Low Resistance Ohmic Contacts Research Articles

Low Resistance Contact to P-Type Monolayer WSe2.

Advanced microelectronics in the future may require semiconducting channel materials beyond silicon. Two-dimensional (2D) semiconductors, with their atomically thin thickness, hold great promise for future electronic devices. One challenge to achieving high-performance 2D semiconductor field effect transistors (FET) is the high contact resistance at the metal-semiconductor interface. In this study, we develop a charge-transfer doping strategy with WSe2/α-RuCl3 heterostructures to achieve low-resistance ohmic contact for p-type monolayer WSe2 transistors. We show that hole doping as high as 3 × 1013 cm-2 can be achieved in the WSe2/α-RuCl3 heterostructure due to its type-III band alignment, resulting in an ohmic contact with resistance of 4 kΩ μm. Based on that, we demonstrate p-type WSe2 transistors with an on-current of 35 μA·μm-1 and an ION/IOFF ratio exceeding 109 at room temperature.

Low-Power Transistors with Ideal p-type Ohmic Contacts Based on VS2/WSe2 van der Waals Heterostructures.

Achieving low-resistance Ohmic contacts with a vanishing Schottky barrier is crucial for enhancing the performance of two-dimensional (2D) field-effect transistors (FETs). In this paper, we present a theoretical investigation of VS2/WSe2-vdWHs-FETs with a gate length (Lg) in the range of 1-5 nm, using ab initio quantum transport simulations. The results show that a very low hole Schottky barrier height (-0.01 eV) can be achieved with perfect band offsets and reduced metal-induced gap states (MIGS), indicating the formation of p-type Ohmic contacts. Additionally, these FETs also exhibit an impressive low subthreshold swing (SS) (69 mV/dec) and high Ion/Ioff (>107) with an appropriate underlap (UL) structure consisting of pristine WSe2. Furthermore, even when the Lg is scaled down to 3 nm, the device can still meet the low-power (LP) requirements of the International Technology Roadmap for Semiconductors (ITRS) by controlling the UL. Consequently, this study provides valuable insights for the future development of LP 2D FETs.

Al‐Rich AlGaN Transistors with Regrown p‐AlGaN Gate Layers and Ohmic Contacts

AbstractEpitaxial regrowth processes are presented for achieving Al‐rich aluminum gallium nitride (AlGaN) high electron mobility transistor (HEMTs) with p‐type gates with large, positive threshold voltage for enhancement mode operation and low resistance Ohmic contacts. Utilizing a deep gate recess etch into the channel and an epitaxial regrown p‐AlGaN gate structure, an Al0.85Ga0.15N barrier/Al0.50Ga0.50N channel HEMT with a large positive threshold voltage (VTH = +3.5 V) and negligible gate leakage is demonstrated. Epitaxial regrowth of AlGaN avoids the use of gate insulators which can suffer from charge trapping effects observed in typical dielectric layers deposited on AlGaN. Low resistance Ohmic contacts (minimum specific contact resistance = 4 × 10−6 Ω cm2, average = 1.8 × 10−4 Ω cm2) are demonstrated in an Al0.85Ga0.15N barrier/Al0.68Ga0.32N channel HEMT by employing epitaxial regrowth of a heavily doped, n‐type, reverse compositionally graded epitaxial structure. The combination of low‐leakage, large positive threshold p‐gates and low resistance Ohmic contacts by the described regrowth processes provide a pathway to realizing high‐current, enhancement‐mode, Al‐rich AlGaN‐based ultra‐wide bandgap transistors.

Establishing Ohmic Contact of a Radial Compressed CNT Bundle with High Work Function Metal.

Establishing low-resistance ohmic contact is critical for developing electronic devices based on traditional silicon and new low-dimensional materials. Due to unprecedented electronic and mechanical properties, the one-dimensional carbon nanotubes (CNTs) have been used as source/drain, gate, or tunnel to fabricate transistors. However, the mechanism causing low-resistance ohmic contact is not clear yet. Here, the hybrid atomic force microscopy-scanning electron microscopy (AFM-SEM) instrument was developed to establish lower-resistance ohmic contact between a radial compressed deformed multiwalled CNT bundle and high work function metal (platinum and gold). The radial compression structure under strong van der Waals attraction was in situ characterized through the SEM image to obtain the diameter and width and through AFM to get height and to perform nanoindentation, indicating that Pt has the smaller radial compression deformation. Molecular dynamics simulations exhibit that compared to Pt, a wider ribbon-like graphene layer formed when the radial compressed CNTs contacted with Au. The bond forming and electron orbital overlapping between C atoms of deformed CNTs and the high work function metal atom is beneficial for good electrical contact.

Structural and electrical investigation of Al/Ti/TiN/Au based N-face n-GaN contact stack

In this work, the structure of Ti/Al/TiN/Au contact layer stack on the N-face of a single-crystal n-GaN substrate is studied after heat treatment at 750 °C. Since TiN is widely regarded as a diffusion barrier in the stack, the formed structures with three different initial TiN thicknesses (15, 60 and 90 nm) in the contact layers are investigated in detail. The primary tool used for the structural investigations was a (scanning) transmission electron microscope ((S)TEM). In all three samples a low resistivity ohmic contact was formed. However, the TiN layer has not completely blocked the diffusion for any of the samples and both Al and Au diffused through the TiN layer at the temperature of 750 °C. As a result of the heat treatment, a complex AlN/TiN/Au2Al/TiN/Au + Al2.67O4 stack was formed on the surface of the GaN substrate. The measurements have not shown the often reported Ti–Al alloy phase in the samples. The formation of the AlN and TiN layers can be explained by the separation of N from GaN creating N vacancies in the GaN substrate which could help the formation of the ohmic behavior.

Unraveling the Transformation from Type-II to Z-Scheme in Perovskite-Based Heterostructures for Enhanced Photocatalytic CO2 Reduction.

The ability to create perovskite-based heterostructures with desirable charge transfer characteristics represents an important endeavor to render a set of perovskite materials and devices with tunable optoelectronic properties. However, due to similar material selection and band alignment in type-II and Z-scheme heterostructures, it remains challenging to obtain perovskite-based heterostructures with a favorable electron transfer pathway for photocatalysis. Herein, we report a robust tailoring of effective charge transfer pathway in perovskite-based heterostructures via a type-II to Z-scheme transformation for highly efficient and selective photocatalytic CO2 reduction. Specifically, CsPbBr3/TiO2 and CsPbBr3/Au/TiO2 heterostructures are synthesized and then investigated by ultrafast spectroscopy. Moreover, taking CsPbBr3/TiO2 and CsPbBr3/Au/TiO2 as examples, operando experiments and theoretical calculations confirm that the type-II heterostructure could be readily transformed into a Z-scheme heterostructure through establishing a low-resistance Ohmic contact, which indicates that a fast electron transfer pathway is crucial in Z-scheme construction, as further demonstrated by CsPbBr3/Ag/TiO2 and CsPbBr3/MoS2 heterostructures. In contrast to pristine CsPbBr3 and CsPbBr3/TiO2, the CsPbBr3/Au/TiO2 heterostructure exhibits 5.4- and 3.0-fold enhancement of electron consumption rate in photocatalytic CO2 reduction. DFT calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy unveil that the superior CO selectivity is attributed to the lower energy of *CO desorption than that of hydrogenation to *HCO. This meticulous design sheds light on the modification of perovskite-based multifunctional materials and enlightens conscious optimization of semiconductor-based heterostructures with desirable charge transfer for catalysis and optoelectronic applications.

Evolution of surface microstructure of Re‐Al‐Ni‐Au based ohmic contacts on n‐type GaN

AbstractRecently, rhenium (Re) based ohmic contacts to GaN have been studied for their low resistivity, smooth surface morphology, and sharp edge acuity at low annealing temperatures. In this work, we discuss the evolution of surface microstructures for Re‐Al‐Ni‐Au ohmic contacts on n‐GaN as a function of Re layer thickness and annealing temperature. For all Re thicknesses, the Al and Ni segregate into agglomerates that increase in size with increasing annealing temperature. These agglomerates are surrounded by Al‐Au films. Along with the underlying Re layer, they form different crystallographic phases of Re‐Al‐Ni, Al6Re, AlAu2, and Al2Au5. This, along with the formation of Re‐N phases at the metal‐semiconductor interface leads to low resistivity ohmic contacts on n‐GaN. Investigating the evolution of the contact microstructure is an important step in understanding the behavior of the Re‐based ohmic contact system.

Low resistance ohmic contact of multi-metallic Mo/Al/Au stack with ultra-wide bandgap Ga2O3 thin film with post-annealing and its in-depth interface studies for next-generation high-power devices

Ultra-wide band gap materials like β-Ga2O3 is considered as a potential candidate for power devices owing to their high breakdown voltage in contrast to the conventional semiconductors. The performance and the figure-of-merit for power devices critically depend on resistance offered by ohmic contact of metal with ultra-wide gap materials. However, high-quality ohmic contacts with Ga2O3 presents a significant challenge due to Fermi-level pinning and formation of Schottky barriers for the majority of metal contacts. Here, we investigated the electrical characteristics and interfacial reactions of Au/Al/Mo metal stack with β-Ga2O3 thin film at various annealing temperatures to demonstrate the ohmic contact formation mechanism. In-depth surface-sensitive XPS and UPS are employed to study the interface of metallic stack/Ga2O3 thin film. Our results indicate that post-annealing at high temperatures led to the intermixing of the metallic stack and results in the formation of binary intermetallic compounds of Al and Au. The intermetallic compound diffuses into the Ga2O3 layer and resulting in the creation of an oxygen-deficiency near the interface and helping to achieve contact resistance of 7.15 × 10−6 Ω cm2. This study establishes a robust foundation for the expanded utilization of Ga2O3 in power electronics and optoelectronics devices, emphasizing the vital role of interface engineering.

Electronic properties, interface contact and transport properties of strain-modulated MS2/borophosphene and MSeS/borophosphene (M = Cr, Mo, W) heterostructure: Insights from first-principles

Controlling the interface contact performance to form low-resistance ohmic contact is of great importance in designing low-power two-dimensional (2D) devices. Here, we explore a 2D semi-metal with Dirac cones, borophosphene, for different types of contact with MS2, MSeS/borophosphene (M = Cr, Mo, W) using density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. The results indicate that under no strain, different van der Waals heterojunctions (vdWHs) exhibit an n-type (p-type) Schottky contact type, with the CrS2/borophosphene vdWHs exhibiting a lower barrier height (Φn = 0.08 eV). Next, we find that the in-plane strain is more efficient than the vertical strain. Under in-plane strain modulation, the contact type will change from Schottky to Ohmic contacts as the tensile strain increases. Similarly, the contact type will change from Schottky to semiconductor as the compressive strain increases. Under the influence of vertical strain, the CrS2/borophosphene vdWHs transition into ohmic contact only when the interlayer spacing increases to above 0.4 Å. Finally, we study the ohmic contact formation and transport characteristics based on the device angle. These findings demonstrate that interface contact types can be effectively regulated by strain, which is crucial for the design and manufacturing of novel nano electronic devices composed of borophosphene based vdWHs.

Transition from Schottky to Ohmic contacts in 2D Ge/GaAs heterostructures with high tunneling probability.

The metal-semiconductor (M-S) contact is usually an Ohmic contact or a Schottky contact, which greatly affects the electronic properties of devices, and it remains a huge challenge to realize a low-resistance Ohmic contact in a metal-semiconductor junction (MSJ). Herein, we systematically studied the band structures, electrostatic potential, charge transfer, Schottky barrier height of carriers, effective carrier masses, and tunneling probability of carriers of a germanene (Ge)/GaAs MSJ. The transition from the Schottky to the Ohmic contact can be caused by applying certain biaxial strains or electric fields, which weakens the Fermi level pinning (FLP) effect and reduces contact resistance. Meanwhile, the electron injection efficiency of Ge/(GaAs)As MSJ (PTB > 27%) is far superior to that of other two-dimensional (2D) vdW MSJs. This work indicates that Ge/GaAs heterostructures are the most compatible for applying high-effective 2D electronic nanodevices under controllable conditions.

Pulsed sputtering selective epitaxial formation of highly degenerate n-type GaN ohmic contacts for GaN HEMT applications

This study describes the selective formation process of highly degenerate n-type GaN (d-GaN) ohmic contacts for the source and drain regions of GaN high electron mobility transistors (HEMTs) using pulsed sputtering deposition (PSD). The selective formation process using SiO2 masks and PSD epitaxial growth enabled the uniform formation of d-GaN in micron-meter size. The optimally formed d-GaN exhibited a minimum resistivity as low as 0.16 mΩ·cm, an electron concentration of 3.6 × 1020 cm−3, and a mobility of 108 cm2 V−1 s−1. Transmission-line-method measurements demonstrated that the contact resistance of GaN HEMTs with d-GaN regrowth contacts was remarkably low at 0.28 Ω·mm, leading to the reasonable DC output characteristics with an on-resistance of 2.8 Ω·mm and a maximum current density of 850 mA mm−1. These findings suggest that PSD epitaxial regrowth of d-GaN is a promising approach for the high-throughput formation of low-resistivity ohmic contacts on large-area GaN HEMT wafers.

Ohmic contact in two-dimensional WS2/Hf2CX2 (X = F/OH) and WS2/graphene/Hf2C heterojunctions

Low-resistance Ohmic contacts are the key to improving the performance of electronic devices. By performing ab initio electronic calculations and quantum transport simulations, we systematically investigate the contact types of the original WS2/Hf2C, the functionalized WS2/Hf2CX2 (X = F/OH), and the graphene-doped WS2/graphene/Hf2C heterojunctions. The obtained results suggest that the strong interfacial coupling occurs in the WS2/Hf2C system, leading to the metallization of the WS2 layer. Functionalization of the Hf2C layer can significantly weaken the interlayer interactions. Thus, the semiconductor characteristics of the WS2 layer can be maintained in the WS2/Hf2CX2 (X = F/OH) and WS2/graphene/Hf2C heterojunctions. Stable Ohmic contacts are formed in these heterojunctions. The heights of the vertical electron Schottky barrier are −0.13 eV, −0.43 eV, and −0.12 eV for WS2/Hf2CF2, WS2/Hf2C(OH)2, and WS2/graphene/Hf2C, respectively. Based on these heterojunctions, we construct a two-probe field-effect transistor model and explore its quantum transport properties. In the lateral direction, all systems form n-type Schottky contacts. This investigation provides a basis for the selection of electrode materials based on WS2 devices.

Tunneling current through non-alloyed metal/heavily-doped SiC interfaces

In this paper, tunneling phenomena at metal/heavily-doped SiC Schottky (non-sintered) interfaces are reviewed, and a design guideline for low-resistance non-alloyed SiC ohmic contacts is proposed. Carrier transport at Schottky contacts on heavily-doped SiC epitaxial layers is quantitatively described by direct tunneling (DT) including both of the thermionic field emission (TFE) and field emission (FE) when the doping density (Nd) is higher than mid-1017cm−3. As for p-type SiC, tunneling of holes in the split-off band is the dominant conduction mechanism due to its light effective mass (0.21m0). Phosphorus ion (P+) implantation makes the interface tunneling current larger by several orders of magnitude compared with that at contacts on epitaxial layers with almost the same Nd, which is plausibly explained by trap-assisted tunneling (TAT) through implantation-induced defect levels. The contact resistivity (ρc) at non-alloyed ohmic contacts formed on heavily P+-implanted SiC (Nd>4×1018cm−3) is significantly reduced thanks to the contribution of TAT especially when Nd is lower than about 1020cm−3, while ρc decreases with increasing Nd according to the DT theory in a higher Nd range (>1020cm−3). At a very high Nd of 2×1020cm−3, an extremely low ρc of 1–2×10−7Ωcm2 is demonstrated for non-alloyed Mg and Ti contacts. Through the experiment and calculation of tunneling current, a model to predict ρc at non-alloyed ohmic contacts taking account of the contributions of TAT and DT is proposed, and a design guideline for low-resistance non-alloyed ohmic contacts is presented as follows; a very low ρc (<10−6Ωcm2) is achievable without sintering by lowering the interface barrier height (≤1eV) and utilizing high-dose ion implantation (≥3×1019cm−3).

Effect of strain on electronic properties of tri-layer MoS2/h-BN/graphene van der Waals heterostructures

The two-dimensional materials have grabbed the attention of researchers because of their high functionality and widely controllable properties. Due to the dimensionality reduction, the external factors have a significant influence on their properties. The density functional theory calculations have been performed on the heterostructures of single-layer graphene (G), boron nitride (BN) and molybdenum disulfide (MoS2) to investigate the alteration in their electronic properties. Our calculations predict that electronic properties such as bandgap, work function, Schottky barrier height and tunnelling barrier probability can be modulated by applying mechanical strain. The application of strain resulted in a band gap opening of about 1 eV in trilayer heterostructures. At low values of strain, heterostructures maintained a low resistance ohmic contact. At −9% strain, the p-to-n-type transition of Schottky contact was observed indicating the flow of electrons from semiconductor to metal. The MoS2 monolayer was found to be more electron deficient and exhibited a greater charge accumulation. MoS2/BN/G heterostructure can be used for the fabrication of capacitive devices. Whereas BN/G/MoS2 and BN/MoS2/G heterostructures can act as potential candidates for trilayer Schottky devices due to the presence of low-barrier metal-semiconductor junction.

Physical mechanism of oxygen diffusion in the formation of Ga2O3 Ohmic contacts

The formation of low-resistance Ohmic contacts in Ga2O3 is crucial for high-performance electronic devices. Conventionally, a titanium/gold (Ti/Au) electrode is rapidly annealed to achieve Ohmic contacts, resulting in mutual diffusion of atoms at the interface. However, the specific role of diffusing elements in Ohmic contact formation remains unclear. In this work, we investigate the contribution of oxygen atom diffusion to the formation of Ohmic contacts in Ga2O3. We prepare a Ti/Au electrode on a single crystal substrate and conduct a series of electrical and structural characterizations. Using density functional theory, we construct a model of the interface and calculate the charge density, partial density of states, planar electrostatic potential energy, and I–V characteristics. Our results demonstrate that the oxygen atom diffusion effectively reduces the interface barrier, leading to low-resistance Ohmic contacts in Ga2O3. These findings provide valuable insights into the underlying mechanisms of Ohmic contact formation and highlight the importance of considering the oxygen atom diffusion in the design of Ga2O3-based electronic devices.

Low Ohmic contact resistance and high on/off ratio in transition metal dichalcogenides field-effect transistors via residue-free transfer.

Beyond-silicon technology demands ultrahigh performance field-effect transistors. Transition metal dichalcogenides provide an ideal material platform, but the device performances such as the contact resistance, on/off ratio and mobility are often limited by the presence of interfacial residues caused by transfer procedures. Here, we show an ideal residue-free transfer approach using polypropylene carbonate with a negligible residue coverage of ~0.08% for monolayer MoS2 at the centimetre scale. By incorporating a bismuth semimetal contact with an atomically clean monolayer MoS2 field-effect transistor on hexagonal boron nitride substrate, we obtain an ultralow Ohmic contact resistance of ~78 Ω µm, approaching the quantum limit, and a record-high on/off ratio of ~1011 at 15 K. Such an ultra-clean fabrication approach could be the ideal platform for high-performance electrical devices using large-area semiconducting transition metal dichalcogenides.

Structural investigation of ultra-low resistance deeply recessed sidewall ohmic contacts for AlGaN/GaN HEMTs based on Ti/Al/Ti-metallization

This study presents a novel approach to forming low-resistance ohmic contacts for AlGaN/GaN HEMTs. The optimized contacts exhibit an outstanding contact resistance of approximately 0.15 Ω·mm. This is achieved by firstly recessing the barrier of the heterostructure to a depth beyond the channel. In this way, the channel region is exposed on the sidewall of the recess. The coverage of the Ti/Al/Ti ohmic metalization on the sidewall is ensured through tilting of the sample during evaporation. The annealing process is performed at a low temperature of 550 °C. The approach does not require precise control of the recess etching. Furthermore, the method is directly applicable to most barrier designs in terms of thickness and Al-concentration. The impact of recessed sidewall angle, thickness and ratio of Ti and Al layers, and the annealing procedure are investigated. Structural and chemical analyses of the interface between the ohmic contacts and epi-structure indicate the formation of ohmic contacts by the extraction of nitrogen from the epi-structure. The approach is demonstrated on HEMT-structures with two different barrier designs in terms of Al-concentration and barrier thickness. The study demonstrate large process window in regard to recess depth and duration of the annealing as well as high uniformity of the contact resistance across the samples, rendering the approach highly suitable for industrial production processes.

Ultrathin stable Ohmic contacts for high-temperature operation of β-Ga2O3 devices

Beta gallium oxide (β-Ga2O3) shows significant promise in high-temperature, high-power, and sensing electronics applications. However, long-term stable metallization layers for Ohmic contacts at high temperatures present unique thermodynamic challenges. The current most common Ohmic contact design based on 20 nm of Ti has been repeatedly demonstrated to fail at even moderately elevated temperatures (300–400 °C) due to a combination of nonstoichiometric Ti/Ga2O3 interfacial reactions and kinetically favored Ti diffusion processes. Here, we demonstrate stable Ohmic contacts for Ga2O3 devices operating up to 500–600 °C using ultrathin Ti layers with a self-limiting interfacial reaction. The ultrathin Ti layer in the 5 nm Ti/100 nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability. This novel contact design strategy results in an epitaxial conductive anatase titanium oxide interface layer that enables low-resistance Ohmic contacts that are stable both under long-term continuous operation (&amp;gt;500 h) at 600 °C in vacuum (≤10−4 Torr), as well as after repeated thermal cycling (15 times) between room temperature and 550 °C in flowing N2. This stable Ohmic contact design will accelerate the development of high-temperature devices by enabling research focus to shift toward rectifying interfaces and other interfacial layers.

Enhanced Performances of Quantum Dot Light-Emitting Diodes with an Organic–Inorganic Hybrid Hole Injection Layer

PEDOT:PSS (polyethylene dioxythiophene:polystyrenesulfonate) is a commonly used hole injection layer (HIL) in optoelectronic devices due to its high conductive properties and work function. However, the acidic and hygroscopic nature of PEDOT:PSS can be problematic for device stability over time. To address this issue, in this study we demonstrated the potential of an organic–inorganic hybrid HIL by incorporating solution-processed WOx nanoparticles (WOx NPs) into the PEDOT:PSS mixture. This hybrid solution was found to have a superior hole transport ability and low Ohmic contact resistance contributing to higher brightness (~62,000 cd m−2) and current efficiency (13.1 cd A−1) in the manufactured quantum-dot-based light-emitting diodes (QLEDs). In addition, the resulting devices achieved a relative operational lifetime of 7071 h, or approximately twice that of traditional QLEDs with PEDOT:PSS HILs. The proposed method is an uncomplicated, reliable, and low-cost way to achieve long operational lifetimes without sacrificing efficiency in optoelectronic devices.

Ge-ion implantation and activation in (100) β-Ga2O3 for ohmic contact improvement using pulsed rapid thermal annealing

In this work, we analyze the optimum annealing conditions for the activation of Ge-implanted β-Ga2O3 in order to reach low ohmic contact resistances. The experiments involved the use of a pulsed rapid thermal annealing treatment at temperatures between 900 and 1200 °C in nitrogen atmosphere. Our investigations show remarkable changes in the surface morphology involving increased surface roughness after high-temperature annealing above 1000 °C as well as a significant redistribution of the implanted Ge. Nevertheless, the specific contact resistance is strongly reduced by one order of magnitude after annealing at 1100 °C, reaching a record value of 4.8 × 10−7 Ω cm2 at an implantation activation efficiency of 14.2%. The highest activation efficiency of 19.2% and lowest sheet resistances were reached upon annealing at 1200 °C, which, in turn, showed inferior ohmic contact properties due to a severe increase of the surface roughness. Our results verify the high potential of applying high-temperature annealing processes above 1000 °C after Ge implantation for reaching low ohmic contact resistances to β-Ga2O3.