Low Contact Resistance Research Articles

Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules

In order to reduce manufacturing costs, the design of silicon-based solar modules is changing from a super-multi-busbar design to a zero-busbar (0BB) design. In this study, two different 0BB technologies based on heterojunction with intrinsic thin-layer solar cells—conventional soldering, and Integrated Film Covering (IFC)—were investigated. IFC-based 0BB technology was found to have a lower contact resistance, which well matches the theoretical calculations and module power testing results. To further measure module reliability, a series of tests on solders and silver pastes were carried out. The results show that Sn43Pb43Bi14 solder is more suitable for soldering-based 0BB technology, whereas Sn32Pb42Bi26 solder is more suitable for IFC-based technology. Additionally, silver paste, which is used for solder ribbon contact areas (SRCAs), is suitable for soldering-based 0BB technology. When Ag@Cu paste is used in SRCAs with IFC-based 0BB technology, a reliable connection can also be achieved. After optimization, modules using both techniques were subjected to and passed lifetime tests, including the thermal cycling, humidity freeze, and hot-spot tests required in IEC standards, as well as more rigorous tests such as thermal–dynamic and thermal–static mechanical loading. The results show that the two technologies have great potential for future mass production.

Thermally stable Bi2Te3/WSe2 Van Der Waals contacts for pMOSFETs application

Novel van der Waals (vdW) contacts formed by layered Bi2Te3 are found effective in improving the performance of WSe2 pMOSFETs. As compared with conventional transition metal-based Ni/Au S/D contacts, over 103 times on-state current improvement is achieved. vdW interface formation between Bi2Te3 and WSe2 is confirmed by X-ray diffraction analysis and scanning transmission electron microscope observation. An atomically flat Bi2Te3/WSe2 vdW interface, where the number of defects could be reduced as small as possible, contributes to the suppression of Fermi-level pinning caused by defect-induced gap states. Moreover, the semimetal-like characteristics of Bi2Te3 are also effective in minimizing the impact of metal-induced gap states. These features offer WSe2 pMOSFETs with exceptional S/D junction characteristics, including suppressed off-state leakage and a higher on–off ratio. In addition, it is found that WSe2 pMOSFETs with Bi2Te3 S/D contacts have excellent thermal stability, maintaining device performance even after 400 °C annealing, which is very promising for CMOS back-end-of-line application. The layered tellurides, reconciling low contact resistance and high thermal stability, are promising, particularly from the perspective of their application in the manufacturing process.

Low Resistivity n‐type GaN Ohmic Contacts on GaN Substrates

In this study, a report is prepared on significantly low specific contact resistivity of alloyed and non‐alloyed ohmic contacts fabricated on an as‐grown n+‐GaN layer and measured with the transfer length method. A low ρc = 8 × 10−8 Ω cm2 is extracted for the alloyed Ti/Al/Ni/Au, and ρc = 4 × 10−7 Ω cm2 for the unannealed Ti/Pd/Au. To achieve these, a highly doped n+‐GaN layer with ND = 1.5 × 1019 cm−3 is used. The results are derived from a study of three different metal contact stacks, namely Ti/Al/Ni/Au (20 nm/300 nm/20 nm/400 nm), Ti/Pd/Au (2 nm/5 nm/200 nm), and Mo/Au (30 nm/200 nm). The Ti/Al/Ni/Au metal contact is studied in both annealed and non‐annealed conditions, whereas for the Ti/Pd/Au and Mo/Au ohmic contacts, a study is conducted without annealing. Their performance and thermal stability are evaluated with a four‐probe TLM, with temperatures ranging from 25 to 150 °C. Finally, a theoretical model based on thermionic emission theory is employed to gain a deeper understanding of the physical mechanisms governing the behavior of the ohmic contacts.

Record Low Contact Resistivity of 10-8 Ω cm² Ohmic Contacts on Oxygen-Terminated Intrinsic Diamond by Transition Metals Metallization

Record Low Contact Resistivity of 10^-8 Ω cm² Ohmic Contacts on Oxygen-Terminated Intrinsic Diamond by Transition Metals Metallization

Exploration of High‐Pressure Annealing and Microwave Annealing in Palladium Germano‐Silicide Formation for Si0.8Ge0.2‐Based Complementary Metal‐Oxide–Semiconductor Transistors

In this study, forming palladium germano‐silicide on Si0.8Ge0.2‐based complementary metal‐oxide semiconductor (CMOS) transistors by high‐pressure annealing compared to microwave annealing is investigated. Boron‐doped Si0.8Ge0.2 layers are epitaxially grown on n‐type Si wafers, achieving an initial boron concentration of 5 × 1015 cm−3, which increase to ≈6 × 1020 cm−3 after microwave annealing, reducing sheet resistance. Palladium is deposited using electron beam evaporation to form a 15 nm layer on Si0.8Ge0.2 (200 nm)/Si (100) substrates. High‐pressure annealing is conducted from 300 to 500 °C in N2 ambiance at 5 kg cm−3, while microwave annealing is performed at 5.8 GHz and 1800–3000 W for 100 s. X‐ray diffractometer confirms high‐intensity Pd2Si phase formation, but scanning electron microscope and atomic force microscope reveal increased surface roughness and clustering after annealing. Sheet resistance increases from 10.35 Ω sq−1 (unannealed) to 131.8 Ω sq−1 (high‐pressure annealing at 300 °C) and 85.8 Ω sq−1 (microwave annealing at 1800 W). In these results, the trade‐offs between annealing methods and metal choices for achieving low contact resistance and Schottky barrier heights in p‐type Si0.8Ge0.2 CMOS circuits are highlighted.

Laterally Actuated Si-to-Si DC MEMS Switch for Power Switching Applications.

Electrothermal actuators are highly advantageous for microelectromechanical systems (MEMS) due to their capability to generate significant force and large displacements. Despite these benefits, their application in reconfigurable conduction line switches is limited, particularly when employing commercial processes. In DC MEMS switches, electrothermal actuators require electrical insulation between the biasing voltage and the transmission line to prevent interference and maintain the integrity of the switch. This work presents a chevron-type electrothermal actuator utilizing a stack of SiO2/ Al thin films on a silicon (Si) structural layer beam to create a DC MEMS switch. The design leverages a thin film Al heater to drive the actuator while the SiO2 layer provides electrical insulation, suppressing crosstalk with the Si layer. The electrical contact resistance of a Si-to-Si interface was evaluated by applying a controlled current and measuring the resultant voltage. A low contact resistance of 150 Ω was achieved when an initial contact gap of 2.52 μm was closed using an actuator with an actuation voltage of 1.2 V and a current of 205 mA, with a switching speed of less than 5 ms. Factors such as the contact force, the temperature, and the residual device layer etching angle significantly impact the Si-to-Si contact resistance and the switch's longevity. The switch withstands a breakdown voltage up to 350 V at its terminal contacts. Thus, it will be robust to self-actuation caused by unwanted voltage contributions, making it suitable for high-voltage and harsh environment applications.

Low Contact Resistance WSe2 p-Type Transistors with Highly Stable, CMOS-Compatible Dopants.

High contact resistance has been a bottleneck in developing high-performance transition-metal dichalcogenide (TMD) based p-type transistors. We report degenerately doped few-layer WSe2 transistors with contact resistance as low as 0.23 ± 0.07 kΩ·μm per contact by using platinum(IV) chloride (PtCl4) as the p-type dopant, which is composed of ions compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication process. Top-gated devices with a gate length of 200 nm showed good switching behaviors, implying that the dopant diffusion into the gate stack is not significant. The devices showed nearly identical performance after being kept in air for 86 days without any encapsulation while retaining the degenerately doped states at 78 K with pressure lower than 10-5 Torr, highlighting the stability of the dopants. The presented method sets forth the availability of highly stable methods for pattern doping the transistors with a thinned Schottky barrier width for low contact resistance devices.

Corrosion behavior of TiZrNbC coatings on TA1 substrates in simulated PEM water electrolysis

A TiZrNbC coating was deposited on the TA1 substrate by magnetron sputtering technology, and the corrosion behaviors of the TiZrNbC coating in a simulated proton exchange membrane water electrolysis (PEMWE) environment under fluctuating potentials were investigated. The microstructures and elemental distributions of the coating were characterized by SEM and EDS. The PEMWE environment was simulated using a dilute H2SO4 solution with pH = 3 and a 5 ppm of fluoride (F-) ions addition, and polarization tests were conducted under the potentials with constant, square and triangular waveforms. The corrosion resistances of the TiZrNbC-coated and bare TA1 substrates were characterized by potentiodynamic polarization, electrochemical impedance spectroscopy and interfacial contact resistance tests. Compared to the bare TA1 substrates, the TiZrNbC-coated TA1 substrates exhibited lower corrosion current densities, higher polarization resistances and lower contact resistances, demonstrating a superior corrosion resistance.

Understanding the Impact of Contact-Induced Strain on the Electrical Performance of Monolayer WS2 Transistors.

Two-dimensional (2D) electronics require low contact resistance (RC) to approach their fundamental limits. WS2 is a promising 2D semiconductor that is often paired with Ni contacts, but their operation is not well understood considering the nonideal alignment between the Ni work function and the WS2 conduction band. Here, we investigate the effects of contact size on nanoscale monolayer WS2 transistors and uncover that Ni contacts impart stress, which affects the WS2 device performance. The strain applied to the WS2 depends on contact size, where long (1 μm) contacts (RC ≈ 1.7 kΩ·μm) show a 78% reduction in RC compared to shorter (0.1 μm) contacts (RC ≈ 7.8 kΩ·μm). We also find that thermal annealing can relax the WS2 strain in long-contact devices, increasing RC to 8.5 kΩ·μm. These results reveal that thermo-mechanical phenomena can significantly influence 2D semiconductor-metal contacts, presenting opportunities to optimize device performance through nanofabrication and thermal budget.

Regulated charge transfer by cascade dual Z-scheme heterojunction of HCdS@ZnIn2S4-Cobalt porphyrin for efficient photocatalytic solar fuel production

Dual Z-schemed photocatalysts possess superior charge-separation efficiency and powerful redox capabilities, which have attracted significant attention in photocatalytic CO2 reduction (PCR). However, most of the dual Z-scheme systems are focused on inorganic semiconductors, which remain a challenge to control the selectivity of PCR products. Herein, we designed a cascade dual Z-scheme structured organic/inorganic heterojunction of HCdS@ZnIn2S4-Cobalt porphyrin (HCdS@ZIS-CoTPPS). HCdS@ZIS-CoTPPS demonstrated excellent PCR activity (CO rate: 93.64 μmol·g−1·h−1) and adjustable syngas ratio when compared to HCdS, ZIS, CoTPPS, and HCdS/ZIS. Experimental characterizations and density functional theory (DFT) calculations reveal that the superior PCR activity of HCdS@ZIS-CoTPPS is contributed by the broad light absorption range, short charge carrier migration path, low interface contact resistance, and strong redox ability induced by the hollow structured cascade dual Z-scheme heterojunction. This article provides valuable inspiration for the design and preparation of atomistic heterojunctions for solar-driven fuel generation.

Growth of Ta-doped SnO2 on GaN as a UV-transparent conducting electrode and band alignment properties of the heterojunction

GaN-based ultraviolet light emitting diodes (UV LEDs) have attracted considerable attention in recent years and are required in various applications such as healthcare, light illumination, and optical communication. However, the limited UV transparency of the electrodes like indium-doped tin oxide has hindered the external quantum efficiency of current UV LEDs. In this work, we present the growth of UV-transparent Ta-doped SnO2 (TTO) thin films on GaN as a promising UV-transparent electrode for LEDs. TTO thin films with a thickness of 200 nm exhibit optical transmission exceeding 80% at the wavelength of 300 nm, with a low resistivity of 2.5 × 10−4 Ω·cm and a low contact resistance of 1.7 × 10−2 Ω cm2 to n-type GaN. High-resolution x-ray photoemission spectra were employed to reveal insight into the electronic structure of TTO and the interfacial band alignment of TTO/GaN heterojunction. The wide optical bandgap (∼4.6 eV) and high UV transparency of TTO films stem from a significant Burstein–Moss shift due to degenerate doping, giving rise to metal-like characteristics and a small barrier height at the interface of TTO/GaN. These findings imply the origin of low contact resistivity of TTO to n-type GaN and may be applicable to the development of UV-transparent electrodes of optoelectronic devices.

Exploring low temperature-sputtered indium tin oxide (ITO) as an anti-reflection layer for silicon solar cells

Abstract This paper tackles challenges in silicon (Si) solar cells, specifically the use of hazardous Phosphorus Oxychloride (POCl3) for emitter formation and silane/ammonia for the Anti-Reflective Coating (ARC) layer, accompanied by high-temperature metallization. The study proposes an eco-friendly ARC layer process, replacing toxic materials. Indium Tin Oxide (ITO) with a refractive index of ∼2.0 is suggested as a non-toxic substitute for SiN x in the ARC layer. ITO enables fine-tuning of optical parameters and, with its electrical properties, supports low-resistivity contacts through efficient, low-temperature metallization processes. ITO-passivated solar cells with Ag polymer paste as a front contact exhibit promising characteristics: a commendable photocurrent density (J sc) of 20 mA cm−2 at 850 °C, low series resistance (R s) of 1.9 Ω, and high shunt resistance (R shunt) of 28.9 Ω, as demonstrated by illuminated I–V measurements. Implementing ITO as the ARC on a less toxic emitter junction enhances Si solar cells’ current density gain, minimizing current leakage during high-temperature processing. In conclusion, adopting less toxic materials and employing low-temperature processing in passive silicon solar cell fabrication presents an attractive alternative for cost reduction and contributes to environmentally sustainable practices in green manufacturing.

Unveiling Versatile Electronic Properties and Contact Features of Metal-Semiconductor Graphene/γ-Ge2SSe van der Waals Heterostructures.

Recently, searching for a metal-semiconductor junction (MSJ) that exhibits low-contact resistance has received tremendous consideration, as they are essential components in next-generation field-effect transistors. In this work, we design a MSJ by integrating two-dimensional (2D) graphene as the metallic electrode and 2D Janus γ-Ge2SSe as the semiconducting channel using first-principles simulations. All the graphene/γ-Ge2SSe MSJs are predicted to be energetically, mechanically, and thermodynamically stable, characterized by the weak van der Waals (vdW) interactions. The graphene/γ-SGe2Se MSJ-vdWH form the n-type Schottky contact (SC), while the graphene/γ-SeGe2S MSJ-vdWH form the p-type one, suggesting that the switching between p-type and n-type SC in the graphene/γ-Ge2SSe MSJ-vdWHs can occur spontaneously by simply altering the stacking patterns, without requiring any external conditions. Notably, the contact features, including contact types and barriers of the graphene/γ-Ge2SSe MSJs are significant in versatility and can be altered by applying electric gating and adjusting interlayer spacing. Both the applied electric gating and strain engineering induce switchability between p- and n-type and SC to OC in the graphene/γ-Ge2SSE MSJs. This versatility underscores the potential of the graphene/γ-Ge2SSe MSJ for next-generation applications that require low-contact resistance and high performance.

Boron‐Doped Zinc Oxide Electron‐Selective Contacts for Crystalline Silicon Solar Cells with Efficiency over 22.0%

The exploration of wide‐bandgap metal compound films with excellent passivation and contact properties on crystalline silicon (c‐Si) surface, as alternatives to traditional‐doped Si thin films, holds significant promise for the future enhancement of c‐Si solar cell efficiency. Herein, conductive boron‐doped zinc oxide (ZnO:B) films are deposited by atomic layer deposition (ALD) process and investigated as electron‐selective contacts, in combination with a thin SiOx passivating interlayer. This combination demonstrates a relative low contact resistivity of ≈2 mΩ cm2 and improved passivation quality. Further application of this SiOx/ZnO:B stack as a full‐area electron‐selective passivating contact in proof‐of‐concept n‐type c‐Si solar cells results in a satisfactory power conversion efficiency of over 22.0%. This electron‐selective passivating contact structure, prepared via low‐temperature, simplified, and the compositionally controlled ALD process, offers a promising pathway for the development of high‐efficiency and low‐cost c‐Si solar cells.

High-Performance p-Type Quasi-Ohmic of WSe2 Transistors Using Vanadium-Doped WSe2 as Intermediate Layer Contact.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs), such as tungsten diselenide (WSe2), hold immense potential for applications in electronic and optoelectronic devices. However, a significant Schottky barrier height (SBH) at the metal-semiconductor (MS) interface reduces the electronic device performance. Here, we present a unique 2D/2D contact method for minimizing contact resistance and reducing the SBH. This approach utilizes vanadium-doped WSe2 (V-WSe2) as the drain and source contacts. The fabricated transistor exhibited a stable operation with p-type quasi-ohmic contact and a high on/off current ratio surpassing 108 at room temperature, reaching 1011 at 10 K. The device achieved an on-current of 68.87 μA, a high mobility of 103.80 cm2 V-1 s-1, a low contact resistance of 0.92 kΩ, and remarkably low SBH values of 1.51 meV for holes at VGS = -120 V with fixed VDS = 1 V. Furthermore, a Schottky photodiode has been fabricated, utilizing V-WSe2 and Cr as the asymmetric contact platform, showing a responsivity of 116 mA W1-. The findings of this study suggest a simple and efficient method for improving the performance of TMDC-based transistors.

Polymer hydrogel electronics with adhesive, self-healing, and anti-ultraviolet capabilities for machine learning-promoted electrophysiological detection

While adjustable mechanical properties, diverse biochemical features, and good ionic conductivity enabled by molecular engineering, polymer hydrogels as ionic skins (i-skins) are still limited by multiple challenges in the practical human health monitoring and health diagnosis, such as undesirable ductility and contact adhesion, poor functionality, and insufficient sensing ability. In view of this, the strategy of multiple dynamic interactions was proposed, and a mussel characteristic double-network polymer composite hydrogel (PG-BT2) with tannic acid and borax as dynamic media was reasonably designed. Due to the dynamic reversible interactions from the crosslinked network combined with special functional groups, the obtained PG-BT2 hydrogel attains remarkable stretchability with >1200 % elongation and good toughness of 268.6 kJ/m−3, and also possesses favorable versatility (e.g., self-healing time <2 s, adhesion strength of 4.62 kPa to pigskin, and UV transmittance of 15.3 % at 365 nm). Additionally, the i-skin derived from the PG-BT2 hydrogel exhibits excellent sensing properties, including a low interface contact resistance of 8.3 kΩ at 1 kHz, fast response/recovery time of 0.657 s/0.348 s, and a wide sensing range of 0–1100 % strain. By combining the hydrogel-based i-skin patch with the wireless circuit connected to a Bluetooth module, the constructed sensor system is capable of accurately monitoring micro-expressions and multi-joint movements. The portable system is also demonstrated to capture high-fidelity signals of electrocardiograms available for training artificial intelligence, achieving differentiation and recognition of blood pressure status, which will greatly benefit health management and intelligent medicine.

A novel dual functional gradient Zn(O,S)/Mg electron-selective contact for dopant‐free silicon solar cells with efficiencies approaching 21 %

Dopant-free carrier-selective contacts have the potential to mitigate or eliminate parasitic absorption, auger recombination losses, etc. Therefore, the vigorous development of dopant-free carrier-selective contact materials is an important way to enhance the performance of crystalline silicon (c-Si) photovoltaics. This study presents the concept of magnetron sputtering gradient deposited zinc oxide (Gd-Zn(O,S)) combined with a low work function magnesium metal layer. Subsequently, the Gd-Zn(O,S)/Mg bifunctional layer was further investigated and optimised to determine its suitability as a contact layer for electron transfer in c-Si solar cells. This strategy simultaneously enables a good passivation and a low contact resistance. The c-Si(n)/a-Si:H(i)/Gd-Zn(O,S)/Mg devices are constructed and achieved an optimal contact recombination current density (J0) of approximately 1.51 fA cm−2, along with a minimum ρc of approximately 35.89 mΩ.cm2. As a consequence, the power conversion efficiency of a prototype silicon heterojunction solar cell is 20.92 %. This study demonstrates that the strategy of replacing the a-Si:H(n) electron transport layer with Gd-Zn(O,S) is an effective approach for improving the SHJ cell. This study also provides new perspectives for designing novel dopant-free carrier-selective contact structures.

Role of electrostatic doping on the resistance of metal and two-dimensional materials edge contacts

In this theoretical study, we compare electrostatically doped metal-transition metal dichalcogenide (TMD) edge-contacts versus substitutionally doped edge-contacts in terms of their contact resistance. Our approach involves the utilization of electrostatic doping achieved by applying back-gate bias to the metal-TMD edge contacts, where carrier injection is primarily governed by the Schottky barrier at the interface. To analyze these contacts, we employ the Wentzel-Kramers-Brillouin (WKB) approximation to calculate the transmission coefficient and use density functional theory (DFT)-derived band structures. We numerically solve the Poisson equation to capture the electrostatic potential. We also account for the impact of the image force using Green's function for the Poisson equation with boundary conditions appropriate to our specific geometry. Our findings reveal that electrostatically doped TMD edge contacts exhibit higher contact resistance compared to impurity-doped edge contacts at equivalent carrier concentrations. At the same time, we find that, among the electrostatically doped edge contacts, a low-κ back-gate oxide in conjunction with low-κ top oxide is preferable in terms of improvement in contact resistance. For instance, in a metal-TMD edge contact scenario involving a monolayer MoS2 as the channel, SiO2 as the infinitely thick top oxide, and a SiO2 back-gate oxide with an equivalent oxide thickness (EOT) of 1nm, we demonstrate that it is possible to achieve an impressively low contact resistance of 50Ωµm when the back-gate bias exceeds or equals 2 V. Published by the American Physical Society 2024

2D Gr/WSe2/MoTe2 vertical heterojunction for self-powered photodiode with ultrafast response and high sensitivity

Two-dimensional materials without lattice constraints can be combined into form heterojunctions via van der Waals forces, providing opportunities for the future development of novel high-performance photodetectors. In non-vertical heterojunction devices with long carrier transport channels, SRH recombination due to defect and interface states in the heterojunction and Langevin recombination due to Coulomb interactions induce large amounts of photogenerated carrier recombination, leading to low quantum efficiencies of the heterojunction. At the same time, defect and interface trap states, as well as the long channel of the non-vertical heterojunction device, lead to a slow response speed of the device. In this work, a 2D WSe2/MoTe2 vertical heterojunction photodiode with a transparent graphene top electrode has been designed to simultaneously achieve high sensitivity and fast response speed of photodetectors. Benefiting from the vertical device structure, high-quality interface and low contact resistance, the photogenerated electron-hole pairs can be efficiently separated and transported. The photodiode exhibits remarkable rectification characteristics with a rectification ratio as high as 1.4 × 104, and provides a broadband and self-powered photodetection from the visible to NIR bands (405–1064 nm) with a maximum responsivity of 0.33 A/W at 785 nm. In particular, the photodiode achieves an ultra-fast rise/fall time of 6.49/6.22 μs at 0 V and a further reduction of the response time to 1.68/1.2 μs at a reverse bias of −1 V. This ultra-fast response allows the photodiode to detect switching signals with a cutoff frequency of more than 70 kHz and 200 kHz at 0 V and −1 V, respectively. This work opens up new opportunities for the development of integrated 2D photodetectors with low power consumption, high sensitivity, and high speed.

Au-free V/Al/Pt Contacts on n-Al0.85Ga0.15N:Si Surfaces of Far-UVC LEDs

The feasibility of replacing the V/Al/Ni/Au contact on n-Al0.85Ga0.15N:Si commonly used in far-UVC LEDs with a Au-free V/Al/Pt contact has been investigated. It is shown that the V and Pt layer thicknesses play an important role for achieving a low contact resistivity and a smooth contact surface at a low annealing temperature. The specific contact resistivity of V(10 nm)/Al(120 nm)/Pt(40 nm) annealed at 700 °C for 5 min is comparable with that of V(15 nm)/Al(120 nm)/Ni(20 nm)/Au(30 nm) annealed at 850 °C for 30 s. Furthermore, the root mean square roughness of the optimized V/Al/Pt contacts was 8.3 nm as compared to 30 nm for V/Al/Ni/Au.