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Measurements of optical activity can be readily performed in transparent matter by means of a rotation of transmitted light polarization. In the case of opaque bulk materials, such measurements cannot be performed, making it difficult to assess possible chiral properties. In this work, we present full angular polarization dependencies of the Raman modes of bulk 1T-TaS

Hydrogen is an important building block in global strategies towards a future green energy system. To make this transition possible, intense scientific efforts are needed, also in the field of materials science. Two-dimensional crystals, such as hexagonal boron nitride (hBN), are very promising in this regard, as it was demonstrated that micrometer-sized exfoliated flakes are excellent barriers to molecular hydrogen. However, it remains an open question whether large-area layers fabricated by industrially relevant methods preserve such promising properties. In this work we show that electron beam-induced splitting of water creates hBN bubbles that effectively store molecular hydrogen for weeks and under extreme mechanical deformation. We demonstrate that epitaxial hBN allows direct visualization and monitoring of the process of hydrogen generation by radiolysis of interfacial water. Our findings show that hBN is not only a potential candidate for hydrogen storage, but also holds promise for the development of unconventional hydrogen production schemes.

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 MOVPE growth of h-BN (more specifically sp2-BN) layers encounters problems concerning growth efficiency, crystallographic quality and a large quantity of optically active point defects of undisclosed nature. Our results show that high quality layers can be grown with continuous or modulated flow methods. Depending on the conditions, the synthesis is limited by insufficient ammonia dissociation, boron diffusion on the surface or probability of chemisorption on atomic steps. Despite the common belief, gas phase parasitic reactions were not observed. It is crucial to control the balance between mass transport, diffusion and desorption from the surface. Supersaturation of TEB leads to a fast nucleation of randomly oriented crystal grains and 3D growth. Epitaxial, two-dimensional (2D) layers can be only obtained with a very low growth rate. The amount of point defects, influencing the optical properties of the layers, depends on the growth conditions and, especially, ammonia supply.

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 electronic structure of $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$ showing a metal-insulator transition and a sequence of different charge density wave (CDW) transformations was discussed in the frame of variable temperature angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. For the commensurate charge density wave phase (CCDW) the Mott gap was estimated to be 0.4 eV and energy gaps ${\mathrm{\ensuremath{\Delta}}}_{\text{CCDW},1},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{\text{CCDW},2},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{B3\ensuremath{-}\mathit{HHB}},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{B4\ensuremath{-}B3}$ were observed. For the nearly commensurate charge density wave phase (NCCDW), the reminiscent of higher and lower Hubbard bands and a very pronounced electronic state associated with the parabolic band at the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point in the Brillouin zone were identified. The incommensurate charge density wave phase (ICCDW) showed a high value of local density of states at the Fermi level and a very pronounced edge of the metallic surface state located in the range of 0.15--0.20 eV above the Fermi level. The obtained STS and ARPES results were consistent with our theoretical calculations performed within DFT formalism including spin-orbit coupling.

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.

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.

1T-TaS2 is a material that belongs to the family of transition metal dichalcogenides and features a rich phase diagram including different charge density wave states. So far, most techniques used to study this material focus on either very local properties on the atomic scale or use global measurements of the whole sample like the electrical resistance. Here, we report on the influence of micrometer scale laser illumination on the global electric properties of a bulk sample. Thermoelectric effects are found to be responsible for a large electric response to the laser illumination. This effect is used to develop a method that allows to spatially map the thermoelectric properties of a 1T-TaS2 sample on the millimeter scale with micrometer resolution. We show that a specific temperature sweep can impede the phase transition, allowing to study long-lived mixed states, consisting of two phases. With this method we are able to image metastable states occurring in the transition between the nearly commensurate and commensurate charge density wave state in bulk 1T-TaS2 and compare the results to Raman spectroscopy.

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.

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 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.

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.

Time resolved photoluminescence of double quantum well structure was investigated versus electric and magnetic fields applied across the sample. The emission due to direct excitons (electron and hole are localized within the same quantum well) decays fast at the nanosecond timescale, whereas the recombination kinetics of indirect excitons is much slower and spreads over microseconds. The time evolution of indirect exciton emission is shown to be altered by application of either electric or magnetic field. This reflects the non-trivial effects of exciton localization which leads to the nonexponential decays of the indirect exciton emission.

Microphotoluminescence of low-density GaN/AlxGa1−xN quantum dots grown by metal-organic vapor phase epitaxy using in situ etching of AlGaN is presented. The detailed analysis of the emission from these structures enables the observation of pairs of lines separated by the energy up to 3 meV. They behave in a different way under different excitation power that suggests that this doublet structure can be associated with the exciton and trion (or biexciton recombination). It is observed that for different quantum dots the energy of the charged exciton complex emission could be higher or lower than the neutral exciton one. It is discussed in terms of a competition between attractive e–h and repulsive e–e (h–h) Coulomb interaction that occurs because of the existence of the built-in electric field that separates electrons and holes in the dot.

Polarization encoded single photons and photon pairs are expected to play a major role in future quantum communication schemes [1]. Therefore, control of polarization properties of single quantum dot emission is of a great value. We study magnetic field induced transition between anisotropy controlled and Zeeman controlled emission from individual GaAs/AlAs quantum dot (QD). We demonstrate the utility of these studies, involving polarized photon correlation measurements, for determination of the anisotropic exchange splitting of excitonic states in quantum dots. The sample was mounted directly on a microscope objective [2] and placed inside a pumped helium cryostat. Continuous wave excitation was performed with use of tunable Ti:Al2O3 laser at wavelength of 718 nm assuring excitation below the energy gap of the barrier material. The typical power of excitation beam was 100μW over the spot of a diameter of about 1μm. Temperature was kept constant at 1.8K. Photoluminescence was excited and collected through the same microscope objective. Correlation measurements were performed in a Hanbury-Brown and Twiss setup. The signal after polarization and spectral filtering was detected using two avalanche photodiodes. Polarization resolved crosscorrelations between biexciton (XX) and exciton (X) photons emitted subsequently in a radiative cascade were measured in magnetic field ranging from 0 to 0.5 T, applied in Faraday configuration. Polarization correlation of photon pairs in parallel linear polarizations appeared as a bunching peak in histograms of correlated counts in the absence of magnetic field. No polarization correlation in circular basis was observed at B = 0T. This is expected for QDs exhibiting in-plane anisotropy, since intermediate X state of the cascade is split by energy of electron-hole exchange interaction into two components emitting in orthogonal linear polarizations. Application of magnetic field transforms Anisotropic Exchange Splitting (AES) into Zeeman splitting of the X level, reducing thus the influence of the anisotropy. In a sufficiently high magnetic field pure exciton eigenstates of angular momentum M = +/1 are observed [3]. Expected conversion of linearly polarized excitonic states to circularly polarized ones is demonstrated by increase of the degree of circular polarization correlation with increasing magnetic field. We apply a simple model proposed by Besombes et al. [4] to describe the progressive change of the polarization correlation with increasing field. The model allows calculating of the degree of polarization correlation as a function of Zeeman splitting to AES ratio. By fitting the model to the experimental data, we were able to determine the anisotropic exchange splitting AES value of order of tens of μeV. This method of the AES determination can be used even in cases when direct measurement by polarization resolved microphotoluminescence is not sensitive enough.

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.

The growth of low-density GaN quantum dots on Al x Ga 1− x N by metalorganic vapor phase epitaxy (MOVPE) using in situ etching of the Al x Ga 1− x N surface in the presence of SiH 4 , is reported. Subsequent growth stages have been verified by atomic force microscopy and micro-photoluminescence ( μ PL). The low temperature μ PL 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 Ga 1− x N quantum dots.

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.

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)

Magneto-photoluminescence (PL) studies up to 28T of the GaN/AlGaN structure are presented. PL spectra in the energy range below the excitonic bulk transitions show periodic energy and line width oscillations what is interpreted as a fingerprint of two dimensional electron gas confined at GaN/AlGaN interface. Observed oscillations are a direct consequence of Landau quantization and known as optical Shubnikov de Haas Effect if related to the Fermi level oscillations. This observation allows us to determine the electron concentration to 3 · 1012cm-2. Appearance of the oscillations is discussed in terms of magnetic field induced potential redistribution in the structure and theory including many-body effects. Surprisingly, it was observed that magneto-PL intensity oscillates also in other energy regions of the spectra. It was found that this emission originates from the second electric energy level and the oscillations are a direct consequence of the interaction with the first electronic subband Landau levels. (© 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.

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.

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.

Magnetic field studies of circularly polarised emission due to free and bound excitons in high quality homoepitaxial GaN layers are presented. The conventional analysis of the band structure of the wurtzite-type symmetry allowed us to determine both the signs and the values of the effective Lande g-factors for electrons and holes participating in the recombination of free excitons originating from Γ9 and Γ7 valence band components as well as for electrons and holes weakly bound to donor and acceptor centres. The magnetic field splitting scheme of the conduction and the valence band is examined. The fine structure of the excitonic emission is also discussed.

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 paper we report results of magnetooptical measurements done on standard InAs MOCVD layers grown on GaAs. Extremely narrow lines (half-widths of the order of 20 mT) — narrower than found by other authors in high quality MBE InAs epilayers on GaAs — as well as the lines of typical half-widths have been found both in the photoconductivity spectra and in the transmission spectra. A detailed comparison with the theoretical dependence of shallow donor and Landau level energies on magnetic field leads to the conclusion that they originate from cyclotron resonance and impurity-shifted cyclotron resonance transitions in that material. PACS numbers: 72.40.+w, 71.55.-i, 78.30.-m

The photoluminescence of homoepitaxial and heteroepitaxial GaN layers is reported. It is shown that the coupling between LO phonons and neutral acceptor bound excitons is much stronger than the coupling between LO phonons and neutral donor bound excitons. In undoped homoepitaxial layer, in spite of that the no-phonon emission due to donor bound excitons is one order of magnitude stronger than the acceptor bound excitons emission, the predominant structure in the LO phonon replica of the excitonic spectrum is related to optical transitions involving acceptor bound excitons. Temperature studies showed that at higher temperature the LO phonon replica is related to free excitons.

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.

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).

Complex charge variation processes in low-density, direct-type GaAlAs quantum dots embedded in a type-II GaAs/AlAs 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.

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