Low Contact Resistance Research Articles

Laser‐Induced Phase Transition and Patterning of hBN‐Encapsulated MoTe2 (Small 17/2023)

hBN-Encapsulated MoTe2 In article number 2205224, Gwan-Hyoung Lee and co-workers investigate phase transition in a laser-irradiated MoTe2 with various stacked geometries. Depending on the passivation and temperature of the laser-irradiated sample, 2H-MoTe2 transforms into various phases, such as Td, 1T′, Mo3Te4, and Te. In the hBN-encapsulated MoTe2, 2H-1T′ heterophase can be patterned by local laser-irradiation, leading to a low contact resistivity of 1.13 kΩ μm.

Vacancy Assisted Bilayer Graphene Contact for Monolayer Graphene Channel Devices

Contact engineering of monolayer graphene channel FETs has been a fundamental limiting factor hindering its intrinsic benefits for applications such as THz electronics, quantum sensing, quantum computing/communication devices, etc. This experimental work presents a unique technique to achieve record low contact resistance using carbon vacancy-assisted bilayer graphene contact. The bilayer graphene contact with engineered carbon vacancies lowered the metal-graphene interfacial distance and enhanced the atomic orbital overlap, eventually lowering the contact resistance. The interfacial properties and orbital interactions are investigated using density functional theory and non equilibrium Green’s function based transport computations. The reduction in contact resistance is then experimentally validated using unique Kelvin probe structures with monolayer and bilayer contacts. The captured contact resistance is as low as ~36 Ω.μm, which is a record low contact resistance value reported to date.

Ex-situ phosphorus-doped polycrystalline silicon passivating contacts for high-efficiency solar cells by physical vapour deposition

We present ex-situ phosphorus-doped polycrystalline silicon (poly-Si) passivating contacts fabricated by the physical vapour deposition method, specifically sputtering. This technique is currently of great interest to the photovoltaics (PV) community due to its low deposition temperature, small footprint, high throughput, and use of low-hazard, solid targets and gases. We optimise the performance of the passivating contacts by adjusting various parameters from the sputtering of amorphous Si films to the formation of the poly-Si layers by a high-temperature thermal diffusion (with phosphorus oxychloride – POCl3, or phosphorus spin-on glass as the dopant sources). For the case of POCl3 diffusion, a high iVoc of ∼720 mV together with a low J0 of 2.6 fA/cm2 after a hydrogenation treatment by atomic layer deposition (ALD) AlOx and forming gas annealing (FGA), and a low contact resistivity ρc of less than 10 mΩ cm2 were achieved. Meanwhile, with spin-on doping, a low average iVoc of ∼600 mV was observed. This variation in performance could be explained by the difference in the crystallinity and the diffusion mechanism of the sputtered Si films after different subsequent thermal diffusion processes. Quokka3 simulations show that cell efficiencies as high as 25% are possible using the optimised passivating contacts. The results demonstrate the potential of these PVD poly-Si films as a promising candidate for future industrial solar cells.

Nonalloyed ohmic contact development with n+ InGaN regrowth method and analysis of its effect on AlGaN/GaN HEMT devices

In this study, the DC performance of AlGaN/GaN based HEMT devices of different geometries (designed to operate in the S, X and Ka-band frequency ranges) with regrown degenerately doped n + In0.12GaN nonalloyed ohmic contacts on different epitaxial structures were investigated. Once the optimal recess etch depth and regrowth thickness for drain and source contacts were determined, the effects of alloyed and nonalloyed ohmic contacts on the maximum drain current (IDS,max), ON‐resistance (Ron), maximum DC transconductance (gm), pinch-off voltage (Vth), drain leakage current (ID,leak), and gate leakage current (IG,leak) were investigated for S, X and Ka-band HEMT devices. The results showed that the use of nonalloyed ohmic contacts resulted in decreasing Rc with a better surface morphology. Additionally, the nonalloyed ohmic contact structure with low contact resistance caused an increase in the IDS,max and gm values by reducing the Ron resistance, and also reducing the ID,leak and IG,leak leakage currents by preventing the surface distortions and trap formations due to the absence of high temperature. Although there was no dramatic change in Vth for S, X and Ka-band HEMT devices, Vth shifts towards positive in S and X-band devices, and towards negative in Ka-band devices.

Ballistic two-dimensional InSe transistors.

The International Roadmap for Devices and Systems (IRDS) forecasts that, for silicon-based metal-oxide-semiconductor (MOS) field-effect transistors (FETs), the scaling of the gate length will stop at 12 nm and the ultimate supply voltage will not decrease to less than 0.6 V (ref. 1). This defines the final integration density and power consumption at the end of the scaling process for silicon-based chips. In recent years, two-dimensional (2D) layered semiconductors with atom-scale thicknesses have been explored as potential channel materials to support further miniaturization and integrated electronics. However, so far, no 2D semiconductor-based FETs have exhibited performances that can surpass state-of-the-art silicon FETs. Here we report a FET with 2D indium selenide (InSe) with high thermal velocity as channel material that operates at 0.5 V and achieves record high transconductance of 6 mS μm-1 and a room-temperature ballistic ratio in the saturation region of 83%, surpassing those of any reported silicon FETs. An yttrium-doping-induced phase-transition method is developed for making ohmic contacts with InSe and the InSe FET is scaled down to 10 nm in channel length. Our InSe FETs can effectively suppress short-channel effects with a low subthreshold swing (SS) of 75 mV per decade and drain-induced barrier lowering (DIBL) of 22 mV V-1. Furthermore, low contact resistance of 62 Ω μm is reliably extracted in 10-nm ballistic InSe FETs, leading to a smaller intrinsic delay and much lower energy-delay product (EDP) than the predicted silicon limit.

Low resistance electrical contacts to few-layered MoS2 by local pressurization

The performance of electronic and optoelectronic devices is dominated by charge carrier injection through the metal–semiconductor contacts. Therefore, creating low-resistance electrical contacts is one of the most critical challenges in the development of devices based on new materials, particularly in the case of two-dimensional semiconductors. Herein, we report a strategy to reduce the contact resistance of MoS2 via local pressurization. We fabricated electrical contacts using an atomic force microscopy tip and applied variable pressure ranging from 0 to 25 GPa. By measuring the transverse electronic transport properties, we show that MoS2 undergoes a reversible semiconducting-metallic transition under pressure. Planar devices in field effect configuration with electrical contacts performed at pressures above ∼15 GPa show up to 30-fold reduced contact resistance and up to 25-fold improved field-effect mobility when compared to those measured at low pressure. Theoretical simulations show that this enhanced performance is due to improved charge injection to the MoS2 semiconductor channel through the metallic MoS2 phase obtained by pressurization. Our results suggest a novel strategy for realizing improved contacts to MoS2 devices by local pressurization and for exploring emergent phenomena under mechano-electric modulation.

N-Type Doping of Naphthalenediimide-Based Random Donor–Acceptor Copolymers to Enhance Transistor Performance and Structural Crystallinity

An integrated strategy of molecular design and conjugated polymer doping is proposed to improve the electronic characteristics for organic field effect transistor (OFET) applications. Here, a series of soluble naphthalene diimide (NDI)-based random donor-acceptor copolymers with selenophene π-conjugated linkers and four acceptors with different electron-withdrawing strengths (named as rNDI-N/S/NN/SS) are synthesized, characterized, and used for OFETs. N-type doping of NDI-based random copolymers using (12a,18a)-5,6,12,12a,13,18,18a,19-octahydro-5,6-dimethyl-13,18[1',2']-benzenobisbenzimidazo[1,2-b:2',1'-d]benzo[i][2.5]benzodiazocine potassium triflate adduct (DMBI-BDZC) is successfully demonstrated. The undoped rNDI-N, rNDI-NN, and rNDI-SS samples exhibit ambipolar charge transport, while rNDI-S presents only a unipolar n-type characteristic. Doping with DMBI-BDZC significantly modulates the performance of rNDI-N/S OFETs, with a 3- to 6-fold increase in electron mobility (μe) for 1 wt % doped device due to simultaneous trap mitigation, lower contact resistance (RC), and activation energy (EA), and enhanced crystallinity and edge-on orientation for charge transport. However, the doping of intrinsic pro-quinoidal rNDI-NN/SS films exhibits unchanged or even reduced device performance. These findings allow us to manipulate the energy levels by developing conjugated copolymers based on various acceptors and quinoids and to optimize the dopant-polymer semiconductor interactions and their impacts on the film morphology and molecular orientation for enhanced charge transport.

Bi-polysilicon passivating contact technique for crystalline silicon solar cell

Polysilicon (poly-Si) passivating contacts overcome the direct metal–semiconductor contact drawback of traditional industrial crystalline silicon (c-Si) solar cells by inserting a layer stack of poly-Si and silicon oxide layers at the rear full-area metal/c-Si interface, which is well-known as a tunnel oxide passivating contact (TOPCon). In conventional industrial TOPCon devices, the direct contact problem affects the emitter, which deteriorates the passivation quality and suppresses the open-circuit voltage (Voc). We herein introduce an innovative bi-poly-Si technique (Bi-TOPCon) featuring rear full-area passivated poly-Si and emitter locally passivated poly-Si to improve the passivation quality of the TOPCon device. The local emitter poly-Si is introduced using metal mask alignment, and its properties are optimised toward high passivation quality and low contact resistance. The Bi-TOPCon device shows a significant improvement in Voc (Voc > 700 mV). Bi-TOPCon is a promising technology for high-efficiency, next-generation industrial TOPCon devices.

Advances of sulfide‐type solid‐state batteries with negative electrodes: Progress and perspectives

AbstractAll‐solid‐state battery (ASSB) technology is the focus of considerable interest owing to their safety and the fact that their high energy density meets the requirements of emerging battery applications, such as electric vehicles and energy storage systems (ESSs). In light of this, current research on high‐energy ASSBs harnesses the benefits of solid‐state battery systems by employing anode materials with high energy densities. Owing to the excellent physical safety of solid electrolytes, it is possible to build a battery with high energy density by using high‐energy negative electrode materials and decreasing the amount of electrolyte in the battery system. Sulfide‐based ASSBs with high ionic conductivity and low physical contact resistance is recently receiving considerable attention. This review provides a summary on various anode materials for ASSBs operating under electrochemically reducing conditions from the perspective of electrochemical and physical safety. The electrochemical and physical properties of sulfide electrolytes used for lithium (Li) metal and particle‐type anode materials are presented, as well as strategies for mitigating interfacial failures in solid‐state cells through interlayer and electrode design.image

Modulation of Schottky barrier at metal/Ge contacts by phosphoric acid coating and excimer laser annealing

Germanium is an attractive material for advanced devices such as high speed transistors and high efficiency optical devices. To realize the high performance operation, low resistive contacts to Ge are required. However, the Fermi level in Ge at metal/Ge contacts is often pinned near the valance band edge. This Fermi level pinning generates high Schottky barrier heights at metal/Ge contacts regardless the work function of the metals, which results in low resistive contacts to p-type Ge but high resistive contacts to n-type Ge. Thus, a technique for modulating the Schottky barrier heights at metal/Ge contacts is desired. For this purpose, effects of surface treatments of Ge using phosphoric acid coating and excimer laser annealing (LA) were investigated. After LA without phosphoric acid coating, the reverse current of the Al/n-type Ge contacts is increased by an order of magnitude. On the other hand, the reverse current is significantly increased by three orders of magnitude after LA with phosphoric acid coating. Here, the Schottky barrier heights are decreased from 0.56 to 0.50 eV and from 0.59 to 0.49 eV after LA for samples without and with phosphoric acid coating, respectively. These phenomena are attributed to formation of very thin GeOx surface layers and very shallow doped layers having very high P concentrations, respectively. This technique is useful for modulation of the Fermi level pinning for metal/Ge contacts with the low thermal budget.

Editorial: Passivating contact solar cells

In the last decade, significant attention has been given to the concept of passivating contact solar cells. Such cells are characterised by very low contact recombination and low contact resistance, a combination that is challenging to achieve with conventional diffused silicon layers. These cells demonstrate a very high open-circuit voltage and therefore higher efficiency. Passivating contacts can be classified into three main technologies according to the material used for the charge-carrier selection: (i) doped amorphous silicon; (ii) doped polycrystalline silicon; and (iii) metal compounds and organic materials. Due to the impressive progress of passivating contacts and the wide variety of technologies and approaches, Progress in Photovoltaic decided to dedicate an entire issue to reviewing the status of this concept. The issue starts with a review of the three main technologies of passivating contacts, summarising their current efficiencies and discussing their distinctive features, advantages, and limitations. While doped amorphous silicon has been used for the manufacturing of industrial heterojunction solar cells for decades, polycrystalline silicon-based solar cells have only recently entered mass production. Interestingly, this technology was introduced to bipolar transistors as early as the 1970s and adapted into silicon solar cells in the 1980s. However, because some aspects other than the contacts limited the obtainable solar cell efficiencies, this technology was not actively investigated until 2013. Nevertheless, since then, a substantial amount of research has been done by several research institutes and companies, enhancing the performance of polycrystalline silicon-based solar cells to efficiencies above 26%. More importantly, significant effort has been made to commercialise this technology with significant progress already reported by several photovoltaic companies. As doped polycrystalline silicon seems to be the most mature passivating contact technology, we review the status of the three leading approaches: The POLO of ISFH, the TOPCon of Fraunhofer ISE, and the monoPoly of SERIS. With several large manufacturers announcing their plans to transfer polycrystalline silicon-based technology into mass production, we also review the activities that have been taken to push the boundaries of this technology and enabled its integration into existing production lines. Manufacturing options, as well as necessary technological advances in cell metallisation and module integration, are discussed from the industrial perspective. We then review the progress of passivating contact technologies that employ metal compounds as the carrier-selective layer. The interest in this technology has recently increased with a few promising families of materials having demonstrated excellent results, specifically alkali/alkaline-earth metal compounds and transition-metal oxides. The number of successful demonstrations of selective-contact materials within these families is increasing fast, with the best solar cell efficiencies now exceeding 23%. The challenges of this technology to be considered for industrial adoption are then discussed, together with its prospects in the context of silicon photovoltaics. In a separate paper, we provide a detailed techno-economic analysis of the use of atomic layer deposited transition-metal oxides for the fabrication of silicon solar cells. The special issue concludes with a paper discussing the challenges and opportunities in the metallisation of industrial perovskite/silicon tandem solar cells. We want to thank all the authors for their great contributions. It was a pleasure to work with such a talented and diverse group of researchers. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

Research Progress of Interfacial Design between Thermoelectric Materials and Electrode Materials.

Intensive efforts have been conducted to realize the reliable interfacial joining of thermoelectric materials and electrode materials with low interfacial contact resistance, which is an essential step to make thermoelectric materials into thermoelectric devices for industrial application. In this review, the roles of structural integrity, interdiffusion, and contact resistance in long-term reliabilities of thermoelectric modules are outlined first. Then interfacial reactions of near-room-temperature Bi2Te3-based thermoelectric materials and various electrode materials are reviewed comprehensively. We also summarized the joining behavior of the mid-temperature PbTe-based thermoelectric materials and commonly used electrode materials. Subsequently, for other thermoelectric materials systems, i.e., SiGe, CoSb3, and Mg3Sb2, previous attempts to join with some electrode materials are also recapitulated. Finally, some future prospects to further improve the joint reliability in thermoelectric device manufacturing are proposed. We believe that this review will provide guidance for preparing thermoelectric devices and optimizing thermoelectric device design.

Temperature-dependent adhesive polymer-based composites with shape adaptability and low thermal contact resistance for thermal management

Temperature-dependent adhesive polymer-based composites with shape adaptability and low thermal contact resistance for thermal management

Electron‐Selective Strontium Oxide Contact for Crystalline Silicon Solar Cells with High Fill Factor

Extensive efforts have been made to develop wide‐bandgap metal compound‐based carrier‐selective contacts to improve the performance of crystalline silicon (c‐Si) solar cells, by mitigating the deleterious effects of metal–Si contact directly. Herein, thermally evaporated wide‐bandgap strontium oxide (SrO x ) is exploited as an electron‐selective contact for c‐Si solar cells. Benefiting from a lower work function (3.1 eV) of SrO x , a strong downward band‐bending is achieved at the n‐type c‐Si/SrO x interface, enabling the electron‐selective transport characteristic. Thin SrO x films simultaneously provide moderate surface passivation after annealing and enable a low contact resistivity on c‐Si surfaces. By the implementation of a single‐dielectric‐layer SrO x ‐based rear contact, a champion power conversion efficiency of 20.0% is realized on the n‐type c‐Si solar cell featuring an intriguing fill factor of 82.8%. Moreover, electron‐selective SrO x contact is demonstrated to show high thermal stability up to 500 °C. The SrO x layer formed by a facile thermal evaporation process presents a unique opportunity to develop highly efficient and low‐cost c‐Si solar cells.

Improving the High-Temperature Gate Bias Instabilities by a Low Thermal Budget Gate-First Process in p-GaN Gate HEMTs

In this study, we report a low ohmic contact resistance process on a 650 V E-mode p-GaN gate HEMT structure. An amorphous silicon (a-Si) assisted layer was inserted in between the ohmic contact and GaN. The fabricated device exhibits a lower contact resistance of about 0.6 Ω-mm after annealing at 550 °C. In addition, the threshold voltage shifting of the device was reduced from -0.85 V to -0.74 V after applying a high gate bias stress at 150 °C for 10-2 s. The measured time to failure (TTF) of the device shows that a low thermal budget process can improve the device's reliability. A 100-fold improvement in HTGB TTF was clearly demonstrated. The study shows a viable method for CMOS-compatible GaN power device fabrication.

Improvements in 2D p-type WSe2 transistors towards ultimate CMOS scaling

This paper provides comprehensive experimental analysis relating to improvements in the two-dimensional (2D) p-type metal–oxide–semiconductor (PMOS) field effect transistors (FETs) by pure van der Waals (vdW) contacts on few-layer tungsten diselenide (WSe2) with high-k metal gate (HKMG) stacks. Our analysis shows that standard metallization techniques (e.g., e-beam evaporation at moderate pressure ~ 10–5 torr) results in significant Fermi-level pinning, but Schottky barrier heights (SBH) remain small (< 100 meV) when using high work function metals (e.g., Pt or Pd). Temperature-dependent analysis uncovers a more dominant contribution to contact resistance from the channel access region and confirms significant improvement through less damaging metallization techniques (i.e., reduced scattering) combined with strongly scaled HKMG stacks (enhanced carrier density). A clean contact/channel interface is achieved through high-vacuum evaporation and temperature-controlled stepped deposition providing large improvements in contact resistance. Our study reports low contact resistance of 5.7 kΩ-µm, with on-state currents of ~ 97 µA/µm and subthreshold swing of ~ 140 mV/dec in FETs with channel lengths of 400 nm. Furthermore, theoretical analysis using a Landauer transport ballistic model for WSe2 SB-FETs elucidates the prospects of nanoscale 2D PMOS FETs indicating high-performance (excellent on-state current vs subthreshold swing benchmarks) towards the ultimate CMOS scaling limit.

Design and Fabrication of Segmented GeTe/(Bi,Sb)2Te3 Thermoelectric Module with Enhanced Conversion Efficiency

AbstractGeTe and (Bi,Sb)2Te3 are two representative thermoelectric (TE) materials showing maximum performance at middle and low temperature, respectively. In order to achieve higher performance over the whole temperature range, their segmented one‐leg TE modules are designed and fabricated by one‐step spark plasma sintering (SPS). To search for contact and connect layers, the diffusion behavior of Fe, Ni, Cu, and Ti metal layers in GeTe is studied systematically. The results show that Ti with a similar linear expansivity (10.80 × 10−6 K−1) to GeTe, has low contact resistance (3 µΩ cm2) and thin diffusion layer (0.4 µm), and thus is an effective metallization layer for GeTe. The geometric structure of the GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module and the ratio of GeTe to (Bi,Sb)2Te3 are determined by finite element simulation method. When the GeTe height ratio is 0.66, its theoretical maximum conversion efficiency (ηmax) can reach 15.9% without considering the thermal radiation and thermal/electrical contact resistance. The fabricated GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module showed a ηmax up to 9.5% with a power density ≈ 7.45 mW mm−2, which are relatively high but lower than theoretical predictions, indicating that developing segmented TE modules is an effective approach to enhance TE conversion efficiency.

A Bioinspired Polymer‐Based Composite Displaying Both Strong Adhesion and Anisotropic Thermal Conductivity

AbstractThe integration and functionality of high‐power electronic architectures or devices require a high strength and good heat flow at the interface. However, simultaneously improving the interfacial bonding and phonon transport of polymers is challenging because of the tradeoff between the cross‐linked flexible chains and high‐quality crystalline structure. Here, a copolymer, poly(dopamine methacrylate‐co‐hydroxyethyl methacrylate [P(DMA‐HEMA)] is designed and synthesized, inspired by the snail and mussel adhesion. The copolymer achievs a high surface adhesion up to 6.38 MPa owing to the synergistic effects of hydrogen bonds and mechanical interlocking. When the copolymer is introduced into vertically aligned carbon nanotubes (VACNTs), the catechol groups in P(DMA‐HEMA) formed strong bonding with the nanotubes through π‐π interactions at the interface. As a result, the P(DMA‐HEMA)/VACNTs composite shows a high through‐plane thermal conductivity (21.46 W m−1 K−1), an in‐plane thermal conductivity that is 3.5 times higher than that of pristine VACNTs, and an extremely low thermal contact resistance (20.27 K mm2 W−1). Furthermore, the composite forms weld‐free high‐strength connections between two pieces of various metals to bridge directional thermal pathways. It also exhibits excellent interfacial heat transfer capability and high reliability even under zero‐pressure conditions.

High‐throughput screening of phase‐engineered atomically thin transition‐metal dichalcogenides for van der Waals contacts at the Schottky–Mott limit

AbstractA main challenge for the development of two‐dimensional devices based on atomically thin transition‐metal dichalcogenides (TMDs) is the realization of metal–semiconductor junctions (MSJs) with low contact resistance and high charge transport capability. However, traditional metal–TMD junctions usually suffer from strong Fermi‐level pinning (FLP) and chemical disorder at the interfaces, resulting in weak device performance and high energy consumption. By means of high‐throughput first‐principles calculations, we report an attractive solution via the formation of van der Waals (vdW) contacts between metallic and semiconducting TMDs. We apply a phase‐engineering strategy to create a monolayer TMD database for achieving a wide range of work functions and band gaps, hence offering a large degree of freedom to construct TMD vdW MSJs with desired contact types. The Schottky barrier heights and contact types of 728 MSJs have been identified and they exhibit weak FLP (−0.62 to −0.90) as compared with the traditional metal–TMD junctions. We find that the interfacial interactions of the MSJs bring a delicate competition between the FLP strength and carrier tunneling efficiency, which can be utilized to screen high‐performance MSJs. Based on a set of screening criteria, four potential TMD vdW MSJs (e.g., NiTe2/ZrSe2, NiTe2/PdSe2, HfTe2/PdTe2, TaSe2/MoTe2) with Ohmic contact, weak FLP, and high carrier tunneling probability have been predicted. This work not only provides a fundamental understanding of contact properties of TMD vdW MSJs but also renders their huge potential for electronics and optoelectronics.image

Achieving low contact resistivity in PEDOT:PSS/n-Si solar cells

Good conductivity and transparency in the visible spectrum along with low processing temperatures and ease of fabrication make Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) a widely accepted polymer for organic-inorganic hybrid heterojunction solar cells. Although the overall conductivity of the PEDOT:PSS is high, the PSS segregates more at the surfaces of the deposited film. This leads to high contact resistivity of PEDOT:PSS film with metal and silicon. In this report, we explore the effect of the spin coating rates on the contact resistivity of PEDOT:PSS with the metal and silicon and the associated performance of PEDOT:PSS/n-Si solar cells. Two different spin speeds of 1000 rpm and 4000 rpm were used to deposit the PEDOT:PSS films over silicon. The PEDOT:PSS films were also deposited in single- and double-layer forms. We could achieve very low contact resistivity of PEDOT:PSS with silicon through spin speed optimizations. Hence, the overall performance of the PEDOT:PSS/n-Si solar cells improves with the spin speed for both single- and double-layer PEDOT:PSS film depositions.