Low Resistance Ohmic Contacts Research Articles

High Performance CMOS-Compatible RF GaN-on-Silicon HEMTs With Low-Resistive and Highly-Conformal Ohmic Contacts

High Performance CMOS-Compatible RF GaN-on-Silicon HEMTs With Low-Resistive and Highly-Conformal Ohmic Contacts

Ohmic Contact Resistance in SiC Diodes with Ti and NiSi P<sup>+</sup> Contacts

Ohmic contacts play a major role in the signal transfer between the semiconductor device and the external circuitry. One of the main technological issues to develop high-performance SiC-based devices is the control of metal/SiC contact properties to fabricate low resistance and high stability SiC Ohmic contacts to p-type SiC. This is mostly due to intrinsic SiC characteristics like large work function, low dopant activation for p-type materials and low hole mobility. These limits are even more emphasized in SiC JBS or MPS diodes, where Schottky and Ohmic contacts on the P doped regions embedded in the active area to improve surge ruggedness are usually formed by using the same metallization process. This naturally results either in a high Schottky barrier height in the Schottky contact with consequent increase of the conduction loss at low currents or in a poorly conductive Ohmic contact, leading to reduced IFSM capability. Therefore, the optimization and control of the process parameters like for example the P+ doping concentration peak underneath the metallization layer and the annealing process temperatures is crucial to obtain a good Ohmic contact and enhance the device´s robustness against surge current.

Si-implantation for Low Ohmic Contact Resistances in RF GaN HEMTs

Abstract In this work, Si implantation and activation for lowering the ohmic contact resistance (Rc) of mm-wave GaN HEMTs has been investigated. Various combinations of annealing temperature/duration and implantation doses were tested. Dopant activation was performed using a modified procedure in an MOCVD tool, involving fast temperature ramping and annealing the samples for 8 minutes at 1150 °C. Thereby, ~ 0.02 ± 0.01 Ω·mm contact resistance was achieved on a fully doped region and ~ 0.1 ± 0.02 Ω·mm when only the source and drain contact region was n-type doped. For comparison, a well-established alloyed Ti/Al/Ni/Au ohmic contact scheme without implantation, was used as reference resulting in an average Rc ~ 0.34 ± 0.12 Ω·mm on the same wafer. Besides the three times lowered contact resistance the implanted contacts also showed a significantly improved on-wafer homogeneity.

Large-area and few-layered 1T′-MoTe2 thin films grown by cold-wall chemical vapor deposition

This study employs cold-wall chemical vapor deposition to achieve the growth of MoTe2 thin films on 4-inch sapphire substrates. A two-step growth process is utilized, incorporating MoO3 and Te powder sources under low-pressure conditions to synthesize MoTe2. The resultant MoTe2 thin films exhibit a dominant 1T′ phase, as evidenced by a prominent Raman peak at 161 cm−1. This preferential 1T′ phase formation is attributed to controlled manipulation of the second-step growth temperature, essentially the reaction stage between Te vapor and the pre-deposited MoO x layer. Under these optimized growth conditions, the thickness of the continuous 1T′-MoTe2 films can be precisely tailored within the range of 3.5–5.7 nm (equivalent to 5–8 layers), as determined by atomic force microscopy depth profiling. Hall-effect measurements unveil a typical hole concentration and mobility of 0.2 cm2 Vs−1 and 7.9 × 1021 cm−3, respectively, for the synthesized few-layered 1T′-MoTe2 films. Furthermore, Ti/Al bilayer metal contacts deposited on the few-layered 1T′-MoTe2 films exhibit low specific contact resistances of approximately 1.0 × 10−4 Ω cm2 estimated by the transfer length model. This finding suggests a viable approach for achieving low ohmic contact resistance using the 1T′-MoTe2 intermediate layer between metallic electrodes and two-dimensional semiconductors.

InAlGaN-based HEMT with very low Ohmic contact resistance regrown at 850 °C by MOVPE

Regrown Ohmic contacts have been widely studied for high millimeter-wave applications. However, few were applied to InAl(Ga)N-based HEMT despite the lattice match benefits with GaN channel because of the poor thermal stability of the quaternary barrier. In this article, we use relatively low temperature (850 °C) MOVPE technique for the regrowth of heavily Si doped GaN (1 × 1020 cm−3) to avoid deterioration of the channel's electrical characteristics. Moreover, high selectivity of the regrowth is obtained, thanks to large opening ratio of the hard mask. The state-of-the-art total Ohmic contact resistance Rc = 0.06 Ω mm is reached with high homogeneity on a 4-in. wafer. This result reflects the combined contribution of the doped GaN interfaces with both the metal contact and the channel. 2 × 50 μm transistors featuring 100 nm gate length with regrown Ohmic contacts present remarkable improvements of DC and RF performances. At 40 GHz, the highest Power Added Efficiency (PAE) of 54% is attained at an output power of 3.8 W mm−1, while the maximum output power of 9.4 W mm−1 is achieved at a PAE of 48%, corresponding to bias voltages of 15 and 30 V, respectively.

Research on the robust electrical contact properties and favorable transport characteristics of two-dimensional WB4/MoSi2N4 van der Waals heterostructures

Ensuring optimal interface contact performance is crucial for achieving low-resistance ohmic contact in the design of two-dimensional (2D) electronic devices. In this study, we systematically investigate the contact properties of van der Waals (vdW) heterojunctions formed by compositing the semi-metal WB4 with Dirac cones and the semiconductor MoSi2N4 through first-principles calculations. A thorough investigation has been carried out on the electronic structure and transport characteristics at the interface. Our research findings indicate that the WB4/MoSi2N4 combination demonstrates robust ohmic contact performance, maintaining ohmic or near-ohmic p-type Schottky contacts over a wide range when exposed to electric field, varying layer spacing, and biaxial strain. This unique characteristic is associated with the Fermi level pinning (FLP), where there are gap states present at the interface. Here, the FLP plays a positive role in maintaining ohmic contact. In addition, different derivatives of MoSi2N4 are combined with WB4 to create heterostructures and study the changes in contact performance. Our research shows that the robust ohmic contact of WB4/MoSi2N4 will have excellent prospects for application in future electronic devices.

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.

Low-resistivity Ohmic contacts of Ti/Al on few-layered 1T'-MoTe2/2H-MoTe2 heterojunctions grown by chemical vapor deposition.

This study explores the phase-controlled growth of few-layered 2H-MoTe2, 1T'-MoTe2, and 2H-/1T'-MoTe2 heterostructures and their impacts on metal contact properties. Cold-wall chemical vapor deposition (CW-CVD) with varying growth rates of MoOx and reaction temperatures with Te vapors enabled the growth of continuous thin films of either 1T'-MoTe2 or 2H-MoTe2 phases on two-inch sapphire substrates. This methodology facilitates the meticulous optimization of chemical vapor deposition (CVD) parameters, enabling the realization of phase-controlled growth of few-layered MoTe2 thin films and their subsequent heterostructures. The study further investigates the influence of a 1T'-MoTe2 intermediate layer on the electrical properties of metal contacts on few-layered 2H-MoTe2. Bi-layer Ti/Al contacts directly deposited on 2H-MoTe2 exhibited Schottky behavior, indicating inefficient carrier transport. However, introducing a few-layered 1T'-MoTe2 intermediate layer between the metal and 2H-MoTe2 layers improved the contact characteristics significantly. The resulting Al/Ti/1T'-MoTe2/2H-MoTe2 contact scheme demonstrates Ohmic behavior with a specific contact resistance of around 1.7 × 10-4 Ω cm2. This substantial improvement is attributed to the high carrier concentration of the 1T'-MoTe2 intermediate layer which could be attributed tentatively to the increased tunneling events across the van der Waals gap and enhancing carrier transport between the metal and 2H-MoTe2.

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).