Er-doped Si Research Articles

Optical Spectroscopy of Er-doped Si Nanostructures

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New approaches for enhancing light emission from Er-based materials and devices

In this work, we present some approaches recently developed for enhancing light emission from Er-based materials and devices. We have investigated the luminescence quenching processes limiting quantum efficiency in light-emitting devices based on Si nanoclusters (Si nc) or Er-doped Si nc. It is found that carrier injection, while needed to excite Si nc or Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. A strong light confinement and enhancement of Er emission at 1.54 μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO 2 with Er-doped Si nc at the center of the Si core has been obtained. By measuring the guided photoluminescence from the cleaved edge of the sample, we have observed a more than fivefold enhancement of emission for the transverse magnetic mode over the transverse electric one at room temperature. Slot waveguides have also been integrated with a photonic crystal (PhC), consisting of a triangular lattice of holes. An enhancement by more than two orders of magnitude of the Er near-normal emission is observed when the transition is in resonance with an appropriate mode of the PhC slab. Finally, in order to increase the concentration of excitable Er ions, a completely different approach, based on Er disilicate thin films, has been explored. Under proper annealing conditions crystalline and chemically stable Er 2Si 2O 7 films are obtained; these films exhibit a strong luminescence at 1.54 μm owing to the efficient reduction of the defect density.

On the effects of double-step anneal treatments on light emission from Er-doped Si-rich silicon oxide

We have studied photoluminescence (PL) from an Er-doped Si-rich Si oxide (SRSO) film thermally annealed under different conditions. Compared to the case of annealing in N2 alone, double-step annealing the film at 875°C in N2 and then at ∼850°C in O2 or vice versa increases Er PL intensities by 10%–15%; while double-step annealing in N2+5%H2 (FG) and then in O2 or vice versa yields significant enhancements of the PL from the SRSO matrix and the Er PL intensity decreases differently by exchanging the processing order. Fourier transform infrared spectroscopy indicates that silicon oxynitride forms after annealing in FG ambient, and for the samples initially oxidized, the increase of Er PL intensity after secondary annealing in N2 (or FG) is due to more Si nanoclusters being formed. The PL spectra exhibit different annealing behavior with increasing the FG annealing temperature and the processing order. Weak oxygen bonds and silicon oxynitrides are believed to form upon O2 and FG annealing, respectively, and play important roles in the PL.

Er-doped silicon nanowires with 1.54μm light-emitting and enhanced electrical and field emission properties

Erbium-doped silicon nanowires have been grown via a vapor transport and condensation method with ErCl3∙6H2O powder as part of the source in one step. The Er-doped silicon nanowires exhibit the room temperature photoluminescence at a wavelength of 1.54μm, ideal for optical communication. From I-V measurements, the resistivity of 4.2at.% Er-doped Si nanowires was determined to be 1.5×10−2Ωcm. The Er-doped silicon nanowires were found to possess excellent field emission properties with a field enhancement factor as high as 1260. The rich variety of enhanced physical properties exhibited by the Er-doped silicon nanowires points to versatile applications for advanced devices.

Light emitting devices based on silicon nanostructures

In this paper, we summarize the results of an extensive investigation on the properties of MOS-type light emitting devices based on silicon nanostructures. The performances of crystalline, amorphous and Er-doped Si nanostructures are presented and compared. We show that all devices are extremely stable and robust, resulting in an intense room temperature electroluminescence (EL) at around 900 nm or at 1.54 μm. Amorphous nanostructures may constitute an interesting system for the monolithic integration of optical and electrical functions in Si ULSI technology. In fact, they exhibit an intense room temperature EL with the advantage to be formed at a temperature of only 900 °C, remarkably lower than the temperature needed for the formation of Si nanocrystals (1100 °C or higher). To improve the extraction of the light, we coupled the emitting system with a 2D photonic crystal structure properly fabricated with ULSI technology to reduce the total internal reflection of the emitted light. We demonstrate that the extraction efficiency is increased by a factor of 4. Finally, the light emission from devices based on Er-doped Si nanoclusters has been studied and in particular we have investigated the luminescence quenching processes limiting quantum efficiency in these devices. In fact the carrier injection, that determines the excitation of Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. These data are presented and the implications on the device performances discussed.

Local environment of Er3+ in Er-doped Si nanoclusters embedded in SiO2 films

The local environment of Er3+ in heavily Er-doped (Er, 2.5at.%) Si nanoclusters embedded in SiO2 films annealed at various temperatures was investigated by using the fluorescence-extended x-ray absorption fine structure spectroscopy. The results show that annealing caused a large effect on the local environment of Er3+ surrounded by O atoms and the 1.54μm photoluminescence intensity. The correlation between the local environment around Er3+ and the corresponding 1.54μm photoluminescence was discussed.

Silicon-Based Light-Emitting Devices: Properties and Applications of Crystalline, Amorphous and Er-Doped Nanoclusters

In this paper, we summarize the results of an extensive investigation on the properties of MOS-type light-emitting devices based on silicon nanostructures. The performances of crystalline, amorphous, and Er-doped Si nanostructures are presented and compared. We show that all devices are extremely stable and robust, resulting in an intense room temperature electroluminescence (EL) at around 900 nm or at 1.54 mum. Amorphous nanoclusters are more conductive than the crystalline counterpart. In contrast, nonradiative processes seem to be more efficient for amorphous clusters resulting in a lower quantum efficiency. Erbium doping results in the presence of an intense EL at 1.54 mu m with a concomitant disappearance of the 900-nm emission. This suggests that under electrical pumping Er is excited through an efficient energy transfer from the silicon clusters which hence become dark. We have identified an Auger de-excitation of Er with trapped carriers as the main process competing with radiative emission and limiting EL efficiency. This process is particularly severe in presence of unbalanced carrier injection (electrons versus holes) and can be controlled in properly designed structures. These data are presented and their implications are discussed

1.54 μm room temperature emission from Er-doped Si nanocrystals deposited by ECR-PECVD

In this work, silicon nanocrystals (Si-nc) embedded in a silicon-rich silicon oxide (SRSO) matrix doped with Er 3+ ions for different erbium and silicon concentrations have been deposited by electron-cyclotron resonance plasma-enhanced chemical-vapor-deposition (ECR-PECVD) technique. Their optical properties have been investigated by photoluminescence (PL) and reflectance spectroscopy. Room temperature emission bands centered at ∼1.54 and at 0.75 μm have been obtained for all samples. The most intense emission band at ∼1.54 μm was obtained for samples with concentrations of 0.45% and 39% for erbium and silicon, respectively. Moreover, it has been found that the broad emission band centered at ∼0.75 μm for all samples shows a very strong interference pattern related to the a specific sample structure and a high sample quality.

Interface effect on emission properties of Er-doped Si nanoclusters embedded in SiO2 prepared by magnetron sputtering

Er-doped Si nanoclusters embedded in SiO2 (NCSO) films were prepared by radio frequency magnetron sputtering on either silicon or quartz substrates. A 1.16μm (1.08eV) photoluminescence (PL) peak was observed from an Er-doped NCSO film deposited on a Si substrate. This 1.16μm peak is attributed to misfit dislocations at the NCSO/Si interface. The emission properties of the 1.16μm peak and its correlation with the Er3+ emission (1.54μm) have been studied in detail. The observed behavior suggests that the excitation mechanism of the 1.16μm PL is in a fashion similar to that shown for Er-doped Si nanoclusters embedded in a SiO2 matrix.

Defect Formation in MBE Er-Doped Si Light-Emitting Structures

Structural defects in Si:Er layers grown by molecular beam epitaxy have been studied by transmission electron microscopy. Two kinds of second phase precipitates are the main defects in the layers with Er concentration ≥ 2х1019 cm-3: ball-shaped precipitates (4-25 nm) of metallic Er localized at the layer-substrate interface and platelet precipitates of ErSi2 extending through the whole layer. We studied the effect of Er concentration (8х1018 - 4х1019 cm-3) and growth temperature (400 - 700°C) on the defect generation. The peculiarities of defect generation in MBE Si:Er layers implanted with B+ ions were also studied.

Revealing the sequential nature of the Si-nanocluster–Er interaction by variable pulse duration excitation

The sequential nature of the Si-nanocluster--Er interaction is investigated by studying the time evolution of the Er luminescence intensity at $1.54\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, after exciting the system through high-power optical pulses with different time duration, in the range between 200 ns and $55\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$. It is shown that while Si nanocrystals and Er ions separately embedded in the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix can be approximated by an effective two level model, the Er-doped Si nanocluster system behaves differently under short pulse excitation. Indeed, for time durations shorter than the mean time between two successive excitations of a Si nanocluster, the number of excitable Er ions is shown to be limited by the total number of sensitizing centers and by the finite energy transfer time.

Absorption cross section and signal enhancement in Er-doped Si nanocluster rib-loaded waveguides

Pump and probe experiments on Er3+ ions coupled to Si nanoclusters have been performed in rib-loaded waveguides to investigate optical amplification at 1.5μm. Rib-loaded waveguides were obtained by photolithographic and reactive ion etching of Er-doped silica layers containing Si nanoclusters grown by reactive sputtering. Insertion losses measurements in the infrared erbium absorption region allowed to gauge an Er3+ absorption cross section of about 5×10−21cm2 at 1534nm. Signal transmission under optical pumping at 1310nm shows confined carrier absorption of the Si nanoclusters. Amplification experiments at 1535nm evidence two pump power regimes: Losses due to confined carrier absorption in the Si nanoclusters at low pump powers and signal enhancement at high pump powers. For strong optical pumping, signal enhancement of about 1.2dB∕cm was obtained.

Ge nanoparticle formation and photoluminescence in Er doped SiO2 films: influence of sputter gas and annealing

The Er3+ luminescence at 1.54μm is of current interest as silica-based optical fibers have low loss at that wavelength. The effect of placing Er in matrixes that are compatible with Si-integrated circuit technology has been explored and Er-doped Si-rich Si oxide has created much interest recently. In this work, we report on photoluminescence (PL) from Er-doped Ge-rich SiO2 films deposited on Si substrates and the effect of Ar+O2 vs. Ar as sputtering ambience is compared. The most intense PL is reached after annealing at 700°C for the Ar sputtering ambience. These films contain Ge rich amorphous clusters observed by transmission electron microscopy. Higher annealing temperatures yield Ge nanocrystals while lower the PL intensity. For films sputtered in an Ar+O2 gas the PL intensity increases with annealing temperature up to 1000°C while no Ge nanocrystals form.

Hydrogenation Effect on the Er Luminescence in Amorphous Silicon Quantum Dot Films

The hydrogenation effect on the Er luminescence at 1.54 μm in an Er-doped amorphous Si quantum dot film was investigated. After hydrogenation, the luminescent properties were different between large-dot (2.5 nm) and small-dot (1.4 nm) samples. In particular, the number of optically active Er ions was increased in a large-dot sample, but decreased in a small-dot sample. We propose that the hydrogenation causes the Er migration toward an Si dot, and the luminescent property depending on the dot size is originated from the number of Er ions near an Si dot before hydrogenation.

Array of luminescent Er-doped Si nanodots fabricated by pulsed laser deposition

Ordered arrays of Er-doped Si nanodots were fabricated by pulsed laser deposition using an anodic aluminum oxide (AAO) nanopore membrane. Er-doped Si (Si:Er) nanodots with an average diameter of 60–80nm and a periodicity of 100nm were uniformly assembled into hexagonally ordered nanopores of AAO membrane. Photoluminescence (PL) spectra taken from Si:Er nanodots showed an emission peak at 1.54μm, which is due to intra-4f shell transition (I13∕24→I15∕24) of Er3+ ions. Postannealing of nanodot arrays up to 500°C resulted in the enhancement of PL intensities without enlargement or aggregation of the nanodots.

Synthesis of very long ErSi 2 nanowires on Si substrates

ErSi 2 nanowires with a length of several micrometers have been fabricated on Si substrates. The starting samples were highly Er-doped amorphous Si thin layers formed by laser ablating an Si:Er 2O 3 mixture target. After post-annealing in vacuum, the nanowires were formed on the Si(0 0 1) surfaces along 〈 1 1 0 〉 directions. The average length and width of the wires depended on the annealing temperature and time. The height of the wires, in contrast, remained equal to be 0.6 nm. The longest wire achieved here was 20 μm. A large aspect ratio of 500 was obtained. X-ray diffraction and TEM observation indicated the hexagonal ErSi 2 formation. Our results provide a way to synthesize Si-based nanowires having enough length for further device applications without an UHV system.

Fabrication of Nanodot Arrays on Si by Pulsed Laser Deposition Using Anodic Aluminum Oxide Nanopore Membrane as Mask

We demonstrate the fabrication of metal and semiconductor nanodot arrays on Si by pulsed laser deposition using an anodic alumina membrane as a mask. A Hexagonal-close-packed nanopore membrane mask was fabricated by the anodic oxidation of aluminum sheet. Pulsed laser deposition of Ag, Ni, ZnO, and Er-doped Si (Si:Er) nanodots was performed on a Si substrate to which an alumina membrane mask adheres by van der Waals interaction. Highly ordered arrays of Ag, Ni, ZnO, and Si:Er nanodots with average diameters ranging from 55 to 86 nm and a periodicity of ∼100 nm were obtained. Also, the time-resolved images of laser-produced plasma plumes were analyzed to examine the dynamical properties of the ejected materials.

Comparative study of photoluminescence of undoped and erbium-doped size-controlled nanocrystalline Si∕SiO2 multilayered structures

Spectra and transients of the photoluminescence (PL) of undoped and Er-doped size-controlled nanocrystalline Si∕SiO2 multilayered structures with mean nanocrystal size of 1.5–4.5nm have been comparatively investigated. The Er-doped structures exhibit a strong Er-related PL band at 0.81eV, while the efficiency of the intrinsic PL band of Si nanocrystals at 1.2–1.7eV decreases by several orders of magnitude in comparison with the undoped structures. At low temperature the PL spectra of the Er-doped structures show several dips separated by the energy of Si TO-phonon and bound to the transition energies between the second and third excited states to the ground state of Er3+. The Er-related PL is characterized by lifetimes of around 3–5ms, a weak temperature quenching, and a high efficiency, which is comparable or even stronger than that of the intrinsic PL in the corresponding undoped samples. This efficient sensitizing of the Er-related luminescence is explained by the structural properties of the samples, which favor a strong coupling between the excitons confined in Si nanocrystals and upper excited states of the Er3+ ions in the SiO2 matrix.

Effect of the erbium dopant architecture on the femtosecond relaxation dynamics of silicon nanocrystals.

Femtosecond pump-probe absorption spectroscopy is used to investigate the role of Er(3+) dopants in the early relaxation pathways of photoexcited Si nanocrystals. The fate of photoexcited electrons in three different Si nanostructures was studied and correlated with the effect of Er-doping and the nature of the dopant architecture. In Si nanocrystals without Er(3+) dopant, a trapping component was identified to be a major electron relaxation mechanism. Addition of Er(3+) ions into the core or surface shell of the nanocrystals was found to open up additional nonradiative relaxation pathways, which is attributed to Er-induced trap states in the Si host. Analysis of the photodynamics of the Si nanocrystal samples reveals an electron trapping mechanism involving trap-to-trap hopping in the doped nanocrystals, whereby the density of deep traps seem to increase with the presence of erbium. To gain additional insights on the relative depths of the trapping sites on the investigated nanostructures, benzoquinone was used as a surface adsorbed electron acceptor to facilitate photoinduced electron transfer across the nanocrystal surface and subsequently assist in back electron transfer. The established reduction potential (-0.45 V versus SCE) of the electron acceptor helped reveal that the erbium-doped nanocrystal samples have deeper trapping sites than the undoped Si. Furthermore, the measurements indicate that internally Er-doped Si have relatively deeper trapping sites than the erbium surface-enriched nanocrystals. The electron-shuttling experiment also reveals that the back electron transfer seems not to recover completely to the ground state in the doped Si nanocrystals, which is explained by a mechanism whereby the electrons are captured by deep trapping sites induced by erbium addition in the Si lattice.

The role of microstructure in luminescent properties of Er-doped nanocrystalline Si thin films

In this contribution, we present a structural and photoluminescence (PL) analysis of Er-doped nanocrystalline silicon thin films produced by rf magnetron sputtering method. We show the strong influence of the presence of nanocrystalline fraction in films on their luminescence efficiency at 1.54 µm studied on a series of specially prepared samples with different crystallinity, i.e., percentage and sizes of Si nanocrystals. A strong increase, by about two orders of magnitude, of Er-related PL intensity in these samples with lowering of the Si nanocrystal sizes from 7.9 to about 1.5 nm is observed. The results are discussed in terms of the sensitization effect of Si nanocrystals on Er ions.