Wide bandgap semiconductors for LEDs, solar and related energy technologies

Resume : Silicon carbide (SiC) is a synthetic non-oxide ceramic, extremely heat- and corrosion stable. As a semiconductor, its high charge mobility or breakdown electric field, and operational pn-junctions up to 600°C, make it desirable for high power and high temperature electronics. Like well-known transparent conducting oxides (TCOs), also SiC comprises a wide band gap, enabling its use in optoelectronics. Despite these advantages, it is rarely used for this purpose on a large scale, due to its costly common production via sublimation single crystal growth or thin film epitaxy. Additionally, the introduction of selected dopants, especially p-type, is challenging. Here, we present an approach to silicon carbide, using an alternative sol-gel processing and carbothermal reduction reaction based approach. This method enables not only the preparation of material in versatile forms, such as spherical particles, porous structures, nanowires or bulk materials, but even enables infiltration and solution deposition techniques, just like common sol-gel techniques for oxides. If required, n- and p- doping of SiC is achieved by addition of suitable compounds at the wet-chemical stage. We will show details on the properties of this sol-gel derived SiC material, such as its morphology and surface termination, defects and impurities and how these depend on the introduced dopant. In terms of applications, we will demonstrate its functionality exemplarily for photovoltaic energy conversion in organic-inorganic hybrid solar cells, where SiC qualifies as inorganic acceptor in combination with a photoactive polymer donor, and for photocatalytic electrochemical carbon dioxide back-conversion into fuel, as an alternative to fossil resources.

Resume : Calcium indate (CaIn2O4) is an important wideband gap (3.9 eV) semiconductor that can be explored for optical doping such as transition metals and/or rare-earths useful to make light-emitters, optical data storage, and other devices. Here, we synthesize Cr4+ doped CaIn2O4–C hybrid nanostructure (1-10 mol% Cr4+) in tuning visible-light emission and dielectric properties. When grafting a C-sp2 surface layer on Cr4+: CaIn2O4 of small crystallites it yields a core-shell structure of tailored dielectric, optical and other properties. A green route using fresh nectar from aloe vera leaves is used to produce this structure from Cr6+, Ca2+ and In3+ salts. The aloe vera nectar embeds the cations in a gel so as it controls an ionic conversion Cr6+ Cr4+ in the ambient air [1,2]. When self-dried at room temperature, the gel was burnt with camphor (a fuel) in air to yield a hybrid phase. Free carbon could be burnt out by annealing the powder at 400C in air. In 1 mol% Cr4+ doped sample (measured at room temperature), a dielectric permittivity εr ~ 97 lasts at low frequencies near 1 Hz, and it decays progressively on raising the frequency in a fairly steady εr ~ 3 above 1 kHz. At 105 to 106 Hz frequency, the ac conductivity picks-up suddenly by nearly three orders of magnitude, e.g., 34.5x10-7 Scm-1. A semi-circle that appears of the Nyquist plots at small 1 mol% Cr4+ doping converts into a linear path intrinsic of an insulating behavior at higher Cr4+ contents. As-prepared Cr4+:CaIn2O4–C exhibits two bands at 265 nm and 360 nm, which are shifted to at 270 and 370 nm when it is annealed at 400-600 ºC for 2 h. These are the ligand-to-metal O2- Cr4+ charge transfer bands. The light-emission is studied in analyzing migration, transfer and recombination processes of light-induced e--h+ pairs. Upon exciting at a 270 nm wavelength from a xenon lamp, two broad emission bands appear at 395 nm and 680 nm as intense blue and red lights. The intensity decreases in both the bands on annealing the sample as a result of a suppressed e--h+ recombination. The results are described in correlation to a hybrid Cr4+:CaIn2O4–C structure.

Resume : The chemical bath deposition has potential as a cost-efficient fabrication technique for transparent electrodes for thin film photovoltaics. In this work we investigate chemical bath-deposited ZnO films, undoped or doped with the group-13 metals Al, Ga and In. The study shows marked differences in dopant incorporation in the films, depending on the dopant type. The addition of dopant salt in the solution modifies the deposition rate, with maximum achieved rate close to 100 nm/min. X-Ray diffraction showed that all films have the (002) texture, whereas the lattice stress suggests substitutional incorporation at Zn sites for specific dopants and concentrations. The average visible transmittance is higher than 80%, while the infrared reflectivity increases with the free carrier density, which is the lowest for undoped ZnO (10^19 cm^-3) and increases up to 1 order of magnitude for doped films. Optical measurements reveal an inverse correlation between carrier density and mobility. Doping enlarges the bandgap and the Urbach energy that is related to the film disorder. The lowest electrical resistivity, measured by four-point probe, is 0.017 Ω∙cm. The stability of the films upon exposure to high doses of UV irradiation is also investigated.

Resume : The paucity of high performance p-type semiconductors has been a stumbling block for the electronics industry for decades, effectively hindering the route to efficient p-n junctions. Through the “chemical modulation of the valence band” (CMVB) pioneered by Hosono et al.,(1) copper based oxides and subsequently copper based chalcogenides have been a focal point for designing efficient p-type semiconductors, particularly transparent semiconductors. Our work extends this concept further into the pnictides with a group of ternary copper phosphides, M(II)CuP (where M(II) = Mg, Ca, Sr and Ba) and explores the relationship between coordination and band dispersion alongside explaining the coordination preference of Cu(I) based materials. Using hybrid density functional theory (DFT) we examine the structural and electronic properties of these four materials where we find hole effective masses matching the industry standard n-type TCOs: In2O3, ZnO and SnO2 paving the way towards the design of high performance p-type materials. (1) Kawazoe, H. et al. P-type electrical conduction in transparent thin films of CuAlO2. Nature 389, 939–942 (1997). (2) Williamson, B. A. D. et al. Engineering Valence Band Dispersion for High Mobility P-Type Semiconductors. Chem. Mater. (2016). doi:10.1021/acs.chemmater.6b03306

Resume : Because of the low temperature processing, high carrier mobility, good transparency over a wide spectral range as well as large area uniformity and excellent mechanical flexibility, amorphous transparent conducting oxides (a-TCOs) have attracted increasing attention recently especially in flexible electronics applications. In comparison with other TCOs, CdO has the highest reported mobility of 200-500 cm2/Vs at an electron concentration of 10^20-10^21 cm-3. However, CdO has a relatively low intrinsic bandgap (~2.2 eV), resulting in a compromised transparency window in the UV region. Alloying of CdO with other wide bandgap semiconductor may result in a-TCOs with high mobility and high transparency. In this work, we have synthesized In2O3-CdO alloy thin films at room temperature using radio frequency magnetron sputtering on glass over the entire alloy composition range. Rutherford backscattering and x-ray diffraction show that In1-xCdxO films with 0.1

Resume : Bandgap engineering plays an important role in designing novel functionalities in semiconductor materials. ZnO and GaN are applicable for optoelectronic devices and exist in the wurtzite hexagonal structure (WZ) with similar lattice constants. Interestingly, combining the two wide-bandgap semiconductors, both having a band gap of ~3.4 eV, results in a strong band bowing effect where the band gap of a 50% alloy can reach 2.4 eV [1]. In the present work, (ZnO)1-X(GaN)X thin films with X ranging from 0 to about 0.2 are deposited on sapphire substrates using RF magnetron sputtering. X-ray diffraction on the as-deposited samples shows a shift in the (0002) peak for increasing GaN content, while transmission measurements show a shift to longer wavelengths, in accordance with a reduced bandgap and previously reported results[2]. The structural properties remain similar during post-deposition annealing up to 800oC for 1 hour, whilst a steeper on-set in the transmission measurements is observed. Moreover, photoluminescence measurements on as-deposited samples exhibit a broad near band edge luminescence, while post growth annealing at 800oC restores the luminescence properties to those of a ZnO like film. Further, transmission electron microscopy measurements and Rutherford backscattering spectrometry results will be discussed. [1] Chen et al. J. Phys. Chem. C 114, 1809 (2010). [2] K. Maeda et.al. J. Phys. Chem. B. 2005, 109, 20504-20510.

Resume : Oxygen-containing yttrium hydride (YH_aO_b) thin films deposited by magnetron sputtering possess wide band gap and high luminous transmittance in the visible spectrum. Studies carried out in our group have shown how the luminous transmittance (Tlum) of the YH_aO_b thin films decreases significantly when illuminated with light of adequate wavelength and intensity. This decrease in the transmittance is reversible, i.e., the films recovered gradually their initial transparency once the illumination was interrupted, and thus YH_aO_b emerged as a new T-type positive photochromic material. Photochromic materials have many applications in smart fenestration, ophthalmic lenses, agricultural films, security markings and many more. The objective of this work is to explore, from an optical perspective and aiming to a technological application, the possibilities that this photochromic material can provide in terms of transparency in the clear state and modulation of the optical transmittance. On this basis, the optical properties of a set of YH_aO_b thin films, deposited by magnetron sputtering under different conditions, has been studied by spectrophotometry and ellipsometry before and after being illuminated during 1 hour at 1 Sun. Our results show how YH_aO_b thin films with excellent transparency in the visible spectrum, Tlum above 80 %, can reduce reversibly their initial luminous transmittance by more than 30 %. In addition, a theoretical model able to reproduce quantitatively the optical properties of the films, both in their clear and photodarkened state, is presented.

Resume : Al4SiC4 is a refractory ceramics with a band gap of about 2.5eV, making it an interesting semiconductor material for various applications in the field of energy. The synthesis of Al4SiC4 single crystals has so far not been investigated. In this study, the sublimation growth method is explored as a potential route for getting single crystals. Combining a thermodynamic analysis with an extensive experimental approach, vaporization and condensation phenomena in the Al4C3 - SiC system are described. Initial composition, temperature and temperature gradient are investigated and optimized regarding the crystallization of Al4SiC4.

Resume : A rapid, 10 min microwave-assisted synthesis of ZnAl2O4 spinel using metalorganic precursors is reported. Nanocrystalline ZnAl2O4 of high phase purity was formed in situ at temperatures as low as 185 °C. The effect of annealing nanoparticles at temperatures in the range 500- 1200 °C on their structural and optical properties was investigated. Crystallite sizes, determined from the XRD patterns, varied from 5 nm in the as-prepared form, to 31 nm when annealed at 1200 °C. The crystallites did not display any definite shape; polyhedral crystallites with well-defined grain boundaries can be seen in the HR-TEM image of annealed samples. Diffuse reflectance spectroscopy (DRS) gives insight into the structural development of the spinel, whereas photoluminescence spectroscopy shows that antisite defects - in the form of zinc interstitials caused by cationic redistribution in a partially inverted spinel and in the form of oxygen and zinc vacancies - governs the different emission bands of the spinel. The investigation of optical properties shows that inhomogeneity in cationic distribution, caused by the rapid synthesis method, is prevalent even after annealing at 1200 °C. Microwave-assisted synthesis opens new opportunities for the rapid synthesis of doped ZnAl2O4 phosphors suitable for display and sensing applications.

Resume : Rare earth-doped III-nitrides such as GaN and AlN, attract great attention in lightening and optoelectronic applications. Particularly, AlN with large optical bandgap (6.2 eV) can offer an opportunity to cover wide range of light spectrum from UV to IR regions. In addition, AlN with high thermal conductivity (300 W/mK) is a very suitable material for high power working optoelectronic devices. Exploiting the large bandgap and the thermal conductivity features of AlN lead to open the door for generating blue emission with low thermal quenching effect even at high power operation. These are very interesting prerequisites for establishing stable white light emitting diodes (w-LED). Seeking for realization the blue emission from AlN, cerium-doped aluminum nitride (Ce-AlN) thin films were prepared at room temperature using radio frequency (RF) reactive sputtering. X-ray diffraction and high resolution transmission electron microscopy (HRTEM) revealed a well crystalline textured microstructure with single <002> out-of-plane orientation. Strong blue emission from the prepared samples was detected when excited by 325 nm laser. Electron energy loss spectroscopy (EELS) has been used to reveal the dominant oxidation state of Ce atoms, which undergoes a change from of Ce(IV) to Ce(III) ions after annealing. The chemical composition was analyzed by simulation of Rutherford backscattering spectrometry (RBS) and compared to HRTEM images. A clear correlation between microstructure, composition and sample photoluminescence (PL) was established. It was found that surface oxidation during post-deposition annealing plays an important role in the PL response of the samples. The role of oxygen in the excitation mechanism was explained from the photoluminescence excitation measurements. We believe that the strong blue emission in this new (oxy)-nitride material holds great potentials for solid state lighting applications due to its thermal and chemical stability as well as the luminescence efficiency. Moreover, the comprehensive approach conducted within this study could serve as a guideline for better understanding and the design of the luminescence behavior in rare earth-doped (oxy)-nitride thin films.

Resume : Eu/O co-doped GaN is a promising material for light emitting diodes (LEDs) in the red. However a further increase of the emission intensity is required for commercial application. The emission intensity of the currently available state-of-the-art materials is limited due the relatively long radiative lifetime (~200 μs) of the Eu ions as well as the low optical activity of Eu ions (typically 1%-10% [1]). In this work, we demonstrate that it is possible to enhance emission intensity by use of a carefully designed nanostructuring of Eu,O-codoped GaN layers grown by organometallic vapor phase epitaxy (OMVPE) – by creating a metamaterial. By using this approach we demonstrate up to two fold increase of emission intensity for the nanostructured material, where up to 87% of the total number of Eu3+ ions have been removed. We explore the different mechanisms which could account for this enhancement, and conclude that it is most probably due to the higher effective excitation cross-section of Eu. [1] Scientific Reports 4, 5235 (2014)

Resume : White LEDs are currently an energy saving lighting solution that constitutes an integrated solution for environmental lighting and communication technologies which constitutes the technology Visible Light communication (VLC). In this paper we propose the use of RGB white LEDs in an integrated lighting/VLC system for indoors navigation. The constituent red, green and blue chips are modulated at specific frequencies and bit rates. The spatial distribution of the signals is dependent on the geometrical configuration of the LEDs and enables a correspondence among cardinal directions and intensity/wavelength/frequency of the optical signals. The detection is done with a photodetector based on two stacked pin heterostructures of a-Si:H and a-SiC:H that works as an optical filter in the visible range. Optoelectronic characterization of the device is presented and includes spectral response, transmittance and I-V characteristics. Results show that under front bias the photocurrent is amplified in the range of longer wavelengths and quenched in the shorter wavelengths. The use of a back optical bias produces the opposite effect. This feature is used for the identification of the optical excitation. Fourier transform is used for signal frequency identification. Analysis of the photocurrent signal dependence on the LED intensity is done to infer the accurate position of the photodetector. The viability of the system was demonstrated through the implementation of an automatic algorithm.

Resume : Light-emitting diodes (LEDs) are broadly used in microelectronic devices that operate in space conditions and at nuclear power plants. It is necessary to compare combined and complex effects of ionizing radiation and long-term operation factors both simultaneous and separately for solving problem of these influences on criterion parameters. The purpose of this research is comparison of mechanisms of emissive power change of IR-LEDs based upon AlGaAs double heterostructures under influence of ionizing radiation and long-term operation. Irradiation was implemented using 60Co gamma-quanta in passive power mode. Long-term operation conditions were realized by step-by-step tests. Operating current was increased from step to step with constant ambient temperature (+65 C). Duration of each step was 24 hours. We have established that change of emissive power of LEDs in both cases can be described by three stages. On first stage the emissive power of LEDs decreases due to reconstruction of initial defect structure of LED crystal. On second stage the emissive power of LEDs goes down due to introduction of new defects. Second stage is limited to transition of LED to low injection mode and further catastrophic failure (third stage). Correlations between irradiation dose and step number have been determined for first and second stage of decrease of emissive power of LEDs. The problem of evaluation of reliability indexes of LEDs based on results of their radiation resistance has been discussed.

Resume : Organic metal halide perovskites (CH3NH3PbX3) have been widely studied for various applications such as light-emitting diodes (LEDs), photovoltaics, lasers or sensors. In particular, organic metal halide perovskites have emerged as a next generation light emitter in LED application because of their outstanding color purity (full width at half maximum of electroluminescence spectra as narrow as 20 nm), easily tunable bandgap through adjustment of the halide element ratio, and low materials cost. However, methylammonium lead bromides (CH3NH3PbBr3) which are suitable for green light emission, have a low exciton binding energy and have a tendency to form cubic shaped crystals, making it difficult to obtain a uniform film. Therefore, the device performance of perovskite LEDs has not yet approached that of OLEDs or QLEDs. Here, we studied the effect of varying molar ratio of the perovskite precursors (MABr and PbBr2) on the properties of synthesized MAPbBr3 thin films as well as on LED device performance. The molar ratio (MABr/PbBr2) was adjusted from 1 (stoichiometry) to 2 (excess MABr) in precursor solution. Structural characterization revealed that the excess MABr condition led to the formation of a new crystalline phase, which appeared to be one of methylammonium compounds, within the MAPbBr3 films. The excess MABr sample also exhibited highest PL intensity, which suggested less occurrence of non-radiative recombination compared to other molar ratios. Based on the PL, the excess MABr was expected to produce the best LED performance, however, it was when MABr/PbBr2 is 1.05 found that led to the best performance. We explain these results by other electronic properties such as Urbach tail energy that we found relevant to the operation of LEDs. With further optimization of hole injection layer, we achieved highly efficient green perovskite LEDs with luminance = 1032.1 cd/m2, current efficiency = 0.905 cd/A and external quantum efficiency (EQE) = 0.198%. Details of characterization results with varying molar ratios and optimization of LED device structures will be presented.

Resume : High-power white-light-emitting phosphor plate excited by blue lase diode is presented. The yellow Y3Al5O12:Ce phosphor films on various substrates were formed through a thermal diffusion of phosphor compositions into the substrates. For the best blue-to-yellow conversion efficiency, the thickness of phosphor film was optimized with varying the diffusion parameters; temperature, time, pressure. The corelation between surface morphology and optical powder was expected by a light simulation by Lighttools. For the practical applications such as car headlamp and beam projector, the excitation power and temperature dependences was analyzed together with Raman measurement for photon-phonon coupling effect.

Resume : Purpose: As a number of buildings are increasing day by day due to the opening of new industries, colleges, hostels, hospitals, etc. Therefore there is continuously increase in energy consumption by these buildings and this scarcity of electricity is compensated by limiting the electricity supply to our villages for irrigation and other usages.As also people who want lands for plotting purpose are looking towards agricultural lands and using solar panels for electricity purposes. Mostly the agricultural lands experience the destruction due to the rise in temperature of the particular region where solar panels are installed. The purpose of the research is to use green energy(solar energy) in the way that it doesn't harm the mother nature. And this can be done by directly transferring the incoming photons to the targeted area using a special class of fiber optics. Methods: 1. Effects and properties of agriculture land before and after installation of solar panels. 2. Testing the heat radiation of Solar panels in static mode (stationary points are installed). 3. Testing for the orientation of fiber optics to catch the maximum of the photons. 4. Testing the suitable way to eliminate heat after illumination by using fiber optics. Results: There is some heat evolved when using the static or dynamic model of solar panels to convert light energy into electricity while no heat is evolved when photons are transferred directly to the targeted area using a special class of fiber optics. Conclusion: The static and dynamic mode solar panels dissipate heat while converting photon's energy into electric energy whereas while transporting the photons directly to the targeted area does not dissipate or evolve any heat energy. Thus global warming can be reduced by up to small extent by decreasing the consumption of solar panels for such uses. References: 1. Jeff Hecht, City of light: the story of fiber optics, OXFORD UNIVERSITY PRESS, 14-24, 1999. 2. K. C. Kao, Physical aspects, Dielectric-fibre surface waveguides for optical frequencies, Vol. 113, No. 7, 1156-1159, 1966. 3. Dr. N.Chandrashekar, Sun Tracking Device, Patent.No.251549, July-2012 4. Tarujyoti Buragohain, Impact of Solar Energy in Rural Development in India, IJESD, Vol. 3, Pg:334-338, 2012

Resume : GaS(x)Se(1-x) mixed crystals are formed from the GaSe and GaS semiconducting materials with x ranging from 0 to 1. By varying the composition its energy gap covers a wide range of the visible spectrum. Taking into consideration the technological applications of GaSe and GaS, GaS(x)Se(1-x) crystals can be important candidate in the usage areas of constituent compounds. Ellipsometry is a powerful and non-destructive optical characterization technique based on the change of the amplitude and phase of the polarized light after reflection from the sample. Analyses of the ellipsometric data are used to get detailed information about the optical constants and electronic band structure of samples. Spectroscopic ellipsometry measurements were performed on GaS(x)Se(1-x) mixed crystals (0 ≤ x ≤ 1) in the 1.2-6.2 eV range. Spectral dependence of optical parameters; real and imaginary components of pseudodielectric function, pseudorefractive index and pseudoextinction coefficient were reported. Critical point (CP) analyses on second-energy derivative spectra of the pseudodielectric function were accomplished to find the interband transition energies. The revealed energy values were associated with each other taking into account the fact that band gap energy of mixed crystals rises with increase of sulfur composition. Peaks in the spectra of studied optical parameters and CP energy values were observed to be shifted to higher energy values as sulfur concentration is increased in the mixed crystals.

Resume : GaSe and GaS are attractive materials in technological applications such as field-effect transistors, photovoltaic, optoelectronic, energy conversion and storage, topological insulators, nonlinear optical devices and photodetectors. Moreover, investigations on GaSe/GaS revealed the potential usage of the structure as ultrathin layer transistors. GaSe and GaS crystals form GaS(x)Se(1-x) mixed crystals with x ranging from 0 to 1. Taking into consideration the technological applications of GaSe and GaS, GaSxSe1-x mixed crystals have potential to be used in the fabrication of long-pass filter, light emitting devices and optical detecting systems. Transmission and reflection measurements on GaS(x)Se(1-x) mixed crystals were performed in the 400-1000 nm spectral range. Band gap energies of the studied crystals were obtained using derivative spectrophotometry analyses. Compositional dependence of band gap energy revealed that as sulfur composition is increased in the mixed crystals, band gap energy increases quadratically from 1.99 eV (GaSe) to 2.55 eV (GaS). Spectral dependencies of refractive indices of the mixed crystals were plotted using the reflectance spectra. It was observed that refractive index decreases nearly in a linear behavior with increasing band gap energy for GaS(x)Se(1-x) crystals. Atomic compositions of the studied crystals obtained from the energy dispersive spectroscopy measurements are well-matched with composition x increasing from 0 to 1 by intervals of 0.25.

Resume : Light-emitting diodes (LEDs) may be operated under conditions of various ionizing radiation. In these operating conditions reliability of LEDs is determined by combined influence of ionizing radiation and long-term operation factors. The purpose of this work is research of the influence of preliminary irradiation by gamma-quanta and neutrons on the reliability of IR-LEDs. Matter of the research is IR-LEDs based upon AlGaAs double heterostructures. Preliminary irradiation was implemented using 60Co gamma-quanta and fast neutrons without electric supply. Then long-term operation conditions were realized by step-by-step increase of operating current with constant ambient temperature (+65 C). Duration of each step was 24 hours. Two characteristic radiation levels were chosen for preliminary irradiation by both gamma-quanta and neutrons. The first irradiation level corresponded to first stage of emissive power decrease of the LEDs under the influence of ionizing radiation (the radiation-stimulated reconstruction of the initial defect structure of the LED crystal). The second irradiation level corresponded to second stage of emissive power drop of the LEDs (the introduction of radiation defects). The preliminary irradiation by gamma-quanta has increased useful lifetime of the LEDs, which allows to use gamma-quanta in the manufacturing technology of LEDs for rise their reliability. Preliminary irradiation by fast neutrons has led to reduce threshold of catastrophic failures appearance due to accelerate degradation of ohmic contacts. Under specific conditions the irradiation by fast neutrons may be recommended for manufacture of LEDs with improved reliability. Probable causes of observable results are discussed.

Resume : Silicon based flexible UV-visible photodetectors can play an important role in optoelectronic applications due to its compatibility with existing CMOS technology and its adaptability where flexibility and agility of the device is required. Here we report the development of a n-ZnO/p-Si membrane heterojunction based flexible photodetector sensitive to both UV and visible photons. The 3 µm thick Si membrane was fabricated by the alkaline etching method followed by deposition of ZnO thin film (120 nm) by RF sputtering. At zero applied bias, a peak responsivity of 0.20 AW-1 with detectivity of 4.8 × 1011 cm Hz1/2 W-1 has been obtained. The flexibility of the Si membrane as substrate has motivated us to utilize the piezo-phototronic effect in ZnO to improve the performance of the fabricated device. Strain induced piezo-potential developed in piezoelectric ZnO thin film has been utilized to modulate the transport property of the photo generated carriers thereby positively affecting the device performance. With the gradual increase in the external tensile strain the photocurrent has increased from 5.5 mA to 6.7 mA. The accompanying simulation analysis reveals the piezopotential distribution developed in the ZnO film on application of stress to the heterojunction.

Resume : The device performance and stability of staggered bottom gate structure tungsten-indium-zinc oxide thin film transistors (WIZO-TFTs) were fabricated by a radio frequency (RF) co-sputtering system using the WIZO and indium-tin oxide (ITO) source/drain (S/D) electrode materials on a same WIZO active channel layer. The electrical device characteristics of the WIZO film as the S/D electrode and the active channel layer, were adjusted by the oxygen partial pressure in the deposition process, respectively. The improvement of the device performance and stability with WIZO and ITO S/D electrodes, and their correlations between the device performance and the physical properties at the interface region were investigated in terms of contact resistance, interface roughness, interfacial trap density, interfacial energy level alignments. The WIZO-TFT with WIZO electrodes exhibited the lower contact resistance, lower interfacial trap density, and more flat interface roughness than the WIZO-TFT with ITO electrodes. Furthermore, the electron barrier of the WIZO-TFT with WIZO electrodes is 0.09 eV, which is lower than the value of 0.21 eV was obtained for the WIZO-TFT with ITO electrodes. This reduced electron barrier could be attributed to the improvement of device performance and stability, because of the electron transport.

Resume : Light and sound-emitting electroluminescent (EL) device has been achieved by constructing phosphor layer on piezoelectric dielectric layer. The EL device consists of screen-printed ZnS:Cu,Al phosphor on high piezoelectric material Pb(Zr,Ti)O3 (PZT). The PZT dielectric layer showed the high dielectric effect reducing threshold voltage up to 10 V, and its high strain effect produced a high sound level up to 60 dB. As the increasing of voltage (10 V - 200 V), the light intensity increased without a spectral change and the sound level increased without tone frequency change. However, as the increasing of frequency (10 Hz – 1 MHz), the light intensity increased with a blueshift, and it sound level increased with a change of tone frequency. The tone frequency was same with the frequency of applied voltage, as a result, all diatonic tones was achieved by adjusting the applied frequency into diatonic frequencies. Furthermore, the temperature dependence of this device showed that the optical and acoustical functions are so different. It is the reason why the former results from the phosphor, but the latter comes from the PZT.

Resume : Ti0.5Al0.5O-dielectric Al0.26Ga0.74N/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) grown on a Si substrate are investigated. The Ti0.5Al0.5O gate dielectric was grwon by using a cost-effective non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique. The effective oxide thickness (EOT) was determined to be 2.68 nm with high dielectric constant k = 9.01. Improved interfacial quality is studied by comparisons of Hooge coefficient (αH), low-frequency noise spectra (1/f), pulse I-V characteristics. As compared to a reference Schottky-gate HEMT, superior improvements have been achieved for the present MOS-HEMT design, including 23% in maximum drain-source current density (IDS, max), 31% in gate-voltage swing (GVS) linearity, 3-order increases in on/off current ratio (Ion/Ioff), 46.3% in two-terminal off-state gate-drain breakdown voltage (BVGD), 65% in three-terminal drain-source breakdown voltage (BVDS), 28% in unity-gain cut-off frequency (fT), and 46% in maximum oscillation frequency (fmax). Enhanced power performances and thermal stability at 300-450 K are also obtained. The present design is advantageous to high-gain and high-power circuit applications. KEYWORDS: MOS-HEMT, Ti0.5Al0.5O, ultrasonic spray pyrolysis deposition.

Resume : We discuss optical reflection loss at intermediate adhesive layer for mechanically stacked multi junction solar cells. Good electrical connection between top and bottom semiconductor substrates has been achieved with a connecting resistivity lower than 0.5 ohmcm2 by dispersing 0.02-mm-ITO particles in an epoxy adhesive layer. We propose conductive and transparent IGZO layers as optical anti-reflection layer formed between the semiconductor substrate and intermediate adhesive to reduce optical reflection loss. 130-nm-thick 0.01-ohmcm IGZO layers were formed on surfaces of GaAs and Si substrates by plasma sputtering. The IGZO surfaces of two substrates were pasted with epoxy adhesive jell dispersed with 3.8-wt%-ITO particles and stacked to make a structure of GaAs/IGZO/adhesive/IGZO/Si. The optical reflectivity was 0.33 for wavelength between 902 and 1020 nm, where the top GaAs is transparent and bottom Si is opaque. This value was close to the optical reflectivity at the top GaAs surface. A low optical reflection loss was estimated as 0.06 because of anti-reflection condition of IGZO layers. Similar low optical reflectivity was observed with different incident angles of light. The optical reflection loss was 0.04 for an incident angle of 20 degrees. In contrast, the optical reflectivity was high of 0.41 for GaAs/adhesive/Si with no IGZO. This paper will also report optical reflection loss for different sample of GaP/IGZO/adhesive/IGZO/Si.

Resume : We propose the use of Visible Light Communication (VLC) for vehicle safety applications, creating a smart vehicle lighting system that combines the functions of illumination, signaling, communications, and positioning. The proposed coding is based on SiC technology. The system is a self-positioning system. Trichromatic Red-Green-Blue LEDs (RGB-LED) are used together for illumination proposes and individually, each chip, to transmit the channel location and data information. The chips of the RGB-LEDs can be switched on and off individually, in a desired bit sequence to transmit the information. The receivers consist of two stacked amorphous cells [p(SiC:H)/i(SiC:H)/n(SiC:H)/p(SiC:H)/i(Si:H)/n(Si:H)] sandwich between conductive transparent contacts. The receiver and the transmitters are characterized through spectral response, I-V characteristics and transmittance measurements under different optical bias. The spectral sensitivity of the receiver as a function of the distance from the transmitters is analysed. An optoelectronic model gives insight into the physics of the system. The receivers join the simultaneous demultiplexing operation with the photodetection and self-amplification. The information and the code position of each LED are transmitted simultaneously through the RGB pulsed transmitted channels. A violet LED is used for error control. Free space is the transmission medium. An on-off code is used to transmit data. An algorithm to decode the information is established.

Resume : ZnO has received much renewed interest as a wide band gap semiconductor for its variety of applications. For certain applications, such as thin film transistors, it is important to have highly crystalline ZnO with few defects, as a high defect concentration introduces too many charge carriers and can contribute to source-drain leakage. In this paper, we present a new roll-to-roll process, namely UV-Assisted Atmospheric Pressure Spatial Atomic Layer Deposition, for synthesizing high quality, crystalline ZnO. Using X-ray diffraction techniques, we show that the UV-activation of diethylzinc allows us to deposit c-axis oriented ZnO at temperatures as low as 50 C with significantly improved crystallinity. This temperature is significant as it is below the glass transition temperature of polyethylene terephthalate (PET), a popular substrate in the field of flexible electronics. Our new process allows us to overcome the tendency of ZnO to be amorphous when grown below 100 C. We also present the effect of growth under UV-illumination at different wavelengths on the defect states in ZnO with the use of X-ray photoemission spectroscopy, photothermal deflection spectroscopy and photoluminescence.

Resume : The dislocation studies on GaN, wide band-gap semiconductor of 3.4eV, have been performed for them normal to (0001). But in real the dislocation are inclined from <0001> especially where the strain varies distinctively. They are the area near the active layer where high amount of Si, Mg, In and Al are doped, and the case of GaN grown on Si (111). Jog or kink accompany when a dislocation is inclined, and the core structure of the dislocation would be deformed from them of parallel to <0001>. We studied the structure of the dislocation with the GaN grown on Si(111) using the transmission electronic microscopy techniques. In the BF view, most of the dislocation were characterized to be edge type. As the inclined angle is about 12 degree from <0001>, it is seen dislocation climb would happen frequently. We have changed the focus in STEM view and acquired a series of images with the respective focus, that is called optical sectioning. As the result, the structure similar to (0001) stacking fault was observed most frequently. And highly strained open-core dislocation also have been observed. In the side of electronic structure, the highly strained open-core edge dislocation might contain Ga metallic bond. From the present result, even it need be more deeply studied, there is great possibility that the inclined edge type dislocation introduce much severe defect-level in band gap degrading the efficiency of the GaN devices. (NRF-2016R1C1B1013667)

Resume : For a long time light-emitting diodes (LEDs) based on gallium phosphide widely applied in optoelectronics. Furthermore, they stay advanced for modern optoelectronics devices manufacturing. Lack of full description of ionizing radiation influence on these LEDs limits their using. The purpose of this investigation is to research parameters changes of LEDs based on GaP epitaxial structure with 590 nm emission wavelength under irradiation by 60Co gamma-quanta. Irradiation was realized by 60Co gamma-quanta without imposition of external electric fields. Before and after irradiation volt-ampere (V-I) and watt-ampere (W-I) characteristics were measured. Based on LEDs V-I characteristics analysis the resistance of ohmic contacts for the investigated LEDs is equaled to (2.28±0.5) Ohm and it practically remains constant up to 2 MGy. W-I characteristics changes under irradiation allow to emphasize low, average and high electron injection fields. Dependences of boundary currents changes between the marked areas on irradiation doses are shown. Emissive power decrease of LEDs with gamma-quanta influence has three stages in the same way that it was observed for LEDs based on AlGaAs, AlGaInP heterostructures with multiple quantum wells. So, the emissive power drop under irradiation by gamma-quanta is described by similar mechanism for various types of LEDs based on different semiconductor structures.

Resume : GaN layers were grown by metal organic vapor phase epitaxy (MOVPE) on GaAs (110) substrate at growth temperature varied in the range of 750-900°C. Low growth temperature (550°C) 50 nm-thick GaN layer was used as buffer layer. The characterizations results obtained by scanning electron microscope (SEM), high resolution X-ray diffraction (HRXRD) and room temperature cathodoluminescence (RT-CL) showed high growth temperature dependence of crystalline and optical properties of GaN. Mixture of cubic and hexagonal crystal structure of GaN were obtained on GaAs (110) surface. The 2?/? spectra are dominated by two broad peaks associated with c-GaN (220) and h-GaN(11.0) reflections. The better alignment of cubic GaN according to (220) orientation was obtained for growth temperature of 850°C. For this growth temperature, morphological observations by SEM showed columnar structure of c-GaN(220) that inclined with respect to the sample surface. The RT-CL spectra showed only cubic GaN emission for this sample whereas both emissions of c-GaN(3.23 eV) and h-GaN (3.40 eV) were observed for 900 °C as extreme growth temperature. Keywords: MOVPE, c-GaN(220), GaAs(110) substrate, HRXRD, Cathodoluminescence.

Resume : A silica- graphitic-C3N4-dots gels was prepared by a one-pot hydrothermal method, the obtained si-g-CDs gels can illustrates white light and has a highly quantum yield of 27% at 365 nm, better than most of carbon nanostructures reported before. The si-g-CDs can be used in White-LEDs and fluorenscent inks. Moreover, wearable device can be used due to its high color index and non-toxicity. The Si-g-CNDs gels also have excellent transparency and flexibility. This work presents a very facile way to fabricate a non-metal high efficiency white LEDs which have potential to largely scale synthesis and applied

Resume : Nickel oxide (NiO) is a wide band-gap p-type semiconductor which has attracted attention as a stable hole-transport/electron-blocking layer in organometal halide perovskite solar cells, demonstrating power conversion efficiencies of over 16%. However, the highest performing perovskite/NiO devices reported to date use NiO formed by vacuum deposition methods such as sputtering and pulsed laser deposition (PLD). Atmospheric pressure spatial atomic layer deposition (AP-SALD), developed in our group, involves the reaction of metal- and oxygen- containing precursor vapours which run through separate channels onto a heated substrate with controlled movement. This enables layer-by-layer deposition of a metal oxide film in air, removing the need for lengthy vacuum-processing steps whilst the technique can be potentially scaled up for high-throughput manufacturing. In this work, we demonstrate the deposition of high-quality NiO films by AP-SALD for the first time, with growth rates on the order of 2.5 Å/cycle. Pinhole-free films with <1 nm RMS roughness are achieved on glass with areas as large as 65 x 65 mm2, enabling reliable deposition of the subsequent perovskite photovoltaic absorbers. Our prototypical perovskite solar cells using AP-SALD NiO show promising power conversion efficiencies of over 12%. Additionally, the NiO films grow conformally to high aspect ratio substrates, making them compatible with textured silicon cell surfaces in tandem photovoltaic devices; a promising area of development for perovskite solar cells. The structural and electrical properties of AP-SALD NiO films will be presented in detail. Deposition conditions optimised for device applications will also be investigated. We will demonstrate the applicability of AP-SALD NiO films in high efficiency perovskite solar cells, for both individual cells and tandem structures.

Resume : Gadolinium oxide doped with rare-earth ions are of interest as functional material for nanophotonics, optoelectronics, alternative energy and conversion system devices. It is known that erbium ions are one of the most suitable emitters in green and red spectral regions. Moreover, specialties of structure of Er3+ energy levels provide an opportunity to convert of UV and near infrared radiation to visible light. The determination of optimal concentration of dopant is a key factor to achieve of highest energy conversion efficiency. The aim of this work is to investigation the luminescent behavior of nanostructured Gd2O3:Er as a function of different Er3+- concentration (0.25-8 mol%). To this purpose the quantum yields, kinetics, temperature dependences of Gd2O3:Er photo-stimulated luminescence under the direct (intracenter) and indirect (energy transfer from «defective» Gd3+-ions) excitation channels have been studied. The results allowed to discuss the nature of concentration quenching mechanisms and establish the quantitative composition of Gd2O3:Er – based materials for the enhancement of energy conversion parameters.

Resume : Organic–inorganic hybrid materials have been broadly investigated owing to their various properties used in optoelectronics and solar cell [1]. Here, we report synthesis of two new organic-inorganic hybrid materials ([C10H10N2]2[Bi2Cl9]Cl and [C10H10N2]2[SbCl4]2Cl2) which are obtained by slow evaporation at room temperature using the same organic cation and two different metals. These two compounds are characterized by X-ray diffraction, infrared and Raman spectroscopy, optical absorption and photoluminescence measurements. Regards the first material, the atomic arrangement revealed that each inorganic entities shares two chlorine atoms to establish the neutrality of charges between anions and protonated cations, Hirshfeld surfaces analysis confirms that Hydrogen bonds dominates to make the stability of the crystal. The structure of the second compound, is formed by parallel layers of 2,2'-dipyridyl cations, anions of (SbCl4)- located in cavities between these layers and isolated chlorine atoms which make the building of the crystal. Hydrogen bonding interactions between anions and organic cations assure the stability of the structure. The assignment of the bands observed in infrared and Raman spectra were predicted from the calculated intensities which shows well agreement with experimental data. Additionally, we demonstrate that these compounds can be used in solar cells. In fact, the energy band gap of these materials was found to be closed to that used in interfacial layers [2] of some organic solar cells. By optimizing optical, electrical, and morphological properties of these new wide bandgap materials, bulk heterojunction solar cells with conversion efficiency exceeding 7 % are obtained in normal device structures with all-solution-processed interlayers. [1] Vitalii Yu. Kotov et al., New J. Chem., 2016, 40, 10041--10047 [2] Sadok Ben Dkhil et al., Adv. Energy Mater. 2014, 4, 1400805

Resume : The need for a high performance, earth abundant transparent conducting oxides (TCOs) is ever present in the expanding electronics and photovoltaics industry. TCOs possess large band gaps (> 3.2 eV) in order to avoid the absorption of visible light, resistivities as low as ~10^-5 ? cm as well as large carrier concentrations (~10^20 cm^-3). Arguably the flagship TCO material is In2O3:Sn (Tin doped Indium Oxide, ITO) with resistivities as low as 10^-5 ? cm making this material ideal for consumer electronics. A main limitation of this material however, is the high cost of indium and the rarity of the element in the earth?s crust which drives the search for a cheaper alternative. SnO2 is an obvious choice due to desirable properties such as high mechanical strength and environmental stability as well as possessing an ideal electronic structure. It is commonly found as the transparent contact layer in photovoltaics.(1) High conductivities can be achieved by doping with Antimony (Sb) or Fluorine (F) which are the most common choice for donor dopant in SnO2 resulting in resistivities as low as 10^-4 ? cm(1) similar to that of ITO yet these dopants are inherently self-limiting. In this presentation we detail the detrimental limitations of Sb and F doped SnO2 using accurate hybrid density functional theory (DFT) and give insights into new screened and intuitive donor dopants which do not succumb to the shortfalls of the past. (1) Bhachu, D. S.; Waugh, M. R.; Zeissler, K.; Branford, W. R.; Parkin, I. P. Chem. - A Eur. J. 2011, 17 (41), 11613.

Resume : Among the various SiC polytypes, cubic 3C-SiC is much more difficult to grow in high crystalline quality than the state of the art hexagonal 6H-SiC and 4H-SiC counterparts. Beside some benefits of 3C-SiC for transistor applications related to a greater electron mobility and a lower metal-oxide-semiconductor interface trap density compared to 4H-SiC, new optoelectronic applications have been discussed in literature very recently. Boron doped 3C-SiC may act as an ideal candidate for an intermediate band (IB) solar cell material. Aluminum doped p-type 3C-SiC could lead to the development of efficient opto-electrochemical water splitting cells. Finally, 3C-SiC with its various intrinsic point defects has been is considered as a suitable candidate for future spintronic-applications. In the presented study we investigated free standing n-type and p-type 3C-SiC material grown in our lab. Temperature dependent photoluminescence measurements revealed the presence of carbon vacancy related V_C(+/++) and V_C C_Si (+/++) intrinsic point defects in the p-type material but not in the n-type materials. This observation is believed to have significant impact on the above mentioned optoelectronic applications.

Resume : The morphology of the SiC powder source material during sublimation growth of SiC crystals has a great impact on the stability of the growth process in terms of constant growth conditions as well on the ability to grow long boules. In this work we studied the influence of three different SiC powder size distributions and the sublimation behavior in 3 inch growth configuration. The evolution of the source material as well as of the crystal growth interface was carried out using in-site X-ray computed tomography. Two SiC powders exhibited a single modal D90 size distribution of ca. 50 µm and ca. 200 µm, respectively. In both cases the average SiC powder density was 1.2 g/cm3. The third powder was a mixture of the above mentioned source materials and exhibited a bimodal particle size distribution. The corresponding average SiC powder density was 1.7 g/cm3. In this latter case the in-situ X-ray computed tomography study revealed an improved growth interface stability that enabled a much longer crystal growth process. During process time, the sublimation-recrystallization behavior of the mixed SiC powder showed a much smoother morphology change and slower materials consumption as well as much more stable shape of the growth interface than in the case of the less dense SiC source. By adapting the size distribution of the SiC source material we achieved to significantly enhance stable growth conditions. The latter is believed to be an important step towards a better polytype stability as well as towards a more constant dopant incorporation during bulk growth of SiC.

Resume : Transparent conducting oxide (TCO) films were widely used in optoelectronic devices such as electromagnetic shielding, gas sensors, information displays, and photovoltaic cells . Zinc oxide (ZnO) has been actively investigated as an alternate material to replace ITO due to its wide direct band gap (3.3 eV), an exciton binding energy of 60 meV , and other desirable properties such as low cost, abundance and nontoxicity. A few reports suggest that Titanium (Ti) is a possible dopant for ZnO films because Ti is a quadrivalent cation and has a radius of 68 pm which is closing to that of Zn (74 pm). The quadrivalent cation may provide one more valence than the trivalent cation as it substitutes Zn in ZnO films. Titanium-doped zinc oxide (TZO) films were prepared on glass substrates by DC magnetron co-sputtering using two individual Zn and Ti metallic targets in a mixture of oxygen and argon gases. Evolutions of the structural, morphological, electrical and optical properties of the TZO thin films were investigated by X-ray diffraction, UV–visible spectrophotometer, scanning electron microscopy (SEM) and four-point probe. The results show that all the films demonstrated strong preferential orientation and good electrical and optical proprieties. The optical constants such as absorption coefficient (α) and the optical bandgap (Eg) were determined from the optical transmission spectra

Resume : Recently, transition metal Mn4 doped materials showing intense narrow red emission have attracted considerable interest due to their potential application as red-emitting phosphors in warm LEDs as cost-effective alternatives to rare-earth doped phosphors. Titanate compounds are known to be relatively cheap, easy-to-obtain and demonstrate excellent stability. Here we present the results of optical and structural investigations of lightly Mn doped magnesium titanate screen printed films and ceramics. The samples were produced through a traditional high temperature solid-state reaction method at 800-1150 °C using TiO2 and lightly Mn doped MgO powders as the raw materials. Electron spin resonance study showed Mn impurity to be about 1017 cm-3 in the raw MgO powder. The Raman spectra of the films showed that the formation of Mg2TiO4 with a cubic structure sets in at 900 °C. The evident red emission appeared in the films annealed at 1000 °C. In the photoluminescence (PL) spectra of the films, two sets of PL bands centered at 660 and 702 nm were observed. Corresponding excitation spectra consisted of two bands with maxima at 328 and 490 nm and at 335 and 542 nm. As the annealing temperature was increased from 1000 to 1150 °C, the intensities of the band at 660 nm in the PL spectra and the signals from Mg2TiO4 crystal phase in the Raman spectra increased. The intense red emission at 660 nm is ascribed to Mn4 in Mg2TiO4 phase, while the origin of 702 nm band is discussed.

Resume : The ultra-small triply doped oleic acid coated hexagonal LaF3:xCe3 ,xGd3 ,yEu3 (x = 5; y = 1, 5, 10 and 15 mol%) nanoparticles were synthesized via a low cost non-aqueous polyol method. The XRD and HRTEM analysis confirmed the size between 5-7 nm. The FTIR measurement indicated chemi-absorbed coating of oleic acid. The auto-existence of Eu2 ions due to reduction was confirmed via XAS and detected by photoluminescence spectra. It had influence in emission property of LaF3:xCe3 ,xGd3 ,yEu3 nanomaterials, showing blue-white-red colors. The Judd-Ofelt parameters (Ωλ) were calculated extensively by intensities of the 5D07FJtransitions (J = 2 and 4) of the Eu3 ion and the forced electric dipole and dynamic coupling mechanisms are considered simultaneously. The greatest efficacy of 85% was achieved for the sample x= 5, y= 10 mol%. The luminescence decay profile suggest cross relaxation process in Eu3 ions when molar concentration of Eu dopant increased from 10 to 15 mol%. A energy transfer mechanism involving circulation of energy over Gd3 ion sub-lattice as bridge and finally trapping by Eu2/3 has been proposed. The DC susceptibility of the displayed ferromagnetically coupled behavior at low temperature. The materials have potential to act as multiplex detector, white LED and scintillators through downshift process, the reason being Ce3 /Eu2 as gamma ray sensitizer, Gd3 as highest stable neutron captor and Eu2 /Eu3 as luminescence activator in the visible region.

Resume : III-Nitride compound semiconductors have attracted much interest and achieved tremendous progress in recent years. Conventional LEDs, LDs and other RF/power transistors are usually based on the six-fold symmetric hexagonal phase GaN (h-GaN). Moreover, h-GaN and related alloys possess high enough polarization fields (∼MV/cm) along the common growth direction <0001>, which are detrimental to transport properties, recombination efficiency and final device performance. However, these polarization fields don’t exist in cubic phase GaN (c-GaN). The biggest problem hindering the application of c-GaN is the phase instability, tending to transform into the more stable h-GaN. Thus, it is essential to achieve the stable existing c-GaN by some reliable growth methods [1]. Here, we proposed a novel nano-scale-patterned Si(100) substrate with inverted-pyramid-shaped holes, by which we can obtain the maximum coverage of cubic-phase GaN. A 2inch-Si(100) wafer coated with patterned SiO2 film was selectively wet-etched in KOH solution. The diameter of the inverted-pyramid-shaped holes is around 300 nm and the sidewalls with Si(111) crystal faces were exposed. The growth of cubic GaN was conducted in a Thomas Swan MOCVD system using TMGa, TMAl and NH3 as precursors. AlN buffer was adopted to protect the Si substrate from the Si-Ga meltback phenomenon. The cross-sectional SEM image of GaN grown in the inverted-pyramid-shaped hole is shown in Figure 1a. The flat top surface of GaN grain is observed. The red and blue rectangles show the regions investigated under Titan-CsP probe-corrected STEM. Corresponding high resolution TEM images are seen in Figure 1c and 1f, Figure 1d and 1g. GaN in marked regions are both proved to be cubic-phase through the FFT diffraction patterns in Figure 1b and 1d. The diffraction peak at about 40° of the XRD (002)-plane 2θ-ω curve in Figure 1h, and the CL peak at 387 nm in Figure 1i also indicate the existence of c-GaN.

Resume : Hydrogen sensor based on MgZnO thin film which was deposited by an RF co-sputtering is presented in the research. Both the MgO and ZnO targets are used to deposit the MgZnO film and its power toward is fixed at 100 and 125 W, respectively, with various times. The result shows a best sensing response of 2.46 for hydrogen concentration of 1000 ppm is obtained at the deposition time of 40 min with substrate heating at 300 C. The sensing response increase with the film thickness is due to the increase of oxygen vacancies which results from the defect of a thicker film. While, the sensing response decrease with further increase of thickness is due to grains growth. The x-ray diffraction analysis and the field emission scanning electron microscopy are also use to observe the film quality of the MgZnO thin film.

Resume : InGaN/GaN based visible light emitting diodes (LEDs) are commercially successful and play a dominant role in solid state lighting [1]. As a way to get white light emission for lighting applications, phosphor-based down-conversion results in low efficiency due to Stoke’s loss and also yields low colour rendering index (CRI) [2]. Hence, to obtain high efficiency and high CRI white light, generation of white light by RGB combination is necessary. InGaN/GaN based blue LED has achieved good efficiency [3] and III-phosphides based red LED has also got high efficiency [2]. However, at green emission wavelengths, the light emission efficiency of devices grown in the commonly used c-direction of GaN decreases with increasing indium (In) content in the active region [4]. Primarily, this is attributed to the increased piezoelectric field in the InGaN/GaN quantum wells (QWs) under biaxial compressive strain that eventually leads to local separation of electrons and holes within the QWs and consequently decreases the emission efficiency [4]. Even though growing stable semi-polar or non-polar planes [5] has received much attention to minimize the polarization problem, good crystal quality and processes that are mask-free and result in fewer processing steps make optimized structure design for c-direction growth still commercially promising. We have grown MQW structures by MOVPE in c(0001) direction on a ~70 nm thick high quality semi-bulk (SB) InGaN buffer (4 periods 15 nm InGaN and 2 nm GaN layers in between two successive InGaN layers) with InGaN barriers other than on conventional GaN buffer with GaN barriers and studied morphologically and optically in details. MQW with indium content of 5% - 7% in the buffer and barrier and 15% - 18% in the wells has yielded blue to cyan emission with a potential to extend to green emission by increased In. Our designed structure gives almost doubled optical emission along with a red-shift in emission wavelength compared to those of a conventional MQW structure. This improvement is due to the screening of built-in field by surface polarization field that eventually reduces the net built-in electric field and also due to more homogenous carrier distribution among the MQWs, when grown on semi-bulk InGaN buffer. These effects result in reduced band bending that increases the emission efficiency by more overlapping of electron and hole wave functions. Less droop is observed by simulation and a 67.5% internal quantum efficiency at an emission wavelength of ~480 nm has been experimentally obtained for MQWs structure with semi-bulk InGaN buffer and InGaN quantum barrier. Detail temperature and excitation power dependent photoluminescence (PL) experiments were carried out to explain the wavelength red-shift and band-filling scenario of localized carriers for our particular MQW in comparison with a conventional MQW. This detail analysis and study of photo excited luminescence is useful for realization of green-gap range emission by an InGaN/InGaN MQW using the semi-bulk InGaN buffer technique. [1] Daniel D. Koleske et al., Report of Sandia National Laboratories, Albuquerque, New Mexico 87185 and Livermore, California 94550, June 2013. [2] Auf Der Maur, M. et al., Physical Review Letters 116(2), 1–5 (2016). [3] Narukawa, Y. et al., Journal of Physics D: Applied Physics, 43(35), 354002 (2010). [4] Joachim Piprek, Physica Status Solidi (a), 207(10) 2217-2225 (2010). [5] T. Wunderer et al.,Physica Status Solidi (b), 248(3) 549-560 (2011).

Resume : Recently optoelectronic devices and electronic power devices using GaN-based materials have attracted much attention because they exhibit high optical and electrical power with high efficiency due to the wide band-gap. These power devices are generally used in high temperature operation by the heat dissipation. For high device reliability, it is important to design the mounting structure for heat sink to obtain efficient heat spreading. Carbon nanotube is expected to be excellent heat conductor for heat spreading because of extremely high thermal conductivity. Therefore, it is thought that carbon nanotube is very useful for electrode material with high thermal conductivity in the GaN device. Recently, we reported on the large workfunction of metallic multi-wall carbon nanotubes (MWCNTs). And we concluded that there is possibility to exhibit ohmic property to p-GaN. In this paper, we demonstrate low contact resistance of the MWCNTs ohmic contact to p-GaN film and LED operation with low voltage. We confirmed the work function of the metallic MWCNTs was determined to be 4.84 eV as high as that of Au, Pd and Ni which are generally used for the p-GaN contact. The specific contact resistance values of the MWCNTs/p-GaN interface were measured by the transfer length method. The MWCNTs electrode layer was prepared on the p-GaN. Finally, the specific contact resistance of the MWCNTs electrode was as low as 2.6×10-3 Ωcm2. We confirmed low contact resistance of the MWCNTs ohmic contact to p-GaN film. We also characterized the properties of LEDs using the MWCNTs ohmic contact for p-GaN. Low operation voltage was successfully obtained. Threshold voltage was about 2.7 V, and high optical power of about 1 W was also obtained.

Resume : The present work aims to investigate the control of self-assembled monolayers (SAM) of Thiol- Porphyrin on the electrical characteristics of GaN based Schottky barrier diode (SBD). It has been reported earlier that the adsorption of SAM of organic molecules onto the semiconductor side or the metal side of the metal-semiconductor interface leads to the tuning of either the semiconductor work function or the metal-gate work function and hence tuning the barrier height in the SBD. This phenomenon may assist in overcoming the problem of Fermi-level pinning on different semiconductors, without regard to actual distribution of gap states. In this work, SAM of Thiolated Porphyrin (TTPSH) organic molecules were sandwiched between Copper (Cu) metal and Gallium Nitride (GaN) semiconductor to tune the work function. The chemisorption of TTPSH SAM on GaN surface was confirmed by using Water contact angle measurements, X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). Kelvin Probe Force Microscopy (KPFM) revealed that the GaN surface potential was reduced from 950 mV to 750 mV after the adsorption of SAM on GaN. A decrease in the surface potential of semiconductor side of the metal-semiconductor interface implies decrease in work function of the semiconductor which may lead to an increase in Schottky barrier height. Cu metal was deposited on the molecularly modified GaN surface and was electrically characterized by current-voltage (I-V) measurements. A significant increase in Schottky barrier height and a decrease in reverse bias leakage current by four orders of magnitude was obtained. An increase in the photoluminescence (PL) intensity of GaN at 365 nm wavelength shows that surface passivation of GaN is occurring, which leads to the improvement of electrical characteristics of the diodes.

Resume : A selector based on a tandem SiC wide band gap optical filter with near-ultraviolet steady states optical switches to select the visible wavelengths is presented. Spectral response and transmittance measurements are performed and show the feasibility of tailoring the wavelength and bandwidth of a polychromatic mixture of different wavelengths. Frequency dependence is analyzed. The selector is realized by using a two terminal double pi’n/pin a-SiC:H photodetector. Five communication channels having different wavelengths and bit sequences are transmitted simultaneously. The combined optical signal is analysed by reading out the photocurrent, under near-UV front steady state background. Data show that 25 different levels in the photocurrent are detected and correspond to the transmission of the thirty-two on/off possible states. The proximity of the magnitude of consecutive levels causes occasional errors in the decoded information. To minimize the errors, four parity bit are generated and stored along with the data word. Results show that the background works as a selector in the visible range, shifting the sensor sensitivity and together with the parity check bits allows the identification and decoding of the different input channels. A transmission capability of 60 Kbps was achieved. The relationship between the optical inputs and the output signal is established and an algorithm to decode the MUX signal presented. An optoeletronic model gives insight on the system physics.

Resume : Aluminium nitride (AlN) is a staple piezoelectric material used in the MEMS industry. Of particular interest is the use of this material in piezoelectric energy harvesting devices. Here we report the impact of the inclusion of rare earth elements (scandium, yttrium and lanthanides) in AlN as substitutionnal alloys. The d33 coefficients of AlN and its alloys were obtained with Ab-initio density functional theory simulations with Abinit and a SQS supercell. The 4f electrons of the lanthanides were represented with a large core in order to facilitate the calculations. The obtained d33 coefficients of the alloys vary from 28 pm/V for scandium to 12-18 pm/V for the lanthanides. We discuss how the elements impact this coefficient and how the use of alloys could benefit the energy harvesting efficiency of devices. We conclude that scandium bearing alloys present the most gain in terms of the electromechanical coupling factor and point out other requirements, such as resistivity of the material, for efficient AlN alloy harvesters.

Resume : Nowadays, the World need to face increasing demand for electric energy. Solar cells could be the alternative source of energy. It is really important to invent material, which would allow us to produce cheap solar cells with high efficiency. In this paper, the one dimensional (1D) ZnS nanocrystals were synthesized for PV solar cells based on poly(3-hexylotiophene) P3H. ZnS nanowires were obtained using zinc stearate, thiourea and octadecylamine (ODA) as a stabilizer and solvent. The obtained material was characterized with the use of XRD, FT-IR, NMR and TEM analysis. The ZnS nanowires have wurzite structure with diameter about of 20 nm. The surface of obtained nanocrystals was modified with derivatives of aminobenzene and aminonaphatlene. The products were characterized with the previously mentioned methods. The original ZnS nanorods and the ones obtained as a result of ligand exchange were used to produce solar cells with P3HT as a conductive polimer. Solar cells with 1D ZnS nanocrystals have a efficiency about 0,5%. This work was financially supported by National Centre for Research and Development under Lider Program, contract no. LIDER/009/185/L-5/13/NCBR/2014.

Resume : CuGaS2 / Cd1-xZnxS based solar cell is proposed in this research work. The performance of a solar cell is simulated and analyzed on simulation software called SCAPS. The structure of a photovoltaic cell is based on Cr doped CuGaS2 semiconductor as the absorber layer and n-dopped Cadmium Zinc Sulphide (CdZnS) as the buffer layer. Analysis is perform on different parameters such as the thickness of the CuGaS2 absorber layer and its carrier density, working temperature, Cd1-xZnxS buffer layer thickness and its carrier density to analyze their effects on the cell performance. The simulations show that the optimized thickness for the absorbing layer and the buffer layer should be from 0.1µm to 5 µm and 0.01µm to 0.7µm, respectively. The optimal photovoltaic properties has been achieved with an efficiency of 28.83% with FF= 84.19%, VOC = 1.0528 V and JSC = 32.52 mA/cm2 when the thickness and carrier density of the CuGaS2 is 2.7µm and 1 ×〖10〗^15 〖cm〗^(-3), respectively, the thickness and the carrier density of the Cd1-xZnxS is 0.5µm and 1 ×〖10〗^16 〖cm〗^(-3), respectively. The above results will give some important guide for feasibly fabricating higher efficiency CuGaS2 solar cells.

Resume : Copper oxide (CuxO) thin films have many applications in heterojunction solar cells, electrochromic devices, oxygen and humidity sensors and attracted significant attention owing to their excellent properties including high absorption coefficient, non-toxic nature and low cost production. The aim of this work is to study the effect of the oxygen flow ratio on the electrical, optical, structural and morphological properties of copper oxide films deposited on Silicon and glass substrates using direct current (DC) reactive magnetron sputtering. The results showed that The crystalline orientation and film's properties were dependent on the oxygen flow ratio and annealing temperatures. A better crystallinity of cuprite and tenorite deposited at the oxygen flow ratio of 20 sccm and 35 sccm respectively was observed and confirmed by photoluminescence. In order to study the morphology of the grown structures a field emission scanning electron microscope was used. The phase structures of films were investigated by X-ray diffractometer. The electrical and optical properties were systematically investigated using Optical absorption UV–visible, four-point probe methodand photoluminescence (PL)

Resume : We have been investigating wide gap solar cells for the top of a tandem solar cell. Processing at low temperatures, which allows the use of glass substrates, is to be beneficial for the applications in terms of lowering the fabrication cost. We employed p-type Cu-Al-O (CAO) and n-type In-Ga-Zn-O (a-IGZO) as active layers. The pulse DC sputtering is performed on a glass substrate in 0-40% oxygen diluted argon atmosphere at temperatures as low as 250°C. The devices were annealed at less than 400°C in air for one hour. The optical band gap of a-CAO and a-IGZO is ~1.95 eV and ~3.21 eV, respectively. ITO/a-CAO/a-IGZO/ITO/substrate solar cells were fabricated and inverted structure solar cells (ITO/a-IGZO/a-CAO/ITO/superstrate) were fabricated for comparison. The light was illuminated from the p-layer side. I-V characteristics and conversion efficiency were quite different between substrate and superstrate cell. The maximum efficiency was around 0.87% under the AM1.5 illumination for substrate type and that of the superstrate was about 0.19%, which was mainly caused by the interface defects. According to a device simulation, a large amount of interface defects in the superstrate cell still remained even after annealing at 380°C. From the modulated admittance method, the main defects located at above 100-150 meV from the a-CAO valence band edge. Although the poor performance of the solar cell, we have demonstrated the possibility of all sputtered top cell.

Resume : The alloying of ZnO with GaN can substantially reduce the band gap, allowing the material to absorb in the visible light region. In this work we further investigate ZnO-GaN alloys in order to better understand the optical properties to allow an optimization of the material for solar energy usage. Our theoretical analyses are based on the density functional theory (DFT), and we improve the energy gaps by means of the meta-GGA modified Becke-Johnson exchange potential and also the post-DFT approach of HSE hybrid functionals [1]. Pair distribution over the several used cells and concentration were analyzed. A correlation of the ratio of Ga-N (or Zn-O) bonds with the total energy was found, indicating that bonds favoring the proper valence match provide more stability. The dielectric function and absorption were analyzed. In the alloy, the onset for the absorption of light with respect to the band gap presents a change when in comparison to the binaries, leading to an optical band gap higher than the one obtained from the band structure. This can help to explain the differences found in different experimental methods [2]. References: [1] M. Dou et al., J. Crystal Growth 350(2012). [2] K. Lee et al, J. Mater. Chem. A 4,2927 (2016).