Materials Science Reports最新文献

The use of ion bombardment for the creation of resistive layers in III-V semiconductors is reviewed. There are two complementary methods to achieve the removal of free carriers in these materials. The first is to create damage-related deep levels by ion bombardment. These levels trap the charge carriers, and are not significantly thermally ionized at room temperature. The resultant high-resistivity material is stable to the temperatures at which the damage-related levels anneal out. The second method relies on implanting a species that creates a chemical deep-level state in the particular semiconductor. Thermally stable high-resistivity material is achieved at temperatures at which the implanted ion becomes electrically active. We also review the device applications in which implant isolation provides significant advantages over other techniques such as the etching of mesas.

GaAs is a very attractive material for special devices such as high-frequency microwave and optoelectronic devices which perform functions unattainable by Si devices. GaAs digital integrated circuits can operate at speeds beyond the capability of Si devices. In addition, compared with Si devices, the GaAs devices operate at lower power, are more radiation tolerant, and the device fabrication process is simpler. Two distinct types of contacts are fundamental components for GaAs devices: Ohmic (low-resistance) and Schottky (rectifying) type contacts. Performance of GaAs devices is strongly influenced by the electrical properties of these contacts. A variety of metallization systems for these contacts has been developed which provide promising device performance. With increase of the integration level of devices, thermal stability during device fabrication process and operation, control of the diffusion depth of the contact metals into the GaAs, and smooth surface morphology have become important issues as well as the electrical properties. The purpose of the present article is to review the development of Ohmic contact materials for GaAs devices prepared by conventional evaporation and annealing techniques and to discuss compatibility of these contact materials with highly integrated circuits.

The ion-beam-induced epitaxial crystallization (IBIEC) and planar amorphization of amorphous Si (a-Si) layers onto single-crystal Si substrates is reviewed. In particular, the dependence of the process on substrate temperature, on substrate orientation and on the energy deposited by the impinging ions into electronic and elastic collisions is treated in detail and discussed. Emphasis is also given to the influence of impurities on IBIEC, where a variety of different phenomena are observed. For instance, fast diffusers, such as Au, are seen to be swept by the moving c-a boundary and present intriguing segregation profiles. Slow diffusers such as As or O, on the other hand, have not enough mobility to move over long-range distances even in the presence of irradiation, but they can strongly modify the kinetics of IBIEC. Dopants such as B, P and As, for example, enhance the ion-induced growth rate by a factor of 2–3, while O retards it. It is also shown that by decreasing the substrate temperature (or by increasing the ion flux) IBIEC can be reversed resulting in a planar layer-by-layer amorphization. This phenomenon evidences the unique non-equilibrium features of ion-assisted phase transitions in silicon which are the result of a dynamic balance between defect production rate and defect annihilation rate. These data are discussed, mainly in comparison with the purely thermally activated growth of a-Si and a possible explanation of the observed phenomena is presented in terms of a simple model. Finally, new possible applications of the phenomenon, such as the ion-induced regrowth of deposited Si layers and of deposited GeSi heterostructures, are illustrated, demonstrating the high potentialities of ion-beam processing in producing epitaxial layers in a non-conventional manner.

With the continuing drive toward greater device densities and finer dimensions in the microelectronics industry, the required proprerties of the metallization layers have become increasingly stringent. Transition metal aluminides are of great interest due to the use of transition metals as diffusion barriers, to suppress hillock formation, and to increase electromigration resistance. This review seeks to collect all the relevant work on transition metal aluminide formation: initial phase formation, temperature of formation, uniformity of growth, and microstructure of the phase formation. Where data are available, the subsequent phases formed, the growth kinetics, and dominant moving species are included. The Ni-Al system is discussed in detail as it is one of the best understood systems and exhibits typical behavior. The Ti-Al, Al(Cu) system and TiW diffusion barriers are also discussed individually due to their technological importance. As has been found for silicide formation, there are some general patterns of behavior with aluminide formation. In general, the initial aluminide phases to grow are the most Al-rich phases: Co₂Al₉, Cr₂Al₁₃, HfAl₃, MoAl₁₂, NbAl₃, NiAl₃, TaAl₃, TiAl₃, WAl₁₂, and ZrAl₃. There are exceptions, Pd₂Al₃, Pt₂Al₃, and VAl₃ are the initially growing phases, but are not the most Al-rich phases. Where marker experiments were performed, Al has been identified as the dominant diffusing species during the growth of the initial phase. It has been suggested that the Al-rich initial phase results from the greater supply of Al (relative to transition metal) to the growing interface with exceptions caused by complex (and hence difficult to nucleate) phases. The initial reaction temperatures ranged from 225–250°C for Pd₂Al₃ and Pt₂Al₃ to 500–525°C for WAl₁₂ formation. In general, the phase formation is planar, though impurities and grain sizes can modify this. For metals forming high-melting-point compounds, the reaction is more likely to be non-uniform. Though generalized rules have been proposed, there are still many open questions. Our understanding of aluminide formation lags behind that of silicides.

Long before the advent of nanofabrication and quantum-effect devices, the technological limitations imposed by polycrystalline, multiphase and thermally unstable contacts to III-V semiconductors were of concern to forward-looking materials scientists. In the early 1980s, efforts to elucidate the complex behaviour of reactive metal/III-V systems were initiated. These early efforts evolved slowly and culminated in the recent achievement of stable and epitaxial metallizations to III-V semiconductors. In this review, we first describe the criteria that must be met for the fabrication of metal/III-V heterostructures. Bulk phase equilibria are useful guides for selecting metal/semiconductor combinations which will not react during growth at moderate temperatures or during subsequent processing steps. We show, however, that phase stability is not sufficient for the fabrication of ultrathin metal overlayers or buried metal heterostructures. Growth conditions must be carefully optimized and combined with the appropriate selection of metallic phases with high melting points in order to suppress the strong tendency for island formation during growth and film agglomeration during overgrowth or processing. In our discussion of metal/semiconductor hetero-structures we highlight the relationship between symmetry differences and defects (domain boundaries) with particular emphasis on semiconductor overlayers grown on high-symmetry metals. Our work and that of others has shown that stable and epitaxial metallizations to III-V semiconductors as well as more complex metal/III-V heterostructures can be achieved with two classes of metallic materials; the transition-metal gallides and aluminides with the CsCl structure (TM-III) and the rare-earth monopnictides with the NaCl structure (RE-V). We discuss and compare the growth of these III-V/TM-III/III-V and III-V/RE-V/III-V heterostructures by molecular beam epitaxy, focusing special attention to the initial stages of growth of metallic films on III-V substrates and III-V overlayers on metallic films. Going beyond the strictly materials issues, we describe the electrical properties of such heterostructures, including stable enhanced-barrier Schottky contacts and semiconductor-clad metallic quantum wells, structures which may be the basis for exciting and novel electronic, photonic and magnetic devices.

A review of the method of self-propagating high-temperature synthesis (SHS) is presented. The review emphasizes the mechanisms of the rapid, non-isothermal reactions associated with this method. Theoretical analyses pertaining to such reactions are presented and examples of experimental observations on solid-solid and solid-gas interactions are discussed.

A pseudoline electron beam (e-beam) which is produced by scanning a spot e-beam along a line faster than the thermal response time of the substrate, has an advantage for recrystallization of silicon-on-insulator (SOI) structures in that the temperature profile along the line can be precisely controlled by the waveform of the scanning signal. In this paper, the current status of the pseudoline e-beam recrystallization method is reviewed, being focused on two essential problems: void generation and subboundary generation in the recrystallized films. First, in order to predict the temperature distribution during heating, the heat equation is solved for both static and dynamic cases using the Kirchhoff transformation and the Green function analysis, respectively. Then, generation mechanisms of voids and subboundaries are experimentally studied and the respective generation models are set up. It is concluded that voids are generated from isolated molten spots in the SOI film and a perforation seed structure in which rectangular seed regions are separately arranged along a line, is effective to suppress them. It is also concluded that subboundaries are generated at the interior corners of the folded 11facets which are formed at the solid-liquid interface. Other topics included in this paper are oblique scanning of the pseudoline e-beam, optimization of the scanning direction and scanning waveform, optimization of the seed direction, generation of twin defects, relation between the subboundary direction and the scanning velocity of the beam, and so on. As a result, a single-crystal SOI area of 100 μm square has been obtained.

A microprobe reflection high-energy electron diffraction (RHEED) technique and its applications to crystallographic analyses and observations of surface micro-areas are reported. Microprobe RHEED is a kind of scanning electron microscopy which uses reflection diffraction spot intensities as an image signal. It was used to analyze crystalline states of lateral epitaxial Si films on SiO₂ substrates (silicon on insulator: SOI). It was found that laser-induced lateral epitaxial regrowth was initiated near the SiO₂ edges. The technique was also used to observe atomic layer structures on crystalline material surfaces. The changes in surface topography of metal-deposited Si(111), and of Si(111) and Si(001) substrates during Si molecular beam epitaxial (MBE) growth, were observed. Growth of ultrathin metal films on Si(111) surfaces was found to be strongly affected by atomic steps on the substrate. Observation of Si MBE growth provided the first known direct evidence that RHEED intensity oscillations occur as a result of layer-by-layer two-dimensional nucleation growth. These results show that microprobe RHEED analysis is a powerful method for characterizing crystalline material surfaces and for studying surface reaction processes with atomic-layer depth resolution.

This review discusses the development and present status of atomic layer epitaxy (ALE), a technology for growing layers of crystalline and polycrystalline materials one atomic layer at a time. Atomic layer epitaxy was originally developed to meet the needs of improved ZnS thin films and dielectric thin films for electroluminescent thin film display devices. Accordingly, early work on ALE was mainly carried out for thin films. During the 80s there has been a growing interest in applying ALE in the growth of single crystals of III–V and II–VI compounds and ordered heterostructures such as layered superalloys and superlattices. ALE has also been extended to the growth of elemental single crystals. A basic advantage of atomic layer epitaxy is in the increased surface control of the growth. This is achieved by combining a sequential reactant interaction with a substrate at a temperature which prevents condensation of individual reactants on the growing surface. This results in a stepwise process where each reactant interaction is typically saturated to a monolayer formation. Accordingly, the rate of the growth in an ALE process is determined by the repetition rate of the sequential surface reactions, and the thickness of the resulting layer is determined by the number of reactant interaction cycles. This self-controlling feature of atomic layer epitaxy ensures excellent uniformity of the thickness over large substrate areas even on non-planar surfaces. Owing to its principle of operation, ALE is especially suitable for producing layered structures of III–V and II–VI compounds. Superlattice structures of both these material groups have already been demonstrated. As a limiting case of superlattices, layered superalloys have also been grown. In ALE, chemical reactions producing a material, are divided into separate subreactions between a vapor and a solid surface, each of which results in a new atomic layer of the material. From the theoretical point of view ALE offers a unique link between theoretical and experimental chemistry by permitting direct observations of subreactions under conditions where the chemical environment is more precisely determined than in conventional continuous reactions.

A review of the method of self-propagating high-temperature synthesis (SHS) is presented. The review emphasizes the mechanisms of the rapid, non-isothermal reactions associated with this method. Theoretical analyses pertaining to such reactions are presented and examples of experimental observations on solid-solid and solid-gas interactions are discussed.