Contact Formation Process Research Articles

Realization of low specific-contact-resistance on N-polar GaN surfaces using heavily-Ge-doped n-type GaN films deposited by low-temperature reactive sputtering technique

We developed a low-temperature ohmic contact formation process for N-polar GaN surfaces. Specific-contact-resistances of 9.4 × 10−5 and 2.0 × 10−5 Ω cm2 were obtained using Ti/Al metal stacks on heavily-germanium-doped GaN films, which were deposited at 500 °C and 600 °C using a radical-assisted reactive sputtering method, respectively. The electrode sintering temperature was as low as 475 °C. Carrier concentrations for the 500 °C and 600 °C samples were 2.6 × 1020 and 1.8 × 1020 cm−3, respectively. These results suggest that this method is highly effective in reducing the contact resistance of GaN devices with low thermal budgets.

Regime features of austempered ductile iron cutting

Machining regimes and sub-regimes of dry straight finish-turningof structural austempered ductile iron (ADI) with low cBN-content PcBN-based cutting inserts were examined. The global and local characteristics of TC contact and chip formation processes tied to this turning, as estimated by Katuku [Journal of Manufacturing Processes 78, 2022, 288–307], were used in this attempt. The significance of each prominent feature of these regimes was discussed from multiple perspectives. Across primary deformation zones (PDZs) and secondary deformations zones (SDZs), drag-controlled (or shock-induced) and defect-controlled plasticity regimes are, in turn, dominant and secondary. Coupled shear localisations in PDZs and SDZs occur via strain softening for cutting speeds beyond ~175 m min−1. BUE softening turns active at ~650 °C (~125 m min−1). Two bordering machining regimes emerged in close link with this cut-off temperature alongside a ⁓16 % cut-off SDZPDZ softening-degree, to name a few. Higher SDZPDZ softening-degree and moderately low mean contact stresses (high TC contact-length factor), TC contact temperature and sliding-friction coefficient, prevailing for Machining-regime-I (50–150 m min−1), hint at lower-pace wear regimes. The opposite conditions, standing for Machining-regime II (200–800 m min−1), suggest temperature-affected and stress-backed higher-pace wear regimes. Mean total energy levels (3476–7122 and 4364–4596 MPa) imply residual stresses built-up and microstructure damage below the machined surface. Machining sub-regimes are linked to solid-state phase transformations in PDZs/SDZs. Shear energy stored in PDZs/SDZs (166–291/1.5–43 MPa and 159–134/33–72 MPa) matches these transformations. Machining sub-regimes are sensitive to shallow variations in TC settings past ~175 m min−1. Powder lubrication by smeared-out graphite nodules is worthless in Machining-regime II. Cutting speeds of 150–400 m min−1 (TC contact temperature of ~685–930 °C and chip compression ratio of 3.4–2.32), sampled at the confluence of Machining-regimes I and II, are recommended for dry straight finish-turning structural ADI with low cBN-content PcBN-based cutting inserts.

Study of Laser Backside Ohmic Contact Formation of SiC-Ni Interface to Evaluate the Process Influence on the Electrical Characteristics

Within this paper, we will present the results of a study on the ohmic contact formation process with nanosecond (ns) pulsed UV lasers. For the study we compared two laser processes: The base line process with a 100-300 ns pulsed laser with Gaussian beam profile and the 3D-Micromac AG process with a 50-100 ns pulsed laser with top hat beam profile. The forward voltage characteristics at wafer level was analyzed and proves a clear benefit of the top hat laser process. Besides, the forward voltage characteristics of a second run was performed to analyze the influence of increasing energy density to the electrical characteristic of heat sensitive front side structures. Also with high energy density no negative influence could be detected.

Dopant Activation Depth Profiling for Highly Doped Si:P By Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM)

Heavily P-doped epitaxial Si layers were deposited over counter-doped Si substrates. P dopant activation was studied as a function of various processes used to prepare the samples. For electrical characterization, depth profiles of carrier concentration values were obtained using SSRM and DHEM techniques. Samples studied included an in-situ doped sample, a sample that was spike annealed at 1000ºC, a sample that was first coated with a Ti/TiN stack and then annealed for silicide formation before the Ti/TiN stack was removed, and a sample that was spike annealed + coated with Ti/TiN + annealed for silicide formation before the removal of the Ti/TiN stack. DHEM analysis showed substantial increase in dopant activation for spike annealed samples. SSRM results suggested lower carrier concentration values and more limited degree of activation. Formation of Ti/TiN contacts and its removal did not affect the carrier concentration values for the as-deposited epi layer. Sample that was first spike annealed and then subjected to contact formation process displaced a reduction in carrier concentration values as measured by DHEM.

Microscopic Understanding of the Growth and Structural Evolution of Narrow Bandgap III-V Nanostructures.

III–V group nanomaterials with a narrow bandgap have been demonstrated to be promising building blocks in future electronic and optoelectronic devices. Thus, revealing the underlying structural evolutions under various external stimuli is quite necessary. To present a clear view about the structure–property relationship of III–V nanowires (NWs), this review mainly focuses on key procedures involved in the synthesis, fabrication, and application of III–V materials-based devices. We summarized the influence of synthesis methods on the nanostructures (NWs, nanodots and nanosheets) and presented the role of catalyst/droplet on their synthesis process through in situ techniques. To provide valuable guidance for device design, we further summarize the influence of structural parameters (phase, defects and orientation) on their electrical, optical, mechanical and electromechanical properties. Moreover, the dissolution and contact formation processes under heat, electric field and ionic water environments are further demonstrated at the atomic level for the evaluation of structural stability of III–V NWs. Finally, the promising applications of III–V materials in the energy-storage field are introduced.

Comparison and Mechanism Analysis of Three-Phase Contact Formation onto Hydrophilic/Hydrophobic Mineral Surfaces in the Presence of Cationic/Anionic Surfactants during Flotation Process

This article presents the dynamic process of the three-phase contact (TPC) formation by colliding bubbles onto muscovite and talc surfaces in water and two types of solutions including cationic CTAC surfactant (cetyltrimethyl ammoniumchloride) and anionic NaOL surfactant (sodium oleate). The TPC formation process was observed through the high-speed camera between bubbles and layered silicate minerals (hydrophilic muscovite and hydrophobic talc). It was found that the rupture of the liquid film between the bubbles and the mineral surface is a prerequisite for TPC formation. In the case of muscovite, TPC was formed only with cationic CTAC, and as the surfactant concentration increased, the time needed for TPC formation was shortened. Due to electrostatic repulsion, TPC did not occur in water and NaOL. However, for talc, TPC occurred both in water and in surfactant solutions. In contrast to muscovite, the time of TPC formation on the talc surface was prolonged with the increase in the surfactant concentration. It was concluded that hydrophobic attraction and electrostatic attraction between mineral surfaces and bubbles can significantly promote the localized foam film rupture, which was the main reason for the TPC appearance in water and surfactants. For the hydrophilic muscovite, CTAC adsorption improved the surface hydrophobicity; I3/I1 in fluorescence spectroscopy increased, and the micro-polarity faded, making TPC formation need more time. However, for the natural hydrophobic talc, the increasing surfactant adsorption decreased I3/I1 values and enhanced the local micro-polarity, causing the extension of time for TPC. Therefore, TPC formation for different minerals resulted from different reasons.

Cortical tension initiates the positive feedback loop between cadherin and F-actin

Adherens junctions physically link two cells at their contact interface via extracellular binding between cadherin molecules and intracellular interactions between cadherins and the actin cytoskeleton. Cadherin and actomyosin cytoskeletal dynamics are regulated reciprocally by mechanical and chemical signals, which subsequently determine the strength of cell-cell adhesions and the emergent organization and stiffness of the tissues they form. However, an understanding of the integrated system is lacking. We present a new mechanistic computational model of intercellular junction maturation in a cell doublet to investigate the mechanochemical cross talk that regulates adherens junction formation and homeostasis. The model couples a two-dimensional lattice-based simulation of cadherin dynamics with a reaction-diffusion representation of the reorganising actomyosin network through its regulation by Rho signalling at the intracellular junction. We demonstrate that local immobilization of cadherin induces cluster formation in a cis-less-dependent manner. We then recapitulate the process of cell-cell contact formation. Our model suggests that cortical tension applied on the contact rim can explain the ring distribution of cadherin and actin filaments (F-actin) on the cell-cell contact of the cell doublet. Furthermore, we propose and test the hypothesis that cadherin and F-actin interact like a positive feedback loop, which is necessary for formation of the ring structure. Different patterns of cadherin distribution were observed as an emergent property of disturbances of this positive feedback loop. We discuss these findings in light of available experimental observations on underlying mechanisms related to cadherin/F-actin binding and the mechanical environment.

Reliable metal-graphene contact formation process flows in a CMOS-compatible environment.

The possibility of exploiting the enormous potential of graphene for microelectronics and photonics must go through the optimization of the graphene-metal contact. Achieving low contact resistance is essential for the consideration of graphene as a candidate material for electronic and photonic devices. This work has been carried out in an 8'' wafer pilot-line for the integration of graphene into a CMOS environment. The main focus is to study the impact of the patterning of graphene and passivation on metal-graphene contact resistance. The latter is measured by means of transmission line measurement (TLM) with several contact designs. The presented approaches enable reproducible formation of contact resistivity as low as 660 Ω μm with a sheet resistance of 1.8 kΩ/□ by proper graphene patterning, passivation of the channel and a post-processing treatment such as annealing.

Scalable fabrication and electrical contact formation process for vertically oriented silicon nanopillars in trenches

Vertical silicon nano-pillars (Si N-Pls) inside the cylindrical trenches have been fabricated as a prospective platform for the development of silicon nanowire based devices. Deep reactive ion etching (DRIE) has been used to etch these features in a silicon substrate. In the present case, the fabricated Si N-Pls are ~15 μm in length and 200–500 nm in diameter, while the trenches are of the diameter 12–25 μm. Alumina (Al2O3) discs of 50 nm thickness and diameter in the range of 200–500 nm, obtained via e-beam evaporation, have been used as a hard mask for defining the Si N-Pls during DRIE. Further, a proof of concept has been provided for obtaining electrical contact to the top-ends of these vertically oriented nano-pillars. The proposed platform involving nano-pillars in trenches can be extended for realizing microfluidic-channels containing an array of vertically oriented and electrically addressable silicon nanowires for several applications including chemical sensing.

Acoustic Determination of Properties Possessed by Porous Titanium

To impart the optimal structure to a powder material and promote high mechanical properties, the production process is to be monitored at each step using nondestructive methods. Acoustic methods are believed to be promising in this respect. The parameters of acoustic fields are influenced by many factors that determine the structure and properties of powder materials. A relevant task is to improve acoustic methods for the identification and use of this influence. A model material is taken as an example to show the capabilities of a nondestructive acoustic method relying on integrated measurement of the propagation velocities of longitudinal, transverse, and rod elastic waves in studying the patterns peculiar to the powder structurization at the molding and sintering stages. The following problems that can be solved using this method are considered and experimentally confirmed: identify how the properties of the starting powders influence the characteristics of the final material, monitor the compaction and contact formation processes in the material, and identify features of these processes, which are manifested as uneven distribution of properties over the material and aftereffects and are stepwise. Additional criteria are proposed to increase the reliability and descriptiveness of the results. Analysis of the experimental data shows that the propagation velocities of bulk and rod elastic waves in different ways reflect the compaction and contact formation in powder materials. The differences are explained by the peculiarities of acoustic fields generated by various types of elastic waves.

1.2 kV silicon carbide Schottky barrier diode embedded MOSFETs with extension structure and titanium-based single contact

Recently, to avoid forward-voltage degradation of body diodes, which is caused by the expansion of stacking faults due to hole-current conduction through the body diodes, silicon carbide (SiC) Schottky barrier diode (SBD) embedded metal-oxide-semiconductor field effect transistors have been actively studied. However, most studies focus on designs that do not allow hole current to flow when reverse current flows from the source to drain, and there are few reports that mention the contact-formation process or contact reliability. A low-resistance ohmic contact and a highly reliable Schottky contact were formed simultaneously by combining the recrystallization of an N+ heavily implanted region to 3C-SiC, opening of a contact hole by dry etching and formation of a titanium electrode followed by post-annealing. Moreover, gate leakage current can be reduced by improving the crystallinity and morphology near the channel by introducing an extension structure, even if the process for simultaneously forming the ohmic and Schottky contacts is applied to the MOSFETs fabrication process. The developed process minimizes the increase in cell pitch due to the incorporation of the SBD and suppresses the conventional increase in on-resistance to about 10%. Regarding diode characteristics, expansion of the stacking faults could be suppressed by eliminating the hole-current conduction, even in the case of a large current density of 500 A cm−2 at 175 °C.

New insights into the contact angle and formation process of nanobubbles based on line tension and pinning

In this paper, the effect of a probe tip on the contact angle measurements and the formation process of nanobubbles based on line tension and pinning theory was investigated. In this investigation, the images of nanobubbles and micropancakes on mica(001) and molybdenite(0001) surfaces were obtained during an alcohol-water exchange by means of atomic force microscopy (AFM) in tapping mode (TMAFM). Based on the image characteristics and the theory of line tension and pinning, a new point of view is presented: the contact angle of a nanobubble is not always a large obtuse angle but is an uncertain value; its true value is affected by the curvature radius of the AFM probe tip, and the value is close to the macroscopic contact angle. Meanwhile, the formation of nanobubbles is affected by the surface tension, line tension and pinning.

Rapid and accurate characterization of silver-paste metallization on crystalline silicon solar cells by contact-end voltage measurement

Contact-end voltage measurement was applied to characterize the contact-formation process of silver paste metallization on p- and n-type crystalline silicon solar cells under different temperatures with well-designed fixtures and test patterns based on the circular transmission line model. The contact-end voltage values were found to be sensitive to sintering temperature, and the current density and contact end voltage curves of both contacts were linear, stating that the contacts were ohmic contacts. Their symbols on the n-type emitter reversed from negative to positive under the established connection mode, which indicated conductive-path changes manifested in the form of macroscopic electrical properties under insufficient, optimal, and over-fired conditions. We inferred that the conductive channel variations were mainly caused by the silver crystallites that precipitated on the emitter surface from the combination of the cross-sectional and interface morphology analyses. No similar phenomenon was observed on the p-type emitter for the few silver crystallites or silver-aluminum alloy without conductive-path alternation. Their values were much greater than those of the n-emitter, which agreed with the present industrial n-type cell characteristics. The measurement improved our understanding of the contact formation process, and can be used as a flexible approach for researchers to optimize the silver-paste formula and sintering processes for high-efficiency solar cells.

Improved Device Performance in AlGaN/GaN HEMT by Forming Ohmic Contact With Laser Annealing

An AlGaN/GaN high-electron-mobility transistor with a novel ohmic contact formation process based on conventional pulsed laser annealing is proposed. Compared with the conventional rapid thermal annealing process, the laser annealing (LA) process not only improves the surface morphology of ohmic contact metal but also exhibits a higher OFF-state breakdown voltage. An excellent rms roughness of 16.5 nm is observed by atomic force microscope which is 16.3% of the reference one. In a device with a gate-to-drain distance of 5 $\mu \text{m}$ , a forward blocking voltage of 442 V is obtained at ${V}_{\textsf {GS}} = -\textsf {8}$ V which has a 37.3% improvement compared with the conventional device. The device with LA process has lower contact resistance at actual working temperature (above 125 °C). The current transport mechanism is dominated by thermionic field emission through the ohmic contact. Moreover, the degradation of dynamic ON-resistance is significantly suppressed due to lower surface states by protecting the bare AlGaN layer from directly exposing under high temperature ambient.

Electrical characterization of single nanometer-wide Si fins in dense arrays.

This paper demonstrates the development of a methodology using the micro four-point probe (μ4PP) technique to electrically characterize single nanometer-wide fins arranged in dense arrays. We show that through the concept of carefully controlling the electrical contact formation process, the electrical measurement can be confined to one individual fin although the used measurement electrodes physically contact more than one fin. We demonstrate that we can precisely measure the resistance of individual ca. 20 nm wide fins and that we can correlate the measured variations in fin resistance with variations in their nanometric width. Due to the demonstrated high precision of the technique, this opens the prospect for the use of μ4PP in electrical critical dimension metrology.

Direct Observation of the Impurity Gettering Layers in Polysilicon-Based Passivating Contacts for Silicon Solar Cells

The formation of certain types of doped polysilicon passivating contacts for silicon solar cells is recently reported to generate very strong impurity gettering effects, revealing an important additional benefit of this passivating contact structure. This work investigates the underlying gettering mechanisms by directly monitoring the impurity redistribution during the contact formation and subsequent processes, via a combination of secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and minority carrier lifetime techniques. Microscopic features of the phosphorus and boron diffusion-doped polysilicon passivating contacts are also presented. Iron is used as a marker impurity in silicon to enable direct quantification of its concentration change in the bulk of the silicon wafers and in the surface layers that compose the contact structure. The results conclusively show that, for phosphorus-doped polysilicon passivating contacts, impurities are relocated from the silicon wafer bul...

Role of first-neighbor geometry in the electronic and mechanical properties of atomic contacts

We study in detail, by experimental measurements, atomistic simulations and DFT transport calculations, the process of formation and the resulting electronic properties of atomic-sized contacts made of Au, Ag and Cu. Our novel approaches to the data analysis of both experimental results and simulations, lead to a precise relationship between geometry and electronic transmission. We reestablish the significant influence of the number of first neighbors on the electronic properties of atomic-sized contacts. Our results allow us also to interpret subtle differences between the metals during the process of contact formation as well as the characteristics of the resulting contacts.

Improved Turn-On and Operating Voltages in AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

While good ohmic contact formation has been achieved on both p-GaN and n-AlGaN surfaces, the turn-on and operating voltages of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) remain very high. We find that this critical problem is mainly caused by the large difference between the annealing temperatures required for ohmic contact formation on the p-GaN and high Al-fraction n-AlGaN surfaces. We studied the effects of the high-temperature annealing treatments required for n-ohmic contact formation on the subsequent p-ohmic contact formation process in DUV-LEDs. The results show that post-annealing treatment at high temperature is necessary to form an ohmic contact on n-Al0.7Ga0.3N, but a treatment temperature of 900°C or more could cause severe degradation of the specific contact resistivity and the bulk resistivity of p-GaN. We conclude that 900°C is the optimum temperature to form an ohmic contact on n-Al0.7Ga0.3N in DUV-LEDs, where p-GaN and n-Al0.7Ga0.3N act as the p- and n-ohmic contact layers, respectively. We also found that the specific contact resistivity of p-GaN can be reduced by an additional low-temperature annealing treatment after the high-temperature annealing step; this effect can be attributed to the enhancement of the hole concentration in the p-GaN surface contact region. Finally, DUV-LEDs that emit at 280 nm were fabricated using four different annealing treatments during processing. A considerable reduction in the series resistance and thereby in the operating voltage was confirmed using the annealing process proposed above, consisting of a high-temperature anneal at 900°C followed by a low-temperature anneal at 500°C for 3 min.

In-depth investigation of spin-on doped solar cells with thermally grown oxide passivation

Solar cell industrial manufacturing, based largely on proven semiconductor processing technologies supported by significant advancements in automation, has reached a plateau in terms of cost and efficiency. However, solar cell manufacturing cost (dollar/watt) is still substantially higher than fossil fuels. The route to lowering cost may not lie with continuing automation and economies of scale. Alternate fabrication processes with lower cost and environmental-sustainability coupled with self-reliance, simplicity, and affordability may lead to price compatibility with carbon-based fuels. In this paper, a custom-designed formulation of phosphoric acid has been investigated, for n-type doping in p-type substrates, as a function of concentration and drive-in temperature. For post-diffusion surface passivation and anti-reflection, thermally-grown oxide films in 50–150-nm thickness were grown. These fabrication methods facilitate process simplicity, reduced costs, and environmental sustainability by elimination of poisonous chemicals and toxic gases (POCl3, SiH4, NH3). Simultaneous fire-through contact formation process based on screen-printed front surface Ag and back surface through thermally grown oxide films was optimized as a function of the peak temperature in conveyor belt furnace. Highest efficiency solar cells fabricated exhibited efficiency of ∼13%. Analysis of results based on internal quantum efficiency and minority carried measurements reveals three contributing factors: high front surface recombination, low minority carrier lifetime, and higher reflection. Solar cell simulations based on PC1D showed that, with improved passivation, lower reflection, and high lifetimes, efficiency can be enhanced to match with commercially-produced PECVD SiN-coated solar cells.

The formation mechanism for printed silver-contacts for silicon solar cells.

Screen-printing provides an economically attractive means for making Ag electrical contacts to Si solar cells, but the use of Ag substantiates a significant manufacturing cost, and the glass frit used in the paste to enable contact formation contains Pb. To achieve optimal electrical performance and to develop pastes with alternative, abundant and non-toxic materials, a better understanding the contact formation process during firing is required. Here, we use in situ X-ray diffraction during firing to reveal the reaction sequence. The findings suggest that between 500 and 650 °C PbO in the frit etches the SiNx antireflective-coating on the solar cell, exposing the Si surface. Then, above 650 °C, Ag+ dissolves into the molten glass frit – key for enabling deposition of metallic Ag on the emitter surface and precipitation of Ag nanocrystals within the glass. Ultimately, this work clarifies contact formation mechanisms and suggests approaches for development of inexpensive, nontoxic solar cell contacting pastes.