Silicon Interface Research Articles

Atomic‐Scale Structural Dynamics at a‐Si:H/c‐Si Heterointerface During Low‐Temperature Thermal Annealing

AbstractThe integrity of the hydrogenated amorphous silicon/crystalline silicon (a‐Si:H/c‐Si) interface is essential for the enhanced performance of a‐Si:H/c‐Si heterojunction‐based devices. However, during annealing processes aimed at passivating silicon dangling bonds, unexpected Si epitaxy and nanotwin formation tend to emerge, even under low‐temperature conditions. Therefore, understanding the influence of such annealing on the a‐Si:H/c‐Si interfacial structure is therefore pivotal for device optimization. In this study, the atomic‐scale structural transformation of the a‐Si:H/c‐Si heterointerface subjected to low‐temperature annealing is delved into. The dynamic evolution of this interface is captured by employing in situ spherical aberration (CS)‐corrected transmission electron microscopy (TEM), molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. The TEM observations indicated that Si epitaxy initiated before Si nanotwin formation, and these nanotwins are inclined to revert to epitaxial structures upon sustained annealing. Through MD and DFT insights, the thermodynamic and kinetic intricacies driving the concerted tri‐layer atomic shift characterizing the Si nanotwin‐to‐epitaxy transition are decoded. The findings shed light on the thermal behavior of a‐Si:H/c‐Si interfaces, offering new perspectives on the thermal management in silicon heterojunction devices.

Development and Characterization of N2O-Plasma Oxide Layers for High-Temperature p-Type Passivating Contacts in Silicon Solar Cells.

Full-area passivating contacts based on SiOx/poly-Si stacks are key for the new generation of industrial silicon solar cells substituting the passivated emitter and rear cell (PERC) technology. Demonstrating a potential efficiency increase of 1 to 2% compared to PERC, the utilization of n-type wafers with an n-type contact at the back and a p-type diffused boron emitter has become the industry standard in 2024. In this work, variations of this technology are explored, considering p-type passivating contacts on p-type Si wafers formed via a rapid thermal processing (RTP) step. These contacts could be useful in conjunction with n-type contacts for realizing solar cells with passivating contacts on both sides. Here, a particular focus is set on investigating the influence of the applied thermal treatment on the interfacial silicon oxide (SiOx) layer. Thin SiOx layers formed via ultraviolet (UV)-O3 exposure are compared with layers obtained through a plasma treatment with nitrous oxide (N2O). This process is performed in the same plasma enhanced chemical vapor deposition (PECVD) chamber used to grow the Si-based passivating layer, resulting in a streamlined process flow. For both oxide types, the influence of the RTP thermal budget on passivation quality and contact resistivity is investigated. Whereas the UV-O3 oxide shows a pronounced degradation when using high thermal budget annealing (T > 860 °C), the N2O-plasma oxide exhibits instead an excellent passivation quality under these conditions. Simultaneously, the contact resistivity achieved with the N2O-plasma oxide layer is comparable to that yielded by UV-O3-grown oxides. To unravel the mechanisms behind the improved performance obtained with the N2O-plasma oxide at high thermal budget, characterization by high-resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) is conducted on layer stacks featuring both N2O and UV-O3 oxides after RTP. A breakup of the UV-O3 oxide at high thermal budget is observed, whereas the N2O oxide is found to maintain its structural integrity along the interface. Furthermore, chemical analysis reveals that the N2O oxide is richer in oxygen and contains a higher amount of nitrogen compared to the UV-O3 oxide. These distinguishing characteristics can be directly linked to the enhanced stability exhibited by the N2O oxide under higher annealing temperatures and extended dwell times.

Amorphous shear band formation in crystalline Si-anodes governs lithiation and capacity fading in Li-ion batteries

The cycling stability of Li-ion batteries is commonly attributed to the formation of the solid electrolyte interphase (SEI) layer, which is generated on the active material surface during electrochemical reactions in battery operation. Silicon experiences large volume changes upon the Li-insertion and extraction, leading to the amorphization of the silicon-interface due to the permeation of the Li-ions into the silicon. Here, we discover how generated non-hydrostatic strain upon electrochemical cycling further triggers dislocation and eventually shear band formation within the crystalline silicon core. The latter boosts the non-uniform lithiation at the silicon interface affecting the SEI reformation process and ultimately the capacity. Our findings are based on a comprehensive multiscale structural and chemical experimental characterization, complemented by molecular dynamics modelling. This approach highlights the importance of considering electrochemical, microstructural and mechanical mechanisms, offering a strategy for developing improved anode materials with enhanced cycling stability and reduced capacity loss.

Predicting buckling regimes during first lithiation of a crystalline silicon cylindrical anode particle: Influence of size and charging rate

A mathematical model for buckling analysis during the first lithiation of cylindrical crystalline silicon (c-Si) anode particles is presented using the finite deformation framework. A reaction–diffusion model that incorporates a reversible alloying–dealloying reaction (ADR) captures the lithiation in the expanding amorphous zone, while an addition reaction models the dynamics of the crystalline-amorphous silicon interface. A “movable” lithium part and an “immovable” lithium part make up the full lithium. In sharp contrast to the case of amorphous silicon (a-Si), the axial force evolution in c-Si is found to show two different peaks during the initial and final stages of lithiation, respectively. These peaks form the basis of two different buckling criteria, which, in turn, are shown to be sensitively influenced by the influx of lithium and a crucial size-dependent parameter. This framework gives an insight towards investigating the buckling phenomena for lithiation with an evolving amorphous silicon zone due to the interface movement, which is still absent in the present literature. Additionally, the present model enables us to obtain a quantitative and mechanistic insight into and capture the previous experimental observation that the mechanical stability of a cylindrical silicon particle may be improved by maintaining a crystalline silicon core inside the amorphous shell through controlled first lithiation. It is shown that such a crystalline-amorphous core–shell structure can be advantageous to more complicated designs involving carbon fiber as the core. It is thus hoped that this model will contribute towards improving the theoretical underpinnings of an improved design of mechanically robust electrodes for next-generation batteries.

Continuous distribution of interface states in n-type double-side poly-Si/SiOx passivating contact solar cells

Advanced passivation contact in high-efficiency silicon solar cells plays an important role for the sake of minimizing recombination losses. A stack of heavily doped polycrystalline silicon (poly-Si) and tunnel SiOx contact has attracted much attention, benefitting from its excellent characteristics of carrier selectivity and passivation, and which has been successfully applied in double-side poly-Si/SiOx passivating contact solar cells. Note that characteristics analysis of defects at tunnel SiOx/c-Si (crystalline silicon) interface remains an issue of concern. Herein, we observe capacitance transient spectroscopy arise from both electron and hole traps in the passivating contact structure of poly-Si(p+)/tunnel SiOx/c-Si(n). Subsequently, we propose a skillful procedure of deep-level transient spectroscopy (DLTS) measurement to confirm that the observed electron and hole traps are located at the tunnel SiOx/c-Si interface but not in the bulk of the c-Si substrate. Finally, we show a clear physical picture of continuous energy distribution for interface states.

Correction to “Excitons and Singlet Fission at the Crystalline Tetracene–Silicon Interface”

Correction to “Excitons and Singlet Fission at the Crystalline Tetracene–Silicon Interface”

Deeper Insight into the Mechanisms Behind Sputter Damage in Silicon Solar Cells Based on the Example of Nanocrystalline Silicon Carbide

AbstractTransparent conducting oxides, like indium tin oxide, enable lateral charge carrier transport in silicon heterojunction solar cells. However, their deposition can damage the passivation quality in the solar cell. This damage during the sputter deposition is a complex issue that has not been fully understood, particularly in various silicon‐based materials like amorphous silicon, polycrystalline silicon, or nanocrystalline silicon carbide. The degradation in passivation quality observed in, for example, amorphous silicon is not only explainable by UV light degradation. This study explores the origin of this degradation based on the example of hydrogenated nanocrystalline silicon carbide by combining simulations with experimental analyses. It delves into potential sources of damage during the sputtering process and determines that neither primary nor secondary effects from plasma luminescence or electron bombardment are likely contributors to the damage. Similarly, the implantation of ions, as well as the creation of vacancies and ionization of lattice atoms, are also considered improbable causes. It is, however, proposed that the transfer of energy to the crystalline silicon interface via phonons can factor into the degradation of the passivation quality. This transfer might be a plausible explanation for the damage observed in the passivation layers during the sputtering process.

Improvement of silicon and epoxy phase interface by constructing rigid interpenetrating networks based on bismaleimide to enhance ablative, mechanical and thermal properties

Polysiloxanes are widely used in the preparation of ablative-resistant resins due to their tailored reactivity and ceramisable properties. Nevertheless, the mechanical and thermal properties may be negatively impacted by the presence of flexible chain segments and poor compatibility. In this study, the phase interface of silicon between epoxy was improved by a rigid interpenetrating crosslinked network constructing by bismaleimide. The pyrolysis of the silicone was inhibited and the oxidation resistance of the resin was enhanced. Both graphitization and ceramisation of the char layer during ablation are significantly improved. This endows the modified resin with outstanding ablative properties, with mass and linear ablation rates reduced to 0.0666 g/s and 0.0512 mm/s, respectively, which exceed those of some conventional ablative-resistant resins. Remarkably, the thermal and mechanical properties are significantly enhanced benefiting from the rigid crosslinked network and improved phase interfaces. Moreover, the destruction of mechanical properties of composites in high temperature environments caused by organosilicones has been effectively addressed. Bismaleimide/silicone co-modified epoxy resins can be used not only as ablative-resistant coatings, but also for the preparation of fire-resistant carbon fibre composites, providing a novel approach to the production of cost-effective, recyclable structural integrated materials for thermal protection.

Impact of Surface States in Graphene/p-Si Schottky Diodes.

Graphene-silicon Schottky diodes are intriguing devices that straddle the border between classical models and two-dimensional ones. Many papers have been published in recent years studying their operation based on the classical model developed for metal-silicon Schottky diodes. However, the results obtained for diode parameters vary widely in some cases showing very large deviations with respect to the expected range. This indicates that our understanding of their operation remains incomplete. When modeling these devices, certain aspects strictly connected with the quantum mechanical features of both graphene and the interface with silicon play a crucial role and must be considered. In particular, the dependence of the graphene Fermi level on carrier density, the relation of the latter with the density of surface states in silicon and the coupling between in-plane and out-of-plane dynamics in graphene are key aspects for the interpretation of their behavior. Within the thermionic regime, we estimate the zero-bias Schottky barrier height and the density of silicon surface states in graphene/type-p silicon diodes by adapting a kown model and extracting ideality index values close to unity. The ohmic regime, beyond the flat band potential, is modeled with an empirical law, and the current density appears to be roughly proportional to the electric field at the silicon interface; moreover, the graphene-to-silicon electron tunneling efficiency drops significantly in the transition from the thermionic to ohmic regime. We attribute these facts to (donor) silicon surface states, which tend to be empty in the ohmic regime.

Thermal Boundary Conductance of Direct Bonded Aluminum Nitride to Silicon Interfaces.

Heat accumulation and self-heating have become key issues in microelectronics owing to the ever-decreasing size of components and the move toward three-dimensional structures. A significant challenge for solving these issues is thermally isolating materials, such as silicon dioxide (SiO2), which are commonly used in microelectronics. The silicon-on-insulator (SOI) structure is a great demonstrator of the limitations of SiO2 as the low thermal conductivity insulator prevents heat dissipation through the bottom of a device built on a SOI wafer. Replacing SiO2 with a more thermally conductive material could yield immediate results for improved heat dissipation of SOI structures. However, the introduction of alternate materials creates unknown interfaces, which can have a large impact on the overall thermal conductivity of the structure. In this work, we studied a direct bonded AlN-to-SOI wafer (AlN-SOI) by measuring the thermal conductivity of AlN and the thermal boundary conductance (TBC) of silicon (Si)/AlN and Si/SiO2/aluminum-oxygen-nitrogen (AlON)/AlN interfaces, the latter of which were formed during plasma-activated bonding. The results show that the AlN-SOI possesses superior thermal properties to those of a traditional SOI wafer, with the thermal conductivity of AlN measured at roughly 40 W m-1 K-1 and the TBC of both interfaces at roughly 100 MW m-2 K-1. These results show that AlN-SOI is a very promising structure for improving heat dissipation in future microelectronics.

Anti‐Reflective Graded‐Index Metasurface with Correlated Disorder for Light Management in Planar Silicon Solar Cells

AbstractRecently, many research efforts have been dedicated to improving light coupling into solar cells and reducing optical losses. Promising candidates regarding scalability include direct nano‐structuring of the absorber layer, anti‐reflective (AR) coatings, or combining both, e.g., pyramidal textures with a conformal coating. However, many of these methods are either insufficient or infeasible for application in thin solar cells. Moreover, approaches based on directly texturing the silicon interface simultaneously strongly increase surface recombination, thus degrading the electronic properties of the solar cell. To circumvent these issues, conformal graded‐index metasurfaces with a correlated positional disorder for light trapping in solar cells are proposed and experimentally demonstrated in this contribution. When considered as a part of a prototypical solar cell geometry, a broadband reduction in reflection is observed that results in photocurrent enhancement. The combined consideration of disorder and conformal graded‐index layers outperforms structures containing only one of these components. The computational guidance toward optimized designs promises to adjust the framework to other settings. The possibility for large‐scale fabrication of the samples paves the way toward a future generation of supporting photonic structures in solar cells.

Three-Step Process for Efficient Solar Cells with Boron-Doped Passivated Contacts

Crystalline silicon (c-Si) solar cells with passivation stacks consisting of a polycrystalline silicon (poly-Si) layer and a thin interfacial silicon dioxide (SiO2) layer show high conversion efficiencies. Since the poly-Si layer in this structure acts as a carrier transport layer, high doping of the poly-Si layer is crucial for high conductivity and the efficient transport of charge carriers from the bulk to a metal contact. In this respect, conventional furnace-based high-temperature doping methods are limited by the solid solubility of the dopants in silicon. This limitation particularly affects p-type doping using boron. Previously, we showed that laser activation overcomes this limitation by melting the poly-Si layer, resulting in an active concentration beyond the solubility limit after crystallization. High electrically active boron concentrations ensure low contact resistivity at the (contact) metal/semiconductor interface and allow for the maskless patterning of the poly-Si layer by providing an etch-stop layer in an alkaline solution. However, the high doping concentration degrades during long high-temperature annealing steps. Here, we performed a test of the stability of such a high doping concentration under thermal stress. The active boron concentration shows only a minor reduction during SiNx:H deposition at a moderate temperature and a fast-firing step at a high temperature and with a short exposure time. However, for an annealing time tanneal = 30 min and an annealing temperature 600 °C ≤ Tanneal≤ 1000 °C, the high conductivity is significantly reduced, whereas a high passivation quality requires annealing in this range. We resolve this dilemma by introducing a second, healing laser reactivation step, which re-establishes the original high conductivity of the boron-doped poly-Si and does not degrade the passivation. After a thermal annealing temperature Tanneal = 985 °C, the reactivated layers show high sheet conductance (Gsh) with Gsh = 24 mS sq and high passivation quality, with the implied open-circuit voltage (iVOC) reaching iVOC = 715 mV. Therefore, our novel three-step process consisting of laser activation, thermal annealing, and laser reactivation/healing is suitable for fabricating highly efficient solar cells with p++-poly-Si/SiO2 contact passivation layers.

Sluggish Electron Transfer of Oxygen-Terminated Moderately Boron-Doped Diamond Electrode Induced by Large Interfacial Capacitance between a Diamond and Silicon Interface.

Boron-doped diamond (BDD) has tremendous potential for use as an electrode material with outstanding characteristics. The substrate material of BDD can affect the electrochemical properties of BDD electrodes due to the different junction structures of BDD and the substrate materials. However, the BDD/substrate interfacial properties have not been clarified. In this study, the electrochemical behavior of BDD electrodes with different boron-doping levels (0.1% and 1.0% B/C ratios) synthesized on Si, W, Nb, and Mo substrates was investigated. Potential band diagrams of the BDD/substrate interface were proposed to explain different junction structures and electrochemical behaviors. Oxygen-terminated BDD with moderate boron-doping levels exhibited sluggish electron transfer induced by the large capacitance generated at the BDD/Si interface. These findings provide a fundamental understanding of diamond electrochemistry and insight into the selection of suitable substrate materials for practical applications of BDD electrodes.

Use of silicon or carbonitriding interface in the adhesion of Ag-DLC film on titanium alloy: a comparative study

Use of silicon or carbonitriding interface in the adhesion of Ag-DLC film on titanium alloy: a comparative study

Interface and surface segregation of germanium in the SiGe semiconductor

The concentration and distribution of germanium in the silicon crystal or SiGe alloy can play critical roles in affecting the performance of prepared semiconductor devices. We quantitatively investigate the segregation behavior of germanium in silicon and SiGe semiconductor based on molecular dynamics simulation using the Stillinger-Weber (SW) and Tersoff empirical potentials. The value predicted by the SW potential is 0.35 which agrees with some reported experimental results. Both potentials predict that the Ge equilibrium segregation coefficient at the solid/liquid interface of silicon is smaller than one. The germanium atoms tend to segregate to the surface in the SiGe nano structures in order to lower the surface energy. We define the surface segregation coefficient Ks to quantitively describe this behavior. It is found that Ks decreases with the increase of total germanium atomic fraction in the liquid slab. The size dependence of the surface segregation coefficient in the nanodroplet is studied and it scales with 1/R3 linearly. The germanium distribution in the SiGe solid slab and nanoparticle is largely affected by the Ge distribution in the liquid slab and nanodroplet before crystallization.

(Invited) Development of Binders, Surface Coatings and Electrolytes to Stabilize Silicon Anodes for Lithium Batteries

Silicon is a desirable anode material for lithium ion batteries due to the high lithiation capacity of silicon. However, the lithiation/delithiation process results in large volumetric changes to the silicon particles. The volumetric changes result in instability of the solid electrolyte interphase (SEI) on silicon. Investigations of the effect of binders, surface modifying agents and electrolyte formulations on the stability of the SEI on silicon anodes will be presented. Methods to modify the silicon interface to improve cycling performance will also be discussed.

Healing rate of hospital-acquired skin tears using adhesive silicone foam versus meshed silicone interface dressings: A prospective, randomized, non-inferiority pilot study.

A skin tear is a traumatic wound that occurs in up to one in five hospitalized patients. Nursing care includes application of a dressing to create a moist wound healing environment. To compare the effectiveness of two standard dressings (adhesive silicone foam vs. meshed silicone interface) to heal hospital-acquired skin tear. An intention-to-treat pilot study was designed using a randomized, non-inferiority trial in an Australian tertiary hospital setting. Consenting participants (n = 52) had acquired a skin tear within the previous 24 h and had agreed to a 3-week follow-up. Data were collected between 2014 and 2020. The primary outcome measure was wound healing at 21 days. Baseline characteristics were similar in both arms. Per protocol, 86% of skin tears were fully healed at 3 weeks in the adhesive silicone foam group, compared to 59% in the meshed silicone interface group. Greater healing was observed across all skin tear categories in the adhesive silicone foam dressing group. In the intention-to-treat sample, healing was 69% and 42%, respectively. Results suggest the adhesive silicone foam dressing may be superior, as it produced clinically significant healing of skin tears at 3 weeks compared to the meshed silicone interface dressing. Accounting for potential loss to follow-up, a sample of at least 103 participants per arm would be required to power a definitive study.

Plasma deposited amorphous silicon passivation layers on InAs surfaces

The compound semiconductors (InAs, GaSb, AlSb) having nearly matched lattice constants (≈6.1 Å) are of great interest in fabrication of infrared micro/optoelectronics, but are hampered significantly by a high density of interface states that lead to degenerately doped surface layers. Native oxides are a cause of interface states, as well as a barrier to the effective passivation of dangling bonds, and must be removed. Re-oxidation must then be prevented prior to deposition of a passivant. We demonstrate effective removal of the native oxide of InAs through utilization of an amorphous silicon plasma enhanced chemical vapor deposition process. A hydrogen-diluted silane plasma provides a reducing environment to react and remove indium and arsenic oxide species while a thin layer of amorphous silicon is grown to passivate and prevent re-oxidation of the InAs surface. The reduction of native oxide species via hydrogen-diluted argon plasma as a pretreatment prior to amorphous silicon was also explored. The surface chemistry is verified via depth profiling X-ray photoelectron spectroscopy, and the impact of a hydrogen-argon plasma pretreatment investigated to further reduce oxygen concentration at the InAs/amorphous silicon interface.

Excellent crystalline silicon surface passivation by transparent conductive Al-doped ZnO/ITO stack

The surface passivation materials commonly used in crystalline silicon (c-Si) solar cells are either electrically insulating or opaque to the solar spectrum, which poses the electrical loss or optical loss. Hence it is of great significance to explore functional films that are simultaneously transparent, conductive and passivating for c-Si. In this study, we successfully tune two most well-known transparent conductive oxides, i.e. Al-doped ZnO (AZO) and indium tin oxide (ITO), as excellent passivation layers for c-Si surface. The AZO film is grown by atomic layer deposition and is capped with a magnetron-sputtered ITO film. Dynamic secondary ion mass spectrometry reveals that such stack design can retain higher hydrogen concentration on c-Si surface than a single AZO film during annealing. In addition, the AZO should be highly doped to reduce the energy barrier height around c-Si surface, thereby increasing the charge carrier asymmetry and thus reducing surface recombination. It is also highly crucial to optimize the annealing temperature and interfacial silicon oxide. As such, the effective minority carrier lifetime reaches 2.0 ms and the implied open-circuit voltage approaches 700 mV. Besides, the AZO/ITO stacked films possess negligible optical absorption, low sheet resistance (∼90 Ω/sq), and outstanding antireflection effect on c-Si surface.

Insights into the Si─H Bonding Configuration at the Amorphous/Crystalline Silicon Interface of Silicon Heterojunction Solar Cells by Raman and FTIR Spectroscopy.

In silicon heterojunction solar cell technology, thin layers of hydrogenated amorphous silicon (a-Si:H) are applied as passivating contacts to the crystalline silicon (c-Si) wafer. Thus, the properties of the a-Si:H is crucial for the performance of the solar cells. One important property of a-Si:H is its microstructure which can be characterized by the microstructure parameter R based on Si─H bond stretching vibrations. A common method to determine R is Fourier transform infrared (FTIR) absorption measurement which, however, is difficult to perform on solar cells for various reasons like the use of textured Si wafers and the presence of conducting oxide contact layers. Here, it is demonstrated that Raman spectroscopy is suitable to determine the microstructure of bulk a-Si:H layers of 10nm or less on textured c-Si underneath indium tin oxide as conducting oxide. A detailed comparison of FTIR and Raman spectra is performed and significant differences in the microstructure parameter are obtained by both methods with decreasing a-Si:H film thickness.