Showing papers in "Materials Science & Engineering R-reports in 1995"

Liquid immiscible alloys

Abstract: The microstructure formation, during casting, of alloys being immiscible in the liquid state such as copperlead or aluminium-lead has gained renewed scientific and technical interest during the last fifteen years. Especially, a new experimental tool, research under reduced gravity conditions, was able to cast new, unexpected results and theories into the discussion on the nature of the complex process of microstructure evolution in such alloys. Prior to the first experiments performed at reduced levels of gravity acceleration, it was generally agreed that the process of phase separation during cooling through the miscibility gap is dictated solely by gravity-induced effects such as natural convection and sedimentation. Fundamental and applied research in space and in earth laboratories could show that there are other mechanisms operating concurrently and under suitable conditions with equal strength. In addition applied research was able to utilize the often unexpected results from space experimentation to develop new casting processes which allow one to produce microstructures on earth suitable for bearings in automotive applications. Therefore this article describes the extensive progress that has been made during the last decade and also the fundamentals of immiscibles. In addition it will be shown that the combination of classical laboratory research, research under reduced gravity conditions and a newly developed computational modelling technique seems to be just becoming available to solve the problems of decomposition, spatial phase separation and microstructure evolution during cooling of an alloy through the liquid miscibility gap.

Surface induced self assembly in thin polymer films

Abstract: The effect of boundary surfaces on phase separation and microphase separation in binary polymer blends and block copolymers respectively, has gained increasing attention over the last 5 years. It has been realized that the complex interplay between wetting and phase separation may severely influence the phase separation process thereby leading to near-surface domain structures, which differ distinctly from the respective bulk morphologies. In the present article, we try to summarize the basic features of surface directed (micro-) phase separation in immiscible polymer systems. For both polymer blends and block copolymers a brief review of the historical development is given, followed by a list of selected examples representing the large number of current research activities in these fields. Particular attention is given to the possibility to actively control the domain morphologies via surface interactions in view of possible technological applications.

Hydrogen-related defects in crystalline semiconductors: a theorist’s perspective.

Abstract: Hydrogen is a common impurity in all semiconductors. Although it is sometimes deliberately introduced, hydrogen often penetrates into the crystal during device processing. It interacts with broken or weak covalent bonds, such as those found at extended and localized defect centers. The main results of these covalent interactions are shifts of energy levels out of (or into) the gap and new optical activity (infrared absorption and Raman scattering). The shifts in energy levels lead to the passivation (or activation) of the electrical activity of various centers. Hydrogen can also interact with the perfect crystal and with itself, sometimes leading to the formation of extended structures known as platelets. Finally, H also acts as a catalyst, dramatically enhancing the diffusivity of interstitial oxygen in Si. The consequences of these interactions are substantial changes in the electrical and optical properties of the crystal, and in the lifetime of charge carriers. The thermal stability of the complexes containing hydrogen varies from room temperature up to several hundreds of degrees Celsius, and the diffusion of H is trap-limited up to rather high temperatures. Hydrogen normally exists in more than one configuration and charge state in semiconductors. A range of experimental and theoretical techniques have been used to investigate the rich properties of hydrogen in semiconductors, and several extensive reviews focusing mostly on the experimental side of these issues have been published in the past five years. The present review focuses mostly on the theoretical work performed in this field. However, the most recent experimental results are also discussed, and the current understanding of hydrogen interactions in semiconductors summarized.

Metal-oxide interfaces

Abstract: Present knowledge on metal-oxide interfaces is reviewed, emphasizing fundamental properties and behaviour. Methods have become available to fabricate ‘ideal’ metal-oxide interfaces with precisely controlled chemistry, special crystallography and high structural perfection. Recent experimental investigations of such model systems have advanced the understanding of adhesion, structure, mechanical behaviour and diffusion reactions at metal-oxide interfaces. In parallel, physical theories and powerful methods of computer modelling have been pushed forward to predict and to interpret the experimental observations. As an important development, recent work increasingly explores the correlation between microscopic properties of metal-oxide interfaces and their technically important macroscopic behaviour.

Fullerenes, nanotubes, onions and related carbon structures

Abstract: Fullerenes, containing five- and six-membered carbon rings, of which C 60 and C 70 are the prominent members, exhibit phase transitions associated with orientational ordering. When C 60 is suitably doped with electrons, it shows novel superconducting and magnetic properties. We review these and other properties of fullerenes in bulk or in film form along with the preparative and structural aspects. Carbon nanotubes and onions (hyperfullerenes) are the other forms of carbon whose material properties have aroused considerable interest. Besides discussing these new forms of carbon, we briefly introduce other possible forms, such as those involving five-, six- and seven-membered rings and hybrids between diamond and graphite.

Continuous cooling transformation kinetics versus isothermal transformation kinetics of steels: a phenomenological rationalization of experimental observations

Abstract: The existing literature has been examined and rationalized to test the general validity of a number of generally accepted concepts concerning the overall transformation kinetics of ferrous alloys. Considerable confusion exists because of the mixup of the continuous cooling kinetics with the isothermal transformation kinetics. Therefore, these two topics are discussed separately. For the continuous cooling process, the following topics are examined: (1) the suppressibility of the martensite transformation at high cooling rates; (2) the cooling rate dependence of M s (martensite transformation-start temperature) and B s (bainite transformation-start temperature); (3) the formation conditions of lath martensite and twinned martensite; and (4) the various features of continuous cooling transformation (CCT) diagrams. For the isothermal transformation process, the following issues are examined: (1) the isothermal transformation kinetics of martensite; (2) the relationship between athermal transformation of martensite and isothermal transformation of martensite; (3) the general features of time-temperature-transformation (TTT) diagrams; (4) the validity of the “isothermal martensite” concept; and (5) the definition of M s and B s for isothermal transformations. Among the main conclusions are: (1) twinned martensite can be formed in all steels, even in pure iron and low-carbon and/or low-alloy steels; (2) isothermal transformation of martensite always follows C-curve kinetics; and (3) B s and M s for isothermal transformations are different from those obtained from cooling transformations. Comparison of literature results with the present assessment of isothermal B s and M s is made and good agreement is observed. The weakness of using TTT diagrams to analyze the continuous cooling kinetics is also discussed. Moreover, (metastable) product diagrams for austenite decomposition are established for both the continuous cooling process and the isothermal transformation process in order to develop a clearer paradigm for both processes.

Compound semiconductor growth by metallorganic molecular beam epitaxy (MOMBE)

Abstract: The replacement of elemental sources with gaseous precursors allows many of the advantages of metallorganic chemical vapour deposition (MOCVD) to be combined with those of molecular beam epitaxy (MBE). This technique has come to be known as metallorganic molecular beam epitaxy (MOMBE) or alternatively chemical beam epitaxy (CBE). While retaining many desirable properties of the parent techniques, however, new challenges are created by this combination, particularly in the area of growth chemistry. This article will review the status of semiconductor deposition by MOMBE, paying particular attention to the areas of impurity contamination and precursor selection. At present, MOMBE has been applied primarily to deposition of III–V materials; consequently this article will focus on these materials. Many of the issues reviewed in this article are applicable to other material systems as well. Basic equipment design, growth efficiency, contamination control, dopant incorporation, selective epitaxy and the role of hydrogen are all discussed.

Crystal growth of ZnSe from the melt

Abstract: High quality zinc selenide (ZnSe) substrate crystals which can be grown at low cost are required for a new generation of laser diodes emitting in the blue-green region of the spectrum The most efficient growth method, which is successfully applied in the production of other semiconductor crystals, has not yet provided a sufficient yield Melt growth of bulk ZnSe crystals is impaired by unfavorable material properties In addition to the high melting point and deviation from stoichiometry due to incongruent evaporation the pronounced tendency for twinning is the most limiting property This review gives a survey of the various experimental approaches adopted to master these obstructions and summarizes the available physicochemical and thermodynamic data relevant for crystal growth Both composition and thermal history of the melt are shown to have a decisive influence on the defect formation during crystallization and cooling of the grown crystal Post growth thermal treatment can be used to affect the equilibrium of point defects and the related electronic and optical properties as well as the crystal structure Based on a close look at the mechanism of twin formation in ZnSe and the first empirical results on twin reduction a hypothesis for the prevention of multi-twinning is given

High-Tc superconductors on buffered silicon: materials properties and device applications

Abstract: The discovery of ceramic superconductors with critical temperatures that overlap the operating temperatures of most silicon devices has led to efforts to incorporate these materials into current silicon technology. Recent progress in the growth of the cuprate thin film superconductor YBa 2 Cu 3 O 7 − δ (YBCO) on buffered silicon has resulted in the development of new electronic devices. The advances in superconductor microelectronics have been remarkable compared with most other materials technologies. This paper begins with a review of the processing of the silicon substrate surface using hydrogen termination technology. Several film deposition techniques are described, namely pulsed laser deposition, metal-organic chemical vapor deposition and multi-target sputtering. The in situ growth of YBCO and buffer film on silicon substrates is also reviewed. The structural and superconducting properties of YBCO thin films are summarized. The p-type nature of YBCO gives rise to an internal electric field which can provide a basis for applications in a new class of field effect transistors. A buffer layer is necessary to prevent barium and copper in the stoichiometric YBCO compound from diffusing into the silicon substrate. The structures and electrical properties of different oxide and conducting buffers, such as yttria-stabilized zirconia (YSZ), MgO, SrTiO 3 , CeO 2 , CoSi 2 and RuO 2 , are presented. The use of these buffer layers between YBCO and Si not only serves to stabilize the YBCO/Si structure, but also provides electrical insulation or conduction in this multilayer system. The stability of this layered structure is determined to a large extent by the properties of the buffer layer and its interfaces with YBCO and Si. A detailed description of the buffer/Si and buffer/YBCO interfacial structures is given. Interfaces in a YBCO-on- buffered-Si system have been shown to impact critically on its electrical properties. The superconductor-insulator-semiconductor (SuIS) capacitor can serve as a building block for fabrication of three-or four-terminal devices such as SuIS field effect transistors (SuISFETs). The applications of superconducting thin films on buffered silicon range from single-layer superconducting interconnects to those based on fabrications of complex multilayer circuits. This paper gives a detailed description of the synthesis, processing and device physics of the SuIS capacitor or diode. Comparisons with other superconducting thin film devices, such as Josephson junctions and microwave filters and resonators, are also made. Finally, results from the fabrication and characterization of SuISFETs are presented.

Ranges in Si and lighter mono and multi-element targets

Abstract: In the present review we describe experimental range studies for ions implanted into Si and lighter mono and multi-element targets. The experimental results are compared with current theories, in particular with the Ziegler, Biersack and Littmark (ZBL) calculations. It is found that for Si targets at implanted energies from 10 to 390 keV and for a large set of ion-Si combinations (29 ≤ Z 1 ≤ 83) there is overall a good agreement (better than 10%) between the experimental data and the ZBL calculations. However, for Au, Yb and Eu, significant theoretical-experimental differences are found when these ions are implanted at energies lower than 70 keV. The disagreement is removed when a cut-off in the interatomic potential is performed. On the other hand, systematic range studies performed in C, B, Be, SiC and polymer target films have shown that whenever medium-heavy ions are implanted in an energy range of 10 keV–7.5 MeV the experimental data are underestimated by the theory by as much as 40%. Using a simple model which takes into account the influence of the inelastic collisions on the nuclear stopping power this last difference is removed and a very good agreement is achieved between the calculated and experimental results. Finally the status for H, B and Li deep implants into Si at energies where the electronic stopping power reaches its maximum, is also reviewed.

Recent progress in low-energy electron diffraction: theory and application to semiconductor surfaces

Abstract: The atomic arrangements of semiconductor surfaces, which are important for an understanding of surface properties, are determined by the dynamic low-energy electron diffraction (LEED) method. Real-time control of epitaxial growth is possible via the measurement of intensity variations of LEED spots. The size and distance distribution of islands are clarified by spot profile analysis of LEED (SPA-LEED). Direct imaging of surface processes such as phase transition and epitaxial growth is possible using low-energy electron microscopy (LEEM).