Rositsa Yakimova

Surface functionalization and biomedical applications based on SiC

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.

Dislocation evolution in 4H-SiC epitaxial layers

4H-SiC commercial wafers and sublimation grown epitaxial layers with a thickness of 100 μm have been studied concerning crystalline structure. The substrates and the epitaxial layers have been separately investigated by high-resolution x-ray diffraction and synchrotron white beam x-ray topography. The results show that the structural quality was improved in the epitaxial layers in the [1120] and [1100] directions, concerning domain distribution, lattice plane misorientation, mosaicity, and strain, compared with the substrates. Misoriented domains have merged together to form larger domains while the tilt between the domains was reduced, which resulted in nonsplitting in diffraction curves. If the misorientation in the substrate is large, we can only see a slight decrease in the misorientation in the epilayer. At some positions on the substrates block structures (mosaicity) were observed. ω-rocking curves showed smaller full width at half maximum values and more uniform and narrow peaks, while the curvat...

Mapping of Local Electrical Properties in Epitaxial Graphene Using Electrostatic Force Microscopy

Local electrical characterization of epitaxial graphene grown on 4H-SiC(0001) using electrostatic force microscopy (EFM) in ambient conditions and at elevated temperatures is presented. EFM provides a straightforward identification of graphene with different numbers of layers on the substrate where topographical determination is hindered by adsorbates. Novel EFM spectroscopy has been developed measuring the EFM phase as a function of the electrical DC bias, establishing a rigorous way to distinguish graphene domains and facilitating optimization of EFM imaging.

Growth of 6H and 4H—SiC by sublimation epitaxy

The epitaxial sublimation growth process of SiC has been investigated. Layers with specular surfaces and growth rates up to 2 mm/h have been obtained. No step bunching is observed by optical microscopy even on very thick layers which indicates a stable step growth mechanism. Under certain growth conditions the morphology degrades. The morphological stability is investigated and discussed in relation to the growth kinetics. Impurities in the epitaxial layers are investigated by secondary ion mass spectroscopy and low-temperature photoluminescence. The carrier concentration is measured by capacitance–voltage measurements. The structural quality of the grown material is improved compared to the substrate as shown by X-ray diffraction measurements.

Anomalously strong pinning of the filling factor nu=2 in epitaxial graphene

We explore the robust quantization of the Hall resistance in epitaxial graphene grown on Si-terminated SiC. Uniquely to this system, the dominance of quantum over classical capacitance in the charge transfer between the substrate and graphene is such that Landau levels (in particular, the one at exactly zero energy) remain completely filled over an extraordinarily broad range of magnetic fields. One important implication of this pinning of the filling factor is that the system can sustain a very high nondissipative current. This makes epitaxial graphene ideally suited for quantum resistance metrology, and we have achieved a precision of 3 parts in 1010 in the Hall resistance-quantization measurements.

Current status and advances in the growth of SiC

Recent achievements in crystal growth and homoepitaxy of SiC, mainly of the 4H polytype, have been discussed. Several growth techniques, such as seeded sublimation growth, high temperature chemical vapor deposition, sublimation epitaxy and liquid phase epitaxy have been utilized to develop technological procedures and understand the growth processes better. The advantages of either method have been stressed. The main target has been the reproducible growth of micropipe free substrate material and thick epitaxial layers for device applications. The purity of the layers has been of special interest. The results obtained are indicative for the massive progress that has been achieved in SiC crystal growth during recent years.

Large homogeneous mono-/bi-layer graphene on 6H-SiC(0001) and buffer layer elimination

In this paper we discuss and review results of recent studies of epitaxial growth of graphene on silicon carbide. The presentation is focused on high quality, large and uniform layer graphene growth on the SiC(0 0 0 1) surface and the results of using different growth techniques and parameters are compared. This is an important subject because access to high-quality graphene sheets on a suitable substrate plays a crucial role for future electronics applications involving patterning. Different techniques used to characterize the graphene grown are summarized. We moreover show that atomic hydrogen exposures can convert a monolayer graphene sample on SiC(0 0 0 1) to bi-layer graphene without the carbon buffer layer. Thus, a new process to prepare large, homogeneous stable bi-layer graphene sheets on SiC(0 0 0 1) is presented. The process is shown to be reversible and should be very attractive for various applications, including hydrogen storage.

Structural improvement in sublimation epitaxy of 4H-SiC

The sublimation epitaxy growth process has been studied. The structural quality of the grown layers improves compared with the substrate mainly due to a diminished domain structure misorientation. ...

Helicity-dependent photocurrents in graphene layers excited by midinfrared radiation of a CO(2) laser

We report the study of the helicity-driven photocurrents in graphene excited by midinfrared light of a CO(2) laser. Illuminating an unbiased monolayer sheet of graphene with circularly polarized radiation generates-under oblique incidence-an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. We show that the current is caused by the interplay of the circular ac Hall effect and the circular photogalvanic effect. By studying the frequency dependence of the current in graphene layers grown on the SiC substrate, we observe that the current exhibits a resonance at frequencies matching the longitudinal optical phonon in SiC.

Small epitaxial graphene devices for magnetosensing applications

Hall sensors with the width range from 0.5 to 20.0 mu m have been fabricated out of a monolayer graphene epitaxially grown on SiC. The sensors have been studied at room temperature using transport and noise spectrum measurements. The minimum detectable field of a typical 10-mu m graphene sensor is approximate to 2.5 mu T/root Hz, making them comparable with state of the art semiconductor devices of the same size and carrier concentration and superior to devices made of CVD graphene. Relatively high resistance significantly restricts performance of the smallest 500-nm devices. Carrier mobility is strongly size dependent, signifying importance of both intrinsic and extrinsic factors in the optimization of the device performance