Formation Of Silicon Quantum Dots Research Articles

Synchrotron-Excited Luminescence and Converting of Defects and Quantum Dots in Modified Silica Films

Optically active defects in modified silicon oxide films on a silicon substrate have been studied by low-temperature photoluminescence (PL) spectroscopy using excitation by synchrotron radiation in the vacuum ultraviolet region. Films of dry thermal silicon oxide obtained by treatment of stoichiometric SiO2 in hydrogen plasma, films of wet thermal silicon oxide, and films with low dielectric constant (so called "low-k" dielectrics) have been studied. Investigations of various type SiO2 films detect the PL centers, which can be conditionally divided into two groups. The first group includes intrinsic point defects, the different variants of oxygen-deficient centers (ODCs). The second group contains centers like the spatially confined excitons in silicon quantum dots (SiQDs). It is shown that SiQDs differ in spectral characteristics, are formed in different ways, due to the transformation and clustering of point defects such as E'-centers, ODC(I) and ODC(II). In this case, the size and spectral properties of quantum dots depend on the mechanism of their formation. Schemes for the conversion of these defects are proposed.It was found that for wet films and nonstoichiometric SiOx films treated in hydrogen plasma, "metastable" centers ODC(I) and ODC(II) arise only under the action of synchrotron radiation in the region of interband or exciton absorption and then they decay because of radiative relaxation. The absence of stable ODC centers in these films is due to their clustering and the formation of SiQDs. In "low-k" mesoporous films, on the contrary, silicon quantum dots SiQDs are not formed, but ODC(I) and ODC(II) centers are clearly manifested. A scheme of electronic transitions upon indirect excitation of silicon quantum dots through exciton states of the SiO2 matrix is considered. The presented results demonstrate the possibility of controlling the optical properties of silicon thin-film structures by creating in them both oxygen-deficient point defects and quantum dots of various sizes.

Formation of Silicon Quantum Dots Sheet on a Nonmetallic CaF2 Surface

AbstractThe research on the atomic layer 2D materials employed in ultrahigh‐speed electronic devices has attracted significant attention and silicene, a monoatomic layer allotrope of Si, is considered the most effective material. For practical device applications, it is necessary to grow silicene on a nonmetallic substrate; however, only the growth of silicene on metallic or semiconductive substrates is reported. In this study, Si quantum dot sheets are grown on a nonmetallic epitaxial CaF2 surface by the deposition of monolayer thick Si. By depositing Si at a temperature ranging from room temperature to 650 °C, a 2D sheet composed of high‐density quantum dots (7.8 × 1012 cm−2) is obtained. Scanning tunneling spectroscopy and X‐ray photoelectron spectroscopy measurements reveal that Si forms a continuous film containing quantum‐sized dots on the CaF2 surface.

First synthesis of silicon nanocrystals in amorphous silicon nitride from a preceramic polymer

We report the first synthesis of silicon nanocrystals embedded in a silicon nitride matrix through a direct pyrolysis of a preceramic polymer (perhydropolysilazane). Structural analysis carried out by XRD, XPS, Raman and TEM reveals the formation of silicon quantum dots and correlates the microstructures with the annealing temperature. The photoluminescence of the nanocomposites was investigated by both linear and nonlinear measurements. Furthermore we demonstrate an enhanced chemical resistance of the nitride matrix, compared to the typical oxide one, in both strongly acidic and basic environments. The proposed synthesis via polymer pyrolysis is a striking innovation potentially allowing a mass-scale production nitride embedded Si nanocrystals.

Enhanced Electroluminescence from Silicon Quantum Dots Embedded in Silicon Nitride Thin Films Coupled with Gold Nanoparticles in Light Emitting Devices.

Nowadays, the use of plasmonic metal layers to improve the photonic emission characteristics of several semiconductor quantum dots is a booming tool. In this work, we report the use of silicon quantum dots (SiQDs) embedded in a silicon nitride thin film coupled with an ultra-thin gold film (AuNPs) to fabricate light emitting devices. We used the remote plasma enhanced chemical vapor deposition technique (RPECVD) in order to grow two types of silicon nitride thin films. One with an almost stoichiometric composition, acting as non-radiative spacer; the other one, with a silicon excess in its chemical composition, which causes the formation of silicon quantum dots imbibed in the silicon nitride thin film. The ultra-thin gold film was deposited by the direct current (DC)-sputtering technique, and an aluminum doped zinc oxide thin film (AZO) which was deposited by means of ultrasonic spray pyrolysis, plays the role of the ohmic metal-like electrode. We found that there is a maximum electroluminescence (EL) enhancement when the appropriate AuNPs-spacer-SiQDs configuration is used. This EL is achieved at a moderate turn-on voltage of 11 V, and the EL enhancement is around four times bigger than the photoluminescence (PL) enhancement of the same AuNPs-spacer-SiQDs configuration. From our experimental results, we surmise that EL enhancement may indeed be due to a plasmonic coupling. This kind of silicon-based LEDs has the potential for technology transfer.

Double stack layer structure of SiNx/pm-Si thin films for downshifting and antireflection properties

In this work, we propose a double stack layer design of SiNx/pm-Si thin films by using dichlorosilane in plasma enhanced chemical vapor deposition (PECVD). Adequate deposition condition has been found to attain the average minimal reflectance corresponding to two different layers of SiNx and pm-Si at the suitable refractive index and thickness of the respective films. The average reflectance of the structure is reduced to ∼3% at normal incidence for a wavelength range from 190 to 1100nm as in comparison to single layer of SiNx thin film structure. Furthermore, using transmission electron microscopy (TEM) presence of double layer stack and formation of silicon quantum dots (QD’s) in the size regime of 3–4nmhas been found. Photoluminescence (PL) diminishment in the double layer structure as in comparison to the single layer of SiNx thin film confirms the effective absorption and total internal reflection induced due to the gradient refractive index. However, visible photoluminescence observed in the present work also remarks the downshift property from the thin film which could subsequently improve the efficiency in silicon solar cells. Present work highlights the prospective of novel structure for downshifting, antireflection and as an passivation coating in new generation of silicon solar cells.

Microwave annealing enhances formation of silicon quantum dots in oxide matrix

In this work, an effective strategy for the controlled synthesis of Si-QDs embedded Si-rich silicon oxide/SiO2 (SRO/SiO2) multilayers using susceptor-assisted microwave annealing (MWA) is introduced. Both MWA and rapid thermal annealing (RTA) were compared to study the growth of Si-QDs embedded in SRO/SiO2 multilayers, and found that the temperature required for forming Si-QDs decreases by 200 °C under MWA. SRO single-layer films were used for the investigation of growth mechanism during MWA. It is shown that MWA annealing could enhance the phase separation in SRO films and more Si-QDs precipitated in SRO film annealed by MWA than RTA one. These results demonstrate that the microwave electromagnetic field has unique influence on the generated Si-QDs, can be considered as an archetype of a non-thermal microwave effect.

Experimental and theoretical rationalization of the growth mechanism of silicon quantum dots in non-stoichiometric SiNx: role of chlorine in plasma enhanced chemical vapour deposition

Silicon quantum dots (Si-QDs) embedded in an insulator matrix are important from a technological and application point of view. Thus, being able to synthesize them in situ during the matrix growth process is technologically advantageous. The use of SiH2Cl2 as the silicon precursor in the plasma enhanced chemical vapour deposition (PECVD) process allows us to obtain Si-QDs without post-thermal annealing. Foremost in this work, is a theoretical rationalization of the mechanism responsible for Si-QD generation in a film including an analysis of the energy released by the extraction of HCl and the insertion of silylene species into the terminal surface bonds. From the results obtained using density functional theory (DFT), we propose an explanation of the mechanism responsible for the formation of Si-QDs in non-stoichiometric SiNx starting from chlorinated precursors in a PECVD system. Micrograph images obtained through transmission electron microscopy confirmed the presence of Si-QDs, even in nitrogen-rich (N-rich) samples. The film stoichiometry was controlled by varying the growth parameters, in particular the NH3/SiH2Cl2 ratio and hydrogen dilution. Experimental and theoretical results together show that using a PECVD system, along with chlorinated precursors it is possible to obtain Si-QDs at a low substrate temperature without annealing treatment. The optical property studies carried out in the present work highlight the prospects of these thin films for down shifting and as an antireflection coating in silicon solar cells.

Formation of silicon quantum dots by RF power driven defect control

We investigated the turning behavior of luminescence origins from the defect to the silicon quantum dot for silicon nitride (SiNx) films synthesized by changing the applied radio frequency (RF) power in plasma-enhanced chemical vapor deposition.

Crystallization Behavior of Silicon Quantum Dots in a Silicon Nitride Matrix

Silicon quantum dot superlattice was fabricated by alternating deposition of silicon rich nitride (SRN) and Si3N4 layers using RF magnetron co-sputtering. Samples were then annealed at temperatures between 800 and 1,100 degrees C and characterized by grazing incident X-ray diffraction (GIXRD), transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). GIXRD and Raman analyses show that the formation of silicon quantum dots occurs with annealing above 1,100 degrees C for at least 60 minutes. As the annealing time increased the crystallization of silicon quantum dots was also increased. TEM images clearly showed SRN/Si3N4 superlattice structure and silicon quantum dots formation in SRN layers after annealing at 1,100 degrees C for more than 60 minutes. The changes in FTIR transmission spectra observed with annealing condition corresponded to the configuration of Si-N bonds. Crystallization of silicon quantum dots in a silicon nitride matrix started stabilizing after 60 minutes' annealing and approached completion after 120 minutes'. The systematic investigation of silicon quantum dots in a silicon nitride matrix and their properties for solar cell application are presented.

Fabrication and Characterization of Silicon Quantum Dots in Si-Rich Silicon Carbide Films

Amorphous Si-rich silicon carbide films were prepared by magnetron co-sputtering and subsequently annealed at 900-1100 degrees C. After annealing at 1100 degrees C, this configuration of silicon quantum dots embedded in amorphous silicon carbide formed. X-ray photoelectron spectroscopy was used to study the chemical modulation of the films. The formation and orientation of silicon quantum dots were characterized by glancing angle X-ray diffraction, which shows that the ratio of silicon and carbon significantly influences the species of quantum dots. High-resolution transmission electron microscopy investigations directly demonstrated that the formation of silicon quantum dots is heavily dependent on the annealing temperatures and the ratio of silicon and carbide. Only the temperature of about 1100 degrees C is enough for the formation of high-density and small-size silicon quantum dots due to phase separation and thermal crystallization. Deconvolution of the first order Raman spectra shows the existence of a lower frequency peak in the range 500-505 cm(-1) corresponding to silicon quantum dots with different atom ratio of silicon and carbon.

Fabrication of Silicon Quantum Dots in Si3N4 Matrix Using RF Magnetron Co-Sputtering

Films consisting of a silicon quantum dot superlattice were fabricated by alternating deposition of silicon rich silicon nitride and <TEX>$Si_3N_4$</TEX> layers using an rf magnetron co-sputtering system. In order to use the silicon quantum dot super lattice structure for third generation multi junction solar cell applications, it is important to control the dot size. Moreover, silicon quantum dots have to be in a regularly spaced array in the dielectric matrix material for in order to allow for effective carrier transport. In this study, therefore, we fabricated silicon quantum dot superlattice films under various conditions and investigated crystallization behavior of the silicon quantum dot super lattice structure. Fourier transform infrared spectroscopy (FTIR) spectra showed an increased intensity of the <TEX>$840\;cm^{-1}$</TEX> peak with increasing annealing temperature due to the increase in the number of Si-N bonds. A more conspicuous characteristic of this process is the increased intensity of the <TEX>$1100\;cm^{-1}$</TEX> peak. This peak was attributed to annealing induced reordering in the films that led to increased Si-<TEX>$N_4$</TEX> bonding. X-ray photoelectron spectroscopy (XPS) analysis showed that peak position was shifted to higher bonding energy as silicon 2p bonding energy changed. This transition is related to the formation of silicon quantum dots. Transmission electron microscopy (TEM) and electron spin resonance (ESR) analysis also confirmed the formation of silicon quantum dots. This study revealed that post annealing at <TEX>$1100^{\circ}C$</TEX> for at least one hour is necessary to precipitate the silicon quantum dots in the <TEX>$SiN_x$</TEX> matrix.

Fabrication of silicon quantum dots in SiNx multilayer using hot-wire CVD

A hot-wire chemical vapor deposition (HWCVD) procedure for growing 40 alternating layers of Silicon quantum dots (Si-QD) and SiNx in a single silicon nitride deposition chamber is presented in this paper. Films of 140–160nm thickness were deposited with a substrate temperature of 250°C and post-annealed between 800 and 950°C. Transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS) and Raman analysis techniques were used to characterize the samples. The cross-sectional TEM analysis confirms the formation of Si-QD in the range 3–5nm with a density of 5×1012/cm2 in the Si-QD/SiNx multilayer. The SIMS measurements indicate the variation in Si and N content as the alternating layers of Si and SiNx grow on the c-Si substrate. Deconvolution of the first order Raman spectra shows the presence of a lower frequency peak in the range 517–518cm−1 corresponding to Si-QD annealed with an increase in the temperature. The intensity ratio at the center frequency of the second order Raman spectrum increases from 0.52 to 0.88 with an increase in the Si-QD size and the annealing temperature.

Self-assembling formation of silicon quantum dots with a germanium core bylow-pressure chemical vapour deposition

The formation of nanometre-scale silicon dots with a germanium core on an ultrathin SiO2layer has been studied by controlling the early stages of low-pressure chemicalvapour deposition (LPCVD) alternately using pure monosilane and 5% germanediluted with helium. From atomic force microscope observations and x-rayphotoelectron spectroscopy measurements, the selective growth of Ge on pregrownSi dots and subsequent complete coverage with a Si cap have been confirmed.Cross-sectional transmission electron microscope images have shown theformation of isolated spherical nanocrystallites with Ge cores in contrastwith hemispherical pregrown Si dots, implying a high structural strain atthe interface between cladding Si and the Ge core. For multiply stackedstructures of the dots with a Ge core, Raman-scattering spectra indicate thatcompositional mixing occurs partly at the Si/Ge-core interface during LPCVD.

Self-assembling formation of silicon quantum dots and their luminescence properties

Self-assembling formation of silicon quantum dots and their luminescence properties

Self-Assembling Formation of Silicon Quantum Dots by Low Pressure Chemical Vapor Deposition

AbstractThe growth of Si dots on thermally-grown SiO2/c-Si from a thermal decomposition of pure silane has been systematically studied in the temperature range from 500 to 650°C. It has been suggested mat the Si dot height and dot diameter on as-grown SiO2 are rate-limited by the cohesive action of adsorbed precursors and the thermal decomposition of silane on Si nucleation sites, respectively. The nucleation site on as-grown SiO2 is likely to be generated by the thermal dissociation of surface Si-O bonds. It has been also found that in Si dot formation on OH-terminated SiO2 surface the nucleation density is dramatically enhanced and consequently the dot size and its distribution become small. This implies that surface OH bonds can provide the nucleation sites.