Semiconductor Contacts Research Articles

A universal resist-assisted metal transfer method for 2D semiconductor contacts

Abstract With the explosive exploration of two-dimensional (2D) semiconductors for device applications, ensuring effective electrical contacts has become critical for optimizing device performance. Here, we demonstrate a universal resist-assisted metal transfer method for creating nearly free-standing metal electrodes on the SiO2/Si substrate, which can be easily transferred onto 2D semiconductors to form van der Waals (vdW) contacts. In this method, polymethyl methacrylate (PMMA) serves both as an electron resist for electrode patterning and as a sacrificial layer. Contacted with our transferred electrodes, MoS₂ exhibits tunable Schottky barrier heights and a transition from n-type dominated to ambipolar conduction with increasing metal work functions, while InSe shows pronounced ambipolarity. Additionally, using α-In2Se3 as an example, we demonstrate that our transferred electrodes enhance resistance switching in ferroelectric memristors. Finally, the universality of our method is evidenced by the successful transfer of various metals with different adhesion forces and complex patterns.

Constructing of n‐Type Semiconductor Heterostructures for Efficient Hydrazine‐Assisted Hydrogen Production

AbstractHydrazine‐assisted water electrolysis presents a promising approach toward energy‐efficient hydrogen production. However, the progress of this technology is hindered by the limited availability of affordable, efficient, and durable catalysts. In this study, a feasible strategy is proposed for interface modulation that enables efficient hydrogen evolution and hydrazine oxidation through the construction of n‐type semiconductor heterostructures. The metal–semiconductor contacts are rationally designed using ruthenium nanoclusters and a range of metal oxide (M–O) semiconductor heterostructures, including p‐type semiconductor substrates (NiO, Co3O4, NiCo2O4, CuCo2O4, ZnCo2O4) and n‐type semiconductor substrate (Fe2O3). Intriguingly, Ru nanoclusters supported on p‐type M–O substrates induce a transition from p‐type M–O to n‐type M‐O/Ru. The design of n‐type semiconductor heterostructures can significantly reduce space‐charge regions and increase charge carrier concentration, thereby improving the electrical conductivity of electrocatalysts. Moreover, Ru atoms can serve as highly efficient active sites for hydrogen evolution reaction and hydrazine oxidation reaction. The NiO/Ru heterostructure can drive current densities of 10 and 100 mA cm−2 with only 0.021 and 0.22 V cell voltages for hydrazine‐assisted water electrolysis. This work provides new insights for the development of highly efficient semiconductor catalysts, enabling energy‐saving hydrogen production.

Giant tunneling resistance and robust switching behavior in ferroelectric tunnel junctions of WS2/Ga2O3 heterostructures: The influence of metal–semiconductor contacts

The ferroelectric tunneling junctions (FTJs) are widely recognized as one of the non-volatile memories with significant potential. Ferroelectricity usually fades away as materials are thinned down below a critical value, and this problem is particularly acute in the case of shrinking device sizes, thus attracting attention to two-dimensional ferroelectric materials (2DFEMs). In this work, we designed 2D ferroelectric Ga2O3-based FTJs with out-of-plane polarization, and the influence of metal–semiconductor contact in the electrode region on the system is considered. Here, using density functional theory combined with the non-equilibrium Green's function approach to quantum transport calculations, we demonstrate robust ferroelectric polarization-controlled switching behavior between metallic and semiconducting states in Ga2O3/WS2 ferroelectric heterostructures. The potential barrier of the metal–semiconductor contact in the electrode region is lower than that of the intrinsic material, thereby resulting in an increased probability of electron tunneling. Our results reveal the crucial role of 2DFEMs in the construction of FTJs and highlight the significant impact of electrode contact types on performance. This provides a promising approach for developing high-density ferroelectric memories based on 2D ferroelectric semiconductor heterostructures.

Enhancing Oxygen Evolution Reaction Performance of Metal-Organic Frameworks through Cathode Activation.

Due to their abundant active sites and porous structures, metal-organic frameworks (MOFs) have garnered significant interest as oxygen evolution reaction (OER) electrocatalysts. Nevertheless, the development of MOF-based electrocatalysts with efficient OER activity and excellent stability simultaneously still faces challenges. Herein, a cathodic activation strategy was used to enhance the OER electrocatalytic performance of M-HHTP for the first time, where M refers to Ni, Cu, Co, Fe, while HHTP denotes 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene. As a prototype, the activated Ni-HHTP (HA-Ni-HHTP) demonstrates outstanding OER performance, with an overpotential as low as 140 mV at 20 mA cm-2 and a small Tafel slope of 78.7 mV-1, surpassing commercial RuO2 and rivaling state-of-the-art MOFs-based electrocatalysts. Characterizations and density functional theory calculations reveal that the superior performance of HA-Ni-HHTP is primarily ascribed to changes in semiconductor type, contact angle, and oxygen vacancy content induced by cathodic activation. Electrochemical impedance spectroscopy analysis using the transmission line model confirms that cathodic activation accelerates charge transport, enhancing the OER process. Furthermore, the cathodic activation strategy holds promise for improving the water oxidation performance of other MOFs such as Fe-HHTP, Co-HHTP, and Cu-HHTP.

The study on influence factors of contact properties of metal-MoS2 interfaces

The metal and two-dimension (2D) semiconductor contact interfaces have a more considerable contact resistance hindering carrier injection, which makes the performance of 2D semiconductor devices less than the theory. The contact properties of Ni, Au, and Mo with MoS2 are simulated by the first-principles method. The interface dipole caused by the interface charge redistribution changes the work function difference at the metal-MoS2 interface, so the interface charge redistribution is one of the important factors for correctly evaluating the contact properties. Due to the metal-induced gap states (MIGS) at metal-monolayer (ML) MoS2 interfaces, the Fermi level is strongly pinned to fixed energy, and the Schottky barrier height (SBH) cannot be regulated efficiently by the metal work function. Although the work function of Au is bigger than Ni, the Fermi level of Au is pinned at a higher position. In the meantime, the bandgap of MoS2 narrows and metallization occurs due to the larger MIGS. In the Mo-MoS2 interface, the Fermi level is pinned near the conduction band minimum of MoS2. The contact resistances (Rc) of the three structures are tested by the Circular Transfer Length Method (CTLM), which is consistent with the prediction of the simulation. The Mo-MoS2 has the smallest Rc. The results indicate that contact resistance of 2D semiconductors cannot be simply predicted by soled work functions or Fermi level pinning, but is determined by several factors.

Toward Ideal Low-Frequency Noise in Monolayer CVD MoS2 FETs: Influence of van der Waals Junctions and Sulfur Vacancy Management.

The pursuit of sub-1-nm field-effect transistor (FET) channels within 3D semiconducting crystals faces challenges due to diminished gate electrostatics and increased charge carrier scattering. 2D semiconductors, exemplified by transition metal dichalcogenides, provide a promising alternative. However, the non-idealities, such as excess low-frequency noise (LFN) in 2D FETs, present substantial hurdles to their realization and commercialization. In this study, ideal LFN characteristics in monolayer MoS2 FETs are attained by engineering the metal-2D semiconductor contact and the subgap density of states (DOS). By probing non-ideal contact resistance effects using CuS and Au electrodes, it is uncovered that excess contact noise in the high drain current (ID) region can be substantially reduced by forming a van der Waals junction with CuS electrodes. Furthermore, thermal annealing effectively mitigates sulfur vacancy-induced subgap density of states (DOS), diminishing excess noise in the low ID region. Through meticulous optimization of metal-2D semiconductor contacts and subgap DOS, alignment of 1/f noise with the pure carrier number fluctuation model is achieved, ultimately achieving the sought-after ideal LFN behavior in monolayer MoS2 FETs. This study underscores the necessity of refining excess noise, heralding improved performance and reliability of 2D electronic devices.

Integration of Multijunction Absorbers and Catalysts for Efficient Solar‐Driven Artificial Leaf Structures: A Physical and Materials Science Perspective

Artificial leaves could be the breakthrough technology to overcome the limitations of storage and mobility through the synthesis of chemical fuels from sunlight, which will be an essential component of a sustainable future energy system. However, the realization of efficient solar‐driven artificial leaf structures requires integrated specialized materials such as semiconductor absorbers, catalysts, interfacial passivation, and contact layers. To date, no competitive system has emerged due to a lack of scientific understanding, knowledge‐based design rules, and scalable engineering strategies. Herein, competitive artificial leaf devices for water splitting, focusing on multiabsorber structures to achieve solar‐to‐hydrogen conversion efficiencies exceeding 15%, are discussed. A key challenge is integrating photovoltaic and electrochemical functionalities in a single device. Additionally, optimal electrocatalysts for intermittent operation at photocurrent densities of 10–20 mA cm−2 must be immobilized on the absorbers with specifically designed interfacial passivation and contact layers, so‐called buried junctions. This minimizes voltage and current losses and prevents corrosive side reactions. Key challenges include understanding elementary steps, identifying suitable materials, and developing synthesis and processing techniques for all integrated components. This is crucial for efficient, robust, and scalable devices. Herein, corresponding research efforts to produce green hydrogen with unassisted solar‐driven (photo‐)electrochemical devices are discussed and reported.

Competitive behavior between the piezoelectric and piezoresistive effects in monolayer WS2 photodetector

The strain induced piezoelectric and piezoresistive effects have been regarded as promising methods to regulate the photoelectric properties of two-dimensional transition metal dichalcogenides. However, the distinction between the influence of piezoelectric and piezoresistive effects on devices is ambiguous. Here, piezo-phototronic photodetectors based on monolayer WS2 were fabricated to investigate the competitive behavior of the piezoelectric and piezoresistive effects. We have shown that the piezoresistive effect dominates the photocurrent enhancement through narrowing the bandgap under small strain. With increasing strain, the influence of piezoelectric effect became more and more obvious, and it dominated the photo-induced carrier transport behavior through polarization charges accumulated at the metal–semiconductor contact interface when the strain exceeded 0.78%. Under the strain condition, the modulation of strain on photocurrent reached as high as 1400%.

Low-power MoS2 metal–semiconductor field effect transistors (MESFETs) based on standard metal–semiconductor contact

With the popularization of electronic devices and the demand for portability, low-power consumption has become crucial for integrated circuit chips. Two-dimensional (2D) semiconductors offer significant potential in constructing low-power devices due to their ultrathin thickness, enabling fully depletion operation. However, fabricating these 2D low-power devices, such as negative-capacitance transistors or tunneling transistors, often requires multiple layers of gate dielectrics or channel band engineering, adding complexity to the manufacturing process and posing challenges for their integration with silicon technology. In this work, we have developed low-power MoS2 metal–semiconductor field effect transistors utilizing a standard metal–semiconductor contact, which eliminates the need for gate dielectrics and semiconductor heterojunctions. It demonstrates a sharp subthreshold slope (SS ∼ 64 mV/dec), a minimum operating gate voltage range (−0.5 ∼ 1 V), a minimum current hysteresis (3.69 mV), and a stable threshold voltage close to 0 V (Vth ∼ −0.27 V). Moreover, we implemented an inverter circuit with a high voltage gain of 47.

A Dual‐Mode Programing Nonvolatile Floating‐Gate Memory with Convertible Ohmic and Schottky Contacts

AbstractResearch on van der Waals heterostructures based on stacked two‐dimensional atomic thickness crystals has received considerable attention because of their unique characters and great potential applications in flexible transparent electronics and optoelectronics. In this study, a nonvolatile memory device with a few‐layer bipolar material WS2 as channel and charge‐trapping layers is designed with a floating‐gate structure in which charges (electrons and holes) can be stored in the charge‐trapping layer using a dual‐mode processing program by changing the metal–semiconductor contact type. The device exhibits different programming currents during programming, in particular, the device has a low programming current for programming voltages ˂20 V. Moreover, the heterostructure exhibits a remarkable long retention time (≈10,000 s), with no apparent degradation and a strong endurance, retaining its original performance even after 1,000 programming/erasing cycles. This study proposes a novel method for reducing power consumption while programming to facilitate artificial synapse applications.

Effect of ZnO and PEDOT:PSS charge selective layers on photovoltage of cuprous oxide (Cu2O) heterojunction solar cells

1.0 cm2 large copper oxide solar cells are fabricated using a solution-based approach and the contributions of the semiconductor contacts to the photovoltage are observed with Kelvin probe surface photovoltage spectroscopy.

Light-Induced Platinum Deposition on Silicon-Based Semiconductor Devices

Metal-semiconductor compounds are used in large numbers in microelectronics and need to be constantly improved. Platinum-silicon semiconductor contacts are often used in sensors and detectors, especially infrared detectors and cameras. Recent developments enable applications in medical technology, such as implanted pressure sensors in the human body.Currently, platinum coatings on silicon-based semiconductor devices are mostly realized by sputtering techniques or platinum-containing printing pastes with subsequent heat treatment.This paper will discuss recent results from the authors' lab on light-induced platinum electrodeposition. Special focus will be given to the pre-treatment of the substrates for the subsequent platinum deposition under illumination. In a systematic approach, all influencing parameters were identified and the most influential ones (pre-treatment, current density, electrolyte type, additives, etc.) were incorporated into an experimental plan. The challenges include a homogeneous platinum coating with low contact resistance and good adhesion. Last but not least, the characterization of the layers (with SEM, EDX, laser scanning microscope, etc.) will be discussed.

Investigating the Polarity Dependence of Multilevel Cell Operation in Conventional Mushroom Phase‐Change Memory Cells

Phase‐change materials have long been employed for rewriteable optical data storage at the industrial scale and are hailed as one of the most mature technologies for their applications in emerging nonvolatile memories. Memristors based on these materials have the potential to circumvent the long‐standing von Neumann bottleneck and offer significant computational advantage through neuromorphic computing. A very core fundamental concept that lies at the crux for such realization is their ability to offer multilevel cell (MLC) storage, in contrast to the binary counterpart. Yet, numerous challenges still remain to be tackled for their successful implementation in this regard. This work is a finite element analysis that particularly reports the polarity dependence of such MLC operation in the conventional mushroom geometry of devices employing this technology. The mechanism lying underneath is discussed and a perspective combining thermoelectric effects with the energy band diagrams resulting from metal–semiconductor contact formation is additionally put forth.

Universal transfer of full‐class metal electrodes for barrier‐free two‐dimensional semiconductor contacts

AbstractMetal–semiconductor contacts are crucial components in semiconductor devices. Ultrathin two‐dimensional transition‐metal dichalcogenide semiconductors can sustain transistor scaling for next‐generation integrated circuits. However, their performance is often degraded by conventional metal deposition, which results in a high barrier due to chemical disorder and Fermi‐level pinning (FLP). Although, transferring electrodes can address these issues, they are limited in achieving universal transfer of full‐class metals due to strong adhesion between pre‐deposited metals and substrates. Here, we propose a nanobelt‐assisted transfer strategy that can avoid the adhesion limitation and enables the universal transfer of over 20 different types of electrodes. Our contacts obey the Schottky–Mott rule and exhibit a FLP of S = 0.99. Both the electron and hole contacts show record‐low Schottky barriers of 4.2 and 11.2 meV, respectively. As a demonstration, we construct a doping‐free WSe2 inverter with these high‐performance contacts, which exhibits a static power consumption of only 58 pW. This strategy provides a universal method of electrode preparation for building high‐performance post‐Moore electronic devices.

Extending Schottky–Mott rule to van der Waals heterostructures of 2D Janus materials: Influence of intrinsic dipoles

The Schottky–Mott (S–M) limit based on the S–M rule is often used to evaluate the Schottky barrier height (SBH) at metal–semiconductor (MS) van der Waals (vdW) contacts but fails at the polar interfaces. In order to extend the S–M rule to the polar interfaces, we here modify the S–M equation to predict the SBH at vdW interfaces of 2D Janus materials, taking into account the effects of intrinsic and interface dipoles. The modified S–M equation is verified based on the first-principles calculations of the MoSi2As2P2/HTaSe2F vdW interfaces, showing a sharp dependence of SBH on the dipole amplitude and direction. Specifically, n-type Schottky barriers tend to form when a semiconductor contacts with a low-work-function surface of Janus metal or a metal interfaces to the high-electron-affinity surface of Janus semiconductor; otherwise, a p-type one is preferable. Interestingly, the smallest n(p)-type SBH could be attained when both intrinsic dipole directions are the same. This work demonstrates that the S–M rule can be extended to the polar interfaces and dipole engineering is an effective strategy to tune the SBH at the MS interface.

Recent Progress in Source/Drain Ohmic Contact with β-Ga2O3

β-Ga2O3, with excellent bandgap, breakdown field, and thermal stability properties, is considered to be one of the most promising candidates for power devices including field-effect transistors (FETs) and for other applications such as Schottky barrier diodes (SBDs) and solar-blind ultraviolet photodetectors. Ohmic contact is one of the key steps in the β-Ga2O3 device fabrication process for power applications. Ohmic contact techniques have been developed in recent years, and they are summarized in this review. First, the basic theory of metal–semiconductor contact is introduced. After that, the representative literature related to Ohmic contact with β-Ga2O3 is summarized and analyzed, including the electrical properties, interface microstructure, Ohmic contact formation mechanism, and contact reliability. In addition, the promising alternative schemes, including novel annealing techniques and Au-free contact materials, which are compatible with the CMOS process, are discussed. This review will help our theoretical understanding of Ohmic contact in β-Ga2O3 devices as well as the development trends of Ohmic contact schemes.

ZrTe2 Compound Dirac Semimetal Contacts for High-Performance MoS2 Transistors.

Recent investigations reveal elemental semimetal (Bi and Sb) contacts fabricated with conventional deposition processes exhibit a remarkable capacity of approaching the quantum limit in two-dimensional (2D) semiconductor contacts, implying it might be an optimal option to solve the contact issue of 2D semiconductor electronics. Here, we demonstrate novel compound Dirac semimetal ZrTe2 contacts to MoS2 constructed by a nondestructive van der Waals (vdW) transfer process, exhibiting excellent ohmic contact characteristics with a negligible Schottky barrier. The band hybridization between ZrTe2 and MoS2 was verified. The bilayer MoS2 transistor with a 250 nm channel length on a 20 nm thick hexagonal boron nitride (h-BN) exhibits an ION/IOFF current ratio over 105 and an on-state current of 259 μA μm-1. The current results reveal that 2D compound semimetals with vdW contacts can offer a diverse selection of proper semimetals with adjustable work functions for the next-generation 2D-based beyond-silicon electronics.

Fermi level depinning via insertion of a graphene buffer layer at the gold–2D tin monoxide contact

Two-dimensional (2D) tin monoxide (SnO) has attracted much attention owing to its distinctive electronic and optical properties, which render itself suitable as a channel material in field effect transistors (FETs). However, upon contact with metals for such applications, the Fermi level pinning effect may occur, where states are induced in its band gap by the metal, hindering its intrinsic semiconducting properties. We propose the insertion of graphene at the contact interface to alleviate the metal-induced gap states. By using gold (Au) as the electrode material and monolayer SnO (mSnO) as the channel material, the geometry, bonding strength, charge transfer and tunnel barriers of charges, and electronic properties including the work function, band structure, density of states, and Schottky barriers are thoroughly investigated using first-principles calculations for the structures with and without graphene to reveal the contact behaviours and Fermi level depinning mechanism. It has been demonstrated that strong covalent bonding is formed between gold and mSnO, while the graphene interlayer forms weak van der Waals interaction with both materials, which minimises the perturbance to the band structure of mSnO. The effects of out-of-plane compression are also analysed to assess the performance of the contact under mechanical deformation, and a feasible fabrication route for the heterostructure with graphene is proposed. This work systematically explores the properties of the Au–mSnO contact for applications in FETs and provides thorough guidance for future exploitation of 2D materials in various electronic applications and for selection of buffer layers to improve metal–semiconductor contact.

A new line tunneling SiGe/Si iTFET with control gate for leakage suppression and subthreshold swing improvement

This article presents a new line tunneling dominating metal–semiconductor contact-induced SiGe–Si tunnel field-effect transistor with control gate (CG-Line SiGe/Si iTFET). With a structure where two symmetrical control gates at the drain region are given a sufficient negative bias, the overlap of the energy bands at the drain in the OFF-state is effectively suppressed, thus reducing the tunneling probability and significantly decreasing leakage current. Additionally, the large overlap area between the source and gate improves the gate’s ability to control the tunneling interface effectively, improving the ON-state current and subthreshold swing characteristics. By using the Schottky contact characteristics of a metal–semiconductor contact with different work functions to form a PN junction, the need to control doping profiles or random doping fluctuations is avoided. Furthermore, as ion implantation is not required, issues related to subsequent annealing are also eliminated, greatly reducing thermal budget. Due to the different material bandgap characteristics selected for the source and drain regions, the probability of overlap of the energy bands in the source region in the ON-state is increased and that in the drain region in the OFF-state is reduced. Based on the feasibility of the actual fabrication process and through rigorous 2D simulation studies, improvements in subthreshold swing and high on/off current ratio can be achieved simultaneously based on the proposed device structure. Additionally, the presence of the control gate structure effectively suppresses leakage current, further enhancing its potential for low-power-consumption applications.

Ionic accelerator based on metal–semiconductor contact for fast electrode kinetics and durable Zn-metal anode

The development of Zn metal anodes with high reversibility and fast kinetics is critical for achieving high-performance aqueous Zn-metal batteries. Herein, we report a Zn@SiC electrode with an Ohmic contact that exhibits excellent durability and enhanced electrode process kinetics. The Ohmic contact barrier (Φb) inhibited electron transfer from the metal to the SiC passivation layer and effectively regulated Zn deposition under the passivation layer, thus suppressing dendrite growth. The built-in potential (Vbi) accelerated the Zn transfer and reaction kinetics and decreased the nucleation energy, thereby enhancing the charge–discharge platform potential (∼40 mV) in full cells. Coupled with the good anti-side reaction ability of the SiC layer, the Zn@SiC electrode successfully delivered a highly reversible stripping/plating process after 3770 h under a 1.0 mA cm−2 current density with a low overpotential of 33.7 mV. Furthermore, by analyzing the energy band structure of the Ohmic contacts, the critical current density was quantitatively determined. This clarified the relationship between the deposition position and the current density, thus providing a standard for practical semiconductor design.