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We present a solution-gated in situ Raman spectroscopy approach, which enables the electrical characterization of graphene on a copper substrate without the need of a transfer process. The application of a voltage across the solution resulted in a shift of the Raman G-band without a significant shift of the 2D band. This observation allowed for the separation of the effects of strain and doping. Based on the G and 2D band shifts we show that we can manipulate the n-type carrier concentration of graphene directly on the copper substrate in a range from about 8× 1012 cm-2 to about 1.5× 1013 cm-2. [ABSTRACT FROM AUTHOR]

Magneto- and piezo-spectroscopy of high quality bulk samples have been used to clarify the origin of the characteristic, V1, V2 and V3 luminescence lines observed in 6H-SiC polytypes, correspondingly, at 1.433, 1.398, and 1.368 eV. The uniaxial stress, up to 500 MPa, was applied along [11-20] and [10-10] directions, perpendicular to the c-axis of the investigated 6H-SiC crystals. Magnetic field was applied in different directions with respect to the c-axis. The obtained results allow us to analyze in detail the symmetry of the initial and final states responsible for the V2 emission line. This analysis leads to a conclusion which contradicts the existing identification of the defect responsible for this characteristic emission. [ABSTRACT FROM AUTHOR]

Abstract: Different emission channels, due to recombination of neutral donor-bound excitons (D0X) in GaN, are discussed from photoluminescence experiments performed in magnetic fields up to 28T for a freestanding sample and a heteropepitaxial layer doped with silicon. For these samples, in addition to the principal recombination channel of D0X, in which donors are left in their ground states, the well-resolved two-electron satellites (TES) involving different donor excitations were observed. A detailed analysis of the magneto-optical data provides important knowledge about the energy structure of the oxygen and silicon donors in wurtzite GaN. The application of high magnetic fields allowed us to tune the intra-donor excitations across the LO-phonon energy. A strong enhancement of TES intensity and the appearance of several anti-crossing processes in the vicinity of the LO-phonon replica of the principal D0X transition have been observed. These effects are discussed in terms of resonant interaction between LO phonons and donor-bound electrons (magneto-polaron effect). [Copyright &y& Elsevier]

Luminescence experiments in magnetic fields up to 28 T on a freestanding gallium nitride (GaN) sample and a heteroepitaxial GaN layer lightly doped with silicon are presented. In these samples, the principal D0X recombination channel is accompanied by two electron satellites (TES), which involve the excited donor states as well as by the longitudinal-optic (LO) phonon replica of the principal transitions. When the internal donor excitations are magnetically tuned into resonance with the LO-phonon excitation, the intensity of TES is strongly enhanced and clear avoided-crossings between the LO-phonon replica of the principal D0X transition and TES involving highly excited states of oxygen and silicon are resolved. The observed behavior is explained in terms of resonant interaction between LO-phonons and donor-bound electrons. It is found that the resonant magnetopolaron interaction is stronger for the oxygen as compared to the silicon donor. [ABSTRACT FROM AUTHOR]

Graphene flow sensors not only hold great prospects for applications but also encounter many difficulties, such as unwanted electrochemical phenomena, low measurable signal, and limited dependence on the flow direction. This study proposes an approach allowing for the detection of a flow direction-dependent electric signal in aqueous solutions of salts, acids, and bases. The key element in the proposed solution is the use of a reference electrode which allows external gating of the graphene structure. Using external gating enables to enhance substantially the amplitude of the flow-generated signal. Simultaneous measurement of the reference electrode current allows us to recover a flow-direction-sensitive component of the flow-induced voltage in graphene. The obtained results are discussed in terms of the Coulomb interaction and other phenomena, which can be present at the interface of graphene with the aqueous solution. [ABSTRACT FROM AUTHOR]

Photoluminescence from bulk HfS2 grown by the chemical vapor transport method is reported. A series of emission lines is apparent at low temperature in the energy range of 1.4–1.5 eV. Two groups of the observed excitonic transitions followed by their replicas involving acoustic and optical phonons are distinguished using classical intensity correlation analysis. The emission is attributed to the recombination of excitons bound to iodine (I2) molecules intercalated between layers of HfS2. The I2 molecules are introduced to the crystal during the growth as halogen transport agents in the growth process. Their presence in the crystal is confirmed by secondary ion mass spectroscopy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The possibility to fabricate van der Waals heterostructures on the wafer-scale would open up a wide range of potential applications. Hexagonal boron nitride (hBN) is one of the most important components in many structures of this kind. Obtaining high quality hBN and the ability to transfer large areas to arbitrary surfaces is a must for future applications of many less durable and less stable van der Waals materials. In this work, we point out key factors that allow us to delaminate large areas of hBN layers grown by metal-organic vapor phase epitaxy on two-inch sapphire substrates. Using scanning electron microscopy and X-ray diffraction, we also compare hBN layers before and after the delamination process. [ABSTRACT FROM AUTHOR]

Raman piezospectroscopy of high quality 6H-SiC crystals is presented. The crystals used in experiments were grown by the seeded physical vapor transport method. Uniaxial stress up to 0.9 MPa, obtained using a spring apparatus, was applied along [11-20] and [10-10] directions. It was found that the application of uniaxial stress led to different energy shifts of the observed phonon excitations in the investigated 6H-SiC crystals. The obtained pressure coefficients vary in the range 0.98-5.5 cm-1 GPa-1 for different transverse optical phonon modes. For longitudinal optic phonon modes pressure coefficients in the range 1.6-3.6 cm-1 GPa-1 were found. The data obtained could be useful in evaluation of local strain fields in SiC based structures and devices including epitaxial graphene. [ABSTRACT FROM AUTHOR]

The principal structural defects in graphene multilayers synthesized on the carbon-terminated face of a 4H-SiC (0001) substrate were investigated using the high-resolution transmission electron microscopy. The analyzed systems include a wide variety of defected structures such as edge dislocations, rotational multilayers, and grain boundaries. It was shown that graphene layers are composed of grains of the size of several nanometres or larger; they differ in a relative rotation by large angles, close to 30°. The structure of graphene multilayers results from the synthesis on a SiC (0001) surface, which proceeds via intensive nucleation of new graphene layers that coalesce under various angles creating an immense orientational disorder. Structural defects are associated with a built-in strain resulting from a lattice mismatch between the SiC substrate and the graphene layers. The density functional theory data show that the high-angular disorder of AB stacked bilayers is not restoring the hexagonal symmetry of the lattice. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC [formula] surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 Å from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC [formula] 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 Å, which is characteristic for graphite, up to 3.7 Å. Graphene grown on 8° off-axis SiC [formula] 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 [formula] substrate. [ABSTRACT FROM AUTHOR]

Graphene/1T-TaS2 hybrid structures were measured using the Raman spectroscopy at room temperature. Spatially resolved Raman maps show that the region for which an effective graphene G band shift was observed correlates well with the tantalum disulfide flake, suggesting a change of the carrier concentration in graphene induced by 1T-TaS2. This effect was observed only for the sample exfoliated in vacuum, which signifies the necessity of preventing tantalum disulfide from oxidation during sample preparation. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance spectroscopy has been applied to study the surface potential barrier in Al xGa1− xN/n-Al xGa1− 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 Al xGa1− 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 Al xGa1− xN(60 nm)/n-Al xGa1− 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 Al xGa1− 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. [ABSTRACT FROM AUTHOR]

Complex charge variation processes in low-density, direct-type GaAIAs quantum dots embedded in a type-fl GaAs/AIAs bilayer are studied by single-photon correlation measurements. Two groups of excitonic transitions are distinguished in the single quantum dot (QD) photoluminescence spectra, namely due to recombination of neutral and charged multiexcitonic complexes. The radiative cascades are found within each group. Three characteristic time scales are identified in the QD emission dynamics. The fastest one (of the order of 1 ns) is related to excitonic radiative recombination. The two remaining ones are related to the QD charge state variation. The one of 100-ns range (typical blinking time scale) corresponds to random capture of single carriers under a quasiresonant excitation. The slowest processes, in the range of seconds, are related to charge fluctuations in the surrounding of the dot. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The growth of low-density GaN quantum dots on Al x Ga1− x N by metalorganic vapor phase epitaxy (MOVPE) using in situ etching of the Al x Ga1− x N surface in the presence of SiH4, is reported. Subsequent growth stages have been verified by atomic force microscopy and micro-photoluminescence (μPL). The low temperature μPL shows sharp emission lines originating from single quantum dots, without any artificial masks or mesa structures. This proves that the proposed growth technique offers unique possibility for detailed optical studies of energetic structure and recombination processes of single GaN/Al x Ga1− x N quantum dots. [Copyright &y& Elsevier]

Boron nitride layers were grown on sapphire substrate by metal organic vapor phase epitaxy system that was originally designed for growth of GaN. Structures were characterized by scanning electron microscopy, atomic force microscopy, the Raman spectroscopy, absorption and time resolved photoluminescence. Presented results confirm successful deposition of BN layers and gives information about basic properties of the material. The Raman line at 1370 cm-1 and absorption edges at 5.6-5.9 eV were observed which is related to hexagonal phase. [ABSTRACT FROM AUTHOR]

We report on attempts to produce a graphene based liquid flow sensor. Our results indicate that modifications of the electric double layer, formed in the vicinity of the graphene surface, dominate over mechanisms responsible for liquid flow-induced voltage/current generation. Several graphene structures were tested in different measurement configurations, aimed to maximize the generated signal amplitude and its stability. Some realizations of working devices in water as well as in aqueous solutions of NaCl or HCl are presented. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Optical absorption and Raman scattering studies of few-layer epitaxial graphene obtained by high temperature annealing of carbon terminated face of 4H-SiC(000-1) on-axis substrates are presented. Changing the pressure and annealing time, different stages of the graphene formation were achieved. Optical absorption measurements enabled us to establish average number of graphene layers covering the SiC substrate. Raman scattering experiments showed that integrated intensity of the characteristic 2D peak positively correlated with the number of graphene layers deposited on the SiC substrate. The spectral width of the 2D peak was found to decrease with the number of the deposited graphene layers. [ABSTRACT FROM AUTHOR]

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). [Copyright &y& Elsevier]

The Raman scattering studies of multi-layer graphene obtained by high temperature annealing of carbon terminated face of 4H-SiC(000-1) substrates are presented. Intensity ratio of the D and G bands was used to estimate the average size of the graphene flakes constituting carbon structures. The obtained estimates were compared with flake sizes from atomic force microscopy data. We found that even the smallest structures observed by atomic force microscopy images are much bigger than the estimates obtained from the Raman scattering data. The obtained results are discussed in terms of different average flake sizes inside and on the surface of the multi-layer graphene structure, as well as different type of defects which would be present in the investigated structures apart from edge defects. [ABSTRACT FROM AUTHOR]