Ohmic Contact Performance Research Articles

Ohmic Contact Formation to β-Ga2O3 Nanosheet Transistors with Ar-Containing Plasma Treatment

Effective Ohmic contact between metals and their conductive channels is a crucial step in developing high-performance Ga2O3-based transistors. Distinct from bulk materials, excess thermal energy of the annealing process can destroy the low-dimensional material itself. Given the thermal budget concern, a feasible and moderate solution (i.e., Ar-containing plasma treatment) is proposed to achieve effective Ohmic junctions with (100) β-Ga2O3 nanosheets. The impact of four kinds of plasma treatments (i.e., gas mixtures SF6/Ar, SF6/O2/Ar, SF6/O2, and Ar) on (100) β-Ga2O3 crystals is comparatively studied by X-ray photoemission spectroscopy for the first time. With the optimal plasma pre-treatment (i.e., Ar plasma, 100 W, 60 s), the resulting β-Ga2O3 nanosheet field-effect transistors (FETs) show effective Ohmic contact (i.e., contact resistance RC of 104 Ω·mm) without any post-annealing, which leads to competitive device performance such as a high current on/off ratio (>107), a low subthreshold swing (SS, 249 mV/dec), and acceptable field-effect mobility (μeff, ~21.73 cm2 V−1 s−1). By using heavily doped β-Ga2O3 crystals (Ne, ~1020 cm−3) for Ar plasma treatments, the contact resistance RC can be further decreased to 5.2 Ω·mm. This work opens up new opportunities to enhance the Ohmic contact performance of low-dimensional Ga2O3-based transistors and can further benefit other oxide-based nanodevices.

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 TiAl3/Au ohmic contact and enhanced performance on AlGaN/GaN HEMT

The advent of an era characterized by intelligent, interconnected systems of AlGaN/GaN High Electron Mobility Transistors (HEMTs) electronic devices is underway. Within this context, the performance of ohmic contacts in HEMTs bears substantive influence on the electrical characteristics of power devices, notably impacting parameters such as on-resistance and output current. Therefore, the improvement of ohmic contacts attributes in AlGaN/GaN HEMT devices has perennially constituted a focal point of considerable industry. Based on the AlGaN/GaN heterostructure, a novel TiAl3/Au ohmic contact has been successfully developed for HEMTs. A low contact resistance of 0.23 Ω·mm (2.33 × 10-6 Ω·cm2) is obtained, which proposes a new interface contact for AlGaN/GaN heterojunction. Transmission electron microscope (TEM) illustrates that the contact mechanism for low resistance is direct contact with two-dimensional electron gas (2DEG) through the substitution of the TiN compound with a smaller area Au dominated penetration. Concurrently, in contrast to TiN, Au exhibits a diminished capacity for Ga dissolution, thereby substantiating its efficacy in preserving the structural integrity of the crystal lattice. In addition, the reduction of Al can impede the diffusion notably, further showing the smooth surface morphology of the electrode with atomic force microscope (AFM) and scanning electron microscopy (SEM) measurements. The ohmic contacts of HEMTs are individually fabricated with Ti/Al/Ni/Au and TiAl3/Au configurations. The HEMTs with TiAl3/Au ohmic contacts demonstrate superior DC characteristics.

Evaluation of Ti/Al/Ni/Au ohmic contact to n-AlGaN with different Ti/Al thickness for deep ultraviolet light emitting diode

Ohmic contacts were formed by varying the Ti/Al thickness ratios in the Ti/Al/Ni/Au metal stack. Based on the Ti/Al/Ni/Au metal electrode in n-AlGaN, several deep ultraviolet light emitting diode (DUV LED) chips were fabricated. Annealed at different temperatures, the different structures and ohmic contacts in these Ti/Al/Ni/Au electrodes were correlated by X-ray diffraction (XRD) pattern, scanning electron microscope (SEM) and atomic force microscope (AFM). The XRD results show that the TiN and AlTi3 phases were formed after two-step high-temperature annealing process. The influence of ohmic contact on the Ti/Al/Ni/Au metal stack on the n-AlGaN performance was explored by optimizing the Ti thickness of contact layer. It demonstrated that when the Ti/Al thickness ratio is 5.41 (Ti = 35 nm), the ohmic contact on Ti/Al/Ni/Au metal stack exhibited the lowest specific contact resistance which is about 3.21x10-5 (Ω·cm2). The I-V curve and I-L curve results of the DUV LED devices fabricated by Ti/Al/Ni/Au metal stacks demonstrate the ohmic contact performance. These results indicated the ohmic contact formation by varying Ti/Al thickness ratios on the Ti/Al/Ni/Au metal electrode is favorable to the application in the n-AlGaN-based DUV LED.

Enhanced p-type Ohmic contact performance in FCLEDs by manipulating thermal stress distribution to suppress Ag agglomeration

The implementation of reflective p-type Ohmic contact is an effective way to solve current crowding and improve the optoelectronic performance of flip-chip light-emitting diodes (FCLEDs). Here, we investigate the effects of annealing temperature, annealing time, and N2 flow rate on the formation for Ag/p-GaN Ohmic contact and determine the optimal annealing process parameters. After inserting an indium-tin oxide layer between Ag and p-GaN, the specific contact resistance decreases from 6.66 × 10−3 to 1.86 × 10−3 Ω cm2. In addition, we discover the appearance of a “black line” around the edges of the chips after high-temperature annealing. Finite element analysis and experiments show that the “black line” is related to Ag agglomeration under high temperatures due to stress concentration at the edges of the chips. A strategy for manipulating the stress concentration by adjusting the thickness of the TiW diffusion barrier layer is proposed based on insight obtained by modeling the stress distribution at the edge of the chips. The electrical properties of the fabricated FCLEDs show that the proposed stress manipulation strategy solves the problem of “black line” effectively and maintains the performance of the chips well.

Low-Resistance Ta/Al/Ni/Au Ohmic Contact and Formation Mechanism on AlN/GaN HEMT

In this article, we systematically investigated the Ta/Al/Ni/Au and Ti/Al/Ni/Au ohmic contacts, and contact formation mechanism on ultrawide bandgap (UWBG) AlN barrier heterostructure without using the source–drain regrowth technique. The excellent ohmic contact performance of the Ta-based scheme was observed. The transmission line model (TLM) results depicted an ultralow contact resistance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.08 \Omega \cdot $ </tex-math></inline-formula> mm and a specific contact resistance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.06\times 10^{-{7}} \,\, \Omega \cdot $ </tex-math></inline-formula> cm2. Atomic force microscope (AFM) shows that the Ta/Al/Ni/Au sample presents a surface morphology improvement compared with the Ti/Al/Ni/Au sample. Transmission electron microscope (TEM) illustrated that the dominant contact mechanism for ultralow resistance is direct contact through TaxAlyAuz alloy penetration. Meanwhile, the difficulty of contact formation for the Ti-based sample was also discussed through the microstructural analysis. These results demonstrate that the proposed Ta/Al/Ni/Au metal scheme is a high-performance and cost-effective ohmic contact technique well suited for AlN/GaN HEMT fabrication process.

Effect of post-metallization anneal on (100) Ga2O3/Ti–Au ohmic contact performance and interfacial degradation

Here, we investigate the effect of post-metallization anneal temperature on Ti/Au ohmic contact performance for (100)-oriented Ga2O3. A low contact resistance of ∼2.49 × 10−5 Ω·cm2 is achieved at an optimal anneal temperature of ∼420 °C for (100) Ga2O3. This is lower than the widely-used temperature of 470 °C for (010)-oriented Ga2O3. However, drastic degradation of the (100)-oriented contact resistance to ∼1.36 × 10−3 Ω·cm2 is observed when the anneal temperature was increased to 520 °C. Microscopy at the degraded ohmic contact revealed that the reacted Ti–TiOx interfacial layer has greatly expanded to 25–30 nm thickness and GaAu2 inclusions have formed between (310)-Ga2O3 planes and the Ti–TiOx layer. This degraded interface, which corresponds to the deterioration of ohmic contact properties, likely results from excess in-diffusion of Au and out-diffusion of Ga, concurrent with the expansion of the Ti–TiOx layer. These results demonstrate the critical influence of Ga2O3 anisotropy on the optimal post-metallization anneal temperature. Moreover, the observed Ti/Au contact degradation occurs for relatively moderate anneal conditions (520 °C for 1 min in N2), pointing to the urgent necessity of developing alternative metallization schemes for gallium oxide, including the use of Au-free electrodes.

Exploiting the Nanostructural Anisotropy of β-Ga2O3 to Demonstrate Giant Improvement in Titanium/Gold Ohmic Contacts

Here we demonstrate a dramatic improvement in Ti/Au ohmic contact performance by utilizing the anisotropic nature of β-Ga2O3. Under a similar doping concentration, Ti/Au metallization on (100) Ga2O3 shows a specific contact resistivity 5.11 × 10-5 Ω·cm2, while that on (010) Ga2O3 is as high as 3.29 × 10-3 Ω·cm2. Temperature-dependent contact performance and analyses suggest that field emission or thermionic field emission is the dominant charge transport mechanism across the Ti/Au-(100) Ga2O3 junction, depending on whether reactive ion etching was used prior to metallization. Cross-sectional high-resolution microscopy and elemental mapping analysis show that the in situ-formed Ti-TiOx layer on (100) Ga2O3 is relatively thin (2-2.5 nm) and homogeneous, whereas that on (010) substrates is much thicker (3-5 nm) and shows nanoscale facet-like features at the interface. The anisotropic nature of monoclinic Ga2O3, including anisotropic surface energy and mass diffusivity, is likely to be the main cause of the differences observed under microscopy and in electrical properties. The findings here provide direct evidence and insights into the dependence of device performance on the atomic-scale structural anisotropy of β-Ga2O3. Moreover, the investigative strategy here─combining comprehensive electrical and materials characterization of interfaces on different semiconductor orientations─can be applied to assess a variety of other anisotropic oxide junctions.

HVPE growth of bulk GaN with high conductivity for vertical devices

The electrical properties of gallium nitride (GaN) substrate are crucial to the performance of vertical power devices. Bulk GaN substrates with carrier concentrations in the range from 6.7 × 1017 to 1.7 × 1019 cm−3 are grown by hydride vapor phase epitaxy. All samples show no obvious tensile stress regardless of the carrier concentration. Moreover, the mobility of Si-doped high-quality bulk GaN is superior to the GaN template with higher dislocation density at the same carrier concentration. The influence of carrier concentration on the performance of ohmic contact on N-face of Si-doped GaN is also carefully studied by circular transfer length measurement and rapid thermal annealing methods. The specific contact resistivity decreases monotonically with increase of carrier concentration, while it increases with the annealing temperature. The N-face contact becomes non-ohmic when the annealing temperature exceeds the limit value, which increases with the carrier concentration. The sample with carrier concentration of 1.7 × 1019 cm−3 still showed ohmic behavior after annealing at 450 °C. These results are not only useful to improve the electrical properties of N-type bulk GaN substrate, but also provide a potential solution for improving the efficiency of vertical devices in the future.

Study on the effect of thermal annealing process on ohmic contact performance of AuGeNi/n-AlGaInP

In this paper, Ni/Au/Ge/Ni/Au laminated metals were deposited on the n-(Al&lt;sub&gt;0.27&lt;/sub&gt;Ga&lt;sub&gt;0.73&lt;/sub&gt;)&lt;sub&gt;0.5&lt;/sub&gt;In&lt;sub&gt;0.5&lt;/sub&gt;P by electron beam evaporation, the ohmic contact with low contact resistance was successfully prepared by optimized annealing process. The specific contact resistance reached 1.4 × 10&lt;sup&gt;–4&lt;/sup&gt; Ω·cm&lt;sup&gt;2&lt;/sup&gt; when annealed at 445 ℃ for 600 s. The result of the secondary ion mass spectrometer shows that the solid-state reaction takes place at the interface between the laminated metal Ni/Au/Ge/Ni/Au and n-AlGaInP, then the germanium atoms and indium atoms diffuse outwards due to thermal decomposition and leave vacancies in the lattice. In this paper, the reason for the formation of ohmic contact is attributed to the fact that the germanium vacancy and indium vacancy are occupied by gallium atoms as donors to increasing the N-type doping concentration. The optimized annealing process can improve the ohmic contact performance of n-(Al&lt;sub&gt;0.27&lt;/sub&gt;Ga&lt;sub&gt;0.73&lt;/sub&gt;)&lt;sub&gt;0.5&lt;/sub&gt;In&lt;sub&gt;0.5&lt;/sub&gt;P with low doping concentration, but the specific contact resistivity has no obvious relationship with the annealing process when the of doping concentration of n-(Al&lt;sub&gt;0.27&lt;/sub&gt;Ga&lt;sub&gt;0.73&lt;/sub&gt;)&lt;sub&gt;0.5&lt;/sub&gt;In&lt;sub&gt;0.5&lt;/sub&gt;P increased. The Schottky barrier is high at low doping concentration of n-(Al&lt;sub&gt;0.27&lt;/sub&gt;Ga&lt;sub&gt;0.73&lt;/sub&gt;)&lt;sub&gt;0.5&lt;/sub&gt;In&lt;sub&gt;0.5&lt;/sub&gt;P. So inter diffusion in the annealing process could significantly increase the doping concentration of n-(Al&lt;sub&gt;0.27&lt;/sub&gt;Ga&lt;sub&gt;0.73&lt;/sub&gt;)&lt;sub&gt;0.5&lt;/sub&gt;In&lt;sub&gt;0.5&lt;/sub&gt;P and reduce the Schottky barrier height. Nevertheless, the Schottky barrier of the sample with high doping concentration is low enough what is not sensitive with inter diffusion. It provides a new method for the preparation of N-electrode of AlGaInP thin film light-emitting-diodes chip, as so as avoid the problem of n&lt;sup&gt;+&lt;/sup&gt;-GaAs absorption in the conventional N-electrode preparation method and the problem of electrode dropping. However, there are still some shortcomings in this paper. The disadvantage is that the high doping concentration of n-AlGaInP will affect the crystal quality, which will reduce the luminous efficiency of LED. Therefore, in order to prepare ohmic contact with excellent properties on n-AlGaInP with lower doping concentration, optimizing the electrode design and the surface treatment of semiconductor materials are the keys to follow-up research.

Performance Improvement of GaN Based Laser Diode Using Pd/Ni/Au Metallization Ohmic Contact

We report an investigation of the effects of different metal systems and surface treatment on the contact performance of GaN lasers. We found that multi-element metal alloy and surface chemical treatment are the keys to achieve good ohmic behavior contacts on GaN laser diodes. Pd/Ni/Au contact demonstrates excellent thermal stability and lowest specific contact resistivity in these metal systems. Properly adjusting the thickness of the Pd and Ni layer and pretreating with the KOH solution can further improve the ohmic contact performance. The improved ohmic behavior of the KOH solution pretreated Pd/Ni/Au contact is attributed to removing surface oxides and the reduction of the schottky barrier heights due to the metal Pd has a high work function and the interfacial reactions occurring between the Pd, Ni, Au, and GaN extends into the GaN film. As a result, a low contact resistivity of 1.66 × 10−5 Ω·cm2 can be achieved from Pd(10 nm)/Ni(10 nm)/Au(30 nm) contacts with KOH solution pretreated on top of the laser diode structure. The power of the GaN based laser diode with the Pd/Ni/Au metallization ohmic contact can be enhanced by 1.95 times and the threshold current decreased by 37% compared to that of the conventional ohmic contact Ni/Au.

Recovery of p-GaN surface damage induced by dry etching for the formation of p-type Ohmic contact

P-type Ohmic contact on the dry-etched p-GaN surface is highly desired for various device applications with the p-GaN buried underneath by an epitaxial layer or a dielectric layer. However, GaN dry etching process usually induces surface damage and inevitably degrades the p-type Ohmic contact performance. In this work, p-type Ohmic contact with a low specific resistivity has been successfully realized on a dry-etched p-GaN surface by using a wet chemical etching combined with a thermal treatment in NH3/N2 ambient. Furthermore, the recovery mechanism of the p-GaN surface damage was studied through an in-depth surface characterization by atomic force microscopy and X-ray photoelectron spectroscopy.

Comparative study of n-GaN transition group refractory metal Ohmic electrode

Ohmic contact is directly related to the performance of GaN device and is one of the important factors affecting device performance. In recent years, many research groups have studied the electrode materials and annealing conditions of n-type GaN Ohmic contacts. In this paper, the ohmic contact properties and structural characteristics of the Hf/Al electrode of a transition group metal refractory metal Hf system under different annealing conditions are studied, and compared with those of the Ti-based ohmic contact Ti/Al electrode. The specific contact resistivity of each electrode is measured by a dot-type transmission line model, and the structural characteristics of the electrode are analyzed by using an Auger electron spectrometer which can be analyzed in depth. The results show that the Hf/Al electrode under the same annealing condition exhibits superior ohmic contact performance compared with the conventional Ti/Al electrode. At the same time, the lowest specific contact resistivity of the Hf/Al electrode annealed in an N&lt;sub&gt;2&lt;/sub&gt; atmosphere at a low temperature of 650 ℃ for 60 s is 4.28×10&lt;sup&gt;–5&lt;/sup&gt; Ω·cm&lt;sup&gt;2&lt;/sup&gt;. The in-depth analysis of Auger electron spectrum shows that the Hf/Al electrode has a solid phase reaction with the n-type GaN material. In addition, the cross section of each electrode is observed by auger electron spectroscopy. In the Hf/Al electrode sample, the metal-semiconductor interface does not show voids after annealing. This situation occurs at the sample interface where the Ti/Al electrode is annealed at 650 ℃ for 60 s in N&lt;sub&gt;2&lt;/sub&gt; atmosphere and annealed at 850 ℃ for 30 s in N&lt;sub&gt;2&lt;/sub&gt; atmosphere. This is one of the reasons why the Hf/Al electrode sample has a lower specific contact resistivity. At the same time, the surface of Hf/Al electrode and Ti/Al electrode annealed at 850 ℃ are characterized by using scanning electron microscope. It is found that the surfaces of both electrodes subject to high temperature annealing show a similar granular rough surface, and this rough surface has a certain influence on the electrical properties of the GaN device. The rough surface formed by the electrode under such high temperature annealing conditions is an urgent problem to be solved in the future research. In summary, the study in this paper indicates the use of Hf/Al to form an ohmic contact with n-type GaN under a low temperature annealing condition.

Improving contact performance of graphene on p-type GaN thin films with V-pits microstructures

This study presents the electrical properties of graphene that directly is contact on two types of p-type GaN thin films. The diameter of several hundred nanometer V-pits were formed on the p-GaN thin films by adjusting the NH3 flow rate during the metal organic chemical vapor deposition epitaxial process. The single-layer graphene with a high transmittance of 97% in the visible range was transferred on p-GaN thin films to form an Ohmic contact. The V-pits provide more carrier transport paths that promote the carrier tunneling into p-GaN thin films, resulting in a better Ohmic contact performance. In addition, the increased current value was attributed to the presence of V-pits on the p-GaN thin films.

(Invited) Interface Engineering of High-k Dielectrics and Metal Contacts for High Performance Top-Gated MoS2 FETs

The 2D materials, in particular Transition Metals Di-chalcogenides (TMD), are potential candidates to scale CMOS technology beyond Silicon. The ability to deposit ultrathin TMD films, down to a mono-layer, offers excellent electrostatic control of the channel by the gate electrode. This enables aggressive scaling of the channel length of transistor, due to superior short channel performance. In particular, few layer Molybdinum Disulphide (MoS2), with a bandgap comparable to Silicon, has emerged as a promising candidate to to enable continued transistors scaling. However, high performance and reliable gate dielectric and contact formation continues to be an open problem. Since the impurity doping strategies for MoS2 are still not clear, the ohmic contact formation is limited by the nature of metal-semiconductor schottky contact. Despite the Fermi level pinning close to conduction band of MoS2, a low source/drain contact resistance value for NMOS transistor is limited by the surface state effects. The problem is exasperated due to large variations in contact resistance values within the wafer, possibly due to surface defects and stoichiometric variations. We demonstrate that appropriate surface preparation can drastically enhance the performance of ohmic contacts on MoS2. In particular, we present a surface-state engineering technique using sulfur treatment of MoS2 before the contact formation. The transistors (L=W= ~1μm) are formed on exfoliated few layer MoS2 flakes (5-7 nm thick). The flakes are treated with ammonium sulphide solution (NH4)2S (Sigma Aldrich, 40% solution in H2O) for 5 min at a temperature of 50 °C, followed by De-ionized water rinse and N2 blow-dry. Nickel (Ni) and Palladium (Pd) contacts are formed using lift-off process. Through extensive material and electrical characterization, we demonstrate that the sulfur treatment reduces to schottky barrier height by 81 meV (Ni) and 135 meV (Pd) due to stronger pinning of Fermi level. This in turn lowers the contact resistance by as much as 10 times with concomitant increase in mobility by a factor of 2. We demonstrate record drain current of 170μA/μm, with complete mitigation of contact resistance and hence drain current variability. Due to the inert basal plane of 2D materials, it is difficult nucleate the growth of dielectrics using CVD and/or ALD techniques, which are routinely used for high-k dielectric integration on Silicon. Although some techniques have been proposed to nucleate the dielectric growth on MoS2, the device performance is inferior due to interface defects. To circumvent this issue, we develop a very high performance HfO2 thin film using electron beam evaporation. The O2 flow rate is optimized during e-beam evaporation to achieve 30nm HfO2 films with k=19 and EOT=6.1 nm. The interface trap density is estimated to be 8.7x1011 /cm2, resulting in a very high field effect mobility value of 63 cm2/V-s. The HfO2 gate oxide results in a very low leakage current of 7x10-7 A/cm2. The top gated transistors fabricated with this dielectric yield one of the highest reported drain current value of 180 μA/μm, with near ideal subthreshold behaviour (60 to 75 mV/decade). We also utilize 30nm HfO2 dielectric as a substrate to deposit MOS2 flakes, instead of conventional 300nm SiO2 substrate. Finally we demonstrate an NMOS inverter circuit using HfO2 top-gated devices using a depletion mode NMOS transistor as a resistive load. The Inverter exhibits an output to input gain of about 9. Figure 1

Influence of residual carbon impurities in a heavily Mg-doped GaN contact layer on an Ohmic contact.

The influence of residual carbon impurities incorporated into a heavily Mg-doped GaN layer has been studied systematically according to the relation between the carbon concentration and specific contact resistance. Furthermore, the results of temperature-dependent current-voltage characteristics and the photoluminescence spectra indicate that a proper concentration of residual carbon impurities can improve the performance of Ohmic contact by introducing deep-level defects to enhance the variable-range-hopping conduction.

The effects of Cu-doped ZnTe intermediate layer on the Ohmic contact to CdZnTe films

Polycrystalline CdZnTe films with thickness about 300μm were deposited by close spaced-sublimation (CSS) method. The as-deposited CdZnTe samples were polished by mechanical polishing (MP) process. Cu-doped ZnTe films were sputtered by RF magnetron sputtering as an intermediate layer to improve the performance of Ohmic contact of Au electrodes to CdZnTe films. The effect of Cu-doped ZnTe intermediate layers on the electrical characteristics of Au/CdZnTe and Au/ZnTe:Cu/CdZnTe was investigated by the circular transmission line model (CTLM). The results indicate a good Ohmic contact of Au/ZnTe:Cu/CdZnTe with low contact resistivity of about 0.79Ω·cm2.

Recent Progress in Ohmic Contacts to Silicon Carbide for High-Temperature Applications

During the past few decades, silicon carbide (SiC) has emerged as the most promising wide-bandgap semiconductor for high-temperature, high-frequency, and high-power applications. All its attractive properties depend critically on and are often limited by the formation of Ohmic contacts to SiC. Although impressive progress has been made, improvements to further reduce the specific contact resistance without lowering the stability of Ohmic contacts during long-term aging are still indispensable. In this regard, we present a review of recent progress in Ohmic contacts to n- and p-type SiC reported in literature. The mechanism of Ohmic contact to SiC, surface preparation methods, and performance of Ohmic contacts are discussed. Emphasis is placed on the thermal stability of Ohmic contacts to SiC. To date, extremely low specific contact resistance values (10−7 Ω cm2 to 10−6 Ω cm2) have been obtained after high-temperature (>800°C) annealing. Moreover, great efforts have been made to achieve good Ohmic contact performance (10−6 Ω cm2 to 10−5 Ω cm2) under low-temperature annealing (<600°C) or even without annealing. During long-term aging at high temperature (300°C to 1000°C), surface morphology degradation, formation of unwanted Kirkendall voids, interdiffusion between metallization stacks and/or SiC substrates, and especially severe oxidation in oxygen-containing atmospheres can be responsible for electrical degradation of Ohmic contacts. These critical issues are discussed along with future perspectives.

Schottky diodes and high electron mobility transistors of 2DEG AlGaN/GaN structures on sapphire substrate

We report Schottky diodes (SDs) and High Electron Mobility Transistors (HEMTs) fabricated of 2DEG AlGaN/GaN structures grown by Metalorganic Chemical Vapour Phase Epitaxy (MOVPE) on sapphire substrate. The SDs and HEMTs were designed intentionally without surface passivation and were successfully fabricated at the Center for Physical Sciences and Technology (CPST), using standard UV photolithography procedures. The performance of Ohmic contacts formed of quaternary Ti/Al/Ni/Au stack was optimized varying the temperature of rapid thermal annealing process. Deposited on the semiconductor metal Ni/Au stack was used to form 0.75 eV height Schottky barriers. The fabricated SDs demonstrated low reverse current and high electric current switching ratio while the HEMTs showed high transconductance and drain saturation currents performance with good transistor channel closing. This work paves a way to develop advanced AlGaN/GaN based HEMT structures as well as new electronic components for operation at high powers and high frequencies.

Effects of nitrogen flow rate in ohmic contacts on InAlN/GaN heterostructures

The effects of nitrogen (N2) flow rate during high-temperature annealing were investigated for Ti/Al/Ni/Au ohmic metallisation on InAlN/GaN heterostructures. The samples annealed at 900°C for 100 s at a 30 SCCM N2 flow rate had the highest ohmic contact performance: a contact resistance of 0.12 Ω·mm, transfer length of 0.31 μm and specific contact resistivity of 3.42 × 10−7 Ω·cm2.