produces unique new phenomena. The essential thermodynamic instability of the very thin crystalline layer (called the template layer) resting on an oxide can cause this layer, under appropriate conditions, to dewet, agglomerate, and self-organize into an array of Si nanocrystals. Using low-energy electron microscopy (LEEM), we observe this process and, with the help of first-principles total-energy calculations, we provide a quantitative understanding of this pattern formation. The Si nanocrystal pattern formation can be controlled by lithographic patterning of the SOI prior to the dewetting process. The resulting patterns of electrically isolated Si nanocrystals can in turn be used as a template for growth of nanostructures, such as carbon nanotubes (CNTs). Finally we show that this growth may be controlled by the flow dynamics of the feed gas across the substrate.

Pattern Formation on Silicon-on-Insulator

ABSTRACTThe strain driven self-assembly of faceted Ge nanocrystals during epitaxy on Si(001) to form quantum dots (QDs) is by now well known. We have also recently provided an understanding of the thermodynamic driving force for directed assembly of QDs on bulk Si (extendable to other QD systems) based on local chemical potential and curvature of the surface. Silicon-on-insulator (SOI) produces unique new phenomena. The essential thermodynamic instability of the very thin crystalline layer (called the template layer) resting on an oxide can cause this layer, under appropriate conditions, to dewet, agglomerate, and self-organize into an array of Si nanocrystals. Using low-energy electron microscopy (LEEM), we observe this process and, with the help of first-principles total-energy calculations, we provide a quantitative understanding of this pattern formation. The Si nanocrystal pattern formation can be controlled by lithographic patterning of the SOI prior to the dewetting process. The resulting patterns of electrically isolated Si nanocrystals can in turn be used as a template for growth of nanostructures, such as carbon nanotubes (CNTs). Finally we show that this growth may be controlled by the flow dynamics of the feed gas across the substrate.

Pattern Formation on Silicon-on-Insulator

ABSTRACTThe strain driven self-assembly of faceted Ge nanocrystals during epitaxy on Si(001) to form quantum dots (QDs) is by now well known. We have also recently provided an understanding of the thermodynamic driving force for directed assembly of QDs on bulk Si (extendable to other QD systems) based on local chemical potential and curvature of the surface. Silicon-on-insulator (SOI) produces unique new phenomena. The essential thermodynamic instability of the very thin crystalline layer (called the template layer) resting on an oxide can cause this layer, under appropriate conditions, to dewet, agglomerate, and self-organize into an array of Si nanocrystals. Using low-energy electron microscopy (LEEM), we observe this process and, with the help of first-principles total-energy calculations, we provide a quantitative understanding of this pattern formation. The Si nanocrystal pattern formation can be controlled by lithographic patterning of the SOI prior to the dewetting process. The resulting patterns of electrically isolated Si nanocrystals can in turn be used as a template for growth of nanostructures, such as carbon nanotubes (CNTs). Finally we show that this growth may be controlled by the flow dynamics of the feed gas across the substrate.