Ultrathin Silicon Oxide Films Research Articles

Modelling of insulating potential in ultra-thin (42 å) silicon oxide film

Based on previously conducted measurements of the tunneling current-voltage characteristics of metal-SiO2-Si (MOS) structures, modeling of the insulating potential in an ultra-thin (4.2 nm) silicon oxide film was performed. The potential in the dielectric was defined in the shape of a trapezoid, with the lateral slopes simulating transition layers and the top base representing the bulk of SiO2. The model parameters – the barrier height and the coordinates of the trapezoid's corner points – were calculated to achieve the maximum match between the experimental and theoretical voltage derivatives of the current logarithm. Common features of the insulating potential, similar to those in thinner silicon oxide films (3.7 nm), were identified: the barrier occupies up to half of the nominal volume of the dielectric gap and is shifted towards the gate electrode, with its slope towards the semiconductor substrate being much more gradual compared to the slope adjacent to the gate.

Interface Effects in the Stability of 2D Silica, Silicide, and Silicene on Pt(111) and Rh(111).

Ultrathin two-dimensional silica films have been suggested as highly defined conductive models for fundamental studies on silica-supported catalyst particles. Key requirements in this context are closed silica films that isolate the gas phase from the underlying metal substrate and stability under reaction conditions. Here, we present silica bilayer films grown on Pt(111) and Rh(111) and characterize them by scanning tunneling microscopy and X-ray photoelectron spectroscopy. We provide the first report of silica bilayer films on Rh(111) and have further successfully prepared fully closed films on Pt(111). Interestingly, surface and interface silicide phases play a decisive role in both cases: On platinum, closed films can be stabilized only when silicon is deposited in excess, which results in an interfacial silicide or silicate layer. We show that these silica films can also be grown directly from a surface silicide phase. In the case of rhodium, the silica phase is less stable and can be reduced to a silicide in reductive environments. Though similar in appearance to the "silicene" phases that have been controversially discussed on Ag(111), we conclude that an interpretation of the phase as a surface silicide is more consistent with our data. Finally, we show that the silica film on platinum is stable in 0.8 mbar CO but unstable at elevated temperatures. We thus conclude that these systems are only suitable as model catalyst supports to a limited extent.

Growth of metastable 2H-CaSi2 films on Si(111) substrates with ultrathin SiO2 films by solid phase epitaxy

The Si-nano dot substrates formed using the ultrathin silicon oxide films were applied to fabricate CaSi2 films. The CaSi2 formed by this process was identified as the metastable phase 2H as the main component, and the 1H structure existed partially at the grains of the 2H phase. Although no experimental reports exist for the formation of 2H-CaSi2 crystal, the Si-nano dot substrates are considered as the high-entropy substrate to form the metastable phases. We experimentally determined the lattice parameter of the 2H phase by the annular dark field–scanning transmission electron microscopy observations using the Si as an internal standard sample.

Application of Si-related Ultrathin (∼1 nm) Films to Crystalline Silicon Solar Cells

Ultrathin (∼1 nm) films have been widely used for high-efficiency crystalline silicon (c-Si) solar cells such as tunnel oxide passivated contact (TOPCon) solar cells. In this article, we present our recent results for the applications of ultrathin silicon oxide (SiOx) and silicon nitride (SiNx) films to c-Si solar cells. The following topics are reviewed. 1) Ultrathin SiNx can be utilized for passivating contacts instead of SiOx in the TOPCon structure. 2) The addition of SiOx between c-Si and thick catalytic-chemical-vapor-deposited (Cat-CVD) SiNx significantly improves the quality of surface passivation. 3) Ultrathin Al-doped SiOx films formed just by dipping in Al(NO3)3 solution on c-Si provide strong upward band bending due to negative fixed charges, which can be used for hole-selective contacts.

Plasma Functionalization of Silica Bilayer Polymorphs.

Ultrathin silica films are considered suitable two-dimensional model systems for the study of fundamental chemical and physical properties of all-silica zeolites and their derivatives, as well as novel supports for the stabilization of single atoms. In the present work, we report the creation of a new model catalytic support based on the surface functionalization of different silica bilayer (BL) polymorphs with well-defined atomic structures. The functionalization is carried out by means of in situ H-plasma treatments at room temperature. Low energy electron diffraction and microscopy data indicate that the atomic structure of the films remains unchanged upon treatment. Comparing the experimental results (photoemission and infrared absorption spectra) with density functional theory simulations shows that H2 is added via the heterolytic dissociation of an interlayer Si-O-Si siloxane bond and the subsequent formation of a hydroxyl and a hydride group in the top and bottom layers of the silica film, respectively. Functionalization of the silica films constitutes the first step into the development of a new type of model system of single-atom catalysts where metal atoms with different affinities for the functional groups can be anchored in the SiO2 matrix in well-established positions. In this way, synergistic and confinement effects between the active centers can be studied in a controlled manner.

Two-Dimensional Ultrathin Silica Films.

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO2 stoichiometry. It consists of precisely two layers of tetrahedral [SiO4] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Local Enhancement of Dopant Diffusion from Polycrystalline Silicon Passivating Contacts.

Passivating contacts consisting of heavily doped polycrystalline silicon (poly-Si) and ultrathin interfacial silicon oxide (SiOx) films enable the fabrication of high-efficiency Si solar cells. The electrical properties and working mechanism of such poly-Si passivating contacts depend on the distribution of dopants at their interface with the underlying Si substrate of solar cells. Therefore, this distribution, particularly in the vicinity of pinholes in the SiOx film, is investigated in this work. Technology computer-aided design (TCAD) simulations were performed to study the diffusion of dopants, both phosphorus (P) and boron (B), from the poly-Si film into the Si substrate during the annealing process typically applied to poly-Si passivating contacts. The simulated 2D doping profiles indicate enhanced diffusion under pinholes, yielding deeper semicircular regions of increased doping compared to regions far removed from the pinholes. Such regions with locally enhanced doping were also experimentally demonstrated using high-resolution (5-10 nm/pixel) scanning spreading resistance microscopy (SSRM) for the first time. The SSRM measurements were performed on a variety of poly-Si passivating contacts, fabricated using different approaches by multiple research institutes, and the regions of doping enhancement were detected on samples where the presence of pinholes had been reported in the related literature. These findings can contribute to a better understanding, more accurate modeling, and optimization of poly-Si passivating contacts, which are increasingly being introduced in the mass production of Si solar cells.

Ultrathin Sulfonated Mesoporous Silica Film with Well-Ordered Perpendicular Mesochannels: In-Situ Preparation, Characterization and Permselectivity

In this paper, a new type of ultrathin sulfonated mesoporous silica film (SMSF) with well-ordered perpendicular mesochannels was in-situ synthesized by Stöber approach and co-condensation method. The mesostructure of the synthesized SMSF was characterized. The results of small-angle XRD, high-resolution TEM, and cyclic voltammetry (CV) exhibited that, after loading of MPTMS, SMSF had an ordered mesostructure with a perpendicular orientation and open ends. SEM showed that SMSF could be entirely transferred and easily handled. FT-IR presented that sulfonic groups were successfully added to the surface of the nanochannels of silica film. Compared with mesoporous silica film (MSF) and commercial cation exchange membrane (CEM), SMSF had the highest permselectivity. The permselectivity of SMSF was not lined with the loading of the sulfonic groups (–SO3H). The highest permselectivity of SMSF to Na[Formula: see text] was 94% when the loading of MPTMS was 5.98% (wt.%). SMSF is a promising material in the application of salinity gradient energy harvest.

Frontispiece: Growth and Atomic‐Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

Frontispiece: Growth and Atomic‐Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

Growth and Atomic-Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support.

The present review reports on the preparation and atomic‐scale characterization of the thinnest possible films of the glass‐forming materials silica and germania. To this end state‐of‐the‐art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side‐by‐side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth.

Investigation of polysilicon passivated contact's resilience to potential-induced degradation

We present clear evidence of excellent resilience to potential-induced degradation (PID) from polysilicon passivated contacts implemented on the rear of n-type solar cells. Under the stress conditions of −1000 V, 50 °C, 30% relative humidity with aluminum foil, no damage was caused to the passivated contact consisting of an ultrathin silicon oxide (SiOx) film and an n+-doped polysilicon (poly-Si) layer after 168 h. With +1000 V bias and under the same chamber conditions, the SiOx/poly-Si (n+) passivated contact showed a slight change that translated into about 1% module power loss after 168 h, which is significantly lower than the 5% threshold recommended by IEC 62804-1 PID test standard. Furthermore, the SiOx/poly-Si (n+) passivated contact, even when encapsulated with ethylene-vinyl acetate copolymer films having a low volume resistivity in the range of 5 × 1014 Ω⸱cm, exhibited good stability under high-voltage stress. The experimental results were also validated by a generic device simulation, where the SiOx/poly-Si (n+) stack was shown to be immune to the surface polarization effect. In addition, a promising cell-level solution (i.e., using a stack of aluminium oxide and silicon nitride) to the polarization-type PID for n-type passivated emitter rear totally diffused silicon solar cells was also demonstrated.

Reliability improvement of pad-extended on chip light-emitting diodes using an ultra-thin passivation layer

The pad-extended on chip (PEC) light-emitting diodes (LEDs) were fabricated and characterized by using an ultra-thin silicon oxide (SiO2) film as a passivation layer. The SiO2 thin film was deposited on the GaN mesa edge using plasma-enhanced chemical vapor deposition (PECVD) by varying deposition temperature from 150 to 350 °C. Atomic force microscopy (AFM), a scanning electron microscope (SEM) and leakage current were employed to analyze the influence of deposition temperature on the structural properties of SiO2 films. AFM measurements demonstrate that the SiO2 surface roughness was larger at a lower deposition temperature than a higher temperature. Forward voltage (Vf) characteristics of these PEC LEDs were detected and recorded for continuous ten thousand probe times. The curve of Vf remained stable at temperatures of 150, 200 and 250 °C. A metal-insulator-metal structure using 250 °C-deposited SiO2 as the dielectric indicates the leakage current density was close to 10−8 A cm−2 with the minimum one lower than 10−12 A cm−2. The relative dielectric constant of the SiO2 (20 nm) was 3.65. The ultra-thin SiO2 film efficiently passivated the GaN mesa edge and prevented charge accumulation at the surface. Reliability has been improved for PEC LEDs using the ultra-thin SiO2 film as a passivation layer.

Ultrathin silica films on Pd(111): Structure and adsorption properties

We studied the preparation of thin silica films on Pd(111) using low energy electron diffraction (LEED), infrared reflection-absorption spectroscopy (IRAS), and scanning tunneling microscopy (STM). The films grow from the onset as a double-layer (bilayer) silicate and show no long-range ordering as judged by LEED, thus bearing close similarities to the silicate films grown on a Pt(111) support. The results provide further evidence that the principal structure (monolayer vs bilayer) of ultrathin silica films on metal substrates is primarily governed by the affinity of a metal substrate to oxygen. Individual adsorption of CO and D2 on the prepared films showed that both molecules penetrate through the film and chemisorb on the Pd(111) surface. Density functional theory (DFT) calculations showed that CO bonding on Pd(111) underneath the silica film becomes weaker as compared to the bare Pd(111) surface, but the vibrational frequencies remain unaffected, that is in nice agreement with the experimental results.

Ultrathin silica film derived with ultraviolet irradiation of perhydropolysilazane for high performance and low voltage organic transistor and inverter

二氧化硅是一种常见且非常重要的介电材料, 但是其传统的制备方法例如物理气相沉积, 化学气相沉积等无法适应大规模生产以及有机电子工业. 本论文介绍了一种简单的制备二氧化硅超薄膜的方法, 即利用紫外光辐照全氢聚硅氮烷, 使其转化为二氧化硅. 这种方法所制备的二氧化硅超薄膜具有超平的表面以及非常低的漏电. 此外, 我们还将该二氧化硅超薄膜应用于有机晶体管和反相器电路中, 这些器件均表现出良好的电学性能. 这些结果表明该方法制备的二氧化硅超薄膜具有很好的实际应用前景.

Ultimate Confinement of Phonon Propagation in Silicon Nanocrystalline Structure.

Temperature-dependent thermal conductivity of epitaxial silicon nanocrystalline (SiNC) structures composed of nanometer-sized grains separated by ultrathin silicon-oxide (SiO_{2}) films (∼0.3 nm) is measured by the time domain thermoreflectance technique in the range from 50 to 300K. The thermal conductivity of SiNC structures with a grain size of 3 and 5nm is anomalously low at the entire temperature range, significantly below the values of bulk amorphous Si and SiO_{2}. The phonon gas kinetic model, with intrinsic transport properties obtained by first-principles-based anharmonic lattice dynamics and phonon transmittance across ultrathin SiO_{2} films obtained by atomistic Green's function, reproduces the measured thermal conductivity without any fitting parameters. The analysis reveals that mean free paths of acoustic phonons in the SiNC structures are equivalent or even below half the phonon wavelength, i.e., the minimum thermal conductivity scenario. The result demonstrates that the nanostructures with extremely small length scales and a controlled interface can give rise to ultimate classical confinement of thermal phonon propagation.

Introducing pinhole magnification by selective etching: application to poly-Si on ultra-thin silicon oxide films

Carrier selective junctions formed by polycrystalline silicon (poly-Si) on ultra-thin silicon oxide films are currently in the spotlight of silicon photovoltaics. We develop a simple method using selective etching and conventional optical microscopy to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. We characterize the selective etching of poly-Si versus ultra-thin silicon oxide. We use test structures with deliberately patterned openings and 3 nm thin oxide films to check the feasibility of magnification by undercutting the interfacial oxide. With the successful proof of our concept we introduce a new method to access the density of nanometer-size pinholes in POLO-junctions with excellent passivation properties.

(Invited) High-K Evolution: Subnanometer EOT Challenges and Future Perspectives for Scaling

CMOS gate dielectric scaling was one of the key technology options for boosting the device performance and for keeping the device downsizing on track up to the recent technology nodes. The conventional homopolar dielectric films such as silicon oxide or silicon nitride were used and scaled down to about 1 nm which was well below the direct tunneling limit of 3 nm thick and the gate oxide thickness was the first key device parameter being shrunk into the nanometer scale. However, the physical limit, the exponentially increase in the gate leakage current, and the poor film uniformity control of ultrathin oxide had inhibited the further downscaling of this kind gate dielectric film. High dielectric constant (HiK) heteropolar metal oxides such as Hf-based or La-based dielectric materials, with several nanometers thick, are able to achieve a capacitance value that is equivalent to a silicon oxide (known as Equivalent Oxide Thickness, or EOT) with thickness in the nanometer or even subnanometer scale and with a gate leakage current of several orders of magnitude smaller. The use of EOT makes the gate dielectric scaling beyond the atomic scale be possible. However, just a couple generations since Intel introduced Hf-based HiK in the 45 nm technology processor production, HiK scaling has already lost its momentum. The last ITRS roadmap for EOT scaling was 0.03 nm reduction per generation and the EOT used the latest fabrication processes was still thicker than 0.7 nm which is the physical limit of bulk silicon oxide. For HiK scaling down to half nanometer EOT range, we shall encounter most of the nonscalabilities as found in the ultrathin silicon oxide film and the challenges is now even tougher. The surface roughness and the interface layer are not only non-scalable, they impose the lower bound for smallest achievable EOT for maintaining device scaling. HiK metal oxides have much poorer properties and less thermally stable when they interface with the silicon substrate and gate electrode. Depending on the processing temperature, partial pressure of oxygen, several different chemical reactions may take place at the interfaces and even in the bulk of HiK layer. The thermally-induced interfacial silicate layer is rougher than the native oxide prepared by atomic-layer deposition and it leads to the significant increase in EOT. The gate electrode layer on the HiK layer has even larger roughness than HiK and will lead to both EOT and electrical characteristic degradation. The thickness variations result in both gate capacitance and local electrostatic field fluctuation which are insignificant with the present fabrication processes when the film is not too thin and the device size is not too small; they will become significant in the nanoscale devices. In addition, Fowler-Norheim and direct tunneling current of HiK films will occur at a much thicker physical thickness as HiK materials usually have much smaller band offsets with silicon and sometimes have heavier carrier effective masses. In principle, a half nanometer EOT gate dielectric film might be achieved with a 3.5 nm thick La2O3 film. At this thickness, direct tunneling should occur and it is expected that the La2O3 film should have a large gate leakage current not to mention the further conduction enhancement due to oxide traps. HiK materials are often found to have much higher bulk traps than the silicon oxide because of the presents of oxygen vacancies and grain boundary states of nanocrystalline phases of metal oxides. These non-ideal factors have been the major constraints for further EOT scaling in the half nanometer range. There are no much options for getting rid of these technology issues. Better uniformity control and low thermal budget process can help to alleviate some of the aforementioned issues. Regarding the alternate HiK candidates, comprehensive studies had been conducted on almost every elemental dielectric materials. The most favorable and yet feasible candidates are still limited to HfO2 and La2O3 and unfortunately they are not good enough in the half nanometer EOT range. Multilayer stack is not likely be a feasible option from the EOT point of view. Yet the only possible option is the use of complex oxides. We shall discuss some recent achievements and the future perspectives on these issues.

Solution-processable precursor route for fabricating ultrathin silica film for high performance and low voltage organic transistors

Silica is one of the most commonly used materials for dielectric layer in organic thin-film transistors due to its excellent stability, excellent electrical properties, mature preparation process, and good compatibility with organic semiconductors. However, most of conventional preparation methods for silica film are generally performed at high temperature and/or high vacuum. In this paper, we introduce a simple solution spin-coating method to fabricate silica thin film from precursor route, which possesses a low leakage current, high capacitance, and low surface roughness. The silica thin film can be produced in the condition of low temperature and atmospheric environment. To meet various demands, the thickness of film can be adjusted by means of preparation conditions such as the speed of spin-coating and the concentration of solution. The p-type and n-type organic field effect transistors fabricated by using this film as gate electrodes exhibit excellent electrical performance including low voltage and high performance. This method shows great potential for industrialization owing to its characteristic of low consumption and energy saving, time-saving and easy to operate.

A dry process for forming ultrathin silicon oxide film on gold nanoparticle

A simple dry process for preparing an ultrathin SiO2 shell on gold nanoparticle (Au NP) has been developed. The adsorption and reaction of 1,3,5,7-tetramethylcyclotetrasiloxane on Au NP-loaded ZnO (Au/ZnO) from gas phase at 353 K yields a multilayer of polymethylsiloxane (PMS) on the Au surface, while a monolayer is formed on the ZnO surface. The postheating in the air at 773 K transforms the PMS layer to a uniform SiO2 layer with thickness (lSiO2) of ∼2 nm on the surface of every Au NP (Au@SiO2/ZnO). UV-visible absorption spectra show that the SiO2 shell enhances the localized surface plasmon resonance of Au NP with its peak redshifted from 530 nm to 571 nm. The 3D finite-difference time-domain calculations for Au@SiO2(lSiO2 = 2 nm)/ZnO indicate that a strong local electric field is generated at the Au-SiO2-ZnO three-phase interface along the peripheral edge of Au NP with an enhancement factor of ∼107.

Single-Molecule Electrochemistry on a Porous Silica-Coated Electrode.

Here we report the direct observation and quantitative analysis of single redox events on a modified indium-tin oxide (ITO) electrode. The key in the observation of single redox events are the use of a fluorogenic redox species and the nanoconfinement and hindered redox diffusion inside 3-nm-diameter silica nanochannels. A simple electrochemical process was used to grow an ultrathin silica film (∼100 nm) consisting of highly ordered parallel nanochannels exposing the electrode surface from the bottom. The electrode-supported 3-nm-diameter nanochannels temporally trap fluorescent resorufin molecules resulting in hindered molecular diffusion in the vicinity of the electrode surface. Adsorption, desorption, and heterogeneous redox events of individual resorufin molecules can be studied using total-internal reflection fluorescence (TIRF). The rate constants of adsorption and desorption processes of resorufin were characterized from single-molecule analysis to be (1.73 ± 0.08) × 10-4 cm·s-1 and 15.71 ± 0.76 s-1, respectively. The redox events of resorufin to the non-fluorescent dihydroresorufin were investigated by analyzing the change in surface population of single resorufin molecules with applied potential. The scan-rate-dependent molecular counting results (single-molecule fluorescence voltammetry) indicated a surface-controlled electrochemical kinetics of the resorufin reduction on the modified ITO electrode. This study demonstrates the great potential of mesoporous silica as a useful modification scheme for studying single redox events on a variety of transparent substrates such as ITO electrodes and gold or carbon film coated glass electrodes. The ability to electrochemically grow and transfer mesoporous silica films onto other substrates makes them an attractive material for future studies in spatial heterogeneity of electrocatalytic surfaces.