Reliable Contacts Research Articles

In situ via Contact to hBN-Encapsulated Air-Sensitive Atomically Thin Semiconductors.

Establishing reliable electrical contacts to atomically thin materials is a prerequisite for both fundamental studies and applications yet remains a challenge. In particular, the development of contact techniques for air-sensitive monolayers has lagged behind, despite their unique properties and significant potential for applications. Here, we present a robust method to create contacts to device layers encapsulated within hexagonal boron nitride (hBN). This method uses plasma etching and metal deposition to create 'vias' in the hBN with graphene forming an atomically thin etch-stop. The resulting partially fluorinated graphene (PFG) protects the underlying device layer from air-induced degradation and damage during metal deposition. PFG is resistive in-plane but maintains high out-of-plane conductivity. The work function of the PFG/metal contact is tunable through the degree of fluorination, offering opportunities for contact engineering. Using the in situ via technique, we achieve ambipolar contact to air-sensitive monolayer 2H-molybdenum ditelluride (MoTe2) with more than 1 order of magnitude improvement in on-current density compared to previous literature. The complete encapsulation provides high reproducibility and long-term stability. The technique can be extended to other air-sensitive materials as well as air-stable materials, offering highly competitive device performance.

Implementation of Highly Reliable Contacts for RF MEMS Switches.

A contact is the key structure of RF MEMS (Radio Frequency Microelectromechanical System) switches, which has a direct impact on the switch's electrical and mechanical properties. In this paper, the implementation of highly reliable contacts for direct-contact RF MEMS switches is provided. As a soft metal material, gold has the advantages of low contact resistance, high chemical stability, and mature process preparation, so it is chosen as the metal material for the beam structure as well as the contacts of the switch. However, a Pt film is used in the bottom contact area to enhance the reliability of the contact. Three kinds of contacts with various shapes are fabricated using different processes. Particularly, a circular-shaped contact is obtained by dry/wet combined processes. The detailed fabrication process of the contacts as well as the Pt film on the bottom contact area are given. The experimental test shows that the contact shape has little effect on the RF performance of the switches. However, the circular contact shows better reliability than other contacts and can work well even after 1.2 × 109 cycles.

A Rational Strategy to Assemble Porphyrins and Phthalocyanins into Crystalline Arrays: The SURMOF Approach

Realizing molecular “Designer Solids” by programmed assembly of building units taken from libraries is a very appealing objective. Recently, metal-organic frameworks (MOFs), or porous coordination polymers (PCPs) have attracted a huge interest in this context. Here, we will focus on MOF-based electrochemical, photoelectro-chemical, photovoltaic, and sensor devices built using porphyrin- and phthalocyanine-containing linkers. Internal interfaces in MOF heterostructures are also of interest with regard to photon-upconversion and the fabrication of diodes.Since the fabrication of reliable and reproducible contacts to MOF-materials represent a major challenge, we have developed a layer-by-layer (lbl) deposition method to produce well-defined, highly oriented and monolithic MOF thin films on appropriately functionalized substrates. The resulting films are referred to as SURMOFs [1,2] and have very appealing properties in particular with regard to optical applications [3]. The fabrication of hetero-multilayers is rather straightforward with this lbl method. In this talk, we will describe the principles of SURMOF fabrication as well as the results of systematic investigations of electrical and photophysical properties exhibited by empty MOFs and after loading their pores with functional guests. Band structure effects occurring in crystalline porphyrin arrays assembled using the SURMOF approach will also be discussed. [5] We will close with discussing further applications [4] realized by loading MOFs with nanoparticles or quantum dots and by creating molecular solids lacking inversion symmetry for second harmonic generation (SHG).[5]In the last part of the presentation we will point out that SURMOFs are very well suited for unattended experimentation approaches. Using robot systems controlled by machine learning (ML) algorithms allows to efficiently optimize thin film properties (orientation, crystallinity, roughness). We will discuss this point in particular in connection with the fabrication of SURMOF-based sensor systems.

Defect Engineering of Graphene for Dynamic Reliability.

The interface between two-dimensional (2D) materials and soft, stretchable polymeric substrates is a governing criterion in proposed 2D materials-based flexible devices. This interface is dominated by weak van der Waals forces and there is a large mismatch in elastic constants between the contact materials. Under dynamic loading, slippage, and decoupling of the 2D material is observed, which then leads to extensive damage propagation in the 2D lattice. Herein, graphene is functionalized through mild and controlled defect engineering for a fivefold increase in adhesion at the graphene-polymer interface. Adhesion is characterized experimentally using buckling-based metrology, while molecular dynamics simulations reveal the role of individual defects in the context of adhesion. Under in situ cyclic loading, the increased adhesion inhibits damage initiation and interfacial fatigue propagation within graphene. This work offers insight into achieving dynamically reliable and robust 2D material-polymer contacts, which can facilitate the development of 2D materials-based flexible devices.

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.

Electrical Characteristics of Thermally Stable Ag–Pd–Cu Alloy Schottky Contacts on n-Al0.6Ga0.4N

We report the fabrication of high-barrier-height and thermally reliable Schottky contacts to n-Al0.6Ga0.4N by using an Ag-Pd-Cu (APC) alloy. The Schottky barrier heights (SBHs) and ideality factors computed using the current-voltage (I–V) model ranged from 0.82 to 0.97 eV and from 3.15 to 3.44, respectively. The barrier inhomogeneity model and capacitance-voltage (C–V) method yielded higher SBHs (1.62–2.19 eV) than those obtained using the I–V model. The 300 °C-annealed APC sample exhibited more uniform electrical characteristics than the 500 °C-annealed Ni/Au Schottky samples (each with the best Schottky behavior). Furthermore, the scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) results indicated that the APC Schottky contacts were more thermally stable than the Ni/Au contacts. On the basis of the X-ray photoemission spectroscopy (XPS) results, the improved Schottky characteristics of the APC alloy contacts are described and discussed.

Rhenium-based low resistivity and low annealing temperature ohmic contacts to n-GaN

We report an ohmic contact metallization scheme for n-GaN based on a Re–Al–Ni–Au multilayer stack that offers low-resistivity and provides better edge sharpness and lower surface roughness at reduced annealing temperature compared to standard Ti–Al-based metallization. We studied three sets of samples with Re thicknesses of 10, 30, and 60 nm and measured specific contact resistances using the circular transmission line method. We obtained reliable ohmic contacts with specific contact resistance of the order of 10−6 Ω cm2 at 550 °C temperature with the contact stack having a large annealing temperature window of >300 °C for resistivity below 5 × 10−6 Ω cm2. The lowest contact resistivity, below 10−7 Ω cm2, is achieved at annealing temperature around 650–700 °C. A reduction in surface roughness by a factor of 4, with excellent edge definition for Re >30 nm thickness is observed as compared to Ti–Al contacts. Grazing incidence x-ray diffraction and electron dispersive x-ray spectroscopy (EDS) show intermetallic phases of RexNy, Re–Al–Ni, Al–Re, and Al–Au being formed. Atomic force microscope and EDS measurements show the formation of crystalline Re–Al–Ni agglomerates, surrounded by phases of AlAu2 and Al2Au5. The presence of Re seems to suppress the formation of viscous AlAu4 phase, thus minimizing the lateral flow of the metals and providing better edge acuity. The temperature dependence of contact resistivity suggests a field-emission mechanism for current transport across the contact. Our results show that Re-based ohmic contacts, with their lower annealing temperature and excellent edge definition, may offer a promising alternative to Ti–Al contacts.

Urban crime and livelihood implications among the motorcycle taxi riders in Dar Es Salaam City- Tanzania

The motorcycle taxi business is increasingly becoming one of the key livelihood strategies in the urban settings of Sub-Saharan Africa. However, crime against operators is threatening the subsector and jeopardizes the livelihood of motorcycle riders. This study examines urban crime and its implications on the livelihoods of motorcycle taxi riders in Dar es Salaam Tanzania in a bid to recommend strategies for alleviating the looming crime. Based on the qualitative design, data were collected using in-depth interviews, focus group discussion and indirect observations with a sample of 100 motorcycle taxi-riders. Thematic data analysis was employed based on the five themes [aka assets] pre-determined from the sustainable livelihood framework namely natural, physical, human, social and financial assets. Findings show that crime has negatively impacted the motorcycle taxi riders’ livelihoods through erosion of the livelihood assets. Accordingly, in order to alleviate the crime, relevant authorities should devise a well-defined mechanism of sharing crime information, use of technology such as the GPRS system and security cameras around the city, ensure the availability of the police and prompt response to crime incidence through night security patrols and sharing reliable contacts, provision of security and crime education and awareness to the riders. Others are ensuring an effective, communication system between and among the motorcycle riders e.g. through phones, WhatsApp groups, and other social media, and use of ID cards by the riders for easy identification and traceability whenever they fall victims of crime.

Understanding Printed Hexagonal Contacts for Large Area Solar Cells through Simulation and Experiments

Since their conception, organic electronic semiconductors have promised large area optoelectronic devices that can be mass produced at a fraction of the cost and embodied energy of devices made from traditional semiconductors. However, upscaling from small area lab‐scale fabrication techniques to large area roll‐to‐roll (R2R) production has proved a substantial challenge. At the heart of this upscaling problem is the need for low cost, reliable contacts which can be readily printed. Device performance is often limited by the contacts, in terms of charge extraction efficiency and morphological compatibility of the sequentially deposited layers. Herein, high‐speed R2R flexographic and gravure printing are combined with numerical device modeling to understand the performance of printed silver hexagonal contacts, and how contact design can affect final device performance. A strategy is presented which we dub “virtual upscaling” whereby the performance of the printed contact is virtually evaluated with an active layer material/device structure before the full device stack is printed. Through this methodology, a set of general design rules is developed which can be applied when experimentally optimizing contacts of optoelectronic devices. This approach has the potential to significantly reduce the number of design iterations and thus print runs when upscaling a structure.

Grasp Planning With a Soft Reconfigurable Gripper Exploiting Embedded and Environmental Constraints

Grasping in unstructured environments requires highly adaptable and versatile hands together with strategies to exploit their features to get robust grasps. This letter presents a method to grasp objects using a novel reconfigurable soft gripper with embodied constraints, the Soft ScoopGripper (SSG). The considered grasp strategy, called scoop grasp, exploits the SSG features to perform robust grasps. The embodied constraint, i.e., a scoop, is used to slide between the object and a flat surface (e.g., table, wall) in contact with it. The fingers are first configured according to the object geometry and then used to establish reliable contacts with it. This work introduces an algorithm that, given the object point cloud, computes the best pre-grasp gripper configuration from which to start the scoop grasp strategy. Several experimental trials in different scenarios confirmed the effectiveness of the proposed method.

Two-step chemical vapor deposition synthesis of NiTe2-MoS2 vertical junctions with improved MoS2 transistor performance

The primary challenge for the widespread application of two-dimensional (2D) electronics is to achieve satisfactory electrical contacts because, during the traditional metal integration process, difficulties arise due to inevitable physical damage and selective doping. Two-dimensional metal–semiconductor junctions have attracted attention for the potential application to achieve reliable electrical contacts in future atomically thin electronics. Here we demonstrate the van der Waals epitaxial growth of 2D NiTe2-MoS2 metal–semiconductor vertical junctions where the upper NiTe2 selectively nucleates at the edge of the underlying MoS2. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning transmission electron microscope (STEM) studies confirmed that NiTe2-MoS2 metal–semiconductor vertical junctions had been successfully synthesized. The electrical properties of the NiTe2-contacted MoS2 field-effect transistors (FETs) showed higher field-effect mobilities (μ FE) than those with deposited Cr/Au contacts. This study demonstrates an effective pathway to improved MoS2 transistor performance with metal–semiconductor junctions.

(Invited) Programmed Assembly of Designer Solids from Molecular Building Blocks: A Route Towards Digitalization of Materials Research?

Realizing multifunctional “Designer Solids” by programmed assembly of building units taken form libraries is a very appealing objective in connection with Materials Research entering the Information Era. Metal-organic frameworks (MOFs) have attracted a huge interest in this context, owing to their enormous wealth of different compounds – the number of characterized elements of this rapidly growing class of materials has just exceeded 100.000. Here, we will focus on a subset of MOF-based functional materials, including electrochemical, photoelectrochemical, photovoltaic and optical devices. In addition to 3D structures, also 2D interfaces in MOF heterostructures are of interest, e.g. with regard to photon-upconversion. Since the fabrication of reliable and reproducible contacts to MOF-materials represent a major challenge, we have developed a layer-by-layer (lbl) deposition method to produce well-defined, highly oriented and monolithic MOF thin films on a number of different substrates. The resulting films are referred to as SURMOFs [1,2]. The fabrication of hetero-multilayers (see Fig. 1) is rather straightforward with this lbl method. In this talk, we will describe the principles of SURMOF fabrication as well as the results of systematic investigations of electrical and photophysical properties exhibited by empty MOFs and after loading their pores with functional guests. As a final point, we will discuss further applications realized by loading MOFs with nanoparticles or quantum dots. We will close with a more general discussion on the potential of programming functionalities into MOFs using in-silico screening methods.[3]

Fuel-burning thermoelectric generators for the future of electric vehicles

Ragone plot showing power density (kW/kg) and energy density (kWh/kg) is widely used for batteries, fuel cells and other energy conversion technologies. This is particularly useful for transportation and mobile applications where the weight is a key limiting factor. Here, we present for the first time, Ragone plot for scalable fuel-burning thermoelectric power generators. Optimizing thermoelectric leg geometry, the highest power and energy densities nearly scale with the square root of the material figure-of-merit (ZT). Internal heat recovery can improve fuel economy by 350% (reaching an efficiency >20% for a temperature independent ZTavg ~0.5), however, there is a trade-off due to the additional mass of heat exchanger hence the reduced power density. Prospects for electrification of particularly compact and unmanned vehicles are discussed. We foresee the potential of using fuel to drive fully-electric vehicles with high temperature thermoelectric generators. Development of the technologies for high heat transfer, reliable and robust contacts, and high ZT over a wide temperature range presents opportunities in the future energy landscape for mobile applications.

Fabrication of layer-by-layer graphene oxide thin film on copper substrate by electrophoretic deposition

In this paper, we investigate the fabrication of graphene oxide (GO) films on copper substrates by means of an electrophoretic technique, with the aim of realizing conductive thin films that could be applied as reliable electrical contacts for use in various applications. Uniform films were fabricated, and the film thickness measured using atomic force microscopy. The graphene oxide film thickness is estimated to range from a few manometers to several tens of manometers, depending on the deposition time. Cross-sectional transmission electron microscopy images indicate that the films possess a multi-layered structure, and that the distance between layers is approximately 0.5 nm. Furthermore, the electrical contact resistance of the films was measured, using conductive probe atomic force microscopy. The prepared GO films exhibit electrical conductivity due to their layered structure, which affects their electrical properties.

Fabrication of deep-sub-micrometer NbN/AlN/NbN epitaxial junctions on a Si-substrate

We have developed a novel fabrication process for ultra-small, full-epitaxial NbN Josephson junctions on a TiN buffered silicon (Si) substrate. We use an electron beam lithography for junction definition followed by a reactive-ion-etch (RIE). A chemical mechanical polishing process and an RIE by using CHF3 gas formed reliable electrical contacts for the junction electrodes. All junctions, with a junction size down to 0.27 μm in diameter, showed excellent current–voltage characteristics with a clear gap and small sub-gap leakage current. We have confirmed that the quality of the junctions was maintained after the removal of the SiO2 dielectric interlayer.

Alcohol-Based Sulfur Treatment for Improved Performance and Yield in Local Back-Gated and Channel-Length-Scaled MoS₂ FETs

Achieving good ohmic contacts, with minimum variability, has remained a challenge for field-effect transistors (FETs) using molybdenum disulfide (MoS2). Surface state engineering using ammonium sulfide is a great way to obtain superior and reliable contacts on MoS2 using high-work-function metals such as nickel. However, the process is aggressive to thin gate dielectrics hampering device yield. We report on an improved sulfur treatment process in an alcoholic medium that is less aggressive to thin gate dielectrics and offers a record low Schottky barrier height of 131 meV on Ni–MoS2 contacts. We demonstrate the suitability of this process with channel length scaling ( ${L}_{\text {G}}$ from 500 to 80 nm) on global back-gated FETs with a record low contact resistance of $1.3~\text {k}\Omega \cdot \mu \text {m}$ for 80-nm FET. We also demonstrate the compatibility of the proposed process with local back-gated FETs on thin high- ${k}$ HfO2 gate dielectrics, which is crucial for scalable process integration in realizing integrated circuits on 2-D 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.

Impact of nanosecond laser energy density on the C40-TiSi2 formation and C54-TiSi2 transformation temperature

The formation of Ti based contacts in new image sensor complementary metal–oxide–semiconductor technologies is limited by the requirement of a low thermal budget. The objectives of these new 3D-technologies are to promote ohmic, low resistance, repeatable, and reliable contacts by keeping the process temperature as low as possible. In this work, ultraviolet-nanosecond laser annealing was performed before classical rapid thermal annealing (RTA) to promote the formation at lower RTA temperatures of the low resistivity C54-TiSi2 phase. The laser energy density was varied from 0.30 to 1.00 J/cm² with three pulses in order to form the C40-TiSi2 phase and finally to obtain the C54-TiSi2 phase by a subsequent RTA at low temperatures. The formed Ti-silicides were characterized by four-point probe measurements, x-ray diffraction, transmission electron microscopy, and atom probe tomography. A threshold in the laser energy density for the formation of the C40-TiSi2 is observed at an energy density of 0.85 J/cm² for the targeted TiN/Ti stack on blanket wafers. The C40-TiSi2 formation by laser annealing prior to RTA enables to reduce the formation temperature of the C54-TiSi2 phase by 150 °C in comparison to a single RTA applied after the Ti/TiN deposition. This specific phase sequence is only observed for a laser energy density close to 0.85 J/cm². At higher energy densities, the presence of C49-TiSi2 or a mixture of C49-TiSi2 and C54-TiSi2 is observed. The underlying mechanisms of the phase sequence and formation are discussed in detail.

Characterizing carrier transport in nanostructured materials by force-resolved microprobing

The advent of novel nanostructured materials has enabled wearable and 3D electronics. Unfortunately, their characterization represents new challenges that are not encountered in conventional electronic materials, such as limited mechanical strength, complex morphology and variability of properties. We here demonstrate that force-resolved measurements can overcome these issues and open up routes for new applications. First, the contact resistance to 2D materials was found to be sensitively depending on the contact force and, by optimizing this parameter, reliable contacts could be repeatably formed without damage to the fragile material. Moreover, resistance of three-dimensional surfaces could be investigated with high accuracy in spatial position and signal through a force-feedback scheme. This force-feedback approach furthermore permitted large-scale statistical characterization of mobility and doping of 2D materials in a desktop-sized automatic probing system that fits into glove boxes and vacuum enclosures using easily available and low-cost components. Finally, force-sensitive measurements enable characterization of complex electronic properties with high lateral resolution. To illustrate this ability, the spatial variation of a surface’s electrochemical response was investigated by scanning a single electrolyte drop across the sample.

Role of polarity in SiN on Al/GaN and the pathway to stable contacts

Despite being the most widely used dielectric for passivation of GaN-based lateral devices, amorphous silicon-nitride still faces many stability challenges, which arise from its complex bulk electronic and interface properties on the polar (Al)GaN surfaces. In this investigation, SiN has been applied as an ultra-thin interlayer (∼3–5 nm) in vertical contact structures on Ga-polar and N-polar GaN templates to study the metal–insulator–semiconductor- (MIS-) like system and better understand the interaction between the polar surface and its dielectric overlayer. We describe the role of amphoteric ≡Si centers in SiN in passivating and providing the polarization countercharge to Al/GaN of different polarities. The consequent requirements of the concentration profile of the amphoteric defects and the corresponding chemical profile of SiN is discussed. The importance of SiN surface termination and their influence on the interface potential on Al/GaN that determines device performance and reliability is also shown. Finally, a pathway to highly stable and reliable ohmic contacts to n-type Ga-polar GaN without instabilities associated with metal directly alloying with GaN as in the case of traditional contacts is proposed.