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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.

The possibility of modification of the local properties of hexagonal boron nitride (h-BN) by laser irradiation is investigated. Investigations conducted using both Raman spectroscopy and electrostatic force microscopy were performed. Laser light induced modifications are found to cause no structural changes. However, they have impact on Raman spectra and local charge state of the material. They are also shown to be stable in time and during electrical grounding of the sample. The mechanism of photoionization of deep defects present in h-BN is proposed to explain the observed phenomenon. The discussed effect opens up new method of nanostructurization of h-BN based planar heterostructures., Comment: 9 pages, 5 figures

Contactless electroreflectance spectroscopy has been applied to study the surface potential barrier in AlxGa1−xN/n-AlxGa1−xN structures with 0 ≤ x ≤ 0.25 grown by metaloorganic vapor phase epitaxy. A strong band-to-band transition followed by Franz–Keldysh oscillation (FKO) was clearly observed for all samples. The value of built-in electric field was determined from the period of FKO, and the surface potential barrier was calculated with knowledge of the thickness of undoped AlxGa1−xN layer. For a set of GaN/n-GaN structures with different thickness of undoped GaN layer (30–120 nm) an analysis of built-in electric field gives the surface potential barrier of 0.59 ± 0.03 eV. For AlxGa1−xN(60 nm)/n-AlxGa1−xN structures with 0.09 ≤ x ≤ 0.25 the surface potential barrier does not vary with Al concentration within the experimental accuracy. In this case the position of the Fermi level at the surface has been estimated to be ∼1.4 eV. The increase of surface potential barrier from 0.59 eV for GaN to 1.4 eV for AlGaN with 0.09 ≤ x ≤ 0.25 is attributed to low concentration of n-type non-intentional residual dopants in the undoped AlxGa1−xN layer as well as the presence of Al atoms at the surface and their higher sensitivity to oxidation and/or creation of defects states which are not present at GaN surfaces.

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 present the study of metalorganic vapor phase epitaxy hexagonal boron nitride (MOVPE-h-BN) and graphene/MOVPE-h-BN heterostructures under the illumination with monochromatic light. The process of illumination makes the modification visible by both the shifting of Raman lines and the emergence of local electric fields. These changes are related to the modification of the charge state of the deep defect centers and can be used to control other 2D materials within heterostructures. The scanning tunneling spectroscopy study of the graphene/MOVPE-h-BN heterostructure allowed us to observe the defect states associated with the h-BN and evaluate the effect of illumination on them as well as the electronic structure of graphene.

The paper provides a deeper understanding of key-parameters of epitaxial graphene growth techniques on SiC. At 16000C, the graphene layer is continuous and covers a large area of the substrate. Significant differences in the growth rate could be observed for different reactor pressures and the polarity of SiC substrates as well as for the substrate miscut and surface quality. In addition, graphene thickness uniformity and mechanism of ridges creation was examined.

Microphotoluminescence of low-density GaN/AlxGa1ixN quantum dots grown by metal-organic vapor phase epitaxy using in situ etching of AlGaN is presented. The narrow lines in the microphotoluminescence spectra due to the single quantum dots are observed. Both energy and intensity of these lines show temporal ∞uctuations. Statistical analysis based on the correlation matrix allowed us to identify objects, which are afiected by photo-induced electric fleld ∞uctuations. Relations between emission lines participating in the spectrum are discussed.

We report studies on electric field built in GaN/Al0.09Ga0.91N structure of nominally 6 nm wide quantum well. The sample was grown in horizontal metal-organic chemical vapor deposition reactor using innovative technology that decreases the density of screw dislocations. Firstly, using visible and mid infra-red interference pattern along the sample, the layer thickness and consequently the quantum well width was determined to vary linearly with the position. Secondly, photoluminescence spectra was taken at different positions. Correlation of those two measurements allows us to determine the built-in electric field to be 0.66 MV/cm, which is considerably larger than previously reported for similar structures.

We present a novel measurement approach which combines the electrical characterization of solution-gated field effect transistors based on epitaxial bilayer graphene on 4H-SiC (0001) with simultaneous Raman spectroscopy. By changing the gate voltage, we observed Raman signatures related to the resonant electron-phonon coupling. An analysis of these Raman bands enabled the extraction of the geometrical capacitance of the system and an accurate calculation of the Fermi levels for bilayer graphene. An intentional application of higher gate voltages allowed us to trigger electrochemical reactions, which we followed in-situ by Raman spectroscopy. The reactions showed a partially reversible character, as indicated by an emergence / disappearance of peaks assigned to C-H and Si-H vibration modes as well as an increase / decrease of the defect-related Raman D band intensity. Our setup provides a highly interesting platform for future spectroelectrochemical research on electrically induced sorption processes of graphene on the micrometer scale.

Layers of InAs quantum dots grown on [100] GaAs substrates were characterised by photoluminescence and investigated by transmission electron microscopy. Two types of InAs islands were observed in these layers. The islands of the first type had mainly a form of big, elongated pyramids. Most of them were found to be dislocated. On the other hand, the islands of the second type were real self-assembled, coherent quantum dots giving rise to . a characteristic photoluminescence band. PACS numbers: 68.55.—a, 61.16.Bg

In this work we report results of photoluminescence (PL) and reflectivity measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapour deposition on GaN substrates. At low temperature (4.2K), very narrow (FWHM = 1.0meV) PL lines related to excitons bound to neutral acceptor (3.4666eV) and neutral donor (3.4719eV) were observed. The energies of free excitons from reflectivity and PL measurements were found to be: E A = 3.4780eV, E B = 3.4835eV and E C = 3.502eV.

Reflection measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on GaN single crystals are reported. At low temperature (4.2 K) three free exciton lines have been found with energies; EA=3.4776 eV, EB=3.4827 eV, and EC=3.502 eV. The spin‐orbit parameter Δso=19.7±1.5 meV and the crystal field parameter Δcr=9.3±0.3 meV have been obtained. From temperature dependence of exciton spectra the energy gap dependence has been found: E(T)=E(0)−λ/[exp(β/T)−1] (λ=0.121 eV, β=316 K).

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.

Far infrared magnetooptical investigations of shallow donors in epitaxial MOCVD GaN layers show two types of shallow donors. In relaxed layers, a donor with an ionization energy of 35 meV was found. In strained, undoped and Si doped samples, a donor with ionization energy 32.5 meV was observed. From the p state splitting in magnetic field, the cyclotron effective mass for conduction electrons was found to be m*=0.222 m0.

Graphene grown by chemical vapor deposition on 4H-SiC (0001) was studied using micro-Raman spectroscopy and atomic force microscopy (AFM). AFM revealed that the graphene structure grown on on-axis substrates has a stepped morphology. This is due to step bunching, which results from etching in hydrogen as well as from the process of graphene formation itself. It was shown by micro-Raman spectroscopy that the properties of graphene present on step edges and on terraces are quite different. Graphene on terraces is uniform with a relatively small thickness and strain fluctuations. On the other hand, graphene on step edges has a large thickness and strain variations occur. A careful analysis of micro-Raman spatial maps led us to the conclusion that the carrier concentration on step edge regions is lowered when compared with terrace regions.

The study of epitaxial graphene layers grown on SiC by two techniques, namely, the traditional Si sublimation method and the recent chemical vapor deposition (CVD) using temperature induced shift of the Raman 2D line, is presented. The measurements of thermal shift rate of 2D line on 4 H-SiC(0001) allowed us to determine notable differences in interaction of graphene with SiC substrate. The obtained results show that graphene layers grown by Si sublimation of 4 H-SiC(0001) are pinned strongly to the substrate. In contrast, the layers of graphene grown on 4 H-SiC(0001) substrates by CVD showed much weaker pinning. It was found that the film consisting of two or three graphene layers grown by CVD was already unpinned and thus showing Raman shift expected for freestanding graphene. The obtained differences in pinning of epitaxial graphene layers are explained in terms of basic growth mechanism differences between these two methods: graphene growth by Si sublimation is a “bottom-up” process and by CVD—a “top-...

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.

The review of structural and optical properties of homoepitaxial layers grown by MOVCD on single crystals GaN substrates is presented. The TEM technique is used to characterise the structural properties of epi-layers. It is found that the structural properties of GaN homoepitaxial layers are determined by the polarity of the substrate surface on which the growth takes place. It is shown that threading dislocations are present only in the layers grown on the [0001] “smooth” surface. On the other hand the layers grown on the [0001] “rough” surface are free from vertical defects. The characteristic feature of the growth on the “rough” surface are pinholes. The optical properties of homoepitaxial layers are predominantly determined by the growth polarity as well. It is shown also that the reflectivity measurement is the most precise way to determine the exciton energies and that emissions due to free excitons are strongly affected by polariton effects.