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Showing papers in "Physica Status Solidi (a) in 2010"


Journal ArticleDOI
TL;DR: In this paper, the authors provide a snapshot of the current state of droop research, reviews currently discussed droop mechanisms, contextualizes them, and proposes a simple yet unified model for the LED efficiency droop.
Abstract: Nitride-based light-emitting diodes (LEDs) suffer from a reduction (droop) of the internal quantum efficiency with increasing injection current. This droop phenomenon is currently the subject of intense research worldwide, as it delays general lighting applications of GaN-based LEDs. Several explanations of the efficiency droop have been proposed in recent years, but none is widely accepted. This feature article provides a snapshot of the present state of droop research, reviews currently discussed droop mechanisms, contextualizes them, and proposes a simple yet unified model for the LED efficiency droop. Illustration of LED efficiency droop (details in Fig. 13).

778 citations


Journal ArticleDOI
TL;DR: In this paper, the development and properties of titanium dioxide (TiO 2 )-based transparent conducting oxides (TCO), which exhibit properties comparable to those of In 2-x Sn x O 3 (ITO), are reviewed.
Abstract: The development and properties of titanium dioxide (TiO 2 )-based transparent conducting oxides (TCO), which exhibit properties comparable to those of In 2-x Sn x O 3 (ITO), are reviewed in this article. An epitaxial thin film of anatase Ti 0.94 Nb 0.06 O 2 exhibited a resistivity (p) of 2.3 x 10 -4 Ω cm and internal transmittance of ~95% in the visible light region. Furthermore, we prepared polycrystalline films with p of 6.4 x 10 -4 Ω cm at room temperature on glass substrates by using sputtering. We focus on characteristics unique to TiO 2 -based TCO, such as a high refractive index, high transmittance in infrared, and high stability in reducing atmospheres. Possible applications of TiO 2 -based TCOs, as well as the mechanism of the transparent conducting properties found in this d-electron-based TCO, are discussed in this review.

167 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of copper salt and thiourea concentrations on the formation of Cu 2 ZnSnS 4 thin films is investigated, which are found to be polycrystalline in nature with kesterite structure.
Abstract: Thin films of Cu 2 ZnSnS 4 (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been deposited by spray pyrolysis technique onto soda-lime glass substrates held at a substrate temperature (T s ) of 643 K. The effect of copper salt and thiourea concentrations on the formation of Cu 2 ZnSnS 4 thin films is investigated. CZTS films formed under optimized conditions are found to be polycrystalline in nature with kesterite structure. The lattice parameters are found to be a = 0.543 nm and c = 1.086 nm. The optical band gap of these films is found to be 1.43 eV. It is found to increase with decrease in copper salt concentration in the solution.

149 citations


Journal ArticleDOI
TL;DR: In this paper, the authors provided atomic and electronic structures, formation energies of defects and bond energies in a-IGZO calculated by first-principles density functional theory (DFT), and confirmed that oxygen deficiencies having small formation energies (2-3.6 eV) form either deep fully occupied localized states near the valence band maximum or donor states, which depend on their local structures.
Abstract: Amorphous In-Ga-Zn-O (a-IGZO) is expected for channel layers in thin-film transistors (TFTs). It is known that a-IGZO is sensitive to an O/H-containing atmosphere; therefore, it is important to clarify the roles of oxygen and hydrogen in a-IGZO. This paper provides atomic and electronic structures, formation energies of defects and bond energies in a-IGZO calculated by first-principles density functional theory (DFT). It was confirmed that oxygen deficiencies having small formation energies (2-3.6 eV) form either deep fully-occupied localized states near the valence band maximum or donor states, which depend on their local structures. All the hydrogen doping form -OH bond and work as a donor. The stable -OH bonds have small formation energy of ~0.45 eV and consist of three metal cations coordinated to the O ion. The bond energy of Ga-O is calculated to be ~2.0 eV, which is the largest among the chemical bonds in a-IGZO (1.7 eV for In-O and 1.5 eV for ZnO). This result supports the idea that the incorporation of Ga stabilizes a-IGZO TFTs.

139 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that MESFETs exhibit high performance and low voltage operation of oxide electronics, compared to results reported for TSO MISFET-based circuits.
Abstract: Transparent conductive oxides (TCOs) are a well-known material class allowing Ohmic conduction. A large free carrier concentration in the 10 21 cm -3 range and high conductivity (beyond 10 4 S/cm) is feasible simultaneously with high transparency. Applications are manifold and include touch screens and front contacts for displays or solar cells. Transparent semiconducting oxides (TSO) are oxides with an intermediate free carrier concentration (typically 10 14 -10 18 cm -3 ) allowing the formation of depletion layers. We review recent results on TSO-based transistors and inverters. Most work has been reported on MISFETs. We show that MESFETs exhibit high performance and low voltage operation of oxide electronics. MESFET-based inverters offer superior performance compared to results reported for TSO MISFET-based circuits. Optical image of inverter based on thin film MESFETs with Mg 0.003 Zn 0.997 O channels (left) and experimental inverter characteristic for supply voltage of V DD = +2.0 V (right).

137 citations


Journal ArticleDOI
TL;DR: In this article, the direct elaboration of graphene by molecular beam epitaxy (MBE) has been studied, using a solid carbon source. Successful growth has been achieved on both the carbon and silicon terminated faces of silicon carbide substrates in the temperature range 1000-1100 °C, as confirmed by low energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS) analysis.
Abstract: The direct elaboration of graphene by molecular beam epitaxy (MBE) has been studied, using a solid carbon source. Successful growth has been achieved on both the carbon and silicon terminated faces of silicon carbide substrates in the temperature range 1000-1100 °C, as confirmed by low energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS) analysis. Atomic force microscopy (AFM) observations show that the initial substrate structure, i.e. flat atomic terraces and half-period high steps, remains almost unaffected during the growth, contrary to what is observed following the graphitization process.

94 citations


Journal ArticleDOI
TL;DR: In this paper, the photoresponse and characteristics of the ZnO nanowire-device demonstrates that focused ion beam process offers a way to fabricate novel nanodevices on a single ZnOs with diameters as small as 100 nm.
Abstract: ZnO nanowires were synthesized by the CVD procedure and have been investigated by SEM, TEM, SAED, Raman, and cw PL spectroscopy. The fabrication of an ultraviolet (UV) photoconductive detector based on single ZnO nanowire (100 nm in diameter) is presented. This nanostructure detector is prepared in an Focused Ion Beam (FIB) set-up by using nanodeposition for metal electrodes. The photoresponse of the UV sensor are studied using a UV source with an incident peak wavelength of 365 nm. It was demonstrated that the output signal of the sensors is reproducible under UV irradiation. The photoresponse and characteristics of the ZnO nanowire-device demonstrates that focused ion beam process offers a way to fabricate novel nanodevices on a single ZnO nanowire with diameters as small as 100 nm. The presented single ZnO nanowire sensor proves to be promising for application in various processes.

90 citations


Journal ArticleDOI
TL;DR: In this article, the authors study recombination at the amorphous/crystalline Si (a-Si:H/c-Si) heterointerface, the amphoteric nature of silicon dangling bonds is taken into account.
Abstract: To study recombination at the amorphous/crystalline Si (a- Si:H/c-Si) heterointerface, the amphoteric nature of silicon (Si) dangling bonds is taken into account. Modeling interface recombination measured on various test structures provides insight into the microscopic passivation mechanisms, yielding an excellent interface defect density reduction by intrinsic a-Si:H and tunable field-effect passivation by doped layers. The potential of this model's applicability to recombination at other Si heterointerfaces is demonstrated. Solar cell properties of a-Si:H/c-Si heterojunctions are in good accordance with the microscopic interface properties revealed by modeling, that are, e.g., slight asymmetries in the neutral capture cross-sections and band offsets. The importance of atomically abrupt interfaces and the difficulties to obtain them on pyramidally textured c-Si is studied in combination with transmission electron microscopy.

87 citations


Journal ArticleDOI
TL;DR: In this paper, a worldwide collaboration has been launched to produce ∼5 kg of 28 Si single crystal with an enrichment factor greater than 0.99985 and of sufficient chemical purity so that it can be used to determine the Avogadro constant with a targeted relative measurement uncertainty.
Abstract: A metrological challenge is currently underway to replace the present definition of the kilogram by the mass of a certain number of silicon atoms. A prerequisite for this is that the Avogadro constant, N A , which defines the number of atoms in a mole, is determined with a relative uncertainty of better than 2 x 10- 8 . Silicon crystals are used for this determination, the difficulty arising thereby is the measurement of the average molar mass of natural Si. Consequently, a worldwide collaboration has been launched to produce ∼5 kg of 28 Si single crystal with an enrichment factor greater than 0.99985 and of sufficient chemical purity so that it can be used to determine N A with the targeted relative measurement uncertainty mentioned above. In the following, the development and first successful tests of all technological steps are reported, and the new equipment for the production of high-purity 28 Si with an enrichment of not less than 0.9999 is described. All steps are defined by a Technical Road Map (TRM28) mandatory for all partners, and all key results are measured by calibrated and certified means, e.g. the C content of the final material is less than 10 15 atoms/cm 3 and the specific resistance is in the range from 400 to 1000 Ωcm. New applications based on this highly enriched and purified 28 Si, and on 29 Si and 30 Si monocrystals produced in parallel, are reported briefly in the fields of solid state spectroscopy, spintronics, quantum computing, cooling of highly loaded SYS optics, superlattice structure (SLS), terahertz laser.

83 citations


Journal ArticleDOI
TL;DR: In this paper, the role of Ga in solution-processed InGaZnO thin-film transistors was investigated, and it was verified that the vacancy-related oxygen Is peak was decreased when the Ga content increased.
Abstract: We investigated the role of Ga in solution-processed InGaZnO thin film transistors (TFTs). The incorporation of Ga into a InZnO compound system results in a decrease in the carrier concentration of the films and an off-current of TFTs. This is a result of the Ga ions forming stronger chemical bonds with oxygen, as compared to the Zn and In ions, acting as a carried suppressor. It was verified, using X-ray photoelectron spectroscopy (XPS), that the vacancy-related oxygen Is peak was decreased when the Ga content increased.

81 citations


Journal ArticleDOI
TL;DR: In this article, the as-grown tin monosulphide (SnS) films were characterized using appropriate techniques and the obtained results are discussed by comparing them with the properties of SnS films grown on amorphous substrate under the same conditions.
Abstract: The development of high-quality tin monosulphide (SnS) layers is one of the crucial tasks in the fabrication of efficient SnS-based optoelectronic devices. Reduction of strain between film and the substrate by using an appropriate lattice-matched (LM) substrate is a new attempt for the growth of high-quality layers. In this view, the SnS films were deposited on LM Al substrate using the thermal evaporation technique with a low rate of evaporation. The as-grown SnS films were characterized using appropriate techniques and the obtained results are discussed by comparing them with the properties of SnS films grown on amorphous substrate under the same conditions. From structural analysis of the films, it is noticed that the SnS films deposited on amorphous substrate have crystallites that were oriented along different directions. However, the SnS crystallites grown on Al substrate exhibited epitaxial growth along the 101] direction. Photoluminescence (PL) and Raman studies reveal that the films grown on Al substrate have better optical properties than those of the films grown on amorphous substrates. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Journal ArticleDOI
TL;DR: In this paper, a 2methoxyethanol solution of gallium isopropoxide and indium polyoxide stabilized by monoethanolamine on (0001) sapphire substrates was used to construct (Ga 1-x In x In x ) 2 O 3 thin films.
Abstract: We prepared (Ga 1-x In x ) 2 O 3 thin films at 900 °C using a sol-gel method, from a 2-methoxyethanol solution of gallium isopropoxide and indium isopropoxide stabilized by monoethanolamine on (0001) sapphire substrates. X-ray diffraction showed that films prepared from solutions with an indium content below 0.4 are single phase with the same monoclinic structure as β-Ga 2 O 3 and that increasing the indium content increases the lattice constants. Optical absorption showed that the band gap decreased linearly to 4.2 eV as indium content increased to 0.3. Planar geometry photoconductive detectors have been fabricated on the (Ga 1-x In x ) 2 O 3 thin films prepared from the solution with an indium content below 0.2. They showed photosensitivity in the solar-blind region, and the long wavelength threshold of spectral response shifted toward longer wavelengths with an increasing indium content. Furthermore, heterojunction structures composed of Ga 2 O 3 and (Ga 1 -x In x ) 2 O 3 were also fabricated and characterized by transmission electron microscopy and optical absorption.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the origins of InN conductivity, considering point defects, impurities, and surface states, and found that the accumulation layers are an intrinsic property of the material that can be attributed to the fact that on polar surfaces occupied surface states are located above the conduction-band minimum (CBM).
Abstract: InN has emerged as a highly promising material for a number of technological applications, but progress is still hampered by lack of control over its conductivity. The material exhibits a high tendency for unintentional n-type conductivity, both in the bulk and on the surface. We discuss the origins of this conductivity, considering point defects, impurities, and surface states. First-principles calculations based on density functional theory, with suitable band-gap corrections, have been fruitfully applied to investigate doping issues. The results indicate that native point defects are unlikely to be the source of bulk conductivity and that attention should be focused on unintentional incorporation of impurities. In particular, hydrogen is a prime candidate for shallow donors in InN. Both interstitial and substitutional hydrogen have high solubility and give rise to n-type conductivity. Substitutional hydrogen consists of H sitting on a nitrogen site, bonding equally to the four In nearest neighbors in a multicenter-bond configuration (a highly unusual type of chemical bond). Substitutional hydrogen, somewhat counterintuitively, is a double donor. With regard to p-type doping, Mg is a promising acceptor in InN. Its ionization energy is ∼0.2 eV and it has lower formation energy in InN than in GaN. The problems encountered in p-type doping of InN are therefore not due to the properties of the acceptor, but to compensation by native defects or unintentional impurities acting as donors. In addition to the bulk conductivity, accumulation of electrons has been almost universally observed on InN surfaces. While donor impurities adsorbed on the surface could of course contribute to this conductivity, we have proposed that the accumulation layers are an intrinsic property of the material that can be attributed to the fact that on polar surfaces occupied surface states are located above the conduction-band minimum (CBM). Fermi-level pinning occurs due to occupied surface states above the CBM, for all In/N ratios, thus explaining the observed electron accumulation. Interestingly, we have found an absence of electron accumulation on nonpolar surfaces of InN at moderate In/N ratios, in agreement with experimental observations on cleaved surfaces. Consequences for growth as well as for devices will be discussed.

Journal ArticleDOI
TL;DR: In this paper, density functional theory is used to show that electronegative molecules like tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and electropositive metals like K dope graphene p-and n-type, respectively.
Abstract: We distinguish three mechanisms of doping graphene. Density functional theory is used to show that electronegative molecules like tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and electropositive metals like K dope graphene p- and n- type, respectively. These dopants are expected to lead to a decrease in carrier mobility arising from Coulomb scattering but without any hysteresis effects. Secondly, a novel doping mechanism is exhibited by Au which dopes bilayer graphene but not single layer. Thirdly, electrochemical doping is effected by redox reactions and can result in p-doping by humid atmospheres and n-doping by NH 3 and toluene.

Journal ArticleDOI
TL;DR: In this paper, the chemical and electrical properties of ZnO thin film transistors were fabricated and their electrical properties were investigated after the oxygen remote plasma treatment, and they found that the I on/Ioff ratio increased from 73 x 10 4 to 86 × 10 6, the subthreshold swings improved from 167 to 045 V/decade, and the saturation mobility (μ sat ) decreased from 163 to 072 cm 2 /V s as plasma exposure times were increased.
Abstract: We deposited ZnO thin films by atomic layer deposition (ALD) and then investigated the chemical and electrical characteristics after plasma treatment The chemical bonding states were examined by X-ray photoelectron spectroscopy (XPS) The XPS spectra of O 1s showed that the intensity of oxygen deficient regions of the ZnO film decreased from 276 to 194%, while the intensity of the oxygen bound on the surface of the ZnO film increased from 150 to 219% as plasma exposure times increased The ZnO film exhibited a decrease in carrier concentration from 49 × 10 15 to 1,2 × 10 14 cm -3 and an increase in resistivity from 12 × 10 2 to 98 × 10 3 Ω cm as the plasma exposure times increased To verify the changes in the chemical and electrical properties of the ZnO films caused by the oxygen remote plasma treatment, ZnO thin film transistors were fabricated and their electrical properties were investigated We found that the I on/Ioff ratio increased from 73 x 10 4 to 86 × 10 6 , the subthreshold swings improved from 167 to 045 V/decade, and the saturation mobility (μ sat ) decreased from 163 to 072 cm 2 /V s as plasma exposure times were increased

Journal ArticleDOI
TL;DR: In this article, the authors presented modulated surface textures on glass substrates for improved light trapping in thin-film silicon solar cells, where the surface morphology, the roughness spectral distribution, and the scattering properties of the modulated surfaces-textured substrates were characterized.
Abstract: We present modulated surface textures on glass substrates for improved light trapping in thin-film silicon solar cells. The surface morphology, the roughness spectral distribution, and the scattering properties of the modulated surface-textured substrates were characterized. We deposited solar cells on the modulated surface-textured substrates and observed enhancement in the performance compared to a solar cell deposited on textured ZnO:Al (reference) substrates.

Journal ArticleDOI
TL;DR: In this article, the electrophoretic deposition of single-walled carbon nanotube hybrid films onto flexible, transparent and conducting indium tin-oxide-coated polyethylerie terephthalate substrates is described.
Abstract: We report the electrophoretic deposition of graphene/single-walled carbon nanotube hybrid films onto flexible, transparent and conducting indium tin-oxide-coated polyethylerie terephthalate substrates. This room-temperature solution process is completely compatible with polymeric substrates and does not require a sophisticated transfer process, allowing uniform and controllable deposition of such carbon thin films with tailored optical and electrical properties over large areas. In particular, the film thickness, optical transparency and electrical conductivity can be modified by the deposition time and the obtained films span several square centimetres in area.

Journal ArticleDOI
TL;DR: In this paper, two hundred-nano metre thick ZnO films were deposited on glass substrates at low growth temperatures varied between 120 and 240 °C, and the results showed that the high electrical conductivity was achieved by playing with the sample stoichiometry and growth conditions.
Abstract: Atomic layer deposition (ALD) was used to fabricate transparent and conductive thin films of ZnO. Two hundred-nano metre thick ZnO films were deposited on glass substrates at low growth temperatures varied between 120 and 240 °C. As zinc and oxygen precursors we used diethylzinc (DEZn) and deionized water, respectively. To find optimal film parameters, the structure, surface morphology, optical and electrical measurements were carried on. The films obtained at 200 °C show the highest carrier concentration (~10 20 cm -3 ) and the lowest resistivity (2 x 10 -3 Ω cm). The films exhibit mobilities up to 37 cm 2 /Vs that we associate to the process technology used. An important point of our approach was that the films studied were not intentionally doped (with Al or other group III elements) but the high electrical conductivity was achieved by playing with the sample stoichiometry and growth conditions.

Journal ArticleDOI
TL;DR: In this article, a-Si:H was used as a low bandgap absorber material to substitute for aSiGe:H alloys in multi-junction solar cells.
Abstract: This paper reviews our progress of using nc-Si:H as a low bandgap absorber material to substitute for a-SiGe:H alloys in multi-junction solar cells. We have focused on three topics: (1) high deposition rate, (2) large area uniformity of thickness and material properties, (3) high solar cell and module efficiencies. Initially, we investigated various deposition methods, including RF, VHF, and microwave glow discharges. After several years of systematic studies, we have been convinced that VHF glow discharge is an applicable method to attain high rate and large-area uniform nc-Si:H depositions. We also studied the stability of nc-Si:H solar cells and observed various metastability phenomena in nc-Si:H solar cells. We have reported an initial active-area cell efficiency of 15.4% using an a-Si:H/a-SiGe:H/ nc-Si:H triple-junction structure. Subsequently, we have increased the deposition rates to around 1.0-1.5 nm/s and achieved an initial active-area efficiency of 13.4% using an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure. Recently, a stable total-area efficiency of 12.5% was measured at NREL on our a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cell. We have also developed large-area VHF deposition systems and demonstrated encouraging module efficiencies using a-Si:H/ nc-Si:H/nc-Si:H triple-junction structures. Initial and stable aperture-area (400 cm 2 ) efficiencies of 11.0 and 10.1 % have been achieved with fully encapsulated modules with a-Si:H/nc-Si:H/nc-Si:H triple-junction structures. In this paper, various aspects of nc-Si:H solar cells are discussed, including the material structures, device design, light trapping, metastability, high efficiency solar cell optimization, and large-area deposition.

Journal ArticleDOI
TL;DR: Bongaers et al. as discussed by the authors proposed a method to improve the efficiency of the IMEC-based IMEMEV system, which is the state-of-the-art in the IMEvi system.
Abstract: [Bongaers, E.; Alenus, J.; Horemans, F.; Weustenraed, A.; Vanderzande, D.; Cleij, T. J.; Wagner, P.] Hasselt Univ, Inst Mat Res, B-3590 Diepenbeek, Belgium. [Lutsen, L.; Vanderzande, D.; Wagner, P.] IMEC, Div IMOMEC, B-3590 Diepenbeek, Belgium. [Troost, F. J.; Brummer, R. -J.] Maastricht Univ, NUTRIM, Dept Internal Med, NL-6229 ER Maastricht, Netherlands. Evi.Bongaers@uhasselt.be; Patrick.Wagner@uhasselt.be

Journal ArticleDOI
TL;DR: In this article, the effect of core-shell structure on dielectric permittivity in the Ag@TiO 2 /PVDF nanocomposites was studied using TEM images.
Abstract: The Ag@TiO 2 core@shell nanoparticles filled ferroelectric poly(vinylidene fluoride) (PVDF)-based composites (0-3 connectivity) were prepared in order to study the effect of core@shell structure on dielectric permittivity in the Ag@TiO 2 /PVDF nanocomposites. The Ag@TiO 2 core@shell nanoparticles were synthesized via a water-thermal meting route. Transmission electron microscope (TEM) images showed the nanoparticles were about 60-120nm in diameter and the thickness of the shell layer was about 6-12 nm, which can be adjusted by changing the fabrication conditions. Results of dielectric measurements showed that the shell-layer thickness of the Ag@TiO 2 core@shell nanoparticles played a key role in deciding the final values of the dielectric properties of the nanocomposites. The variation of dielectric properties of the nanocomposites with temperature also depends on the shell-layer thickness of the Ag@TiO 2 core@shell nanoparticles.

Journal ArticleDOI
TL;DR: In this paper, a lead-free perovskite Ba(Fe 1/2Nb1/2)O3 (BFN) was prepared by conventional ceramic fabrication technique at 1200 °C/5 h in air atmosphere.
Abstract: Lead-free perovskite Ba(Fe1/2Nb1/2)O3 (BFN) was prepared by conventional ceramic fabrication technique at 1200 °C/5 h in air atmosphere. The crystal symmetry, space group, and unit cell dimensions were determined from the experimental results using FullProf software whereas crystallite size and lattice strain were estimated from Williamson–Hall approach. X-ray diffraction (XRD) analysis of the compound indicated the formation of a single-phase monoclinic structure with the space group P2/m. EDAX and SEM studies were carried out in order to evaluate the quality and purity of the compound. To find a correlation between the response of the real system and idealized model circuit composed of discrete electrical components, the model fittings were presented using the impedance data. Complex impedance analyses suggested the dielectric relaxation to be of non-Debye type. The correlated barrier hopping (CBH) model was employed to successfully explain the mechanism of charge transport in BFN.

Journal ArticleDOI
TL;DR: In this article, an improved gallium nitride (GaN) nanowire (NW) based light emitting diodes (LEDs) were studied and improved with and without InGaN/GaN multiple quantum wells.
Abstract: We studied and improved gallium nitride (GaN) nanowire (NW) based light emitting diodes (LEDs). PIN nanodiodes with and without InGaN/GaN multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) under N-rich conditions on n-doped Si(111) substrates. Thanks to the coalescence of the p-type region of the NWs grown at low temperature, an autoplanarization process has been performed to obtain LEDs. Ni/Au top contacts have been deposited and patterned in order to bias the devices. A multiple-scale characterization approach has been carried out through the comparison of localized cathodoluminescence (CL) and macroscopic electroluminescence (EL) spectra. It shows that the EL emission of PIN-based LED at room temperature is related to defects in the p-type region of the NWs. In order to enhance the radiative recombinations of NW-based LEDs, we have first added InGaN/GaN MQWs, and secondly an electron blocking layer (EBL) has been inserted between the MQWs and the p-type zone of the NWs. The LED with EBL exhibited an emission band at 420 nm. The blue-shift of this emission band with increasing injected current is attributed to quantum confined Stark effect (QCSE) and evidences the radiative emission of InGaN/GaN MQWs. At 50 mA dc current, this improved NW-based LED emits about 500 times more light than the heterostructure without EBL. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal ArticleDOI
TL;DR: In this article, the phase purity of Ni0.3Fe2O4 was investigated by X-ray diffraction (XRD), which revealed the formation of single-phase cubic spinel structure.
Abstract: The ferrite samples of Ni0.7−xMnxZn0.3Fe2O4 (where x = 0.0–0.7 in steps of x = 0.1) were synthesized by a sol–gel autocombustion method using nitrates of respective metal ions. The synthesized samples were annealed at 600 °C for 4 h. The phase purity of the samples was investigated by X-ray diffraction (XRD). An analysis of XRD patterns reveals the formation of single-phase cubic spinel structure. The crystal lattice constant increases gradually with increasing x from 8.389 to 8.473 A. The cation distribution of constituent ions shows linear dependence of Mn substitution. Based on the cation distribution obtained from XRD data, structural parameters such as lattice parameters, ionic radii of available sites, and the oxygen parameter “u” have been calculated. The trend of theoretically calculated lattice parameter with Mn content matches well with the experimentally obtained values. An initial increase followed by a subsequent decrease of saturation magnetization with increase in x is observed. Possible explanations for the observed structural and magnetic behavior with various Mn content are discussed.

Journal ArticleDOI
TL;DR: In this article, the authors show that low lateral growth rate is a limiting parameter for efficient double-step growth process of indium nitride and also report that the use of CBrCl3 during InN growth enhances the lateral growth.
Abstract: Since a few years, indium nitride promising properties for device applications have attracted much attention worldwide. Huge efforts are dedicated to optimize indium nitride growth. However, this growth is extremely challenging, in particular using the metal organic vapor phase epitaxy (MOVPE) technique which exhibits very low growth rates. This may explain why most of the samples available in the scientific community, which also present the best electrical properties, were grown by molecular beam epitaxy (MBE). However, up to date, no intrinsic indium nitride layer was obtained. InN epilayer crystalline quality suffers from the lack of lattice matched substrates, leading to the use of double-step growth process. In this paper, we show that InN low lateral growth rate is a limiting parameter for efficient double-step growth process. But, we also report that the use of CBrCl3 during InN growth enhances the lateral growth rate. To improve the growth rate along the c-axis, we investigate alternative precursors for both nitrogen and indium species. We show that ammonia remains the best precursor, but combined with triethylindium, the growth rate increases with optimum crystalline quality. Finally, we compare MOVPE-grown and MBE-grown InN layers in order to understand the difference observed on the electrical properties. We show how thermal annealing can improve the MOVPE-grown InN layers leading to similar electrical properties than reported in MBE-grown samples. The role of ammonia as source of hydrogen is also discussed.

Journal ArticleDOI
TL;DR: In this article, the problem of parasitic modes emerges due to the reduced refractive index difference between the GaN waveguide and AlGaN cladding layers for green compared to blue emitting laser diodes.
Abstract: In this paper we investigate the waveguiding (WG) of direct green InGaN laser diodes grown on c-plane GaN substrates. The problem of parasitic modes emerges due to the reduced refractive index difference between the GaN waveguide and AlGaN cladding layers for green compared to blue emitting laser diodes. We discuss several approaches to avoid substrate modes. We investigate different materials and designs for optimized WG of green InGaN laser diodes using a 1D transfer matrix simulation tool.

Journal ArticleDOI
TL;DR: In this article, the effect of adding excess silver to the melt on the microstructure and thermoelectric properties of Mg 2 Sn was investigated in the temperature range 10-700 K.
Abstract: The electronic and thermal transport properties of Mg 2 Sn crystals prepared by radio-frequency (RF) induction melting, were investigated in the temperature range 10-700 K to study the effect of adding excess silver to the melt on the microstructure and thermoelectric properties. As the Ag-content increases the amount of silver incorporated at doping site tends to saturate so that the carrier concentration increases asymptotically to 6.3 x 10 19 cm -3 . The excess amount of silver reacts with magnesium to form a fine MgAg + Mg 2 Sn eutectic structure. The temperature dependence of the carrier mobility varies from ~T -1.5 for the heavily doped samples in the extrinsic range to ~T -2.8 for the undoped sample in the intrinsic range. From electrical conductivity and Hall measurements on the undoped samples we find the energy gap to be 0.36-0.38 eV. Doped samples show broad Seebeck peaks of 150-210 μVK -1 at T= 350-550 K. The thermal conductivity decreases sharply as the addition of Ag is increased, and reaches a minimum value of κ = 3.4 W m -1 K -1 at 450 - 500 K for the doped sample containing a uniformly distributed eutectic phase, leading to a maximum figure of merit ZT max = 0.30.

Journal ArticleDOI
TL;DR: In this paper, the p-type doping of InN films using Mg-acceptors by MBE, the demonstration of successful P-type control and conduction, and their electrical and optical properties as well as epitaxy behaviors of InNs on GaN and effects of threading dislocations on high-purity undoped InN epilayers grown on the GaN template are also discussed.
Abstract: InN is still the least studied material among III-nitrides and there are several problems to be overcome for better understanding of its material properties and also its material control for device application. In particular, successful p-type doping is a quite important issue but quite difficult in InN. In this article, we present a study on the latest advances in material control paying special attention to the p-type doping of InN films using Mg-acceptors by MBE, the demonstration of successful p-type control and p-type conduction, and their electrical and optical properties as well. Furthermore, as a prerequisite of successful p-type doping of InN, epitaxy behaviors of InN on GaN and effects of threading dislocations on high-purity undoped InN epilayers grown on GaN template are also discussed.


Journal ArticleDOI
Abstract: Stabilized gold nanoparticle (AuNP) colloids have been fabricated by nanosecond pulsed laser ablation of a pure gold plate in cyclohexanone. The AuNPs colloid exhibits a UV-Vis absorption spectrum with a surface plasmon absorption peak at about 540 nm. Scanning electron microscopy has shown the formation of spherical AuNPs with average size about 53 nm. The shift of 24 cm -1 is observed in the carbonyl band of the colloid using FTIR spectroscopy. This shift indicates that the monomer carbonyl group of cyclohexanone interacts with the surface of the AuNPs and leads to stabilizing the colloid. A large nonlinear refractive index of -2.92 x 10 -7 cm 2 /W is measured using the Z-scan technique under continuous wave laser irradiation at 532 nm. Our results show that the large induced nonlinear refraction is attributed to the surface plasmon resonance (SPR) enhancement effect of AuNPs, high thermo-optic coefficient and low thermal conductivity of cyclohexanone. Observation of far-field diffraction ring patterns confirm a thermally induced negative lens effect and spatial self-phase modulation in the laser beam as it traverses the colloids.