Graphene Capping Research Articles

Graphene capping layer for improving the photovoltaic stability of ZnO nanorods solar cells under temperature variations

The present study evaluates the role of the graphene capping layer on the optoelectronic performance of ZnO/Si solar cells at different ambient temperatures. The n-ZnO buffer thin film was sputter-deposited on the p-Si substrates to form a p-n heterojunction. Next, ZnO nanostructures were chemically synthesized on top of the ZnO/Si substrates, and few-layer graphene was deposited on ZnO nanostructures using an electrophoretic deposition technique. The planar and cross-sectional electron microscopy analyses exhibited that the graphene capping layer entirely covered the surface of ZnO nanostructures, and their morphology remained intact after graphene transfer. Their photoluminescence (PL) properties before and after graphene deposition were analyzed at various temperatures to evaluate the graphene role in the optical behavior of ZnO nanostructures. The PL intensity was reduced for the graphene-containing sample because of the slight light absorption in each graphene layer. However, the stability of PL spectra at elevated temperatures was improved after graphene transfer. Next, a heterojunction solar cell was developed by depositing top and back contacts to study the photovoltaic performance of nanostructures at various temperatures. The current-voltage results, recorded under a solar light simulator, illustrated that the efficiency of the bare ZnO device was reduced at high temperatures because of the available interstitials and oxygen vacancies. After graphene transferring, this decay was moderated, which enhanced the photovoltaic stability of ZnO-based solar cells. Therefore, the graphene capping layer can act as a stabilizing thin film to enhance the photovoltaic durability of ZnO nanostructures under harsh conditions.

Improving the Electromigration Life of Advanced Interconnects through Graphene Capping

Since the discovery of graphene, 2D materials are establishing a rapidly growing and promising field, with great potential for diverse applications given their excellent conductivity and high transparency. In particular, graphene can isolate external chemical reactions when applied to back-end-of-line (BEOL) interconnect metals, thereby preventing the oxidation of the interconnect metals. In addition, it can improve the conductivity and breakdown current density of interconnects. However, the thermal budget remains an important problem in BEOL interconnects. We demonstrate an advanced graphene deposition method using an in-house plasma plasma-enhanced chemical vapor deposition system, which offers low thermal budget and high stability. We also optimize carbon precursors to improve the quality of graphene on Ru and Co thin films. We show the improvement of electrical conductivity, electromigration lifetime, and maximum breakdown current density after capping Ru and Co interconnects with graphene. The electromigration lifetimes of the Ru and Co interconnects increase by 4 and 4.5 times, respectively, and their maximum breakdown current density increases by 17.6 and 10.6%, respectively. The results show that capping with graphene has a high potential in BEOL applications.

Graphene Capping of Cu Back-End-of-Line Interconnects Reduces Resistance and Improves Electromigration Lifetime

Graphene Capping of Cu Back-End-of-Line Interconnects Reduces Resistance and Improves Electromigration Lifetime

Effective Passivation of Anisotropic 2D GeAs via Graphene Encapsulation for Highly Stable Near-Infrared Photodetectors.

Germanium arsenic (GeAs) as a promising two-dimensional (2D) semiconducting material has attracted extensive attention. The high carrier mobility and tunable bandgap of GeAs offer broad prospects in electronic and optoelectronic device-related applications. The unique intrinsic anisotropy arising from the low-symmetry structure can be applied in the design of new devices. However, the rapid degradation of mechanically exfoliated GeAs in the environment poses a challenge to its practical development in scalable devices. Here, an approach to stabilize the sensitive material without isolation from the ambient environment is reported. The graphene capping layer effectively suppresses environmental degradation, enabling the encapsulated GeAs photodetectors to maintain the key electronic properties for more than 3 months under ambient conditions. In addition, the regulation of the work function of graphene significantly improves the device performance. An improved responsivity of 965.07 A/W is 20 times higher than that of pure GeAs. This work provides opportunities for the practical application of GeAs and other environmentally sensitive 2D materials.

Ultrafast photoresponse in ultraviolet detectors based on zinc oxide nanorods: the effect of a graphene capping layer

Ultrafast photoresponse in ultraviolet detectors based on zinc oxide nanorods: the effect of a graphene capping layer

Atomic structures and interfacial engineering of ultrathin indium intercalated between graphene and a SiC substrate.

Two-dimensional metals stabilized at the interface between graphene and SiC are attracting considerable interest thanks to their intriguing physical properties, providing promising material platforms for quantum technologies. However, the nanoscale picture of the ultrathin metals within the interface that represents their ultimate two-dimensional limit has not been well captured. In this work, we explore the atomic structures and electronic properties of atomically thin indium intercalated at the epitaxial graphene/SiC interface by means of cryogenic scanning tunneling microscopy. Two types of surfaces with distinctive crystalline characteristics are found: (i) a triangular indium arrangement epitaxially matching the (√3 × √3)R30° cell of the SiC substrate and (ii) a featureless surface of more complex atomic structures. Local tunneling spectroscopy reveals a varying n-type doping in the graphene capping layer induced by the intercalated indium and an occupied electronic state at ∼-1.1 eV that is attributed to the electronic structure of the newly formed interface. Tip-induced surface manipulation is used to alter the interfacial landscape; indium atoms are locally de-intercalated below graphene. This enables the quantitative measurement of the intercalation thickness revealing mono and bi-atomic layer indium within the interface and offers the capability to tune the number of metal layers such that a monolayer is converted irreversibly to a bilayer indium. Our findings demonstrate a scanning probe-based method for in-depth investigation of ultrathin metal at the atomic level, holding importance from both fundamental and technical viewpoints.

Suppression of Surface Defects and Vibrational Coupling in GaN by a Graphene Monolayer

The optical transitions and random walking dynamics of the three types of excitons, namely, free, donor‐bound, and acceptor‐like surface‐defect‐bound excitons, in both the graphene−GaN hybrid structure and bare GaN are comparatively examined using variable‐temperature steady‐state photoluminescence and time‐resolved photoluminescence. The results reveal that the effective suppression of surface defects may be realized by a graphene capping monolayer, evidenced by the strong reduction of the luminescence signal of the surface‐defect‐bound excitons and the simultaneous enhancement of the free exciton luminescence. More interestingly, the coupling strengths between all the three kinds of excitons and the longitudinal optical phonons tend to weaken, especially for the surface‐defect‐bound excitons, which exhibit a reduction of ≈40%. These findings indicate that capping GaN by graphene could be an effective way to passivate harmful surface defects and address the heating problem in GaN‐based devices.

Metal induced charge transfer doping in graphene-ruthenium hybrid interconnects

To enable graphene-integrated interconnects in modern VLSI circuits, a major roadblock is developing an efficient Back End of Line (BEOL) compatible doping technique. In this paper, we demonstrate metal-induced doping of graphene in graphene-ruthenium hybrid structures. We study doping by systematically performing different material characterization techniques – Internal Photoemission Spectroscopy (IPE), Raman Spectroscopy and Kelvin Probe Force Microscopy (KPFM) to gain a deeper understanding on the charge transfer at the graphene-Ru interface. In IPE, we measure the relative band alignment of graphene and Ru, the interface potential barrier and effective work function of 4.9eV. With Raman spectral mapping, we report p-type doping in single layer graphene on Ru film with carrier density 1.9E13cm−2. And with KPFM, Fermi-level shift of ∼420 meV (wrt intrinsic graphene) is observed implying downward shift of Fermi level in the graphene valence band. Electrically, graphene capping results in ∼19 % drop in sheet resistance of Ru accompanied by significant decrease in contact resistance. Moreover, the temperature coefficient of resistance reduces after graphene capping indicating better response to thermal fluctuations. By performing an extensive study using different material and electrical techniques, our results provide a viable and practical basis for integrating graphene as a conductor in advanced interconnects.

Enhanced Exciton-Exciton Collisions in an Ultraflat Monolayer MoSe2 Prepared through Deterministic Flattening.

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of Δn ∼ 2 × 1012 cm-2 μN-1, resulting in the significant intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "graphene-capping-assisted AFM nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual line width of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that the formation of biexciton occurs even at extremely low excitation power (Φph ∼ 2.3 × 1019 cm-2 s-1) due to the enhanced collisions between excitons.

(Invited) Scaling of Copper Interconnection As Opposed to Alternative Conductors

Cu/low-k interconnection is facing to challenges in its extension per scaling in the 3nm node and beyond where the metal pitch is less than 32 nm. As is listed in Table.1, the difficulties lie in securing (1) Cu gap fill capability in scaled via in the dual damascene process flow due to the limited step coverage of PVD Cu seed layer, (2) Vx-Mx breakdown voltage and TDDB reliability due to scaled spacing and misalignment in lithography, (3) Electromigration reliability when the barrier/liner is scaled, (4) Low line resistance due to decreased Cu volume when the barrier/liner scaling is limited, (5) Low via resistance when the barrier/liner scaling is limited, (6) Selectivity in selective CVD deposition of cobalt capping layer which is required for electromigration reliability and (7) Low interconnect parasitic capacitance when thickness of the plasma-induced damage of the intra-layer low-k dielectric becomes comparable to the inter-metal spacing per scaling.Because of these difficulties in extension of dual damascene Cu interconnection scheme, various alternative metallization unit processes, materials (conductors and dielectrics) and schemes (alternative to dual damascene) have been investigated. These new processes and materials include both (a) those for extension of Cu technology such as Co_Cu hybrid metallization (i.e., Co for via and Cu for wires) and (b) those for entire replacement of Cu to other conductors. Basically extension of Cu interconnects is the preference in the industry rather than entire replacement to alternative conductors and/or metallization schemes, because entire replacement needs to be accompanied with alignment of all unit processes to the new metal schemes such as CMP and/or RIE of the new conductor, deposition of the conductor, wet cleaning compatible to the conductor, and nucleation/adhesion layer formation for the conductor on dielectrics, in all fine dimensions. In addition, even when fine Cu lines are replaced to alternative conductors such as Co, wide power rails need to stay with Cu for the required low line resistance (i.e., Cu_Co composite metallization).In this talk, process candidates which have potential to resolve the difficulties listed in Table 1 and enable extension of Cu interconnection scheme are reviewed such as (i) via pre-fill processes with vertical ALD of low-k dielectrics [1], (ii) plasma damage resistant low-k dielectric materials [2], (iii) Fully Aligned Via integration process [3], (iv) PVD-assisted ALD barrier formation [4, 5], (v) through cobalt self-forming barrier process barrier/liner scaling processes without loss in their integrity [1, 6 - 8], (vi) graphene capping material on top of Cu interconnects for electromigration reliability 10]. Then, the talk covers (vii) the composite metallization scheme [11] which is an enabler for the post-Cu alternative conductor metallization. Figure 1

Suppressed Interdiffusion and Degradation in Flexible and Transparent Metal Electrode-Based Perovskite Solar Cells with a Graphene Interlayer.

Metal-based transparent conductive electrodes (TCEs) are attractive candidates for application in indium tin oxide (ITO)-free solar cells due to their excellent electrical conductivity and cost effectiveness. In perovskite solar cells (PSCs), metal-induced degradation with the perovskite layer leads to various detrimental effects, deteriorating the device performance and stability. Here, we introduce a novel flexible hybrid TCE consisting of a Cu grid-embedded polyimide film and a graphene capping layer, named GCEP, which exhibits excellent mechanical and chemical stability as well as desirable optoelectrical properties. We demonstrated the critical role of graphene as a protection layer to prevent metal-induced degradation and halide diffusion between the electrode and perovskite layer; the performance of the flexible PSCs fabricated with GCEP was comparable to that of their rigid ITO-based counterparts and also exhibited outstanding mechanical and chemical stability. This work provides an effective strategy to design mechanically and chemically robust ITO-free metal-assisted TCE platforms in PSCs.

Phonon thermal transport in encapsulated copper hybrids

We use molecular dynamics simulations to characterize the effect of various surface terminations on phonon thermal transport in nanoscale Cu slabs. Specifically, we studied Cu slabs approximately 4 nm in thickness with atomistically flat (111) surfaces, slabs with ∼30% surface vacancies to mimic atomic-level roughness, and slabs with a surface oxide. Motivated by recent experimental observations, we study the effect of capping these surfaces with single layer graphene. From the thermal conductivity of the various samples as a function of length, we extracted conductivity and phonon mean free paths in the absence of boundary scattering other than that originating from the surfaces under study. As expected, both surface vacancies and an oxide layer reduce thermal conductivity and we characterize the effect in terms of the specularity parameter. While capping the slabs with graphene increases the conductivity, the poor thermal contact between Cu and graphene results in less than ideal performance of the hybrid material. Interestingly, the simulations reveal that the graphene capping layer reduces surface scattering on the Cu slabs, and this effect is significantly more pronounced in the case of a defective surface. The results provide insights into the use of graphene capping to improve transport in nanoscale interconnects for nanoelectronics.

Epitaxial and atomically thin graphene–metal hybrid catalyst films: the dual role of graphene as the support and the chemically-transparent protective cap

Revolutionary catalyst protection by single layer graphene capping, tremendous catalyst lifetime longevity and activity enhancement towards oxygen reduction reaction.

Co film stretching induced by lattice mismatch and annealing: The role of Graphene

Thin Co films intercalated between a Graphene capping layer and the Ir(111) surface are of interest for spintronics applications due to their peculiar magnetic properties and to their chemical stability. The structure, and then the magnetic properties, of the Co films depend on the intercalation process which is strongly influenced by the temperature, total amount of Cobalt and quality of the capping Graphene layer. In order to identify and disentangle the effects of these contributions, we report on the structural characterisation of four Co films as a function of thickness, annealing temperature, and Graphene capping. From the structural point of view, the deposition of Co on a hot Ir substrate mimics quite well the intercalation process proving the validity of the colander model describing the Graphene role during the process.

Preventing Thin Film Dewetting via Graphene Capping.

A monolayer 2D capping layer with high Young's modulus is shown to be able to effectively suppress the dewetting of underlying thin films of small organic semiconductor molecule, polymer, and polycrystalline metal, respectively. To verify the universality of this capping layer approach, the dewetting experiments are performed for single-layer graphene transferred onto polystyrene (PS), semiconducting thienoazacoronene (EH-TAC), gold, and also MoS2 on PS. Thermodynamic modeling indicates that the exceptionally high Young's modulus and surface conformity of 2D capping layers such as graphene and MoS2 substantially suppress surface fluctuations and thus dewetting. As long as the uncovered area is smaller than the fluctuation wavelength of the thin film in a dewetting process via spinodal decomposition, the dewetting should be suppressed. The 2D monolayer-capping approach opens up exciting new possibilities to enhance the thermal stability and expands the processing parameters for thin film materials without significantly altering their physical properties.

Protecting Silicon Film Anodes in Lithium-Ion Batteries Using an Atomically Thin Graphene Drape.

Silicon (Si) shows promise as an anode material in lithium-ion batteries due to its very high specific capacity. However, Si is highly brittle, and in an effort to prevent Si from fracturing, the research community has migrated from the use of Si films to Si nanoparticle based electrodes. However, such a strategy significantly reduces volumetric energy density due to the porosity of Si nanoparticle electrodes. Here we show that contrary to conventional wisdom, Si films can be stabilized by two strategies: (a) anchoring the Si films to a carbon nanotube macrofilm (CNM) current collector and (b) draping the films with a graphene monolayer. After electrochemical cycling, the graphene-coated Si films on CNM resembled a tough mud-cracked surface in which the graphene capping layer suppresses delamination and stabilizes the solid electrolyte interface. The graphene-draped Si films on CNM exhibit long cycle life (>1000 charge/discharge steps) with an average specific capacity of ∼806 mAh g-1. The volumetric capacity averaged over 1000 cycles of charge/discharge is ∼2821 mAh cm-3, which is 2 to 5 times higher than what is reported in the literature for Si nanoparticle based electrodes. The graphene-draped Si anode could also be successfully cycled against commercial cathodes in a full-cell configuration.

Metal-Free Carbon-Based Nanomaterial Coatings Protect Silicon Photoanodes in Solar Water-Splitting.

The decreasing cost of silicon-based photovoltaics has enabled significant increases in solar electricity generation worldwide. Silicon photoanodes could also play an important role in the cost-effective generation of solar fuels, but the most successful methods of photoelectrode passivation and performance enhancement rely on a combination of precious metals and sophisticated processing methods that offset the economic arguments for silicon. Here we show that metal-free carbon-based nanomaterial coatings deposited from solution can protect silicon photoanodes carrying out the oxygen evolution reaction in a range of working environments. Purified semiconducting carbon nanotubes (CNTs) act as a hole extraction layer, and a graphene (Gr) capping layer both protects the CNT film and acts as a hole exchange layer with the electrolyte. The performance of semiconducting CNTs is found to be superior to that of metallic or unsorted CNTs in this context. Furthermore, the insertion of graphene oxide (GO) between the n-Si and CNTs reduces the overpotential relative to photoanodes with CNTs deposited on hydrogen-passivated silicon. The composite photoanode structure of n-Si/GO/CNT/Gr shows promising performance for oxygen evolution and excellent potential for improvement by optimizing the catalytic properties and stability of the graphene protective layer.

Effects of multi-layer graphene capping on Cu interconnects

The benefits of multi-layer graphene (MLG) capping on Cu interconnects have been experimentally demonstrated. The resistance of MLG capped Cu wires improved by 2–7% compared to Cu wires. The breakdown current density increased by 18%, suggesting that the MLG can act as an excellent capping material for Cu interconnects, improving the reliability characteristics. With a proper process optimization, MLG capped Cu interconnects could become a promising technology for high density back end-of-line interconnects.