Showing papers in "Materials Science & Engineering R-reports in 2001"

Recent advances in Schottky barrier concepts

Abstract: Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Polymeric biomaterials for tissue and organ regeneration

Abstract: This paper reviews recent work involving polymeric biomaterials used for skin, cartilage, bone, vascular, nerve and liver regeneration. Skin trauma involves damage to the epidermal, dermal and/or subdermal tissues. Epithelial, dermal and full-thickness replacements are considered. Cartilage research is mainly focused on replacing hyaline cartilage. Researchers investigate both nondegradable polymers, which must provide mechanical stability, and degradable polymers, which must support cartilage regeneration. Natural healing in large bone defects often fails. Materials for bone reconstruction must be biocompatible, offer mechanical properties similar to those of bone and support tissue regeneration. The area of vascular grafts draws attention as improvements to the patency of existing materials are needed. Studies to improve current vascular graft polymers as well as develop new polymers are reviewed below. The design and testing of materials for nerve regeneration, to repair damage caused by illness or accident, is an active area of research. Directional nerve guidance via tubulation is discussed, as are matrix materials to enhance axonal extension. Finally, liver transplantation remains one of the only options for chronic liver disease and the demand for liver transplants far exceeds the number of available organs. The complexity of parameters involved in liver regeneration is presented here.

Cadmium zinc telluride and its use as a nuclear radiation detector material

Abstract: We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd1ˇxZnxTe) with zinc content xa 0:1‐0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. # 2001 Elsevier Science B.V. All rights reserved.

Recent developments in lithium ion batteries

Abstract: Lithium ion rechargeable batteries are used as the power supply of cellular phones and several other portable electrical devices at present, and demand appears to increase exponentially. The concern about energy sources in the near future, either for electric vehicles (EV) or for large-scale batteries for electricity power storage, has made lithium ion rechargeable battery development into a growth area which has gained high momentum for its research activities. Here, while presenting the state of the art of lithium ion battery technology, the current research into materials, which constitute anode, electrolyte and cathode is described and the underlying problems associated with their development, advantages and drawbacks is analyzed. Both polymer electrolytes, which is a recent topic, as well as conventional organic liquids electrolyte, are also described.

Tin–lead (SnPb) solder reaction in flip chip technology

Abstract: Solder reactions between SnPb and one of the four metals, Cu, Ni, Au, and Pd have been reviewed on the basis of the available data of morphology, thermodynamics, and kinetics. The reactions on both bulk and thin film forms of these metals have been considered and compared. Also the two kinds of reactions, above and below the melting point of the solder, have been considered and compared. The rate of intermetallic compound formation in wetting reactions between the molten solder and the metals is three to four orders of magnitude faster than those between the solid state solder and the metals. The rate is controlled by the morphology of intermetallic compound formation. In the wetting reaction between molten SnPb and Cu or Ni, the intermetallic compound formation has a scallop-type morphology, but in solid state aging, it has a layer-type morphology. There are channels between the scallops, which allow rapid diffusion and rapid rate of compound formation. In the layer-type morphology, the compound layer itself becomes a diffusion barrier to slow down the reaction. Similar morphological changes occur between SnPb and Au or Pd. The stability of scallop-type morphology in wetting reaction and layer-type morphology in solid state aging have been explained by minimization of surface and interfacial energies. The unusually high rate of scallop-type intermetallic compound formation has been explained by the gain of rate of free energy change rather than free energy change. Also included in the review is the use of a stack of thin films as under-bump-metallization, such as Cr/Cu/Au, Al/Ni(V)/Cu, and Cu/Ni alloyed thin films.

Developments in the chemistry and band gap engineering of donor-acceptor substituted conjugated polymers

Abstract: This paper reviews the tools to manipulate and minimize the band gap of conjugated (co)polymers. The effects of minimization of the bond length alternation and of the incorporation of donor-π-acceptor units are discussed in particular. A systematic study of a series of alternating donor–acceptor oligomers has revealed the achievable limits and allowed for a prognosis of the performance of corresponding copolymers. The decisive influence of the LUMO eigenvectors of the acceptors on the band gap is highlighted. A new synthesis of benzo[1,2-c:4,5-c]bis[1,2,5]thiadiazole containing donor–acceptor monomers and their acid-catalyzed polymerization are described. Finally, band gap tuning by appropriate substitution, conformational adjustment and mesoscopic ordering is discussed.

Materials Processing by Gas Cluster Ion Beams

Abstract: This paper discusses the principles and experimental status of gas cluster ion beam (GCIB) processing as a promising surface modification technique for practical industrial applications. Theoretical and experimental characteristics of GCIB processes and of related equipment development are described from the moment of neutral cluster formation, through ionization, acceleration and impact upon a surface. The impact of an accelerated cluster ion upon a target surface imparts very high energy densities into the impact area and produces non-linear effects that are not observed in the impacts of atomic ions. Unique characteristics of GCIB bombardment have been found to offer potential for various industrial applications that cannot be achieved by conventional ion beam processing. Among prospective applications are included shallow ion implantation, high rate sputtering, surface cleaning and smoothing, and low temperature thin film formation. Sputtering effects produced by cluster ion impact are particularly interesting. High sputtering yields and lateral distribution of sputtered atoms cause surface smoothing effects which cannot be achieved with monomer ion beams. Surface smoothing to atomic levels is expected to become the first production use of GCIB.

Ion implantation into GaN

Abstract: The current status of ion beam processing of GaN is reviewed. In particular, we discuss the following aspects of ion implantation into GaN: (i) damage build-up and amorphization, (ii) preferential surface disordering and loss of nitrogen during ion bombardment, (iii) ion-beam-induced porosity of amorphous GaN due to material dissociation, (iv) anomalous surface erosion during ion bombardment at elevated temperatures, (v) the effect of implantation disorder on mechanical properties, (vi) current progress on annealing of implantation disorder, (vii) electrical and optical doping, and (viii) electrical isolation. Emphasis is given to current problems which may hinder a successful application of ion implantation in the fabrication of GaN-based devices.

Science and technology of ferroelectric films and heterostructures for non-volatile ferroelectric memories

Abstract: We present in this article a review of the status of thin film ferroelectric materials for nonvolatile memories. Key materials issues relevant to the integration of these materials on Si wafers are discussed. The effect of film microstructure and electrode defect chemistry on the ferroelectric properties relevant to a high density nonvolatile memory technology are discussed. The second part of this review focuses on approaches to integrate these capacitor structures on a filled poly-Si plug which is a critical requirement for a high density memory technology. Finally, the use of novel surface probes to study and understand broadband polarization dynamics in ferroelectric thin films is also presented.

Guide to references on III–V semiconductor chemical etching

Abstract: The literature on chemical etching of III–V semiconductors is reviewed with the intent to organize citations in categories useful to device and materials investigators. Descriptive citations are grouped by the intended etch application and subgrouped by specific semiconductors for both wet and dry etching. A separate section groups citations by the various chemical compositions used as etchants so that a broad view of results and issues can be accessed. The final section lists references by author, with complete titles and notes of their relevance to etching.

Comprehensive characterization of hydride VPE grown GaN layers and templates

Abstract: GaN community has recently recognized that it is imperative that the extended, and point defects in GaN and related materials, and the mechanisms for their formation are understood. This is a first and an important step, which must be followed by defect reduction before full implementation of this material and its allied binaries/ternaries in devices. This review is based on a recent concerted effort to establish benchmarks as far as defects are concerned, and identify the basic issues involved. Samples were analyzed for extended defects by TEM and chemical etches, for polarity by electric force microscopy and convergent beam electron diffraction (CBED), for point defects by DLTS, for optical quality and deep defects by photoluminescence (PL), for vacancies by positron annihilation, for donor and acceptor like states within the gap by ODMR and EPR, and for carrier transport targeted for defects and impurities by variable temperature and magnetic field-dependent Hall measurements. Hydride VPE samples grown at Lincoln Laboratories with 1.5, 5.5 and 55 μm thicknesses were investigated for defects by TEM, and their polarity was found to be Ga-polarity, as expected, by CBDE combined with simulations. The density of misfit dislocations at the substrate/EPI interface was determined to be on the order of 10 13 cm -2 based on high-resolution electron microscopy images. The threading dislocation density decreased gradually with distance from the interface, reaching a value of about 10 8 cm -2 at the surface of a 55 μm film. A 200 μm thick laser separated and free-standing HVPE grown GaN template grown at Samsung was also characterized similarly. The free surface and substrate sides were confirmed to be Ga- and N-polarity, respectively. The density of dislocations near the N-face was determined to be, in order, (3 ± 1) x 10 7 and (4 ± 1) x 10 7 by cross-sectional TEM and plan-view TEM, respectively. Identical observations on the Ga-face revealed the defect concentration to be less than 1 x 10 7 cm -2 by plan-view TEM and 5 x 10 5 cm -2 by cross-sectional TEM. Defects in a 10 μm thick GaN layer grown by HVPE at Lincoln Laboratory have been investigated by photo electrochemical (PEC) etching, and by wet etching in hot H 3 PO 4 acid and molten KOH. Threading vertical wires (i.e. whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic force microscopy, one finds the density of whisker-like features to be 2 x 10 9 cm -2 , the same value found for the etch-pit density on samples etched with both H 3 PO 4 and molten KOH. Values agree well with TEM results. A free standing GaN template has been characterized for its structural and optical properties using X-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy (AFM). The Ga-face and the N-face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga- and N-faces were about 5 x 10 5 cm -2 for the former and about 1 x 10 7 cm -2 for the latter, again in good agreement with TEM results mentioned above. High resolution X-ray rocking curves (omega scans) were measured. The [0 0 2] symmetric and [104] asymmetric peaks in 10 μm thick HVPE films had FWHM values between 5.8 and 7.9 arcmin, and 3.9 and 5.2 arcmin, respectively. The Samsung template investigated had wide diffraction peaks (20.6 and 24 arcmin for [0 0 2] and [1 0 4] diffractions, respectively) on the Ga-face, similar for the N-face, when a 2 mm slit size was used. When the slit size was reduced to 0.02 mm, the Ga- and N-face [0 0 2] peaks reduced to 69 and 160 arcsec. A bowing radius of 1.2 m was calculated to account for increased broadening with wider slits.

The electronic structure of polymer-metal interfaces studied by ultraviolet photoelectron spectroscopy

Abstract: Ultraviolet photoelectron spectroscopy has come of age. UPS can take its place beside its older, better-known sister, ESCA (or XPS) as a surface sensitive method which has become more useful in learning certain specific things about interfaces at distances significantly larger than the typical electron elastic mean-free-paths dictated by the photon energies employed. In particular, the emergence of UPS as a real tool for interfacial studies has been applications driven, evolving after needs within polymer-based electronics applications. The situation is clarified through the use of several examples, drawn from the applications-spectroscopy literature.

Near-field scanning optical microscopy studies of electronic and photonic materials and devices

Abstract: Probing optical properties of materials and optical characterization of crystallographic defects at the nanometer scale have been inaccessible until recently due to the diffraction limit of light. With the invention of nearfield scanning optical microscopy (NSOM), resolution at the 50‐100 nm level using visible or near infrared light is now practical. In addition to describing the NSOM technique, this review focuses on the application of NSOM to the characterization of electronic and photonic materials and devices, with particular emphasis on defects. The unique capability of NSOM to simultaneously measure surface topography and local optoelectronic properties, thereby eliminating the need to perform cross correlation analysis on results obtained using different techniques, is particularly useful in this area. Several examples are discussed. By performing near-field photocurrent (NPC) measurements, NSOM is used to probe electrical activities associated with individual threading dislocations and dislocation networks in strain relaxed, compositionally graded GeSi films. The non-destructive nature of NSOM helps elucidate how microstructural defects in the SrTiO3 bicrystal substrates affect YBa2Cu3O7 film growth and GBJJ performance. Characterization of III‐V and II‐VI semiconductors, guantum dots grown by strain epitaxy, laser diodes, waveguides, and photonic crystals is also included. The advantages and disadvantages of NSOM in each application will be outlined. Throughout the review, emphasis is placed on how NSOM complements existing materials characterization techniques, as well as how quantitative results can be obtained from NSOM measurements. # 2001 Elsevier Science B.V. All rights reserved.

Oxygen diffusion in the superconductors of the YBaCuO family: isotope exchange measurements and models

Abstract: The thermodynamic and kinetic aspects of the oxygen exchange in the solid oxide-oxygen phase system are discussed for the YBaCuO compounds. For both YBa 2 Cu 3 O 7− x (Y123) and Y 2 Ba 4 Cu 7 O 15− x (Y247) a vacancy model can successfully be used to describe the phase equilibrium. The dependence of some properties (lattice constants, Raman scattering, magnetic properties, and local structure EXAFS) of the nearly-equilibrium Y123 samples on the oxygen-stoichiometry is briefly presented. Studies of the carbon dioxide content in the YBaCuO family compounds show that apart of YBa 2 Cu 4 O 8 (Y124), a complete removal of the carbon without decomposition of the compounds is not possible. In the case of Y247 the influence of the CO 2 content on T C was found to be large. The oxygen-isotope exchange kinetics (tracer diffusion) for all three YBaCuO compounds is discussed in detail. In the structure of Y123 and Y247 the CuO chains are the ‘fastest paths’ for the 18 O diffusion, from which further exchange with the apex and planar sites takes place. A three-dimensional diffusion model for the oxygen exchange is presented. The model and the experimentally determined values of the diffusion coefficients allow the calculation of the isotope distribution for all three oxygen sites as a function of temperature, time and grain size. The kinetics of the low temperature ( 2 Cu 3 O 7− x and Y 0.7 Pr 0.3 Ba 2 Cu 3 O 6.98 . The total OIE in underdoped oxygen depleted Y123 increases with decreasing T C . It has been shown that the OIE in site-selectively substituted samples is mainly due to the oxygen in CuO 2 sites.

Use of excitons in materials characterization of semiconductor system

Abstract: The objective of this review article is to provide an overview of the use of excitons in the characterization of semiconductor alloys and quantum well structures In particular, it is shown how the measurements of the excitonic linewidth at low temperatures using a variety of optical spectroscopic techniques such as photoluminescence (PL), cathodoluminescence (CL), and absorption can be used to provide information about the structural quality of alloys and quantum well structures The results of several theoretical approaches that have been developed to calculate the excitonic linewidth in semiconductor alloys as a function of compositional disorder, which is primarily responsible for excitonic line broadening, are reviewed The measurements of the excitonic linewidths in a variety of III–V and II–VI based semiconductor alloys are described and compared with the calculated values to obtain information about their quality This is followed by a review of the results of a theoretical formalism, which has been used to calculate the excitonic linewidth due to interfacial disorder in quantum well structures The combined effects of both the compositional and the interfacial disorders on the excitonic linewidth in quantum well structures are discussed The results of the measurements of excitonic linewidth in several III–V and II–V semiconductor based quantum well structures are reviewed and compared with those calculated to gain insight into their quality This article is intended to provide a balanced review of both the theoretical and experimental developments in this field

Isotope effects on the lattice dynamics of crystals

Abstract: It seems likely that isotope effects are most clearly manifested in crystal lattice dynamics, which is evidenced by works in this field that have been published for more than half a century. A great number of stable isotopes and well-developed methods of their separation has made it possible to date to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu2O, Si, Ge and α-Sn with a controllable isotopic composition. The accumulated voluminous theoretical and experimental data suggest that the isotopic composition of a crystal lattice exerts some influence on the thermal, elastic, and vibrational properties of crystals. These effects are quite large and can be readily measured by modern experimental techniques (ultrasound, Brillouin and Raman scattering, and neutron scattering). For example, the change in the lattice constant is Δa/a=10−3 to 10−4, while the change δcik in the elastic constants amounts to several percent. The maximum km (where km is the maximum of thermal conductivity) measured for the most highly enriched 70 Ge (99.99%) sample is 10.5 kW/mK, one order of magnitude higher than for the natural Ge (analogous for C and Si). In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between a LiH crystal and its deuteride exceeds hundred degrees. Of the same order of magnitude is the difference between Debye temperatures for diamond crystals. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The scattering lines in isotopically mixed crystals are not only shifted (the shift of LO lines exceeds 100 cm−1) but are also broadened. This broadening is related to the isotopic disorder of a crystal lattice. It is shown in this review that the degree of change in the scattering potential is different for different isotopic mixed crystals. In the case of germanium and diamond crystals, phonon scattering is weak, which allows one to successfully apply the coherent potential approximation (CPA) for describing shift and broadening of scattering lines. In the case of lithium hydride, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments. Capture the thermal neutrons by isotope nuclei followed by nuclei decay produces new elements in a very large number of possibilities for isotope selective doping of different materials. The review closes with a section describing future developments and applications of isotope technology and engineering.