Stable Ohmic Contacts Research Articles

Optimization of Non-Alloyed Backside Ohmic Contacts to N-Face GaN for Fully Vertical GaN-on-Silicon-Based Power Devices.

In the framework of fully vertical GaN-on-Silicon device technology development, we report on the optimization of non-alloyed ohmic contacts on the N-polar n+-doped GaN face backside layer. This evaluation is made possible by using patterned TLMs (Transmission Line Model) through direct laser writing lithography after locally removing the substrate and buffer layers in order to access the n+-doped backside layer. As deposited non-alloyed metal stack on top of N-polar orientation GaN layer after buffer layers removal results in poor ohmic contact quality. To significantly reduce the related specific contact resistance, an HCl treatment is applied prior to metallization under various time and temperature conditions. A 3 min HCl treatment at 70 °C is found to be the optimum condition to achieve thermally stable high ohmic contact quality. To further understand the impact of the wet treatment, SEM (Scanning Electron Microscopy) and XPS (X-ray Photoelectron Spectroscopy) analyses were performed. XPS revealed a decrease in Ga-O concentration after applying the treatment, reflecting the higher oxidation susceptibility of the N-polar face compared to the Ga-polar face, which was used as a reference. SEM images of the treated samples show the formation of pyramids on the N-face after HCl treatment, suggesting specific wet etching planes of the GaN crystal from the N-face. The size of the pyramids is time-dependent; thus, increasing the treatment duration results in larger pyramids, which explains the degradation of ohmic contact quality after prolonged high-temperature HCl treatment.

(Invited) Ga2O3 Semiconductor Devices for High-Temperature Operation

Beta phase of gallium oxide (Ga2O3) has attracted a lot of attention as an ultra-wide band gap semiconductor for applications in high-power electronic devices. High power operation often comes with increased temperatures of the devices due to joule heating, especially in semiconductors like Ga2O3with low thermal conductivity. However, electrical performance and reliability of the Ga2O3based diodes and transistors at very high temperature, above 400C, is not well established.In this talk, I will present our recent experimental results on fabrication of Ga2O3semiconductor devices for applications as high-temperature diodes and hydrogen gas sensors. The fabricated Ga2O3/NiO p-n heterojunctions allow for device operation with >100-1000 rectification ratio up to 400-600C [1]. Despite the stable ultrathin Ohmic contact [2], the device performance degrades over tens of temperature cycles and hundreds of hours at these elevated temperatures, likely due to instability of the rectifying contact. We propose and implement alternative interfacial rectifying contact layers, that enable both high performance and long reliability of these high temperature Ga2O3 semiconductor devices.[1] Sohel, Zakutayev et al Physica Status Solidi (a) 220 2300535 (2023)[2] Callahan, Zakutayev et al J. Vac. Sci. Technol. A 41, 043211 (2023)

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.

Pseudo‐Vertical Schottky Diode with Ruthenium Contacts on (113) Boron‐Doped Homoepitaxial Diamond Layers

Electrical properties of pseudo‐vertical Schottky barrier diodes (pVSBDs) prepared on (113)–oriented boron‐doped diamond (BDD) layers using ruthenium (Ru) for both the ohmic and Schottky contacts are investigated. First, Ru ohmic contacts are evaporated on homoepitaxial BDD layers with different resistivity, and their specific contact resistance is measured using circular transfer length method structures after annealing at various temperatures up to 750 °C. Then, pVSBD structures are fabricated on the boron‐doped bilayer consisting of a lower, heavily boron‐doped layer ensuring an ohmic contact and an upper, lightly doped layer providing a rectifying Schottky contact. After necessary mesa etching, both contacts are formed by the Ru evaporation. The results show that Ru forms a stable ohmic contact with very low contact resistance (10−5–10−6 Ω cm2) when deposited on BDD layers with metallic conductivity. It also provides an acceptable Schottky contact on low‐doped (113) homoepitaxial BDD. Both contacts, which are made simultaneously, realize pVSBDs with low on‐state resistance and low forward voltage drop. However, the lower barrier of the ruthenium contacts results in higher leakage. Ru pVSBDs thus show a lower rectification ratio, higher leakage, and a worse ideality factor compared to analogical pVSBDs using molybdenum contacts.

Realization of Flexible Large-Sized GaInP/GaAs/InGaAs Solar Cells With Stable Low-Temperature Ohmic Contact Technique

Realization of Flexible Large-Sized GaInP/GaAs/InGaAs Solar Cells With Stable Low-Temperature Ohmic Contact Technique

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 (>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.

The high thermal conductivity of graphene prevents ZnO nanorod-graphene interface from degradation

The electrical and thermal properties of the junction between individual ZnO nanorods and conductive electrodes are investigated. It is demonstrated that a stable ohmic contact can be prepared between ZnO nanorods and graphene by eliminating contamination on the graphene surface by local Joule heating. Moreover, it is shown that, for conventional metal electrodes, such as copper, Joule self-heating forms hot spots and subsequently causes thermal damage to the ZnO nanorod near the interface. This issue can be eliminated in graphene contacts because of their excellent thermal conductivity. The experimental results are validated by the numerical modeling of heat dissipation near the ZnO/graphene interface using COMSOL Multiphysics software. To characterize the interface between individual nanorods and graphene, a novel technique was developed using a nanomanipulator in the chamber of the scanning electron microscope. In contrast to conventional approaches, the method eliminates the use of extensive processing chemistry and allows observation of morphological changes in situ when performing electrical characterization.

Formation mechanism, interface characteristics and the application of metal/SiC thin-film ohmic contact after high-temperature treatment

SiC, as a representative of the third generation semiconductors, is widely investigated in power devices and sensors. However, ohmic contacts, an important component for signal output of various SiC chips, have always faced challenges with unclear formation mechanism and difficulty to withstand high temperature. To solve such problems, two sets of NPN-type SiC ohmic contact were prepared, including Ti/TaSi2/Pt and Ni/W. Experimental results show that whether ohmic contact can be formed mainly depends on the contact metal. The alloy phase with lower work function is formed after high temperature annealing, which reduces the height and width of the Schottky barrier. For the prepared SiC ohmic contacts, It was demonstrated that the invasion of oxygen atoms cause contacts failure, and incorporation of barrier layer (Ta-based layer) and effective protective layer (Pt layer) determine whether high temperatures can be tolerated. Finally, piezoresistive 4H–SiC pressure sensor chips with dimensions of 3000 μm × 3000 μm × 200 μm were fabricated using the obtained ohmic contacts. Sensor chips output resistance were monitored up to 600 °C in air to show stability of the ohmic contact. The highly stable and high temperature ohmic contact can be applied in a variety of SiC devices.

Development of Low-Resistance Ohmic Contacts with Bilayer NiO/Al-Doped ZnO Thin Films to p-type GaN

The fabrication of low-resistance and thermally stable Ohmic contacts is essential for the realization of reliable GaN power devices. In the particular case of p-type GaN, a thin Ni/Au bilayer is commonly used for Ohmic contacts. However, Au metal contacts are quite expensive, are incompatible with the complementary metal oxide-semiconductor foundries, and also have poor thermal stability. Thus, seeking an alternative that is affordable and thermally stable is crucial. In the present study, we investigate Au-free Ohmic contact formation on p-type GaN using a bilayer Ni/Al-doped ZnO (AZO) thin film. Careful studies were focused on identifying the role of process parameters such as annealing parameters: temperature, time, and atmosphere in order to obtain an excellent Ohmic contact on p-GaN. Our results show that the contact resistance can be significantly reduced using a Ni/AZO bilayer with a suitable rapid thermal process. We demonstrate that the specific contact resistance for Ni/AZO on p-GaN can reach the lowest value of 1.85 × 10-4 Ω·cm2 for a sample with a 5 nm Ni layer annealed at 500 °C in air for 5 min. Our work demonstrates that the bilayer Ni/AZO contact could be suitable for efficient GaN power diodes or transistors.

A high thermal stability ohmic contact for GaN-based devices.

Co-integration of gallium nitride (GaN) power devices with Si logic ICs provides a way of applying high power and high efficiency circuits on a single chip. In order to co-integrate GaN devices with Si ICs, an ohmic contact for GaN devices has to be Si compatible and durable at the same or higher temperature of the back-end process in the conventional complementary metal oxide semiconductor (CMOS) industry. In this work, an Au-free ohmic junction with high thermal stability for AlGaN/GaN high electron mobility transistors (HEMTs) was presented. The proposed titanium nitride (TiN) contacts on AlGaN/GaN HEMTs retained their ohmic characteristics and stayed stable at temperatures even higher than 1000 °C. The interface chemistry analysis using STEM EELS revealed the enhancement of the binding energy of Ga-N and Al-N and invisible diffusion of Ti during treatment below 1000 °C. This clarifies the origin of the highly stable ohmic contact. Thus, our work provides a new pathway and thought for forming reliable contacts for HEMTs or another GaN-based devices.

Enhancement of the light output efficiency and thermal stability of AlGaN-based deep-ultraviolet light-emitting diodes with Ag-nanodot-based p-contacts and an 8-nm p-GaN cap layer.

The efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is limited by the high absorption issue of the p-GaN contact layer or poor contact properties of the transparent p-AlGaN contact layer. Enhancement of the light output efficiency and thermal stability of DUV LEDs with an emission wavelength of 272 nm was investigated in this work. Ag nanodots on an 8-nm p-GaN cap layer were used to form ohmic contact, and Al and Mg reflective mirrors were employed to enhance the light output power (LOP) of DUV LEDs. However, serious deterioration of LOP occurred after the high-temperature process for the LEDs with Al and Mg reflective mirrors, which can be attributed to the damage to the ohmic contact properties. A Ti barrier layer was inserted between the Ag/p-GaN and Al layers to prevent the degeneration of ohmic contact. The wall-plug efficiency (WPE) of DUV LEDs fabricated by the Ag-nanodot/Ti/Al electrode is 1.38 times that of LEDs fabricated by adopting a thick Ag layer/Ti/Al at 10 mA after a high-temperature process. The Ag-nanodot/Ti/Al electrode on thin p-GaN is a reliable technology to improve the WPE of DUV LEDs. The experimental and simulated results show that the ohmic contact is important for the hole-injection efficiency of the DUV LEDs when p-GaN is thin, and a slight increase in the contact barrier height will decrease the WPE drastically. The results highlighted the importance of thermally stable ohmic contacts to achieve high-efficiency DUV LEDs and demonstrated a feasible route for improving the LOP of DUV LEDs with a thin p-GaN cap layer and stable reflective electrodes.

In Situ Grown Cuboidal MnCo2O4/h Boron Nitride Heterojunction: A Noble Metal-Free Approach Based on Efficient Hole Extraction for Electrochemical Oxygen Evolution Reaction

Rational design of complex spinel oxides directly onto the substrate as a highly stable electrocatalytic anode material for oxygen evolution reaction (OER) is a crucial step to replace a noble metal-based electrocatalyst. Herein, surfactant-free cuboidal MnCo2O4 (C-MCO) was synthesized in situ over FTO by a green and simple synthetic protocol. Stable and good ohmic contact between C-MCO and the substrate ensures efficient transfer of the charge carrier at their interface. Hexagonal boron nitride (h-BN) nanosheets coupled with C-MCO provides a surface for easy deposition of the co-catalyst, making active sites easily available and accessible for enhanced OER activity. The surface-modified C-MCO/h-BN electrode shows a Tafel slope value of 66 mV/dec with an overpotential of 240 mV at a current density of 10 mA/cm2. C-MCO/h-BN displays ∼2.8-fold enhancement in turnover frequency compared to its pristine counterpart, with a faradaic yield of 97%. This study can be used for large-scale clean energy production owing to its low-cost, efficient, eco-friendly, and long-term stability of the C-MCO/h-BN electrocatalyst.

Improvement of the Sensitivity and Selectivity of Gas Molecules of Graphene-base Gas Sensor by Applying Carbon Nanotubes

Chemical biosensors using graphene as a sensing element have been developed for a new generation of highly-sensitive miniature exhaled multi-gas sensors for early detection of various diseases. In this study, carbon nanotubes (CNTs) were synthesized directly on graphene to increase the surface area of the molecular adsorption in order to improve the sensitivity and selectivity of the graphene-based gas sensor. The synthesis of graphene was performed in a low-pressure chemical vapor deposition apparatus on a very pure copper catalyst substrate. The grown graphene was transferred onto a Si/SiO2 substrate. A CNT layer was also formed directly on the graphene sensor element using a thermochemical vapor deposition method. Stable ohmic contact was observed between the synthesized CNTs and the base graphene layer. It was also confirmed that the sensitivity of the graphene sensor with the CNT layer to gas molecules was improved by about 4 times compared with the conventional graphene sensor.

Piezo-resistive effects of Area-arrayed Dumbbell-Shape Graphene Nano-ribbon Structure for Highly Sensitive Micro-Scale Tactile Sensor

In order to develop a highly-sensitive small-scale tactile sensor, piezoresistive effect of graphene nanoribbon was applied to the sensor. It was confirmed that the graphene nanoribbon narrower than about 70 nm showed semiconductive properties and thus, large piezo-resistive constant. The dumbbell shape was applied to the stable junction between metallic graphene and semiconductive graphene. It was easy to form stable ohmic contact between the metallic graphene and metallic electrode. An area-arrayed nanoscale GNR structure with varying width was fabricated for a highly sensitive tactile sensor. Graphene was synthesized inside a copper pocket using a low-pressure chemical vapour deposition (LP-CVD) method. PMMA- assisted wet transfer process was used to transfer graphene grown on the copper to the target substrate. Then, the electrodes were deposited using electron-beam deposition. A four-point bending test was carried out on the sensor to determine the piezo-resistive effect of the DS-GNR. Finally, the gauge factor was determined under the application of various amounts of strain.

Performance and reliability of β-Ga2O3 Schottky barrier diodes at high temperature

Beta-gallium oxide (β-Ga2O3) is an ultrawide bandgap semiconductor that has potential for power electronic applications and devices operating at high temperatures. Particularly important for these applications are its 4.9 eV bandgap, facile electron doping, and the ability to grow β-Ga2O3 crystals from the melt. In this work, vertical β-Ga2O3 Schottky barrier diodes were fabricated using Pt Schottky and Ti-based Ohmic contacts and Au contact pads on unintentionally doped n-type, (2¯01)-oriented single crystal substrates. The diode’s temperature-dependent electrical properties up to 400 °C were investigated, and the Pt/Ga2O3 Schottky barrier height was determined to be close to 1.2 eV. The degradation of the contacts over multiple cycles up to 400 °C was observed, resulting in a significant increase in series resistance of the diodes by 1000× at ambient temperature after they were cycled. According to electron microscopy measurements, this degradation is likely due in part to the migration and oxidation of Ti at the top surface of the Au contact pads. This degradation highlights the need for further research and development to ensure stable Ohmic and Schottky contacts to Ga2O3 at temperatures above 400 °C.

Accelerated Aging Stability of β-Ga2O3-Titanium/Gold Ohmic Interfaces.

Stable ohmic contacts are critical to enable efficient operation of high-voltage electronic devices using ultrawide bandgap semiconductors. Here we perform, for the first time, thermally accelerated aging of Ti/Au ohmic interfaces to (010) β-Ga2O3. We find that a heavily doped semiconductor, doped n-type by Si-ion implantation, treated with reactive ion etch (RIE), results in a low specific contact resistance of ∼10-5 Ω cm2 that is stable upon accelerated thermal aging at 300 °C for 108 h. The low resistance interface is due to thermionic field emission of electrons over an inhomogeneous barrier. Scanning/transmission electron microscopy indicates that the multi-layered structure and elemental distribution across the contact interface, formed during a 1 min 470 °C post-metallization anneal, do not change noticeably over the aging period. A ∼1 nm interfacial layer is observed by high-resolution microscopy at the Ti-TiOx/Ga2O3 interface on all samples exposed to RIE, which may contribute to their excellent stability. In addition, longer-range facet-like interfacial features are observed, which may contribute to the inhomogeneous barrier. In contrast, Ti/Au junctions to moderately doped (010) Ga2O3 made with no RIE treatment exhibit a high contact resistance that increases upon accelerated aging, along with a partially lattice-matched interface. The methods used here to understand the process, structure, and electrical property relationships for Ti/Au contact interfaces to β-Ga2O3 can be applied to assess and tune the stability of a variety of other oxide-semiconductor interfaces.

Review of electrical contacts to phase change materials and an unexpected trend between metal work function and contact resistance to germanium telluride

Devices based on the unique phase transitions of phase change materials (PCMs) like GeTe and Ge2Sb2Te5 (GST) require low-resistance and thermally stable Ohmic contacts. This work reviews the literature on electrical contacts to GeTe, GST, GeCu2Te3 (GCuT), and Ge2Cr2Te6 (GCrT), especially GeTe due to the greater number of studies. We briefly review how the method used to measure the contact resistance (Rc) and specific contact resistance (ρc) can influence the values extracted, since measurements of low contact resistances are susceptible to artifacts, and we include a direct comparison of Au-, Pt-, Ni-, Mo-, Cr-, Sn-, and Ti-based contacts using a systematic approach. Premetallization surface treatment of GeTe, using ex situ or in situ approaches, is critical for minimizing contact resistance (Rc). Transmission electron microscopy reveals that interfacial reactions often occur and also clearly influence Rc. The lowest Rc values (∼0.004 ± 0.001 Ω mm) from the direct comparison were achieved with as-deposited Mo/Ti/Pt/Au (Ar+ plasma treatment) contacts and annealed Sn/Fe/Au (de-ionized H2O premetallization treatment). In the case of Sn-based contacts, low Rc was attributed, in part, to the formation of SnTe at the contact interface; however, for Mo-based contacts, no such interfacial reaction was observed. Comparing all contact metals tested beneath a cap of at least 100 nm of Au, Mo/Ti/Pt/Au offered the lowest contact resistance as-deposited, even though the work function of Mo is only 4.6 eV, and the low contact resistance remained stable even after annealing at 200 °C for 30 min. This trend is surprising, as high work function metals, like Ni and Pt, would be expected to provide lower Rc values when they are in contact with a p-type semiconductor like GeTe. Through materials’ characterization, an inverse relationship between the metal work function and Rc for higher work function metals can be attributed to the reactivity of many of the metals with GeTe. Studies of contacts to GST in the literature involve only a small number of contact materials (Ti, TiN, TiW, W, Pt, and graphene) and employ varied geometries for extracting contact resistance. For hexagonal GST, TiW is reported to provide the lowest ρc of ∼2 × 10−7 Ω cm2, while TiN provided the lowest reported ρc of ∼3 × 10−7 Ω cm2 to cubic GST. For the ternary PCMs GCuT and GCrT, contact resistance studies in the literature are also limited, with W being the only metal studied. While more extensive work is necessary to draw wider conclusions about trends in current transport at metal/GST, metal/GCuT, and metal/GCrT interfaces, reduction of Rc and high thermal stability are critical to engineering more efficient and reliable devices based on these materials.

Refractory W Ohmic Contacts to H-Terminated Diamond

A major challenge facing diamond electronics is creating reliable and stable ohmic contacts to a hydrogen-terminated diamond (D:H) conducting surface. In this work, we explore a novel contact-first approach for creating refractory, scalable, and self-aligned ohmic contacts to D:H. Our contacts are based on W sputtered on O-terminated diamond followed by surface hydrogenation. We show that the H-plasma treatment provides a suitable thermal step to create the ohmic contact that relies on the formation of WC at the diamond surface. This thermal step also causes an order of magnitude increase in the W sheet resistance though the metallization that remains mechanically stable. The best contact resistance obtained in this work is $2.6~\Omega $ -mm. We further demonstrate that the electrical contact takes place exclusively at the edge of the metal.

Effects of Grounding Bottom Oxide Protection Layer in Trench-Gate SiC-MOSFET by Tilted Al Implantation

A trench gate SiC-MOSFET with BPW grounded by tilted Al implantation is developed in order to optimize the cell design and process for grounding the BPW in a more simple manner. From evaluation of static characteristics, the MOSFETs with sidewall region can improve the trade-off relationship between Ron,sp and Vbd by the variation of dSRs, and is superior than that of conventional BPW ground contact structure. From evaluation of dynamic characteristics, the MOSFETs with sidewall region can realize stable p-type ohmic contact for BPW compared with conventional BPW ground contact structure. Furthermore, the trade-off between Ron,sp and tsc of this device is adjusted to optimized layout of the p-type sidewall regions without degradation of the dV/dt dependence of turn-on and turn-off losses.

Stable and reliable ohmic contact on p-type 4H-SiC up to 1500 h of aging at 600 °C

The stability and reliability at high temperature of Ti3SiC2 based ohmic contacts on p-type 4H-SiC (0001) 4°-off substrates were studied. The contact was grown from Ti100-xAlx alloys annealed at high temperature (from 900 °C to 1200 °C). The Specific Contact Resistance (SCR) at room temperature and at high temperature (up to 600 °C) was in the 10−4–10−5 Ω.cm2 range. A Schottky barrier height of 0.71 to 0.85 eV was calculated for the set of samples. After aging period at 600 °C for 1500 h, the SCR was very stable for Al contents x < 80 at.%. This was correlated with chemical and physical stability of these contacts, where the residual stress located on 4H-SiC/Ti3SiC2 interface decreased after aging, for which the Ti3SiC2 phase was preserved. Whereas, in the case of x = 80 at.%, the Ti3SiC2 phase disappeared and the contacts were not ohmic anymore after long time aging. The obtained results showed that Ti3SiC2/4H-SiC system is thermodynamically stable at high temperatures and can therefore be a good candidate, with high potential, for high power and high temperature electronic applications.