Size-controlled Silicon Nanocrystals Research Articles

Erbium-doped slot waveguides containing size-controlled silicon nanocrystals

Silicon based slot waveguides with a slot containing Si nanocrystals (Si-nc) and Erbium ions (Er3+) inside a silica matrix were prepared using sputter deposition and low-energy ion implantation. This sequence enabled independent optimization of nanocrystal formation and Er3+ incorporation parameters. Using a superlattice approach, the size of the Si-nc inside the slot could be controlled and optimized for maximum Er3+ luminescence yield at 1.54 μm. Er3+ is found to be efficiently pumped by Si-nc of sizes around 3 to 4 nm. Increasing Er3+ photoluminescence at 1.54 μm with increasing post-implantation annealing temperatures up to 1000 °C is attributed to annealing of matrix or Si-nc interface defects mainly. Additionally, a dependence of the Er3+ luminescence intensity on both the excitation and emission linear polarization orientation is shown, which demonstrates efficient field enhancement in sputtered slot waveguide structures.

Picosecond dynamics of photoexcited carriers in silicon nanocrystal/Si3N4 superlattices: Presence of K centers

We report in detail on the picosecond dynamics of photoexcited charge carriers in size-controlled silicon nanocrystals in silicon nanocrystal/Si3N4 superlattices. The samples were prepared using plasma enhanced chemical vapor deposition and subsequent thermally induced phase separation. The pump and probe transmission technique was used to monitor directly the initial (picosecond) carrier dynamics. The transient transmission signal (decay time about 2 ps) was found to be independent of the nanocrystal size, pump pulse intensity and wavelength from the interval 400 to 700 nm. The signal is attributed to the dynamics of photoexcited carriers captured on the K0 centers in Si3N4.

Formation of size controlled silicon nanocrystals in nitrogen free silicon dioxide matrix prepared by plasma enhanced chemical vapor deposition

This paper reports the growth of silicon nanocrystals (SiNCs) from SiH4–O2 plasma chemistry. The formation of an oxynitride was avoided by using O2 instead of the widely used N2O as precursor. X-ray photoelectron spectroscopy is used to prove the absence of nitrogen in the layers and determine the film stoichiometry. It is shown that the Si rich film growth is achieved via non-equilibrium deposition that resembles a interphase clusters mixture model. Photoluminescence and Fourier transformed infrared spectroscopy are used to monitor the formation process of the SiNCs, to reveal that the phase separation is completed at lower temperatures as for SiNCs based on oxynitrides. Additionally, transmission electron microscopy proves that the SiNC sizes are well controllable by superlattice configuration, and as a result, the optical emission band of the Si nanocrystal can be tuned over a wide range.

(Invited) Transport and Electroluminescence Properties of Size-Controlled Silicon Nanocrystals Embedded in SiO2 Matrix Following the Superlattice Approach

In this invited talk, we will report our latest advances in the field of nanostructured silicon-based light emitters with special emphasis in those that are rare earth doped. We have recently shown that it is possible to push external quantum efficiencies over the 10% by fine tuning the material composition and by using a new device concept that we have called hot electron engineering.Most of the rare earth amount introduced in the host material must be optically active. This can be accomplished by using a deposition method that introduces the rare earth during film growth and by a suitable passivating annealing procedure that: i) does not promote rare earth precipitation and clustering; ii) brings the rare earth to the 3+ oxidation state and iii) reduces non radiative defects. Additionally, for electroluminescent devices the matrix composition must be engineered so that it allows conduction-band transport in contrast with defective systems in which transport is hopping-like.The hot electron engineering concept relies on the fact that the rare earth excitation process in Si-based dielectric matrices proceeds mainly via impact excitation by hot electrons. Thus, devising suitable structures for which electrons can be accelerated to average energies that are resonant to the rare earth excitation energies, will improve both efficiency and maximum power emission. Additionally, driving the device with pulsed polarization and short duty cycle will improve device efficiency and lifetime. Finally, the structures must allow at the same time significant light extraction efficiency. This can be accomplished by improving the antireflective properties of the whole stack and/or by nanostructuring the ITO or polysilicon electrode.We will be show silicon oxides, nitrides and oxynitrides as hosts for the rare-earth species and, additionally, we will display the advantages and drawbacks of introducing small percentages of carbon and aluminum. The rare-earth species incorporated in those matrices are Er, Ce, Tb, Eu and Nd and their light emission efficiency depend in a complex way on matrix composition, material processing, device structure and device polarization. The simplest hot electron engineering structures are bilayers and trilayers with silicon oxide as accelerator. More complex structures like multilayers and superlattices will be also introduced and optimal designs will be proposed. Some reliability issues and potential solutions for them will be also presented.We acknowledge existing collaborations in this field with teams of the McMaster University at Hamilton (Canada), Institut de Microelectrònica de Barcelona (Spain), Nanophotonics Technology Center of Valencia (Spain), Instituto de Óptica of Madrid (Spain), Ion Beam Institute at Rossendorf (Germany), IMTEK at the University of Freiburg (Germany) and CIMAP-CNRS at Caen (France).

(Invited) Transport and Electroluminescence Properties of Size-Controlled Silicon Nanocrystals Embedded in SiO2 Matrix Following the Superlattice Approach

We report a throrough analysis of the electrical and optical properties of silicon nanocrystals (Si-NCs)/SiO2 superlattices (SL), deposited on Si substrate by means of plasma-enhanced chemical-vapor deposition. A post-deposition annealing treatment was performed to induce phase separation and, therefore, precipitate the Si excess in the form of Si NCs. An excellent size control is reached by the limited thickness of the silicon rich layer of the SL. A discussion on the NC size effect on the injection and transport (electrical), and on absorption and electroluminiscence (electro-optical) is presented. An strong correlation between charge transport and electroluminescence is revealed. This work demonstrates that Si-NCs are a promising photovoltaic and luminescent material system; especially in the case of a SL-based material.

Photoexcited charge carrier dynamics in silicon nanocrystal/SiO2 superlattices

We report in detail on the dynamics of photoexcited charge carriers in size-controlled silicon nanocrystals in silicon nanocrystal/SiO2 superlattices. The samples were prepared using plasma enhanced chemical vapor deposition and subsequent thermally induced phase separation. This unique approach allows preparation of well-defined Si nanocrystals. Experimental techniques of time-resolved absorption and photoluminescence were used to monitor the carrier dynamics on a wide time scale from picoseconds to milliseconds for a set of samples with different parameters (nanocrystal size, hydrogen annealing). The initial fast decay (tens of picoseconds) dependent on pump intensity for excitation levels exceeding one electron–hole pair per nanocrystal can be interpreted in terms of the bimolecular recombination with constant B=(2–3)×10−10cm3s−1. The slow pump intensity independent decay (microseconds) can be reproduced well by a stretched-exponential function. The dependence of stretched-exponential parameters on photoluminescence photon energy and sample properties agrees well with the picture of trapped carriers.

Charge transport in Si nanocrystal/SiO2 superlattices

Size-controlled silicon nanocrystals in silicon oxynitride matrix were prepared using plasma-enhanced chemical vapor deposition following the superlattice approach. A combination of current transport and charge trapping studies is carried out on a number of samples with varied structural configuration. We demonstrate that at low electric fields, trapping of injected carriers dominates, if the coupling between the silicon nanocrystals is strong. In contrast, we show that at higher electric fields, the charge distribution within the films is essentially governed by charge separation within the superlattice. This effect can be well explained by a two-step electric field ionization of silicon nanocrystals that proceeds via defect-assisted band-to-band tunneling of silicon valence electrons to the conduction band and is mediated by silicon surface dangling bonds. The defects are dominating the charge transport even if the defect density is reduced to a minimum by efficient hydrogen passivation.

Donor ionization in size controlled silicon nanocrystals: The transition from defect passivation to free electron generation

We studied the photoluminescence spectra of silicon and phosphorus co-implanted silica thin films on (100) silicon substrates as a function of isothermal annealing time. The rapid phase segregation, formation, and growth dynamics of intrinsic silicon nanocrystals are observed, in the first 600 s of rapid thermal processing, using dark field mode X-TEM. For short annealing times, when the nanocrystal size distribution exhibits a relatively small mean diameter, formation in the presence of phosphorus yields an increase in the luminescence intensity and a blue shift in the emission peak compared with intrinsic nanocrystals. As the mean size increases with annealing time, this enhancement rapidly diminishes and the peak energy shifts further to the red than the intrinsic nanocrystals. These results indicate the existence of competing pathways for the donor electron, which depends strongly on the nanocrystal size. In samples containing a large density of relatively small nanocrystals, the tendency of phosphorus to accumulate at the nanocrystal-oxide interface means that ionization results in a passivation of dangling bond (Pb-centre) type defects, through a charge compensation mechanism. As the size distribution evolves with isothermal annealing, the density of large nanocrystals increases at the expense of smaller nanocrystals, through an Ostwald ripening mechanism, and the majority of phosphorus atoms occupy substitutional lattice sites within the nanocrystals. As a consequence of the smaller band-gap, ionization of phosphorus donors at these sites increases the free carrier concentration and opens up an efficient, non-radiative de-excitation route for photo-generated electrons via Auger recombination. This effect is exacerbated by an enhanced diffusion in phosphorus doped glasses, which accelerates silicon nanocrystal growth.

Doping efficiency of phosphorus doped silicon nanocrystals embedded in a SiO2 matrix

Strongly size controlled silicon nanocrystals in silicon oxynitride matrix were prepared using plasma enhanced chemical vapor deposition following the superlattice approach. Doping was achieved by adding diluted phosphine as a precursor gas. Phosphorus quantification was done by secondary ion mass spectrometry. A model based on Poissonian distributions of interface defects and dopants is proposed to calculate the defects and the dopants per silicon nanocrystal as a function of phosphorus concentration. The model requires the comparison between the photoluminescence spectra from passivated and unpassivated samples. Finally, the doping efficiency of silicon nanocrystals embedded in silicon oxynitride is estimated to be >20%.

Optical constants and band gap expansion of size controlled silicon nanocrystals embedded in SiO 2 matrix

Silicon nanocrystals (Si-NCs) with different sizes embedded in SiO 2 matrix were synthesized by phase separation and thermal crystallization of SiO x/SiO 2 supperlattice approach. The optical constants and band gap expansion of Si-NCs have been investigated by spectroscopic ellipsometry, based on the Maxwell–Garnett effective medium approximation and the Forouhi–Bloomer optical dispersion model. Similar spectra shapes but smaller values of Si-NCs optical constants with respect to bulk crystalline Si is observed. With the size of Si-NCs decreasing from 6 nm to 2 nm, the band gap increases from 1.64 eV to 2.56 eV. The band gap expansion, as compared to bulk crystalline Si, which agrees with the prediction of first-principles calculations based on quantum confinement effect, is presented in this paper.

Formation of size-controlled silicon nanocrystals in plasma enhanced chemical vapor deposition grown SiO xN y/SiO 2 superlattices

Size controlled silicon nanocrystals (SiNC) in silicon oxynitride matrix were prepared using plasma enhanced chemical vapor deposition. The as-deposited superlattices (SLs) and the corresponding bulk films were treated by thermal annealing. Hydrogen effusion was performed during the heating up by choosing a sufficiently low heating ramp. The phase separation of the layers into SiNCs and surrounding oxynitride matrix was studied at temperatures of up to 1150 °C. The influence of the annealing temperature on SiO xN y/SiO 2 – SLs with varying SiO xN y layer thickness was investigated by several analytical techniques including variable angle spectroscopic ellipsometry, photoluminescence (PL) spectroscopy, x-ray photoelectron spectroscopy, Fourier transform infrared spectrometry (FTIR) and transmission electron microscopy (TEM). Before annealing FTIR investigations show in addition to the expected Si-O bonds also the formation of nitrogen and hydrogen related bonds. The shift of the Si-O-Si stretching vibration to higher wave numbers after annealing indicates phase separation. The disappearance of the hydrogen related bonds indicates the hydrogen effusion. The PL signal is rising significantly with increasing annealing temperature and the PL peak position is strongly related to the thickness of the SiO xN y sublayers due to quantum confinement effects. TEM investigations confirm the size-controlled growth of SiNCs within the oxynitride matrix. The role of incorporated nitrogen and hydrogen is discussed.

Structural and optical characterization of size controlled silicon nanocrystals in SiO2/SiOxNy multilayers

Abstract We offer a complete structural and optical study of samples containing silicon nanocrystals (Si-NCs) embedded in SiO2/SiON multilayers, varying the oxynitride layer thickness from 2.5 to 7 nm. Using energy-filtered transmission electron microscopy we have determined the size distribution of the precipitated Si-nanoaggregates. Raman scattering measurements were used to investigate the Si-NC size and crystalline quality. By combining both techniques, the nanoaggregate crystalline degree has been evaluated, with values around 50% for all the samples. Photoluminescence spectroscopy has shown a blueshift of the emission at smaller NC sizes, presenting the sample with Si-NCs of 3.9 nm the best emission properties.

Selective Formation of Size-Controlled Silicon Nanocrystals by Photosynthesis in SiO Nanoparticle Thin Film

The SiOx thin film with a thickness of about 1 mum was formed on a GaAs substrate by bar-coating with the organic solution of the SiOx nanoparticles (~40 nm). The as-formed SiOx thin film consists of the SiOx nanoparticles; thus the thin film is macroscopically discontinuous and is referred to as a nanoparticle thin film. Although there were no silicon (Si) nanocrystals in the as-formed SiOx nanoparticle thin film, Si nanocrystals were observed by Raman scattering measurement after the thin film was exposed to the laser beam. The growth of Si nanocrystals by laser irradiation is referred to as photosynthesis. The photosynthesis of Si nanocrystals is found to be a self-limiting process. After the average size reaches a certain value, further increase of irradiation time or laser power does not increase the average size. The photosynthesis is similar to the thermal synthesis of Si nanocrystals from SiOx but much faster and low-temperature growth of Si nanocrystals from SiO x. Furthermore, the laser irradiation makes nanoparticles larger by merging. This suggests a possibility of low-temperature formation of a Si-nanocrystal array embedded in a SiO2 thin film. Such a structure has many potential device applications

Luminescence efficiency at 1.5μm of Er-doped thick SiO layers and Er-doped SiO∕SiO2 multilayers

The luminescence from Er-doped thin films is studied in two different systems. The first one is a SiO single layer. The second one is a SiO∕SiO2 multilayer allowing us to obtain size-controlled silicon nanocrystals. In both systems, the annealing-temperature dependence of the luminescence is investigated. It is shown that the optimal annealing temperatures are equal to 700 and 1050°C for the single layer and the multilayer, respectively. Moreover the luminescence efficiency at 1.5μm is one order of magnitude higher in the single Er-doped SiO layer. These results are discussed in relation to the formation of silicon nanoparticles with annealing treatments.

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.

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.

Charge Transport in Silicon Nanocrystal Arrays

ABSTRACTSingle layers of isolated, size-controlled silicon nanocrystals were prepared by thermal crystallization of a thin amorphous silicon layer sandwiched between silicon dioxide layers. A subsequent oxidation treatment ensured controlled increase in their lateral separation. The size of the nanocrystals, separation of the nanocrystals (from < 1 nm to ∼ 4 nm), stoichiometry of the resulting oxide and surface morphology were monitored with transmission electron microscopy, scanning transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. Mesoscopic charge transport studies performed with an electrostatic force microscope (EFM) revealed rapid lateral transport of charges when the nanocrystals were tightly packed (< 1 nm average separation) and interconnected. As the inter-nanocrystal separation was increased, lateral charge transport was rapidly suppressed. Nanocrystals separated by up to 3.6 nm retained the injected charges in a well-defined localized region (∼ 62 nm diameter region) for a time of the order of several days. The ability to switch from a very short to a very long retention time using the same structure by simply changing the post-growth processing conditions is attractive for various applications involving charge transport and localization.

Fabrication and photoluminescence properties of erbium doped size-controlled silicon nanocrystals

Er doping of dense arrays of nearly monodisperse Si nanocrystals produced by a new SiO/SiO 2 superlattice approach is presented. This synthesis method is fully compatible with standard Si technology. The prepared samples show a strong increase of Er-related luminescence by a factor of up to 5000 in comparison with Er-doped bulk SiO 2. Photoluminescence (PL) spectra and transients of the undoped and Er-doped size-controlled nanocrystalline Si/SiO 2 superlattices have been comparatively investigated as a function of implantation dose and Si nanocrystal size. It is shown that the energy of the optically excited Si nanocrystals can almost be completely transferred to the Er 3+ ions, resulting in a very strong emission at 1.54 μm. The efficiency of the energy transfer increases for smaller crystal sizes and an increasing overlap between the emission energy of the Si-related luminescence and the absorbing Er 3+ energy levels. For superlattices with nanocrystal sizes of 2 nm a transfer efficiency to the Er 3+ ions is found to be nearly one at room temperature under an excitation with photons of 3.7 eV. This properties of a highly efficient excitation of the Er ions via the Si nanocrystals is in agreement with the Förster mechanism.

Ordering and self-organization in nanocrystalline silicon

The spontaneous formation of organized nanocrystals in semiconductors has been observed during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 A into brick-shaped crystallites oriented along the (111) crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.