Ultrathin SiO2 Films Research Articles

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.

High‐performance perovskite/silicon heterojunction solar cells enabled by industrially compatible postannealing

AbstractIn recent years, developing dopant‐free carrier‐selective contacts, instead of heavily doped Si layer (either externally or internally), for crystalline silicon (c‐Si) solar cells have attracted considerable interests with the aim to reduce parasitic light absorption and fabrication cost. However, the stability still remains a big challenge for dopant‐free contacts, especially when thermal treatment is involved, which limits their industrial adoption. In this study, a perovskite material ZnTiO3 combining with an ultrathin (~1 nm) SiO2 film and Al layer is used as an electron‐selective contact, forming an isotype heterojunction with n‐type c‐Si. The perovskite/c‐Si heterojunction solar cells exhibit a performance‐enhanced effect by postmetallization annealing when the annealing temperature is 200–350°C. Thanks to the postannealing treatment, an impressive efficiency of 22.0% has been demonstrated, which is 3.5% in absolute value higher than that of the as‐fabricated solar cell. A detailed material and device characterization reveal that postannealing leads to the diffusion of Al into ZnTiO3 film, thus doping the film and reducing its work function. Besides, the coverage of SiO2 is also improved. Both these two factors contribute to the enhanced passivation effect and electron selectivity of the ZnTiO3‐based contact and hence improve the cell performance.

Electroluminescence in metal-oxide-semiconductor tunnel diodes with a crystalline silicon/silicon dioxide quantum well

We investigated electroluminescence (EL) in metal-oxide-semiconductor (MOS) diodes with an indium tin oxide (ITO) film/ultrathin silicon dioxide (SiO2) film/ultrathin crystalline-silicon (Si) layer/buried SiO2 layer/Si substrate structure at a negative voltage applied to the ITO film. The EL peak shifted toward higher energy with decreasing Si-layer thickness. The dependence of the EL-peak energy on the Si-layer thickness shows that the EL is due to one-dimensional quantum confinement. Characterization of the MOS diodes suggested that electrons tunneling through the ultrathin SiO2 film from the conduction band (CB) of the ITO film are injected into higher energy states in the CB of the ultrathin Si layer with a negative voltage applied to the ITO film, and the tunneling electrons radiatively recombine with holes accumulated near the Si/SiO2 interface in the ultrathin Si layer.

Density Functional Theory Study on Dependence of Stability of Fe, Cu, and Ni Atoms on Surface Orientation of Si Crystal

The deep levels related to metal impurities degrade the yield of the complementary metal oxide semiconductor (CMOS) image sensors due to the increase in white pixels. Regarding the gettering technology, the stability of major contaminated metals, such as Fe, Ni, and Cu atoms near Si (100) wafer surface was clarified. However, recent high-performance devices including the latest CMOS image sensors involve complex surface structures including (110) and (111) surfaces. In this study, density functional theory calculations were performed to investigate the dependence of the stability of Fe, Ni, and Cu atoms on Si surface orientation. Our key findings are as follows. (1) The stability of Fe, Ni, and Cu atoms is in the order of (110) > (111) > (100) Si surface. (2) The stability of each metal is in the order of Cu > Fe > Ni regardless of surface orientation. (3) Fe, Ni, and Cu are unstable in the ultra-thin SiO2 film formed on the surface. These results suggest that controlling metal contamination, e.g., by using a chemical solution that removes the oxide film on the (110) and (111) surface, is very important in the manufacturing process for CMOS image sensors. In addition, thermal oxidation should be performed immediately when the Si surface is exposed.

Sub-2 nm boron doping in silicon using novel ultra-thin SiO2 film produced by sol–gel dip coating as a capping layer

Advancements in semiconductor electronics and sensor technologies demand a more robust doping approach and process to fabricate ultra-shallow doping with abrupt depth, particularly on complex structured surfaces. Molecular monolayer doping (MLD) is a potential approach, but is limited by the difficulty in depositing a high-quality SiO2 capping film without damaging the dopant molecular layer. In this work, a relative humidity condition during deposition of the film in the sol–gel dip-coating process is found to produce continuous, dense and stoichiometric ultra-thin SiO2 films of quality equivalent to that produced by complicated thermal oxidation/etching and sophisticated chemical vapor deposition and atomic layer deposition methods. Applying these ultra-thin SiO2 films as capping layers in the molecular MLD method, B doping in Si with sub-2 nm effective depth and sub-5 nm abrupt depth is achieved. Remarkably, around 82%–86% of doped B atoms in Si are found to be electrically active as estimated from sheet resistance measurements. The established sol–gel dip coating conditions to deposit ultra-thin SiO2 films are generic and can be extended to produce high-quality ultra-thin films of other metal oxide materials required for advanced technological applications.

Ultrathin Heterogeneous Inorganic Solid Electrolyte Interphase for Dendrite-Free and Improved Lithium Battery Performance

Next-generation lithium metal batteries are expected to drive the practical applications that require high energy-density storage, such as, portable electronics and electric vehicles. However, the direct use of metallic lithium causes harsh safety issues, low-grade rate, and cycling performances due to unstable solid electrolyte interphase (SEI) and undesired lithium dendrite growth. Here, the ultrathin bilayer of graphite and SiO2 is deposited on Li foil (Graphite.SiO2@Li) using Radio Frequency (R-F) sputtering to address the inherent problem of lithium metal anode. The R-F sputtered electrodes can be cycled for more than 1300 h and 550 h at a current density of 0.5 mA cm-2 and 1 mA cm-2 respectively to reach a capacity of 1 mAh cm-2. Graphite.SiO2@Li anode coupled with Lithium titanate (LTO) and Lithium Nickel Cobalt Manganese oxide (NMC111) reaches higher capacity retention (96.78% and 98.57%) compared to bare Li (92.31% % and 82.73%) respectively. Higher capacity at higher rates and long stable cycling performances with low voltage hysteresis was achieved with the use of graphite.SiO2@Li anode. The graphite layer lowers the impedance, buffers the volume expansion issue, and helps to electrically connect the plated Li with the bulk Li electrode underneath. The higher electrolyte wettability of SiO2 provides much better Li-ion transportation, and ultrathin SiO2 film allows fast Li-ion diffusion and lithiation/delithiation to slow down the growth of Li dendrites. Besides, its high Young’s modulus form robust SEI, the insulating property of SiO2 blocks the electron transport, and chemical stability prevent side reaction with the liquid electrolyte.

The negative fixed charge of atomic layer deposited aluminium oxide—a two-dimensional SiO2/AlO x interface effect

The origin of the commonly observed negative fixed charge density (Q fix) in atomic layer deposited (ALD-)aluminium oxide is still a matter of debate despite its widespread applications in (opto-)electronics, particularly in silicon photovoltaics. Q fix plays a crucial role for excellent Si surface passivation, which is mandatory for high efficiency solar cells. Often, Q fix is believed to originate from structural or compositional specifics of the first few nanometres of ALD-AlO x adjacent to the Si-interface. Here, we demonstrate that the negative Q fix is solely an interfacial effect of ALD-AlO x and the SiO2 ultra-thin film that grows inevitably during ALD on Si. Furthermore, it is proven that a second Q fix-layer exists at the upper AlO x /SiO2 interface of SiO2/AlO x /SiO2-stacks, which can carry up to a quarter of the total Q fix. We show that both SiO2/AlO x interfaces can be separated by a charge-lean material such as HfO2 (rather than AlO x ) without significant impact on the measured Q fix. This renders the location of Q fix exactly at the two-dimensional interface of SiO2 and AlO x , rather than in the near-interfacial AlO x volume. The origin of Q fix is discussed in detail. The possibility to obtain very high charge densities of around −5 × 1012 cm−2 by sub-nm thick ALD-AlO x enables advanced applications such as passivating hole-selective contacts for Si solar cells or nanoelectronic Si-doping strategies via Al-induced SiO2 modulation doping.

PH-Responsive Ultrathin Nanoporous SiO2 Films for Selective Ion Permeation.

Ultrathin nanoporous (NP) films are an emerging field for selective and effective ion/molecular separation and electrochemical sensing applications. We describe selective ion permeation in surface-functionalized ultrathin NP SiO2 films (NP SiO2-NH2). The ultrathin NP SiO2 films with ca. 8 nm thickness were prepared from silsesquioxane-containing blend polymer Langmuir-Blodgett films (nanosheets) using the photo-oxidation method. The porous SiO2 surface was modified with a pH-responsive amine-containing silane coupling agent. Selective ion permeation was demonstrated under acidic pH conditions (pH ≤ 6) using two equally sized redox probes: negative (Fe(CN)63-/4-) and positive (Ru(NH3)62+/3+) ions. The current density for Fe(CN)63-/4- decreased as the pH value increased to pH = 6, whereas it increased for Ru(NH3)62+/3+. Control measurements revealed that the probes can penetrate the pores of nonfunctionalized SiO2 films irrespective of pH values, indicating that both the size and the surface charge response contributed to selective ion permeation. Results obtained from this study pave the way for new applications in molecular separation and sensing applications based on ultrathin nanoporous films (<10 nm) and tailored surfaces.

Application of Phosphorus‐Doped Polysilicon‐Based Full‐Area Passivating Contact on the Front Textured Surface of p‐Type Silicon

A p‐type crystalline silicon (c‐Si) passivated emitter and rear contact (PERC) nowadays have become mainstream in the highly competitive photovoltaic market. Herein, the recently popular passivating contact concept on the front textured surface of p‐type c‐Si PERC solar cells is applied. The full‐area textured passivating contact consists of an ultrathin SiO2 film of ≈1.5 nm thickness grown with thermal oxidation and phosphorus‐doped polysilicon (poly‐Si) contact layer by low‐pressure chemical vapor deposition. A detailed investigation of poly‐Si with different crystalline structures, doping conditions, and thicknesses on the passivation effect and parasitic absorption loss is carried out. Preliminary achievement of 21.3% efficiency is realized in large‐area (244.3 cm2) p‐PERC c‐Si solar cells without the need for additional laser selective redoping. Theoretical calculation expects that the cell efficiency can be enhanced to 23.4% by decreasing the recombination current to a reasonable level. It is demonstrated that further improvement of low‐cost p‐PERC c‐Si solar cells is feasible using the full‐area textured passivating contact processes which are fully compatible with existing production lines.

Effect of Nucleation Parameters of Ge Quantum Dots Grown over Silicon Oxide by LPCVD

Germanium quantum dots (Ge-QD) were grown by Low Pressure Chemical Vapor Deposition (LPCVD) on Si nucleus previously grown on 3 nm thick SiO2 ultra thin film. Samples were analyzed by atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). We report the analysis of the influence of the nucleation parameters on size and spatial distribution of Ge-QD. AFM images show a Ge-QD density of around 3.6x1010 cm-2, with an 11 nm mean size and 2.9 nm height. Finally, HRTEM investigation shows that the Ge-QD have a crystalline structure, i.e., they are nanocrystals.

Tuning the photocatalytic activity of TiO2 nanoparticles by ultrathin SiO2 films grown by low-temperature atmospheric pressure atomic layer deposition

We employed atomic layer deposition (ALD) to deposit ultrathin SiO2 layers on P25 TiO2 nanoparticles to fabricate TiO2/SiO2 core/shell nanostructures. The ALD process was carried out in a fluidized bed reactor working at atmospheric pressure using SiCl4 and H2O as precursors, enabling the deposition of SiO2 at 100 °C with the ability to control the thickness at the sub-nanometer level. By controlling the thickness of the SiO2 in a very narrow range, i.e., below 2 nm, the photocatalytic activity of TiO2 can be tuned. In particular, an enhancement was obtained for the SiO2 layers with a thickness below 1.4 nm, in which the layer with a thickness of about 0.7 nm exhibited the highest photocatalytic activity; for SiO2 layers thicker than 1.4 nm, the photocatalytic activity was strongly suppressed. The photocatalytic activity enhancement and the degradation mechanism of RhB by the TiO2/SiO2 photocatalysts were investigated by combining X-ray photoelectron spectroscopy, UV–Vis absorption spectroscopy, photoluminescence spectroscopy and the aid of charge carrier and radical scavengers. Our findings have revealed an improvement of photogenerated charge separation due to the SiO2 coating and the dominating role of hydroxyl radicals in the degradation of RhB.

Self-Aligned Top-Gate Amorphous InGaZnO TFTs With Plasma Enhanced Chemical Vapor Deposited Sub-10 nm SiO2 Gate Dielectric for Low-Voltage Applications

In this letter, self-aligned top-gate (SATG) amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) with an 8.62-nm SiO2 gate dielectric grown by conventional plasma enhanced chemical vapor deposition (PECVD) at 300 °C are fabricated. The results show that the ultrathin PECVD SiO2 film has excellent dielectric properties, such as a low leakage current density of 10−8 A/cm2 at 2 MV/cm, a high breakdown electric field of 9.3 MV/cm, and an almost independent capacitance of bias frequency. The fabricated SATG a-IGZO TFTs with the ultrathin SiO2 gate dielectric exhibit high electrical performances, with major parameters, including a low gate leakage current of 10−13 A, a small subthreshold swing value of 124 mV/decade, a decent field-effect mobility of 10.13 cm2/ $\text{V}\cdot \text{s}$ , and a high ON/OFF current ratio up to 108. Furthermore, the TFTs also show a good electrical stability under positive and negative gate bias stresses.

Modulation of lattice constants by changing the composition and strain in incommensurate Nowotny chimney-ladder phase FeGeγ epitaxially grown on Si

The single phase FeGeγ nanocrystals (NCs) with incommensurate Nowotny chimney-ladder structure were epitaxially grown on Si substrates by reaction between deposited Fe atoms and Ge nanostructured layers on Si. The Ge nanostructured layers worked as blocking layers of the Fe diffusion to Si substrates. We used the two kinds of the Ge nanostructured layers: Ge wetting layers formed by direct Ge deposition on Si and Ge NCs formed by Ge deposition on ultrathin SiO2 films with ultrasmall nanowindows on Si. In the case of Ge wetting layers, the FeGeγ NCs were directly formed on Si, where there was a-axis strain due to the 2.9% lattice mismatch between FeGeγ and Si. On the other hand, in the case of Ge NCs, formed FeGeγ NCs were elastically-strain-relaxed due to the small contact with Si substrates through nanowindows in the ultrathin SiO2 films. Furthermore, the c-axis lattice constant in the epitaxial FeGeγ NCs changed due to the variation of the Fe-Ge composition. These results demonstrated that a-axis lattice constant was modulated by strain control of FeGeγ NCs due to the change of contact area with Si substrate and that c-axis lattice constant of FeGeγ NCs with incommensurate Nowotny chimney-ladder structure was modulated by manipulating Fe-Ge composition. These modulation of lattice constants will open a road for manipulating phonon group velocity related to the lattice constant and managing the waste heat in various devices implemented on Si substrates.

Thin-Film Silicon MEMS for Dynamic Mass Sensing in Vacuum and Air: Phase Noise, Allan Deviation, Mass Sensitivity and Limits of Detection

Microelectromechanical sensors have extreme mass sensitivities, making them promising for mass sensing, gas sensing, and biosensing applications. Despite the numerous examples of MEMS/NEMS mass sensors in the literature, the characterization of the resonator or oscillator system is often incomplete. Resonant mass sensor studies should include phase noise (or Allan deviation), as the use of these metrics allows for the identification of the noise mechanisms, the optimization of the measurement and actuation parameters, the prediction of the limit of detection, and the comparison of different types of resonators or vibration modes. In this paper, we summarize the metrics we consider to be the most important in the characterization of dynamic mass sensors, and then apply them to low-cost, hydrogenated amorphous silicon (a-Si:H) thin-film MEMS cantilever resonators. The effects of the actuation voltage on the phase noise were studied. The mass sensitivities of these resonators were calibrated using patterned thin-films localized at the tip of the cantilevers as well as uniform ultra-thin films of SiO2. The experimental mass sensitivities in vacuum ranged −98 Hz/pg to −4.6 kHz/pg. The limits of detection were calculated to be 100-833 fg in vacuum and 37-846 pg at atmospheric pressure. [2019-0006]

Thermoelectric power factor enhancement based on carrier transport physics in ultimately phonon-controlled Si nanostructures

We thoroughly investigated carrier and phonon transports related to thermoelectric properties using absolutely-controlled Si nanostructures, namely Si films containing epitaxial nanodots (NDs). In the Si nanostructure, various ND materials were introduced selectively such as Ge and β-FeSi2. This brings the ND/Si hetero-interface control; interface energy band offset and lattice mismatch strain in Si films. Ultrahigh density stacking faults can also be intentionally introduced using ultrathin SiO2 films. We reduced thermal conductivity (<2 Wm−1K−1) close to amorphous Si due to phonon transport control in absolutely-controlled Si nanostructures: scattering both by NDs and intentionally-doped atomic-scale impurities, namely wide-scale length phonon scattering. The electron transport related to Seebeck coefficient and electrical conductivity was manipulated by intentionally-introduced nanostructures: ND/Si hetero-interfaces and stacking faults. In the epitaxial Si thin films containing β-FeSi2 NDs, energetic carrier transports through stacking faults with several tens of meV barrier height enhanced the power factor. In the strained epitaxial Si thin films containing Ge NDs, it was found that strain-induced band splitting leading to high electron mobility enhanced the power factor. These results demonstrated that thermoelectric performance determined by electron and phonon transports were strongly dependent on the crystal characters (strain, crystal defects and the interfaces) relating to intentionally-introduced nanostructures. This suggests that it is important to design nanostructured materials on nanoscale and atomic-scale for enhancement of thermoelectric performance: introduction of stacking faults with proper energy barriers, NDs and dopants scattering phonons, and the less point defects. This also demonstrates that its nanostructuring methodology opens a load to realization of thermoelectric Si thin films.

Development of Interfacial Nanoassembly Techniques in Functional Nanomaterials

The precise integration of functional materials, such as organic–inorganic hybrid building blocks or π-conjugated units, is a key issue in the development of bio- and nanoelectronic devices. This focus review highlights the author’s recent study of the nanoscale assembly of polymer materials based on the Langmuir–Blodgett (LB) technique, which allows high-density integration of functional units. (1) The nanoassembly of silsesquioxane-containing polymers can be used to produce uniform monolayers, which are precursors of ultrathin SiO2 films with controlled porosity. The SiO2 films were used as resistive switching devices with low set/reset voltages. (2) The nanoassembly of inorganic nanoparticles and metal-organic framework films on polymer films demonstrates the importance of the spatial alignment of anchoring units in the polymer films depending on adsorption mechanisms. (3) The nanoassembly of π-conjugated units can be used for high-density and low-dimensional assembly, using newly proposed amphiphilic homopolymers with π-conjugated units in the side chains. These examples demonstrate the potential of LB-based fabrication as a candidate strategy for functional device fabrication. The nanoscale assembly strategies based on the Langmuir–Blodgett (LB) technique were highlighted to demonstrate the potential for functional device fabrication. (1) The assembly of silsesquioxane-containing polymers can be used to produce uniform monolayers, which are precursors of ultrathin SiO2 films with controlled porosity. (2) The assembly of inorganic nanoparticles and metal-organic framework films on polymer films demonstrates the importance of the spatial alignment of anchoring units. (3) The assembly of π-conjugated units can be used for high-density and low-dimensional assembly.

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.

Growth of epitaxial FeGeγ nanocrystals with incommensurate Nowotny chimney-ladder phase on Si substrate

We developed the growth technique of epitaxial nanocrystals of FeGeγ (γ ∼ 1.52), a recently-found new crystal with incommensurate Nowotny chimney-ladder phase, on Si(001) substrates. This bulk material is promising for thermoelectric application, and now their films and nanostructures are expected to be fabricated. However, it was difficult to grow single phase epitaxial FeGeγ directly on Si substrates by conventional growth method. On the other hand, we successfully formed almost single phase epitaxial FeGeγ nanocrystals on Si substrates covered with ultrathin SiO2 films with nanowindows. Reflection high energy electron diffraction indicated that the nanocrystals had the Nowotny chimney-ladder structure, the lattice constants of which were approximately consistent with those of FeGeγ bulk. This indicated strain relaxation in nanocrystals due to the hemispherical shape. This will open a road to realize high performance thermoelectric nanomaterials on Si substrates.

Reliability factors of ultrathin dielectric films based on highly controlled SiO2 films

Two main factors determine the reliability of ultrathin SiO2 films thermally grown on Si when subjected to long-term electrical stress. One of them is two-dimensional thickness uniformity for suppressing the decrease in Weibull slope. The other is film density distribution, which affects the degradation rate during application of high electric stress. The importance of thermal process optimization in inhibiting density fluctuation is demonstrated.

Resistive switching of organic–inorganic hybrid devices of conductive polymer and permeable ultra-thin SiO2 films

We propose a resistive switching device composed of conductive polymer (PEDOT:PSS) and SiO2 ultra-thin films. The SiO2 film was fabricated from silsesquioxane polymer nanosheets as a resistive switching layer. Devices with metal (Ag or Au)∣SiO2∣PEDOT:PSS architecture show good resistive switching performance with set–reset voltages as low as several hundred millivolts. The device properties and the working mechanism were investigated by varying the electrode material, surrounding atmosphere, and SiO2 film thickness. Results show that resistive switching is based on water and ion migration at the PEDOT:PSS∣SiO2 interface.