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GaAs
Kinds of GaAs Terms modified by GaAs Selected AbstractsBandgap characters in GaAs-based ternary alloysCRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2010N. Tit Abstract The existence and origins of the bowing character in the bandgap variation of GaAs-based ternary alloys are theoretically investigated based on two different computational methods. Within the framework of the virtual crystal approximation (VCA), both the empirical sp3s * tight-binding (TB) method with, and without, the inclusion of the spin-orbit coupling effects, and the first-principle full-potential linear augmented plane wave (FP-LAPW) technique are applied on both the common-cation GaSbxAs1-x and the common-anion Ga1-xInxAs alloys. These methods are used to calculate the bandgap energy, the partial and total densities of states and the constituent charge ionicity versus the composition x. The results show that the bowing behavior exists in the case of common-cation alloys (GaSbxAs1-x) as a manifestation of a competition between the anion atoms (As and Sb) in trapping the made-available-cationic charges. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (,anion). Consistent with this in the case of common-anion alloys (Ga1-xInxAs), as due to the lack of anion competition, the bowing is just absent and the variation of bandgap energy is found to be rather linear. The excellent agreement between our theoretical results and recent photoluminescence data has corroborated our claim. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Large area lateral overgrowth of mismatched InGaP on GaAs(111)B substratesCRYSTAL RESEARCH AND TECHNOLOGY, Issue 12 2005S. Uematsu Abstract Application of InGaAs/InGaP double-heterostructure (DH) lasers increases the band offset between the cladding layer and the active layer more than the use of conventional 1.3 µm InGaAsP/InP lasers. As a first step in realizing 1.3 µm InGaP/InGaAs/InGaP DH lasers, we proposed InGaP lattice-mismatched epitaxial lateral overgrowth (ELO) technique and successfully carried out the InGaP growth on both GaAs (100), (111)B and InP (100) substrates by liquid phase epitaxy. In this work, we grew the InGaP crystal on GaAs (111)B substrate by adjusting Ga and P composition in In solution, to obtain In0.79Ga0.21P (, = 820 nm) virtual substrate for 1.3 µm InGaAs/InGaP DH lasers. To grow the InGaP all over the lateral surface of the substrate, the growth time was extended to 6 hours. The amount of InGaP lateral growth up to 2 hours was gradually increased, but the lateral growth was saturated. The InGaP lateral width was about 250 µm at the growth time of 6 hours. We report the result that optical microscope observation, CL and X-ray rocking curve measurements and reciprocal lattice space mapping were carried out to evaluate the crystal quality of the grown InGaP layers. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Hydrogen- and carbon-related defects in heavily carbon-doped GaAs induced degradation under minority-carrier injectionELECTRONICS & COMMUNICATIONS IN JAPAN, Issue 5 2010Hiroshi Fushimi Abstract GaAs/AlGaAs heterojunction bipolar transistors (HBTs) have attracted much attention because of their high-speed performance. However, long-term operation seriously degrades the device characteristics: the current gain decreases and the low-bias-leakage current increases. This degradation has long been an issue in GaAs-based devices operated under minority-carrier injection, such as laser diodes. The cause of degradation is thought to lie in the carbon-doped base, but this is not yet certain. In this paper the degradation of HBTs is described, especially that of GaAs/AlGaAs HBTs with a heavily carbon-doped base layer. Two types of device degradation are found, namely, hydrogen-related degradation and carbon-related degradation. The mechanisms governing the degradation are discussed in the framework of the recombination-enhanced defect reaction (REDR) and charge state effect (CSE). © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(5): 33,41, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10208 [source] Analysis of GaInAsP laser diodes degraded by light absorption at an active layer of the facetELECTRONICS & COMMUNICATIONS IN JAPAN, Issue 2 2010Hiroyuki Ichikawa Abstract Electrostatic discharge-induced degradation is one of the serious reliability problems of GaInAsP/InP laser diodes. The authors have conducted an analysis of electrostatic discharge-induced degradation, and have elucidated the principal degradation mechanism. The main cause of degradation is heating by light absorption at the active layer of the facet. This phenomenon is similar to the catastrophic optical damage that occurs in GaAs-based high-power laser diodes. The problem has become more serious with the recent tendency to high power demand. Therefore, technology to suppress against degradation is extremely important. Focusing on facet coating, which is one of the key processes to suppress facet degradation, we demonstrated that facet degradation can be successfully suppressed by inserting an ultrathin aluminum layer between the semiconductor and the dielectric coaling films. This effect is caused by a reduction of surface recombination. This degradation suppression technology has the potential to be applied not only to GaInAsP/InP laser diodes, but to any InP-based laser diodes. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(2): 32,38, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10196 [source] New Developments in Vertical Gradient Freeze Growth,ADVANCED ENGINEERING MATERIALS, Issue 7 2004O. Pätzold The Vertical Gradient Freeze (VGF) technique is an important method for growing high quality compound semiconductors such as GaAs. Results obtained with a novel VGF set-up developed for the growth under influence of a rotating magnetic field (RMF) and under vapour pressure control are presented in this paper. The RMF is shown to be a powerful tool to affect the heat and mass transport within the melt in a definite way. In GaAs:Si growth, RMF induced flow results in a decreased curvature of a nominally concave-shaped interface, i.e., it contributes to an axial heat transfer at the solid-liquid interface. The axial dopant segregation of Ga in Ge is found to be improved under continuous RMF action due to better mixing of the melt. The set-up also allowed to determine the influence of carbon and the arsenic vapour pressure on the dopant incorporation and crystal quality. [source] Electronic Contact Deposition onto Organic Molecular Monolayers: Can We Detect Metal Penetration?ADVANCED FUNCTIONAL MATERIALS, Issue 13 2010Hagay Shpaisman Abstract Using a semiconductor as the substrate to a molecular organic layer, penetration of metal contacts can be clearly identified by the study of electronic charge transport through the layer. A series of monolayers of saturated hydrocarbon molecules with varying lengths is assembled on Si or GaAs and the junctions resulting after further electronic contact is made by liquid Hg, indirect metal evaporation, and a "ready-made" metal pad are measured. In contrast to tunneling characteristics, which are ambiguous regarding contact penetration, the semiconductor surface barrier is very sensitive to any direct contact with a metal. With the organic monolayer intact, a metal,insulator,semiconductor (MIS) structure results. If metal penetrated the monolayer, the junction behaves as a metal,semiconductor (MS) structure. By comparing a molecule-free interface (MS junction) with a molecularly modified one (presumably MIS), possible metal penetration is identified. The major indicators are the semiconductor electronic transport barrier height, extracted from the junction transport characteristics, and the photovoltage. The approach does not require a series of different monolayers and data analysis is quite straightforward, helping to identify non-invasive ways to make electronic contact to soft matter. [source] Cover Picture: Photolithographic Route to the Fabrication of Micro/Nanowires of III,V Semiconductors (Adv. Funct.ADVANCED FUNCTIONAL MATERIALS, Issue 1 2005Mater. Abstract The cover shows a patterned assembly of GaAs nanowires with their ends tethered to a bulk single-crystal wafer as described on p.,30 by Rogers and co-workers. These wires, which have triangular cross-sections, were fabricated via a top,down process that combines photolithography and anisotropic chemical etching. Nano/microwires of semiconducting materials (e.g., GaAs and InP) with triangular cross-sections can be fabricated by "top,down" approaches that combine lithography of high-quality bulk wafers (using either traditional photolithography or phase-shift optical lithography) with anisotropic chemical etching. This method gives good control over the lateral dimensions, lengths, and morphologies of free-standing wires. The behaviors of many different resist layers and etching chemistries are presented. It is shown how wire arrays with highly ordered alignments can be transfer printed onto plastic substrates. This "top,down" approach provides a simple, effective, and versatile way of generating high-quality single-crystalline wires of various compound semiconductors. The resultant wires and wire arrays have potential applications in electronics, optics, optoelectronics, and sensing. [source] Soft Contact Deposition onto Molecularly Modified GaAs.ADVANCED FUNCTIONAL MATERIALS, Issue 11-12 2002Electrical Effects, Thin Metal Film Flotation: Principles Abstract We describe and analyze a process to position a ,,1 nm thick molecular layer between two solid surfaces without damage to the molecules. The method is used to deposit a metal film in a soft, gentle manner on a semiconductor, yielding functional semiconductor/molecule/metal junctions. It is a combination of the lift-off procedure, known from, for example, lithography, and the bonding process, known from, for example, wafer bonding. The combined method may find application also outside the area described here. We point out its major difficulties as well as solutions to overcome them. For this we rely on concepts from the physics of liquid and solid surfaces and interfaces. Conditions are found, in terms of choice of solvents, under which the method will be effective. The efficacy of floatation as a soft contacting procedure is demonstrated by the preparation of Au and Al contacts on GaAs single crystal surfaces, modified by a self-assembled monolayer of small organic molecules. The resulting electrical properties of the contacts depend crucially on how the molecular interface with the contacting metal is formed. This type of wet contacting procedure to make dry devices may be advantageous especially if biomolecules are used. [source] Adsorption-Induced Magnetization of PbS Self-Assembled Nanoparticles on GaAs,ADVANCED MATERIALS, Issue 13 2008Alexander Zakrassov PbS nanoparticles attached to a GaAs substrate via organic molecules have magnetic properties. When the organic molecules are aligned parallel or at some angle relative to the surface normal, the magnetic anisotropy vector K coincides with the molecular orientation. [source] Restricted and unrestricted Hartree,Fock approaches applied to spherical quantum dots in a magnetic fieldINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2006C. F. Destefani Abstract The Roothaan and Pople,Nesbet approaches for real atoms are adapted to quantum dots in the presence of a magnetic field. Single-particle Gaussian basis sets are constructed, for each dot radius, under the condition of maximum overlap with the exact functions. The chemical potential, charging energy, and total spin expected values are calculated, and we have verified the validity of the quantum dot energy shell structure as well as Hund's rule for electronic occupation at zero magnetic field. At finite field, we have observed the violation of Hund's rule and studied the influence of magnetic field on the closed and open energy shell configurations. We have also compared the present results with those obtained within the LS-coupling scheme for low electronic occupation numbers. We focus only on ground-state properties and consider quantum dots populated up to 40 electrons, constructed by GaAs or InSb nanocrystals. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Isotope and disorder effects in the Raman spectra of LiHxD1,x crystalsJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2001V. G. Plekhanov Most of the physical properties of a solid depend on its isotopic composition in some way or another. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. A great number of stable isotopes and well-developed methods for their separation have made it possible to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu2O, Si, Ge and ,-Sn with a controllable isotopic composition. Among these compounds, LiH possesses the largest value of the isotope effect. The great number of theoretical and experimental data suggest that the isotopic composition of a crystal lattice exerts some influence on the vibrational properties of crystals. These effects are fairly large and can be readily measured by modern experimental techniques (ultrasound, Brillouin and Raman scattering and neutron scattering). In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between an LiH crystal and its deuterated analogue exceeds a 100 K. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The scattering lines in isotopically mixed crystals are not only shifted (the shift of LO lines exceeds 100 cm,1) but are also broadened. This broadening is related to the isotopic disorder of the crystal lattice. It is shown that the degree of a change in the scattering potential is different for different isotopic mixed crystals. In the case of germanium and diamond crystals, phonon scattering is weak, which allows one to apply successfully the coherent potential approximation (CPA) for describing the shift and broadening of scattering lines. In the case of lithium hydride, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments. The common nature of the isotopic and disorder effects in a wide range of crystals is emphasized. Copyright © 2001 John Wiley & Sons, Ltd. [source] Diode-pumped doubly passively Q-switched Nd:LuVO4 laser with Cr4+:YAG saturable absorber and GaAs output couplerLASER PHYSICS LETTERS, Issue 10 2009K. Cheng Abstract A diode-pumped doubly passively Q-switched Nd:LuVO4 laser with Cr4+:YAG saturable absorber and GaAs output coupler is realized. This laser can generate a shorter and more symmetric pulse profile when compared with pure GaAs. By using two Cr4+:YAG saturable absorbers with different small-signal transmissions T0 = 0.71 and T0 = 0.81, the maximum average output power are 1.15 W and 1.31 W at the incident pump power of 11.23 W, corresponding to optical conversion efficiencies of 10.2% and 11.7%. At the same incident pump power, the minimum pulse widths of 4.18 ns and 5.42 ns can be obtained at T0 = 0.71 and T0 = 0.81, respectively. The pulse repetition rate and the single-pulse peak power of the laser are also measured. (© 2009 by Astro Ltd., Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Optimization of Bragg reflectors in AlGaAs/GaAs VCSELsLASER PHYSICS LETTERS, Issue 5 2005V. M. N. Passaro Abstract In this paper a detailed investigation of the distributed Bragg reflectors in GaAs-based vertical cavity surface emitting lasers is presented. The influence of layer doping concentration, number of periods, oxide aperture and AlxGa1,xAs alloy composition on output emission power and threshold current has been found. Both oxidized and non oxidized structures have been considered. A number of interpolation curves are extracted and presented for design and fabrication purposes. Although the results are presented for GaAs-based structures, the theoretical approach is very general. (© 2005 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] High-field optically detected EPR and ENDOR of semiconductor defects using W-band microwave Fabry,Pérot resonators,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005J.-M. Spaeth Abstract The designs of W-band (,95 GHz) Fabry,Pérot microwave resonators for optically detected EPR and ENDOR using the magnetic circular dichroism of the optical absorption (MCDA) as well as for photo-luminescence-detected EPR are briefly described. We report on the first MCDA-detected high-field EPR/ENDOR investigation of the paramagnetic EL2+ defect in semi-insulating GaAs. The higher-order effects, which prevented the unambiguous analysis of previous MCDA-detected K-band EPR/ENDOR experiments could be suppressed in W-band. The analysis of the ENDOR spectra showed that an extremely precise alignment of the samples is necessary. The paramagnetic El2+ defect turned out to be an As antisite defect, which has four almost equivalent nearest 75As neighbours differing less than 1.5% in the superhyperfine interactions suggestive of an isolated As antisite, while the third 75As shell (fifth neighbour shell) is clearly of lower symmetry than expected for an isolated As antisite. We discuss as a possible solution to this paradoxical situation that EL2+ is an isolated antisite at room temperature, which at low temperature, where all magnetic resonance experiments are performed, associates itself with shallow acceptors such as ZnGa, more than two nearest neighbour distances away. According to recent theoretical calculations, such ,loose' complexes with binding energies between 0.01 eV and 0.05 eV and disturb the equivalence of the nearest neighbour superhyperfine (shf) interactions less than 1.5%. Also, W-band EPR was measured using the photo-luminescence for detection to investigate P dopants in 6H-SiC. Copyright © 2005 John Wiley & Sons, Ltd. [source] A 80,100 GHz image-reject passive-HEMT mixerMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 12 2006John W. Archer Abstract This paper describes a single-sideband, subharmonically pumped, passive-HEMT integrated circuit mixer developed for use in transceivers for point-to-point telecommunications in the 83,87 GHz band. The gallium arsenide (GaAs) monolithic microwave integrated circuit (MMIC) can readily be fabricated using a standard commercial process. The mixer performs equally well for either up- or down- conversion. For RF signals in the range 80,100 GHz, the conversion loss is typically 20 dB with LO drive in the range 40,50 GHz. Under these conditions, high rejection of the undesired sideband (>18 dB), relative to the desired signal, is achieved. For up-conversion, 1 dB compression of the mixer gain typically occurs at ,12 dBm RF output power. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2429,2433, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21953 [source] Three-dimensional coplanar waveguide lowpass filters for MMIC applicationsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 6 2003Laleh Lalehparvar Abstract Shielded single and multilayer coplanar waveguide lowpass filter on GaAs and Si substrates in MMIC technology is presented and their performance is investigated with electromagnetic simulations. The filters are both Chebyshev type. The stepped-impedance method is used in the design of the lowpass filter, where high- and low-impedance line sections are cascaded together. The shielded single-layer lowpass filter consists of three low- and two high-impedance sections with characteristic impedances of 20, and 100,. The multilayer filter makes use of three low- and two high-impedance lines with characteristic impedances of 5, and 120,, respectively. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 37: 403,405, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10931 [source] Photoluminescence properties of GaAs nanowire ensembles with zincblende and wurtzite crystal structurePHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 7 2010B. V. Novikov Abstract Self-standing III,V nanowires (NWs) are promising building blocks for future optoelectronic devices such as LEDs, lasers, photodetectors and solar cells. In this work, we present the results of low temperature photoluminescence (PL) characterization of GaAs NWs grown by Au-assisted molecular beam epitaxy (MBE), coupled with the transmission electron microscopy (TEM) structural analysis. PL spectra contain exci- ton peaks from zincblende (ZB) and wurtzite (WZ) crystal structures of GaAs. The peaks are influenced by the quantum confinement effects. PL bands corresponding to the exciton emission from ZB and WZ crystal phases are identified, relating to the PL peaks at 1.519 eV and 1.478 eV, respectively. The obtained red shift of 41 meV for WZ GaAs should persist in thin NWs as well as in bulk materials. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Critical diameters and temperature domains for MBE growth of III,V nanowires on lattice mismatched substratesPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 4 2009G. E. Cirlin Abstract We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au-assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation-free III,V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Surface morphology of highly mismatched InSb films grown on GaAs substrates by molecular beam epitaxyPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 2 2007Min Xiong Abstract InSb films on GaAs(001) substrates with and without GaAs buffer layer have been grown by molecular beam epitaxy. Rather than surface undulations, aligned ripples and pyramidal hillocks along the orthogonal ,110, directions were observed on the surface of InSb films. Both the preferential growth and the termination of ripples were proved to be related to strain-driven mass transport. A model was proposed to elucidate the formation of the hillocks, which are more efficient to relax strain than ripples. Due to the strain relaxation through hillocks with small bases predominantly, the surfaces of the InSb films grown without a GaAs buffer layer are smoother than those of films grown with a GaAs buffer layer. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Excitonic effects and Franz,Keldysh oscillations in photoreflectance of ultrapure GaAs epilayersPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2009O. S. Komkov Abstract A model based on the exciton theory has been presented for lineshape analysis of photoreflectance (PR) spectra. Experimental PR spectra of high-quality thick GaAs epilayers of different purity were measured to verify the theoretical results. It has been shown that excitonic effects completely prevail over the one-electron theory in the formation of photoreflectance spectrum even at high electric fields and room temperature, giving evidence of the existence of Franz,Keldysh oscillations enhanced by excitonic effects. A comparison of PR spectra simulated using the exciton theory and the one-electron approximation has shown that the semiperiods of the oscillations were practically similar. This demonstrates the validity of using the simple "high-field" model of Aspnes and Studna to determine surface electric fields. In photoreflectance of thick ultrapure GaAs epilayers an oscillating structure was experimentally observed in the region between the excitonic and the fundamental bandgap. This provides a possibility for contactless characterization not only for doped (N , 1014,1018 cm,3) but also for ultrapure (N , 1011,1013 cm,3) GaAs epilayers as well as other semiconductor materials. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Fast measurements of photoreflectance spectra by using multi-channel detectorPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2009R. Kudrawiec Abstract Photoreflectance (PR) measurements were performed on GaAs- and GaN-based structures in the so called ,bright configuration' where the sample was illuminated by white light (probing beam) instead of monochromatic light as it takes place in the standard configuration, i.e., so-called ,dark configuration'. Within this concept the whole PR spectrum can be measured/processed at the same time using a multi-channel detector (i.e., CCD camera) instead the phase sensitive lock-in detection system with the one-channel detector. In this work PR spectra for Si ,-doped GaAs structure have been measured using both the CCD detector system as well as the standard lock-in technique with the one-channel detector system. GaAs-related Franz,Keldysh oscillations, which are typical of Si ,-doped GaAs structure, have been clearly observed in PR spectra measured by using the two detection systems. In addition, the PR system with the CCD detector has been used to measure PR spectra in the UV spectral region for an InGaN/GaN/Al2O3 structure. In this case, PR resonances related to InGaN and GaN band gap absorption have been clearly observed. Using the PR system with CCD detector the time of PR measurements was reduced to few seconds for both GaAs- and GaN-based structures. It shows that the bright configuration of PR set-up with multi-channel detection system is very promising and perspective in the fast diagnostic of semiconductor structures. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] On the doping of graphenePHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 9 2008R. Jones Abstract Density functional theory is used to show that charge transfer occurs between chemical dopants in GaAs and adsorbates composed of C60 and graphene lying on the (110) surface of GaAs. In the case of C60, charge transfer only occurs for n-type GaAs in agreement with previous experimental results. However, the calculations show that transfer between graphene and both n- and p-type GaAs can occur which offers a simple way of doping graphene. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Changes in optical properties of MnAs thin films on GaAs(001) induced by ,- to ,-phase transitionPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 4 2008B. Gallas Abstract MnAs layers with 45 nm thickness were grown epitaxially on GaAs(001) substrates. Ellipsometry measurements were made in the spectral range 0.045 eV to 6 eV as a function of temperature (between ,10 °C and 50 °C) at 70° of incidence. In this way the transition from the hexagonal ,-phase to the orthorhombic ,-phase could be monitored. Non-zero off-diagonal elements of the Jones matrix for an azimuth of 38° off the [10] axis of the substrate indicate that the optical functions of MnAs are anisotropic in both phases. The optical conductivity exhibits low-energy interband transitions around 0.3 eV, more clearly seen in the ,-phase than in the ,-phase. Extrapolation of the optical conductivity to zero frequency confirms that the ,-phase is about two times more conducting than the ,-phase. A broad structure is observed in the visible range around 3 eV. The ,-phase is characterised by an anisotropy induced energy difference of this structure with a maximum at 2.8 eV for the extraordinary index and at 3.15 eV for the ordinary index. This difference vanishes in the ,-phase in which anisotropy mainly induces changes in amplitude of the 3 eV structure. The assignment of the structures will be discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Monitoring the ,- to ,-phase transition in MnAs/GaAs(001) thin films as function of temperaturePHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 4 2008B. Gallas Abstract MnAs layers with a 5 nm thick amorphous GaAs capping layer were grown epitaxially on GaAs(001). Generalized ellipsometric measurements were made on a 45 nm thick layer in the spectral range 1.5,4 eV at temperatures between ,10 °C and 50 °C in steps of 5 °C. By using both the diagonal and off-diagonal elements of the Jones matrix, the in-plane unixial anisotropy of MnAs was determined in terms of the ordinary and extraordinary complex dielectric functions. The measurements at each temperature could be well reproduced by modeling using the optical properties of the two limiting phases ,-MnAs and ,-MnAs determined at ,10 °C and 50 °C, respectively. The best sensitivity to the volume fractions of the two phases was obtained near 2.2 eV by monitoring the generalized ellipsometric parameter ,p for which the variations reached 30°. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mechanism of excitation and relaxation in Er,O-codoped GaAs for 1.5 ,m light-emitting devices with extremely stable wavelengthPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2008Yasufumi Fujiwara Abstract Energy-transfer processes in Er,O-codoped GaAs (GaAs:Er,O) have been investigated by means of a pump and probe reflection technique. Time-resolved reflectivity exhibited a characteristic dip; a negative signal due to bandgap renormalization in less than 1 ps and then a gradual recovery. In the recovery process, there were two components, fast and slow. The fast recovery time (several ps) was inversely proportional to Er concentration. The analysis based on a rate equation indicated that it is due to the capture of electrons by charged traps. The slow recovery (30,60 ps) was well coincident with the time (54 ps) predicted theoretically in the framework of a multiphonon-assisted model. Optical excitation cross section of Er ions in GaAs:Er,O has also been studied in various samples and shown to depend strongly on Er concentration. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Numerical simulation of anisotropic elastic fields of a GaAs/GaAs twist boundaryPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 9 2007Salah Madani Abstract Self-assembled nanostructures are particularly interesting for optoelectronic and photonic applications, especially on silicon and GaAs substrates. Nevertheless, their long-range spatial distribution is random, their density is difficult to control, their size distribution can be large and their shapes can be different. By overcoming these drawbacks, it should be possible to improve the performances of existing devices or to fabricate new ones. This work studies the possibility to order on a long range self-assembled nanostructures on a GaAs substrate, by means of the elastic fields induced at the surface by shallowly buried periodic dislocation networks. The needed strain and stress fields, generated by a square network of screw dislocations located between a finite layer of GaAs bonded onto a semi-infinite GaAs substrate, are calculated using anisotropic elasticity. The results obtained are compared to those obtained using isotropic elasticity. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Atomic layer epitaxy of GaMnAs on GaAs(001)PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 4 2007M. Ozeki Abstract A self-limiting mechanism in atomic layer epitaxy (ALE) has been investigated for the heterogrowth of GaMnAs on GaAs(001) substrate. In the ALE, trimethylgallium, bismethylcyclopentadienylmanganese and trisdimethylaminoarsine were used as source materials of gallium, manganese and arsenic atoms, respectively. Although the growth of GaMnAs was carried out at a high growth temperature of 500 °C, a distinct self-limiting mechanism was observed for the manganese alloy composition up to 6% and the epitaxial layer had no indications of including MnAs phase. The layer showed an atomically flat surface morphology reflecting the self-limiting growth. The self-limiting mechanism was largely affected by the lattice mismatch between GaMnAs epitaxial layer and GaAs substrate. When the manganese alloy composition exceeded 7%, the self-limiting mechanism was broken and MnAs precipitates were observed in the epitaxial layer. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] InGaAs/GaAs quantum wells and quantum dots on GaAs(11n) substrates studied by photoreflectance spectroscopyPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2007J. S. Rojas-Ramirez Abstract Pseudomorphic InGaAs/GaAs quantum wells (QWs) and self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on GaAs(11n)A substrates. Photoreflectance spectroscopy was employed to investigate the transitions in the heterostructures. The transitions in QWs have two contributions, a blue shift due to the compressive strain, and a red shift due to the quantum confined Stark effect produced by the piezoelectric field. A traditional theoretical interpretation of the QWs transitions employing a simple well model with sharp interfaces shows discrepancies with the experimental results. In order to satisfactorily explain the transitions we proposed to include segregation effects of Indium at the wells interfaces. The matrix transfer method was implemented to numerically solve the Schrödinger equation taking into account In segregation effects by including an asymmetric potential well with a profile depending on the details of the In incorporation. With segregation effects included, the calculated transitions fit very well the PR spectra. On the other hand, the transitions in self-assembled QDs were obtained by fitting the PR spectra employing a first derivative line-shape function. For n = 2, 4, 5, two functions were required to fit the spectra. For n = 3 only one function was required, in agreement with the more uniform QDs size distribution observed by atomic force microscopy on GaAs(113)A. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] MOVPE growth and optical characterization of GaAsN films with higher nitrogen concentrationsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2006F. Nakajima Abstract We have successfully grown high-N-content GaAsN films up to 5.1% on GaAs(001) substrates using tertiarybutylarsine (TBAs) as the As precursor by metalorganic vapour phase epitaxy (MOVPE). The narrow X-ray diffraction (XRD) peaks and clear Pendellosung fringes indicate that the GaAsN/GaAs interface is fairly flat and the GaAsN layers are uniform. By the photoluminescence (PL) measurement at 10 K, the clear PL peaks related to the near-band-edge transition could be detected and the bandgap energy was red-shifted to 1.16 eV in 1.9%-N GaAsN film. But, in higher N-content films no peak could be detected. So, post growth annealing in the reactor was applied to 4.7% and 5.1%-N films, and resulted in an enhancement of the PL peak intensity, and the bandgap energy of 5.1%-N film was consequently determined to be 0.95 eV at room temperature. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Preparation and characterization of pulse electrodeposited GaAs filmsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2006K. R. Murali Abstract GaAs is a III,V compound possessing high mobility and a direct band gap of 1.43 eV, making it a very suitable candidate for photovoltaic applications. Thin GaAs films were prepared by plating an aqueous solution containing GaCl3 and As2O3 at a pH of 2 and at room temperature. The current density was kept at 50 mA cm,2 and the duty cycle was varied in the range 10,50%. The films were deposited on titanium, nickel and tin oxide coated glass substrates. Films exhibited polycrystalline nature with peaks corresponding to single-phase GaAs. Optical absorption measurements indicated a direct band gap of 1.40 eV. Atomic force microscope measurements indicated uniform coverage with large crystallites for the films deposited at higher duty cycles. Photoelectrochemical cells were made using the films as photoelectrodes and graphite as counter electrode in 1 M polysulphide electrolyte. At 60 mW cm,2 illumination, an open-circuit voltage of 0.5 V and a short-circuit current density of 5.0 mA cm,2 were observed for the films deposited at a duty cycle of 50%. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||||||||