Ultrathin Silicon Oxide 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.

Enhanced Corrosion Protection of Printed Circuit Board Electronics using Cold Atmospheric Plasma-Assisted SiOx Coatings.

The miniaturization and widespread deployment of electronic devices across diverse environments have heightened their vulnerability to corrosion, particularly affecting copper traces within printed circuit boards (PCBs). Conventional protective methods, such as conformal coatings, face challenges including the necessity for a critical thickness to ensure effective barrier properties and the requirement for multiple steps of drying and curing to eliminate solvent entrapment within polymer coatings. This study investigates cold atmospheric plasma (CAP) as an innovative technique for directly depositing ultrathin silicon oxide (SiOx) coatings onto copper surfaces to enhance corrosion protection in PCBs. A systematic investigation was undertaken to examine how the scanning speed of the CAP deposition head impacts the film quality and corrosion resistance. The research aims to determine the optimal scanning speed of the CAP deposition head that achieves complete surface coverage while promoting effective cross-linking and minimizing unreacted precursor entrapment, resulting in superior electrical barrier and mechanical properties. The CAP coating process demonstrated the capability of depositing SiOx onto copper surfaces at various thicknesses ranging from 70 to 1110 nm through a single deposition process by simply adjusting the scanning speed of the plasma head (5-75 mm/s). Evaluation of material corrosion barrier characteristics revealed that scanning speeds of 45 mm/s of the plasma deposition head provided an effective coating thickness of 140 nm, exhibiting superior corrosion resistance (30-fold) compared to that of uncoated copper. As a proof of concept, the efficacy of CAP-deposited SiOx coatings was demonstrated by protecting an LED circuit in saltwater and by coating printed circuits for potential agricultural sensor applications. These CAP-deposited coatings offer performance comparable to or superior to traditional conformal polymeric coatings. This research presents CAP-deposited SiOx coatings as a promising approach for effective and scalable corrosion protection in miniaturized electronics.

Understanding and Mitigating Atomic Oxygen-Induced Degradation of Perovskite Solar Cells for Near-Earth Space Applications.

Combining high efficiency with good radiation tolerance, perovskite solar cells (PSCs) are promising candidates to upend expanding space photovoltaic (PV) technologies. Successful employment in a Near-Earth space environment, however, requires high resistance against atomic oxygen (AtOx). This work unravels AtOx-induced degradation mechanisms of PSCs with and without phenethylammonium iodide (PEAI) based 2D-passivation and investigates the applicability of ultrathin silicon oxide (SiO) encapsulation as AtOx barrier. AtOx exposure for 2h degraded the average power conversion efficiency (PCE) of devices without barrier encapsulation by 40% and 43% (w/o and with 2D-PEAI-passivation) of their initial PCE. In contrast, devices with a SiO-barrier retained over 97% of initial PCE. To understand why 2D-PEAI passivated devices degrade faster than less efficient non-passivated devices, various opto-electrical and structural characterications are conducted. Together, these allowed to decouple different damage mechanisms. Notably, pseudo-J-V curves reveal unchanged high implied fill factors (pFF) of 86.4% and 86.2% in non-passivated and passivated devices, suggesting that degradation of the perovskite absorber itself is not dominating. Instead, inefficient charge extraction and mobile ions, due to a swiftly degrading PEAI interlayer are the primary causes of AtOx-induced device performance degradation in passivated devices, whereas a large ionic FF loss limits non-passivated devices.

Novel Passivation and Gettering Strategy for Silicon Wafer by Al2O3/n‐poly‐Si/SiOx Stack for High‐Efficiency p‐Type Passivating Tunnel Oxide Contact Solar Cells

This study focuses on the enhanced passivation and gettering of boron‐doped p‐type solar grade silicon wafers by incorporating carrier‐selective and passivating tunnel oxide contact (TOPCon). A symmetrical stack of aluminum oxide (Al2O3)/p‐doped n‐type polysilicon (n‐poly‐Si)/ ultrathin silicon oxide (SiOx) in conjunction with long cycles of forming gas annealing is used for enhancing the silicon wafer quality with a novel approach. Multilayer of n‐poly‐Si/SiOx on p‐type crystalline silicon wafer exhibits an implied open‐circuit voltage (iVoc) of 726 mV, effective carrier lifetime (τeff) of 857 μs, and a low recombination current density (Jo) of 1.9 fA cm−2 when subjected to a postdeposition annealing (PDA) of phosphorus‐doped hydrogenated amorphous silicon (n‐a‐Si:H) at 820 °C. To boost passivation and gettering quality, 10 nm‐thick Al2O3 layers on both sides of n‐poly‐Si/SiOx samples are added. This leads to improved τeff (962 μs), reduced Jo (1.1 fA cm−2), and higher iVoc (728 mV). Herein, a thinner 50 nm n‐poly‐Si layer for improved properties is applied. The experiments show improved passivation and gettering. A Quokka‐3 simulation examines the potential of high‐efficiency p‐type TOPCon cells. A novel solar‐grade p‐type wafer quality enhancement approach is introduced, amalgamated with Quokka‐3 results, which could be a milestone in high‐efficiency p‐type TOPCon solar cell production.

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.

Ultra-thin silicon oxide tunnel layer passivated contacts for screen-printed n-type industry solar cells

Effects of the ultra-thin silicon oxide (SiOx) on tunnel oxide passivated contact solar cells (TOPCon) are investigated in this paper. The experiment is fabricated to study the TOPCon electrical performance with different SiOx thickness and the champion efficiency is achieved by the TOPCon with a 1.5 nm SiOx layer, with Voc 698.93 mV, Jsc 40.14 mA/cm2, FF 81.03 and efficiency 22.73 %. For futher understanding, EDNA2 and AFORS-HET are used to simulate the relationship between the SiOx thickness, rear SRV and the doping concentration in n+ layer. It indicates that the total J0 at the SiOx-(c-Si) interface (J0, interface) can be affected by the phosphorus concentration at the SiOx-(c-Si) interface. Besides, due to the variation of SRV or P-doped concentration, different recombination mechanisms, including auger recombination and surface recombination, will dominant the total J0, interface. In addition, the sensitivity of TOPCon solar cells electrical performance to SiOx thickness can be affected by rear SRV, and the sensitivity of TOPCon solar cells electrical performance to rear SRV can be affected by SiOx thickness.

Optimization of SIS solar cells with ultra-thin silicon oxide layer

Due to the simple process, low energy consumption and stable performance, semiconductor/insulating layer/semiconductor (SIS) solar cells have attracted lots of research interests. However, the device physics of SIS solar cells needs further clarification and improvement. In this paper, the effects of TCO work function on the performance of TCO/SiO2/n-Si heterojunction solar cells were simulated by AFORS-HET and the physics mechanisms for these effects were clarified. It is found that for a work function of 5.2 eV or higher of TCO, the SIS devices possess high ƞ of 22.9% or higher, while the devices show the S-shape J-V curve with low FF when the work function work function of TCO is under 5.1 eV. Further analysis indicates that the work function difference between TCO and n-Si substrate is so small that there is an insufficient band bending at the interface leading to a lower built-in electric field that degrades the photogenerated carriers’ separation, which results in the deformation of J-V curve. Moreover, the simulation results show that the performances of TCO/SiOx/n-Si devices are significantly affected by the band offsets at the SiOx/n-Si interface. When the conduction band offset is higher than 0.4 eV, it can effectively prevent electrons from passing through the SiOx layer to reduce the recombination at the interface, resulting in the excellent output performances of the SIS device. But for a low conduction band order of 0.4 eV or less, the blocking effect of electrons weakens, leading to the large recombination at the interface. Finally, we propose two mechanisms of S-shape J-V curve of SIS solar cells: one is the built-in electric field is too small to separate photogenerated carriers, and the other one is that the carrier transmission is blocked by the thick tunneling layer or insulating layer. This work deepens the understanding of the device physics of SIS solar cells and paves the way for enhancing the output performance of industrial Si-based solar cells.

Effect of plasma-assisted N2O/Ar oxidation on TOPCon solar cells

N2O/Ar gas mixture are used to deposit ultra-thin silicon oxide (SiOx) layer by plasma enhanced chemical vapor deposition (PECVD), which is used as the tunneling oxide layer in the TOPCon (tunnel oxide passivated contacts) structure. It is found that the N2O/Ar flow ratios strongly affect the properties of SiOx layer, such as deposition rate, the resistance of high temperature, passivation quality, etc., thus significantly affect the properties of polysilicon passivated contact structures and the efficiency of TOPCon solar cells. The deposition rate of SiOx layer and contact resistivity of SiOx/poly-Si(n+)/SiNx:H structure decrease with the increases of Ar flow. However, there is an optimal N2O/Ar flow ratio to obtain the best passivation quality and the champion efficiency. The optimal flow ratio of N2O/Ar= 5:2 is found, and obtain the optimum thickness of SiOx and uniform polysilicon passivated contact. The highest minority carrier lifetime (τ) of 5751 μs, highest implied open circuit voltage (iVoc) of 741.3 mV, lowest single side recombination saturation current density(J0) of 4.79 fA/cm2, contact resistivity(ρcontact) of 2.23 mΩ/cm2 and champion efficiency of 25.06% are obtained at the flow ratio of N2O/Ar= 5:2.

Optimization of activated phosphorus concentration in recrystallized polysilicon layers for the n-TOPCon solar cell application

The pivotal structure of n-TOPCon (tunnel oxide passivating contact) solar cells is the passivating contact structure composed of a heavily doped polysilicon (poly-Si) layer and an ultrathin silicon oxide (SiOx), which can provide excellent selectively carrier transport. The activated phosphorus concentration in the passivating contact structure plays a crucial role in the passivation quality and contact resistivity performance. By adjusting PH3 flow rate, SiOx thickness and activation temperature, three different activated phosphorus concentrations in poly-Si were achieved in this work, the phosphorus concentration from SiOx to silicon bulk decreased at the same rate. It can be found that the passivation quality decreases with increasing concentration and the contact resistivity decreases significantly. With the aid of ellipsometry, we successfully extracted the refractive index n and extinction coefficient k of three poly-Si with different activated phosphorus concentrations. The extracted n and k values can well feedback the influence of concentration change on short-circuit current density in the simulation. Due to the high concentration led to the decrease of the open circuit voltage and short circuit current of the final TOPCon solar cell, but the obvious filling factor benefit can be obtained, only moderate doping concentration can obtain the best efficiency performance. Finally, the activated phosphorus concentration of about 3 × 1020 cm−1 can obtain the best efficiency, >24.8% on average.

Engineering Heterointerface for High-efficiency Silicon Solar Cells

High-performance passivating contacts are crucial to realizing high-efficiency crystalline silicon (c-Si) solar cells. The passivating contacts can efficiently transport photogenerated carriers from c-Si to metal electrodes. A representative passivating contact is the stack of ultra-thin silicon oxide (SiOx) and doped polycrystalline Si. In the structure, the layer thickness of silicon oxide is limited to 1～2 nm to collect photogenerated carriers. For further improvements, we have developed NAnocrystal Transport path in Ultra-thin dielectrics for REinforced passivating contact（NATURE contact）as a new passivating contact. The NATURE contact consists of Si nanocrystals (NCs) and SiOx matrix. The Si NCs and the SiOx matrix function as carrier transport paths and passivation layers for the c-Si surface, respectively. The NATURE contact can attain good passivation performance and conductive property. In this paper, we briefly explain the fundamentals of the passivating contacts and introduce the properties of the NATURE contact.

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.

Ultrathin (3.7 nm) Silicon Oxide Layers with a Low Concentration of Broken Bonds on the Contact with a Semiconductor

Ultrathin (3.7 nm) Silicon Oxide Layers with a Low Concentration of Broken Bonds on the Contact with a Semiconductor

A novel passivating contact approach for enhanced performance of crystalline silicon solar cells

Passivated contacts based on ultrathin silicon oxide (SiOx) layers and phosphorus-doped nanocrystalline silicon oxide (nc-SiOx(n)) layers have been examined for their application in tunnel oxide-passivated contact (TOPCon) solar cells. Passivated contact nc-SiOx(n)/SiOx, is accomplished by implementing a thermally grown SiOx tunnel layer and a plasma-enhanced chemical vapor deposited (PECVD)-grown nc-SiOx(n) layer, which are subsequently transformed into a more crystalline phase by annealing at a higher temperature. In this research, a 3.2 × 3.2 cm solar cell was fabricated, where the base material was n-type crystalline silicon (c-Si(n)), and an aluminum oxide (Al2O3) acts as a passivation layer which helps to enhanced the passivation properties and indium tin oxide (ITO) layer was used on the front side, which could serve as an anti-reflection coating (ARC), respectively. The influence of the temperature, doping level, and thickness of nc-SiOx(n) on the surface passivation of the contacts was investigated. Superior recombination current density (Jo) values of up to 2.9 fA/cm2 were assessed for the nc-SiOx(n)/SiOx contacts. TOPCon solar cells with top boron-doped emitter, Al2O3, and ITO/rear stack of nc-SiOx(n)/SiOx passivation contacts were formed and resulted in Voc = 650 mV and FF = 78%. Furthermore, we focused on ameliorating the achievements of solar cells using a transparent passivating contact-based nc-SiOx(n), as well as the passivation process and operating principle.

Pinhole‐Free Ultrathin Silicon Oxide Layer by Ozone‐Dissolved Deionized Water

The pinhole‐free silicon oxide (SiO x ) layer grown using dissolved ozone in deionized water (DI‐O3) is demonstrated. An ultrathin SiO x layer of thickness 1.53 nm ± 1.5% is grown on silicon wafer using 7 ppm DI‐O3. A four‐step methodology, including 25 wt% aq. tetramethylammonium hydroxide (TMAH) solution at 80 °C, is employed to magnify the pinhole signatures in the underlying silicon and visualized with dark‐field optical microscopy (DFOM) and scanning electron microscopy (SEM). The DFOM images show spots on the surface of wafer that originate from airborne particles, contamination, etc. SEM images reveal the absence of etch pits in silicon even after etching of SiO x /silicon in TMAH for 300 s. The minority carrier lifetime and interface states density in AlO x ‐capped SiO x /Si structure are ≥2 ms at carrier density of 1 × 1015 cm−3 and less than 2 × 1011 cm−2 eV−1, respectively.

A thermally erasable silicon oxide layer for molecular beam epitaxy

We present a systematic study of the oxidation and deoxidation behaviours of several kinds of ultrathin silicon oxide layers frequently used in silicon (Si) technology, which in this work serve as surface protecting layers for molecular beam epitaxy (MBE). With various characterization techniques, we demonstrate that a chemically grown silicon oxide layer is the most promising candidate for subsequent removal in an ultra-high vacuum chamber at a temperature of 1000 ∘C, without making use of a reducing agent. As a demonstration, a tensile-strained Ge(100) layer is epitaxially grown on the deoxidised wafer with an atomically flat surface and a low threading dislocation density of 3.33 × 108 cm−2. Our findings reveal that the ultra-thin oxide layer grown using a chemical approach is able to protect Si surfaces for subsequent MBE growth of Ge. This approach is promising for the growth of III/V-on-Si (using Ge as a buffer) and all group-IV related epitaxy for integration on the Si photonics platforms.

Comparing the Gettering Effect of Heavily Doped Polysilicon Films and Its Implications for Tunnel Oxide‐Passivated Contact Solar Cells

In addition to excellent surface passivation and carrier selectivity, the structure based on the heavily doped polysilicon layer on an ultrathin silicon oxide interlayer also demonstrates strong impurity gettering effects. Herein, the gettering strength of a range of phosphorus‐ or boron‐doped polysilicon films from different fabrication techniques is assessed and compared. Iron, one of the most common metallic impurities in silicon, is used as a tracer impurity to quantify the gettering strength (segregation coefficient). A comparison of the experimental results to the literature, combined with measurements of the electrically active and inactive dopant concentrations, enables us to suggest the main gettering mechanisms in different polysilicon films. The differences in the segregation coefficients of the phosphorus‐doped polysilicon films for iron are within one order of magnitude, in spite of their different combinations of gettering mechanisms. On the other hand, boron‐doped polysilicon films show a large variation in their gettering effects, although the predominant gettering mechanisms are all attributed to electrically inactive boron, according to the current understanding of the gettering mechanisms from the literature. Finally, the impact of different polysilicon gettering effects on the efficiency of tunnel oxide‐passivated contact (TOPCon) cells is simulated and discussed.

A novel hardmask-to-substrate pattern transfer method for creating 3D, multi-level, hierarchical, high aspect-ratio structures for applications in microfluidics and cooling technologies

This letter solves a major hurdle that mars photolithography-based fabrication of micro-mesoscale structures in silicon. Conventional photolithography is usually performed on smooth, flat wafer surfaces to lay a 2D design and subsequently etch it to create single-level features. It is, however, unable to process non-flat surfaces or already etched wafers and create more than one level in the structure. In this study, we have described a novel cleanroom-based process flow that allows for easy creation of such multi-level, hierarchical 3D structures in a substrate. This is achieved by introducing an ultra-thin sacrificial silicon dioxide hardmask layer on the substrate which is first 3D patterned via multiple rounds of lithography. This 3D pattern is then scaled vertically by a factor of 200–300 and transferred to the substrate underneath via a single shot deep etching step. The proposed method is also easily characterizable—using features of different topographies and dimensions, the etch rates and selectivities were quantified; this characterization information was later used while fabricating specific target structures. Furthermore, this study comprehensively compares the novel pattern transfer technique to already existing methods of creating multi-level structures, like grayscale lithography and chip stacking. The proposed process was found to be cheaper, faster, and easier to standardize compared to other methods—this made the overall process more reliable and repeatable. We hope it will encourage more research into hybrid structures that hold the key to dramatic performance improvements in several micro-mesoscale devices.

Reliable Memristor Based on Ultrathin Native Silicon Oxide.

Memristors based on two-dimensional (2D) materials can exhibit great scalability and ultralow power consumption, yet the structural and thickness inhomogeneity of ultrathin electrolytes lowers the production yield and reliability of devices. Here, we report that the self-limiting amorphous SiOx (∼2.7 nm) provides a perfect atomically thin electrolyte with high uniformity, featuring a record high production yield. With the guidance of physical modeling, we reveal that the atomic thickness of SiOx enables anomalous resistive switching with a transition to an analog quasi-reset mode, where the filament stability can be further enhanced using Ag-Au nanocomposite electrodes. Such a picojoule memristor shows record low switching variabilities (C2C and D2D variation down to 1.1 and 2.6%, respectively), good retention at a few microsiemens, and high conductance-updating linearity, constituting key metrics for analog neural networks. In addition, the stable high-resistance state is found to be an excellent source for true random numbers of Gaussian distribution. This work opens up opportunities in mass production of Si-compatible memristors for ultradense neuromorphic and security hardware.

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.

Process–Structure–Properties Relationships of Passivating, Electron‐Selective Contacts Formed by Atmospheric Pressure Chemical Vapor Deposition of Phosphorus‐Doped Polysilicon

Herein, we investigate the process–structure–properties relationships of in situ phosphorus (P)‐doped polycrystalline silicon (poly‐Si) films by atmospheric pressure chemical vapor deposition (APCVD) for fabricating poly‐Si passivating, electron selective contacts. This high‐throughput in‐line APCVD technique enables to achieve a low‐cost, simple manufacturing process for crystalline silicon (c‐Si) solar cells featuring poly‐Si passivating contact by excluding the need for vacuum/plasma environment, and additional post‐deposition doping steps. A thin layer of this P‐doped poly‐Si is deposited on an ultrathin (1.5 nm) silicon oxide (SiO x ) coated c‐Si substrate to fabricate the passivating contact. This is followed by various post‐deposition treatments, including a high‐temperature annealing step and hydrogenation process. The poly‐Si films are characterized to achieve a better understanding of the impacts of deposition process conditions and post‐deposition treatments on the microstructure, electrical conductivity, passivation quality, and carrier selectivity of the contacts which assists to identify the optimal process conditions. In this work, the optimized annealing process with post‐hydrogenation yields passivating contact with a saturation current density (J 0) of 3 fA cm−2 and an implied open‐circuit voltage (iV OC) of 712 mV on planar c‐Si wafer. Junction resistivity values ranging from 50 to 260 mΩ cm2 are realized for the poly‐Si contacts processed in the optimal annealing condition.