Showing papers in "Materials Science & Engineering R-reports in 2004"

Bulk metallic glasses

Abstract: Amorphous alloys were first developed over 40 years ago and found applications as magnetic core or reinforcement added to other materials. The scope of applications is limited due to the small thickness in the region of only tens of microns. The research effort in the past two decades, mainly pioneered by a Japanese- and a US-group of scientists, has substantially relaxed this size constrain. Some bulk metallic glasses can have tensile strength up to 3000 MPa with good corrosion resistance, reasonable toughness, low internal friction and good processability. Bulk metallic glasses are now being used in consumer electronic industries, sporting goods industries, etc. In this paper, the authors reviewed the recent development of new alloy systems of bulk metallic glasses. The properties and processing technologies relevant to the industrial applications of these alloys are also discussed here. The behaviors of bulk metallic glasses under extreme conditions such as high pressure and low temperature are especially addressed in this review. In order that the scope of applications can be broadened, the understanding of the glass-forming criteria is important for the design of new alloy systems and also the processing techniques.

Surface modification of titanium, titanium alloys, and related materials for biomedical applications

Abstract: Titanium and titanium alloys are widely used in biomedical devices and components, especially as hard tissue replacements as well as in cardiac and cardiovascular applications, because of their desirable properties, such as relatively low modulus, good fatigue strength, formability, machinability, corrosion resistance, and biocompatibility. However, titanium and its alloys cannot meet all of the clinical requirements. Therefore, in order to improve the biological, chemical, and mechanical properties, surface modification is often performed. This article reviews the various surface modification technologies pertaining to titanium and titanium alloys including mechanical treatment, thermal spraying, sol–gel, chemical and electrochemical treatment, and ion implantation from the perspective of biomedical engineering. Recent work has shown that the wear resistance, corrosion resistance, and biological properties of titanium and titanium alloys can be improved selectively using the appropriate surface treatment techniques while the desirable bulk attributes of the materials are retained. The proper surface treatment expands the use of titanium and titanium alloys in the biomedical fields. Some of the recent applications are also discussed in this paper.

Carbon nanotubes: properties and application

Abstract: Carbon nanotubes are unique tubular structures of nanometer diameter and large length/diameter ratio. The nanotubes may consist of one up to tens and hundreds of concentric shells of carbons with adjacent shells separation of ∼0.34 nm. The carbon network of the shells is closely related to the honeycomb arrangement of the carbon atoms in the graphite sheets. The amazing mechanical and electronic properties of the nanotubes stem in their quasi-one-dimensional (1D) structure and the graphite-like arrangement of the carbon atoms in the shells. Thus, the nanotubes have high Young’s modulus and tensile strength, which makes them preferable for composite materials with improved mechanical properties. The nanotubes can be metallic or semiconducting depending on their structural parameters. This opens the ways for application of the nanotubes as central elements in electronic devices including field-effect transistors (FET), single-electron transistors and rectifying diodes. Possibilities for using of the nanotubes as high-capacity hydrogen storage media were also considered. This report is intended to summarize some of the major achievements in the field of the carbon nanotube research both experimental and theoretical in connection with the possible industrial applications of the nanotubes.

Scaling, dimensional analysis, and indentation measurements

Abstract: We provide an overview of the basic concepts of scaling and dimensional analysis, followed by a review of some of the recent work on applying these concepts to modeling instrumented indentation measurements. Specifically, we examine conical and pyramidal indentation in elastic-plastic solids with power-law work-hardening, in power-law creep solids, and in linear viscoelastic materials. We show that the scaling approach to indentation modeling provides new insights into several basic questions in instrumented indentation, including, what information is contained in the indentation load-displacement curves? How does hardness depend on the mechanical properties and indenter geometry? What are the factors determining piling-up and sinking-in of surface profiles around indents? Can stress-strain relationships be obtained from indentation load-displacement curves? How to measure time dependent mechanical properties from indentation? How to detect or confirm indentation size effects? The scaling approach also helps organize knowledge and provides a framework for bridging micro- and macroscales. We hope that this review will accomplish two purposes: (1) introducing the basic concepts of scaling and dimensional analysis to materials scientists and engineers, and (2) providing a better understanding of instrumented indentation measurements.

Nanocrystalline materials and coatings

Abstract: In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

ZnO nanowire growth and devices

Abstract: The large surface area of ZnO nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for micro-lasers or memory arrays. In addition, they might be doped with transition metal (TM) ions to make spin-polarized light sources. To date, most of the work on ZnO nanostructures has focused on the synthesis methods and there have been only a few reports of the electrical characteristics. We review fabrication methods for obtaining device functionality from single ZnO nanorods. A key aspect is the use of sonication to facilitate transfer of the nanorods from the initial substrate on which they are grown to another substrate for device fabrication. Examples of devices fabricated using this method are briefly described, including metal-oxide semiconductor field effect depletion-mode transistors with good saturation behavior, a threshold voltage of ∼−3 V and a maximum transconductance of order 0.3 mS/mm and Pt Schottky diodes with excellent ideality factors of 1.1 at 25 °C and very low (1.5 × 10 −10 A, equivalent to 2.35 A cm −2 , at −10 V) reverse currents. The photoresponse showed only a minor component with long decay times (tens of seconds) thought to originate from surface states. These results show the ability to manipulate the electron transport in nanoscale ZnO devices.

Properties of lead-free solder alloys with rare earth element additions

Abstract: Due to the inherent toxicity of lead (Pb), environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies. This has prompted the development of “Pb-free” solders, and has enhanced the research activities in this field. In order to become a successful solder material, Pb-free alloys need to be reliable over long term use. Although many of these alloys possess higher strength than the traditional Sn–Pb ones, there still exist reliability problems such as electromigration and creep. Also, the solderability of many Pb-free alloys is inferior to that of Sn–Pb and any improvement or replacement will be welcomed by industry. In order to develop new Pb-free solders with better properties, trace amounts of rare earth (RE) elements were selected by some researchers as alloying additions into Sn-based solders. These solder alloys are mainly Sn–Ag, Sn–Cu, Sn–Zn and Sn–Ag–Cu. In general, the resulting RE-doped solders are found to have better performances than their original ones. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep resistance in some RE-doped alloys gives creep rupture time increases by over four times for Sn–Ag and seven times for Sn–Cu and Sn–Ag–Cu. Like other Sn-based alloys, their creep rates are controlled by dislocation pipe diffusion in the Sn matrix. Also, it was found that the creep rate of these Sn-based alloys can be represented by a single empirical equation. With the addition of RE elements, solders for bonding on difficult substrates such as on semiconductors, diamond, and optical materials have also been developed. This report summarizes the effect of RE elements on the microstructure, mechanical properties, wetting behavior of certain Pb-free solder alloys. As an illustration of the advantage of RE doping, interfacial studies were carried out for electronic interconnections with RE-doped Pb-free alloys. It was found that the intermetallic compound (IMC) layer thickness and the amount of interfacial reaction were reduced in a Ball Grid Array (BGA) package. These results indicate that RE elements would play an important role in providing better electronic interconnections.

Chemical mechanical planarization for microelectronics applications

Abstract: The progressively decreasing feature size of the circuit components has tremendously increased the need for the global surface planarization of the various thin film layers that constitute the integrated circuit (IC). Global planarization, being one of the major solutions to meet the demands of the industry, needs to be achieved following the most efficient polishing procedure. Chemical mechanical polishing (CMP) is the planarization method that has been selected by the semiconductor industry today. CMP, an ancient process used for glass polishing, was adopted first as a microelectronic fabrication process by IBM in the 80 s for SiO2 polishing. To achieve efficient planarization at miniaturized device dimensions, there is a need for a better understanding of the physics, chemistry and the complex interplay of tribo-mechanical phenomena occurring at the interface of the pad and wafer in presence of the fluid slurry medium. In spite of the fact that CMP research has grown by leaps and bounds, there are some teething problems associated with CMP process such as delamination, microscratches, dishing, erosion, corrosion, inefficient post-CMP clean, etc.; research on which is still developing. The fundamental understanding of the CMP is highly necessary to characterize, optimize and model the process. The CMP process is ready to make a positive impact on 30% of the US$ 135 billion global semiconductor market. This paper presents an overview of CMP process in general, the science and mechanism of polishing, different metal and dielectric CMP processes. The impact of consumables on the CMP process, post-CMP cleaning, modeling of different CMP processes as well as the future trends are also discussed.

Formation and growth of tracks in nuclear track materials

Abstract: This paper reviews some aspects of solid-state nuclear track detectors (SSNTDs) and their applications in the radon and other research fields. Several geometrical models for the track growth given in the literature are described and compared. It is found that different models give close results for the dimensions of track openings. One of the main parameters that govern track formation is the bulk etch rate Vb. Dependences of Vb on different parameters such as the preparation procedures, etching conditions, irradiation before etching, etc. are examined. A review of existing methods for determination of the bulk etch rate and track etch rate Vt is also given. Examples of Vt functions for some detectors are presented. Some unsolved questions related to Vt and some contradictory experimental results published in the literature are also summarized in the paper. Applications of SSNTDs for radon and progeny measurements are discussed. New designs of diffusion chambers that have appeared in the last few years are portrayed. A review of analytical and Monte Carlo methods for the calculation of the calibration factors in radon measurements is presented. Particular attention has been given to methods of long-term passive measurements of radon progeny with SSNTDs. These measurements are rather difficult and there is not yet a widely accepted solution. One possible solution based on the LR 115 SSNTD is outlined here. Methods for retrospective radon measurements are also described. Various applications of SSNTDs in other fields of physics and other sciences are briefly reviewed at the end of the paper. # 2004 Elsevier B.V. All rights reserved.

Synthesis and properties of epitaxial electronic oxide thin-film materials

Abstract: The growth and properties of electronic oxide thin films are reviewed. In particular, the synthesis and properties of superconducting, insulating, conducting, magnetic, and semiconducting epitaxial oxide structures are discussed. Crystal structures and functional properties common to many oxide materials are briefly reviewed. A description of film-growth techniques follows. Finally, an extensive overview of the epitaxial growth for specific oxide material systems is given. This includes the epitaxial growth of electronic oxide thin films on oxide and non-oxide substrates.

Nanometric superlattices: non-lithographic fabrication, materials, and prospects

Abstract: A non-lithographic technique that utilizes the self-organized, highly ordered anodized aluminum oxide (AAO) porous membrane as a template is employed as a general fabrication means for the formation of vastly different two-dimensional lateral nanometric superlattices. The fact that material systems as different as metals, semiconductors, and carbon nanotubes (CNT) can be treated with the same ease attests to the generality of this nano-fabrication approach. The original alumina nanopore membranes determine the uniformity, packing density, and size of the nanostructures. The flexibility of using a variety of materials, the accurate control over fabrication process, and the command over the alumina template attributes give us the freedom of engineering various physical properties determined by the shape, size, composition, and doping of the nanostructures. The novel nanomaterial platform realized by this unique technique is powerfully enabling for a broad range of applications as well as for uncovering new physical phenomena such as the collective behavior of arrays of nano-elements that may not be intrinsic to individual nano-elements.

Multiphase solidification in multicomponent alloys

Abstract: Multiphase solidification in multicomponent alloys is pertinent to many commercial materials and industrial processes, while also raising challenging questions from a fundamental point of view. Within the past few years, research activities dedicated to multiphase solidification of ternary and multicomponent alloys experienced considerable amplification. This paper gives an overview of our present understanding in this field and the experimental techniques and theoretical methods research relies on. We start with an introduction to thermodynamic databases and computations and emphasize the importance of thermophysical property data. Then, we address pattern formation during coupled growth in ternary alloys and cover microstructure evolution during successive steps of phase formation in solidifying multicomponent alloys. Subsequently, we review advances made in phase field modeling of multiphase solidification in binary and multicomponent alloys, including various approaches to crystal nucleation and growth. Concluding, we address open questions and outline future prospects on the basis of a close interaction among scientists investigating the thermodynamic, thermophysical and microstructural properties of these alloys.

Self-organized growth on GaAs surfaces

Abstract: GaAs(001) has been one of the most intensively studied surfaces for the past 30 years due both to its importance as a substrate for epitaxial growth and to the challenge its phase diagram of complex structures presents to computational methods. Yet despite substantial experimental and theoretical effort, a number of fundamental questions remain concerning growth kinetics and mechanisms on this surface, even for homoepitaxy, but more especially in the formation of heterostructures. These issues have acquired a renewed timeliness because the quantum dots that are formed during the Stranski–Krastanov (SK) growth of InAs on GaAs(001) can be used for optoelectronic applications and have potential in quantum dot-based architectures for quantum computing. In this review we survey the current state of understanding of growth kinetics on GaAs surfaces, beginning with the simplest case, homoepitaxy on GaAs(001). We compare interpretations of recent reflection high energy electron diffraction measurements taken during the initial stages of growth with predictions of ab initio density functional calculations. We also consider the extent to which snapshot scanning tunnelling microscopy images from rapidly quenched samples truly reflect the growing surface structure as revealed by in situ real-time methods. We then examine the present experimental and theoretical status of the SK growth of InAs quantum dots on singular orientations of low-index GaAs surfaces, focussing on such issues as the importance of substrate orientation and surface reconstruction of the substrate, wetting layer formation, the nucleation kinetics of quantum dots, their size distributions and the role of strain. The systematics and anomalies of the phenomenology will be highlighted, as well as the current understanding of quantum dot formation.

Characterization of electrically active dopant profiles with the spreading resistance probe

Abstract: Since its original conception in the 1960s, the spreading resistance probe (SRP) has evolved into a reliable and quantitative tool for sub-micrometer, electrically active dopant, depth profiling in silicon. Its application limit has in recent years even been pushed down to ultra-shallow (sub-50 nm) structures. In this review, a systematic discussion is presented of all issues of importance for a high quality raw data collection and subsequent high accuracy data analysis. The main focus will be on the new developments over the last two decades. The qualification requirements to be fulfilled for 10% profile accuracy (in the absence of carrier spilling) and some of the main fields for SRP application in today's industry will be covered. Finally, a critical assessment of the technique will be made, and its future roadmap will be discussed.

Novel insulators for gate dielectrics and surface passivation of GaN-based electronic devices

Abstract: The use of gate insulators for compound semiconductor electronics would alleviate many of the problems encountered in current Schottky based devices. Further, circuit design can be simplified since enhancement-mode MOSFETs can be used to form single supply voltage control circuits for power transistors. The use of MOSFETs also allows the use of complementary devices, thus producing less power consumption and simpler circuit design. A critical need is to develop reliable methods for deposition of these insulating films. This will enable the development of a new class of compound semiconductor electronics for high-speed communication and data processing applications. Both MgO and Sc 2 O 3 are shown to provide low interface state densities (in the 10 11 eV −1 cm −2 range) on n- and p-GaN, making them useful for gate dielectrics for metal-oxide semiconductor (MOS) devices and also as surface passivation layers to mitigate current collapse in GaN/AlGaN high electron mobility transistors (HEMTs). Clear evidence of inversion has been demonstrated in gate-controlled MOS p-GaN diodes using both types of oxide. Charge pumping measurements on diodes undergoing a high temperature implant activation anneal show a total surface state density of ∼3 × 10 12 cm −2 . On HEMT structures, both oxides provide effective passivation of surface states and these devices show improved output power. The MgO/GaN structures are also found to be quite radiation-resistant, making them attractive for satellite and terrestrial communication systems requiring a high tolerance to high energy (40 MeV) protons.