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We demonstrate quantum emission capabilities from boron nitride structures which are relevant for practical applications and can be seamlessly integrated into a variety of heterostructures and devices. First, the optical properties of polycrystalline BN films grown by metalorganic vapour-phase epitaxy are inspected. We observe that these specimens display an antibunching in the second-order correlation functions, if the broadband background luminescence is properly controlled. Furthermore, the feasibility to use flexible and transparent substrates to support hBN crystals that host quantum emitters is explored. We characterise hBN powders deposited onto polydimethylsiloxane films, which display quantum emission characteristics in ambient environmental conditions.

The implementation of graphene in nanoelectromechanical systems and electronic applications requires not only techniques to fabricate high-quality monolayers, but also methods to process these layers. Nondestructive processing is especially challenging in the case of fragile suspended graphene membranes. In this work, we present a direct writing method for graphene grown on germanium that yields suspended layers without the need to transfer the graphene layer. To this end, we employ laser-induced photoelectrochemical etching which is highly selective and dissolves only germanium leaving the graphene layer intact. Only a focused continuous wave laser beam and water (or an aqueous solution) are required for the etching to proceed. Raman spectroscopy measurements were performed in-situ to monitor the etching process. These measurements reveal a dramatic increase of the graphene-related Raman bands as the graphene layer detaches from the substrate. This substantial increase indicates that the commonly observed weak Raman signal for graphene on germanium is not an inherent material property but is due to the interaction of the germanium substrate with graphene. Together with the established graphene growth on germanium, the presented direct writing method builds a complete toolbox for graphene membrane-based applications.

Graphene oxide suspension in various solvents was spin coated on metal organic vapor phase epitaxy grown GaN/saphire layers. Samples were characterised using the Raman spectroscopy and atomic force microscopy, before and after high temperature treatment. We found that graphene oxide was modifed by high temperature treatment, however a considerable modification was also observed as a result of impinged laser light incident due to the measurements. The Raman spectra were decomposed into two contributions showing different behaviour during the Raman scattering measurements.

Graphene is chemically and mechanical resistant and can therefore readily be used for applications in harsh environments. This makes graphene, also owing to its excellent electrical properties, a very promising material for use in medicine, chemistry, food industry as well as in electronics, optoelectronics, robotics and nanotechnology. One of the areas of research, which is currently being widely investigated, is the use of graphene as a liquid flow sensor. The reports addressing such sensors, which were published so far, are very contradictory 1) . This indicates that the underlying physics is still not well understood and that more in-depth studies are needed in order to shed more light on this subject. In this communication we present results on our research on graphene flow sensor structures. We were able to obtain sensors, which showed, in certain range, a linear dependence on the liquid flow velocity. We show that this sensors functioned in different liquids like deionized water (DI) or acid solutions. The main element of the investigated sensors is graphene on different substrates immersed in a flowing liquid. We used epitaxial graphene on SiC with different resistivities and graphene grown on Cu which was then transferred to SiO2 or PET (polyethylene terephthalate). Different kinds of electric contacts, which were made of Pd/Au, Cr/Au or Ti/Au were fabricated and studied. Furthermore, we investigated samples with Ti/Au metallization and an additional SiO2 contact passivation. To completely exclude possible effects on the generated signals due to the metallization, we constructed a sample for which the electrical connections were not immersed in the liquid. Various liquids like DI, aqueous solutions of NaCl and HCl and organic liquids (isopropanol and ethanol) were investigated. Two types of configurations, with and without additional gate were used in our experiments. As a gate, we used Pt or Cu wires and in some cases we extended the setup to work with an additional Ag/AgCl electrode. We performed measurements in a passive system, which allowed for the investigation of voltage/current generation caused by the flow of liquids (as reported in ref 1). In order to broaden the picture we also studied configurations with external voltages applied to the gate, as well as a configuration in which current was forced through the graphene sample. To understand the phenomena we have to take into account the presence of a double layer at the graphene/liquid interface. A double layer is made of two layers of oppositely charged ions forming at the interface inside the liquid. We conclude that the main reason responsible for the observed generated signals could be the perturbation of this double layer. Our studies provide new important information that help to gain a better understanding of the observed phenomena.

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.

The so-called “growth” of graphene was performed using a horizontal chemical vapor deposition (CVD) hot-wall reactor. In-situ etching in the mixture (H2-C3H8) was performed prior to growth at 1600oC temperature under 100 mbar. Systematic studies of the influence of the decomposition temperature and time, substrates roughness, etching of the substrates, heating rate, SiC dezorientation and other process parameters on the graphene thickness and quality have been conducted. Morphology and atomic scale structure of graphene was examined by Scanning Tunnelling Microscopy (STM), Transmission Electron Microscopy (TEM) and Raman scattering methods.

A new method of the growth of low-density GaN quantum dots on AlGaN by MOVPE is reported. The method bases on in-situ etching of the AlGaN surface in the presence of silane (SiH 4 ) and subsequent growth of randomly nucleated GaN nano-crystallites. Investigation of successive growth stages with atomic force microscopy (AFM) shows that density of the GaN crystallites is of the order of 10 8 cm -2 . Micro-photoluminescence (μPL) measurements show sharp emission lines originating from single quantum dots without any artificial masks or mesa structures. This gave unique possibility for advanced studies on optical and electrical properties of single GaN quantum dots in AlGaN.

Time-resolved photoluminescence measurements of high-quality GaN show that the spectra of two-electron satellites (TES) in GaN include also lines coming from excited states of a donor-bound exciton (D 0 X) complex. The lines connected with recombination from the ground and excited states have generally similarly long lifetimes (1.1–1.4 ns, in the case of an exciton bound to oxygen donor). However, analysis of initial dynamics (between 0 and 0.5 ns) shows some transfer of energy between the lines. In fact, the ground-state-related line reaches its maximum 0.1 ns after the excited-state–related line. A rate-equation model taking into account internal transitions in the D 0 X complex gives a characteristic internal time constant of about 0.2 ns.

We present optical properties of GaN/AlGaN structures containing quantum dots, which were obtained in a low-pressure MOVPE reactor on a SiH4-treated surface. Photoluminescence (PL) shows two bright PL bands (P1, P2) and one weak PL band (P3) at energies of 3.48 eV, 3.44 eV and 3.35 eV, respectively. Time-resolved PL shows that the P1, P2 and P3 bands have lifetimes of about 0.05 ns, 0.4 ns and 1.3 ns, respectively. Micro-photoluminescence measurements reveal that these bands consist in fact of many sharp lines coming from small spots on the sample. Such behaviour is characteristic for quantum dots or islands. The P2 shows significant spectral shift with time. As confirmed by power dependent measurements, the spectral shift is due to excitation of high-energy quantum dots followed by diffusion of excitons between QDs to lower energies. Excitation distribution parameters found at low temperature allow explaining of the behaviour of the PL at higher temperatures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature and excitation power dependencies of photoluminescence from GaN/AlGaN quantum well structures are studied. We show that depending on the well width, the increase of the excitation power results in the blue or red shift of the emission energy. Temperature affects the observed emission energy shifts in a way which is entirely different from that for unstrained GaN bulk. Surprisingly, the observed differences are larger for wider wells. These results can be understood assuming that not only potential fluctuations but also built-in electric field and strain effects must be taken into account. The role of the particular mechanisms is discussed on the basis of performed calculations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Successful growth of the ZnO and ZnO:Mn crystals by the CVT method and their characterisation are reported. The source material was synthesized at 650 °C from oxygen and zinc vapours and subsequently – baked (as a powder) to achieve stoichiometry. Crystal growth (at ∼1100 °C, with the rate ∼1–2 mm/day) proceeded in graphite-covered quartz ampoules containing pure (6N) hydrogen or nitrogen or chlorine and a little of water vapour and carbon. The crystals, both as-grown and annealed in pure oxygen, were characterised by the measurements of: transmission spectra in energy gap and far infrared regions, photoluminescence and reflectivity spectra, electrical transport properties and EDXRF – the energy dispersive X-ray fluorescence (for Mn content determination). In the surface region – the impurities and the Mn content were studied by the secondary ion mass spectroscopy (SIMS). Manganese and native defects (VO) were investigated by electron paramagnetic resonance (EPR) and Raman spectroscopy. Results of the measurements are presented and discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the photoluminescence experiments on high quality GaN samples in high magnetic fields. A detailed analysis of the recombination due to excitons bound to neutral donors and acceptors is presented. Special attention is focussed on transitions for which the impurity is left in the excited state (so called two electron satellites). These results show a rich energetic structure of excited states of the impurity involved in such a recombination process. The magnetic field dependence of the energy structure of the shallow neutral donor and acceptor in GaN is then discussed. The validity of the effective mass approximation for shallow impurities in wide gap semiconductors is examined.

There are three exciton branches in wurzite-GaN: A, B and C. Here, time-resolved photoluminescence and magneto-optic spectroscopy of the B-exciton are presented, both free (FX B ) and bound to donor (D 0 X B ) in homoepitaxial GaN. The free B-exciton has an energy of about 3.483 eV and a very short lifetime of about 20 ps. The B-exciton bound to donor (D 0 X B ) has an energy of 3.475 eV and a lifetime longer than the free excitons but shorter than the lifetime of the A-exciton bound to donor. It is shown that a simple model based on an invariant expression describes properly the behavior of D 0 X A and D 0 X B in a magnetic field. Results of resonance excitation experiments suggest that in many cases the process of binding a free exciton to a donor conserves the exciton type and that the relaxation of B- to A-exciton takes place later. Moreover, thermal excitation of D 0 X A to D 0 X B has been observed.

Recently, ZnO attracts a wide interest as a promising material for the application in optoelectronic devices working in the blue and ultraviolet region and (when doped with magnetic impurities) in spintronic devices. Unfortunately, the technology of good, large (e.g.: 2.5–5 cm in diameter) single crystals is very difficult, even as compared with other II–VI compounds. We report on the successful growth of the ZnO crystals with a chemical vapour transport (CVT) method and on the characterisation of them. The source material is synthesised at 650 °C from oxygen and zinc vapours and subsequently, baked (as a powder) to achieve stoichiometry. The crystals grow (with the rate 1–2 mm per day) in the graphite-covered quartz ampoules containing pure (6N) hydrogen or nitrogen and a small amount of water vapour. The crystals, both as-grown and annealed in pure oxygen, are characterised by the measurements of photoluminescence spectra, transmission spectra, far infrared transmission, X-ray diffraction and electrical transport. The surface region is analysed by the secondary ion mass spectroscopy (SIMS).

We have studied the transport properties of graphene grown on silicon side of SiC. Samples under study have been prepared by two different growth methods in two different laboratories. Magnetoresistance and Hall resistance have been measured at temperatures between 4 and 100 K in resistive magnet in magnetic fields up to 22 T. In spite of differences in sample preparation, the field dependence of resistances measured on both sets of samples exhibits two periods of magneto-oscillations indicating two different parallel conducting channels with different concentrations of carriers. The semi-quantitative agreement with the model calculation allows for conclusion that channels are formed by high-density and low-density Dirac carriers. The coexistence of two different groups of carriers on the silicon side of SiC was not reported before., Comment: 5 pages, 6 figures, accepted for publication in the "IOP Journal of Physics: Conference series" as a contribution to the proceedings of the 20th International Conference on "High Magnetic Fields in Semiconductor Physics", HMF 20

The principal structural defects in graphene layers, synthesized on a carbon-terminated face, i.e., the SiC(000$\overline{1}$) face of a $4H$-SiC substrate, are investigated using microscopic methods. Results of high-resolution transmission electron microscopy (HRTEM) reveal their atomic arrangement. The mechanism of such defects' creation, directly related to the underlying crystallographic structure of the SiC substrate, is proposed. The connection between the $4H$-SiC(000$\overline{1}$) surface morphology, including the presence of the single atomic steps, the sequence of atomic steps, and also the macrosteps, and the corresponding emergence of planar defective structure (discontinuities of carbon layers and wrinkles), is revealed. It is shown that the disappearance of the multistep island leads to the stress-related creation of wrinkles in the graphene layers. The density functional theory (DFT) calculation results show excess carbon atoms convert a topmost carbon layer to the sp${}^{2}$-bonded configuration, liberating Si atoms in the barrierless process. The DFT results show that the diffusion of carbon atoms is essentially impossible at the C-terminated SiC surface. On the contrary, DFT results prove that diffusion of the silicon atoms is possible on the C-terminated SiC surface at a high temperature close to 1600 \ifmmode^\circ\else\textdegree\fi{}C. Because, according to TEM studies, at the carbon-terminated SiC surface, the buffer layer is absent, that creates a channel for effective horizontal diffusion of both silicon atoms under the graphene layer. Ultimately the silicon atoms escape could be facilitated by the channels created at the bending layer defects (wrinkles). The sp${}^{2}$-bonded carbon atoms are incorporated into the growing graphene layers, which contribute to stress in the growing layers, detachment from SiC support, and partial contribution to the creation of wrinkles. These results explain the phenomenon of the growth of thick undulated graphene layers by subsequent creation of the new layer underneath the existing graphene cover and also the creation of the principal defects in graphene at the C-terminated SiC(000$\overline{1}$) surface.

Optical absorption and Raman scattering studies of epitaxial graphene structures obtained by annealing of carbon terminated face of $4H\text{-SiC}(000\text{\ensuremath{-}}1)$ on-axis substrates using standard chemical-vapor deposition reactor are presented. Two series of samples grown at different argon pressures in the reactor and different annealing times were studied. Optical absorption and Raman scattering were used to determine the number of graphene layers formed on the substrate surface. The observed dependence of the number of graphene layers formed on annealing time and argon pressure strongly indicates that the growth kinetics of graphene is limited by Si evaporation and two-dimensional Si diffusion.

Properties of polaritons (free excitons coupled with photons of similar energy) in gallium nitride are investigated by performing calculations based on dielectric function theory including all three excitons A, B and C (char- acteristic for the wurtzite structure). Moreover the excited states of excitons have been taken into account by adding Elliott's components to dielectric function. Energies, polarizabilities and damping constants of excitons are determined. It is shown that due to inter-exciton interactions the B and C excitons are strongly damped. It is estimated that the characteristic time of B to A relaxation is tBA = 1 ps. The exciton C lifetime is estimated τc= 0.2ps.

In their Comment on our publication [Phys. Rev. B 66, 245317 (2002)], Freitas et al. criticize our identification of donor-bound exciton transitions and the analysis of two-electron satellite emission lines. We show that some arguments given in the Comment are in conflict with our time-resolved and magneto-optical data. We present a discussion of the intradonor transition energies and donor chemical shifts to avoid misunderstanding.

Magneto-luminescence experiments performed on a thick, freestanding GaN layer in magnetic fields up to 28 T are presented. The photoluminescence spectrum of this material shows narrow emission lines due to neutral donor-bound excitons (D°X). In addition to the principal recombination channel of D°X in which the donor is left in its ground state, two-electron satellites (TES) involving different excited donor states, as well as LO phonon replica of the principal and satellite transitions are observed. For the magnetic field range in which internal donor excitations are tuned into resonance with the LO phonon excitations, the intensity of TES is strongly enhanced. A clear resonant interaction between TES corresponding to the highly excited donor states (n > 4) and LO phonon replica of D°X is observed. The observed effect is discussed in terms of electron-phonon interaction.

Structural investigations of hydrogenated epitaxial graphene grown on SiC(0001) are presented. It is shown that hydrogen plays a dual role. In addition to contributing to the well-known removal of the buffer layer, it goes between the graphene planes, resulting in an increase of the interlayer spacing to 3.6 A–3.8 A. It is explained by the intercalation of molecular hydrogen between carbon planes, which is followed by H2 dissociation, resulting in negatively charged hydrogen atoms trapped between the graphene layers, with some addition of covalent bonding to carbon atoms. Negatively charged hydrogen may be responsible for p-doping observed in hydrogenated multilayer graphene.

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC (0001¯) surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 A from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC (0001¯) are loosely bound to the SiC substrate. Moreover, the structural observations reveal a certain degree of disorder between the graphene planes, which manifests itself in a rotation of the layers and in an increase in the interplanar spacing between certain carbon layers from 3.35 A, which is characteristic for graphite, up to 3.7 A. Graphene grown on 8° off-axis SiC (0001¯) substrates covers the steps of SiC and as a result disorder seems to be not as pronounced as it is on the on-axis SiC (0001¯) substrate.

Transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and micro-Raman investigations of epitaxial graphene on 4H-SiC on-axis and 4° off-axis are presented. The STM images show that there is superimposed on 1×1 graphene pattern the carbon nanomesh of honeycomb 6×6 structure with the lattice vector of 17.5 A. The TEM results give evidence that the first carbon layer is separated by 2 A from the Si-terminated SiC surface and that subsequent carbon layers are spaced by 3.3 A. It is also found in TEM that the graphene layers cover atomic steps, present on 4° off-axis SiC(0001) surface, indicating a carpetlike growth mode. However, a bending of graphene planes on atomic steps of SiC apparently leads to generation of stress which leads to creation of edge dislocations in the graphene layers.

Experiences de photoluminescence sur une variete de puits quantiques multiples de GaAs-Ga 1-x Al x As dopes selectivement par le silicium, dont la croissance est obtenue par epitaxie de faisceau moleculaire. Etude comparative de la photoluminescence des structures similaires non dopees, et de la matrice d'AsGa dopee par le Silicium. Attribution des structures de photoluminescence obtenue par analyse attentive de la dependance des spectres avec: la temperature, la force de l'excitation et le champ magnetique

Hydrogenic donors in confined semiconducting materials are worth studying because they exhibit interesting similarities with the hypothetical two-dimensional (2D) hydrogen atom /1/. A detailed understanding of their properties is also required for the material doping in view of device applications. Far-infrared magneto-spectroscopy — especially in high magnetic fields - has long proved to be a valuable tool for the study of impurity states in bulk semiconductors and also more recently in quasi-2D materials /2,3/. In the present communication, we summarize our magneto-optical studies carried out on the Silicon impurity in GaAs-GaA1As multiquantum-wells (MQW). Special emphasis is given to the optical phonon energy region where 2D bound resonant polarons are observed.