Size-controlled Si Nanocrystals Research Articles

Low-power wavelength modulation in size-controlled Si nanocrystals using quantum confined Stark effect

We report on the quantum confined Stark effect coupled with a permanent built-in electric dipole moment in size-controlled Si nanocrystals (SiNCs) investigated under steady state conditions by photoluminescence spectroscopy. The study was conducted on samples with SiNC layer thicknesses between 2 nm and 5 nm. The emission spectra of the samples subjected to electric field magnitudes of up to 5 × 108 V/m were analyzed in terms of the dependency of the spectral shift on field magnitude and SiNC size. A clear trend of red shift along with increasing dipole separation was observed with increasing SiNC size. Experimental results show a high degree of accuracy to the model based on the time independent perturbation theory approximation for a biased quantum well. We propose a potential application for size-controlled SiNCs in photon detection in the near infrared band based on the studied phenomena as well as the use of SiNCs as a model platform for further study of the Stark effect in quantum confined structures as a whole.

Size-tunable electroluminescence characteristics of quantum confined Si nanocrystals embedded in Si-rich oxide matrix

Tunable electroluminescence properties of size-controlled Si nanocrystals embedded in silicon rich oxide films are demonstrated at room temperature, using an active light emitting layer in the metal oxide semiconductor device structure. Plasma enhanced chemical vapor deposited Si-rich oxide films were annealed at elevated temperatures to form Si nanocrystals of varying diameters. A typical redshift in the photoluminescence peak is observed with increasing annealing temperature, confirming the formation of quantum confined Si nanocrystals. The carrier transport and light emission mechanism have been studied in detail through current–voltage characteristics and ultrafast transient spectroscopy, respectively. The origin of electroluminescence and the size-tunable emission peak have been analyzed and attributed to the radiative recombination of carriers within Si nanocrystal quantum wells. The fabricated Si nanocrystal-based metal oxide semiconductor light emitting diode and the resultant size-dependent tunable electroluminescence are very attractive as a potential CMOS compatible optical source for future photonic integrated chips.

Formation of Si nanocrystals in LP CVD semi-insulating polycrystalline silicon films

Influence of oxygen on silicon nanocrystals formation peculiarities and size in semi-insulating polycrystalline silicon thin films has been demonstrated by the means of local atomic surrounding sensitive electronic structure experimental studies. Low-pressure chemical deposition from the gas phase at a relatively low temperature leads to formation of semi-insulating polycrystalline silicon film containing nanocrystals with the grain size of 20–40 nm. At the same time, films oxygen doping result in formation of silicon nanocrystals arrays with the mean particle size less than 10 nm. Possibility of size-controlled Si nanocrystals formation followed by electronic structure reconstruction of amorphous films with nanocrystalline inclusions is discussed and can be found prospective for a range of possible applications.

Formation of size-controlled and luminescent Si nanocrystals from SiOxNy/Si3N4 hetero-superlattices

Silicon nanocrystals formed in the annealed SiNx/Si3N4 superlattices are attractive for research due to the smaller band offsets of Si3N4 matrix to Si in comparison with commonly used SiOx/SiO2 superlattices. However, the annealed SiNx/Si3N4 structures contain an increased number of nanocrystal interface defects, which completely suppress nanocrystal emission spectrum. In this work, we study a novel SiOxNy/Si3N4 hetero multilayer combination, which compromises the major issues of SiOx/SiO2 and SiNx/Si3N4 superlattices. The annealed SiOxNy/Si3N4 superlattices are investigated by TEM, demonstrating a precise sublayer thicknesses control. The PL spectra of the annealed SiOxNy/Si3N4 superlattices are centered at 845–950 nm with an expected PL peak shift for silicon nanocrystals of different sizes albeit the PL intensity is drastically reduced as compared to SiO2 separation barriers. The comparison of PL spectra of annealed SiOxNy/Si3N4 superlattice with those of SiOxNy/SiO2 superlattice enables the analysis of the interface quality of silicon nanocrystals. Using the literature data, the number of the interface defects and their distribution on the nanocrystal facets are estimated. Finally, it is shown that the increase of the Si3N4 barrier thickness leads to the increased energy transfer from the Si nanocrystals into the Si3N4 matrix, which explains an additional drop of the nanocrystal PL intensity.

Absence of quantum confinement effects in the photoluminescence of Si3N4–embedded Si nanocrystals

Superlattices of Si-rich silicon nitride and Si3N4 are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 °C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si3N4. Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 105 times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si3N4 reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si3N4 band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si3N4 interface defect literature. In addition, the role of Si3N4 valence band tail states as potential hole traps is discussed. Most strikingly, the PL peak blueshift with decreasing NC size, which is often observed in literature and typically attributed to quantum confinement (QC), is identified as optical artifact by transfer matrix method simulations of the PL spectra. Finally, criteria for a critical examination of a potential QC-related origin of the PL from Si3N4-embedded Si NCs are suggested.

Structural and optical properties of size controlled Si nanocrystals in Si3N4 matrix: The nature of photoluminescence peak shift

Superlattices of Si3N4 and Si-rich silicon nitride thin layers with varying thickness were prepared by plasma enhanced chemical vapor deposition. After high temperature annealing, Si nanocrystals were formed in the former Si-rich nitride layers. The control of the Si quantum dots size via the SiNx layer thickness was confirmed by transmission electron microscopy. The size of the nanocrystals was well in agreement with the former thickness of the respective Si-rich silicon nitride layers. In addition X-ray diffraction evidenced that the Si quantum dots are crystalline whereas the Si3N4 matrix remains amorphous even after annealing at 1200 °C. Despite the proven Si nanocrystals formation with controlled sizes, the photoluminescence was 2 orders of magnitude weaker than for Si nanocrystals in SiO2 matrix. Also, a systematic peak shift was not found. The SiNx/Si3N4 superlattices showed photoluminescence peak positions in the range of 540–660 nm (2.3–1.9 eV), thus quite similar to the bulk Si3N4 film having peak position at 577 nm (2.15 eV). These rather weak shifts and scattering around the position observed for stoichiometric Si3N4 are not in agreement with quantum confinement theory. Therefore theoretical calculations coupled with the experimental results of different barrier thicknesses were performed. As a result the commonly observed photoluminescence red shift, which was previously often attributed to quantum-confinement effect for silicon nanocrystals, was well described by the interference effect of Si3N4 surrounding matrix luminescence.

Rapid thermal annealing of size-controlled Si nanocrystals: Dependence of interface defect density on thermal budget

Photoluminescence properties of size-controlled Si nanocrystals (NCs) formed by various annealings have been studied in detail. The thermal treatments involve rapid thermal annealing (RTA, 10 to 180 s) as well as conventional tube furnace annealing (1h) at 1100 °C. Whereas the photoluminescence (PL) peak positions and the TEM images indicate only minor changes in NC size, the PL intensity varies over more than two orders of magnitude. A correlation between the total thermal budget applied by the different annealing treatments and the PL intensity is demonstrated. In addition, the PL improvement of interface defect passivation by post-annealing in H2 ambient is investigated. RTA with H2 passivation is not able to achieve the PL intensity and NC interface quality of conventionally annealed and passivated samples. The combination of these results with our previous electron spin resonance studies allows to estimate the interface defect densities. Tube furnace annealed samples after H2 treatment have less than 2% defective NCs. In contrast, more than 95% defective NCs are assumed for a 180 s RTA.

Nitrogen at theSi-nanocrystal/SiO2interface and its influence on luminescence and interface defects

The influence of the high-temperature annealing ambient, i.e., ${\text{N}}_{2}$ and Ar on size controlled Si nanocrystals (NCs) ranging from $\ensuremath{\sim}2$ to $\ensuremath{\sim}6\text{ }\text{nm}$ embedded in ${\text{SiO}}_{2}$ has been investigated in detail. Generally, ${\text{N}}_{2}$ annealing is proven to be beneficial as the dangling bond density (${\text{P}}_{\text{b}}$ defects at the $\text{NC}/{\text{SiO}}_{2}$ interface) is about half, accompanied by a doubled photoluminescence (PL) intensity. The PL blueshift of ${\text{N}}_{2}$ annealed samples compared to Ar-annealed samples (N-blueshift) was found to be pronounced only for small NCs whereas it appears to be insignificant for larger NCs. The origin of this N-blueshift was previously attributed to a growth suppression of the NCs by the presence of N during the annealing process. However, no evidence for this assumption is found by time-resolved PL, as the luminescence decay times are similar despite considerable N-blueshift. The exact location of the N incorporated during annealing was investigated by time-of-flight-SIMS and electron-spin resonance. Besides the distinct N enrichment in the NC layer, the ${\text{K}}^{0}$ center $(^{\ifmmode\bullet\else\textbullet\fi{}}\text{S}\text{i}\ensuremath{\equiv}{\text{N}}_{3})$ was detected indicating the formation of an interfacial N layer at the $\text{NC}/{\text{SiO}}_{2}$ interface. Elastic recoil detection analysis enabled the quantification of the incorporated N as well as the excess Si. Combined with transmission electron microscopy analysis (determination of NC size) the calculation of the NC density per superlattice layer and the thickness of the interfacial N layer were achieved. It turns out that $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{N}\text{ }\text{atoms}\text{ }{\text{cm}}^{\ensuremath{-}2}$ exist at the NC surface, which is well in accordance to the optimum value of the bulk $\text{Si}/{\text{SiO}}_{2}$ interface. These results strongly support our recently suggested explanation for the N-blueshift that is based on an increased NC band gap by the influence of interfacial N on the polarity of the surface terminating groups.

(Invited) Nanocrystalline Materials: Optimization of Thin Film Properties

The approach of superlattices to enable the size control of different nanocrystalline materials was investigated. In case of size controlled Si nanocrystals synthesized by SiO/SiO2 superlattices this approach enables a precise bandgap engineering of well passivated nanocrystals. Which results in a tunable luminescence signal, a resonant energy transfer to Er3+ ions and the confirmation of the quantum confined origin of the luminescene and the prove of optical amplification. In case of Zr based dielectrics the size control by the superlattice approach enables the stabilization of the tetragonal phase, which is favorable for high capacitance applications. For HfO2 an additional doping of the films, e.g. by Si atoms for the phase stabilization is necessary. Doping or interlayer additionally improves the electrical performance of the dielectrics.

P b ( ) centers at the Si-nanocrystal/SiO2 interface as the dominant photoluminescence quenching defect

The correlation of paramagnetic defects and photoluminescence (PL) of size controlled Si nanocrystals (NCs) has been studied as a function of annealing ambient (Ar or N2) and subsequent H2 treatment. The dominant defects measured by electron spin resonance are interfacial Pb(0) and Pb1 centers. Whereas the latter appears to play only a minor role in PL quenching, a pronounced correlation between Pb(0) density and PL intensity is demonstrated. Annealing in N2 is found to be superior over Ar both in terms of PL performance and defect densities. The origin of the PL blueshift found for N2 annealing compared to Ar was previously interpreted as a growth suppression of the Si clusters due to incorporation of N atoms or a silicon consuming nitridation at the NC/SiO2 interface. The results presented here, demonstrate the blueshift to be more pronounced for small NCs (∼2 nm) than for larger ones (∼4.5 nm). Therefore, we suggest an alternative interpretation that is based on the influence of the polarity of surface terminating groups on the electronic properties of the NCs.

Fabrication and luminescence properties of Si quantum dots based on Si-rich SiNx/N-rich SiNy multilayer

SiN-based multilayers were prepared in a plasma enhanced chemical vapor deposition system followed by subsequently thermal annealing and laser irradiation with the aim of fabrication three-dimensional constrained, size-controlled and well-regulated Si nanocrystals. The experimental results show that Si nanocrystals grow in the Si-rich SiN sublayer. Furthermore, the grain size can be controlled according to the thick of Si-rich SiN. It is also found that the crystalline fraction of the multilayers irradiated by laser is significantly higher than that by thermal annealing. The devices that employing the laser-irradiated multilayer as luminescent active layer exhibit an enhanced visible electroluminescence and the external quantum efficiency is improved by 40% in comparison with the device without annealing.

Production of size-controlled Si nanocrystals using self-organized optical near-field chemical etching

We demonstrate the selective photochemical etching of Si in a self-organized manner, which strongly depends on the distribution of the optical near field. This dependence was described by the virtual exciton-phonon-polariton model. The photoluminescence (PL) spectra from the etched Si exhibited a blueshifted PL peak at 1.8 eV, corresponding to Si nanocrystals of 2.8 nm diameter.

Size Controlled Si Nanocrystals: From Photonic to Electronic Applications

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Hopping conduction in size-controlled Si nanocrystals

We investigate the temperature dependence of conduction in size-controlled silicon nanocrystals. The nanocrystals are ∼8nm in diameter, covered by ∼1.5nm thick SiO2 shells. In 300nm thick films for temperatures T from 30to200K, the conductivity σ follows a ln(σ) vs 1∕T1∕2 dependence. This may be associated with either percolation-hopping conductance or Efros-Shklovskii variable range hopping. Assuming hopping sites only on the nanocrystals, the data agree well with the percolation model.

Visible range whispering-gallery mode in microdisk array based on size-controlled Si nanocrystals

Microdisks based on in-plane embedded size-controlled Si nanocrystal∕SiO2 superlattices (Si-NCs∕SiO2 SLs) were mass-fabricated arranged in well-ordered arrays. The microdisks were fabricated with size irregularity of below 0.2% over large-scale areas. Overlapping whispering-gallery modes (WGM) of the visible nanocrystalline-silicon luminescence were observed. A comparison between analytical calculation and experimental results is reported. We found that only one axial and one radial WGM exist due to their thin disk thickness and birefringence characteristic of Si-NCs∕SiO2 SLs, and that the mode spacing is 15nm and 6nm for microdisks with a diameter of 8.8μm and 23.7μm, respectively. The advantages of such size-controlled Si-NCs∕SiO2 embedded in microdisk arrays for Si-based photonic application will be discussed.

Multilevel charge storage in silicon nanocrystal multilayers

The feasibility of multilevel charges in layered arranged Si nanocrystals in a metal-oxide-semiconductor structure is investigated. The structures are created with up to three layers of size-controlled Si nanocrystals having a size of around 3.9nm (±0.4nm). Using a suitable write bias, the apparent states of charge storage are evident in the series of capacitance-voltage (C-V) curves. These memory effects are due to the successive charging of a varied number of Si nanocrystal layers in the floating gate. The widths of the memory windows were estimated by a modified charge equation for the multilayered nanocrystal samples, i.e., each memory window is quantified by charging a different number of Si nanocrystal layers. The static current-voltage (I-V) curve can be fitted by Fowler–Nordheim tunneling which represents the dominant tunneling behavior through the Si nanocrystal multilayers separated by thin silicon dioxide. Time retention investigations demonstrate the stability of the programming states. The results demonstrate the feasibility for using such a stack structure in multibit∕cell nonvolatile memories. In addition, due to the fact that more then one Si nanocrystal layer can be arranged and completely charged, the charge density can be easily increased to 3×1012cm−2 as required for device applications.

Electrical behavior of size-controlled Si nanocrystals arranged as single layers

A metal–oxide–semiconductor structure containing a single layer of size-controlled silicon nanocrystals embedded into gate oxide was fabricated. Size control for the silicon nanocrystals was realized by using a SiO2/SiO/SiO2 layer structure with the embedded SiO layer having the thickness of the desired Si nanocrystals and using a high-temperature annealing for forming the silicon nanocrystals. Current–voltage, capacitance–voltage, and conductance–voltage characteristics were measured for a sample containing 4-nm-sized crystals. From the Fowler–Nordheim plot an effective barrier height of 1.6 eV is estimated for our silicon nanocrystals. Electron trapping, storing, and de-trapping in silicon nanocrystals were observed by capacitance–voltage and conductance–voltage measurements. The charge density was measured to be 1.6×1012 /cm2, which is nearly identical to the silicon-nanocrystal density measured approximately via a transmission electron microscopy image. Conductance measurements reveal a very low interface charge of our structure.

Size-Controlled Si Nanocrystals for Photonic and Electronic Applications

Size-Controlled Si Nanocrystals for Photonic and Electronic Applications

Carrier Transport and Lateral Conductivity in Nanocrystalline Silicon Layers

AbstractWe have studied carrier transport and lateral electrical properties of nanocrystalline Si layers containing size controlled Si nanocrystals. Using results from direct current (dc) and alternating current (ac) conductivity measurements, the charging of Si nanocrystals and Coulomb blockade effect are discussed.