Table of contents

Silicon became the material of the 20th century. However, during the last decades, new needs have driven research and development of a new class of semiconductors, the wide band gap materials, for high power, high temperature, high voltage and high frequency devices and sensors. For these applications, wide band gap semiconductors have figures of merit that are several orders of magnitude higher compared with conventional semiconductors. In addition, some of them also exhibit such interesting characteristics as (i) outstanding mechanical properties, (ii) resistance to radiation damage and/or (iii) biocompatibility, a very useful feature for interfacing with biology. In order to have wide band gap semiconducting materials that can be used for electronics, photonics, sensors, microelectromechanical devices and other high-tech applications, some of the mandatory requirements include: (i) the availability of high quality and large wafers, (ii) having p- and n-type doping capability providing usable Fermi level positions, (iii) understanding and controlling surfaces and interfaces, (iv) the ability to fabricate self-organized nanostructures and (v) the potential to achieve miniaturization and integration.

The reviews in this Cluster Issue of Journal of Physics D: Applied Physics cover several of these important issues including growth, doping, engineering surfaces and interfaces, defects, nanotechnology and bio-functionalization. Achievements, progress and prospects are addressed, along with the difficulties, forming a good basis to evaluate the present status and future prospects of this promising and exciting field of science and technology.

While previously published experimental results have shown that the step-free (0 0 0 1) 4H–SiC mesa growth surface uniquely enables radical improvement of 3C–SiC and 2H–AlN/GaN heteroepitaxial film quality (>100-fold reduction in extended defect densities), important aspects of the step-free mesa heterofilm growth processes and resulting electronic device benefits remain to be more fully elucidated. This paper reviews and updates recent ongoing studies of 3C–SiC and 2H–AlN/GaN heteroepilayers grown on top of 4H–SiC mesas. For both 3C–SiC and AlN/GaN films nucleated on 4H–SiC mesas rendered completely free of atomic-scale surface steps, TEM studies reveal that relaxation of heterofilm strain arising from in-plane film/substrate lattice constant mismatch occurs in a remarkably benign manner that avoids formation of threading dislocations in the heteroepilayer. In particular, relaxation appears to occur via nucleation and inward lateral glide of near-interfacial dislocation half-loops from the mesa sidewalls. Preliminary studies of homojunction diodes implemented in 3C-SiC and AlN/GaN heterolayers demonstrate improved electrical performance compared with much more defective heterofilms grown on neighbouring stepped 4H–SiC mesas. Recombination-enhanced dislocation motion known to degrade forward-biased 4H–SiC bipolar diodes has been completely absent from our initial studies of 3C–SiC diodes, including diodes implemented on defective 3C–SiC heterolayers grown on stepped 4H–SiC mesas.

The present paper gives an overview of the different routes to grow SiC single crystals. The focus is put on the new emerging processes compared with the well established ones. A review of the process engineering modelling is given. Finally, some selected results are pointed out as they should be considered for the future development of SiC material.

Cubic BN (cBN) has a set of extreme properties similar or even superior to diamond. The advance of science and technology of cBN has however been severely hampered by the poor quality of the material available (random orientation, limited film thickness, poor crystallinity and adhesion with substrates due to a non-cubic BN interlayer). This paper reviews the recent progress in the nucleation, growth and characterization techniques of cBN films. It describes various successful approaches in interface engineering and growth techniques in increasing film thickness, improving crystallinity and adhesion of cBN films to the substrate, which are the major issues hindering cBN films for both mechanical and electronic applications. Based on observations of the surface and interface structures, we further discuss the growth mechanisms of cBN films via physical and chemical routes.

The growth of monocrystalline diamond films of electronic quality and large thickness (>few hundreds of microns) is an important issue in particular for high-power electronics. In this paper, we will describe the different key parameters necessary to reach this objective. First, we will examine the deposition process and establish that only microwave assisted diamond deposition plasma reactors can achieve the optimal growth conditions for the efficient generation of the precursor species to diamond growth. Next, we will consider the influence of the monocrystalline diamond substrate orientation and quality on the growth of the epitaxial layer, especially when the deposited material thickness exceeds 100 µm. The need to use a specific pre-treatment procedure of the substrate before the growth and its impact will also be discussed. Finally we will look at the growth conditions themselves and assess the influence of the process parameters, such as the substrate temperature, the methane concentration, the microwave power density and the eventual presence of nitrogen in the gas phase, on both the morphology and quality of the films on the one hand and the growth rate on the other hand. For this, we will introduce the concept of supersaturation and comment on its evolution as a function of the process parameters.

The properties of phosphorus incorporation for n-type doping of diamond are discussed and summarized. Doping of (0 0 1)-oriented diamond is introduced and compared with results achieved on (1 1 1) diamond. This review describes detailed procedures and conditions of plasma-enhanced chemical vapour deposition (CVD) growth and characteristics of electrical properties of phosphorus-doped diamond. The phosphorus incorporation was characterized by SIMS analysis including mapping. n-type conductivity is evaluated by Hall-effect measurements over a temperature regime of 300–1000 K. The crystal perfection of (0 0 1)-oriented n-type diamond is also evaluated by x-ray diffraction, Raman spectroscopy, reflection high-energy electron diffraction and cathodoluminescence analyses. The results show that phosphorus atoms are incorporated into the diamond network during (0 0 1) CVD diamond growth and that phosphorus acts as a donor as in (1 1 1)-oriented diamond. This result eliminates the restriction on substrate orientation, which had previously created a bottleneck in the development of diamond electronic devices.

Understanding the achievable degree of homogeneity and the effect of surface structure on semiconductor surface chemistry is both academically challenging and of great practical interest to enable fabrication of future generations of devices. In that respect, silicon terminated SiC surfaces such as the cubic 3C–SiC(1 0 0) 3 × 2 and the hexagonal 6H–SiC(0 0 0 1) 3 × 3 are of special interest since they give a unique opportunity to investigate the role of surface morphology on oxygen or hydrogen incorporation into the surface. In contrast to silicon, the subsurface structure plays a major role in the reactivity, leading to unexpected consequences such as the initial oxidation starting several atomic planes below the top surface or the surface metallization by atomic hydrogen.

The ability to manipulate semiconductor surfaces and interfaces has gained significant attention in the past few years, in particular to design novel routes for the development of sensors and new materials, like intelligent tissues, for bio-physics applications. In this paper, we review some recent results on the atomic manipulation of cubic silicon carbide (SiC) surfaces, as obtained by ab initio simulations, and compare them with the available experimental data. The main focus will be the study of the interaction and adsorption mechanism of simple molecules on SiC surfaces. SiC, and in particular the cubic polytype, is a wide-gap semiconductor, that for its inertness and transparency has been and still is considered a leading candidate material for biocompatible devices.

Using complementary surface analysis techniques, we study the Ge growth on distinct SiC(0 0 0 1) reconstructions and elucidate complex mechanisms occurring by thermal activation. Two Si-rich reconstructions, (3 × 3) and , and one C-rich, , are concerned, on which Ge is found to grow in Stranski–Krastanov and Volmer–Weber modes, respectively. The best Ge-wetting layer is favoured on the (less Si-rich) because closest to a perfect truncated SiC(0 0 0 1) termination. At sufficiently high temperature, the Ge-wetting layer is organized in the form of a (4 × 4)Ge reconstruction for which we propose a first atomic model that is based on the 3 × 3 structure. Annealing Ge on the (3 × 3) and surfaces provokes spectacular successive 2D/3D and unusual 3D/2D transitions not only of Ge but also of Si and C, respectively, coming from the surface initial richness. In both cases, a phase separation is observed either in the 2D or 3D structures, which is unexpected for the Ge/Si binary system and somewhat usual for the Ge/C one. In the case of Ge on , a special 2D heterostructure graphite/Ge/SiC is achieved at the atomic level. This acts as a Schottky barrier and then can be promising for future possible applications.

From the viewpoint of application in power electronics, SiC possesses the greatest advantage of having SiO₂ as its native oxide. Unfortunately, the usual thermal oxidation produces an unacceptably high density of interface states, with a complex energy distribution. Deep states are assumed to be caused by carbon excess at the interface, while the slow electron traps, called NIT, with especially high density near the conduction band of 4H-SiC (which would be the best polytype for power devices), are expected to originate from oxide defects near the interface. Unlike the case of the Si/SiO₂ interface, simple hydrogen passivation does not help to reduce the high trap density. A possible passivation method for both deep states and NIT is post-oxidation annealing or oxidation in the presence of NO or N₂O molecules. Here we present systematic and sophisticated theoretical calculations on a model of the 4H-SiC/SiO₂ interface, in order to establish the main reaction routes and the most important defects that are created during dry oxidation, and may give rise to the observed interface traps. We also investigate the effect of nitrogen in suppressing them.

SiC is unique amongst the wide bandgap semiconductors in that the natural thermal oxide is stoichiometric SiO₂, as is the case for silicon. The possibility of producing devices such as MOSFET in which thermal SiO₂ is used as the gate insulator has motivated substantial work aimed at understanding the morphology and electrical properties of the SiO₂/SiC interface and the processes responsible for thermal oxide growth. The oxide growth kinetics are quite different, parallel and anti-parallel to the crystal polar direction. We review the experimental study of the nature of the thermal oxide grown in ultra-dry oxygen and of the extended interfacial region at the SiO₂/SiC interface on the nominally Si-terminated and C-terminated polar surfaces of hexagonal polytypes of SiC, highlighting how the use of stable isotopic tracing has helped to clarify processes for which kinetics measurements alone do not prove to be sufficiently incisive.

We present a review of the different optical techniques that can be used to investigate the presence of as-grown and/or process-induced stacking faults (SFs) in 4H–SiC epitaxial layers. A SF is always a finite admixture of different polytypes, and we begin with a brief review of the systematic of SiC polytype structure and electronic properties. Next, we discuss the optical signature and compare with the results of several model calculations, taking successively into account the effect of valence band offset, internal polarization and non-homogeneity of the potential well. Finally, we consider cathodo-luminescence and micro-photoluminescence techniques and show that, in both cases, some screening of the built-in electric field can be achieved.

Structures of (0 0 0  ) aligned carbon nanotubes (CNTs) formed by surface decomposition of the SiC(0 0 0  ) C-face in vacuum were investigated by cross-sectional, plan-view transmission electron microscopy and electron diffraction methods. Zigzag-type CNTs were confirmed to be selectively formed by this method and the formation mechanism was proposed by crystallographic analysis. The wall number of CNTs was found to be directly proportional to the diameter of CNTs. Compared with theoretical calculation, it was revealed that all carbon atoms remained on the surface after selective evaporation of Si atoms by the decomposition of each mono-layer of SiC(0 0 0  ) and then constructed the CNT walls with a minimum diffusion distance at the interface. For actual application of some nanotube devices, patterning of CNTs on the SiC substrate by the photoresist technique and fabrication of a large-area uniform CNT film on a 12 in. CVD-SiC substrate were demonstrated.

Novel properties of energy absorption and anti-erosion wear affected by the flexibility of the CNT film were revealed by nano-indentation and erosion wear experiments.

Well-ordered two-dimensional functional nanostructure arrays have a broad range of potential applications in molecular electronics, ultra-high density data storage, biosensors, single-electron, single-photon and quantum computation devices. Various surface nanotemplates that are naturally or artificially patterned at the nanometre scale have been used to guide the formation of highly periodic nanostructure arrays. In this paper, we review recent progress in the development of various surface nanotemplates as well as the self-assembly of nanostructures on them, with particular emphasis on the unique SiC nanomesh template. The formation mechanism of this nanomesh template is attributed to the self-assembly of accumulated carbon atoms into well-ordered honeycomb superstructures at the nanometre scale on the 6H–SiC(0 0 0 1) surface. The size and spacing of unit cells of the SiC nanomesh can be tailored in the 2–2.5 nm range by adjusting the annealing time. This nanomesh is demonstrated to be an effective template for the formation of well-ordered molecular arrays of copper(II) phthalocyanine (CuPc). The growth of C₆₀ on the nanomesh follows a typical Stranski–Krastanov mode, and a complete C₆₀ wetting monolayer can be epitaxially grown on this nanotemplate. It is effective in isolating metal nanoclusters, resulting in the formation of monodispersed Co nanoclusters with a narrow size distribution. The adsorption and desorption of Co nanoclusters and C₆₀ do not change its atomic structure, making it a chemically and thermally stable nanotemplate for the formation of well-ordered nanostructures arrays as well as monodispersed metal nanoclusters.

The review is devoted to nanodiamond as a member of new nanocarbon allotropes. The past results related to the main features of detonation technology for producing nanodiamond are highlighted. Effects of technology on the structure of nanodiamond particles as well as functionalization of nanodiamond surface to chemical properties are discussed. The real structure of single nanodiamond particles has been critically reviewed and its aggregation problem emphasized. Several applications of nanodiamonds mainly as precursors for CVD diamond film growth, for forming new magnetic nanomaterials and field electron emitters are reviewed. As a result, the availability of nanodiamonds as attractive building blocks for nanotechnology is concluded.

ZnO is a unique material that offers about a dozen different application possibilities. In spite of the fact that the ZnO lattice is amenable to metal ion doping (3d and 4f), the physics of doping in ZnO is not completely understood. This paper presents a review of previous research works on ZnO and also highlights results of our research activities on ZnO. The review pertains to the work on Al and Mg doping for conductivity and band gap tuning in ZnO followed by a report on transition metal (TM) ion doped ZnO. This review also highlights the work on the transport and optical studies of TM ion doped ZnO, nanostructured growth (ZnO polycrystalline and thin films) by different methods and the formation of unique nano- and microstructures obtained by pulsed laser deposition and chemical methods. This is followed by results on ZnO encapsulated Fe₃O₄ nanoparticles that show promising trends suitable for various applications. We have also reviewed the non-linear characteristic studies of ZnO based heterostructures followed by an analysis on the work carried out on ZnO based phosphors, which include mainly the nanocrystalline ZnO encapsulated SiO₂, a new class of phosphor that is suitable for white light emission.

We report on the current properties of Al_1−xIn_xN (x ≈ 0.18) layers lattice-matched (LM) to GaN and their specific use to realize nearly strain-free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state-of-the-art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of (1–5) × 10¹⁸ cm⁻³ and a large Stokes shift (∼800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified through the properties of GaN/AlInN multiple quantum wells (QWs) suitable for near-infrared intersubband applications. A built-in electric field of 3.64 MV cm⁻¹ solely due to spontaneous polarization is deduced from photoluminescence measurements carried out on strain-free single QW heterostructures, a value in good agreement with that deduced from theoretical calculation. Other potentialities regarding optoelectronics are demonstrated through the successful realization of crack-free highly reflective AlInN/GaN distributed Bragg reflectors (R > 99%) and high quality factor microcavities (Q > 2800) likely to be of high interest for short wavelength vertical light emitting devices and fundamental studies on the strong coupling regime between excitons and cavity photons. In this respect, room temperature (RT) lasing of a LM AlInN/GaN vertical cavity surface emitting laser under optical pumping is reported. A description of the selective lateral oxidation of AlInN layers for current confinement in nitride-based light emitting devices and the selective chemical etching of oxidized AlInN layers is also given. Finally, the characterization of LM AlInN/GaN heterojunctions will reveal the potential of such a system for the fabrication of high electron mobility transistors through the report of a high two-dimensional electron gas sheet carrier density (n_s ∼ 2.6 × 10¹³ cm⁻²) combined with a RT mobility μ_e ∼ 1170 cm² V⁻¹ s⁻¹ and a low sheet resistance, R ∼ 210 Ω/□.

Silicon carbide has attracted considerable attention in recent years as a potential material for sensor devices. This paper reviews the current status of SiC technology for a wide range of sensor applications. It is shown that SiC MEMs devices are well-established with operational devices demonstrated at high temperatures (up to 500 °C) for the sensing of motion, acceleration and gas flow. SiC sensors devices using electrical properties as the sensing mechanism have also been demonstrated principally for gas composition and radiation detection and have wide potential use in scientific, medical and combustion monitoring applications.

A review of properties of wide gap semiconductor materials such as diamond, diamond-like carbon films, SiC, GaP, GaN and AlGaN/GaN that are relevant to electronic, optoelectronic and microwave applications is presented.

We discuss the latest situation and perspectives based on experimental and theoretical results obtained for wide gap semiconductor devices. Parameters are taken from the literature and from some of our theoretical works. The correspondence between theoretical results and parameters of devices is critically analysed.

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

The search for materials and systems, capable of operating long term under physiological conditions, has been a strategy for many research groups during the past years. Silicon carbide (SiC) is a material, which can meet the demands due to its high biocompatibility, high inertness to biological tissues and to aggressive environment, and the possibility to make all types of electronic devices.

This paper reviews progress in biomedical and biosensor related research on SiC. For example, less biofouling and platelet aggregation when exposed to blood is taken advantage of in a variety of medical implantable materials while the robust semiconducting properties can be explored in surface functionalized bioelectronic devices.

A summary of photo- and electrochemical surface modifications applied on single-crystalline chemical vapour deposition (CVD) diamond films is given. The covalently bonded formation of amine- and phenyl-linker molecule layers is characterized using x-ray photoelectron spectroscopy, atomic force microscopy (AFM), cyclic voltammetry and field-effect transistor characterization experiments. Amine- and phenyl-layers are very different with respect to formation, growth, thickness and molecule arrangement. We detect a single-molecular layer of amine-linker molecules on diamond with a density of about 10¹⁴ cm⁻² (10% of carbon bonds). Amine molecules are bonded only on initially H-terminated surface areas to carbon. In the case of electrochemical deposition of phenyl-layers, multi-layer formation is detected due to three-dimensional (3D) growths. This gives rise to the formation of typically 25 Å thick layers. The electrochemical grafting of boron-doped diamond works on H-terminated and oxidized surfaces.

After reacting such films with hetero-bifunctional crosslinker molecules, thiol-modified ss-DNA markers are bonded to the organic system. Application of fluorescence and AFM on hybridized DNA films shows dense arrangements with densities of up to 10¹³ cm⁻². The DNA is tilted by an angle of about 35° with respect to the diamond surface. Shortening the bonding time of thiol-modified ss-DNA to 10 min causes a decrease of DNA density to about 10¹² cm⁻². Application of AFM scratching experiments shows threshold removal forces of around 75 nN for DNA bonded on phenyl-linker molecules and of about 45 nN for DNA bonded to amine-linker molecules. DNA sensor applications using Fe(CN₆)^3−/4− mediator redox molecules, impedance spectroscopy and DNA-field effect transistor devices performances are introduced and discussed.

Diamond is a wide band gap semiconductor with unique electrical, physical, chemical and mechanical properties, opening the possibility to realize, in diamond, electronic devices with unprecedented properties. However, this uniqueness also poses some difficulties specific to diamond. Amongst these are the wide band gap of diamond, the presence of electrically active defects due to sp² bonded carbon and unintentionally present impurities, the absence of shallow dopants in diamond (in particular donors), the presence of extended defects and grain boundaries (in CVD polycrystalline diamond) and difficulties related to material availability and device processing.

Nevertheless, diamond-based prototype devices with promising properties have been realized in the last few years. These include, amongst others, Schottky diodes, field effect transistors, cold electron emission devices and detectors for ionizing radiation, demonstrating the huge potential that this wide band gap semiconductor bears.

The uniqueness of diamond, the promise it bears as an unusual material for specific electronic devices and the difficulties related to its application are briefly reviewed. Reference is given to some recent reviews published on specific topics and to some of the more recent breakthroughs and potential applications in the field.