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An evaporated CdSe layer was incorporated with a very thin CdS layer (40 nm) to optimize the performance of CdTe solar cells. Se alloys with narrower bandgaps were formed in the CdTe deposition and annealing process. With the incorporation of the CdSe layer at the optimal thickness, the highest short-circuit current density was obtained because of the simultaneous high carrier collection in the short-wavelength region and extended carrier collection in the long-wavelength region. Although narrow bandgap Se alloys were introduced, the open-circuit voltage was also significantly increased. As suggested by dark current–voltage measurements, the current leakage paths induced by the thin CdS layer can be significantly suppressed by the incorporation of the CdSe layer. The redshift of PL emission confirms the formation of the Se alloys. As revealed by transient photo-capacitance measurement, the Se alloys are less defective compared with that of CdTe, which is also in accordance with longer carrier lifetimes extracted from photoluminescence decays. The formation of Se alloys prior to the CdTe layer is favorable for the collection of photogenerated carriers. We attribute the performance enhancement to the formation of Se alloys, which are helpful to suppress the leakage current and enhance carrier collection. The incorporation of CdSe with thin CdS layers is validated to improve the diode quality and promising for further device performance improvement.

Silicon (Si) films were deposited on low-cost graphite substrates by the electrochemical reduction of silicon dioxide nanoparticles (nano-SiO2) in calcium chloride (CaCl2), melted at 855 °C. Cyclic voltammetry (CV) was used to analyze the electrochemical reduction mechanism of SiO2 to form Si deposits on the graphite substrate. X-ray diffraction (XRD) along with Raman and photoluminescence (PL) results show that the crystallinity of the electrodeposited Si-films was improved with an increase of the applied reduction potential during the electrochemical process. Scanning electron microscopy (SEM) reveals that the size, shape, and morphology of the Si-layers can be controlled from Si nanowires to the microcrystalline Si particles by controlling the reduction potentials. In addition, the morphology of the obtained Si-layers seems to be correlated with both the substrate materials and particle size of the feed materials. Thus, the difference in the electron transfer rate at substrate/nano-SiO2 interface due to different applied reduction potentials along with the dissolution rate of SiO2 particles during the electrochemical reduction process were found to be crucial in determining the microstructural properties of the Si-films.

This study presents a novel chemical process for the extraction of high purity silica from diatomite. The process involves leaching of raw diatomite with acid prior to alkali extraction, solubilization of treated diatomite with alkali solution to form sodium silicate solution, and precipitation of silica gel by adding sulfuric acid. Besides, the process includes washing of silica gel with hot solution of 6 M hydrochloric acid for three hours. The morphology and the chemical composition of the raw diatomite, the derivative products, and the final purified silica were determined by scanning electron microscopy, X-ray fluorescence, X-ray diffraction, and inductively coupled-plasma optical emission spectroscopy. The results indicate that raw diatomite contains over 80 wt% of amorphous micro-porous silica (SiO2) with particle size, 10–35 μm. We were able to enrich the content of silica in diatomite to 90.2 wt% by step-wise acid leaching. Moreover, in this study, boron (B) was removed from silica slurry using mannitol as a complexing compound. The elimination ratio of boron based on this method was about 70%. Incorporation of mannitol in the final stage of the acid washing of silica gel led to a silica product with lower B content. The final purified silica is 99.99 wt% (four nines) pure.

For copper indium gallium selenide [Cu(In1−x,Gax)Se2, CIGS]-based solar cells, defect states or impurity phase always form due to both the multinary compositions of CIGS film and the difficulty of controlling the growth process, especially for high Ga concentration. To further improve device performance, it is important to understand such formation of impurity phase or defect states during fabrication. In the work presented herein, the formation mechanism of impurity phase Cu2−δSe and its depth profile in CIGS film with high Ga content, in particular CuGaSe2 (i.e., CGS), were investigated by applying different growth conditions (i.e., normal three-stage process and two-cycle three-stage process). The results suggest that impurity phase Cu2−δSe is distributed nonuniformly in the film because of lack of Ga diffusion. The formed Cu2−δSe can be removed by etching the as-deposited CGS film with bromine-methanol solution, resulting in improved device performance.

International audience; Atomic layer etching (ALE) of Ga-polar GaN (0001) using a standard inductively coupled plasma-reactive ion etching system is achieved in this work. The sequential process is using Cl2 to modify the surface in the adsorption step. For the activation step, the authors compare two rare gas plasmas, namely, Ar and Kr, and show a much larger and well-defined ALE window for the latter. The ALE of GaN is demonstrated by etching mesa structures masked with a photoresist. A constant etching rate per cycle of two monolayers is obtained. The experimental conditions of this self-limited process are found by changing both the adsorption and activation times, together with the source power. This provides an atomic-scale process for nanofabrication, with significant improvements to the GaN surface.

Hydrogen isotope tracer experiment is taken on the a-C:H + D films prepared by the reactive RF magnetron sputtering including CH 4 and D 2 . The nine different samples were prepared with changing the partial pressure of D 2 in the process gas. The concentrations of H and D relative to C in the films were determined by ERDA and RBS. It was found that the total amount of H + D concentration is constant at ~ 22.7%. This means that hydrogen makes a chemical bond with one of four hands of C, and little amount of H atoms exist in the interstitial site. The D concentration in the films linearly increases with increasing the relative partial pressure P D 2 /( P D 2 + P CH 4 ). This implies that the activated CH 4 and D 2 molecules stick to the growing films and release the H and D atoms. XPS results show that all the prepared samples have the same sp 3 -rich structure.

Structural and electrical properties of polycrystalline CuGaSe2 thin films have been studied by changing the Ga/Cu ratio in the films. CuGaSe2 thin films with various Ga/Cu ratio were grown over Mo-coated soda-lime glass substrates. With the increase of Ga content in CuGaSe2, morphology of the films was found to deteriorate which is associated with the smaller grain size and the appearance of impurity phases presumably due to the phase transition from the chalcopyrite structure to the defect-related phase on the surface of the films. Properties of the Ga poor films were affected by the Cu rich secondary phases. Electrical properties of the films were strongly influenced by the structural properties and degraded with increasing the Ga/Cu ratio in the film. Device performances, fabricated with the corresponding CuGaSe2 films, were found to be correlated with the Ga/Cu ratio in the films and consistent with the observed structural and electrical properties.

A single crystalline Eu-doped GaN was grown by gas-source molecular beam epitaxy and photoluminescence (PL) properties were studied. The PL spectra show red-emission at 622 nm originating from intra 4f–4f transition of Eu3+ ion without band-edge emission of GaN. The peak shift of the red-emission with the temperature variation from 77 K to room temperature is less than 1.6 meV, and thermal quenching of the luminescence was found to be small compared with the band-to-band transition. Fourier transform infrared spectra showed an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the intensity was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu3+ ion in GaN and the optical process for the luminescence at 622 nm was discussed with the relation to the defect.

(a) Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai,Tsukuba 305-8573, Japan(b) Photon Factory, Institute of Materials Structure Science,High Energy Accelerator Research Organization, Oho, Tsukuba 305-0801, Japan(Received July 22, 2002; accepted October 1, 2002)PACS: 61.10.Ht; 68.55.Ln; 71.20.Eh; 78.55.CrSingle crystalline Er-doped GaN was grown by ammonia source molecular beam epitaxy, and lumi-nescence properties, structural properties were studied. It was found that the intensity of the lumi-nescence in green spectral region from Er-doped GaN is two orders of magnitude stronger thanthat of Tb-doped GaN, which shows luminescence in green spectral region as well, and that thecoordination symmetry of N around Er in GaN was lower than that of Tb. The lower coordinationsymmetry may be one of the reasons for the improved luminescence of Er-doped GaN.

Properties of deep-level defects in CuGaSe2 thin-film solar cells were investigated using transient photo-capacitance (TPC) spectroscopy. Two Gaussian-shaped deep-level defects centered at around 0.8 eV and 1.54 eV above the valence band were identified. The electronic structure of the two defects was illustrated by a configuration coordinate model to explain the thermal quenching effect in the two defects, which considered a large lattice distortion for the 0.8 eV defect while no distortion for the 1.54 eV defect.

application/pdf, The effects of group-IV element dopants on the structural and electrical properties of Cu2O thin films were studied. Similar dopant-induced behavior was found in the observed variations of the growth orientation and electrical properties of Si- and Ge-doped Cu2O thin films. Ge doping was found to induce electrically active acceptors with an activation energy of 0.18 eV, comparable to the 0.19 eV value of Si-doped Cu2O. These results suggest that locally formed silicate and germanate have the same effect on the structural and electrical properties of Cu2O. On the other hand, Sn and Pb likely act as donors when incorporated substitutionally onto Cu-lattice sites, although further study may be required to suppress self-compensation effects in Cu2O to achieve n-type conductivity.

application/pdf, The dependence of Europium (Eu)-related luminescence intensity on the Eu concentration in Eu-doped GaN was studied. This luminescence is observed at 622 nm and originates from the intra-4f transition of the Eu3+ ion. The intensity of the luminescence increased with increasing Eu concentration, up to about 2 at. %, and then abruptly decreased. It was found that polycrystalline growth began to be induced at Eu concentrations of more than 2 at. %. In addition, clear evidence for the formation of EuN compounds was obtained by x-ray diffraction and extended x-ray absorption fine structure analysis. The cause of the concentration quenching is likely to be related to the polycrystalline growth as well as EuN formation.

The photocarrier dynamics of CuIn1−xGaxSe2 (CIGS) thin films were studied using white-light transient absorption (TA) measurements, as an understanding of this behavior is essential for improving the performance of solar cells composed of CIGS thin films. A characteristic double-peak structure due to the splitting of the valance bands in the CIGS was observed in the TA spectra under near-band-gap resonant excitation. From a comparison of the TA decay dynamics monitored at these two peaks, it was found that the slow-decay components of the electron and hole relaxation are on the nanosecond timescale. This finding is clear evidence of the long lifetimes of free photocarriers in polycrystalline CIGS thin films.

This volume of the Journal of Physics: Conference Series contains papers presented at the Tunisia-Japan Symposium: R&D of Energy and Material Sciences for Sustainable Society (TJS 2014) held at Gammarth, Republic of Tunisia on November 28-30, 2014. The TJS 2014 is based on the network of the Tunisia-Japan Symposium on Science, Society and Technology (TJASSST) which has been regularly organized since 2000. The symposium was focused on the technological developments of energy and materials for the realization of sustainable society. To generate technological breakthrough and innovation, it seems to be effective to discuss with various fields of researchers such as solid-state physicists, chemists, surface scientists, process engineers and so on. In this symposium, there were as many as 109 attendees from a wide variety of research fields. The technical session consisted of 106 contributed presentations including 3 plenary talks and 7 key-note talks. We hope the Conference Series and publications like this volume will contribute to the progress in research and development in the field of energy and material sciences for sustainable society and in its turn contribute to the creation of cultural life and peaceful society.

Admittance spectroscopy has been performed to investigate the effect of antimony (Sb) on GaInNAs material in relation to the deep-level defects in this material. Two electron traps, E1 and E2 at an energy level 0.12 and 0.41 eV below the conduction band (E C), respectively, were found in undoped GaInNAs. Bias-voltage dependent admittance confirmed that E1 is an interface-type defect being spatially localized at the GaInNAs/GaAs interface, while E2 is a bulk-type defect located around mid-gap of GaInNAs layer. Introduction of Sb improved the material quality which was evident from the reduction of both the interface and bulk-type defects.

To clarify the role of buffer layer in organic solar cells (OSCs), the electronic properties of bathocuproine (BCP)/Mg interface were systematically investigated by using ultraviolet photoemissions spectroscopy, high-resolution X-ray photoemission spectroscopy, angle-resolved X-ray photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results show there are gap states at the interface, which are caused by the interaction between BCP and Mg. The formation of Mg-N bond was found at the interface. The NEXAFS measurements show that BCP molecules for 1-2 monolayers are lying-down on the substrate, whereas orient randomly for thick BCP layer. It was supposed that the gap states and the highly-ordered orientation of thin BCP layer are the reasons for improving the performance of OSC with BCP buffer layer and low work function metal cathode.

A facile synthesis method was developed to produce hexagonal-phase of NaYF4 nanocrystals (NCs) doped with Er3+ in different concentration, which showed upconversion (UC) emission from infrared to visible spectral region. This proposed method is simple and less toxic compared with generally used method so far. It was found that up-conversion emission spectra of NaYF4:Er3+ NCs, excited at 1550 nm, included four peaks at about 980 nm, 800 nm, 660 nm and 540 nm. The effect of Er3+ concentration on UC in β-phase NaYF4:Er3+ NCs were discussed based on the excitation power dependence. The optimum Er3+ concentration for 2-step and 3-step UC was found to be around 10~30%.

The electronic properties of fullerene (C60)/bathocuproine (BCP)/Ag heterostructures were studied as a function of the BCP layer thickness by photoemission spectroscopy. For the thin BCP layer, the energy levels are flat and gap states exist at the interface. In contrast, energy band bending occurs at the C60/BCP interface when the BCP layer is thick, resulting in a considerable barrier for electron transport and therefore causing charge accumulation in organic solar cells. The results reveal that a thin BCP layer gives a much more favorable energy level structure and conform that charge accumulation is responsible to the anomalous current-voltage (I-V) curve.

The Cu-dependent phase transition in polycrystalline CuGaSe2 thin films has been studied by an electron probe micro-analyzer (EPMA) and the synchrotron x-ray diffraction method. A Cu-deficiency parameter, Z, defined as (1 − Cu/Ga) was used to study the phase transition. Upon increasing the Z-value, the composition of the films on the Cu2Se-Ga2Se3 pseudo binary tie line was found to shift from the stoichiometric CuGaSe2 (1:1:2) (Z = 0) to the Ga-rich composition through the formation of several ordered defect compounds.The structural modification in the Cu-poor CuGaSe2 film has been investigated by the synchrotron x-ray diffraction method. The existence of the Cu-poor surface phase over the near-stoichiometric bulk CuGaSe2 film was confirmed by the fitting of the accelerated voltage dependent EPMA data.The Cu-dependent phase transition in polycrystalline CuGaSe2 thin films has been studied by an electron probe micro-analyzer (EPMA) and the synchrotron x-ray diffraction method. A Cu-deficiency parameter, Z, defined as (1 − Cu/Ga) was used to study the phase transition. Upon increasing the Z-value, the composition of the films on the Cu2Se-Ga2Se3 pseudo binary tie line was found to shift from the stoichiometric CuGaSe2 (1:1:2) (Z = 0) to the Ga-rich composition through the formation of several ordered defect compounds.The structural modification in the Cu-poor CuGaSe2 film has been investigated by the synchrotron x-ray diffraction method. The existence of the Cu-poor surface phase over the near-stoichiometric bulk CuGaSe2 film was confirmed by the fitting of the accelerated voltage dependent EPMA data.

Erbium-doped GaN with different doping concentrations were grown by ammonia-source molecular beam epitaxy. The intra-4f-shell transitions related green luminescence were observed by both photoluminescence (PL) and cathodoluminescence (CL) measurements. It was found that concentration quenching of Er-related luminescence was observed in PL measurements while not in CL measurements. The different excitation and relaxation processes are suggested as the cause of the concentration quenching characteristics between PL and CL. The strong Er-related CL intensity in highly doped GaN demonstrates that high energy excitation is a promising approach to suppress the concentration quenching in Er-doped GaN.

application/pdf, Electron emission spectra resulting from thermal collision of He*(23S) atoms with 2,9-demethyl-4,7-diphenyl-1,10-phenanthroline (BCP) films deposited on metal substrates were measured to characterize gap states emerged at the organic-metal interface. For BCP on Au, the gap state is originated from weak chemisorption and serves as a mediator of metal wave functions to the first layer. For BCP on K, organic-metal complex is formed by spontaneous diffusion, yielding the gap states delocalized over the film. In the interfacial region, all the gap state reveals an incommensurate shift with the valence band top of the film, indicating the breakdown of the Schottky–Mott model as evaluating the transport characteristics in organic-metal system.

application/pdf, The systematic variations in the structural, optical, and electrical properties of polycrystalline Cu(In,Ga)Se2 (CIGS) thin films with Na doping level were investigated. Precise control of the Na concentration in CIGS films was demonstrated using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to CIGS growth. The CIGS grain size was observed to decrease with increasing Na concentration, although the surface morphology became smoother and exhibited a stronger (112) texture, which has been demonstrated consequence of larger grain size. The Ga composition gradient in the CIGS films was found to become large due to the presence of Na during growth, which in turn led to a decrease in the nominal band gap energy. Variations in the photoluminescence spectra and electrical properties suggested that the formation of an acceptor energy state, which may originate from OSe point defects, was enhanced in the presence of Na. This result suggests that not only Na, but also the presence of O in combination with Na contributes to the compensation of point defects and enhances p-type conductivity in CIGS films.

application/pdf, A relationship between intra-4f transitions of Er and vacancy-type defects in Er-doped GaN was studied by using a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured for Er-doped GaN grown by molecular beam epitaxy. A clear correlation between the defect concentration and the photoluminescence (PL) intensity was observed. The major defect species detected by positrons was identified as a Ga vacancy VGa, and its concentration increased with increasing Er concentration [Er]. For the sample with [Er]=3.3 at. %, the maximum integrated intensity of PL was observed. The VGa concentration was above 1018 cm−3 and additional vacancies such as divacancies started to be introduced at this Er concentration. For the sample with higher [Er], the PL intensity decreased, and the mean size of vacancies decreased due to an introduction of precipitates and/or metastable phases.

application/pdf, We have studied photoluminescence (PL) properties of Eu3+-doped GaN (GaN:Eu3+) epitaxial films by microscopic PL imaging spectroscopy. The GaN:Eu3+ epitaxial films exhibit efficient red luminescence related to intra-4f transitions of Eu3+ ions. The intensity and the spectral shape of the Eu3+-related PL are sensitive to the Eu3+ concentration, the excitation wavelength, and the monitored position. Microscopic PL imaging spectroscopy revealed that efficient red luminescence of GaN:Eu3+ epitaxial films is due to Eu3+ ions around point defects and dislocations.

application/pdf, Hole traps in p-type Cu2O were studied by means of deep level transient spectroscopy in the heterostructure of p-Cu2O/i-ZnO/n-ZnO. In addition to the trap level at about 0.45 eV from the valance band edge, which is already reported as being due to Cu vacancy, we found a new trap level at about 0.25 eV. The new trap is tentatively assigned as Cu-di-vacancy from the trap concentration dependence on oxygen flow rate and substrate temperature.

Employing p/sup +/-ZnTe/ZnSe quantum wells whose sub-bands are aligned in energy so that resonant tunnelling of holes can occur through the multiquantum well region, Au/Pt/Pd nonalloyed ohmic contacts to p-type ZnSe have been realised. A specific contact resistance as low as 5.0*10/sup -2/ Omega cm/sup 2/ has been achieved for N-doped ZnSe with a hole concentration of 7.0*10/sup 16/ cm/sup -3/. >

Room temperature pulsed operation of ZnCdSe/ZnSe/ZnMgSSe SCH lasers has been achieved. Blue-green stimulated emission is observed at a wavelength of 498.5 nm. The threshold current density is 2.8 kA/cm/sup 2/ for the diode made by molecular beam epitaxy.

Au(Pt)Pd ohmic contacts to p-ZnTe are reported, with a determination of the optimum annealing temperature and the optimum Pd layer thickness. The specific contact resistance, measured by the transmission line method, was as low as 5*10/sup -6/ Omega cm/sup -2/. This value of the specific contact resistance is two orders of magnitude lower than that of Au or Au/Pt contact to p-ZnTe. >

The continuous wave operation of an ZnSe/ZnMgSSe laser diode was achieved for the first time at 77K. Blue stimulated emission was observed at a wavelength of 447nm and the threshold current density was 225A/cm2.