Nickel dissolution into AuGe in alloyed AuGe/Ni/Au Ohmic contacts on GaAs/AlGaAs multilayer structures

Magnetic properties of alloyed Ohmic contacts of the type AuGe/Ni/Au on GaAs/AlGaAs multilayers with n⁺ cap layer with different AuGe compositions and Ni-layer thicknesses are examined. Magnetization data indicate that the annealed structures are non-magnetic, at room temperature for commonly used anneal temperatures (∼ 400-430 °C) and Ni-layer thicknesses (10-100 nm). The transformation of Ni to non-magnetic phase begins at ∼ 100 °C, well below temperatures at which extensive alloying with the GaAs substrate takes place. The fraction of Ni transformed to non-magnetic phase on annealing appears to scale with AuGe layer thickness, has a quadratic dependence on anneal temperature and is time independent for time scales of minutes. The data indicate that the Ni layer dissolves into the AuGe layer at temperatures well below that at which alloying between AuGe and GaAs substrate takes place. The dissolved Ni concentration is limited by a solubility that increases with anneal temperature and decreases with decreasing Ge content from that of the AuGe eutectic composition.     

Preface

The reliability of GaAs microwave devices is directly related to the integrity of Schottky and ohmic contacts for Schottky barrier devices and metal/semiconductor field effect transistor devices. The interface analysis of these device structures using surface analysis techniques has become extremely important in the study of the degradation of these devices. The research reported here focuses on three different metallic systems, namely Au/In, Au-12 wt.% Ge and Ni/AuGe, for both Schottky and ohmic contacts. The three metallic systems were evaporated onto 〈100;〉- oriented GaAs substrates (N_D=3×10¹⁷cm^-3) in an ultrahigh vacuum system. These samples were thermally aged by keeping them at 150°C for 500h. Current-voltage and capacitance-voltage measurements were made on as-deposited and thermally aged samples. The ideality factor decreased in all the samples. There was an apparent large increase in barrier height in AuGe/GaAs and Ni/AuGe/GaAs Schottky diodes. There was an insignificant change in the contact resistivity of ohmic contacts after thermal aging.The changes in the electrical characteristics of these device structures are explained on the basis of the formation of an oxide layer after thermal aging. A comparison of the Auger depth profiles of the as-deposited and the thermally aged samples substantiates the electrical observations. However, Au/In/GaAs Schottky diodes do not show the existence of an oxide layer at the interface. The out-diffusion of indium to the surface might have removed the oxygen from the interface to result in an Au-GaAs interface in the thermally aged sample. A slight increase in the barrier height of this sample is due to the Au-GaAs interface rather than the In-GaAs interface.     

石墨烯在量子霍尔电阻中的应用

基于砷化镓的量子霍尔电阻自然基准需要在约1.5K的温度条件下运行,存在成本高和操作复杂等诸多问题。随着石墨烯材料独特电性能的发现,因其可以在约4.2K的温度复现量子霍尔效应而成为制作量子霍尔电阻的理想材料。各国专家围绕石墨烯在电学计量领域的应用开展了大量的工作,取得了可喜的进展。对当前石墨烯在量子霍尔电阻中应用的进展和存在的问题进行了总结,并对未来的发展进行了展望。     

Capacitance theory of open quantum systems with classical contacts

We consider semiconductor devices composed of a small quantum structure as the active device region and two classical environments constituting the source- and the drain contact. The contacts are taken as free electron gases with infinite conductivity defining the chemical potentials in the contacts. The transport through the quantum structure is described in the Landauer–Büttiker formalism using electronic scattering wave functions which determine the electron density in the quantum system. In our Hartree approximation these charges and the induced charges in the contacts are the sources of the self-consistent Coulomb field. As a particular quantum structure we study a GaAs heterostructure device consisting of a two-dimensional electron gas sandwiched between a gate contact and an AlGaAs blocking barrier [see V.T. Dolgopolov et al., Phys. Low-Dim. Struct. 6 (1996) 1]. We demonstrate the quantitative agreement of our theory with the experimental results.     

Beyond Gold: Spin‐Coated Ti3C2‐Based MXene Photodetectors

2D transition metal carbides, known as MXenes, are transparent when the samples are thin enough. They are also excellent electrical conductors with metal‐like carrier concentrations. Herein, these characteristics are exploited to replace gold (Au) in GaAs photodetectors. By simply spin‐coating transparent Ti₃C₂‐based MXene electrodes from aqueous suspensions onto GaAs patterned with a photoresist and lifted off with acetone, photodetectors that outperform more standard Au electrodes are fabricated. Both the Au‐ and MXene‐based devices show rectifying contacts with comparable Schottky barrier heights and internal electric fields. The latter, however, exhibit significantly higher responsivities and quantum efficiencies, with similar dark currents, hence showing better dynamic range and detectivity, and similar sub‐nanosecond response speeds compared to the Au‐based devices. The simple fabrication process is readily integratable into microelectronic, photonic‐integrated circuits and silicon photonics processes, with a wide range of applications from optical sensing to light detection and ranging and telecommunications.     

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.     

Thermal reliability of n-GaAs/Ti/Pt/Au Schottky contacts with thin Ti films for reduced gate resistance

The thermal stability of Ti/Pt/Au Schottky contacts on n-GaAs with Ti films 0–60 nm is investigated. The contacts with Ti films as small as 10 nm remain thermally stable with annealing up to 400°C. The changes induced by thermal treatment in the electrical characteristics of the contacts are correlated with the Rutherford backscattering and microscopic analysis of the annealed samples. It shows profuse interdiffusion and interfacial reaction with 300°C anneal for the GaAs/Pt/Au system. It has been found that introducing the Ti film between GaAs and Pt/Au, the interdiffusion of Pt and Au is also prevented. These results are useful for reducing the gate metallisation resistance of metal semiconductor field effect transistors.     

Thermally stable Pd/Sn and Pd/Sn/Au ohmic contacts to n-type GaAs

Thermal stability of novel Pd/Sn and Pd/Sn/Au Ohmic contacts to n-GaAs has been investigated and compared to the non-alloyed Pd/Ge and alloyed Au–Ge/Ni metallizations. Metallization samples are furnace annealed at various temperatures and systematically characterized utilizing Scanning Electron Microscopy (SEM) and current–voltage (I–V) measurements. Contact resistivities, ρ_c, of the proposed metallization are measured using a conventional Transmission Line Model (cTLM) method. The Pd/Sn Ohmic contacts display superior thermal stability at 410°C when compared to the Pd/Ge contacts. After annealing at 410°C for 4 h, ρ_c of the Pd(50 nm)/Sn(125 nm) metallization remains in the low 10⁻⁵ Ω cm² range, whereas ρ_c values increase to 10⁻⁴ Ω cm² for the Pd(50 nm)/Ge(126 nm) contacts. At 410°C, the Pd/Sn/Au metallizations also display better thermal stability than that of non-alloyed Pd/Ge and alloyed Au–Ge/Ni metallizations. The long-term stability at 300°C of the Pd/Sn and Pd/Sn/Au Ohmic contacts is also reported.     

Development of wear resistant composite surface on mild steel by laser surface alloying with silicon and reactive melting

The present study concerns laser surface alloying with silicon of mild steel substrate using a high-power continuous wave CO₂ laser with an objective to improve wear resistance. The effect of surface remelting using nitrogen as shrouding environment (with and without graphite coating) on microhardness and wear resistance has also been evaluated. Laser surface alloying leads to formation of a defect free microstructure consisting of iron silicides in laser surface alloyed mild steel with silicon and a combination of silicides and nitrides when remelted in nitrogen. Carbon deposition prior to remelting leads to presence of a few martensite in the microstructure. A significant improvement in microhardness is achieved by laser surface alloying and remelting to a maximum of 800 VHN when silicon alloyed surface is melted using nitrogen shroud with carbon coating. A detailed wear study (against diamond) showed that a significant improvement in wear resistance is obtained with a maximum improvement when remelted in nitrogen atmosphere followed by carbon coating.     

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires

The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Unlike Ni/Au contacts to planar GaN, current-voltage (I-V) measurements of Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N(2)/O(2). This degradation originates from the poor wetting behavior of Ni and Au on SiO(2) and the excessive void formation that occurs at the metal/NW and metal/oxide interfaces. The void formation can cause cracking and delamination of the metal film as well as reduce the contact area at the metal/NW interface, which increases the resistance. The morphology and composition of the annealed Ni/Au contacts on SiO(2) and the p-GaN films were investigated by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) measurements. Adhesion experiments were performed in order to determine the degree of adhesion of the Ni/Au films to the SiO(2) as well as observe and analyze the morphology of the film's underside by SEM. Device degradation from annealing was prevented through the use of a specific adhesion layer of Ti/Al/Ni deposited prior to the nanowire dispersal and Ni/Au deposition. I-V measurements of NW devices fabricated using this adhesion layer showed a decrease in resistance after annealing, whereas all others showed an increase in resistance. Transmission electron microscopy (TEM) on a cross-section of a NW with Ni/Au contacts and a Ti/Al/Ni adhesion layer showed a lack of void formation at the contact/NW interface. Results of the XRD and TEM analysis of the NW contact structure using a Ti/Al/Ni adhesion layer suggests Al alloying of the Ni/Au contact increases the adhesion and stability of the metal film as well as prevents excessive void formation at the contact/NW interface.     

Selective MBE growth of nonalloyed ohmic contacts to 2D electron gas in high-electron-mobility transistors based on GaN/AlGaN heterojunctions

Specific features of how nonalloyed ohmic contacts to the 2D conducting channel of high-electron-mobility transistors based on AlGaN/(AlN)/GaN heterostructures are fabricated via deposition of heavily doped n ⁺-GaN through a SiO₂ mask by ammonia molecular-beam epitaxy have been studied. The technique developed makes it possible to obtain specific resistances of contacts to the 2D gas as low as 0.11 Ω mm on various types of Ga-face nitride heterostructures, which are several times lower than the resistance of conventional alloyed ohmic contacts.     

激光熔敷钴合金涂层的抗空蚀和冲刷磨损性能

采用激光熔敷方法在20SiMn低合金钢表面制成了Co合金涂层，以两种目前水轮机转用材20SiMn低合金钢和0Cr13Ni5Mo不锈钢作为对比材料，对涂层进行了空蚀，液固两相流冲刷磨损及空蚀和冲刷磨损联合作用实验，采用失重法并结合能谱分析，X射线衍射分析及显微硬度测量等手段研究了涂层的显微结构和在不同形式损伤下涂层的性能并分析了原因。结果表明：在20SiMn低合金钢表面激光熔敷形成的Co合金涂层有良好的抗空蚀性能，这与涂层在空蚀过程中发生的应力诱发马氏体相变密切相关。良好的抗空蚀性能有利于提高涂层抗空蚀和冲刷磨损联合破坏，从而使涂层表现出与0Cr13Ni5Mo不锈钢接近的抗联合损伤性能。     

2.4 Formation of various metal semiconductor junctions by means of ion evaporation

The paper deals with the result of some preliminary experiments of applying ion-evaporated metal layers to silicon and heat treatment of the samples. Ion evaporation is based on the electron beam evaporation, namely, the ions arising from the 180° deflection type e-gun are accelerated to cause implantation effects during the condensation of the neutral particles. The accelerating voltage is typically 20 kV, the ion current 5–20 mA.Four metals have been chosen for the experiments: Al, Ni, Cu and Mo. The I–V-characteristics of the contacts deposited by ion evaporation were ohmlc. The curves appreciably changed after heat treatment of the samples at 400, 500 and 580°C.The AlSi contacts had very high resistivity after the treatment at 400°C, but then Al alloyed diode type contacts were achieved at 500°C.Ni and Mo formed alloyed type contacts at 580°C, and similar results were obtained in case of Cu. Very interesting results were obtained for Ni and Mo at 500°C, where we obtained I–V-characteristics of the Schottky-diode type, but with the reverse polarity.     

Effect of active metal coatings on the mechanical properties of silicon nitride-based ceramics

The effects of titanium, zirconium, hafnium and tantalum coatings on the mechanical properties of three silicon nitride ceramics were studied. The titanium coatings was found to cause a 50% decrease in the four-point bend strength of one of the silicon nitride ceramics while the effects of the zirconium, hafnium and tantalum coatings on all three silicon nitride ceramics were moderate. The reactions at a high temperature (940–980°C) between titanium and the grain-boundary glassy phase was the major cause for the degradation of the ceramic properties by the titanium coating. Residual tensile stress developed at the reaction interface replaced the glassy grain-boundary phase. Analytical electron microscopy showed the formation of a 180 nm thick Ti₅Si₃ layer and the crystallization of the amorphous grain-boundary phase. An indentation technique was used to measure qualitatively the residual stress developed at the reaction interface.     

新型量子霍尔电阻样品计量应用研究

采用AsGa砷化镓系材料研制了多种结构的新型量子霍尔电阻样品,介绍了该样品的特点及应用领域。使用常温电流比较仪和低温电流比较仪,用过渡比对法对新型的量子霍尔电阻与标准量子霍尔电阻样品进行了双重测量比对验证,在实验室内测量结果的相对偏差小于4×10^-8,验证了新型的量子霍尔电阻样品和测量系统的准确度满足当前计量应用需求。     

Ni/Au、Pd/Au与p-AlGaN材料的接触特性

在AlGaNpin型日盲紫外探测器结构中的p-AlGaN层上生长了Ni/Au和Pd/Au,并在600～850℃温度下进行快速热退火,测量其退火前后传输线模型中各金属接触间的电学性质。实验发现,Ni/Au与Pd/Au在p-AlGaN上表现出了不同的接触性能。为了更好的说明金属与p-AlGaN材料接触之间在退火后电流的变化,还测量了p-AlGaN材料裸片两点之间I-V曲线在退火前后的变化。实验表明,比起Ni/Au来,Pd/Au在p-AlGaN材料上制备欧姆接触具有一定的优势,并在文中进行了分析。     

钛合金表面加弧辉光离子渗镍铬及其性能研究

采用加弧辉光离子渗金属新技术处理钛合金Ti5Al2.5Sn表面,研究了渗层的相组成特点,成分分布情况,评价了改性层的磨擦摩损性能,及与钛合金基体间的接触腐蚀相容性等。结果表明加弧辉光离子渗技术可以快速地在钛合金表面获得NiCr镀渗复合层。渗层由Ni3Ti等金属间化合物组成,其硬度、耐磨性能均高于离子注氮层,具有较高的抗含Cl^-1水溶液腐蚀性能,在含Cl^-1腐蚀环境中与钛合金基体接触相容。     

Preservation of Surface Conductivity and Dielectric Loss Tangent in Large‐Scale,Encapsulated Epitaxial Graphene Measured by Noncontact Microwave Cavity Perturbations

Regarding the improvement of current quantized Hall resistance (QHR) standards, one promising avenue is the growth of homogeneous monolayer epitaxial graphene (EG). A clean and simple process is used to produce large, precise areas of EG. Properties like the surface conductivity and dielectric loss tangent remain unstable when EG is exposed to air due to doping from molecular adsorption. Experimental results are reported on the extraction of the surface conductivity and dielectric loss tangent from data taken with a noncontact resonance microwave cavity, assembled with an air‐filled, standard R100 rectangular waveguide configuration. By using amorphous boron nitride (a‐BN) as an encapsulation layer, stability of EG's electrical properties under ambient laboratory conditions is greatly improved. Moreover, samples are exposed to a variety of environmental and chemical conditions. Both thicknesses of a‐BN encapsulation are sufficient to preserve surface conductivity and dielectric loss tangent to within 10% of its previously measured value, a result which has essential importance in the mass production of millimeter‐scale graphene devices demonstrating electrical stability.     

Microstructure and electrical characteristics of Ni-Cr thin films

Microstructure of thin films of different compositions of Ni-Cr alloy, over the range from 40 wt.% to 80 wt.% Ni, deposited on silicon nitride coated GaAs substrates, is investigated under Transmission Electron Microscope (TEM) in as-deposited and annealed conditions. The procedure of wet etching vias through GaAs substrates is used for preparing samples for TEM. Microstructures of as-deposited films of different Ni-Cr alloys are related to the nature of the parent alloy in the binary phase diagram. The microstructural transformations in annealed films of single and two-phase alloy materials are interpreted based on the comparison of TEM micrographs in conjunction with the X-ray diffraction and electron diffraction data. The changes in the electrical characteristics such as resistivity and temperature coefficient of resistance (TCR) of the films in relation to the microstructural transformations are discussed. The effect of superimposing a polyimide layer on the TCR of Ni-Cr films is also discussed.     

The characterization of plasma-sprayed/rapid omnidirectional compaction — processed tantalum coatings as applied to densified ceramic substrates

The application of rapid omnidirectional compaction (ROC) processing to plasma-sprayed tantalum coatings in order to produce dense wear-resistant tantalum carbide coatings on a variety of predensified ceramic substrates is reported. Substrate materials included aluminium nitride, silicon carbide, silicon nitride, and WC-6Co. In particular, the phase chemistry which evolves during ROC processing, the surface morphology of the coating as-deposited and after ROC processing, and the erosive wear resistance of the coatings, are discussed.