bluuebunny/crossref_metadata_2025 huggingface dataset:
Functional Behaviour of Thin Film Dielectric Superlattices
Authors: Gregg J. M. , Corbett M. H. , O'Neill D. , Catalan G. , Bowman R. M.
Published: April 2011

Pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is a superlattice. The properties of two superlattice systems were investigated as a function of superlattice wavelength (δ) – one based on barium strontium titanate and the other on lead-based relaxor electroceramics. In both systems the dielectric constant was significantly enhanced at stacking wavelengths of a few unit cells. However, the dielectric enhancement seen in the barium strontium titanate superlattices was found to be due to Maxwell-Wagner effects, whereas in the relaxor superlattices Maxwell-Wagner behaviour was not evident; rather, the dielectric enhancement was associated with the onset of polar coupling aroundδ ∼ 10nm.
Compositional Disorder Induced Relaxor Behavior of (PbLa)TiO3 Thin Films
Authors: Bhaskar S. , Majumder S. B. , Katiyar R. S.
Published: April 2011

Temperature dependent dielectric behavior of sol-gel derived ferroelectric Pb1−xLaxTiO3 (PLT) (x = 0.05 to 0.30) thin films on Pt/Si substrates has been studied. The characteristics of the diffuse phase transition and possible relaxor behavior of PbTiO3 thin films doped with different amounts of La are investigated. Room temperature X-ray and micro Raman results indicate that the crystal structure of the PLT films was strongly influenced by the La content. The softening of the E(1TO) mode with increasing La content indicates that the incorporation of La in the PT lattice results in a structural disorder in the material. The dielectric permittivity and loss tangent of the PLT thin films were measured in the temperature range of 80 –700 K at frequencies between 1 kHz and 1 MHz. Transition temperatures (Tm) for PLT (x = 0.05, 0.20, and 0.30) are 640, 460, and 254 K respectively, and are higher in comparison to reported values of bulk ceramics. The permittivity maximum broadened, and showed relaxor- type frequency dependent permittivity characteristics for PLT (x = 0.30) films. The broadening parameter was significantly influenced by La doping and our results indicate that PLT thin films undergo a normal-to-relaxor ferroelectric transformation for La concentrations of 25 at% in PLT films.
Initial Growth Stages of Epitaxial BaTiO3 Films on Vicinal SrTiO3:Nb (001) Substrates
Authors: Visinoiu A. , Alexe M. , Lee H. N. , Zakharov D. N. , Pignolet A. , Hesse D. , Gösele U.
Published: April 2011

The growth mechanism of epitaxial BaTiO3 films on vicinal Nb-doped SrTiO3 (SrTiO3:Nb) (001) substrate surfaces was studied in terms of surface morphology, crystalline orientation, microstructure, and film/substrate interface. Well-oriented BaTiO3 thin films were grown on SrTiO3 substrates with well-defined terraces by pulsed laser deposition. The regularly terraced TiO2-terminated surfaces of vicinal SrTiO3:Nb (001) substrates were prepared by a definite chemical and thermal treatment. Under our conditions, BaTiO3 seems to grow with a layer-then-island (Stranski-Krastanov) growth mechanism. In order to investigate the orientation and crystallinity of the BaTiO3 films, x-ray diffraction and high-resolution transmission electron microscopy were performed. Ferroelectricity of the BaTiO3 films was proved by electrical measurements performed on Pt/BaTiO3/SrTiO3:Nb heterostructures.
Monopolar Spin Orientation and Determination of Spin Relaxation Times in Quantum Well Structures
Authors: Ganichev Sergey D. , Danilov Sergey N. , Sollinger Martin , Weiss Dieter , Wegscheider Werner , Prettl Wilhelm , Bel'kov Vasily V. , Ivchenko Eugenius L.
Published: April 2011

It is shown that monopolar optical spin orientation of free carriers in zinc-blende structure based quantum wells (QWs) causes an electric current which reverses its direction upon changing the helicity of the radiation from left to right circular polarization resulting in a circular photogalvanic effect. The monopolar non-equilibrium population of spin-up and spin-down states has been achieved by far-infrared optical excitation ofp- andn-type GaAs/AlGaAs QWs structures. Two methods are introduced allowing to determine spin relaxation times. One is based on the Hanle effect in magnetic field induced circular photogalvanic effect, the other is spin sensitive bleaching of absorption. In contrast to usually applied methods of optical spin orientation, in the present case of terahertz excitation only one kind of charge carriers is involved in spin orientation and relaxation processes.
Interdiffusion in Exchange Biased NiFe/IrMn/CoFe Electrode in Magnetic Tunnel Junctions
Authors: Lee Jay H. , Jeong Hee D. , Rho Il C. , Yoon Chong S. , Kim Chang K.
Published: April 2011

Extent of Mn diffusion to the plasma-oxidized AlOx tunnel barrier of magnetic tunnel junction was examined using Auger Electron Spectroscopy (AES) and X-Ray Photoelectron Spectroscopy (XPS). A magnetic film stack consisting of Ta/AlOx/CoFe/IrMn/NiFe/Ta was deposited with the AlOx layer treated under different plasma oxidation durations. AES depth profiles showed that Mn diffusion to the AlOx/CoFe interface increased with increasing oxidation after annealing at 300°C. XPS analysis indicated that Mn found at the CoFe/AlOx interface in the over-oxidized electrode was in the form of MnO2. Our research suggests that Mn diffusion was accelerated by preferential oxidation of Mn at the CoFe/AlOx interface.
Phase Separation at Interfaces in La2/3Ca1/3MnO3 Thin Films
Authors: Fontcuberta J. , Bibes M. , Wojcik M. , Jedryka E. , Nadolski S. , València S. , Balcells Ll. , Martínez B.
Published: April 2011

We present an extensive physical characterization of La2/3Ca1/3MnO3 (LCMO) epitaxial films grown on SrTiO3 (STO), LaAlO3 (LAO) and NdGaO3 (NGO) substrates. The main difference among these substrates is that they have a different mismatch (∼ 1.2%, -1.8% and - 0.1%, respectively) with the manganite. The films can be coherently grown up to 180 nm for STO and NGO but partial relaxation is observed for LAO. The magnetotransport data indicate that very thin films (<27nm) display non-conventional magnetoresistive properties: a substantial magnetoresistance develops which can be progressively appreciated for STO and LAO films. 55Mn-Nuclear Magnetic Resonance experiments on all these films reveal the presence of non-homogeneous electronic states. These experiments in fact provide clear evidence of the presence of two distinguishable ferromagnetic Mn states and a non-ferromagnetic phase. The possible role of strain-induced charge localization is discussed. It turns out that electronic phase separation occurs in all films irrespectively of the particular substrate used; thus we conclude that strain is not the unique driving force for charge localization.
Spin-Valve Effect in Magnetic Resonant Tunneling Devices
Authors: Chantis A. N. , Demchenko D. O. , Petukhov A. G.
Published: April 2011

We propose a new electronic device utilizing resonant tunneling between two magnetic materials. The device is comprised of a semiconductor quantum well sandwiched between two insulating barriers and two ferromagnetic electrodes. The situation in which a resonant level fits in the energy interval where the minority density of states of a ferromagnetic emitter is zero can be considered as an almost ideal spin valve and leads to a great enhancement of magnetoresistance. This situation can be achieved by tuning the width of the quantum well. As an example we will consider GaMnAs/AlAs/GaAs/AlAs/GaMnAs double-barrier heterostructure. We can demonstrate that at a certain thickness of the quantum well and the barriers this system can significantly outperform conventional tunneling junctions comprised of one insulating barrier sandwiched between two ferromagnetic electrodes.
Gas Source MBE Growth and Characterization of TlInGaAs/InP DH Structures for Temperature-independent Wavelength LD Application
Authors: Asahi Hajime , Lee Hwe-Jae , Mizobata Akiko , Konishi Kenta , Maeda Osamu , Asami Kumiko
Published: April 2011

TlInGaAs/InP double-hetero (DH) structures were grown on (100) InP substrates by gas source MBE. The photoluminescence (PL) peak energy variation with temperature decreased with increasing Tl composition. For the DH with a Tl composition of 13%, the PL peak energy varied only slightly with temperature (−0.03 meV/K). This value corresponds to a wavelength variation of 0.04 nm/K and is much smaller than that of the lasing wavelength of InGaAsP/InP distributed feedback laser diodes (0.1 nm/K). TlInGaAs/InP light emitting diodes with 6% Tl composition were fabricated and the small temperature variation of the electroluminescence peak energy (−0.09 meV/K) was observed at the wavelength around 1.58 μm. The results are promising to realize the temperature-independent wavelength laser diodes, which are important in the wavelength division multiplexing (WDM) optical fiber communication systems.
Luminescent Characteristics of InGaAsP/InP Multiple Quantum Well Structures by Impurity-Free Vacancy Disordering
Authors: Zhao J. , Zhang X. D. , Feng Z. C. , Deng J. C. , Jin P. , Wang Y. C. , Xu G.
Published: April 2011

InGaAsP/InP multiple quantum wells have been prepared by Impurity-Free Vacancy Disordering (IFVD). The luminescent characteristics was investigated using photoluminescence (PL) and photoreflectance (PR), from which the band gap blue shift was observed. Si3N4, SiO2 and SOG were used for the dielectric layer to create the vacancies. All samples were annealed by rapid thermal anne aling (RTA). The results indicate that the band gap blue shift varies with the dielectric layers and annealing temperature. The SiO2 capping was successfully used with an InGaAs cladding layer to cause larger band tuning effect in the InGaAs/InP MQWs than the Si3N4 capping with an InGaAs cladding layer. On the other hand, samples with the Si3N4-InP cap layer combination also show larger energy shifts than that with SiO2-InP cap layer combination.
Airplane and Drop Experiments on Crystallization of InxGa1−xSb Semiconductor under Different Gravity Conditions
Authors: Balakrishnan Krishnan , Hayakawa Yasuhiro , Komatsu Hideki , Murakami Noriaki , Nakamura Tetsuo , Kimura Tadashi , Ozawa Tetsuo , Okano Yasunori , Miyazawa Masafumi , Dost Sadik , Dao Le. H. , Kumagawa Masashi
Published: April 2011

Melting and crystallization experiments of InGaSb were done under the reduced gravity condition (10−2G) in an airplane and at the normal gravity condition (1G) in the laboratory. Crystallized InGaSb was found to contain many needle crystals in both the cases. Reduced gravity condition was found to be more conducive for crystal growth than the normal gravity condition. Formation of spherical projections on the surface of InGaSb during its crystallization was in-situ observed using a high speed CCD camera in the drop experiment. Spherical projections showed dependence of gravity during its growth. Indium compositions in the spherical projections were found to vary depending on the temperature.
InP Self Assembled Quantum Dot Lasers Grown on GaAs Substrates by Metalorganic Chemical Vapor Deposition
Authors: Dupuis R. D. , Ryou J. H. , Heller R. D. , Walter G. , Kellogg D. A. , Holonyak N. , Reddy C. V. , Narayanamurti V. , Mathes D. T. , Hull R.
Published: April 2011

We describe the operation of lasers having active regions composed of InP selfassembled quantum dots embedded in In0.5Al0.3Ga0.2P grown on GaAs (100) substrates by MOCVD. InP quantum dots grown on In0.5Al0.3Ga0.2P have a high density on the order of about 1–2×10 cm−2 with a dominant size of about 10–15 nm for 7.5 ML growth.[1] These In0.5Al0.3Ga0.2P/InP quantum dots have previously been characterized by atomic-force microscopy, high-resolution transmission electron microscopy, and photoluminescence.[2] We report here the 300K operation of optically pumped red-emitting quantum dots using both double quantum-dot active regions and quantum-dot coupled with InGaP quantum-well active regions. Optically and electrically pumped 300K lasers have been obtained using this active region design; these lasers show improved operation compared to the lasers having QD-based active regions with threshold current densities as low as Jth ∼ 0.5 KA/cm2.
Raman and Photoluminescence Mapping of Lattice Matched InGaP/GaAs Heterostructures
Authors: Attolini G. , Fallini P. , Germini F. , Pelosi C. , Martínez O. , Sanz L. F. , González M. A. , Jiménez J.
Published: April 2011

The influence of the substrate on composition and CuPt-type spontaneous order of MOVPE lattice matched InGaP/GaAs layers was studied. The study was carried out by microRaman and microphotoluminescence. The order was determined by the band gap, while the Raman parameters were also contributed by the surface topography that was also related to the type of substrate. The spontaneous order increases with Si- doping of the substrates. Doping the layers with Zn randomises the alloy.
InAsN Grown by Plasma-Assisted Gas Source MBE
Authors: Shih Ding-Kang , Lin Hao-Hsiung , Chu Tso-Yu , Yang T. R.
Published: April 2011

We report the structural, electrical and optical properties of bulk InAsN alloy with various nitrogen contents deposited on (100) InP substrates by using plasma-assisted gas source molecular beam epitaxy. It is found that the fundamental absorption edge of InAsN, as compared to that of InAs, shifts to higher energy due to Burstein-Moss effect. A dramatic increase of the electron effective mass in a nitrogen-containing III-V alloy is also observed from infrared reflectivity and Hall measurement on these degenerate InAsN samples. The sizeable increase on electron effective mass is consistent with the theoretical predictions based on band-anticrossing model.
Deep Centers and Their Capture Barriers in MOCVD-Grown GaN
Authors: Johnstonea Daniel K. , Ahoujjab Mohamed , Yeoc Yung Kee , Hengeholdc Robert L. , Guidod Louis
Published: April 2011

GaN and its related alloys are being widely developed for blue-ultraviolet emitting and detection devices as well as high temperature, high power, and high frequency electronics. Despite the fast improvement in the growth of good quality GaN, a high concentration of deep level defects of yet unconfirmed origins are still found in GaN. For both optical and electronic devices, these deep carrier traps and/or recombination centers are very important and must therefore be understood. In the present work, deep level defects in GaN grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) have been investigated using Isothermal Capacitance Transient Spectroscopy (ICTS) and Current Voltage Temperature (IVT) measurements. Several deep level electron traps were characterized, obtaining the emission energy, concentration, and capture cross section from a fit of exponentials to the capacitance transients. ICTS was also used to reveal information about the capture kinetics involved in the traps found in GaN by measuring the amplitude of the capacitance transient at each temperature. At a reduced filling pulse where the traps were not saturated, several of them showed marked reduction in capacitance transient amplitude when compared to the transient amplitude measured under conditions where the filling pulse saturates the traps. This reduction in transient amplitude indicates that there is a barrier to carrier capture, in addition to the emission barrier. It has been found that several traps had capture barriers that were significant fractions of the emission energies up to 0.32 eV. These capture barriers may lead to persistent photoconductivity and reduced trapping. In this paper, deep level emission energies as well as capture barrier energies found in MOCVD-grown GAN will be discussed.
Quantum Dot Long-Wavelength Detectors
Authors: Bhattacharya Pallab , Stiff-Roberts Adrienne D. , Krishna Sanjay , Kennerly Steve
Published: April 2011

Long-wavelength infrared detectors operating at elevated temperatures are critical for imaging applications. InAs/GaAs quantum dots are an important material for the design and fabrication of high-temperature infrared photodetectors. Quantum dot infrared photodetectors allow normal-incidence operation, in addition to low dark currents and multispectral response. The long intersubband relaxation time of electrons in quantum dots improves the responsivity of the detectors, contributing to better hightemperature performance. We have obtained extremely low dark currents (Idark = 1.7 pA, T = 100 K, Vbias = 0.1 V), high detectivities (D* = 2.9×108cmHz1/2/W, T = 100 K, Vbias = 0.2 V), and high operating temperatures (T = 150 K) for these quantum-dot detectors. These results, as well as infrared imaging with QDIPs, will be described and discussed.
A Theoretical Study of Structural Disorder and Photoluminescence Linewidth in InGaAs/GaAs Self Assembled Quantum Dots
Authors: Lin Yih-Yin , Jiang Hongtao , Singh Jasprit
Published: April 2011

The past few years have seen considerable efforts in growth and device application of selfassembled quantum dots. However, the photoluminescence (PL) linewidth, which represents structural fluctuations in dot sizes, is still in the range of 30–50 meV. This large linewidth has deleterious effects on devices such as lasers based on self-assembled dots. In this paper we will examine the configuration-energy diagram of self-assembled dots. Our formalism is based on: (1) an atomistic Monte Carlo method which allows us to find the minimum energy configuration and strain tensors as well as intermediate configurations of dots; (2) an 8-band k·p method to calculate the electronic spectra. We present results on the strain energy per unit cell for various distributions of InAs/GaAs quantum dots and relate them to published experimental results. In particular we examine uncovered InAs/GaAs dots and show that in the uncovered state a welldefined minimum exists in the configuration energy plot. The minimum corresponds to the size that agrees well with experiments.
PbTe Flash Evaporation on Si <100> Substrates for Heterojunction Infrared Detectors
Authors: Guimarães Sonia , Silva Sabrina de C. F. F. da , Assis João M. K. de
Published: April 2011

This work is to present results of flash evaporation of PbTe directly over single crystals p-type Si substrates, in order to produce heterojunction infrared detectors (HIRD), working at 4.3μm IR wavelength. The evaporation was performed on modified JEOL vacuum equipment, model JEE4B(a), working with vacuum pressure around 10−5 torr, using diffusion pump. The HIRDs produced with this method presented the same detectivity (D*) values of HIRDs made with Hot Wall Epitaxial System (HWE)(b), in which PbTe epitaxial layers were grown directly over the same Si substrates, where an ionic pump reached about 10−7 torr as vacuum pressure. The best results, were obtained with PbTe layers grown with Molecular Beam Epitaxial (MBE) method(c) directly over Si substrates, where the vacuum pressure is around 10−9 torr, also using an ionic pump. The advantage of growing PbTe directly over Si wafers is that the HIRDs perform at room temperature. The detectivity values of HIRDs obtained with methods (a) and (b), were D*≈4,8 × 105 cm.Hz1/2 W−1 and with method (c), D* ≈ 6,7 × 106 cm.Hz1/2 W−1. Different technologies: (a) very low costs, (c) high technology; not very different results.
Calculations of Dielectric Constant for AlGaInAs Quaternary Semiconductor Alloy in the Transparent Region and Above (0.4-4.0eV)
Authors: Linnik M. , Christou A.
Published: April 2011

The modeling of the spectral behavior of the refractive index of AlGaInAs quaternary IIIV semiconductor alloy in the energy range from 0.4 to 4eV, including the transparent region, is presented. The extended model of interband transition contributions incorporates not only the fundamental absorption edge contribution to the dielectric function, but also contributions from higher energy and indirect transitions. It is demonstrated that indirect energy transitions must be included in the calculations of the complex dielectric function of the material in the transparent region. Indirect transitions from different critical points in the Brillouin zone are treated separately. The comparison between the theoretical refractive indices and the experimental data for AlGaInAs alloy is presented. These calculations have been applied to the design of Bragg mirrors with the highest refractive index contrast for heterostructure lasers.
Annealing of some II-IV-V2 crystals in the vapor of volatile constituents
Authors: Voevodin Valeriy G. , Voevodina Olga V. , Bereznaya Svetlana A. , Korotchenko Zoya V. , Fernelius Nils C. , Goldstein Jonathan T. , Ohmer Melvin C.
Published: April 2011

Experiments on annealing of CdGeAs2-, CdSnAs2- and ZnGeP2-crystals in the vapor of volatile constituents were carried out. Conductivity and Hall effect measurements were performed to characterize the modification of electrical properties, caused by the interaction of the crystal with the gas phase during annealing. Literature data and the results of the present work are discussed based on the results of a quasi-chemical analysis. This yielded that the results of annealing depends essentially on both the conditions of the experiment and the initial imperfection of the crystal. The most probable native structural defects becoming apparent under the annealing were the following: for CdSnAs2 - SnCd, VAs; for CdGeAs2 - VAs, VCd, CdGe, GeCd; for CdSiAs2 - SiAs, VAs ; for CdSiP2 - VCd, VP; for ZnGeP2 - ZnGe, GeZn, VZn, VP; and for ZnSnP2- ZnSn, SnZn, VZn, VP.
Annealing effect on the nonradiative carrier recombination in AlGaAs/GaAs investigated by a piezoelectric photothermal spectroscopy
Authors: Fukuyama Atsuhiko , Nagatomo Hiroaki , Akashi Yoshito , Ikari Tetsuo
Published: April 2011

Electron non-radiative recombination process of photoexcited carriers in as-grown and annealed n-Al0.2Ga0.8As/GaAs hetero-structure samples are investigated by using a piezoelectric photothermal spectroscopy (PPTS). The PPT signal above the band-gap energy of GaAs substrate decreased when the sample was annealed at 815°C. In the frequency dependent measurements, the deviations from 1/f linear function are clearly observed in the AlGaAs/GaAs samples. This critical deviation frequency was found to shift to the lower frequency region by annealing. Our experimental results are explained by assuming that the sample annealing generates an unknown deep level in AlGaAs epitaxial layer region and this level effectively traps the photoexcited carriers non-radiatively.
On the scaling of exciton and impurity binding energies and the virial theorem in semiconductor quantum wells and quantum-well wires
Authors: Leyva-Dios M. de , Oliveira L. E.
Published: April 2011

We have used the variational and fractional-dimensional space approaches in a study of the virial theorem value and scaling of the shallow-donor binding energies versus donor Bohr radiusin GaAs-(Ga,Al)As semiconductor quantum wells and quantum-well wires. A comparison is made with previous results with respect to exciton states. In the case the donor ground-state wave function may be approximated by a D-dimensional hydrogenic wave function, the virial theorem value equals 2 and the scaling rule for the donor binding energy versus quantum-sized Bohr radius is hyperbolic, both for quantum wells and wires. In contrast, calculations within the variational scheme show that the scaling of the donor binding energies with quantum-sized Bohr radius is in general nonhyperbolic and that the virial theorem value is nonconstant.
Luminescence of quasi-2DEG in heterostructures based on PbS films
Authors: Khlyap G.
Published: April 2011

The paper deals with the problem of luminescence due to non-equilibrium charge carriers transfer between the quasi-2D electron system localized in the space-charge region of the heterostructures based on lead sulfide films (up to 3 μm thickness) and zinc selenide substrates surrounded by the wide gap semiconductor region. The processes of electro- and photoluminescence are studied, the band diagram is proposed and the main parameters of the structure PbS/quasi-2DEG/ZnSe are calculated.
Optical Characterization of Iv-VI Mid-Infrared Vcsel
Authors: Zhao F. , Wu H. , Zheng T. , McCann P. J. , Majumdar A. , Jayasinghe Lalith , Shi Z.
Published: April 2011

PbSe/PbSrSe multiple-quantum-well (MQW) structures and PbSrSe thin films were grown on BaF2 (111) substrates by molecular beam epitaxy (MBE) and characterized by Fourier transform infrared (FTIR) spectrometer. Strong photoluminescence without Fabry-Perot interference fringes was observed even at room temperature from the MQW structures. The peak energies for the MQW structures with different well widths shifted to high energy with increasing temperature. The absorption edge of PbSrSe layer was determined by transmission spectra. Meanwhile, we designed and fabricated λ=4.1 μm MQW vertical cavity surface emitting laser (VCSEL). A power output of 40 mW was obtained at room temperature. The room temperature threshold pump density is 200 kW/cm2.
Sb-Based Mid-Infrared Diode Lasers
Authors: Mermelstein C. , Rattunde M. , Schmitz J. , Simanowski S. , Kiefer R. , Walther M. , Wagner J.
Published: April 2011

In this paper we review recent progress achieved in our development of type-I GaInAsSb/AlGaAsSb quantum-well (QW) lasers with emission wavelength in the 1.74–2.34 μm range. Triple-QW (3-QW) and single-QW (SQW) diode lasers having broadened waveguide design emitting around 2.26 μm have been studied in particular. Comparing the two designs we have find that the threshold current density at infinite cavity length as well as the transparency current density scale with the number of QWs. Maximum cw operating temperature exceeding 50°C and 90°C has been obtained for ridge waveguide lasers emitting above and below 2 μm, respectively. Ridge waveguide diode lasers emitting at 1.94 μm exhibited internal quantum efficiencies in excess of 77%, internal losses of 6 cm−1, and threshold current density at infinite cavity length as low as 121 A/cm2 reflecting the superior quality of our diode lasers, all values recorded at 280 K. A high characteristic temperature TOof 179 K for the threshold current along with a value of T1 = 433 K for the characteristic temperature of the external efficiency have been attained for the 240–280 K temperature interval. Room temperature cw output powers exceeding 1.7 W have been demonstrated for broad area single element devices with highreflection/ antireflection coated mirror facets, mounted epi-side down. The latter result is a proof for the high power capabilities of these GaSb-based mid-ir diode lasers.
Field Effect Controlled Photoresistors Based on Chemically Deposited PbS Films
Authors: Pentia Eugenia , Pintilie Lucian , Matei Ion , Pintilie Ioana
Published: April 2011

MOS-like structures were obtained by chemical deposition of a polycrystalline PbS thin film on top of a silicon dioxide/Si substrate. Gold ohmic electrodes in coplanar configuration were subsequently deposited by vacuum evaporation on PbS surface (drain and source electrodes). The gate aluminum electrode was deposited on the back of the Si substrate. The dependence of the photoconductive signal, generated in the PbS film, on the gate voltage was studied for wavelengths ranging between 800 nm and 3000 nm at room temperature as well as at low temperatures. It was found that the relative variation of the signal could be as high as 50 % for gate voltages ranging between −30 V and +30 V. Two possible mechanisms are proposed to explain the signal variation with the gate voltage: 1) Variation of the depleted region's thickness in the PbS film, that leads to a variation of the conduction channel's resistance (the reference resistance called, also, the dark resistance), 2) The possible variation of the majority carriers (holes) life-time due to the electron blocking at the PbS/oxide interface when positive gate voltages are applied on the back electrode. Integrated IR detectors with controlled sensitivity in the 800–3000 nm range can be manufactured at a relatively low cost using the PbS/oxide/Si MOS-like structure.
Photo-stimulated Rebuilding of Structure in Semiconductors
Authors: Rashidova S. S. , Oksengendler B. L. , Turaeva N. N. , Aripov I. M.
Published: April 2011

A strong electron-phonon interaction observed in semiconductors allows to realize a special kind of atomic rebuilding under photo-irradiation of the medium. We consider three types of defective structures: one atomic, two-atomic (“dump-bell”) and multi-atomic (kinks) defects. It is shown that an inversion effect of the defect potential term may be realized if there are non-equivalent neighboring positions of one-atomic and two-atomic defects. In this case the photo-excitation of electronic subsystems of these defects leads to an inversion of potential terms and a consecutive athermal rebuilding of the defective structures. The Yahn-Teller effect and psevdo-effect play a special role here. It is shown for kink-structures that the photo-excitation of the electronic subsystems results in the alteration of kink nonius and the successive exponentially strong decrease of an activation barrier. Especially effects of athermal atomic rebuilding of U-negative defects are considerable.
Effect of Zn atom diffusion in the active layer of InGaAlP visible-LED investigated by the Piezoelectric Photothermal Spectroscopy
Authors: Ohno Ryuji , Taiji Yoshihito , Sato Shoichro , Fukuyama Atsuhiko , Shigetomi Shigeru , Ikari Tetsuo
Published: April 2011

It has been reported that Zn atoms diffused from the Zn-doped p-InAlP cladding to the active layer in InGaAlP visible-light-emitting diodes cause a degradation of light output efficiency. A doping effect of the Zn atoms was then investigated using a Piezoelectric Photothermal Spectroscopy from a nonradiative transition point of view. The results indicate that the Zn-doping unexpectedly induces a decrease of the nonradiative component of the electron transitions above the band gap of the active layer. The experimental results are explained by considering that Zn doping cause the increase of both shallow and deep acceptor levels at the same time with the different rate for generation.
CVD Diamond Thin Films for Alpha Particle Detector Application
Authors: Wang S. G. , Zhang Q. , Yoon S. F. , Ahn J.
Published: April 2011

In this paper, CVD diamond thin films for alpha particle detector have been prepared using hot-filament chemical vapor deposition (HFCVD) technique. Alpha particle detectors with coplanar and sandwich electrode geometry have been fabricated on the diamond films using liftoff technology. The detector performance has been tested using a 241 Am source with a characteristic energy of 5.5 MeV. An average charge collection efficiency of 32 % for the detector with coplanar and of 21 % for the detector with sandwich geometry is obtained.
Improved Routes towards Solution Deposition of Indium Sulfide Thin Films for Photovoltaic Applications:
Authors: Govender Kuveshni , Smyth-Boyle David , O'Brien Paul
Published: April 2011

Conditions necessary for the reproducible deposition of pristine In2S3 thin films on TO-glass substrates by low temperature solution deposition have been identified. Baths containing carboxylic acids yield adherent, specular and crystalline films, within a defined pH range, dependent on the particular acid employed. Films have been characterised by XRD, SEM, XPS and electronic spectroscopy. As–deposited films were found to crystallise as tetragonal β-In2S3, no evidence for incorporation of hydroxy-indium species was found by XPS measurements.
Properties of 2D and 3D Dielectric Structures Fabricated by Electrochemical Dissolution of III-V Compounds
Authors: Tiginyanu I.M. , Langa S. , Christophersen M. , Carstensen J. , Sergentu V. , Foca E. , Rios O. , Föll H.
Published: April 2011

Porous layers and membranes representing 2D and 3D dielectric structures were fabricated on different III-V compounds (GaAs, InP, GaP) by electrochemical etching techniques. Nonlithographically fabricated ordered nanopore arrays in InP are reported for the first time. We show that the reflectance from nanostructured InP is lower than that from bulk InP in the spectral interval 1.5–2.2 eV. The artificial anisotropy induced by nanotexturization was studied in porous GaP membranes and the refractive indices for ordinary and extraordinary beams were evaluated.
Fabrication of Microstructures for Microphotonic Circuit
Authors: Chakraborty Subhasish , Hasko D G , Mears R J
Published: April 2011

We describe fabrication of sub-micron photonic bandgap structures on Si/SiO2 optical waveguide, which could be used at λ=1.54μm.
Current spreading in AlGaN:Mg cladding layers of laser structures
Authors: Figge Stephan , Böttcher Tim , Zellweger Christoph , Ilegems Marc , Hommel Detlef
Published: April 2011

The consequences of the anisotropic resistance in AlGaN/GaN strained layer superlattices for the operation of laser diodes were studied for structures driven in LED mode. A series of laser structures containing different Mg-doped AlGaN bulk and AlGaN/GaN strained layer superlattice cladding layers was compared to estimate the current spreading in the cladding due to the formation of 2D-hole gases. Current-voltage measurements revealed a significant spread of the current path in the superlattices, whereas no spreading was seen for the bulk cladding. In contrast to this, investigations of the electroluminescence showed no significant differences.
Electrical Characterization of GaN Metal Oxide Semiconductor Diodes Using MgO as the Gate Oxide
Authors: Kim J. , Gila B. P. , Mehandru R. , Johnson J.W. , Shin J. H. , Lee K.P. , Luo B. , Onstine A. , Abernathy C. R. , Pearton S.J. , Ren F.
Published: April 2011

GaN metal oxide semiconductor diodes were demonstrated utilizing MgO as the gate oxide. MgO was grown at 100°C on MOCVD grown n-GaN in a molecular beam epitaxy system using a Mg elemental source and an electron cyclotron resonance oxygen plasma. H3PO4 based wet-chemical etchant was used to remove MgO to expose the underlying n-GaN for ohmic metal deposition. Electron deposited Ti/Al/Pt/Au and Pt/Au were utilized as ohmic and gate metallization, respectively. An interface trap density of low-to-mid 1011 eV-1cm-2was obtained from temperature conductance-voltage measurements. Terman method was also used to estimate the interface trap density and a slight lower number was obtained as compared to the conductance method. Results from elevated temperature (up to 300°C) conductance measurements showed an interface state density roughly three times higher(6x1011 eV–1 cm-2 ) than at 25°C.
Proton Irradiation Effects on Scandium Oxide/Gallium Nitride MOS Diodes
Authors: Allums K. , Luo B. , Mehandru R. , Gila B. P. , Dwivedi R. , Fogarty T.N. , Wilkins R. , Abernathy C. R. , Ren F. , Pearton S. J.
Published: April 2011

The stability to proton radiation of GaN metal oxide semiconductor (MOS) diodes fabricated using the novel gate dielectric Sc2O3 was investigated. The MOS diodes were fabricated by depositing the dielectric with molecular beam epitaxy onto MOCVD-grown GaN on sapphire. The stability of GaN Schottky diodes was also investigated for comparison. Current-voltage (I-V) and capacitance-voltage (C-V), were employed to monitor any change in the electrical characteristics of the diodes. Preliminary testing indicates that the GaN-based diodes are in fact affected by proton irradiation, but only at fairly high doses. Doses equivalent to 10 years in low earth orbit, ~5x109cm-2, produce a decrease in the reverse breakdown field in both Schottky and MOS diodes. However, even after irradiation, the GaN MOS diodes showed twice the reverse breakdown voltage of non-irradiated Schottky diodes. Further, while the Schottky diodes showed reduced forward breakdown voltage, the MOS diodes showed no change in forward breakdown. These results suggest that the oxide/GaN interface is stable and is not being damaged by the radiation. The change in reverse breakdown is most likely due to generation of damage in the GaN resulting in the formation of shallow donors.
Tailorable Rectification: A study of Vertical Transport in AlGaN/GaN Heterostructures
Authors: Singh Madhusudan , Singh Jaspirt , Mishra Umesh
Published: April 2011

In this paper, we report calculations that show that a metal-polar semiconductor heterostructure can exhibit highly controllable non-linear current-voltage (I-V) characteristics. Change in barrier thickness can alter the characteristics from Schottky-like to ohmic in different bias regimes. The origin of these unusual effects is a large electric field (> 106 V/cm) and high sheet charge (∼1013 – 1014 cm-2) without doping, in the polar heterostructure. Theoretical calculation of the tunneling current density in these systems indicates that very interesting non-linear behaviour is shown by theseystems, ev en in the undoped case. Choice of suitable compositions of the materials and thicknesses can be used to tailor devices with desired characteristics.
Self-Heating Effects in High-Power AlGaN/GaN HFETs
Authors: Kuball M. , Uren M.J. , Hayes J.M. , Martin T. , Birbeck J.C.H. , Balmer R.S. , Hughes B.T.
Published: April 2011

We report on the non-invasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN heterostructure field effect transistors (HFETs). Micro-Raman spectroscopy was used to produce temperature maps with ≈1 μm spatial resolution and a temperature accuracy of better than 10°C. Significant self-heating effects in the source-drain opening of AlGaN/GaN HFETs were measured. Devices grown on sapphire and SiC substrates were compared. Three-dimensional finite-difference heat dissipation calculations were performed as function of device geometry.
A High-Power AlGaN/GaN Hetero Field-Effect Transistor
Authors: Yoshida Seikoh , Ishii Hirotatsu
Published: April 2011

An AlGaN/GaN hetero field effect transistor (HFET) was operated at 20 A. Its on-state resistance was lower than that of a Si-based FET. GaN and related materials were grown by gas-source molecular beam epitaxy (GSMBE). Sapphire substrates were used for GaN growth. An undoped GaN/Al0.2Ga0.8N heterostructure wasgrown on the substrate. The sheet carrier density of two dimensional electron gas was 1x1013 cm2 and the mobility was 1200 cm2/Vs at room temperature. The breakdown field of undoped high resistive GaN layer was about 2.0 MV/cm. Al0.2Ga0.8N/GaN HFET for a large current operation was fabricated. The gate width was 20 cm and thegate length was 2000 nm. The electrode material of the source and the drain was Al/Ti/Au and the Schottky electrodes were Pt/Au. The distance between the source and the drain was 6000 nm. The maximum breakdown voltage of gate and source was 600 V. The on-state resistance of the HFET was about 2 mohm•cm2 at 100 V. The transconductance (gm) of this HFET was about 120 mS/mm. It was confirmed that the HFET with a gate width of20 cm could be operated at the condition of a large current operation.
Back-Doping Design in AlGaN/GaN Heterostructure Field-Effect Transistors for High-Power Applications
Authors: Maeda Narihiko , Tsubaki Kotaro , Saitoh Tadashi , Kobayashi Naoki
Published: April 2011

A novel doping design has been proposed that yields high two-dimensional electron gas (2DEG) densities in the AlGaN/GaN heterostructure field-effect transistors (HFETs) even when the AlGaN barrier layers are designed to be very thin. In the novel doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. High 2DEG densities can thus be obtained. Moreover, relatively high 2DEG mobilities can be obtained for high 2DEG densities, because back-doped donor atoms are sufficiently remote from the position of the 2DEG so that the 2DEG is less subjected to the ionized impurity scattering due to the relevant donor atoms. By using this back-doping design, a very high 2DEG density of 2.8x1013 cm-2 (2DEG mobility is 850 cm2/Vs) has been obtained at 300 K in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is as thin as 120 Å. Thus, the back-doping design is effective to obtain high 2DEG densities in the HFETs with thin barrier layers, and promising for high-power applications.
High Quantum Efficiency AlGaN/GaN Solar-Blind Photodetectors Grown by Metalorganic Chemical Vapor Deposition
Authors: Chowdhury U. , Wong M. M. , Collins C. J. , Yang B. , Zhu T. G. , Beck A. L. , Campbell J. C. , Dupuis R. D.
Published: April 2011

We report the growth, fabrication and characterization of back-illuminated AlGaN p-i-n photodetectors operating in the solar-blind wavelength range. A peak external quantum efficiency (EQE) of 45.5% (0.1A/W) was obtained at a wavelength of 270 nm. This EQE was obtained at zero volt bias and without the use of any anti-reflection coating on the back of the substrate. To our knowledge, this is the highest EQE reported to date for p-i-n photodetectors operating in the solar-blind wavelength range.
Novel conguration of Mg-H complexes in GaN
Authors: Limpijumnong Sukit , Northrup John E. , Walle Chris G. Van de
Published: April 2011

We describe a novel conguration of the Mg-H complex in GaN, which is not the lowest-energy conguration at T =0 but is stabilized at elevated temperatures by the large entropy associated with a set of low-energy rotational excitations. We focus on a comparison with two types of experimental results: (1) vibrational spectroscopy using polarized light [B. Clerjaud et al., Phys. Rev. B 61, 8328 (2000)]; and (2) ion-channeling [W. Wampler et al., J. Appl. Phys. 90, 108 (2001)]. New results on the predicted quantum-mechanical delocalization of the hydrogen atom and on the ect of replacing hydrogen with deuterium are reported.
GaNxAs1-xGrowth by Molecular Beam Epitaxy with Dispersive Nitrogen
Authors: Wang S.Z. , Yoon S.F. , Ng T.K. , Loke W.K. , Fan W.J.
Published: April 2011

The effect of energetic nitrogen ion bombardment during growth may have a deleterious effect on the material quality. To avoid the bombardment effect of energetic nitrogen ions, a modified mode for GaAsN growth using dispersive nitrogen is reported. High quality GaAsN epilayers and good GaAsN/GaAs interface were achieved using this growth mode. The results suggest that the surface of samples grown using dispersive nitrogen has fewer defects than those grown using direct nitrogen beam. The optical property of GaAsN samples grown using the dispersive nitrogen technique was found to improve, due to the lower ion bombardment effect. This growth technique is expected to be advantageous for growing high quality GaAsN materials for optoelectronic applications.
Surface Instability and Associated Roughness of Pendeo-epitaxy GaN (0001) Films Grown via Metalorganic Vapor Phase Epitaxy
Authors: Roskowski Amy M. , Miraglia Peter Q. , Preble Edward A. , Einfeldt Sven , Davis Robert F.
Published: April 2011

A growth process route that results in thin film GaN templates with a smooth surface morphology at the optimum temperature of 1020°C has been developed. Atomic force microscopy (AFM) reveals hillocks on films grown above 1020°C. Hillocks resulted from the rotation of heterogeneous steps formed at pure screw or mixed dislocations which terminated on the (0001) surface. Growth of the latter feature was controlled kinetically by temperature through adatom diffusion. The 106 cm-2 density of the hillocks was reduced through growth on thick GaN templates and regions of pendeo-epitaxy (PE) overgrowth with lower pure screw or mixed dislocations. Smooth PE surfaces were obtained at temperatures that reduced the lateral to vertical growth rate but also retarded hillock growth that originated in the stripe regions. The (1120 ) PE sidewall surface was atomically smooth, with a root mean square roughness value of 0.17 nm which was the noise limited resolution of the AFM measurements.
White Beam Synchrotron X-ray Topography and X-ray Diffraction Measurements of Epitaxial Lateral Overgrowth of GaN
Authors: Chen W.M. , McNally P.J. , Jacobs K. , Tuomi T. , Danilewsky A.N. , Lowney D. , Kanatharana J. , Knuuttila L. , Riikonen J.
Published: April 2011

The Epitaxial Lateral Overgrowth (ELO) of GaN on Al2O3 using a SiO2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with White Beam Synchrotron X-ray Topography (WBSXT) and X-ray rocking curve analysis. The sapphire substrate was identified with a dislocation density of the order of ~106cm-2. WBSXT in both transmission and back reflection mode is used to image the ELO GaN and confirms that crystal planes in the lateral overgrown part (wing) are tilted, and that the wing tilt increases as the fill factor increases. X-ray rocking curve and WBSXT measurements confirm the same wing tilt tendency as the fill factor changes. The WBSXT method provides a measure of the maximum wing tilt, while the X-ray rocking curve method gives the average wing tilt. The average wing tilt reaches about 1602 arcsec at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2372 arcsec when the fill factor is only 0.571. This study shows that WBSXT is an effective method in dislocation and wing tilt determination for the GaN on Al2O3 ELO epilayer system or indeed for similar systems. The tilted wings induce a slightly lower compressive stress in the coalesced region of the GaN epilayer.
Removal of 6H-SiC substrate influence when evaluating GaN thin film properties via x-ray
Authors: Preble Edward A. , Miraglia Peter Q. , Roskowski Amy M. , Einfeldt Sven , Davis Robert F.
Published: April 2011

Non-uniformity in GaN thin films deposited on 6H-SiC can make determining the effects of growth variables difficult. Results presented in this work show the effects of the SiC substrates on the GaN films, and how to correct for these effects to obtain meaningful data about the properties of the thin film rather than the substrate underneath. Rocking curve values of GaN thin films are found to track almost 1:1 with the values of the underlying SiC. Plotting rocking curves with respect to the substrate, as well as a variable of importance can therefore yield more meaningful and reliable comparisons instead of plotting the data for the variable alone. This procedure is used to demonstrate the effects of thickness and AlN and AlGaN buffer layers on GaN thin films.
Influence of substrate orientation on photoluminescence in InGaN/GaN multiple quantum wells
Authors: Chen P. , Chua S.J. , Wang W.
Published: April 2011

In this study, the photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells (MQWs) grown on sapphire substrates with different orientation at 720°C were investigated at room temperature. Four different substrates were used, which have different surface step structures. Their orientations are: c-plane, a-plane, c-plane with off-set of 2 degree and c-plane with off-set of 6 degree towards a-plane. PL spectra from InGaN/GaN MQWs grown on c-plane substrates showed double-peak emission, while those from the MQWs grown on the others substrates showed single-peak emission. The strongest emission was found on the substrate with off-set of 2 degree towards a-plane. A Shift of PL peak position to long wavelength was observed on the substrates with an off-set angle. Meantime, all emissions were also investigated at different excitation power density. Experimental results indicate that the regular step structure on the substrates can seriously affect the growth of InGaN/GaN MQWs. Segregation effect in InGaN will lead to the high In composition regions (quantum-wire like structure) in the substrates with an off-set angle due to the regular steps on them, as observed by atomic force microscopy. It can be concluded that the surface steps on substrate play an important role in the formation of the In-rich InGaN quantum-wire like structure.