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Semiconductor Interfaces (semiconductor + interface)
Selected AbstractsReflectance and photoluminescence studies of InGaN/GaN multiple-quantum-well structures embedded in an asymmetric microcavityPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2006D. Y. Lin Abstract Using reflectance (R) and photoluminescence (PL) measurements InGaN/GaN multiple-quantum-well (MQW) structures embedded in an asymmetric microcavity with different thickness of stacking pairs have been studied. The asymmetric microcavity structures are composed of a cavity sandwitched between the air/semiconductor interface and a mirror using distributed Bragg reflector (DBR). For the DBR with thinner AlN layers the high-reflectivity stop band locates at higher photon energy. The luminescence efficiency and the spectrum of InGaN/GaN multiple-quantum-well structures will be modified by the microcavity. A comparison of PL with R spectra shows that the emission efficiency can be enhanced by matching up the luminescence spectrum coming from the MQW and the high-reflectivity stop band. From the blue shift of the cavity modes as a function of incident angles the refractive index and cavity length can be determined. By measuring the PL spectra as a function of emission angle, it is found that the PL spectra were predominatly determined by microcavity resonances. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spontaneous and stimulated emission in InAs LEDs with cavity formed by gold anode and semiconductor/Air interfacePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2005B. Matveev Abstract The paper presents results on spectral and power measurements in InAsSbP/InAs double heterostructure flip-chip LEDs with cavity formed by bottom anode mirror and air/semiconductor interface in the temperature range of 77,573 K. Data on near and far field patterns in the 3 µm range together with the threshold characteristincs of the L-I curves are discussed with respect to resonant cavity effects at 77,573 K and stimulated emission at 77 K in the direction perpendicular to the p-n junction. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Alkyl-Chain-Length-Independent Hole Mobility via Morphological Control with Poly(3-alkylthiophene) NanofibersADVANCED FUNCTIONAL MATERIALS, Issue 5 2010Wibren D. Oosterbaan Abstract The field-effect transistor (FET) and diode characteristics of poly(3-alkylthiophene) (P3AT) nanofiber layers deposited from nanofiber dispersions are presented and compared with those of layers deposited from molecularly dissolved polymer solutions in chlorobenzene. The P3AT n -alkyl-side-chain length was varied from 4 to 9 carbon atoms. The hole mobilities are correlated with the interface and bulk morphology of the layers as determined by UV,vis spectroscopy, transmission electron microscopy (TEM) with selected area electron diffraction (SAED), atomic force microscopy (AFM), and polarized carbon K -edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The latter technique reveals the average polymer orientation in the accumulation region of the FET at the interface with the SiO2 gate dielectric. The previously observed alkyl-chain-length-dependence of the FET mobility in P3AT films results from differences in molecular ordering and orientation at the dielectric/semiconductor interface, and it is concluded that side-chain length does not determine the intrinsic mobility of P3ATs, but rather the alkyl chain length of P3ATs influences FET diode mobility only through changes in interfacial bulk ordering in solution processed films. [source] Strain-engineered novel III,N electronic devices with high quality dielectric/semiconductor interfacesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003M. Asif Khan Abstract Since the early demonstration of 2D-electron gas [M. A. Khan et al., Appl. Phys. Lett. 60, 3027 (1992)] and a heterojunction field effect transistor (HFET) [M. Asif Khan et al., Appl. Phys. Lett. 63, 1214 (1993)] in III,N materials, rapid progress has been made to improve the DC and RF performance of GaN,AlGaN based HFETs. Stable and impressive microwave powers as high as 4,8 W/mm have been reported for device operation frequencies from 10 to 35 GHz. The key reason for these high performance numbers is an extremely large sheet carrier densities (>1 × 1013 cm,2) that can be induced at the interfaces in III,N hetereojunction [A. Bykhovsk et al., J. Appl. Phys. 74, 6734 (1993); M. Asif Khan et al., Appl. Phys. Lett. 75, 2806 (1999)]. These are instrumental in screening the channel dislocations thereby retaining large room temperature carrier mobilities (>1500 cm2/Vs) and sheet resistance as low as 300 ,/sq. These numbers and the high breakdown voltages of the large bandgap III,N material system thus enable rf-power approximately 5,10 times of that possible with GaAs and other competitor's technologies. We have recently introduced a unique pulsed atomic layer epitaxy approach to deposit AlN buffer layers and AlN/AlGaN superlattices [J. Zhang et al., Appl. Phys. Lett. 79, 925 (2001); J. P. Zhang et al., Appl. Phys. Lett. 80, 3542 (2002)] to manage strain and decrease the dislocation densities in high Al-content III,N layers. This has enabled us to significantly improve GaN/AlGaN hetereojunctions and the device isolation. The resulting low defect layers are not only key to improving the electronic but also deep ultraviolet light-emitting diode devices. For deep UV LED's they enabled us to obtain peak optical powers as high as 10 mW and 3 mW for wavelengths as short as 320 nm and 278 nm. Building on our past work [M. Asif Khan et al., Appl. Phys. Lett. 77, 1339 (2000); X. Hu et al., Appl. Phys. Lett. 79, 2832 (2001)] we have now deposited high quality SiO2/Si3N4 films over AlGaN with low interface state densities. They have then been used to demonstrate III,N insulating gate transistors (MOSHFET (SiO2) and MISHFET (Si3N4) with gate leakage currents 4,6 order less than those for conventional GaN,AlGaN HFETs. The introduction of the thin insulator layers (less then 100 Å) under the gate increases the threshold voltage by 2,3 V. In addition, it reduces the peak transconductance gm. However the unity cut-off frequency, the gain and the rf-powers remain unaffected as the gm/Cgs (gate-source capacitance) ratio remains unchanged. In addition to managing the defects and gate leakage currents we have also employed InGaN channel double heterojunction structures (AlInGaN,InGaN,GaN) to confine the carriers thereby reducing the spillover into trappings states. These InGaN based MOS-DHFETs exhibited no current-collapse, extremely low gate leakage currents (<10,10 A/mm) and 10,26 GHz rf-powers in excess of 6 W/mm. We have also demonstrated the scalability and stable operation of our new and innovative InGaN based insulating gate heterojunction field effect transistor approach. In this paper we will review the III,N heterojunction field-effect transistors progress and pioneering innovations including the excellent work from several research groups around the world. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The Effects of Moisture in Low-Voltage Organic Field-Effect Transistors Gated with a Hydrous Solid ElectrolyteADVANCED FUNCTIONAL MATERIALS, Issue 16 2010Nikolai Kaihovirta Abstract The concept of using ion conducting membranes (50,150 ,m thick) for gating low-voltage (1 V) organic field-effect transistors (OFETs) is attractive due to its low-cost and large-area manufacturing capabilities. Furthermore, the membranes can be tailor-made to be ion conducting in any desired way or pattern. For the electrolyte gated OFETs in general, the key to low-voltage operation is the electrolyte "insulator" (the membrane) that provides a high effective capacitance due to ionic polarization within the insulator. Hydrous ion conducting membranes are easy to process and readily available. However, the role of the water in combination with the polymeric semiconductor has not yet been fully clarified. In this work electrical and optical techniques are utilized to carefully monitor the electrolyte/semiconductor interface in an ion conducting membrane based OFET. The main findings are that 1) moisture plays a major part in the transistor operation and careful control of both the ambient atmosphere and the potential differences between the electrodes are required for stable and consistent device behavior, 2) the obtained maximum effective capacitance (5 ,F cm,2) of the membrane suggests that the electric double layer is distributed over a broad region within the polyelectrolyte, and 3) electromodulation spectroscopy combined with current,voltage characteristics provide a method to determine the threshold gate voltage from an electrostatic field-effect doping to a region of (irreversible) electrochemical perturbation of the polymeric semiconductor. [source] Fluctuation model for a rough metal/semiconductor interfacePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2003N. L. Dmitruk Abstract The fluctuation model with Gaussian-type of barrier heights distribution that was recently applied successfully to explain the dark current,voltage characteristics of Au/GaAs barrier structure with microrelief interface, in this paper is extended to photoelectric characteristics. In addition to the change of the Richardson constant and the apparent temperature coefficient of barrier height we predict the decrease of the open-circuit voltage of photodetectors or solar cells. The theoretical predictions have been verified experimentally on the Au/GaAs Schottky barriers with dendrite-like or quasigrating interface prepared by wet anisotropic etching. [source] Organic Polyaromatic Hydrocarbons as Sensitizing Model Dyes for Semiconductor NanoparticlesCHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 4 2010Yongyi Zhang Abstract The study of interfacial charge-transfer processes (sensitization) of a dye bound to large-bandgap nanostructured metal oxide semiconductors, including TiO2, ZnO, and SnO2, is continuing to attract interest in various areas of renewable energy, especially for the development of dye-sensitized solar cells (DSSCs). The scope of this Review is to describe how selected model sensitizers prepared from organic polyaromatic hydrocarbons have been used over the past 15 years to elucidate, through a variety of techniques, fundamental aspects of heterogeneous charge transfer at the surface of a semiconductor. This Review does not focus on the most recent or efficient dyes, but rather on how model dyes prepared from aromatic hydrocarbons have been used, over time, in key fundamental studies of heterogeneous charge transfer. In particular, we describe model chromophores prepared from anthracene, pyrene, perylene, and azulene. As the level of complexity of the model dye-bridge-anchor group compounds has increased, the understanding of some aspects of very complex charge transfer events has improved. The knowledge acquired from the study of the described model dyes is of importance not only for DSSC development but also to other fields of science for which electronic processes at the molecule/semiconductor interface are relevant. [source] High-Performance Organic Field-Effect TransistorsADVANCED MATERIALS, Issue 14-15 2009Daniele Braga Abstract With the advent of devices based on single crystals, the performance of organic field-effect transistors has experienced a significant leap, with mobility now in excess of 10,cm2 V,1 s,1. The purpose of this review is to give an overview of the state-of-the-art of these high-performance organic transistors. The paper focuses on the problem of parameter extraction, limitations of the performance by the interfaces, which include the dielectric,semiconductor interface, and the injection and retrieval of charge carriers at the source and drain electrodes. High-performance devices also constitute tools of choice for investigating charge transport phenomena in organic materials. It is shown how the combination of field-effect measurements with other electrical characterizations helps in elucidating this still unresolved issue. [source] Thin Film Solar Cells: Materials Science at InterfacesADVANCED ENGINEERING MATERIALS, Issue 10 2005J. Fritsche Abstract Interfaces are important for the efficiencies of thin film solar cells. In particular for polycrystalline chalcogenide semiconductors as Cu(In,Ga)(S,Se)2 and CdTe the existing physical concepts, which describe the electronic properties of semiconductor interfaces, are not sufficient. The increased complexity is mostly due to the non-abruptness of the interfaces and the strong tendency for the formation of defects. For the CdTe thin film solar cell a very relevant interface for their operation and efficiency is the CdTe/CdS semiconductor hetero junction. The properties of the semiconductor interfaces have been characterised systematically with photoelectron spectroscopy (XPS/UPS) in integrated ultra high vacuum (UHV) systems for sample preparation and analysis. Withal the key topic is the experimental determination of the band alignment at the semiconductor interfaces. For high efficiency CdTe solar cell production CdCl2 activation is of major importance. The effects of the CdCl2 treatment step on CdTe solar cells had been not completely understood so far. To investigate its influence the activation process has been transferred into the integrated UHV system. We will report about chemical and electronic modifications of the CdTe/CdS hetero interface due to in-situ CdCl2 activation performing sputter depth profiles in combination with X-ray photoelectron spectroscopy (XPS). [source] Interface Engineering for Organic ElectronicsADVANCED FUNCTIONAL MATERIALS, Issue 9 2010Hong Ma Abstract The field of organic electronics has been developed vastly in the past two decades due to its promise for low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, and ease of processing. The performance and lifetime of these devices, such as organic light-emitting diodes (OLEDs), photovoltaics (OPVs), and field-effect transistors (OFETs), are critically dependent on the properties of both active materials and their interfaces. Interfacial properties can be controlled ranging from simple wettability or adhesion between different materials to direct modifications of the electronic structure of the materials. In this Feature Article, the strategies of utilizing surfactant-modified cathodes, hole-transporting buffer layers, and self-assembled monolayer (SAM)-modified anodes are highlighted. In addition to enabling the production of high-efficiency OLEDs, control of interfaces in both conventional and inverted polymer solar cells is shown to enhance their efficiency and stability; and the tailoring of source,drain electrode,semiconductor interfaces, dielectric,semiconductor interfaces, and ultrathin dielectrics is shown to allow for high-performance OFETs. [source] Molecules on Si: Electronics with ChemistryADVANCED MATERIALS, Issue 2 2010Ayelet Vilan Abstract Basic scientific interest in using a semiconducting electrode in molecule-based electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test-bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without, to a first approximation, altering the interfacial chemistry. To investigate semiconductor-molecule electronics we need reproducible, high-yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power. To consider these issues and questions we will, in this Progress Report, review junctions based on direct bonding of molecules to oxide-free Si. describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified. investigate to what extent imperfections in the monolayer are important for transport across the monolayer. revisit the concept of energy levels in such hybrid systems. [source] Computational study of titanium (IV) complexes with organic chromophoresINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2006Ivan Kondov Abstract A computational study of small titanium complexes with the chromophores catechol, alizarin, and coumarin 343 is presented. Employing density functional theory (DFT), the ground-state geometries, energies, and harmonic frequencies of the different compounds are calculated. Furthermore, time-dependent DFT and the configuration interaction singles (CIS) method are used to determine excitation energies and excited-state gradients. Based on these results, the character of the excited states as well as electronic-vibrational coupling strengths are analyzed, and the implications for electron-transfer reactions at dye,semiconductor interfaces are discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Engineering the line up of electronic energy levels at inorganic,organic semiconductor interfaces by variation of surface termination and by substitutionPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2008Thomas Mayer Abstract In the Anderson model the alignment of electronic energy levels follows from the assumption that the vacuum-levels of the contacting phases are at equal height. Engineering of the line up may be attempted by changing the ionization energy of the substrate and/or of the adsorbate. We report on the variation of the Si(111) ionization energy by induced surface dipoles of ,CH3, ,H, and ,GaSe terminations and the induced variation of the HOMO line up of PTCDA and ZnPc layers. In addition the variation of the line up by changing the organic molecule ionization energy is exemplified by F substitution in ZnPc. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||