We demonstrate that Franz-Keldysh oscillations (FKOs) observed by photoreflectance (PR) spectroscopy are highly sensitive to the surface morphology of AlxGa1−xN layers in AlxGa1−x N heterostructures. Three Al0.2Ga0.8N/GaN heterostructures with different surface-morphology profiles, which are confirmed with atomic force microscopy, have been investigated. The X-ray-diffraction patterns are hardly affected by the Al0.2Ga0.8N/GaN-layer morphology. In contrast, it is revealed that cracks and pits dominating the morphology remarkably reduce the amplitude of the FKOs from the Al0.2Ga0.8N/GaN layer, which is attributed to the following two mechanisms related to the cracks and pits. One is lifetime broadening due to carrier scattering, and the other is the suppression of the modulation magnitude for the built-in electric field, which is caused by the trapping and recombination of photogenerated carriers at the surface.

Thin films of CoPc of various thickness have been deposited onto glass substrates using thermal evaporation technique at room temperature. The dark electrical resistivity measurements were carried out at different temperature range (298–423 K). An estimation of mean free path ($\ell_{0}$) of charge carriers in CoPc thin films and bulk resistivity, $\rho_{B}$ was attempted. Measurements of thermoelectric power confirm that CoPc thin films behave as p-type semiconductors. The ac conductivity (σac) has been investigated in the frequency range (102–106 Hz) and temperature range (298–407 K). σac is found to be proportional to ωs where s ≈ 0.879 which is frequency and temperature independence. The ac conductivity interpreted by the correlated barrier hopping (CBH) model with centers of intimate valence alternation pairs type with a maximum barrier height, WM ≈ 1.594 eV.

Poly(2,5-thienylene vinylene), PTV and its soluble alkyl derivatives were synthesised using the dithiocarbamate precursor route (DTC-route) in a good yield, with high molecular weight (Mw), low polydispersity (PD), and with a high maximum absorption peak, $\lambda_{max}$. The polymers were characterised by absorption spectroscopy, cyclic voltammetry and, X-ray diffraction.

Temperature and frequency dependence of a.c. conductivity have been studied in glassy Se70Te30−xSbx (2 ≤ x ≤ 10). An agreement between experimental and theoretical results suggests that the a.c. conductivity behaviour of Se-Te-Sb system can be successfully explained by correlated barrier hopping (CBH) model. The observation of Meyer-Neldel rule in case of a.c. conductivity at high temperatures is also reported for glassy Se70Te30−xSbx (2 ≤ x ≤ 10) alloys.

The optical study of transmission T(λ) and reflection R(λ) spectra has enabled us to determine compounds that possess gaps close to 1.5 eV. The conditions for preparation of a ternary compound whose gap is 1.48 eV have been used to create a Pb1−xCdxS-n/Si-p heterojunction. The heterojunction thus obtained presented a low yield (0.5%), as determined from the study of the J–V characteristic. In order to improve its efficiency, we have studied the effect of thermal treatment on the prepared heterojunction. X-ray diffraction has been used to determine the variation in the crystalline parameter and the ratio of cadmium in the Pb1−xCdxS compound, and the dependence of its gap on the cadmium concentration. We have also used the C–V technique to determine the average concentration of impurities in the Pb1−xCdxS compound.

It is known that the structure dependence of the lifetimes of quasi-bound states in rectangular multi-barrier structures is significant but still ambiguous. In this paper, we report the elaborate dependence of the lifetimes of higher quasi-bound states in symmetrical double-barrier structures on structure parameters, such as well width, barrier width and composition mole fraction. Based on the energy uncertainty condition at quasi-bound states method, numerical calculations are carried out to investigate the structure dependence of the lifetimes of higher quasi-bound states. The calculation results show that the lifetimes of higher quasi-bound states vary with structure parameters as a simple and exact function, and that the lifetimes of higher quasi-bound states increase exponentially with structure parameters.

This paper presents a commercial high voltage thyristor used as a switch allowing a tank capacitor to discharge in a load. In classical high power pulse technology applications the output voltage pulse has to be characterized mainly by its crest value, its rise-time, the period the thyristor is held in the on-state and the fall-time. These parameters are studied as a function of the power circuit and of the trigger circuit. The thyristor presents two behaviours: the main current is either higher or lower than the latching current. The “low current” behaviour is extensively investigated as it allows repetitive operation of the device. Two pulse power applications triggering electrical discharges are presented. Each one necessitates a specific pulsed power supply using series thyristor stacks or Marx structures. The first pulsed source delivers negative pulses with a crest voltage $V_{oM}=-35$ kV, a turn on capability of $T_{r}=90$ ns and a repetition rate F = 900 Hz. The second is built using Marx structure and is characterized by $V_{oM}=60$ kV, $T_{r}=250$ ns, F = 900 Hz.

We report continuous wave and time resolved photoluminescence studies of self-assembled InP quantum dots grown by metalorganic chemical vapor deposition. The quantum dots are embedded into indirect band-gap In0.5Al0.5P layers or In0.5Al0.3Ga0.2P layers with a conduction band line-up close to the direct-to-indirect crossover. As revealed by photoluminescence spectra, efficient interdiffusion of species from the barrier layers produces (Al,In)P or (Al,Ga,In)P-dots. This interdiffusion creates potential barriers that are repulsive for electrons of X valleys around the QDs. Both samples show a fast exponential decay component with a time constant between 0.5 and 0.7 ns. In addition, the sample with indirect band gap matrix shows a slow non-exponential time-decay, which is still visible after more than 100 µs. The fast component is attributed to direct recombination of electron-hole pairs in the dots whilst the slow component, which follows a power law t−0.75 results from recombination of holes in the dots and electrons in metastable states around the dots.

We investigated mobility in the regioregular poly(3-octylthiophene) (RR-P3OT) and regioregular poly(3-hexylthiophene) (RR-P3HT) by the Charge Extraction by Linearly Increasing Voltage method (CELIV). The samples were produced by the drop-casting and spin-coating methods on the Indium-Tin-Oxide substrate and provided with Al contacts. In both materials we had observed the “negative” mobility dependence at low electric fields, i.e., the mobility used to decrease with increasing electric field (in P3HT, this was only observed here in the drop-casted sample). Such behaviour was never observed by CELIV in P3OT before. Data were analysed within the Poole-Frenkel and Gaussian disorder models. The higher energetic disorder parameters were obtained in P3OT as compared to P3HT. This could be related to the longer side-chains of P3OT. We had demonstrated that in both materials with increasing temperature conductivity grows due to the thermally activated mobility.

Electrical and photoelectrical measurements are made at different temperatures on heterojunction photovoltaic cells fabricated by vacuum deposition of n-(Ga2Se3-Ga2S3) thin films onto p-Si single crystals. A complete study of the current density as a function of voltage and temperature is carried out in order to gain fundamental information on trap depth, trap distribution and position of the Fermi level. At high forward bias, the results are consistent with space-charge-limited conduction due to an exponentially decreasing distribution of traps. At low voltages, the dark current density in the forward direction varies exponentially with voltage. The devices exhibit strong photovoltaic characteristics with an open-circuit voltage of 0.70 V, a short-circuit current density of 9.42 mA cm−2 and a power conversion efficiency of 5.20%. These parameters have been estimated at room temperature and under light illumination provided by a halogen lamp with an input power density of 50 mW cm−2.

A slab two-dimensional photonic crystal in diamond is being considered for quantum information processing. In view of the low refractive index of diamond, the question whether a high Q cavity is achievable has to be addressed. Here, various geometrical designs of single point defects and double heterostructures for the formation of high Q nanocavities in diamond are modeled. The highest value of vertical quality factor $Q_{v}\sim 70\,000$ was achieved in double heterostructures. This result seems to be significant for further consideration of photonic crystals in diamond for quantum electrodynamics applications. 


We have investigated properties of poly(9-vinylcarbazole) (PVK) doped with 30% wt 4-dibutylamino-4'-nitrostilbene (DBANS), depending on the orientation of the polar DBANS molecules. Appearance of the orientation-induced built-in electrical field was proven optically by the Solid Electric Field Induced Second Harmonic Generation and electrically by Current-Voltage characterization. Modification of optical properties was evidenced by the spectral dependencies of absorption coefficient. The Thermally Stimulated Currents spectra demonstrated that carrier transport and trapping are affected, too.

We have demonstrated debundling of molydenym-sulphur-iodine nanowires simply by diluting nanowire dispersions in isopropanol. Using atomic-force-microscopy we observe the bundle diameter distribution to decrease dramatically with concentration. Detailed analysis of the data suggests the presence of an equilibrium bundle number density. The population of individual nanowires increases with decreasing concentration until almost half of all dispersed objects are individual nanowires at a concentration of 4 × 10−3 mg/ml. The partial concentration of individual nanowires peaks at a concentration of ~7 × 10−3 mg/ml. This debundling also occurs spontaneously without the input of sonic energy, suggesting thermodynamic solubility. The absorbance of the nanowire dispersions, measured in the visible region increases linearly with concentration indicating a concentration independent absorption coefficient. However, for the infra-red feature that has been associated with band edge transitions, the absorption coefficient increases with increasing concentration for both stoichiometries. This suggests that this transition may be quenched by the inter-nanowire interactions associated with bundling. Finally, nanowire re-aggregation can be induced by the addition of small quantities of non-solvents.

Thermal evaporation technique was used to prepare As20Se80−xTlx films from bulk materials; ($5\leqslant x\leqslant 35$ at.%). The effect of TI addition on the thermoelectric properties and mobility activation energy of As-Se-Tl chalcogenide semiconductors has been studied in the homogeneous glass-forming region through temperature range (300–380 K). The thermoelectric power, TEP had a positive sign over the whole temperature range investigated, indicating p-type conductivity for As20Se80−xTlx films. TEP activation energy, $\Delta E_{s}$ could be calculated from TEP measurements. It was found that $\Delta E_{s}$ decreases with increasing Tl-content and is found to vary between 0.814 and 0.517 eV. The mobility activation energy, $\Delta E_{Q}$ could be calculated.

A theoretical model is presented for the study of the scattering and the localisation of spin-waves at an extended inhomogeneous structural and magnetic boundary separating ultrathin Heisenberg ferromagnetic films. The model system consists of two different magnetic materials A and B with different thickness of two and three atomic layers, on either side of a defect atomic step. The matching technique is used with nearest neighbour magnetic exchange to analyse both the localisation and the scattering spin dynamics. The localised spin states that manifest themselves as Rayleigh branches, and the local densities of spin states are calculated on this boundary. The coherent reflection and transmission scattering properties of spin-waves incident from the interior of the ultrathin films on the inhomogeneous boundary are also calculated. The numerical calculations are applied in particular to a system of three Fe ferromagnetic atomic layers and two Gd ferromagnetic atomic layers, across a defect atomic step. The results illustrate the occurrence of Fano resonances in the transmitted spectra due to the localised spin states on the inhomogeneous boundary. An interesting physical effect is observed for this magnetic and atomic step boundary, namely the frequency selective conductance of the spin-waves via Fano resonances, by an appropriate choice of the angle of spin-wave incidence on the boundary.

High resolution spectroscopy of quantum systems is a key point in the quantum information field. Rare earth ions in crystals are interesting candidates for this application because of the long coherence lifetime of some of their transitions. These ions can also be manipulated by optical excitation which has, however, to be produced by ultra-stable laser sources. This proves to be a very difficult task in the case of dye lasers which are necessary to excite Pr3+ doped crystals and especially Pr3+:Y2SiO5. This compound is by far the most used host in rare earth based quantum information studies. In this paper, we discuss the use of Pr3+,Yb3+ codoped materials to build an infrared pumped solid state laser suitable to excite Pr3+:Y2SiO5 around 606 nm. We show by the analysis of a rate equation model that avalanche upconversion is not very efficient to obtain a high power laser. This is more easily obtained if a second laser is set to pump resonantly Yb3+ ions. The spectroscopic properties of a new matrix Pr3+,Yb3+:PbF2 are also investigated. We found that this compound emits at 606.18 nm with a width of 5 nm and would be therefore suitable to excite Pr3+:Y2SiO5. Moreover, it can be excited at 857 nm and 975 nm, i.e. in the range of high power laser diodes.

The optical absorption in organic solar cells, and hence their efficiency to convert sunlight into electricity, is limited by both a confined spectral absorption range and the very thin active layers which are required since polymers have short charge carrier diffusion lengths. We propose to make the most of this apparent 'drawback' by using transparent contacts on both sides of the diode, leading to brilliant red-coloured semitransparent solar cells. Such cells are interesting for new, aesthetically appealing applications where coloured glasses or foils with the additional benefit of providing electric power are desired. Maybe even more importantly – such semitransparent solar cells are essential for mechanically stacked tandem arrays where two subcells are combined. We investigated different cathodes based on Al-doped ZnO and thin LiF and Al layers. Apparently, the different cathode layer preparation conditions strongly influence the cell performance. Our standard LiF/Al cells do not usually require annealing for improved efficiencies, in contrast to cells with sputtered cathodes. For the latter, a thermal post-treatment can greatly enhance the performance, depending on the sputtering process. We could demonstrate power conversion efficiencies of up to 3.4% for single cells and up to 4.2% for stacked tandem arrays.

An analytical expression of the free energy consisting of the strain energy, surface energy and interfacial energy for the coherent island/substrate system, as well as the evolving relations of aspect ratio against volume of the island and misfit of the system, which provides a broad perspective on island behavior, is obtained, and used to study the equilibrium shapes of the systems. Under certain growth conditions in systems with a film/substrate lattice misfit, deposed material is known to aggregate into islandlike shapes with geometries having triangular shaped cross-sections. A two-dimensional model assuming liner elastic behavior is used to analyze an isolated triangular shaped island with elastic properties similar to those of the substrate assumed to be semi-infinite. The results show that in order to minimize the total free energy, a coherent island will adopt a particular height-to-width aspect ratio that is a function of only the island volume. The effect of a misfit dislocation on the equilibrium shape of an island is in passing examined. These can serve as a basis for interpretation of experiments.

Thin lead sulfide films were grown on single crystal GaAs(100) substrates by chemical deposition using Pb(NO3)2 and CS(NH2)2 with excess of NaOH in aqueous solution at a range of deposition temperatures 0–50 °C. The microstructure and morphology evolution were studied as a function of the deposition conditions, resulting in a wide range of microstructures. Ultrahigh resolution scanning electron microscopy and atomic force microscopy indicated a systematic change in particle shape and surface morphology as a function of deposition temperature and deposition time. X-ray diffraction of 200–500 nm thick films indicated a dominant $\langle 110\rangle$ texture throughout the deposition temperature range. At deposition temperatures above 40 °C, single crystal films were obtained. Cross-sectional transmission electron microscopy analyses showed a unique (011)PbS||(100)GaAs and [100]PbS||[011]GaAs orientation relationship.

In order to fabricate integrated imaging and particle sensors a hydrogenated amorphous silicon (a-Si:H) p+-i-n+ photodiode technology has been developed, optimized and modelled. The realisation of a one-dimensional model of this by Chatterjee has given us a precious modelling reference. In this paper we propose a bi-dimensional model realized in finite elements approach with Atlas-Silvaco Software of particle sensor achieved by a 4 µm p+-i-n+ amorphous silicone photodiode. We will validate it by comparison with the above-mentioned one-dimensional model. The interest of the second dimension lies not only in the calibration of different structures but also in the influence between neighbouring pixels of the sensor and how this will affect their sensibility. Thus a three pixel bi-dimensional structure has been realized. The leakage and crosstalk currents between two neighbouring pixels have been studied and characterized using a uniform AM1.5 light source.