Table of contents

Nanostructured surfaces can be broadly defined as substrates in which the typical features have dimensions in the range 1–100 nm (although the upper limit of 100 nm may be relaxed to greater sizes in some cases, depending on the material and the specific property being investigated). The recent surge of interest in these systems stems from the remarkable effects that may arise from the critical size reduction. Interesting novel properties (catalytic, magnetic, ferroelectric, mechanical, optical and electronic) occur as we reduce the dimensions from a practically infinite (and periodic) solid crystal to a system composed of a relatively small number of atoms. So far, nanostructured materials or nanomaterials are perhaps the only sub-field of nanoscience that has made the transition from fundamental science to real world applications, thus becoming a technology (a good example of this are nanostructured surface coatings). This paper describes some selected examples of recent progress in the study of nanostructured surfaces. Surface reconstructions, which occur either naturally or as a consequence of the interaction with adsorbates, are discussed because of their importance in model chemical reactions and for their potential use as templates for the ordered growth of nanostructures. Supramolecular assemblies and molecular nanostructures, resulting from the balance between molecule–molecule and molecule–surface interactions, are described because of their fundamental interest and their potential use in nanoelectronic devices. Recent progress in the growth of semiconductor nanostructures, in particular Ge–Si and InAs–GaAs, is briefly reviewed. A few selected examples of nanostructured functional materials, such as ferroelectric and magnetic nanostructures, are discussed in view of their potential for applications in future data storage devices. Nanostructured materials used in catalysis and gas sensor applications are briefly described. Finally, perspectives and future challenges in this emerging field of research are also discussed.

We review the theoretical description of spin–orbit (SO) scattering and electron spin resonance (ESR) in carbon nanotubes. Particular emphasis is laid on the effects of electron–electron interactions. The SO coupling is derived and the resulting ESR spectrum is analysed using both the effective low-energy field theory and numerical studies of finite-size Hubbard chains and two-leg Hubbard ladders. For single-wall tubes, the field theoretical description predicts a double-peak spectrum linked to the existence of spin–charge separation. The numerical analysis basically confirms this picture, but also predicts additional features in finite-size samples.

In a tunnelling experiment across a quantum dot it is possible to change the coupling between the dot and the contacts at will, by properly tuning the transparency of the barriers and the temperature. Gate voltages allow for changes of the relative position of the dot addition energies and the Fermi level of the leads. Here we discuss the two limiting cases: weak and strong coupling in the tunnelling Hamiltonian. In the latter case Kondo resonant conductance can emerge at low temperature in a Coulomb blockade valley. We give a pedagogical approach to the single-channel Kondo physics at equilibrium and review the Nozières scattering picture of the correlated fixed point. We emphasize the effect of an applied magnetic field and show how an orbital Kondo effect can take place in vertical quantum dots tuned both to an even and to an odd number of electrons at a level crossing. We extend the approach to the two-channel overscreened Kondo case and discuss recent proposals for detecting the non-Fermi liquid fixed point which could be reached at strong coupling.

Islands grown during semiconductor heteroepitaxy are in a thermodynamically metastable state. Experiments show that diffusion at the surface region, including the interior of the islands, is fast enough to establish local equilibrium. I review here applications of a Monte Carlo method which takes advantage of the quasi-equilibrium nature of quantum dots and is able to address the issue of intermixing and island composition. Both Ge islands grown on the bare Si(100) surface and C-induced Ge islands grown on Si(100) precovered with C are discussed. In the bare case, the interlinking of the stress field with the composition is revealed. Both are strongly inhomogeneous. In the C-induced case, the interplay of strain and chemical effects is the dominant key factor. Islands do not contain C under any conditions of coverage and temperature.

We discuss the self-aggregation process of InAs and Si–Ge quantum dots (QDs) on natural and patterned GaAs(001) and Si(001) and Si(111) surfaces, with reference to our recent studies with scanning tunnelling and atomic force microscopy and current experimental and theoretical works. Various methods for obtaining naturally structured surfaces are briefly surveyed, as the patterning formed by the surface instability and by the strain in mismatched heteroepitaxy, and the latest methods of pre-patterning and growth at selected sites are discussed. Basic topics are also addressed that determine the final morphology of QDs, such as the wetting layer formation, the elastic strain field and the two-dimensional to three-dimensional phase transition.

The growth of kinetically self-organized 2D islands in Si/Si(111) epitaxy is described. The island size distribution for this system was measured using scanning tunnelling microscopy (STM). The influence of surface reconstructions on growth kinetics is studied directly using a method of simultaneous deposition and STM scanning. For the case of growth of Si islands on Si(111), lateral growth of rows of the width of the 7 × 7 reconstruction unit cell at the edges of two-dimensional islands leads to the formation of 'magic' island sizes. The evolution of the size and shape of individual {105} faceted Ge islands (hut clusters) on Si(001) is measured during growth. A slower growth rate is observed when an island grows to larger sizes. This behaviour can be explained by kinetically self-limiting growth. The potential formation of thermodynamically stable strained islands of a specific size is discussed. The formation of 2D Si/Ge nanostructures at pre-existing defects is studied. The step flow growth mode is used to fabricate Si and Ge nanowires with a width of 3.5 nm and a thickness of one atomic layer (0.3 nm) by self-assembly. One atomic layer of Bi terminating the surface is used to distinguish between the elements Si and Ge. A difference in apparent height is measured in STM images for Si and Ge. Also different kinds of two-dimensional Si/Ge nanostructure such as alternating Si and Ge nanorings having a width of 5–10 nm were grown.

The growth of different silicon carbide (SiC) polytypes on each other is possible by control of the surface structure and the appropriate thermodynamic parameters. Special ultrahigh vacuum conditions, like those used in solid source molecular beam epitaxy, allow the determination of the species on the surface and also the in situ characterization of the growing polytype by electron diffraction methods. The surface reconstruction which favours the growth of a certain polytype can be controlled by reflection high energy electron diffraction. For a non-destructive determination of the polytype of a grown thin SiC film, methods like x-ray photoelectron diffraction (XPD) and electron channelling can be used. The interaction length of electrons near 1 keV kinetic energy is in the range of 1 nm and therefore sensitive to the stacking sequence of the most common SiC polytypes 3C, 4H, 6H with c-axis dimensions between 0.75 and 1.5 nm. To prepare polytype heterostructures like 4H/3C/4H or 6H/3C/6H, untwinned 3C SiC films without double-positioning boundaries have to be grown. On-axis α-SiC substrates with uniform surface stacking termination are a prerequisite for this. Such surfaces can be prepared using high temperature hydrogen etching, sublimation etching or step-flow growth. These equally terminated crystals with threefold surface symmetry are particularly suitable for detailed studies of the atomic-geometric structure and their changes during growth or after certain treatments. Results of surface-sensitive characterization methods like scanning tunnelling microscopy, XPD and low energy electron diffraction are presented.

High temperature epitaxial processes for SiC bulk and thin films by physical vapour transport and chemical vapour deposition are reviewed from an academic point of view using heat and mass transfer modelling and simulation. The objective is to show that this modelling approach could provide information on fabrication and characterization for the improvement of the knowledge of the growth history. Recent results of our integrated research programme on SiC, taking into account the fabrication, process modelling and characterization, will be presented.

A brief survey of the development of the sublimation growth of SiC is given. The growth equipment and especially the hot zone of the furnace for the physical vapour transport (PVT) technique are described in detail. In order to grow micropipe-free SiC crystals, near-thermal-equilibrium growth is developed and the individual processing steps are revealed. The essential parameters for the growth of 4H-, 6H-, 15R- and 3C-SiC single crystals are discussed and a survey of the incorporation of donors (N, P) and acceptors (Al, B) during the PVT growth is presented.

The atomic scale ordering and properties of cubic silicon carbide (β-SiC) surfaces and nanostructures are investigated by atom-resolved room and high-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS), synchrotron radiation-based valence band and core level photoelectron spectroscopy (VB-PES, CL-PES) and grazing incidence x-ray diffraction (GIXRD). In this paper, we review the latest results on the atomic scale understanding of (i) the structure of β-SiC(100) surface reconstructions, (ii) temperature-induced metallic surface phase transition, (iii) one dimensional Si(C) self-organized nanostructures having unprecedented characteristics, and on (iv) nanochemistry at SiC surfaces with hydrogen. The organization of these surface reconstructions as well as the 1D nanostructures' self-organization are primarily driven by surface stress. In this paper, we address such important issues as (i) the structure of the Si-rich 3 × 2, the Si-terminated c (4 × 2), the C-terminated c (2 × 2) reconstructions of the β-SiC(100) surface, (ii) the temperature-induced reversible metallic phase transition, (iii) the formation of highly stable (up to 900 °C) Si atomic and vacancy lines, (iv) the temperature-induced sp to sp³ diamond like surface transformation, and (v) the first example of H-induced semiconductor surface metallization on the β-SiC (100) 3 × 2 surface. The results are discussed and compared to other experimental and theoretical investigations.

The current understanding of the relaxation and reconstruction of low-index cubic SiC surfaces, as it derives from first-principles calculations, is briefly reviewed in comparison with surface-sensitive experimental data. The calculated structural properties are obtained from ab initio total energy and grand canonical potential minimization in the framework of the local density and generalized gradient approximations of density functional theory. Characteristic surface structural properties are related to the surface electronic structure and to the ionicity of the underlying bulk crystal. For a number of cubic surfaces, there is good agreement between first-principles results and the data. In other cases, most noticeably for Si-terminated SiC(001) surfaces, there is still considerable controversy with respect to the atomic and electronic structure in both experiment and theory.

The basal surfaces of hexagonal SiC exhibit a large variety of surface reconstructions that develop under a similarly rich variety of sample preparations. A subset of these surface phases, which have been investigated in structural detail using scanning tunnelling microscopy and quantitative low-energy electron diffraction, is described and shown to offer the scope to be used for the formation of SiC-based semiconductor devices. The phases discussed are the (3 × 3) and reconstructions for the (0001) surfaces of 4H- and 6H-SiC and the oxygen-uptake-driven reconstructions of these polytypes for both the (0001) and the (000) surface orientations. We show that the (3 × 3) reconstruction corresponds to a highly passivated surface that facilitates hexagonal single-crystal growth, while suitable preparation of the reconstruction favours a switch to cubic growth and hence to the formation of a heterojunction. The reconstructions promise to form defect-free interfaces to insulating silicon oxide films, which is important for device applications.

The possibility is discussed that, independent of the polytype, large electronic correlation effects give rise to semiconducting SiC surfaces with [111] or [0001] orientations. The most important surface translational symmetries and 3 × 3 are considered. The discussion is based on the detailed knowledge of the surface atomic geometries and the electronic structures derived for these geometries using a local density approximation for exchange and correlation. It is argued that the resulting half-filled, weakly dispersive dangling-bond bands within the fundamental gaps are split according to a Hubbard interaction parameter U, and that the surface systems undergo a Mott–Hubbard transition. Such a physical picture provides a qualitatively correct account of the single-particle excitation spectra either inferred from angle-resolved photoemission and inverse photoemission experiments or obtained from scanning tunnelling spectroscopy. The gap opening and hence the Hubbard U parameter are discussed in terms of their dependence on the surface reconstruction as well as the SiC polytype.

Recent advances in probing the electronic structure of SiC with electron-excited luminescence techniques reveal the presence of localized electronic states near its surfaces and interfaces. These localized states form not only as a result of interface chemical bonding but also due to the formation of new lattice polytypes. Such electronic features are sensitive to the conditions under which the SiC is processed, as well as the application of electrical or mechanical stress. These localized changes on a nanometre scale provide a new perspective to Schottky barrier formation, band alignment, and polytypism in SiC as well as its performance in electronic devices.

Surface hydrogenation is a well established technique in silicon technology. It is easily accomplished by wet-chemical procedures and results in clean and unreconstructed surfaces, which are extremely low in charged surface states and stable against oxidation in air, thus constituting an ideal surface preparation. As a consequence, methods for hydrogenation have been sought for preparing silicon carbide (SiC) surfaces with similar well defined properties. It was soon recognized, however, that due to different surface chemistry new ground had to be broken in order to find a method leading to the desired monatomic hydrogen saturation. In this paper the results of H passivation of SiC surfaces by high-temperature hydrogen annealing will be discussed, thereby placing emphasis on chemical, structural and electronic properties of the resulting surfaces. In addition to their unique properties, hydrogenated hexagonal SiC {0001} surfaces offer the interesting possibility of gaining insight into the formation of silicon- and carbon-rich reconstructions as well. This is due to the fact that to date hydrogenation is the only method providing oxygen-free surfaces with a C to Si ratio of 1:1. Last but not least, the electronic properties of hydrogen-free SiC {0001} surfaces will be alluded to. SiC {0001} surfaces are the only known semiconductor surfaces that can be prepared in their unreconstructed (1 × 1) state with one dangling bond per unit cell by photon induced hydrogen desorption. These surfaces give indications of a Mott–Hubbard surface band structure.

Results of recent photoemission studies of oxidation of hexagonal SiC surfaces and SiO₂/SiC interfaces are reviewed and discussed. These investigations have focused on two main questions thought to have a significant effect on MOS device characteristics: the existence of carbon clusters or carbon-containing by-products and the existence of sub-oxides at the SiO₂/SiC interface. The presentation is focused on Si-terminated surfaces of hexagonal n-type SiC(0001) crystals since they to date have been considered the most promising for device applications. The results reviewed show that no carbon clusters or carbon-containing by-product can be detected at the interface of in situ or ex situ grown samples with an oxide layer thickness larger than about 10 Å. Since the presence of carbon clusters was suggested in a recent scanning microscopy study it appears that they may exist, possibly depending on the sample preparation method used, but in such low concentrations that they are not detectable using photoemission. The presence of sub-oxides at the SiO₂/SiC interface has been revealed in recorded Si 2p core level spectra by several groups. The results were not unanimous, however. The number of sub-oxides present and the shifts reported were different. The results of a recent study including also the Si 1s core level and Si KLL Auger transitions are therefore examined. These together with earlier Si 2p data show the presence of only one sub-oxide, assigned to Si¹⁺ oxidation states, besides the fully developed SiO₂ (Si⁴⁺). Possible reasons for the differences obtained earlier are discussed. That the sub-oxide is located at the interface is concluded from the relative intensity variations observed for the different components versus electron emission angle. An oxide thickness dependence of the SiO₂ chemical shift in the core levels and Auger transitions is shown, similar to but smaller in magnitude than the thickness dependence revealed earlier for SiO₂/Si. On cooled SiC(0001) substrates, adsorption of metastable molecular oxygen is suggested to occur in the initial oxidation stage like on the Si(111)-7 × 7 surface. Oxidation results from the C-terminated surface and some preliminary results for the non-polar and surfaces are included and they show distinct differences both as regards the sub-oxides present and the amount of carbon-containing by-products at the interface in the initial oxidation stage compared to the Si-terminated SiC(0001) surface.

Remote plasma-assisted oxidation of SiC is a low temperature process, 300 °C, for the formation of device quality interfaces on SiC. This paper discusses two aspects of the process: (i) the motivation for eliminating high temperature oxidation processes that can generate silicon oxycarbide, Si–O–C, interfacial regions which can be a source of interfacial defects and (ii) the kinetics of the remote plasma-assisted oxidation process that effectively eliminates interfacial Si oxycarbide transition regions. The differences between interfacial relaxation at Si–SiO₂ and SiC–SiO₂ are based on the relative stabilities of the suboxides of Si and SiC, SiO_x and (Si,C)O_x, respectively.

Comparative analysis of the electronic structure of thermally oxidized surfaces of silicon and silicon carbide indicates that in both cases the fundamental (bulk-band-related) spectrum of electron states is established within less than 1 nm distance from the interface plane. The latter suggests an abrupt transition from semiconductor to insulator. However, a large density of interface traps is observed in the oxidized SiC, which are mostly related to the clustering of elemental carbon during oxide growth and to the presence of defects in the near-interfacial oxides. Recent advancements in reducing the adverse effect of these traps suggest that the SiC oxidation technology has not reached its limits yet and fabrication of functional SiC/oxide interfaces is possible.

This paper describes the present state of a nitrogen-based passivation for SiO₂ layers on 4H-SiC. Interface state density, oxide breakdown field, channel mobility and gate oxide reliability have been characterized following nitric oxide (NO) passivation anneals. The kinetics of nitrogen incorporation and the quantitative modelling between nitrogen content and interface trap density with NO anneals have also been discussed.

Photocurrent relaxation was investigated in pure and Pr-doped amorphous As₂S₃ films after constant illumination had been switched off. It is shown that, in the temperature range from 290 to 370 K and for the time interval 0.5–150 s, the photocurrent relaxation might be approximated as an algebraic function. An analytic model of the recombination-controlled photocurrent decay in amorphous semiconductors is formulated. For the As₂S₃ samples with Pr impurity, the relaxation rate is lower and the form of the decay profile varies only lightly in comparison with non-doped samples. The results are discussed in terms of the band tails and deep acceptor-like centres present in a-As₂S₃ films.

Magnetic anisotropy measurements performed in a (110) (Tb_0.27Dy_0.73)Fe₂ (Terfenol-D) film epitaxially grown on a sapphire substrate are presented. The magnetic torque curves have been determined by using a vectorial vibrating sample magnetometer, which allows us to measure the angular dependence of magnetization components parallel, , and perpendicular, , to the applied field up to 2 T. The fourfold symmetry associated with the cubic structure within the (110) plane is clearly observed. The analysis of the experimental torque has been carried out considering magnetocrystalline anisotropy up to sixth order and magnetoelastic energy up to second order; so, the magnetocrystalline anisotropy constants in the (110) plane of the film, K₁ and K₂, have been obtained. This allows us to determine the direction of the magnetization easy axis for (110) Terfenol-D thin-film: it is at RT, passes through at 140 K and then changes to at 40 K. It was completely impossible to explain the angular dependence of the experimental magnetic torque without including shear and tetragonal magnetoelastic stress parameters, b₂ and b₁, respectively. This confirms the paramount role of the strain in the determination of the magnetic properties in this kind of Terfenol-D thin film.

Electrical and optical properties and the spectra of deep hole traps in p-GaN films implanted with high doses (3 × 10¹⁶ cm⁻³) of Co,Mn, Fe and Cr and annealed at 700 °C are reported. The dominant deep traps generated by this implantation are of the same type as observed in similar films heavily implanted with protons. The magnitude of the changes in the conductance and transmission of the GaN correlates with the atomic mass of the transition metal ions and the density of primary radiation defects. For fabrication of spintronic devices such as spin-LEDs using implantation of the TM species into the top p-GaN contact layer, the best results should be expected using Cr implantation since this produces both room temperature ferromagnetism and the smallest reduction in conductivity of the p-GaN.

Both hexagonal and cubic films were grown on c-plane (0001) sapphire substrate at low temperature. X-ray diffraction measurements show that the cubic Mg_xZn_1−xO films grow along the [111] direction while the hexagonal ZnO films grow along [0001]. The temperature-dependent optical properties of Mg_xZn_1−xO films were measured by ultraviolet optical transmission with temperature variation from 10 to 300 K and analysed by theoretically fitting the optical absorption spectra. For Mg_xZn_1−xO with , only stable hexagonal phase was observed and the optical absorption edge red shifts with temperature increase monotonically. For Mg_xZn_1−xO with , the crystal structure is cubic at 300 K. However, as measurement temperature decreases from 300 to 10 K an abnormality of the optical absorption is observed, which is attributed to the possible phase transition from cubic to hexagonal structure. The underlying physical mechanism for the crystal phase transition is attributed to the interaction of stress with stacking faults in the cubic Mg_xZn_1−xO.

The effect of interface conditions on the giant magnetoresistance (GMR) is studied. It is shown that the GMR changes drastically in the presence of extra magnetic monolayers at the ferromagnetic electrodes. The monolayers may make the spin-dependent conductance channels nearly or completely closed if they are antiferromagnetically exchange coupled to the electrodes. It is shown that the conductance spectrum of the device reflects, to a large degree, the internal band structure of the nanotube. Therefore the GMR effect is quite sensitive to the relative bands' line-up of electrodes and the nanotube and it may be tuned by a gate voltage applied to the nanotube.

Powders of n-diamond can be synthesized by pyrogenation of carbon black and nanometre-sized iron catalyst at atmospheric pressure and at a temperature of 1100 °C. The stability of n-diamond was investigated with x-ray diffraction, thermal gravimetric analysis and differential thermal analysis. The results indicated that n-diamond was a metastable phase: it can decompose at room temperature slowly. Thermal decomposition of n-diamond begins at 150 °C and is complete at 400 °C, and the decomposition of n-diamond was an exothermic reaction.

Using molecular dynamics simulations combined with kinetic Monte Carlo methods we have studied the evolution of copper nanoclusters on a copper (100) surface. We have developed a method for relaxing the clusters into a suitable configuration for input into the kinetic Monte Carlo method using molecular dynamics. Using kinetic Monte Carlo methods we have simulated the evolution of clusters with sizes of 22–2045 atoms at temperatures of 220–1020 K. We found that the Cu clusters on the surface will be reduced to one monolayer if given enough time to relax, and that this process shows an Arrhenius behaviour. In this paper we present the relaxation method that we developed and our observations for the evolution of the clusters.