Formation Of Si Nanocrystals Research Articles

Atomic-scale study on the dopant distribution in phosphorus and boron-doped Si nanocrystals/SiO2 multilayers

Understanding the distributions and behaviors of dopants in Si nanocrystal are the primary and necessary issues to realize the controllable doping at nanoscale and develop the next generation of optoelectronic devices. This work reports the atomic-scale distributions of phosphorus and boron dopants in Si nanocrystals multilayers. The phosphorus-doped and boron-doped Si nanocrystals/SiO2 multilayers are fabricated by PECVD and subsequently annealed at 1000 °C. It is found that the locations of phosphorus are redistributed after the formation of Si nanocrystals due to the combined effects of formation energy and self-purification. Phosphorus dopants are mainly distributed at the Si nanocrystals surfaces to passivate the dangling bonds, while part of them incorporate into Si nanocrystals lattice sites to provide free electrons. However, boron dopants exhibit different distributions in contrast to phosphorus. The concentration of boron on Si nanocrystals surfaces can reach as high as 40.0 at. %, which forms a dopant-shell covering on Si nanocrystals. Meanwhile, the boron dopant-shell can modify the surface states of Si nanocrystals like Si-oxide related emission centers and dangling bonds, which is responsible for the luminescence properties. Moreover, the boron-aggregations with concentration near 74.8 at. % are appeared inside Si nanocrystals and led to the damage of crystalline lattice.

Effect of the Graded Silicon Content in SRN/SRO Multilayer Structures on the Si Nanocrystals and Si Nanopyramids Formation and Their Photoluminescence Response.

Two multilayer (ML) structures, composed of five layers of silicon-rich oxide (SRO) with different Si contents and a sixth layer of silicon-rich nitride (SRN), were deposited by low pressure chemical vapor deposition. These SRN/SRO MLs were thermally annealed at 1100 °C for 180 min in ambient N2 to induce the formation of Si nanostructures. For the first ML structure (MLA), the excess Si in each SRO layer was about 10.7 ± 0.6, 9.1 ± 0.4, 8.0 ± 0.2, 9.1 ± 0.3 and 9.7 ± 0.4 at.%, respectively. For the second ML structure (MLB), the excess Si was about 8.3 ± 0.2, 10.8 ± 0.4, 13.6 ± 1.2, 9.8 ± 0.4 and 8.7 ± 0.1 at.%, respectively. Si nanopyramids (Si-NPs) were formed in the SRO/Si substrate interface when the SRO layer with the highest excess silicon (10.7 at.%) was deposited next to the MLA substrate. The height, base and density of the Si-NPs was about 2–8 nm, 8–26 nm and ~6 × 1011 cm−2, respectively. In addition, Si nanocrystals (Si-ncs) with a mean size of between 3.95 ± 0.20 nm and 2.86 ± 0.81 nm were observed for the subsequent SRO layers. Meanwhile, Si-NPs were not observed when the excess Si in the SRO film next to the Si-substrate decreased to 8.3 ± 0.2 at.% (MLB), indicating that there existed a specific amount of excess Si for their formation. Si-ncs with mean size of 2.87 ± 0.73 nm and 3.72 ± 1.03 nm were observed for MLB, depending on the amount of excess Si in the SRO film. An enhanced photoluminescence (PL) emission (eight-fold more) was observed in MLA as compared to MLB due to the presence of the Si-NPs. Therefore, the influence of graded silicon content in SRN/SRO multilayer structures on the formation of Si-NPs and Si-ncs, and their relation to the PL emission, was analyzed.

Role of secondary etching of silicon nanowires towards quantum confinement effect

The quantum confinement effect (QCE) plays a significant role in tuning the bandgap of the silicon nanowires (SiNWs) by converting from indirect to direct bandgap semiconductor. The QCE makes the silicon (Si) semiconductor suitable for efficient solar cell and optoelectronic devices. The QCE only occurs for the nanostructure of Si, whose diameter is lesser than Bohr's diameter (≈9.8 nm). Metal-assisted chemical etching (MACE) fabricates columnlike SiNWs array on the Si substrate through primary etching. Length of the fabricated NWs is in the micrometer range, and its diameter is much higher (d > 70 nm) than Bohr's diameter, which contradicts the existence of QCE in the NWs. However, secondary etching etches the sidewalls of the SiNWs forming Si nano-crystals and Si quantum dots on the NWs whose average size is lesser than Bohr's diameter. HRTEM image and mathematical analysis of the SiNWs, using Raman, XRD, and Photoluminescence (PL) characterization, calculate the average size of the nanostructures formed on the sidewalls. The analysis indicates nanostructures of size to be less than five nm, formed on the SiNWs due to secondary etching, the sole reason behind the origin of QCE.

A light-influenced memristor based on Si nanocrystals by ion implantation technique

We fabricated light-influenced resistive random access memory (RRAM) based on Si nanocrystals (NCs) with a configuration of Au/Si NC@SiO2/Si device by ion implantation. The Si NCs were self-assembled synthesized through the Si ion implantation into the SiO2/Si substrates followed by thermal annealing process. The resistance states of the Au/Si NC@SiO2/n-Si devices can be regulated by electric field and light irradiation, which can be distinguished and evaluated by the photoluminescence spectra. It was speculated that a series of SiOx (x ≤ 2) nanoinclusions were bridged by more conductive oxygen vacancies, incorporating with Si NCs to form the continuous conductive pathway. Besides, the forming Si NCs and electrons from the substrates play key roles in the formation of the conductive filaments. The light-influenced resistive switching memory based on the implanted Si NCs provides the possibility to integrate the light and electric fields into integrated circuits in the future.

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.

Optical properties of Si nanocrystals in SiO2 matrix synthesized by reactive pulsed laser deposition

Si ion implantation into SiO2 is widely used to synthesize specimens of SiO2 containing supersaturated Si. We also prepared specimens of supersaturated Si in SiO2 by reactive pulsed laser deposition (PLD) in an oxygen atmosphere. After high temperature annealing of these specimens induces the formation of embedded luminescent Si nanocrystals in SiO2. In this work, the potentialities of excimer UV-light (172 nm, 7.2 eV) irradiation and rapid thermal annealing (RTA) to enhance the photoluminescence and to achieve low temperature formation of Si nanocrystals have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluence of 7.5 x 1016 ions/cm2. We also prepared Si nanocrystals embedded in a SiO2 by reactive pulsed laser deposition (PLD) in an oxygen atmosphere by using conventional PLD system with 2nd-harmonic YAG laser (532 nm, 10 Hz, 80 J/cm2) under controlled low oxygen pressure. Samples were subsequently irradiated with an excimer-UV lamp. After the process, the samples were rapidly thermal annealed before furnace annealing (FA). Photoluminescence spectra were measured at various stages at the process. We found that the luminescence intensity is enhanced with excimer-UV irradiation and RTA. Moreover, effective visible photoluminescence is found to be observed even after FA below the annealing temperature at 1000 °C, only for specimens treated with excimer-UV lamp and RTA. We will make clear the similarities of photoluminescence with the way of preparation techniques.

Forming Si nanocrystals on insulator by wet anisotropic etching

Implementing the Silicon-On-Insulator platform, we reveal an approach to fabricate Si nanocrystals on insulator by a single step, wet anisotropic etching of thin crystalline layers. The evolution of the Si layer was investigated from the onset of the dissolution process up to the development of isolated nanocrystals. Our results also provide several insights into the understanding of the early stages of the hillocks formation characteristic for the anisotropic etching of Si(001). We discuss several related mechanisms and we give experimental evidences that support the stabilization of the pyramidal hillocks apices by short-lived silicate particles.

Analysis of the conduction mechanisms responsible for multilevel bipolar resistive switching of SiO2/Si multilayer structures

Abstract We report on the bipolar resistive switching behavior of a SiO2/Si multilayer (ML) structure measured without current compliance for non-volatile memory applications. The SiO2/Si ML structure was thermally annealed to induce the formation of Si nanocrystals, which act as conductive nodes. It was found that the SET/RESET voltages and the ON/OFF resistance ratio (107-102) depend on the current-voltage measurement cycles. Moreover, the low resistance state from the initial cycle can be recovered by increasing the stop voltage in a subsequent cycle. The space charge limited conduction (SCLC) is the mechanism responsible for the charge transport in the high resistance state (HRS) during the first measurement cycle, while the Ohmic conduction predominates in the low resistance state (LRS). This suggest the formation of a conductive path within the SiO2/Si ML, which breaks apart and becomes reconstructed upon switching between the LRS to HRS and vice versa. Performing a sequence of switching cycles, one can observe that the SCLC becomes predominant for the both LRS and HRS, suggesting the partial formation and breakdown of several different conductive paths.

UV Sensitivity of MOS Structures with Silicon Nanoclusters.

Selective UV sensitivity was observed in Metal-Oxide-Semiconductor structures with Si nanoclusters. Si nanocrystals and amorphous Si nanoparticles (a-Si NPs) were obtained by furnace annealing of SiOx films with x = 1.15 for 60 min in N2 at 1000 and 700 °C, respectively. XPS and TEM analysis prove phase separation and formation of Si nanocrystals in SiO2, while the a-Si NPs are formed in SiO1.7 matrix. Both types of structures show selective sensitivity to UV light; the effect is more pronounced in the structure with nanocrystals. The responsivity of the nanocrystal structure to 365 nm UV light is ~ 4 times higher than that to green light at 4 V applied to the top contact. The observed effect is explained by assuming that only short wavelength radiation generates photocarriers in the amorphous and crystalline nanoclusters.

Swift heavy ion irradiation assisted Si nanoparticle formation in HfSiOx nano-composite thin films deposited by RF magnetron sputtering method

Silicon co-deposited HfOx thin films were prepared by RF magnetron sputtering method. The average size of Si co-deposited nanocomposites is around 10–12 nm. 100 MeV Ag ion irradiation induces the Si nano-crystal formation within the nanocomposite. The average grain size of the Si nano-crystals increases with the increase of ion fluence. The observed luminescence at 430 nm is due to oxygen related defect states and the luminescence at 530 nm may be attributed to the emission from Si nanoparticles. Red shift in emission wavelength indicates either the increase in the size of the nanoparticles or ion induced change in the dielectric constant of surrounding medium. The increase in the intensity of the luminescence peak at 440 nm indicates the creation of defect states by ion irradiation. Raman measurements further confirm the formation of Si NPs that are embedded in the composite medium.

One-Pot Synthesis of Gel Glass Embedded with Luminescent Silicon Nanoparticles.

Preparation of highly luminescent glasses involves expensive and complicated processes and usually requires high temperature. In this work, we show that luminescent silicon (Si) nanoparticle (NP)-embedded silicate gel glasses can be developed under near-ambient conditions by a remarkably simple, one-pot strategy, without using any sophisticated instrumentation or technique. Simultaneous hydrolysis and reduction of (3-aminopropyl)triethoxysilane leads to the formation of colloidal Si nanocrystals that can be transformed to a glassy phase upon slow evaporation followed by freezing. Structural investigations reveal the formation of a sodium silicate gel glass framework having discernible shear bands, along with embedded Si NPs. High photoluminescence quantum yield (ca. 35-40%), low glass-transition temperature ( Tg ≈ 66-73 °C), strain-tolerant mechanical stability, and inexpensive preparation make the glass attractive for applications as display materials and photonic converters.

Site-controlled formation of single Si nanocrystals in a buried SiO2 matrix using ion beam mixing.

For future nanoelectronic devices – such as room-temperature single electron transistors – the site-controlled formation of single Si nanocrystals (NCs) is a crucial prerequisite. Here, we report an approach to fabricate single Si NCs via medium-energy Si+ or Ne+ ion beam mixing of Si into a buried SiO2 layer followed by thermally activated phase separation. Binary collision approximation and kinetic Monte Carlo methods are conducted to gain atomistic insight into the influence of relevant experimental parameters on the Si NC formation process. Energy-filtered transmission electron microscopy is performed to obtain quantitative values on the Si NC size and distribution in dependence of the layer stack geometry, ion fluence and thermal budget. Employing a focused Ne+ beam from a helium ion microscope, we demonstrate site-controlled self-assembly of single Si NCs. Line irradiation with a fluence of 3000 Ne+/nm2 and a line width of 4 nm leads to the formation of a chain of Si NCs, and a single NC with 2.2 nm diameter is subsequently isolated and visualized in a few nanometer thin lamella prepared by a focused ion beam (FIB). The Si NC is centered between the SiO2 layers and perpendicular to the incident Ne+ beam.

Laser-Stimulated Phase Transformations in Thin Layers of SiOx and CNx –Ni

We demonstrate the possibility of laser-assisted phase transformations in thin layers of SiOx from the amorphous nonstoichiometric phase into a nanocomposite layer with Si nanocrystals embedded in the silicon-oxide matrix. The formation of Si nanoparticles (NPs) in SiOx films occurs due to their irradiation with nanosecond pulses of a Nd+3:YAG laser with base wavelength λ = 1.064 μm and the second harmonic λ = 0.532 μm. The nanostructurization of the surface in the form of nanoparticles with mean sizes varying within the range 5–85 nm was detected with the help of atomic force microscopy and optical transmission spectra. The sizes of nanoparticles and their surface distribution depend on the intensity of the laser and its wavelength λ. We determined the mechanism of formation of Si nanocrystals by laser-induced thermal shocks. By analyzing the results of atomic force microscopy, as well as the data of Raman and polarization-modulation spectroscopies, we detected the increase in the amount of single-crystal phase in the film of CNx–Ni nanocomposite with simultaneous increase in the sizes of CNx–Ni nanoparticles caused by the action of laser pulses. The mechanism of laser-stimulated nanocrystallization is proposed. This mechanism is based on the local annealing of the CNx shell as a result of laser heating of the metallic Ni core of CNx–Ni nanoparticles and a more intense sublimation of the amorphous phase of CNx–Ni due to the lower temperature of dissociation of the CNx shell as compared with the Ni core.

Resistive switching control for conductive Si-nanocrystals embedded in Si/SiO2 multilayers

In this paper, we report on the enhanced control of resistive switching in multilayer Si/SiO2 structures, which permit the formation of Si nanocrystals with a typical size of 5.88 nm and overall good shape homogeneity. The deposition of a different number of Si and SiO2 bilayers (6, 8 and 10) allowed control of SET/RESET voltages in negative bias ranges 4.5–10 V and 6.3–13 V for six- and ten-bilayer devices, respectively. The corresponding resistance ratio between ON/OFF states varied in the ranges 107–105 for the aforementioned number of bilayers. Based on the result of XPS measurements, we suggest that the resistive switching in the studied system occurs due to the formation and annihilation of Si−Si and Si−O bonds, which serve as conductive pathways and isolating material, respectively.

Single-crystalline Si nanowires fabrication by one-step metal assisted chemical etching: The effect of etching time and resistivity of Si wafer

The one-step metal assisted chemical etching (1-MACE) of p-Si wafers with different resistivities and etching time in HF/AgNO3/H2O2 aqueous solution, resulted in large-area vertical Si nanowires (SiNWs). The field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) revealed that the diameters and lengths of nanowires are decreased with increasing the doping level of Si wafer, while their roughness and porosity are increased. The selected area electron diffraction (SAED) patterns showed that SiNWs retain their single-crystal structure of starting wafers. The reflectance spectra indicated that the etched samples have a very low reflectance (∼0.1% and less) in the visible range acting as an anti-reflecting and high absorption layer in solar cells. Furthermore, broadband PL emissions are observed only in samples etched for 60 and 80 min, that are well consistent with the TEM images and Raman shift spectra analysis considering the formation of Si nanocrystals (SiNCs) (∼2.3–3.5 nm) decorated on the sidewalls of the nanowires.

Improvement of Laser Processing for Colloidal Silicon Nanocrystal Formation in a Reactive Solvent

We report the highly improvement of colloidal Si nanocrystal (Si-nc) formation by pulsed-UV-laser irradiation of porous silicon in a HF-contained organic solvent. The Si-nc in such reactive organic solvent exhibits higher photoluminescence quantum yield (∼50–70%) than that in an ordinary organic solvent without HF (∼20%). This enhancement of the quantum yield is caused by HF-induced removal of the surface oxidation layer and subsequent hydrogen termination of the Si surface. These reaction kinetics promote an efficient hydrosilylation between the hydrogen-terminated surface and an organic solvent, resulting in oxygen-free surface terminated by alkyl groups. Furthermore, the preparation yield in the HF-contained solvent is higher and the size distribution of Si-nc becomes more homogeneous than those in the ordinary solvent. These results can be attributed to efficient pulsed-laser-induced fragmentation by the removal of the oxidation layer on the porous Si target surface.

Local epitaxy from the silicon substrate in silicon–rich SiC during Si–nanocrystals formation

This paper reports on the structural evolution of the silicon–rich SiC/c–Si interface upon 1100°C, 30min thermal treatment, which is typically used when fabricating c–Si based devices that incorporate Si–nanocrystals in an SiC matrix. The onset of local interfacial epitaxy is revealed and analyzed. The inhibitory effect of a thin SiC interfacial buffer layer is reported. The investigation is based on Transmission Electron Microscopy and UV–Visible Reflectance spectroscopy. We show that a minimum thickness is required for the SiC buffer layer to preserve the presence and quality of the interface. The investigation also indicates that the epitaxial regrowth occurs in competition with random nucleation, which turns out to be the dominant crystallization mechanism in silicon rich SiC. A value 3÷10nm is given for the diffusion distance of carbon in silicon upon 1100°C, 30min thermal treatment, which confirms previously reported results. The investigation shows that qualitative as well as quantitative structural information can be rapidly and routinely obtained by careful analysis of UV–Visible Reflectance spectral data.

Formation of silicon nanocrystals in silicon carbide using flash lamp annealing

During the formation of Si nanocrystals (Si NC) in SixC1−x layers via solid-phase crystallization, the unintended formation of nanocrystalline SiC reduces the minority carrier lifetime and therefore the performance of SixC1−x as an absorber layer in solar cells. A significant reduction in the annealing time may suppress the crystallization of the SiC matrix while maintaining the formation of Si NC. In this study, we investigated the crystallization of stoichiometric SiC and Si-rich SiC using conventional rapid thermal annealing (RTA) and nonequilibrium millisecond range flash lamp annealing (FLA). The investigated SixC1−x films were prepared by plasma-enhanced chemical vapor deposition and annealed at temperatures from 700 °C to 1100 °C for RTA and at flash energies between 34 J/cm2 and 62 J/cm2 for FLA. Grazing incidence X-ray diffraction and Fourier transformed infrared spectroscopy were conducted to investigate hydrogen effusion, Si and SiC NC growth, and SiC crystallinity. Both the Si content and the choice of the annealing process affect the crystallization behavior. It is shown that under certain conditions, FLA can be successfully utilized for the formation of Si NC in a SiC matrix, which closely resembles Si NC in a SiC matrix achieved by RTA. The samples must have excess Si, and the flash energy should not exceed 40 J/cm2 and 47 J/cm2 for Si0.63C0.37 and Si0.77C0.23 samples, respectively. Under these conditions, FLA succeeds in producing Si NC of a given size in less crystalline SiC than RTA does. This result is discussed in terms of nucleation and crystal growth using classical crystallization theory. For FLA and RTA samples, an opposite relationship between NC size and Si content was observed and attributed either to the dependence of H effusion on Si content or to the optical absorption properties of the materials, which also depend on the Si content.

Transition of Carrier Transport Behaviors with Temperature in Phosphorus-Doped Si Nanocrystals/SiO2 Multilayers.

High-conductive phosphorus-doped Si nanocrystals/SiO2(nc-Si/SiO2) multilayers are obtained, and the formation of Si nanocrystals with the average crystal size of 6 nm is confirmed by high-resolution transmission electron microscopy and Raman spectra. The temperature-dependent carrier transport behaviors of the nc-Si/SiO2 films are systematically studied by which we find the shift of Fermi level on account of the changing P doping concentration. By controlling the P doping concentration in the films, the room temperature conductivity can be enhanced by seven orders of magnitude than the un-doped sample, reaching values up to 110 S/cm for heavily doped sample. The changes from Mott variable-range hopping process to thermally activation conduction process with the temperature are identified and discussed.